Nia Design Manual For Diversion Dams.pdf
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Nia Design Manual For Diversion Dams.pdf as PDF for free.
More details
- Words: 89,405
- Pages: 343
DESIGN MANUAl for DIVERSION DAMS
Copyright 1987 ,(Under Process) by the NATIONAL IRRIGATION ADMINISTRATION
No part of this manual may be reproducer] by any mec11anical, photograph 'c. or electronic process, or in the form of a phonographic or tape rec•>l'rl.n<>:, n · r '"'11'' i: be restored in a reh·'eval sy3tem. t;·a:osmit.ted or othenu'se copied fer n:·: i·· ''l' p1·i\·ate use, Y.'i'!:ont VvTi~trn p;: --:· :: 7 --, f:·-··a1 the N:-~ :~~:;:.:1. h··-~.r:-~1tf--n 1\d --~ ·.= ~">
RIEPUBUC OiF THE PHIUPPUNES NATiONAl IRRIGATION ADMINiSTRATiON
...~
:.. >..::..._c:: ..
u\ce 1nanual which ern!Jv•
current design c ,-. design practices and procedures will· Nat!onal Iniga~ion Adminisirati,, .. adopted from intemationally acc0, .. teria and modified to suit the Agenc:/:' . C
•
•
uf
.-
Edited by the DESIGN HES!':ArtCH SECT•GN DESIGN AND SPECIFICATION DroP:·. NATIONAL l!UUGATION MJ:,! :.
in consult:tti0:1 "'.-·1 ,_ WATEI: & 1'0'"' (' .
T 'f
MANUAL ON DIVERSION DAMS CONTENTS
Description
Para No. Chapter 1 -
Page No.
INTRODUCTION
1.0
Introduction
1
1.1
Scope
1
Chapter 2 - PLANNING AND INVESTIGATION
2.0 2.1
General
Extent of Studies
2 2
2.2
Stages of Investigations
2
Reconnaissance Stage Preliminary Investigation
3 3
Study of Available Maps Regional and Site Geology Study of Foundation Strata Study of Available Run off and Flood Flow Data Water Studies Availability of Construction Material Communication to the Site of Work Finalisation of Alternative Sites
3 3 3 3
Detailed Investigations
4
2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.5 2.2.2.6 2.2.2.7 2.2.2.8 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.3.4 2.2.3.5 2,2.3.6 2.2.3.7 2.2.3.8
Field Survey Hydrological Data Sediment Studies Surface and Sub-surface Investigation Diversion Requirements Construction Materials Communication System Other Miscellaneous Studies
4 4 4 4
4
5 5 6 7 7
8 8
Chapter 3 - ALIGNMENT AND LAYOUT
3.0
Function of Diversion Works
9
3.1
Location
9
3.1.1 3.1.2 3.1.3 3.2 3.2.1 to 3.2.6 3.2.7 3.3 3.4 -3.5 3.6
Location Versus River Stages Advantages Disadvantages
9 10 10
Layout
10
General Requirements Misc. Requirements
10 11
Alignment Layout of Training Wall
11 12
Guide Banks
12
River Diversion Scheme
13
iii
Para No.
Description
Page No.
Chapter 4 -TYPES OF DIVERSION WEIRS
4.0 4.1
15 15
Introduction Diversion Dams
4.Ll
Classification According to Crest Shape Vertical Drop Type Glacis Type Ogee Type
15 16 16 16
Vertical Drop versus Sloping Type Classification by Material
17 17
4.2
Barrages
17
4.3
Gated Weirs
17
4.1.1-1 4.1.1.2 4.1.1.3 4.1.2 4-1.3
Chapter 5 - DAMS ON PERVIOUS FOUNDATION
5.1 5.2 5.3
Introduction
21
Different Theories
Example
21 23 23 23 24
Lane's Weighted Creep Theory
26
Rationale Example
26 28
Potential Flow Theory
29 29
Methods Based on Creep Theory
5.3.1 5.3.2
Creep Theory Bligh Theory
5.3.2.1 5.3.3 5.3.3.1 5.3.3.2 5.4 5.4-2 5.5 5.6 5.7
Method of Solution Theory of Seepage Flow
5.7-1 5.7.2 5.7.3 5.7.4 5.7.5 5.8
Exit Gradient as Related to Dam Design
33 33
Khosla's Theory of Independent Variables Correction for Mutual Interference of Piles Correction for Floor Thickness Correction for the Slope of Floor Exit Gradient Example Illustrating Use of Khosla Curves
38 38 40 40 42 42
Structures on Permeable Foundation of Finite Depth
47
Chapter 6 - COMPONENTS OF DIVERSION WORKS
6.1 6.2 6.3 6.3.1 6.3.2
Layout of
Diver~ion
Works
48
Overflow Dam
48 48
Sluiceways
Functions Discharge Capacity
48 48
iv
Para No.
6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.5
Description
Training Wall
52
Functions and Elements of Training Wall Hydraulic Requirements Role of Upstream Training Wall Width of Pocket and Model Studies Length of Training Wall Top Level of Training Wall Conclusions
Intake Work
52 52 52 52 54 54 54 55 55 55 55 55 57
Location and Layout Type of Structure Crest Level and Waterway
57 57 57
General Necessity of Breast Wall
57 60 60
Fish ladder
6.5.1 6.5.2 6.5.3 6.5.4 6.6 6.6.1 6.6.2 6.6.3 6.6.3.1 6.6.3.2 6.7
Page No.
Necessity Location Investigations and Data Design Requirements
River Training Works
Chapter 7 - HYDRAULIC DESIGN
7.1 7.2
Design Data
7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.1.1 7.3.1.2 7.3.1.3 7.3.1.4
Field Data
61 61
Topographical Data Soil Foundation Data Hydrological Data Miscellaneous Data
61 61 62 62
Design Parameters - Terminology
62
Design Flood
63
General Status of Design Flood Determination of Design Flood Selection of Design Flood
63 63 63 66 66 66 68 68 68 70 70 70 72 75
7.3.2 7.3.3
Maximum Permissible Afflux Scour and Silt Factor
7.3.3.2 7.3.4 7.3.5 7.3.6 7.3.7 7.3.7.1 7.3.7.2 7.3.7.3 7.3.8
Extent of Scour Safe Exit Gradient Roughness Coefficient Retrogression of Levels Coefficient of Discharge
General Nappe·Shaped Crest Broad Crest Pond Level
77
v
Description
Para No.
Page No.
Hydraulic Design of Spillway and Sluiceway
77
Economic Design Waterway
Shape of Crests
77 80 82
Ogce Crest Profile
82
Crest and Floor Elevations of Spillway and Sluiceway
85
7.1.4.1
Spillway
85
7 .4.4 .2
Sluiceway
85
7 .4.5
Width of Under Sluiceway
7.4.6
Length and Level of Downstream F'loor
86 86
Energy Dissipators
86
General Requirements
87 87 87 89
7.4 7.4.1 7.4.2 7.4.3 7.4.3.1 7.4.4
7.5 7.5.1 7.5.2 7.5.3 7.5.4
Horizon tal Type Hydraulic Jump with Sloping Apron Design Criteria
7.5.4.1 7 .5.4.2 7 .5.4.3 7.5.5
7.5.6 7.5.7
7.5.8
Terminology Stilling Basin Parameters
89 89 89
Stilling Stilling Stilling Stilling
92 92 93 94
General
Basin Basin Basin Basin
Type Type Type Type
I II III IV
7.6
Total length of Floor
98
7.7
Depth of Cut-off
98
7.8
Floor Thickness
98
7.9
Protection Works
101
Upstream Block Protection
101 101
7.9.1 7.9.2 7.9.3 7.9.4
Downstream Block Protection
Design of Graded Filter Loose Stone Protection 7.9.4.1 7.9.4.2 7.9.4.3 7.9.4.4
7.9.5 7.10 7.10.1 7.10.2 7.10.3 7.11
103
103
General Hcquircmcnts
103
Velocity VS Size of Stone
103
Slope of Launched Apron Thickness and Length of Material
105
Protection Around Divide Wall
105
Free Board
105
105
Free Board in Abutment
105
Free Board in Stilling Basin
107
Free Board in Piers
107
Design Procedure in Brief Chapter 8 - STRUCTURAL DESIGN
107
A - Spillway Section
8.1
General
115 vi
Description
Para No.
Page No.
- - - - - - -----· ·--8.2
Requirement of Stability
8.3
Loads and Forces
115
Forces for Design Forces Inducing Stability and Instability Design Assumptions
8.3.1 8.3.2 8.3.3 8.4
Load Combinations
8.5
Design Criteria
8.5.1 8.5.2 8.5.3 8.5.4 8.5.4.1 8.5.4.2
Dead Load Live Load Pressure of Pond Water, Tail Water Earth and Silt Pressures General Criteria
8.5.5.1 8.5.5.2
General Effect of Earthquake
8.5.6.1 8.5.6.2
8.5.8.1 8.5.8.2
118
General Inertia Forces
120 120 123 123 123
Assumptions General Procedure Safety Criteria
8.6 8.6.1 8.6.2 8.6.3 8.6.4
Sliding Resistance Resistance Against Overturning Safety Against Foundation Failure Safety Against Failure of Material
8.7.1 8.7.2
123 123 126 127 128 128
Contraction Joint
8.7
118
118
Hydrodynamic Forces • H.caction of Foundation
8.5.7 8.5.8
117 117 117 117 117 117
118
Earthquake Forces
8.5.6
116
118 118
Uplift Pressure
8.5.5
115 115 116 116
128 128
Longitudinal Joint Transverse Joint B - Sluiceway
8.9.1 8.9.2
Cutoff Impervious Floor
129 129 129 129
8.9.2.1 8.9.2.2 8.9.3
General Requirements Design Cri tcria
129 130'
Abutments
130
General Top Width Design Criteria Safety Criteria Value of Friction
130 131 131 131 132
General
8.8 8.9
Design Criteria
8.9.3.1 8.9.3.2 8.9.3.3 8.9.3.4 8.9.3.5
vii
Para No.
Description
8.9.4 8.9.4.1 8.9.4.2 8.9.4.3 8.9.4.4 8.9.4.5 8.9.5 8.9.6 8.9.7 8.9.7.1 8.9.7.2 8.9.7.3 8.9.7.4 8.9.7.5 8.9.8 8.9.9 8.9.10 8.9.1::>.1 8.9.19.2 8.10 8.10.1 8.10.2 8.10.3 8.10.4 8.10.5
Page No.
Piers
132
General Thickness of Piers Length of Pier Height of Pier Design Criteria
132 132 132 132 133
Loading Combination Evaluation of Forces Effect of Earthquake on Earth Pressure
133 134 135
Active Pressure Without Surcharge Passive Pressure Without Surcharge Active Pressure Due to Uniform Surcharge Passive Pressure Due to Uniform Surcharge Effect of Earthquake in Sub soil Water
135 136 136 138 138
Hydrostatic Pressure Force Due to Water Current Longitudinal Forces in Road Bridge
139 139 139
Due to Braking Effect Miscellaneous Details
139 140 140
Longitudinal Joints Transverse Contraction Joints Weep Holes Backfill Material Backfill Drainage
140 140 140 140 141
Due to reactions
C - Divide Wall
8.11 8.11.1 8.11.2 8.12 8.12.1 8.12.2 8.12.3 8.12.4
General
142
Location Position, Length and Height
142 142
Design Criteria
142
Load Combination Evaluation of Loads and Forces Wave Pressure Foundation Design
142 142 144 144
Chapter 9 - INTAKE WORK
9.1
General
9.2
Layout
9.3
T ypas of Structure
9.4
Hydraulic Oasign
9.4.1 9.4.2
145 145 145 147
Pond Level Water way and Sill Level
147 147
viii
Para No.
Description
Page No.
Open Type Structure
147
Water Way Width and Shape of Sill Shape of Approaches and Other Component Parts
148 149
Upstream Transition Downstream Transition
150 150
9.5.4 9.5.4.1 9.5.4.2 9.5.4.3 9.5.4.4
Design Criteria from Surface, Flow Consideration Depth of Upstream Cut Off Stilling Basin Dimensions and Appurtenances Thickness of Floor VS Hydraulic Jump Downstream Protection
150 150 150 150 151
9.5.5
Design Criteria from Sub Surface Flow Consideration
151
Exit Gradient at the End
Length of Impervious Floor Floor Pressure
151 151 151
Structural Design Hoist Bridge Free Board
152 152 152
Barrel Type Structure
152
General Evaluation of Losses
152 152
Transition Losses Friction Losses
152 155 156
9.5 9.5.1 9.5.2 9.5.3 9.5.3.1 9.5.3.2
9.5.5.1 9.5.5.2 9.5.5.3 9.5.6 9.5.7
9.5.8 9.6
9.6.1 9.6.2 9.6.2.1 9.6.2.2 9.6.2.3 9.6.2.4 9.6.3 9.6.4
Bend Losses Loss in Change of Direction
150
156
Contraction and Expansion
Calculation of Waterway
159 159
9.7
Energy Dissipation
159
9.8
Trashracks
161
Chapter 10- SILT EXCLUDER
10.1
General
10.2
Location and Alignment
10.3
Design Criteria
10.3.1 10.3.2
Number and Size of Tunnels Spacing and Shape
10.3.3
10.3.4
Roof and Bed of Tunnels Exit and Exit Channel
10.3.5
Transition
164 164
164 164 166 166 166 166
10.4
Escapage Discharge and Minimum Head
10.5
Losses in Tunnels
10.6
Structural Design
10.7
Control Structures & Misc. Factors
10.7.1 10.7.2
166
167 167
Control Structure Miscellaneous Factors
168
168 168 ix
--------~·---·-·-----~------------~---~··--~~----------~---
Description
Para No.
168
10.8
Efficiency of Silt Excluder
10.9
Field Study Based on Model Experiments
Chapter 11
~RIVER
Page No.
--~~---~
--~--~------··-······-----------
168
TRAINING WORKS
11.1
General
173
11.2
Types of Training Works
173
11.3
Guidebanks
173
Classification Form in Plan Geometrical Shape Layout of Guidebanks Length of Guidcbanks
174 174 174
11.3.1 11.3.1.1 11.3.1.2 11.3.2 11.3.3
174 174
11.4
Approach Embankments
177
11.5
Afflux Embankments
177
11.6
Design of Guidebanks
177
Section
179
Top Width F'rec Board Side Slope & Protection
179
11.6.1 11.6.1.1 11.6.1.2 11.6.1.3
11.6.4.1 11.6.4.2 11.6.4.3 11.6.4.4
11.7.1 11.7.2 11.7 .3 11.7.4 11.7 .5 11.7.6
179 179 179
Thickness of Pitching Fillers Loose Stone Apron
11.6.2 11.6.3 11.6.4
11.7
179
181 181
Size of Stone Depth of Scour Thickness of Launched Apron Slope of Launched Apron
181 181
Design of Approach Embankments
182
181
182
Top Width Sedion Size of Stone Thickness of Pitching Filter Launching Apron
184 184 184 184
184
Design of Afflux Embankment
184
Top Width Slope and Protection VI orks
184
11.9
General Behaviour of Launching Aprons
184
11.10
Other Training Works
185
11.8 11.8.1 11.8.2
Chapter 12
~
184
INSTRUMENTATION
12.1
General
186
12.2
Objects of Instrumentation
186
X
Para No.
Page No.
Description
12.3
Instrumentation for Structures on Permeable Foundation
186
12.4
Uplift Pressure Pipes
186
Numbers and Location
187
Design of Pressure Pipes
187
Filter Points General Requirements
187 190
12.7
Installations
190
12.8
Precausions
190
12.9
Maintenance
190
12.10
Observations
191
General Record of Observation Time Lag Frequency of Observations
191 191 191 191
Presentation & Analysis of Data
192
12.5 12.5.1)
I 12.5.2) 12.6 12.6.1 12.6.2
12.10.1 12.10.2 12.10.3 12.10.4 12.11
Chapter 13- DAMS ON IMPERVIOUS FOUNDATIONS 13.1
General
194
13.2
Design Analysis
194
13.3
Safety Criteria
195
13.4
Internal or Uplift Pressu-re
195
Criteria for Design
195
Foundation
196
13.4.1 13.5 13.5.1 13.6 13.6.1 13.6.2 13.6.2.1 13.6.3 13.6.3.1 13.6.3.2 13.6.4
Structural Competency
196
Foundation Treatment
196
General Methods of Treatment
196 196
Surfn.ce Preparation
196
Procedure for Grouting
197
General Type of Grouting Drainage
197 197 197
13.7
Waterway - Crest Length
13.8
Stilling Basin - Floor Thickness
13.9
Protection Works
197 198 198
Chapter 14 - OPERATION & MAINTENANCE 14.1
General
199 XI
:,
·>--..,"'!\!"'!\!2=.--!!lif=:tm-r.;,..tll!@li:·Ol'\im"o"l®rn·=m=M=i::!limJR~~~~.gq:z:JJJJL _!£#!'&, CW
. mxt®&liiiliiiiliii£&9
o
Description
Para No.
Hydro - Mechanical Installation
14.2
Operation of Gates Maintenance
14.2.1 14.2.2 14.2.2.1 14.2.2.2 14.2.2.3 14.2.2.4 14.2.2.5
14.3 14.3.1 14.3.2 14.3.3 14.3.4 14.3.5 14.3.6 14.3.6.1 14.3.6.2 14.3.6.3 14.3.6.4 14.3.6.5 14.3.6.6
14.3.7
14.4.2.1
14.1.2.2 14.4.2.3
199 199 200
Gate Groove and Seals Steel Wire Ropes Winches and Hoists Lighting and Other Arrangement
200 200 200 201 201
Civil Work
201
Aprons Impervious Floors Sediment Excluder Intake Work Instrumentation Data Other Observations Pressure Release Pipes Standing Wave Profile Sediment Observation, Aggradation of River Bed Retrogression Settlement
202 202 202 202 202 203 203 203 203 203 203 204
Hiver Training Works
204
Regulation and Operation
204
General Requirements Operation - Periods
204 204
Pre Monsoon Operation Monsoon Operation Post Monsoon Operation
204 204
History of Diversion Works
205
Gates
14.4 14.4.1 14.4.2
Page No.
14.5
Chapter 15 -
205
DESIGN OF DIVERSION DAM
15.1
General
15.2
Illustrative Example
206 206
Mag-Asawang Tuhig Diversion Dam (Location) Geology Afflux & Crest Levels
206 206 206
Hydraulic Design
207
Design Data Determination of Waterway, afflux elevation, d/s basin elevation & dis floor length under different flow conditions
207
15.2.1 15.2.2 15.2.3 15.3 15.3.1
15.3.2
A.
Water way L Looseness factor 2. Width of sluiceway & ogee portion 3. Length - ogee portion
xii
207 207 207 213 214
Para No.
Description
B.
Determination of afflux Elevation, Downstream Basin Elevation and Downstream Floor Length I First Condition - High Flood condition, spillway not silted up
Length and Level of basin floor
II Second Condition - High Flood with dam silted up to crest level
III Third Condition - Medium flood Condition Water level on upstream is just 0.30 m. (one foot) above the ogee crest
IV Fourth Condition - Low flood condition Water flowing in the sluice way is just 1.00 m. above the sluiceway crest
E,
F.
:::;:p
244
Table 15.1 (Sluiceway Portion) Table 15.2 (Spillway Portion)
245
Depth of cut off
242
a. Spillway Portion i) Upstream cutoff ii) Downstream cutoff
242
b. Sluiceway Portion
246
242 246
I) Upstream cutoff li) Downstream cutoff
246 247
Length of work & exit gradient
247 247 248 249
Protection Works
xiii
j -
238
242 242
a. Spillway Portion 1. UiS block protection 2. UiS launching apron 3. DiS block protection 4. DiS launching apron
'Y(
230
241 241
Governing Values Sluiceway Portion Spillway Portion Crest Shape
a. Spill way Portion b. Sluiceway Portion G.
222
230 231
a. Spillway Portion b. Sluiceway Portion
D.
218
222 225
a. Spillway Portion b. Sluiceway Portion
15.3.3.4 15.3.3.5
214
218 220
a. Spillway Portion b. Sluiceway Portion
15.3.3
214
214 216
a. Spillway Portion b. Sluiceway Portion C.
Page No.
249 249 250 251 251
Description
Para No.
251
b. Sluiceway Portion
254 254 256 256 258
1. UIS block protection
2. UIS launching apron 3. DIS block protection 1. DIS launching apron H.
Training Wall
258 260
a. Hydraulic design b. Protection Works
15.4
261 261
Structural Design
I.
Pressure calculations a. Spillway portion i) U/S pile line ii) Intermediate pile line iii) Downstream pile line b. Sluicev·.ray portion
i) Upstream pile line ii) Downstream pile line
J.
266 267 268
1. Hydraulic gradient and jump profile
268 268
Table J -1 Table ,J- 2 Table J -· 3
268 271 272
273
3. Uplift due to hydraulic jump
268
Jump Profiles
274
Table J -4 Table J --5 Table J -6
275 276
274
277
1. Floor thickness
277
a. Spilhvay Portion b. Sluiceway Portion
L.
261 262 263 264 266
Floor thickness 2. Uplift pressure under static head Hydraulic gradient line elevations under different flow conditions
K.
Page No.
281
Stability Analysis
283
Load Combination A - Dam completed but no water in pond & no tail water
285
Load Combination C - Maximum flood with dam silted up to crest level
288
Load Combination E - Water level on upstream at crest level, normal dry weather tailwater, full uplift, silt deposit up to crest level with earthquake
290
Design of Guide Banks
295
a. Layout b. Protection Works
295 296 xiv
LIST OF FIGURES
Description
Figure
Page No.
Chapter 3- ALIGNMENT AND LAYOUT
3.1
Layout. of a Typical Diversion Works
14
Chapter 4 -TYPES OF DIVERSION WEIRS
4.1
V ert.ical Drop Diversion Dams
18
4.2
Sloping Glacis Diversion Dam
19
4.3
Ogec Diversion Dam
20
Chapter 5- DAMS ON PERVIOUS FOUNDATION
5.1} 5.2
Effect of Seepage
22
5.4) 5.5J
Bligh Method
25
5.6
The Flow Net
34
5.7
Standard Forms - Khosla's Method of Independent Variables
37
5.8
Mutual Interference of Piles -- Definition Sketch
37
5.9
Percentage Pressure Curves
39
5.10
Khosla's Safe
5.11
Design Example
5.3
l~xit
Gradient Curve
41 43
Chapter 6- COMPONENTS OF DIVERSION WORKS
6.1
Components of Dam
49
6.2
Effect of Pocket Width on Flow Conditions
53
6.3
Divide Wall
56
Chapter 7 - HYDRAULIC DESIGN
7.1
Discharge Coefficients for vertical faced ogee crest
73
7.2
I\atio of head on crest to design head
7.3
Coefficient of discharge for ogee-shaped crest with sloping upstream face
7.4
Coefficient with respect to position of downstream apron
73 74 76
7.5
Ratio of discharge coefficient due to apron effect
78
7.6
Ratio of discharge Coefficient due to tail water effect
79
7.7
Broad crested weir ~ relative coefficients
81
7.8
Ogee crest - downstream profile
83
7.9
Ogee crest -
84
7.10
Tail Water Conditions Classification
upstream profile
XV
88
Description
Figure
Page No.
90
7.11
Definitions and Notations
7.11A
Length of jump & ratio · of conjugate depth
7.12
Curve for determination of sequent depth
7.13 A&B
Froude number and dimension sketch for Basin 1.
7.14 A&B
Recommended length of Basin II Appurtenances for Basin II
7.15
Energy loss in Hydraulic jump
7.16
Uplift pressure on floor
7.17
Protection of Works
91
95 96 97 97
99 100 104
a) Launching Apron b) Upstream protection works c) Downstream protection works.
7.18
Maximum stone size in rip rap mixture
7.19
Step wise procedure
7.20
Head losses in pipe bends
106 108
(Structure Design) Chapter 8 - STRUCTURAL DESIGN 8.1
Map of Philippines showing Seismic Zones
119
8.2
Base and maximum pressure for constant sloping faces
121
8.3
Coefficients of pressure distribution for constant sloping faces
122
8.4
Water pressure and uplift pressures
124
8.5
Earth pressure due to earthquake
137
8.6
Distribution of ratio Dynamic increment Vertical effective pressure
137
8.7
Wind pressure and Wave diagram
143
Chapter 9- INTAKE WORK 146
9.1
Layout of Canal Head Regulator
9.2. A & B
Open Type Intake Works
153-154
9.3. A & B
Barrel-Tupe Intake Works
157-158
Chapter 10- SILT EXCLUDER
10.1
Silt Excluder - Plant & Section
165
A- 1
Narora Silt Excluder (Plan & Sections)
170
xvi
·---····---·-----·------·--·--·--------
Page No.
Description
Figure
-----··----·--·-------·
A- 2
Narora Silt Excluder (a field study & its efficiency)
172
Chapter 11 --RIVER TRAINING WORKS 11.1
11.2 11.3
145
Guide Banks - F'orm in Plan
176
GcomeLrical Shape of Guide Banks
178
Typical Layout of River Tr<~.ining Works
11.4
Size of Pitching Vs. Velocity
11.5
Details of Guide Bank
180 183
Chapter 12- INSTRUMENTATION
12.1
Dam on Permeable Foundation
12.2
Location of Pressure Tapping Points
15.1
Location Plan
15.2
River bed profile of Hivcr "A"
15.3
River bed profile of River "B"
15.4
River bed profile of !bolo River
15.5
Cross·section @Dam axis (facing upstream)
15.6
Tailwater Rating Curve
15.7
Crest Profile
15.8
Energy of flow curves
15.9
Relation between the length & the height of jump Structural Design
15.10
Hydraulic jump and uplift pressures (Spillway)
15.11
Hydraulic jump and uplift pressures (sluiceway)
188 189
208 209 209 210
211
212 243 269 270
279 280
Chapter 16- DESIGN OF INTAKE WORK
298
Canal Section
L
1.1
1.2
298
For canal maintenance For land soaking
300
301
Design of Intake Structures
2. A.
B.
c.
301
Design Data Sill level of Intake \Vaterway
301
302
xvii
m:aw
Description
Figure
Page No.
····---···---~------------~
Determination of basin level & length
D.
Pond at crest. level \Vater at crest. level
a.
b. c.
During maximum flQ:Od
303 303 305 306
F.
Depth of cutoff Total length of floor
G.
Analysis for percentage of pressure
307 308 309
H.
Floor thickness
311
Notes
312
E.
LIST OF PLATES
Plate 1
Development Plan
Plate 2
General Plan
Plate 3
Section Sho\ving right and left sluiceway
Plate 4
Ogce sedion & appurtenant details
Plate 5
Right & left sluiceway showing training wall
Plate 6
Hight intake structure
Plate 7
Left intake structure
Plate 8
Position of Pressure Pipes in ogee
Plate 9
Pressure observation pipes - Left Sluiceway
Plate 10
Details of eonstruct.ion joint
APPENDIX
9A
1
~
!2A
The Gumbel method
114
Head Losses in Pipe Bends
16Cl
Hcgister of Uplift Pressure Pipe Observations
193
A
1
Introduction
169
A
2
Efficiency of Excluder
171
A
3
Discussion on Test Results & Conclusions
171
xviii
CHAPTER I
INTRODUCTION
• Water flowing in natural streams and rivers is the wealth of a nation. This wealth can be utilized only harn(~sscd and controlled. \Vith the development of agriculture through irrigation, rivers have served as the principal sources of water for the production of crops.
if rivers are properly
The Philippines has immense water resources, most of which still remain to be utilb;ed to beneficial use for purposes of irrigation and power generation. This can be achieved by constructing storage dams in the upper reaches of rivPrs and diversion dams in the lower reaches. By building a series of dams, basin wide control of the rivers in a val!ey ean be ::tchieved. Building of dams and diversion of river \Vater as an art is as old as the recorded history of mankind. In the early days of civilization, earth dams were constructed, later on rubble masonry was used for building dams. The first masonry dams were built in Spain in the sixteenth-seventeenth centuries. It was in the nineteenth eentury that stability of the section against overturning, slid~ng, etc. was considered. This led to the development of rational design of gravity dams. Though div(;rsion of river watc'r is an old art. the science of design and construction of diversion dams and weirs as economical and hydraulically efficient structures is of recent origin. The design practice has, however, had a rapid evolution with the development of Lacey's silt theory followed by Khosla's theory or design of vveirs on permeable foundation. With t.he rapid evolution in theory, analysis, design and techni(]UCS described in various books/reports the need for a single text. that will illustrate the art of design in a unified manner IS obviously acute. The present. manual for fulfilling the objective is an endeavour in that direction.
1.1
SCOPE
This manual deals with divPrsion dam on permeable and other types of foundation limiting the design flood to the order of 2500-3000 eumecs with the following dimcnsions:a) not more than 1() metres (50 ft.) high if on rock foundation such as hard rock mass of granite, sand stone, lime stonp, ete. b)
low overfal! dams on foundation consisting of pervious strata of sand, gravels, talus, etc. depending upon t.he bearing capacity of soil and the net head i.e., difference in upstream water level and down stream water level is not more than about 9 metres (30ft.)
In the manual, an allcmp\ has been made to bring out the concept of sub-soil flow through the permeable foundation and various theories relevant to the design of weirs (overflow dams) on permeable foundation. ThP drsign procedures based on t.he modern knowledge of hydraulic engineering especially in case of alluvial rivers and the widely accepted method of independent variables devis(~d by Dr. Khosla havp been discussed in the manual. The design standard thus evolved, lays down the criteria for planning, layout, and design of dam and appurten
1
CHAPTER 2
PLANNING AND INVESTIGATIONS
2.0
GENERAL
Diversion dams ar(~ eonstructed for diverting water from rivers for various uses like irrigat.ion, power generation. water supply, navigng-ineering-economic considerations.
2.1
EXTENT OF STUDIES
Ttw magnitude of the project and the obvious physical appearance of such surface conditions usually govern t.he extent of investigation to he made. As such no hard and fast rule for determining the extent of investigations which may he neeessary in each ease can be laid down. A dam that is to be constructed on a pervious or \VC:ak foundation lo creai.P a depth of water of 5 meters will require much more foundation investigation than a 15 meters high dam to be buill on solid and unfractured rock. For !.he height of diversion dams discussed in this manual, the size of the physical structure has little rebtion to the extent of the investigation necessary. The maximum justifiable investigation cost is however limited by !.he magnitude of the project. Th(~ ulLirnat.e objf'Cti\'P of a!\ investigations for diversion schemes is to enable engineers to locate. desig-n and tonstruct. the most cconomieal structures \vhich vvill meet the irrigation/power requirements for which the projeet is envisaged. The survey and investigations of dam sites will require an adequate knowledge of the requirements of oestgn, considerable experience and exercise of good engineering jurlgmcnL The more f.h<' d('sign(~r understands the types of structures and the project layout towards which end h(' is working, the more he will be :1b!c to do an intelligent and selective job.
2.2
INVESTIGATIONS
For clPLcrmining the most. desirable ~uld economic site for a darn, the investigations shall include surveys, topographic mapping, geologic studies, extent of areas to be submerged, possible location of protection dikes and subsurface explorations. Investigation if ('.arried to completion, is an expensive and time consuming phase of project develop. project is not economically or technically sound. Hence, an investigation should be planned and exeeutPd so t!lat the probable soundness of the project will be determined as (~arly and as inexpensively as possible. To accomplish this objective, the investigation may be divided into three stages: mcnt. Moreover, it may indicate that the
1. Reeonnaissance
2. Preliminary investigations
:J. Detailed investigations 2
2.2.1
Reconnaissance Stage
This stage is desigrwd primarily tn support a decision on whether to proceed with more detailed investigations on UH~ basis of rough data and short cut studies. Heeonnaissancc is carried out. to get information about. i.he feasibility of the proje<~t. preliminary to the choice of site; the type of structure suitable with respect Lo foundation, and lhc ava~ilahle materials for construction and the tentative estimates of alternative types of structure. This can bc~ done with the help of study of the basin using some maps, rainfall statistics. st.atenwnt. of discharges at existing works in the river ami statistical atlas of the tract hffcctcd, supplemented by a few inspection of t.hp relevant and important sites including similar works in adjacent localities. In the Philippines, topographic maps prepared by photogrammet.ric method with a scale of 1:50,000 m and 10 nwter cont.our interval can he obtained from Lhc office of the Board of Technical Surveys and Maps at. Tanduay, lVlanila.
It is important. Lo record all t.he data obtained in t.hc reconnaissance stage. Even if from economic (:onsirleration. the project is not considPred viable, changing economic conditions later may result in a reevaluation of Lhc projc<'L Lhus costly duplic:at.ion of the earlier investigation can be avoided. 2.2.2
Preliminary Investigations
These are undertaken in J~TPal('r dct.a_i] until a t.ent.ative sekcLion of site is made warranting a detailed investigation thereafter. Foundation and sub-surface explorations are extended and in general, programmes of work is suffi<'iently hro;H1cncd t.o coliPel information necessary for preliminary design. Preliminary a) Study of
inv(~stigat.ions available~
should in('lud(' tht' following:
maps,
b) Hegional and site geology, e) Stu·dy of foundation st.raLa.
d) Study of available run off and flood flow data.
e) Wat.er studies, f)
Availabilit-y of eonslruct.inn rnaterials, Jnd
g} i\c-:cessibili!.y of th(~ sit.e of work.
2.2.2.1
Study of Available Maps
Study of available> maps should be made to have a general ide;:~. of t.he topography. river bc~haviour, possible sit.cs and t.heir eat.rhment. arra. Earlier maps, if avail;:~.hle, should he consulted as tlwy would provide useful information regarding stability of river at. the site. 2.2.2.2
Regional and Site Geology
It should be studied wit.h particular reference to adverse geological indications su<:h as faults. fractured zones, shear zones solution cavities, c1.c. 2.2.2.3
Study of Foundation Strata
Data should be <'ollC'('t('d hy digging trial pil.s and drilling boreholes and studies of existing nearby deep eut.s and \>v"ells and rc~pnrLs of other projeet.s in the nearby zone. 2.2.2.4
,., ..
Study of Available Run-off and Flood Flow Data ',.
,,
... ,~'
3
2.2.2.5
Water Studies
It is necessary to assess the extent of water requirements for diversion during the various periods and the feeding level.
In that behalf two categories of preliminary water studies shall be carried out: i) l\cquircmcnt of water for irrigation and power, if any. ii) Availability of supplies
2.2.2.6
Availability of Construction Material
Preliminary investigation should include assessment of the requirements of construction materials and their availability in the vicinity of the sites under consideration.
2.2.2.7
Accessibility of the Site
\Vhik deciding on the choice of suitable sit.c, due consideration should be given for easy accessibility and economie transportation of matrrial to the site of \Vorlc
2.2.2.8 Pos~iblc sites for location of the diversion dam should be marked out on the basis of investigation carried out in a(':cordanee with the above requirement. It may he p_ossib!e to eliminate some of these sites on topographical and other considerations by site inspection. Further investigation should be carried out for the remaining sites hy sub-surface explorations. Gauge discharge observations should also be started, if not presently available. Thereafter, considering the merits and demerits of the different alternate sites, the sites can he graded in the order of their relative suitability.
2.2.3
Detailed Investigation
i\fLPr preliminary sdection of site·, these investigations shall be carried out in much greater detail \.vith a vi<~w of eolkcting data for the design of the main structure and its appurtenant works for the site chosen. These should include the following: a)
Field surveys,
b) Hydrological data, c) Sediment studies, d) Surface and suhsurfaec investigation,
e) Field and laboratory tests, f)
Diversion requirerncnts,
g) Construction materials, h)
Communication systems. and
i) Other misr.:ellancous studies.
2.2.3.1
Field Survey -~-
-;-'i'llli11 •;(j'
.' ' ,«.
-
-
The survey should be plotted to a suitable scale. It should show all the salient features like firm banks. rock outcrops, deep channcis. larg-e shoals and islands, deep pools and important land marks, etc. The length of the survey may depend upon Lhe nature of the stream. the size of the diversion dam and the purpose of diversion. If the river course on the upstream and down~tream of the site is straight. the length of survey can be shortened whereas in case of rncandcring rivers the length of survey may be increased so as to cover at least two fully developed meanders on the upstream of the dam axis and one meander length on the downs~ream or as may be required for detailed model studies.
If a permanent bench mark is not availab\P ncar the site of work, the same should be established and value ascertained by double levelling. It should be as close to the work as possible but not so close as to be eroded by the river or stand as an obstruc:-Uon in the execution of the work.
it~
ii) For model studies it is desirable to observe the cross sections at closer intervals. say 100 m, up to 2 km upstream and 1 krn downstream of the proposed dam. In the rcrnaining reach the cross-sections may be observed at an interval of 200m. The cross-levels in the river bed may be spaced 10 to ~30 m depending upon the topography of the river. The cross-sections should be extended on both banks up to about 2.5 m above the high flood level, if possible, otherwise to an extent such that proper layout of guide and afflux banks may be decided. In hilly streams cross-sections at closer intervals may be taken.
2.2.3.2
Hydrological Data
The aim of the collection of hydrological data is twofold:
a) for romputing the design flood, b) for assessing the available weekly and rnonthly run-off on more realistic basis. F'or these studies it is necessary to obtain r::tinfall and run-off data.
The pertinent hydrologieal dakt arc as follows:
a) Design flood ··- The following data should be collected for estimating the maximum anticipated flood: i} Hainfal! Daily rainfall recorded at different stations in the catchment area and data regarding storms with respect to sucrcssive positions of Lhe centre of the storm in the catchment should be collected. Storms causing peak discharges should be separated for unit hydrograph analysis;
ii) Flood hydrographs for isolated rain storms for working out unit hydrograph; iii) CatehmcnL characU'risties, such as shape, slope, orientation, drainage system and infiltration capacity for developing synthetic hydrograph, if adequate data are not available; iv) Peak flow data for the river for as many years as possible for frequency analysis; v) F'lood marks by local inquiry to estimate maximum flood by slope area methOd; vi} Daily river gauge should he observed and stage and discharge data at or near the site of the pro. posed \vork, with stages and observed dischar.ges during flood, for as many years as possible should be eollcclcd.
b) Run·off --- A gaugP disehargc sitP should be established at a suitable point in the vicinity of the darn site. The gauge discharge dat.a should he utilized to evolve a gauge-discharge curve for computing !.he discharges for lhl~ period for which river gauge data ara available. The run-off data thus obtained should lw utilized for estimating depPndabk yield. If the data available for the sitt' is inadequate, a correlation coul(l be established by utilizing the long term data available for a nearby site on the river.
2.2.3.3
Sediment Studies
If no data of sediment load carried by the river arc available, sediment observations should be started immediately with the gaugc~-disehargc· observations as soon as the project is contemplated. The quality
5
and quantity of sediment carried by the water, especially during flood season, is necessary for planning sediment excluding or preventing worl\s and to frame a suitable mode of regulation. The observations may be of two types; il Detailed ii) Representative In the detailed observations the samples may be taken at several cross-sections at every 30 em. depth, at an interval of one week. The representative samples should be taken daily and at 3 cross-sections, one at the centre and two one third width from the edges at 0.60 of the depth. In c:1se lhe sediment eharge brought by the river is excessive, due margin shall he given for sedimentation whiie fixing the pond capacity·, particularly in case where pondage is proposed to be provided to meet diurnal power fluctuations.
2.2.3.4
Surface and Sub-surface Investigation
a) General Bore holes should he drilled at specified intervals covering the dam area and appurtenant structures. The location of borings shall he correctly marked and numbered on the survey sheets. These borings .should be earried to hard rock level or to a depth of 15 to 25m below the deepest river bed level depending on the strata and the structure like piers. abutments, floor, etc. Trial pits mi1y be exeavated to determine the depth of the overburden and loose deposits, if hard rock level is at a .c;;hzdlow depth. In case of large depths of overburden comprising large size boulders where ordinary boring method may prove inadequate, geophysical method may be employed Lo locale the rock surface. b)
Alluvial Foundation
The following investigations should be carried out in alluvial foundation: i) Borings at suitable loc:at.ions should be made in lhc dam area and borclogs prepared showing details of various strata encountered. The spaeing of boreholes should be planned in a manner so as to cover the foundations of various structures. ii) For sandy foundation, dynamic and static penetration tests should be performed below each structure (abutments, piers, etc.) and in each bay to estimate bearing pressure, likely settlement and ncccssit_y of settlement joints. For boulder strab., plate bearing tests shall he required. iii) Soil classific.:ttion and determination o[ unit weight of soil, angle of internal friction of soil, void ratio and specific gravity up to foundation level should he done. iv) In case of clayey and silty foundations undisturbed sampling should be done and tests conducted for determination of unconfirmed comprcs_sivc strength and consolidation characteristics. v) Modulus of subgrade reaction in case of raft foundation should be determined. c) Impermeable Foundation
In casf' of rock found;1tion at shallow depth, drill holes should he carried out. to ascertain the depth of weathered zone, extent of joints and fissures and to determine the necessity or otherwise of grouting to avoid exccssivQ seepage losses. The depth of impervious layer should he loeated in the entire width of the dam and intake area. While the upstream sheet piles may be embedded into the impervious layer, adequate open area shall he provided below thP downstn~am shept pile depth to allow rcleas{' of pressure~ on Uw downstream side. Similarly. ?"·'J~f;>rd~nn<: •-:-r Ot!trrop:- of lmpervious ~trat.um, if any, which may be touching- t.he bottom of the floor ha~c toO\':
<"·x<:dv!o'.!-·r"i
.
_ ;:-·:-:'
:'"~·-
-~;:;'~ ... _.
.
__ , __, __ . . . ,_
·.-.,-,-~--~.i~,.·-·_t_;-; ...
-~t-.. 0 ,_!1(~
··--'~n
:->_~:u·r~:t.:!.n.
wl-.0.thc:·
the iinpervious boundery is horizonLLl, .-;\oping HfJ 1.ovvanJK UH· ciovvn,-;u·P:<m nr llfi:·n.t·c·nn<. i\n 1 tnperv·1 ~~H-" boundary sloping up on the do\vnstrcam will cause eonstriction of seep.:1g-c and should be a voided by shifting the location of the dam slightly upstream, taking help of model studies if necessary .
::1 .-~
<
:··I'-'-....
6
d) Water Table Observations of water table in the area adjacent to the location of the dam should also be carried out; The likelihood of higher levels in the pond leading to rise of water table with consequent waterlogging in the adjacent area should also he investigated for suitable measures. e) Field and Laboratory Tests i) Detailed foundation investigation comprising penetration tests in addition to the tests outlined in foregoing paragraph should be conducted in the dam and appurtenant structure arc:J. to mark out soft foundation area, if any, for special treatment or for examining the possibility of avoiding it by suitably changing the layout.
ii) Field permeability tests should be earried and thereby the dewatering requirement. If the carried out to reduce the excavation requirement should be studied in detail based on economic and foundation, etc.
2.2.3.5
out to assess the order of seepage losses from the pond latter works out to be to excessive, studies should be by resorting to a reinforced raft structure \Vhich aspect other considerations, like availability of material, type of
Diversion Requirements
Diversion requirements should be worked out in accordance with the needs of the project. Detalled water studies, under the following two categories should be carried out: 1) Hequirements of water for irrigation, and power if any. 2)
Availability of Supplies
Water Requirements
For irrigation, consideration should be given to climatic conditions, soil types, types of crops, crop distribution, irrigation efficiency and conveyance losses, etc. For details refer to the Manual on Canal & Canal Structures.
Availability of Supplies Data for proper assessment of supplies is sometimes not available. Where a systematic record is available, careful examination is needed before it can be used. The records should i ncl udc: a) Daily, monthly and yearly rainfalls at various stations observed by the concerned department. b) Gauge readings at v,1rious sites of rivers and its tributaries. c) Discharge observations at some selected sites.
Maximum flood discharge for the design of the structure may be obtained from the data collected by the method detailed in the chapter on Hydraulic Design. Determination of low water level is ncc:cssary to determine the maximum head acting on the diversion dam. It is difficult to determine the low water level in the absence of records. Observation for a few years before construction and local inquiries will help in getting an approximate idea. 2.2.3.6.
Construction Materials
Survey of the availability of construetion materials like sand, gravel and boulders, suitable earth, etc., their availability which is necessary for determining the type of construction in preparing comparative 7
estimates. Availability of hard stone may make masonry preferable to concrete. If limestone is available, its hydraulicity, strength and durability should be investigated. Laboratory and field tests should be carried out for determining the quality of aggregates and earth materials.
2.2.3.7 Communication System
Investigation should include dislocation and relocation of the existing facilities and additional facilities re(]uircd during construction and operation.
2.2.3.8
Other Miscellaneous Studies
a) Pond Survey i) The area submerged up to normal pond level or ·within the afflux embankment that shall be surveyed and all immovable properties corning within it should be recorded and valued before the works arc started to avoid disputes at. a later stage.
ii) The extent and charaetcr of relocation of existing communities, railroads and public highway on account of the raising of water level should be determined.
iii) Existing water rights and government regulations to be observed should be ascertained.
b) F'ish Pass These arc required for migratory fish and should be provided for in consultation with experts of the fisheries department. Necessary data, as may be advised, should be collected for assessing the need for provision of a fish pass. c) Log- Chute
To justify the necessity of a Jog cHute, statistics of logs, such as their numbers, si?;es and periods in which they arc handled, cost of handling and other relevant data should be collected. d) 1\ail/roadhridgc across the dam Sometimes a roadhridgc may be provided across a dam for providing communication facilities between two sides of the river. In such a case, data regarding the type of bridge, width of roadway, footpaths, class of loading, etc. should be collected (if railway bridge is also required, the data should be eolle<:ted in consultation 1,vith railway authorities).
8
CHAPTER 3
ALIGNMENT AND LAYOUT
3.0
FUNCTIONS OF DIVERSION WORKS
As the name implies, diversion works mainly serve to direct the required supply into the canal from t.he river. Permanent canals. taking off from rivers are provided with such permanent diversion structures. Their functions can be summarized broadly as follows:a) To raise the water level in the river to the required extent for diverting the supplies into the canal. b) To regulate intake of water into the can3.1 and control silt entry.
3.1 3.1.1
LOCATION
Location Versus River Stages
Diversion dap1s can be located i~1 any of the four stages of a river. In the Philippines the rivers which originate from mountains have generally the following stages of flow. il Rocky stage ~ In this stage the river stream is in the hills. The bed slope and velocities are high. The cross section of the stream is made up of rock or very large boulders. ii) Boulder stage/Sub-mountainous - As the river emerges from Lhc hills, its slow~ and V(dotily an~ reduced. The bed and sides are composed of boulder or gravel. The river cross section is usually W(:ll defined and confined between non submersible banks on either side which arc close to the main current of the river. These is strong sub-soil flow in the boulder region because of the high permeability of the material. iii) Trough stage ~ F'rom the boulder stage the river passes on to the alluvial plain created by itself. Its cross section is generally made of alluvial sand silt. The bed slope is small and the velocities are not high. During high floods the river spreads outs over a wide area as banl<:s higher than the high flood level are relatively far away from the main current of the river.
iv) Delta stage ·- As the river approaches the ocean, the country slope and velocity fall down so much that the water is unable to carry its sediment load. It drops down its sediment and divides into the channel on either side of the deposit resulting in the formation of a delta. For the construction of diversion works, the Rocky and Delta stages are unsuitable. The Rocky stage will present no difficulty so far as construction of diversion works are concerned but it would be prohi-
bitively costly to construct the channel to lead the water from the dam to its commanded area. In the delta stage, the area available is small and irrigation requirements are not significant. The choice would thus lie between the boulder stage and the trough stage and should he made on merits. The following are the advantages and disadvantages of a diversion dam in Boulder stage in comparison to that in an alluvial stage.
9
I
I J
#
3.1.2 Advantages i) The length of the darn is generally shorter in boulder reaches.
ii) Since the silt factor is high in boulder reaches, the requirements of cut off and proteetion works are reduced. iii) Since high banks arc available in boulder reaches, the cost of training works is reduced.
iv) Construction material like sand and aggregate arc locally available in boulder stage.
3.1.3
Disadvantages
i) In the boulder area there is strong sub soil flow in the river bed, which reappears on the surface in the trough stage. During periods of shor.L supply, there would be loss of considerable percentage of total river discharge. ii) In the canal. which has t.o run in subhill tract with sand and boulder formation to large depths. there would be heavy seepage losses.
iii) A large number of cross drainage works would normally be required on canals in subhill tracts.
iv} Surface protection is necessary against erosive action of rolling boulders in floods.
3.2.1.
General __.. A typieal diversion works eonsists of the following parts: 1. The main dam or t.hc weir divided into bays by piers - This provides the obstruction across the river required to raise its water level and divert the water into the canal.
2. Sllliceways -- ThcsP ar(' gate eon trolled opening in the weir \Vith crest-S at a low level. They arc located on the same side as the off.take canals. Their basic functions include the preservation of a -clear and defined river channel approaching the canal intake; scouring of silt deposited in front of canal intake and control of silt entry in the canal. :1. Canal Intak(~ \Vork -·· These arc gate-controlled openings normally aligned between 90° 'to) 120° wit.h respect. to the axis of thr weir. The intake work serves to regulate the supply of water and eont.rol the entry of silL in the canal. It. is a wall between l.hc weir and the sluiceways aligned at right angle to the weir axis and ext.end.c; a lit.t.le upstream of the intake and in the downstream up to end of loose protedion of the sluirrways. The training wall serves to separate the floor of the sluiceways which is at lov·n~r level than the weir proper; to isolate the pockets upstream of the intake to facilitate scouring operation; and, to prevent formations of cross currents to avoid their damaging cff<'els.
1. Training W;lll
1
Tlwsf~ are required to cheek the meandering tendency of the river and guide it to flow axially through the dam.
:5. Guide Banks
3.2.2 Location for a diversion work involves consideration of its main components. An ideal site should be one which satisfies t.hC' rpquiremcnts of all the components as discussed in the parag-raphs which follow. Each feat.un~ of the river shall be considered in detail upon arrival at the final sit.e. For irri_gation purposes, the divPrsion works shall be so planned that. full command may he obtained by a dam of reasonable height. Tlw combined cost. of construction of the diversion works and the canal starting from the dam to the part when~ irrigation <·ommcJl('_es should be as small as is consistent. with the efficiency of the Project. While deciding !.he proper location the points noted below also need careful consideration.
3.2.3 The riVf~r rea('h should. as far as possible. be straight so that velocities may he uniform and the sectional ~u-ea of t.hc st.rcarn fairly constant.. This will obviate oblique approach as well as non-uniform dis
10
tribut.ion of flow on to the dam. The hanks should preferably be high, well defined and incrodablc. If such a site is available, it may need very small or pradically no guide banks. In case of high banks, the locality will not he submerged during high floods and a considerable saving in the cost of flood protective embankments can be effected. In the case of a meandering river the dain should be located at the nodal point.
3.2.4 A slight curvature at th(~ site may be advantageous for the off-taking channel located on the down· stream end of the outer <:urvature since it will cause less sediment deposition in the canal. However, cross eurrents may be produced due to the curvature and may endanger the foundation. Moreover, if canals take-off from both the hanks, the (~anal taking off on the inner side of the curve will draw com paratively more s<~dimcnt.. Therefore, proper J·udgment should be exercised in deciding the dam location in a eurved reach of the river.
3.2.5 The sluiceways should be located in the deep channel in order to ensure adequate supply to the canal head at all times. When canals take-off from both sides, a site with deep channels on both banks and low water in the centre is the most suitable.
3.2.6 VVhere proposed, due eonsidt-rat.ion should be given in the layout lo all pos.c;iblc locations of the sediment ejector and to the availability of heads for effective operation of the escape channel {for details of silL ejector refer t.o the Manual on Canal & Canal Structures).
3.2.7 Miscellaneous requirements the final darn loc::ttion.
The following factors would also need consideration before deciding
i) No considerable damage due Lo inundation of public and private property and bcilitics upstream will result. after the dam is constructed.
ii) The value of neecssary land and right of way is cheap. iii) The site is accr~ssible to transportation.
iv) No water right.s guarantee is affe<:ted. v) No heLory or rninlng activity upst.rearn of the dam .site should be present as mill or mine tailing will )lollute t.hc good quality of irrigation \Vater.
3.3
ALIGNMENT
3.3.1
Alignment of main dam
The alignment of a divprst.ion dam should be such as to ensure normal and uniform flow through all the bays as far as possible. As discussed in th(" preceding paragraph, as a rule, the most suitable lor.at.ion for t.he diversion st.ruc:t.ure and intake works of a canal is on the straight reach of the river, the velocity being uniform, and the section::tl area of the stream fairly consbnt. There arc two ways of aligning a dam at a site with respect to the direction of the current. a)
Right angled to the conrse of the n·ver
A diversion dam :digrwd JWrpcndicu\ar lo lhP coursr of the river will have the minimum length and • -, • · .• :-. .. ~~·~{" <el...~,,! £"(!'~'~---~·-' " - ·' ~'-.!;r""''"> £l,_.,,, rn;..,[' f 1 ;<erl,7,...'7ifl1T Cal){!;('] tv
h)
Ohlique to t.h(' rnrrent
A skew alig-nrncnt should be avoided unless availability of foundations. The Tliv<~rsion dam may The adva1~tagP of the former is that with the fJll of shall he maintained along the face of Intake works of
othenvisc necessitated by site conditions such as be skCwcd towards the canal side or away from it. wat.f!r taking place in that direction, a deeper channel the canal.
In the lat.t.er case, i.e., skewed alvay from the canal side. the main current is deflected away from the canal which is a dPsirablr' feature \vhere boulders and heavy gravel arc carried by the river. c)
Cra?!ed aJiqmnen t
If the c:oursc· of the river is not. straight, the diversion structure ma.Y havP to he provided along a curved ali?:nmcnt.. In this ca:w :-tdequate training works upstream and downstream should he provided to lead the current to and from Lht! struct.urr smoothly. Tlw c:-tnal taking off from the innPr side will draw more si!L It. is. how(~V('r, ,l;('ncr·ally prokrab](' t.o adopt straight line alignnwnt. 1vhercvcr possible.
3.3.2
Alignment of Intake Works
The- inlakc work for t.lw ('.~tnal is usually aligned at an angle of 90° to 110° to the dam axis to minirnizp sediment. Pnt.ry into t.Jw earn] and avoid haddlow and form::ttion of st:-tgnant. zones in the pocket. The upstream abutment of t.hc- work should be skewed from the line at right.
3.4
LAYOUT OF TRAINING WALL
3.4.1 J\ dividP \vall (training wall} is constructed at right angle to the axi:.; of the d:-tm in front of the int.akP sJ.ru(·lurc. Linder adverse flow conditions, a divide wall may he required in the main darn portion, to help minimizP cross flow in Lh(' river scd.ion, thereby allowing entry of sill free water in !.hC' can.:d.
3.4.2
OLlwr principal functions of LhP training wall may be classified as follows: l. to scparat(• tlw s!uicew:t.Y po(~kd. floor from the ogee spillway floor.
2. to enabiP the sluir('way' to function as scouring sluice and to isolate LlH' r.omparat.ively still pocket upst.re;:un of the intake 1vork for deposition of sediment in thP pocket.
:J. to pr('vcnt formation of cross flow to facilitate entry of silt free water into the intake structure. It is tlf'('('SS:lry to continue Uw training wall on the downstream to ensure adequacy of tail water depth at. lo\v flows in the sluiceway hays for the formation of jump and to avoid cross flow in the close vicinity of the st.ructurP. The crossflow may result in seours. The training wall is generally extended to the onrl of the impPrvious floor. To dPLerminP t.lw position and length of upstream training wall for most. effcdivc functioning, model studies are advised t.o be carriNl out. F'or detailed description on the role and layout of divide 1vall, refer to Chapter No. G.
3.5
GUIDE BANKS
3.5.2 The layout of the guide banks should be such as to guide the flood smoothly through the diversion dam. The guide banks arc provided generally in pair symrncntrical in plan and may either be kept parallel or converged slightly towards the work and usually extend a little distance downstream from the abutments of the work.
3.5.3 In case of wide alluvial banks. the length and curvature of the head of guide banks should be kept such that the worst mPand0r loop is well away from either the canal embankment or the approach embankment. If the alluvial bank is close to the dam, the guide banks may be tied to it by providing suitable curvature. lf there arc outcrops of hard strata on the banks, it is desirable to tie the guide banks to such control points.
The most effective alignment, length and shape of guide banks should be decided by model studies. (For details refer to Chapter No. 11) Layout of a typical diversion dam is shown in Fig. 3.L
3.6
RIVER DIVERSION SCHEME -
While deciding the location and layout of the diversion structure due consideration should also be given to the river diversion and flood handling arrangement during the construction. At times the h_ydraulic requin~mcnt may havt' to he compromised to obtain a workable diversion scheme, i.e., sometimes the dam is construct.erl in a spill of the river and the river diverted to it by providing suitable river training work. It is necessary that such alternate scheme arc supported by adequate model studies.
13
~------------------------------------~
DAM PORTION
GUIDE BANK
LAYOUT OF A TYPICAL DIVERSION WORKS FIG. 3.1
i' ~------------------------------------------------------------~
CHAPTER 4
TYPES OF DIVERSION WEIRS
4.0
INTRODUCTION:
Diversion weirs may be divided into two classes: (1) Temporary (2) Permanent
Temporary structures sueh as brushdams are co~structcd every year after floods. At certain locations, such temporary structures are possible with due economy.
Permanent weirs arc those which arc designe_d and constructed to withstand the onslaught of floods. These can be further classified into: (1) Fixed-crested weirs
(2) Movable-crested weirs, which can be further divided into: a) Barrag-e b) Gated weirs
Fixed·crestcd weirs present a solid obstruction across a river over which crest flood water is allowed to pass. In this manual, these are hereafter referred to as diversion dams. Barrages arc gate conlrol!cd weirs in which the entire ponding up is effected by gates and no heading up is attempted by any solid obstruction. Gated weirs consist of diversion dams over which crests collapsible gates, drumgates or other types of gates are installed to raise the normal operating water surface. During flood conditions, the gates will be at collapsed or raised position, depending on their design, so that the flood water will pass unimpeded, thus preventing the "overtopping of the banks. A weir may also be cL:t~sificd as broad-crested or sharp-crested. It is said to be broad-crested when the head pr()ducing the flow does not reach a value of twice the crest width. \Vhen said limit is exceeded, the overflowing sheet of water termed the nappe becomes detached and the weir becomes essentially sharp crested.
4.1 4.1.1 .
\ .'
J:
t
j
l'i
hi
~I l~'1
l 1i /j
,.,.,.\
·w
l;:.<>'
DIVERSION DAMS Classification According to Crest Shape
Diversion dams may be categorized according to the shape of the crest. The types of profiles usually adopted are as follows:
(1) Vertical drop type or over-fall type
15
l- -~-=i. ,i~· =-~= = =~" " "'" " "'" " "'" " " " "'" '" " "'" " "'" '" "____________, _______.,..__,
{2) Sloping t_ype, furl.her divided into: (a) glacis
(b) O_!;Ct'
4.1.1.1
Vertical Drop Type
Flows may br fr<>e diseharging or !.hey may be supported along a narrow section of the cn'sf, \vhich has a vert.ic;_d or ne~u·\y vertical downstream faec. Oeeasionally. the crest is extended in the form of an overhanging lip t.o din~ct. small discharges away from the face of the overfalt section. In this type, the nappe is ventilated sufficiently to prevent a pulsating, fluctuating jet. The vertical drop type is not. adoptable for high drops on yielding foundation, because of the large impaet forees \vhich must. be absorbed by the apron at the point of the jet. Typical section of this type is shown in F'ig. 1.1.
4.1.1.2
Glacis Type
Glacis is Llw name givcn to the surface \vhich slopes downward from the crC'st to the downstr(~am apron. On :1 sloping glacis, it is only Lhe horizontal component of the velocity of hypercritical jet which takes part. in t.hP impact. the vertical component remains unaffected. However, even if the energy dissipation is less cffirient because of the vertical component of velocity remaining int.acl, the position of the jump is st.ahlP and predictable. Furthermore, knowing the range of the hydraulic jump for different discharge intensities, the floor within that range can be designed to withstand the uplift pressure in the jump trough. Typical section of a glacis type diversion dam is shown in Fig. 1.2.
4.1.1.3
Ogee Type
Tll(' cn'st is oge(~ nf S shaped in profile. The upper curve of the ogce ordinariiy is made to conform closely to t.lw profile of t.h(: !0\ver nappe of a ventilated sheet falling from a sharp-crested weir. The profi!P !wlow t-he upper (llrvc of t.hc o_t~ec is eontinuecl tangent along a slope to support the sheet. on the face of the ov0rf!ow. A reverse curve at the bottom of the slope turns the flow onto the apron of a stilling basin.
Flow over t.lw <:r~;st. is made to adhere t.o the fac(; of t.he profile by preventing access of air to Lhc underside of the slwe!.. For discharges at. designed head, t.he flow glides over the crest v. . ith no interference from the boundary surface and attains near rnaxirnum diseharge efficiency. The uppPr curve at tlw cTes!. may be made <'it.hcr broader or sharper than the nappe profile. A broader shape vvill supporl. t.hc shp(~L and posit.ivP hydrost:-tLie pressure will occur along the contact surface. The supported sheet thus ('rcat.cs a hackwat.cr effect and reduces the efficiency of discharge. F'or a sharper shape. the sheet. tcncl.s to pull away from the crest and to produce sub-at.mosplwric pressure along the contact surfaee. This negative prc~ssure effect inercases the effective head and thereby increases the discharge. This negative pressure, however, induces eavitation \vithin the surface. I-IencP, lw1'au~(' nf its high disd1;lrge efficiency, LIH' nappe·shaped profile is used for most overflow erest. Typic;d section is shown in Fig-..1.::3.
4.1.2
Vertical Drop Versus Sloping Type No definitt> rule can be laid down in the choiee of vertical drop and sloping types. If statical con
16
=====
siderations and material requirements alone were allowed to govern the choice, the vertical drop would likely be chosen. However, to determine the more suitable type, the motion of the water, river bed materials and suspended particles must also be taken into account. A particular advantage> of the vertical drop is that the energy of the overfall jet for small dis. charges is dissipated for most part on the apron due to its almost vertical impingement on the apron. For larger discharges and \vater cushions of the required depth necessary for energy dissipation, aprons long enough to function as dissipaters are impractical for economical reasons. Furthermore, if the river carries sediments, the space between the river bed and the crest gets completely filled with sqlimcnts during high discharges, and the dam becomes, in effect, a sloping type.
In a sloping apron dam, the sediments carried by water roll over the downstream faCe. At the end of the apron. the jet emerges \Vit.h high velocity corresponding to the fall. but with proper shaping of the apron it can c:1sily be passed over the river bed without causing any appreciable scour. I~xit vclodty in the ease of vcrtic.al drop is less than that of the sloping apron type. Hence, erosion of the downstream river bcrl is not rnuc.h in the vertical drop and lesser downstream bed protection is required.
4.1.3
Classification by material
The dams rn::ty also he ebssified basPd on the materials compnsmg the structure. The more com mon types of small dams arc the earthfi\1, roekfill, solid masonry and concrete gravity dams. Timber has also been used in the eonst.ruetion of darns but because of the amount of labor involved and the short life of the structure, timber dams have proven uneconomical.
4.2
BARRAGES
In case of barrages, the crest is kept at a low level and the ponding up of the river for diversion is accomplished primarily by means of gat.es. These gates can he raised clear off the high flood level and thus enable the high flood to be passed with a minimum of afflux. By suitable manipulation of the gates, the flow conditions above tlw barrage can he closely controlled and shoal formation or cross currents upstream of the work minimized. A barrage provides maximum control on the river.
4.3 GATED WEIRS
In th(~ case of gat cd weirs, LhP larger part of the ponding is carried out by the solid obstruction or main hody of the dam. Additional hPad is obtained by installing gates on the crest of the diversion dam. During floods. t.lwse gates are in collapsed or raised positions, depending on their design and the weir functions in a manner similar to the fixed-crested weir.
17
--~ ·-
H. F. L. EL. 180.06 C'
H.F.L. EL. 179.46
"""~ 9 ~·~lijfl;=-~~
DWARFWALL
NEW
PUDDLE
.17
12\ll
NOTES: 1. ALL DIMENSIONS ARE iN CENTIMETERS UNLESS OTHERWISE INDICATED.
VERTICAL
DROP DIVERSION DAMS
2. ALL ELEVATIONS ARE IN METERS-
F1g. 4. I
RL 66_46 RL 6.3
R.l. 65.76
2713
H OTE S: L)
ALL
DIMENSIONS
UNLESS 2.)
ALL
ARE IN CENTIMETERS
OTHERWISE
ELEVATIONS
INDICATED. ARE IN METERS.
Mo:t.. Pond R.L. 2 !1.51 m. I Top d <;ate
Normal Pond
~1210.~:-_____............---
!R.L.210.67
Apron
,v-"" SUty d~
------
SLOPING
\ R.L.194.~1 '.
-~--
Sheetpi!e
-Compacted S<3nd
~lfl.!S 9 .-~__.
R.C.Roft
GLACIS DIVERSION DAM
F
g. 4.2
420
195 [1051
800
I :
,.
I
_Lji_JS W.S. EL. 58.90
-
--···~-i.::;.--~,;;:.:::_--.•-.-
·CHUTE
BLOCK
lli
:r::
WEEP HOLE
~
\
';:J~------,-..::E_::L::_.c:5~2c..:.·_28:::_0_·-j__._._,._ _ _ ___.J[:··
I
'2:;
-EL. 52
~~~~~_;~~~----~----~~--~--~~~·E~L~.5~1~:6~~~-~:~;-~·,§···~~f~~[~~!~~r;~~~::~~:~;J PILE .80
1SHEET
L_:..El_c,'\9_~- --FILTER
_ _j
DRAINS--······
1730
325 0
- - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __j
I NOTEs:
OOEE
l.)
ALL D!MENS!OHS ARE IN C!:NT!METERS UNLESS 0THC:lhV1SE INDICATED.
2.)
ALL ELEVATIONS ARE IN
METERS.
DIVERSION DAM FIGURE 4. 3
-------------
CHAPTER 5
DAMS ON PERVIOUS FOUNDATION
5.1
INTRODUCTION
For Jack of favorable sites, diversion structures are often built on alluvial soil foundation. However, unlike dams constructed on rock which present relatively few difficult foundation problems, the design of dam on pervious foundations involves problems of erosion of the foundation material, settlement and seepage under the structure. In the case of pervious foundation, little resistance is offered by the soil and percolation may reach the downstream toe of the dam without any substantial Joss of head. In such a situation, the percolating water may carry the soil particles with it and thus undermine the structure. This undermining is supposed to result to what is known as "piping". In the 19th century some of the major works constructed were mainly designed on experience and intuition. Based on experiments which were carried out to identify the damages which had occurred to some such works constructed, the accuracy of Darcy Law [V ~ k ilfi], where V is the velocity of seepage water, k is a soil property called the transmission constant or coefficient of permeability and h is the head loss in the distance 1 was confirmed. The forces developed on the weir (dam) and its sub-structure by the water seeping through the foundation can cause two types of failures (Fig. 5.1).
i) Uplifting of floor The percolating water exerts an upward pressure on the work till it emerges at the downstream, and if the pressure is not counter-balanced by the weight of the concrete or masonry, the work will fail by rupture of part of its floor. The floor is thus to be designed to resist the uplift pressure. ii) Undermining of the subsoil Failure by undermining also known as "piping· failure" is caused by the percolating water, retaining sufficient force, to lift up soil grains at the emerging end. This leads to increased porosity of the soil and hence progressive intensification and undermining, and ultimately removal of soil under· neath.
5.2
DIFFERENT THEORIES
Several methods have been developed for the analysis and design of weirs on permeable foundation. These methods can be grouped under the following two heads: a) methods based on creep theory b) methods based on potential theory Both theories are based on the validity of Darcy's Law for flow of water under pervious foundations. Darcy's law can be stated as V = ki
I
21
(, I'
UPSTREAM WATER LEVEL- \
---=-;oc.--o"c:-;::_~.,..;:-~~~~~~-:;::0-----.
CREST , ---~~-L . _. H
~----
'
I
G.L,
G. L.
/
\
\
-.:?:
SHEET
\
PILE~"-/
"~
- --
/
/
I FIG. 5. I
--,.._-
EFFECT
OF
SEEPAGE CREST
CREST
SHEET PILE - PILES/-___:;,
l
I
C/~ {
l
---->-
J.- ~->- It
r=··-----·- b -
( a )
t(~?~~:: -
·-?-
l
(b )
FIG. 5. 2
c \ a l
cw ( b )
FIG. 5. 3
where: V k i
superficial velocity of seeping water, obtained as the ratio of the quan.tity of seeping water and the superficial area of seepage. ~
~
coefficient of permeability of soil hydraulic gradient
~
..h. 1
It is known that Darcy's Law is not valid for small velocities in clays and generally in gravels. But it is customary to assume the validity of this law for all types of homogenous pervious foundation. 5.3 METHODS BASED ON CREEP THEORY 5.3.1
Creep Theory
The creep theory is based on the assumption that concentrated percolation of water may take place along the line of contact between solid structure and the previous foundation material, known as the liJ.?.e yt~<;Eg.£1?. It is also possible such concentrated seepage can occur through layers of higher permeability which might help to develop a passage or vein between upstream and downstream sides of the work. The creep length must be sufficiently long, so that the foundation material cannot be carried away by the seeping water i.e. the structure cannot fail by piping. For this purpose, a minimum creep ratio (given by the length of creep divided by head causing seepage) is specified by experience. Furthermore, along the creep length, the ratio of loss of head is taken to be uniform. The above phenomenon was expressed in a quantitative form by Bligh. It was presumed that the percolating water creeps along the contact of the loose profile of the structure. The length of path thus traversed by water is called the creep length. This theory came to be known as Bligh's creep theory. Lane after analyzing a large number of clams and weirs both with failures and without failures brought out deficiences in Bligh's theory. He proposed a new theory on statistical basis which is known as Lane's Weighted Creep Theory. Investigations and further experiments carried out by Dr. A.N. Khosla on the existing diversion works led to the rational solution to the problem of sub-surface flow. These developments took place with special reference to weirs on permeable foundations but are applicable to all hydraulic structures on permeable foundations.
5.3.2 Bligh Theory According to Bligh, the percolating water could creep along the contact of the base profile of the river into the sub-soil, losing head en route, proportional to the length of its travel. In the case of percolation flow. Bligh assumes that instead of following the short cut as indicated in Fig. 5.2(a) by ABCDE, the percolation flow clings to the line of contact between solid work and permeable soil as shown by polygon ABF.JGDJD. Thus the length of creep is not 2a + 2d + b but is longer as 2a + 4d + b. The increase is therefore 2d. Bligh himself fixed a limit to the application of his assumption. He realised that in the case of sheet p~les driven at too close a clistanec from one another, this assumption became illogical. He stipulated that h1s _method holds good so long as the horizontal distance between the pile lines, b, was greater than twice their depth. On the other hand, for smaller values of b, the line of creep followed the path as shown in Fig. 5.2(bl.
23
sm' mmm_-
]dtWZ: Q
MZUUJP
G L
Z...Z.d
In Fig. 5.4, Length of travel would be: L
~
b 1 + d 1 + d 1 + b2 + d2 + d 2 + b:1
If H is the total head over the weir, the loss of head per unit length of creep would be:
This loss of head per unit length represented as the average hydraulic gradient (C) was termed as the percolation coefficient. He assigned a safe value of C for each of the different classes of soil. If the hydraulic gradient is within the safe values, it is assumed that there is no danger to the structure against piping. Safe hydraulic gradients for different soils recommended by Bligh are given in the. Table 5.1
Table 5.1 Recommended Safe Hydraulic Gradients S. No.
Type of Soil
Value of Hydraulic Gradient
1 2.
Light sand and mud
1118
F'ine micaceous sand
1115
3
Coarse grained sand
1112
4
Sand mixed with boulder & gravels
1/9 to 115
No discrimination was made by him for horizontal and vertical creeps in assessing their effectiveness against undermining or piping. Because of its simplicity, Bligh's theory found general acceptance.
5.3.2.1 1)
The example given below would illustrate his recommendations.
Uplift Pressure
Creep leng-th up to point C, 11 metres from the upstream face ~1 + 2 x 5 + 2 x 3 + 14 Residual uplift pressure at C
~
7(1 - 31/60)
~
31m.
3.38 m.
The thickness of Hoor at any point should be sufficient to resist the residual uplift pressure.
If H1 is the residual uplift pressure at any point then the thickness of floor at that point should be:
where
G = specific gravity of concrete
24
z::_i:w .li.,'f11!!l.
I
SHEET PILE---·~
I
d1
dz
SHEET PILE-
-- J,..
-I-
FIG. 5.4
NORMAL WATER LEVEL
~
0
SHEET ~-
M. ·-··-------··-··---- ... -·-14.00 --·-·-
t--··················· ·-····'1 o,,.,.. oc_o:..:"'.c:· .......
PILE -·-
_j_
·-··---~~~~--+
--···--·--+-·-· -·· --····· .......... ......
20
()()· "'~ ·-----·------~ FIG. 5. 5
BLiGH
METHOD
0
····--·--~
In the above case, thickness of floor shown at C
3.38 T24
3.38 G - 1
~
2.73·m.
assuming G for concrete = 2.24
2)
Hydraulic Gradient Total creep length for the water emerging at Bin Fig. 5.5. L
~
1 + 2 x 5 + 10 + 2 x 3 + 20 + 2 x 6 + 1
Head on the structure
~
~
60 m.
7.0 m.
Therefore: Hydraulic Gradient
7.0 60
1/8.57
According to the Bligh Theory, the structure would be safe on gravel and sand but not on coarse or fine sand.
5.3.3
Lane's Weighted Creep Theory
As for Bligh Theory, the subsoil water follows the line of creep i.e. the line of contact of the dam and cutoff wall of the foundation. According to Lane. water travels along the line of creep because the resistance to percolation along this line may be less on account of the difficulty of securing an intimate contact than it directly through the foundation materials. In practice, however, the contact between vertical or steeply sloping surface is more likely to be intimate than that along horizontal or slightly sloping surface, likewise the contact between earth and deep sheet piles is more likely to be intimate (~ompared to that of a concrete found;1Lion cast over a flat bedding. Thus in caleulating the length of creep, distinction should be made between the vertical and horizontal surfaces and greater weight should be attributed to the vertical surface than to the horizontal.
5.3.3.1
Rationale
Lane,* after a study of more than 200 existing dams. both failures and non-failures recommends using a weighted line of creep, in which horizontal contacts with the foundation and slopes flatter than 45', being less liable to have intimate contact are given only one third the value of steeper and vertical contacts. That is, his line of creep is the summation of all the flatter and horizontal lines of contact between head and tail water following along the contact surface of the base of the dam and the cutoffs. In equation, the weighted creep length (Lw) is given as Lw
~
Ul N + V
N
~
sum of all horizontal contact and all the sloping contact less than 45°
V
=
"
sum of all vertical contact plus the sloping, greater than 45°
•ASCE Transaction VoL 100. 1935.
26
To ensure safety against piping ''Lw"should not be less than CJ-L, where His the seepage pressure head i.e. the difference of water levels on the upstream and downstream, while C is an empirical coefficient dependent on the nature of the soiL Values of Cor safe weighted creep ratio for different kinds of soils are given in Table 5. 2.
Table 5.2 Lane's Creep Coefficient Material
C (Safe Weighted Creep Ratio)
Very fine sand or silt
8.5
}<'ine sand
7.0
Medium sand
6.0
Coarse sand
5.0
Fine gravel
4.0
Medium gravel
3.5
Coarse gravel ineluding cobbles
3.0
Boulders with some cobbles & gravel
2 ..1
Soft clay
3.0
Medium c:lay
2.0
Hard clay
1.8
Very hard clay or hardpan
1.6
This weighted-crec~p theory, as d(~vclopcrl by Lane. is the method being used by many designers as a means of designing lo\v (:oncrcte dams on pervious foundation to be snfe against uplift pressures and piping.
The following contlusions made hy Mr. Lane should, however, be observed in applying his weightedcreep theory; 1) to avoid "short path" condition. the distance between tlw bottom of two successive cutoffs should not normally be less than one-half the weighted creep distnnce between them. Should the distance
between llw bottoms be kss t.han onc·h::llf the weighted creep distance, then twice the distance between them should be used instead of the actual line of creep between them . .~) the v·.leightpd.c:rcep head ratio is t.hc weighted·crecp distance divided by the effective head. 3) revers(~ filter drains. weep holes an~l pipe drains arc aids in relieving seepage pressures under the dam and. if properly provided, Lhe rec-ommended snfe weighted creep head ratios may be reduced by as much as 10 per cent even without performing flow net analysis - but in no case shall the ratio be less t.ha n l.G. 4) c:trP rnnst be exercised to ensure that cutoffs are properly tied in at the ends so that the water will noL outflank them. ··'·-' --- ,,,,J '····
27
··"'""'''~_.,;.~~ ''·~
• •>., A.. n.l •··
'l:r:~"''' .. e
5.3.3.2
Illustrative example: Lane's Weighted Creep Theory (Refer to Fig. 5.5)
A. ·Determination of type of foundation material on which the structure shall be safe: 1/3 (10 + 20 + 1) + 2
Total length of Weighted Creep (Lwl
5 + 2
X
X
3 + 2
X
6 +
39.00 m 7.0 m.
Head of Water (HI
iJ.~,oo
m. 7.00 m.
Weighted Creep Ratio ICI
5.57
The structure is safe on coarse sand where C
=
5.0, based on Lane's recommended weighted cr
head ratio. According to USB!\ Criteria (Design of Small Dams), with the provision of inverted filter drains
weep holes, the above calculated weighted creep head ratio can be increased by 10 per cent; accordingly 5.57
Hcvised Weighted Creep Hatio ICl
X
1.10
6.13
The .:d.ruet.urf' is sah• even if to rest. on medium sand where C B.
=
6.0.
Check for "short pat.h" condition: Length of weighted creep from a to b 1/3 (10) + 1 + 2
X
5 + 3
17.33 m.
Fi2:o)2~
Distance between a and b
lo.zo m.
>
lio o)z17.33
2 C.
8.6G5
Determine uplift. head from A t.o B: 10/3 + I + 2
Len1_;th of creep up to point D
X
5 + 2
X
3
20.33 m. 7 (1 -
Uplift at point D
20 .3;3_ 39.00
14.00
--3·--t-
Length of erecp up to Point C
1 2x5 + 2x3
21.66 m.
7 II -
Uplift at point C
2L()§ I :19.00
3.11 \Vhllf' this LhPory was an improvement of the original Bligh Throry, it too suffers the limitatlo; an empirieai approach. As would be seen in F'ig. 5.3 (a) & {b) although the weighted creep ratio of both
28
section shown are identical with respect to Lane criterion, the design shown in (b) would have a high probability of failure due to piping in the vicinity of point C. 5.4
POTENTIAL FLOW THEORY
In the creep theory, it is assumed that loss of head is proportional to the length of creep. Modern and field experiments have established that this is not true. The rate of loss of head is found to be non-uniform. Therefore, the creep theory is not eonsidcrcd reliable for finding uplift pressure in permeable foundations. The potential theory on the other hand, yielded results conforming with the findings of the model and field experiments. The methods based on this theory are now widely used instead of the methods based on creep theory. In potential theory it is assumed that: i) The soil medium is fully saturated
iii Darcy's law is valid
iii) Soil particles and water arc incompressible iv) No consolidation and expansion of soil takes place v) The flow is two dimensional \.I The surface f!ow equation following the Darcy's Law V = IT.
dx, dy & dz can be compared mathematically to d2 o
,-Jx--2
2
+
d o ····dy2·
=
kdh dX' t h roug h a so1'I me d'Ium o f e I emcnt,
_d!0 _ which is the well known Laplace + dz 2 - 0
equation diseussed under seepage flow (para 5.5) 5.4.2
Method of solution:
The problem was solved for simple weir profiled by several investigators adopting analytical procedures. It was found that analytical so.lut.ions for practical weir profiles were either not possible in the present state of mathematical knowledge or too complicated Lobe workable. Hence the investigators turned to the following alternatives: i) Graphical iii Analogy Melhod iii) Model studies
iv) Numerical methods v) Approximate analytical method Tolal leng-th of floor (b)
(
~ ')2
m
piles ~ 11.50 m.
b'
distance hctwccn upstream
d
depth of pile> on which the effc~d. is to be determined
D
the dcpt.h of Lhc pile line, the influence of which has to he determined on the neighbouring pile of
t\vO
depth d
c
JJB
d ! ..J2 h
SOLUTION:
% PHESSURE AT KEY POINTS i) UPSTH.EAM PILE LINE:
d
~
118.00 - 113.00
~
5.00 m.;
29
5.00
rJ
1
f)
'
c
fi2ilii
.0%2
from fiR. 5.9 (upper curve)
Vln
19
viDl
100
vic
28
0c 1
- 100 -
·· 0n
0c
in fig. 5.11
V) D - 1 00 - l 9 - 81%
vic -
100 ..
zs -
0n
vlc -
9%
72%
~ correction for depth v'r
118.00 - 147.00 5
'~
---~------------
-
1.80% (+vel
INTE!tFE!tENCE OF 2ND PILE LINE D
•• d - 1H ··· 143.00 - <\.00 m.
h'
c-o
14.50 m.
4.00
4.00
I·
52
(;jc corrected _.., 72
-f-
1.80
1-
1.5-1
--o:
75)~4°/o
iii INTERMEDIATE PILI•; LINE: d
- 1-18.00 ···· 113.00 - 5.00 m.;
a:
-
~~
-· :JZ/5 -
HJAO
•;r1 Jll_ 1.~~:-~!2.. l)l - 1 ."> -~· ' b . !i2 - .Z\):3
(1 - -b' I - 0.707 b frorn fig. 5.9 (lower curve)
for (1 -
b' ·b)-
.707
0c - zg 01]) - 37 30
X
9
In Fig. 5.11 =
¢lE1
100 ·- 29
71%
100 - 37
63%
Vim
0El
ViE!
-·
0m
0c1
=
58°/o
71 - 63
...
~
8%
J48,00 - 147.00 5
correction for depth
1.60% ( -- ve) r/Jc1
~ 1/5
correction for depth
X
8
1.60% ( + ve) Interference of U/S pile line on 0E1 : -~- 1.54°/o as the two pile lines arc equal and placed at the same level. Interference of DIS pile on 0cl
]) -
117.00 - !39.00
8.00 m
d
147.00 - 113.00
4.00 m
-
8.00 + 4.00 ······· 5z:oo -
iii)
2.07% ( + ve)
ViE!
corrected
71 - UiO - 1.54 - 67.86%
0cl
corrccled - 58 + 1.60 + 2.07 + 61.67%
DOWNSTRF:AM PILE LINE d - 145 -· 139 - 6.00 m.
cl
b
6.00
11 c
"52.00 - . "
with d!b = .115 from fig. 5.9 -- lower curve 0E 2
32%
vlnz
23% (Curve for 0Dl
0~<~2 -
0nz -
9%
31
X
8
0E2
145.00 - 143.50 6
corrcetion for depth
----~-·····-··~~-
x 9- 2.25% (+ vc)
Intcrfcrcnee of intermediate Pile line:-
D "" 113.50 ·- 143.00 - .50 m. ci
-
lilEz
143.50 ·- 139.00 - 4.50
corrected - 32 + 2.25- .215 - 34.04% (-vc)
Based on above calculations, pressures at key points are: -
_[JL5_1'[L_L'
Ljl'[I~.
INTEJ]l>£JjJ2IA_1'!fLJ'!1'"E
Ll.N!I. .. (OD2
81.00%
67.86%
7:3.34%
63.00%
61.67%
23.00%
EXIT GJIAJ)JENT: Maximum \V.L. on upstream = ILL. 151.00 m. Maximum seepage hear! II!) - 151.00 -
H5.00 - 6.00 rn.
Depth of D.S. Cutoff (d) - 145.00 -· 1:19.00 - 6.00 m. Total Length of Floor (b) - 52.00 m b
d
0:
52.00 6.00
8.67
Hefcrring to the graph for Exit gradient, F'or o: - 8.67;
7r
1. .rx.
"'"' .15
GE - (Hfrll
G "
ir
1
j/\.=
C;;,l .1 5 Exit gradient
6.67 _,., 1 in G.G7
Tlw structure is safe for a foundation material of fine sand for which the safe exit gradient is 1/G to 117 as per t.ablc :5.G.
l
l 32
i' i
I *
Th~re arc no detailed design procedures based on the first four methods. However, such procedures are available based on approximate analytical methods. These methods can he grouped under the follo\ving two heads: i) Method of fragments
ii) Method of independent variables The method of frag-m('nt was devised by Pavlovaky and later modified by Chugacv. The method of independent variahlcs is due to Khosla. Br.forc Khosla's theory of independent variables and its application in the design of dams on permeable foundation is described, a brief representation of the theory of seepage flow is explained hereunder:
5.5
THEORY OF SEEPAGE FLOW
According t.o Laplace equation 'l''
-~~:YL. f1x~
0
j.
.
(5.1)
The above cqu::1.tion deals with three dimension::J.\ flow. In \VCirs and other similar struet.urcs where t.hc width of the river is eon::;iderah\e, the flow may he considered as hvo dimensional, as the flow al any cross section of the st.ruct.urp is not. apprcriab\y influ(~need by any cross flow from the sides. It h:"ts also lwen PX{Wrimenblly dc>monst.rat.ed that for a homogeneous soil which obeys Darcy's Law. lhc conditions of slca
0
{5.2)
The differential equation governs the distribution of the"flow potential" 0 = kh wheD.· k is the per· meability of the soil as defined by Darcy and h is the head, at any point within the soil. Graphically, equation (5.2) can be represented hy two sets of curves that intersect each other at right angles (F'ig. 5.6). i) Streamline
A set of streamlines indieate the path adopted by particles of water flowing through thr:.~ sub-soil. Every particle of water entering the subsoil would trace out its own path and will represent a streamline. The first streamline \Vould follov., the outline of the base of the work. other succeeding flowlines will gradually transition out to a semi ellipse if tlw pervious soil extends to a very large depth. In case there is ,1n impervious boundary al ;l certain depth. the last streamline will follow the impervious boundary and the intermediate streamlines \Vil\ present a smooth transition from the first streamline to the last. ii) Equipot.cn.Unl.l.in.cs
Every streamline has a head h 1 (F'ig. 5.6) if the downstream bed is considered as datum. As il crncrges into t.he atmosphere, the head is zero. The head h 1 is entirely lost through the seepage. At the inter· mediate stage, it has a residual head h. 0ince this applies to every streamline, it follows that there will be points in different. streamlines having the same value of residual head h. By joining such points, the curve thus obtained is c;d\cd an equipotential line.
5.6
EXIT GRADIENT AS RELATED TO DAM DESIGN
Dam (weir) failure from seepage flow can occur because of: a) Undermining of the subsoil b) Uplift due t.o pressure under the floor
33
-
-~--------
(7 ,rEOUIPOTENTIAL
LINES __j
v
h
w
y
z STREAM
LINESJ7 ;
I
EQUIPOTENTIAL LINES
,, ,, .
--------- ----
I THE
FLOW NET FIG. 5.6
As has been discussed earlier, in the case of flow through sandy soil, flow net is formed under the foundations of a dam or weir. The streamlines and equipotential lines thus formed intersect each other at right anglc.s as the particles of flowing water take the path of steepest pressure gradient which is normal to the e[juipotential lines
Net seepage force in the direction of flow pdA - lp + dpl dA dpdA -dpdA
The seepage force per unit volume
dAC!r-
-dp
Jr··
As the head or the pressure decreases in the direction of flow ~J? is negative and force is positive dl . ]I· cncc [ orcc I,,
.. dp actwg . . t I d. t. f fl ow. 111 .1c 1rec wn o
~:.:..-ar
=
If the soil at exit end is to remain stable and resist undermining, the downward force exerted by the submerged weight of the soil should be more than the upward force of seepage. Let G be the specific gravity of soil grains and n the porosity of soil, the submerged weight of a unit volume of soil
o
·"
w 11 - nl IG ·- 11
For critical condition 1(*1
-
w (1 -
Dividing hoth sides by
cih rll vvherc p
.
c,,
n) (G -
11
w
11 -- n) IG - 11
wh
represents the rate of ioss of head or the gradient at exist end
If critical gradient is reached, there would be failure by undermining. Assuming G for river sands is 2.G5 and the approximately average value of porosity is 0.4 Value of critical gradient. - 11 - 0.4) 12.65 .... 1) - 0.99 say LO
35
~-·
_, ····-·-·-·---------
The actual gradient at exit. must be much less depending upon the factors of safety. Thus. for the class of soil mentioned above, the flotation gradient (critical gradient) ~ Unity or 1/1. This flotation gradient \vill vary with the pore space of the sub-soil and the density of the soil partides. The density of the soil particles, though generally in the neighbourhood of 2.6 to 2.7 for most of the sands in Philippines, may vary in extreme cases from 1.8 to 2.8. Similarly the pore space {porosity) may range between 20 pereenL to 45 percent. \Vorking on these limits, value of Flotation Gradient may range from 0.14 to 1.44.
Table 5 Flotation Gradient 2.0 2.2
Pore Space/ Density
2.8
2.6
2.4
0.20
1.44
1.28
1.12
0.96
0.80
0.64
0.25
1.35
1.20
1.05
0.90
0.75
0.60
0.30
1.26
1.12
0.98
0.84
0.70
0.56
o.:Js
1.17
1.04
0.91
0.78
0.65
0.62
0.10
1.08
0.96
0.84
0.72
0.60
0.48
0.88
0.77
0.66
0.55
0.44
0.15
0.99
1.8
In any ordinary structure, hovvcver, the criticai value of 1.1 or nearly so is almost impossible to occur if other factors do nto intervene.
The other factors which create conditions suitable for the development of critical value of exit gradient are: i) Scour
extending down to the bottom of the toe wall
ii) Surges/waves occurring- in the soil with sudden application or reduction of head.
iii) SuddPn ponding upstream of the work by closing off flow downstream. In addition, some other uncertainties ~in nature such as non homogeniety of soil, intrusion of clay heels and zones of very porous material etc. may call for application of factor of safety to the critical value of the exit gradient.
Dr. Khosla has recommended the following factors of safety to critical values of exit gradient. This will not apply to days which arP more or less impervious.
Table 5.3 S.No. 1
Factor of Safety Type of Material ·-·--·--------··------------------shingle 4 to 5
2
coarse sand
5 to 6
1
fine sand
5 to 7
36
I ~
WS.
~~r H
. I
.,.-
-
W.S ~..----
H
!;I
- - - ' - - - - - E - , c __
- (
c1 b
I
d
l_
I
c:.c-~:-Sheet Pile
I
b
Sheet
dl
I
Pile-2::"--
D
D1 (i)
T
_j_
( i i)
FIG. 5. 7 .
W.S.
-r ----;_.- I H
r=--
b
-----------~
-1- - - - - - - - L - - , - - - - - -
c
E
;:::_::_Sheet Pile
(iii)
( iv )
STANDARD FORMS- KHOSLA'S METHOD OF INDEPENDENT VARIABLES
e c
D
)
/ li._:::___ Pile Line I
E C
E C 1
------···-·-- ______ b_s __ -·-·-·--------·
l _____
?--Pile Line 2
i'
! 2--Pile Line 3
ol
D
12' ------------------1
.... ----·-- --···-
b
----------
·--·
-----····----.
------------, FIG. 5.8
MUTUAL INTERFERENCE OF PILES -
DEFINITION
SKETCH
5.7
KHOSLA'S THEORY OF INDEPENDENT VARIABLE FOR UPLIFT AND EXIT GRADIENT
Khosl::t found the solution for simple cases such as a simple floor, a single sheet pile, Door with a single sheet pile etc. Practical weir profiles arc more complicated a·nd may have several piles. Khosla postulated that a floor with multiple piles can be considered to be made up of several simple weir profiles, consisting of only one pile and the full floor length. The effect of piles is essentially local. As long as the distance between adjacent piles is not small compared to the pile length, the effect due to one pile on the potentially close to the other pile is small and can be estimated. Thus, in Khosla's method a weir profile is broken down into simple units consisling of: a) floor and only one sheet pile at either end (Fip;. 5.7 & iii b) A straight horizontal floor depressed below the bed but with no vertical cut off. Fig 5.7 (iii) c) A str~ight horizontal floor of negligible thickness with a sheet pile at some intermediate thickness Fip;. 5.7 (iv) Each elementary form is tlwn treated as independent of others. The pressure at the key points are cal· eulat.ed with the usc of curves given in Fig. 0.9. These key points arc the junction points of the floor and the pile line of that particular elementary form, the bottom point of that pile line and the bottom curves in the case of a depressed floor. The percentage pressure observed from the curves for the simple forms with whid1 the profile is broken up have been demonstrated to hold for the assembled profile as a whole subject to certain corrections as given b~low:
il Mutuallnlcrfcrcnce of piles ii) Thickness of floor
iii) Slope of floor The potentials, as fraction of I-I
J
Cos
1
Cos
'h;;
71"
<J!c
7T
1
0n
71"
c-o
(head causing seepage), at salient point at exit are given by:
;)
---- ·;i--l
Cos-·
1
;, ;)
( ~_1
a
The solutions of these equations in the form of curves are given in Fig. 5.9.
5.7.1
Correction for Mutual Interference of Piles Tlw corrPct.ion of i\-1utua! lntt~r-fcrenec of f'iles is given by the following formula.
c "
~
Where C b'
19
cL1. .D... b
b ~ total floor lenp;th.
Correction to he applied as percentage of head. ~
ll·
Distance between two piles Depth of pile line, the influence of which has to be determined on neighbouring pile line of depth d.
d .., Depth of pi!(' on which the effect is to he determined.
38
PLATE VII· 6 OF C. B.\. PUBUCATIOH ~0.12 100
Shut pile not at end
Value
.!.... cos~ 1()..!-1)--Eqn 7.36
rr
0
of.!....=~
0.4
--,:--
"'
b o.s
..!.. cos· 1 ()...Jfi)--Eqn.7.37
rr
-,1
-:} CO:S-
\
~~
)--- Eqn 7.38
,,
90 To find
~E
0:::: for
. ba5CI rotto
subtract
20
for ony value~ of cc and bose~ rolio-b.read
,,
''I U- b
far that vo\ue of oc and 30
IOO.Thvs 0£ for~~ ~0.4 end cx;.::4;'
from
o..i:c
0C (or-b': 0.6 and oc~ 4= 100-29.1=70.9% To get QIO for values of
~
,, b
less than 0. 5
read CD for bose~ ratio 1-
,,
tn~btrocl
":§
and
•8
!ram 100. Thus, ¢0
foe b=Q.4ond cr..= 4~\oo.¢0 b•
60
0
'?
"'
for "f=0.60fld o::-4
=100-· 44.8 =55.'2.
ec,=
oo,=
74 100-¢0
100- ~D (Depro,~C~d) f'D': 00- - ( 0E-0Dl ~ (Eqn7 701 3 ~~~ +oc2 2
PD'=
'
0
"" i;'f
2
Deve~:)C(l.d floorl
'
50
50
u
"'0
40 ~ .i
•",
.
0
0
0
> 30
30
.,"
''
0
20
>
i 10
0.3 RATIO
PERCENTAGE
0.4 b'/b
PRESSURE
07
CURVES
FIG. 5,9
This correction i.s positive for points in the rear of the back water and negative for points forwar in the dire(·tion of flow. This equation is valid if the foi!owing conditions are satisfied. i) Tht~ Intermediate pile is longer or equal to the outer pile.
ii) The distance beL,vcen pi!cs is more than twice the depth of the outer pile. In Fig. 5.8 the dimPnsions have been marked as they apply to point e or pile line 1 owing to thP ii fluence of pile !inc 2. The dfcd of inv~rfercncc of a pile is to he determined only for the face of the adjaren pile t.owards the interfering pile; e.g. pile line 2 will interfere with the downstream face of pile line 1 an· upstream faec of pile line :3.
5.7.2
Correction for Floor Thickness
In Uw standard forms with vertical eutoffs, the thickness of the floor is assumed to he ncgligihl\ Thus as obscrv(~d from the r.11rvPs. the pressure at llw junction points E & C pertain to the level at t.h1 Lop of UH~ floor whereas t.Jw actual .iunetion is with the bottom of the floor. The pressures al actual points E g. C arC' int.prpolatcd by a::surning st.raight line variations from the hypothetical point E to I and also from D t.o C. It. is positive at. lovver elevations.
5.7.3
Correction for the Slope of Floor
A suit:thle percentage correction is to be applied for a sloping floor, the corrcetion being plus for lhe down and minus for the up slopes following the direction of flow. The values of corrections arc given in Table 5.5.
Table 5.5 Correction for Floor Slope
vert.ieal
Slope horizont.<1l
Corre(~tion
°/o of Pressure
in 1
11.2
in 2
6.5
in :l
1.5
in 1
3.3
1 n i)
2.8
in fi
z.;,
in 7
2.:3
in 8
----·-······-·-·-··-··
--------
2.0
Tlw coiTection is appiieahle to t lw key points of the pile line fixed at. Lhr beginning or the end of the slopt'. Thus. in fig. S.8 Uw slope correction is applieahle only to point E of pile line 2. The percentage eorrec.t.ion given by Llw abov(' table is Lo bP further multiplied hv the proportion of the horizontal lenvth of slope to LhP disL~tncp lwt.wP('ll th(' t\vo pile lines in between which the sloping floor is located. In fig. tlw cnTTP{'Lion to hP applied at E. for t.Jw pi!P linr> 2 will lw obtained by multiplying the appropriate figurP from Uw above' tabie by bs The corrcei.ion is minu::; for the up and plus for the down slo!WS in the dirt.'C b' Lion of flow.
5.8,
40
·war{
o the in :t(l ·,lCCnl
rH
_ ano
r;l=---ible: I the 25
:ll.dlal.
F; to D
_L
·H'
-- ~·"'-"=.fe-
EXI::"'DIE~~ r,±d=:====2-/r::l==::!~
•it,,, for JIJS
are 0.3.
a< - ? c(
...\
- , t ;;;-;;{' 2
++-H-
G•<.:. • d.,11
'
TT/T
£o•(H5) I
'
d ·f
5
0'
0
0
:age ·.yf ll
I
FIG. 5.10
I
. 5.7.4 ' Exit Gradient According to Dr. Khosla, undermining can start only with the particles at exit being carried away by seeping water. This can happen if the velocity of seeping water at exit end is high or in other words if the gradient ;:tt exit end is high. It has be(~n determined that for a standard form consisting of flow or length b, with a vertical cutoff of depth, d the exit gradient at its downstream end is given by the equation,
Jl
(5.3)
d
Where
~ ~
1 +
l_i o-:::'!2~
and
o:
2
~ ~ d"
F'rom the curves in Fig. 5.10, for any value of o:: or .!? , the corresponding value of --}~ can be read d -~~ off_ Knowing the values H and d, the value of GE is pasily, calculated. It is obvious from the equation (5.3) that if rl ~ 0, GE is infinite. It is therefore, essential that a vertical cutoff should be provided at the downstream end of the floor. To safeguard against piping, the exit gradient must not be allowed to exceed a c:ertain safe limit for different soils as given in Table 5.6. The uplift pressures must be kept as low as possihle consistent with the safety at the exit, so as to keep the floor thickness to the minimum.
Table 5.6 SAFE EXIT GRADIENT ~---····-------
SNo.
Type of Material
Safe Exit-Gradient
1
Shingle
1/4 to 1/5
2
Coarse
1/5 to 1/6
3
Fine sand
1/6 to 1/7
EXAMPLE ILLUSTRATING USE OF KHOSLA CURVES
4.5
With rcfcrcnec to Fig. 5.11, determine the corrected percentage pressures at the key points. Deter· mine also the exit gradient. Total length of floor (b) - 52 m b' = distance behvcen upstream two piles
=
14.50 m.
d ,. . depth of pile on which the effect is to be determined D
the depth of the pile line, the influence of which has to be determined on the neighbouring pile of depth cl
c-
19
ns
J
b'
d
I
I)
b
42
n
POND LEVEL
J'
EL~1!31.00~
! RL. 1413.00
_i1
.~
3
·,
::::::11
t R. L.
I4 5 .0 0
u n. 147. oo
INTERMEOIAH:
Ii II
-J 0/S SHEET PILE
jn. fU.I43.00 ~0 I .I
EL. 14:5.50
PILE
fr:t.L.14:S. 00
II ill I
:s 6. 0 0
14.50
52.00
·---
DESIGN KHOSLA'S
EXAMPLE THEORY
FIG. 5.11
SOLUTION: % PRr~SSURE AT KEY POINTS i) UPSTREAM PILE LINE: ~
d
148.00 - 143.00
5.00 m.;
J:>
L
.0962
b
0:
from fig. 5.9 (upper curve) 0D
19
100 - 0D
0c 0c 1
28
100 -
0c
in fig. 5.11 0D
100 - 19 ~ 81%
0c
100 - 28 ~ 72%
}D
~ 9%
148.00 - 147.00
Oc correction for depth =
5
x9
1.80% (+vel
INTERFERENCE OF 2ND PILE LINE D ~ d ~ 147.00 - 143.00 ~ 4.00 m.
14.50 m.
b'
~ 19
c
.j-4.oo 14.50
x 1.00 ±-.1cQQ_ 52
1.54 ~ ( + ve)
0c corrected
~ 72 + 1.80 + 1.54 ~ 75.34%
iii INTERMEDIATE PILE LINE:
148.00 - 143.00 ~ 5.00 m.;
d
b d
cr
hi
=
15.25;
52/5
~
10.40 15.25
bi
52
b (1-
b'
b)
~
.293
0.707
44
-~' :.
from fig. 5.9 (lower curve) .707
lor
f/Jc
~
zg
0D ~ 37 In fig. 5.11 0E 1 ~ 100- 29 ~ 71% 5 - 25 0Dl ~ 100 - 37 ~ 63%
0E 1 0c 1
~
0m
~
n -
83
58o/o 148.00 - 147.00
0El correction for depth
8%
X
8
5 1.60% (- ve)
Ocr correction
1/5
lor depth
X
8
1.60% (+vel Interference of U/S pile line on OE1' - 1.54% as the two pile lines are equal and placed at the same level. Interference of DiS pile on \1c1 147.00 - 139.00
~
8,00 m
d
147.00 - 143.00
~
4.00 m
c
19
D
~
j 36,00 8,00
X
8.00_ + 4,00 52.00
2.07% ( + vel 0E1 corrected
~
71 - 1.60 - 1.54
~
67.86%
0c1 corrected ~ 58 + 1.60 + 2.07 ~ 61.67% iiil DOWNSTREAM PILE LINE
d
~
145 - 139
d
6.00 m.
6.00 ~ 52.00 ~ ·115
b
with dib.
~
~
.115 from fig. 5.9 - lower curve
0E2 ~ 32% 0D2 ~ 23% (Curve for 0n) 0E2 - 11iD2 ~ 9%
45
0E2 correction for depth ~
s c
Interference of intermediate Pile line:~
D
143.50
143.00
d
143.50 ~ 139.00 ~ 4.50
c
19
(0.50:_ J . 36
0E2 corrected
t
.50 m.
l
I
~50__:>:___4,50 ~ .215% ( ~ ve) 52.00
X
e
32 + 2.25 ~ .215 ~ 34.04%
Based on above calculations, pressures at key points are:~
£1'/J]i:JIME_.QIA'J'E PIL]j: LINE
[J(:IJ'JLEf!NE
DIS PILE LINE_
glp~
~.Q.
.0_E 1
~I.ll.
\IJ C1..
1lEz..
1lDz
81.00%
75.34%
67.86%
63.00%
61.67%
34.04%
23.00%
XIT GRADIENT:
Maximum W .L. on upstream
=
R.L. 151.00 m.
Maximum seepage head(!!) ~ 151.00 ~ 145.00 ~ 6.00 m. Depth of D. S.Cutoff (d) ~ 145.00 -· 139.00 ~ 6.00 m. Total Length of Floor (b) ~ 52.00 m. b d
--
~
<X
-
52.00 6.00
---
~
8.67
Referring to the graph for Exit gradient, For o:
~
8.67; =
GE -
.5
(!'..! .15- ___L_ 6
.. 6.67
exit gradient - 1 in 6.67 The structure is safe for a foundation material of fine sand for which the safe exit gradient is 1/6 to 117 as per table 5.6.
46
5.8
STRUCTURES ON PERMEABLE FOUNDATION OF FINITE DEPTH
'
In the methods explained in the preceding sections, it is assumed that the pervious foundation is homogeneous and infinite. In a number of practical cases however, there may be sites where the impervious strata occurs at shallow depth and pervious foundation is finite and comparable to the width of the dam. In such cases, it is known that the results of Dr. Khosla's theory as outlined in the previous sec· tions, do not agree with those of the model experiments. Mathematical solutions have been attempted by several investigators. The solutions are more complicated than those for ·the infinite depth case. It has been found that in cases of weirs with short piles, weirs with long piles (5 times depth of sheet piles) or when the depth of pervious foundation is approximately 2.5 times the width of the weir, the case is equivalent to that of infinite depth for purposes of computing potentials and exit gradients. As the design organizations are not at present recognizing the effect of this particular aspect, the procedures are not discussed in the Manual.
47
CHAPTER 6
COMPONENTS OF DIVERSION WORKS
6.1
lAYOUT OF DIVERSION WORKS
A typical diversion head works consist of the following parts Wig. G.l).
1) The overflow dam 2) Sluiceways
3) Training wall 4) Fish ladder
5) Intake work
6) River training works 6.2 THE OVERFLOW DAM The overflow dam provides the obstruction across the river required to raise up its water level and divert the water into the canal. It is aligned at right angles to the. direction of flow of the river.
6.3 6.3.1
SLUICEWAYS Function
The sluiceways are bays in continuation of the diversion dam with a crest at a lower level. These are located on the same side as the off take canal of the river. If the canals take off on either side, it would be necessary to provide sluiceways on either side. The crest of the sluiceways is generally kept at the lowest level of the cross section of the river during low supplies at the proposed site of the dam. It is also related to the crest level of the off laking canal. Main functions of the sluiceways are:
i) to maintain clear and well defined river channel towards the canal regulator. Because of its crest at lower level, a deep channel develops towards the intake work, capable of easy diversion of low supply discharge into the canal.
iii to scour silt deposited in front of the canal intake and control silt entry into the canal. iii) where applicable, to pass low floods without manipulation of gates or shutters installed on the dam crest. As long as surplus water to be discharged does not exceed the discharging capacity of the sluiceways, it is passed through the sluiceways. iv) to lower the afflux level by providing greater discharge per meter length than the overflow darn.
6.3.2
Discharge Capacity
Discharge capacity of sluiceways is fixed on the following consideration.
48
1.
RIGHT AFFLUX-
BANK
.J
RIGHT D.S. GUIDE .
BANK
COMPONENTS
~~
DAM FIG. 6.1
i)
ii)
at. least twice the maximum discharge in t.he offtake canal to ensure good scouring capacity about. 100/o or higher of the design flood discha;:ge at afflux elevation considered
iii) a throrctical bottom velocity required to sluice sediments of a given diameter may be obtai.ncd from fig. {).1 and used in the computation of the discharge· (this method also takes into account the settling velocity with regards to sediments to be excluded). The procedure for the last method sl
Given the following: Operating water surface elcvution
(m)
Sluicew;-ty and intake sill elevations
(m)
Discharge
(em/sec)
o[
main c:tnal (Qmci
Silting velocity (vsilting "~ 0.'30 MPS
(mps)
Diameter of sediments to he excluded (al)
(rnml
Diameter of sediments to be .c;Juiccd (a2)
(mm)
( ~~~~!~-~L- )
Slope of sidewall (s)
horizontal
L Compute H ~ Opcr, W,S, Clov, -- Sluiceway Sill Elcv, 2. Compute Ar and solve for Min. B
mequircd areal
Ar -- Clmc V silting i\r
Min, B -
]f -
H
(Required B)
2s
(Supplied area)
H0
Opcr,
-
W_ S,
f~lcv_
-Intake Sill Clcv,
,J_ Dctt~rmine required settling velocity (W) from a 1 vs. W graph (Figure 6.4) and compute required length of channei (L).
L
=
5. From Figure fl.5 determine cornpctcnt bottom velocity (VI) required to move sediments of a2 diameter size. ()_ Cornpute for t.he mean vc!ncity (V 1 ) lhc mean \~clocit.y head (hv 1) required at the sluiceway channel.
v1
-
v2
vll, __ ()_70
2g
50
7. Compute for d 1 and A1
8. Compute for l.hc discharge required to sluice particles of a 2 diameter size {Q5 )
9. From the tailwater ratin;s curve of the river. determine the tailwater elevation for the corrcs· ponding Qs. 10. Compute
11. Determine
g.sc from
the fo!lownrY table h
0.00
1.000
0.10
0.991
0.20
() .983
0.30
0.972
0.40
0.956
0.50
0.9:l7
O.GO
fUll f) Hll
090
J
I 00 ----~
0.907
ll.WiG
0 77H
0621 0 000
----
12. Compute Bs
51
6.4 TRAINING WALL Functions and Elements of Training ~~vaH
6.4.1
As briefly described eariicr in Chapter 3, main functions of the training wall are: i) to separate the flO\V of the scouring sluices which is at lower level than in the main dam.
ii) to isolate the pocket upstream of the canal head regulator to facilitate scouring operation. iii) to prevent formation of cross currents to avoid damaging effect. The training wall extends both on the upstream as well as on the downstream of the dam. It is the upstceam length of the training wall that governs the flow curvature and needs to be studied in greater detail. Normally, it is a concrete or masonry structure of. suitable top width aligned at right angles to the darn axis.
6.4.2
Hydraulic Requirements
From hydraulic considerations, a.n ideal training wall is one which: i) ii)
ensures a deep river channel in front of the sluiceway bays for proper feeding of offtaking canal. eliminates parallel flow
iii) minimizes sediment entry into the cana'1. iv) makes the riverflow approach the dam uniformally. The oblique approach of flow to the dam will not only result in making some of the main dam bays inactive while increasing the flow concentration through the others, but may also cause development of islands on the upstream of the dam. l"urthermon~. if the quantity of sediment drawn by the offtaking channel exceeds its carrying capacity, the excess sediment load would be deposited in the channel thereby reducing its discharging capacity.
The above requirements for the efficient functioning can be met to a very large extent, with the provision of suitable training wall in conjunction with proper river training works, optimum angle of offtake channel and raised crest of intake works.
6.4.3
Role ot upstream training wall
The upstream training wall ~crves to ge:nerate a convex curvature towards the intake which results in spiral flow normal to curvature thereby diverting heavily sediment laden bedlines away from the intake and relatively clear water into the offtaking channel. The efficacy of the training wall in the exclusion of sediment depends on its length as well as upon the width of the sluice pocket.
6.4.4 Width of Pocket and Model Studies
ln t.he case of gated type of diversion work, the gencrai criterion in vogue for deciding the width of the pocket is that the ratio of the average velocity of flow in the river to that in the pocket (VnNp) s!1ould be greater than unity during maximum fiood when appreeiablc bed movement takes place j~ the nver. It may, how(~vcr, lw cmphasizC'd that this, by itself, is not sufficient guarantee for effective sand exclusion unless ass~wiaLccl with favourabh: curvature.
w ~
u
-"" ~I-.~1 0
"' .hS_GENO.
-" 0
Surfac$ lines of flow Bad
El
linfls of flow
0 0 _Q
.6
.2
--
~:: ~ 0
~--------
""-
:::>
a- Lines of flow with divide wall after 4 bays
~
0 Q_
E 0
0
~
u
~
·; .::!
]i
~~-..::-·:::-----
Flow--+------~--=:-:·:::·-:'-="-~-..=---::-~--:=.--=-=-~--=--
\2
.
~
--~:;L_
-
-----
~onol
~
"
-"'
b - Lines of fiowwith divide wall after 5 bays Dam
Crest---
u
0
"'
__c-t o!,
-" 0
"-
" "
~
0
E 0
0
"u "' " ~ "-,o ~ -·o ~-
c"-
::0
c- Linss of flow with divide wall after 6 bays
EFFECT OF POCKET
FIG. 6.2
WIDTH ON FLQ\>V CONDITIONS
For the pfficienL functionin!~ of a diYcrsion dam, 1t. is essential t.o provide an optimum width of the pocket.. /\ sm:tll('r width of t.h'(' poC'kt't t·psuli.s in higher v('\ocities :lnd conscqucn_Uy rnore sediment (~nt.rv into !.hP pnckr.'t. and t!ltim:li.Ply into the offtaking· channel, \Vhcreas a very w1dc pock('!. may result ·in parallel fl<"lv; :llo:1g tile siuiccways and t.he training wall. For exam piP, model studies conducted to ch~t.ermirw Uw optimum width nf the :-:luin' poda't for t.h(' LO\\-·cr Sarda Diversion \Vork in India, indicated t.hal ilw S(~dinH~nl. cnt.rv into t.lw canal was minimum wlwn the t.raini!li( wall was placf'd after the fifth b;n'. t.he total of t.lw m:tin hav.s being 20. \Vhen t.hc training wall was t.est.Pd afLf'r the sixth hay, parallel f\0\~ was observNl along Uw ~1uie(:way bays and the training wall. This caused more turbulence in the pod;.et. and th1~ quant.ily of sPdim('nL ~·nt.Pring into the can;1l was found to increase. Thus. the pockeL with four bays was found in;tdc(.JUatc v;hercas with six bays was l.oo large. Therefore, Lhc width of t.lw pocket \'>':l.S kept ('qual t.o G kt_ys. Fig. G.2 shows the flow lines for ;:dl Uw three conditions.
6.4.5
Len~j'ch
of Training VVa!!
6.4.5.1 As pointed out Parli(·r, t.he lPnf'.( h of Llw 1.r·aining wall upsl.!'(':tnl of the divrrsion dam phy.s a very important. role~ in gcnerat.in,e; l":tv(JUI·abic ('lJrvat.ure (Jf flow for feeding Lh~~ offtaking channel. Earlier the practicr \vas l.n providP a lraining \.Vall much longC'r t.han the width of Lhe c:-tnal intake. One of the main argum(•nts advaneed in favour of a long training wall was t.hal it served a~ a t.rap for coarser bed material. Hnv-n'vPr, it. h~ts hH'n r:st.ahlishcd !.hal. a very long training: wali dews not nC'ccssarily hrlp in rninimi7;ing ilH· S1'dimcnt. entry into the off·t.aking chall!wi. On the olhcr hand it. may vitiate How eurvature and pr('Sr,'nL difficuiliC's in flushinv. Furt.lwrmor(', a very long divide w;1il may indun: parallel flow along it, and even slight obliquity of f!O\v !llay mask a few of l.he other main bays adjacent t.o iL 6.4.5.2 From Llw :thov(' it is ('vid('nt !lut Loo lnnR a training wall is hydraulirally disadvanLlfU'OUS. On the otht'r hand if th('!T is nn t r:tinin~~ w:dl or if t!w lcngt.h of training wall is irndequ:lt,c, paraikl fiow could tkvf'lop along !.hC' sluic('way ha.vs causing considC'rablP velocities and turbulanee in t !w poekct, t.hen'hy inercasing t.hc sediment. entry into t.hc offt.aking channeL In order, therefore to achit:'H~ satisfactory flo\v conditions near t.he head regulator, it. i.s nt:ecssary lo provide adequate length of training waiL
6.4.6
Top Level of
Trainin~J
VVal!
Tlw l.op !Pvt'l of t.hc train in;:~ w:dl also a ffc~ct.s ii.s pcrform:tnce. I[ the top oi the trainin,s: wall is Ju:pt. lower !han t.lw pond levPl in the par!. or ('ilt..ire kngt.h of i.he t.raining wall, the water \·viil spill over introducing t.tw silting dfC'et. in l.hc pockpt.. This may lead to jump formation and consequent scour near t.h(' training· wall in t.hC' ol.hl'r main bays. This phenom(~non is based on model studies. Such conditions lllilJ' also occur ;\1 water lf'Vf~ls higJlCr !.han t.hc pond Jc:ve! if Uw sluice g-:tl<'::> arc partly open and the gaU: of t hf· adj;u·cnt hays of main darn are fully opt:n. Th(~rdore, it. is considered advisable to keep t.hc Lop of the divide \Val\ above high flood level or pond level whichever is higher.
6.4.7
Conclusions
il TIH: t~xpr:rir.'flf"P of modPl <;iudi(~s and prnt.ot.ype lwhaviour C;:trried out in India on sonw of the diversion work." havP indica!Pd th:ti. in <'asc of a singiP int.ak(', a training wall covering 2./:J width of the int.ake gcncr:1l!y giv('S satisfaelor_y c.ondiLions, ;.v/wreas in L·win intakes, it. has bPen g-enerally found necessary l.o provid(' a dividr.' wall covr:rin.1< t.hc' ('nt.irt: \.vidt.h of the intake to obtain saLisf;ct.ory h~'draulic conditions. If t.hl' nl'ftakin;~ ('hann('l." an' r('quired t.o run only during non monsoons wiH'n the ;ivc.r water is compar:tt.iv\:iy sedimt~n\. frer·. t.hr~ requiren~Pnt.s of opt.imum.lf'ngl.h ~)f training \Vail can be relaxed.
iii) The ratio of the optimum length of the training wall to the optimum width of the potket V generally from 0.8 to 1.6. A curve, Fig·. G.:J(a), has been proposed to determine the :1pproxirnate value of this r.1.tio. Similarly another curve, li'ig. G.3 (b), h:1.s hcen suggested to determine the approximate width of the undcrsluicc pockcL These an· based on studies carried out on the data pert.~tining to various diversion rlams (barrages) in India. The length of the training wall was determinul with the hcip of the model studies in most of the cases and the width of the pocket in a few eases.
6.5 FISH LADDER 6.5.1
Necessity
Large rivers support various types of fish, Jnany of \vhich ;tre migratory. Tlwy move from one part of the river to another according to srttsons. Gencraily speaking, they move upstream to downsl.rcam in the beginning of \Vintcr in sC'are.h of warmth and return upstream before t.hC' monsoon for clearer water. If there is no arrangement for their migration, large scale destruction of the~ fish may Lake place in the river.
6.5.2
Location
The entranee of a fish\vay should be loeated at a point beyond \Vhich they are unable to pa~s. in other words, at or ncar the b,1.sc of the dam or obstruction. It. is generally advisable to locale the entrance somewhat out of range of a heavy overf.:lll of water from the weir/dam. F'ish ladders at >vcirs arc generally incated !War the training wall due to the availability of water throughout the yc·ar in Lh<:' rivedwd downslrC'am of Lhe undcrsluieeways. These arc usually located adjacent to the training wail near the unckrsluicewa.ys.
6.5.3
Investigations and Data
It is cssenlihould lw c:onsulted regarding Lype of fish, correct depth for a specific area and other requirements.
6.5.4
Design Requirements
In order that fish ladders may funetion successfully, it is necessary that they be so located as to attrar:t. fish. A number of factors sueh as physical features of the l~1cation, overeornc and the available \Vater supply may influence Lhc design of a fishway. 1\ larger abundant flow of \vat<~r may prove advant.ag-cous a5 the oulfall will provide a Rreater fish than that frorn a small one. Sometimes conditions may limit both the dimensions and quantity of water.
designed and the height to fishway with extraction to the available
ThP -'lc!u<11 {l,._<:)t-rn fnr ·1 fi~bw:1.v ;r,cl,Hl\'S ~' \:tr•,r!o(l c.:pdion of _"'-lH:h icnzth and dimcn~ions as will provide a scric~ of compan.rncnts f.hr;;ug-·n ,. ..-,,J,~f, L(if .,.,. -._.-" ';~~ ·h,-- ·.. ,,._ ,_,,.,,"ri "f'h,-. ~!''"!":·:oin 11 1._,,,factor for cffieiency is thai.. if the .supply of water is incrca~wd it is desirable t.o <~nt.:u·ge llH~ oinwn:~ion .., a.nd u.:ually lessen the grade in order to reduce the Lurhulencc in the eompanmc:nts to a point where fish wlll not become fatigued. 55
z
~-----t--1
o.-•-,---11
--,-------1
1.5 ~- ~+-- +-~--- --J-----~--~!
I I II 1.0 ~;:"1170. ~u tO -v,_o--+- ---1-----1-l 16•
L/W
•12
i
9
·004 (a)~
·006
-0012
·01 ·OH
Plot of L/W vorsus Qc/OR LEGEND: L
x
=
VI Qc,.
Qrz
'C'tu
0
.QQ2
·004
'006
·OOI.ll
·01
LlOHrTH OF DIV!Orl WALL 'NtOTM OF UMDirft SLvtCl!3 POCKlT CAHAI.. OE-:5\GN Dl3CHAR0E PHV!!~ OESlQH OlSCHARBE WIDTll rJf CANAL lfi.AO MOVLATOI\
·Ott
Oc/0 0 (b) Plot of W/Wh verou• Qc/O•
DIViDE FIG. 6. 3
The following points may serve as a guide in the design of fishways:a) The entrance 1.o the fish way should be from a pool in which fish would naturally collect when further progress upstream is prevented by the obstruction. b) The entrance should be well submerged at all stages of water when fish are seeking the ascent through it. c) ThP size of the free cross-section of fishway should be sufficient for unhampered swimming movements eonsidering the size and number of fish using it. The width should not be too narrow. e) Water should be available in the ladders during the periods of migration. f)
L,ongitudinal slopes may vary from 8:1 for low lifts to 10:1 for high lifts to ensure a current of velocity not. exceeding :3 m per second, main consideration being to dissipate energy in such manner as to provide smooth flow at sufficiently low velocity.
6.6 INTAKE WORK 6.6.1
Location and Layout
The intake work also termed as eanal head regulator is provided just on the upstream of the diversion dam. It serves the following functions:~ i) to regulate the supply of water in the canaL ii) to control the entry of silt in the eanal and avoid back flow and formation of stagnant zonps in the pocket upstream of the sluicev·lay. The intake structure is normally aligned at an angle of 90°"110° with the axis of the diversion dam. In case of major projects, the alignment should be fixed on the basis of model studies. Regulation is done by means of gates. According to the old practice, the intake structure had a large number of small span gates, but modern trend is t.o use steel gates of spans ranging between 8 to 12 meters and operated by electric winches.
6.6.2 Type of Structure Pipe or box culvert may be adopted at sites when chance of silting up is remote and when open trough structure is found to be uneconomical.
5.6.3
Crest Level and Waterway
6.6.3.1
General
The crest level and waterway of the head regulator are inter-related. Control on the site entering the canal is provided by keeping the crest of the intake work about 1 m to 1.5 m. higher than the crest level of the sluice,vays. The prindple invo!verl. being that the required discharge is to be passed into the canal with the designed "pond level" (or level to which water can be raised in the river at. low stage by ffi(~ans of a diversion dam). The crest level is fixed from the following considerations: a) Head over the crest measured below pond level should be sufficient to push the design discharge of the canai, without. lowering the canal water level to satisfy the command water level over se;vice area. b) Crest of intake should he higher than crest. of the sluiceway as per guidelines discussed in Chapter 7.
57
Q
:; ·}
iJ
:~
<0
<;f
.,.
{_() (9
·.,4
lL
>j N
i
·.;~
;j 1'1
~
I
I
1
I
<>< 0
.g
E\
<0
1,,
0 .,. II
::1
;~ ·,~
0
:'!: :>.
~
·u
N
0
j
<::!+
0
ro
0 N
0
ro
I
I
N
OOJ
~
N
"'
0
(DIAMETER
OF
SE01iv1[NT$
<0
0
.,.
N
0
0
TO BE EXCLUDED, 01 , mm.
0
"' 0
0
<0
.,.
0 0
0 0
N
0 0
0 0
-"'
(/)
.,.0
0
C>
.s ~
i3
------ N
.,> 0
0
~'
'IE1E,
GQ.l<4PETENT
BOTTOM
VELOClTY
( Vb -
MPS :
c) If a silt ejector is provided, the crest level of the intake shall be fixed at the roof level of the silt excluder. d) Crest level should be sufficiently low for economy in waterway length.
6.6.3.2
Necessity of Breast Wall
As the required discharge into the canal can be passed at pond level, it is necessary to provide a gate controlled opening only from the
The principles involved in the design are described in chapter 12.
6.7
RIVER TRAINING WORKS
River training works at the diversion head works are required to guide the river and provide a smooth appro::tch to the work and also to prevent the river from outflanking the banks. This purpose is usually aecomplishcd by guide banks on either side. In addition. m;lrginai embankments
60
CHAPTER 7
HYDRAULIC DESIGN 7.1
DESIGN DATA
In the design of diversion dam, the investigations to be carried out and the data required have been discussed in Chapter 2. Before taking up the hydraulic design, it is imperative that the designer is fully conversant with the projeet as a whole, the field conditions, and the data necessary for the proper design. The collection of design data should be given extra attention.
The data which arc essential for hydraulic design of a diversion dam, can be categorized under:a) Field data bl Design parameters
7.2
FIELD DATA
The following data have to be collected by field measurements and on the basis of investigations carried out.
7.2.1
Topographical Data i) An index map with salient features described in Chapter 2, para. 2.2.3.1
ii) A topographic map of the area around the proposed site of diversion dam. Ttle contour plan of the area around the proposed site of the dam with contour intervals of 0.5 m. on flat area and one metre on steep area. The contour plan should extend to about 2 km. on the upstream and l km, on the downstream of the site and up to 1 km. on each bank of the river. iii) Cross section of the river at the proposed site and at intervals of 50 downstream and at intervals of 1001200 metres beyond up to GOO site both on the upstream and on the downstream respectively. If the ciablc fall in the river slope, cross sections may be taken at closer conditions.
m. for 200 m. upstream and metres from the proposed topography indicates appre· intervals depending on site
iv) Profile of the river bed with observed water levels along the deep current for a distance of 1 km upstream or farther if the backwater effect is likely to extend, and 1 km. on the downstream of the proposed site.
7.2.2
Soil Foundation Data ~~r
•. .
• <, , •
L'
-.,. 1- • . . . -
··-''-
·'- ,.
-'- '
61
For diversion damsi\vcirs with rnore than 3000 cumecs dischargr the spacing and depth of bon~boles may be deeidNl based on rl'quin'nH~Jds of individual cases and si!e requirements. Such log ch;-~rt.s of holes along t.hP upstream and downstream cut off lines may be taken Ln decide t.he bot.t.om levci of cutoff, in case impermcablr: strata ar(' met. with. 7.2.3
Hydrological D<1ta
A gauging st-ation at or nt'ar tlw proposPd site of t.he \VOrk should lw established ;ls soon a:-;; the proposal for the work i.s first. initiated. and sLa.ge dischargp dat.a should be ro\!cet.ed. Two gauges should be ps!.ahlished upst._reanl of UH· axis of the proposed work, one on the upstream and Olll~ downstream nf
the axis. i'vlorc gauge:::; may he installed
wiH~n
necessary.
Other hydroloRit';d dJta may be cniicct.ed in aeeordance with t.be details given in
7.2.4
chapt.(~r
2.
Miscellaneous Data i)
Samples of sudan: and sub-surface wai.cr should be tested for sui Lability of concrC'Ling.
iil Wind ve!o(·ity should lw obtairwd for working out wave wash etc. This may howevc:r, he rvquircd in case of darns wiH·re thP water depth is large. iii) Others as per details in Chapter 2
7.3
DESIGN PAI1AMETERS
Terminology For the design p:trarnt-!.Prs. discuss(•d lwrcunckr, Lhe following terminology {ddinitinns) sh;lll apply in this chapter
Llw ;:;a me ;ts it. \vas
LhP construction nf Lhe dam.
ii) ConrPntrat.ion heLm· Tlw factor by which the dischargP JWr unit. length of a diversion dam assuming uniform dis! rihut.inn is rcql)ired to be mult.iplipd to get !.he design per unit kngl h for designing" it.s various ('knwnt.s. iii) Exit gTadif'n( foundation soii at is morl~ than the hydraulic r;radicni
iv)
The upw:nd S('epag<~ force per unit t.lw exit Pnd ni a diversion dam, Lc·nding suhmPq(cd wPight of a tl!lit volume of of Pmcrging st.reamlirws at. the end of an
vnlti!'llP of pcrcolai.in;~ watl'r t hrntl_t.;h to \ifL up Llw soil particles it' t hl· i'nrce the part.icks. It is also de lined ;ts the impervious apron.
factor Tlw ral.io or Uw ovr~rall length of Uw dam provifh:d t.n tiw tht'Oreli('a]iy computed minimum stab](' width of Lhe river ::tt. the design flood obt::tirwd on Liw basis of Lacey's
LO()Sf'[H'S.<.;
C([\Iation.
The kv~_·l of wa\.cr. immediately upstream of a dam. t.o facilit.aLe withdrawal intn t.hc canal or for any ot iwr purpnsr:. vi) Iktrogrcssion It is t.h(• pr·ogTcss'ive rlPgradation, of the downstream lines and \cveis as a rTsuit of construction ol a dam, c;tusing low(~ring of tlH· downstream river stage. Ttw main p;lram('LC'rs \vhi('h shotlld lw dPeidcd on Llw basis of available data and by Px.t·rcising engineering judgc'illt'nL an~ as under: i) Dcsi:;n rlnod i) Pcrmissihl<: afflux
iii) Scour and silL factor
02
iv) Safe ex it g r adient v) Coefficient of roughness of river channel vi) Retrogressio n of levels vi i) Coefficie nt of discha r ge under different disc harge conditio ns viii) Pond level For economic design of d iversion dams , two factors p lay predominant r ole - afflux and pond leve l. In addition, the design flood to be adopted in the design of the stru cture is also a very impor tant factor wh ich h as to be decided after consideration of economic and hydrological facto r s.
7.3.1 Design Flood 7.3 .1.1
Ge n e ral
D es ign flood is the flood adopted for the design purposes after consid e r ations of e conomic and hyd r o· logic factor s . I t may be the maximum probable flood or the sta ndard project flood or a flood corres· ponding to some des ired fr eCJuency of occurrence depending upon the sta nda rd of security that shou ld be provided against possible failur e of the structure. The p rop e r se lc<"tion of t h e d esig n flood is of utmost importance as th is has a hearing on both the fac to rs of s afe t y a nd cos t of th e str udu r e . .If t he selected design fl ood is too low it mak es for r unn ing too high a risk tha t involves not onl y t he Lo t
S tatus of Design Flood
Diversion dam s/weirs have usually sma ll s torage capacities and the risk of life and p rope r ty downstr eam could rar ely he en hanced by fa ilure of the structure. Nevertheless, div e r s ion dams over r ivers arc fa irly la r ge and costly structures . Quite often a road or r a il way bridge is also in corporated into the diversion s t r ue t u re. i\ part fr om th e failure, irrigation and com m u ni ca Lion t hat a r e depen d·e n t on the wor ks would he disrupted. In consideration of t h ese ri sks , divers ion dams s hould be designed for t h e foll owing design flood . a) 500 year flood for calculation of free boa rd of abutments, gu ide banks and afflux embankme nts. h) 100 year flood for all othe r calculations. 7.3.1.3 Determination of Desig n Fl ood The pertinen t factors on whic h the des ign flood ca n be b ased arc t h e str ea mflow r eco rd s whi c h are com pu ted from the precipitation r ecords. In the Phi li ppines. str eam fl ow re co rd s arc ava ila ble in t h e Natio na l Wate r Resources Cou n ci l (NWRC) a nd arc co n taine d in the a n nual publication entit led "Surface Wate r Su ppl y B ulletin" . Precip i tation records and oth er meteo r ologica l da ta can be obtained from the Weather Bureau or from the annua l publication e nti tled "Annual Cl imatological Re view." T he more commonly used methods for estimating design flood include: i) hydrological methods;
ii) empirical flood formulae; iii) enve lope curves;
63
•
iv) flood frequency analysis; v) rational methods involving unit hydrograph; i)
Jiydrolo giw/. Method
The design flood is obtained by applying a safety factor which depends upon the judgment of the designer to the observed or estimated maximum historical flood at the site or nearby site on the same stream.
In some cases, after ~he flood subsides, it may be possible to determine the magnitude of the peak discharge on the basis of the cross sectional area and the water surface profile indicated by water marks on the banks of the stream. Such peak discharge determination can be made by Slope Area Method. This method can be ut.iliz.Nl primarily to determine the discharge of a stream from specific field data. In this method, Manning's formula may be used for the determination of velocity but it would be subject to the uncertainty of the value of the roughness coefficient. iii
Empirical Flood Fo,-mulae
The empirical formulae commonly used in some countries are the Dicken's formula, Rave's formula and Inglis' formula in which the peak flow is given as a function of the catchment area and a coefficient. For Philippine conditions, A. A. Villanueva and A.B. Peleiia in their article "Notes on Intense Rainfall and Hun·off Luzon" (published in the Philippines Engineering News Record in December, 19391 has been able to derive from cnveiopc curves Empirical Flood Formulas for Extreme, Hare, Occasional and Frequent events. The first three arc as follows: 210 A jA + 17
150
)A
85 A jA + 9
Q.Ocosionai
where: Q
=
+ 13
A
=
Drainage Area in Square kilometres, and
Discharge in cu. miscc.
Discharge values should be used keeping in view the limitations described in the 'article'. In designing irrigation ·diversion dams which are usuaily low and of the ovcrfall type, it has been the practice of th(' National Irrigation Administration to adopt the average of the rare and occasional flows as the design flood for dam sites without streamflow records. However, the values of the coefficients used in such cm'pirical formulae, generally vary within rather wide limits and have to be selected on the basis of judgment. They have limited regional application, should be used with caution and only when a more accurate method can not be applied for lack of data.
iii)
Envelope
Cnrm~s
In tlw envelope curve method, maximum floorl is obtained from thP PnveloiW rurves of all observed maximum floods for a number of catchments in a homogenous meteorological region plotted against
64
drainage area. This method, although useful for generalizing the limits of flood actually experienced in the region. cannot be reiicd upon for estimating maximum probable floods for the spillway capacity except as an aide to judgment. Thus. the methods at i), ii) and iii) are helpful in arriving at rough preliminary estimate of the maximum flood.
iv)
Flood Freqnency 1iiethods a) The probable fn~(jU('!WY of a flood of a given magnitude can be determined on a mathematical basis
~1y the law of probability provided that sufficient. records arc available for the study to represent truly
average conditions in t.he river. FrequPr1ey analysis methods may he used provided t.hal the design flood is within the aec('pt.Pd degree of extrapolation of avail:lb!c streamflow records. It may also be used as an end to the estimati0n of design floods for projects when the unit hydrograph method cannot be used for want of adequate data. Many methods of flood frequenc_y determination based on streamflow data have hecn published namely "Gumbel ivlethnd", "HazPn l'v1('Lhnd" and "Goodrich Method". More eommonly used in the Philippines is ihc Gumbel Ml't.hod. This method giv('s fairly ac<·cptabl(~ valu<'S for flood estimates up to 2:1 years when 20-ycar streamflow data is available. For bigger pPriods like 50-year and lOO·year. corresponding theoretical values ran he cxLrapolat.ul by mathematical computation. The method is illustrated in Appendix
I. b) Flood Frequcn\'y on Hegional Basis \Vhen there is v('ry little hydrologic or hydromet.corological data available in the basin itself, regional flood frequency analysis is undertaken. The techniqtH' !"or making rq~ional frequency analysis \Viii involve coilecting annual peak discharges of all rivers in t.lw n~gion v. :it.h avai!ab]p historical rr:cords or gauging stations along with the respective watershed an~as. t.hen undC'rt.aklng tlw rlnod ·frequency analysis for each gauging station hy Gumbel
method to determine the estimated peak flow for 2, 5, 10, 50, 100 and 1000 year flood making a homogeneity (Least. Sf]uare l\1ethod) computing the ratio of peak discharge for each gauging station to the corn~s· ponding Mean Annual P(~ak diseharge; <·omputing the Confidence interval (Cl) and plotting the Regional F'requency curve, plotting th(' Drainage Arca·Mcan Annual Flood Relationship curve for the region; and finally deciding or selecting the d(~sign fiood to be adopted in the diversion dam.
v)
Ra.t.ionoll~Iethods
Jn.-oolm:ng Un£! !Iydrograph
In this method of det.l~rmining Lhc design flood. the steps involved are:a) Analysis of rainfall versus runoff data fcir derivation of loss rates under critical condition; b) Derivation of unit. hydrograph by analysis (or hy synthesis} in cases where data are not available; e) Derivation of (ksip;n storm; and d) Derivation of design flood from the d(~sign storm by the application of the rainfall increments to the unit hydrogr·aph. Um:t I-Iydroymph. f,imit.oJions
al The uni~ h_y. dror;raph principlP is not. applicable for drainage basin having an area more than SOOO km::: \vlWrt' v:tlley storage effects arc noL reflected. h) Unit h:{drograph principle is not. reeornmt>ndcd for raf.\'hmcnLs having an an'a le.;:;s than about
25 km2_
65
c) Large number of rain ga u ges s uitably loca te d s hould be available in the entire catchment to reflect the t r ue weightcci rainfall of the catchm~nt. d) Unit hydrogra ph principle is n ot appli cable when s ignificant catc hment a r ea is s now bound.
7.3.1.4
Selection of Design Flood
In t he P hilippin es. t he re gions arc subj ected to ty phoo ns . w hi c h generally, a r e accompanied by he avy ra infall s giv in g rise to fl oods , extr e m ely he a vy in ma g nitud e hut s hor t in duration. Occur r e nc e of s uch flood s ha ve to be taken into consid e ration when ca rryi ng out hydrolog ical analysis. W hile deciding th e 'design flood to be a dopted in t h e stability a n a lysis and other as pects of de s ig n during flood conditi o ns. th e design enginee r should therefore evaluate and com pare all th e different results obtained from various methods since unfortunately, no m ethod is perfec t in predicting with ce rtain ty t h e t im e and size of a ny floods t hat may take pla ce in the future. Thei r occurrence a nd mag· nitude arc as uncertain a s the meteorological phenomenon and continuity factors which cause t hem. By exerc isi ng good judgment and r e-exami na t ion of t he p hys ica l c haracte ri stics of the water s hed , he ca n select the m ost acce ptabl e flood d ischarge to be used in -t he d es ign.
7 .3 .2
M aximu m Permissible Afflux
The widt h of the di ve rsion dam is gove r n ed by the value of afflux to be permitted at t he des ign flood and t h e proposed cr est le ve ls . Ma ximum pe rmissib le afflux is a ls o im portant for the desi g n of ciowns tr cam, cistern , flood protection and ri ver t raining works, upstre am an d downstr ea m protections an d cul offs. S in ce the h ngth of water way , corr es pon ding d isc harge pe r meter and afflu x are correlated, by proviciing hi gher afflux t.hc kn g lh of th e dam ca n be r educed but the cost of the dam and training works would inc r ease due to t he increase d head o f water. Maximum p e rmiss ible a fflu x s hould therefo r e, he decided based on the foll owing cons iderations: a) loss of rev e nue clu e to suhmNgence s hould be as s mall a s possible. b) cost of a lflu x em bankments and other t rainin g works for protecting property on the upstream s h ould b e minimum. Wit h re ga rcl s to Lhe limita t ion on t he disc harge conce nt ration per m etr e w idth of dam, th ere are instances wh en d i!>r hargc intensity ha ~ been provided as 25 cu mecs/m le n gth or so. Theoretically also, th e r e s houlc! be no lim it lo LIH' dis charge intensity hut it ca nno t be increased b eyo nd certain limits du e to the co nstr aints of perm issible afflux a nd the level of t he down s tream basin required from hydrau lic jump con si d e ration. Furthcrmore, in so me ca~es whe r e drivin g depth o f s heet piles is limited due to existence of b oulde rs, robbles and gravels in the unrl e rl y in g strata. th e value of afnux ma y have to be limitc d based on t he ro ns id c r al.ion o f pract icable d e pth to wh ic h s heet piles can be d riv en. In some cases. high cost of block pro tect ion and ripra p m ay dictate to limiting t h e va lue of the afflux. The ex tent of afflux to h e provided is a lso go verned by co ns iderations for s ilt exclusion on divers ion dams. E nde avour should be made to attai n the most eco nomi cal com bina t ion between e xte nt of afflux and leng th of wate rway. Thi s can h e done by trial and erro r . For prel imi na r y calcul ation s , a valu e of 1.5 rn to 2.0 m in fo ot hills . i.e., in t he steep rearhe s of t he ri ve r with b ou ld e r or rock bed and O.G t o 1 m in plains m ay be adopte d depending upon e levatio n of the submergence area.
7.3.3
Scour a nd silt factor
River srour is like-ly to 0('\llr in erodiblr «oil« lil
66
...
tions. If actu~tl ohsPn';llinns ;ne not :tvail:1hlP. it may b(' assumed th;d. thf' scour dof's not extc·nd !wyond 2.0 m with the· c·oiwsivP maic·rial. Extent. of scour in a river \Vit.h cohcsion!cs.s bed matcrlai may be cal· eulatcd hy La\'cy'_c; ror:11ULL H
CJ.rn (
H
1.:):) (
v)
I'"\
'N
lH'll
]oOS\:lh'SS
bel. or is TllOt'(' than
or ~(
11 -: 1 whr·n loosr·rwss fa\Lor is less than 1
\Vhcre T(
d(•pth of
:>COlli'
h(']nw t.iH' dc:signPd ma:
:;ilL factor
mr
avt·rage part.i\h' diameter in mm.
q - norm;li inL·n;-;;ity of d!sch:1rgc in m: 1/s('C pPr metre \vidLh S
7.3.3.1
\Valr·r stlrhc(' slopr·
ThP foi]O'.Vii1J~- l:lhk ,L;l\'(':-; vaiur·s off for v:lrinus types
or soils
Table 7.1
Soil
Sit.(' of particle;;
1
0.3:J
{).()f\
0.50
0.1;)
O.GR
0.3
0.9(i
(1.()
0.,,
1.21
0.7
1.-17
1.0
1.76
2.0
2.19
5.0
3.89
l (). ()
5.56
20.0 Bo11ldc:rs
7.88
:JO
12.3
7:)
15.2
19()
G7
7.3.3.2 The extent of seour in a river with erodible bed material varies at different pbccs along the structure as indkated in Table 7.2 below:~
Table 7.2
S.No.
Loeation
Design Scour Range
Mean
----··---------·--··-· -------·---------- ··-------·
1.
Upstrt>anl of impervious floor
1.2') to 1.75 R
1.5!\
2.
Do\vnstre.:lm of irnperviou.s floor
1.75 to 2.25R
2.0R
3.
Noses of guide han};::;;.
2.0 to 2.5R
Z.25R
1.
Nos(~s
2.0 lo 2.5i\
2.25H
S.
Transition from nosc t.n straight
1.25 to 1.7 5H
1.5R
G.
Straight rca('h o[ guide hanks
1.0 to l.5R
1.25R
Not(~:
of training \vall {Divide \Vall)
The ealcuiat.ed values of H. are without 20°/o concentration in the values of q (intensity of discharge)
Depth of cut off The depth of scour lo dct.erminc dPpth of cutoff wall or sheet pile can be worked out by the formula discusspd under para 7.;l.:l. Tlw ('Utoff dept-h should generally l)(' provided for a scour up to lR for the upstream and 1.2sn. for t-he dov,,nstrC'am. \Vhile working out H., 20°/o concentration in the value of q (intensity of disehargc) should be taken into aceount..
7.3.4
Safe Exit Grudient
Exit gr;ldicnt. is the gradiPnt. of sub.soi! flo,_v line at exit from the structure. By providing adequate cut. off depth in relation to floor !"ngt.h, this gradiPnt has to be kept within iimit to prevent di;;placPmcnt of soil p;~.rticles. As diseusscd in Chapter ;) allhough the crit.ieal value of exit is 1.1 or so, in view of other unccrtaint.iPs in n:1turc' of soil, it is dcsir~lble to .:tpply a factor of safety as given in the table for various types of soils.
7.3.5
Roughness Coefficient
In using Manning's fonnula for dPt.(•rmining flood dischargL' of the river hased on cross sections and available water sudaec slope~, thP folimving table is a guide for adopting a suitable value of "n" (coefficient. of roughness)
v
GS
Table 7.3 Channel condition
Value of "n"
1. Natural streams with good alignment. fairly constant section
om
2. Mountain streams in clean loose cobbles. Hivers with variable section and some vegetation growing in banks
0.04 to 0.05
3. Rivers with fairly straight alignment. and cross-section, badly nbstructecl by small trees, very little underbrush or aquatic growth--·
0.06 to 0.075
4. !livers with irregular alignment and cross-section moderately obsstructcd by small trees and underbrush. Hivcrs with fairly regular alignment. and cross-scdion, heavily obstructed by small trees and underbrush
0.100
5. Rivers with irregular alignment. and cross-section, covered with growth of virgin
timber
and
occa.<:::ional
dense· particles
of bushes
and
small trees, some logs and dead fallen trees .
0.125
6. Rivers with very irregular alignment and cross-section, many roots, trees, bushes. large logs. and other drift on bottom, trees continually falling into channel due to bank caving "-·
0.15 to 0.20
If the dc~cript.ion of channel condition docs not fit in with the actual condition obtaining at site, the value ofn should be worked out as follows:··i) Assume a basic value of 0.010 for channels in earth, 0.015 for channels in rock, 0.014 for channels
in gravel, and 0.028 for channels in coarse gravel. ii)
iiit
iv)
Add as follows for degree o[ irregularity -smooth
0.000
minor ·- ·- ·-·
0.005
moderate
0.010
severe
0.020
Add for change in size and
shapt~
of cross·Sec!.iorl
gTadual
0.000
occasional
0.005
frequeni
0.010 to 0.015
Add for obstruction such as debris,
roots.~etc.
negligible effect -··
minor
c~ffecl.
appreciable effect severe effcet v)
- 0.000 0.0 10
O.o:JO - O.OGO
Add for vegetation low effect
0.005 to 0.010
medium effect
0.010 to 0.025
high effect -·· ··- ~ ---- --·
0.025 to 0.050
very high effect
0.050 to 0.100
69
vi)
Add for dntnnel Tneander --(Lm = rnca nder length of reach) (Ls '"""" straight length of reaeh Lm/Ls 1.0 to 1.2
0.000
1.2 lo 1.:0
0.15 times n:"
1.5
where n..,
7.3.6
=
n
0.3 times
n~
(i) + (ii) + (iii) + (iv) + (v) above
Retrogression of Levels
As a result of Lhc construction of a dam and (~onsequent ponding: up of supplies, rciaLivcly silt free water escaping over the crest picks up bed silt dmvnsLream of the dam. This cause dcgradalion or retrogression of downstream levels. In the first few years the rclrogrcssion of kvcls is rapid and progressive. The lowering of level known as retrogression is found Lo be high at low river stages and small at high river stages. The pro<:C'SS of retrogression and shoal forrnation on tlw upstream develop and continue for a number of yeJ.rs but a stage is gradually reaclwd vv·hen upstream pond absorbs no co;1rse silt & rcversf~ process starts. Thr designs of downstream works have. h(nvever, to be tested under maximum retrogressed condition of the river for cnsu_ring safety of the structure. Hetrogression of water levels is more pronounced in alluvial rivers carrying more silt, having finer bed material and steep slope. A va!ur of 1.2~ to 2.25 meters maybe adopted as retrogrc~s:;ion for ai!uvial rivers at iower river stages dqH>nding upon Uw amount of ::;ill in the river and its slope. and 0.:3 m to 0.5 m in high siagcs. For intermediate disch::trgcs, t.hc effect of retrogression may be obtained by ploLting the retrogressed high flood lpvp]s on a log --- log graph. For tht' purpose of cksign, low slag:e retrogression value may he ::tpplir~d for Zl~ro discharge elevation for a strc::trn with morc' or l('SS uniforrn bed lev(~]. F'or a stream with unpven bed 1Pvel, low stage retrogression value may be appiicd to the st::tgc~ corresponding to S-10°/o of design dis('harg(-! or eorresponding to t.hP base flov•/ whichever is lower. High stage retrogression valtw ma_y lw applied Lo mean annual flood diseharge or in the absenre of discharge data to 60-70°/o of design flood discharge. _ In case the proposed diversion work is situated dO\vnstrcam of a dam, the possibiiiL_v of heavier ret.ro-
d
gi~c-:ssion than normal should be eonsid('recl for the design of downstream flood level and downstream
protection \VOrks.
7.3.7
7.3.7.1
Coefficient of Discharge
General
The flow over tilt' divt:rsion dam may be r.ont.ro!ied by means of gat.es or allowed freely without restrictions. The upper surface: of the freely fiowing \vatcr is at atmospheric pressure. The lo\vcr surface is generally guided b_y t.hC' .surfaec' of the spilhv::ty crest itsC'lf. In the oger type of spillway, tlw overflo\v crest. is shaped Lo conform as nearly as possible to tlH~ profile of t.h(: lower nappe of a ''!'!nt.il:J.lPd jet of w<~ler· issuing· nver ;1 sharp·crestPd '-'--'cir. 'J'hC' ,-.rr.st 9r0file. thus ha"' tp lw desif!nr>rl r~_ot l~ ·,., ...;ui? ·'he int.PndP(.l hvdr::l:lli;: ,•nnr.!i!-ion~ bu: -.:ht!uirl ai:c:c) fit til(' intendco ciownsu·(•;Hn profill~ oi the spilhvay body wi1ich is deter~rniTH:d by structural conditions. The basic equation for Lhe discharge over an overfali dam Js
70
IH
where
Q
.,.,
L
total discharge in cumec length over the crest in m
H
=
head over the crest in m
C
=
coefficient of discharge
Coefficient of discharge for flow over the spillway (ovcrfall dam), and sluicewaY would depend upon the following featurcs:a) whether the top of the structures acts as sharp crested or broad crested; h) whether the crest is submerged or free; c:) shape of crest and upstream slope,
The discharge coefficient, C, is further influenced by 1) depth of approaeh 2) ratio of darn height to the total head on crest. Head due to approach vclocit.y can be obtained from the equation h
where
va ,. . , g
:;
=
v 2 _J_
2g
velocity of approach in meters
,., 9.80 meters/see.
In the c~lsc of ogee crest, the dlscharge is given by the formula
when
L
effective length of crest
He . . .,
Lola! !wad on the crest. including velocity of approach
i) Effcctim;; length of omnflow cn;st
When crest piers arr~ provided between the abutments in ease of gated spillway or bridge over the dam, the piers contract the flow and reduce the effective length of the spillway as follows; L = L' --· 2 (N kP + K) II(, when
L
-~
effective length of the
L'
~
net length of the crest
N -
(TCSL
total number of piers
kp "" pier contraction codfficient k:l
~
abutment contraction c:ocffieicnt
ii) Pier and ahutment contraction cocffin:ent The pier contraction eoeffi('iPnt. kn, is affedcd hy the shape and location of the pier nose, the thickness ');,,,_. 1\~n hp·:rl ;" ~~,j.,t;()n l ( ' f)~,, '~rH>~~P' hp::d ':~!'rl t\,..,. .-..~)r);'l'"'h "f~]()('itv l<'nr '"()flrlii_i()!~"i f)f desi\!n
.r f!•r··
71
1' or squarc-nns('(l piers with <:orne:rs rounded on ~~ r;tdius pqual to abou! OJ of the pier thickness
0.02
For round-nosed p1ers
0.0 l
For point.(•d nosP pic1·s
0.0
TIH~ ahut.mPnL rontract.ion eoeffieicni is affc~ded b_y the shape of the ;:~hutment. the angle hct.weu1 the upstream appro:u:h w:d! y.nd the axis of flow, the head in n>\aLion to the design head. and Lhe approach velocity. F'or \"Onditions of dcsi_t;n lw;l(l, H 0 , average coefficient may be assumed as follow;;:
K, For squar(' ahul.nwnLs with heachvall ::tt Lo din~d-ion of flow
~H) 0
0.20
For rounded ahu! ments with lwadwal! at 90° 1o dirt>cl-ion of flow, wlwn 0.5 H 0 >
I!.IGh,.
0.10
>
For rnundr·d abu!tll('nL whPre r 0.5 l-Ie :~nd hr·adw:tll is plaeed not more than '15°
() .0
i.o dirc('Lion oi" fiow
,.
ndius o[ ahuLnH'nL rounding
Thl' splliwa_v with Ol:(l'l' shaped (Tesi will beh:lV(' likt~ a sharp crested structure. In ::;uch weirs/dams with sharp <'i'l'Si.Hi prnfiie and who::;(' hPights are not iess than about one fifth Lhe lwad producing flow over t.hem. the codfiriPnt of discharge remains fairly constant.. F'or weir height. less than about one fifth the lwad, the ("OnLract.ion of flmv lwronws incrcasingi_y suppressed and Uw (Test coefficient. decreases. V\-'hcn it f(l~Ls silh~d up on t.hc upstream and sediment rises up to crest, it \Vil! behavf' like a broad crested structure. The sluiceway, lwcausc of large crest \vidth provided Lo accommod:tfc· gates ;1nd stop logs \Vil! behavc like a broad crested structure exeept under very high head exc(~eding 2.:) times the crest width, where th(~ flow \Vill be as for sharp crested condition.
Cerwraily, all cas('s \viwre Lailwat.er is below the CJ(~si are cases of free flow. \Vherc l.aihvat.er is abOVC tlw Cl'l'.S\., ;[ ll1a)' ::;iil\ \w :t fn~C flow it' there is hydraulic jump which ('Otdd be checked by dr~l\Ving the water sud"acP profile or when the depth of Lailwat.er above the crest. is less than 2/311{'. l•>.:rcpL in c:tsr• of Uw iaq(l' structur<'S where fiP!.erminat.ion of coe·fficient of dis('hargc by mode! tests are justifi('d t.hc st.:lndard shape of crest with known coefficient. of discharge as described hereunder should tw adopted.
7.3.7.2 a)
Nappe-Shaped Crest Verlin!.lfa.cerl oyee
For n;tppe sh:tpr·d tTC·st w·ith v('rt.ical face upstream, Llw basif" eoeffi('iPnt. of dischar_gc should lw (';tl('nl:Jtl'd frnrn ih(• C\l!"V(' in [<'i;~-. 7.1. The cor:fficient.s are valid only when tlw (H(l'C is rortned tn the idral napiH' :->lupc. ikpvndin.:; upon Lhe ('Ondit.ions of flow on ih(' downstream of i.hC'
Wlwn t iw ('l'(•st h:ls twcn shapr~d for a he: ad larger
72
(H"
smalh~r than Liw head under consideration,
VALUES DISCHARGE
\_ I
COEFFICIENTS
OF
p
FIG. 7.1
Ho
FOR VERTICAL-FACED OGEE CREST
I ~-·---1
uj3 VJ
1---
z
w
u
t.o ,_____ ,_
,1:-f ',
'I
+-+ -
w
""
u "-
j
0
0
-11
1-1-
""-
,_....v
v
v :__...;.-- -r_l • I
' , -i- i-----1 - +--+- -I- -i----1
' ! ! LL L_j_L
I
0.9
0 1---
"cr
RATIO
Ho OF HEAD ON CREST TO DESIGN HEAD'--
COEFFICIENT OF DISCHARGE
Ha
FOR OTHER THAN THE
DESIGN
:
j_---1-
HEAD
FIG. 7_ 2
UPSTREAM FACE SLOPE
SLOPE
ANGLE WITH THE VERTICAL 1
!8° 25
33°'41
1
45° 00
1
I
I
i I
VALUES
OF
p
Ho
COEFFICIENT OF DiSCHARGE FOR OGEE- SHARPED CREST WITH SLOPING UPSTREAM FACE
FIG. 7.3
the eoetiH'lPnt o! disTharge will differ from that shown on Fig. 7.1. A widened shape will result in positive pressure along the crest. eonLact. surface whilP a narrower crest shape ·will produce negative pressure along he contaet surface rc~sult.ing in an increased discharge. In F'ig. 7.2 is shown the variation of th-e coefficient as related t.o values of .LD.· lin
\Vhere H{. is Uw actual head being considered and H 0 is the head to which Uw shape of the crest has been designed.
cl Effect of Upstrcmn Face Slope In Fig. 7.?, is shown Uw dfect of upstream sloring face on the ratio of coefficient for an ovcrfiow
1
cn~st. It is observL~d that Lhc codficicnt. of discharge is reduced for large ratios of t_l~-- and is increased for • 0
srnall ratios. d)
E';Jcr:t. of Downst.rcmn /ipnm and Snhmergcnce
D<)\V!Lc;lrcam tai!w.:-tter level
iiil a true h_ydraulic: jump will OC{:ur.
iv) a dro\vncd j1nnp will occur in vvhich the hig-h velocity jet will follow the face of the overflow. v) no jump will on·ur !.he je-t will break a\-vay from the face of the overflow and finally intermingle with slow moving water underneath.
In this figure, t.he decrease in t.hc coeffi{~icnt is also indicated. After the apron and taihvatcr level have bPPn t.entat.ively fixed, the type of flow ean be known and the c~xtcnt of decrease in t.he coefficient of discharge can be rpad off from the figure. \Vhcn it i.e; known that t.he flow on llw downstream side is super critical or \Vhen the hydraulic jump occurs, t.hc rkcreasc in ! he <'oeffieicnt. of discharge is due principally to the back water effect of dmvnst.rearn ::tpron and is indepcndcn1 of t.lH~ Lailwater level. In that case, the extent of modification in the basic value of roc:ffieient. of discharge ran he read directly from Fig. 7.5 which plots the same data represented by t.he vprt.ieal cbshf'd lines of Fig. 7.-'1 in a slightly different form.
' l.i, ":the
/"·--....,\
\Vhen the value of
h
...
d u~-
exceeds ahou{
downstream floor position has little effect on
thP coefficient of discharge but i.lwrr~ is a dN.:reasP in the coefficient caused by t.ailwat.er submergence. In this the rnodifir:u.ion in the value of t.hc coefficient cctn be read directly from Fig. 7.G which has been prepared from fig. 7 Jl. 7.3.7.3
Broad Crest
As discussPd e:trliPr. a weir is b:·oad-
75
4.0
4.2
4.6
4.6
0.0
1.0
}-
z
w
()
1.4
0:
w
()_
1.0
I
" lI
~
w
u
w
O.l
ij!
1.2
"'
:r: ()
1.1
<J)
z
0
w
"'"'w ::< al ::l
"'"0
u.
1.0
0
}-
z w
0.9
Q 0.6
1::
0.7
8
w
"':
w w 0:
"'w 0
w
0.6
2
"'"'0:w
4
0
0.0
L)
w 0.4 0.0 0.2 0.1 0
POSITION OF DOWNSTREAM APRON
FIG. 7' 4
Effects of downstrecim influen08s on f!OV'I over weir crests
:J.087 I 11 :1'"
Q
WHEN
q
1.811
~
'
discharge in eum/sec
L
crest length in metres
II
hc;Hf producing flow in met-res (including head due to velocity of approach)
Coefficient of discharge is 3.087 in English units ancl1.70G in metric units. In cases \V\wn the upstn~am of the dam has been silted up to crest. the basic coefficient of discharge should be taken as for h:coacl crcstNl weir. b) In the design of sluiceway crest with freP overfa\1, the basic coefficient of dischaq;c shouid he taken as 3.087 in English units and 1.70G in metric units. Effect of submergence should he calcuLltcd as per values calculated under Fig. 7.7.
7.3.8
POND LEVEL
The pond level is t.hc· wa\.er ]pvel required in the undcrsluiec pocket upst.rc:1m of thf' intake wat.rr to feed full ~uppiy diseharg:(; in the c,1.nai. The pond lcvtd .o;hould be such that the command level in the can~d can be attained with Uw (ksign dischargt' after meeting all t.hc losses in head incurred in passing through the Intake. Various losses al t.he lntake structure arc enumerated as follows:-i) Change in an1;ie of flow in,; water from approach channel towards Intake structure. ii) Loss due to convcrg<'ncc at inlcttrltnsition (or entrance loss at the sill arcai. iii) Trash rack loss. iv) Sudden constriction loss at entry of the gate. v) Gradual contraction or transition Joss from the gate opening up to the st.arl of Lhe uniform barred section. vi) Friction loss along pipe/ba1Tel. vii) Bend loss, if any. viii) Sudden expansion loss at. exit. ix) Loss due Lo
diveq{t~nce
al the otttict transition.
Methods of solving for each kind of losses have br:cn detailed out in chapter 9 of the lvlanual.
If under certain situations t.hert' is a limitation of pond level, the full supply lc·.,·e\ shall be fixed by substracting the working head from the pond level. 7.4
7.4.1
HYDRAULIC DESIGN OF SPILLWAY AND SLUICEWAY Economic Design
In the desir;n of diversion dam, l.wo t'act.ors pla.Y predominant. role ·- "afflux" :tnd "pond \evrl''. As clis.r.usscd under paras. 7 ..1.2 & 7.53.R afflux is fixed from considerations of submergence. habitation of valuable land. site conditions eLc. The p(;rrnissible afflux may or may not include consideration of cost only. Pond levr:l, however. is determined hy topography of the eommand area and Lhere is no scope for adjustment. of pond level on consideration of cost of the structure. \Vhcn permissible affiux is decided, the maximum intensity of discharge is fixed and hence the
77
------------------------------· 1.00
-,---,---
.--,---~-..----,-,---r·--,-----,--,--,,--,~-------,-___.-=-·"'~·
f---1---1---+---+--t----t·-----t--+---t- ;:::<"f~t----t---t----t--t·-----j
----'-----~--
f-----1----
,, ""n1 )>
-l
0
, 0
()
0 0
l>
I
"
n1
()
()
0
0
0
1
0 0 iii rn
z
-l
i2d
/
/
1
0.80
lL
I I
'
He
'
1
~d
:
1---+---+---1+--+---~--.
., ,, -l
:
~
'
rn rn -n
"'
:
I
,
Gl n1
+-----+--+---+---c'l-··-- -
r·---+--+~---==v----~~"-_J__V______J.____L_···_·_·=r~= __j ____L_----j[---r-1-~ V ~~--~11;;----------------: l
o.9o 1-----+---+--+-/-___.F---t---1
Ui ;:: I
I
I
0
n1
_- _.
'
i
-~
:
1
~~
1
i -~77//777777777
7/J/7///7///::
/I
---+---1---!---l---1--- ---t---1----+---+---t---+----- - - - ---1----1
0.76 1.0
1.1
1.2
1.3 P 0 SIT ION
1.4 OF
DOWNSTREAM
I. 5 APRON
Ratio of discharge coeificient$ due to apron
1.6 -
1.7
hd+d
efh ct.
FIG. 7.5
I .8
1.0
r---~~--~----ll----r---~-----r-~=-=r====1=====t====i=====f=====r~~r=--~-----r----l ------~---- - - r - - - ------+----+----::;,.-f""---t--+-----+----l--+----+----+--l---+---+-----4
'" 1-=----~---+//v i
.,;o "' rn 0 rn 0.., ::2 ;: "' 0 :r: )>
;o
"
0 ()
01
c
0.4
I
'r------+1+---+---+----+------l----------+-f---------1
rn :)!
__ 4
1
;;;
~e
0
l
:
~
rL/72/ / '/
r
:
I
.,z .,;;;z ~~
!
i
////////
1'i (l
()
-- -r ---- -r:ha- - - - - - - - - - - - -r· -l_hd
0
rn
'l1
0.6 --
::! m
0
0
i~ _____+----+--~1--+---+----~L_-______.__+--_:=::1_--t-+--1:
I-----------t---+-~1-----+----
)>
-1
0
/v
02
I
~-----+---+--~~------r------+----+----~---+---+--+-----+---+--+-----1----+--0 0
0.1
0.2
0-4
0.3 DEGREE
OF
SUBMERGENCE
0.5
0.6
0.7
h d He
Ratio of discharge coefficients due to toilwater effect.
FIG. 7.6
0.8
minimum waterway. \Vlwn pond level is decided, the height of gates and depth of dam floor get fixed. At this stage, attempt t.o reduce the cost of diversion darn consists in finding out:--a) whether inrreasing the waterway frorn the minimum value (which has been fixed by maximum allowable discharge intensity) would reduce the cost and if so whaL should be the waterways for minimum· cos t.s? b) to what extent. Lbe length of floor may be reduced for minimurn cost? One considc-~ration for fixing minimum wal.envay is the rnaximurn discharge intensity consistent. with allo\-vabie afflux. Another consideration is the maximurn depth of cut-off, which is ·worked ouL on the basis of scour dc'pLh. The practicability of Lht~ (~utoff should be considered from view poinl of driving sheetpi!c in alluvial soil or excavating cut.off pit in boulder reach. The maximum discharge intensity which satisfies both these criteria is adopted and thc: minimum waterway is calculated. Though it would appPar that. minimum \Vall~rway \VO\l!d give minimum cost, this is not. always true because dccrcasP in \vatenvay is increase in discharge intensity and would mean interaction of the following two opposite f:ld.ors: i) decrease in e~ost of gaLc~s. bridges and piers;
ii) inerease in ("(}SL on account of increase in,_lcngth and thickness of downstream floor, dt'pth of cutoff, volume of p(:rvious prot PeLion. work at ends ·or floor, cost of dewatering operaLions, cost of launching apron in guide banks and afflux embankments. lL is the net n'sult of lhc.'>c~ two farlors which dc~tcrminp to what extent the minimum waterway fixed from above eonsickratinns should be ineecascd for maximum economy. IL has bc:c~n found Lhat incrc:asp in discharge~ intensity (decrease in waU~rway) at first reduces the eosl. i\flcr l"P.:-tching a minimum, thC' cost increases with increase in discharge intensity, specially in those cast~s where gates are of bigger siz('S.
7.4.2
Waterw"J.y
As discusse~d in t.lw prc't:C'ding para.f~Taphs. in a diversion dam, the length of waterway and afflux are eorreht.cd. For thr c'conomical and hydr:rulicall:y e[ficiE~nt design, the combination bet\-;1een the two is requi1·ed to be mark by trial and error. Vi/hcn~as, in deep and confined riv('rs v•.'it.h stable banks. t.hc: overall wat.crway (between abut.ment.s including t.hirkrwss of piers) should he approximately cqual Lo the actual widt.h of the river at. the design flood, for shailow and meandering rivers with pcrl11l:':thlc: foundations, Lhc minimum stable width of Lhl~ river at. Uw design flood. may be worked out. from Lacey's formula:
\vhcre
F
minimum st.ablf' widi h of t.hc river in m,
design flood discharge in cunwcs. For determining the~ preliminary value of the waterway, stable width so determined can be taken as a guide.
__ . At._:~ sit~c .\~h~~rt~. due~ l:n 0xiste~1cc .ol gravels. boulders and cobbles in the foundations, depth for :--twP;._ p:ltn1~ _Js.. tlml_lPO: maxJ!:ltrn: u~1!. discharge t.h;tt would not. scour the bed below the practical depth of ("ULoti sl1oul(i ill·st. tw (!ct.ennnwcJ w1th Lace_y's formub discussed in section 7.3.3 and given as foilenvs:
80
----·-o BROi-\0 CRESTED WEIR !. DiSCH.ARGE
FOf~
Q= 3.087 LH3/2 Q
= i . 706
I L DiSCHARGE
ji AFFLuX·Eci.-9
(METRIC)
I
SUBMERGED CREST CONDITION:
EL.
-~
I
_,__ I
'
( ENGLISH UNITS)
L H 3/ 2 FOR
I
FREE FLOVI CONDITION:
:LT. W. EL. (M. FU
I
:he I
'1-
,i'l~
-'--CREST LEVEL
REL!-\TiVE SU3l',IEHGED
COEFFiCIENTS s
CREST AND
FREE
FLOW CONDITIONS
-·----~-
h 11
~· ~-·-·
.,
3 1 nc
0.0 0. t 0.2 0. 3 0. 4 0.5
I
tI
I +- '
I i --+i
r:r 4 v 1.000 0.991 0.983 0.972 0.956 0. 937
··--·-··t·-----~--
' /'nc ns
f
I
1=-:~
I
·""'
0.6
~---------
I
'1
------
L
·······-----~--.
..
"'
'
'.-
I
'lI
0.':3_07 -0.856 --------0.778 ---··---·-·--------0. 62i . 0.000 -' ----- - -
0.7 0.8 0.9 i.O
~ !'">
'v-£1 .....
--
C 5 = COEFFiCIENT FOR SUBMERGED CREST C =COEFFICIENT FOR FREE FLOW
NOTE: ABOVE IS AN EXCERPT FROM 1 TABLE 2a, ENG G. FOR DAMS, P. 373 1 VOL. II, BY CREAGER, HINDS & JUSTIN.
""I G. 7.7
R
1.35 (i-)l t J (metric unit)
q
unit discharge (allowing 20% concentration)
f
silt factor corres ponding to grain siz~ at a minimum depth of 1 m : or deeper where feasible
R
scour depth below des ign flood level.
Generally, in bo~J!dcr r eaches, it would be economical to reduce the waterway to about 0.6 to 0.80 times thr> Lacey's waterway. In plains where the silt factor is in the neighbourhood of unity, it is generally economical to keep the waterway 1.0 to 1.2 times the Lacey's Waterway . It is preferable to have a narrower waterway, which also reduces shoal formations.
7.4.3
Shape of C rests
The ove rfl ow portion of LIF' dn.m (sp ill way) may be ogee-shaped w ith upstream face ve r tical or inclined. The sluiceway shou ld be of slab type with an upstream slope of 2:1 to 3:1 depending on site cond itio ns. The downstream slope for sluiceway shall be as required for the glacis of stilling basin and is generally 3:1. For major projects it is advisable to undertake moclcl studies for obtaining the best shape of the crests. For weirs with falling shutlers, top width of the crest shall be limited to that r equired for accommodating the shutter in fallen position. 7.4.3.1
Ogee Crest Profile
To bC' acceptable. the ogee profile should provide maximum possible hydraulic efficiency, structural stability and economy and also avoid the formation of objectionable sub-atmospheric p r essu r e at the surface . In general. the majority of low overflow dam cross sections ai-e designed with a vertical upstream face. Model tests have shown that the effect of the approach velocity is negligeble when the depth of flow (P) below the crest is greater than about 1-l/3 t imes the head of flow over the crest (Hd). With the ratio P/Hd less than 1 -1/3, the approach velocity is seen to have a noticeable effect on the discharge and nappe profile. The crest shape recommended helow is based on the studies by the United States Army Corps of Engineers after comparison with the ex perimental data of the Uni t ed States Bureau of Reclamation . a)
Downstream Profile
The downstream profile may conform to the equation:-
---
1.85
X
=
where Hd X,Y
~
=
2.0 Tid design head
coordinate shown in Fig. 7.8
. Th~ profile plotted hasecl on the ahovC' equation is designed for one value of des ign head (hd) only whtch IS generally rhosen to give the maximum practicable hydraulic efficiency. For lower heads of flow the press ur e on Lhe rresl will be above atmospheric and [or higher heods of flow the pressure will be 1 " " " ~h-a n atrnosnh~ r ir . T hr> lo\~r>r thP desig n head of the f'rp s t profi!.-.. . t hf' grea ter wi ll bC' the disch a rge coefficient for the lull ran g e ot head s . Model tests show that the design head may be exceeded by 25 percent up to which limit no harmful cavitation is seen to develop. The data for xUl!) ancl 2Hd 0·85 are tabulated on F'ig. 7 .8 for the convenience of the designer.
82
-··-----·------------·
0
~_I c:-1---~
! !
i
~i
-d
li
::t:
:::r: i
I . ii
CREST
I
;\ 1.8 5
v
"
X
xLB5
H~
0.10
0.0141
6
27.0!!5
0.15 0.20 0.25 0.30
0.0299 0.0509 0.0769 0.1078
7
36.596
B
46.831
9
58.257
10
70.795
0.35 0.40
0.!434 0.183G
12
S-9.!94
6
i31.928
168.897 210.017
7 ·8 9
2 3 4 5
.H~t
2.000
26
31.896
1.00
1.096
3.605 5.01.38 6.498 7.855
27 28
32.93 7 3 3.97! 35.000 3 6.02t"t
1.05 1.10 1.15 1.20
I. I 6 I 1.226 1.291 I. 3 58
1.25 1.30 1.3 5 1.40
I .425 1.492 1.560
!0.460 !L713
31 32 33
1.45
1.697
335.64"0
ii
37
42.062 43.053
i.~O
12
t5.354 16.532
36
£5 40.349
1.766 1.835
38 39 40
44.040 46.023 46.002
1.60 1.65 1.70
1.905 I. 975 2.045
41
46.978 4 7. 9 50
1.75 1.80
2.116 2.188
0.70 0.80
o.r)tG9
0.6516
25 30
9.172
0.8229 1.000
3~
7!8:664
13
17.696
1.00 1.20
00
920.049
it'~
18.847'
1.40!
45
! i44.046
!5
IS.S85
1.40 LSO
1.86•\ 2.336
50
l6
55
i390.253 i653.330
1.80 2.00 2.50 3.00 3.50
2.967 3.t505
60
!947.959
65
5.447
70 75
2253.863 2590.785
!2.996
100
5.00
19.638
30
41.067
255.215
16.160
29
33
20
50
X/Hct
34
0.3887
4.00
ay/ox
14.159
0.60
so so
2.H~O.tJ5
12.:>48
0.2233 0.2-(74
7. 6\$3 10.151
h,;
10
0.4~
0.50
0.90
2
0.85
37.041 38.054 39.063 40.0-'>6
l·'i 16 18
~1-.
SLOPE DATA
DO\'INSTRE/l.M QUADRANT DP.TP.
I r\
AXiS·
29<'.3.496
33! 6.77'9 4!24.235
5011.872
21.112 22 .2 2 9
17 18 19
23.33!) 24.(.3~
20
25.:521
2i 22 23 24
20.602
27.674 213.741 29.7'39
25
30.852
42 43 44 45
46 47 48
!.:35
48.919 49.884 50.846 51.607 5<:.761
49
53.714 54.663
50
55.610
OGEE DO\VNSTFtEA~\~1
PHOFiLE) FiG. 7.8
1.629
I
,_""ft, 1I
J Ha _ _ _ _
T
l I I
-
1
:11
~
1
(
----
0.270Hd
"------ORiGIN
OF COORDINATES
i
I
~ 1-]'
. i
L
UPSTREAM QUADRANT
(
j
I
,y
EQUATION: y
= 0.724
I 85 0.375 0 625 (X+0.270H1.!' (Xt0.270Hdl. 1•0.i26H·a-0.4315Hcl H 0.85 ~
COORDINATEs: X/H.
Y/H.
G
0
-o.oooo -o.ozoo -o.o40o -o.osoo -o.osoo
0.0000
2
2
2
-0.1600
Y/H d
0.0296
-0.2400
c
0.0787
0.0004
-0.1700
0.0339
-0.2450
0.0836
0.0016
-0.1800
0.0386
-0.2500
0.0889
0.0038
-0.1900
0.0437
-0.2550
0.0948
0.0068
-0.2000
0.0494
-0.2600
0.1016
-0.1000
O.OlOS
-0.2100
0.0556
-0.2650
0.1099
-0.1200
0.0156
-0.2200
0.0624
-o.2sao
0.1165
-0.1400
0.0221
-0.2300
0.0701
-0.2703
0.1261
OGEE ( UPSTREAM
CREST PROFILE FIG. 7.9
II
i!
h
For pr;::~.ctical purposes and to facilitate the fabrication of templates during construction, the curve developed from the above equation can bu u·ansferrcd into tiw nearest corn pound circular curve.
bl Upstream Proj?},.; The upstream profilf' has to be tangent to the vertical face and tangent to the horizontal at crest point to ensure that there is no discontinuity along the surface of flow. The recommended curve incorporates the ordinates computed by 'M_C,_~9-~Y_N" and others, in the region close to the sharp crested weir where accurate experimental data mcasureiTicnts arc difficult and fits the USER experimental data over the remaining portion. A table of ordin:ttes are given on Fig. 7.9 for the convenience of the designer.
7.4.4 Crest and Floor Elevations of Spillway and Sluiceway 7.4.4.1
Spillway
a) Upstream floor level of spillway should b'c fixed from the following considcrations:i) it should he fixed at the general river bed level which is higher than upstrearn floor level of a sluiceway. This is generaily in a deep. well-defined channel in front of the intake.
ii) foundation on fiJi should be avoided as far as possible which reduces to a minimum dewatering and excavation.
b) Crest levpi of dam can be fixed slightly above pond level to avoid wastage of pond water by \vave wash and anticipated siltation upstrc;un of the dam. A free board of 0.2 m. is recommended.
7.4.4.2
Sluiceway
a) The crest and upstream floor levr.\ of sluiceway should be kept equal to the elevation of the deep section of the river aL darn axis.
b) In fixing crest level of sluiceway Lhe following points should be considered: i) it should be as low as possible for proper flushing of silt deposited in front of intake structure. But it should not he too low to producP a high discharging head causing intensity of discharge in excess of pcrmis.sihle flow for the given t_ype of foun
ii) as the intak(~ sill levei is to hP kept. above the siuieeway crest to avoid silt. entry into the canal, an inercase in the height of sluiceway crest will raise the intake sill higher and reduce depth of flow over intake sill thereby requiring greatPr length of v.:aterway for the intake. lL is necessary to restrict the height of sluiccwa_y crest for economy of intake structure.
I
----j
I
O.llO h
nPond level
i
!.________
h 0
Jj_____,L._h__________
85
•
nnlakc sill level
.ia:!."lt'.' i.<:.c ~'w
ay crest lc vel
From ! ~: sb'Leh showing t.h(' rel:ttive positions of different. components of dlvt'rsion \vorks, it is obvious t.h:1i t.iwr:c is ;t limit !.o adjustment. of sluin~way crest lc~vcl and intake sil! lcvc~l. His recommended that. for /r:::l nkubtinns. l.hP sluiceway crest !c~vel maybe fixed O.S m above the upst.n:am floor level of sluic('W
VVidth of Undersluiceway
The~ fuiF!.ion and discharging l':tjla\'iLy of a sluicr;way has been dealt. with in Chapter G, under "Com· ponent.s or diversion dam". As Llle discharging capnciLy' of Uw siuicev.ray portion is CO·related with the ()Vf'ra!i discharge in the river. t.h(' width t.he sluice\V
or
/i) it. shouid he {'apable of passing at. least double the' canal discharge at Pond lr:vel to ensure good scouring cap;u-ily;
ii) it shonld lH' widr> enough to accomnwdatr> an approach chanrwl and also to keep the approach vcloci\.ir:s. sufr\{'if·tll.ly l(l\ver than crii ical Lo rnsun~ maximum settling of suspended silL; iii) it. should br· n;11-:-ow enough l.o crc~at.e a bottom vclo{'iL.Y in t.lw approarh ,-hanncl which wili rnovc the maximum diallL('L(·r ol· panklcs cxpPc!.cd t.o be dt'posiU!d with t.1w avaiLibl\~ flushing disc.harge. This is nec('ssar.v t.o m:1inLiin approach channel in good co:Hlition so that canal c:u: draw necessary supply Lhroug;h l he inLtk('. iv) in ,·a:--:e it. lwnmws ncl'l'Ssary to hring down t.he afflux ('!cvat.ion for kccpiilg it. ..,~·it.hin pcrroissible limit or for- limliil!{ down.i'dingly. No h:1rd :1nd fa.-:i ruk can lw i:1id down in'twe('li width of sluie('way and w;ttl'l'W
(spiihv;~.y
vl wiwr(' ;--;ili. r'xr:lud\'rs ar1' pr\)vidul, t.lw width of t.hc poekr:t should lw dd.c'rmincd by the velocity required in \.h(' po('kl'·l- \n induce silt:\i.ion; vi) wiwn tlw widLh of the spillway portion is ;:ppn~ciablr_~. riV('r sluices adjoining t.he pockets can
be provid('d t.o take can· of lO\\. floods and freshets thprehy economizing on t.ht cost.; vii) For other guidclin(·s in regarding \he width of th(~ pocket rc~fr:rence may be made Co para 6.6 of
Chapter G. 7.4.5.1 On all import.:tnt works Uw widj h :-ol· t.he sluiceway pod.ion and the Jen,1;l.h of training wall shall be fixed on Uw basis of model n:pcrimcnt.s.
7.4.6
Length and Level of Downstream Floor
The lengih :\iHl lr~vP! of downstream finor should be fhlermined from consickr:ti.ions of dissipalin~ the ('ncrgy of surf:u'r' flow, so Lhal. t ht' i'l'sidual (·~ncq;·y af!.e;- leaving the con(·.ret.e floor i;; too lit.t.lc to cause any C'rosion ni' th(' s!r(';Jm hr~d. ;-;t.;lnding w;l_V(' is eonsidered as one of t.lw rnost. cfre{'t.iv·c: means of killing t.lw surplus vrH~rg_y in t.lw hydraulic structure. The desi,t;n nf dnwnst.n:am proter!ivf' works or energy dissipai.ors is t~ssenLi:dly one of reducing the high veloeiLy flow low enough to minimize erosion of the natural river iwd.
7.5
ENERGY DISSIPATORS
From a pl·;u·ti(·:ti view point-. h.rdr;wlic .1:nmp i..:.: a use:·u) mr·;tns nf di.c.::-.;ipai.iil.l;· t·:..:Cl':;s crwrg_1.· in suppr criLil:al fiow or w;\i.vr discl1arging OV('i' a dattl. il.s nlct'it is in prc·\'\'nLing possible erosion lwimv the overflow
8i)
spillway and sluices, for it quickly reduces the velocity of f1ow on a paved apron to a point when the flow becomes incapable of scouring the downstream bed of the river. Hydraulic jump basin is an effective device for reducing the exit velocities to a tranquil state. Hydraulic jump type slilling basins arc of two types:i)
horizontal apron type
ii) sloping apron lypc
7.5.1
Genera! Requirements
Among various factors which nffect the design of energy dissipaters, the most important. one is the depth discharge relationship of the exit channel at the site of the slructurc. To know its impiicat.ions on the design of energy dissipator is very necessary. The general formula for Lhe hydraulic jump on a hori:r:.ontal channel of rectangular section is as follows:
Where D1 and D:! :tre the conjugate depths upstream and downstream of the jump and V 1 & V2 are the corresponding velocities. The height. of the t.ailwat.er for each discharge may or may not correspond to the height of jump. The jump height curve may be related to tailwatcr rating curve in four ways. il Jump height cur·vc always above lhc tail water rating curve. ii) .Jump height curve always below the Lailwater rating- curve. iii) .Jump heig-ht curve above the tail\,,atcr rating curve at lov..' discharges and belo·w at higher discharges. iv) Jump height curve bclov,r the tail water curve at low discharges and above aL high discharges. All the stages ;:1.rc shown in F'ig. 7.10 in respective order.
7.5.2
Horizontol Type
When the tailwater rating {:ur·ve approxirn;:J.Lcly follows the hydraulic jump curve or is only slightly above or below it, then hydraulic jump type stilling basins vdth hori7,ontal apron provides Lhc best solution for encr!{y dissipation. In this case the requisite depth may be obtained on a proper apron near or at the ground level so that it is quite cconornic:.li. For overflow dams spillway on weak bedrock conditions and diversion dam/weirs on sand and loose gravel, hydraulic jump Lypc stilling basins arc recommended.
7.5.3
Hydrauiic Jurnp With Sloping Apron
\Vhen the tai\watcr is too deep as compared to the sequent depth, the jet left. at the natural ground level would continue Lo go as a strong current near the bed forming a drowned jump, which is harmful to the river hcci. In such a case, a hydraulic jump type stilling basin with sloping apron should be preferred as it would allow an efficient jump to he formed at suitable level on the sloping apron . .
··
).\.
87
rj
/
I
·;
-·
/
TAlL WATER DEPTH
/////////////
D = C 0 NJUG ATE DEPTH 2 (SEQUENT DEF'TH)
-·JUMP RATING I
~I
l-i ow G. I zO
I· !
<(
0::
l!J 1--i--
!
o~l
~a:l
I
Z
W3: 0-
w
I
i/
RATING
Ul 1-- !L_ II _ _ _ _ __
DISCHARGE Q
__ JUMP RATING
I
I
(
Sl z 01 51 Z
15
!
0
o zo
8 'I~~
I
"/\·--TAIL WATER
w
IDl S CHARGE 0
I
• /
5::::' l
::> ...J
!1
. _/
~--~,
Z
(f)
//
r-
<(
::l...J
<(
0-
Ull--
w
DISCHARGE Q
/-///
/./
w3:
/i
/
(_TAIL WATER RATING
I
(l_
(//
i--
I I-
\.··-TAIL WATER RATING
D I S C H P.R G E Q
w
I/-
oo Zo::
I
~/
I-I-
z
::>...J
o-
w
(/)I-
(LUMP
DISCHARGE Q
1
! lI
I
I
'l
RATING
TJ..\!L 'vVATEfi CONO!T!ONS CLASSiF!C,I\T!ON FIG. 7.10
7.5.4
Design Criteria
7.5.4
General
On Uw basis of eomprehcnsiv(; series of tests conduded by lJ.S. Bureau of Reclamation properties of hydraulic jump have been d('Lermin(~d . .Jump form and the flo1v characteristic can be related t.o t.he kinetic flow factor (V 2 /gd) of t.hr. discharge entering the basin, the critical depth of flow {d,), or the Froude numbt~r {vJJi~·d).
/\ccordingl_y, h~ydraulk jump type stilling basin with horizontal apron can be classified into the foliowing eategories: a) Stilling basins whr~re t.he incoming flow Froudc factor is less than 1.7 arc designated as Type L h) Stilling h:\sins where the incoming flnw~-Froude fador is greater than 4.5 and the incoming velocity (V 1) exceeds ·15 m/scc. are designated as Type IL c) Stilling basins vvlwr(: the' incorning flow Froude factor is above 4.5 and the incoming (v 1) does not exceed -1.5 m/sec. arc designated as Type III. d) Stilling basins where the incoming flow Froudc factor-is between 2.5 and 4.5 arc designated as Type IV. 7.5.4.2
Terminoiogy
For the JHlrpost~ of t.hP (ksign criteria described hereunder, the following terminology and parameters (indicated in Fig. 7.11) shall apply: L Hydraulic jump ··- It is an abrupt transition of flow from water depth D 1I\ 2. Length of Hydraulie .Jump The dist.aner- irom the beginning of the jump to a point downstream when either the high velocit.y j('t. begins Lo le~tve the floor or to a point on the surface immediately downstream of the rollPr, whichever is longer. This can be determined from Fig. 7.111\. 3. Conjugate Depths ·~ \Vater depths at the beginning and at the end of the hydraulic jump related
hy the formula.
!
2
'!. Chute Block
rJ1 ; sF1 2 -- 1 ]
For horizontal apron
l
Triangular blocks inst.alltd at the upstream end of the stilling basins.
5. Basin Hloeks/Bafrlc Blocks/Baifle Piers -~ Blocks installed on the basin floor between chute bloeks
and end silL G. I~nd Sill ~- Solid or dent.at.ed 1val! const.ruet.cd at the downstream end of the stilling basin. 7. F'roude Number -- 1\ dimensionless number eharaderising the vertical and gravitational forces.
v
I;;,-a when
7.5.4.3
F - FroudP Number V -
velocity of flow
D
Depth of flow
Stilling Basin Parameters
The main factors for Uw basin required t.o be determined arc basin floor cievation, it.s appurtenances. /----~-.
89
and
r-
l l
r:t ;;~ >-~
,:; '-{
;{;
I I
I
------------------:
1----Vl
I 02
i
(-i-i
"~;!
..L
···~
FIG. 7.11
NOTATIONS
II
'D;= DEPTH OF FLOW AT THE BEGINNING OF THE JUMP, MEASURED PERPENDICULAR TO THE FLOOR.
IDz =DEPTH
I'
02 ' ~DEPTH CONJVGATE(Sequont)TO D2 FOR
LENGTH OF BASIN
Lj=
LENGTH OF
HYDRAULIC JUMP
q = DISCHARGE INTENSITY Sh= SPACING OF BASIN BLOCKS
DEPTH OF BASIN
Sd= SPACING OF DENTS IN DENTATED SILL
F1 = FROUOE NUMBER OF FLOW AT THE BEGiNNING OF THE l
IV
Lb=
SLOPING APRON( OR PARTLY SLOPING AND PARTLY HORIZONTAL)
Dc=CRITICAL WATER DEPTH
JUMP.
g =ACCELERATION DUE TO GRAVITY
I hb
SHAPE FACTOR
I= LENGTH OF THE INCLINED PORTION IN BASIN
CONJUGATE(Saqu•nt)TO D1 FOR
HORIZONTAL APRON.
Db~
K=
=HEiGHT
se= SPACING OF CHUTE BLOCKS
V; =VELOCITY OF FLOW AT THE BEGINNING OF THE JUMP Vz= VELOCITY
OF FLOW AT THE
END
OF
JUMP
0 F BASiN BLOCI<S
Wb= WIDTH
OF BASIN BLOCKS
OF THE CHUTE BLOCKS
We= WIDTH
OF CHUTE BLOCKS
l
he =HE! GHT
HL =HEAD LOSS IN HYDRAULIC JUMP
Wd= WIDTH OF DENTS IN DENTATED SILL
ho =HEIGHT OF END SILL
-8- =ANGLE OF THE SLOPING APRllN WITH
THE HORIZONTAL.
f'
'
<;
8
·I
I
I
7 6
Lj
D2
Ic-~----~--I I
--+- -
---<--<
5 ~~l /~-·--I v I
<
4 r-:~----------
~--- - - - -
2i 0
4
2
~
I
--1---
/HORIZONTAL APRON
-
'-=- t 1-~
I
3
tt I
\ " -I~ _,.J~
.J"-'"
DI
8
6
10
fj=~
12
'
~-- '-i _ j
e
14
18
16
20
YiiDI"
LENGTH OF JUMP IN TERMS OF CONJUGiiJE DEPTH Dz 0.25 TAN e=o.3o
0.15
I
0<20
I
0<10
o
26
-----~-<-f----------+-----+-----r'--1
22
-----L----~----t----,--Tt----1
18
I
I
I 1" --------1-< . <---- --·--j----
Dz
!yo+
'<-
I
D I I £.•
I
--<--1----l----,'I?L...._I----l---------1
10-------
. !
2
v. F!=-'
~
RATIO OF CONJUGATE
DEPTH Dz TO
D1
FIG< 711- A
i)
Hasi·n Floor
J~'lc1:at.ion
D can be c:1lcuiated from the foliowlng equaLio:1 knowing HL, q, D(' ~lnd D 1 or from Fig. 7.15 . 2
De
., (
)1 ':\
g
DI 2
+
II:tvinl{ obt.airwd 1) 1 and l}), Lhr rk·v;1t.ion of tlw basin floor may be calculated b_v eiUwr clPduct.ing the specific energy at _..;(·\_·t.i()n i --from Lhc t.otai energy line at. that. section or thaL at sect-ion 2 -·- 2 from the downstream tot-al energy LinH'.
1
To eaicuhtr- Hr, from the known upstream and do1vnstrcam toLtl energy lines, the foilowing procedure maybe adopted: :d \Vhcre t.he basin j:-; dirr-cl.iy dnwnstr('afll from the crest or where t.hr ('hute in noi longer than t.hc hydr·aulie head. IIL may be assunH~d equal t.o the diffcren<:P in the upstream and downstream total energy lines. hl \Vlwre the chute• lt~n~~-t.h c·xccPds thP h_ydraulir head, D 1 should be first. dctcrmin~d by Laking· HL equal t.o the dilfcn·n('(' br-twr•en tlw upst.n~am and downstream total Pnergy lines. Next calculate friction losses in Llu~ chui.l' hy :\Lulning·'s formula and determine the more accurate value of Hr, and consequently that of I\. The process may be r(~pc:>. Led till t.he dr>sired accuracy is achieved.
7.5.5
Stilling Basin Type !
This typP does not tl\'(~d a spc<"ial stilling basin t.o still f!O\VS, except Lhat. the channel lengths beyond the poinl whr-n' the depth starts to change should not. he less than about ,1cJ~. No baffles or other dissipating devices are nu·d('d. 7.5.6
Stilling Basin Type il
This typt· of st iiiing h;l:::in i.'-' usu;lliy us<·d on high dam and earth dam spihvays and largT canal st; :wLtJrvs. It ronLains ch;!i(' h]{)('_i~s :1t. the upstream end and a den La Led sit! nc~ar the dow;1::;trearn end. No baffle pier::: an· usc~d lweauS(' of th(~ relatively high velocities entering the jump. T!w following
rul(~s
arP rccommcndPd for genPralizaLion of Basin II. Figure 7.12:
I. Set. apron elevation t.o ut.iiize full {'onjugate (.aihvatcr depth, plus an added factor of safety if needed. An additional L:H·.t.or of safeLy is advisahk for both low and high values of the Froudc number. A minimurr. margin of safety of G perr(~nt of D:~ is recommended.
2. Basin II may he dfecLivr: down to a Froude number of 4. 3. The kngLh of basin c:u1 lw obtained from the eurve on Figure 7.J2 (c). 1. ThP !wight of chuLP blocks is ('qu:li Lo the depth of flow entering Uw basin, or Dl. F'igurc 7.12 {A). Tht• width and spacing should he cqu:l! to approxinwt.cly D 1: ho\VPvcr. i.his may lJp variC'd to eliminate t.r;tditional blocks. :\ span~ l'(jt!al to D 1 /'2 is pr;_cferabie along each wall to reduce spray ~1nd maintain desirable prcssur(·s.
92
;). The hc,igh! of t.he dentated sill is equal to 0.2D~.? and the maximum \vidth and spacing recommended is approximately O.Fi D~~· On the· si!! a dentate is •eeonnnendcd adjacent to each walL Figure 7.12 (A). The slope of Uw continuous portion of the sili is 2:1. For narrow basins. \Vhich contain only a few dent.at.cs according to th(' above rulP. it is advisable to reduce the width and tlw spacing. Howevpr, \Vidths and spaces should remain c·qual. H.Pducing- Uw width and spacing ad.ually improvps Lhc performance in narrow basins; thus, the minimum width and spacing of lhc clcnLatcs is governed only by structural considera lions. G. It is not. nccessary Lo stagger the rhulc blocks with respect t.o the sill dentate.:;. In fact, this prarlice is usually inadvisable from a construction standpoint. 7. lt. is rr.commcndc·d that t.hr sharp int.erseet.ion hrhvcen chute and basin apron, F'igurP 7.12 (Al. be replacl'd wit.h a curvr· of reasonable radius (H ill) 1 ) when the slope of Lhe chute is 1:1 or greater. Chut.e hior.k.:; can be incorporat.('d on t.hc curved bee as readily as on the plane surf:lccs.
>
Following t.hc abnvf' ruk•s sill rcsuit. in a safe. conservative stilling basin L)r spillways up loGO meters high :tnd for fiows up to 4S Ul. m.isc~c. per rncd.er of basin width, provickd t.hc jet entering Lh(: basin is reasonably uniform hot.h as to velocity and depth. For greater falls, larger unit disch:trgcs or possible asymmetry, a modC'l st.udy of t.hP spPcifi(~ design is recommended.
7.5.7
Stilling Basin Type 1!!
This type of h;tsin wa::: cil'VC!lopPd for a class of smallPr st.rudures in which vplocitiC's ai t.he entrance t.o t.lw basin arc moderate or low (up t.o 15·18 meters per second} :tnd disclnrges per meter of \Vidt.h are less than tB cubic mct.cr per .:;econd. The fo!lO\ving
rt~les
pertain t.o tlw design or this type of basin:
1. The stilling b:\sin operat.PS best. ..-d. full con_jug;tt.c~ tailwat.c~r depth. D~. A l'l':lSOnablc fZJ.ct.or or safety is inhen•n\ in t.h(' eonju)~·:ll.c~ depth for all values of the Froudc number and it. is rc~commcnded that. this margin of saf!.•l.y (Figun' 7.t:nn no! be reduced.
2. ThP length of ln:::in which i.s lc.:;s than OIW·half the kngi.h or the natural jump, can he obt.ain('d by consulting t.hP cu;·vp for Ba.sin III in Figure 7.1:3 (D). As a reminder, an excess of tailwaLer depth does not. substitute for pool length or vice versa. :.L Stilling Basin III may be effcct.ive for values of Froude number as low as .-1.0. ·1. Height, width and spacing nf chu!.e bloeks should equal t.he avrragc depth of fiow entering l.hc basin, or D 1• Width nf hlol'ks may lw dc:<"n~ased, provich:d spacing is reduced a likc> amount. Should D 1 pro\·c· to be lPss !.han 20 em., t.!w b]()('ks sho11lrl he made 20 centimeters high. Larger chute blocks t.end lo t.hrow a portion of Lhc hig:h velocity jcL over Uw baffk piers.
!5. The blocks may be vertical also shown not appear eommendcd
height of Lhc bafi'lc piers varir·s wit.h the Froude number and is given in Figure 7.1:3 (r.l. The be ('Ulws nr t.hcy may be constructed as shown in Figure 7.1;3 (A): the upstream face should and in one plane. Tlw vpr!.ical face is important. The width and spacing of baffle piers are in figure 7.i/} (/\l. 1n narrow strudun~s where the specified width and spacing of blocks do practical, bl(wk ·.vidt.h and spacing may be reduced a lik(~ amount. A half space is re· a{ljaccnt. t.n the· \valls.
G. The upst.rc~am face of t.he b:tfflc piers shc;uld be set. at a distance of 0.8 D 2 from Lhe downstrc~am face of t.he chute blod::5 (Figure 7.]:}/\). The reeornmPlHied po5it.ion, hr~ight. and spacing of the baffle piers on t.hP apron shouid be adhered t.o cardttll.y. as i.hPs(: dim(~nsions ;u·(' irnport.ant.. For example, if t.hc blocks are set. appreciabiv upstream from i.lw poc:i!inn shown. t.hc~' will prndr.H'I.' a casc:uie with rcsnit-ing wav0 ;1.('(ion. lf the: baffle~') ~·':'~
f;:·;·h, •.
~':-,,,,_,, .. " " ' "
'h.,,
c:hn\'.'!~
.,
ln•;•Yr•-
<~ .. "'":~
,,.;ll
t •.•
- , r , , ; ..."l
1 .:1,. .,.,,;"'~'
.. ,,,,,,,.. ,
93
;r ~•-"
1, .. cn".'
t-he hafflc piers on the pool floor at randorn and expect anything like the cxcc!tcnt action ot.hcnvisc associated with Type Ill b<1sin. 7. The height of t.hc solid sill at the end of the basin is given in Figure 7. 13(C). The siopc is 2:1 upward in the direction of flov-.:. \Vith this slope, rninirnum wave heights and erosion could be expected. 8. Il is und('sirable t.o round or sLrcamlirw t.hP edges of lhC" chute blocks. end sill or baffk piers. St.n;amiining of baffle piPrs may result in loss of half of their effectiveness. Small chamfers t.o prevent chipping of the edges may be usr:d. 9. It is recommendt~d t.hat a radius of reasonable length (H ~:,:_~ 1D 1) be used at. t.h1~ intersection of t.lw ehut.e and basin apron for slopPs at -1S 0 or greater. 10. As a general rule, the slope of Lhc~ chute has little effect on Lhe jump unless long flat. slopes arc involved.
7.5.8
Stilling Basin Type IV
This type of basin is :uleq~1a!r' for FroudP numbers lwtwpen 2.:} and '!.:). The low Froude nurnber range is enrounLercci prin<'ipally in t lw (ksig-n of canal struct.ures, hut oceasionally low dams and outiet works Ldl in this <":liegory. For !.his range of Froude nurnhcr, the entering jet. oscillates intermit.!.('llt.\y from bottom to surLu'C' with no partirttlar pC'riod. Each osc~illation gcn(~rat.cs a wave whieh is difficult t.o dampen. In narrow st.ntet.ures, sw'h as eana!s, waves may persist to some degree for miles. As they encounter oh:-d.rudions in the canal. surh as hridgP piers, turnouts, C':hccks and t.ransit.ions, reflected \vaves may he gcrwrated whid1 t.enrl to dampen, modify or intensify the origin::d w;1vc. ·wav('s arc dest.ru\Jive t.o e:trt.h-lined canais and ripr:1p and producl' con:-:;iderable surges at gaging stations and in measunng dPviees. St.ruci.Urt's in this range of Froud(~ nurnbers are lhc ones which have been found to require t.he most. maintenance. On \·vidr' St.l'U\·tun•s. S\!(~h as div('rsion dams. \vavt~ acl.ion is not. as pronounecd when the waves t'all \ravel iatf'rally as well as par;tllPl Co t.lw direction of flow. The eornbincd action produces ~ome dampening dfr:c.t. but. also r('sult.s in a d10ppy wat.er surface. The waves may or may not. be dissipated in a short. distance.
Thr: dimensions and proportions for Type IV Ba:;;;in arc shown in Figure 7.11. The width of t.he blocks is shown equal t.o D. and t.his is Lhc maximum n'commended. From a llydraulic standpoint, it. is d(:slrahle th;:tl. t.hc: blocks lw consl.ruct.('d narrower t.han inditaLed, preferably 0.75 D 1 . The ratio of block widt.h to spacing should be m:tinLlirwd as 1:2.:). The extreme Lops of i.he blocks are 2D 1 above Lhe floor o[ Lhe stilling basin. The blocks may appear t.o he ratiwr high. and in some cas<~s. Pxt.remcly long, hut. this is essrn!.ial as the jet lc:1.ving· LhP top of t.he blocks rnust. play at. the base of t.he roller t.o be cffcdivc. To accommodate the various slop<:s or chutes and ogc~c shapes encountered, t.he horizontal top length of the blocks should be at. least. 2D 1. The upper surfacL~ of each hlnck is sloped <:tt :) 0 in a downstream din:clion. A t.ai!wal.cr depth S t.o 10 percent. greater t.han the conjugate depth is strongly recommended for Basin IV. Since the jump i:> V('ry S('n:>it.ivc t.o LaihvaLc'r dc~pt.h at· these low values of t.he froude number, a slight. ddi<'iency in t.ailwat.er depth ma_y allow the jump to sweep eomp!etcly out. of t.he basin. The jump performs much hPLL<'r and \vavc act.ion is diminished if the tai!wat.cr depth is increased t.o ap proximately L2 ]):?·
Tlw kngt.h of Hasin IV, \vhich is rC'lat.ively short, can be obtained from Figure 7.1:1 {c). No baffles pipr_c; are n('edc:d in ihc basin, as Lhes<' will prove a grcat.c~r ri<:trimcnt. than aid. The addition of a smali triangular sill ph\·('d :l.! t.he end of t.hc apron for scour c:ont.rol is desirable. An end sill of t.he t.ypc used on Basin III is saLisi"ac!nry. H flPsi.l;nu! foo· tlw m; xirnum disch:Hgt_', \)ac;in IV will lH'rfnrm ::;at.isfacloriiy for 'jnw:1.;;i_r·pnn1
only.
i:·n:•'
:t'C• <.:j_)j] tll.>"
h:tsin
will
c.;i_il\
hi';~
~""''!!
l\'SSl'!'
flows. \Vav•,'S
'\f;q bl' of !_j,p ('"(lir~"-!'" V'>.r,·irt.v
I,!St!td\v
Dentated 'li!!Chu ts bloclls-
,, J. ~~;L-~~~~------~~~7r~v ,):.,:.··~·~
o.o
.0
o
~._._·_:_-
·a.'
0
.: L
0
. 0 .
·
0
·.oo_
(A) TYPE li BASIN DiMENSIONS·
,l:±JTi:J±i T-=f0f11E7GiH~oriiJ7f'it~-ti ++F :··:· B 4
6
FJG, 7.12
8
!0 FROUDE
12 NUMBER
14
3 18
18
StillllliJ basin choroctaristics for ust with Froud\! numbers
ohov~
4.!5
End
sill~
0
·' 0
r
, 0
r
0
r
-- -L-
(A) TYPE lli BASIN DIMENSIONS FROUOE
F\G. 7.13
Stillino bo3io
cfHlract~rl~lic,
NUMBER
(or uM• "?.llh Froude; numb:H; obove 4.5 whero incominc velocity (V 1 )
50-60 foet por
:~ocond.
doll~
not exceed
.
Chut~
_}_- r
block~
~-
Ends!!!---, ~QCC
Fractiorwl
-r......_- ~'If -Max. tooth
wi
0
/ \........ -- Sp<JCG" Z..5'1t
.
-
0
0
: 2d, • ., • .
0
~-
0
0
opt!OTKll
-~~. 1
'
n
-Top surfoc•
"
0
n
0
. .<> . .
r
·--
-L-·-·~
·.: C' ••
.:. .:::
:.. ._- ._.
FROUOE NUMSER
i
-
dz
.
'!
I
l -I
r:-c· 1':
~:..:d=z{ljl+SF
,
1
I
I
'l
-
.
-!) _ _
'
~-·
3-
4
3
; { 81 MINIMUM TAILWATER DEPTHS
I
2
2
i
(C 2
:
'
I
I LEhGTH I
4
!
5 ..J!~
'
« JU'MP !
3
4
0
4
FROUDE HUMSER
FIG. 7.14
St!liino;~
basin choract.orl:tlca 1or Froudo
num~rc bet~oon
Z..ti on
7.5.9
Conce;1tration of Discharge
While working- out the size of stilling basin, the incoming unit discharge should be increased by 20 per cent in all calculations in order to cater for various fac~ors such as formation of islands, obliquity of flow and river stage etc.
7.6
TOTAL LENGTH OF FLOOR
'Total length of impervious floor which consists of downstream basin length glacis/spiliway/chute and upstream floor should be fixed from the following consideration: a)
Exit gradient should not exceed the safe limit.
bl The structure should be safe against sliding.
c) The downstream floor length considering dissipation of energy, width of crest, slope length upstream and downstream should he accommodated in the total floor length.
7.7
DEPTH OF CUTOFF
Depth of cutoff should be determined both from scour consideration and checked from safe exit gradient requirements. for determination of scour depth, unit average discharge should be increased by 20 per cent to allow for concentration of flow and depth as per provision under para 7.3.3. The cutoffs shall be suitably extended into the banks on both sides up to at least twice their depth from top of the floors. The depth of the downst.rc<1m cutoffs along the total length of impervious floor shouid also be sufficient to reduce the exit gradient within safe limits for the governing design condition. For caleuht.ion of exit gradient, reference should be made to the detailed procedure given in Chapter 5. The cutoffs may be of masonry, concrete or sheet piles depending upon the type of soil and depth of the cutoffs to suit construction convenience.
7.8
7.8.1
FLOOR THICKNESS
The floor should be designed for the following conditions. i) Uplift. pressure due to static head
Thickness of the floor should be adequate to resist uplift and soil reaction. Uplift pressure due to static head shall be as per the procedure laid down in Chapter 5 para 5.7. ii) Uplift pressure due to hydraulic jump
Due t.o the formation of the jump, high unbalan<~ed pressures are developed m the trough. The floor should he of adequate thickness to \vithstand the effect of the jump. For determining thr: uplift pressure at and upstream of the jump section, it would be necessary to plot t.hP water surhce profih~ of !.he jump for particular flow c:ondition. This can be plotted by using Figure 7.1G. The uplift pressure is the ordinate measured from the hydraulic gradient line to water surface.
The floor at. every section should be designed for the bigger of the two uplift pressures, i.e .. one with hydraulic jump and other for no flow condition. A factor of safety of not lCss than 1.1 should be provided in caleulating the floor thickness. 7.8.2 The U~)i!fl prr>~<::ur~_~<:: nn. t hP jump lentTt.h and thr :equin~rnent of floor thickness are however moderated to sorne extent riue to the· follo\ving factors. a) The uplift due:: to maximum depth of the trough operates only at the deepest point of the trough.
98
0
HL/Dc
.-
:oz:£:.
IDe
I
3
:s.1s
ii:6e '311
raoo
4
.. ,.....
1.425 .301 ~ 1308
·:zr.
r28l '9D'
l1~ -:241 i"i':1ii 240
6
·• D•fo, 'rz;;, o;;~
·3
2
·0
' o:;;D '0.1;, f21;, 0'/o, ""/o, o,
Jo
....' c.o.·
..
·7
o%,/o,;;;; ~~~!o,;o, ,o 2 ;~ jo, /o,
00 i l l :397 I!S.i6 1.360 6.83 .381 .69 1.378 .415 . :»5 ~ 6:79 336 io.94 :331 ·. 0' 1.327 7.23 .323 1.292 aa1 0 I a21 .296 18.3 4 i .290 .3"' \1.07 '9:20 '.276 '9.39 [.274 '9.5 '.2" 19.64 23! '9.7 . 207 '9.69 I .26.5
.82
••••
::3
I·•••
s
II
'"
,.
·,w }j£ G1 ra
,1:9o 235 1<2.01 23' '12.00 ''"-01 '222 13-Oil 1003 !.202 115.13 fi<.n .193 116.20 .193 110:74 ToT m:il4 .196 18.95 .198 16.00 " ' 15 .!87 .188 110.74 17. . '7 91 1 .Iii flU' 75 18.86 174 IMIS .174 119.14 '17( .,.-;-ro ]iC ...:79 .iao i1i.oo . ISS lzoiJ7 .16 :-;<0 0.71 120.60 .1M 120.89 I20.9E .160 1200 l2t4j is1 Lo2 .IOC 21.6 JOO 21.70 '.159 121.88 100
~·7
~~~
..
17~ 22.41
""'
I
~
2
lfi
182 23.3
-;;-;; = = rn:iii :>13 = 2.>--:63 ~ = 'TIT =
~142
22 23 24 20
•• .....••
fMlf
12S•o
..... -
37
••
'
!TIT .
.120 131.,..
,,
13"-78
: .",; [;.,
"" •... ,
142 '6:>.
.12
3:21 J2;t.ii'2.2' :12.5 sUi. ;w :ru50 -:121 [~3.00 -.;;r
--;;-;,
--.-.--
'2,_.-i
T27 lici7il .i J2< i31.4S
rn .12•
i8 14:
.
.134 ~0.38 .134 lza,. .132 ~&18 I .131 [2920 .131 129.34 .129 130.ct I
1232; .134 '2&30 l2«c:i1 1·.13> .(30 29.89
..... ~:'"
"
.102123.46 '24.31
=
.139 26.75 -:1'
"""' 27 30:1
30
y.s· i2.6
.I
?US
I ,I:,•:
I
.113 f5&oc
' ... 131.60
'""
r:::l ·:.~ I"""
!121
233
~1M 12275
,.,
~
"'"'
I
10
12'<49 1.147
*::
1~'.20! 1<.41
,.,., .... ·' "' "'·". I
~,..., Q6.17 T4l i2&z .141 J3o ~ I .138 27.01l .
'Elf 27; .33 [20,82
I»Mz
l348i
I 114
[3!100
:Jo•
~~
Relation ofener!Jy lou, c:honn~ltl'rlth
"''"
or
r.m:o
1~£~
~
!26.33
II~ ~ I::~! r.~~
critical depth and dop!h before
"" I•
.231 1 .21o [13.
.22R
216
,.zo:
I:~:: 1:!.~ ":" '·'"-' ''·"'''"'
_,..,,
.in [R.74
.
,,.. 126<2
.14( 126.0-c
..
I"'
·"· ""' ~
~ ."2 m
.117134.96
.132
I"'"' 13429
.IIi I
I
-::"
.118
I"• ;'! I·~
1
·".
I .I
_1.£!,2..1.-T----T:I:...J
£-
baud on poolwlrlthDc<3~ D 1= Depth at up~lloum ond of jump
C3
Da;-Dopth at down~! roam ~:nd of jump_ K::: Oz-. D 1
El. Cz::: EL.C,-{~ +h '1}:: E."J Cr(Di h"2:*t\l 3
Dz.
1 2
-J K{X+!)
HL,.f~3
3~-~--
0C4K
"I
ENERGY
LOSS IN
HYDRAULIC
JUMP
FIG.7.15
NOTE: Data
from msrnoror.dum by C.R. Bun.y,Aprll 16,1939
''~"' l>&n
'"'"
~~ 'J"'-""rl ~~~~ 1:·~~ ~~-~~~
sur~ac:o
·'"'
(3!25 .112 Jll 137.98 [.11 138.0! 138.70 .100 1387
,..0
De= Ccl tical do pH• for flo w corn !aorrt-d,
.!?.! -
.100 .128 J26
20 .II 6
:lis
,,.,,
.113
and after ji.Sllp, for hydrqullc jumps in rectangular
Water
.146
:1~
l?l'-86
16~ 1:;;;;;; "~ >f
lov!ll! floor.
HL=Difterenceln energy levels at Up41rllam and dawnstroom ond$ of j~ST:p_
....
... I~
.....
I·''" ~7.2·
I
J33 [zan i.
, I 114 l>sz:
;::-_.~
.-
]2 .20
~""
32
...
~:rr i 3~~~ "•·
1<0.13
1 _,., 123:<: .131 [2M8 1.130 J31 iZO-"" .129 13021 .128 130.20 I .128 13031 .128 I30AO !71:11< :.127 [31.3 131. 131.9
·"
·'''I·'""'
~~
;.:
.1141362: .11413640 137.11 .12131.18 13 i>S9a .112 136:0' i l l ~ '36.81 : ·"' IY>.OO [57.4< [Ji2 '3i'<7 T!i :~m ~~~.-:til im9 .II IOZ70 13ii>< i3791 !5aJ3 .110 13331< 1310. .i1c 1'31!.:!< jj 1334: :iTo [3849 .I. 138M .108 l38iiii l.i09 i3a92 J()9 ~ J09 ~ [lw [31>.38 56.06
)I
l::·::r:Jii 16.3!)i 192 1"-48 .191 f16.t14 '.191 17.43 IME 176 118.48 I·' I 173 1&.33 .172 19.<: .I fi•Jl2 .166 ~016 H7 20.20 .167 120.34 121.34 I·'" ~·o 1-'"" i 21JO I .1az 121.25 21.9: 122.2 23.02 .154 123JI .103 .lti() 123.71 •23.8C .1:><: t3.1l9
l3l23 I .ll I 133.31 . I .II 9 133.09 . 119 IHO
', . 136.10
I
110
' :24<
·1'! ~ li: ·"'"~.-, ~~~~ 1:~~~ 1.~~01 •••• "'. '\82 ii:;,: ... ...... i;~c~1 r:::~ r::::~ ~·~ ;:;; 1:~ i:~~~:r•• -,, r••·"'' .....-= '"" :- , •••• 8
!0.66 -368 la.oz .362 7.!8 .319 i '6.7.-
I
l12.bl .2Z7 i2.u .226 12.79
!o2~~ o,;J
.112
.10
:.o
'::~ ' .108
"
- = 0 !162 Ho
_, '----'-- __Lj .• >·!. ,,·-1 . -.''
-'-··: :.-L L1
0., y
"'
0
·--!-•
----!.
_,
0
(i)
•
• ~
"'
=0.083
0
00
_,
f.O
0., y
"' ·0.!1
_,
---
(D To@ condition
of flow as indicaTed in fiQvre 7.4
EXPLANATlON ~
WATER SURFACE
PRESSURE
®
F JG L.f1£ 7. I 6- FlOH OVER $l..BiV.ERGED CAMS-TYP!CAL PRESSURE ANJ SURFACE PROFILES.
It becomes suddenly small on either side. Due to beam action the floor slab over a length resists the uplift. As such the average intensity of effective uplift is Jess than maximum point intensity. b) Jet of water flowing over the downstream slope exerts a downward force, which counteracts the unbalanced head to some extent. In view of the above factors, the uplift pressure (unbalanced head) for design purposes are taken 2/3 of the calculated value.
7.9 PROTECTION WORKS
As discussed under para 7.3.3 "scour and silt factor", scour depth is related to silt factor of the bed material. The predominant factors which determine the maximum depth of scour downstream of diversion structure arc the discharge intensity and the turbulence (as indicated by I the effect of sides being negligible because of the wider section of the river. Other important factors are the flow pattern and the grade of bed material. \Vhcnever a rigid structure constricting the waterway is built across a natural channel, the velocity and discharge intensity being greater than the channel, when the water leaves the structure and flows over the bed of the natural channel, the depth of flow gradually increases downstream of the masonry or concrete floor. inducing a scour hole and then gradtqJly decreases till the stable depth is reached. Since the scour below the structure is affected by width to depth ratio and in case of overflow dam/weir, the waterway provided is longer compared to channels, the maximum scour downstream at various locations is as given in table 7 .2.
f
-;N
,~
t.l-.-
··
7.9.1
Upstream Block Protection
Beyond the upstream end of the impervious floor pervious protection comprising of cement concrete blocks of adequate size laid over loose stone should be provided. Cement concrete blocks should be of adequate size so as not to get dislodged. These can be of 1500 x 1500 x 900 mm for diversion dams/ weirs in alluvial reaches. The size of blocks can be bigger for such structure in boulder stage of the river.
,>{
,L~
The length of upstream block protection should be nearly equal to D, which can be determined as under D
~
XR - !High flood level - floor level) where X is the multiplier in Fig. 7.17 (a) & R is the scour depth as given in table 7.2 under para 7.3.3 without taking into account 20°/o concehtration in the value of q (discharge intensity)
7.9.2 Downstream Block Protection
Beyond the downstream endof the impervious floor as well pervious block protection should be provided. The blocks should he of adequate size laid over a suitably designed inverted filter for the base material in the river bed. The cement concrete blocks ·should be generally not smaller than 1500 x 1500 x 900 rum size to be laid with joints of 60 mm to 75 mm width packed with gravels. Heavier blocks are preferred since wide shallow blocks are apt to be carried away by high velocity (current of water). Deep blocks get wedged in n.nd resist disloeation. The length of downstream block protection should be approximately equal to 1.5 d. If the downstream length is such that construction of inverted filter with blocks becomes costly or unmanageable. part of calculated length may be provided with inverted filter with block protection and the remaining part only block protection should be provided. It is, however, desirable that length of block protection over filter should not be less than the depth of cutoff. 79.2.~
Ulhf!P IN()rk_inq 0'!+ 1)
n_0 ~-~lnw:>n('p ~0"' "')::C'?~~.".:!,~_;_,~ <:-hn,l!j J,p ~.'l(~_p ;~ +.!">P (~~<:;chp_~Q'C Qn: '.l!"~t
iength vn.!ul) of \ql, in (:.ornp\lt.ing t.hr: normai s<:OlJr h. Ior r.hf~ up~fTPrtm :lnrl (iownst.rnam prote<'.f.ion works;
101
-
.,,""'
FIG. 7.16
FLOW OVER SUBMERGED DAM -TYPICAL PRESSURE AND SURFACE PROFILES- CONTINUED
(13)
~~~; d, 1.7(; L
1[.;-=0.619
o.._,_~
LO
- ~- ~",. _L
1:-.- f
0
7"
~ ~t~ f::,.
- ~-f£ft ~£ --r'
\
~---
1--\-·
-·
,_
-
~~--
-
\/
0
1'1<1 .. <1
L
110 =2
@)
14
ON-...!
'-
!
I
'-
'
t"·t4m
I '~t'l--f--: 'r/
;1-n
ro~o.9~5
L
i I' 1'-N ' I ~n --+:- ' :~LL-1\ .I ~+- I • .r I -.: I
0
" '
I
I '
f__J.-· -- -- ...--I
--1- .-
I
,,[,
bii-- -= :;:-II /::::::; -1-j i- -~---' I J- I
++ I
['.I
--
/
i I
•
7
7 1.0
y
Ho
..
7.9.2.2 A toe wall of masonry or concrete extending to about 500 mm below the bottom of the filter should be provided at the end of the inverted filter to prevent it from getting disturbed. Arrangement is shown in Fig. 7.17(b) & 7.17(c). 7.9.3 Design of Graded Filter
The graded filter should roughly correspond to the design criteria.
D15 of the filter D85 of base material
= 5 or less
where Dl5 & D8.5 denote grain sizes and
D15 is the size such that 15 percent of the soil grains is smaller than that particular size.
D85 is the size such that 85 percent of the soil grain is smaller. than that particular size. The above percentages are by weight as determined by mechanical analysis. ii} The filter may be provided in two or more layers. The grain size curves of the filter layers and the base material should be roughly parallel.
7.9.4
Loose Stone Protection
7.9.4.1 Beyond the block protection on the upstream and downstream of a dam or weir located on per· meable foundation, loose boulder or stones shall be provided as a launching apron, to spread uniformly over scoured slopes.
Design Considerations for Stone Protection Work should be as under:a) Individual stone size should be heavy enough to resist displacement by highest calculated velocity at the end of the floor. b) Thickness of stone layer should be adequate to prevent escape of river bed material. c) The size of stone used should not be less than 300 mm size and no stone should weigh less than 40 kg. In case the velocity is too high or when somewhat smaller size stones arc used, the loose stone apron should be provided in the form of blocks of suitable size depending on cost. In boulder reaches of the river, cement concrete blocks are preferred. d) Volume of riprap work should be adequate to cover the slope of scour hole to calculated depth. e) The calculated volume should be placed in a limited length such that riprap after launching shall not move beyond the slope to be protected.
fl Individual stone size to be determined from the curve described hereunder. 7.9.4.2 The Corps of engineers, Civil Works Investigators Vicksburg USA have prepared design charts correlating the bottom velocity against stone, on the bed of channel and rivers. Fig. 7.18 shows the curve prepared by the United States, Bureau of Heclamation, correlating stable size of stone and its weight in flowing water with average bottom velocity against stone (bottom velocity is 0.7 of average velocity). The chart can be used as a guide for the selection of stone for the riprap.
103
--------..-;;~·;;-;_y~,~
_Jj.f''.-.!·L:=--------------.---,,--
'\SPACES TO !IE FILLED
?>
i\ITH GRAVEL
~f-----'cc...·5_:D:.___ _ _--l
·· ... · •... ,, 11 -u 11 v. i'·; ·..... ~:\'! .. , ... ·.,....., . . ··:·.~· .·:.·. ·-'.'"•"
>'
Ul I
....
I I
LOOSE STONE PRQ.TECTION_J GRADED FILTER ......_ ....... ·SHEET PILE .......
.....
XR" DtY D
I \
.....
' ....
I \
\ ....... ......_,
"'- ..... ·;~
• _, __._@_
LAUNCHING
FIG. 7.17 a
APRON
'','~'~ (LOOSE STONE i PROTECTION
ku/S
I
END OF IMPERVIOUS FLOOR
·>.···:_... ,· ... ,."'-··:..o·. . ·._. ; •··. ·, ·:.
GRADED FILTER
j
-SHEET
UPSTREAM PROTECTION WORKS ·~
••. ·i~
r
PILE
FIG.7.17b
D/S END OF IMPERVIOUS FLOOR OPEN JOINTED CONCRETE BLOCKS SPACES TO BE FILLED
STONE PROTECTION
TOE WALL '---~- GRADED FILTER
·SHEET PIUE
DOWN STREAM PROTECTION WORKS
PROTECTION
WORKS FIG. 717 c
7.9.4.3
Slope of Launched Apron
The slope of the launched apron is dependent on the grade and size of material in the river bed. Slope length should be calculated taking the slope of scour hole as 1.5:1 in boulder and gravel, 2:1 in gravel and coarse sand, 2.5:1 in fine sand and silt for downstream protections. For upstream protection, the slopes may be assumed as 1:1 for boulder and gravel, 1.5:1 for gravel and coarse sand, and 2:1 for sand and silt. 7.9.4.4 Thickness and Length of Material on Launched Slopes
Volume of material calculated to cover slope length should be increased by 25% to account for packing and losses. If D be the depth of scour hole and n is the slope (H:V), then horizontal distance of deepest scour is nD. The calculated volume is generally spread over a length equal to 1.5D. Model experiences indicate that the performance is better if it is laid in gradually increasing thickness starting from the floor end towards river ward end, where severity of attack and hence, possibility of loss of stone is greater. 7.9.5
Protection Around Divide Wall
The protection arrangement in the form of block protection and loose stone - riprap should be on the same design principles as indicated in the preceding paras. Factor of safety against scour should also be the same without taking into consideration the concentration while working out the discharge intensity, (para 7 .9.2.1 ).
7.10
FREE BOARD
Free board to be provided for various components should be as under:7.10.1
Free board in abutment
Top level should be fixed on the basis of 500-year flood with the provision that:a) There should be no overtopping by waves. b) There should be no overtopping due to surface roughness. c) In the bridge portion, the top level be fixed to offer sufficient clearance for floating material under the bndges dunng h1gh flood with 100-year frequency.
Determination of Wave Height
Wave height can be calculated by Molitor's formula:hw and
0.032 JFv + 0.763 - 0.271 0.032 FV for F
where F
=
1
W
for F
< 32 kms
> 32 kms
Fetch
-~ ~or
Fetch p_erpendicula~ to the abutments_ ~hould be taken as distance between abutments on two banks. calcu!atmg wave he1ght for pond cond1tton, maximum wind velocity as experienced in the past should e taken. In the absence of data, 80 km/hr for 500 year flood condition may be adopted. A..Jter!"!.:Jtively
~-Jrr-e ?o~.rd
of I
~o
LS m, s,houid be
;",ood or above the aiiJuxea water leveL
-~H·ov!ded
above the flood 1eve! for J _tn 500 vear .· ' ··
105
STREA M OF STILLING BASINS
13 5
~~~S~IZ~E~O~F~R~I~P~R~A~P~TO~B~E~U;S~E~D~DCM~T'N~~~~~-t~li=rJCJ:JC1 I --I~J,L\--\-~-+-t--1 -+-- -l-+-l-1---t--i-H+-H_lr-~-H_j-_t+-~t-_tt-:-+--rH--lf-1-t-tlt__ _ ~--~--~-~_j~~~,N~00TiE~_J-~
f-l-+---t--ITh• rlprap should bo compos•d of a f-J--t-~t--1 ws !I gradsd mixture but moat of the
'-1---f---\--t--1-1/+-+--irlt-11
atones should be of the size indicated r
120 f-l-+--t--1 by th• curv• -R ;prop ahould be plac•d \-l--~+--+-+f-l--4--1r--Jr-t--t--1 +-+-+--t--1 over a filter blanket or bedd.~ng of \--~'--+-1---1 graded grav€11 in a Jayar 1.5 t!ml!s I
1--i-+--t--1 105
l-+-t-t-t-lft-t-t-H-tl-- -19oo.2s
(or more) asthlck as the largest &tone diameter.
I
~l~tr-~',-=~~~11~=~~-l=~~-l--~+--+~Jt-+~-t-itl--1t_ti-;1t-t'c1fjt_j~~J=~;=~f=b2-1+-l-l~-t-t-t-t-t-t-llll7-r-_ ___ !-r~~--- ~-
1~-+-+-+--r-r-J,_I
1320.57
::1!
ttr=r~l--4-~J-r-+-+----~~+-=tt=.t1~~tJri=t1t-t-n-tii=tu ~ 9 o '~ - -\- J--1~-l--1-4--h-1-+-+~5sr-+--b/r-.'r-: +t-t-HIIt1::: _ 11-- ____ -1--++-+'- -\-.-1-++-+--'H--t--t-1 i1 <-+- 1 ·1~ -~,-1~~=--~-~CC~--1--++++--+--+-A 1 ~rt-rt-t-l!lll 1 ..... -- - ---1- ---+-v,·I+~+--~--~'
'
<3)
896.86
--1--
f=
~ 75 h '
I
!:
,
!
i .
1
.. '
•
1
_;__~-'-- _LJIS_.o --+L-1-~L'-''~-1--l---l--Hr+, -t-'H-t-1 _ t--1~' __ _I I sL-t--t-H-++H---t-l
i . ; :
+ -'- ..,
w t-o %1
·
r-f-~~i-
\ f \ ' •
. .•
- t-1 i ' I !
'
(')()
!
,
"
,
~ t±J~Tr~:=r=:~l-T··-,-L-'-l~~····~~ Ll i_:_l, . i-,--r--- ',~Az'"' I ' 1
'
'
.
;
g
~+-·-t-'---~-t·--r--
"'45
l i : '· I ! ~f---f--+·-f---t~+--f---t-! 1 ' - - -1--
-
1
~rr
1---t
30
___
:
--+- - - --f---- -t-1-Vf---i-, 7W-
--
4S->
1
~-
f--\--'1-/ ~
1j
---
_~--~-~--
1-- ---- --15 1- -- ...
1 ,
-~-
--- -----
v\ Y •
.,
F
--1~ -~
=-t,-j= j-~.---+-+--H-+-+--ir-2<~4.9<> I
1-F--- f·ZFF --1--1--lc--t--1---- --~
0
I
,
-1--
1 i
-r
-~1-+-++-+--t--i
Q
I,
'
- - . . . . ·····/' -....... - - -
I -- -
subJ&Cttochoogeasa
result of further tests '-4L78 or oporatingexperlencos F points ore prototype riprap Installations ,_20.63 which fallod.
s
3
4
0 .J
"'3 w
"'1"' 0
<(
0
"'~ 1:X:
"'
I4J
lit
fno;~:~a~~:n•satistactwy ' - 9.23
: ~~r ·~~.~ -J~t=~·2
CD
it -71L89
Curve _is tentative and
L
:1!c::
a::
----~~ ;~:~:;a,::~:.':":n•;ory ra
F
0
"'"'1""
w
Curve shows minimum
\
-=
..J
t-139.21
NOTES
•
" cs
U)
+-1-+--t--t--+-+-+++-t--l-;~aO.O
1
1/
652.92
1
--- i 1 1 • :
i
I
:
j
;L -r-T--t ,
_
I
I
10
5
.~::{ 6
1
BOTTOM VELOCITY IN METER PER SECOND CURVE TO DETERMINE MAXIMUM STONE SIZE IN RIPRAP MIXTURE FIG_ 7.18
I
I I
'
II
I I
!
•
7.10.2
Free Board in Stilling Basin
Free board is provided against overtopping by surges, splash, spray and wave action set up by the turbulence of the jump. The following empirical formula expressions provides values which have proved to be satisfactory Free board (in ftl ~ 0.1 (V 1 + dz) where
vl
~ velocity before formation of jump
d2 ~ depth below the jump 7.10.3
Free Board in Piers
For the bridge portion, height of piers will be the same as abutment. For the upstream, 1 m free board may be adopted above afflux elevation corresponding to 100-year flood. For the downstream, it would be based on surface roughness.
7.11
DESIGN PROCEDURE IN BRIEF
For the facility of the designer, various steps involved in the design of dam are summarised as under:
I. Data Required i) Design flood ii) Stage discharge rating curve at the site iii} Maximum water level
iv) River cross sections at the site, upstream and downstream
II. Parameters to be assumed i) Lacey's silt factor (f) iii Safe exit gradient iii) Afflux III. Following to be designed i) Pond level iii Shape of crest, glacis slope. coefficient of discharge iii) Water way, discharge coefficient, crest levels, afflux iv) Depth of sheet piles in relation to scour depth and exit gradient v} Level and length of downstream floor in relation to the energy dissipation arrangement
vi) Thickness of downstream apron (floor) with respect to a) uplift pressure b) hydraulic jump vii) Design of protection works their length, thickness etc. The procedure is illustrated in Fig. 7.19.
107
Appendix A The Gumbel Method
This method will give a fair assurance of yielding acceptable values, especially for estimates up to 25year floods, if available streamflow records covers at least' 20 years. For such short recoid, graphical linearization by plotting on Gumbel probability paper is adequate. However, for bigger floods like the 50year, 100-year or 200-year flood, corresponding theoretical values can be extrapolated by mathematical computation. Briefly, the Method involves the collection of streamflow records for a damsite under consideration from the annual publications of the "Surface Water Supply Bulletin" prepared by the Hydrology Division of the Bureau of ~ublic Works covering at least twenty (20) years, picking out the biggest flood discharge in each year (Annual Peak Flow), thence rearranging the data is descending magnitude, followed by computing the probability that a flood of specified magnitude will not occur in a given year and/or calculating the reduced variate and corresponding flood discharge for any selected return period, and finally plotting in the Gumbel probability paper. The following is a step-by-step procedure of performing flood frequency distribution analysis by Gumbel Method: A.
Graphical Linearization (Good up to 20-year flood only) (1) After compiling the annual or yearly peak discharges (designated as Q in the tabulation), rearrange them in descending magnitude and in another column designated as "m" (order number) number them from 1 toN (N being the number of years of record). (2) Calculate the probability, P, that an event will be equalled or exceeded in any one year as given by the formula p
_!]1_
~
(Record this in another column.)
N + 1
(3) In the next column, calculate the prol)_ability that the event will not occur in a given year, as given by the formula · ~
P,
1
~
P
(4) Plot the points on the Gumbel probability paper with P, ~ values as abscissas and Q ~ values as ordmates.
(5) Draw with a straight edge the mean line that best ~ fit through the plotted points.
B.
By Mathematical Computation using Statistical Principle (For 25-year, 50-year, 100-year and 200-year flood)
(6)
Sum up all the yearly flood discharges and take the mean, i.e., L;Q
Q~N (7) In another column place the heading. Q
Q value.
Q. This means that you have to subtract Q from each
Q) ~ values and sum up to obtain L;(Q ~ QJZ.
(8)
Square all the (Q
(9)
Calculate the standard deviation, D,, according to the equation.
Do
·'
~
j L:
(Q
N
~- Q)Z ~
1
108
DIVERSION
HYDRAULIC
FOR
t
DESIGN
DESIGN
r
SUBSOIL FLOW
DETERMINATION OF UPLIFT PRESSURE
DAM
I I
STRUCTURAL
t
FOR SURFACE FLOW
J, DETER Ml NATION OF EXIT GRADIENT
DESIGN
l
CREST LEVELS SPILLWAY AND UNDERSLUICES
WATERWAY AFFLUX AND DISCHARGE/ M.
+
DESIGN WITH EVAWATlON OF RESPECT TO EFFECT OF R E TROGPE SSION HYDRAUUC JUO'P
~
DESIGN WITH RESPECT TO SCOUR
f DETERMINimON OF DOWNSTREAM
DETERMINATION
OF
FLOOR LEVEL LENGTH OF DOWSTREAM
DETERMINATION OF UPLIFT PRESSURE
FLOOR
DETERMINATION
OF
MINIMUM
DEPTH OF UPSTREAM AND DOWNSTREAM SHEET
STEPWISE
DESIGN OF DOWNSTREAM PROTECTION WORKS
PILES
PROCEDURE FIG. 7.19
(10)
Compute the value of the reduced variate, "y", for the required return periods, T, (say T, 25 years, 50 years and 100 years)
according to the equation y
~
-
Ln
- 2.3
1) log /2.3 log T \ T,- 1
(11) By substituting computed values o! "y" from step 10, calculate the corresponding flood discharge Qr, for each return period, according to the relation:
y
~
-a'
D,
(Q, -
-
Q) +
c
where the factors a' and C are a function of N (see Table 2 ·- 1) Table 2 - 1
N
a'
c
10
0.970
0.500
15
1.021
0.513
20
1.063
0.524
25
1.092
0.531
30
1.112
0.536
35
1.129
0.540
40
1.141
0.544
50
1.161
0.549
100
1.206
0.560
1,000
1.269
0.574
1.283
0.577
(12) Plot the points on the Gumbel probability paper with the return periods, T., or reduced variates, y, as abscissas and the respective calculated flood discharges, Q., as ordinates.
··-~-:~
110
Numerical Example: Flood Frequency Distribution Computation by qumbel Method
Year
1951 1952
Annual
Peaks (cu. m./scc)
Q
m
60 46 29
83
1 2
1953 1954 1955 1956 1957
31 27
1958 1959 1960
36 57 83
60 57 46
5 18
3 4
36 31 29 27 18 5
p
m N + 1 0.091
5 6 7 8 9 10
0.637 0.727 0.818 0.909
1- p
0.909 0.818
0.182 0.273 0.363 0.455 0.546
P,
0.727 0.637 0.545 0.454 0.363 0.273 0.182 0.091
Plot the values of Q and P, on the Gumbel probability paper and draw a straight line thru the points plotted. To determine the magnitude of say, the floods, proceed as follows:
25-year, 50-year and 100-year flood and/or other bigger
(Q- Q)
(Q- Q)2
83
43,8
1920.0
60
20.8 17.8
435.0
Q
57 46
Total N
~ ~
Q~
6.8
319.0 46.0
36 31 29
3.2 8.2 - 10.2
27
- 12.2
104.0 149.0
18 5 392 10 392 10
- 21.2 - 34.2 (Q - Q)2 ~
450.0 1170.0 4667.0
-
39.2
111
10.2 63.8
j
~
Standard Deviation, Ds
(Q- QP N- 1
j 46967
~
520
For return period, T r
25
Reduced Variates, y
-Ln(Ln ~) T,- 1 - 2.3 log (2.3 log
22.8
25 ) 24
- 2.3 log (2.3 x 0.01703) - 2.3 (8.59829 - 10)
~
~
- 2.3 log 0.0392
- 2.3 ( -1.4067)
3.23 For return period, Tr
50:
Reduced Variates, y
50 - 2.3 log (2. 3 log - ) 49 - 2.3 (8.296 - 10)
~
3.92
100:
For return period, T r
- 2.3 log (2.3 log
y
- 2.3 (7.997 - 10)
100
99' ) ~
4.61
The other equation for reduced variate is:
a'
y
From Table 2-1, for N
10: 0.97 22.8
y
(Q,- Q) +
a'
c
~ ~
c
0.97 0.50
(Q, - 39.2) + 0.50
0.0425 Q, - 1.666 + 0.50 0.0425 Q, - 1.166
Finally, ForT,
~
25:
3.182
0.0425 Q25 - 1.66
Q25
3.182 0.0+ 1.166 425
~
1 0 2. 30 cu. m./sec.
112
ForT, Qso
For Tr QJOO
50: 3.92 + 1.166 0.0425
119.50 cu. m./sec.
100: 4.61 + 1.166 0.0425
~
135.91 cu. m./sec.
The plotted flood frequency curve on Gumbel probability paper is shown in Figure A·l.
113
:----·--Gil• '·~
'POBABILITY
·r , . ' . .. ·~: .,; :
o:: .. ,·I'.H. ,,
.. :
··.:.L.·
·-
'···.· ·,. '.' '
1
u· ;I
CJ (Jl
"
11
I
' :I I I '
!
..
I':
1
:····.·.·.,
'I''
,,
i'H•
· .. .
.
'" ....
'
L
'
.
. ' i, ... y
.
.....
------
' :
'
A'
·
:. , · 1'-.',._ r1.•
+.·.·
!li:i
·c
'
I
..
_
1
.I
iri :: I
,._·-····H!··t+~···"j- 1+~+
·
'·--.'.J..!--+J--.. ,..,...,¥V::_;..l_.f..;...''-4---·i-''''---+---C.,.'~ 'H+mic'-itlttti++'+'+1·tti . ' '/ ; . I: I I I H+i++'+'+ .. -.' _:_ ;A' '·'·' ' ::!.it:!!·
~ .· t:.'if'l,·'··!''.·,'.. ; ' '' -:-1'-H~•H+;+tti:+thct+•-,;-t-i-7-·•t ··---; r~f
:
:'·tl·i•·..
E • c':, ..· ,l. ,.:·, :·, ·-- ~=-~+·-'rcc-~·1--'-++-+---'• :.·~"-···'+· 'r' -:·. ·.· :[.~ I? ' · . ··. ..:• ;·, ': '· .' ---:·
· ·· · · · · · · · ·• · · · · · · · · ·
..
'lh
•.c..·
---'_+'",",-·,+,-.. '
'•
,.F
1
:I !TiTTT: ',.· ··,',• . . L,,·
. ... I
m
.. 1:
+
...... •. . .'.•·.·•_.··.t;,,·,, I ~ fL+t.l.r.·.H•··Ul\.. 'r!-l. ... • :::l·f.'l ···!'''!···•······:.·. ' I·! 1.11\ ~: ITtll·.
•... f
'
"
':
,.,
::·tr
·'. .·~ . + . . · · -· .. '
, , -. - - · · •. ·••·•,,·••,. c·.·-.·.',I , ,·.•. . . •.·.· ·-· ', . , .........
H+ 'itl~-7-'i-•-H+t+•·r- ·
Cl
::
trlil: ,'I ·.. ·• ' l!J i · ' HlfHrH ,.• • ,.... · · · · ·
·• • '
: :'
J:
~
': .·:
', , :'' I'
:
(l
[: ; . ' :
' I i !.'·.. Iii H: I
·
·+1.0<".,7+(-+~:.;.·~•H+:...~++H-1......;-++-'-++++__L_,+."H. ' ' ·'.,
z (l
c
:;:: "tJ
m :n
.
(Jl
m
1
~~#l~t~~~ r~:rt~ t :~-:;f_=H-:'=·,~· _··=:~::~:-·L~_+1-+ -+ "'-:•1:~ ',-,·-~:,+-':;'~_:;:L~. it.,·+~t~I~-:-"+i;f+.~ ~f.Hf fi:H~r ;!it.+ HHit1t-~t-·
tJt1h±H
i 1:::' ! 1 :Ti ti~ K rr~ ~+ . tPiTj; 1, : rt4i++L~.·.irQ .J[_! ' 1 ~ . r, ·'+::4·.~....., ·.+--+,-_F,--. ' ' 1 I I I I ~J. ; :1 f iTt!+.~IJJ..LLL dlfL!Jli TTl 1 .~ '. LLLL_I'L- .!·'..,:~.;.·.·.:r: l....: •-.~-1---.~-.,!_.;.. ,.---~--·! · +1H'l·tU-J%.:. .~.·1cL.·_ j.r' .• ,: __.: .:.:_..~ .L...LLLlJ_,: ', H 1 +H.-r-m·"~ 'ly,j;+: 'I,,;,; '•LflLLHl'il.:,.,._:u_:,.thl.' '"'r·+-··~i--ri+tL_C:~: '•, i 1 TTITI 11 1;LUi• 111 1111l 1 11U4ll LL:U : : : " ..1. 1: 1 '
(l
0
z
1
CJ
I
1
,!
" " " +.'- , :
!
•
1
Ill
Pr -.Oil G
-2.0
y~
'--
I
lilil
.10 .20 .30
Ill
.50
I'
ilililll !iT':;
.70
.80
:
-r-
;
I
.90
'
.95
.97
1
..t-H
~~-~
L,_
th-l+l+IHH+I--t+t++-H++-H
JiriJnL ~t 1 !-I! 1 l i ' ! : '1 -~'- iHH-t'fHHriH+tn 1 :'L!i :.LU!',I·II.,I'LI' '-+-~~T'.,•.-i,--llrl_~,_11llliT ·,rr ~T-·.::.T'-h-:~·~···.~,~:,. ··Hi'HfHttt+i ' .:',! TTTT ' r-+'+1+++ 1
: ;
. 1
.
-
1
:
1
!
:
!
! :
1
1
.'·,··It'.'
+i'
!+'
1
II
.98
.99
.995
.9975
.999
_.fL.o_________o_L.o__~g~e_m_.__LLo_________2.J.._o________3~._o_________4~._o______~_5L.o _________6L,.o ________~7.0y m 1! •8ABILITY, Pr • (I- N +I I
.. ---........
I
·
1
~c URE__ ~"".-~ I
FREQUENCY
REDUCED DISTRIBUTION
ANALYSIS
BY
VARIATE , y.- ln ( Ln GUMBEL
METHOD
Tr
-:r;:::[ I ---·- · - - - - · · · - - - - - - - - - '
.CHAPTER 8
STRUCTURAL DESIGN A. (SPILLWAY SECTION) Solid Gravity
8.1
GENERAL
In the structural design of the solid gravity overflow section of the dam, it is essential that all design loads are carefully chosen to represent as nearly as can be determined, to actual loads that will act on the structure, and that all the resistive forces used in design represent as accurate an evaluation as possible. The design can be made more rational by utilizing fully the data available from the investigations. A gravity dam as the name implies derives its stability from the force of gravity of the material in its section and thus should have sufficient weight to withstand the forces and the overturning moment carried by the water impounded behind it. The loads in turn are transferred to the foundations.
8.2
REQUIREMENT FOR STABILITY
The design shall satisfy the following requirements of stability. a) The section should be safe against sliding on any plane or combination qfplanes. b) The section should be safe against overturning. c) Unit stresses in the material of the structure (body of the dam) or pressure on the foundation should not exceed the permissible limit.
8.3 8.3.1
LOADS AND FORCES
The following forces shall be considered in the design:a) Dead load (and liveload, if any) b) Pressure of pond water and tail water c) Earth and silt pressure d) Uplift pressures e) Earthquake forces i) Horizontal Inertia Forces
iD Vertical Inertia Forces iii) Hydrodynamic Forces fl Reaction of the foundation 115
There would be no occasion for ice pressure. Wind pressure and wave pressure would be negligible compared to other forces and hence this need not be considered in the design of spillway section. Stresses due to variation of temperature being insignificant, are also ignored.
8.3.2
Forces Inducing Stability and Instability
a) Factors which promote instability are:
i) Pond level and tailwater loads iii Earthquake forces and silt pressure iii) Uplift pressure b) Factors which promote stability are: i) iii iii) iv)
Dead load Shear resistance of the dam and foundation Quality and strength of concrete Strength characteristics of foundation and abutments. These factors also vary. Most important are dimensions and weight.
Whereas, factors as dead load and static reservoir load can be calculated accurately, other factors such as earthquake, silt and uplift pressure, should be based on assumptions. In case of dams on permeable (soil) foundation concrete cutoff walls are generally provided. The cutoff, if properly located provides an additional ,factor against sliding.
8.3.3
Design Assumption
The following assumptions are made for calculation of stability:a) The base width determined by projecting the spillway slope to the foundation line will act as effective foundation width and all concrete downstream of this point is disregarded. b) Loads coming on any transverse section is carried entirely by this section without transfer of load to adjacent transfer section or to the abutment. c) Vertical stresses vary linearly from upstream face to downstream face.
d) The material in the foundation and in the body of the structure is homogeneous. e) The stresses in the foundation and the body of the structure are within elastic limits. f)
No movements are caused in the foundation due to transference of load.
g) The resistance offered by steel sheet piles or cutoffs against sliding and overturning are disregarded. 8.4
LOAD COMBINATIONS
Design should normally be tested for the following load combinations. Any other combination of loads which have reasonable probability of simultaneous occurrence depending upon peculiar loading conditions and/or special site conditions, should also be considered: Load Combination A
Dam completed but no water in pond and no tailwater.
Load Combination B
Water level on upstream at crest level. normal dry weather tailwater, full uplift, silt deposit up to crest level.
Load Combination C
Upstream water at afflux elevation, tailwater at retrogressed flood elevation, full uplift, silt on upstream at crest level.
116
·'*lNA\ii&iL&. 2
Load Combination C1
Same as C - with no silt deposit on upstream
Load Combination D
Combination A with earthquake.
Load Combination E
Combination B with earthquake.
8.5 DESIGN CRITERIA 8.5.1
Dead Load
This shall comprise of weight of concrete/masonry plus the weight of piers, bridges and other appurtenances, if any. For preliminary design unit weight of reinforce concrete shall be taken as 2400 kg./cu.m. and of masonry 2300 kg/cu.m. The total weight of dam acts vertically through the centre of gravity of the cross section.
8.5.2
Live Load
Liveload, in case a bridge is combined with diversion dam, should be taken as per standard practice in !fridge design.
8.5.3 Pressure of Pond Water, Tailwater and Flowing Water Upstream i)
Static Pressure
Unit weight of water shall be assumed as 1000 kg/cum. Static water pressure shall be assumed to act normal to the face of the section and to vary linearly from top to bottom. Tailwater pressure shall be based on water surface elevation as per retrogressed rating curve. ii) Dynamic pressure
The force of current intensity of pressure due to water current may be calculated by the formula:
where P V
=
intensity of pressure in kg./sq.m. due to water current.
=
velocity of the current in metre per sec. at the point where the pressure intensity is being calculated.
The value of v2 shall be assumed to vary linearly from zero at the point of deepest scour to the square of the maximum velocity at the free surface of water. The maximum velocity may be assumed to be .[2 times the maximum mean velocity of the current. 8.5.4 Earth and Silt Pressures 8.5.4.1
General
Gravity dams are subjected to earth pressures on the upstream and downstream sides where the foundation trench is so backfilled. In the low dams, earth pressure are insignificant and should be ignored. Silt deposited on the upstream of the dam, for purpose of design, is treated as a saturated cohesionless soil having full uplift and whose value of internal friction is not materially changed on account of submergence. The experiments carried out indicate that silt pressure and water pressure exist together in a submerged fill and that the silt pressure on the dam is reduced in the proportion that the weight of the fill gets reduced by submergence.
117
8.5.4.2
Criteria
For calculating forces due to silt, the following criteria is recommended:
a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid weighing 1360 kg .1m3 b) Vertical "silt and water" pressure with a combined density of 1925 kg/m 3 c) Increase of silt pressure due to earthquake being insignificant may be neglected.
' 8.5.5 Uplift Pressure 8.5.5.1 Uplift pressure under a concrete dam on pervious (soil) foundation arc related to seepage through permeable materials. In such type of foundation, little resistance is offered by the soil to retard water percolation. The forces developed on the dam and its sub·structurc by the seeping water can cause two types of failures. i) Uplifting of floor ii) Undermining of the sub-soil termed as "piping" The concept of this type of failure has been discussed in Chapter 5 (Dams on Pervious Foundation). The methods developed for the analysis of design of structure against the above two types of failures have been detailed in Chapter 5, para. 5.7. Uplift pressure at any point can be calculated by the procedure discussed therein taking into account the effect of the upstream and downstream cutoffs, interference of cutoffs (piles), thickness of floor and slope etc.
8.5.5.2
Effect of Earthquake
During an earthquake, the water pressure is changed by the hydrodynamic effect. However, the change is not considered effective in producing a corresponding increase or reduction in the uplift farce; Because of its transitory nature, it shall be assumed not to have any effect on the uplift.
8.5.6 8.5.6.1
Earthquake Forces General
Earthquake imparts acceleration to the dam which usually increases the effective loadings in the structure. In the stability analysis therefore because of the additional force, both horizontal and vertical earthquake loads should be applied in the direction which produces the least stable structure. In order to determine the total forces due to an earthquake, it is necessary to establish the earthquake intensity or acceleration. This is usually expressed in relation to the acceleration due to gravity. Seismic coefficient used in the, design of any structure, is dependent on many variable factors including the variation in the soil conditions. It is necessary to indicate broadly the seismic coefficients that could be adopted in different parts of the country, though, a rigorous analysis considering all the factors involved, has got to be made in the case of all important projects to arrive at suitable seismic coefficients for design.
Fig. 8.1 shows the seismic map of the Philippines showing the different zones and the different soils on the basis of which the country can be classified.
118
td
.•
II •
•
18°
14~------·
•
10•
s• ll
LEGEND : ZONE 2 3
D ~
llililllll
MAP OF THE PHILIPPINES SHOWING SEISMIC ZONES
FIG. 61
8.5.6.2 Inertia Forces For Philippine conditions and size of the structure, in tl;w analysis of low dams should be followed:
followiJ;~g
criteria
a) A horizontal acceleration of 0.15 g shall be assumed and vertical acceleration of 0.1 g unless modified by seismic observations. b) Vertical acceleration should be considered only in stability analysis and not in calculation of stresses. c) Inertia forces should be considered to act at the centre of gravity of the structure. d) The horizontal inertia force shall be assumed to act from upstream to downstream or downstream to upstream to get the worst condition in design. Similarly, vertical inertia force shall be assumed to act upwards or downwards. Also, horizontal inertia force and vertical inertia force shall be assumed to act simultaneously. e) Full weight of the mass and not buoyant weight should be used in calculation of inertia forces. 8.5.7 Hydrodynamic Forces Due to horizontal acceleration of the foundation and dam, there is an instantaneous hydro· dynamic pressure (or suction) exterted against the dam in addition to the hydro-static forces. The direction of hydro-dynamic force is opposite to the direction of earthquake acceleration. Based on the assumption that water is incompressible, the hydro-dynamic pressure at depth y below the reservoir surface may be calculated by the formula: Py ~ C o: h wh
hydrodynamic pressure in kg{m 2 at depth y
where Py
c
coefficient which varies with shape and depth. design horizontal seismic coefficient
o:h
w
unit weight of water in kg/m3
h
total depth of water in m.
The value of C for vertical face or constant upstream slopes may be obtained from the equation:-
c where
y
~
Cm ~
depth below surface maximum value of C obtained from figure 8.2
The value of C can also be obtained directly from figure 8.3 The approximate value of total horizontal shear and moment about the centre of gravity of a section are given by the relations:vh
0.726 Py
Mh
0.299 Py h2
120
"'·
'
I
J ..J
< u
...
...a;> ... ... %
;I
90 0
2
0
..."' ...u ...< ... 0
~t"---- . . .......,
A ..
v. '~ ~ ~
'-......
60 •
..........
0
f1 Roa. W.S.-·-.
'·
~
)...
...
.....
1 I 1
~:·:-,:~
..... TYPICAL ..... t'--.-,SECTION F" <1( ~ ~ 'N; ,--.....
--.t
.,
--~max.
,
-P. baoo E
TYPICAL PRESSURE 0 lAG RAM
i
I I
I : '~
..J
<
0.1
0 . 2.
0.3
0.4
PRESSURE
0.5
-r--
I
~
ZO"
::>
>
~I ~
I
/}'·':.:., :. ;~: •. :
--1'..:1
""<
0
:::;_
...~ f...
.,~
~
.<'N~-'f"
'
I
i : '~ 0.7
0.6
COEFFICIENT
I
I
I 0.8
0.9
C
BASE AND MAXIMUM PRESSURE COEFFICIENTS FOR CONSTANT SLOPING Fti:ES
FIG. 8. 2
0
'
)-...
0. I
.... !!':
g
0.2
"'"' ..,...,
0.3
~
ILO::
::> .,
.. a: 0
IL
-'0
.... m:>:
1-
'\ ~ ....... t-.- 1--. f\ ~ ~ ~ r:::-- 1'--.. \ [\ ~ ~ t' ~"-....
0.4
\
(.)ILl
~_J <(
- 1-
"'o
0.6
I-
ll
0.6
,.,j.:
4!30
\ ~
75°
~
"" """ 1\.- "
\ c60 ~
.
0.7
........
"<; '30
\
\
ILIQ.
z c
1\
\
0.5
"""
\
\
\ \
l ...
•· Res. W.S. ~.-- '.S:9
·oo
'
~
"'
\
.·. :·
--. ·.. :
0
0.2
0.3
0.4
-
;
[\
\
1\
0.1
I
""" !'\. \
1\ \
I 0
I
0 .··:
0.9 1.0
·,.
1-
l
h
't l
:_'_ --~ - - !_ -:
\ \ 1\
\
1
1-
0.5
0.6
0.7
0.6
PRESSURE . COEFFICIENT C I c--c -(2--J+ jY -(2--l -2mh h h h
(y
y
y]
wh.ere: Cm is maximum C value from figure
COEFFICIENTS FOR PRESSURE DISTRIBUTION FOR CONSTANT SLOPING FACES
FIG.8.3
:t
i
j
Mti\\t'M%! tAb
IU¥
I
2
0.9
where
8.5.8
vh
hydrodynamic shear in kg./m at any depth y
Mh
moment in kg. m due to hydro-dynamic force at any depth y.
Reaction of Foundation
8.5.8.1 Under stable conditions, the resultant of the horizontal and vertical loads on the dam shall be balanced by an equal and appositive force which constitutes the vertical reaction of the foundation. In the stability analysis the following shall be the assumptions. i) It will be assumed that resultant of all vertical and horizontal forces including uplift is balanced by an equal and opposite reaction at the foundation, consisting of the total vertical reaction and the total horizontal shear and friction. It will also be assumed that locations of these forces are such that summation of their moments is equal to zero. ii) Distribution of vertical reaction along the base shall be assumed as linear and calculated by eccentric loading formula. The foundation reaction shall be computed taking into account the uplift pressures and to this computed foundation reaction, uplift pressure should be added to determine the total upward force at any point. iii) Internal stresses and foundation pressures should be calculated both with and without uplift to determine the worst condition. ','k
-;~
8.5.8.2
General Procedure
\}
The general procedure in calculating the foundation reaction should be as follows: i) In fig. 8.4, by analysing one metre strip of the dam, moments of all the forces should be taken about the toe i.e. point D. ii) Take the summation of all overturning moments and resisting moments (M 0 & MR) iii) Compute net moments LM
~
LMR - LM 0
iv) Take the summation of all vertical forces T-V v)
Distance from the toe where resultant meets the base of the dam.
X the resultant of all forces should be within the middle third. vi) Foundation Reaction at the toe and at the heel fo (at the toe)~
fA(attheheel)~
I:V(1-6e) B B
z;Y( 1
+
6e)
B
B
where e is the eccentricity
8.6 SAFETY CRITERIA 8.6.1 Sliding Resistance a)
General
. Many of the loads on the dam are horizontal or have horizontal components which are resisted by fnctwnal or sheanng forces along horizontal or nearly horizontal planes in the body of the dam. In order
123
MAXIMUM
n~~---~-=! _t_
I
WATER SURFACE
a
b
---·--/, ..
i
'\
,
,'
'
'
>..~,.,.._-.,.,.,.-~
j_~--- ~--- --..·-·_.:.__,;;_;___=:_·_. ·_._· I
... •
.. ·..
=--"+'---'---'-'--"'
c
WATER
d
CONTRACTION JOINT
PRESSURES ACTING ON
_j'{!'!'_TER
AN
OVERFLOW CONCRETE DAM
S~f!F~ACE... ;
• . . . ". : ...·.' -
-~
..
I '
B
BASE
AND
'o
UPLIFT PRESSURES FIG. 8.4
t~
;u wa
*Ed •Rt-kl
9b
8.
f4 IQ
that the dam may not slide on any planes, the total forces tending to slide the dam should not exceed a certain ratio of the normal force on the planes. Sliding resistance is also a function of the cohesion inherent in the materials and at their contacts and the angle of internal friction of the materials at the surface of sliding. b)
Criteria for Design
i) If cohesion is absent or insignificant and friction is the only force resisting sliding the factor of safety against sliding shall be calculated as follows: F1 ~ (W- U)tanQ p where:
F1
factor of safety against sliding
W
total weight of the dam
U
total uplift force
0
coeff. of internal friction of the material
p
~
total horizontal force
The factor of safety thus calculated shall not be less than that specified below:Loading Combination
11
-~
;E
Factor of Safety Against Sliding
2.0 1.5 1.2
A.B.C,C 1 D ··:
E
Shear friction factor
'< 4 3 1.5 'I
ii) In case where cohesion, in addition to friction, resist sliding, the factor of safety against sliding shall be calculated as follows: (W -
U) tan p
0+
CA
where
Fz
il
shear friction factor
c
cohesion of the foundation material
A
area of base considered
The factor of safety calculated shall not be less than the value mentioned in the table above. iii) The value of C and 1/J may be assumed for the purpose of preliminary design on the basis of availabe data on similar or comparable materials. The values given in the table below may serve as a guide. For final design, these should be determined from actual investigation at the site. iv) In the stability analysis resistance offered by cutoff shall be ignored. v) Factor of safety against overturning should be more than 1.5 under normal condition and more than 1.1 under seismic condition.
125
J
vi) Jn case shear keys are required, the forces induced by the keys i.e. passive pressure etc. should be accounted for. ,
Table No. 8.1
Material
C (KN/m 2 or 100kg/cm2)
1. Hard boulder clay, hard fissured clay, hard weathered mudstones ....................... .
300
2. Very stiff bolder clay, , ...................... .
150-300
3. Stiff fissured clay, stiff weathered boulder clay
75-150
4. Firm normally consolidated clay, fluvioglacial and lake clay ............................... .
40-75
5. Soft normally consolidated clay (e.g. marine, river and estuarine clay) ..................... .
20-40
6. Medium uniform sand 0.58 0.62 0.72
a) relative density 38% b) relative density 50% c) relative density 72% 7. Coarse uniform sand
0.65 0.75
a) relative density 50% b) relative density 70% 8. Gravelly sand a) 20% gravel, maxim urn size 3/4" i) relative density 50% ii) relative density 70%
0.68 0.77
b) 20% gravel, maximum stze 1 1/z", R.D. 70%
0.78
c) 35% gravel, maximum size 3/4" R.D. 70%
0.81
d) 50% gravel, maximum size 3/4", R.D. 70%
0.85
e) 50% gravel, maximum size 3"' R.D. 70%
0.87
9. Crushed rock, maximum size 3" a) R.D. 50% b) R.D. 90%
8.6.2
0.85 0.98
Resistance Against Overturning
Before the structure overturns, other type of failure would occur, such as cracking of upstream material due to tension, crushing of toe material and sliding. The structure may therefore, be considered safe against overturning, if the following conditions are fulfilled; i) No tension on the upstream face
ii) Compressive stress on the downstream is within allowable limit in the foundation material. iii) Factor of safety is adequate against sliding.
126
8.6.3
Safety Against Foundation Failure
8.6.3.1
Design Criteria
The unit stress in the foundation must be kept within prescribed maximum values. The foundation should be investigated and maximum allowable stress established. i) Maximum vertical pressure on foundation material near the toe shall not exceed the allowable
bearing pressure of the soil.
iii The allowable bearing pressure in final design should be based on field soil investigation.
De termination of Bearing Pressure: Allowable bearing pressure
Q
5 - 4 N2 B + 16 (100 ;. N 21 D; where:
Q
Allowable bearing pressure in kg./sq. m.
N
Number of blows per 30 em. in the standard penetration test.
=
B
Smaller dimension of the well cross·section in metres
])
Depth of foundation below scour level in m.
For preliminary design the allo~;vablc bearing pressure may be assumed as follows:-
Material
Allowable Tons/sq. ft.
Bearing pressure kg./cm.Z
1. V cry dense sand and gravel (N > 50)
4.0
3.6
2. Dense sand & gravel· (N ~ 30 - 491
3.5
3.2
3. Medium dense sand and gravel (N ~ 10 - 29)
3.0
2.7
0.25 0.50
0.23 0.66
1.00
0.91
1.50
1.37
4. Silt and clays Soft (N
~
6)
Medium (N
~
5 to 101
~
11 to 20) Hard (N > 201
Soft (N
where: N
=
is the standard penetration value, for silt and clays.
iii) During operation and after construction of the dam, the foundation will remain submerged. The value of allowable bearing pressure mentioned above should be halved for use in design calculations.
iv) Under seismic condition allowable bearing pressure shall be increased by 25 per cent. In case of raft it can be increased by 50 per cent.
3.6.3.? !~the "::'!"" of N foro q~,.. s0il '-" !es~ the.:!"!. 1.0 v~lJ:?.~.:n:: r:1useC bv ee.::thoua_ke may cause liaue· factior.. ln ·cna·c (:a.st:, adeqtWH: comp;H:tion is to or, don(; r,o aehir:v(: suitai1i<: vallle of N.
127
8.6.4 Safety Against Failure of Materials of the Dam 8.6.4.1
Quality and Strength of Concrete Ma;>onry
The strength of concrete/masonry shall exceed the stresses anticipated m the structure by a safe margin. Tensile stresses at the upstream face of the dam shall not be permitted even under seismic conditions. Tensile stresses not exceeding 5 kg./cm 2 may be permitted on the downstream face.
8.6.4.2 Allowable stress under earthquake i) In all calculations involving earthquake forces, either the permissible stresses shall be increased by 33 1/30fu or the total forces or moments should be reduced to 75°/o. ii) The permissible increase in bearing pressure is 250Jo.
8.7 CONTRACTION JOINTS
A contraction joint is a formed vertical or inclined surface between masses of concrete/masonry placed at different times. If the concrete dam is appreciably more than about 15 metres in length it is necessary to divide the structure into blocks by providing transverse contraction joints. Contraction joints in diversion dam structure requires careful consideration. They must be water tight in spite of differential settlement and must be provided for contraction and expansion of concrete to prevent cracking.
8.7.1
Longitudinal Joint (Joint parallel to Dam axis)
No contraction joint shall be provided parallel to dam axis, except at the junction of the toe and the stilling basin: This longitudinal joint should be provided with double water stop of metal or of rubber. In addition, a drainage arrangement should be provided just upstream of the toe to create pressure relief. 8.7.2 Transverse (Joints perpendicular to dam axis)
Transverse contraction joints shall be provided at regular spaces. Locations may be selected to suit the phasing of contraction. In no ease should the spacing exceed 15 m to 20 m. According to latest practice transverse joints are not keyed .or grouted. If considered necessary to avoid seepage loss, waterstop. may be provided near the upstream face of the joint.
8.7.3 Transverse contraction joints form seepage planes along which the pond water can flow to the downstream. To check this seepage, watertop is to be provided horizontally from the upstream sheet pile to the longitudinal contraction joint to prevent the possibility of piping.
128
STRUCTURAL DESIGN B -SLUICEWAY
8.8
GENERAL
Sluiceway portion of the diversion structure consists of the following essential components. i)
Impervious floor cutoffs
ii) Abutments iii) Piers
iv) Divide wall
v) Energy dissipating arrangement vi) Cutoffs VIII Protections viii) Flank and flared out walls ix) Gates and gate bridge, hoist and hoist bridge, cte.
8.9
DESIGN CRITERIA
8.9.1
Cutoffs
The upstream and downstream cutoffs of the diversion structure maybe of the steel sheet piles or of masonry or reinforced cement concrete. The depth of the cutoff shall be fixed as per provisions given under Hydraulic Design.
8.9.2 Impervious Floor
I ~ .)~ ·.._·
·_J- ~._-
Generally there arc two types of floor, viz: a) Gravity type: where the uplift is balanced by the self weight of the floor only considering unit length of floor. b) Reinforced concrete raft: In this type, the uplift pressure is balanced by the weight of the floor, piers and other self imposed dead loads, considering the span as single unit. While the gravity type of floor can be of either plain concrete or masonry, the raft type should be of reinforced concrete only.
8.9.2.1
General Requirements
The floor shall be designed against the following forces: o.) Uplift pressure Uplift pressure at any point shall be calcnlated as per procedure detailed in Chapter 5 "Dams on Pervious Foundation", taking into account the upstream and downstream cutoffs, 10tcrmcdiatc cutoffs If any, interference of cutoffs (piles), thickness of floor and shape of the glacis. bl Hydraulic Jump The adequacy of the thickness of downstream glacis and downstream floor shall be checked under unbalanced head condition with the formation of hydraulic jump, as laid down under para 7.8 (ii). For this purpose, the gate operation chart and corresponding downstream water level may be made usc of.
8.92.2 a)
Design Criteria Gra.1Jity floor
The following should be the design requirements. il The thickness of floor should be at least 10% more than the thickness calculated to counteract the uplift pressure at that point under the worst combination of loads under different conditions considering submerged weight of floor. ii) The floor may be independent of piers and abutments {in which case water tight seals can be pro· virled) or rnonolithic with piers and abutments. iii) When the floor is of plain concrete, suitable temperature reinforcement shall be provided. b)
Reinforced Concrete Raft
i) The floor shall be designed considering as a raft as per the theory of beams on elastic foundations. Theoretically, the soil reaction is not uniform but for simplicity in calculation, it may be taken as uniform except for large spans on flexible foundations in· which case it should be evaluated by Heteyni's method of beam on elastic foundation. ii) For shorter spans the floor shall be designed as a continuous beam resting on homogeneous foundation. The abutment, if necessary can be made independent by providing a joint in the raft with water seals of acceptable standards. The raft shall be designed for the moments caused by the worst combinations of forces
a) Uplift b) Soil reaction c) Moments transferred from abutments d) Seismic forces iii) For the purpose of analysis, the entire width of the raft may be divided into different sections, dependmg on the loads and moments anticipated to act over the different sections, such as upstream section (including glacis), gate, downstream glacis and downstream floor sections. 8.9.3 Abutments 8.9.3.1
General
Abutments are vertical walls or inclined walls. Mostly they are kept vertical from the crest for a sufficient length, then flared or warped.
130
Abutments ar(~ gPIH~r:dly dcsi);Iwd :ts rcl31nmg '-'Valls separated from the main floor hy an ex~ pansion _joint. \Vil.h sells. In somp cases, if t.hP totZ~l \vaterway between the abutmcots docs no\. exceed -10-50 met.res, the abu!mi:n(s ;~n: !'onsi.r·u<"tcr.l monolithic with the main floor and the whole section is designed as
8.9.3.2 Top Width Top widl.h of !.he ahut.rncn\ in (':tch hlo<'k shall be fixed as per tlw requirements due to the loads and moments, minim urn width rpquir('d for b\oekouls of main gale, stop log grooves and bridge bearing etc. Minimum width of ahut.ment.s clear of bJ·ock outs should be about 600 mm. In the gate and road/ rail bridge blotk it. is g(~tH:rally kt~pl as l2SO t.o 1100 rnm.
8.9.3.3
Design Criterb Loads and Forces
Abutment. should he (ksigncd !.o withstand !.he following loads and forces a) Dead load
b) Live load due t.o t.hc moving t.r·al"fic
ovr~r
the bridge where, provided
c) Impacl and Braking effect d) Earth pr('-'>surc, livl' load surcharge and sat.urat.ion pressure~. e) Earthquake
i) Horizontal
forces:-··
inertia fore<·
ii) Dynamic increment. of earth pressures. Water lev(:\ on sluiceway sid(~ .will he governed by the river stage. Water level on the backfill side will be governed hy hydraulic gradient line from upstream to downstream prevailing, due U; the river st.1gc during the post. construction period, an·d subsoil water level during construction period. In order to find out the "vorst. effect, cakulations should he done under the foilowing conditions:-
t·omplekd, full backfill, sat.urat.ion level at. subsoil \Vater level or ;)ond level \Vhichever is higher, sluiceway side dry. "
i) Construction
ii) Design floor! co:;rlit.ion iii) Annual or norm{tl flood tondit.ion Un case annual flood data is not available, it. may be taken a.s 60°/o Lo 70°/o of cksign flood).
8.9.3.4 Safety Criteria i) The abulment scr:lion shall he checked for sarcty against allow
and sliding. Allowable bearing pressure should be determined by field and laboratory tests:-ii) Factor of safety againsl overturning and sliding should be as under:
F.O.S.
Loading Condition ----~-·~-~·---
Sliding
-1
I
1
j
l
!l :t
Overturning
I
I
under seismic condition
1.50
under normal condition
1.20
under seismic condition
i) coarse sandy soil
containing no silt or clay
ii) Coarse sandy soil containing silt
!
l
1.20
8.9.3.5 The value of friction shall be determined by field investigations, However, following values may serve as a guide.
1 I
Ij
1.50
under normal condition
8.9.4 8.9.4.1
0.55 0..15
Piers
General
In diversion dams constructed as reinforced cement concrete structure, the piers are constructed monolithic with the floor of the diversion structure. This is so in the case of floor designed as a raft, in the case of gravity type of floor, piers are generally constructed independent of the floor. Proper joint and sections arrangement between the gravity floor and all round should be provided. 8.9.4.2 Thickness of Pier
The thickness of the pier shall be fixed from consideration of (i) forces and moments transferred by the pier to the floor/foundation (ii) minimum thickness required at the block-outs for the main gate and stop log grooves and also (iii) the weight of the pier required for counteracting the uplift pressure. The thickness of the pier for reinforced cement concrete structures generally varies from 1.5 to 2.5 meters
8.9.4.3
i
Length of Pier
In the case of a raft type floor of the diversion structure, the piers shall generally be extended up to full width of the raft to avoid cantilever action of the raft at the ends. In the case of gravity type floor, the length of pier can, however, be restricted according to the minimum requirement from con· siderations of road/rail bridges, hoist bridge space required for housing instruments if any, main gate groove, stoplog grooves, space for storage of stoplogs, adequate length to prevent cross flows occurring which may cause damages to the floor and beyond. 8.9.4.4
Height of Pier
On the upstream side, the pier shall generally be constructed above the pond level with adequate free board. The height shall be fixed as per requirement of the weight of the pier in counteracting uplift pressure. On the downstream side, the piers shall generally be constructed above the high flood level up to about 3 m beyond the end of the bridges and instrumentation platform, if any, and thereafter the height could
I
132
be reduced according to low flood levels on the downstream side. In the portions where road/rail bridges are provided the height of the piers shall be fixed such that the bearings of the bridges are not affected by the high flood. Adequate allowance shall also be made so that the beams of the bridges are not hit by floating debris during high floods. In the main gate portion, the height of the pier shall be fixed so that during high flood, the bottom of the gate is at least 1 metre clear of the affluxed high flood level. In earthquake regions, however, the top level of the pier could be restricted to the top level of the abutments and steel trestles provided over the piers for housing the hoist bridges for operation of the gates and stoplogs. This arrangement would reduce the loads and moments due to inertia during earthquakes.
8.9.4.5 Design Criteria General
Pier shall be designed to withstand the following loads and forces: a) Dead loads b) live loads due to road/railway bridges.
c) Force due to the braking effects of live loads on road bridge. d) Dead and live loads of gates, stop logs counterweights and the hoist bridge. e) Force due to water current flowing obliquely.
fl Differential hydrostatic pressure with one side gate open and the other adjacent gate closed. g) Seismic forces. h) Hydrodynamic forces due to seismic conditions.
Wave pressure is insignificant compared to other forces and may be neglected. Forces due to tractive effort on road bridge is less than that due to braking effect and thus need not be considered. Where the height of pier is more than 12M, tractive force or braking effect should be considered.
8.9.5 Loading Combination While considering earthquake forces, the following points should be kept in view:-
il Earthquake need not be considered to act simultaneously with design flood. Earthquake may be considered to occur along with mean annual flood. In the absence of data on annual flood, the value of mean annual flood may be taken as 60% to 70% of design flood. iii Vertical inertia forces should be considered while testing stability of the structure. It need not be considered in calculating stresses. iii) While evaluating inertia forces, full weight of the mass should be taken, ignoring reduction due to buoyancy or uplift. Only in case of evaluation of dynamic increment or decrement of lateral earth pressure, buoyant weight should be adopted. iv) Horizontal earthquake forces are to be calculated for full dead load, but not for full live load. Fraction of live load for which earthquake force is to be calculated, is indicated below:For stability calculation -
l~ ~
Railway Bridge
50%
Road Bridge
25°/o
133
For stress calculatian 100%
Hailway Bridge
50°/o
Iload Bridgl'
For working out. live load or hridg(', seismic force due to live loarl shall he ignored when acting in the direction of traffic but shall iw LakPn int.o eon~idcration \vhen actin~ in a direction perpendicular to traffic. v) I\~rrnissible stresses maybe irwn~ased by ;-n 1;:-J 0/o or Uw forees/monwnts m;tyhe reduced to 75°/o.
Subj('d to the considerations inclie;~t.cd above, t.hP loadinJ-?: eornbinaLions which givP t.he \vorsl effect, should be used in t.he design.
8.9.6
Evaluation of Forces a) Dead Load
F'or preliminary design the following data maybe adopted:--·· 62.5 lbs/c.ft. (1000 kp;lm")
Unit weight of water Unit weight of dry eart-h
=
100 lhs!c.ft. (lGOO kg/m:3) J 20 lhs/c.ft (Hl20 1q;lm'1)
~
1:JG lbs/e.ft (21 GO kr;/m 1 )
Unit \Veight of <·ompact.ed ('Jrl h Unit wl'i~~ht. of s:lt.urat.Pd c:1rth Unit
w(~ight.
72.:51 lbs/c.ft (llGO kg/nr3)
of suhnwrg('d l'arth
Unit wPight of conerdt'
~
1:JO lbs/c.ft (2:100 kp;lm 1)
For
Gat.P Brid,t;(' ··- It. should he designed for the rated load of gates, hoists and hoisting equipmcnts. In abscnee of data, prclin1inary calculations maybe based on following assumptions il 100 Jbs/ft." 1-188 lZgim 2) -~ for opcratinr; platforms without sloplogs, for operating platforms \Vith sloplogs. Detailed design should be cheeked using actual loading ::tftcr the gates, hoists and hoisting cquipmenl.s have been designed. In road bridges, standard bridge loading should be used, c)
Earth Pressure
Earth pressure rnay be evaluated by the procedure detailed in para 8.9.7. The following assumptions should be made when working out the earth pres~ure. i) If the backfill is compacted. water load <.can be ncglecterl. If the backfill is not. compacted. hydro· static pressure up to maximum saturation level should be considered in addition to earth prc;surc which will r.onsisL of two parts:~
134
a) Pressure of submerged earth below saturation level b) Pressure of moist earth above saturation level ii) Weep holes, though provided, should b'e considered as inactive when analysis is done under nonscisrnic condition. d)
Inertia Forces
Unless otherwise specified in relevant earthquake code of the country, the seismic coefficient should he adopted as under:~ Horizontal seismic coefficient
-- 0.15 g 0.1 g
V erlical
The inertia force which is obtained by multiplying the full dead load (without reduction due to bouyancy) by the seismic coefficient maybe considered unchanged along the height of the structure. The inertia forces will be assumed to act at the center of gravity of the structure. The horizontal inertia force shall be assumed to act in any horizontal direction. For calculating stability, it shall be combined with verticil inertia force acting upward, and [or calculating foundation pressure it shall be combined with vertical inertia force acting downward. As discussed under para 8.5.6.2 for the design of spillway, vertical inertia force need not be considered while calculating concrete stresses. Vertical inertia force should be calctilated not only on concrete masonry, steel etc. but also on water load. c)
Hydrodynamic Forces
Method of evaluation of hydrodynamic forces due to earthquake has already been elaborated in para 8.5.7.
8.9.7 Effect of Earthquake on Earth Pressure 8.9.7.1
Active Pressure Without Surcharge
Active pressure due to earth[ill under seismic condition shall be calculated by the formula:--- This is illustrated in Fig. 8.5.A.
where:
Pa
active earth pressure in kg/n1eter length of the wall
w ~ unit weight o[ soil in kg/m3 h
c,
height o[ backfill (1 - cxv) cos2 cos
i\ cos 2
A cos ( J" + o: + ?t )
[
(r/J-1
horizontal seismic coefficient
o:v
vertical seismic coefficient
rl)
sin((/) + J ) sin((/) - i - A ) + --- ( cos (i -cx:l cos (! + o: + A )
The maximum of the two being the value for design. o: h
ex -
135
yJ
angle of internal frielion of soil
J
angle of friction belv·/een the wall and the carthfill
ex::
angle which earth face of wall makes with the vertical slope of earthfill
Point of /\pplication
From thP total pressure calculated as above. the static active pressure, calculated by putting '-< " = ,1, '"' 0, is subtraetcd. This gives the dynamic increment. The static component is assumed t.o act at h/:~ above the base. The dynamic increment. is assumed to act at mid-height. c< h
=
3.9.7.2
Passive Pressure Without Surcharge
Passive pressure due to earthfi!l may be calculated as follows: - This is illustrated in fig. 8.5B.
(l.oC
where A
COS I! CO!)
~-~--:-~(. ,····· COS fJ --
c
o-
vi cos2 ((/J + « - A I + I!A I [ 1 - ( sin(~ ·- J cos (i
-o<.
l
I sin. 1
Point of Application
Minimum of the two values being adopted in design.
n
From the static passive pressure obtairied by putting oe h = c<. v = 0, the dynamic passive pressure as obtained above is to be deducted to get the dynamic decrement. The static passive pressure will act at h/:J and the dynamic decrement shall be assumed to act at an elevation 0.66 h above the base.
8.9.7.3
Active Pressure Due to Uniform Surcharge
where: q
=
uniform surcharge per unit area of inclined earthfill.
Point of Application
Static component will act at mid·height and dynamic increment will act at 0.66 h.
136
jj
;&J.
1J££ill&k iii.
CU£1 JWi
DIRECTION Of HORIZONTA~,.,-- EARTHQUAKE ACCI!:LERATION
B ACTIVE
PRESSURE
EARTH
(FIG. 8. 5 A)
PASSIVE
PRI!:SSUI'lE
(FIG. 8. 58)
PRESSURE DUE TO EARTHQUAKE ON RETAINING WALLS !ABUTMENTS) FIG.8.5
r - - - - - - , - - - -··--- ., ' / /
''
'
/ /
/
h
'
'
Dl STRIBUTION OF THE RATIO WITH HEIGHT OF WALL
LATERAL DYNAMIC INCREMENT VERTICAL EFFECTIVE PRESSURE
FIG. 8.6
8.9.7.4
Passive Pressure Due to Uniform Surcharge
,:.;
1
•••••
Point of Application
Static component will act at mid·lwight and dynamic decrement at 0.66 h.
8.9.7.5
Effect of Earthquake on Subsoil Water in the Backfill
Dynamic increment in active earth pressure and dynamic decrement in passive earth pressure, in case of submerged backfill shall be calculated as indicated above with the following modifications:a) /
for submerged fill - 'h x
J for dry fill
where:
W s = saturated unit weight of soil in gm/c.c. c) For w use buoyant weight while calculating static pressure as well as dynamic pressure.
Hydrodynamic pressure on account of water, contained in earthfill shall not be considered separately as the effect of acceleration on water has been considered indirectly. In partially submerged backfill, the ratio of the lateral dynamic increment in active pressures to the vertical pressure at various depths along the height of wall may be taken as shown in figure 8.6.
··~.· -~ .. ••
The pressure distribution of dynamic increment in active pressures may be obtained by multiplying the vertical effective pressures by the coefficients in the figure, at corresponding depths. This procedure may also be used for determining the distribution of dynamic pressure increments. In the fig. 8.6, h
height of wall
h' ,.- height of submergence
•.:!'·• .•'}§
Ca
value of dynamic earth ptessure for saturated backfill
Ca'
value of dynamic earth pressure for submerged backfill
Ka
value of Caw hen« h ~
Ka'
value of Ca' when« h
~
«. v «:
~
il
v ~
il
~ ~
0 0
To find the value of dynamic increment in active pressure at any depth (d), calculate the vertical pressure at d. Also from the diagram find out the value of horizontal intercept (X). Then dynamic increment = vertical pressure x (X).
138
8.9.8 Hydrostatic Pressure
When one of the adjacent gate is jamncd and water is static on that side of the pier, while water is flowing on the other side, a hydrostatic pressure equivalent to velocity head will act on the pier. The worst effect will be when velocity is highest i.e. during design flood condition.
8.9.9
Force Due to Water Current
Piers intended to be parallel to the direction of the current shall be designed for a deviation of 20 degrees from the normal direction of the current. Intensity of pressure normal to the pier and acting on the area of the side elevation of the pier can be solved by taking the component of velocity of the current in a direction normal to the pier and applying the following formula:-
where:
v2
p
78
p
intensity of pressure in kg./sq. m.
V
~
component of the velocity at the surface (normal to the pier) in m/s
The value of V2 shall be assumed to vary linearly from zero at the junction of pier with floor to the square of maximum velocity at the free surface of water. For the purpose assume maximum velocity to be [2 times the maximum mean velocity of the current.
The value of P should be integrated to full height to get the total head on the pier. The effect of current shall in no case be taken as less than that of a static force due to a difference
of head of 0.25 metre between the opposite [aces of a pier at high flood leveL
8.9.10
Longitudinal Forces in Road Bridge Force Due to Braking Effect on Road Bridge
Unless otherwise specified by road bridge' code of the country, the braking effect shall be evaluated as follows:' a) In case of a single lane or a two.Jane bridge. Braking effect shall be twenty percent of the first train load plus ten percent of the loads of the succeeding train or part thereof. Considering loads in one lane only when the entire first train is not on the full span, the braking force shall be taken as equal to twenty percent of the loads actually on the span. b) In case of bridge having more than two lanes As in case {a) above, for the first two lanes plus five percent of the loads on the lanes in excess of two.
c) The loads shall not be increased on account of impact. d) The force due to braking effect shall be assumed to act along a line parallel to the roadway and 1.829 m above it. While transferring the force to the bearings, the change in the vertical reaction at the bearings should be taken into account.
139
.§@
&W
zp
tm
;c
8.9.10.2
Longitudinal Forces Due to Reactions in Bearings
The longitudinal force at any free bearing shall be limited to the sum of dead and live load reactions at the bearing, multiplied by the appropriate coefficient of friction as under:-
Roller bearing
0.03
Sliding bearings of hard copper alloy
0.15
Sliding bearing of
steel on cast iron or steel on steel
0.25
Longitudinal force at the fixed bearing shall be taken as the algebraic sum of the longitudinal forces, at the free bearings in the bridge unit under consideration and the force due to the braking effect on the wheels. 8.10 MISCELLANEOUS DETAILS 8.10.1
Longitudinal Joints
No longitudinal joints parallel to the dam axis should be provided in the sluiceway portions except under special circumstances. As the downstream toe of the sluiceway, where piers get discontinued, a longitudinal joint may be provided for convenience of construction. This joint shall however be protected by a double line of water stops. In addition, suitable arrangement for pressure release should be provided just upstream of the toe. The joint so provided should be continued up to the abutment and end of the pier of sluiceway. Vertical joint in the abutment shall also be backed by a suitable graded filter.
8.10.2
Transverse Contraction Joints
The transverse contraction joints form seepage planes along which reservoir water can flow. For providing such joints the procedure to be followed shall be the same as discussed under "Spillway".
8.10.3 Weep Holes
·.·.~
. Weepholes shall be provided in the abutments to facilitate drainage of the backfills. The spacing and Size of weep holes would depend upon the amount of drainage, type of soil in backfill and sub-base and the extent to which the structure should be protected against uplift. Graded filters at the back of the weep holes should be provided to prevent washing of the backfill materiaL 8.10.4
Backfill Material
Expansive soils should not be used for backfill behind abutments. Wherever possible free drainage materials should be used, because: '
il The higher angle of internal friction of the free-draining material will result in lower lateral earth pressures.
ii) Saturation pressure can be reduced or eliminated by adequately draining the backfilL
:;~
i, ·.• ...·.·1··
140
8.10.5
Backfill Drainage
Backfill should be maintained in a drained condition. There are many techniques available to achieve this objective such as: i) Preventing water from entering the backfill. ii) Installation of sub-surface drainage system. iii) Providing weep holes as discussed above.
141
STRUCTURAL DESIGN C -DIVIDE WALL (TRAINING WALL)
8.11
GENERAL
Location
8.11.1
Undersluices, river sluices and spillway of the diversion dam are separated by properly designed divide walls (lraininp; walls). Their function lay out and hydraulic requirements have been discussed in chapters 6 and 7. As described thereunder, the divide wall may extend both upstream and downstream to the end of the impervious floor. In some. cases, however, the length of downstream portion of the divide wall can be curtailed/omitted if so indicated by hydraulic model tests.
Position, Length and Height
8.11.2
Lengths and height and width of pocket to be determined as detailed out under para 6.4.
8.12
DESIGN CRITERIA
Divide wall shall be designed as a retaining wall against the following forces. a) Hydrostatic pressure due to difference in water level on the two sides created with one side gale open and other side gate closed. b) Force due to water current flowing obliquely. c) Wave pressure
d) Inertia force due to earthquake. e) Hydrodynamic force due to earthquake. f)
8.12.1
Earth pressure assuming pond silted up to spillway crest.
Load Combination
Load combination shall be the same as specified for the design of pier described under para 8.9.5 and para 8.12.5. This should also be tested when spillway portion is silted up.
8.12.2
Evaluation of Loads and Forces
Evaluation of loads and forces shall be due according to para 8.9.3.3.
••.·.;..(J
'~
142
l----1<~" rb•tructed
"
ere st
\ \ \
i I~ hw
I
PW
Unobstructed crest
------r-------pproxtmate pressure line
= 2/3 hw
StIll
ter level
l-----2.4hw
WIND
PRESSURE
AND WAVEDIAGRAM
FIG. 8. 7
8.12.3 Wave Pressure Wind blowing over the pond area causes a drag on the surface. The effect of the drag is to pull the top surface along the direction of wind and thus ripples and waves are formed. Wave pressure can be calculated as under:-
hw ·~ 0.032 1/VF + 0.76:1 - 0.271 Fli1 where: hw
height of wave in m.
F
fetch of reservoir in km.
V
velocity of wind on water surface in km./hr.
Maximum unit pressure Pw due to wave (in t/m 2l is given by the formula. (Fig. 8.7} Pw
~
2.4 whw. This occurs at 0.125'hw above the still water level.
The total pressure (in t} is given by the relation: Pw
2400 hw2 kgJm2 ~ 2000 h2w (approximately} represented by triangle ABC acting at 0.357 h2 w
8.12.4 Foundation Design The foundation of the divide wall should be extended below the scour depth. It can be limited to a depth of 1.25 H below the HFL at the nose portion in view of the protection of cement concrete blowing and riprap all round the nose. In case of masonry divide walls, it is necessary to provide well foundation, otherwise it can be of steel sheet piles where it is possible to drive the sheet piles to sufficient depths, taking them well below the deepest possible scour. The depth of the foundation may be decreased progressively towards the impervious floor of the dam up to a minimum depth of the main cutoff. The upstream and downstream walls shall be separated out from the main pier beyond the impervious floor of the dam/weir by joints ..Joints shall also be provided in the divide walls at places where there are changes from sheetpile/cutoff foundation to wall foundation.
.·.il ~]
8.12.5 The divide wall is likely to be subjected to a maximum differential pressure when the full discharge of the river is passing through the undersluices and no discharge is passing over the spillway. In this case there will be difference of water level on two sides. Upstream wall shall be designed to resist the differential head on account of the velocity of flow in the pocket due to the gates adjacent to the divide wall on either side being kept open and closed position. The downstream wall shall be designed to resist the moments due to the differential head caused by the closure of gate on the side and opening of gate on the other side. It shall be based on model studies. As it is difficult to assess the hydrostatic forces discussed under 8.12, it is customary to design the wall for a differential head of 2 m for preliminary designs. This can be combined with silt pressure where such a combination becomes critical.
I
144
CHAPTER 9
INTAKE WORK
9.1
GENERAL
The intake works also known as Head Hegulators or Head \Vorks is provided ::tl the head of the off taking canal, immediately upstrearn of a diversion dam. Its detailed funrtion, location and layout have heen discussed in Chapter 6. para G.G. As discussed therein, the int:d'\e work should he suitably aiigncd so as to reduce silt entry into the ('anal to a minimum and avoid back flow and formation of stagnant zone in the pocket.
9.2 LAYOUT
The axis of the work is thus normally aligned at <:~n angle 90° - 110° with the axis of diversion dam as shown in Fig. 9.1. Because of its inter-relation with the diversion worl\, the layout should be established with model studies, if considered necessary. Where possible, intake works should be located on the outside of the bend or the eoncavc bank of the river for effective sediment exclusion from the offtaking eanal. At the outside of a river bend, the top high velocity water flows outwards and is then defleeted towards th(~ head whereas the slow moving bed water is deflected across the bed to\vards the inner convex side of the curve causing deposition of sediment at that side. A canal taking off from this convex side will draw excessive charge of bed materials. In the case where a eanal is to be considered on each side of the river, the dam and head works structure should be located on a straight portion of the river. It is also preferable to locate the dam and intake work at a point along the river where the offtaking canal will be along natural ground above designed flood level. Such a location will require a higher dam, but will avoid the need for expensive ·protection work along the canal to protect it against damage from flood flows in the river. In most of the eases the intake works will be constructed on the lower reaches of the river on per· meable foundation comprising of sand, gravel or silt. Mostly, intake loeation and surface topography rather than geology would thus determine the site to be adopted. The engineer in charge should therefore review carefully the possible sites before any extensive foundation exploration \'>'ork is undertaken.
9.3 TYPES OF STRUCTURE
Type of Intake Works may be open type or pipeibox conduits. The form of the control structure for the intake works depends on the type of gate used which arc most distinguishing feature of this structure. The type of gate to be selected would depend on the flow condition of the river under which they arc to operate and the amount of water to be diverted. Barrel (Box) type may be adopted \vhen chance of silting up is remote and where open type trough structure is uneconomical.
145
DAN
AXIS
i
01'" CANAL INTAKE
i ALIGNMENT OF
INTAKE
DAN AXIS
.J9____ fi
BLOCK PROTECTION
l
j' r--1. - . /
"~
--,-
l
SHEET PILl!"
UPSTREAM INPflf;VIOU8 FL.O<m
! Plli:R
.SLOPE IIH 2
DO'ilNSTFEAM FLOOf\ DOWHST£'".!:1\N SH!:ET PILE
n==---·-· .
.
INVERTED FILTER TOE WALL
L
LOOSE APRON
CANAL
TYPICAL PLAN
OF
INTAKE
LAYOUT OF CANAL HEAD REGULATOR FIG.9.1
In the open type, the ~ontrol structure takes the shape of a rectangular flume subdivided into a number of bays by piers which are surmounted by an operating deck. On some of the larger installations where an operaLing road is involved, a bridge is provided in addition to the operating deck. This type of control structure is generally preferred because of its acccssability for repairing the gates and for the removal of debris and drift. At some locations submerged gates arc employed. The vertical-lift gates or the top-scaled gates are the customary submerged types. To close the remaining portion of the structure that is not closed by the gate panel walls, also termed as breast, walls arc used. These walls are designed as reinforced-concrete slabs supported against the piers or buttresses. At reservoirs where the intake structure is constructed through an earth embankment or a small earth dam, the so-called closed type of structure is used. This type is essentially a buried conduit consisting of a single or mtdtiplc·barrcl reinforced-concrete section which extends through the embankment. The flow through the structure is controlled by top-scaled radial gates or vertical-lift gates which arc located near either the center of the embankment or the upstream end of the conduit. To avoid having the conduit under full reservoir pressure, a condition which would increase the danger from leakage if cracks should develop in the \valls of the conduit, it is desirable to have the control gates located ncar the intake to the structure. The disadvantage of this location is that an access bridge would be required from the top of the embankrnent to the operating deck of the eontrol structure. Trashracks should always be provided at the intake to prevent trees and other clrift from getting lodged in the closed conduit.
9.4
9.4.1
HYDRAULIC DESIGN
Pond level
Pond level upstn~am of Lhe intake is generally obt11incd by adding the working head to the designed fuH supply level in the canal. The working head is the head required for passing the designed discharge into the canal and includes the head losses in the regulator.
' 9.4.2 Water way and Sill level
'
Sill level is fixed by subtracting from pond level the head over the sill required to pass the full supply discharge in the canal at. specified pond level. Considerations for fixing sill level (crest level) and water way, have been detailed in Chapter G, para 6.6.3. The hydraulics of intake 'vorks usually involves either one or both of two conditions of flow - i) open channel or (free flow) ii) full conduit or (pressure flow). Analysis of open channel flow either in open water way or in a part full conduit, is based on the principle of steady non"uniform flow conforming to the law of conservation of energy. Full pipe flow in closed conduit is based on pressure flow, which involves a study of hydraulic losses to determine the total heads needed to produce the required discharges.
il*
Hydraulic jump, basins and impact. block clissipators are normally employed to dissipate the energy of flow at the downstream end of the intake work. These devices arc designed on the basis of the law of conservation of momentum_
:!~
9.5 OPEN TYPE STRUCTURE ?:ii
>$
;q
A
'·~
DESIGN PROCEDURES
The following design procedure is generally adopted in determining various components of the (open type) intake work. i) Crest level and waterway and the number of piers and span width are first determined. ii)
A sloping glacis is provided and continued to the level for the formation of hydr<1ulic jump under different diScharge conditions. 147
iii) A length of horizontal floor beyond the glacis to satisfy requirements of stilling basin discussed in chapter 7.5 "Energy dissipation" is provided. At the end of the basin a vertical cutoff is provided. iv) The glacis and downstream floor is checked against uplift and exit gradient as per procedure ou tlincd in chapter 5.
i)'
Normally, under static condition, the worst condition for checking the floor would be, when the canal is dry and design flood is passing in the river. If the uplift pressures are too high, a reinforced concrete mat maybe necessary from economic consideration and in such a case the piers may have to be extended down the glacis to provide for the end support against bending of the slab.
-?) '.'.'.1'
v) At the time of the formation of hydraulic jump when part discharge is passing into the canal, the glacis should be checked for the unbalanced head for different discharge intensities. vi) For reducing pressure, the impervious floor upstream of undersluices is sometimes extended to the end of the intake work.
vii) In the case of gate controlled structure, when the high flood level is much higher than the pond level, provision of a breast wall i~ necessary, as discussed in para 9.3. The breast wall spans from pier to pier and is designed to support its own weight in addition to the water pressure acting against it. When fully closed, it shuts the opening from crest to bottom of breast wall. Without the breast wall it is sometimes uneconomical to provide gates right up to high flood level.
.'I'
;"f
.··
~X }; ·.·,···
Various parameters/components involved in the design should be determined as under:-
I j
9.5.1
Waterway
Flow in an open channel intake work shall be similar to that in open channel spillway discussed in the foregoing chapters. When the submerged slide gates are used, discharges through the control with the gates completely raised will be open crest flow.
The required head over the sill H, for passing a discharge Q, with an effective waterway L, shall be worked from the following formula: Q
CLc H c3 l2
where Q C
=
discharge in m:3/s,
a coefficient effective waterway in m; and
II,
required head over the crest for passing a discharged Q, in m.
. In the formula given above the exact value of C depends on many factors including the head over the stll, shape, wtdth of the stl!, tts hetght over the upstream floor and roughness of its surface. It is, therefore, recommended that for large mtakc structure. the value of C be determined by model studies where values l~ascd on prototype observations on similar structures arc not available. The value of C should be modifled for submerged conditions as outlined in Chapter 7.
After having decided the effective waterway, the total waterway between the abutments including piers shall be worked out from the following formula: L, ·~ L c + 2 (N K p + K.,\ I H.t +. W
148
I
where LL
Le
total =
N
w~terway,
effective waterway number of piers pier contraction coefficient
Ka
9.5.1.1
=
abutment contraction coefficient
II(,
head over erest,
W
total width of all piers.
Recommended values of KP are as follows:
a) For square nose piers with corners roun?ed with a radius equal to about 0.1 of the pier thickness:
KP
*
~
0.02
rR
-
O.lt
I
t
b) For rounded nose piers: KP
~
O.Ql
~-R
0.5t
j t
c) For pointed nose piers: Kr ~ 0.00
\ ):60°
9.5.1.2 Recommended values of Ka are as under: a) In square abutments with head walls at goo to the direction of flow:-
Ka
~
0.2
b) For rounded abutments with head walls at goo to the direction of flow: for 0.5 H 0 .> r > 0.15 H,,
K0
~
0.1
9.5.2 Width and shape of sill - Width of sill shall be kept according to the requirements of the gates, trash racks and stop logs subject to a minimum of 2/3 H, The edges of sill shall be rounded off with a radius equal to H 8 • The upstream face shall generally be kept vertical and the downstream sloped at 2:1 or flatter.
14g
9.5.3 Shape of Approaches and Other Component Parts
The shape of approaches and other component parts may prcf(~rabiy he fixed by means of model studies. However, for works of medium size, the eritcria given as under may be adopt.cd.
9.5.3.1
Upstream Transition
Joining the vertical abutment of slui(~eway with upstream verLical abutment of intake is made hy providing a circular curve of radius equal to three times the depth of flow over sill excludei roof or over intake sill, if there is no silt excluder. This is done for smooth entry of \Vater. The other upstream vertical abutment. of intake is joinC'd to vertical wing wall in similar manner. The transition from vertical \ving- wall to guide bank slope is done by means of warped retaining wall or broken back transition.
9.5.3.2
Downstream transitions
Wing walls shall normally lw kept vertical up to the end of t.lw Impervious floor beyond which they shall be flared from vertical to the actual .slope of the canal section. However, in order to obtain greater economy the wing walls may be kept. ycrtieal up to the t.cc of glacis beyond which they may be flared g-radually to l/2H: l Vat the end of impervious floor. In the remaining length, wing walls may be flared from 1/z:l to the actu~d shape of the canal section. The wing wall section shall be designed as retaining wall only up to the \Vater side sloPe of 1h:l beyond which it shall he assumed resting on compacted backfill and designed accordingly. Vertical joints should be provided al the end of impervious floor and on the self-supporting scdion.
9.5.4
Design Criteria from Surface
Flow cons£deraiion - In the case of intake work on permeable foundations, the following factors need to be determined. (In case of downstream non-erodible beds protective measures may not he necessary).
9.5.4.1
Depth of Upstream Cut·Off
On the upstream .side of the intake .work, cut·off shall be provided and taken to the same depth as the cutoff upstream of diversion work. 9.5.4.2
Stilling Basin Dimension and Appurtenances
Thes0 shall be provided on the same principles as adopted in case of sluiceway.
9.5.4.3 Thickness of Floor on Sloping Glacis with Reference to Hydraulic Jump - The hydraulic JUmp profile shall be plotted under different conditions of flow. Average heig-hl of the jump trough shall th~n b~ obtamed by deducting the levels of the jump profile from corresponding hydraulic gradient line. Th_Is will be taken as the unbalanced head for which safety of glacis rloor shall be ensured. As a rough g-erde the unbalanced head may be assumed to be 'h ld 2 - d 11. where d 1 and d 2 arc conjugate depths at the bcg1nmng and end of the hydraulic jump.
150
9.5.4.4
Downstream protection
At the end of concrete floor on the downstream an inverted filter 1.5 to 2 D long (D being the depth of scour below bed), consisting of 600 to 900 mm deep concrete blocks with open joints laid over 500 to 800 mm graded filter should be provided. Downstream of the inverted filter, loose apron 1.5 D long of boulders of stones in wire boulder crates shall be provided so as to ensure a minimum thickness of lm launched position. The individual stone size should be heavy enough to resist displacement by the highest calculated velocity at the end of the floor. Where the working head and velocity are too small, small size blocks or required stone size can be considered as rip rap. Upstream of the impervious floor, blocks and loose apron shall be provided which should be similar to that provided upstream of the dam.
9.5.5
Design Criteria from Sub·surface
Flow Considerations - The factors enumerated below should be considered. 9.5.5.1
Exit Gradient at the end of Impervious Floor
Value of exit gradient should be determined as per the procedure formulae given m Chapter 5. The factors of safety for exit gradient for different types of soils shall be as follows: Shingle
4 to 5
Coarse sand
5 to 6
Fine sand
6 to 7
9.5.5.2 Total Length of Impervious Floor As the length of downstream floor and downstream cut off are inter-related, total floor can be decreased by increasing the depth of downstream cut-off and vice versa but increase in depth of cut off would result in the concentration of uplift pressure, specially in the lower half of the floor. A balance between the two should be worked out on the basis of economic studies. Total floor length required shall be the sum of: a) horizontal floor in the downstream from the surface flow considerations; b) length required to accommodate sloping glacis and crest; c) about 3m upstream of the crest or length required from other considerations. Depth of downstream cut-off should be worked out for this floor length lo ensure safe exit gradient. If depth of downstream cut·off so calculated is excessive, it can be reduced by increasing upstream floor length. As a rough guide depth of downstream cut-off should nol be less than (cl/2 + 0.5), where d is the water depth corresponding to full supply discharge.
9.5.5.3
Floor Pressure
Uplift pressures at key points on the floor should be determined as per procedure discussed in
151
-----~~-~·~................ ~------
Chapter :), coiTl'sponding- t.o the condition t.h,1t there: is hi.Rh flood level in the river upstream of head regulator and no water in tlw canal downstream of the intake \Vork. Upstream of sill, only nornimal floor thickness of about l m. should lw provided.
Structural design
9.5.6
The componl'nl.s of the intake work being· similar to those of sluiceway their structural design shall lw done in Lhe s;1nw manner and as for sluiceway.
9.5.7
Hoist Bridge or Operating Platform
/\ bridge or working platform shall be provided for operation o[ g::J.Les and stop logs. of \Vorking platform should dqwnd on the travel of gates.
9.5.8
The length
Free Bourd
J\cl(:quatc· free bo:trd ;1s pPr n·quirPnlent disrussed in Chapter 7 should be provided while fixing height of abutments. which on the upsLn~am sick should be at the same level as for diversion dam.
A typical open type Intake \York is sho\vn in Ii'igurc 9.2A & 9.ZB.
9.6
BARREL TYPE STRUCTURE
9.6.1
General In this type of work. the hydr<1ulics usually involves the following conditions of flow i) Part full conduit (free Dow) iii Full conduit (pressure flow)
Analysis of parl.ial!y flo\ving flow is based on the principle of conversion of energy and that for the ful! condud. flm\· involve'S sl.udy of hydraulic losses to determine the total head needed to produce the requirpd disl·harges. Tlw siJ.(' of tlw water way would thus be fixed from the following considerations. a) Total head loss should remain within !.he permissible limits. b) VPlocit.y should lw mon· than flushing velocity and less than eroding velocity. c\ ?vlinimun1 dinH'nsinn of opening should In~ such as t.o p('rmit. clearance of debris inside the barre~ manually.
9.6.2
Evaluation of Losses
9.6.2.1
Transition losses
Loss at. inl<'L :1nd ouLll't due- to ch:tngP in flow section ·-
.,
This should hl' ctlcu!al.t'd :1s K (
v,)~
tg·
152
•
where
v
~
the higher of the velocities of flow before and after transition
=
lOwer of the velocities of flow
1
v
2
g ""' acceleration due to gravity K ~ a coefficient, its value depending on different types of transition, given in the table 9,1,
Table 9.1 Types of transition
Coefficient at inlet
Coefficient at outlet
0.1
0.2
0.2
0.3
0.3
0.4
0.3
0.5
oA
0.7
0.1
0.2
--~-----·---··--·--·---------
a) Open transition to closed conduit i) stream line warped to rectangular opening ii) straight warped to rectangular opening iii) straight warped with bottom corner fillets to pipe opening iv) broken back to rectangular opening v) broken back to pipe opening b) Closed transition square or rectangular to round (maximum angle with center line ~
7'h 0 )
If the submergence at outlet exceeds one sixth of the depth of opening, the head loss coefficient would be 1.0 at the outlet. 9.6.2.2
Friction Losses
a) Friction loss in Barrel (conduit) ·'_-~ ·~~
Friction loss should be calculated by Manning's Formula:y2 L n2 R 4/3 where:
V = average velocity in barrel in m/s
length of barre I
L n R
=
coefficient of rugosity of barrel surface hydraulic radius of barrel section.
Value of n for concrete surface may be selected from the table 9.2.
155
Table 9.2 Concrete with rough joints ................... .
0.016 to 0.017
Concrete with dry mix, rough' forms .. _........ .
0.015 to 0.016
Concrete with wet mix, steel forms ........... .
0.012 to 0.014
Concrete, very smooth ....................... .
0.011 to 0.012
For design calculations, n may be taken as 0.015 unless otherwise specified. b) Loss in pipe
L y2 f ·--!) 2g f
where
9.6.2.3
Darcy's friction factor - its relationship with Manning's n, is given in Fig. 9.2 C
=
D
Diameter of pipe in metres
V
Velocity in m/scc.
L
Length in metres
Bend Losses
Coefficient of loss at bend in circular section may be adopted from figure 9A·1 (Head Loss in pipe bends) • Head loss at bend in rectangular section may be calculated from the following formula:?
hb ~ F I :!:.. 2g where
G ) 180
hb = head loss due to bend in metre F
0.124 + 3.104 I Z~
S
width of the barrel in metre
)1!2
R
=
radius of bend along centre line of box in metre
G
~
angle of bend
g
9.6.2.4
X
acceleration of deviation in.dcgrces
Loss due to change in direction of flow while entering from approach channel toward the intake:
This can be calculated by the formula given in para 9.6.2.3 above.
156
'~----···->''·•' '''"'/!": •,__:,_-,,_.
;;x;~!_,~:'!-~\5:""-;-;.,~;'.'J>.':;;.;:,<.o'""<-
_;c:,i;;:~:i;&;{;:,;0":f)~;;:lfiiliL::-,-;;:--" ' "-'""' ;~:;,,,-. "·'"''' ,·,..... ~ .... Y·'.
,,,,__._,.
' , •. ,
· ,.,,<~;.,:,;;;
'
•>'.->
'-H"'~.'uo>•~·"'·~-"•'
---------------------"'·----------------~-------------"""""11
OPERATING SHED AND PLATFORM
"
EL 123.40
I:r==1::J 1=1"'" EL 12o,wj__
--
'
::
::
~G-1. PIPE 'RAILINGS
~EL.I22.00
EL .121.00
-,
[
~c-
>OO,HCOO~ FFLUX ;L,
(6 .S;t
M. THK. RICH CRETE
&
I I ..
I DRAIN
PIPE
GRADED
FILiER
.
~
.. -·
l
I 3oo
'~'
3oo
LEAN CONC
t:
350_W_o_
'
I
"·-4-4.30X 1,55 INTAKE STEEL GATE
8
I
2020
I I
410
1.50X 1.50X .90CONC.BLJ)CKS
GOKGS.BOULDER RIP RAP' 90 CM. THICK
I 21so
PART-IAL SE:CTION OF SLUICEWAY SHOWING FACE OF INLET INTAKE
LEL.121.00
!!Il==
J.
'I
r--"'
•
.
•,:._-
•
....
•
• -
.
'j
j
"'
y
REINFORCEMENT ARE NOT SHOWN, SECTION ARE OF CONCRETE OUTLINE ONLY.
/-
,,
'
·,
·'
.. "-·:·
NOTE:
..
-~
.'
~
'.
'-·
....
,..:--
----
.....___
"j:
j
y
·>
_,
0 Q
...
I
0 Q
lJ
lJ_
jLEL.I12.45
430
•.. P,
.. ·.
,
·.l
.'
*' .• )
't>
~-
.-v.
•\
I·
••
I 1100
430
I
SECTION
430
w•~N-'""'---·---
1ool l
• ••
• I>
•••
'
~
\
'
MAG·ASAWANG TUBIG RIVER IRRIG. PROJ.
lGJ
430
Is~\
PROVINCE OF ORIENTAL 114 I ND 0 R 0
-,
_________________
1-1
-----------------·-----,.,, 0
-~ ~'
I
LoJ 11
'' .
,.
·a
' t'
INTAKE WORKS
11
FIG,9.28
SECTIONS
SHEET
®
_,.,,_,,,,.,..•
~.----
. ---·..
···~··~
~~···""'"'''·" ""~"' .,"·'""""' ...~'=· ·-'·~''"·"-· ,0.,,.
..
,:.. ;.;;;;.i;;;.:;;~;:,;,;t.!£fi0Pri£''o::': ;·: .;,-;;,,;::.A-;;:._, ::;.).:.;.•::;~C·-·:c.:;~;; ,•:;,\'.:l
OPERATING SHED AND PLATFORM
'
EL. 123.40
~G.I. PIPE RAILINGS '122.00 EL 12o.wj_,
:;
::
[
J
tEL.I16.00
CREST
30CM. THK. RICH CONCRETE
:-
CONC+E I :4:8 DRAIN
PI f'E--------"
GRA OED
I
300
I
J
300
FILTER
350
-+-------
410
~
PART-IAL SE:CTIO N OF
LEL.121.00
....
''
.- .
,i
~:
.I ;;
'"'
,.'
'. ' ..,:.,. I
0
Q
• )
.,
''
l EL.I16.00
Ii
II_~
I· ...
:.
0
Ul"C" " ---~6J
Ij
. '.<.·.·"
\.,: -~· '-:,.' ·, .. ·., ·,' :~. '
·',.'
~
Q
y
',''
·-
430
430100
SECTION
"'·-·-----------
'
·..
..
~- ~·--·-·-····
'' '
~
. ,.,
\IOC
II
--~·'·
··i
;
.. "
_______,,,_'""""""'"""""''"'""".
-~-~~~------------·
''"'"'"'"'·"~~-;.;.=~~">if{,;.h,
~
/
OPERATING
SHED
a
PLATFORM
~ Il[l[ljljljljllJi,IJ Jl [ 1
1 1 1 [ [
THICK CONC. WEARING SURFACE DRAIN PIPE
EL.II2.00
..
I
1111
"(;! ''
IIIII•
DRAIN PIPE
1,
'"~' ·~· " '"'"
STO~E SPAWLS WITH 75 GAP FILLED W/GRAVEL TAIL WATER
E L .112.33 .
GRADED F I~ TER
"-- --t-;--BLQCK PROTECT[ Mo...---'
I
!''
1' 1'1 1 1' I' 11 111' 1'1 ' 1'1 ' '" r , ,·, ,." ,
I I
840 CONCRETE CLrr -OFF WALL OR SHEET PILE
OPEN - TYPE
INTAKE
MAG-ASAWANG TUBIG RIVER IRRIGATION PROJECT
WORKS
PROVINCE OF ORIENTAL
INTAKE
MINDORO
WORKS SHEET
SECTION
FIG.9.2A
OF !!
~;.
~;.."'=-".
~£~ 1
=~~·~
n;wrv""=-
"'"
~=,...,.,.._,.,,~o
'_,_., ·-e
'>''f",~~'~"-'"·'~'" ~·?''" <'o''C',',;>":_"'"'""'""{•,Wfo=,.,~~,.,.~,,~"""'"'·"""""~~~.'0'-"-"'~"'''""' --~~---~.~-,.~-~-"'-"'o'<'N~-·
··>~-~~~1
--·--·----· '
-.J';;;e~'"W''t'
;,.~==""""-~"""'U~'<~-=~''":I".!:l"'""•="~'·''"''''·'·''"'
.
:·' .
'
I
I I
I I I I OPERATING
I
SHED
a
PLATFORM
I I I
I
'---
BLOCK PROTECT! 0
I I
I OPEN -
I
TYPE
INTAKE
i
I ~-!
1i'
..~ ""'""''""'~,,...,,,"~"'~-
,,,..,, .,
-~It?
'10(!::1-~~M~~""~".l..'l"
It$'
"f
1H
~~~"""""'~ '<
-·-
'"'-"·'''~-----~
9.6.3 Contraction and expansion
To minimize head losses and to avoid cavitation tendencies along the conduit surface, contraction and expansion on transition to and from gate control sections in the pressure conduit should be gradual. For contractions, the maximum convergent an?le should not exceed that given below:tan
oC
1
= u
angle of the conduit wall surfaces with respect to its center line
where u
v&D
arbitrary parameter
fgD
the average velocities and the diameters at the beginning and end of the transition.
Expansion should be more gradual than contraction because of the danger of cavitation, where sharp change in the side wall occur. Normally tan o<: ~ 1/2 u. o<: should not exceed 10°.
9.6.4 Calculation of waterway
In order to minimize deposition of debris within the barrel, it is necessary to provide a m1mmum velocity of 1.5 m./sec. Size of the barrel should be approximately determined on the basis of required discharge and assumed velocity and then tested for head loss. If the calculated head loss is more than permissible, the size and velocity should be adjusted. The size of the gate opening may be approximately determined by using the submerged orifice flow as follows and then tested for head losses. Q ~ 0.75 A J2g H
where
H
available head in difference between the upstream and downstream water level
A
Area of opening, Q
~
Discharge
9.7 ENERGY DISSIPATION
To avoid undesirable disturbance from the jump and to maintain free flow conditions, ample free board should be provided in the closed conduit from the control gate to the outlet. Where sufficient head is available it would be desirable to have the water flow at super critical velocity through the conduit and to have the stilling pool located beyond the outlet to avoid objectionable vibration of the structure under the embankment. An example illustrating barrel type intake work is shown in Fig. 9.3 A & 9.3 b. 9.8 TRASHRACKS
Trashracks arc desirable at the entrance to some head works structures and arc essential when desilting works are involved or where a lot of detritus/debris is coming in the river. Racks usually arc constructed of flat steel bars which are set on edge and arc joined by bolts or welded to the edges of the cross bars. The welded type provides more space teeth to pass between the bars. The rakes can be made in panels for ease in handling. For the facility of clearing, these arc generally inclined at a slope of 1(H) to 4(V). Velocity of flow through the gross area should not exceed about 1 m/sec. Bar spacing is generally 25 mm to 100 mm for fine & 10 em to 30 em. for coarse racks. · On small Intake works, the racks are usually raked by hand but on larger works, mechanical rakes arc generally provided.
159
-'=·
NOMOGRAPH FOR DARCY's " 0.055
$
0.10
0.050
0.09
0.045
llfll
f "
----"nj' }(
-------"
D OJ1 11
•y
7
,,
30.0 It
20.0
0.08
·.'.·lk
0.040 0.07 0.035
0.06
10.0 9.0 -8.0 . 7.0
0.030
0.05
. 6.0
0.04 20
.. 0.03
0.02
= c = en
-
= = _en >u a:
Cl)
a: u
<(
...J
::E
:::::J <(
0.010 DARCY'S EQUATION FOR FRICTION LOSS IN CIRCULAR PIPES L V2 ht • t- D 2g
0.01
h ,I~ .h. f D 3 D 0.005
0.003
0.004
=Cl
= a:
3
0.5
LJJ
0.4
<(
Cl
y2 2g
RELATIONSHIP BETWEEN 11 11 MANNING'S n AND DARCyt$ II f II
!•~ • D I
0.2
116.5n2
1/3 0.4-
FLOW IN CIRCULAR CONDUITS REFERENCE: SMALL
1.0 a: 0.9 Cl 0.8 >0.7 :I: 0.6
LJJ
1-
::E
MANNING'S EQUATION FOR FRICTION LOSS IN PIPES
DAMS, P. 451
FIG.9.2C
•
<(
2.0
4
OF
Cl
10
z z z
DESIGN
4.0 3.0 :::::J
(!)
<(
._!£1
15 -
-
Cl
;~ •
-0.020
5.0
0. I
Appendix- 1
Loss of Heads in a Structure
Loss of Head in a structure may consist of the following:a) Loss at inlet transition and outlet transition. b) Loss inside the structure due to skin friction. c) Loss at bends due to change in direction of flow. d) Loss due to obstruction, e.g. in trash racck. Method of estimating the different types of losses are indicated in the following paragraphs: 1. Loss at inlet transition and outlet transitionv 2
This should be calculated ask ( -2~
-
V?
2
)
2g
velocity of flow before transition
where v2
velocity of flow after transition
g
acceleration due to gravity
k
a coefficient value of which is given in table 9.1 for different types of transition.
2. Loss inside the structure due to friction-
Friction loss shall be calculated by Manning's formula:-
where
v
=
average velocity in the structure in m/s
L
length of the structure in m
n
coefficient of rugosity of barrel surface
R
hydraulic radius of barrel section
Value of n for concrete surface may be selected from the table 9.2:-
For calculation of loss inside structure n may be taken as 0.013 unless otherwise specified. For determining depth of [low in a concrete lined channel, a value of n of about 0.018 should be assumed in order to account for air swell, wave action etc. For determining specific energies of flow needed for designing the dissipating device, a value of
n of about 0.008 should be assumed.
161
3. Loss at Bend Coefficient of Joss of head at bend in a circular conduit may be adopted from Figure 9A-1. Head loss at bend in rectangular section may be calculated from the following formula ~ y2 GF'•··-·2g 180 hh
head loss due to bend in meter
F
0.121 + 3.101 ( z~ )'I'
S
~
width of barrel in meter
H
radius of bend along central line of barrel in meter
g
acceleration due to gravity in m/sec.2
0 ~ angle of deviation in degrees
4. Loss in trashrack
Head loss in trashrack may be calculated from the formula:H ~ 1.32 ( where:
i;' )2 (sin A) (sec 1518 B)
in~hes
H
Head loss in trashrack in
T
thickness of trashrack in inches
V
velocity below trashrack in It/sec.
A
angle of inclination of the rack with horizontal
B
angle of approach
D
center to center spacing of trashrack bar in inches.
162 ._-_a
~
n 'L,~
.26
I I
Loh cb•tilcH•nt!. f•r dioe
'1
en~s
.24
'I,
Re
4-*-
.22 Y
. 20
,..,:X: )>
0
5 (/) (/)
·<
m (/)
,.., :X:
)>
0
.181~
r
.16
0
"U "U
.14
,z
.
.08
"
.06
0
(/)
tD
I
1 I I± I
1
•
I I+-+ i
i
':l 4
I
•
I
i
I'
1
I 1
I II II.J-..+ ~~ J.--1-:tJ I I
1
1
k'l ' I, /i. ._ . :
I
I
= . .+
'
it
,
1
I~
~~-~
.04
!
J~
L.J:j
(', 0
&(0
10'
15'
20'
I'
•.
· _I
i.
25'
u 30'
!
I
r±' t=ti'j!t ----r= -- - · · I
I
1 -
I
',
_
'I
y
"
-
: I I L ~~~ ' !,--r·r· ··;i ~. . I i I I I ~
I
-.;:;-~ I
I
c __
.,
I
!
I I i I l I I I I ITq_: -~,Q-~~m=ffftft r-r-t
...02
I
1
1
!Y/1 1 I I I ; I / IY i i i I I "1'- .I J I I I I l V ,---:-}" I I 1_...-Yi
.10
.0
opg11 . 1' ~··r. brndr i I I I
iSing~
l_ __ _
.12
z
...;
i /j ,
-E-
I
,..,
()
i
Y
~
:,
I
,..,..,0 .,.,
!'
[,'I
I II I I jI'-~f{',I I II
(/)
Ii
lin
(/)
,.., Ol
I I 1/
i
I.
F•~nol~s ruribe
I
()
2
J---t- I
I
s+>oolh ihte~i or
df
._
_ _' :_:::-_-
-j--
• -
75°
80'
+-
I I 35'
40'
45'
DEFLECTION
50'
55'
60'
65'
70°
8 5°
ANGLE
FIG
~i
:t1
90°
9A- I
CHAPTER 10
SILT EXCLUDER
10.1
GENERAL
River flowing in erodible v3.llcys C
10.2
LOCATION AND ALIGNMENT
The excluder tunnels are required to be placed in front of the intake structure and are aligned parallel to the intake. Change in the alignment if found necessary, should be in a smooth curve of radius 10 to 15 times the width of the individual tunnel. River approach also plays an important part and it should be kept straight to the mouth of the tunnel as far as possible.
10.3
DESIGN CRITERIA
Silt excluder comprises a number of rectangular tunnels running parallel to the axis of intake work and terminating close to the undersluices. The principle being to pass the slow moving bed water, con· taining heavy charge of the coarse grades of material through the tunnels whose tops act as platforms at canal sill level. These tunnels lead to pocket gates in the dam/weir which are so regulated that: i) the tunnel does not get clogged with sand ii) the bed water and top water are divided without causing eddies, which would throw the coarse grades into suspension.
10.3.1
Number and Size of Tunnels
The number of tunnels is determined by the available discharge for escapage. Number of tunnels is also fixed from following considerations:a) With the discharge available for flushing, the velocity through the tunnels shall exceed the self clearing velocity.
164
EL.221.34
I.
ExlstlnQ floor
EL. 217.68
PLAN OF TUNNt:.LS
H.f.L. EL. 224 62
EXISTING POND LEVEL EL. 223.17
R.C.SLAB
13M
2.13M
TOP
2.13 M.
EL. 221.34
2.l3M
2.13M
EL. 2
CROSS
SECTION OF TUNNELS
SILT
EXCLUDER
PLAN S. SECTION
FIG. 10.1
b) Size of tunnel should be such as to permit manual cleaning.
d Height of tunnel ~hould be more than the height of silt deposit in order to avoid choking. The number of tunnels usually provided arc 4 to 6. Size of tunnels depends upon the number of tunnels, self-clearing velocity of flow required to be provided which for the alluvial rivers be 3 m./sec. and 4.0 to 1.5 m.{scc. for the~ boulder stage river and the discharge available for escapage. Generally, 1.8 x 2.1 m. size excluder tunnels are adopted for river in alluvial stage and 1.8 x 2.4 m. size tunnels for river in boulder stage.
10.3.2
Spacing and Shape
The tunnc~l nearest to the head regulator has to be of the same length as that of regulator. The con· sccutive tunnels should he spaced at distances such that the mouth of the one nearer to the head regulator comes within the suction zone of the succeeding tunnel so that no dead zone is left between the two to permit sediment. to deposit. Generally, a distance of about 12 m ..may be adequate for head regulators of longer depth. The tunnels shall be suitably bell mouthed at the inlet to mm1m1ze entry losses and improve suction. Bell mouthing shall be done .within the thickness of divide wall and may be done on any suitable elliptical curve. 10.3.3
Roof and Bed of Tunnels
The roof slab of the tunnel should be kept flush with sill of the canal regulators and the bed kept at the upstream floor level of sluiceway. It is also helpful to draw graph of velocity distribution and the corresponding integrated graph of discharge derived therefrom. 10.3.4
Exit and Exit Channel
All the tunnels outfall into the stilling basin through one or two undcrsluice bays adjacent to the intake work. The tunnels should be suitable, throttled laterally or vertically or bothway so as to produce accelerating velocities in the tunnels; maximum being at the exit end so that sediment material once extracted docs not deposit anywhere in the tunnels. The exit channel to take the silt bearing water into the river should be properly maintained downstream of the excluder. A separate outfall channel is generally not required when the bed material is sand. If, however, boulder and gravels arc to be excluded and the main channel is away, a lined channel may be necessary, or a channel may have to be created with suitable training measures.
10.3.5 Transitions
Transitions to piers, in bell mouthing at top or sides, should preferably be elliptical, the major axis being in the direction of flow and two to three times the minor axis.
10.4
ESCAPAGE DISCHARGE AND MINIMUM WORKING HEAD
Eseapage discharge (flushing discharge) of 15 to 20 per cent of the canal discharge is generally reqmred. A m1mmum of 0.5 to 0.75 m. of working head is required for sediment excluders on sandy rivers and minimum of 1.0 to 1.25 m. is required for excluders on shingle or boulder beds.
1
l
166
l!lU
10.5 LOSSES IN TUNNELS
These shall comprise of friction losses and losses at the bends and transitions and shall be computed
by the following formula: a) Friction loss
where hr
head loss in m.
=
V
velocity in m/s
L
length of tunnel in m
N
rugosity coefficient depending on type of surface
R
hydraulic mean depth.
b) Loss due to bend
where
Hn
loss due to bend
F
0.124 + 3.134 (S/2r) 'lz
g
acceleration due to gravity
0
angle of deviation in degrees
S
width of tunnel in m
r ~ radius of bend along centre line of tunnel in m. c) Transitional loss due to change of velocity in contraction
vzo
v2
1 ) h" ~ 0.1 (-"- - - 2g 2g
where he
=
transitional loss due to change of velocity in contraction.
V and V 2 ~ the velocities before and after the transition. 1
I
g ~ acceleration due to gravity. 10.6 STRUCTURAL DESIGN
i) The tunnels should be stable against buoyancy and uplift ii) Roof of the. tunnel should be designed for water load and silt load. iii) Floor should be designed against uplift and foundation reaction.
167
!1:((;; .·; ; ·;
3
2&
2
iv) Walls should be designed against hydrostatic pressure assuming adjacent tunnel to be empty. Hydrodynamic effect and inertia forces to be taken into consideration.
j I
10.7 CONTROL STRUCTURES & MISCELLANEOUS FACTORS
1i
10.7.1
l
The excluder tunnels are operated by the undersluice gates. These should be regulated either for the tunnels to run full bore or to remain completely closed.
]j j
Il l
Control Structure
10.7.2 Miscellaneous Factors The following draw backs should be kept in veiw while designing a silt excluder.
l
i) In case the excluder is unfavorably located, tunnels do not flush the incoming debris and may get partly choked resulting in a large,percentage of bed material to be drawn by the canals.
l
ii) Higher discharge in the pocket brings in relatively more sand and thus counteract favorable curvature. In view of the above factors, it is recommended that before finalizing a theoretical design of work of this nature, hydraulic model tests be carried out to check the performance of the proposed design. 10.8 EFFICIENCY OF SILT EXCLUDER Efficiency of silt excluder is given by E
so- s,
~---..::.__
where
S0
total silt content that would enter the canal if there is no silt excluder provided.
S,
silt actually entering the canal
In practice efforts are made to exclude everything larger than 0.2 mm in diameter and efficiency is worked out on material of coarse grade. The same excluder may be expected to work effectively, where the proportion of coarse silt is greater. On the other hand, the coarser the heaviest grade of silt carried, the greater will be the slope and velocity and consequently less concentration of silt in the lower layers.
10.9
I
FIELD STUDY BASED ON MODEL EXPERIMENTS
A silt excluder along the barrage at Narora in U.P. Province, India, across river Ganga, was con structed in 1967. A brief description of the work along with model studies carried out at the Research institute and comparison of the results with the prototype performance of the excluder has been given in Appendix A. The study of field performance of the silt excluder highlights the fact that the excluder functions well and that the qualitative studies on hydraulic model do bear a relationship with the quantitative results expected on prototype.
168
Appendix A
ILLUSTRATION OF NARORA SILT-EXCLUDER
A-1
Introduction
A 924 m long barrage designed for a flood discharge of 14.150 m3/sec was constructed across River Ganga at Narora in U.P. State in India. A six tunnel excluder was also constructed to reduce the sediment entry into the offtaking canal from its right bank and sited just upstream of the barrage at an angle of 107° (Figure A-ll. The excluder covers extreme right under sluice bay of 15.3 m width infront of the head regulator. The mouth of the tunnel has been staggered by a distance of 10.8 m so as to cover the entire width of the head regulator. The size of the tunnels has been kept such that loss through each tunnel is the same. Top of the crest of head regulator is at R.L. 176.36 and the floor of the pocket is at R.L. 174.63. Thus, a clear depth of 1.43 m is available for excluder tunnels allowing 0.3 m as thickness for top scale. The water depth which is available for most of the time above this floor when pond level is maintained at R.L. 178.96, is 4.33 m. The excluder thus covers 1/3 water depth of heavy sediment concentration. Other salient features of the barrage are given in Table A - 1. TableA-1 1. Total length of Barrage
924.2 m
2. Spans for weir portion 54 No. of 12.2 m
658.4 m
3. Spans for sluiceway 7 No. of 15.2 m
106.7 m
4. 52 piers of 2.44 m for weir portion
126.8 m
5. 6 piers of 3.03 m for undersluices
18.2 m
6. 2 cross wall of 2.44 each
6.9 m
7. 1 divide wall between under sluice and weir bays
3.0 m
8. Fish ladder
6.2 m
9. Designed flood discharge
141.50 m3/sec.
10. Barrage upstream floor level R.L.
175.6 m
11. Barrage bay crest top R.L.
176.2 m
12. Under sluice upstream floor R.L.
174.6 m
Model studies were carried out at the Irrigation Research Station. Sand of size (Dsol 0.23 mm was injected into the model manually upstream of the dam axis. The efficiency of the excluder was determined by measuring the quantity of silt entering into the canal without excluder and with excluder with the following formula:Silt Collected without excluder-Silt Efficiency of Excluder ~ _ _ __:C_:o_ll_:e..:.c_te_:d_:_w_i_th__::E:.:x:.:c..:.lu:_d:_e:_r_:__ _ 0.01 x (Silt collected without excluder)
169
iliii'?¥
NARORA
SILT
12
0
6 !m
SCALE
SCALE
#'¥¥¥&
m
FOR PLAN
FLOW
I R=
I
DAM AXlS (BARR AGE)
R=12.42
10.82
I R = 9.22
FOR SECTION
i@!"
EXCLUDER
H0.668-H0.668----H0.6 68 -+-10. 66&-+ 10.6 68--1 0
fi%4
A 6
R =aDO!
~~
,~-----.\=:==2~3~d~~~ AXIS OF HEAD REGULATOR
O.S.FLOOR
R. L 175.25
' - - - - - - - - A PRO._.N_,__ _ _ _ _ _ _ _ _ __L__
P L
"'&PIER
•
. . SECTION
A N
• • __
;~:i;:::;ili
THROUGH TUNNEL NO. I
·~· ' 1'1Fill'~j;~~ll '~' ~,,:;;: SECTION
- '·~
ON
f
~'.'~;~~;t"' '""
A- A FIG.
A- I
~-
A-2
Efficiency of Excluder
As the effieierwy of a silt excluder is the percentage reduction in the quantity of sediment wh~ch would enter the canal, if there is no such structure, for determining efficiency the silt concentratiOn shall need to he observed as under:~ i)
in the channel upstream of the excluding tunnels
ii) in the canal downstream of the regulator; or i)
in the excluder out fa\\ or ::tt the exit of excluder tunnels
ii) in the downstream of the regulator
In the first case, the pcrr.entage efficiency would be:Percentage Efficiency
=
Ir - I,
x 1 00
(1)
I, where
Ir -= silt intensity in parts per thousand of water by volume Ic -= silt intensity in parts per thousand of water by volume in the canal downstream of canal regulator
In the second case, the silt intensity in the river pocket can be calculated from the formula:(2)
where Ie
Silt intensity passing through the excluder
Qr,
Discharge through the excluder
QC
Discharge in canal
From the equation (1) above, the efficiency of silt excluder can be determined. The function of the excluder being in removal of the bed sediment, its efficiency can also be directly evaluated with the formula.
Percentage efficiency
where
X
br
intensity of bed load in the river
be
bed load in the canal
100
(3)
A·3 Discussion on test results & Conclusions The efficiency of the excluder on the basis of formulae (1) & (2) which are based on total load carried by the river is not possible to be determined in view of taking observations in turbulent flow and thus Equation (3) based on bed load was used for working out the efficiency. The efficiency so worked out
•
171
I
I
I j
collected from field data, varies from 51 percent to 87 percent. The variation of efficiency percentage, discharge and bed load of the river have been plotted against time in Fig. A-2. It can be seen that the general average value of exclusion efficiency remains above 60 percent for most of the discharges. The maximum value of efficiency of the bed load exclusion works out to 87 percent against a corresponding figure of 91.6 percent (as per equation 3) obtained on model. This is an indication of the fact that results of model studies of silt exclusion are fairly representative of Prototype performance, and thus high light the fact that the qualitative studies on hydraulic model do bear- a relationship with the quantitative result~ expected on prototype.
j l
l
NARORA
J
SILT
A FIELD STUDY OF
EXCLUDER ITS
EFFICIENCY
II '
I'
...
l
l j' '
>
100
~
l
.
l
!
j
l1
J j
.."" ..'• • • ".' " ."
<.>
:I<JOO
.,""
...
60
1000
.. c;
40
'!:
c
"'<.>
.;
0
0
.
-' 0
20 0
r;; ""-' <.>
..,."".
..u,. ....... "' ".. r
DISCHARQE
<.>
8£0
LOAD
10
0
_j
I 1•
40
20
EI""I""ICI £HCY
0 $~7·71
14-T-71
19-7-71
24-7-71
Z9-7-71
TIME
VARIATION OF EFF!CIENC!ES 7BED LOAD AND DiSCHARGE WITH TIME
FlG.A-2
•_,_®
il
172
CHAPTER 11
RIVER TRAINING WORKS
11.1
GENERAL
Rivers in the alluvial planes generally flow over a flat country and meander widely within their permanent river banks. River training works are provided guide the river to flow exially through the dam/weir and to check out.flanking. Besides, these are reqmred to:
:o
minimize possible cross flows through the structur·e which may endanger the structure and protection works. ii) prevent flooding of the riverine lands upstream of the work due to the afflux caused by the construction of the obstruction (dam) in the rivBr. iii) provide favourable curvature of flow at the intake from the consideration of sediment entry into the canal. i)
Though the planning and design of training works is done largely by empirical methods, reliance has to be placed on the intuition and judgment of experienced engineers. Model experiments are extremely useful and valuable in the layout of these works.
11.2 TYPES OF TRAINING WORKS
River training works generally provided at diversion dams may be classified as below: a) Guide banks embankments b) Approach embankments c) Afflux embankments
,11.3 GUIDEBANKS
In a flat country in view of wide alluvial belt through which the river flows, it is necessary to narrow down and restrict its course and help the river flowing centrally through the dam constructed across it. Guide banks, as the name implies, are artificial embankments meant for guiding the river flow past the dam without causing any damage to the structure and other appurtenant works. The guidebanks usually consist of two heavily built embankments in the river in the form of bell mouth. The portion of the river between the normal river banks and the guidebanks is closed by simple embankments. Sometimes, if the river has high scour resistant marginal lines, it becomes economical to dispense with one of the guidebanks. Guidebanks are constructed in pairs generally symmetrical in flow. These are constructed both•upstream and downstream in the direction of flow on one flank or both flanks according to the requirements at site. Proper alignment of guidebanks induces favorable curvature of flow, which in turn helps in minimizing sediment entry into the canal system. Guidebanks should be
173
'""''
so designed that even if the river swings, a safe marginal distance is available between its extreme swing an9 approach embankments.
Classification
11.3.1
Guidebanks can further be classified: i) according to their form in plan and ii) according to their geometrical shape. The latter are named for the shape of their curved heads such as straight, elliptical guide banks with circular or multi radii curve heads. These are however applicable to wide flood plains and longer diversion works. For low dams, the former are generally adopted. 11.3.1.1
Form in Plan
Guide banks can either be divergent, convergent or parallel (Fig. 11.1) Divergent guidebanks exercise attracting influence on the flow. With such guidebanks, the approach embankment gets relatively less protection in worst possible embayment as compared to the equal bank length of parallel guidebanks. Divergent guidebanks would thus require a longer length compared to parallel guidebanks for the same degree of protection. Besides, divergent guidebanks induce oblique flow on to the diversion dam and give rise to tendency of shoal foundation because of larger waterway between curved heads . '~
.• ' M_.
.
The convergent guidebanks have a disadvantage of and tendency of shoaling along the bank. Sometimes of these draw backs, convergent guidebanks are rarely suitable. They provide uniform flow from their head to the
excessive attack and heavy scour at the head the end bays are rendered inactive. Because used. Parallel guidebanks are considered most axis of the dam.
11.3.1.2 Geometrical Shape
-i~.·
According to the shape of the curved head, the guidebanks can be classified as straight and elliptical as shown in Figure 11.2. In the case of elliptical guidebanks, the elliptical curve is up to the quadrant of the ellipse and followed by single or ~ometimes multi radii circular curves. Due to gradual change in curvature, the flow, in the elliptical shape hugs the guide banks all along its length as against separa· tion of flow occurring in case of straight banks. Elliptical guide banks are however more suitable in wide flood plain rivers. 11.3.2
Layout of Guidebank
The layout of the guidebanks should be such as to guide the flood smoothly through the work. This is however dependent on local topography, site of the structure and alignment of approach embankment. The exact shape of the guidebank is dictated by model studies.
11.3.3
Length of Guidebank
Length of the guide banks should be considered from the following requirements:i) The maximum obliquity of current should be limited to reasonable value. iii Approach embankment or both sides should be fully protected in the event of main channel of the river em baying considerably behind the training works.
··~,!1
Based on experiments, the length of guidebanks can be co-related with the length of the structure 4 between abutments (L).
'.·~ ;%\
The length of guidebanks on the upstream can thus be kept 1.0 L to 1.5 L. Obliquity of flow be
174
Wont
..... 0
b l
/
"'
I
I
~
•
'
'\
a::
sulca
bank
' '
{i
f
~
i
-"
''
c 0
"'
\ \
0
Divergent~~
.?
pouiblo lmbaymont
I
I
1/
I
'·
!
0::
/
~
--Diatance between the approach bank and the wont p011lble embayment.
a) DIVERGENT I PARALLEL GUIDE BANK
.... c
) ~
~
u::::
•i
...c
~ ~
0
.,
"'
~
~
a:
Shoal
•
JLAxlo
b) CONVERGENT
GUIDE
UPSTREAM
BANKS- FORM IN PLAN FIG. II. I
r
R•0.46f>w
...1
"'-
li2 0
-
L
I I
1I
~g~
0
0
AI STRAIGHT GUIDE BANK
5•To eo•
0.3 TO 0.!5R
Bl ELLIPTICAL GUIDE BANK
GEOMETRICAL SHAPE OF GUIDE BANKS FIG. II. 2
limited to 30° to protect the bank from severe river action. In case of wide alluvial belt, the length is generally determined by fitting meander loops between the guidebank and afflux embankment. The radius of worst possible loop should be ascertained from available data. In case no data is available, the 3 radius can be determined by dividing the radius of the river by 2.5 for discharges up to 5000 m /sec. an
11.4 APPROACH EMBANKMENTS
In the construction of diversion dams on alluvial rivers, the natural waterway is restricted for better flow conditions and economy through the diversion dam. The unbridged width can be blocked by con· structing embankments called as approach embankments shown in fig. 11.3. The approach embankments on sides between the guidebank and alluvial belt edge are generally aligned in line with the axis of the dam/weir up to a point beyond the range of worst anticipated loop.
11.5 AFFLUX EMBANKMENTS
Afflux embankments are the embankments extending from the abutments or the approach embank· ment as per such toporgraphy of the dam site and connected on the upstream to the ground above affluxed high flood level. On the downstream, it should be taken to a length necessary to protect the canal or approach embankment from high flood. Afflux embankments should be located outside the zone of em· bayment of the meandering river as indicated in fig. 11.3 showing typical layout of river training works. In case a road exits near the site of dam, the afflux embankment may be continued with the road lor economy. Where flood embankment already exist, same can be used as afflux embankments. The section may however be strengthened when necessary, The alignment of the embankments need to be reviewed from time to time when river flow conditions change.
11.6 DESIGN OF GUIDEBANKS
Guidebanks have to b~ perm~nent structures like the dams/weirs they protect; and warrant great care ~n de~1gn and executwn. Gmdebanks should be so designed that even if the river swings, a safe margmal distance should be available between its extreme swing and approach embankment.
177
••
- 22
..D&J
., -..." ...;
=II""" I/
s
(/')
0
:li!
e
5~
0
...; !i<
l5
xi ~ 31:
"'iii
z
0
"'
g
~
1LLJ
a:>
12
IL.
?
<(" K 0
a::
0
~
0:: 0..
)P=-
~
~-' ...
-'
>
~
!i
"'
0::
~
~ ~
!:(:I
...3
"'
0
~
0
~
!;i!E :;:o !11~
_J
_J
(.)
a.. >-
1-
•
.&LM¥
.Q
&;;;;.;,
11.6.1 11.6.1.1
Section Top Width
In Lh<~ straight portion of the guidehank, also known as "shank", the top width be kept about 6 metres aL formation level (the height of the embankment) to permit carriage and storage of material. At the noses, the width may be inereased suitably to enable the vehicles to take turn. On small banks lesser width ean be provided keeping- in view olh(~r means of carriage of material along the guidebanks.
11.6.1.2 Free Board A free hoard of 1 to 1.5 m. should be provided above the highest flood level (HFL) for 1 in 500-year flood or ahove the afflux water level and as per r:ritcria discussed in Chapter 7.
11.6.1.3 Side Slope & Protection Side slopes of guidchanks depend upon the material they are to he made of and the height. Side ·slope of 2:1 to 2:5:1 is considered adequate and slope 2:1 is generally adopted. Due to surge waves, current v
guide banks between high flood and low water level, therefore, requires protection. Stone pitching should normally be done all along the shank on the river side and continued round the back of the head. Turfing can he done on the rear side of the shank. Full allowance for settlement should also be made. The size cf stone required on the sloping surface may be worked out from the curves given in fig. 11.4. Generally, stone pitching on slopes is done manually. Pitching with stone weighing from 40 to 70 kg (0.3 to 0.4 ml for an average velocity up to 3.5 m/s should be done. Cement concrete blocks of depth equal to the thickness of pitching for higher velocity may be used, but round and smooth boulders should be avoided. Pattern grouting in blocks of required size can also be done, where necessary. Hand packing & careful gradation of stone between sand and the large stone are necessary to prevent the sand being sucked
out by high velocity flow.
11.6.2 Thickness Of Pitching The thickness of pitching is generally adopted as equal to the size of stone but not less than 0.25 m. For higher velocities where size of stone works out more than 0.4 meter, cement concrete blocks of 0.4 to
0.5 m thickness can be used. 11.6.3 Filters A graded filter of about 20 em to 30 em in thickness and generally satisfying criteria as described in chapter 7 - para 7.9.3 should be provided below the pitching to protect the embankment material from being sucked out.
Thickness of stone pitching can also be worked out with the help of formula:
where Q
Discharge in cumec.
T
Thickness of pitching in m
' This would however be applicable to rivers with high discharges. In this ease, the stone pitching should be patch grouted covering 25% to 30% of the area and in suitable number of panels. Hand packing
179
:s
I
I
I
I
I /;
2
II I f-
I~
·st NK SLOPE 1:1~
~
0.6
~LOPE 2:
BANK
1
0.4
0.2
LIJ
:::E
-
<{
-
I 1/
= -
3: I ----._ I
0
0 .I
f-
0 0.0 G fIll)
VI
I
I
I
E
// ::
i"/j_
v
I()
Ul
N
IJ..
"
0
1-
..
Ul
!
(!)
-=
1.0
w :1: i
-
!
Ii
i-
0.0
I
I 4
2
=
G
4
I=
I
G
10
VELOCITY, m /s
SIZE OF PITCHING STONE
Vs. VELOCITY
FIG.
;:~&Q¥4 . .29!¥12\
-~- Q ~- 1&1&1@4 4
i
i
:z:
f-
0.7
10
=
I II
ill/ : ; I/,
;z 0 1-
-
0.06
o.o
LIJ
-
BANK SLOF
w
w z
~X
I
=
-
-
0::
1-
-
-
I I II
f-
E
/=
./
0.8
f-
I
.&
Q£;)5J;l$
!\§;.Qk
2
11.4
and careful gradation of the stone, with smaller stuff, as quarry refuse between the sand and Jarg(~ stones should be used to prevent the sand from being sucked out by high velocity flow. VVith this arrangement, the thickness of pitching can be suitably reduced.
11.6.4 Loose stone Apron. (Launching Apron) The face of the guidebank which is protected up to the river bed level with stone pitching so that the sloping portion is not damaged, is alsO subjected to scour which would occur at the t.oc with consequent undermining and collapse of the stone pitching. To obviate such damage to the slopes, stone cover, in the form of apron is laid beyond the toe on the horizontal river bed so that scour would undermine the bed first, starting at its farthest end and works backwards towards the slope. The apron then launches to cover the face of the scour with stone forming a continuous carpet below the permanent slopes of the guidebanks. Adequate quantity of stone should thus be required to ensure complete protection. The quantity would depend on the apron thickness, depth of scour and slope of the launched apron.
11.6.4.1
Size of stone
The size of stone for the launching apron can be determined with the help of curve given in Fig. 7.18. In case required size of stones arc not economically available, cement concrete blocks or smaller size stones in wire crates may be used. The crates (gabions) '3hould be made out of 14 mm Gl wire with double knots and should be closely knit and securely tied.
11.6.4.2
Depth of Scour
The depth of scour at different locations of the guidcbank shall be different as discussed under para 7.3.3. Scour depth may be adopted as given below:Noses of guidcbanks
2.0 to 2.5 R
Transition from noses to straight
1.25 to 1.75
Straight reach
1.0 to 1.5 R
The status of flood for which the scour should be calculated, should be 100-year flood and the apron be designed accordingly.
11.6.4.3 Thickness of Launched Apron In launching, the apron will not form a uniform thickness as by hand packing. The thickness should be
25 to 50 percent more than thickness of pitching on slopes. 11.6.4.4 Slope of Launched Apron The materials of the armour on the slope of the shank and in apron act only as a "face wall". Since the guideb.~nk slopes arc to be stable by themselves, the slope should have the same angle as the "angle of repose of th~ mater~al of which It IS constructed. Model experiments have shown that an apron does not launch sat1sfactorrly unless the angle of repose of the underlying material is flatter than that of the protection work. Observatwns at guidebanks on various rivers in alluvial beds have shown that the actual slope of a launched apron ranges from 1:5 to 3:1. However, in most cases the average approximate to 2:1.
181
The face slope of the launched apron should not therefore be assumed steeper than 2:1 for loose boulders and stones and 1.5:1 for concrete blocks or stones in wire crates. 11.6.4.5 Shape and Size of Launching Apron
It has been observed that whatever be the type of apron section, a certain dispersion, which will be the maximum at the outer edge, is unavoidable where the apron launches. As a general practice, an adequate thickness of apron should be provided at the toe of the slope to ensure a strength after launching equal to that of the stone pitching on the slope face. Additional stone out of the total apron quantity worked out, should be provided for irregularity of launching and washing away of stones; this can be better distributed in the apron in triangular wedge shape with maximum thickness at the outer edge. It has been observed that shallow and wide aprons launch evenly if the scour takes place rapidly. If the scour is gradual, the effect of the width on the launching apron is marginal. As discussed under para 7.9 of chapter 7 under "Protection Works", depth of scour "D" should be worked out from the normal scour "R". A width of launching apron equal to 1.5D has been found adequate and should be provided in the shank portion as well as in the curved portion of the guidebank. A minimum thickness of 1.5T (where T is the thickness of slope stone) should be provided in the straight portion. For the curved head where the apron has to cover a wider area, the thickness should be gradually increased to 2.25 T. It would be desirable to extend the apron from the shank to the head in a length L 1 equal to one fourth of the radius of the curved head. To minimize the loss of stones, the apron should be placed at the lowest possible bed level. A complete design of the guidcbank has been illustrated in Fig. 11.5.
'
Volume of Stone i)
Slope Portion For a slope of 2:1 with F as free board
I
total length ~
..f5 (Y
+ F)
If T be the thickness and allowing 25% extra as losses
I Il i
Quantity of stone/unit length ~ {5 x 1.25 x T (Y + F) ii) Apron Portion a) Straight reach Volume of unit length b) Curved reach volume
f5 X
D
l
Il
1
I
11.7
1.25T ~ 2.25D
X
1.251'
'h (1.51' + 2.251') x 1.5D
2.81D
1
X
X
T
DESIGN OF APPROACH EMBANKMENTS
In the design of the approach embankment, same guidelines as applicable to the design of guidcbanks should be followed with the modification given hereunder: 11.7.1
Top Width
Top width_ of the approach embankment should be from 6 m to 9 m at the form>tt - 1'011 leve 1. ') , n sma 11 wor k s Iesser WI d t h be provided keeping in view other means of transport of material.
182
45° TO 60°
0.3 TO 0.5R
/
-----
'
' \\ '
'
R" 0.45 Pw Pw" 4.83
"\JO
,r
.E L.
T
-,-_J.-L.j--.x R "I D
s LEGEND:
']
;~ ~ /ill
f y D X R
" " " " " T s ""
+Y
1.5 T
FREE SOARD DEPTH OF WATER ABOVE LOW WATER DEPTH OF DEEPEST SCOUR MULTIPLIER NORMAL SCOUR DEPTH THICKNESS OFSLOPE STONE THICKNESS OF SOLING (FILTER)
D
SECTION X- X (NOT TO SCALE)
';~ 'di
~···H.
F L.
··--·-···· ____ j --..,------·,..
xR
' ' ' ' I ' ..............',
DEEPEST
- - -
SECTION (Nill
SCOURJL _ _J
---~
_
'-......_ '-....::_ _ -, _ _;;,.
D
__
Y-Y
TO SCALE)
DETAILS OF GUIDE BANK
FIG. II, 5
D-tY
11.7.2 Section The section of the approach embankment on canal side shall he designed for the condition when there is high flood on the upstream and no water on the downstream. On the other side, the embankment should be safe against differential head with affluxed level on the upstream and retrogressed level on the downstream. 11.7.3 Size of Stone In workinR out the size of stone, the velocity should he calculated with 40 per cent of the design discharge.
11.7.4 Thickness of Pitching Thickness shall he based on 40 per cent of the design discharge.
11.7.5 Filter Generally, no filter should be needed. In case, however, a wider and higher section is required, filter shall be provided for as in the case of guide banks.
11.7.6
I
l
I I
I
Launching Apron
Size of stone shall be same as for Ruidebanks. Depth of scour shall however be 0.5 D to 1.0 D, as indi· cated in Fig. 11.3. Thickness and slope should be the same as for guidebank as per para 11.6.4.3 and 11.6.4.4. A width of launching apron equal to 0.5 D should be provided from the abutment up to the distance covered by the curved head of the guidebank and beyond it where it is exposed, the width of the launching apron should be increased to 1.0 D.
11.8 DESIGN OF AFFLUX EMBANKMENT 11.8.1
Top Width
Top width and also other parameters should be same as for approach embankment.
11.8.2 Slope and Protection Works As afflux embankments are generally constructed away from the main channel of the river, these are not subjected to strong river currents. The provision of pitching of slopes and launching apron is not considered necessary. Reaches which are likely to be subjected to strong river currents, paving of slopes may be necessary, otherwise it is desirable to provide turfing only.
11.9 GENERAL BEHAVIOUR OF LAUNCHING APRONS Results of model tests and field observations show that for satisfactory launching, bed material should scour easily and evenly. With an apron laid on the river bed consisting of alternate layers of sand & clay, stones slide down as sand layers scour and clay layers subside & thus stones from clay layer are
184
washed away. Clay bands cannot thus be used as dependable foundation for aprons. Where unavoidable, heavy mainte;nance of such apron is required. 11.10
OTHER TRAINING WORKS
In the case of very wide rivers, it may be found that necessary extension of the guidebanks to provide protection for the entire length of the approach bank is not possible on consideration of economy.
In such cases, the approach banks across/along the river banks, have to be protected by additional protective works such as revetment, spurs, etc. If, howeveor, the groyne can he avoided by a moderate increase in the length of the guidebank, it should be done. If suitable points are available for the con· struction of training measures like groynes or pitched islands, the length of the guidebanks can be considerably reduced and security of diversion structure increased under post construction conditions.
185
CHAPTER 12
1
INSTRUMENTATION
l
l 12.1
\
GENERAL
Diversion dams and related works form key structures to the development of river basin development for irrigation as discussed in the earlier chapter. These structures are built to last for many decades. Du~ to the pondagc created upstream, there arc tremendous forces inherent, which the dam is expected to Withstand throughout its operation life. Effect of failure of such a dam and consequent negation of planned benefits of the project, make it imperative that means should be available in the dam for providing informa-
J
I l
tion on continued assurance of its safety.
l l
12.2 OBJECTS OF INSTRUMENTATION
1
The objectives of instrumentation are two fold. Firstly, the instruments installed at the damsite keep a constant watch over their performance in service and indicate the distress spots which call for remedial measures. Secondly, observations from the instruments form a cumulative record of the structural behaviour. The information obtained through measurements promotes the understanding of the influence of various parameters and other assumption in the design on the structural behavior and thus leads to formulation of more realistic design criteria . .
I
12.3
INSTRUMENTATION FOR STRUCTURES ON PERMEABLE FOUNDATIONS
In hydraulic structure on permeable foundations water stored percolates below the foundation of the structure. The pressure gradient acting along the direction of flow is a critical design parameter at the exit end of the structure. Design of the structure, as outlined in the previous chapters involves calculation of these pressures and gradient on the basis of certain assumptions. Actual observations of the pressure, during the operation therefore becomes important. For the observations of pressures, instruments (pressures pipes), which are installed in the structure serve the following purposes:
i) These act as tell tales watching the stability and to predict undesirable developments.
ii) To investigate if the actual pressures at various points on the structure are in conformity with those assumed for the purposes of design.
A systematic record of these observations, apart from its scientific value, will be as necessary for the maintenance of the structure as a record of usual sub-surface sounding and probings.
12.4
UPLIFT PRESSURE PIPES
In dams on permeable foundations, the pond water percolates below foundation due to the difference
186
·~-m.MLJ&
w
of head "H" (Fig. 12.1). At intermediate points along the bottom profile at A. B residual pressures d"ve\op, which act upwards and tend to lift the structure unless counter weight i.e. dead weight of the structure itself counteracts the same. The pressure gradient at the exit and at C is also a critical design parameter. At this, if the force of water is in excess of the effective weight, piping can occur. The measurement of these pressures at all such critical locations iS thus very important. 12.5 NUMBER AND LOCATION 12.5.1 The pressure points to be installed can be divided into three groups: i)
Along and beneath the horizontal floor.
iil Along the vertical cutoffs. iii) At different depths under the foundations. For very low dams where the foundation strata consists of boulders & gravels installation of pressure points at lower depths at (ii) & (iii) can be eliminated. This shall however depend on individual site.
The location of these points should however be pbnncd carefully taking into account the presence of stratification in sub·soil and other geological and design features specifically presence of clay bands. A typical arrangement of the pressure points under a structure is shown in Fig. 12.2. On a diversion dam, suitable locations under the horizontal floor are:-
i) Upstream and downstream aprons ii) Immediately upstream and downstream of vertical cutoffs. iii) Intermediate points at regular intervals.
Under the floor at the selected locations, uplift pressures are helpful from stand point of safety of the structure, whereas tapping points located along the faces of vertical cut offs at suitable depths help in the correct evaluation of the effect of the depth and spacing of cutoffs in addition to that of stratification of the sub·soil on uplift pressures. In the sub·soil the pressure points should be located at suitable depths and intervals under the pervious and impervious floors. Normal distribution of pressures ':lot effected by the vertical cut offs shall be indicated at these points. Pressure points along the downstream vertical cutoff on the downstream face, at C shown in figure 12.1. shall indicate the pressure required for computing the exit gradient. 12.5.2 Location Pressure tapping points should generally be located along the abutments of the structures and at a member of intermediate sections between the abutments, at -suitable intervals. Normally, at a minimum number of three intermediate sections, pressure tapping points should be provided. This shall however be dependent on the importance and magnitude of the structure. The pipes from the filter points (described in para 12.6) are led to the piers or abutment walls to enable water level readings to be taken throughout the year.
12.6 DESIGN OF PRESSURE PIPES 12.6.1 12.6.1.1
Filter Points Pressure tapping points consist of filter points made of brass at the base, of 50 mm inner dia·
187
,-~-'-~""'"' ---.--~-~---·""---~-~-~--··-·"-----··-·-- -~·~.:c.~~--~~------"'-"'-'"""-''-"~--~-"~--"""-'>"-''"""''" ·'·~------~'·""-'"'-"'-"4~,~-0>..C"""'~·-···="'~'-"~'''~·"""""-":."'"''"'"".,:"''-"·"'·'"--'~-"'''''""'-=;,~,~-'-.~''"~"""-·''"·'./-~~---~-, -··----~
UPSTREAM WATER LEVEL
j
STAND PIPE PEIZOMETER)
•
_j__ p
t
-r
~DOWNSTREAM
WATER LEVEL
A
DAM ON PERMEABLE FOUNDATION
FIG.
11
12. I
. "':ik
}.~!4%
'fj;!i&,&_.-i,:
iW'*4
;l.~;i;Oi\~·f
"'"""*''·><· •F0,' 0
'
NOT!:S: L OC A TICN OF TAPP!NG POINTS WOULD VARY WITH THE SHAPE a SIZE.
---i~ "'...
UPSTREAN
Al.L DIMENSIONS ARE IN MM.
k~
FIG. 12.2
meter and 100 em in length with 500 micron wire gauge strainer as shown ln fig. 12.2. These arc further connected by 40 mm G.!. pipes to suitable stand pipes located in the dam structure for measurement of water level. The filter point shall be fitted with a driving point at one end and a threaded blind pipe of 50 mm diameter and 75 mm length at the other end. 12.6.1.2 The filter points shall be laid horizontal where excavation permits or alternatively these <:an he
driven down to the desired levels. In case of foundation soil being too hard and difficulty experienced in driving to larger depth, the filter point along with blind pipe can be inserted in a bore hole of about 100 mm diameter. 12.6.2 Generally graded filter material around the filter points is not required since no flow through these points is expected. In case of very fine sub-soil material, however, graded filter can be provided to avoid choking up of the point. From economic considerations, where the above arrangement is costly porous tube piezometers, where porous ceramic tube acts as a filter tip, can be provided. 12.7 INSTALLATION
The filter points are laid horizontally or driven vertically and connected to pipe laid vertically in piers and abutments. Points directly under a pier or abutment wall shall be connected to the observations platform by a single vertical lengths of piping. But those away from piers and abutment walls shall be connected by horizontal lcnghts of piping. The horizontal lengths shall be placed well below the lowest pressure level that is likely to occur at the respectivP lJOints. Otherwise, no observations will be possible during certain water level conditions~when the observations stand pipe will be dry. The horizontal piping between the filter point and the observation point on the superstructure shall be slightly inclined downwards in the direction of the filter point to avoid any possible air lock. When more than one pipe are driven at the same place to different depths, they shall be spaced not closer than 30 em so as to avoid direct connection between any two filter points. During the installation of pressure pipes the properties of soil around the tip, should be observed, particularly when the tips are located in soil with different properties and permeabilities recorded. This may be of help in subsequent analysis and interpretation of observations. During erection, the ends of all pipes shall be kept closed by caps to avoid foreign matter finding its way into the pipes making observation of water level unreliable.
12.8 PRECAUTIONS
i) All vertical pipes shall be kept dead vertical and no link of any sort shall be allowed. Failure in this requirement may make it impossible to lower the bell sounder to the right place for observations. iii Each pressure~tapping point shall be given a distinct number and that number shall be marked on the filter point and on each length of connecting pipe. These distinctive numbers shall be stamped on the caps at the end of the stand pipes and on the masonry or concrete platform where these are located. 12.9
MAINTENANCE
i) Alter installation every year before the onset of monsoons, each pipe shall be tested to see that the fi_lter pornt rs not choked. If any choking has occurred remedial measures like using compressed air or water under pressure by jetting through the pipe, shall be taken; iii E ac 11 vertica . I stand pipe shall be provided with a screw cap to avoid bird nests and tampering. iii) All m1ssmg . . screw caps on the tops of stand pipes shall be replaced with their original numbers stamped. 190
if!Ld. iJ _
:a
iv) All the piping including the stand shall be coated with a good quality anti-corrosive paint, taking care that original pipe numbers are not obliterated.
12.10 OBSERVATIONS 12.10.1 i) ii)
The following observations should be made: Upstream and downstream water level should be read from water level gauges suitably fixed. The water levels in all the stand pip~s shall be read by means of a bell sounder lowered into the pipe by a steel tape or by electrical devices.
iii) The depth of sediment on the upstream and downstream floors and the soil characteristics of the sediment shall be observed. Depth of sediment can be measured by sounding. 12.10.2 Record of Observation The observation shall be recorded suitably in the Registers & Forms as per the format given in Appendix 12 A Following observations shall be recorded. a) Date of observations. b) Upstream and downstream water level. c) Total head H - the difference between upstream and downstream levels. d) Temperature of river water and temperature of water in selected pipes.
e) Depth of sediments on upstream and downstream floors. f)
Water levels in all pipes.
g) Residual pressure (P) level.
difference of water level in stand pipe and downstream river water
h) Velocity potential percentage,
1:5
(P/Hl x 100
While recording all the above observations and their analysis, pipes should be grouped by lines laid in a single section from upstream to downstream. For proper maintenance record, one page shall be earmarked for one line and sufficient pages shall be reserved for subsequent record of observations for at least one year. Record is generally prepared in the field in duplicate, i.e., one additional copy for the central office. 12.10.3 Time Lag When there are large fluctuations in the upstream and downstream water level in the river, during rising or falling floods or when river supply is ponded up, the results are likely· to be influenced by time lag. A rise in the upstream level will give relatively lower readings and when falling, higher readings are expected. With sufficient and regular observations, it shall be possible to make due allowance. A sudden rise in downstream level will give relatively lower pipe readings and vice versa. Similarly, in rising flood when upstream and downstream levels arc rising, the pipes will read relatively low and in falling flood, when upstream and downstream levels are falling, they will read high.
12.10.4 Frequency of Observations The frequency of observations shall depend on local requirements. In case of special problem, obscrva tions can be more frequently recorded.
191
I j
Generally, once a week at key points and once a fortnight for other points shall be enough for watching stability of structure. Daily observations shall be made during such periods when water fluctuations arc more, and specially during floods.
I '
12.11
1 l I l
.I
l
I
PRESENTATION & ANALYSIS OF DATA
For a given pressure point, 0 ie P/H values remain constant for any structure provided temperature of water and depth of sediment, scour on upstream and downstream do not alter. The function.ll should form the basis for plotting, for any variation in its normal value needs to be explained by temperature and sedimentation. Following graphs shall be prepared to have a quick idea about the behaviors of different pipes at any time of the year and bring to light the abnormality .
Date of observation should be plotted on X axis and variable on Y axis. Variable being: a) value of 0 b) river temperature
c) value of H d) downstream water level e) depths of sediment/scour at the pervious floors, upstream and downstream. These graphs should be kept plotted up to date for all key points to help ascertaining unfavourable developments.
192
:::WMSZZ::,
Z:.¥
till
·~
~';'$
"'--~'-\1;;'•.;-.--·<·~··cc/y/i,-l,i';>"
''
• .,,. ~-~,:-F\'-" -
tii-~;;:';;f;
•.-,••'A0i,f;.•:t::;f,··~·'-''' "''~"-~"·"-""'"''"',/,
d''·'''"-,,;;,;;,·-
•'«'"''
·APPENDIX
REGISTER
Of UPLIFT PRESSURE NamG
of
Nome
of hydrou11c structure :
s
K
I
l
OBSERVATIONS
River:
E T
(ALONG
PIPE
12A
PIPE
c
*
H
UNEI
i' •..
: !
Name of observer:
•
t,'
,[)ate
of observatIon :
Time
of o bservotion :
·<
Upstream water leveJ:
(m)
Downstream water level:
(m)
Head " H Snode temperature:
(m)
;
•c •c
River water temperature:
De p!ll of sed lment on upstream pervk>LtS tloor: Dep!ll of sediment on downstream pervk>us tlooc
Line Distance t ron Reduced level RedUC
PIPE
( II
NO. Upstroo m Eod of Bottom ot of Impervious Pipe floor ( 2)
( 3)
NOTE :lf Sketch to show: i}Foundation prot lie : II) Uplift pressure pipe
0
Bond ~ipos,
It
Any ( 4)
No.
(m) (m)
Toto! floor lenotn :
Reduced level Pipe water Depth of water Reduced le.,.. p io m of Top of pipe, Temperature In Pipa, of w:rter In (col.8- d/$ m. •c m. river W.L.) Pipe, m. ( 5)
( 6)
( 7)
( 8)
( 9)
0 ' ( P/H) DESIGNED (I 0)
X
100
REMARKS
OBSERVED ( II)
( 12)
with rumbers: .
~
~
r.·_j_ .
}I :~
CHAPTER 13
~
..Jg····
DAMS ON IMPERVIOUS FOUNDATIONS
13.1
GENERAL
To make water flowing in a stream or river available for irrigation use, it must be diverted by means of a diversion dam, intake works and other appurtenant works. Such diversion works are generally
l~ ~
'I~.
·.•.•
located in or close to the channel of an alluvial river. Though the location of a diversion dam is established by the requirements of project plan, it may be required to place dam on almost any type of foundation material. Very few sites for diversion dams. however, permit construction on rock foundation, since bed rock is usually at such depth that its use as a foundation is impracticable and uneconomical. At some locations, however, the project plan may so dictate as to found the dam on impermeable (rock) foundation. Considerations in the design of diversion dam founded on impervious material {rock) being similar to those in the design of gravity dam, its analysis has been discussed in this chapter as applicable to design of gravity dam.
13.2
DESIGN ANALYSIS
For the design of gravity dam, it is necessary to determine the forces which may be expected to effect the stability of the structure. The forces which should be considered are: i) The external water pressure. iii Internal water pressure (pore pressure or uplift) in the dam and foundation. iii) Silt pressure. iv) Earthquake forces. vi Weight of the structure. vi) Forces from gates or other appurtenant structures. Design Assumptions for calculation of stability, evaluation of load and forces promoting stability and inducing instability and load combination shall be the same as adopted for the spillway section (solid
gravity) on permeable foundation discussed in chapter 8. The forces to be evaluated are however, briefly described as under:a) Forces which induce instability are:i) Pond level and tail water loads. iii Earthquake and silt pressure. iii) Uplift pressures. b) Factors that promote stability are:i) Dead load.
194
•
ii) Shear resistance of dam and foundation.
iii) Quality and strength of concrete.
iv) Strength characteristics of the foundation and abutments.
13.3 SAFETY CRITERIA
Safety criteria against sliding and overturning shall be the same as for dams on permeable foundation discussed in Chapter 8 of the manual. In the evaluation of the forces and design analysis the following features for dams on rock foundation shall, however, need special consideration.
1. Uplift pressure. 2. Foundation and its treatment
3. Water way - crest length
4. Stilling basin 5. Protection Works
13.4 INTERNAL OR UPLIFT PRESSURE
Uplift forces occur as internal pressures in pores, cracks and seams within the body of the dam, at the contact between the dam and its foundation and within the foundation. It is recognized that there are two constituent clements in uplift pressure; the area factor or the percentage of the area on which uplift
acts and the intensity factor or the ratio which the actual intensity of uplift pressure bears to the intensity gradient extending from head water to tail water at various points.
Effective downstream drainage, whether natural or artificial, will generally limit the uplift at the toe of the dam to tailwater pressure. Drainage holes drilled subsequent to grouting in the foundation or other drainage arrangement, where maintained in good repair; are effective in giving a partial relief
to the uplift pressure intensities under and in the body of the dam. The degree of effectiveness of the system will depend upon the character of the foundation and the dependability of tho effective maintenance of the dr;oinage system.
13.4.1
Criteria for Design
The following criteria are recommended for calculating uplift forces: a) In case of highly jointed & broken foundation, the pressure distribution may be required to be based on electrical analogy. For low diversion dams, however, where there are no formed drains, it is safe to assume the straight line variation from head water to tail water pressures as a measure
of uplift. b) The uplift is assumed to act over 100 per cent of the area. c) It is assumed that uplift pressures are not affected by earthquakes. Uplift pressures can be reduced by forming drains through the concrete of the dam and by drilling drainage holes into the foundation rock. Such drains arc usually provided near the upstream wall of the dam·exercising proper care to ensure that direct piping from the reservoir will not result. Other methods
used to reduce the uplift at the contact of the dam with the foundation include construction of cutoff wall under the upstream face, construction of drainage channels (usually of sewer type) between the dam and the foundation. These methods usually are considered only additional safety factors in the design of low dams and should not be considered in reducing the design and stability requirements.
195
13.5
FOUNDATION
13.5.1
Structural Competency
The foundation should be investigated and the maximum allowable stress established though the structural competency of the foundation is not usually amenable to precise and complete evaluation. The data from exploration and tests should be supplemented by engineering and geological judgment. Where the quality of the foundation materials approach or falls below that of the concrete/masonry of the structure on it, the foundation competency becomes a controlling factor and should be appraised in full detail. 13.6
FOUNDATION TREATMENT
13.6.1
General
The rocks near the bed of the river are characterized by numerous structural distressed conditions which may be undesirable in dam foundation. It is of great importance that all important geologic condi· tions affecting the feasibility of the dam site are ascertained before commencement of construction.
In the case where geology exhibits extensive discontinuities such as faults, clay filled joints or bedding planes, strength is governed solely by the resistance against shearing along these discontinuities. In the case of soft, uncemented shales or slates having weak, near horizontal planes, the toe may not be considered strong enough to provide sufficient assurance against sliding. In such cases a deep heel trench below the upstream side of the dam, is provided to be filled with reinforced concrete securely anchored to the dam. 13.6.2 METHODS OF TREATMENT
Several methods of treatment of foundation, such as excavation of seams of decayed or weak rock by open excavation, mining of objectional material and back filling with concrete, or by drilling and grouting can be resorted to. For small dams foundation of rock, including hard shale, generally do not present any problem of bearing strength. Rock foundation, should however, be carefully investigated to determine their per· meability. If erosive leakage, excessive uplift, high water loss occur through crevices of faultp]anes, the foundation should be grouted. The determination of whether or not a foundation should be grouted is made by examining the site geology and by analyzing the water loss quantities of foundation exploration holes. A great deal of experience is required to make this decision since every foundation is unique. Normally in the case of storage dam and in its estimation, foundation grouting is provided. For detention dams or for extremely low diversion dams, grouting of rock foundation is not required. The treatment of foundation for such type of dams may only consist of low pressure blanket grouting, which can be divided into two parts. a) surface treatment b) sub surface operations.
13.6.2.1
Surface Preparation
Where surface rock is weathered and distressed, it would be necessary to remove over burden and shattered mass before laying concrete. 13.6.2.2 Subsurface operation where required. This consist of grouting & drainage treatment.
196
13.6.3 Procedure for Grouting 13.6.3.1 The principal objectives of foundation grouting arc to establish an effective barrier to ·seepage and to consolidate the rock under the structure.
13.6.3.2 a)
Low Pressure Grouti·ng The general plan for treating consists of low pressure blanket grouting. rl:hc _low pressure grouting aims at improving the foundation condition by providing a general co~sol!dat1_o~ of th_e sur face rock and to fill and seal major surface scams and crevices. The surface IS thus JOinted tnLo a monolithic mass.
b) High Pressure Grouting
High pressure grouting aims affecting a relatively water tight seal across leaky zones in the foundation. Procedures diff(.~r significanlly according to the conditions of the foundation and irnpor tance of the structure. c)
Depth of grout holes The required depth of the deepest holes depends upon nature of the rock in foundation. Actual assessment of depth can be made by water testing up to varying depth to determine extent of leakage. Generally, depth can be determined with the following formula:~ d
~
0.33 h + c
where d
depth of holes in metres.
h
height of dam in metres.
c
constant varying from 7.!) to 25.
In a hard dense foundation, the depth may vary from 30 to 40 per cent of the head and in a poorer foundation, the holes may reach deeper as much as 70 per cent of the head.
13.6.4 Drainage The final treatment of the foundation is to provide drainage. Suitable drainage measures, which should be maintained under constant repair, contribute significantly to the reduction of uplift. Drainage is accomplished by drilling one or more lines of holes immediately downstream of the glacis. Sizes, spacing and depth of holes can be assumed on the basis of judgment and characteristics of rock.
I
13.7 WATERWAY- CREST LENGTH
In a diversion dam, the water way and afflux are co-related and for an economically and hydraulically efficient design the combination between the tWo is made by trial and error. In the case of diversion dams on permeable foundations the waterway is fixed taldng into consideration the stable width of the river worked out from the Lacey equation. For dams on impervious foundations, major factor governing the crest length of the overflow weir is the cost. The most economical length for a given flood can be determined by comparing the cost of weirs of various lengths with the cost of other features effected by crest length. Some of the important features to be considered in making the cost comparisons can be public utility services affected by back water, like
197
roads, rails, telegraph, pipe lines, etc.; Cost of appurtenant works - sluiceway, head works, stilling basins, protection and river training works. Though it may not always be practical to use the economical crest length because of other controlling features, cost studies can he used as a guide. 13.8 STILLING BASIN - FLOOR THICKNESS
Thickness of floor for the stilling basin (downstream apron) shall be dependent on uplift pressures which in turn would he different from those worked out in case of dam or pervious foundation by Khosli Theory discussed in Chapter 5. In the case of basins of dams on impervious (rock) foundation, the floor (which is at a lower level than river channel) shall be subjected to an uplift equal to the tail water heador higher depending on the sur· face. During spillway,. running, the basin flow is subjected to higher head because of the slope of jump profile. The basin floor must be' heavy enough to withstand the unbalanced head. For design, the stilling basin floor is considered to be a free body in equilibrium with foundation reactions balancing active loads. Uplift forcc.s caused by hydrostatic head on the bottom of the slab are counter balanced by the weight of concrete and effective weight of water in the basin.
13.9
PROTECTION WORKS
In view of the non-erosive characteristics of the river channel, protection works as required on dams on permeable foundation, may not be required in case of those on impermeable {rock) foundations. Curtain walls downstream of the stilling basin may however be taken to suitable depth. Necessary rip rap material shall be provided consistent with the velocities both upstream and downstream of the dam.
198
CHAPTER 14
OPERATION AND MAINTENANCE
GENERAL
Every dam and the water retained structure presents a potential danger. A probable failure of the dam means not only the loss of structure and the water stored, but the stored water though in small measure, may also cause damage to life and property in the areas immediately downstream of the dam on its failure. The necessity for proper inspection and maintenance of dams and appurtenant structures is evident. The risks of damage to the works can be increased as much by neglect of proper and timely operation and maintenance as by inadequacies in design and construction. Diversion dams which are founded on sandy or gravelly stream bed materials have their stability ensured by a broad base with cutoff walls, be safeguarded by frequent inspections for evidence of piping or boils. An increase in the volume of seepage or piping would call for suitable remedial measures. Proper operation and maintenance of such dams is thus of great importance. It is necessary that the engineer incharge makes themselves familiar with the basis of design and manual of work. Operation and maintenance of diversion dam and appurtenant works can be divided into two categories:~
a) Hydro-mechanical installations b) Civil Works
14.2 HYDRO-MECHANICAL INSTALLATION
Main items under this category are generally gates and winches. 14.2.1
Operation of Gates
il All lift gates should be operated at suitable intervals to keep them in working order. In low supplies when opening are not desirable, raising of gates by 15 ems for a few minutes would be sufficient. Such gates which have not been moved for a long time should not be forcibly raised at once but should be lifted slowly by almost 3 em or so and left in that position for about 10 minutes till the silt deposited against the gates is softened and water leaks through. This is necessary to a void strain on the machinery. ii) The operation speed of the gates should be limited to the maximum speed indicated by the manufacturers. iii) The operation of undersluice gate should be based on model studies for optimum hydraulic efficiency, structural safety, and maximum silt exclusion.
I
iv) ?peration of ~eir shutter~, where provided, and their sequence of operation shall be based taking mto account rrver behaviour, shoal formation and scour. It should be decided by the engineer
199
in charge. Generally. the gates starting from the centre are opened first and then moving on either side. v) The gates are opened in installment of 30 em or so at a time. Gate opening should be increased to allow passage of boulders, etc. vi) The head gate of the intake work should be opened equally unless otherwise indicated on model
studies. vii) The openings of the silt excluders, where provided, should be closed slowly to avoid water hammer. viii) The operation of gates should be done in such a manner that the permissible difference in static head is not exceeded on either side of divide wall to endanger safety of the structure.
Maintenance
14.2.2
All machinery and gates at the works should be kept clean, tidy and in proper working order. Care should be exercised that it is properly handled in conformity with the manufacturers instructions.
14.2.2.1
Gates
For proper maintenance of gates, following instructions should be taken into consideration. i) The gates and counter balanced boxes should hang vertical and plumb. Any adjustment if required to keep them in such shape should be made from time to time. ii) For shutters, all the holding devices like chains/anchors holding them should be kept free from rust. iii) All the gate grooves, angles for gates/shutters should be maintained clear of debris, silt, etc. iv) The upstream face which would be coming in contact with water should be provided with a suitable and proper paint to make its life long. Other parts of the gates and shutters not in contact with water can be painted with a suitable paint which can be ready mix paint but be water and heat resistant. v) Machined surfaces, stainless surfaces, brass or bronze do not require any painting.
14.2.2.2
Gate groove and Seals
i) The grooves and their machined faces should be maintained clean and well lubricated. Before application of lubricants, these should be cleaned of all sticky deposits. ii) All seals just after construction and during closures should be tested for their efficiency. The verticality and horizontality of the seal seat and wall plate should be checked. Rubber seals should be suitably tested to press uniformly by inserting paper strips or other means. Seals should be adjusted or replaced as necessary. 14.2.2.3
Steel Wire Ropes
i) All steel wire ropes should be kept clean and well lubricated. Generally, at least once during a year, the ropes should be lubricated after proper cleaning. The lower portion of ropes under water can be wrapped with gunny (jute) bags after proper lubrication. ii) Inspection and maintenance of wire clamps should be done periodically. It is recommended that clamping devices should be tested at least once in three years.
200
14.2.2.4 Winches and Hoists
il All winches and lifting arrangement should be examined to ensure that all gears and axles are clean and properly lubricated. All bearings should be kept lubricated. The alignment of shafts should be checked and coupling bolts tightened. iii All grease cups be kept full of lubricants to protect the bearings against rust. The winches should be tested for workability periodically. These should be operated in right direction and for that properly marked directions and limits should be indicated. iii) For electrically operated hoists, all precautions necessary for ensuring safety and proper operation of motors, switching devices, upkeep of proper insulation of wirings be taken. The electrical and mechanical parts should be periodically inspected. iv) The decking should be properly maintained. Wooden planking where provided in the decking of hoist bridge, should be checked frequently. Loose nuts and bolts be tightened and worn out planks be replaced. v) All lift gates/fall shutters should be tested and the engineers in charge should submit a certificate to the competent authority before the advent of monsoons that all gates/falling shutters are in good operation cOndition.
14.2.2.5 Lighting and other Arrangement
il All the lighting arrangement and illumination where provided, should be checked daily during flood season and once in a week in slack season.
ii) For the road bridge, the bridge bearings should be cleaned and greased once in a year after the monsoon season. iii) Painting of super structure :;hould be done once in 2 years.
14.3 CIVIL WORK Inspection and Maintenance
Periodical inspection of diversion dams is necessary for ascertaining and detection of any damage and to ensure proper functioning. Further objectives are as follows:
a) to ensure adequacy of the structures to serve the purpose for which they are designed. b) to verify the condition and monitor behaviour. c) to investigate conditions which cause distress.
d) to study the extent of damage, if any, and planning of maintenance and repairs. Adequate inspection shall be carried out by competent personnel after an unusual event. All inspection observations should be compared with the design assumptions and results of model studies. Such inspections are generally carried out annually for all underwater works after monsoons. Besides, detailed inspection should be carried out in stages by drying various portions of the dam and inspecting them against crack or damage, at least once in every five years. All such repairs shall however be carried out before on-set of next monsoons.
Inspection and maintenance of the following components of the dam should be given special attention. i) Aprons ii) Impervious floors
201
iii) Sediment excluding devices iv) Intake work v) Instrumentation
l
vi) Other observations vii) River training works
I -i
14.3.1
Aprons
The sounding and probing of the upstream and downstream apron and the arc~tS just upstream and downstream shall be carried out immediately after the monsoon to assess the scour and launching of aprons. The block protection portion particularly on downstream shall be carefully examined against any settlement and effectiveness of the filter. Suitable measures shall be taken to protect against harmful retrogression.
14.3.2 a)
Impervious Floors
Upstream Floor
Upstream and downstream floors upstream floor is not much during lean lamps. In the boulder reaches joints repairs can be carried out under water, b)
I
shall be inspected after the monsoons. When water depth on the water flows, inspections can be done with the help of under water of the stone sets should be carefully inspected. Whereas minor for major repairs the areas may need to be isolated.
Downstream Floor i) Downstream floor of the stilling basin should be inspected carefully. Inspection can he properly
done during winter flows and necessary repairs should be carried out before the monsoons. ii) In case of permeable sandy foundation, for deep cisterns requiring cleaning and dewatering, inspection can be done by probing. In boulder reaches, however, dewatering and cleaning may be carried out in rotation once in three years.
:~ cl
iii) Deep cistern when being dewatered should be ensured against the maximum head for which it was designed. In the operation/regulation manual, this should be specified.
l
14.3.3
I Cj
A complete inspection of the silt excluder including the opening, roofs, tunnel mouths, etc. should be carried out during winter months with the help of divers and under water lamps. Whereas, m1nor repairs can be carried out under water, major repairs may require local isolation.
14.3.4
i
Sediment Excluder
Intake Work
A thorough inspection of the work should be carried out in winter months. The upstream floor can be inspected by probing and downstream cistern when dry or by local isolation. After 3 years or so the upstream floor shou1d be examined thoroughly by isolating the area. Necessary repairs should be carried out in time.
l l
14.3.5
Instrumentation Data
The data observed on the instruments (pressure observation pipes) em~edded for pressure, described
I j
202
Cj
I
·-:g
n_ __ s_ X. MlJiMKJJ?JM ...
iJ@i . _..JLL..
't
in Chapter 12, should be examined and performance report prepared every year. The observed pressures should be compared with the design uplift pressures with the help of graphs recommended m the format. If any additioanl uplift pressure pipe is required at the critical locations, this may be installed. The uplift pipes should be maintained in accordance with the recommendations discussed in the chapter on instrum~ntation taking special care that each pipe is; i) given a separate number
ii) mouths of all pipes are kept closed to avoid clogging with foreign matter iiil each pipe is tested frequently with water jetting. 14.3.6 Other Observations 14.3.6.1
Pressure Release Pipes
For checking efficiency of the drainage system provided downstream of the dam, pressure release pipes should be checked for the quantity and quality of the discharge. The sediment content in the affluent should be observed. It may be possible to take observations during dry season. Suitable remedial measures shall be taken when required. 14.3.6.2 Standing Wave Profile
Hydraulic jump profile shall be recorded with the help of strip gauges which require to be painted on the wing walls and the divide wall. The profile in the propotype should be compared with the design profile for various conditions of flow. 14.3.6.3 Sediment Observation
Water samples both upstream and downstream of the undersluices for sediment content should be taken. In addition, below the canal intake work the samples be collected to assess the suspended sediment. These observations should be made at least once a week during the monsoons to determine the efficiency of the sediment excluders. Such observation would be useful in ascertaining any modification in the regulation procedure. 14.3.6.4 Aggradation of Riverbed
With the construction of dam, river bed upstream is likely to get silted and aggradate resulting in increased afflux and consequently reduced free board. Gauges should be fixed upstream at intervals of 1000 m or so to record water level and to determine increase in the afflux elevation if any. If necessary. , training works may have to be raised suitably for restoring the design free board. 14.3.6.5 Retrogression
The retrogression expected in the river bed downstream of the dam should be observed with the help of gauges fixed downstream of the work at an interval of 10 m. or so. In case of excessive retrogression as provided in the design, remedial measures should be taken when required to ensure safety of the structure.
203
14.3.6.6
Settlement
For measurement of any settlement of the structure, because of presence of clay layer or other soft soil in the foundation, permanent observation points should be established by precise leveling. The levels should be observed in the winter months every year and remedial measures taken, if necessary.
14.3.7
River Training Works
i) A detailed survey of the river training works upstream and downstream of the dam shall be carried out every year. ii) On the upstream, the survey should extend about 1 meter above the design flood level on both the banks. On the downstream, the survey should extend to a length up to which changes with the construction of dam have occurred. iii) A number of permanent bench marks should be established on banks to facilitate survey. iv) Changes in river course should be examined and remedial measures should be taken, if necessary. v) Guidebanks, afflux embankment and approach embankments should be examined in winter months and necessary repairs to the embankment section, pitching and apron be carried out before monsoons. vi) Sufficient stock of stones and boulders should be made available in the close vicinity of the protection works for use in case of emergency. 14.4
REGULATION AND OPERATION
14.4.1
General requirements i) For efficient functioning of the system, it is necessary that adequate regulation staff should be
provided and their duties clearly specified. ii) Sufficient stock of material, tools and plants required to meet emergencies should be maintained at all sites of diversion dams_ A complete list of all such items be maintained and their availability periodically checked. ' iii) The gauge sites should be linked with telephone/telegraph lines to the diversion works. On major rivers, the installation of wireless transmitting stations, for speedy transmission of flood wirings should be considered. 14.4.2 The operation and regulation can be divided into three periods depending upon the flow in the river. i) Pre monsoon
ii) During monsoon iii) Post monsoon
14.4.2.1
Pre monsoon operation
During this period, river flow is low and thus no wastage of water would be desirable. In case of diversion dams where falling shutters are provided, these shall be so regulated to conserve all the a~ailable s~pplies to maintain Pond level. The releases through the Intake into the canals should be done wrth the help of gauges and corresponding discharge tables, which should be checked occasionally.
204
;;;
zan~
14.4.2.2 Monsoon Operation
i) A number of gauges shall be installed sufficiently upstream of the dam to ensure timely information for operation of gates at the dam site. ii) During floods, the recording of gauges and transmission of the same should be done more frequently so that the Engineer and staff in charge of the regulation are posted with the rising flood. iii) To help minimum entry of sediment into the canal, still pond regulation should be done as far as possible. In case of locations where it is not possible for canals to be closed for flushing of the sediment, still pond regulation can be adopted. The attempt should be to resort to still pond regulation as much as possible, depending on flow in the river. iv) The excluders should be kept open during floods taking into consideration safety of the structure. The under sluice gates under no circumstances be allowed to be overstopped. v) Sediment Change Observation
It is important to keep constant watch over the sediment content entering the head work, that being ejected out by the excluder and the sediment deposited in the canal. In this respect following action should be taken.
a) In low floods, sediment charge observation be done and frequency can be fixed depending on the silt charge. b) cross section of the canal be observed at~suitable intervals.
c) water surface slope in the head reach of the canal be kept under observation with the help of gauges.
d) ponding upstream of power station, if any in the canal reach be restricted to the required extent.
vi) Operation of weir shutters, where provided, should preferably be based on model studies. vii) Pond level be kept minimum to feed the canal with the required discharge by suitably opening the gates.
--~~
14.4.2.3 Post-monsoon operation
i) Sediment change observation & cross sections on the canal shall be continued till satisfactory results are obtained. iii Still or semi still pond operation, with sediment excluder operating depending on the surplus water, should be continued till water is reasonably clear.
iii) When canal is opened, it should be run with a low supply for few hours and discharge gradually increased. The rate of filling and lowering of the canal should be prescribed depending upon water table along the canal, type of drainage filter behind lining, etc. Generally, rate of filling is faster than lowering of supply in the canal, to avoid cracks in the lining. 14.5 HISTORY OF DIVERSION WORKS
A continuous history of the river behaviour and overall performance of the diversion dam Intake work and riv~r traini~g works should?~ maintained on all major works. In the history of work, in~pection ~cports. repairs earned out and additional measures adopted for river training works and any other important feature relevant t.o the over all performance of the structure, should be given.
205
0 h
c
CHAPTER 15
"s F t
DESIGN OF DIVERSION DAM AN ILLUSTRATIVE EXAMPLE
15.1
GENERAL
The concept of flow of water through the sub soil below the dams on permeable foundation and consequent attendant hydraulic gradients and uplift pressures generated have been discussed in the fore· going chapters. The design of such a dam thus involves besides the above forces, the determination of the most economical length and section of overflow portion (spillway), sluiceway portion, having the required capacity to pass the maximum allowable flood, maximum afflux elevation; energy dissipation arrange· ment, protection works against expected scour on the downstream and upstream, stability analysis and structural design of all the components of diversion works. Whereas in the preceding chapters the design principles & procedure of designs are covered, in this chapter, an illustrative design example based on the design criteria described in the previous chapters, has been presented.
15.2 15.2.1
ILLUSTRATIVE EXAMPLE MAG-ASAWANG TUBIG DIVERSION DAM LOCATION
The proposed diversion dam will be located on Mag-Asawang Tubig River approximately at the con· fluence of Ibolo and Aglubang Rivers. The dam would be an ogee type, about 2.00 metres high and is proposed to be of concrete and rubble masonry. The intake on the left bank would have capacities of 5.539 ems. during land soaking and 7.959 ems. during crop maintenance and that on the right bank would have a capacity of 33.97 ems. for crop maintenance and 40.998 ems. for land soaking. Based on the command area, the operating water surface of the dam would be at elevation 115.80 m. 15.2.2 GEOLOGY: Shallow boreholes were drilled along the dam line with a combined depth penetration of about 5.60 m. which indicated that bed rock exposure at the dam line is sparse. Gravels, boulders and coarse sands occurring in a highly variable penetration are preponderant in the river bed and in the flood plains. The material at depth is pervious and the relative proportion of sand mix and gravel is highly variable.
15.2.3
AFFLUX & CREST LEVELS
The existing H.F.L. corresponding to the discharge rating curve for a design flood of 5320 cumcc as determined, is E1.117.93 IUR) at the dam site. The usual value of allowable afflux as a result of con· struction of the dam is 1.5 to 2.0 m. in foot hills, i.e., steep reaches of the river with boulders and
206
1
0.60 m. to 1 m. in plains (para. 7.3.2 of the Manual). For the Mag·Asawang Tubig, an afflux of about 1 m. has been provided keeping in view location of the dam and also to avoid excessive pumping during construction. According to the cross sections of the river, the deepest river bed in the portion of the under sluices is El.113.00 m. In the ogee portion, the average level is at El.114.0. Accordingly, the crests of under sluices has been kept at El. 114.00 and for ogee at El. 116.00, allowing a free board of 0.2 m. over the Pond level of 115.80, adopted from consideration of irrigating the service area to be commanded from the proposed dam.
15.3 HYDRAULIC DESIGN 15.3.1
DESIGN DATA
1. Index map showing the river bank 2 km upstream and 1 km downstream. 2. Location of the dam sites shown on the map with river profile and cross sections of the riVer upstream and downstream. (Refer to Fig. 15·1 - 15.5). ·
3. Photographs of the dam site including aerial photo contact prints. 4. Design flood - 5320 cumecs (100.year frequency). 5. Stage discharge rating curve at the site (Fig. 15.6.)
6. Lacey's silt factor f 7. Safe exit gradient
~
~
4 1/5
8. Retrogression of bed gressed)
Fig. 15.6 -
showing tail water rating curve (Retrogressed &
Unretro-
15.3.2 Determination of water way, afflux elevation, downstream basin elevation and downstream floor length under different flow conditions of spillway and sluiceway will be made as detailed out here under: A.
WATERWAY
The total water way lor the dam is decided by trial and error, and the water way for sluiceway as per guidelines under para. 7.4.5. In the present case, based on the actual topography at site, a total length of 440.00 m. has been adopted in order to have suitable alignment of guidebanks. 1. Looseness factor
As per Lacey's formula p
where
P Q
4.83 =
Looseness factor
JQ
minimum st3.blc waterway in. meters maximum flood discharge
p ~ 4.83 J5320
352.29 m
440.00 335.00
1.2
5320 cu. m/sec. Thus,
actual waterway Lacey's waterway
say 335.0 m.
L·I···
207
MAG-ASAWANG
TUB~G
RIVER
IRRIGATION
PROJECT
0
v 0
,'"-
~\
\ \
\
\
DAM
::r:
(!)
NOT CRAWN TO
LOCATION
PLAN FIG. 15.1
SCALE
EL.I25.00
I~
"
l:i: .......
I' I~
lr
z EL.I20 .00
t--
"
1-
"t;}
"'
'" "
0
!"--.
U1
li<; I~
... 10
"'-
-.......
0
I;~; I~
!'--
u
EL.II5 .00
10 I"'
·b.
I
I
"' U1
"- 1'
lo
I(J EL .110.00
otooo
0+200
o+soo
otsoo
o+400
1+000
RIVER BED PROFILE OF RIVER "A' SCALE :
H • I : 8000 MTS. V = 1: 200 MTS.
FIG.
15.2
LJ3 0
8
0
......
2!l
0
!"---
.
·-
~~
..
"'
1'- f-.,
I;';
2
:- 1---
1\\
1-
"'U1
" ~
1\
;; I~
~
£ u
EL .!Ia.
,,....
' r-
... ·Jf
1\
:It
.._ I " I--.
<
c<4
'1li -i
;;,
c- - ·-
c
'\
0 1\
L
u
L
0+000
ot200
0+400
0-+800
1 +ooo
1+200
RIVER BED PROFILE OF RIVER "B" SCALE:
"'
H= I : 8000 MTS. V= I: 200 MTS.
FIG. 15.3
'
EL. 12 5.00
In
~
+
t-......... EL. 120.00
"'
1'-.
...
" )'-., 1'-
!
10
b
...
~
0
.. + XI{)
r--....
!
EL.II!I.OO
- ---
0 0
1-'
~
1\
~0
\
t'--
v
r---
i:"
'\. f'. 1\ !-"
EL. 110,00
' ' !
.I
'
EL 105.00 0+000
0+200
0+400
RIVER BED PROFILE SCALE
0+600
0+800
1+000
OF !BOLO RIVER :aooo
H •
I
V=
1:200
MTS. MTS.
FIG. 15.4
il
t 22
L L &bGWG-MJJAlJZ.'
i
!&% ; &
lt200
EL. I 2!5.00
UNO R Ul ES 10 H: 40. 0
I:!I
WID
H
EE
0
'"
EL. 120.00
e- I'-
" "'
\ ~
\
I
/C fE TL OlE
~ U/!
:i, u
AP ON
EL
V.
a
RE T
""
~~
L
Sl
U/
A RO
S Ul
E
"
Ot400
--
.. -·
1--
--- --
--
~
z
"' ~-
-
!'\
f'\
r-'
E
FC F
OtSOO
! •
I
I Ot200
II~()(
DA
0
~
ELEVi OF
AY'
EL.IIO.OO OtOOO
-~.
t-i
/
E L.ll!5.00
0
[-.
a: '--" ·-
-
«
0
I
I
Ot800
I +000
I t200
I +400
CROSS- SECTION@ DAM AXIS(FACING UPSTREAM) STATION: I t !500 SCALE: H = I: 8,000 mrs V I: 2 00 mts
=
!
FIG. 15.5
!
LOW STAGE - 100 CMS- 1.20 M HIGH STAGE - ~320 CMS .30M
EL.I20.00 EL.II9.00 EL. 118.00 EL.II7.00 EL.II6.00 EL.I15.00 EL. 114.00 E L.ll3.00 EL.II2.00 EL.III.OO
RETROGRESSION RETROGRESSION
LiU~R T 'OE RESS D Tt IL~ A E
R TF OE RES SED T IL NA E R TI~G Cl~VI
1000
2000
!RAT NE
3000
4000
CURV
5000
6000
7000
DISCHARGE (CMS) SCALE
TAILWATER
1:200
MTS.
RATING
CURVE
FfG.
15. G
This falls with in the range specified under para. 7.4.2. 2. Width of undersluice & agee portion For the area to be commanded from two canals, one on each side of the river, two sluiceways are provided to create suitable pond. Intake works for the left and right side canal for discharges 7.959 cumecs, and 40.998 cumecs respectively shall also be provided. Assuming water way for sluiceway as follows:
a) Right portion 33.00 m.
6 bays of 5.50 m. each
7.00 m.
5 piers of 1.40 m. each
40.00 m. b) Left portion 11.00 m.
2 bays of 5.50 m. each 1•
1.40
1 pier of 1.40 m. w~,
12.40 m.
W net ~ 33.00 + 11.00 ~ 44.00 m.
W gross ~ 40.00 + 12.40 ~ 52.40 m.
Effective length of sluice way due to piers:
For the calculation of discharge in the formula, Q ~
C L H e 3t2
L is the effective length taking into account end contractions for the piers and abutments as per para 7.3.7. I
where
and
L
L
~
L"- 2 (N kp
~
effective length
+ ka) H,
L'
net length of the crest
N
number of piers
kp
pier contraction coefficient
ka
abutment contraction coefficient
H,
total head on crest
I}~ 44.00 m. N
kp
6
.V1 (refer to page 7-23 for contraction coefficients) 119.20 - 114.00
5.20 m.
(Where U/S EL.
119.20 & afflux. El.
~
based from the succeeding computations)
213
118.92
L'
~
44 - 2 [ 6 (.01)
~
43.38 m.
8
3. Length - agee portion
Width o[ two training walls of 1 m. each ~ 2.00 m. Length a[ ogee ~ 440 - 52.40 - 2.00 ~- 385.60 m. (Abutment contraction coefficient has been ignored, therefore take the net length of the ogee as the effective length which is equal to 385.60 m) B.
DETERMINATION OF AFFLUX ELEVATION, DOWNSTREAM BASIN ELEVATION AND DOWNSTREAM FLOOR LENGTH
The following abbreviations have been adopted working out the discharges over spillway and sluiceway. h,
for various parameters in the calculations for
head due to velocity of approach energy elev. - afflux elev.
He
head producing flow energy elcv.- crest elev.
da
depth of approach afflux elev.- u/s apron elev.
h,
afflux elev. - crest elev.
h5
tail water elcv. - crest clev. discharge per meter run
q
va
5320 440 ~ 12.09
velocity of approach
qid,
I.
First Condition: High flood condition, spillway not silted up
a)
Spillway Portion In Sketch 1 Try Energy El.
~
119.22 m.
P = crest el. - u/s apron el.
P/he
116.00 - 114.00
2.00 m.
119.22 - 116.00
3.22 m.
2.00/3.22
~
0.62
From Fig. 7 .1; C0 ~ 3.835 (discharge coefficient for free flow condition) hd
U/S Energy level - tail water level 119.22 - 117.63
~
1.59
214
.,
i&
d ~ tailwater level - dis retrogressed bed level 117.63 - 112.80
a)
~
4.83 m.
SPILLWAY PORTION UiS Energy IEL . :_:,~:::.....:1:.:1::,:9•::.22:_-:---- - - - - - - - - - - - - - - - - - - - - - 1 Afflux
EL.I~
..\ ·()
118.92
p~'--~~~~~~--~--DiS T .E.L. ~ 11:,.:7c.:.:.9'-'4'----,---Tailwater EL. ~ 117.63 (R)
:::::.....•c.........:..---'---:-"-- -:/
. Crest EL.
UiS Apron EL.
~
~
116.00:
/
114.00 DiS Bed Level
~
112.80
SKETCH 1
b)
SLUICEWAY PORTION
UiS T.E.L. - 119.20 Afflux EL.
~
118.92
---~D~iS~T~.E~.L~~~~11~8~.0~5_ _ _ __
Tailwater EL.
EL. 114.00
-U: ,:i: ,:S: ,:2Ap::,:r::,:on~E:.:L~.-~~11::,:3::,:.0::,:0_~~
~
117.63 (R)
Crest level
~~-----~~~~~~~~~~--~ DiS Bed Level ~ 112.80
.
SKETCH 2
215
&3
SkWJ
&;g;;g;_a
2
1.59 3.22
hd + d ~ 1.59 + 4.83 ~ 6.42
.49
~ 1.99 :o--1.7
6 .4 2 3.22
the downstream floor position has little effect on the coefficient of discharge which is solely by tailwater submergence.
l
~ ~
with
~
0.49 &
1.99;
c
I
% correction ~ O.S% (from Fig. 7 .4) Where Cs is the correction for submergence or decrease in coefficient for flow over submerged weir.
C,
~
(1.00 - 0.005) C0
~
0.995
X
3.835
~
3.81
In this, the modification in the value of the coefficient can be read directly from Fig. 7.6.
0.975
When
i.e. C,
3.74
q
Adopting value of C, q ~
~
3.74
3.74 (3.22)312 1.811 11.93 cms./m.
Try afflux El.
~
118.92 m.
119.22 - 118.92
0.30 m.
118.92 - 114.00
4.92 m.·
v A ~ ~ ~ 2.42 m.lsec. hA ~ va 2 I 2g ~ (2.42J2 I 2 (9.80) ~ .299 Adopt UIS Total Energy level Q ogee ~
11.93 (385.60)
~
~
119.22 m. & afflux El.
4600.20 ems.
I. First Condition
High flood with dam not yet silted up b)
Sluiceway portion Determination of UIS Energy Elevation Refer to Sketch 2
216
118.92 m.
With afflux El. = 118.92 m. (from the spillway computation) h, = 117.63- 114.00 = 3.63 (where 117.63 is the retrogressed level, assuming a retrogression of 0.3 m.) h, = 118.92 - 114.00 = 4.92 m . .!!s = 3 ·63 = 0 738 h, 4.92 . Refer to Fig. 7.7
C, I C 0.70 .10 [
J
.038
.8x56J
0.738 0.80
y
J
.078
.778
.078
.10 .038
y y
= .30
X
= .856 - .30 = .826 =
c, I c
C, = .826 (C) = .826 (1.706) = 1.409 Try Energy Elevation = 119.39 m. ha
=
119.39 - 118.92
=
.47 m.
H, = 119.39 - 114.00 = 5.39 m. da = 118.92 - 113.00 = 5.92 m. q = 1.409 (5.39)312 = 17.63 cmslm ~
17.63 (43.38) . = 2 47 5.92 (52.40) / . ha = (2.47)2 119.60 = .31 < .47 m. not O.K., since Assumed ha should be equal to computed h•. Try Energy Elevation
=
119.20 m.
ha = 119.20 - 118.92 = .28m. He = 119.20 - 114.00 = 5.20 m. da = 118.92 - 113.00 = 5.92 m. q = 1.409 (5.20)312 = 16.71 cms./m. 16.71 (43.38) . ( .40) 5 92 52
=
2.34 mlsec.
ha = (2.34)2 119.60 = .279 "'- .28 O.K.
217
Adopt U/S Energy Elev. ~ 119.20 m. & Afflux El. ~
Q sluice
16.71 (43.38)
Q ogee
11.93 (385.60)
Q TOTAL
5325.08 >
C.
118.92 m. for sluiceway.
724.88 ems. ~
4600.20 ems.
5320 ems O.K.
LENGTH AND LEVEL OF BASIN FLOOR
a) Spillway portion
From the previous computation;
q ~ 11.93 cms./m. U/S Energy Elevation ~ 119.22 m Afflux Elevation ~ 118.92 m. Tailwater Elevation ~ 117.63 m. (Retrogressed) 1. Determination of downstream energy elevation
Depth of water ~ 117.63 - 112.80 ~ 4.83 m. (112.80 being the bed level) 11.93 4.83
~
2.47 m/sec.
(2.47)2 119.60 ~ .31 m.
hots
Downstream energy elevation
tailwater elev. + hots 117.63 + .31 117.94 m.
I 'j
2. Determination of basin length:
!
HL ~ difference in energy levels at upstream and downstream ends of jumps ~
119.22- 117.94
~
1.28 m.
Accounting for 20% concentration (as per para. 7.5.9) q,
~
1.20 (11.93)
~
q,
Discharge per meter length with 20% concentration
D,
critical depth for flow considered
j
(14.32)2 9.80
2.76 m. 1.28/2.76
.46
218
,) Jf
4U&Z
I J i I
14.32 cms/m.
where
3
r
!JMi
hkiiMkli&ilkLJ&ki£& SMJ£1 I .( £,;;;;;;;QJ£ Jil£2 . ' j j &
Refer to Fig. 715 for values of D2 I D 1 & D1 I De corresponding to computed values of HL I De where D ~ depth at upstream end of jump 1 D ~ depth at downstream end of jump 2 By means of interpolation, exact value~ corresponding to computed HL I D,. which cannot be found from the values listed in Fig. 7.15 can be determined, as the case shown hereunder.
1.40 .46
.10
D 1 1 D,
Dzl D1
HL I DC
.541]
J
3.09] .06 X1 3.35
L.5o
'J
X2 .26 .516
"]
.025
for Dz I D1 .
:1
4$ ;
.025
.10 .06
.26 Y1
.10 .06
Yz
'1
Y1
.06 (.26) 0.10
~
Yz
(.06) (.25) .10
~
.015
.16 X
1
Dzl D1
3.09 + .16
.541 - .015
D1 I D,
xz
.526
3.25 forHLIDc
~
.46;
;
D2 1D 1 ~ 3.25 & D 1 I D, ~ .526
D I D, ~ .526; D 1 ~ .526 (D,l ~ .526 (2.76) ~ 1.45 m. 1
Dzl D1 ~ 3.25; D2 ~ 3.75 (D 1) ~ 3.25 (1.45) ~ 4.71 m. V
1
V
2
(velocity of flow at the beginning of the jump) q, I D1 ~ 14.32 11.45 ~ 9.88 mlsec. (velocity of flow at the end of jump) q, I D2 ~ 14.32 14.71 ~ 3.04 mlsec. Froude's No. (F)
F ~ 2.62 <
~ 9.881
I
2.62
J-9-.8-(_1._4-5)
4.5 Design of Basin Type IV has been adopted as per para 7.5.4.1
Refer to Fig. 7.14
219
•
~
for F Lb
~
~
2.62 ; Lb/D2
4.84
length of basin
4.84 (D 2 l 4.84 (4.71) 22.80
3. Basin floor elevation Asper para. 7.5.4.3, item i, the elevation of the basin floor may be calculated as follows; a)
Basin level where hvl
U/S Total energy line - D 1 - hvl head due to velocity of flow at the beginning of the jump vl2 /2g
Basin level
119.22 - 1.45 - (9.88)2 I 2 (9.80) 112.79 m. ~
b) Basin level where hv 2
DiS Total energy line - Dz - Hvz head due to velocity of flow at the end of the jump
Basin level
117.94 - 4.71 - (3.04)2 /2 (9.80) 112.76 m.
B) Sluiceway Portion:
Determination of Length and Level of basin floor q = 16.71 cms/m. U/S Energy Elev. = 119.20 m. ' Afflux Elcv. = 118.92 m. Tailwater El.
=
117.63 m. (Retrogressed)
1. Computing for energy elevation at downstream; depth of water ~ 117.63 - 112.80 ~ 4.83 m. V D•S ~ 16.71 113.381 ~ 2.86 m/sec., where .
4.83 (52.40)
52.40 m.; is the gross width h 01 s ~ (2.86)2 /19.60 ~ .42 m. ~
DiS Energy Elevation
117.63 + .42
2. Determination of Basin Length
HL
~
119.20 - 118.05
~
1.15 m.
With 20% concentration;
q,
1.20 (16.71)
D ~ 3) c Hr, .' D, ~
120.051' _ 9.80 -
~
20.05 cms/m.
3· 4 D, m.
1.15/3.45 ~ .33
Referring to Fig. 7.15
220
118.05 m.
By interpolation: HL I D,
c-30] .10 0.33 .03 0.40
D2 I D 1
D1 ID,
2.81] ~ Y1 0.28 x 1 3.09
0.572] 2 x 2 · Y .031 0.541
for D1 I D,
for Dz I D1:
Y!
.031 Yz
.10 .03
.28 Y!
.10 .03
J
.03 (.28) .10
Yz
.03 (.031) .10 .009
2.81 + .08 D I D, ~ .563; 1 D I D ~ 2.89; 1 2 v ~ q, I D 1 ~ 1 v 2 ~ q, I D 2 ~
~
2.89 & x 2
~
D1
D,
~
.572 - .009
~
D 1 ~ .563 (D,l ~ .563 (3.45) ~ 1.94 m. D2 ~ 2.89 (D 1) ~ 2.89 (1.94) ~ 5.61 m. 20.05 I 1.94 ~ 10.34 mlsec. 20.05 I 5.61 ~ 3.57 mlsec.
Froude's No. (F)~ V 1
jiT5i
~ 10.34 f9.80 (1.94) ~ 2.37
From Fig. 7.14 For F ~ 2.36; LB I D2 ~ 4.6 L 8 ~ 3.50 D2 ~ 4.60 (5.61) ~ 25.81
3. Basin Floor Elevation; a) Basin Level ~ UIS EE - D 1 - hvl 119.20 - 1.94 - (10.34P I 19.60 111.81 m. b) Basin Level ~ DIS EE - Dz - hvz 111.79 m.
221
.563
Afflux El.
~
119.48
rD/S T.E.L. (
117.92 _ _
Tailwater El. ~ 117.63
116.00
rCrest El.
rU/S Apron El.
~
114.00 DiS Bed level ~ 112.80 (assumed)
Sketch 3
11. Second Condition: High Flood with dam silted a) Spillway Pprtion (s•;e Sketch 3) In this condition wherein the upstream of the dam has been silted up to crest, the basic coefficient of discharge should be taken as for broad-crested weir which is 1.706 in metric units. However, the effect of submergence should be calculated as per values listed under Fig. 7.7. Assume afflux El.
~
119.48 m.
Try U/S Energy Elevation 119.48 - 116.00
3.48 m. ·
117.63- 116.00
1.63 m.
119.60 - 116.00
3.60 m.
119.60 - 119.48
.12m.
1.63 3.48
.17
222
§iW
£ =.Jdii@)@lJ
r
~
119.60 m.
.. ~HL
UiS T.E.L.
119.84
Afflux EL.
119.48 DiS T.E.L
~
.::.1..:::18::.:..2::9:___ _ _ _ _ _ __
Tailwater EL. ~ 117.63 (Retrogressed)
.. EL. 114.00
UiS Apron EL. ~ 113.00
DiS Bed Level ~ 112.80 (Retrogressed)
b) Sluiceway Portion - Second Condition - High Flood with dam silted up to Spillway Crest
Sketch 4
I II
Refer to Fig. 7.7 for values of C,IC By interpolation:
l
I
C, I C
h, I h, .10
.40 .470
[
J
.937
II
.10 .07
~
.019 y
y
~
.013
X
~
.956 - .013
.47; C, I C
c,
yJ .019
X
.50
for h 5 I h,_
J
.956 .07
~
q ~
.943
~
c
c, (H)
~
.943
.943 ~
~
.943 (1.706)
1.609
312
1.609 (3.60fl12 10.99 cmslm. da
~
119.48 - 114.00 10.99 .4 5 8
~
~
5.48 m.
2 .01 m Isec. .
ha ~ (2.01J2 I 19.60 ~ .21
Try UIS Energy Elevations
~
> .12 m.
not O.K.
119.70 m.
ha
~
119.70 - 119.48
~
.22m.
He
~
119.70 - 116.00
~
3.70 m.
q ~ 1.609 (3.70f312 ~ 11.45 cms./m. 11.45 .4 5 8
~
2.089 mlsec.
ha ~ (2.089)2 I 19.60 ~ 0.223 "" 0.22 Adapt UIS Energy Elevation
~
119.70 m.
& Afflux Elevation
~
119.48 m.
O.K.
Q ogee ~ 11.45 (385.60) ~ 4415.12 ems.
224
b) Sluiceway Portion (See Sketch 4)
Length and Level of Downstream Floor 1. Computing Energy Elevation
Adopt afflux El. ~ 119.48, as worked out for spillway portion in silted up condition h, ~ 117.63 - 114.00
3.63 m.
h, ~ 119.48 - 114.00
5.48 m.
h, I h,
~
3.6315..48 ~ 0.660
Refer to Figure 7.7 for values of C, I C computed h, I h, based on formula Q ~ 1.706 L H3i2 are By interpolation:
C, I C
h, I h,
0.907
[0.60 0.660
X
0.856
0.70 By ratio & proportion: .10 .06
X ~
If h, I h,
~
.907 -
C, where C
c,
I
031
~
,876
.660; the interpolated value of
C, I c
I
y
,031
~
y
.052
~
~
~
.876
.876 (CI; 1.706 (coefficient of Discharge for free flow condition)
~
.876 (1.706)
~
1.49
After performing various trials; Try Energy Elevation
~
119.84 m,
hn
~
119.84 - 119.48
~
0.36 m.
H,
~
119.84 - 114.00
~
5.84 m.
~
6.48 m.
•
c, H312 1.49 (5.84)3/2 21.02 cms./m. da
119.48 - 113.00
225
q x W net of sluiceway da x W gross of sluiceway
va
2 1. 02 (43 ·38 ) ~ 2.685 mlsec. 6.48 (52.40) ha ~ v0 2 I 2g ~ (2.685)2 I 2 (9.80) 0.368 "' 0.36 m.
O.K.
Adopt Energy Elevation ~ 119.84 m. & Afflux Elevation ~ 119.48 m .. Q sluice ~ 21.02 (43.38) ~ 911.84 ems. ~
Q ogee
4415.12 ems 5326 ems
Q Total ~ 4415.12 + 911.84
>
5320
0 .K.
2. Basin Length
From the previous computations: q
~
21.02 cms.lm.
UIS Energy Elevation ~ 119.84
Afflux Elevation
~
119.48
Retrogressed Tailwater Elevation ~ 117.63 (assuming 0.30 m. retrogression at high stage flow) Computing for Energy Elevation at Downstream:-
depth of water ~ retrogressed tailwater El.Assumed retrogressed bed level ~
'i
~
117.63 - 112.80
Velocity at downstream
~
4.83 m.
q x W net of sluiceway da x W gross of sluiceway
21.02 (43.38) 4.83 (52.40) 3.60 mlsec.
l
Velocity head, ho 1s
(Vo/sl2 I 2g (3.60) 2 I 2 (9.80)
hors
.66 m.
DIS Energy Elevation
Tailwater Elevation + horo 117.63 + 0.66 118.29 m.
226
HL
difference in energy levels at upstream and downstream ends of jump 119.84 - 118.29
Accounting for 20°/o concentration;
q,
~
~
1.20 (21.021
critical depth D,
25.22 cmslm.
~
4.02 m.
3
HL I D, ~ 1.55 I 4.02 ~ .386 Refer to Fig. 7.15 for values of D2 I D 1 & D 1 I D, corresponding to computed value of HL I d, By interpolation: ).
,,'
HL I D,
l ,;
D2 1 D1
D1 I D,
2.81
0.572
[ 0.30 .10 [086
0.386
J Y! 128
Xj
0.541
' 3.09
0.40
By ratio & Proportions:
..J.Q_ .086
.28
Y!
.24
Xj
Y!
2.81 + .24
..JQ..
.031
.086
Y2
~
Yz
.027
xz
.572 - .027
3.05 ~ D2 I D1
~
.545
~
xz
D 1 I D,
227
J Yz
]""
D1 D2
depth at upstream end of jump depth at downstream end of jump
D,
~
critical depth for flow considered
D1 I D,
~
.545; D1
.545 ID,l
~
.545 (4.02)
~
2.19 m .
3.05; D2
3.05 ID 1)
~
3.05 12.19)
~
6.68 m.
'lc I Dl
25.22 /2.19
q, I D2
25.22 I 6.68
Dz/ Dl
VI Vz
~
Froude's No. (F) ~ V1
JgD 1
From Fig. 7 .14; for F ~ 2.486 L 8 I D2 where L 8 Ln
~
~
length of basin
~
4.80 IDzl
~
~
11.52 m/sec. 3.78 m/sec.
~ 11.52/J 9.8 (2.19) ~ 2.486
4.8
4.80 (6.68)
~
32.06 m.
3. Basin Level
Basin Level
U/S Energy El. - D1 - hvJ h . ; w ere vl2/2g 119.84 - 2.19 - (11.52? /19.60 110.88 m.
Basin Level
Did Energy El. - hv2 - vl/2g 118.29 - 6.68 - (3.78J2 /19,60 110.88 m.
Adopt basin floor level
~
110.88 m,
a) Spillway portion
I
Length and Level of Downstream floor 1. Computing for energy elevation at downstream:
From the previous computation;
q
~
11.45 cms/m.
U/S Energy Elevation
119.70 m.
afflux Elevation
119.48 m,
Tailwater Elev.ation
117.63 m. (Retrogressed)
depth of water
117.63 - 112.80
~
228
4.83 m.
11.45 4.83
~
2 .37 m Isec.
H 01s ~ (2.37)2 /19.60 ~ 0.29
m. 117.63 + .29
Downstream Energy Elevation
117.29 m. HL ~ 119.70 - 117.92 ~ 1.78 m.
2. Basin Length
Accounting for 20% concentration; q, ~ 1.20 (11.45) ~ 13.74 cms/m. 3
j (13.7 4)2 9.80
~ 2.68
m.
H1, I D, ~ 1.78 I 2.68 ~ .664 Refer to Fig. 7.15
By interpolation: HL I D, [ .10
.60] .644
D 1 / D,
D 2 / D1 3.60]
.064
J
.22
Y!
Xj
xz .477
3.82
.70
.494] Yz
J
.017
For D2 I D 1 ;
y1
Y!
~
y2
.14 3:60 + .14 D2/D 1
:1::. '
.10 .064
.22
.10 .064
~
~
.010
x2 ~ 0.494 - .010 ~ .484
3.74
x2 ~ D1 I D, ~ 0.484
3.74 .494 D, ~ .484 (2.68) ~ 1.30 m.
D1 I D,
.484 ; D1
Dz I D1
3.74; D2 ~ 3.74 D 1 ~ 3.74 (1.30) ~ 4.86 m.
v1
~
q,!D 1
~
13.74 /1.30
~
10.57 m/sec.
V2
~
q,/D 2
~
13.7 4 I 4.86
~
2.83 m/sec.
Froude's No. (F) ~ v 1 / JgD1
~
~ 10.57 I )9.8 (1.30) ~ 2.96
229
II
Refer to Fig. 7.14 For F
~
2.96; Ln/D2
~
5.1
Ln ~ Length of Basin ~ 5.1 (4.86) 3.
I
24.79 m.
Basin Level
Basin Level
~
U iS EE - D1 - hvl 119.70 - 1.30 - (10.57)2 i19.60 112.70 m.
Basin Level
DiS EE - Dz - hvz 117.92-4.86- (2.83)2 i19.60 112.65 m.
Adopt Basin floor level
~
112.65 m.
Ill) Third Condition-Medium Flood Condition {See Sketch 5) Water Level on Upstream is just 0.30 m. {one foot} above the ogee crest
I
The 0 gee Crest
I
a) Spillway Portion
Gwux
El.
~
i!
116.30
Urest El.
~
116.00 Tail water El.
i
=
115.00
I
114.00
l
I
DiS bed level
l
l
112.80 (assum!f:
Sketch 5
+
1.0 Determination of UiS Energy Elevation Afflux El.
~
Ii
116 + 0.30 {Crest level + 0.30 m.)
230
116.30
d,
116.30 - 114.00
Try Energy Elevation
~
~
2.30 m.
116.3009 0.0009
116.3009 - 116.30 p
116.00 - 114.00 ~
~
2.00
116.3009 - 116.00
~
He
actual head
.3009
Ho
design head (head during high flood condition, where US!EE
119.22 m.)
3.22 m.
And from Fig. 7.1, discharge coefficient for free flow condition, C0 ~ 3.835 Since the crest has been shaped for a head higher than the head consideration, as per para. 7.3.7.2, item b, and Fig. 7.2, variation of the coefficientas related to values of Hc/H 0 are shown.
-{:-~ 0
c
-c-~ 0
q
d,
v,
.3009 3.22 .82;
c
~
~
3.14 1.811
~
.82 (3.835)
.82 C0 (.3009/312
116.30 . 114.00
~
(0.13)2 19.60
~ .00086
.29 (385.60)
~
3.14
2.30 m .
0.13 m/sec.
~
~
.29 cms./m.
.29 2.30
Adopt U/S Energy El. Q ogee
.093; Refer to Fig. 7.2
;::J
.0009
O.K
116.3009 & Afflux El.
116.30
111.82 ems.
b) Sluiceway Portion
Afflux Elevation
116.30 (since water is just 0.30 m. above the crest level)
(See Sketch on the next page)
231
i
! 116.30 116.00
Crest El.
I
Ots T.E.L. ~
Tail water El.
l l
__
115.42 ~ ·n5.20
El. 114.00'-'--'-----,_
1f]_E~L~11~3~.o~o
__________
~
El. 112.80
Sketch 6 1.0
Determine U/S Energy Elevation
Since tailwater elevation is not yet known, assuming that it is free-flow condition and use the formula, Q
I I
Ii
q
1.706 LH 312 or 1.706 H 312
After various trials;
Try Energy El. ha
~
116.44 m. 0.14
116.44 - 116.30
2.44 m. 116.44 - 114.00 He 312 q ~ 1.706 (2.44) ~ 6.50 cms./m. da
116.30 - 113.00 6.50 (43.38) 3.30 (52.40)
~
~
3.30 m.
1.63 m/sec.
ha ~ (1.63)2 /19.60 ~ .136 ':":! .14 Q sluice Q ogee
~ ~
6.50 (43.38)
~
281.97 ems.
111.82 ems.
Q Total ~ 281.97 + 111.82
393.79 ems.
1.1. Determination of Tailwater Elevation
From the tailwater Rating Curve, for Q ~ 393.79 ems; corresponding Unretrogressed Tailwater El. ~ 115.95 m, & Retrogressed Tailwater El. ~ 115.20 m. These values can be verified by referring to the Tabulation of Discharges at various elevations, through interpolation.
232
+&&
Discharge (ems)
Elevation (m)
.50
["'~"J
[ 198.201 215.29 393.79
195.59
y
413.49
116.00 .50 -y
~
215.29 195.59
y
.50 (195.59) 215.29
=
.45
El. x = 115.50 + .45 = 115.95 m. Unretrogressed tailwater El. = 115.95; & Retrogressed Tail water El. = 115.20 m. As the tailwater elevation is higher than the crest elevation of the sluiceway, therefore effect of submergence should be accounted for: With afflux Elev. = 116.30 m. & Retrogressed Tailwater El.
=
115.20
h, = 115.20 - 114.00 = 1.20 m.
h, ~ 116.30 - 114.00
=
2.30 m.
h,lh, = 1.20 12.30 = .52 Refer to Fig. 7.7
C, I C
"l:::1"' 0.60
.J2Q_
.10
Ji2 X
y
=
;y
=
.006
.02 (.03) .10
= .937 - .006 = .931
for h, I h, = .52 ; C, I C = .931
•
C, = .931 (C) = .931 (1.706) = 1.59
Try UIS Energy El.
=
116.42 m.
233
I .. ···-- :····:::·.. .....:..::
......
-~-..-...._
__
.._ _________ ___ ........_ .................
.,....
~
Tabulation of Discharges at Different Elevations
Section 0 + 550
s
"'""
0.011
n
~
0.04
AREA
ACCUMULATED AREA
PERIMETER
ACCUMULATED PERIMETER
113.50
4.0
4.0
10
10
0.40
2.622
1.423
114.00
16.0
20.0
8
18
1.111
2.622
2.813
56.26
ELEVATION
~
~
R
~AlP
K (ls 1h) n
V (KR213)
Q
5.692
36
1.00
2.622
2.622
94.39
0.765
2.622
2.193
114.04
74
. 142
0.685
2.622
2.037
198.20
(i4
196
0.878
2.622
2.404
413.49
330
526
0.639
2.622
1.945
653.52
712.0
118
714
0.997
2.622
2.617
1863.30
412
1124.0
228
942
1.193
2.622
2.949
3314.68
118.00
492
1676.0
104
1046
1.545
2.622
3.504
5662.46
118.50
468
3084.0
22
1068
1.951
2.622
4.094
8531.90
114.50
16.0
36.0
18
115.00
16.0
52.0
32
115.50
45.30
97.30
116.00
74.70
172.0
116.50
164.0
336.0
117.00
376
117.50
Lowest bed level
Q100
~
113.0. M.
5320 ems.
Tw ELEV. Q500 ~
~
~
117.93 (Unretrogressed)
6910 ems.
TW ELEV.
~
118.22 (Unretrogressed)
Average Lowest bed level Retrogressed bed level
~ ~
114.00 M. 112.80 M.
·•
68
ha
~
116.42 - 116.30
He
0.12 m.
116.42 - 114.00 2.42 m. 2 1.59 (2.42)31 ~ 5.99 cms/m.
q
da ~ 116.30 - 113.00 ~ 3.30 m. 5.99 (43.38) 3.30 (52.40)
1.503 m/sec.
~
ha ~ (1.503)2 /19.60 ~.115m. ~ .12 Adopt U/S Energy El. ~ 116.42 and afflux El.
Q sluice
~
5.99 (43.38)
~
116.30 m.
259.85 ems.
Q ogee
111.82 ems.
Q total
111.82 + 259.85
~
371.67 ems.
2. Determination of length and level of the basin floor.
a) Spillway Portion
116.3009 m.
1. Upstream Energy El.
afflux El.
116.30 m.
Retrogressed Tailwater El.
115.20 m.
depth of water
115.20 - 112.80
Determine Downstream Energy Elevation q ~ 0.29 cms/m, (from the previous computation) VDIS
~
0 2
2 ~ 0 ~ 0.12 m/sec.
~ (0.12)2 /19.60 ~ .0007
h01 s
Downstream Energy El.
115.20 + .0007 115.2007 say 115.20 m.
2. Determine basin length
H 1,
116.30 - 115.20 1.10 m.
With 20% concentration; q0
~
1.20 (0.29)
~
0.35 cms./m.
235
2.40 m.
De
J
3
HL I De
~
L35J2 9.80
~
1.1010.23
~
0.23 m.
4.78
from Fig. 7.15;
4.7.01
.08
.12
X!
4.80
10.01
~
D2 I D1
.08 (.002) .10 .0016
9.89 + .10
~
for HL I De
~
~
.2634 D 0
D2
~
9.99 D 1
~
~
9.99 x2
4.78; Dz I D1
D1
V1
~ ~
0.351 .06
~
~
D 1!Dc
9.99 & D1 I De ~
.2634 (.23) 9.99 (.06)
~
~
~
.265 - .0016 =
0.2634
0.06 m.
0.61 m.
5.83 mlsec.
V 2 ~ 0.351.61 ~ 0.57 mlsec.
hvi ~ (5.83)2 119.60 ~ L73 m.
hv2 ~ (0,57)2 119.60 ~ .02 m. Froude's No. (F)~ V 1 F
lfil\
~ 5.83 IJ9.80 (.06) ~ 7.60
> 4.5; Adopt Design of stilling basin 11
from Fig. 7.12 A: for F ~ 7.60; L 8 I D2 ~ 4.20 Length of basin, L 8
=
.002
Y2
.10 x1
_j
.002
.08 (.12) .10
~
X2
.263
.10 .08
.10 .08
Y!
Y2
Y!
4.78
.10
.265l
9.89]
4.20 (D21 ~ 4.20 (.61) ~ 2.56 m.
236
= .2634
;~ )~
8.28
Froude's No. (F)
~ 9.81 ~
X
0.868
2.84
From Fig. 7.14 C for F
~
2.84;
5.1 (3.06)
3.90 (D2 l 15.60 M 3. Basin floor elevation
i) Basin level
~
U/S EE - D1 - hvl 116.42 - .87 - 3.48 112.08 m.
ii) Basin level
DiD EE - Dz - hvz 115.42 - 3.07 - .28 112.07
Hence adopt basin floor El.
·;ijl.·.~.
IV
112.07 m.
Fourth Condition Low flood conditions i.e. Water flowing in the sluiceway is just 1.00 m. above the sluiceway crest
U /s TEL ~ 115.03 Afflux El ~ 115.00
Dis TEL
EL 114.00
~
114.60
Tailwater EL
EL 113.00
DiS Bed level
~
113.07 ~
112.30
Sketch 7
Since w'ter is just 1.00 m. above the sluiceway crest and the afflux El. will pass through the ogee portion, its crest level being at El. 116.00 m. Assuming various values of Energy El. and checking with ha by trial;
238
115.00 m., no discharge
3. Basin floor elevation
i) Basin level 116.3009 - 0.06 - 1.73 114.51 m.
DID EE - Dz - hvz
ii) Basin level
115.20 - 0.61 - .02 114.57 m. ~
Hence adopt basin level
114.51 m.
b) Sluiceway Portion
•
1. Upstream & downstream energy elevation
Upstream Energy EJ.· ~
Afflux Elevation
~
116.42 m.
116.30 m. ~
Retrogressed Tailwater El.
115.20 m.
Determine downstream energy elevation;
Depth of water
q
~
115.20 - 112.80
~
2.40 m.
5.99 cms/m.
~
5.99 (43.38) 2.40 (52.40)
~
2.07 m/sec.
ho1s ~ (2.07)2 /19.60 ~ 0.22 m. Downstream Energy EJ.
~
115.20 + ,0.22
b) Basin length
HL
~
116.42 - 115.42
1.00 m.
With 20% concentration; qe
~
1.20 (5.99)
a ~
7.19 cms./m.
I (7.19P
~
1.74
1.00 /1.7 4
~
0.575
v
HL/De
~
9.80
Refer to Fig. 7.14; for HL iDe ~ 0.575; D2 JD1 ~ 3.53; and D1iDe ~
~
D1
.4990 De
D2
3.53 D1
V1
7.19/0.868
Vz
7.19/3.06 ~ 2.35 m.isec.
~
.499 (1.7 4)
3.53 (0.868) ~
~
0.868 M 3.06 M
8.28 m/sec.
237
.4990
~
115.42 m.
1.0 Determine U/S Energy Elevation Try U/S Energy Elevation ~ 115.03 m. ha ~ 115.03 - 115.00 ~ .03 m.
He
~
115.03 - 114.00
~
1.03 m.
q ~ CH312 q ~ 1.706 (1.03)312 (adopting a free flow condition)
1.78 cms/m. da ~ 115.00 - 113.00 ~ 2.00 m.
1.78 143.38) 2.00 (52.40)
.74 m/sec.
ha ~ (.7 4)2 /19.60 ~ .028 "'- .03 Adopt U/S T.E.L ~ 115.03 & afflux EL. ~ 115.00 Q sluice ~ 1.78 (43.38) ~ 77.22 ems. Determine Tail water Elevation (See Tabulation of Discharges at various elevations) By interpolation Discharge (ems)
Elevation (m)
56.26l
El. 114.00] y
.50
20.96
El. X
[
[
77.22
38.13
94.39
El 114.50
By ratio and proportion
.50
38.13 20.96
y y
El. x
~
.27
114.00 + .27
~
114.27 m.
for Q ~ 77.22; Tailwater El. ~ 114.27 m (Unretrogressed) Taking into account a retrogression of 1.20 m. (as per para. 7 .3.6)
239
Retrogressed Tailwater EL ~ 114.27 - 1.20 ~ 113.07 m. Compute for the downstream energy elevation; water depth d/s ~ 113.07 - 112.80 ~ 0.27 m. 1.78 (43.38) 0.27 (52.40) hD/S ~
~
5.46 m/sec.
(5.46)2 /19.60 ~ 1.52 m.
DiS Energy Elevation ~ 113.08 + 1.52
114.60 m. Determine length and level of basin floor
2.0 Basin Length
HL
U/S EE - DIS EE 115.03 - 114.60 .43 m.
I
I
With 20% concentration; qc
~
1.20 (1.78)
~
3j (2.14J2 9.80 H L I De
~
.43 I .78
~
2.14 cms/m.
~ 0 78 .
m
.
.551
Refer to Fig. 7.15;
.50] .051 .551
.10
i
I I!
[ .60
.25
.10 .051
Yt
.051 (.25) .10 .12
.10 .051
.022 Y2
Y2
.051 1.022) .10 .011
x1 ~ D !D ~ 3.35 + .12 ~ 3.47 x 2 ~ D1!D, ~ .516 - .011 ~ .505 2 1
for HL!Dc ~ .551; D2/D 1 ~ 3.47 & D1 I D, ~ .505 D D
1
2
V1
~
.505 D, ~ .505 (0.78) ~ .39 m.
~
3.47 D 1 ~ 3.47 (,39) ~ 1.35 m.
~
2.14 I .39 ~ 5.49 m/sec.
V2 - 2.14 /1.35
~ 1.59
m/sec.
5.49
Froude's No. (F)
~
~ J9.80 (.39)
2.80
From Fig. 7.14 C; for F ~ 2.80; L 8 /D2 ~ 5.00 L8
~ 5.00
(Dz)
~ 5.00 (1.35) ~ 6.75
m.
3. Basin floor level
a) Basin level ~ UIS EE - D1 - hvl 115.03 - .39 113.10
(5.49J2 19.60
m.
b) Basin Level ~ DID EE - Dz - hvz 114.60 - 1.35 -
(1.59JZ 19.60
113.12 m. Hence adopt basin level ~ 113,10 m.
15.3.3 GOVERNING VALUES
For governing values to be adopted in the qes!gn (Refer to '!'abies 15.1 & 15.2)
15.3.3.4 Sluiceway
Based on the various con
Length of basin iil Dl iii) D2 iv) Basin floor level i)
110.88 m,
241
say 32.00 m.
Basin appurtenances
D1 & D2 are values required in determining the dimensions of basin appurtenances such as chute blocks, basin blocks, and end sill. For convenience in the calculations of such dimensions, Figure 7.12, 7.13 & 7.14 can be used.
The floor level of 110.88 if adopted, is likely to involve quite an excavation. But para. 7.5.5.1 of. the manual states "that if the raising of the floor becomes obligatory due to site conditions, the same could be raised up to 15% of D2 . The basin in that case should, however, be supplemented by chute and basin blocks. Therefore, revised basin floor level
~
110.88 + 0.15 D2 ::::. 112.00m.
15.3.3.5 Spillway;
The governing condition is the same as fdr the sluiceway.
i) ii) iii) iv)
Length of basin D1 ~ 1.30 m. D 2 ~ 4.86 m. Basin floor level
~
24.80 m.
~
112.65 m.
Due to small difference between retrogressed bed level and governing floor level, the floor can be raised up to retrogressed bed level which is equal to El. 112.80 m. Summary of the results of parameters worked out for both sluiceway and spillway are tabulated as Table 15.1 and 15.2 respectively. D.
CREST SHAPE
Coordinates of the ogee profile adopted for the spillway crest are worked out by the formula discussed in Chapter 7 para. 7.4.3.
I I I
Downstream profile (Based on the U.S. Army Corps of Engineers) Upstream profile
Y
0.724 (X + 0.270 Hd)L 85 + 0.125 Hd - 0.4315 Hct0. 375 (X + 0.270 Hd)0.625 Hdo.ss
X & Y coordinates thus computed are shown in Fig.15.7 along with the shape of the crest. E.
DEPTH OF CUTOFF
a) Spillway Portion
Depth of upstream and downstream cutoff walls from scour considerations. i) Upstream Cutoff In the formula, R
1.35 (
~
)!13 (Refer para 7.3.3) from the previous computation
242
i I
I
I
EQUATION FOR OOWSTREAM PROFILE,
X
X
1.8.5
Xl~!52.0
H4
~ 85
0 0
OF
q'ORIGIN
e
0
~.
COORDINATES ~
8
~
y
8
"'
0
"'~
a,.,
0
"'o
0
,.C\
0
"',.;
8 .;
.,0
,.;
8
"'
• !50 .20
0. 0!509
• 0 10
.2!5
0.0769
• 0 I !5
.30
0. 1078
.3!5
0. 14 34
. 021 . 029
~0
0. 1836
. 037
.45
0.2283
. 046
.50
0. 2174
.056
.60
0. 3887
.. 078
.10
0. 5169
.104
.80
0. 6618
. 133
.90
0. 8229
. 165
1.00
I. 000
.201
1.20
I. 401
.262
1.40
I. 864
4.00
1.60
2. :566
.374 .480
1.80
2. 967
.600
4.!50
2.00
3. 60!5
.124
2.50 3.00
5. 447 1. 633
I. 09!5 1.534
3.50
I 0. I !5 I
4.00
12. 996
2.163
16. 160
3 . 2!50
5.00 5.50
19. 638 23.42!5
3 . 949 4 .710
y
• .126 Hd •. 36
I I
1·50
''. I
'
2.00 'I
2.50 I I I!
3.00
'
,'!
I
'I 'I'' i 1 I'
I I
3.50 ;
''I
I I
I
'' I
I I I'
' i I.
''
I.
''''
!5. 00
2.041
4.50
FOR UPSTREAM PROFILE, X • 270 Hd •. 78
1·00
Hd • AFFLUX. EL.- CREST •118.92 -116.00 • 2.92 lol.
c
R E
s
T
I
p R 0 F I L E FIG.
15.7
I I
I
tI I
TABLE 15.1 SLUICEWAY PORTION Unit
Summary of calculations at sluiceway under different flow conditions
911.84
259.85
77.22
cms/m
16.71
21.02
5.99
1.78
cms/m
20.05
25.22
7.19
2.14
ems
2) Discharge intensity, q 3) q with 20% concentration
119.20
119.84
116.42
118.92
119.48
116.30
115.03 115.00
117.63
117.63
115.20
113.07
114.00
114.00
114.00
114.00
5.92
6.48
3.30
2.00
m/sec . m
2.34
2.685
1.503
.74
.28
.36
.12
.03
m
5.20
5.84
2.42
1.03
m m m
118.05
118.29
115.42
114.60
l
113.00
113.00
113.00
113.00
112.80
112.80
112.80
112.80
m
4.83
4.83
2.40
.27
m/sec,
2.86
3.60
2.07
5.46
I I
m m m
4) Upstream Total Energy Line 5) Afflux Elevation 6) Tailwater Elevation (Retrogressed)
m m
7) Crest Elevation 8) Depth of water upstream, da
•
10) Upstream ha 11) He = head above crest h0 12) Downstream Total Energy Line 13) U/S Apron El. 14) DiS Apron El, (assumed) 15). Depth of water downstream, d 01s 16) V018 17) hws 18) Head Loss, EL 19) D,
(4) (3) (2) Low Flood Medium High flood with Dam Flood Water water level is just 1.00 is .30m. silted up above the m. above the to crest ogee crest sluice crest level
724.88
1) Total Discharge, Q
9) Upstream Va
(1) High flood with Dam not yet silted
m
.42
.66
.22
1.52
m
1.15
1.55
1.00
.43
m
3.45
4.02
1.74
.78
.57
.551
.33
20) El< I D, 21) D2/ D1 22) D1 I D,
2.89
.368 3.05
.563
23) D1
.545
.505
.501
1.94
2.19
24) Da 25) V1 26l v 2 27) hvl
m m/sec. m/sec.
5.61
6.68
.87 3.07
1.35
10.34
11.52
8.26
5.49
3.57
3.78
2.34
1.59
m
5.45
6.77
2.48
1.54
28) hy
m
.65
.73
.38
.13
2.37
2.486
2.83
2.80
m m
3l) Basin floor level
i
3.47
3.53
m
29) froude's No. (F) 30) Length of Basin; L 8
II lI
.39
25.81
32.06
15.60
6.75
111.79
110.88
112.07
113.10
I j '
! I _\
i
I )
:!
l
'
244
-;;
'
=
ilh. L
2£
33
kiU!i144
! &.- 9:11
.£J&_Q Ri
; £&
I2
I
@}.$
TABLE 15.2 SPILLWAY PORTION Summary of calculations at spillway under different flow conditions
1) 21 3) 4) 51
Total Discharge, Q Discharge Intensity, q q with 20% concentration Upstream Total Energy Line Afflux Elevation
Unit
ems
8) Depth of Water, upstream, da 9) Upstream va
(2) (3) (4) High flood Medium Low flood with Dam Flood Water water level silted up is .30m is just 1.00 to crest above the m. above the Level ogee crest sluice crest
4600.20
4415.12
cms/m
11.93 14.32
m
119.22 118.92
11.45 13.74 119.70
• cm·s/m
6) Tailwater Elevation (Retrogressed) 7) Crest Elevation
(1) High flood with Dam not yet silted
m m
117.63
m m m/sec.
116:00 4.92 2.42
116.00
.22. 3.70
119.48 117.63 5.48 2.089
111.82 .29 .35 116.3009 116.30 115.20 116.00 2.30 .13
101 U/S ha 11) He ~ head above crest
m
.30
m
3.22
12) Downstream Total Energy Line 131 U/S Apron El. 14) DiS Apron El. (assumed)
m
117.94 114.00
117.92
115.20
m
114.00
114.00
m m
112.80 4.83
112.80 4.83 2.37
112.80 2.40 .12
151 Depth of water downstream, d 16) Downstrea, vD/S 17), hD/S 18). Head Loss, H 1, 19)/ Critical depth, De 20)! HL I DC 21) Dz/ D1
DiS
m/sec.
m m m
2.47 .31 1.28 2.76 .46 3.25
.29 1.78 2.68 .664 3.74
.0009 .3009
.007 1.10 .23 4.78 9.99
'
22) D1 I De 23) D1 241 D2 251 v1 261 V2 27) 28) 29) 30)
hvl hv 2 Froude's No. (F)
Length of Basin, L 8 31) Basin floor level
.526
.484
.2634
m
1.45
1.30
.06
m
4.71 9.88
4.86
.61
m/sec. m/sec. m m
10.57
3.04 4.98 .47
2.83 5.70 .41
5.83 .57 1.73 .02
m
2.62 22.80
2.96 24.79
m
112.76
112.65
245
7.60 2.56 114.51
q ~ 11.93 cms./m (from Condition 1 - High Flood water with dam not yet silted) f
~
4.0
Accounting for 20% concentration qc
~
1.20 (11.93)
~
14.32 cms.im.i
1.35 [ (14.432)2 ]1i3
Depth of scour, R ~
5.02 m.
·On the upstream side; allow scour
~
xR
~
~
1.1 R
1.1 (5.02)
~
5.52 m.
Required depth of cutoff (D) ~ xR - (Afflux el. - UiS apron el.)
·'- ~
~,
-~
.
p
5.52 - (118.92 - 114.00) 0.60 m. However provide a minimum of 1.50 m. deep cutoff. ii) Downstream Cutoff On the downstream side; allow scour
~
1.25R
~
1.25 (5.02)
~
6.28 m.
Required depth of cutoff (D) ~ 1.25R - (tailwater el. - dis bed level) 6.28 - (117.63- 112.80) 1.45 m. Provide 2.60 m. dis cutoff at El. 110.20 m. b) Sluiceway Portion
Depth of upstream and downstream cutoff walls from scour consideration i)
Upstream Cutoff q ~ 21.02 cms./m. (refer to previous computations)
With 20% concentration 1.20 (21.02)
~
25.22 cms.im. 1i3
R
On the upstream side allow for R Depth of scour (D)
~
7.31 m. ~
1.1 (7.31)
8.04 - (119.48 - 113.00)
Provide 2.00 m. uis cutoff
I
246
~ ~
8.04 m. 1.56 m.
iil Downstream Cutoff On the downstream allow for 1.25 R ~ 1.25 (7.31) ~ 9.14 m.
= 9.14 - (117.63 - 112.80) = 4.31 m.
Depth of scour (D)
However, the dis depth of pile is restricted to 4.00 m. for facility of excavation, being difficult in boulder reach. F.
LENGTH OF WORK & EXIT GRADIENT (Para 7.3.4) a) Spiliway Portion
Safe exit gradient ~ 1/5 (refer to Table 7.2) Maximum statio head (H) ~ 116.00 - 112.80 = 3.20 m. ~
2.60 m.
2.60 ~ 3.20
0.1625
Depth of downstream cutoff (D)
G
E
~ H d
1
X
7f
.{?:
Hence 1
"!i\ 1
7[
[A
GE X d = l. ~ ~H5 ~
0.1625 ~
0.1625 (n) ( J?l)
1
1 [ 0.1625 (n)
j""l;.-;;:;;'l 1 +
·I
o<:2
~ ~
r
2 /l
Q
3.84
- 1
[2 (3.84) - 1] 2
oC
J 44.62
«
b d b
X
- 1
6.60 (2.10)
~
~
6.60
17.16 m
44.62
Figure 5.11 of chapter 5 of Design Manual can also be used in determining any value of oc with the given value of 1 without any detailed calculations.
7fJlf Computed value of total length of floor (B) is less than the computed vlaue of the stilling basin floor; the total length of spillway is then based on other considerations: i.e., from required length of downstream apron and from stability. • ~
Adopted floor length
31.50 m.
The floor length shall be provided as below: 19.20 m.
Downstream horizontal floor
6.10 m
Width of ogee Upstream floor Total
6.20 m. 31.50 m.
b) Sluiceway portion
Total floor length and exit gradient Safe exit gradient
~
1/5
~
Depth of d/s cut-off (d)
4.00 m.
116.00 - 112.00
Maximum Static Head (H)
4.00 m.
__ 1_
4.00 4.00
1f[X'
0.20
From Fig. 5.11, Chapter 5 for
1 7f rx
~
0.20;
.X:
~
3.93;
/\ ~ 2.53
""~ ~ ; b ~ oe d ~ d ~ 3.93 (4.00) ~ 15.72 m. Stilling Basin Length
~
24.10 m.
However, to accommodate the glacis, u/s floor, total length of flow as 39.60 m. has been adopted as per details given on the next page:
248
-.\ ' &£ -.
1Jiik@
s ;;;gg;g; TI_ .:
_a~z.
_.&.. LU¥it
atJ££2
;J£..
The floor length shall be provided as below: 24.10 m.
Downstream horizontal floor
Downstream glacis length with 1:3 slopes ~ 3 (114.00 - 112.00)
6.00 m. 3.00 m.
Crest Width Upstream glacis length with 1:3 slope ~ 3 (114.00 - 113.00)
3.00 m. 3.50 m. (minimum)
Balance shall be for U /S floor Total
39.60 m. 112.80 - 4.00
Elevation of downstream cutoff G.
~
PROTECTION WORKS
As per requirements discussed under para 7.9, beyond the upstream and downstream of impervious floors, cement concrete blocks along with loose boulder or stone aprons are provided to protect the work against scour. a) Spillway Portion
Refer para7.9.2.1, while working out D. no allowance for concentration should be made in the discharge per unit length (q) in computing the normal scour R for the upstream and downstream protection works;
R
1.35 (
02
f
)113
where q
f
11.93 cms./m. (based on the governing condition for spillway portion) ~
4.0
R
4.44 m.
1) Upstream Block Protection
The length of upstream block protection should be nearly equal to D, which can be determined as uilder:
D
~
xR- (High flood level - floor level)
where
~
R
~
scour depth
~
x
~
multiplier (refer to Fig. 7.17)
xR
~
1.5 R ~ mean design scour for upstream portion (Para 7.3.3.1)
4.44 m.
.~ ~
-~
{~
.· ·.·~
:~
249
D
1.5 (4.44) - (118.92 - 114.00) 6.66 - 4.92 1.74 m.
Length of upstream block protection "'- D ~ 1.74 m. say 1.80 m. Provide blocks of 1.5 m x 1.5 m. x 0.9 m. cement concrete blocks, at least 2 blocks So block protection length ~ 2 x 1.5 ~ 3.0 meters Toe Wall ~ 0.6 m. Total length provided ~ 3.0 +. 0.6 ~ 3.60 m.
2) Launching Apron (Upstream)
As per para 7.9.4; beyond the block protection on the upstream and downstream of the dam, loose boulder or stones shall be provided as launching apron. i) Size of Stone: Velocity (v) ~
d
where
d q
,I
vuts
~
afflux el. - upstream apron el. 118.92 - 114.00 ~ 4.92 m. 11.93 cms/m (from the previous computation) 11.93/4.92 ~ 2.42 m/sec.
Referring to Fig. 7.18 Chapter 7; for v ~ 2.42 m/sec., Size of stone ~ 26 em. ~ .26 m. diameter Corresponding weight of stone ~ 25 kg. The weight of the stone can also be calculated by finding the weight of a spherical stone with the required diameter in -neter and using density of 2641.04 kgim3 for boulders.
Weight of stone Ws
.J
i.
7T
r3 x 2641.04
4 3
7T (
~) 3
3
2
X
2641.04
24.30 kg. Size of stone should not, however, be less than 30 em., and no stone should weigh less than 40 kg. Use 40 kg., 30 em 0 Loose stone protection as launching apron.
250
l
4L
.....ZkM&.g;ggg;QQMQZ£ &
ti.
WSL
Required length of launching apron (L)
4.34 - 1.80 2.54 m. say 2.60 m.
Provide 40 kg., 30 em.
Oloose stone protection in a length of 2.60 m.
3) Downstream Block Protection
R
~
4.44 m. (from previous computation)
Anticipated scour depth, xR
2.0 R (para 7 .3.3.1) 2.0 (4.441 8.88 m.
2.0 R - (Tailwater el. - retrogressed bed level)
d
8.88 - (117 .63 - 112.80) 4.05 m.
•
Length of block protection "' 1.5 D (para 7.9.2r 1.5 (4.05) 6.08 m. say 6.50 m.
Provide 6 rows of 1.5 m x 1.5 m x .90m c.c. blocks with 2 sets of 60 em. thick toe wall of concrete extending to about 60 em. below the bottom of the blocks for protecting the filter. The filter three rows should be provided with 7.50 em. gap in between, filled with gravel on 60 em. thick graded filter. Total length beyond impervious floor: Block protection
~
1.50 x 6
9.00 m.
X
2
1.20 m.
Gap between blocks
~
Toe wall
~
.60
.075 x 4 Total
0.30 m. 10.50 m. O.K.
>
6.50 m
4) Downstream Launching Apron i) Size of Stone
v
d
~
q/d tailwater el. - dis retrogressed bed level
251
ii)
'I
Apron Thickness
.90 m. equal to the thickness of the cement concrete blocks used.
Thickness of launching apron It) iii)
I
Apron Length
Required length of launching apron; assuming the slope of scour hole as 1.5 : 1 at the upstream
Possible settling of upstream stone protection due to effect of scour
If D be the depth of scour hole and n is the slope (H:V) then horizontal distance of deepest scour is nD (para 7 .9.4 .4)
Horizontal distance of deepest scour
1Ji.
(1.74)
1
2.61 m. Quantity of launched apron
j3.25 D say 1.80 D cum/m
Volume (V)
~
1.80 (1.74)
~
3.13 cum/m
Accounting for an increase of 25% for packing and losses; Vrequired
~
1.25 (3.13)
~
3.91 cum/m
Required length of launching apron (L)
Volume/m-strip thickness 3.91 cum/m .90 m .4.34 m.
The required length of block protection is 1.80 m. only. Since block protection has been provided in a length of 3.60 m., deduct the excess amount from the computed length of loose stone protection.
252
I
117.63 - 112.80 4.83 m. 11.93/4.83 ~ 2.47 m/sec.
Vct/s
Since Vu;s
r--
vd/s;
therefore. adopt same size
Of stone as
for launching apron on the upstream.
ii) Apron Thickness
Depth of toe wall ~ 1.50 m. Since block protection thickness is equal to .90 m., provide .60 m. thick filter bed underneath . Launching apron thickness (t)
.90 + .60
1.50 m. iii) Apron Length
Assuming a slope of scour hole as 2:1 for dis portion
1-----_::_1.::::...5Do:.,__-----1,
_j_ t
T
From previous computation;
D
~
4.05 m.
Horizontal distance of deepest scour ~
2 T (4.05)
8.10 m.
J5IT
Volume (V)/m-strip
say 2.25 D
With 25% F.S.;
v
2.25 (4.05)
9.11 cum/m.
V
1.25 (9.11)
11.39 cum/m
Required length (L)
11.39 cum/m 1.5 m. 7.59 m. say 8.00 m
253
'
Since actual length of non-launching apron is equal to 10.50 m., and the required length is only 6.50 m., there is an excess of 4.00 m., subtract this excess from the computed length of launching apron. _ ,_\'_!11
-~
Required length of loose stone protection ~ 8.00 - 4.00 ~
Provide 40 kg., 30 em.
•.
, +~L
-.
4.00 m.
0 loose stone protection in a length of 4.00 m. to serve as the launching apron.
__
b) Sluiceway Portion i)
Upstream Block Protection q ~ 21.02 cms/m (based from the governing condition for sluiceway portion) f
~
4.0
R ~ 1.35 [ (21~2)2 )1/3 6.48 m. Anticipated scour depth ~ 1.5 R ~ 1.5 (6.48) ~ 9.72 m. Scour depth (D) ~ 1.5 R - (afflux el. - u/s apron el.) 9.72 - (119.48 - 113.00) 3.24 m.
Length of U /S block protection
D ~ ·3.24 m. say 3.50 m.
Provide 3 - rows of 1.5 m x 1.5 x .9 m. cc. blocks with 60 em. thick toe wall extending to about 60 em. below the bottom of the blocks. Total length provided
~
5.10 m.
(1.50 x 3) + .60
2) Upstream Launching Apron i) Size of Stone q
21.02 cms/m.
d
afflux el. - u/s apron el. 119.48 - 113.00
v
~
6.48 m.
q X Wnct d X W gross
254
where ~
W net
Wgro.c;s
43.38 m. =
52.40
m.
21.02 X 43.38 6.48 X 52.40
v
2.69 m/sec.
Refer to Fig. 7.18 for v
~
2.69 m/sec.
Size of stone ~ 31 em. ~ Corresponding weight of stone
41.20 kg.
Since the corresponding weight of stone is greater than the minimum weight of 40 kg.; adopt 60 kg. stone which is the maximum allowable. -· ~
Size of stone
35 em.
0
ii) Apron Thickness Thickness of launching apron (t) ~
1.50 m.
depth of toe wall
iii) Apron Length Slope of scour hole for u/s portion
1.5:1
1.5D
POSSIBLE SETTLING OF UPSTREAM STONE PROTECTION DUE TO EFFECT OF SCOUR
1.5 D
Horizontal distance of deepest scour
1.5 (3.24) 4.86 m. Volume 'V)
J3.25 D say 1.80 D 1.80 (3.24)
~
5.8iJ cum/m
With 25% F.S.;
v
1.25 (5.83)
7.29 cum/m
255
-
------ ~----------·----~------·-------
7.29 cum/m 1.50 m.
Required length (L)
4.86 m Actual length of non-launching apron provided Excess provide ~ 1.60 m. Required length of launching apron
~
5.10 m. against 3.50 m.
4.86 - 1.60 3.26 m. say 4.00 m.
Provide 60 kg., 35 em. ¢ loose stone protection in a length of 4.00 m. 3) Downstream Block Protection
R
~
6.48
Anticipated scour depth D
~
~
2.0 R
~
2.0 (6.48)
~
12.96 m,
2.0 R - (Tailwater el. - dis retrogressed bed level) 12.96 - (117 .63 - 112.80) 8.13 m.
Length of block protection 1.5 D
1.5 (8.13) 12.20 m.
Provide 8-rows of 1.5 m x 1.5 m x .9 m cc. blocks with 7.50 em. gap, filled with gravels on 60 em. thick graded reverse filter and a toe wall of concrete extending to about 60 em. below the bottom of the blocks. Total length provided; Block protection
1.50
Gap in between
X
.075
Toe Wall
.60
X
8 X
12.00 9
0.675 0.60
1
Total
13.275 m.
4) Downstream Launching Apron i) Size of stone
q
21.02 cms./m.
d
117.63 - 112.80
v
21.02 I 4.83
~
~
4.83
3.60 m/sec.
From Fig. 7.18, Chapter 7; for v
~
3.60 m/sec
Size of stone
~
52 em. fJ
256 . J
The maximum allowable weight of stone is 60 kg. only. When larger size of stones are dictated as the case herein, additional rows of cement concrete blocks may be used. Adopt 60 kg .. 35 em. 0 loose stone protection li) Apron
Thickness .
Required thickness of launching apron (t) ~ 1.50 m. Iii) Apron Length
Slope of scour hole for dis portion
2:1
1.5 D
Horizontal distance of deepest scour ~
f (8.13) ~ 16.26
Volume (V) ~ 2.25 D ~ 2.25 (8.13) ~ 18.29 cum/m With 25% F.S.; V - 1.25 (18.29)
~
22.86 cum/m.
•
Required length (L) ~ 2 2 ·86 1.50 ~
15.24
m.
Providing 4 rows of 1.5 m-x 1.5 m x .9m cc blocks and 60 em. thick toe wall; Additional length of blocks
~
(4
x 1.50)
+ ,60
6.60 m. Actual required leng·th of loose stone
15.24
~
6.60
8.64 m say 9.00 m. Provide 60 kg., 35 em.
¢ loose stone protection in a length of 9.00 m.
257
H.
TRAINING WALL a) Hydraulic Design 1) Top Level
Max. afflux
~
119.48
Keeping the top of the training wall on the upstream at the same level. Top of wall
~
119.40
21 Determination of u/s Cut-Off
EL. 119.40
6.40
1.75 R
EL. 113.00 EL. 111.00
Assuming scour q
~
R
~
1.75 R
~
5320/440 12.09 cms/m 13 1.35 (q2 I !)113 ~ 1.35 [ (12.049) 1 ]
Q/L
7.84 (For 1.25 R, Dis negative)
1.75 R 1.75 R
~
(119.40 - 113.00) + D
D
~
7.84 - 119.40 + 113
D
~
2.00 mts
Bottom Level
~
113.0 - 2.0
Adopt
4.48 m.
1.44 mts.
!I
111.00
1
3) Determination of u/s Cut-Off
At the noses; Assuming scour depth
~
l
2 R, (foundation materials being mostly sand and gravel)
258
EL 119 40
"'
EL. 116.00
.--
2R · 3.00 m.
EL. 113.00
LJ
D
L
EL. 110.40
where:
R ~ 1.35 (q2ll)!i3 ~ 1.35 [ (12.~9)2 ] 1/3 2R
~
8.96 mts.
8.76
~
119.40 - 113.00 + D
D
~
8.96 - 119.40 + 113
~
4.48
2.36 mts. say 2.60 mts.
4) Determination of DIS Cut-Off
Keeping the top of the wall at Ei. 118.00, i.e., above the retrogressed F.S.L., i.e., 117.93 scour depth ~ 2R
,-------
EL 118 00
EL. 117.93 (Unretrogressed)
5.13 m. 2R
'm ;'~'
EL. 112.80
LJ
L
259
D EL. 108.95
where: R
~
1.35 (q2 1 f)113
~
~
2R
~
8.96 mts.
D
~
8.96 - 117.93 + 112.80
1.35
~
UseD
2 (12 ·09 1 4
[
~
]113 ~
4.48 mts.
3.83 mts.
3.85 mts.
b) Protection Works 1 l Downstream Portion
Mean Scour
~
2.0 R
where R ~ 1.35 (q2 1 £)113 2.0 R
~
12.96 mts. ~ 9A3 ftlsec. (2.875 MISec.)
(21.02) (3.28) (44) 5.83 (52.40) ~
size of stone weight
~
270 lbs
~
D
~
I I
0
15" ~
122 kg.
use 65 kgs. or 14" 2.0 R
(21.02P 113 ] ~ 6.48 mts. 4
1.35 [
!.
0
117.93 - 112.80 + D 12.96 - 117.93 + 112.80
~
I
7.83 mts.
I I
L ~ {5(D) ~ 17.51 mts;
tl
~
(1.50
1.25
X
14 ( _
12 3 281
) ~ 0.66 say 0.70 mts.
Volume~
Lt1
~
1.50 D
17.51 (0.70) ~
~
1.50 (7 .83)
Volume 1.5 D Use concrete blocks
~
12.26 ~
11.75 12.26 11.75 ~ l.0 4
1.50 x 1.50 x 0.90
in a length of 1.5 D Upstream Portion
R
~
6.48 mts.
2.0 R
~
12.96 mts. 21.02 (3.28) 119.48 - 113.0
21.02 (3.28) 6.48
260
10.64 ft./sec. (3.24 MISec.)
Stone
~
Weight
15" 0 ~
270 # s
122 kgs.
use 65 kgs. or 14" 2.0 R
119.48 - 113.0 + D
D
12.96 - 119.48 + 113
L
}3.25D ~ 11.68 mts.
1.50 X 14 ) tl ~ 1.25 ( 12 (3.28) 11.68 (0.70)
Vol.
¢
Vol 1.5
6.48
0.67 "" 0.70 mts.
8.18 8 ·18 1.5 (6.48)
~ 0.84 mts.
Use concrete blocks 1.50 x 1.50 x 0.90 in a length of 1.0 D
15.4
STRUCTURAL DESIGN PRESSURE CALCULATIONS: (Spillway Portion)
a} Spillway Portion - For determining uplift pressures, according to Khosla's theory, it is essential to assume the floor thickness at the upstream & downstream cut-off.
The assumed length and floor thickness are indicated in the Fig. given below.
B H=3.20¥.
A
\
~·=":..:·':::'2:.:.·"'=-+-t--+--'1'-
:1·~
~
f-
140 170
EL.112.1!K)
t __
W~! IOOlJOO,I00"-1"''++-,,---'1
.-:-----~-:-----i--'",;J
\ +------·~---.......---'-------------~:..::""=·•cc•_":.:.·i------+------1----l ro I 10'11 •"" ~ 2•o 1- ,..,~~
~
Sketch H
261
Referring to para. 5.7 of the Manual b
~
total length of floor
~
31.50 m.
b'
distance between two piles
d
depth of pile on which the effect is to be determined
D ~ the depth of the pile line, the influence of which has to be determined on the neighboring pile of depth d (adopted as 1.5 m.)
C ~ correction to be applied as percentage of head 19
J~
X
d + D b
H ~ head acting ~ crest level - dis basin floor level
I '
In order to determine uplift pressures acting on the floor, the % pressure at upstream and downstream sheet piles are worked out. The pressure distribution from upstream sheet pile line to downstream sheet pile line is assumed to be linear. % Pressure at key points
i
i)
Upstream Pile Line 1.50 m.
114.00 - 112.50 31.50 m. 1
d b
~
1.50 31.50
~
048 .
Refer to Fig. 5.10, chapter 5 14% 20% % of pressure at the bottom of the sheet pile ~ 100 % of pressure at the boLtom of the floor ~ 100
0E 100 - 0E 100 -
'·
-1·
~
0o
0E
100 - 14 ~ 86%
~ 100 -
20 ~ 80~o
a) Correction for thickness The thickness of the floor at the location of the sheet piles are tentatively assumed for correcting the values of 0, and (1,2 in the upstream and 0E in the downstream. If t is the floor thickness at U/S sheet pile of depth d, correction due to floor thickness ~ (0 01 - 0c 1 l which is positive.
f
1
Assume 0.30 m. floor thickness UIS; f/Jc 1 correction for thickness ~
114.00 - 113.70 1.50 1.20(-r) 262
(86-80)
b) Correction due to mutual interference of sheet piles is worked out by the following formula:
c~ ¢c 1 correction
0c 1 corrected
[f.
19
d
X
~
D
due to interference of the 2.nd pile line; D
113.70 - 111.30
2.40 m.
d
113.70 - 112.50
1.20 m.
b'
1.40 m.
b
31.50 m.
c
19
fflo
(
0
80 + 1.20 + 2.84
1.20 +2.40 31.50
2.84 (+)
84.04%
ii) Intermediate Pile Line
b
31.50 m.
b1
1.70 m.
d2
114.00 - 111.30 m. (including floor thickness)
-,,
~
base ratio
;;
1.70 31.50
horizontal distance between U/S pile and intermediate pile I total floor length
.054
b dz
31.50 2.70
11.67
The value of 0c 2 can be read directly from Fig. 5.10 (Sheet pile not at end) for given values of "'- and base ratio~ . To find 0E 2 for the known vaue of oe. and base ratio~ read for base ratio 1 - ~ for that value of oe and subtract from 100. To get 0o 2 for ~- less than 0.50, read ¢; 0 for base ratio 1 oC and subtract from 100.
)}]. b
Qc C/JE2
.05 ;
11.67 1
oC ~
72.50%
~
bl
b
.95
0c 2
100 - 0c: 0c ~ 7% 100 - 7
~
93%
~D2
100 -
¢D2
100 - 20.50
00
~ 20.50% ~
79.50%
a) Correction for thickness If tz is the floor thickness at downstream sheet pile of depth d2, the correction ~ .,1;- (9E 2 which is negative. -
263
002 )
1::2 ~ 114.00 - 112.30 ~ 1.70 m.
70 (93 - 79.50) ~ 0E2 correction for thickness ~ 1. 2.70 8.50 m. ( - ) b) Correction for interference of UiS pile line, ~
113.70 - 112.50
~
1.20 m.
d
113.70 - 111.30
~
2.40 m.
c
19
D
+ 1.20 {Wo ( 2.4031.50 .
0E 2 corrected For
1.54 (-)
93 - 8.50 - ; .54 ~ 82.96%
0c 2 . '
a) Correction for thickness
d2
1.70 m. d2
1.70 ( ) 2 .70 79.50 - 72.50
~
4.411+)
2.70 m.
b) Correction for interference of DiS pile line
Il I l
~
112.30 - 110.20
2.10 m.
d
112.30 - 111.30
1.00 m.
b'
28.10 m.
b
31.50 m.
D
j
19 ~C2 ~
,
+
2.10 ) 51 ( ) ( 1.00 31.50 ~ . +
2.10 28.10
corrected " 72.50 + 4.41, + .51
~
77.42%
Ill} Downstream pile line
Assuming 1.00 m. thickness for dis floor;
I I
d3
~
112.80 - 110.20
b
~
31.50 m.
~
2.60 m.
28.80 m.
b1
1
,ia_
oC
b
2.60 31.50
~
083 .
Refer to Fig. 5.10 0E ~ 27% ¢E3
Oo
~ 18% ~
903 264
23 2 3 .Ji
.. ;£!1i.Wk11lik j ·~I d•
L2
.&
i&l4tLLL Jiktilmu& 2 EtC 2 i.
I! 1
112.80 - 111.80 (27 - 18) 2.60
a) Correction for thickness
3.46(-) b) Correction for interference of intermediate pile line; d ~ 111.80 - 111.30 ~ 0.50 m. d ~ 111.80 - 110.20 b'
=
28.10 m.
b
~
31.50 m.
c
~ 19
j 28.10 0.50
1.60 m.
( 1.60 + 0.50 ) ~ 0.17 (-) 31.50
c) Slope Correction (Refer to para 5.73) for slope 1 in 1.5
By interpolation (from Table 5.6) 0/o
Slope 1:
'JJ
1:.2
1
/z
1:
1~/z
of correction
1 6.5
1:
1 .50
J
4.7 y
y ~
'
'}'
.50 (4.7) 2.35
X
~
11.2 - 2.35
8.85%
for slope fo 1 : 1/z
c
~
8.85%
L'
~
28.30
actual correction
OE3
corrected
8.85
X
1. 65 -- .~r-2 ( + 28.30
27 - 3.46 - .17 + .52
~
23.89%
265
Thus the pressure calculated at key points are given below;
Intermediate Pile Line
Upstream Pile Line
fiEl 100%
0m
llc1
86%
84.04%
OE2 82.96%
0D2
Downstream Pile Line
flc2
79.50%
0E3
77.42%
23.89%
0os 18%
0cs 0
Pressure Calculations
b) Sluiceway Portion
By ratio &
proportion elevation at different points on the sloping glacis are determined.
t
OF CREST
€L.U2,00
I
23.10
5
.60 m.
b
Sketch I i) Upstream pile line
dl b
1 oC
113.00 - 111.00
2.00 m. (including floor thickness)
39.60 m. dl b
2.00 39.60
.051
266
From Fig. 5.10, chapter 5 ~D ~ 14%
20%
j\E
¢m
100 - 14
~
86%
¢cl
100 - 20
~
80%
Assume U/S floor thickness ~ 1.00 m. a) Corrections for thickness ~
t:gg
(86 - 80) ~ 3.00 ( + )
b) Correction due to interference of downstream pile line D
112.00 - 108.80 ~ 3.20 m.
d
112.00 - 111.00
b'
38.00 m.
~
1.00 m.
39.60 m.
b
~
c
~ 19
j 38.00 3.20
( 1.00 + 3.20 ) ~ 0 58 ( ) + 39.60 .
Since slope correction is only applicable in cases wherein intermediate piles are provided, therefore, this correction will not be treated in here: ~Cl Corrected ~ 80 + 3.00 + .58 ~ 83.58%
ii) Downstream pile line
Assuming 1.50 m. thickness; d2
~
112.00 - 108.80
b
~
39.60 m. d
1
~ b
DC
3.20 m.
3.20 ~ 39.60- ~ ·081
From Fig. 5.10 (chapter 5, Design Manual) ~E
26%
0E2
0D
18%
¢D2
a) Correction for thickness
l.OO 3.20
(26 - 18) ~ 2.50 (-)
267
b) Correction due to interference of U/S pile line
D
~
111.00 - 111.00
~
0
No interference
0Ez corrected ~ 26 - 2.50
23.50%
Downstream Pile Line
Upstream Pile Line
¢£1
001
9cz
100% 86% 83.58%
J.
~E2
¢oz
23.50%
18%
~C2 0
FLOOR THICKNESS
1. Hydraulic gradient & Jump Profile ~;, .·.·.·.;.1
The floor shall be designed for the following two conditions 1) Uplift pressure due to static head.~ 2) Uplift due to hydraulic jump. 2. Uplift Pressure under static head
~' l···
:_~_:
:i
In the first condition, the floor should be adequate to resist uplift and soil reaction. Based on the uplift pressure worked out in the foregoing para., the level of hydraulic gradient lines at key points under different flow condition a) during high flood; b) under normal flow operation with water at the crest level, worked out at key points for both spillway & sluiceway portion. These are tabulated in Table J-1. In view of the varying requirement of concrete sections as indicated in the sketches, the thickness of floor shall be determined at a number of points A, B, C, D, E, F, G, H, E1, Ez, D1 , Dz, etc. as shown in Sketches H & I and as such hydraulic gradient line (sub·soil pressure worked out at then points for spillway & sluiceway are given in table J·2, J-3 respectively. 3. Uplift due to Hydraulic jump.
When water overflows the crests, with the formation of hydraulic jump, high unbalanced pressure are developed in the Stilling Basin Trough. In determining the pressure in the jump limited Complete Water Surface profile of the standing wave is plotted with the help of Fig. 15.8 15.9. Knowing q and Er at different locations where Er is the specific energy in the formula.
. Corresponding values of Dare read from Fig. 15.8 and thus the water profile before jump forma· tlon can be plotted. In plotting water profile after jump F (Froude Number) is determined by
268
15
EIH~RGY OF FL0\!1 CURV~S
'
I \
~
l\
I~ ol.t 0
I
2
3
4 Di"-1'1"H
5
6
7
8
9
{0)-{m)
FIG. 15.8
10
rr;2,,.,
6
•F,~ 6
~
..
~
/_
-p
2
17/ 0
~ /7
0
~
-
,~ !-
/
~ ./~ /
.....
/ / lr'l' ''" -1~u~P
NO
•,2' eJ
!.....;.,~,.~
/ t
~
10
15
20
25
30
L 01
RELATION BETWEEN lliE LENGTH
a THE HEIGHT CF JWP
FIG. 15.9
TABLE J - 1 The level of the hydraulic gradient lines at keypoints under different flow conditions. Spillway Portion
Condition
Downstream water level
Upstream water
(Datum)
level
m.
m.
(1)
(2)
High
117.63
0Dl
iflc1
0E2
ifloz
1/Jcz
iflE3
0D3
0c3
m.
100%
86%
84.04%
82.96%
79.50%
77.42%
23.89%
18%
0
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
119.48
1.85
1.85
1.59
1.56
1.53
1.47
1.43
.44
.33
119.48
119.22
119.19
119.16
119.10
119.06
118.07
117.96
3.20
2.75
2.69
2.65
2.54
2.48
.76
.58
116.00
115.55
115.49
115.45
115.34
115.28
113.56
113.38
.. 112.80
DiS Pile Line
iPEl
Flood Normal
Height/Elevation of subsoil H.G. Line above datum Intermediate Pile Line U/S Pile Line
Head
116.00
3.20
Operation
117.63
112.80
Sluiceway Portion Condition
Downstream water level
Upstream water
Head
(Datum) (1)
(2)
(31
(4)
High
117.63
119.48
1.85
Flood
Normal Operation
112.00
116.00
Height/Elevation of subsoil H.G. line above datum Upstream Pile Line Downstream Pile Line
4.00
QlEl
Q)Dl
1/)Cl
I/JE2
0D2
0cz
100% (51
86% (6)
83.58% (7)
23.50% (8)
18% (9)
0 (10)
1.85
1.59
1.54
.43
.33
119.48
119.22
119.17
118.06
117.96
4.00
3.44
3.34
.94
.72
116.00
115.44
115.34
112.94
112.72
-
117.63
112.00
TABLEJ- 2 Spillway Portion
(1)
(2) Horizontal distance from U/S end of floor
%
(5) H.G. Line Elevation during high flood ~ T.W.E. + col. 4 ~ 117.63 + (4)
(6) Height of sub.soil ~ Hyd. grad. line ~ H x col. 3/100 where H ~ 13.20 m. IN or mal op.)
(7) Hyd. grad. lim elev. during normal condi tion ~ D,S floor level t (6) ~ 112.80 + (6)
m
El
-
100
1.85
119.48
3.20
116.00
Dl
-
86
1.59
119.22
2.75
115.55
C1
.30
84.04
1.56
119.19
2.09
115.49
E2
.30 + 1.40
~
1.70
82.96
1.53
119.16
2.65
115.45
79.50
1.47
119.10
2.54
115.34
2.40
77.42
1.43
119.06
2.48
115.28
70.18
1.30
118.93
2.24
115.04
1.30
118.93
2.24
115.04
68.66
1.27
118.90
2.20
115.00
8.00
66.75
1.23
118.85
2.13
114.93
9.00
64.84
1.20
118.83
2.07
114.87
10.00
62.94
1.16
118.79
2.01
114.81
61.03
1.12
118.75
1.95
114.75
1.70
J
C2 A
1.70 + .70
~
2.40 + 3.80
~
(
B
•
(4) Height of subsoil ~ Hyd. line ~ Hxcol 3/100 where H ~ 1.85 M. (During high flood)
Point
D2
I.
(3) Pressure Percentage
c D
6.20
6.20 6.20 + .80
'
70.18 ~
7.00 + 1.00
7.00
~
~
'
E
8.00 + 1.00
F
9.00 + 1.00
G
10 + 1.00
H
11.00 + 1.30
~
12.30
58.56
1.08
118.71
1.87
114.67
HI I
12.30 + 2.00
~
14.30
54.75
1.01
118.64
1.75
114.55
14.30 + 3.00
~
17.30
49.03
.90
118.53
1.57
114.37
J
17.30 + 5.00
~
22.30
39.51
.73
118.36
:j
1.26
114.06
K
22.30 + 5.00
~
27.30
29.99
.55
118.18
.96
'I'
113.76
L
27.30 + 2.00
~
29.30
26.18
.48
118.11
.83
•
Ea
29.30 + 1.20
~
30.50
23.89
.44
ll8.07
.76
113.56
'-l
Da
30.50 + .40
~
30.90
18.
.33
117.96
.58
113.38
Ca
30.90 + .60
~
31.50
0
-
117.63
-
112.80
J
i ;;
' ,'j
j
~ ~
11.00
.
,,I
1
272
.
113.63
'1&1'1'
. ..
>~<
TABLE J - 3 Sluiceway Portion (1)
(2)
(3)
Horizontal
Pressure
distance from
Percentage
(4) Height of Subsoil
~
Hydraulic Gradient Line Elev.
Hyd.
gradient line
~
U/S end of
High flood
Normal Opera-
floors
condition
ting condition
H
~
119.48 -
H
H x col. 3/100
~
(7)
(6)
(5)
116.00 -
117.63
112.00
1.85 m.
~
~
T.W.E. + 4
~
~
117.63 + col.4 + col. 5
DIS flood el.
High Flood
Normal Opera-
condition
ting Condition
(m)
(m)
Point
(m)
El
-
100
1.85
4.00
119.48
116.00
Dl
-
86
1.59
3.44
119.22
115.44
C1
.60
83.58
1.54
3.34
119.17
115.34
A
.60 + 2.90
~
3.50
78.99
1.46
3.16
119.09
115.16
B
3.50 + 3.00
~
6.50
74.25
1.37
2.97
119.00
114.97
c
6.50 + 3.00
~
9.50
69.50
1.29
2.78
118.92
114.78
D
9.50 + 2.00 11.50 + 3.00
~
E
~
11.50 13.50
66.34 63.18
1.22 1.17
2.65 2.52
118.85 118.80
F
13.50 + 2.00
~
15.50
60.02
1.11
2.40
118.74
114.65 114.52 114.40
Fl
15.50 + 1.00
~
16.50
58.44
1.08
2.33
118.71
114.33
Fz
16.50+1.00
17.50
56.86
1.05
2.27
118.68
114.27
F3
17.50 + 1.00
~
18.50
55.28
1.02
2.21
118.65
114.21
F4 G
18.50 + 1.00
~
19.50
53.70
.99
2.14
118.62
114.14
19.50 + 1.00
~
20.50
52.11
.96
2.08
118.59
114.08
H
20.50 + 5.00
~
25.50
44.21
.81
1.77
118.44
113.77
I
25.50 +. 5.00
~
30.50
36.30
.67
1.45
118.30
113.45
J
30.50 + 5.00
~
35.50
28.40
.52
1.13
118.15
113.13
Ez
35.50 + 3.10
~
38.60
23.50
.43
.94
118.06
112.94
Dz Cz
38.60 + .40
~
39.00
18
.33
.72
117.96
112.72
39.00 + .60
~
39.60
-
-
117.63
112.00
%
~
0
~
.
273
4.00 m.
..
ikii+V
if?it!W
' ''VW1
TABLE J - 4 SPILLWAY PORTION
A) Prejump profiles - High Flood Condition Upstream Total Energy Level ~ 119.70 m.
Distance from Upstream End
Elevation at point of consideration
1
2
C) 7.00 m.
116.00
Energy of flow (Ef) ~ U/S T.E.L. - 2
q
~
13.7 4 cms./m.
3
4 3.70 m
D) 8.00 m D1 ) 8.28 m
115.80 115.68
3.90 m 4.02 m
E) 9.00 m
115.28
4.42 m
F) 10.00 m
114.47
5.23 m
G) 11.00 m
113.39
6.31 m
H) 12.30 m
112.80
6.90
De~
Dl
Location of critical depth De
~
~
,,
5
3.08 m.
116.00 + 3.08
~
119.08
2.70 m 2.68 m
115.80 + 2.70 115.68 + 2.68
~
118.50 118.36
1.99 m 1.64 m
115.28 + 1.99 114.47 + 1.64
~
117.27 116.:l_l
1.40 m
113.39 + 1.40
~
114.79
1.30 m
112.80 + 1.30
~
114.10
El. 115.68; 1.28 m. from C of the crest
274
Elevation of Water surface
Depth of Water, (D) from Energy of flow curves (Fig. 15.8)
~ ~
TABLEJ- 5 SPILLWAY PORTION
b) Post jump profiles F F2
Point
Distance from the toe of glacis ~ X
~
~
DiS T.E.L. = 117.92 m
2.96
DiS Floor Level = 112.80 m.
8.76
X
y
Dl
Dl
y
Elevation of water surface
Hl
2.00
1.54
1.50
1.95
112.80 + 1.95
H2
3.00
2.30
1.81
2.35
112.80 + 2.35
·•
H3
4.00
3.08
2.10
2.73
112.80 + 2.73
I
5.00
0.84
2.30
2.99
112.80 + 2.99
J
10.00
1.69
3.15
4.10
112.80 + 4.10
K
15.00
11.53
3.38
4.39
112.80 + 4.39
L
17.00
13.08
3.43
4.46
112.80 + 4.46
c3
19.20
14.77
3.63
4.72
112.80 + 4.72
275
= = = = = = = =
114.75 115.15 115.53 115.79 116.90 117.19 117.26 117.52
TABLE J -6' SLUICEWAY PORTION a) Prejump Profiles During high flood condition Distance from Elevation at Upstream End point of consideration
Energy of flow (Efl UIS T.E.L. ~
Elevation of water surface
Depth of Water, (D) from energy of flow curves
119.15 m
Bl
6.50 m
114.00
5.84
5.15
114.00 + 5.15
~
Cl
9.50 m
114.00
5.84
4.10
114.00 + 4.10
~
118.10 m
C11 10.04 m
113.82
6.02
4.02
113.82 + 4.02
~
117.84 m
Dl 11.50 m
113.33
6.51
3.07
113.33 + 3.07
~
116.40 m
El 13.50 m
112.66
7.18
2.68
112.66 + 2.68
~
115.34 m
Fl 15.50 m
112.00
7.84
2.19
112.00 + 2.19
~
114.19 m
Location of Critical depth, D
~
De
Dl
~
~
El. 113.82,@ .54 m. away from the end of the sluice crest.
b) Post jump profiles
F2 F Point
~ ~
DIS T.E.L.
2.486 6.18
Distance from the toe of glacis ~ X
~
118.29
DIS Floor Level
X Dl
-
'y
~
112.00
y
Elevation of water surface in meter
Dl
Fl
1.00
.46
1.05
2.30
112.00 + 2.30 ~ 114.30
F2
2.00
.91
1.25
2.73
112.00 + 2.73 ~ 114.73
F3
3.00
1.37
1.43
3.13
112.00 + 3.13 ~ 115.13
F4
4.00
1.82
1.62
3.54
112.00 + 3.54 ~ 115.54
G
5.00
2.28
1.81
3.96
112.00 + 3.96 ~ 115.96 ~
H
10.00
4.57
2.26
4.96
112.00 + 4.96
I
15.00
6.85
2.57
5.63
112.00 + 5.63 ~ 117.63
276
116.96
F
9
jgD 13 Knowing F2, the relation between the abscissa ordinate of the profile can be read from Fig. 15.9. For spillway the values for prejump & post jump profiles are given in Table J4 & J-5, & for sluiceway in Table J -6.
4.
Floor Thickness
With the help of plots of H.G. Line and Water Surface profiles, the design uplift pressures can be evaluated. The ordinates measured from the hydraulic gradient line to water surface are the uplift pressures for which lhe floor is to be designed. The uplift pressure which will occur with the maximum pond level upstream and no flow downstream should also be determined (as shown in fig. 5.10 & 5.11). The requirement of floor thickness is worked out by taking the larger of the two uplift pressures and dividing it by the submerged density of floor material i.e. (G-1) . In this example, specific weight of concrete is taken as .144 pcf. and the specific weight of water as 62.40 pel. Submerged density of the floor material
144 - 62.40 62.40
~
1.30
a) Spillway portion
;I
1. Downstream floor 1
~
It would be clear from the drawing that the static head governs the thickness of the spillway floor up to 9.20 m. from the downstream floor end while beyond it, the dynamic condition governs the thickness because of the larger value of the ordinates after applying the requirement of 2/3 of the head. In view of the tendency of water into the jump trough rolling backwards, thus reducing the unbalanced head, the 2/3 value is considered adequate for working out the floor thickness. i) at point Ea
Unbalanced head
.54 m.
Static head
.76 m.
The static head is more than the unbalanced head. Floor thickness required ~
.76 ~ .58 m. 1.30
Taking into account a 10% factor of safety as per para 7.8.1 Floor thickness
~
1.10 (.58)
~
.63 m.
For construction purposes provide 65 em. floor thickness at 1.00 m. from downstream end of floor. ii) at point K
Unbalanced head
.99 m ..
Static head
.96 m.
277
Static head still governs, since design unbalanced head is 'l/3 of the actual unbalanced head, which is lesser Floor thickness required ~ ·9 6 ~ .73 m. 1.30 With 10.% F.S; t
~
1.10 (.73)
~
I I
.80 m.
Provide 80 em. floor thickness at 4.20 m. from dis end of floor
1··2 i
I
iii) at point J Unbalanced head ~ 1.46 m; 2/3 (1.46)
.97 m.
~ !}.13
Static head 1.26 m. governs Floor thickness required ~ }.26 ~ .97 m. 1.30 . With 10% F.S.; t
~
~
1.10 (.97)
1.07 m.
Provide 110 em. thickness at 9.20 m. from dis end of floor iv) at point I Unbalanced head ~ 2.7 4 m; 2i3 (2.7 4) Static head
~
1.82 m.
1.57 m.
Here, unbalanced head governs Floor thickness required ~ 1.82 ~ 1.40 m. 1.30 With 10% F. S.; t
~
1.10 (1.40)
~
1.54 m.
Provide 160 em. floor thickness at 14.20 m. from dis end of floor
v) at point Ha Unbalanced head Static head
~
~
3.04 m; 2i3 (3.04)
~
2.02 m. governs
1.63 m.
Floor thickness required
~
1.10 x
!:~~ ~
1.70 m.
Provide 170 cm.·floor thickness at 15.20 m. from dis end of floor vi) at point H 2 Unbalanced head Static head
~
~
3.45 m; 2/3 (3.45)
2.30 m. governs
1.69 m.
Floor thickness required
~ 1.10 x i:~~ ~ 1.94 m.
Provide 195 em. floor thickness at 16.20 m. from dis end of floor
278
1-
}
j j j j j j j j j j
'
j
.4$
...
ll .f3
ll .lt-4
1.411
1.11
.
'
·r·
j
Ctn2
j
n
j
II 2.00
Jo
i'
j j
Ita
j j j j
L ·O
000
~·o
40
[)
j j
fj
j j j j
J
j j j
"".so
2e.•o
ao<>
j
••
j j j j j j j j j j
: \y )
j j
----------------------------------------------~F~IG_..~~5~·-"------~,Jj j
-----.!!!!--..
='"'·""'=--~~·-..............------: j j
j j j
U/ S T. £ L.. • I ~I · lA !II
. 118.8!1
118.80
Ill .74
11!1.71
118.&8
.s.ee
118.!19
I
I
t'"""
~ ..
! . 48 4.:55
•5 .!12
nlao ...,....
2 .65
••.o• .a.]···
I TI:S
. 70
I
lUUI
I
..
118, 82
.!12
. 41 1 117.8ll
117.85
117. 80
.. --ll·;:.s·
T. f.L . • Wl.lt
111.
Jlr.w. f. L . • 117 . 81 Ill .
117. 8-ll
3 .08
11 .o8
, 2.21
2 .SI
2.14
I
2.08
l.l ll
c1 112 .u . 72 112.00
4
i
22!1
.14
n.. 112.00
f· 2:!0
II . 84
1.48
I
I
280
.4!1 II . IS
I
1!10
:oo
12!1
...
l e.L.
::c:Q_
200
200
100
100
!!>_0
300
1100
S IO
!100
:S CIO
J
J
-.U~d------;:~------b--_;____~-.t;;;--'1• 21.40
88.!10· 88.8!1
88.54
83 .18
eo.o2
!18 .44
:58 .88
11821
113.70
JUMP
AND
44.21
82 .11
38 . 80
DRAULIC
23.~
341 .30
Ill.
-
UPLIFT
PRESSURES
(SLUICE WAY)
I
FIG . 15 . 11 --------~ - ~------------------~----------------------------___;,...;..;._..;..;;.....;...__ _--1\
--- -....
--
ue.IO 19.08
ua.oo
•••
117.
II&. 1\e..OO
EL.II6.QQ (CREST LEVEL)
-==-
11:34. 11:3.34 _ _ _ _ _ _ _ __
~
" -~•--:r::-.;----'--1. . 17 H 81
2.44
~--....,p~....::.:.........u~o!!YJll cl
2.\8
II S ,00
D
El 100
cl
!)I
200
•oo •.
too
60
•oo
·-
l
I
I
.. a3.&S
••
100
78.88
74.215
ea. oo. 6&. 615
I
•.
·~'·'
I •
J ." ~
0' ..,,
-- ·-:-- ----
•• • • ·,
---
._.., .. --.-~ -·~--,.~-----
..
U/ S
T. E. L. . :
119. 70 M.
HYO.
118.83
118. 79
118.78
118. 71
118 . ~
118. 88
U8 .60
118 . 57
118.53
--
GRA DI ENT
L I NE
FOR
MAX.
118.36
U/ S
:I_
1.46 2.7-4
El.. 114.00
117. 63
II . li
II .iO
3 .04 3. ee
.w. £ L.. •
/
....:
2 .88
RMo\1. CONOITIONl.
11 8 .18
T. £1. . : 117 . 82
3 .45
3. 8i
4.81 4. 53
_j[ I
HYO.
114 . 43
114.37
I. 63
1.117
QRAOIENT
LINE
FOR
NO
A 1.-n!
1. 81
.87
1.6$
F
.: " ( - --
ll l5. 76
1.26 112 .80
11 3 38
I
.88
·"
'
.76
c~
·-------~~----~.'----+--~ 11 2.80
H;s
Hz
HI_
Ho
11 3 . 63 113. 58
·I 0 160 260
EL.. 110. 20~
450
...
'~
245
165
+ t
~
2 00
200
-100
-L
~
000 - ---------
I
I
,•
_1'0. 18
68.00
68.75
64.94
.,•
39.51
8 .37
62 .94
23
26.18
29 . 99 :J 0 1
....
4 20
6 1.03
56 .66
.116
52.114
54.75
!!10.94
'1&.03
520
31.~
HYDRAULIC
JUMP
100
.. .
AND
UPLIFT
PRESSURES
{SPILLWAY) FIG . '15.10
/
..
-··~--·------= --:;;;:=-s~jiji!'!QW!'£::!'•~~~~~~ ~~·.,..,. .-.
.. . . --~--~-~~·~ """="'""'"__,_._.._
....._.4.___ ..._...~-
-
-------~---------
---
-~--4·-·
-
___
.___,_..~
AFFLUX EL . .1 119.48M.J,
----c _. Z.&! M
1.04 EL. 114.00
r
~ +- ,.o 1"" 1
r-~--~~r~o~---t~----------~·~•~o, __________~~-----~•··~·~---
•
n.••
....,.
_10.18
79.!:10
68.00
66.70
82,96
l>G
100
140
uo I
I
vii) at point H 1 Unbalanced head
Static head
=
~
3.89 m; 2i3 (3.89)
~
2.59 m. governs
~-.
1.75 m.
Floor thickness required
~
1.10 x
i:;~ ~
2.19 m.
Provide 220 em. floor thickness at 17.20 m. from d/s end of floor
viii) at point Ho Unbalanced head ~ Static head
~
1.81 m.
Floor thickness required
•·.~·: '!
3.02 m. governs
4.53 m; 213 (4.53)
1.10 x
~:~~ ~ m~
Provide 255 em. floor thickness at 18.20
2.55 m from dis end of floor
ix) at point H Unbalanced head Static head
~
~
4.61 m; 2i3 (4.61)
3.07 m. governs
1.87 m. 1.10
Floor thickness required
X
3.07 1.30
2.60 m.
Provide 260 em. floor thickness at the toe of the glacis.
2) Upstream floor
The subsoil hydraulic gradient line is below the water level, i.e., all the unbalanced head acting on the floor is counterbalanced by the self weight of the water. Thus, only a nominal thickness of about 4·5 ft., i.e., 1.6 to 1.7 m. is provided.
bl Sluiceway Portion
a) Downstream floor i) at point E 2 Unbalanced head Static head
~
~
.43 m.
.94 m. governs
Floor thickness required
~
1.10 x
i~4 ~
0
.80 m.
Provide 80 em. floor thickness at 1.00 m. from dis end of floor. ii) at point
J
Unbalanced head
.52 m.
281
Static head
~
1.13 m.
governs
~ 1.10 x i:~~ ~ .95 m.
Floor thickness required
Provide 95 em. floor thickness at 4.10 m. from dis end of floor. iii) at point I Unbalanced heaC: Static head
~
~
.70 m.
1.45 m.
governs
)
Floor thickness required ~ 1.10 x 1. 45 ~ 1.22 m. 1.30 Provide 125 em. floor thickness at 9.10 from dis end of floor. iv) at point H Unbalanced head Static head
~
~
1.48 m.
1.77 m.
governs
Floor thickness required ~ 1.10 x 1.n 1.30
~ 1.50 m.
Provide 150 em. floor thickness at 14.10 m. from dis end of floor. v) at point G Unbalanced head ~ 2.63 m.; 2i3 (2.63) Static head
~
i ·I
l
1.75 m.
1.10
X
2.08 1.30
1.76 m.
Provide 180 em. floor thickness at 19.10 m. from dis end of floor. vi) at point F 4 Unbalan.ced head ~
l l
2.08 m.
Floor thickness required
Static head
I
~
3.08 m; 2/3 (3.08)
2.14 m.
Floor thickness required
governs
~ 1.10 x
~
i:;t
2.05 m.
l I
I
~ 1.81 m.
Provide 185 em. floor thickness at 20.10 m. from dis end of floor viii) at point F 3 Unbalanced head Static head
~
~
3.52; 2i3 (3.52)
2.34 m.
2.21 m.
Unbalanced head governs Floor thickness required ~ 1.10 x 21.30 ·34 ~ 1 .98 m . Provide 200 em. floor thickness at 21.10 m. from dis end of floor 282
,_:y
viii) at point F 2 3.95 m; 2/3 (3.95)
Unbalanced head Static head
~
• 2.63 m. governs
~·
2.27 m.
Floor thickness required
~ 1.10 x i:~~ ~ 2.22 m.
Provide 225 em. floor thickness at 22.10 m. from dis end of floor. ix) at point F 1 Unbalanced head Static head
~
~
4.41 m; 2/3 (4.41)
2.33 m.
Floor thickness required
~
~
2.94 governs
i:;6
1.10 x
~
2.49 m.
Provide 250 em. floor thickness at 23.10 m. from d/s end of floor. x) at point F Unbalanced head Static head
~
~
4.55 m; 2/3 (4.55)
~
3.03 m governs
2.40 m.
Floor thickness required
1.10
X
3.03 1.30
2.56 m.
Provide 260 em. floor thickness at the toe of the glacis b) Upstream floor· (See procedure for floor thickness as for spillway portion)
K. STABILITY ANALYSIS
As discussed in Chapter 8 of the Manual, in the structural design of the solid gravity over flow dam, the section of the dam shall satisfy the following requirements of stability. i) ii)
The section should be safe against sliding. The section should be safe against over turning.
iii) Unit stress in the material of the structure or pressure on the foundation, should not exceed the permissible limit. Testing the dam section as per sketch on the next page for the following combination of forces.
·~!:~
Combination A
Dam completed but no pond water & no tail water.
Combination C
Maximum flood with dam silted to crest level.
Combination E
Water level on upstream at crest level, tail water at retrogressed floor elevation, silt upstream & with earthquake. 283
. eo
4.50
3.30
2.00
0
Wz
1
----1
w.;
1- ________ "ti:_::2._,Eo=.L:.e.llc-c5".2"'e_Jll__
I I I 3
.•
El.l14.00
WI
2
~.47-1
I I I I I I
.20
I I
0
______ T _______ _"'0
El. 112.30
4
I I I
I El. 111'.3oJ}
iJI::L. 111.20
I 7
I
.:so+~
ol g!
0 N
El.110.20
~------------~6~.~9~5_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __l_ __l~1~.6~5_ _~8i_________________lj9 2.00
Dam completed but no water in pond and
Load Combination A:
no tailwater
Area of the section & C.G. AREA (sq, m) 1.70
X
~
4.5
y (From toe) m.
1.70 2.10 + -2-
7.65 ~
/z (.40) (.80)
1
.16
Ay
~
2.95
2.10 + 1.70 + 1.60 + ~
~
3.30 (.80)
~
(.72) (2.00)
A4
1/z
A5
2.00 (2.98)
As
1
A7
2.64
~
~
1
Ag
2.10 (2.00)
LA
27.728
~
~
~
~
2.10 + 2.98 + 2.98 2.10 +
3.84
1.65
/z (1.10) (1.65)
As
.72
5.96
/z (2.48) (3.10)
.50 (3.30)
2.10 +
3 0 :
4.20
3
5.53
-~2 ~
(.40) .884 9.90
3.75
5.32
3.83
--z:-
3.59
21.40
2.60 +
2.48 3
3.43
13.17
2.10 +
.50 2
~
2.35
2 1.00 + 3(1.10) 2.10 -2~ 1.05
.908
22.568 1
~
1.73
3.88 1.57 4.41 81.61
Y
8 61 ~ 27.728 1. ~ 2.94 m. from the toe
•
285
DAM WEIGHTS & MOMENTS
Taking 1 -metre strip
l 1
l
I
Moment@ Toe (Kg.m.)
Lever Arm (M)
Dam weights (Kg_)
wl
7.65 (2400)
~
18360
4.25 3.30 + 2.00 + .80 + -2-
W2
0.16 (2400)
~
384
5.30 +
-3-~
5.57
w3
2.64 (2400)
~
6336
5.30 +
-~0 ~
5.70
w4
0.72 (2400)
~
1728
3.30 +
w5
5.96 (2400)
~
14304
3.30 +
w6
3.84 (2400)
~
9216
.20 +
32
W7
1.65 (2400)
~
3960
3.:0
~ 1.65
Ws
0.908 (2400)
Wg
4.20 (2400)
LW
~
~
~
2179.2
10080
.80
32
(2.00)
2.00 2
(3.10)
2.00 +
1.65 3
2.00 2
1.00
~
~
~
8.35
153306 2138.88 36115.2
4.63
4.30 ~
~
2.27
8000.64 61507.2 20920.3 6534
~
2.55
5556.4 10080
LM ~ 304158.62 (
66547.2
•
286
Foundation Reactions:
304158.62 66589.88
X=
<x
10.60 ~ 3.53 3
L 3
f toe~
LW -s
~
4.57 m. from the toe
.. within the middle third
( + ~ 1
B
where
e
~
B/2 10.60 2
ftoe
=
fheel =
x - 4.57
~
66589.88 10.60
[
8877.86 Kg./m 2
<
LW
1 +
(1-
6e) B
6.6589.88 10.60
[
3686.26 Kg./m 2
<
If
0.73 6 (0.73) 10.60
32290 Kg./m 2
1 -
] Qb
6 (0.73) 10.60
]
32290 Kg./m 2
O.K.
287
•
§
At
rm
2EZ££LZZL
I
Load Combination C: Maximum flood with dam silted up to crest level Afflux El.
119.48 m.
~
Tailwa tcr El.
~
117.63 m.
A} Static & Uplift Pressure
Point
Net head
Pressure in Kg./m 2
3.48· 3.88
3.48
3480
3.88
3880
5.48 5.48 •
5480 5480
Pressure
Available Head, in m. ~ W.S. el. - elev. of point, ~ h
Percent~
age(%)
119.48 - 116.00 119.48 - 115.60
0 1 2
5 6 7 9 10 11 . 12
l
7.18 7.18
82.96 77.42
117.90 - 112.30
5.60
65.51
3.69
3690
115.77 - 111.20
4.57 5.57
62.37
2.85
62.37
3.47
2850 3470
3.90 1.30 1.40 1.99
58.56
2.28 1.30 1.40 1.99
2280 1300 1400 1990
115.77 114.10 114.10 114.20 117.27
8
~
5.48 5.48 5.96 5.56
119.48 119.48 119.48 119.48
3 4
~
-
-
114.00 114.00 112.30 112.30
110.20 110.20 112.80 112.80 115.28
~ ~ ~
~
5960 5560
B) Silt pressure
It is assumed that silt and water mixture is completely fluid, the resulting horizontal pressure is assumed to be equivalent to that of a fluid having a unit weight of 85 pel (1360 Kg./m3) and the vertical pressure with 120 pel (1925 Kg.Jm 3)
As the water thrust has already been considered, an additional thrust due to the fluid of density 22.60 pel (363 Kg./m3} for horizontal pressure and 57.60 pel (924 Kg./m3) for vertical pressure may be added. Thus, for silt pressure, h
116.00 - 114.00
~
2.00 m.
PH
363.0 x 2.00 ~ 726 Kg./m2
Pv
924 x 2.00 ~ 1848 Kg./m2
(See tabulation of summation of stabilizing and overturning Moments)
288
Summary:
356720.3 + 304158.62 - 347815.1 ~ 313063.82 Kg. m.
2: MR
2830.7 + 66589.88 ~ 69420.58 Kg.
Z:V
~
X=
~
21120.2 Kg.
~ 4.51
313063.82 69420.58
>
3
~ 3.53 O.K.
10 60
(within the middle third)
B
e
- 4 51 -X= 10.60 2 .
2
0.79 m.
Foundation Reactions: ftoe
=
LV B
9477.7 Kg./m2
69420.58 10.60
6e B
( 1 +
<
32290 Kg./m2
B
B
3620.5 Kg./m 2
<
1 + 6 (.79) 10.60
]
O.K.
69420.58 10.60
( 1 - ~)
L,V
[
32290 Kg./m2
1 -
[
6 (.79) 10.60
]
O.K. Safe
(W- U) tan 0
Factor of Safety Against sliding. F 1
p
where:
total external forces acting downward + Total weight of dam itself ~ 46914.5 + 66589.88
w
~
U
~
total uplift force
~
P
~
total horizontal force
113504.38 Kg .
•
44083.8 Kg. ~
21120.2 Kg.
Assuming a gravelly sand material. 20% gravel, maximum size is 3/4" and with a relative density of 70%, the corresponding value of tan fi ~ .77 from Table No. 8.1. F1
~
113504.38 - 44083.8) (0.77) 21120.2 2.53
>
2.00
289
Allowable Factor of Safety for this loading combination ~ 2.00 F1
>
F,
O.K.
Factor of Safety against overturning 660878.92 347815.1 1.90 > 1.5
O.K.
..
Safe
Load Combination E - Water level on upstream at crest level, dry weather tailwater, full uplift, silt deposit up to crest level with earthquake. A)
Static and Uplift Pressures
Available Head, in m. Point
~
W.S. el - of point
Pressure
Net Head
Pressure
Percentage
h in m.
(in Kg{m 2)
(%)
0 1 2
0 116.00 - 115.60 116.00 - 114.00
.40 2.00
3 4
2.00 116.00 - 112.30
3.70
5 6 7
3.70 116.00 - 112.30 116.00 - 111.20
8 9 10 11 12
116.00 116.00 116.00 116.00
-
0
~
82.96 77.42
3.70
65.51 62.37
4.80
110.20 110.20 112.80 112.80
~
5.80 5.80 3.20 3.20
116.00 - 115.28
~
.72
~
62.37 58.56
.40
400
2.00 2.00
2000
3.07
3070 2860
2.86 2.42
2000
2420
2.99
2990
3.62 3.40 3.20
3620 3400 3200
3.20
3200
.72
720
i
!r
I I I
from the previous computation, silt pressure values are:
PH ~ 726 Kg./m2 Pv ~ 1848 Kg./m2
290
I I
l
Taking the statical moment about the toe of the dam, the distance of the center of gravity from the toe will be X ~ _i_ ( pl + 2Pl) 3 Pl + Pz STABILITY ANALYSIS: - Moments due to external forces MOMENT ABOUT TOE !KG-ml LEVER ARM (m)
EXTERNAL FORCES !KG)
RIGHTING
Po-I
-
Water~
pl-2
~ 3880 + 5480 121 ~ 9360
Silt -"--->
pl-2
Water~
Pz-s
t
!
~
Silt
5.30 + -80 ~ 5.70 2
3480 + 3880 1.80) ~ 2944.0 2 .
726 12.00) -
~
)
6882.5
)
28588.5
)
99 1025 . +_lQ_~ 2
41328
)
4.74
~
24660
Pz_s - 1848 14.50)
~
8316
-
44366.4
1452
5480 14.501
~
16780.8 (
3.80 + 2.00 11123.40 + 1590.801 ~ 4.74 3 1918.80
2
6.10 +
4
-~0 ~ 8.35
205911 (
8.35
69438.6 (
2.10 + L70 11221.80 + 2246.80) ~ 2 _9 4 '""'3 2345,20
5480 ; 5960 IL7 0I ~ 9724
OVERTURNING
~
Ps-•
t
r,_5
t
Ps-6 "" 5560 +2 3690 16 .25) ~. 28906,3
3,65 + 6.25 ~ 6.78 2
195984.4
)
t
Ps-7y
3690 ; 2850 IL 651
2.00 + !.65 1584.25 + 1512.9013 29 . 2 1340.70 -
17751.2
)
--';
p6-71!
1 .oo + 1-10 1584.25 + !512.90 I ~ 1 57 1340.7Q . 3
5647.3
)
~
p7-8
~ 2850
1516.8
)
t
Ps-9
~ 3470 + 2280 121
5750
)
t
p!0-11
t
Pn-12y
~
! ..,__
t t
___,..
5960 ;
~
~
3690
~
5560 1.701 -
12850 11.10)
+ 3470 Ill
Pn-12 11 p12-0y
2
~
~
~
~
~
5395.5
~
3597
~
3160
~
5750
2
-
4032
).00 1711.35 + 1168.501 3 1295.1)0
1300 ; 1400 1.20) ~ 270
~1287 + 5331 ~ 0.15
2
553.50
40.5 (
1400 ; 1990 13.!0)
~
5254.5
.20 + 3.10 1407.95 + 57 41 ~ 2.39 2 694.95
12558.3 (
!400 ; 1990 (2.48)
~
4203,6
2 60 + 2.48,407.95 + 57 41 ~ 3 77 64~5 -
15847.5 (
3,30 + 2.00 1713.40 + 815.901 ~ 4 66 2 1121.35 .
25490.2 (
:;,08 + _,1g_ 1713.40 + 815.901 ~ fi.41 3 1121.35 -
10653.4
.
!990 + 3480 121 ~ 5470 ~
.
Fy Net - 46914.5 - 44083,8 Fy Net 2830.7
~ FH -
0.48
. 2~0 ~ 1.00
p12-0l! - !990 ; 3480 1.721 ~ !969.2 ~ ~
~
t
~
21120.2
;
t
s
~ Fy
~
291
Fy
~
~
46914,5 44083.8
c
~MR ~356720.3 ~ M ~ 347815.1 0
~
.)
··~ B.
STABILITY ANALYSIS:
MOMENT ABOUT TOE (KG·ml LEVER ARM (ml
EXTERNAL FORCES (KGI
RIGHTING
Po-I
Water~
pl-2
~
Silt-
pl-2
~
726 121
1
p2-3
~
2000 14.50)
~
9000
J
p2-3
~
1848 (4.501
~
8316
Water Silt
~
.
l
•
·.··1··
•
1 ~
1 ~
~·
..•.
1
-£{4
! l
y
y
....·.·.·.1·
)1.• '~
~
l -'
0 + 400 I 801 - 160 2 . -
l
-
~
P,_,
p6-7
Po-7 P,_g Pg_g
~
2400
2.00
3.80 +
932.8 (,
410 + 164 492
I
3
~
)
4.58
10992 )
450 2
6.10 +
8.35
~
75150
8.35
2000 + 3070 IL701 ~ 4309.5 2 3070 ; 2860 1.701 ~ 2075.5
.70
2
c.
69438.6 (,
629.35 + 820 2 .10 + 170 3 I 1039.35 9.90 +
J
6650.16
4.58
1452
)
2.89
~
12454.5 )
~
10.25
21273.8 J
~
6.78
11187.0)
2800 + 2420 16.251 2
~
16500
3.65 + 6.25 2
2420 + 2990 - - - 2 - - IL101
~
2975.5
2.00 +
~
2420 + 2990 IL651 2
~
4463.3
I 612.95 + 992.20 I 2. OO + L65 3 1109.05
~
2990 + 3620 ILOO) 2
~
3305
I 742.10 + 1225.90 I ~ 2.73 2.00 + 100 1355.05 2
9022.6)
2.00 /2
7020 )
~
~
3620 + 3400 (2) ~ 7020 2
~
pl0-11
3200 1.20)
=;
Pll-12v
~
pll-121-I""
Piz-ov
=
Piz-ou
=
F.vNET =
640
~
.20 2
11101 612.95 + 992.20 ~ 3.06 . 1109.05
1. 3
2.60 + I 147.60 + 1312 I 803.60'
121
~
720 1.72) 2
5146.8 lbs
720 ~
259.2
2
3.30 +
3
5.08 +
T
(2.00)
.72
~
~
~
~
4.10
4.63
5.32
l
LFv
~
24912 lbs
1
LFv
~
30058 lbs
1L
F.vNET ~ 30058 - 24912 ~ 5146 Kg
292
12497.1
J
64 (
3200 2+ 720 2.48 ~ 4860.8
2
2.80
0.10
~
3.10 I 147.60 + 1312 I 0.20 + -2803.60
720
~
9105.0 )
LOO
~
3200 2+ 720 13.101 ~ 6076
F.n - 9322 lbs
r
~
2
~
p4-5 ~
P,_,
400 + 2000 (2)
80x2 583 3 .
5.30 +
OVERTURNING
3.02
18349.5 ( 19929.3 ( 3333.6 ( 1378.94 (
t7MR 188576.74 I
r
Mo200885.16
Forces due to earthquake from the previous computation: 2.94 m. from the toe
Weight of dam ~ 66589.88 Kg. a) Inertia forces (para. 8.5.62) Fw ~ 0.15 WT ~ 0.15 (66589.88) ~ 9988.48 Kg. The effect of vertical earthquake is, however, not considered in this analysis and is neglected. Moment due to earhtquake, Me
Fw
(g
0
9988.48
X
2.94
29366.13 Kg. m.
J
b) Hydrodynamic forces (para 8.5.7) C<{h
wh
hydrodynamic pressure in Kg./m2
Where Py
at depth y <\'h
design horizontal seismic coefficient 0.15
W h
unit weight of water 1000 Kg./m3 ~
total depth of water in meter 2.00 m.
Where y ~ depth below surface
Cm
when
maximum value of C obtained from fig. 8.2
C
Cm
y
h, as the case hereunder
y
2.00 m. 2.00 2.00
Maximum value of C
~
1 .73
Unit weight of water 1000 Kg./m3 py
.73 (.15) (1000) (2.00) 219 kg./m2
Vh
Total horizontal shear
293
Vh
~
0.726 Py . h ~
0.726 (219) (2.00)
317.99 kg./m.
'
Mh ~ 0.299 py h2 0.299 (219) (2.00)2 261.92 kg.m/m. - strip SUMMARY:
From the previous computation: Due to dam weight: MR
~ 304158.62 1
F v ~ 66589.881 ~MR ~
304158.62 + 188576.74 ~ 492735.36 Kg. m.
~M 0 ~
200885.16 + 29366.13 + 261.92 ~ 230513.21 Kg. m.
~M ~ ~MR
~
V
~
~H ~
-
~M 0 ~
262222.15 Kg. m.
66589.88 - 5146
~
61,443.88 Kg. ~
9322 + 9988.48 + 317.99
~ 4,27 m. from the toe
262,222.15 61,443.88
X=
19628.47 Kg.
>
10,60 3
(within the middle third) O.K. e
~
. _ll_ 2
- x
~
10 6
· ~ - 4,27 ~ 1.03 m •
fgundation Reactions;
( 1- ~ ) B
!heel ~
6 (1.03)
61443.88 10.60
[
1-
2417.0 Kg./m2
<
32290 J(g./mz
61443.88 10.60
[
1
9176.11
<
+
10,60
6 (1.03) 10.60
32290 Kg./m 2
Factor of Safety against sliding,
] 0.1<;.
] O.K.
!'\ ~
(W -
where W
~
total weight of the <1am
U
~
total uplift force
P
~
total horizontal force
~
~ 66,58~.88
Rg.
5146 Kg. ~
19,628.47 Kg. 294
¥) tan 0
Assuming a gravelly sand material, 20% grave, maximum ~ 3/4" with a relative density of 70°,\J; Refer to Table no. 8.1 ~
Tan 0 ~
F 1
.77 2.4 1
(66,589.88 - 5146) (.77) 19,628.47
Allowable F .S. for this condition ~ 1.2 F1
>
allow F.S.
O.K.
Factor of Safety against overturning, F 0 492,735.36 230,513.21
L.
~
2.14
>
1.50 O.K.
DESIGN OF GUIDE BANKS a) Layout
Total Width ~ L ~ 440.0 m. As per fig. 11.2, for the elliptical guide banks, U /S Length, a ~ 1.0 L ~ (1.0) 440 ~ 440 m. DiS Length, 0.25 L ~ (440) ~ 110 m .. 0.40 L • Pw ~ 4.83
~
0.40 (440)
JQ ~ 4.83 J5320
~
176 m.
~ 352.29 m.
0.45 (352.29) ~ 158.53 say 160m.
Radius of curve R b
~
R
~
160.0 m.
For layout the coordinates calculated with the formula
y2
+
b2
~
1
295
y
y
X
1
440
80
381
5 10
440
90
364
439
100
343
15
438
105
332
20
436
110
320
25
435
115
306
30
432
120
291
35
429
125
275
40
426
130
257
45
422
135
236
50
418
140
213
55
413
145
186
60
408
150
153
65
402
155
109
70
396
160
0
b) Protection Works
Intensity of discharge d/s of sluiceway ~ 21.02 cms/m. scour depth varies from 1.0 R - 1.5 R Taking Mean depth of scour ~ 1.25 R Where:
R
~
1.35
8.10
(21.02)2 ]1/3 4.00
[
1.25 R ~
D
~
4.00
1.5 D
~
6.00 m.
v
~
X
~
6.48
8.10 mts.
(117.63 - 112.80) + D
21.02 (3.28) _ 4 83 size of stone weight
~
~ ~
4.35 m/sec. 13"
~
16.28 ft./sec.
0
110 lbs
~
50 kgs.
296
[5 D
~
L
~
8.94
. 1.25 ( 1.50 V
Lt1
~
X
12
8.94 (0.62)
v
~
1.5 D
13 3.28
~
X
~
0.62
5.54
5·54 6.00
~ 0 92 say 0.90 mts. .
Use protection works for upstream and downstream portion ~ 1.50 D ~ 6.00 mts.
Nose Protection
Depth of scour ~ 2 R D ~ 2R- (117.63- 112.8) R where
~
1.35
[
l16.71P 4
113
]
~ 5.56
q ~ 16.71 cms./m. D ~ 5.56 x 2 - 4.83 ~ 6.29 1.5 D ~ 1.5 x 6.29 = 10.50 m .
'J :1'·;·.·.·.
.
297
CHAPTER 16
DESIGN OF INTAKE WORK
1.
CANAL SECTION
Main Canal No. 1
DATA:
Qcm
~
33.97 ems.
(Discharge for crop maintenance)
QLS
~
40.998 ems.
(Discharge for land soaking)
Rugosity Coefficient n
0.015
~
Side slope (SS) ~ 1 1/z : 1 Water surface slopeS ~ 0.0002
where
b
~
2d
b
~
bed width
d ~ depth of water
1.1 For crop maintenance X ~
y ~
1.5 d
J(1.5d)2
+ d2
y
~
1.803 d
A
~
bd + (1.50 d) (d) ( 1/z) (2)
298
A~
2d2 + 1.5 d 2 3.5 d2
p
~
b + 2 (2.803 d)
p
~
5.606 d
R
~
AlP~
v
~
A~
-
3.5d2 5.606 d
Rzt3
1
0.624 d
s'h
n
1 1 0.015 (0.624 dJ2'3 (0.0002) /2
v ~ v
~ 0.688 d 213
Q
~
Av 33.97 ~ 3.5 d 2 (0.688 d213)
d ~
33.97 ) . (0. 3 50 688
b
~
2d
~
Using b
~
5.40
d
~
2.70
2 (2.70)
3/8 ~ 2.70 mts. ~
5.40 mts.
A ~ bd + 1.50 d2 A ~ 5.40 (2.70) + 1.5 (2.70J2 A
~
25.515 sqm
p
~
b + 3.606 d
p
~
5.40 + 3.606 (2.70)
P
~
15.14 m
R
~
AlP
v ~
v V
~
25.515 I 15.14
~
1.685
2.. Rzts sl/2 n
~
-
~
1.34 m/sec.
10.015
(1.685)213 (0.0002)112
Q ~ Av ~ 25.515 x 1.34
Q
~
34.19 ems.
299
1.2 For land soaking
QLS
~
40.998 ems.
Using b Try d
~
5.40 mts. 2.97 mts.
Check: A ~ bd + 1.5d 2
A
5.40 (2.97) + 1.50 (2.97P
A
29.27 sq. m.
p
b + 3.606 d
p
5.40 + 3.606 (2.97)
P
~
16.11 mts.
R
~
A!P
~
29.27 /16.11
1.82
_ 1_ (1 82J213 (0 0002) 112 0.015 . . V
~
1.405 m/sec. 29.27 (1.405)
Q
Av
Q
41.12 ems.
~
>
Free board
40.998 ems.
0.25 d + 0.30 0.25 (2.97) + 0.30 1.05 mts.
Canal Elements: Qcm
"7
33.97 ems
QLS
40.998 ems.
dcm
2.70 mts
dLs
2.97 mts
v
1.34 m/sec.
R
1.685 m
p
15.14 m.
b
~
5.40 m.
A~
25.515 sq. m.
de
3.05 m.
~
300
'it£
1&
&
• , 2 2. L Lti4-AU£WMIJJtJ§WJ&%t4
2 LZL.2 2 &%Q_£&4.JQJ!il4&.QL4 St&Z&.Zlt. LL
n % 0.015
s
% 0.0002
;
D% 4.00 m CB
112.33 mts.
TB
116.33 mts.
WSEL% 115.30 (LS) WS EL% 115.03 (em.) 2. DESIGN OF INTAKE STRUCTURES: A. Design Data:
Diversion Dam Pond level % 115.80 mts. Crest level % 116.00 mts. All. Elev. (Dam silted) % 119.48 mts. Sluiceway U/S apron Elev. % 113.00 mts. Sluiceway crest elev. % 114.00 mts. Canal Discharge % 40.998 (LS) 33.970 (em) %
Silt factor % 4.00 Exit Gradient % 1/5 1.15 (33.97)
Additional 15% for the discharge (em) of MC# 1 is needed for the silt ejector ems. Since it is smaller than QLS• LS governs.
39.07
B. SILL LEVEL OF INTAKE
It should be as much as possible above sluiceway crest. Consistent with economy (after carrying out trials). On this consideration sill level is fixed 0.45 m. (1.476 ft.) above sluiceway crest, i.e., at elev. 114.00 + 0.45 % 114.45 mts.
EL. 116.00 EL. 115.80
l
EL. 115.30 (LS) EL 115.03 (CM) - - - _EL
~
~4.451
EL. 113.00
ASSUMED VALUE
+
301
2
LKL&Ad&M4¥
L£&
C. WATERWAY Using Elev. 116.00, i.e., W.L. at crest level hs
~
115.30 - 114.45
~
0.85
he
~
116.00 - 114.45
~
1.55
hslhc
~
0.85 11.55
C, 1 C
~
0.937 -
c,
~
0.9226 (1.706)
Q
~
CLH3i2
L
~
~
0.548
0.048 (0.0 3) ~ 0.9226 0.10 ~
1.57
41 1.57 (1.55)3t2
13.53 mts.
Using Elev. 115.80 mts (pond level) hs
~
115.. 30 - 114.45
~
0.85
he
~
115.80 - 114.45
~
1.35
hslhc
~
0.85 11.35
C5 I C
c,
~ 0.907 ~
~
0.63
~:~~ (0.051) ~ 0.8917
1.706 (0.8917)
~
1.52
41 1.52 (1.35)312 Use LT
~
17.20 mts.
17.20 mts. Provide 4 - 4.30 mts with 3 - 1.00 mt. pier width total width
~
17.2 + 3 x 1
~
20.2 M
302
K4K. 2 .
g_ H.£%Mt&.G.J&0J .iLL
e
I •
A
•
2
&&
.£.
RATING CURVE OF MC #1
Velocity (V) ~ -1.486 x R2!3 X Sll2 n-
' 1
j
~
I I
WATER WATER LEVEL DEPTH
112.33 112.53 112.73 112.93 113.13 113.33 113.53 113.73 113.93 114.13 114.33 114.53 114.73 114.93 115.13 115.30
0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 2.97
AREA A
PERIMETER p
~
s
~
R
0.186 0.351 0.500 0.640 0.766 0.888 1.005 1.117 1.226 1.332 1.436 1.537 1.636 1.734 1.817
6.12 6.84 7.56 8.28 9.01 9.73 10.45 11.17 11.89 12.61 13.33 14.05 14.78 15.50 16.11
1.14 2.40 3.78 5.28 6.90 8.64 10.50 12.48 14.58 . 16.80 19.14 21.60 24.18 26.88 29.27
where n
.015 .0002
v~
R213
0.326 0.498 0.680 0.743 0.837 0.924 1.003 1.077 1.145 1.211 1.273 1.332 1.388 1.443 1.489
.943R 213
0.307 0.470 0.594 0.701 0.789 0.871 0.946 1.016 1.080 1.142 1.200 1.256 1.309 1.361 1.404
Q~AV
0.350 1.128 2.245 3.701 5.444 7.525 9.933 12.680 15.746 19.186 22.968 27.130 31.652 36.584 41.095
D. DETERMINATION OF BASIN LEVEL & LENGTH
This is checked for three conditions: a. Pond at crest level 116.00
and only onegate is partially opened say 0.10 MT Area of length
~
~
0.10 X 4.30
0.43 M.
Depth of water up to MD-Height of opening ~ 116.00 - 114.45 -
h
~
0.1°/2 ~ 1.50 mt. (0.10 being the total opening)
1.50 M or 4.92 ft.
Velocity~ cd j2 gh ; cd ~ 0.60
V
~
0.60 2(32.20) (4.92)
Q
~
AV
~
0.43 (3.26)
~
~
10.68 FPS
~
326 mps
1.40 ems.
303
-::::0::::
-= - -=-
-:ZSS.G
!ZL
22
•.· \.·.1·.' 'i-
From rating curve:
WL
Q
1.128 1.117
J
J
.272 112.73
y ]
X
1.40 [
.20
112.93
2.245
112.73 + 0.272 (0.20) 1.117
X
112.78 m.
X
.i,'!!il
?~
l rEL.116.00 rEL.112.78
r
D
EL.114.45
~~-1
Neglecting velocity head which is very small: ~
116.00 - 112.78 ~ 3.22 1.40/4.30 ~ 0.326 cms/m. 120 q ~ 1.20 (0.326) ~ 0.39 cms/m.
F q
~
De
~ 3po.39J2 ~ o.249 9.80
FiDe
~
3.22/0.249
~
12.93 ~
Refer Fig. 7.15 taking F HLiDe 12.90
.10
[
12.93
HL
•
Dz/ Dl
J
'""] 'J
.03
Xj
13.00
0.176
xz
.09
18.67
0.176
18.58 + 0.03 (0.09) 0.10
18.61 m.
D1/De ~ 0.176 D1
0.176 (0.249)
D2 Vl
18.61 (0.04) q/D 1
~
~ ~
0.04 0.7 4
0.39/0.04
~
9.75 M/sec.
304
J
EL.112.33
Froude No.
9.75
V1
~
J9.80
jqDl
X
15.57
0.04
As from fig. 7·16 A (For Basin II) LB/D2 ~ 3.90 (0.74) ~ 2.89 M. 112.04 mts.
Basin Level ~ 112.78 - 0.74 b) Water at crest level
and all gates opened UIS WE ~ 116.00 M. DIS WE ~ 115.50 M. hs ~ 115.50 - 114.45 ~ 1.05 he ~ 116.00 - 114.45 ~ 1.55 hslhc
1.0511.55 ~ 0.677
e81e
0.907 - 0.077 (0.051) 0.10
e8 q ~
~
0.86773
1.706 ro.86773l ~ 1.48
e5 HcL 50
~ (1.48) (1.55)1. 50
2.86 cmslm.
1.20 q ~ 1.20 (2.86) ~ 3.43 cmslm. (3.43)2 ~ 1.06 M. 9.80
De
116.00 - 115.00 ~ 0.50 ~
0.5011.06
0.4 72
D1 IDe
D2 I D1 0.400
J
.072
.10
[
3.09
0.472
0.541
'
~6
x2
Xj
0.516
3.35
0.50
D /Dc ~ 0.541 1
D1
J
~
0.523 (1.06)
0.072 (0.025) 0.10 ~
0.523
0.55
D 2/D 1 ~ 3.09 + 0.072 (0.26) 0.10 ~
3.2772
D2
3.2772 (0.55)
v1
q/Dl ~ 3.4310.55 ~ 6.24
1.80
(6.24)2 2(9.80)
1.99
305
J
.025
l
v2
~
1.91
~ y22 ~ (1.91J2 ~ 0.19
hv 2
2(9.80)
2g ~
Basin Level
113.51 M.
115.50 - 1.80 - 0.19 V1
Froude No.
jgDl 6.24 )9.80 (0.55) 2.69
~
3.80
LBiD 2 LB
~
6.84 M.
3.80 (1.80)
c) During maximum flood
Gates opened at a height required to flow the design discharge ~
Max. Affl. Elev.
119.48 M.
Intake Design Discharge
~
41 ems
'
EL. 119.48
W.E.
EL. 114.45 EL. 113.00
In partial gate opening, discharge can be calculated with the help of Submerged orifice formula.
Cd A /2gJl 119.48 - 115.30
Q
h
~
If x is the opening & Q
40.998 X
0.6
X
~
17.2
~
4.18 m.
40.998
J2
g
X
40.998 0.6 X 17.2 /19.62 40.998 93.458
~
6.18 X
9.18
0.439 m.
306
So gate opening
~
0.439 m. 40.998 17.2 X 0.439
Velocity through the opening
5.43 misec. 0.5
Loss of head at the entry
vz 2g
0.75 m. 119.84 m. T .E.L. just uis of gate 119.84 - 0.75 ~ 119.09 T.E.L. dis of gate Downstream Wl.L. ~ 115.30 Head Loss HL ~ 119.09 - 115.30 ~ 3.79 q ~ 40.998 ~ 2.38 17.2 HL ~ 3.79 q ~ Q!L ~ 40.998i17 .20 ~ 2.38 cmsim. 1.20 q ~ 1.20 (2.38) ~ 2.86 cmsim. D
~ 3F2.83'r' ~ 0.94
c
9.80
HLiDc ~ 3.79i0.94 ~ 4.03 Dz/Dl ~ 8.4 D1!Dc ~ 0.279 D1 0.279 (0.94) ~ 0.26 D2 8.4 (0.26) ~ 2.20 q/Dl 2.86i0 .26 11.0 q/D2 ~ 2.83i2.20 1.3 Froude No.
mo11.0w.26l
LBiDz ~ 3.4 LB ~ 3.4 x 2.2
~
~
6.89
7.4 m.
Basin Elev. 119.09 - 0.26 -
(11)2
2g
119.09 - 0.26 - 6.173 Basin Elev. 115.30 - 2.2 -
~
112.66 m.
1.32 ~ 113.0 m. 2g
E. DEPTH OF CUT OFF
Determination of uis and dis cut-off depth: q
~
1.20 (4l.Oi17.20)
2.86 cmsim.
R ~ 1.35 (q 2!f)113
1.71 m.
307
Bottom level UiS cut-off
~
116.0 - 1.71
114.29 m.
For DiS Portion: q ~ 1.20 (4l.Oi8) ~ 6.15 cmsim. R ~ 1.35 (6.152 i4)1i3 ~ 2.85 m. DiS cut off ~ 1.25 R ~ 1.25 x 2.85 "" 3.56 Bottom level 115.30 - 3.56 111.74 DiS d ~ 112.33 - 111.7 4
~
0.59 m.
As the UiS cut-off of intake is located adjacent to uis cut-off of sluiceway the bottom level should not be above the bottom level of sluiceway cut off i.e. it should be up to EL. 111.00 ~ 113.00 - 111.000 ~ 2.0 m. upstream depth of cut off.
F. TOTAL LENGTH OF FLOOR:
Maximum static head will act during design flood and when canal is dry. High flood level with Dam silted ~ 119.48 Canal bed level ~ 112.33 Maximum static head, H ~ 7.15 Exit gradient GE ~ 1i5 Using d ~ 4.00 m. 1 - GE 1fji\-
X
..!!_ ~
l.
H
5
X
~ 7.15
~ 7.85 (with exit gradient as 0.202) ot:. ~ J(2A- 1)2 - 1 ~ )(2(7.85)- 12- 1
j\
b/d b
~
oC
~
14.67 (4.0)
~
Adopt length of floor Basin elevation
14.67
58.70 m. 78.70 m. 112.00 m.
308
rEL.112.33
EL. 113.0:
'-----------~E~L~.1~1~2~.00~------=---__/
~ ~11.0
-r
1
rEL. 108.33
3:1
I~ '"
8.70
I
I
7.35
l
8.40
18.30
1
1
4.15 1
7.80
58.70
G. ANALYSIS FOR PERCENTAGE OF PRESSURE
l r"'"'
EL. 114.45 EL. 113.0
~\ I~ 4.0
I .
'I_________________________E=:L.::.·_::1.::.12_.o_::o___________
~
111.0 8.70
7.35
38.65 58.70
I .
309
EL.108.33
First Pile Line: depth of downstream cut-off 112.33 - 108.33 ~ 4.00 m. length of floor ~ 58.70 113.00 - 111.00 ~ 2.00 m.
D d b d1 1 -
dl ~ _bQ__ ~ 0.034 58.70
""' - b
From Fig. 5.10
l
l l
GlD 11% 0E ~ 17% i)JD
1
GlC 1
~
100 - 11
89%
~
100 - 17
83%
i) Correction for thickness
C ~ 113.00 - 112.00 (89 - 83) ~ 3.0% additive 2 ii) Correction for interference of 2nd pile with the first pile.
c ~
[~]
19Flr
Where:
I
112.00 - 108.33 112.00 - 111.00 b ~ 58.70 m.
I c
~ 19
3.67 m. 1.00 m.
j58.70 3.67 [3.67+1.0] ~ 58.70
Final value for 0c
1
~
83+0.38 +3
0.38% ( +) ~
86.38%
Second Pile Line d b
~
1 ~ o<:
112.33 - 108.33 58.70
_i_
~
b
4.00 58.70
~
4.00 m.
0.068
From Fig. 5.10
I
0E2 ~ i)JD
25%
GlD2 ~ i)JD
17%
iii) Correction for thickness
c
~ _1_ (25- 17)
4.0
2% (-)
310
&&Lb
Correction for interference of pile number 1 on pile number 2
~
C
19Jf.
[D~d]
where:
D d b
111.33 - 111.00 111.33 - 108.33 b ~ 58.70 m.
c
19
0.33 m. 3.00 m.
j 58.70 0.33 [0.33+3.0] ~ 58.70
0.08% (-)
I·
iv) Correction for slope: For sS
3:1 4.5% 58.70 m.
c L1
c
~
"'"'%~
4.50 (7.35) ~ 0.56% ( +) 58.70
Corrected (j)E
2
~
·I
58.70
23.48%
23.48%
25 - 0.08 - 2 + 0.56
H. FLOOR THICKNESS i) During maximum flood
EL. 119.48
I
EL. 115.28
EL. 114.45 EL. 113.0
-:. J
EL. 112.0
//1.(<,.){11/0\'
' 12.70
I I
4.35 I
8.4o
4.15
18.30
I
3.0
I
I
c
b
a
L-
38.65
58.70 h
119.48 - 115.28
311
4.2 m.
7.80
Points
a b c d
:II }!
Dist. from U/S end
28.45 46.75 50.90 58.70
Ofo
pressure
55.89 36.28 31.84 23.48
Effective Uplift Head 2.3 1.50 1.33 0.98
Thickness
1.77 1.15 1.02 0.76
d,'f!i
The depth of floor can be checked for other flow conditions as in the case of dam.
X---- X---·-- X - - - - - X - - - - - X - - - - X
NOTES:
1. Design details of the work are given in the appended Plates No.1 to 10 .
·•••• }
•••
•.
2. The design of Mag·Asawang Tubig Diversion Dam was modified on the availability of more field data such as grain size analysis of the bed material, latest survey, etc. and the appended drawings also accordingly modified.
~u~.·
•.··..
312
..~~-.---~"0:-::·~-~~~-::~----: ·. -~~
-
..
"7"
~./.--_:
....
--------
........
PLATE
. 0 +000)
"'o
<
us
\
L
"'"'
\
\I
\\
\ I
I I
-
I
'
l .
\
\
I
\
/
\ \
'\
'
S.!io-... _ _ .
. 2 000
SCALE
I
\ I
"E I'L ;.,, OF 0/S Gli!Q(
./
/
/
/
/
//:/
I
f
.,II'.
I
0• t.J
I
:-
' I
.,
I
I
I I
I
.."" /
I
I I
I I
/ I
__..// /'
/
/
/ I I
'
MAG·ASAWANG TUBIG RIVERlliRIGATON ffiDJE
I
DIVERSION GENERAL
I
'DE: SIGfo!EO:
DEVELOPMENT SCALE
M-- -- -------
ORAIIr'H: ~:.M...;~'!.!!'!,.._t;!'!..--
'
PLAN
WORKS
LAYOUTPLAN c~o: ---------"' EVJilW£0: ._- - - - - - - SU Mlln£0:.--- - - - - - -
RECaM!ii£NOEO
l. 2,000
APPROVED
AsST:- A0NjNI3nUT0RDW'PlO ___ _
A0WINI
SH(fT__OF.--
A
e.J,G[.f't<.J, _..
PLATE
2
NOTES: I. A.LL LEVELS A.f£ IN WETER S AND OIWENSIOHS IN
CENTIMETERS.
2..LINQTH AHD HEIGHT Of THE: TRAIMIMI Wii..LLI1 TH! ~f-T1J(I AHGU: 0' TH! M'CTAKE 111UJCTUUS A MD TH!: t..EHQ!f'~ AHD LAYOOT ~ THE QUID It BAHKI ARE TO BE T£8iED ott TtC NODI:L, I. NOD£L TEST IHCOLO H CARRIED OUT TO CN:ItOC THE WIDTH l'f'OVIMD 'LOW COMOITlOIII AND 'INIEitiY 1)4.1:$-tPATION IYITIM.
IMPORTANT NOTE ! WITH
THt: AVAILAIIUTY 0, WO,.. fiELO DATA
IIZE ANALYStS Of THE fOlJNOA.TIOH WATERIAL, ETC) AHD hiOM:L STUDIES, LEHGTH Of' -..T£RWAY
(GRAIN EL.II!.CO
CONNECTED 'WORKS
..
20
10
30 I
I ; 400 WTS.
SC,t,LE:
EL.112 .DO
EL.Il2.80
U1"11et.IC C1' ntl P"HU.Ir>t"UIU
NATIOfriAL IRftlGATION
ADMINSTR'AT10N
FHILIA"NE MEDIUM SCALE IRRIGATICN PR:lJE I"JtO.,..al: Clf c.aTAL WINDOtte.
~ASAWANG TUIMG RIVER IRRIGATION PROJ..ECT
DIVERSION GENERAL ~IKIA;t);_
GENERAL
PLAN
SCALE :
1:400
________
WMP DUWIII' ; _ _ _ _ _ _ _
WORKS PLAN -----RIIV1rrlfta.:_ _ _ _ _ _ - ·
c::K>:~:
IU!i:llll"TU,;_ _ _ _ _ _
ltECOIEIWEND£0
w.vco.C»iiULiift'
;;JS.JTc'f ~ ~
-··
......-.:
PLATE
1----
le
I
1 . ~.1 . ~•.80C:O.OC!It:TI: LOCK III 000 I'll ) 0\lt:R CW. QRADt:D fiLT£11
(IOOOPSI) 'til 7 ~ Clll. GAP IN IETWE[N I F.ILL ED WI . . A\IEL ) O H .t>O Clol. THI(. QJIAOEO REVt:IISE
I---__, )
l ty,.__--i ~---i
ltll70 fiLTEII
•o
s....£.CTION
11
SCALE : 120 117.'5
NOTES.
11
A •· A r:roo
1. ALL L;.VELS A RE IN t.IETERS ll Dl t.IENSION
I N CMS .
2 . "EFER ' RELEVANT SHEETS SHOWIN G L AY OUT PLAN
311.5
AN D SECT I 0 N S .
3 . Dt S GLA CIS
... OF IRIDGE
DETAIL "Aw ~ALl:
I : 40 MTS.
...
AGA I NST
~.
--==
-
APRON ARE T O BE PI\OTEC TED WIT
ABRAS IO N.
4. ENERGY DISSI~ATION . ARRI.ItG£M ENT CHECKED
El. . ll7.fS3(R!lJtooESSED ) 01 00
a
EXTRA HI\RDENING MATER IALS TO GUARD
TO B E
ON MODEL ST U DI ES
D E SI GN OF BR I DGE
I
I S HO T GIVEN W 1 kEPORT .
0
IZ3
ltCALir IIJI-
4
'!...Ill
: 100 -------~ I
- 1000 l'tl Coti~ITC '
Cot!CIIITI
III'\IIUC Of'Tl11! I'HILIPI'IIIU
NATIONAL IARIGATION
i
·~-+--
....
I",........'" Ill
uo
A OM ... I STRATI ON
PHILFP1NE MEDIUM SCALE I ~ I GAT!~ PROJI' 1'910W.C:C 07 ClttiiiTAL MII OOtiO
MAG-~NG Tuete ~A llilAI ••UlOfl PR OJEC
DIT ~L • A •
DIVERSIO N
10 0
~ -----+-----~eoo ~~---+------=to~o ~----+-
WORKS
-•no - - - ---- ---DllMIC 1!1,.1!_1.!'~--- ·~ ~~~ R ~~~~=-~~----
1000 I'll
s
E
c
T 1o N
S CALI : RII'IIA~
-
"s- a"
1: 100
PLATE
4
AXIS OF SUJICEWAY
NOTE: ALL il\MENSIONS
AHO LEVELS IH
METRES,
CONCRETE ~ l~m.xllSOt.'\. x Q.9Qf1111(10::0 pal) PLACED AS SHOWN w/ 7.ei0 C<''l. G» IN BET\!IEEH {FILLED w/ GR..eVEI...) C*l 60 csn. -~HK. GRACED
DETAIL "B" SCALE
ARE IH CENTIMETRES
1:2!5 M.
REVERSED ALTER.
ICOCN:. THK.
II I.
75CM.VRti.E) ALTER.-~--·-'7
i .
~~ J___g45~~tJ=~±~:~4=~--~-~3~ -~=~~m =:=·~~-·~·~-~~-le20Jii-:~---DE~~!:'~~"'
I I
.
\
l-<13~_::!?0_
___
SECTION "C-C" SCALE :
I :75
WITS.
WATER STOP
.
DETAIL
SCA!...£
"A"
•'
.·:: ~-- .
I
._::1 ' -. -,
t:z!5 MTS
FILTER
L50xi.!Ox.90CONCRETE SLOO<S
r--- OO::Opc!)W/ncm.ALLER ON
:~-
112.00
I
60 om.'I'HK.
A
GRADE
I
WATER~
DETAIL OF GRADED FILTER
220
DET. OF JOINT Ar TOE
Rfi"U3UC
NATIONAL
DETAIL OF DENTATED END SILL (Sluiceway)
Of THE
IRRIGATION
I
S
ADMINISTRATION
PHILIPPINE MEOIUM SCALE IRRIGATION PROJECT P'ft0VINC£0f'<JHENT4L NlMDORO
MAG-ASAW~G 11JBtG RIVER
IRRIGATION
OlVER SlON WORKS OGEE SECTION
8 APPURTENANT OCTAILS
I
·--~
PLATE 38!.W
3100
12~0
1830
1270
OF BRIDGE
1170
100
118.00
SOKG. BOJLDER THICK
30 CM. !5000 PSI CONCRETE
1~0
CM.
1.!50x 1.50x 0.90 BLOCKS WITH 7.0 CM GAP IN
1.50 xl.50x 0.90
CONCf~ETE
CONCRETE BLOCKS OVER
1 BET'WEEN ON A SO CM GRADED FILTER
SQ CM. THICK GRAD EO
FILTER 290
300
300
$00
949
13!50
SECTION "D- 0" SCALE
100
NOTES: l. ALL LEVELS ARE IN METER AND Dltr.IENSIONS IN CENTIMETER. 2. REFER RELEVANT SHEET FOR GENERAL LAYOUT PLAN AND SECTION.
3. DIS GLACIS AND APRON ARE TO BE PROTECTED WITH EXTRA HARDENING MATERIALS TO GUARD AGAINST ABRASION .
..
OF BR!DGE
1-------'•=oo_ _ ~t----~ 600
rrt~Tl
·
1
REPU8LIC
01" THE
PHlL!PPlHES
NATIONAL IRRIGATION
ADMINISTRATION
PHILIPPINE MEDIUM SCALE IRRIGATION PROJECT PROVIHCI!:OI'"OII:II!:ltTAL MlNOOII:O
MAG-ASAWANG TUBIG RIVER IRRIGATION PROJECT
DIVERSION RIGHT
a
WORKS
LEFT SLUICEWAY SHOWING TRAINING WALL
DUI$MI!:O : _. __ • ___ _
CHECK£0: - - - - - - - - - - ·
:~~:~~: :. ·_-:: :: :: :
60 KGS. BOULDER
:.: :
RECOMMENDED
,.
L~Oxi.~Ox0.90
CONCRETE BLOCKS WITH 7.0 ChLOAP IN BETWEEN ON A 60 CM. GRADED FILLER
SECTION "E -E" SCALE
1:100
WA...Co!cOirciuL.tAHT - ~Ci' ~" -ote,iit:iteKa-al'ici. iiidf. APPROVED - As"i-iiiwiiui.,..-MDWt..litO.-- _
'- ic*iiuiTitAToftaMttT-01'"_
f'MI(MO. - - -
PLATE
NOTE;
l REfER GATE DRAWINGS FOR DETAILS OF BLOCK OUT FOR GATE GROOVE AND STOP LOG GROOVE.
I
I
r-oFUE.:Ol£0
73S
sEc Tl oN • G scALE
1
125
G
1830
I
omn~~"::oo ~~:tn~~~
I
-&
:
~
j
I
~.-._,_r~ -;:::-·c:::··~·=··:;:::.-i~=:-~ L-;: ~-:"-~: -;: -~=~- -:<: ::
·~-
' " :I "'
0
Wldddu11;i
EL 1&1 00
•.
,. .
I
'I
'
:#
.
r;
i
-Lt
. --;:::-:::::··:::::"=,·= .. =···=-r·~-=--.-=·-=--=·:::=:-::2··::'1· ·:/ ··
.'7 -'.
;::::;.-.:z,·=--·=···:o:··
1
i
.
·.
•
.
.,
:;
::
11iill 11
! -----t-
0:o, ~·
----1
l----~, _. , '
I '. -+. .
I
EL .!16.33
8
r,
"'
II
! I
~~~uu~~~~mu~!_lw~wW~~n~rr~~~~---
-,-
i EL.II2.00
~ EL.112. 0
El..112.
Ill
Th'
SECTION
" GJ-
I',
111111
I
I
GJ"
I
I
I
'
i
I~
REf>(JBLIC
OF'
THE
NATIONAL IRRIGATION
PHILII'f>INES
AOMINIS':'"RAilON
fHILIPPINE MEDIUM SCALE IRRIGATION PROJECT
I I
PROVINCE OF ORIENTAL
,-1-
MINDORO
MAG·ASAWANG TUBIG RIVER IRRIGATION PROJECT
DIVER Sl 0 N W 0 R K S EL.IIG 33
~-=- RIGHT I.-rAKE ;}[SIGf./£0
"'"" "
-----'--------------~···-·
. ·--------
·_,_LUJ
STR'""'' >< CHECKED.------
i~:~;~~·
---
--- ~ - - -·
~
~~~-~~-~--~~-------~---
PLATE 'I
=·=~·=";±fl,,
r;:,,,;;::;::.
7
=9=
....
!
'
•
'
IJliL I
1------------------SECTIO~N SCALI: :
'F,)-
l: lOO
I
430
I o4:5~ '
F
IIITt.
I
I I --f r---~~
-~
II
T_l.. ' .
'
:' I
12!.00
\
\ I
\
-- -!' ~
'-1 2'
~
I
•
I
~~
~ \
I
J
I
.
eJ:l
I SECTIQN'~-F~·
. -'..
·.-:,:_:·.'
'. ~·
~·
<:',
',, ' '
I
I,!! I,
EL.Il3.Be
EL.!l4.4~-
/
/ //
/
ftl:f'UIIUC
ott
THE PHILIPPIP'I£!1
N~AL IRRIGATION
ADMINISTRATION
PHIUPF'I'JE MEDIUM S:::ALE IRRIGAllO'< PROJECT P'ltCMNCE r::JF OltiEIHAL WliiOOfi.O
MAO"'J\SA~NO TUBIG RIVER IRRIOAT ION
PROJECT
DIVIRSION Vltm&C!i
L EFT INTJJ(! I TRU;;C~T"U~·~·=-----J DESIGNi-t).:... _ _ _ _ _ CHEc o:_ ______ _ .,
0 0 fV"''fl
N" . NWI" FEVlEWED.:., _ _ _ _ ___ :.__ - - - SUBWITTE-9'----...:...
RECOM!.iEH
PLAN SCALE :
CJF
INTAKE #2 1: I 00
MTS.
-Aiir-:-A'DwiiiaTJtAi"OR
Aoliiiili'fttAToR
4~----------------------------------------==~----------------------------------------~~---L--------------------·-------------------------------------------------------------·------------t:~~~~D~W~·~·~O~-~~~-~~T==~~i·A2E~~:J
PLATE
OF BRIDGE
EL.l
EL.Il9.40
EL.
ne.oo
"" ",, "" ""
soo
000
SECTION SCALE:
ii[]
~
I •
:
il'' li jl .' .
•I
..
( ( (
I IHJJ .,
_
..
,..,___,
IiI
I':
1:1'0
I
~
"•
I
'2
' ,,'
;
I
I
I '
:'
'
I
II
II
II
II
II
II
I
)))
,7
7:'
i
I
I :
1:
~.
II
I
I
00"
II
I
0
~~
~3
I
D
'IIIIi
I
~~
: :~
!
11
0
I'.
i
Xo - X a
D D
:Jillllii
!
11
I
,... ...
.
( I
!, ALL LEVELS ARE IN METERS AND Olp.,jENS!ONS IN CM.
H ,,
EL.
I
NOTES:
ltf:MIIl.IC 01" THE
NATIONAL
IRRIGATION
ADMINISTRATION
PHILIPPINE MEDIUM SCALE IRRIGATION PROJECT l"ftOVIMCI:
•
1"1-rll.II"I"IN[J
01" Olti!MTAL
MIMOOfiiO
WAG·ASAWANG TUBlG RIVER IRRIGATION PROJECT
DIVERSION WORK POSITION OF PRESSURE PIPES
I
I
t>U'ISfot[t>: • • • • • • • • • • • pu-.·tt:_i!A_L..~~ •., . •••
IN
OGEE
CHt:CICt:P: - - - - - - - - - · - · ltf:VIf:WI:O: - - ___ • - - · · - · .UOYITTI:D __ • - - - - • ., ••
RECOMMENDED
PLAN OF OGEE SHOWING POSITION OF PRESSURE PIPES SCALE:
irA.OcOic~fANT ;;.o.ii:.;T~ APPROVED
1:120
..; ..
:.
;.<,
•·
OtC.te**'•*""-PIPi"·
PLATE
9
GENERAL NOTES FOR OBSERVATION PIPES!
"'''' ''
8
,,
'''''"'
I 0
'
I""'~
~I 'I
""
::
9
I
----·
RIPRAP
'"""
-~------
I
EACH STANO PIPE SHALL BE FITTED AT THE TOP WITH SCREW CAPS.
2.
ALL CONNj::CTIONS SHALL BE LEAKPRooF ,'\NO ALL PIPELINES SHALL BE COATED WiTH ANT!-CORROSIIJ.E PAINTS.
3.
ALL VERTICAL PIPES SHALL BE DEAD VERTICAL WITHOUT ANY BEND CON_TR iCTION.
4.
WHERE MORE Tl-lAN ONE PRESSURE ·poiNT IS LAID AT SOME DEPTH, THESE 3HALL NOT BE Sf.>ACEO CLOSt:H. THAN 30 CM.
0.
FOR PRECAUTION AND !NSTAL"-ATION PROCEDURE REFER TO RELE'/ANT MANUAL:
'
.-~--~-----~-~-~
I
)1!0 II
•:
' -l
60 KG. BOULDER
ISO[
I.
NOTES: I.
...
ALL DIMENSIONS ARE IN CM. ELEVATIONS IN METERS.
AND
---~----'-----
1-·
r-
1---
I c.i~.:---~---
.-
---------------------------
-.;. ____:L __ -~,---,----,--.-----T--
---··
"-~--~-_J_----1
-.--,-----,-I
;
•
- . ::.-
"""=-~-=---
I PLAN OF SLUICEWAY ~HOWING POSITION OF PRESSURE PIPES (LEFT SIDE) : 120
SCALE
I
r
...•, ,,"
,, ",,
,,,,,,
~
EL.11'!63(REToRGGRESSED) -~-
,,
"•',, .,'
= = ~~· = =
=
'•' ' ::
"
=
l:-
I' I'
,'
REP1.J8LIC OF THE Pl-l!liPI>IH[S NATIONAL IRRIGATION ADMINISTR:ATION
PHILJPPI'JE MEDIUM SCALE IRRIGATION PROJECT PROVINCE'OFQ~li(IH.AL
~INOORO
MAG-ASAWANG TUB!G RIVER IRRIGATION PROJECT
DIVERSION WORKS
I
~&{__
I
eq
330
360
4"0
\
- '--------+
360
I
I
300-L
300
F
i
300
GRADED FIL
600
+
500 500 --r-----.:c=___-----+ ____c:_::.::_______
PRESSURE OSSERVATIOt.l PIPES -LEFT SU;iCEWAY o€SI.wtrD: - - _- .• - - - --
500 !}OO 350 /60j_ ----'-'-.:______-+--_.::::;_:____1~::::_--.+::1,
DltAII .. :
SCALE:
~·v
Ccit..i'~A.T.\HT
l>!i~icT
_ vu 100
: - - - - JtEVIFWrD: ___ .••
:JU.WITTl't>'. ___ ·
--qECOfiYOIOI!D
WAf..CoS
SECTION
ICHI!!~Xf'D
_f!A~~~- ·- ____
W.-HMitPI-0-1¢: ofi_,-~ ~-Sioie-i: OUT.
APPROVED
ASsT. AOi.llHliTitAToii
"'*lfiliTftATO!i-
--------------------------------------------------------------------------------------~----------------------------------------------------------------------------------------------------------------------------------------r=========~o~w~o-~. -~-o~~ _~at~ __ ~
PLATE
10
.r DAN. AXIS ~--·v-oCM .
!SOCO PSI COS:: RETE WEARINJ SURFACE
1 40KGS. BOLLCER Rlf'RAP ' 60CM. THK ON A 30 CM
GRAVEL
_____ 15_00.
-~ _ -··---+---~--
y -Y
"
--~~--.
s
E
cT I0 N
SCALE:
I :715
II
-----
BED
~5.00________ j__3Zu____j_d_ed___ ~--~~-~--~-~~---· __ 4__~_0____ ----~~G'L __ --~4.00~.---'
~=~-=----~
-
.
--~:~'-!
~--------~--2nd ~-~----
STit.GE
POLYTHENE
CONCRETE PA-PER
STAGE CONCRETE
BBER
DETAIL OF TRANSVERSE
JOINT
SElL
DE"IAIL OF CONSTRUCTION JOINT
.----------jI IIEI"!.JtUC 01" THt f"""CUI",.!J<[t:
NA.riONAL IRRIG.t...TION
ADidiNISTRATION
_
~ !U PP1N;R~~~, ~:~L ~~~~~!ON ~-~~~1 IM¥l-ASM'AHG TUB10 RIVER lRRK3-ATlON Pf~O.t:CT
~
DIVERSION
WORKS
~~ er.~1 ~A~~~-~--=~-~--~~-~~:.1S_r~-~~~~L~~{:~ J~?J~!~r.::. rtr:VIr.\r-.n . . . . . . . . . __ .
p.:_":_~_,_!__ rY~~':!.~ _~; :~~~~~J-~t;::;~ ~~~--:-·
t
~i;:.CO.
oo•iu_caiir
,t..~tSTJ:iitMIII~M~
~ttO.--
-~~"~'"""'"''"·.,;&,i,ts·.lili!!.£!l§ll!91i!!i!i!l!li@=£~£~i£!1.1£t!ll£11!![\!l_l!i£1!/!121!.1!1Z!!!.i!O!lt""""'-~v-""--,,,,~.,~-
...- ... . .
;
·i
'"}~~tv':~~~· ,o;r__i~< l) _ _ _ _ AOJ~H:iPTltATO!t lp;cc:L-OF--.ii"AH W0. ;..,,.-.~
~;
..
_
I
.
........
,.,.,..~,...,..~
'
Copyright 1987 ,(Under Process) by the NATIONAL IRRIGATION ADMINISTRATION
No part of this manual may be reproducer] by any mec11anical, photograph 'c. or electronic process, or in the form of a phonographic or tape rec•>l'rl.n<>:, n · r '"'11'' i: be restored in a reh·'eval sy3tem. t;·a:osmit.ted or othenu'se copied fer n:·: i·· ''l' p1·i\·ate use, Y.'i'!:ont VvTi~trn p;: --:· :: 7 --, f:·-··a1 the N:-~ :~~:;:.:1. h··-~.r:-~1tf--n 1\d --~ ·.= ~">
RIEPUBUC OiF THE PHIUPPUNES NATiONAl IRRIGATION ADMINiSTRATiON
...~
:.. >..::..._c:: ..
u\ce 1nanual which ern!Jv•
current design c ,-. design practices and procedures will· Nat!onal Iniga~ion Adminisirati,, .. adopted from intemationally acc0, .. teria and modified to suit the Agenc:/:' . C
•
•
uf
.-
Edited by the DESIGN HES!':ArtCH SECT•GN DESIGN AND SPECIFICATION DroP:·. NATIONAL l!UUGATION MJ:,! :.
in consult:tti0:1 "'.-·1 ,_ WATEI: & 1'0'"' (' .
T 'f
MANUAL ON DIVERSION DAMS CONTENTS
Description
Para No. Chapter 1 -
Page No.
INTRODUCTION
1.0
Introduction
1
1.1
Scope
1
Chapter 2 - PLANNING AND INVESTIGATION
2.0 2.1
General
Extent of Studies
2 2
2.2
Stages of Investigations
2
Reconnaissance Stage Preliminary Investigation
3 3
Study of Available Maps Regional and Site Geology Study of Foundation Strata Study of Available Run off and Flood Flow Data Water Studies Availability of Construction Material Communication to the Site of Work Finalisation of Alternative Sites
3 3 3 3
Detailed Investigations
4
2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4 2.2.2.5 2.2.2.6 2.2.2.7 2.2.2.8 2.2.3 2.2.3.1 2.2.3.2 2.2.3.3 2.2.3.4 2.2.3.5 2,2.3.6 2.2.3.7 2.2.3.8
Field Survey Hydrological Data Sediment Studies Surface and Sub-surface Investigation Diversion Requirements Construction Materials Communication System Other Miscellaneous Studies
4 4 4 4
4
5 5 6 7 7
8 8
Chapter 3 - ALIGNMENT AND LAYOUT
3.0
Function of Diversion Works
9
3.1
Location
9
3.1.1 3.1.2 3.1.3 3.2 3.2.1 to 3.2.6 3.2.7 3.3 3.4 -3.5 3.6
Location Versus River Stages Advantages Disadvantages
9 10 10
Layout
10
General Requirements Misc. Requirements
10 11
Alignment Layout of Training Wall
11 12
Guide Banks
12
River Diversion Scheme
13
iii
Para No.
Description
Page No.
Chapter 4 -TYPES OF DIVERSION WEIRS
4.0 4.1
15 15
Introduction Diversion Dams
4.Ll
Classification According to Crest Shape Vertical Drop Type Glacis Type Ogee Type
15 16 16 16
Vertical Drop versus Sloping Type Classification by Material
17 17
4.2
Barrages
17
4.3
Gated Weirs
17
4.1.1-1 4.1.1.2 4.1.1.3 4.1.2 4-1.3
Chapter 5 - DAMS ON PERVIOUS FOUNDATION
5.1 5.2 5.3
Introduction
21
Different Theories
Example
21 23 23 23 24
Lane's Weighted Creep Theory
26
Rationale Example
26 28
Potential Flow Theory
29 29
Methods Based on Creep Theory
5.3.1 5.3.2
Creep Theory Bligh Theory
5.3.2.1 5.3.3 5.3.3.1 5.3.3.2 5.4 5.4-2 5.5 5.6 5.7
Method of Solution Theory of Seepage Flow
5.7-1 5.7.2 5.7.3 5.7.4 5.7.5 5.8
Exit Gradient as Related to Dam Design
33 33
Khosla's Theory of Independent Variables Correction for Mutual Interference of Piles Correction for Floor Thickness Correction for the Slope of Floor Exit Gradient Example Illustrating Use of Khosla Curves
38 38 40 40 42 42
Structures on Permeable Foundation of Finite Depth
47
Chapter 6 - COMPONENTS OF DIVERSION WORKS
6.1 6.2 6.3 6.3.1 6.3.2
Layout of
Diver~ion
Works
48
Overflow Dam
48 48
Sluiceways
Functions Discharge Capacity
48 48
iv
Para No.
6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6 6.4.7 6.5
Description
Training Wall
52
Functions and Elements of Training Wall Hydraulic Requirements Role of Upstream Training Wall Width of Pocket and Model Studies Length of Training Wall Top Level of Training Wall Conclusions
Intake Work
52 52 52 52 54 54 54 55 55 55 55 55 57
Location and Layout Type of Structure Crest Level and Waterway
57 57 57
General Necessity of Breast Wall
57 60 60
Fish ladder
6.5.1 6.5.2 6.5.3 6.5.4 6.6 6.6.1 6.6.2 6.6.3 6.6.3.1 6.6.3.2 6.7
Page No.
Necessity Location Investigations and Data Design Requirements
River Training Works
Chapter 7 - HYDRAULIC DESIGN
7.1 7.2
Design Data
7.2.1 7.2.2 7.2.3 7.2.4 7.3 7.3.1 7.3.1.1 7.3.1.2 7.3.1.3 7.3.1.4
Field Data
61 61
Topographical Data Soil Foundation Data Hydrological Data Miscellaneous Data
61 61 62 62
Design Parameters - Terminology
62
Design Flood
63
General Status of Design Flood Determination of Design Flood Selection of Design Flood
63 63 63 66 66 66 68 68 68 70 70 70 72 75
7.3.2 7.3.3
Maximum Permissible Afflux Scour and Silt Factor
7.3.3.2 7.3.4 7.3.5 7.3.6 7.3.7 7.3.7.1 7.3.7.2 7.3.7.3 7.3.8
Extent of Scour Safe Exit Gradient Roughness Coefficient Retrogression of Levels Coefficient of Discharge
General Nappe·Shaped Crest Broad Crest Pond Level
77
v
Description
Para No.
Page No.
Hydraulic Design of Spillway and Sluiceway
77
Economic Design Waterway
Shape of Crests
77 80 82
Ogce Crest Profile
82
Crest and Floor Elevations of Spillway and Sluiceway
85
7.1.4.1
Spillway
85
7 .4.4 .2
Sluiceway
85
7 .4.5
Width of Under Sluiceway
7.4.6
Length and Level of Downstream F'loor
86 86
Energy Dissipators
86
General Requirements
87 87 87 89
7.4 7.4.1 7.4.2 7.4.3 7.4.3.1 7.4.4
7.5 7.5.1 7.5.2 7.5.3 7.5.4
Horizon tal Type Hydraulic Jump with Sloping Apron Design Criteria
7.5.4.1 7 .5.4.2 7 .5.4.3 7.5.5
7.5.6 7.5.7
7.5.8
Terminology Stilling Basin Parameters
89 89 89
Stilling Stilling Stilling Stilling
92 92 93 94
General
Basin Basin Basin Basin
Type Type Type Type
I II III IV
7.6
Total length of Floor
98
7.7
Depth of Cut-off
98
7.8
Floor Thickness
98
7.9
Protection Works
101
Upstream Block Protection
101 101
7.9.1 7.9.2 7.9.3 7.9.4
Downstream Block Protection
Design of Graded Filter Loose Stone Protection 7.9.4.1 7.9.4.2 7.9.4.3 7.9.4.4
7.9.5 7.10 7.10.1 7.10.2 7.10.3 7.11
103
103
General Hcquircmcnts
103
Velocity VS Size of Stone
103
Slope of Launched Apron Thickness and Length of Material
105
Protection Around Divide Wall
105
Free Board
105
105
Free Board in Abutment
105
Free Board in Stilling Basin
107
Free Board in Piers
107
Design Procedure in Brief Chapter 8 - STRUCTURAL DESIGN
107
A - Spillway Section
8.1
General
115 vi
Description
Para No.
Page No.
- - - - - - -----· ·--8.2
Requirement of Stability
8.3
Loads and Forces
115
Forces for Design Forces Inducing Stability and Instability Design Assumptions
8.3.1 8.3.2 8.3.3 8.4
Load Combinations
8.5
Design Criteria
8.5.1 8.5.2 8.5.3 8.5.4 8.5.4.1 8.5.4.2
Dead Load Live Load Pressure of Pond Water, Tail Water Earth and Silt Pressures General Criteria
8.5.5.1 8.5.5.2
General Effect of Earthquake
8.5.6.1 8.5.6.2
8.5.8.1 8.5.8.2
118
General Inertia Forces
120 120 123 123 123
Assumptions General Procedure Safety Criteria
8.6 8.6.1 8.6.2 8.6.3 8.6.4
Sliding Resistance Resistance Against Overturning Safety Against Foundation Failure Safety Against Failure of Material
8.7.1 8.7.2
123 123 126 127 128 128
Contraction Joint
8.7
118
118
Hydrodynamic Forces • H.caction of Foundation
8.5.7 8.5.8
117 117 117 117 117 117
118
Earthquake Forces
8.5.6
116
118 118
Uplift Pressure
8.5.5
115 115 116 116
128 128
Longitudinal Joint Transverse Joint B - Sluiceway
8.9.1 8.9.2
Cutoff Impervious Floor
129 129 129 129
8.9.2.1 8.9.2.2 8.9.3
General Requirements Design Cri tcria
129 130'
Abutments
130
General Top Width Design Criteria Safety Criteria Value of Friction
130 131 131 131 132
General
8.8 8.9
Design Criteria
8.9.3.1 8.9.3.2 8.9.3.3 8.9.3.4 8.9.3.5
vii
Para No.
Description
8.9.4 8.9.4.1 8.9.4.2 8.9.4.3 8.9.4.4 8.9.4.5 8.9.5 8.9.6 8.9.7 8.9.7.1 8.9.7.2 8.9.7.3 8.9.7.4 8.9.7.5 8.9.8 8.9.9 8.9.10 8.9.1::>.1 8.9.19.2 8.10 8.10.1 8.10.2 8.10.3 8.10.4 8.10.5
Page No.
Piers
132
General Thickness of Piers Length of Pier Height of Pier Design Criteria
132 132 132 132 133
Loading Combination Evaluation of Forces Effect of Earthquake on Earth Pressure
133 134 135
Active Pressure Without Surcharge Passive Pressure Without Surcharge Active Pressure Due to Uniform Surcharge Passive Pressure Due to Uniform Surcharge Effect of Earthquake in Sub soil Water
135 136 136 138 138
Hydrostatic Pressure Force Due to Water Current Longitudinal Forces in Road Bridge
139 139 139
Due to Braking Effect Miscellaneous Details
139 140 140
Longitudinal Joints Transverse Contraction Joints Weep Holes Backfill Material Backfill Drainage
140 140 140 140 141
Due to reactions
C - Divide Wall
8.11 8.11.1 8.11.2 8.12 8.12.1 8.12.2 8.12.3 8.12.4
General
142
Location Position, Length and Height
142 142
Design Criteria
142
Load Combination Evaluation of Loads and Forces Wave Pressure Foundation Design
142 142 144 144
Chapter 9 - INTAKE WORK
9.1
General
9.2
Layout
9.3
T ypas of Structure
9.4
Hydraulic Oasign
9.4.1 9.4.2
145 145 145 147
Pond Level Water way and Sill Level
147 147
viii
Para No.
Description
Page No.
Open Type Structure
147
Water Way Width and Shape of Sill Shape of Approaches and Other Component Parts
148 149
Upstream Transition Downstream Transition
150 150
9.5.4 9.5.4.1 9.5.4.2 9.5.4.3 9.5.4.4
Design Criteria from Surface, Flow Consideration Depth of Upstream Cut Off Stilling Basin Dimensions and Appurtenances Thickness of Floor VS Hydraulic Jump Downstream Protection
150 150 150 150 151
9.5.5
Design Criteria from Sub Surface Flow Consideration
151
Exit Gradient at the End
Length of Impervious Floor Floor Pressure
151 151 151
Structural Design Hoist Bridge Free Board
152 152 152
Barrel Type Structure
152
General Evaluation of Losses
152 152
Transition Losses Friction Losses
152 155 156
9.5 9.5.1 9.5.2 9.5.3 9.5.3.1 9.5.3.2
9.5.5.1 9.5.5.2 9.5.5.3 9.5.6 9.5.7
9.5.8 9.6
9.6.1 9.6.2 9.6.2.1 9.6.2.2 9.6.2.3 9.6.2.4 9.6.3 9.6.4
Bend Losses Loss in Change of Direction
150
156
Contraction and Expansion
Calculation of Waterway
159 159
9.7
Energy Dissipation
159
9.8
Trashracks
161
Chapter 10- SILT EXCLUDER
10.1
General
10.2
Location and Alignment
10.3
Design Criteria
10.3.1 10.3.2
Number and Size of Tunnels Spacing and Shape
10.3.3
10.3.4
Roof and Bed of Tunnels Exit and Exit Channel
10.3.5
Transition
164 164
164 164 166 166 166 166
10.4
Escapage Discharge and Minimum Head
10.5
Losses in Tunnels
10.6
Structural Design
10.7
Control Structures & Misc. Factors
10.7.1 10.7.2
166
167 167
Control Structure Miscellaneous Factors
168
168 168 ix
--------~·---·-·-----~------------~---~··--~~----------~---
Description
Para No.
168
10.8
Efficiency of Silt Excluder
10.9
Field Study Based on Model Experiments
Chapter 11
~RIVER
Page No.
--~~---~
--~--~------··-······-----------
168
TRAINING WORKS
11.1
General
173
11.2
Types of Training Works
173
11.3
Guidebanks
173
Classification Form in Plan Geometrical Shape Layout of Guidebanks Length of Guidcbanks
174 174 174
11.3.1 11.3.1.1 11.3.1.2 11.3.2 11.3.3
174 174
11.4
Approach Embankments
177
11.5
Afflux Embankments
177
11.6
Design of Guidebanks
177
Section
179
Top Width F'rec Board Side Slope & Protection
179
11.6.1 11.6.1.1 11.6.1.2 11.6.1.3
11.6.4.1 11.6.4.2 11.6.4.3 11.6.4.4
11.7.1 11.7.2 11.7 .3 11.7.4 11.7 .5 11.7.6
179 179 179
Thickness of Pitching Fillers Loose Stone Apron
11.6.2 11.6.3 11.6.4
11.7
179
181 181
Size of Stone Depth of Scour Thickness of Launched Apron Slope of Launched Apron
181 181
Design of Approach Embankments
182
181
182
Top Width Sedion Size of Stone Thickness of Pitching Filter Launching Apron
184 184 184 184
184
Design of Afflux Embankment
184
Top Width Slope and Protection VI orks
184
11.9
General Behaviour of Launching Aprons
184
11.10
Other Training Works
185
11.8 11.8.1 11.8.2
Chapter 12
~
184
INSTRUMENTATION
12.1
General
186
12.2
Objects of Instrumentation
186
X
Para No.
Page No.
Description
12.3
Instrumentation for Structures on Permeable Foundation
186
12.4
Uplift Pressure Pipes
186
Numbers and Location
187
Design of Pressure Pipes
187
Filter Points General Requirements
187 190
12.7
Installations
190
12.8
Precausions
190
12.9
Maintenance
190
12.10
Observations
191
General Record of Observation Time Lag Frequency of Observations
191 191 191 191
Presentation & Analysis of Data
192
12.5 12.5.1)
I 12.5.2) 12.6 12.6.1 12.6.2
12.10.1 12.10.2 12.10.3 12.10.4 12.11
Chapter 13- DAMS ON IMPERVIOUS FOUNDATIONS 13.1
General
194
13.2
Design Analysis
194
13.3
Safety Criteria
195
13.4
Internal or Uplift Pressu-re
195
Criteria for Design
195
Foundation
196
13.4.1 13.5 13.5.1 13.6 13.6.1 13.6.2 13.6.2.1 13.6.3 13.6.3.1 13.6.3.2 13.6.4
Structural Competency
196
Foundation Treatment
196
General Methods of Treatment
196 196
Surfn.ce Preparation
196
Procedure for Grouting
197
General Type of Grouting Drainage
197 197 197
13.7
Waterway - Crest Length
13.8
Stilling Basin - Floor Thickness
13.9
Protection Works
197 198 198
Chapter 14 - OPERATION & MAINTENANCE 14.1
General
199 XI
:,
·>--..,"'!\!"'!\!2=.--!!lif=:tm-r.;,..tll!@li:·Ol'\im"o"l®rn·=m=M=i::!limJR~~~~.gq:z:JJJJL _!£#!'&, CW
. mxt®&liiiliiiiliii£&9
o
Description
Para No.
Hydro - Mechanical Installation
14.2
Operation of Gates Maintenance
14.2.1 14.2.2 14.2.2.1 14.2.2.2 14.2.2.3 14.2.2.4 14.2.2.5
14.3 14.3.1 14.3.2 14.3.3 14.3.4 14.3.5 14.3.6 14.3.6.1 14.3.6.2 14.3.6.3 14.3.6.4 14.3.6.5 14.3.6.6
14.3.7
14.4.2.1
14.1.2.2 14.4.2.3
199 199 200
Gate Groove and Seals Steel Wire Ropes Winches and Hoists Lighting and Other Arrangement
200 200 200 201 201
Civil Work
201
Aprons Impervious Floors Sediment Excluder Intake Work Instrumentation Data Other Observations Pressure Release Pipes Standing Wave Profile Sediment Observation, Aggradation of River Bed Retrogression Settlement
202 202 202 202 202 203 203 203 203 203 203 204
Hiver Training Works
204
Regulation and Operation
204
General Requirements Operation - Periods
204 204
Pre Monsoon Operation Monsoon Operation Post Monsoon Operation
204 204
History of Diversion Works
205
Gates
14.4 14.4.1 14.4.2
Page No.
14.5
Chapter 15 -
205
DESIGN OF DIVERSION DAM
15.1
General
15.2
Illustrative Example
206 206
Mag-Asawang Tuhig Diversion Dam (Location) Geology Afflux & Crest Levels
206 206 206
Hydraulic Design
207
Design Data Determination of Waterway, afflux elevation, d/s basin elevation & dis floor length under different flow conditions
207
15.2.1 15.2.2 15.2.3 15.3 15.3.1
15.3.2
A.
Water way L Looseness factor 2. Width of sluiceway & ogee portion 3. Length - ogee portion
xii
207 207 207 213 214
Para No.
Description
B.
Determination of afflux Elevation, Downstream Basin Elevation and Downstream Floor Length I First Condition - High Flood condition, spillway not silted up
Length and Level of basin floor
II Second Condition - High Flood with dam silted up to crest level
III Third Condition - Medium flood Condition Water level on upstream is just 0.30 m. (one foot) above the ogee crest
IV Fourth Condition - Low flood condition Water flowing in the sluice way is just 1.00 m. above the sluiceway crest
E,
F.
:::;:p
244
Table 15.1 (Sluiceway Portion) Table 15.2 (Spillway Portion)
245
Depth of cut off
242
a. Spillway Portion i) Upstream cutoff ii) Downstream cutoff
242
b. Sluiceway Portion
246
242 246
I) Upstream cutoff li) Downstream cutoff
246 247
Length of work & exit gradient
247 247 248 249
Protection Works
xiii
j -
238
242 242
a. Spillway Portion 1. UiS block protection 2. UiS launching apron 3. DiS block protection 4. DiS launching apron
'Y(
230
241 241
Governing Values Sluiceway Portion Spillway Portion Crest Shape
a. Spill way Portion b. Sluiceway Portion G.
222
230 231
a. Spillway Portion b. Sluiceway Portion
D.
218
222 225
a. Spillway Portion b. Sluiceway Portion
15.3.3.4 15.3.3.5
214
218 220
a. Spillway Portion b. Sluiceway Portion
15.3.3
214
214 216
a. Spillway Portion b. Sluiceway Portion C.
Page No.
249 249 250 251 251
Description
Para No.
251
b. Sluiceway Portion
254 254 256 256 258
1. UIS block protection
2. UIS launching apron 3. DIS block protection 1. DIS launching apron H.
Training Wall
258 260
a. Hydraulic design b. Protection Works
15.4
261 261
Structural Design
I.
Pressure calculations a. Spillway portion i) U/S pile line ii) Intermediate pile line iii) Downstream pile line b. Sluicev·.ray portion
i) Upstream pile line ii) Downstream pile line
J.
266 267 268
1. Hydraulic gradient and jump profile
268 268
Table J -1 Table ,J- 2 Table J -· 3
268 271 272
273
3. Uplift due to hydraulic jump
268
Jump Profiles
274
Table J -4 Table J --5 Table J -6
275 276
274
277
1. Floor thickness
277
a. Spilhvay Portion b. Sluiceway Portion
L.
261 262 263 264 266
Floor thickness 2. Uplift pressure under static head Hydraulic gradient line elevations under different flow conditions
K.
Page No.
281
Stability Analysis
283
Load Combination A - Dam completed but no water in pond & no tail water
285
Load Combination C - Maximum flood with dam silted up to crest level
288
Load Combination E - Water level on upstream at crest level, normal dry weather tailwater, full uplift, silt deposit up to crest level with earthquake
290
Design of Guide Banks
295
a. Layout b. Protection Works
295 296 xiv
LIST OF FIGURES
Description
Figure
Page No.
Chapter 3- ALIGNMENT AND LAYOUT
3.1
Layout. of a Typical Diversion Works
14
Chapter 4 -TYPES OF DIVERSION WEIRS
4.1
V ert.ical Drop Diversion Dams
18
4.2
Sloping Glacis Diversion Dam
19
4.3
Ogec Diversion Dam
20
Chapter 5- DAMS ON PERVIOUS FOUNDATION
5.1} 5.2
Effect of Seepage
22
5.4) 5.5J
Bligh Method
25
5.6
The Flow Net
34
5.7
Standard Forms - Khosla's Method of Independent Variables
37
5.8
Mutual Interference of Piles -- Definition Sketch
37
5.9
Percentage Pressure Curves
39
5.10
Khosla's Safe
5.11
Design Example
5.3
l~xit
Gradient Curve
41 43
Chapter 6- COMPONENTS OF DIVERSION WORKS
6.1
Components of Dam
49
6.2
Effect of Pocket Width on Flow Conditions
53
6.3
Divide Wall
56
Chapter 7 - HYDRAULIC DESIGN
7.1
Discharge Coefficients for vertical faced ogee crest
73
7.2
I\atio of head on crest to design head
7.3
Coefficient of discharge for ogee-shaped crest with sloping upstream face
7.4
Coefficient with respect to position of downstream apron
73 74 76
7.5
Ratio of discharge coefficient due to apron effect
78
7.6
Ratio of discharge Coefficient due to tail water effect
79
7.7
Broad crested weir ~ relative coefficients
81
7.8
Ogee crest - downstream profile
83
7.9
Ogee crest -
84
7.10
Tail Water Conditions Classification
upstream profile
XV
88
Description
Figure
Page No.
90
7.11
Definitions and Notations
7.11A
Length of jump & ratio · of conjugate depth
7.12
Curve for determination of sequent depth
7.13 A&B
Froude number and dimension sketch for Basin 1.
7.14 A&B
Recommended length of Basin II Appurtenances for Basin II
7.15
Energy loss in Hydraulic jump
7.16
Uplift pressure on floor
7.17
Protection of Works
91
95 96 97 97
99 100 104
a) Launching Apron b) Upstream protection works c) Downstream protection works.
7.18
Maximum stone size in rip rap mixture
7.19
Step wise procedure
7.20
Head losses in pipe bends
106 108
(Structure Design) Chapter 8 - STRUCTURAL DESIGN 8.1
Map of Philippines showing Seismic Zones
119
8.2
Base and maximum pressure for constant sloping faces
121
8.3
Coefficients of pressure distribution for constant sloping faces
122
8.4
Water pressure and uplift pressures
124
8.5
Earth pressure due to earthquake
137
8.6
Distribution of ratio Dynamic increment Vertical effective pressure
137
8.7
Wind pressure and Wave diagram
143
Chapter 9- INTAKE WORK 146
9.1
Layout of Canal Head Regulator
9.2. A & B
Open Type Intake Works
153-154
9.3. A & B
Barrel-Tupe Intake Works
157-158
Chapter 10- SILT EXCLUDER
10.1
Silt Excluder - Plant & Section
165
A- 1
Narora Silt Excluder (Plan & Sections)
170
xvi
·---····---·-----·------·--·--·--------
Page No.
Description
Figure
-----··----·--·-------·
A- 2
Narora Silt Excluder (a field study & its efficiency)
172
Chapter 11 --RIVER TRAINING WORKS 11.1
11.2 11.3
145
Guide Banks - F'orm in Plan
176
GcomeLrical Shape of Guide Banks
178
Typical Layout of River Tr<~.ining Works
11.4
Size of Pitching Vs. Velocity
11.5
Details of Guide Bank
180 183
Chapter 12- INSTRUMENTATION
12.1
Dam on Permeable Foundation
12.2
Location of Pressure Tapping Points
15.1
Location Plan
15.2
River bed profile of Hivcr "A"
15.3
River bed profile of River "B"
15.4
River bed profile of !bolo River
15.5
Cross·section @Dam axis (facing upstream)
15.6
Tailwater Rating Curve
15.7
Crest Profile
15.8
Energy of flow curves
15.9
Relation between the length & the height of jump Structural Design
15.10
Hydraulic jump and uplift pressures (Spillway)
15.11
Hydraulic jump and uplift pressures (sluiceway)
188 189
208 209 209 210
211
212 243 269 270
279 280
Chapter 16- DESIGN OF INTAKE WORK
298
Canal Section
L
1.1
1.2
298
For canal maintenance For land soaking
300
301
Design of Intake Structures
2. A.
B.
c.
301
Design Data Sill level of Intake \Vaterway
301
302
xvii
m:aw
Description
Figure
Page No.
····---···---~------------~
Determination of basin level & length
D.
Pond at crest. level \Vater at crest. level
a.
b. c.
During maximum flQ:Od
303 303 305 306
F.
Depth of cutoff Total length of floor
G.
Analysis for percentage of pressure
307 308 309
H.
Floor thickness
311
Notes
312
E.
LIST OF PLATES
Plate 1
Development Plan
Plate 2
General Plan
Plate 3
Section Sho\ving right and left sluiceway
Plate 4
Ogce sedion & appurtenant details
Plate 5
Right & left sluiceway showing training wall
Plate 6
Hight intake structure
Plate 7
Left intake structure
Plate 8
Position of Pressure Pipes in ogee
Plate 9
Pressure observation pipes - Left Sluiceway
Plate 10
Details of eonstruct.ion joint
APPENDIX
9A
1
~
!2A
The Gumbel method
114
Head Losses in Pipe Bends
16Cl
Hcgister of Uplift Pressure Pipe Observations
193
A
1
Introduction
169
A
2
Efficiency of Excluder
171
A
3
Discussion on Test Results & Conclusions
171
xviii
CHAPTER I
INTRODUCTION
• Water flowing in natural streams and rivers is the wealth of a nation. This wealth can be utilized only harn(~sscd and controlled. \Vith the development of agriculture through irrigation, rivers have served as the principal sources of water for the production of crops.
if rivers are properly
The Philippines has immense water resources, most of which still remain to be utilb;ed to beneficial use for purposes of irrigation and power generation. This can be achieved by constructing storage dams in the upper reaches of rivPrs and diversion dams in the lower reaches. By building a series of dams, basin wide control of the rivers in a val!ey ean be ::tchieved. Building of dams and diversion of river \Vater as an art is as old as the recorded history of mankind. In the early days of civilization, earth dams were constructed, later on rubble masonry was used for building dams. The first masonry dams were built in Spain in the sixteenth-seventeenth centuries. It was in the nineteenth eentury that stability of the section against overturning, slid~ng, etc. was considered. This led to the development of rational design of gravity dams. Though div(;rsion of river watc'r is an old art. the science of design and construction of diversion dams and weirs as economical and hydraulically efficient structures is of recent origin. The design practice has, however, had a rapid evolution with the development of Lacey's silt theory followed by Khosla's theory or design of vveirs on permeable foundation. With t.he rapid evolution in theory, analysis, design and techni(]UCS described in various books/reports the need for a single text. that will illustrate the art of design in a unified manner IS obviously acute. The present. manual for fulfilling the objective is an endeavour in that direction.
1.1
SCOPE
This manual deals with divPrsion dam on permeable and other types of foundation limiting the design flood to the order of 2500-3000 eumecs with the following dimcnsions:a) not more than 1() metres (50 ft.) high if on rock foundation such as hard rock mass of granite, sand stone, lime stonp, ete. b)
low overfal! dams on foundation consisting of pervious strata of sand, gravels, talus, etc. depending upon t.he bearing capacity of soil and the net head i.e., difference in upstream water level and down stream water level is not more than about 9 metres (30ft.)
In the manual, an allcmp\ has been made to bring out the concept of sub-soil flow through the permeable foundation and various theories relevant to the design of weirs (overflow dams) on permeable foundation. ThP drsign procedures based on t.he modern knowledge of hydraulic engineering especially in case of alluvial rivers and the widely accepted method of independent variables devis(~d by Dr. Khosla havp been discussed in the manual. The design standard thus evolved, lays down the criteria for planning, layout, and design of dam and appurten
1
CHAPTER 2
PLANNING AND INVESTIGATIONS
2.0
GENERAL
Diversion dams ar(~ eonstructed for diverting water from rivers for various uses like irrigat.ion, power generation. water supply, navig
2.1
EXTENT OF STUDIES
Ttw magnitude of the project and the obvious physical appearance of such surface conditions usually govern t.he extent of investigation to he made. As such no hard and fast rule for determining the extent of investigations which may he neeessary in each ease can be laid down. A dam that is to be constructed on a pervious or \VC:ak foundation lo creai.P a depth of water of 5 meters will require much more foundation investigation than a 15 meters high dam to be buill on solid and unfractured rock. For !.he height of diversion dams discussed in this manual, the size of the physical structure has little rebtion to the extent of the investigation necessary. The maximum justifiable investigation cost is however limited by !.he magnitude of the project. Th(~ ulLirnat.e objf'Cti\'P of a!\ investigations for diversion schemes is to enable engineers to locate. desig-n and tonstruct. the most cconomieal structures \vhich vvill meet the irrigation/power requirements for which the projeet is envisaged. The survey and investigations of dam sites will require an adequate knowledge of the requirements of oestgn, considerable experience and exercise of good engineering jurlgmcnL The more f.h<' d('sign(~r understands the types of structures and the project layout towards which end h(' is working, the more he will be :1b!c to do an intelligent and selective job.
2.2
INVESTIGATIONS
For clPLcrmining the most. desirable ~uld economic site for a darn, the investigations shall include surveys, topographic mapping, geologic studies, extent of areas to be submerged, possible location of protection dikes and subsurface explorations. Investigation if ('.arried to completion, is an expensive and time consuming phase of project develop. project is not economically or technically sound. Hence, an investigation should be planned and exeeutPd so t!lat the probable soundness of the project will be determined as (~arly and as inexpensively as possible. To accomplish this objective, the investigation may be divided into three stages: mcnt. Moreover, it may indicate that the
1. Reeonnaissance
2. Preliminary investigations
:J. Detailed investigations 2
2.2.1
Reconnaissance Stage
This stage is desigrwd primarily tn support a decision on whether to proceed with more detailed investigations on UH~ basis of rough data and short cut studies. Heeonnaissancc is carried out. to get information about. i.he feasibility of the proje<~t. preliminary to the choice of site; the type of structure suitable with respect Lo foundation, and lhc ava~ilahle materials for construction and the tentative estimates of alternative types of structure. This can bc~ done with the help of study of the basin using some maps, rainfall statistics. st.atenwnt. of discharges at existing works in the river ami statistical atlas of the tract hffcctcd, supplemented by a few inspection of t.hp relevant and important sites including similar works in adjacent localities. In the Philippines, topographic maps prepared by photogrammet.ric method with a scale of 1:50,000 m and 10 nwter cont.our interval can he obtained from Lhc office of the Board of Technical Surveys and Maps at. Tanduay, lVlanila.
It is important. Lo record all t.he data obtained in t.hc reconnaissance stage. Even if from economic (:onsirleration. the project is not considPred viable, changing economic conditions later may result in a reevaluation of Lhc projc<'L Lhus costly duplic:at.ion of the earlier investigation can be avoided. 2.2.2
Preliminary Investigations
These are undertaken in J~TPal('r dct.a_i] until a t.ent.ative sekcLion of site is made warranting a detailed investigation thereafter. Foundation and sub-surface explorations are extended and in general, programmes of work is suffi<'iently hro;H1cncd t.o coliPel information necessary for preliminary design. Preliminary a) Study of
inv(~stigat.ions available~
should in('lud(' tht' following:
maps,
b) Hegional and site geology, e) Stu·dy of foundation st.raLa.
d) Study of available run off and flood flow data.
e) Wat.er studies, f)
Availabilit-y of eonslruct.inn rnaterials, Jnd
g} i\c-:cessibili!.y of th(~ sit.e of work.
2.2.2.1
Study of Available Maps
Study of available> maps should be made to have a general ide;:~. of t.he topography. river bc~haviour, possible sit.cs and t.heir eat.rhment. arra. Earlier maps, if avail;:~.hle, should he consulted as tlwy would provide useful information regarding stability of river at. the site. 2.2.2.2
Regional and Site Geology
It should be studied wit.h particular reference to adverse geological indications su<:h as faults. fractured zones, shear zones solution cavities, c1.c. 2.2.2.3
Study of Foundation Strata
Data should be <'ollC'('t('d hy digging trial pil.s and drilling boreholes and studies of existing nearby deep eut.s and \>v"ells and rc~pnrLs of other projeet.s in the nearby zone. 2.2.2.4
,., ..
Study of Available Run-off and Flood Flow Data ',.
,,
... ,~'
3
2.2.2.5
Water Studies
It is necessary to assess the extent of water requirements for diversion during the various periods and the feeding level.
In that behalf two categories of preliminary water studies shall be carried out: i) l\cquircmcnt of water for irrigation and power, if any. ii) Availability of supplies
2.2.2.6
Availability of Construction Material
Preliminary investigation should include assessment of the requirements of construction materials and their availability in the vicinity of the sites under consideration.
2.2.2.7
Accessibility of the Site
\Vhik deciding on the choice of suitable sit.c, due consideration should be given for easy accessibility and economie transportation of matrrial to the site of \Vorlc
2.2.2.8 Pos~iblc sites for location of the diversion dam should be marked out on the basis of investigation carried out in a(':cordanee with the above requirement. It may he p_ossib!e to eliminate some of these sites on topographical and other considerations by site inspection. Further investigation should be carried out for the remaining sites hy sub-surface explorations. Gauge discharge observations should also be started, if not presently available. Thereafter, considering the merits and demerits of the different alternate sites, the sites can he graded in the order of their relative suitability.
2.2.3
Detailed Investigation
i\fLPr preliminary sdection of site·, these investigations shall be carried out in much greater detail \.vith a vi<~w of eolkcting data for the design of the main structure and its appurtenant works for the site chosen. These should include the following: a)
Field surveys,
b) Hydrological data, c) Sediment studies, d) Surface and suhsurfaec investigation,
e) Field and laboratory tests, f)
Diversion requirerncnts,
g) Construction materials, h)
Communication systems. and
i) Other misr.:ellancous studies.
2.2.3.1
Field Survey -~-
-;-'i'llli11 •;(j'
.' ' ,«.
-
-
The survey should be plotted to a suitable scale. It should show all the salient features like firm banks. rock outcrops, deep channcis. larg-e shoals and islands, deep pools and important land marks, etc. The length of the survey may depend upon Lhe nature of the stream. the size of the diversion dam and the purpose of diversion. If the river course on the upstream and down~tream of the site is straight. the length of survey can be shortened whereas in case of rncandcring rivers the length of survey may be increased so as to cover at least two fully developed meanders on the upstream of the dam axis and one meander length on the downs~ream or as may be required for detailed model studies.
If a permanent bench mark is not availab\P ncar the site of work, the same should be established and value ascertained by double levelling. It should be as close to the work as possible but not so close as to be eroded by the river or stand as an obstruc:-Uon in the execution of the work.
it~
ii) For model studies it is desirable to observe the cross sections at closer intervals. say 100 m, up to 2 km upstream and 1 krn downstream of the proposed dam. In the rcrnaining reach the cross-sections may be observed at an interval of 200m. The cross-levels in the river bed may be spaced 10 to ~30 m depending upon the topography of the river. The cross-sections should be extended on both banks up to about 2.5 m above the high flood level, if possible, otherwise to an extent such that proper layout of guide and afflux banks may be decided. In hilly streams cross-sections at closer intervals may be taken.
2.2.3.2
Hydrological Data
The aim of the collection of hydrological data is twofold:
a) for romputing the design flood, b) for assessing the available weekly and rnonthly run-off on more realistic basis. F'or these studies it is necessary to obtain r::tinfall and run-off data.
The pertinent hydrologieal dakt arc as follows:
a) Design flood ··- The following data should be collected for estimating the maximum anticipated flood: i} Hainfal! Daily rainfall recorded at different stations in the catchment area and data regarding storms with respect to sucrcssive positions of Lhe centre of the storm in the catchment should be collected. Storms causing peak discharges should be separated for unit hydrograph analysis;
ii) Flood hydrographs for isolated rain storms for working out unit hydrograph; iii) CatehmcnL characU'risties, such as shape, slope, orientation, drainage system and infiltration capacity for developing synthetic hydrograph, if adequate data are not available; iv) Peak flow data for the river for as many years as possible for frequency analysis; v) F'lood marks by local inquiry to estimate maximum flood by slope area methOd; vi} Daily river gauge should he observed and stage and discharge data at or near the site of the pro. posed \vork, with stages and observed dischar.ges during flood, for as many years as possible should be eollcclcd.
b) Run·off --- A gaugP disehargc sitP should be established at a suitable point in the vicinity of the darn site. The gauge discharge dat.a should he utilized to evolve a gauge-discharge curve for computing !.he discharges for lhl~ period for which river gauge data ara available. The run-off data thus obtained should lw utilized for estimating depPndabk yield. If the data available for the sitt' is inadequate, a correlation coul(l be established by utilizing the long term data available for a nearby site on the river.
2.2.3.3
Sediment Studies
If no data of sediment load carried by the river arc available, sediment observations should be started immediately with the gaugc~-disehargc· observations as soon as the project is contemplated. The quality
5
and quantity of sediment carried by the water, especially during flood season, is necessary for planning sediment excluding or preventing worl\s and to frame a suitable mode of regulation. The observations may be of two types; il Detailed ii) Representative In the detailed observations the samples may be taken at several cross-sections at every 30 em. depth, at an interval of one week. The representative samples should be taken daily and at 3 cross-sections, one at the centre and two one third width from the edges at 0.60 of the depth. In c:1se lhe sediment eharge brought by the river is excessive, due margin shall he given for sedimentation whiie fixing the pond capacity·, particularly in case where pondage is proposed to be provided to meet diurnal power fluctuations.
2.2.3.4
Surface and Sub-surface Investigation
a) General Bore holes should he drilled at specified intervals covering the dam area and appurtenant structures. The location of borings shall he correctly marked and numbered on the survey sheets. These borings .should be earried to hard rock level or to a depth of 15 to 25m below the deepest river bed level depending on the strata and the structure like piers. abutments, floor, etc. Trial pits mi1y be exeavated to determine the depth of the overburden and loose deposits, if hard rock level is at a .c;;hzdlow depth. In case of large depths of overburden comprising large size boulders where ordinary boring method may prove inadequate, geophysical method may be employed Lo locale the rock surface. b)
Alluvial Foundation
The following investigations should be carried out in alluvial foundation: i) Borings at suitable loc:at.ions should be made in lhc dam area and borclogs prepared showing details of various strata encountered. The spaeing of boreholes should be planned in a manner so as to cover the foundations of various structures. ii) For sandy foundation, dynamic and static penetration tests should be performed below each structure (abutments, piers, etc.) and in each bay to estimate bearing pressure, likely settlement and ncccssit_y of settlement joints. For boulder strab., plate bearing tests shall he required. iii) Soil classific.:ttion and determination o[ unit weight of soil, angle of internal friction of soil, void ratio and specific gravity up to foundation level should he done. iv) In case of clayey and silty foundations undisturbed sampling should be done and tests conducted for determination of unconfirmed comprcs_sivc strength and consolidation characteristics. v) Modulus of subgrade reaction in case of raft foundation should be determined. c) Impermeable Foundation
In casf' of rock found;1tion at shallow depth, drill holes should he carried out. to ascertain the depth of weathered zone, extent of joints and fissures and to determine the necessity or otherwise of grouting to avoid exccssivQ seepage losses. The depth of impervious layer should he loeated in the entire width of the dam and intake area. While the upstream sheet piles may be embedded into the impervious layer, adequate open area shall he provided below thP downstn~am shept pile depth to allow rcleas{' of pressure~ on Uw downstream side. Similarly. ?"·'J~f;>rd~nn<: •-:-r Ot!trrop:- of lmpervious ~trat.um, if any, which may be touching- t.he bottom of the floor ha~c toO\':
<"·x<:dv!o'.!-·r"i
.
_ ;:-·:-:'
:'"~·-
-~;:;'~ ... _.
.
__ , __, __ . . . ,_
·.-.,-,-~--~.i~,.·-·_t_;-; ...
-~t-.. 0 ,_!1(~
··--'~n
:->_~:u·r~:t.:!.n.
wl-.0.thc:·
the iinpervious boundery is horizonLLl, .-;\oping HfJ 1.ovvanJK UH· ciovvn,-;u·P:<m nr llfi:·n.t·c·nn<. i\n 1 tnperv·1 ~~H-" boundary sloping up on the do\vnstrcam will cause eonstriction of seep.:1g-c and should be a voided by shifting the location of the dam slightly upstream, taking help of model studies if necessary .
::1 .-~
<
:··I'-'-....
6
d) Water Table Observations of water table in the area adjacent to the location of the dam should also be carried out; The likelihood of higher levels in the pond leading to rise of water table with consequent waterlogging in the adjacent area should also he investigated for suitable measures. e) Field and Laboratory Tests i) Detailed foundation investigation comprising penetration tests in addition to the tests outlined in foregoing paragraph should be conducted in the dam and appurtenant structure arc:J. to mark out soft foundation area, if any, for special treatment or for examining the possibility of avoiding it by suitably changing the layout.
ii) Field permeability tests should be earried and thereby the dewatering requirement. If the carried out to reduce the excavation requirement should be studied in detail based on economic and foundation, etc.
2.2.3.5
out to assess the order of seepage losses from the pond latter works out to be to excessive, studies should be by resorting to a reinforced raft structure \Vhich aspect other considerations, like availability of material, type of
Diversion Requirements
Diversion requirements should be worked out in accordance with the needs of the project. Detalled water studies, under the following two categories should be carried out: 1) Hequirements of water for irrigation, and power if any. 2)
Availability of Supplies
Water Requirements
For irrigation, consideration should be given to climatic conditions, soil types, types of crops, crop distribution, irrigation efficiency and conveyance losses, etc. For details refer to the Manual on Canal & Canal Structures.
Availability of Supplies Data for proper assessment of supplies is sometimes not available. Where a systematic record is available, careful examination is needed before it can be used. The records should i ncl udc: a) Daily, monthly and yearly rainfalls at various stations observed by the concerned department. b) Gauge readings at v,1rious sites of rivers and its tributaries. c) Discharge observations at some selected sites.
Maximum flood discharge for the design of the structure may be obtained from the data collected by the method detailed in the chapter on Hydraulic Design. Determination of low water level is ncc:cssary to determine the maximum head acting on the diversion dam. It is difficult to determine the low water level in the absence of records. Observation for a few years before construction and local inquiries will help in getting an approximate idea. 2.2.3.6.
Construction Materials
Survey of the availability of construetion materials like sand, gravel and boulders, suitable earth, etc., their availability which is necessary for determining the type of construction in preparing comparative 7
estimates. Availability of hard stone may make masonry preferable to concrete. If limestone is available, its hydraulicity, strength and durability should be investigated. Laboratory and field tests should be carried out for determining the quality of aggregates and earth materials.
2.2.3.7 Communication System
Investigation should include dislocation and relocation of the existing facilities and additional facilities re(]uircd during construction and operation.
2.2.3.8
Other Miscellaneous Studies
a) Pond Survey i) The area submerged up to normal pond level or ·within the afflux embankment that shall be surveyed and all immovable properties corning within it should be recorded and valued before the works arc started to avoid disputes at. a later stage.
ii) The extent and charaetcr of relocation of existing communities, railroads and public highway on account of the raising of water level should be determined.
iii) Existing water rights and government regulations to be observed should be ascertained.
b) F'ish Pass These arc required for migratory fish and should be provided for in consultation with experts of the fisheries department. Necessary data, as may be advised, should be collected for assessing the need for provision of a fish pass. c) Log- Chute
To justify the necessity of a Jog cHute, statistics of logs, such as their numbers, si?;es and periods in which they arc handled, cost of handling and other relevant data should be collected. d) 1\ail/roadhridgc across the dam Sometimes a roadhridgc may be provided across a dam for providing communication facilities between two sides of the river. In such a case, data regarding the type of bridge, width of roadway, footpaths, class of loading, etc. should be collected (if railway bridge is also required, the data should be eolle<:ted in consultation 1,vith railway authorities).
8
CHAPTER 3
ALIGNMENT AND LAYOUT
3.0
FUNCTIONS OF DIVERSION WORKS
As the name implies, diversion works mainly serve to direct the required supply into the canal from t.he river. Permanent canals. taking off from rivers are provided with such permanent diversion structures. Their functions can be summarized broadly as follows:a) To raise the water level in the river to the required extent for diverting the supplies into the canal. b) To regulate intake of water into the can3.1 and control silt entry.
3.1 3.1.1
LOCATION
Location Versus River Stages
Diversion dap1s can be located i~1 any of the four stages of a river. In the Philippines the rivers which originate from mountains have generally the following stages of flow. il Rocky stage ~ In this stage the river stream is in the hills. The bed slope and velocities are high. The cross section of the stream is made up of rock or very large boulders. ii) Boulder stage/Sub-mountainous - As the river emerges from Lhc hills, its slow~ and V(dotily an~ reduced. The bed and sides are composed of boulder or gravel. The river cross section is usually W(:ll defined and confined between non submersible banks on either side which arc close to the main current of the river. These is strong sub-soil flow in the boulder region because of the high permeability of the material. iii) Trough stage ~ F'rom the boulder stage the river passes on to the alluvial plain created by itself. Its cross section is generally made of alluvial sand silt. The bed slope is small and the velocities are not high. During high floods the river spreads outs over a wide area as banl<:s higher than the high flood level are relatively far away from the main current of the river.
iv) Delta stage ·- As the river approaches the ocean, the country slope and velocity fall down so much that the water is unable to carry its sediment load. It drops down its sediment and divides into the channel on either side of the deposit resulting in the formation of a delta. For the construction of diversion works, the Rocky and Delta stages are unsuitable. The Rocky stage will present no difficulty so far as construction of diversion works are concerned but it would be prohi-
bitively costly to construct the channel to lead the water from the dam to its commanded area. In the delta stage, the area available is small and irrigation requirements are not significant. The choice would thus lie between the boulder stage and the trough stage and should he made on merits. The following are the advantages and disadvantages of a diversion dam in Boulder stage in comparison to that in an alluvial stage.
9
I
I J
#
3.1.2 Advantages i) The length of the darn is generally shorter in boulder reaches.
ii) Since the silt factor is high in boulder reaches, the requirements of cut off and proteetion works are reduced. iii) Since high banks arc available in boulder reaches, the cost of training works is reduced.
iv) Construction material like sand and aggregate arc locally available in boulder stage.
3.1.3
Disadvantages
i) In the boulder area there is strong sub soil flow in the river bed, which reappears on the surface in the trough stage. During periods of shor.L supply, there would be loss of considerable percentage of total river discharge. ii) In the canal. which has t.o run in subhill tract with sand and boulder formation to large depths. there would be heavy seepage losses.
iii) A large number of cross drainage works would normally be required on canals in subhill tracts.
iv} Surface protection is necessary against erosive action of rolling boulders in floods.
3.2.1.
General __.. A typieal diversion works eonsists of the following parts: 1. The main dam or t.hc weir divided into bays by piers - This provides the obstruction across the river required to raise its water level and divert the water into the canal.
2. Sllliceways -- ThcsP ar(' gate eon trolled opening in the weir \Vith crest-S at a low level. They arc located on the same side as the off.take canals. Their basic functions include the preservation of a -clear and defined river channel approaching the canal intake; scouring of silt deposited in front of canal intake and control of silt entry in the canal. :1. Canal Intak(~ \Vork -·· These arc gate-controlled openings normally aligned between 90° 'to) 120° wit.h respect. to the axis of thr weir. The intake work serves to regulate the supply of water and eont.rol the entry of silL in the canal. It. is a wall between l.hc weir and the sluiceways aligned at right angle to the weir axis and ext.end.c; a lit.t.le upstream of the intake and in the downstream up to end of loose protedion of the sluirrways. The training wall serves to separate the floor of the sluiceways which is at lov·n~r level than the weir proper; to isolate the pockets upstream of the intake to facilitate scouring operation; and, to prevent formations of cross currents to avoid their damaging cff<'els.
1. Training W;lll
1
Tlwsf~ are required to cheek the meandering tendency of the river and guide it to flow axially through the dam.
:5. Guide Banks
3.2.2 Location for a diversion work involves consideration of its main components. An ideal site should be one which satisfies t.hC' rpquiremcnts of all the components as discussed in the parag-raphs which follow. Each feat.un~ of the river shall be considered in detail upon arrival at the final sit.e. For irri_gation purposes, the divPrsion works shall be so planned that. full command may he obtained by a dam of reasonable height. Tlw combined cost. of construction of the diversion works and the canal starting from the dam to the part when~ irrigation <·ommcJl('_es should be as small as is consistent. with the efficiency of the Project. While deciding !.he proper location the points noted below also need careful consideration.
3.2.3 The riVf~r rea('h should. as far as possible. be straight so that velocities may he uniform and the sectional ~u-ea of t.hc st.rcarn fairly constant.. This will obviate oblique approach as well as non-uniform dis
10
tribut.ion of flow on to the dam. The hanks should preferably be high, well defined and incrodablc. If such a site is available, it may need very small or pradically no guide banks. In case of high banks, the locality will not he submerged during high floods and a considerable saving in the cost of flood protective embankments can be effected. In the case of a meandering river the dain should be located at the nodal point.
3.2.4 A slight curvature at th(~ site may be advantageous for the off-taking channel located on the down· stream end of the outer <:urvature since it will cause less sediment deposition in the canal. However, cross eurrents may be produced due to the curvature and may endanger the foundation. Moreover, if canals take-off from both the hanks, the (~anal taking off on the inner side of the curve will draw com paratively more s<~dimcnt.. Therefore, proper J·udgment should be exercised in deciding the dam location in a eurved reach of the river.
3.2.5 The sluiceways should be located in the deep channel in order to ensure adequate supply to the canal head at all times. When canals take-off from both sides, a site with deep channels on both banks and low water in the centre is the most suitable.
3.2.6 VVhere proposed, due eonsidt-rat.ion should be given in the layout lo all pos.c;iblc locations of the sediment ejector and to the availability of heads for effective operation of the escape channel {for details of silL ejector refer t.o the Manual on Canal & Canal Structures).
3.2.7 Miscellaneous requirements the final darn loc::ttion.
The following factors would also need consideration before deciding
i) No considerable damage due Lo inundation of public and private property and bcilitics upstream will result. after the dam is constructed.
ii) The value of neecssary land and right of way is cheap. iii) The site is accr~ssible to transportation.
iv) No water right.s guarantee is affe<:ted. v) No heLory or rninlng activity upst.rearn of the dam .site should be present as mill or mine tailing will )lollute t.hc good quality of irrigation \Vater.
3.3
ALIGNMENT
3.3.1
Alignment of main dam
The alignment of a divprst.ion dam should be such as to ensure normal and uniform flow through all the bays as far as possible. As discussed in th(" preceding paragraph, as a rule, the most suitable lor.at.ion for t.he diversion st.ruc:t.ure and intake works of a canal is on the straight reach of the river, the velocity being uniform, and the section::tl area of the stream fairly consbnt. There arc two ways of aligning a dam at a site with respect to the direction of the current. a)
Right angled to the conrse of the n·ver
A diversion dam :digrwd JWrpcndicu\ar lo lhP coursr of the river will have the minimum length and • -, • · .• :-. .. ~~·~{" <el...~,,! £"(!'~'~---~·-' " - ·' ~'-.!;r""''"> £l,_.,,, rn;..,[' f 1 ;<erl,7,...'7ifl1T Cal){!;('] tv
h)
Ohlique to t.h(' rnrrent
A skew alig-nrncnt should be avoided unless availability of foundations. The Tliv<~rsion dam may The adva1~tagP of the former is that with the fJll of shall he maintained along the face of Intake works of
othenvisc necessitated by site conditions such as be skCwcd towards the canal side or away from it. wat.f!r taking place in that direction, a deeper channel the canal.
In the lat.t.er case, i.e., skewed alvay from the canal side. the main current is deflected away from the canal which is a dPsirablr' feature \vhere boulders and heavy gravel arc carried by the river. c)
Cra?!ed aJiqmnen t
If the c:oursc· of the river is not. straight, the diversion structure ma.Y havP to he provided along a curved ali?:nmcnt.. In this ca:w :-tdequate training works upstream and downstream should he provided to lead the current to and from Lht! struct.urr smoothly. Tlw c:-tnal taking off from the innPr side will draw more si!L It. is. how(~V('r, ,l;('ncr·ally prokrab](' t.o adopt straight line alignnwnt. 1vhercvcr possible.
3.3.2
Alignment of Intake Works
The- inlakc work for t.lw ('.~tnal is usually aligned at an angle of 90° to 110° to the dam axis to minirnizp sediment. Pnt.ry into t.Jw earn] and avoid haddlow and form::ttion of st:-tgnant. zones in the pocket. The upstream abutment of t.hc- work should be skewed from the line at right.
3.4
LAYOUT OF TRAINING WALL
3.4.1 J\ dividP \vall (training wall} is constructed at right angle to the axi:.; of the d:-tm in front of the int.akP sJ.ru(·lurc. Linder adverse flow conditions, a divide wall may he required in the main darn portion, to help minimizP cross flow in Lh(' river scd.ion, thereby allowing entry of sill free water in !.hC' can.:d.
3.4.2
OLlwr principal functions of LhP training wall may be classified as follows: l. to scparat(• tlw s!uicew:t.Y po(~kd. floor from the ogee spillway floor.
2. to enabiP the sluir('way' to function as scouring sluice and to isolate LlH' r.omparat.ively still pocket upst.re;:un of the intake 1vork for deposition of sediment in thP pocket.
:J. to pr('vcnt formation of cross flow to facilitate entry of silt free water into the intake structure. It is tlf'('('SS:lry to continue Uw training wall on the downstream to ensure adequacy of tail water depth at. lo\v flows in the sluiceway hays for the formation of jump and to avoid cross flow in the close vicinity of the st.ructurP. The crossflow may result in seours. The training wall is generally extended to the onrl of the impPrvious floor. To dPLerminP t.lw position and length of upstream training wall for most. effcdivc functioning, model studies are advised t.o be carriNl out. F'or detailed description on the role and layout of divide 1vall, refer to Chapter No. G.
3.5
GUIDE BANKS
3.5.2 The layout of the guide banks should be such as to guide the flood smoothly through the diversion dam. The guide banks arc provided generally in pair symrncntrical in plan and may either be kept parallel or converged slightly towards the work and usually extend a little distance downstream from the abutments of the work.
3.5.3 In case of wide alluvial banks. the length and curvature of the head of guide banks should be kept such that the worst mPand0r loop is well away from either the canal embankment or the approach embankment. If the alluvial bank is close to the dam, the guide banks may be tied to it by providing suitable curvature. lf there arc outcrops of hard strata on the banks, it is desirable to tie the guide banks to such control points.
The most effective alignment, length and shape of guide banks should be decided by model studies. (For details refer to Chapter No. 11) Layout of a typical diversion dam is shown in Fig. 3.L
3.6
RIVER DIVERSION SCHEME -
While deciding the location and layout of the diversion structure due consideration should also be given to the river diversion and flood handling arrangement during the construction. At times the h_ydraulic requin~mcnt may havt' to he compromised to obtain a workable diversion scheme, i.e., sometimes the dam is construct.erl in a spill of the river and the river diverted to it by providing suitable river training work. It is necessary that such alternate scheme arc supported by adequate model studies.
13
~------------------------------------~
DAM PORTION
GUIDE BANK
LAYOUT OF A TYPICAL DIVERSION WORKS FIG. 3.1
i' ~------------------------------------------------------------~
CHAPTER 4
TYPES OF DIVERSION WEIRS
4.0
INTRODUCTION:
Diversion weirs may be divided into two classes: (1) Temporary (2) Permanent
Temporary structures sueh as brushdams are co~structcd every year after floods. At certain locations, such temporary structures are possible with due economy.
Permanent weirs arc those which arc designe_d and constructed to withstand the onslaught of floods. These can be further classified into: (1) Fixed-crested weirs
(2) Movable-crested weirs, which can be further divided into: a) Barrag-e b) Gated weirs
Fixed·crestcd weirs present a solid obstruction across a river over which crest flood water is allowed to pass. In this manual, these are hereafter referred to as diversion dams. Barrages arc gate conlrol!cd weirs in which the entire ponding up is effected by gates and no heading up is attempted by any solid obstruction. Gated weirs consist of diversion dams over which crests collapsible gates, drumgates or other types of gates are installed to raise the normal operating water surface. During flood conditions, the gates will be at collapsed or raised position, depending on their design, so that the flood water will pass unimpeded, thus preventing the "overtopping of the banks. A weir may also be cL:t~sificd as broad-crested or sharp-crested. It is said to be broad-crested when the head pr()ducing the flow does not reach a value of twice the crest width. \Vhen said limit is exceeded, the overflowing sheet of water termed the nappe becomes detached and the weir becomes essentially sharp crested.
4.1 4.1.1 .
\ .'
J:
t
j
l'i
hi
~I l~'1
l 1i /j
,.,.,.\
·w
l;:.<>'
DIVERSION DAMS Classification According to Crest Shape
Diversion dams may be categorized according to the shape of the crest. The types of profiles usually adopted are as follows:
(1) Vertical drop type or over-fall type
15
l- -~-=i. ,i~· =-~= = =~" " "'" " "'" " "'" " " " "'" '" " "'" " "'" '" "____________, _______.,..__,
{2) Sloping t_ype, furl.her divided into: (a) glacis
(b) O_!;Ct'
4.1.1.1
Vertical Drop Type
Flows may br fr<>e diseharging or !.hey may be supported along a narrow section of the cn'sf, \vhich has a vert.ic;_d or ne~u·\y vertical downstream faec. Oeeasionally. the crest is extended in the form of an overhanging lip t.o din~ct. small discharges away from the face of the overfalt section. In this type, the nappe is ventilated sufficiently to prevent a pulsating, fluctuating jet. The vertical drop type is not. adoptable for high drops on yielding foundation, because of the large impaet forees \vhich must. be absorbed by the apron at the point of the jet. Typical section of this type is shown in F'ig. 1.1.
4.1.1.2
Glacis Type
Glacis is Llw name givcn to the surface \vhich slopes downward from the crC'st to the downstr(~am apron. On :1 sloping glacis, it is only Lhe horizontal component of the velocity of hypercritical jet which takes part. in t.hP impact. the vertical component remains unaffected. However, even if the energy dissipation is less cffirient because of the vertical component of velocity remaining int.acl, the position of the jump is st.ahlP and predictable. Furthermore, knowing the range of the hydraulic jump for different discharge intensities, the floor within that range can be designed to withstand the uplift pressure in the jump trough. Typical section of a glacis type diversion dam is shown in Fig. 1.2.
4.1.1.3
Ogee Type
Tll(' cn'st is oge(~ nf S shaped in profile. The upper curve of the ogce ordinariiy is made to conform closely to t.lw profile of t.h(: !0\ver nappe of a ventilated sheet falling from a sharp-crested weir. The profi!P !wlow t-he upper (llrvc of t.hc o_t~ec is eontinuecl tangent along a slope to support the sheet. on the face of the ov0rf!ow. A reverse curve at the bottom of the slope turns the flow onto the apron of a stilling basin.
Flow over t.lw <:r~;st. is made to adhere t.o the fac(; of t.he profile by preventing access of air to Lhc underside of the slwe!.. For discharges at. designed head, t.he flow glides over the crest v. . ith no interference from the boundary surface and attains near rnaxirnum diseharge efficiency. The uppPr curve at tlw cTes!. may be made <'it.hcr broader or sharper than the nappe profile. A broader shape vvill supporl. t.hc shp(~L and posit.ivP hydrost:-tLie pressure will occur along the contact surface. The supported sheet thus ('rcat.cs a hackwat.cr effect and reduces the efficiency of discharge. F'or a sharper shape. the sheet. tcncl.s to pull away from the crest and to produce sub-at.mosplwric pressure along the contact surfaee. This negative prc~ssure effect inercases the effective head and thereby increases the discharge. This negative pressure, however, induces eavitation \vithin the surface. I-IencP, lw1'au~(' nf its high disd1;lrge efficiency, LIH' nappe·shaped profile is used for most overflow erest. Typic;d section is shown in Fig-..1.::3.
4.1.2
Vertical Drop Versus Sloping Type No definitt> rule can be laid down in the choiee of vertical drop and sloping types. If statical con
16
=====
siderations and material requirements alone were allowed to govern the choice, the vertical drop would likely be chosen. However, to determine the more suitable type, the motion of the water, river bed materials and suspended particles must also be taken into account. A particular advantage> of the vertical drop is that the energy of the overfall jet for small dis. charges is dissipated for most part on the apron due to its almost vertical impingement on the apron. For larger discharges and \vater cushions of the required depth necessary for energy dissipation, aprons long enough to function as dissipaters are impractical for economical reasons. Furthermore, if the river carries sediments, the space between the river bed and the crest gets completely filled with sqlimcnts during high discharges, and the dam becomes, in effect, a sloping type.
In a sloping apron dam, the sediments carried by water roll over the downstream faCe. At the end of the apron. the jet emerges \Vit.h high velocity corresponding to the fall. but with proper shaping of the apron it can c:1sily be passed over the river bed without causing any appreciable scour. I~xit vclodty in the ease of vcrtic.al drop is less than that of the sloping apron type. Hence, erosion of the downstream river bcrl is not rnuc.h in the vertical drop and lesser downstream bed protection is required.
4.1.3
Classification by material
The dams rn::ty also he ebssified basPd on the materials compnsmg the structure. The more com mon types of small dams arc the earthfi\1, roekfill, solid masonry and concrete gravity dams. Timber has also been used in the eonst.ruetion of darns but because of the amount of labor involved and the short life of the structure, timber dams have proven uneconomical.
4.2
BARRAGES
In case of barrages, the crest is kept at a low level and the ponding up of the river for diversion is accomplished primarily by means of gat.es. These gates can he raised clear off the high flood level and thus enable the high flood to be passed with a minimum of afflux. By suitable manipulation of the gates, the flow conditions above tlw barrage can he closely controlled and shoal formation or cross currents upstream of the work minimized. A barrage provides maximum control on the river.
4.3 GATED WEIRS
In th(~ case of gat cd weirs, LhP larger part of the ponding is carried out by the solid obstruction or main hody of the dam. Additional hPad is obtained by installing gates on the crest of the diversion dam. During floods. t.lwse gates are in collapsed or raised positions, depending on their design and the weir functions in a manner similar to the fixed-crested weir.
17
--~ ·-
H. F. L. EL. 180.06 C'
H.F.L. EL. 179.46
"""~ 9 ~·~lijfl;=-~~
DWARFWALL
NEW
PUDDLE
.17
12\ll
NOTES: 1. ALL DIMENSIONS ARE iN CENTIMETERS UNLESS OTHERWISE INDICATED.
VERTICAL
DROP DIVERSION DAMS
2. ALL ELEVATIONS ARE IN METERS-
F1g. 4. I
RL 66_46 RL 6.3
R.l. 65.76
2713
H OTE S: L)
ALL
DIMENSIONS
UNLESS 2.)
ALL
ARE IN CENTIMETERS
OTHERWISE
ELEVATIONS
INDICATED. ARE IN METERS.
Mo:t.. Pond R.L. 2 !1.51 m. I Top d <;ate
Normal Pond
~1210.~:-_____............---
!R.L.210.67
Apron
,v-"" SUty d~
------
SLOPING
\ R.L.194.~1 '.
-~--
Sheetpi!e
-Compacted S<3nd
~lfl.!S 9 .-~__.
R.C.Roft
GLACIS DIVERSION DAM
F
g. 4.2
420
195 [1051
800
I :
,.
I
_Lji_JS W.S. EL. 58.90
-
--···~-i.::;.--~,;;:.:::_--.•-.-
·CHUTE
BLOCK
lli
:r::
WEEP HOLE
~
\
';:J~------,-..::E_::L::_.c:5~2c..:.·_28:::_0_·-j__._._,._ _ _ ___.J[:··
I
'2:;
-EL. 52
~~~~~_;~~~----~----~~--~--~~~·E~L~.5~1~:6~~~-~:~;-~·,§···~~f~~[~~!~~r;~~~::~~:~;J PILE .80
1SHEET
L_:..El_c,'\9_~- --FILTER
_ _j
DRAINS--······
1730
325 0
- - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __j
I NOTEs:
OOEE
l.)
ALL D!MENS!OHS ARE IN C!:NT!METERS UNLESS 0THC:lhV1SE INDICATED.
2.)
ALL ELEVATIONS ARE IN
METERS.
DIVERSION DAM FIGURE 4. 3
-------------
CHAPTER 5
DAMS ON PERVIOUS FOUNDATION
5.1
INTRODUCTION
For Jack of favorable sites, diversion structures are often built on alluvial soil foundation. However, unlike dams constructed on rock which present relatively few difficult foundation problems, the design of dam on pervious foundations involves problems of erosion of the foundation material, settlement and seepage under the structure. In the case of pervious foundation, little resistance is offered by the soil and percolation may reach the downstream toe of the dam without any substantial Joss of head. In such a situation, the percolating water may carry the soil particles with it and thus undermine the structure. This undermining is supposed to result to what is known as "piping". In the 19th century some of the major works constructed were mainly designed on experience and intuition. Based on experiments which were carried out to identify the damages which had occurred to some such works constructed, the accuracy of Darcy Law [V ~ k ilfi], where V is the velocity of seepage water, k is a soil property called the transmission constant or coefficient of permeability and h is the head loss in the distance 1 was confirmed. The forces developed on the weir (dam) and its sub-structure by the water seeping through the foundation can cause two types of failures (Fig. 5.1).
i) Uplifting of floor The percolating water exerts an upward pressure on the work till it emerges at the downstream, and if the pressure is not counter-balanced by the weight of the concrete or masonry, the work will fail by rupture of part of its floor. The floor is thus to be designed to resist the uplift pressure. ii) Undermining of the subsoil Failure by undermining also known as "piping· failure" is caused by the percolating water, retaining sufficient force, to lift up soil grains at the emerging end. This leads to increased porosity of the soil and hence progressive intensification and undermining, and ultimately removal of soil under· neath.
5.2
DIFFERENT THEORIES
Several methods have been developed for the analysis and design of weirs on permeable foundation. These methods can be grouped under the following two heads: a) methods based on creep theory b) methods based on potential theory Both theories are based on the validity of Darcy's Law for flow of water under pervious foundations. Darcy's law can be stated as V = ki
I
21
(, I'
UPSTREAM WATER LEVEL- \
---=-;oc.--o"c:-;::_~.,..;:-~~~~~~-:;::0-----.
CREST , ---~~-L . _. H
~----
'
I
G.L,
G. L.
/
\
\
-.:?:
SHEET
\
PILE~"-/
"~
- --
/
/
I FIG. 5. I
--,.._-
EFFECT
OF
SEEPAGE CREST
CREST
SHEET PILE - PILES/-___:;,
l
I
C/~ {
l
---->-
J.- ~->- It
r=··-----·- b -
( a )
t(~?~~:: -
·-?-
l
(b )
FIG. 5. 2
c \ a l
cw ( b )
FIG. 5. 3
where: V k i
superficial velocity of seeping water, obtained as the ratio of the quan.tity of seeping water and the superficial area of seepage. ~
~
coefficient of permeability of soil hydraulic gradient
~
..h. 1
It is known that Darcy's Law is not valid for small velocities in clays and generally in gravels. But it is customary to assume the validity of this law for all types of homogenous pervious foundation. 5.3 METHODS BASED ON CREEP THEORY 5.3.1
Creep Theory
The creep theory is based on the assumption that concentrated percolation of water may take place along the line of contact between solid structure and the previous foundation material, known as the liJ.?.e yt~<;Eg.£1?. It is also possible such concentrated seepage can occur through layers of higher permeability which might help to develop a passage or vein between upstream and downstream sides of the work. The creep length must be sufficiently long, so that the foundation material cannot be carried away by the seeping water i.e. the structure cannot fail by piping. For this purpose, a minimum creep ratio (given by the length of creep divided by head causing seepage) is specified by experience. Furthermore, along the creep length, the ratio of loss of head is taken to be uniform. The above phenomenon was expressed in a quantitative form by Bligh. It was presumed that the percolating water creeps along the contact of the loose profile of the structure. The length of path thus traversed by water is called the creep length. This theory came to be known as Bligh's creep theory. Lane after analyzing a large number of clams and weirs both with failures and without failures brought out deficiences in Bligh's theory. He proposed a new theory on statistical basis which is known as Lane's Weighted Creep Theory. Investigations and further experiments carried out by Dr. A.N. Khosla on the existing diversion works led to the rational solution to the problem of sub-surface flow. These developments took place with special reference to weirs on permeable foundations but are applicable to all hydraulic structures on permeable foundations.
5.3.2 Bligh Theory According to Bligh, the percolating water could creep along the contact of the base profile of the river into the sub-soil, losing head en route, proportional to the length of its travel. In the case of percolation flow. Bligh assumes that instead of following the short cut as indicated in Fig. 5.2(a) by ABCDE, the percolation flow clings to the line of contact between solid work and permeable soil as shown by polygon ABF.JGDJD. Thus the length of creep is not 2a + 2d + b but is longer as 2a + 4d + b. The increase is therefore 2d. Bligh himself fixed a limit to the application of his assumption. He realised that in the case of sheet p~les driven at too close a clistanec from one another, this assumption became illogical. He stipulated that h1s _method holds good so long as the horizontal distance between the pile lines, b, was greater than twice their depth. On the other hand, for smaller values of b, the line of creep followed the path as shown in Fig. 5.2(bl.
23
sm' mmm_-
]dtWZ: Q
MZUUJP
G L
Z...Z.d
In Fig. 5.4, Length of travel would be: L
~
b 1 + d 1 + d 1 + b2 + d2 + d 2 + b:1
If H is the total head over the weir, the loss of head per unit length of creep would be:
This loss of head per unit length represented as the average hydraulic gradient (C) was termed as the percolation coefficient. He assigned a safe value of C for each of the different classes of soil. If the hydraulic gradient is within the safe values, it is assumed that there is no danger to the structure against piping. Safe hydraulic gradients for different soils recommended by Bligh are given in the. Table 5.1
Table 5.1 Recommended Safe Hydraulic Gradients S. No.
Type of Soil
Value of Hydraulic Gradient
1 2.
Light sand and mud
1118
F'ine micaceous sand
1115
3
Coarse grained sand
1112
4
Sand mixed with boulder & gravels
1/9 to 115
No discrimination was made by him for horizontal and vertical creeps in assessing their effectiveness against undermining or piping. Because of its simplicity, Bligh's theory found general acceptance.
5.3.2.1 1)
The example given below would illustrate his recommendations.
Uplift Pressure
Creep leng-th up to point C, 11 metres from the upstream face ~1 + 2 x 5 + 2 x 3 + 14 Residual uplift pressure at C
~
7(1 - 31/60)
~
31m.
3.38 m.
The thickness of Hoor at any point should be sufficient to resist the residual uplift pressure.
If H1 is the residual uplift pressure at any point then the thickness of floor at that point should be:
where
G = specific gravity of concrete
24
z::_i:w .li.,'f11!!l.
I
SHEET PILE---·~
I
d1
dz
SHEET PILE-
-- J,..
-I-
FIG. 5.4
NORMAL WATER LEVEL
~
0
SHEET ~-
M. ·-··-------··-··---- ... -·-14.00 --·-·-
t--··················· ·-····'1 o,,.,.. oc_o:..:"'.c:· .......
PILE -·-
_j_
·-··---~~~~--+
--···--·--+-·-· -·· --····· .......... ......
20
()()· "'~ ·-----·------~ FIG. 5. 5
BLiGH
METHOD
0
····--·--~
In the above case, thickness of floor shown at C
3.38 T24
3.38 G - 1
~
2.73·m.
assuming G for concrete = 2.24
2)
Hydraulic Gradient Total creep length for the water emerging at Bin Fig. 5.5. L
~
1 + 2 x 5 + 10 + 2 x 3 + 20 + 2 x 6 + 1
Head on the structure
~
~
60 m.
7.0 m.
Therefore: Hydraulic Gradient
7.0 60
1/8.57
According to the Bligh Theory, the structure would be safe on gravel and sand but not on coarse or fine sand.
5.3.3
Lane's Weighted Creep Theory
As for Bligh Theory, the subsoil water follows the line of creep i.e. the line of contact of the dam and cutoff wall of the foundation. According to Lane. water travels along the line of creep because the resistance to percolation along this line may be less on account of the difficulty of securing an intimate contact than it directly through the foundation materials. In practice, however, the contact between vertical or steeply sloping surface is more likely to be intimate than that along horizontal or slightly sloping surface, likewise the contact between earth and deep sheet piles is more likely to be intimate (~ompared to that of a concrete found;1Lion cast over a flat bedding. Thus in caleulating the length of creep, distinction should be made between the vertical and horizontal surfaces and greater weight should be attributed to the vertical surface than to the horizontal.
5.3.3.1
Rationale
Lane,* after a study of more than 200 existing dams. both failures and non-failures recommends using a weighted line of creep, in which horizontal contacts with the foundation and slopes flatter than 45', being less liable to have intimate contact are given only one third the value of steeper and vertical contacts. That is, his line of creep is the summation of all the flatter and horizontal lines of contact between head and tail water following along the contact surface of the base of the dam and the cutoffs. In equation, the weighted creep length (Lw) is given as Lw
~
Ul N + V
N
~
sum of all horizontal contact and all the sloping contact less than 45°
V
=
"
sum of all vertical contact plus the sloping, greater than 45°
•ASCE Transaction VoL 100. 1935.
26
To ensure safety against piping ''Lw"should not be less than CJ-L, where His the seepage pressure head i.e. the difference of water levels on the upstream and downstream, while C is an empirical coefficient dependent on the nature of the soiL Values of Cor safe weighted creep ratio for different kinds of soils are given in Table 5. 2.
Table 5.2 Lane's Creep Coefficient Material
C (Safe Weighted Creep Ratio)
Very fine sand or silt
8.5
}<'ine sand
7.0
Medium sand
6.0
Coarse sand
5.0
Fine gravel
4.0
Medium gravel
3.5
Coarse gravel ineluding cobbles
3.0
Boulders with some cobbles & gravel
2 ..1
Soft clay
3.0
Medium c:lay
2.0
Hard clay
1.8
Very hard clay or hardpan
1.6
This weighted-crec~p theory, as d(~vclopcrl by Lane. is the method being used by many designers as a means of designing lo\v (:oncrcte dams on pervious foundation to be snfe against uplift pressures and piping.
The following contlusions made hy Mr. Lane should, however, be observed in applying his weightedcreep theory; 1) to avoid "short path" condition. the distance between tlw bottom of two successive cutoffs should not normally be less than one-half the weighted creep distnnce between them. Should the distance
between llw bottoms be kss t.han onc·h::llf the weighted creep distance, then twice the distance between them should be used instead of the actual line of creep between them . .~) the v·.leightpd.c:rcep head ratio is t.hc weighted·crecp distance divided by the effective head. 3) revers(~ filter drains. weep holes an~l pipe drains arc aids in relieving seepage pressures under the dam and. if properly provided, Lhe rec-ommended snfe weighted creep head ratios may be reduced by as much as 10 per cent even without performing flow net analysis - but in no case shall the ratio be less t.ha n l.G. 4) c:trP rnnst be exercised to ensure that cutoffs are properly tied in at the ends so that the water will noL outflank them. ··'·-' --- ,,,,J '····
27
··"'""'''~_.,;.~~ ''·~
• •>., A.. n.l •··
'l:r:~"''' .. e
5.3.3.2
Illustrative example: Lane's Weighted Creep Theory (Refer to Fig. 5.5)
A. ·Determination of type of foundation material on which the structure shall be safe: 1/3 (10 + 20 + 1) + 2
Total length of Weighted Creep (Lwl
5 + 2
X
X
3 + 2
X
6 +
39.00 m 7.0 m.
Head of Water (HI
iJ.~,oo
m. 7.00 m.
Weighted Creep Ratio ICI
5.57
The structure is safe on coarse sand where C
=
5.0, based on Lane's recommended weighted cr
head ratio. According to USB!\ Criteria (Design of Small Dams), with the provision of inverted filter drains
weep holes, the above calculated weighted creep head ratio can be increased by 10 per cent; accordingly 5.57
Hcvised Weighted Creep Hatio ICl
X
1.10
6.13
The .:d.ruet.urf' is sah• even if to rest. on medium sand where C B.
=
6.0.
Check for "short pat.h" condition: Length of weighted creep from a to b 1/3 (10) + 1 + 2
X
5 + 3
17.33 m.
Fi2:o)2~
Distance between a and b
lo.zo m.
>
lio o)z17.33
2 C.
8.6G5
Determine uplift. head from A t.o B: 10/3 + I + 2
Len1_;th of creep up to point D
X
5 + 2
X
3
20.33 m. 7 (1 -
Uplift at point D
20 .3;3_ 39.00
14.00
--3·--t-
Length of erecp up to Point C
1 2x5 + 2x3
21.66 m.
7 II -
Uplift at point C
2L()§ I :19.00
3.11 \Vhllf' this LhPory was an improvement of the original Bligh Throry, it too suffers the limitatlo; an empirieai approach. As would be seen in F'ig. 5.3 (a) & {b) although the weighted creep ratio of both
28
section shown are identical with respect to Lane criterion, the design shown in (b) would have a high probability of failure due to piping in the vicinity of point C. 5.4
POTENTIAL FLOW THEORY
In the creep theory, it is assumed that loss of head is proportional to the length of creep. Modern and field experiments have established that this is not true. The rate of loss of head is found to be non-uniform. Therefore, the creep theory is not eonsidcrcd reliable for finding uplift pressure in permeable foundations. The potential theory on the other hand, yielded results conforming with the findings of the model and field experiments. The methods based on this theory are now widely used instead of the methods based on creep theory. In potential theory it is assumed that: i) The soil medium is fully saturated
iii Darcy's law is valid
iii) Soil particles and water arc incompressible iv) No consolidation and expansion of soil takes place v) The flow is two dimensional \.I The surface f!ow equation following the Darcy's Law V = IT.
dx, dy & dz can be compared mathematically to d2 o
,-Jx--2
2
+
d o ····dy2·
=
kdh dX' t h roug h a so1'I me d'Ium o f e I emcnt,
_d!0 _ which is the well known Laplace + dz 2 - 0
equation diseussed under seepage flow (para 5.5) 5.4.2
Method of solution:
The problem was solved for simple weir profiled by several investigators adopting analytical procedures. It was found that analytical so.lut.ions for practical weir profiles were either not possible in the present state of mathematical knowledge or too complicated Lobe workable. Hence the investigators turned to the following alternatives: i) Graphical iii Analogy Melhod iii) Model studies
iv) Numerical methods v) Approximate analytical method Tolal leng-th of floor (b)
(
~ ')2
m
piles ~ 11.50 m.
b'
distance hctwccn upstream
d
depth of pile> on which the effc~d. is to be determined
D
the dcpt.h of Lhc pile line, the influence of which has to he determined on the neighbouring pile of
t\vO
depth d
c
JJB
d ! ..J2 h
SOLUTION:
% PHESSURE AT KEY POINTS i) UPSTH.EAM PILE LINE:
d
~
118.00 - 113.00
~
5.00 m.;
29
5.00
rJ
1
f)
'
c
fi2ilii
.0%2
from fiR. 5.9 (upper curve)
Vln
19
viDl
100
vic
28
0c 1
- 100 -
·· 0n
0c
in fig. 5.11
V) D - 1 00 - l 9 - 81%
vic -
100 ..
zs -
0n
vlc -
9%
72%
~ correction for depth v'r
118.00 - 147.00 5
'~
---~------------
-
1.80% (+vel
INTE!tFE!tENCE OF 2ND PILE LINE D
•• d - 1H ··· 143.00 - <\.00 m.
h'
c-o
14.50 m.
4.00
4.00
I·
52
(;jc corrected _.., 72
-f-
1.80
1-
1.5-1
--o:
75)~4°/o
iii INTERMEDIATE PILI•; LINE: d
- 1-18.00 ···· 113.00 - 5.00 m.;
a:
-
~~
-· :JZ/5 -
HJAO
•;r1 Jll_ 1.~~:-~!2.. l)l - 1 ."> -~· ' b . !i2 - .Z\):3
(1 - -b' I - 0.707 b frorn fig. 5.9 (lower curve)
for (1 -
b' ·b)-
.707
0c - zg 01]) - 37 30
X
9
In Fig. 5.11 =
¢lE1
100 ·- 29
71%
100 - 37
63%
Vim
0El
ViE!
-·
0m
0c1
=
58°/o
71 - 63
...
~
8%
J48,00 - 147.00 5
correction for depth
1.60% ( -- ve) r/Jc1
~ 1/5
correction for depth
X
8
1.60% ( + ve) Interference of U/S pile line on 0E1 : -~- 1.54°/o as the two pile lines arc equal and placed at the same level. Interference of DIS pile on 0cl
]) -
117.00 - !39.00
8.00 m
d
147.00 - 113.00
4.00 m
-
8.00 + 4.00 ······· 5z:oo -
iii)
2.07% ( + ve)
ViE!
corrected
71 - UiO - 1.54 - 67.86%
0cl
corrccled - 58 + 1.60 + 2.07 + 61.67%
DOWNSTRF:AM PILE LINE d - 145 -· 139 - 6.00 m.
cl
b
6.00
11 c
"52.00 - . "
with d!b = .115 from fig. 5.9 -- lower curve 0E 2
32%
vlnz
23% (Curve for 0Dl
0~<~2 -
0nz -
9%
31
X
8
0E2
145.00 - 143.50 6
corrcetion for depth
----~-·····-··~~-
x 9- 2.25% (+ vc)
Intcrfcrcnee of intermediate Pile line:-
D "" 113.50 ·- 143.00 - .50 m. ci
-
lilEz
143.50 ·- 139.00 - 4.50
corrected - 32 + 2.25- .215 - 34.04% (-vc)
Based on above calculations, pressures at key points are: -
_[JL5_1'[L_L'
Ljl'[I~.
INTEJ]l>£JjJ2IA_1'!fLJ'!1'"E
Ll.N!I. .. (OD2
81.00%
67.86%
7:3.34%
63.00%
61.67%
23.00%
EXIT GJIAJ)JENT: Maximum \V.L. on upstream = ILL. 151.00 m. Maximum seepage hear! II!) - 151.00 -
H5.00 - 6.00 rn.
Depth of D.S. Cutoff (d) - 145.00 -· 1:19.00 - 6.00 m. Total Length of Floor (b) - 52.00 m b
d
0:
52.00 6.00
8.67
Hefcrring to the graph for Exit gradient, F'or o: - 8.67;
7r
1. .rx.
"'"' .15
GE - (Hfrll
G "
ir
1
j/\.=
C;;,l .1 5 Exit gradient
6.67 _,., 1 in G.G7
Tlw structure is safe for a foundation material of fine sand for which the safe exit gradient is 1/G to 117 as per t.ablc :5.G.
l
l 32
i' i
I *
Th~re arc no detailed design procedures based on the first four methods. However, such procedures are available based on approximate analytical methods. These methods can he grouped under the follo\ving two heads: i) Method of fragments
ii) Method of independent variables The method of frag-m('nt was devised by Pavlovaky and later modified by Chugacv. The method of independent variahlcs is due to Khosla. Br.forc Khosla's theory of independent variables and its application in the design of dams on permeable foundation is described, a brief representation of the theory of seepage flow is explained hereunder:
5.5
THEORY OF SEEPAGE FLOW
According t.o Laplace equation 'l''
-~~:YL. f1x~
0
j.
.
(5.1)
The above cqu::1.tion deals with three dimension::J.\ flow. In \VCirs and other similar struet.urcs where t.hc width of the river is eon::;iderah\e, the flow may he considered as hvo dimensional, as the flow al any cross section of the st.ruct.urp is not. apprcriab\y influ(~need by any cross flow from the sides. It h:"ts also lwen PX{Wrimenblly dc>monst.rat.ed that for a homogeneous soil which obeys Darcy's Law. lhc conditions of slca
0
{5.2)
The differential equation governs the distribution of the"flow potential" 0 = kh wheD.· k is the per· meability of the soil as defined by Darcy and h is the head, at any point within the soil. Graphically, equation (5.2) can be represented hy two sets of curves that intersect each other at right angles (F'ig. 5.6). i) Streamline
A set of streamlines indieate the path adopted by particles of water flowing through thr:.~ sub-soil. Every particle of water entering the subsoil would trace out its own path and will represent a streamline. The first streamline \Vould follov., the outline of the base of the work. other succeeding flowlines will gradually transition out to a semi ellipse if tlw pervious soil extends to a very large depth. In case there is ,1n impervious boundary al ;l certain depth. the last streamline will follow the impervious boundary and the intermediate streamlines \Vil\ present a smooth transition from the first streamline to the last. ii) Equipot.cn.Unl.l.in.cs
Every streamline has a head h 1 (F'ig. 5.6) if the downstream bed is considered as datum. As il crncrges into t.he atmosphere, the head is zero. The head h 1 is entirely lost through the seepage. At the inter· mediate stage, it has a residual head h. 0ince this applies to every streamline, it follows that there will be points in different. streamlines having the same value of residual head h. By joining such points, the curve thus obtained is c;d\cd an equipotential line.
5.6
EXIT GRADIENT AS RELATED TO DAM DESIGN
Dam (weir) failure from seepage flow can occur because of: a) Undermining of the subsoil b) Uplift due t.o pressure under the floor
33
-
-~--------
(7 ,rEOUIPOTENTIAL
LINES __j
v
h
w
y
z STREAM
LINESJ7 ;
I
EQUIPOTENTIAL LINES
,, ,, .
--------- ----
I THE
FLOW NET FIG. 5.6
As has been discussed earlier, in the case of flow through sandy soil, flow net is formed under the foundations of a dam or weir. The streamlines and equipotential lines thus formed intersect each other at right anglc.s as the particles of flowing water take the path of steepest pressure gradient which is normal to the e[juipotential lines
Net seepage force in the direction of flow pdA - lp + dpl dA dpdA -dpdA
The seepage force per unit volume
dAC!r-
-dp
Jr··
As the head or the pressure decreases in the direction of flow ~J? is negative and force is positive dl . ]I· cncc [ orcc I,,
.. dp actwg . . t I d. t. f fl ow. 111 .1c 1rec wn o
~:.:..-ar
=
If the soil at exit end is to remain stable and resist undermining, the downward force exerted by the submerged weight of the soil should be more than the upward force of seepage. Let G be the specific gravity of soil grains and n the porosity of soil, the submerged weight of a unit volume of soil
o
·"
w 11 - nl IG ·- 11
For critical condition 1(*1
-
w (1 -
Dividing hoth sides by
cih rll vvherc p
.
c,,
n) (G -
11
w
11 -- n) IG - 11
wh
represents the rate of ioss of head or the gradient at exist end
If critical gradient is reached, there would be failure by undermining. Assuming G for river sands is 2.G5 and the approximately average value of porosity is 0.4 Value of critical gradient. - 11 - 0.4) 12.65 .... 1) - 0.99 say LO
35
~-·
_, ····-·-·-·---------
The actual gradient at exit. must be much less depending upon the factors of safety. Thus. for the class of soil mentioned above, the flotation gradient (critical gradient) ~ Unity or 1/1. This flotation gradient \vill vary with the pore space of the sub-soil and the density of the soil partides. The density of the soil particles, though generally in the neighbourhood of 2.6 to 2.7 for most of the sands in Philippines, may vary in extreme cases from 1.8 to 2.8. Similarly the pore space {porosity) may range between 20 pereenL to 45 percent. \Vorking on these limits, value of Flotation Gradient may range from 0.14 to 1.44.
Table 5 Flotation Gradient 2.0 2.2
Pore Space/ Density
2.8
2.6
2.4
0.20
1.44
1.28
1.12
0.96
0.80
0.64
0.25
1.35
1.20
1.05
0.90
0.75
0.60
0.30
1.26
1.12
0.98
0.84
0.70
0.56
o.:Js
1.17
1.04
0.91
0.78
0.65
0.62
0.10
1.08
0.96
0.84
0.72
0.60
0.48
0.88
0.77
0.66
0.55
0.44
0.15
0.99
1.8
In any ordinary structure, hovvcver, the criticai value of 1.1 or nearly so is almost impossible to occur if other factors do nto intervene.
The other factors which create conditions suitable for the development of critical value of exit gradient are: i) Scour
extending down to the bottom of the toe wall
ii) Surges/waves occurring- in the soil with sudden application or reduction of head.
iii) SuddPn ponding upstream of the work by closing off flow downstream. In addition, some other uncertainties ~in nature such as non homogeniety of soil, intrusion of clay heels and zones of very porous material etc. may call for application of factor of safety to the critical value of the exit gradient.
Dr. Khosla has recommended the following factors of safety to critical values of exit gradient. This will not apply to days which arP more or less impervious.
Table 5.3 S.No. 1
Factor of Safety Type of Material ·-·--·--------··------------------shingle 4 to 5
2
coarse sand
5 to 6
1
fine sand
5 to 7
36
I ~
WS.
~~r H
. I
.,.-
-
W.S ~..----
H
!;I
- - - ' - - - - - E - , c __
- (
c1 b
I
d
l_
I
c:.c-~:-Sheet Pile
I
b
Sheet
dl
I
Pile-2::"--
D
D1 (i)
T
_j_
( i i)
FIG. 5. 7 .
W.S.
-r ----;_.- I H
r=--
b
-----------~
-1- - - - - - - - L - - , - - - - - -
c
E
;:::_::_Sheet Pile
(iii)
( iv )
STANDARD FORMS- KHOSLA'S METHOD OF INDEPENDENT VARIABLES
e c
D
)
/ li._:::___ Pile Line I
E C
E C 1
------···-·-- ______ b_s __ -·-·-·--------·
l _____
?--Pile Line 2
i'
! 2--Pile Line 3
ol
D
12' ------------------1
.... ----·-- --···-
b
----------
·--·
-----····----.
------------, FIG. 5.8
MUTUAL INTERFERENCE OF PILES -
DEFINITION
SKETCH
5.7
KHOSLA'S THEORY OF INDEPENDENT VARIABLE FOR UPLIFT AND EXIT GRADIENT
Khosl::t found the solution for simple cases such as a simple floor, a single sheet pile, Door with a single sheet pile etc. Practical weir profiles arc more complicated a·nd may have several piles. Khosla postulated that a floor with multiple piles can be considered to be made up of several simple weir profiles, consisting of only one pile and the full floor length. The effect of piles is essentially local. As long as the distance between adjacent piles is not small compared to the pile length, the effect due to one pile on the potentially close to the other pile is small and can be estimated. Thus, in Khosla's method a weir profile is broken down into simple units consisling of: a) floor and only one sheet pile at either end (Fip;. 5.7 & iii b) A straight horizontal floor depressed below the bed but with no vertical cut off. Fig 5.7 (iii) c) A str~ight horizontal floor of negligible thickness with a sheet pile at some intermediate thickness Fip;. 5.7 (iv) Each elementary form is tlwn treated as independent of others. The pressure at the key points are cal· eulat.ed with the usc of curves given in Fig. 0.9. These key points arc the junction points of the floor and the pile line of that particular elementary form, the bottom point of that pile line and the bottom curves in the case of a depressed floor. The percentage pressure observed from the curves for the simple forms with whid1 the profile is broken up have been demonstrated to hold for the assembled profile as a whole subject to certain corrections as given b~low:
il Mutuallnlcrfcrcnce of piles ii) Thickness of floor
iii) Slope of floor The potentials, as fraction of I-I
J
Cos
1
Cos
'h;;
71"
<J!c
7T
1
0n
71"
c-o
(head causing seepage), at salient point at exit are given by:
;)
---- ·;i--l
Cos-·
1
;, ;)
( ~_1
a
The solutions of these equations in the form of curves are given in Fig. 5.9.
5.7.1
Correction for Mutual Interference of Piles Tlw corrPct.ion of i\-1utua! lntt~r-fcrenec of f'iles is given by the following formula.
c "
~
Where C b'
19
cL1. .D... b
b ~ total floor lenp;th.
Correction to he applied as percentage of head. ~
ll·
Distance between two piles Depth of pile line, the influence of which has to be determined on neighbouring pile line of depth d.
d .., Depth of pi!(' on which the effect is to he determined.
38
PLATE VII· 6 OF C. B.\. PUBUCATIOH ~0.12 100
Shut pile not at end
Value
.!.... cos~ 1()..!-1)--Eqn 7.36
rr
0
of.!....=~
0.4
--,:--
"'
b o.s
..!.. cos· 1 ()...Jfi)--Eqn.7.37
rr
-,1
-:} CO:S-
\
~~
)--- Eqn 7.38
,,
90 To find
~E
0:::: for
. ba5CI rotto
subtract
20
for ony value~ of cc and bose~ rolio-b.read
,,
''I U- b
far that vo\ue of oc and 30
IOO.Thvs 0£ for~~ ~0.4 end cx;.::4;'
from
o..i:c
0C (or-b': 0.6 and oc~ 4= 100-29.1=70.9% To get QIO for values of
~
,, b
less than 0. 5
read CD for bose~ ratio 1-
,,
tn~btrocl
":§
and
•8
!ram 100. Thus, ¢0
foe b=Q.4ond cr..= 4~\oo.¢0 b•
60
0
'?
"'
for "f=0.60fld o::-4
=100-· 44.8 =55.'2.
ec,=
oo,=
74 100-¢0
100- ~D (Depro,~C~d) f'D': 00- - ( 0E-0Dl ~ (Eqn7 701 3 ~~~ +oc2 2
PD'=
'
0
"" i;'f
2
Deve~:)C(l.d floorl
'
50
50
u
"'0
40 ~ .i
•",
.
0
0
0
> 30
30
.,"
''
0
20
>
i 10
0.3 RATIO
PERCENTAGE
0.4 b'/b
PRESSURE
07
CURVES
FIG. 5,9
This correction i.s positive for points in the rear of the back water and negative for points forwar in the dire(·tion of flow. This equation is valid if the foi!owing conditions are satisfied. i) Tht~ Intermediate pile is longer or equal to the outer pile.
ii) The distance beL,vcen pi!cs is more than twice the depth of the outer pile. In Fig. 5.8 the dimPnsions have been marked as they apply to point e or pile line 1 owing to thP ii fluence of pile !inc 2. The dfcd of inv~rfercncc of a pile is to he determined only for the face of the adjaren pile t.owards the interfering pile; e.g. pile line 2 will interfere with the downstream face of pile line 1 an· upstream faec of pile line :3.
5.7.2
Correction for Floor Thickness
In Uw standard forms with vertical eutoffs, the thickness of the floor is assumed to he ncgligihl\ Thus as obscrv(~d from the r.11rvPs. the pressure at llw junction points E & C pertain to the level at t.h1 Lop of UH~ floor whereas t.Jw actual .iunetion is with the bottom of the floor. The pressures al actual points E g. C arC' int.prpolatcd by a::surning st.raight line variations from the hypothetical point E to I and also from D t.o C. It. is positive at. lovver elevations.
5.7.3
Correction for the Slope of Floor
A suit:thle percentage correction is to be applied for a sloping floor, the corrcetion being plus for lhe down and minus for the up slopes following the direction of flow. The values of corrections arc given in Table 5.5.
Table 5.5 Correction for Floor Slope
vert.ieal
Slope horizont.<1l
Corre(~tion
°/o of Pressure
in 1
11.2
in 2
6.5
in :l
1.5
in 1
3.3
1 n i)
2.8
in fi
z.;,
in 7
2.:3
in 8
----·-······-·-·-··-··
--------
2.0
Tlw coiTection is appiieahle to t lw key points of the pile line fixed at. Lhr beginning or the end of the slopt'. Thus. in fig. S.8 Uw slope correction is applieahle only to point E of pile line 2. The percentage eorrec.t.ion given by Llw abov(' table is Lo bP further multiplied hv the proportion of the horizontal lenvth of slope to LhP disL~tncp lwt.wP('ll th(' t\vo pile lines in between which the sloping floor is located. In fig. tlw cnTTP{'Lion to hP applied at E. for t.Jw pi!P linr> 2 will lw obtained by multiplying the appropriate figurP from Uw above' tabie by bs The corrcei.ion is minu::; for the up and plus for the down slo!WS in the dirt.'C b' Lion of flow.
5.8,
40
·war{
o the in :t(l ·,lCCnl
rH
_ ano
r;l=---ible: I the 25
:ll.dlal.
F; to D
_L
·H'
-- ~·"'-"=.fe-
EXI::"'DIE~~ r,±d=:====2-/r::l==::!~
•it,,, for JIJS
are 0.3.
a< - ? c(
...\
- , t ;;;-;;{' 2
++-H-
G•<.:. • d.,11
'
TT/T
£o•(H5) I
'
d ·f
5
0'
0
0
:age ·.yf ll
I
FIG. 5.10
I
. 5.7.4 ' Exit Gradient According to Dr. Khosla, undermining can start only with the particles at exit being carried away by seeping water. This can happen if the velocity of seeping water at exit end is high or in other words if the gradient ;:tt exit end is high. It has be(~n determined that for a standard form consisting of flow or length b, with a vertical cutoff of depth, d the exit gradient at its downstream end is given by the equation,
Jl
(5.3)
d
Where
~ ~
1 +
l_i o-:::'!2~
and
o:
2
~ ~ d"
F'rom the curves in Fig. 5.10, for any value of o:: or .!? , the corresponding value of --}~ can be read d -~~ off_ Knowing the values H and d, the value of GE is pasily, calculated. It is obvious from the equation (5.3) that if rl ~ 0, GE is infinite. It is therefore, essential that a vertical cutoff should be provided at the downstream end of the floor. To safeguard against piping, the exit gradient must not be allowed to exceed a c:ertain safe limit for different soils as given in Table 5.6. The uplift pressures must be kept as low as possihle consistent with the safety at the exit, so as to keep the floor thickness to the minimum.
Table 5.6 SAFE EXIT GRADIENT ~---····-------
SNo.
Type of Material
Safe Exit-Gradient
1
Shingle
1/4 to 1/5
2
Coarse
1/5 to 1/6
3
Fine sand
1/6 to 1/7
EXAMPLE ILLUSTRATING USE OF KHOSLA CURVES
4.5
With rcfcrcnec to Fig. 5.11, determine the corrected percentage pressures at the key points. Deter· mine also the exit gradient. Total length of floor (b) - 52 m b' = distance behvcen upstream two piles
=
14.50 m.
d ,. . depth of pile on which the effect is to be determined D
the depth of the pile line, the influence of which has to be determined on the neighbouring pile of depth cl
c-
19
ns
J
b'
d
I
I)
b
42
n
POND LEVEL
J'
EL~1!31.00~
! RL. 1413.00
_i1
.~
3
·,
::::::11
t R. L.
I4 5 .0 0
u n. 147. oo
INTERMEOIAH:
Ii II
-J 0/S SHEET PILE
jn. fU.I43.00 ~0 I .I
EL. 14:5.50
PILE
fr:t.L.14:S. 00
II ill I
:s 6. 0 0
14.50
52.00
·---
DESIGN KHOSLA'S
EXAMPLE THEORY
FIG. 5.11
SOLUTION: % PRr~SSURE AT KEY POINTS i) UPSTREAM PILE LINE: ~
d
148.00 - 143.00
5.00 m.;
J:>
L
.0962
b
0:
from fig. 5.9 (upper curve) 0D
19
100 - 0D
0c 0c 1
28
100 -
0c
in fig. 5.11 0D
100 - 19 ~ 81%
0c
100 - 28 ~ 72%
}D
~ 9%
148.00 - 147.00
Oc correction for depth =
5
x9
1.80% (+vel
INTERFERENCE OF 2ND PILE LINE D ~ d ~ 147.00 - 143.00 ~ 4.00 m.
14.50 m.
b'
~ 19
c
.j-4.oo 14.50
x 1.00 ±-.1cQQ_ 52
1.54 ~ ( + ve)
0c corrected
~ 72 + 1.80 + 1.54 ~ 75.34%
iii INTERMEDIATE PILE LINE:
148.00 - 143.00 ~ 5.00 m.;
d
b d
cr
hi
=
15.25;
52/5
~
10.40 15.25
bi
52
b (1-
b'
b)
~
.293
0.707
44
-~' :.
from fig. 5.9 (lower curve) .707
lor
f/Jc
~
zg
0D ~ 37 In fig. 5.11 0E 1 ~ 100- 29 ~ 71% 5 - 25 0Dl ~ 100 - 37 ~ 63%
0E 1 0c 1
~
0m
~
n -
83
58o/o 148.00 - 147.00
0El correction for depth
8%
X
8
5 1.60% (- ve)
Ocr correction
1/5
lor depth
X
8
1.60% (+vel Interference of U/S pile line on OE1' - 1.54% as the two pile lines are equal and placed at the same level. Interference of DiS pile on \1c1 147.00 - 139.00
~
8,00 m
d
147.00 - 143.00
~
4.00 m
c
19
D
~
j 36,00 8,00
X
8.00_ + 4,00 52.00
2.07% ( + vel 0E1 corrected
~
71 - 1.60 - 1.54
~
67.86%
0c1 corrected ~ 58 + 1.60 + 2.07 ~ 61.67% iiil DOWNSTREAM PILE LINE
d
~
145 - 139
d
6.00 m.
6.00 ~ 52.00 ~ ·115
b
with dib.
~
~
.115 from fig. 5.9 - lower curve
0E2 ~ 32% 0D2 ~ 23% (Curve for 0n) 0E2 - 11iD2 ~ 9%
45
0E2 correction for depth ~
s c
Interference of intermediate Pile line:~
D
143.50
143.00
d
143.50 ~ 139.00 ~ 4.50
c
19
(0.50:_ J . 36
0E2 corrected
t
.50 m.
l
I
~50__:>:___4,50 ~ .215% ( ~ ve) 52.00
X
e
32 + 2.25 ~ .215 ~ 34.04%
Based on above calculations, pressures at key points are:~
£1'/J]i:JIME_.QIA'J'E PIL]j: LINE
[J(:IJ'JLEf!NE
DIS PILE LINE_
glp~
~.Q.
.0_E 1
~I.ll.
\IJ C1..
1lEz..
1lDz
81.00%
75.34%
67.86%
63.00%
61.67%
34.04%
23.00%
XIT GRADIENT:
Maximum W .L. on upstream
=
R.L. 151.00 m.
Maximum seepage head(!!) ~ 151.00 ~ 145.00 ~ 6.00 m. Depth of D. S.Cutoff (d) ~ 145.00 -· 139.00 ~ 6.00 m. Total Length of Floor (b) ~ 52.00 m. b d
--
~
<X
-
52.00 6.00
---
~
8.67
Referring to the graph for Exit gradient, For o:
~
8.67; =
GE -
.5
(!'..! .15- ___L_ 6
.. 6.67
exit gradient - 1 in 6.67 The structure is safe for a foundation material of fine sand for which the safe exit gradient is 1/6 to 117 as per table 5.6.
46
5.8
STRUCTURES ON PERMEABLE FOUNDATION OF FINITE DEPTH
'
In the methods explained in the preceding sections, it is assumed that the pervious foundation is homogeneous and infinite. In a number of practical cases however, there may be sites where the impervious strata occurs at shallow depth and pervious foundation is finite and comparable to the width of the dam. In such cases, it is known that the results of Dr. Khosla's theory as outlined in the previous sec· tions, do not agree with those of the model experiments. Mathematical solutions have been attempted by several investigators. The solutions are more complicated than those for ·the infinite depth case. It has been found that in cases of weirs with short piles, weirs with long piles (5 times depth of sheet piles) or when the depth of pervious foundation is approximately 2.5 times the width of the weir, the case is equivalent to that of infinite depth for purposes of computing potentials and exit gradients. As the design organizations are not at present recognizing the effect of this particular aspect, the procedures are not discussed in the Manual.
47
CHAPTER 6
COMPONENTS OF DIVERSION WORKS
6.1
lAYOUT OF DIVERSION WORKS
A typical diversion head works consist of the following parts Wig. G.l).
1) The overflow dam 2) Sluiceways
3) Training wall 4) Fish ladder
5) Intake work
6) River training works 6.2 THE OVERFLOW DAM The overflow dam provides the obstruction across the river required to raise up its water level and divert the water into the canal. It is aligned at right angles to the. direction of flow of the river.
6.3 6.3.1
SLUICEWAYS Function
The sluiceways are bays in continuation of the diversion dam with a crest at a lower level. These are located on the same side as the off take canal of the river. If the canals take off on either side, it would be necessary to provide sluiceways on either side. The crest of the sluiceways is generally kept at the lowest level of the cross section of the river during low supplies at the proposed site of the dam. It is also related to the crest level of the off laking canal. Main functions of the sluiceways are:
i) to maintain clear and well defined river channel towards the canal regulator. Because of its crest at lower level, a deep channel develops towards the intake work, capable of easy diversion of low supply discharge into the canal.
iii to scour silt deposited in front of the canal intake and control silt entry into the canal. iii) where applicable, to pass low floods without manipulation of gates or shutters installed on the dam crest. As long as surplus water to be discharged does not exceed the discharging capacity of the sluiceways, it is passed through the sluiceways. iv) to lower the afflux level by providing greater discharge per meter length than the overflow darn.
6.3.2
Discharge Capacity
Discharge capacity of sluiceways is fixed on the following consideration.
48
1.
RIGHT AFFLUX-
BANK
.J
RIGHT D.S. GUIDE .
BANK
COMPONENTS
~~
DAM FIG. 6.1
i)
ii)
at. least twice the maximum discharge in t.he offtake canal to ensure good scouring capacity about. 100/o or higher of the design flood discha;:ge at afflux elevation considered
iii) a throrctical bottom velocity required to sluice sediments of a given diameter may be obtai.ncd from fig. {).1 and used in the computation of the discharge· (this method also takes into account the settling velocity with regards to sediments to be excluded). The procedure for the last method sl
Given the following: Operating water surface elcvution
(m)
Sluicew;-ty and intake sill elevations
(m)
Discharge
(em/sec)
o[
main c:tnal (Qmci
Silting velocity (vsilting "~ 0.'30 MPS
(mps)
Diameter of sediments to he excluded (al)
(rnml
Diameter of sediments to be .c;Juiccd (a2)
(mm)
( ~~~~!~-~L- )
Slope of sidewall (s)
horizontal
L Compute H ~ Opcr, W,S, Clov, -- Sluiceway Sill Elcv, 2. Compute Ar and solve for Min. B
mequircd areal
Ar -- Clmc V silting i\r
Min, B -
]f -
H
(Required B)
2s
(Supplied area)
H0
Opcr,
-
W_ S,
f~lcv_
-Intake Sill Clcv,
,J_ Dctt~rmine required settling velocity (W) from a 1 vs. W graph (Figure 6.4) and compute required length of channei (L).
L
=
5. From Figure fl.5 determine cornpctcnt bottom velocity (VI) required to move sediments of a2 diameter size. ()_ Cornpute for t.he mean vc!ncity (V 1 ) lhc mean \~clocit.y head (hv 1) required at the sluiceway channel.
v1
-
v2
vll, __ ()_70
2g
50
7. Compute for d 1 and A1
8. Compute for l.hc discharge required to sluice particles of a 2 diameter size {Q5 )
9. From the tailwater ratin;s curve of the river. determine the tailwater elevation for the corrcs· ponding Qs. 10. Compute
11. Determine
g.sc from
the fo!lownrY table h
0.00
1.000
0.10
0.991
0.20
() .983
0.30
0.972
0.40
0.956
0.50
0.9:l7
O.GO
fUll f) Hll
090
J
I 00 ----~
0.907
ll.WiG
0 77H
0621 0 000
----
12. Compute Bs
51
6.4 TRAINING WALL Functions and Elements of Training ~~vaH
6.4.1
As briefly described eariicr in Chapter 3, main functions of the training wall are: i) to separate the flO\V of the scouring sluices which is at lower level than in the main dam.
ii) to isolate the pocket upstream of the canal head regulator to facilitate scouring operation. iii) to prevent formation of cross currents to avoid damaging effect. The training wall extends both on the upstream as well as on the downstream of the dam. It is the upstceam length of the training wall that governs the flow curvature and needs to be studied in greater detail. Normally, it is a concrete or masonry structure of. suitable top width aligned at right angles to the darn axis.
6.4.2
Hydraulic Requirements
From hydraulic considerations, a.n ideal training wall is one which: i) ii)
ensures a deep river channel in front of the sluiceway bays for proper feeding of offtaking canal. eliminates parallel flow
iii) minimizes sediment entry into the cana'1. iv) makes the riverflow approach the dam uniformally. The oblique approach of flow to the dam will not only result in making some of the main dam bays inactive while increasing the flow concentration through the others, but may also cause development of islands on the upstream of the dam. l"urthermon~. if the quantity of sediment drawn by the offtaking channel exceeds its carrying capacity, the excess sediment load would be deposited in the channel thereby reducing its discharging capacity.
The above requirements for the efficient functioning can be met to a very large extent, with the provision of suitable training wall in conjunction with proper river training works, optimum angle of offtake channel and raised crest of intake works.
6.4.3
Role ot upstream training wall
The upstream training wall ~crves to ge:nerate a convex curvature towards the intake which results in spiral flow normal to curvature thereby diverting heavily sediment laden bedlines away from the intake and relatively clear water into the offtaking channel. The efficacy of the training wall in the exclusion of sediment depends on its length as well as upon the width of the sluice pocket.
6.4.4 Width of Pocket and Model Studies
ln t.he case of gated type of diversion work, the gencrai criterion in vogue for deciding the width of the pocket is that the ratio of the average velocity of flow in the river to that in the pocket (VnNp) s!1ould be greater than unity during maximum fiood when appreeiablc bed movement takes place j~ the nver. It may, how(~vcr, lw cmphasizC'd that this, by itself, is not sufficient guarantee for effective sand exclusion unless ass~wiaLccl with favourabh: curvature.
w ~
u
-"" ~I-.~1 0
"' .hS_GENO.
-" 0
Surfac$ lines of flow Bad
El
linfls of flow
0 0 _Q
.6
.2
--
~:: ~ 0
~--------
""-
:::>
a- Lines of flow with divide wall after 4 bays
~
0 Q_
E 0
0
~
u
~
·; .::!
]i
~~-..::-·:::-----
Flow--+------~--=:-:·:::·-:'-="-~-..=---::-~--:=.--=-=-~--=--
\2
.
~
--~:;L_
-
-----
~onol
~
"
-"'
b - Lines of fiowwith divide wall after 5 bays Dam
Crest---
u
0
"'
__c-t o!,
-" 0
"-
" "
~
0
E 0
0
"u "' " ~ "-,o ~ -·o ~-
c"-
::0
c- Linss of flow with divide wall after 6 bays
EFFECT OF POCKET
FIG. 6.2
WIDTH ON FLQ\>V CONDITIONS
For the pfficienL functionin!~ of a diYcrsion dam, 1t. is essential t.o provide an optimum width of the pocket.. /\ sm:tll('r width of t.h'(' poC'kt't t·psuli.s in higher v('\ocities :lnd conscqucn_Uy rnore sediment (~nt.rv into !.hP pnckr.'t. and t!ltim:li.Ply into the offtaking· channel, \Vhcreas a very w1dc pock('!. may result ·in parallel fl<"lv; :llo:1g tile siuiccways and t.he training wall. For exam piP, model studies conducted to ch~t.ermirw Uw optimum width nf the :-:luin' poda't for t.h(' LO\\-·cr Sarda Diversion \Vork in India, indicated t.hal ilw S(~dinH~nl. cnt.rv into t.lw canal was minimum wlwn the t.raini!li( wall was placf'd after the fifth b;n'. t.he total of t.lw m:tin hav.s being 20. \Vhen t.hc training wall was t.est.Pd afLf'r the sixth hay, parallel f\0\~ was observNl along Uw ~1uie(:way bays and the training wall. This caused more turbulence in the pod;.et. and th1~ quant.ily of sPdim('nL ~·nt.Pring into the can;1l was found to increase. Thus. the pockeL with four bays was found in;tdc(.JUatc v;hercas with six bays was l.oo large. Therefore, Lhc width of t.lw pocket \'>':l.S kept ('qual t.o G kt_ys. Fig. G.2 shows the flow lines for ;:dl Uw three conditions.
6.4.5
Len~j'ch
of Training VVa!!
6.4.5.1 As pointed out Parli(·r, t.he lPnf'.( h of Llw 1.r·aining wall upsl.!'(':tnl of the divrrsion dam phy.s a very important. role~ in gcnerat.in,e; l":tv(JUI·abic ('lJrvat.ure (Jf flow for feeding Lh~~ offtaking channel. Earlier the practicr \vas l.n providP a lraining \.Vall much longC'r t.han the width of Lhe c:-tnal intake. One of the main argum(•nts advaneed in favour of a long training wall was t.hal it served a~ a t.rap for coarser bed material. Hnv-n'vPr, it. h~ts hH'n r:st.ahlishcd !.hal. a very long training: wali dews not nC'ccssarily hrlp in rninimi7;ing ilH· S1'dimcnt. entry into the off·t.aking chall!wi. On the olhcr hand it. may vitiate How eurvature and pr('Sr,'nL difficuiliC's in flushinv. Furt.lwrmor(', a very long divide w;1il may indun: parallel flow along it, and even slight obliquity of f!O\v !llay mask a few of l.he other main bays adjacent t.o iL 6.4.5.2 From Llw :thov(' it is ('vid('nt !lut Loo lnnR a training wall is hydraulirally disadvanLlfU'OUS. On the otht'r hand if th('!T is nn t r:tinin~~ w:dl or if t!w lcngt.h of training wall is irndequ:lt,c, paraikl fiow could tkvf'lop along !.hC' sluic('way ha.vs causing considC'rablP velocities and turbulanee in t !w poekct, t.hen'hy inercasing t.hc sediment. entry into t.hc offt.aking channeL In order, therefore to achit:'H~ satisfactory flo\v conditions near t.he head regulator, it. i.s nt:ecssary lo provide adequate length of training waiL
6.4.6
Top Level of
Trainin~J
VVal!
Tlw l.op !Pvt'l of t.hc train in;:~ w:dl also a ffc~ct.s ii.s pcrform:tnce. I[ the top oi the trainin,s: wall is Ju:pt. lower !han t.lw pond levPl in the par!. or ('ilt..ire kngt.h of i.he t.raining wall, the water \·viil spill over introducing t.tw silting dfC'et. in l.hc pockpt.. This may lead to jump formation and consequent scour near t.h(' training· wall in t.hC' ol.hl'r main bays. This phenom(~non is based on model studies. Such conditions lllilJ' also occur ;\1 water lf'Vf~ls higJlCr !.han t.hc pond Jc:ve! if Uw sluice g-:tl<'::> arc partly open and the gaU: of t hf· adj;u·cnt hays of main darn are fully opt:n. Th(~rdore, it. is considered advisable to keep t.hc Lop of the divide \Val\ above high flood level or pond level whichever is higher.
6.4.7
Conclusions
il TIH: t~xpr:rir.'flf"P of modPl <;iudi(~s and prnt.ot.ype lwhaviour C;:trried out in India on sonw of the diversion work." havP indica!Pd th:ti. in <'asc of a singiP int.ak(', a training wall covering 2./:J width of the int.ake gcncr:1l!y giv('S satisfaelor_y c.ondiLions, ;.v/wreas in L·win intakes, it. has bPen g-enerally found necessary l.o provid(' a dividr.' wall covr:rin.1< t.hc' ('nt.irt: \.vidt.h of the intake to obtain saLisf;ct.ory h~'draulic conditions. If t.hl' nl'ftakin;~ ('hann('l." an' r('quired t.o run only during non monsoons wiH'n the ;ivc.r water is compar:tt.iv\:iy sedimt~n\. frer·. t.hr~ requiren~Pnt.s of opt.imum.lf'ngl.h ~)f training \Vail can be relaxed.
iii) The ratio of the optimum length of the training wall to the optimum width of the potket V
6.5 FISH LADDER 6.5.1
Necessity
Large rivers support various types of fish, Jnany of \vhich ;tre migratory. Tlwy move from one part of the river to another according to srttsons. Gencraily speaking, they move upstream to downsl.rcam in the beginning of \Vintcr in sC'are.h of warmth and return upstream before t.hC' monsoon for clearer water. If there is no arrangement for their migration, large scale destruction of the~ fish may Lake place in the river.
6.5.2
Location
The entranee of a fish\vay should be loeated at a point beyond \Vhich they are unable to pa~s. in other words, at or ncar the b,1.sc of the dam or obstruction. It. is generally advisable to locale the entrance somewhat out of range of a heavy overf.:lll of water from the weir/dam. F'ish ladders at >vcirs arc generally incated !War the training wall due to the availability of water throughout the yc·ar in Lh<:' rivedwd downslrC'am of Lhe undcrsluieeways. These arc usually located adjacent to the training wail near the unckrsluicewa.ys.
6.5.3
Investigations and Data
It is cssenli
6.5.4
Design Requirements
In order that fish ladders may funetion successfully, it is necessary that they be so located as to attrar:t. fish. A number of factors sueh as physical features of the l~1cation, overeornc and the available \Vater supply may influence Lhc design of a fishway. 1\ larger abundant flow of \vat<~r may prove advant.ag-cous a5 the oulfall will provide a Rreater fish than that frorn a small one. Sometimes conditions may limit both the dimensions and quantity of water.
designed and the height to fishway with extraction to the available
ThP -'lc!u<11 {l,._<:)t-rn fnr ·1 fi~bw:1.v ;r,cl,Hl\'S ~' \:tr•,r!o(l c.:pdion of _"'-lH:h icnzth and dimcn~ions as will provide a scric~ of compan.rncnts f.hr;;ug-·n ,. ..-,,J,~f, L(if .,.,. -._.-" ';~~ ·h,-- ·.. ,,._ ,_,,.,,"ri "f'h,-. ~!''"!":·:oin 11 1._,,,factor for cffieiency is thai.. if the .supply of water is incrca~wd it is desirable t.o <~nt.:u·ge llH~ oinwn:~ion .., a.nd u.:ually lessen the grade in order to reduce the Lurhulencc in the eompanmc:nts to a point where fish wlll not become fatigued. 55
z
~-----t--1
o.-•-,---11
--,-------1
1.5 ~- ~+-- +-~--- --J-----~--~!
I I II 1.0 ~;:"1170. ~u tO -v,_o--+- ---1-----1-l 16•
L/W
•12
i
9
·004 (a)~
·006
-0012
·01 ·OH
Plot of L/W vorsus Qc/OR LEGEND: L
x
=
VI Qc,.
Qrz
'C'tu
0
.QQ2
·004
'006
·OOI.ll
·01
LlOHrTH OF DIV!Orl WALL 'NtOTM OF UMDirft SLvtCl!3 POCKlT CAHAI.. OE-:5\GN Dl3CHAR0E PHV!!~ OESlQH OlSCHARBE WIDTll rJf CANAL lfi.AO MOVLATOI\
·Ott
Oc/0 0 (b) Plot of W/Wh verou• Qc/O•
DIViDE FIG. 6. 3
The following points may serve as a guide in the design of fishways:a) The entrance 1.o the fish way should be from a pool in which fish would naturally collect when further progress upstream is prevented by the obstruction. b) The entrance should be well submerged at all stages of water when fish are seeking the ascent through it. c) ThP size of the free cross-section of fishway should be sufficient for unhampered swimming movements eonsidering the size and number of fish using it. The width should not be too narrow. e) Water should be available in the ladders during the periods of migration. f)
L,ongitudinal slopes may vary from 8:1 for low lifts to 10:1 for high lifts to ensure a current of velocity not. exceeding :3 m per second, main consideration being to dissipate energy in such manner as to provide smooth flow at sufficiently low velocity.
6.6 INTAKE WORK 6.6.1
Location and Layout
The intake work also termed as eanal head regulator is provided just on the upstream of the diversion dam. It serves the following functions:~ i) to regulate the supply of water in the canaL ii) to control the entry of silt in the eanal and avoid back flow and formation of stagnant zonps in the pocket upstream of the sluicev·lay. The intake structure is normally aligned at an angle of 90°"110° with the axis of the diversion dam. In case of major projects, the alignment should be fixed on the basis of model studies. Regulation is done by means of gates. According to the old practice, the intake structure had a large number of small span gates, but modern trend is t.o use steel gates of spans ranging between 8 to 12 meters and operated by electric winches.
6.6.2 Type of Structure Pipe or box culvert may be adopted at sites when chance of silting up is remote and when open trough structure is found to be uneconomical.
5.6.3
Crest Level and Waterway
6.6.3.1
General
The crest level and waterway of the head regulator are inter-related. Control on the site entering the canal is provided by keeping the crest of the intake work about 1 m to 1.5 m. higher than the crest level of the sluice,vays. The prindple invo!verl. being that the required discharge is to be passed into the canal with the designed "pond level" (or level to which water can be raised in the river at. low stage by ffi(~ans of a diversion dam). The crest level is fixed from the following considerations: a) Head over the crest measured below pond level should be sufficient to push the design discharge of the canai, without. lowering the canal water level to satisfy the command water level over se;vice area. b) Crest of intake should he higher than crest. of the sluiceway as per guidelines discussed in Chapter 7.
57
Q
:; ·}
iJ
:~
<0
<;f
.,.
{_() (9
·.,4
lL
>j N
i
·.;~
;j 1'1
~
I
I
1
I
<>< 0
.g
E\
<0
1,,
0 .,. II
::1
;~ ·,~
0
:'!: :>.
~
·u
N
0
j
<::!+
0
ro
0 N
0
ro
I
I
N
OOJ
~
N
"'
0
(DIAMETER
OF
SE01iv1[NT$
<0
0
.,.
N
0
0
TO BE EXCLUDED, 01 , mm.
0
"' 0
0
<0
.,.
0 0
0 0
N
0 0
0 0
-"'
(/)
.,.0
0
C>
.s ~
i3
------ N
.,> 0
0
~'
'IE1E,
GQ.l<4PETENT
BOTTOM
VELOClTY
( Vb -
MPS :
c) If a silt ejector is provided, the crest level of the intake shall be fixed at the roof level of the silt excluder. d) Crest level should be sufficiently low for economy in waterway length.
6.6.3.2
Necessity of Breast Wall
As the required discharge into the canal can be passed at pond level, it is necessary to provide a gate controlled opening only from the
The principles involved in the design are described in chapter 12.
6.7
RIVER TRAINING WORKS
River training works at the diversion head works are required to guide the river and provide a smooth appro::tch to the work and also to prevent the river from outflanking the banks. This purpose is usually aecomplishcd by guide banks on either side. In addition. m;lrginai embankments
60
CHAPTER 7
HYDRAULIC DESIGN 7.1
DESIGN DATA
In the design of diversion dam, the investigations to be carried out and the data required have been discussed in Chapter 2. Before taking up the hydraulic design, it is imperative that the designer is fully conversant with the projeet as a whole, the field conditions, and the data necessary for the proper design. The collection of design data should be given extra attention.
The data which arc essential for hydraulic design of a diversion dam, can be categorized under:a) Field data bl Design parameters
7.2
FIELD DATA
The following data have to be collected by field measurements and on the basis of investigations carried out.
7.2.1
Topographical Data i) An index map with salient features described in Chapter 2, para. 2.2.3.1
ii) A topographic map of the area around the proposed site of diversion dam. Ttle contour plan of the area around the proposed site of the dam with contour intervals of 0.5 m. on flat area and one metre on steep area. The contour plan should extend to about 2 km. on the upstream and l km, on the downstream of the site and up to 1 km. on each bank of the river. iii) Cross section of the river at the proposed site and at intervals of 50 downstream and at intervals of 1001200 metres beyond up to GOO site both on the upstream and on the downstream respectively. If the ciablc fall in the river slope, cross sections may be taken at closer conditions.
m. for 200 m. upstream and metres from the proposed topography indicates appre· intervals depending on site
iv) Profile of the river bed with observed water levels along the deep current for a distance of 1 km upstream or farther if the backwater effect is likely to extend, and 1 km. on the downstream of the proposed site.
7.2.2
Soil Foundation Data ~~r
•. .
• <, , •
L'
-.,. 1- • . . . -
··-''-
·'- ,.
-'- '
61
For diversion damsi\vcirs with rnore than 3000 cumecs dischargr the spacing and depth of bon~boles may be deeidNl based on rl'quin'nH~Jds of individual cases and si!e requirements. Such log ch;-~rt.s of holes along t.hP upstream and downstream cut off lines may be taken Ln decide t.he bot.t.om levci of cutoff, in case impermcablr: strata ar(' met. with. 7.2.3
Hydrological D<1ta
A gauging st-ation at or nt'ar tlw proposPd site of t.he \VOrk should lw established ;ls soon a:-;; the proposal for the work i.s first. initiated. and sLa.ge dischargp dat.a should be ro\!cet.ed. Two gauges should be ps!.ahlished upst._reanl of UH· axis of the proposed work, one on the upstream and Olll~ downstream nf
the axis. i'vlorc gauge:::; may he installed
wiH~n
necessary.
Other hydroloRit';d dJta may be cniicct.ed in aeeordance with t.be details given in
7.2.4
chapt.(~r
2.
Miscellaneous Data i)
Samples of sudan: and sub-surface wai.cr should be tested for sui Lability of concrC'Ling.
iil Wind ve!o(·ity should lw obtairwd for working out wave wash etc. This may howevc:r, he rvquircd in case of darns wiH·re thP water depth is large. iii) Others as per details in Chapter 2
7.3
DESIGN PAI1AMETERS
Terminology For the design p:trarnt-!.Prs. discuss(•d lwrcunckr, Lhe following terminology {ddinitinns) sh;lll apply in this chapter
Llw ;:;a me ;ts it. \vas
LhP construction nf Lhe dam.
ii) ConrPntrat.ion heLm· Tlw factor by which the dischargP JWr unit. length of a diversion dam assuming uniform dis! rihut.inn is rcql)ired to be mult.iplipd to get !.he design per unit kngl h for designing" it.s various ('knwnt.s. iii) Exit gTadif'n( foundation soii at is morl~ than the hydraulic r;radicni
iv)
The upw:nd S('epag<~ force per unit t.lw exit Pnd ni a diversion dam, Lc·nding suhmPq(cd wPight of a tl!lit volume of of Pmcrging st.reamlirws at. the end of an
vnlti!'llP of pcrcolai.in;~ watl'r t hrntl_t.;h to \ifL up Llw soil particles it' t hl· i'nrce the part.icks. It is also de lined ;ts the impervious apron.
factor Tlw ral.io or Uw ovr~rall length of Uw dam provifh:d t.n tiw tht'Oreli('a]iy computed minimum stab](' width of Lhe river ::tt. the design flood obt::tirwd on Liw basis of Lacey's
LO()Sf'[H'S.<.;
C([\Iation.
The kv~_·l of wa\.cr. immediately upstream of a dam. t.o facilit.aLe withdrawal intn t.hc canal or for any ot iwr purpnsr:. vi) Iktrogrcssion It is t.h(• pr·ogTcss'ive rlPgradation, of the downstream lines and \cveis as a rTsuit of construction ol a dam, c;tusing low(~ring of tlH· downstream river stage. Ttw main p;lram('LC'rs \vhi('h shotlld lw dPeidcd on Llw basis of available data and by Px.t·rcising engineering judgc'illt'nL an~ as under: i) Dcsi:;n rlnod i) Pcrmissihl<: afflux
iii) Scour and silL factor
02
iv) Safe ex it g r adient v) Coefficient of roughness of river channel vi) Retrogressio n of levels vi i) Coefficie nt of discha r ge under different disc harge conditio ns viii) Pond level For economic design of d iversion dams , two factors p lay predominant r ole - afflux and pond leve l. In addition, the design flood to be adopted in the design of the stru cture is also a very impor tant factor wh ich h as to be decided after consideration of economic and hydrological facto r s.
7.3.1 Design Flood 7.3 .1.1
Ge n e ral
D es ign flood is the flood adopted for the design purposes after consid e r ations of e conomic and hyd r o· logic factor s . I t may be the maximum probable flood or the sta ndard project flood or a flood corres· ponding to some des ired fr eCJuency of occurrence depending upon the sta nda rd of security that shou ld be provided against possible failur e of the structure. The p rop e r se lc<"tion of t h e d esig n flood is of utmost importance as th is has a hearing on both the fac to rs of s afe t y a nd cos t of th e str udu r e . .If t he selected design fl ood is too low it mak es for r unn ing too high a risk tha t involves not onl y t he Lo t
S tatus of Design Flood
Diversion dam s/weirs have usually sma ll s torage capacities and the risk of life and p rope r ty downstr eam could rar ely he en hanced by fa ilure of the structure. Nevertheless, div e r s ion dams over r ivers arc fa irly la r ge and costly structures . Quite often a road or r a il way bridge is also in corporated into the diversion s t r ue t u re. i\ part fr om th e failure, irrigation and com m u ni ca Lion t hat a r e depen d·e n t on the wor ks would he disrupted. In consideration of t h ese ri sks , divers ion dams s hould be designed for t h e foll owing design flood . a) 500 year flood for calculation of free boa rd of abutments, gu ide banks and afflux embankme nts. h) 100 year flood for all othe r calculations. 7.3.1.3 Determination of Desig n Fl ood The pertinen t factors on whic h the des ign flood ca n be b ased arc t h e str ea mflow r eco rd s whi c h are com pu ted from the precipitation r ecords. In the Phi li ppines. str eam fl ow re co rd s arc ava ila ble in t h e Natio na l Wate r Resources Cou n ci l (NWRC) a nd arc co n taine d in the a n nual publication entit led "Surface Wate r Su ppl y B ulletin" . Precip i tation records and oth er meteo r ologica l da ta can be obtained from the Weather Bureau or from the annua l publication e nti tled "Annual Cl imatological Re view." T he more commonly used methods for estimating design flood include: i) hydrological methods;
ii) empirical flood formulae; iii) enve lope curves;
63
•
iv) flood frequency analysis; v) rational methods involving unit hydrograph; i)
Jiydrolo giw/. Method
The design flood is obtained by applying a safety factor which depends upon the judgment of the designer to the observed or estimated maximum historical flood at the site or nearby site on the same stream.
In some cases, after ~he flood subsides, it may be possible to determine the magnitude of the peak discharge on the basis of the cross sectional area and the water surface profile indicated by water marks on the banks of the stream. Such peak discharge determination can be made by Slope Area Method. This method can be ut.iliz.Nl primarily to determine the discharge of a stream from specific field data. In this method, Manning's formula may be used for the determination of velocity but it would be subject to the uncertainty of the value of the roughness coefficient. iii
Empirical Flood Fo,-mulae
The empirical formulae commonly used in some countries are the Dicken's formula, Rave's formula and Inglis' formula in which the peak flow is given as a function of the catchment area and a coefficient. For Philippine conditions, A. A. Villanueva and A.B. Peleiia in their article "Notes on Intense Rainfall and Hun·off Luzon" (published in the Philippines Engineering News Record in December, 19391 has been able to derive from cnveiopc curves Empirical Flood Formulas for Extreme, Hare, Occasional and Frequent events. The first three arc as follows: 210 A jA + 17
150
)A
85 A jA + 9
Q.Ocosionai
where: Q
=
+ 13
A
=
Drainage Area in Square kilometres, and
Discharge in cu. miscc.
Discharge values should be used keeping in view the limitations described in the 'article'. In designing irrigation ·diversion dams which are usuaily low and of the ovcrfall type, it has been the practice of th(' National Irrigation Administration to adopt the average of the rare and occasional flows as the design flood for dam sites without streamflow records. However, the values of the coefficients used in such cm'pirical formulae, generally vary within rather wide limits and have to be selected on the basis of judgment. They have limited regional application, should be used with caution and only when a more accurate method can not be applied for lack of data.
iii)
Envelope
Cnrm~s
In tlw envelope curve method, maximum floorl is obtained from thP PnveloiW rurves of all observed maximum floods for a number of catchments in a homogenous meteorological region plotted against
64
drainage area. This method, although useful for generalizing the limits of flood actually experienced in the region. cannot be reiicd upon for estimating maximum probable floods for the spillway capacity except as an aide to judgment. Thus. the methods at i), ii) and iii) are helpful in arriving at rough preliminary estimate of the maximum flood.
iv)
Flood Freqnency 1iiethods a) The probable fn~(jU('!WY of a flood of a given magnitude can be determined on a mathematical basis
~1y the law of probability provided that sufficient. records arc available for the study to represent truly
average conditions in t.he river. FrequPr1ey analysis methods may he used provided t.hal the design flood is within the aec('pt.Pd degree of extrapolation of avail:lb!c streamflow records. It may also be used as an end to the estimati0n of design floods for projects when the unit hydrograph method cannot be used for want of adequate data. Many methods of flood frequenc_y determination based on streamflow data have hecn published namely "Gumbel ivlethnd", "HazPn l'v1('Lhnd" and "Goodrich Method". More eommonly used in the Philippines is ihc Gumbel Ml't.hod. This method giv('s fairly ac<·cptabl(~ valu<'S for flood estimates up to 2:1 years when 20-ycar streamflow data is available. For bigger pPriods like 50-year and lOO·year. corresponding theoretical values ran he cxLrapolat.ul by mathematical computation. The method is illustrated in Appendix
I. b) Flood Frequcn\'y on Hegional Basis \Vhen there is v('ry little hydrologic or hydromet.corological data available in the basin itself, regional flood frequency analysis is undertaken. The techniqtH' !"or making rq~ional frequency analysis \Viii involve coilecting annual peak discharges of all rivers in t.lw n~gion v. :it.h avai!ab]p historical rr:cords or gauging stations along with the respective watershed an~as. t.hen undC'rt.aklng tlw rlnod ·frequency analysis for each gauging station hy Gumbel
method to determine the estimated peak flow for 2, 5, 10, 50, 100 and 1000 year flood making a homogeneity (Least. Sf]uare l\1ethod) computing the ratio of peak discharge for each gauging station to the corn~s· ponding Mean Annual P(~ak diseharge; <·omputing the Confidence interval (Cl) and plotting the Regional F'requency curve, plotting th(' Drainage Arca·Mcan Annual Flood Relationship curve for the region; and finally deciding or selecting the d(~sign fiood to be adopted in the diversion dam.
v)
Ra.t.ionoll~Iethods
Jn.-oolm:ng Un£! !Iydrograph
In this method of det.l~rmining Lhc design flood. the steps involved are:a) Analysis of rainfall versus runoff data fcir derivation of loss rates under critical condition; b) Derivation of unit. hydrograph by analysis (or hy synthesis} in cases where data are not available; e) Derivation of (ksip;n storm; and d) Derivation of design flood from the d(~sign storm by the application of the rainfall increments to the unit hydrogr·aph. Um:t I-Iydroymph. f,imit.oJions
al The uni~ h_y. dror;raph principlP is not. applicable for drainage basin having an area more than SOOO km::: \vlWrt' v:tlley storage effects arc noL reflected. h) Unit h:{drograph principle is not. reeornmt>ndcd for raf.\'hmcnLs having an an'a le.;:;s than about
25 km2_
65
c) Large number of rain ga u ges s uitably loca te d s hould be available in the entire catchment to reflect the t r ue weightcci rainfall of the catchm~nt. d) Unit hydrogra ph principle is n ot appli cable when s ignificant catc hment a r ea is s now bound.
7.3.1.4
Selection of Design Flood
In t he P hilippin es. t he re gions arc subj ected to ty phoo ns . w hi c h generally, a r e accompanied by he avy ra infall s giv in g rise to fl oods , extr e m ely he a vy in ma g nitud e hut s hor t in duration. Occur r e nc e of s uch flood s ha ve to be taken into consid e ration when ca rryi ng out hydrolog ical analysis. W hile deciding th e 'design flood to be a dopted in t h e stability a n a lysis and other as pects of de s ig n during flood conditi o ns. th e design enginee r should therefore evaluate and com pare all th e different results obtained from various methods since unfortunately, no m ethod is perfec t in predicting with ce rtain ty t h e t im e and size of a ny floods t hat may take pla ce in the future. Thei r occurrence a nd mag· nitude arc as uncertain a s the meteorological phenomenon and continuity factors which cause t hem. By exerc isi ng good judgment and r e-exami na t ion of t he p hys ica l c haracte ri stics of the water s hed , he ca n select the m ost acce ptabl e flood d ischarge to be used in -t he d es ign.
7 .3 .2
M aximu m Permissible Afflux
The widt h of the di ve rsion dam is gove r n ed by the value of afflux to be permitted at t he des ign flood and t h e proposed cr est le ve ls . Ma ximum pe rmissib le afflux is a ls o im portant for the desi g n of ciowns tr cam, cistern , flood protection and ri ver t raining works, upstre am an d downstr ea m protections an d cul offs. S in ce the h ngth of water way , corr es pon ding d isc harge pe r meter and afflu x are correlated, by proviciing hi gher afflux t.hc kn g lh of th e dam ca n be r educed but the cost of the dam and training works would inc r ease due to t he increase d head o f water. Maximum p e rmiss ible a fflu x s hould therefo r e, he decided based on the foll owing cons iderations: a) loss of rev e nue clu e to suhmNgence s hould be as s mall a s possible. b) cost of a lflu x em bankments and other t rainin g works for protecting property on the upstream s h ould b e minimum. Wit h re ga rcl s to Lhe limita t ion on t he disc harge conce nt ration per m etr e w idth of dam, th ere are instances wh en d i!>r hargc intensity ha ~ been provided as 25 cu mecs/m le n gth or so. Theoretically also, th e r e s houlc! be no lim it lo LIH' dis charge intensity hut it ca nno t be increased b eyo nd certain limits du e to the co nstr aints of perm issible afflux a nd the level of t he down s tream basin required from hydrau lic jump con si d e ration. Furthcrmore, in so me ca~es whe r e drivin g depth o f s heet piles is limited due to existence of b oulde rs, robbles and gravels in the unrl e rl y in g strata. th e value of afnux ma y have to be limitc d based on t he ro ns id c r al.ion o f pract icable d e pth to wh ic h s heet piles can be d riv en. In some cases. high cost of block pro tect ion and ripra p m ay dictate to limiting t h e va lue of the afflux. The ex tent of afflux to h e provided is a lso go verned by co ns iderations for s ilt exclusion on divers ion dams. E nde avour should be made to attai n the most eco nomi cal com bina t ion between e xte nt of afflux and leng th of wate rway. Thi s can h e done by trial and erro r . For prel imi na r y calcul ation s , a valu e of 1.5 rn to 2.0 m in fo ot hills . i.e., in t he steep rearhe s of t he ri ve r with b ou ld e r or rock bed and O.G t o 1 m in plains m ay be adopte d depending upon e levatio n of the submergence area.
7.3.3
Scour a nd silt factor
River srour is like-ly to 0('\llr in erodiblr «oil« lil
66
...
tions. If actu~tl ohsPn';llinns ;ne not :tvail:1hlP. it may b(' assumed th;d. thf' scour dof's not extc·nd !wyond 2.0 m with the· c·oiwsivP maic·rial. Extent. of scour in a river \Vit.h cohcsion!cs.s bed matcrlai may be cal· eulatcd hy La\'cy'_c; ror:11ULL H
CJ.rn (
H
1.:):) (
v)
I'"\
'N
lH'll
]oOS\:lh'SS
bel. or is TllOt'(' than
or ~(
11 -: 1 whr·n loosr·rwss fa\Lor is less than 1
\Vhcre T(
d(•pth of
:>COlli'
h(']nw t.iH' dc:signPd ma:
:;ilL factor
mr
avt·rage part.i\h' diameter in mm.
q - norm;li inL·n;-;;ity of d!sch:1rgc in m: 1/s('C pPr metre \vidLh S
7.3.3.1
\Valr·r stlrhc(' slopr·
ThP foi]O'.Vii1J~- l:lhk ,L;l\'(':-; vaiur·s off for v:lrinus types
or soils
Table 7.1
Soil
Sit.(' of particle;;
1
0.3:J
{).()f\
0.50
0.1;)
O.GR
0.3
0.9(i
(1.()
0.,,
1.21
0.7
1.-17
1.0
1.76
2.0
2.19
5.0
3.89
l (). ()
5.56
20.0 Bo11ldc:rs
7.88
:JO
12.3
7:)
15.2
19()
G7
7.3.3.2 The extent of seour in a river with erodible bed material varies at different pbccs along the structure as indkated in Table 7.2 below:~
Table 7.2
S.No.
Loeation
Design Scour Range
Mean
----··---------·--··-· -------·---------- ··-------·
1.
Upstrt>anl of impervious floor
1.2') to 1.75 R
1.5!\
2.
Do\vnstre.:lm of irnperviou.s floor
1.75 to 2.25R
2.0R
3.
Noses of guide han};::;;.
2.0 to 2.5R
Z.25R
1.
Nos(~s
2.0 lo 2.5i\
2.25H
S.
Transition from nosc t.n straight
1.25 to 1.7 5H
1.5R
G.
Straight rca('h o[ guide hanks
1.0 to l.5R
1.25R
Not(~:
of training \vall {Divide \Vall)
The ealcuiat.ed values of H. are without 20°/o concentration in the values of q (intensity of discharge)
Depth of cut off The depth of scour lo dct.erminc dPpth of cutoff wall or sheet pile can be worked out by the formula discusspd under para 7.;l.:l. Tlw ('Utoff dept-h should generally l)(' provided for a scour up to lR for the upstream and 1.2sn. for t-he dov,,nstrC'am. \Vhile working out H., 20°/o concentration in the value of q (intensity of disehargc) should be taken into aceount..
7.3.4
Safe Exit Grudient
Exit gr;ldicnt. is the gradiPnt. of sub.soi! flo,_v line at exit from the structure. By providing adequate cut. off depth in relation to floor !"ngt.h, this gradiPnt has to be kept within iimit to prevent di;;placPmcnt of soil p;~.rticles. As diseusscd in Chapter ;) allhough the crit.ieal value of exit is 1.1 or so, in view of other unccrtaint.iPs in n:1turc' of soil, it is dcsir~lble to .:tpply a factor of safety as given in the table for various types of soils.
7.3.5
Roughness Coefficient
In using Manning's fonnula for dPt.(•rmining flood dischargL' of the river hased on cross sections and available water sudaec slope~, thP folimving table is a guide for adopting a suitable value of "n" (coefficient. of roughness)
v
GS
Table 7.3 Channel condition
Value of "n"
1. Natural streams with good alignment. fairly constant section
om
2. Mountain streams in clean loose cobbles. Hivers with variable section and some vegetation growing in banks
0.04 to 0.05
3. Rivers with fairly straight alignment. and cross-section, badly nbstructecl by small trees, very little underbrush or aquatic growth--·
0.06 to 0.075
4. !livers with irregular alignment and cross-section moderately obsstructcd by small trees and underbrush. Hivcrs with fairly regular alignment. and cross-scdion, heavily obstructed by small trees and underbrush
0.100
5. Rivers with irregular alignment. and cross-section, covered with growth of virgin
timber
and
occa.<:::ional
dense· particles
of bushes
and
small trees, some logs and dead fallen trees .
0.125
6. Rivers with very irregular alignment and cross-section, many roots, trees, bushes. large logs. and other drift on bottom, trees continually falling into channel due to bank caving "-·
0.15 to 0.20
If the dc~cript.ion of channel condition docs not fit in with the actual condition obtaining at site, the value ofn should be worked out as follows:··i) Assume a basic value of 0.010 for channels in earth, 0.015 for channels in rock, 0.014 for channels
in gravel, and 0.028 for channels in coarse gravel. ii)
iiit
iv)
Add as follows for degree o[ irregularity -smooth
0.000
minor ·- ·- ·-·
0.005
moderate
0.010
severe
0.020
Add for change in size and
shapt~
of cross·Sec!.iorl
gTadual
0.000
occasional
0.005
frequeni
0.010 to 0.015
Add for obstruction such as debris,
roots.~etc.
negligible effect -··
minor
c~ffecl.
appreciable effect severe effcet v)
- 0.000 0.0 10
O.o:JO - O.OGO
Add for vegetation low effect
0.005 to 0.010
medium effect
0.010 to 0.025
high effect -·· ··- ~ ---- --·
0.025 to 0.050
very high effect
0.050 to 0.100
69
vi)
Add for dntnnel Tneander --(Lm = rnca nder length of reach) (Ls '"""" straight length of reaeh Lm/Ls 1.0 to 1.2
0.000
1.2 lo 1.:0
0.15 times n:"
1.5
where n..,
7.3.6
=
n
0.3 times
n~
(i) + (ii) + (iii) + (iv) + (v) above
Retrogression of Levels
As a result of Lhc construction of a dam and (~onsequent ponding: up of supplies, rciaLivcly silt free water escaping over the crest picks up bed silt dmvnsLream of the dam. This cause dcgradalion or retrogression of downstream levels. In the first few years the rclrogrcssion of kvcls is rapid and progressive. The lowering of level known as retrogression is found Lo be high at low river stages and small at high river stages. The pro<:C'SS of retrogression and shoal forrnation on tlw upstream develop and continue for a number of yeJ.rs but a stage is gradually reaclwd vv·hen upstream pond absorbs no co;1rse silt & rcversf~ process starts. Thr designs of downstream works have. h(nvever, to be tested under maximum retrogressed condition of the river for cnsu_ring safety of the structure. Hetrogression of water levels is more pronounced in alluvial rivers carrying more silt, having finer bed material and steep slope. A va!ur of 1.2~ to 2.25 meters maybe adopted as retrogrc~s:;ion for ai!uvial rivers at iower river stages dqH>nding upon Uw amount of ::;ill in the river and its slope. and 0.:3 m to 0.5 m in high siagcs. For intermediate disch::trgcs, t.hc effect of retrogression may be obtained by ploLting the retrogressed high flood lpvp]s on a log --- log graph. For tht' purpose of cksign, low slag:e retrogression value may he ::tpplir~d for Zl~ro discharge elevation for a strc::trn with morc' or l('SS uniforrn bed lev(~]. F'or a stream with unpven bed 1Pvel, low stage retrogression value may be appiicd to the st::tgc~ corresponding to S-10°/o of design dis('harg(-! or eorresponding to t.hP base flov•/ whichever is lower. High stage retrogression valtw ma_y lw applied Lo mean annual flood diseharge or in the absenre of discharge data to 60-70°/o of design flood discharge. _ In case the proposed diversion work is situated dO\vnstrcam of a dam, the possibiiiL_v of heavier ret.ro-
d
gi~c-:ssion than normal should be eonsid('recl for the design of downstream flood level and downstream
protection \VOrks.
7.3.7
7.3.7.1
Coefficient of Discharge
General
The flow over tilt' divt:rsion dam may be r.ont.ro!ied by means of gat.es or allowed freely without restrictions. The upper surface: of the freely fiowing \vatcr is at atmospheric pressure. The lo\vcr surface is generally guided b_y t.hC' .surfaec' of the spilhv::ty crest itsC'lf. In the oger type of spillway, tlw overflo\v crest. is shaped Lo conform as nearly as possible to tlH~ profile of t.h(: lower nappe of a ''!'!nt.il:J.lPd jet of w<~ler· issuing· nver ;1 sharp·crestPd '-'--'cir. 'J'hC' ,-.rr.st 9r0file. thus ha"' tp lw desif!nr>rl r~_ot l~ ·,., ...;ui? ·'he int.PndP(.l hvdr::l:lli;: ,•nnr.!i!-ion~ bu: -.:ht!uirl ai:c:c) fit til(' intendco ciownsu·(•;Hn profill~ oi the spilhvay body wi1ich is deter~rniTH:d by structural conditions. The basic equation for Lhe discharge over an overfali dam Js
70
IH
where
Q
.,.,
L
total discharge in cumec length over the crest in m
H
=
head over the crest in m
C
=
coefficient of discharge
Coefficient of discharge for flow over the spillway (ovcrfall dam), and sluicewaY would depend upon the following featurcs:a) whether the top of the structures acts as sharp crested or broad crested; h) whether the crest is submerged or free; c:) shape of crest and upstream slope,
The discharge coefficient, C, is further influenced by 1) depth of approaeh 2) ratio of darn height to the total head on crest. Head due to approach vclocit.y can be obtained from the equation h
where
va ,. . , g
:;
=
v 2 _J_
2g
velocity of approach in meters
,., 9.80 meters/see.
In the c~lsc of ogee crest, the dlscharge is given by the formula
when
L
effective length of crest
He . . .,
Lola! !wad on the crest. including velocity of approach
i) Effcctim;; length of omnflow cn;st
When crest piers arr~ provided between the abutments in ease of gated spillway or bridge over the dam, the piers contract the flow and reduce the effective length of the spillway as follows; L = L' --· 2 (N kP + K) II(, when
L
-~
effective length of the
L'
~
net length of the crest
N -
(TCSL
total number of piers
kp "" pier contraction codfficient k:l
~
abutment contraction c:ocffieicnt
ii) Pier and ahutment contraction cocffin:ent The pier contraction eoeffi('iPnt. kn, is affedcd hy the shape and location of the pier nose, the thickness ');,,,_. 1\~n hp·:rl ;" ~~,j.,t;()n l ( ' f)~,, '~rH>~~P' hp::d ':~!'rl t\,..,. .-..~)r);'l'"'h "f~]()('itv l<'nr '"()flrlii_i()!~"i f)f desi\!n
.r f!•r··
71
1' or squarc-nns('(l piers with <:orne:rs rounded on ~~ r;tdius pqual to abou! OJ of the pier thickness
0.02
For round-nosed p1ers
0.0 l
For point.(•d nosP pic1·s
0.0
TIH~ ahut.mPnL rontract.ion eoeffieicni is affc~ded b_y the shape of the ;:~hutment. the angle hct.weu1 the upstream appro:u:h w:d! y.nd the axis of flow, the head in n>\aLion to the design head. and Lhe approach velocity. F'or \"Onditions of dcsi_t;n lw;l(l, H 0 , average coefficient may be assumed as follow;;:
K, For squar(' ahul.nwnLs with heachvall ::tt Lo din~d-ion of flow
~H) 0
0.20
For rounded ahu! ments with lwadwal! at 90° 1o dirt>cl-ion of flow, wlwn 0.5 H 0 >
I!.IGh,.
0.10
>
For rnundr·d abu!tll('nL whPre r 0.5 l-Ie :~nd hr·adw:tll is plaeed not more than '15°
() .0
i.o dirc('Lion oi" fiow
,.
ndius o[ ahuLnH'nL rounding
Thl' splliwa_v with Ol:(l'l' shaped (Tesi will beh:lV(' likt~ a sharp crested structure. In ::;uch weirs/dams with sharp <'i'l'Si.Hi prnfiie and who::;(' hPights are not iess than about one fifth Lhe lwad producing flow over t.hem. the codfiriPnt of discharge remains fairly constant.. F'or weir height. less than about one fifth the lwad, the ("OnLract.ion of flmv lwronws incrcasingi_y suppressed and Uw (Test coefficient. decreases. V\-'hcn it f(l~Ls silh~d up on t.hc upstream and sediment rises up to crest, it \Vil! behavf' like a broad crested structure. The sluiceway, lwcausc of large crest \vidth provided Lo accommod:tfc· gates ;1nd stop logs \Vil! behavc like a broad crested structure exeept under very high head exc(~eding 2.:) times the crest width, where th(~ flow \Vill be as for sharp crested condition.
Cerwraily, all cas('s \viwre Lailwat.er is below the CJ(~si are cases of free flow. \Vherc l.aihvat.er is abOVC tlw Cl'l'.S\., ;[ ll1a)' ::;iil\ \w :t fn~C flow it' there is hydraulic jump which ('Otdd be checked by dr~l\Ving the water sud"acP profile or when the depth of Lailwat.er above the crest. is less than 2/311{'. l•>.:rcpL in c:tsr• of Uw iaq(l' structur<'S where fiP!.erminat.ion of coe·fficient of dis('hargc by mode! tests are justifi('d t.hc st.:lndard shape of crest with known coefficient. of discharge as described hereunder should tw adopted.
7.3.7.2 a)
Nappe-Shaped Crest Verlin!.lfa.cerl oyee
For n;tppe sh:tpr·d tTC·st w·ith v('rt.ical face upstream, Llw basif" eoeffi('iPnt. of dischar_gc should lw (';tl('nl:Jtl'd frnrn ih(• C\l!"V(' in [<'i;~-. 7.1. The cor:fficient.s are valid only when tlw (H(l'C is rortned tn the idral napiH' :->lupc. ikpvndin.:; upon Lhe ('Ondit.ions of flow on ih(' downstream of i.hC'
Wlwn t iw ('l'(•st h:ls twcn shapr~d for a he: ad larger
72
(H"
smalh~r than Liw head under consideration,
VALUES DISCHARGE
\_ I
COEFFICIENTS
OF
p
FIG. 7.1
Ho
FOR VERTICAL-FACED OGEE CREST
I ~-·---1
uj3 VJ
1---
z
w
u
t.o ,_____ ,_
,1:-f ',
'I
+-+ -
w
""
u "-
j
0
0
-11
1-1-
""-
,_....v
v
v :__...;.-- -r_l • I
' , -i- i-----1 - +--+- -I- -i----1
' ! ! LL L_j_L
I
0.9
0 1---
"cr
RATIO
Ho OF HEAD ON CREST TO DESIGN HEAD'--
COEFFICIENT OF DISCHARGE
Ha
FOR OTHER THAN THE
DESIGN
:
j_---1-
HEAD
FIG. 7_ 2
UPSTREAM FACE SLOPE
SLOPE
ANGLE WITH THE VERTICAL 1
!8° 25
33°'41
1
45° 00
1
I
I
i I
VALUES
OF
p
Ho
COEFFICIENT OF DiSCHARGE FOR OGEE- SHARPED CREST WITH SLOPING UPSTREAM FACE
FIG. 7.3
the eoetiH'lPnt o! disTharge will differ from that shown on Fig. 7.1. A widened shape will result in positive pressure along the crest. eonLact. surface whilP a narrower crest shape ·will produce negative pressure along he contaet surface rc~sult.ing in an increased discharge. In F'ig. 7.2 is shown the variation of th-e coefficient as related t.o values of .LD.· lin
\Vhere H{. is Uw actual head being considered and H 0 is the head to which Uw shape of the crest has been designed.
cl Effect of Upstrcmn Face Slope In Fig. 7.?, is shown Uw dfect of upstream sloring face on the ratio of coefficient for an ovcrfiow
1
cn~st. It is observL~d that Lhc codficicnt. of discharge is reduced for large ratios of t_l~-- and is increased for • 0
srnall ratios. d)
E';Jcr:t. of Downst.rcmn /ipnm and Snhmergcnce
D<)\V!Lc;lrcam tai!w.:-tter level
iiil a true h_ydraulic: jump will OC{:ur.
iv) a dro\vncd j1nnp will occur in vvhich the hig-h velocity jet will follow the face of the overflow. v) no jump will on·ur !.he je-t will break a\-vay from the face of the overflow and finally intermingle with slow moving water underneath.
In this figure, t.he decrease in t.hc coeffi{~icnt is also indicated. After the apron and taihvatcr level have bPPn t.entat.ively fixed, the type of flow ean be known and the c~xtcnt of decrease in t.he coefficient of discharge can be rpad off from the figure. \Vhcn it i.e; known that t.he flow on llw downstream side is super critical or \Vhen the hydraulic jump occurs, t.hc rkcreasc in ! he <'oeffieicnt. of discharge is due principally to the back water effect of dmvnst.rearn ::tpron and is indepcndcn1 of t.lH~ Lailwater level. In that case, the extent of modification in the basic value of roc:ffieient. of discharge ran he read directly from Fig. 7.5 which plots the same data represented by t.he vprt.ieal cbshf'd lines of Fig. 7.-'1 in a slightly different form.
' l.i, ":the
/"·--....,\
\Vhen the value of
h
...
d u~-
exceeds ahou{
downstream floor position has little effect on
thP coefficient of discharge but i.lwrr~ is a dN.:reasP in the coefficient caused by t.ailwat.er submergence. In this the rnodifir:u.ion in the value of t.hc coefficient cctn be read directly from Fig. 7.G which has been prepared from fig. 7 Jl. 7.3.7.3
Broad Crest
As discussPd e:trliPr. a weir is b:·oad-
75
4.0
4.2
4.6
4.6
0.0
1.0
}-
z
w
()
1.4
0:
w
()_
1.0
I
" lI
~
w
u
w
O.l
ij!
1.2
"'
:r: ()
1.1
<J)
z
0
w
"'"'w ::< al ::l
"'"0
u.
1.0
0
}-
z w
0.9
Q 0.6
1::
0.7
8
w
"':
w w 0:
"'w 0
w
0.6
2
"'"'0:w
4
0
0.0
L)
w 0.4 0.0 0.2 0.1 0
POSITION OF DOWNSTREAM APRON
FIG. 7' 4
Effects of downstrecim influen08s on f!OV'I over weir crests
:J.087 I 11 :1'"
Q
WHEN
q
1.811
~
'
discharge in eum/sec
L
crest length in metres
II
hc;Hf producing flow in met-res (including head due to velocity of approach)
Coefficient of discharge is 3.087 in English units ancl1.70G in metric units. In cases \V\wn the upstn~am of the dam has been silted up to crest. the basic coefficient of discharge should be taken as for h:coacl crcstNl weir. b) In the design of sluiceway crest with freP overfa\1, the basic coefficient of dischaq;c shouid he taken as 3.087 in English units and 1.70G in metric units. Effect of submergence should he calcuLltcd as per values calculated under Fig. 7.7.
7.3.8
POND LEVEL
The pond level is t.hc· wa\.er ]pvel required in the undcrsluiec pocket upst.rc:1m of thf' intake wat.rr to feed full ~uppiy diseharg:(; in the c,1.nai. The pond lcvtd .o;hould be such that the command level in the can~d can be attained with Uw (ksign dischargt' after meeting all t.hc losses in head incurred in passing through the Intake. Various losses al t.he lntake structure arc enumerated as follows:-i) Change in an1;ie of flow in,; water from approach channel towards Intake structure. ii) Loss due to convcrg<'ncc at inlcttrltnsition (or entrance loss at the sill arcai. iii) Trash rack loss. iv) Sudden constriction loss at entry of the gate. v) Gradual contraction or transition Joss from the gate opening up to the st.arl of Lhe uniform barred section. vi) Friction loss along pipe/ba1Tel. vii) Bend loss, if any. viii) Sudden expansion loss at. exit. ix) Loss due Lo
diveq{t~nce
al the otttict transition.
Methods of solving for each kind of losses have br:cn detailed out in chapter 9 of the lvlanual.
If under certain situations t.hert' is a limitation of pond level, the full supply lc·.,·e\ shall be fixed by substracting the working head from the pond level. 7.4
7.4.1
HYDRAULIC DESIGN OF SPILLWAY AND SLUICEWAY Economic Design
In the desir;n of diversion dam, l.wo t'act.ors pla.Y predominant. role ·- "afflux" :tnd "pond \evrl''. As clis.r.usscd under paras. 7 ..1.2 & 7.53.R afflux is fixed from considerations of submergence. habitation of valuable land. site conditions eLc. The p(;rrnissible afflux may or may not include consideration of cost only. Pond levr:l, however. is determined hy topography of the eommand area and Lhere is no scope for adjustment. of pond level on consideration of cost of the structure. \Vhcn permissible affiux is decided, the maximum intensity of discharge is fixed and hence the
77
------------------------------· 1.00
-,---,---
.--,---~-..----,-,---r·--,-----,--,--,,--,~-------,-___.-=-·"'~·
f---1---1---+---+--t----t·-----t--+---t- ;:::<"f~t----t---t----t--t·-----j
----'-----~--
f-----1----
,, ""n1 )>
-l
0
, 0
()
0 0
l>
I
"
n1
()
()
0
0
0
1
0 0 iii rn
z
-l
i2d
/
/
1
0.80
lL
I I
'
He
'
1
~d
:
1---+---+---1+--+---~--.
., ,, -l
:
~
'
rn rn -n
"'
:
I
,
Gl n1
+-----+--+---+---c'l-··-- -
r·---+--+~---==v----~~"-_J__V______J.____L_···_·_·=r~= __j ____L_----j[---r-1-~ V ~~--~11;;----------------: l
o.9o 1-----+---+--+-/-___.F---t---1
Ui ;:: I
I
I
0
n1
_- _.
'
i
-~
:
1
~~
1
i -~77//777777777
7/J/7///7///::
/I
---+---1---!---l---1--- ---t---1----+---+---t---+----- - - - ---1----1
0.76 1.0
1.1
1.2
1.3 P 0 SIT ION
1.4 OF
DOWNSTREAM
I. 5 APRON
Ratio of discharge coeificient$ due to apron
1.6 -
1.7
hd+d
efh ct.
FIG. 7.5
I .8
1.0
r---~~--~----ll----r---~-----r-~=-=r====1=====t====i=====f=====r~~r=--~-----r----l ------~---- - - r - - - ------+----+----::;,.-f""---t--+-----+----l--+----+----+--l---+---+-----4
'" 1-=----~---+//v i
.,;o "' rn 0 rn 0.., ::2 ;: "' 0 :r: )>
;o
"
0 ()
01
c
0.4
I
'r------+1+---+---+----+------l----------+-f---------1
rn :)!
__ 4
1
;;;
~e
0
l
:
~
rL/72/ / '/
r
:
I
.,z .,;;;z ~~
!
i
////////
1'i (l
()
-- -r ---- -r:ha- - - - - - - - - - - - -r· -l_hd
0
rn
'l1
0.6 --
::! m
0
0
i~ _____+----+--~1--+---+----~L_-______.__+--_:=::1_--t-+--1:
I-----------t---+-~1-----+----
)>
-1
0
/v
02
I
~-----+---+--~~------r------+----+----~---+---+--+-----+---+--+-----1----+--0 0
0.1
0.2
0-4
0.3 DEGREE
OF
SUBMERGENCE
0.5
0.6
0.7
h d He
Ratio of discharge coefficients due to toilwater effect.
FIG. 7.6
0.8
minimum waterway. \Vlwn pond level is decided, the height of gates and depth of dam floor get fixed. At this stage, attempt t.o reduce the cost of diversion darn consists in finding out:--a) whether inrreasing the waterway frorn the minimum value (which has been fixed by maximum allowable discharge intensity) would reduce the cost and if so whaL should be the waterways for minimum· cos t.s? b) to what extent. Lbe length of floor may be reduced for minimurn cost? One considc-~ration for fixing minimum wal.envay is the rnaximurn discharge intensity consistent. with allo\-vabie afflux. Another consideration is the maximurn depth of cut-off, which is ·worked ouL on the basis of scour dc'pLh. The practicability of Lht~ (~utoff should be considered from view poinl of driving sheetpi!c in alluvial soil or excavating cut.off pit in boulder reach. The maximum discharge intensity which satisfies both these criteria is adopted and thc: minimum waterway is calculated. Though it would appPar that. minimum \Vall~rway \VO\l!d give minimum cost, this is not. always true because dccrcasP in \vatenvay is increase in discharge intensity and would mean interaction of the following two opposite f:ld.ors: i) decrease in e~ost of gaLc~s. bridges and piers;
ii) inerease in ("(}SL on account of increase in,_lcngth and thickness of downstream floor, dt'pth of cutoff, volume of p(:rvious prot PeLion. work at ends ·or floor, cost of dewatering operaLions, cost of launching apron in guide banks and afflux embankments. lL is the net n'sult of lhc.'>c~ two farlors which dc~tcrminp to what extent the minimum waterway fixed from above eonsickratinns should be ineecascd for maximum economy. IL has bc:c~n found Lhat incrc:asp in discharge~ intensity (decrease in waU~rway) at first reduces the eosl. i\flcr l"P.:-tching a minimum, thC' cost increases with increase in discharge intensity, specially in those cast~s where gates are of bigger siz('S.
7.4.2
Waterw"J.y
As discusse~d in t.lw prc't:C'ding para.f~Taphs. in a diversion dam, the length of waterway and afflux are eorreht.cd. For thr c'conomical and hydr:rulicall:y e[ficiE~nt design, the combination bet\-;1een the two is requi1·ed to be mark by trial and error. Vi/hcn~as, in deep and confined riv('rs v•.'it.h stable banks. t.hc: overall wat.crway (between abut.ment.s including t.hirkrwss of piers) should he approximately cqual Lo the actual widt.h of the river at. the design flood, for shailow and meandering rivers with pcrl11l:':thlc: foundations, Lhc minimum stable width of Lhl~ river at. Uw design flood. may be worked out. from Lacey's formula:
\vhcre
F
minimum st.ablf' widi h of t.hc river in m,
design flood discharge in cunwcs. For determining the~ preliminary value of the waterway, stable width so determined can be taken as a guide.
__ . At._:~ sit~c .\~h~~rt~. due~ l:n 0xiste~1cc .ol gravels. boulders and cobbles in the foundations, depth for :--twP;._ p:ltn1~ _Js.. tlml_lPO: maxJ!:ltrn: u~1!. discharge t.h;tt would not. scour the bed below the practical depth of ("ULoti sl1oul(i ill·st. tw (!ct.ennnwcJ w1th Lace_y's formub discussed in section 7.3.3 and given as foilenvs:
80
----·-o BROi-\0 CRESTED WEIR !. DiSCH.ARGE
FOf~
Q= 3.087 LH3/2 Q
= i . 706
I L DiSCHARGE
ji AFFLuX·Eci.-9
(METRIC)
I
SUBMERGED CREST CONDITION:
EL.
-~
I
_,__ I
'
( ENGLISH UNITS)
L H 3/ 2 FOR
I
FREE FLOVI CONDITION:
:LT. W. EL. (M. FU
I
:he I
'1-
,i'l~
-'--CREST LEVEL
REL!-\TiVE SU3l',IEHGED
COEFFiCIENTS s
CREST AND
FREE
FLOW CONDITIONS
-·----~-
h 11
~· ~-·-·
.,
3 1 nc
0.0 0. t 0.2 0. 3 0. 4 0.5
I
tI
I +- '
I i --+i
r:r 4 v 1.000 0.991 0.983 0.972 0.956 0. 937
··--·-··t·-----~--
' /'nc ns
f
I
1=-:~
I
·""'
0.6
~---------
I
'1
------
L
·······-----~--.
..
"'
'
'.-
I
'lI
0.':3_07 -0.856 --------0.778 ---··---·-·--------0. 62i . 0.000 -' ----- - -
0.7 0.8 0.9 i.O
~ !'">
'v-£1 .....
--
C 5 = COEFFiCIENT FOR SUBMERGED CREST C =COEFFICIENT FOR FREE FLOW
NOTE: ABOVE IS AN EXCERPT FROM 1 TABLE 2a, ENG G. FOR DAMS, P. 373 1 VOL. II, BY CREAGER, HINDS & JUSTIN.
""I G. 7.7
R
1.35 (i-)l t J (metric unit)
q
unit discharge (allowing 20% concentration)
f
silt factor corres ponding to grain siz~ at a minimum depth of 1 m : or deeper where feasible
R
scour depth below des ign flood level.
Generally, in bo~J!dcr r eaches, it would be economical to reduce the waterway to about 0.6 to 0.80 times thr> Lacey's waterway. In plains where the silt factor is in the neighbourhood of unity, it is generally economical to keep the waterway 1.0 to 1.2 times the Lacey's Waterway . It is preferable to have a narrower waterway, which also reduces shoal formations.
7.4.3
Shape of C rests
The ove rfl ow portion of LIF' dn.m (sp ill way) may be ogee-shaped w ith upstream face ve r tical or inclined. The sluiceway shou ld be of slab type with an upstream slope of 2:1 to 3:1 depending on site cond itio ns. The downstream slope for sluiceway shall be as required for the glacis of stilling basin and is generally 3:1. For major projects it is advisable to undertake moclcl studies for obtaining the best shape of the crests. For weirs with falling shutlers, top width of the crest shall be limited to that r equired for accommodating the shutter in fallen position. 7.4.3.1
Ogee Crest Profile
To bC' acceptable. the ogee profile should provide maximum possible hydraulic efficiency, structural stability and economy and also avoid the formation of objectionable sub-atmospheric p r essu r e at the surface . In general. the majority of low overflow dam cross sections ai-e designed with a vertical upstream face. Model tests have shown that the effect of the approach velocity is negligeble when the depth of flow (P) below the crest is greater than about 1-l/3 t imes the head of flow over the crest (Hd). With the ratio P/Hd less than 1 -1/3, the approach velocity is seen to have a noticeable effect on the discharge and nappe profile. The crest shape recommended helow is based on the studies by the United States Army Corps of Engineers after comparison with the ex perimental data of the Uni t ed States Bureau of Reclamation . a)
Downstream Profile
The downstream profile may conform to the equation:-
---
1.85
X
=
where Hd X,Y
~
=
2.0 Tid design head
coordinate shown in Fig. 7.8
. Th~ profile plotted hasecl on the ahovC' equation is designed for one value of des ign head (hd) only whtch IS generally rhosen to give the maximum practicable hydraulic efficiency. For lower heads of flow the press ur e on Lhe rresl will be above atmospheric and [or higher heods of flow the pressure will be 1 " " " ~h-a n atrnosnh~ r ir . T hr> lo\~r>r thP desig n head of the f'rp s t profi!.-.. . t hf' grea ter wi ll bC' the disch a rge coefficient for the lull ran g e ot head s . Model tests show that the design head may be exceeded by 25 percent up to which limit no harmful cavitation is seen to develop. The data for xUl!) ancl 2Hd 0·85 are tabulated on F'ig. 7 .8 for the convenience of the designer.
82
-··-----·------------·
0
~_I c:-1---~
! !
i
~i
-d
li
::t:
:::r: i
I . ii
CREST
I
;\ 1.8 5
v
"
X
xLB5
H~
0.10
0.0141
6
27.0!!5
0.15 0.20 0.25 0.30
0.0299 0.0509 0.0769 0.1078
7
36.596
B
46.831
9
58.257
10
70.795
0.35 0.40
0.!434 0.183G
12
S-9.!94
6
i31.928
168.897 210.017
7 ·8 9
2 3 4 5
.H~t
2.000
26
31.896
1.00
1.096
3.605 5.01.38 6.498 7.855
27 28
32.93 7 3 3.97! 35.000 3 6.02t"t
1.05 1.10 1.15 1.20
I. I 6 I 1.226 1.291 I. 3 58
1.25 1.30 1.3 5 1.40
I .425 1.492 1.560
!0.460 !L713
31 32 33
1.45
1.697
335.64"0
ii
37
42.062 43.053
i.~O
12
t5.354 16.532
36
£5 40.349
1.766 1.835
38 39 40
44.040 46.023 46.002
1.60 1.65 1.70
1.905 I. 975 2.045
41
46.978 4 7. 9 50
1.75 1.80
2.116 2.188
0.70 0.80
o.r)tG9
0.6516
25 30
9.172
0.8229 1.000
3~
7!8:664
13
17.696
1.00 1.20
00
920.049
it'~
18.847'
1.40!
45
! i44.046
!5
IS.S85
1.40 LSO
1.86•\ 2.336
50
l6
55
i390.253 i653.330
1.80 2.00 2.50 3.00 3.50
2.967 3.t505
60
!947.959
65
5.447
70 75
2253.863 2590.785
!2.996
100
5.00
19.638
30
41.067
255.215
16.160
29
33
20
50
X/Hct
34
0.3887
4.00
ay/ox
14.159
0.60
so so
2.H~O.tJ5
12.:>48
0.2233 0.2-(74
7. 6\$3 10.151
h,;
10
0.4~
0.50
0.90
2
0.85
37.041 38.054 39.063 40.0-'>6
l·'i 16 18
~1-.
SLOPE DATA
DO\'INSTRE/l.M QUADRANT DP.TP.
I r\
AXiS·
29<'.3.496
33! 6.77'9 4!24.235
5011.872
21.112 22 .2 2 9
17 18 19
23.33!) 24.(.3~
20
25.:521
2i 22 23 24
20.602
27.674 213.741 29.7'39
25
30.852
42 43 44 45
46 47 48
!.:35
48.919 49.884 50.846 51.607 5<:.761
49
53.714 54.663
50
55.610
OGEE DO\VNSTFtEA~\~1
PHOFiLE) FiG. 7.8
1.629
I
,_""ft, 1I
J Ha _ _ _ _
T
l I I
-
1
:11
~
1
(
----
0.270Hd
"------ORiGIN
OF COORDINATES
i
I
~ 1-]'
. i
L
UPSTREAM QUADRANT
(
j
I
,y
EQUATION: y
= 0.724
I 85 0.375 0 625 (X+0.270H1.!' (Xt0.270Hdl. 1•0.i26H·a-0.4315Hcl H 0.85 ~
COORDINATEs: X/H.
Y/H.
G
0
-o.oooo -o.ozoo -o.o40o -o.osoo -o.osoo
0.0000
2
2
2
-0.1600
Y/H d
0.0296
-0.2400
c
0.0787
0.0004
-0.1700
0.0339
-0.2450
0.0836
0.0016
-0.1800
0.0386
-0.2500
0.0889
0.0038
-0.1900
0.0437
-0.2550
0.0948
0.0068
-0.2000
0.0494
-0.2600
0.1016
-0.1000
O.OlOS
-0.2100
0.0556
-0.2650
0.1099
-0.1200
0.0156
-0.2200
0.0624
-o.2sao
0.1165
-0.1400
0.0221
-0.2300
0.0701
-0.2703
0.1261
OGEE ( UPSTREAM
CREST PROFILE FIG. 7.9
II
i!
h
For pr;::~.ctical purposes and to facilitate the fabrication of templates during construction, the curve developed from the above equation can bu u·ansferrcd into tiw nearest corn pound circular curve.
bl Upstream Proj?},.; The upstream profilf' has to be tangent to the vertical face and tangent to the horizontal at crest point to ensure that there is no discontinuity along the surface of flow. The recommended curve incorporates the ordinates computed by 'M_C,_~9-~Y_N" and others, in the region close to the sharp crested weir where accurate experimental data mcasureiTicnts arc difficult and fits the USER experimental data over the remaining portion. A table of ordin:ttes are given on Fig. 7.9 for the convenience of the designer.
7.4.4 Crest and Floor Elevations of Spillway and Sluiceway 7.4.4.1
Spillway
a) Upstream floor level of spillway should b'c fixed from the following considcrations:i) it should he fixed at the general river bed level which is higher than upstrearn floor level of a sluiceway. This is generaily in a deep. well-defined channel in front of the intake.
ii) foundation on fiJi should be avoided as far as possible which reduces to a minimum dewatering and excavation.
b) Crest levpi of dam can be fixed slightly above pond level to avoid wastage of pond water by \vave wash and anticipated siltation upstrc;un of the dam. A free board of 0.2 m. is recommended.
7.4.4.2
Sluiceway
a) The crest and upstream floor levr.\ of sluiceway should be kept equal to the elevation of the deep section of the river aL darn axis.
b) In fixing crest level of sluiceway Lhe following points should be considered: i) it should be as low as possible for proper flushing of silt deposited in front of intake structure. But it should not he too low to producP a high discharging head causing intensity of discharge in excess of pcrmis.sihle flow for the given t_ype of foun
ii) as the intak(~ sill levei is to hP kept. above the siuieeway crest to avoid silt. entry into the canal, an inercase in the height of sluiceway crest will raise the intake sill higher and reduce depth of flow over intake sill thereby requiring greatPr length of v.:aterway for the intake. lL is necessary to restrict the height of sluiccwa_y crest for economy of intake structure.
I
----j
I
O.llO h
nPond level
i
!.________
h 0
Jj_____,L._h__________
85
•
nnlakc sill level
.ia:!."lt'.' i.<:.c ~'w
ay crest lc vel
From ! ~: sb'Leh showing t.h(' rel:ttive positions of different. components of dlvt'rsion \vorks, it is obvious t.h:1i t.iwr:c is ;t limit !.o adjustment. of sluin~way crest lc~vcl and intake sil! lcvc~l. His recommended that. for /r:::l nkubtinns. l.hP sluiceway crest !c~vel maybe fixed O.S m above the upst.n:am floor level of sluic('W
VVidth of Undersluiceway
The~ fuiF!.ion and discharging l':tjla\'iLy of a sluicr;way has been dealt. with in Chapter G, under "Com· ponent.s or diversion dam". As Llle discharging capnciLy' of Uw siuicev.ray portion is CO·related with the ()Vf'ra!i discharge in the river. t.h(' width t.he sluice\V
or
/i) it. shouid he {'apable of passing at. least double the' canal discharge at Pond lr:vel to ensure good scouring cap;u-ily;
ii) it shonld lH' widr> enough to accomnwdatr> an approach chanrwl and also to keep the approach vcloci\.ir:s. sufr\{'if·tll.ly l(l\ver than crii ical Lo rnsun~ maximum settling of suspended silL; iii) it. should br· n;11-:-ow enough l.o crc~at.e a bottom vclo{'iL.Y in t.lw approarh ,-hanncl which wili rnovc the maximum diallL('L(·r ol· panklcs cxpPc!.cd t.o be dt'posiU!d with t.1w avaiLibl\~ flushing disc.harge. This is nec('ssar.v t.o m:1inLiin approach channel in good co:Hlition so that canal c:u: draw necessary supply Lhroug;h l he inLtk('. iv) in ,·a:--:e it. lwnmws ncl'l'Ssary to hring down t.he afflux ('!cvat.ion for kccpiilg it. ..,~·it.hin pcrroissible limit or for- limliil!{ down.
(spiihv;~.y
vl wiwr(' ;--;ili. r'xr:lud\'rs ar1' pr\)vidul, t.lw width of t.hc poekr:t should lw dd.c'rmincd by the velocity required in \.h(' po('kl'·l- \n induce silt:\i.ion; vi) wiwn tlw widLh of the spillway portion is ;:ppn~ciablr_~. riV('r sluices adjoining t.he pockets can
be provid('d t.o take can· of lO\\. floods and freshets thprehy economizing on t.ht cost.; vii) For other guidclin(·s in regarding \he width of th(~ pocket rc~fr:rence may be made Co para 6.6 of
Chapter G. 7.4.5.1 On all import.:tnt works Uw widj h :-ol· t.he sluiceway pod.ion and the Jen,1;l.h of training wall shall be fixed on Uw basis of model n:pcrimcnt.s.
7.4.6
Length and Level of Downstream Floor
The lengih :\iHl lr~vP! of downstream finor should be fhlermined from consickr:ti.ions of dissipalin~ the ('ncrgy of surf:u'r' flow, so Lhal. t ht' i'l'sidual (·~ncq;·y af!.e;- leaving the con(·.ret.e floor i;; too lit.t.lc to cause any C'rosion ni' th(' s!r(';Jm hr~d. ;-;t.;lnding w;l_V(' is eonsidered as one of t.lw rnost. cfre{'t.iv·c: means of killing t.lw surplus vrH~rg_y in t.lw hydraulic structure. The desi,t;n nf dnwnst.n:am proter!ivf' works or energy dissipai.ors is t~ssenLi:dly one of reducing the high veloeiLy flow low enough to minimize erosion of the natural river iwd.
7.5
ENERGY DISSIPATORS
From a pl·;u·ti(·:ti view point-. h.rdr;wlic .1:nmp i..:.: a use:·u) mr·;tns nf di.c.::-.;ipai.iil.l;· t·:..:Cl':;s crwrg_1.· in suppr criLil:al fiow or w;\i.vr discl1arging OV('i' a dattl. il.s nlct'it is in prc·\'\'nLing possible erosion lwimv the overflow
8i)
spillway and sluices, for it quickly reduces the velocity of f1ow on a paved apron to a point when the flow becomes incapable of scouring the downstream bed of the river. Hydraulic jump basin is an effective device for reducing the exit velocities to a tranquil state. Hydraulic jump type slilling basins arc of two types:i)
horizontal apron type
ii) sloping apron lypc
7.5.1
Genera! Requirements
Among various factors which nffect the design of energy dissipaters, the most important. one is the depth discharge relationship of the exit channel at the site of the slructurc. To know its impiicat.ions on the design of energy dissipator is very necessary. The general formula for Lhe hydraulic jump on a hori:r:.ontal channel of rectangular section is as follows:
Where D1 and D:! :tre the conjugate depths upstream and downstream of the jump and V 1 & V2 are the corresponding velocities. The height. of the t.ailwat.er for each discharge may or may not correspond to the height of jump. The jump height curve may be related to tailwatcr rating curve in four ways. il Jump height cur·vc always above lhc tail water rating curve. ii) .Jump height curve always below the Lailwater rating- curve. iii) .Jump heig-ht curve above the tail\,,atcr rating curve at lov..' discharges and belo·w at higher discharges. iv) Jump height curve bclov,r the tail water curve at low discharges and above aL high discharges. All the stages ;:1.rc shown in F'ig. 7.10 in respective order.
7.5.2
Horizontol Type
When the tailwater rating {:ur·ve approxirn;:J.Lcly follows the hydraulic jump curve or is only slightly above or below it, then hydraulic jump type stilling basins vdth hori7,ontal apron provides Lhc best solution for encr!{y dissipation. In this case the requisite depth may be obtained on a proper apron near or at the ground level so that it is quite cconornic:.li. For overflow dams spillway on weak bedrock conditions and diversion dam/weirs on sand and loose gravel, hydraulic jump Lypc stilling basins arc recommended.
7.5.3
Hydrauiic Jurnp With Sloping Apron
\Vhen the tai\watcr is too deep as compared to the sequent depth, the jet left. at the natural ground level would continue Lo go as a strong current near the bed forming a drowned jump, which is harmful to the river hcci. In such a case, a hydraulic jump type stilling basin with sloping apron should be preferred as it would allow an efficient jump to he formed at suitable level on the sloping apron . .
··
).\.
87
rj
/
I
·;
-·
/
TAlL WATER DEPTH
/////////////
D = C 0 NJUG ATE DEPTH 2 (SEQUENT DEF'TH)
-·JUMP RATING I
~I
l-i ow G. I zO
I· !
<(
0::
l!J 1--i--
!
o~l
~a:l
I
Z
W3: 0-
w
I
i/
RATING
Ul 1-- !L_ II _ _ _ _ __
DISCHARGE Q
__ JUMP RATING
I
I
(
Sl z 01 51 Z
15
!
0
o zo
8 'I~~
I
"/\·--TAIL WATER
w
IDl S CHARGE 0
I
• /
5::::' l
::> ...J
!1
. _/
~--~,
Z
(f)
//
r-
<(
::l...J
<(
0-
Ull--
w
DISCHARGE Q
/-///
/./
w3:
/i
/
(_TAIL WATER RATING
I
(l_
(//
i--
I I-
\.··-TAIL WATER RATING
D I S C H P.R G E Q
w
I/-
oo Zo::
I
~/
I-I-
z
::>...J
o-
w
(/)I-
(LUMP
DISCHARGE Q
1
! lI
I
I
'l
RATING
TJ..\!L 'vVATEfi CONO!T!ONS CLASSiF!C,I\T!ON FIG. 7.10
7.5.4
Design Criteria
7.5.4
General
On Uw basis of eomprehcnsiv(; series of tests conduded by lJ.S. Bureau of Reclamation properties of hydraulic jump have been d('Lermin(~d . .Jump form and the flo1v characteristic can be related t.o t.he kinetic flow factor (V 2 /gd) of t.hr. discharge entering the basin, the critical depth of flow {d,), or the Froude numbt~r {vJJi~·d).
/\ccordingl_y, h~ydraulk jump type stilling basin with horizontal apron can be classified into the foliowing eategories: a) Stilling basins whr~re t.he incoming flow Froudc factor is less than 1.7 arc designated as Type L h) Stilling h:\sins where the incoming flnw~-Froude fador is greater than 4.5 and the incoming velocity (V 1) exceeds ·15 m/scc. are designated as Type IL c) Stilling basins vvlwr(: the' incorning flow Froude factor is above 4.5 and the incoming (v 1) does not exceed -1.5 m/sec. arc designated as Type III. d) Stilling basins where the incoming flow Froudc factor-is between 2.5 and 4.5 arc designated as Type IV. 7.5.4.2
Terminoiogy
For the JHlrpost~ of t.hP (ksign criteria described hereunder, the following terminology and parameters (indicated in Fig. 7.11) shall apply: L Hydraulic jump ··- It is an abrupt transition of flow from water depth D 1
hy the formula.
!
2
'!. Chute Block
rJ1 ; sF1 2 -- 1 ]
For horizontal apron
l
Triangular blocks inst.alltd at the upstream end of the stilling basins.
5. Basin Hloeks/Bafrlc Blocks/Baifle Piers -~ Blocks installed on the basin floor between chute bloeks
and end silL G. I~nd Sill ~- Solid or dent.at.ed 1val! const.ruet.cd at the downstream end of the stilling basin. 7. F'roude Number -- 1\ dimensionless number eharaderising the vertical and gravitational forces.
v
I;;,-a when
7.5.4.3
F - FroudP Number V -
velocity of flow
D
Depth of flow
Stilling Basin Parameters
The main factors for Uw basin required t.o be determined arc basin floor cievation, it.s appurtenances. /----~-.
89
and
r-
l l
r:t ;;~ >-~
,:; '-{
;{;
I I
I
------------------:
1----Vl
I 02
i
(-i-i
"~;!
..L
···~
FIG. 7.11
NOTATIONS
II
'D;= DEPTH OF FLOW AT THE BEGINNING OF THE JUMP, MEASURED PERPENDICULAR TO THE FLOOR.
IDz =DEPTH
I'
02 ' ~DEPTH CONJVGATE(Sequont)TO D2 FOR
LENGTH OF BASIN
Lj=
LENGTH OF
HYDRAULIC JUMP
q = DISCHARGE INTENSITY Sh= SPACING OF BASIN BLOCKS
DEPTH OF BASIN
Sd= SPACING OF DENTS IN DENTATED SILL
F1 = FROUOE NUMBER OF FLOW AT THE BEGiNNING OF THE l
IV
Lb=
SLOPING APRON( OR PARTLY SLOPING AND PARTLY HORIZONTAL)
Dc=CRITICAL WATER DEPTH
JUMP.
g =ACCELERATION DUE TO GRAVITY
I hb
SHAPE FACTOR
I= LENGTH OF THE INCLINED PORTION IN BASIN
CONJUGATE(Saqu•nt)TO D1 FOR
HORIZONTAL APRON.
Db~
K=
=HEiGHT
se= SPACING OF CHUTE BLOCKS
V; =VELOCITY OF FLOW AT THE BEGINNING OF THE JUMP Vz= VELOCITY
OF FLOW AT THE
END
OF
JUMP
0 F BASiN BLOCI<S
Wb= WIDTH
OF BASIN BLOCKS
OF THE CHUTE BLOCKS
We= WIDTH
OF CHUTE BLOCKS
l
he =HE! GHT
HL =HEAD LOSS IN HYDRAULIC JUMP
Wd= WIDTH OF DENTS IN DENTATED SILL
ho =HEIGHT OF END SILL
-8- =ANGLE OF THE SLOPING APRllN WITH
THE HORIZONTAL.
f'
'
<;
8
·I
I
I
7 6
Lj
D2
Ic-~----~--I I
--+- -
---<--<
5 ~~l /~-·--I v I
<
4 r-:~----------
~--- - - - -
2i 0
4
2
~
I
--1---
/HORIZONTAL APRON
-
'-=- t 1-~
I
3
tt I
\ " -I~ _,.J~
.J"-'"
DI
8
6
10
fj=~
12
'
~-- '-i _ j
e
14
18
16
20
YiiDI"
LENGTH OF JUMP IN TERMS OF CONJUGiiJE DEPTH Dz 0.25 TAN e=o.3o
0.15
I
0<20
I
0<10
o
26
-----~-<-f----------+-----+-----r'--1
22
-----L----~----t----,--Tt----1
18
I
I
I 1" --------1-< . <---- --·--j----
Dz
!yo+
'<-
I
D I I £.•
I
--<--1----l----,'I?L...._I----l---------1
10-------
. !
2
v. F!=-'
~
RATIO OF CONJUGATE
DEPTH Dz TO
D1
FIG< 711- A
i)
Hasi·n Floor
J~'lc1:at.ion
D can be c:1lcuiated from the foliowlng equaLio:1 knowing HL, q, D(' ~lnd D 1 or from Fig. 7.15 . 2
De
., (
)1 ':\
g
DI 2
+
II:tvinl{ obt.airwd 1) 1 and l}), Lhr rk·v;1t.ion of tlw basin floor may be calculated b_v eiUwr clPduct.ing the specific energy at _..;(·\_·t.i()n i --from Lhc t.otai energy line at. that. section or thaL at sect-ion 2 -·- 2 from the downstream tot-al energy LinH'.
1
To eaicuhtr- Hr, from the known upstream and do1vnstrcam toLtl energy lines, the foilowing procedure maybe adopted: :d \Vhcre t.he basin j:-; dirr-cl.iy dnwnstr('afll from the crest or where t.hr ('hute in noi longer than t.hc hydr·aulie head. IIL may be assunH~d equal t.o the diffcren<:P in the upstream and downstream total energy lines. hl \Vlwre the chute• lt~n~~-t.h c·xccPds thP h_ydraulir head, D 1 should be first. dctcrmin~d by Laking· HL equal t.o the dilfcn·n('(' br-twr•en tlw upst.n~am and downstream total Pnergy lines. Next calculate friction losses in Llu~ chui.l' hy :\Lulning·'s formula and determine the more accurate value of Hr, and consequently that of I\. The process may be r(~pc:>. Led till t.he dr>sired accuracy is achieved.
7.5.5
Stilling Basin Type !
This typP does not tl\'(~d a spc<"ial stilling basin t.o still f!O\VS, except Lhat. the channel lengths beyond the poinl whr-n' the depth starts to change should not. he less than about ,1cJ~. No baffles or other dissipating devices are nu·d('d. 7.5.6
Stilling Basin Type il
This typt· of st iiiing h;l:::in i.'-' usu;lliy us<·d on high dam and earth dam spihvays and largT canal st; :wLtJrvs. It ronLains ch;!i(' h]{)('_i~s :1t. the upstream end and a den La Led sit! nc~ar the dow;1::;trearn end. No baffle pier::: an· usc~d lweauS(' of th(~ relatively high velocities entering the jump. T!w following
rul(~s
arP rccommcndPd for genPralizaLion of Basin II. Figure 7.12:
I. Set. apron elevation t.o ut.iiize full {'onjugate (.aihvatcr depth, plus an added factor of safety if needed. An additional L:H·.t.or of safeLy is advisahk for both low and high values of the Froudc number. A minimurr. margin of safety of G perr(~nt of D:~ is recommended.
2. Basin II may he dfecLivr: down to a Froude number of 4. 3. The kngLh of basin c:u1 lw obtained from the eurve on Figure 7.J2 (c). 1. ThP !wight of chuLP blocks is ('qu:li Lo the depth of flow entering Uw basin, or Dl. F'igurc 7.12 {A). Tht• width and spacing should he cqu:l! to approxinwt.cly D 1: ho\VPvcr. i.his may lJp variC'd to eliminate t.r;tditional blocks. :\ span~ l'(jt!al to D 1 /'2 is pr;_cferabie along each wall to reduce spray ~1nd maintain desirable prcssur(·s.
92
;). The hc,igh! of t.he dentated sill is equal to 0.2D~.? and the maximum \vidth and spacing recommended is approximately O.Fi D~~· On the· si!! a dentate is •eeonnnendcd adjacent to each walL Figure 7.12 (A). The slope of Uw continuous portion of the sili is 2:1. For narrow basins. \Vhich contain only a few dent.at.cs according to th(' above rulP. it is advisable to reduce the width and tlw spacing. Howevpr, \Vidths and spaces should remain c·qual. H.Pducing- Uw width and spacing ad.ually improvps Lhc performance in narrow basins; thus, the minimum width and spacing of lhc clcnLatcs is governed only by structural considera lions. G. It is not. nccessary Lo stagger the rhulc blocks with respect t.o the sill dentate.:;. In fact, this prarlice is usually inadvisable from a construction standpoint. 7. lt. is rr.commcndc·d that t.hr sharp int.erseet.ion hrhvcen chute and basin apron, F'igurP 7.12 (Al. be replacl'd wit.h a curvr· of reasonable radius (H ill) 1 ) when the slope of Lhe chute is 1:1 or greater. Chut.e hior.k.:; can be incorporat.('d on t.hc curved bee as readily as on the plane surf:lccs.
>
Following t.hc abnvf' ruk•s sill rcsuit. in a safe. conservative stilling basin L)r spillways up loGO meters high :tnd for fiows up to 4S Ul. m.isc~c. per rncd.er of basin width, provickd t.hc jet entering Lh(: basin is reasonably uniform hot.h as to velocity and depth. For greater falls, larger unit disch:trgcs or possible asymmetry, a modC'l st.udy of t.hP spPcifi(~ design is recommended.
7.5.7
Stilling Basin Type 1!!
This type of h;tsin wa::: cil'VC!lopPd for a class of smallPr st.rudures in which vplocitiC's ai t.he entrance t.o t.lw basin arc moderate or low (up t.o 15·18 meters per second} :tnd disclnrges per meter of \Vidt.h are less than tB cubic mct.cr per .:;econd. The fo!lO\ving
rt~les
pertain t.o tlw design or this type of basin:
1. The stilling b:\sin operat.PS best. ..-d. full con_jug;tt.c~ tailwat.c~r depth. D~. A l'l':lSOnablc fZJ.ct.or or safety is inhen•n\ in t.h(' eonju)~·:ll.c~ depth for all values of the Froudc number and it. is rc~commcnded that. this margin of saf!.•l.y (Figun' 7.t:nn no! be reduced.
2. ThP length of ln:::in which i.s lc.:;s than OIW·half the kngi.h or the natural jump, can he obt.ain('d by consulting t.hP cu;·vp for Ba.sin III in Figure 7.1:3 (D). As a reminder, an excess of tailwaLer depth does not. substitute for pool length or vice versa. :.L Stilling Basin III may be effcct.ive for values of Froude number as low as .-1.0. ·1. Height, width and spacing nf chu!.e bloeks should equal t.he avrragc depth of fiow entering l.hc basin, or D 1• Width nf hlol'ks may lw dc:<"n~ased, provich:d spacing is reduced a likc> amount. Should D 1 pro\·c· to be lPss !.han 20 em., t.!w b]()('ks sho11lrl he made 20 centimeters high. Larger chute blocks t.end lo t.hrow a portion of Lhc hig:h velocity jcL over Uw baffk piers.
!5. The blocks may be vertical also shown not appear eommendcd
height of Lhc bafi'lc piers varir·s wit.h the Froude number and is given in Figure 7.1:3 (r.l. The be ('Ulws nr t.hcy may be constructed as shown in Figure 7.1;3 (A): the upstream face should and in one plane. Tlw vpr!.ical face is important. The width and spacing of baffle piers are in figure 7.i/} (/\l. 1n narrow strudun~s where the specified width and spacing of blocks do practical, bl(wk ·.vidt.h and spacing may be reduced a lik(~ amount. A half space is re· a{ljaccnt. t.n the· \valls.
G. The upst.rc~am face of t.he b:tfflc piers shc;uld be set. at a distance of 0.8 D 2 from Lhe downstrc~am face of t.he chute blod::5 (Figure 7.]:}/\). The reeornmPlHied po5it.ion, hr~ight. and spacing of the baffle piers on t.hP apron shouid be adhered t.o cardttll.y. as i.hPs(: dim(~nsions ;u·(' irnport.ant.. For example, if t.hc blocks are set. appreciabiv upstream from i.lw poc:i!inn shown. t.hc~' will prndr.H'I.' a casc:uie with rcsnit-ing wav0 ;1.('(ion. lf the: baffle~') ~·':'~
f;:·;·h, •.
~':-,,,,_,, .. " " ' "
'h.,,
c:hn\'.'!~
.,
ln•;•Yr•-
<~ .. "'":~
,,.;ll
t •.•
- , r , , ; ..."l
1 .:1,. .,.,,;"'~'
.. ,,,,,,,.. ,
93
;r ~•-"
1, .. cn".'
t-he hafflc piers on the pool floor at randorn and expect anything like the cxcc!tcnt action ot.hcnvisc associated with Type Ill b<1sin. 7. The height of t.hc solid sill at the end of the basin is given in Figure 7. 13(C). The siopc is 2:1 upward in the direction of flov-.:. \Vith this slope, rninirnum wave heights and erosion could be expected. 8. Il is und('sirable t.o round or sLrcamlirw t.hP edges of lhC" chute blocks. end sill or baffk piers. St.n;amiining of baffle piPrs may result in loss of half of their effectiveness. Small chamfers t.o prevent chipping of the edges may be usr:d. 9. It is recommendt~d t.hat a radius of reasonable length (H ~:,:_~ 1D 1) be used at. t.h1~ intersection of t.lw ehut.e and basin apron for slopPs at -1S 0 or greater. 10. As a general rule, the slope of Lhc~ chute has little effect on Lhe jump unless long flat. slopes arc involved.
7.5.8
Stilling Basin Type IV
This type of basin is :uleq~1a!r' for FroudP numbers lwtwpen 2.:} and '!.:). The low Froude nurnber range is enrounLercci prin<'ipally in t lw (ksig-n of canal struct.ures, hut oceasionally low dams and outiet works Ldl in this <":liegory. For !.his range of Froude nurnhcr, the entering jet. oscillates intermit.!.('llt.\y from bottom to surLu'C' with no partirttlar pC'riod. Each osc~illation gcn(~rat.cs a wave whieh is difficult t.o dampen. In narrow st.ntet.ures, sw'h as eana!s, waves may persist to some degree for miles. As they encounter oh:-d.rudions in the canal. surh as hridgP piers, turnouts, C':hccks and t.ransit.ions, reflected \vaves may he gcrwrated whid1 t.enrl to dampen, modify or intensify the origin::d w;1vc. ·wav('s arc dest.ru\Jive t.o e:trt.h-lined canais and ripr:1p and producl' con:-:;iderable surges at gaging stations and in measunng dPviees. St.ruci.Urt's in this range of Froud(~ nurnbers are lhc ones which have been found to require t.he most. maintenance. On \·vidr' St.l'U\·tun•s. S\!(~h as div('rsion dams. \vavt~ acl.ion is not. as pronounecd when the waves t'all \ravel iatf'rally as well as par;tllPl Co t.lw direction of flow. The eornbincd action produces ~ome dampening dfr:c.t. but. also r('sult.s in a d10ppy wat.er surface. The waves may or may not. be dissipated in a short. distance.
Thr: dimensions and proportions for Type IV Ba:;;;in arc shown in Figure 7.11. The width of t.he blocks is shown equal t.o D. and t.his is Lhc maximum n'commended. From a llydraulic standpoint, it. is d(:slrahle th;:tl. t.hc: blocks lw consl.ruct.('d narrower t.han inditaLed, preferably 0.75 D 1 . The ratio of block widt.h to spacing should be m:tinLlirwd as 1:2.:). The extreme Lops of i.he blocks are 2D 1 above Lhe floor o[ Lhe stilling basin. The blocks may appear t.o he ratiwr high. and in some cas<~s. Pxt.remcly long, hut. this is essrn!.ial as the jet lc:1.ving· LhP top of t.he blocks rnust. play at. the base of t.he roller t.o be cffcdivc. To accommodate the various slop<:s or chutes and ogc~c shapes encountered, t.he horizontal top length of the blocks should be at. least. 2D 1. The upper surfacL~ of each hlnck is sloped <:tt :) 0 in a downstream din:clion. A t.ai!wal.cr depth S t.o 10 percent. greater t.han the conjugate depth is strongly recommended for Basin IV. Since the jump i:> V('ry S('n:>it.ivc t.o LaihvaLc'r dc~pt.h at· these low values of t.he froude number, a slight. ddi<'iency in t.ailwat.er depth ma_y allow the jump to sweep eomp!etcly out. of t.he basin. The jump performs much hPLL<'r and \vavc act.ion is diminished if the tai!wat.cr depth is increased t.o ap proximately L2 ]):?·
Tlw kngt.h of Hasin IV, \vhich is rC'lat.ively short, can be obtained from Figure 7.1:1 {c). No baffles pipr_c; are n('edc:d in ihc basin, as Lhes<' will prove a grcat.c~r ri<:trimcnt. than aid. The addition of a smali triangular sill ph\·('d :l.! t.he end of t.hc apron for scour c:ont.rol is desirable. An end sill of t.he t.ypc used on Basin III is saLisi"ac!nry. H flPsi.l;nu! foo· tlw m; xirnum disch:Hgt_', \)ac;in IV will lH'rfnrm ::;at.isfacloriiy for 'jnw:1.;;i_r·pnn1
only.
i:·n:•'
:t'C• <.:j_)j] tll.>"
h:tsin
will
c.;i_il\
hi';~
~""''!!
l\'SSl'!'
flows. \Vav•,'S
'\f;q bl' of !_j,p ('"(lir~"-!'" V'>.r,·irt.v
I,!St!td\v
Dentated 'li!!Chu ts bloclls-
,, J. ~~;L-~~~~------~~~7r~v ,):.,:.··~·~
o.o
.0
o
~._._·_:_-
·a.'
0
.: L
0
. 0 .
·
0
·.oo_
(A) TYPE li BASIN DiMENSIONS·
,l:±JTi:J±i T-=f0f11E7GiH~oriiJ7f'it~-ti ++F :··:· B 4
6
FJG, 7.12
8
!0 FROUDE
12 NUMBER
14
3 18
18
StillllliJ basin choroctaristics for ust with Froud\! numbers
ohov~
4.!5
End
sill~
0
·' 0
r
, 0
r
0
r
-- -L-
(A) TYPE lli BASIN DIMENSIONS FROUOE
F\G. 7.13
Stillino bo3io
cfHlract~rl~lic,
NUMBER
(or uM• "?.llh Froude; numb:H; obove 4.5 whero incominc velocity (V 1 )
50-60 foet por
:~ocond.
doll~
not exceed
.
Chut~
_}_- r
block~
~-
Ends!!!---, ~QCC
Fractiorwl
-r......_- ~'If -Max. tooth
wi
0
/ \........ -- Sp<JCG" Z..5'1t
.
-
0
0
: 2d, • ., • .
0
~-
0
0
opt!OTKll
-~~. 1
'
n
-Top surfoc•
"
0
n
0
. .<> . .
r
·--
-L-·-·~
·.: C' ••
.:. .:::
:.. ._- ._.
FROUOE NUMSER
i
-
dz
.
'!
I
l -I
r:-c· 1':
~:..:d=z{ljl+SF
,
1
I
I
'l
-
.
-!) _ _
'
~-·
3-
4
3
; { 81 MINIMUM TAILWATER DEPTHS
I
2
2
i
(C 2
:
'
I
I LEhGTH I
4
!
5 ..J!~
'
« JU'MP !
3
4
0
4
FROUDE HUMSER
FIG. 7.14
St!liino;~
basin choract.orl:tlca 1or Froudo
num~rc bet~oon
Z..ti on
7.5.9
Conce;1tration of Discharge
While working- out the size of stilling basin, the incoming unit discharge should be increased by 20 per cent in all calculations in order to cater for various fac~ors such as formation of islands, obliquity of flow and river stage etc.
7.6
TOTAL LENGTH OF FLOOR
'Total length of impervious floor which consists of downstream basin length glacis/spiliway/chute and upstream floor should be fixed from the following consideration: a)
Exit gradient should not exceed the safe limit.
bl The structure should be safe against sliding.
c) The downstream floor length considering dissipation of energy, width of crest, slope length upstream and downstream should he accommodated in the total floor length.
7.7
DEPTH OF CUTOFF
Depth of cutoff should be determined both from scour consideration and checked from safe exit gradient requirements. for determination of scour depth, unit average discharge should be increased by 20 per cent to allow for concentration of flow and depth as per provision under para 7.3.3. The cutoffs shall be suitably extended into the banks on both sides up to at least twice their depth from top of the floors. The depth of the downst.rc<1m cutoffs along the total length of impervious floor shouid also be sufficient to reduce the exit gradient within safe limits for the governing design condition. For caleuht.ion of exit gradient, reference should be made to the detailed procedure given in Chapter 5. The cutoffs may be of masonry, concrete or sheet piles depending upon the type of soil and depth of the cutoffs to suit construction convenience.
7.8
7.8.1
FLOOR THICKNESS
The floor should be designed for the following conditions. i) Uplift. pressure due to static head
Thickness of the floor should be adequate to resist uplift and soil reaction. Uplift pressure due to static head shall be as per the procedure laid down in Chapter 5 para 5.7. ii) Uplift pressure due to hydraulic jump
Due t.o the formation of the jump, high unbalan<~ed pressures are developed m the trough. The floor should he of adequate thickness to \vithstand the effect of the jump. For determining thr: uplift pressure at and upstream of the jump section, it would be necessary to plot t.hP water surhce profih~ of !.he jump for particular flow c:ondition. This can be plotted by using Figure 7.1G. The uplift pressure is the ordinate measured from the hydraulic gradient line to water surface.
The floor at. every section should be designed for the bigger of the two uplift pressures, i.e .. one with hydraulic jump and other for no flow condition. A factor of safety of not lCss than 1.1 should be provided in caleulating the floor thickness. 7.8.2 The U~)i!fl prr>~<::ur~_~<:: nn. t hP jump lentTt.h and thr :equin~rnent of floor thickness are however moderated to sorne extent riue to the· follo\ving factors. a) The uplift due:: to maximum depth of the trough operates only at the deepest point of the trough.
98
0
HL/Dc
.-
:oz:£:.
IDe
I
3
:s.1s
ii:6e '311
raoo
4
.. ,.....
1.425 .301 ~ 1308
·:zr.
r28l '9D'
l1~ -:241 i"i':1ii 240
6
·• D•fo, 'rz;;, o;;~
·3
2
·0
' o:;;D '0.1;, f21;, 0'/o, ""/o, o,
Jo
....' c.o.·
..
·7
o%,/o,;;;; ~~~!o,;o, ,o 2 ;~ jo, /o,
00 i l l :397 I!S.i6 1.360 6.83 .381 .69 1.378 .415 . :»5 ~ 6:79 336 io.94 :331 ·. 0' 1.327 7.23 .323 1.292 aa1 0 I a21 .296 18.3 4 i .290 .3"' \1.07 '9:20 '.276 '9.39 [.274 '9.5 '.2" 19.64 23! '9.7 . 207 '9.69 I .26.5
.82
••••
::3
I·•••
s
II
'"
,.
·,w }j£ G1 ra
,1:9o 235 1<2.01 23' '12.00 ''"-01 '222 13-
~·7
~~~
..
17~ 22.41
""'
I
~
2
lfi
182 23.3
-;;-;; = = rn:iii :>13 = 2.>--:63 ~ = 'TIT =
~142
22 23 24 20
•• .....••
fMlf
12S•o
..... -
37
••
'
!TIT .
.120 131.,..
,,
13"-78
: .",; [;.,
"" •... ,
142 '6:>.
.12
3:21 J2;t.ii'2.2' :12.5 sUi. ;w :ru50 -:121 [~3.00 -.;;r
--;;-;,
--.-.--
'2,_.-i
T27 lici7il .i J2< i31.4S
rn .12•
i8 14:
.
.134 ~0.38 .134 lza,. .132 ~&18 I .131 [2920 .131 129.34 .129 130.ct I
1232; .134 '2&30 l2«c:i1 1·.13> .(30 29.89
..... ~:'"
"
.102123.46 '24.31
=
.139 26.75 -:1'
"""' 27 30:1
30
y.s· i2.6
.I
?US
I ,I:,•:
I
.113 f5&oc
' ... 131.60
'""
r:::l ·:.~ I"""
!121
233
~1M 12275
,.,
~
"'"'
I
10
12'<49 1.147
*::
1~'.20! 1<.41
,.,., .... ·' "' "'·". I
~,..., Q6.17 T4l i2&z .141 J3o ~ I .138 27.01l .
'Elf 27; .33 [20,82
I»Mz
l348i
I 114
[3!100
:Jo•
~~
Relation ofener!Jy lou, c:honn~ltl'rlth
"''"
or
r.m:o
1~£~
~
!26.33
II~ ~ I::~! r.~~
critical depth and dop!h before
"" I•
.231 1 .21o [13.
.22R
216
,.zo:
I:~:: 1:!.~ ":" '·'"-' ''·"'''"'
_,..,,
.in [R.74
.
,,.. 126<2
.14( 126.0-c
..
I"'
·"· ""' ~
~ ."2 m
.117134.96
.132
I"'"' 13429
.IIi I
I
-::"
.118
I"• ;'! I·~
1
·".
I .I
_1.£!,2..1.-T----T:I:...J
£-
baud on poolwlrlthDc<3~ D 1= Depth at up~lloum ond of jump
C3
Da;-Dopth at down~! roam ~:nd of jump_ K::: Oz-. D 1
El. Cz::: EL.C,-{~ +h '1}:: E."J Cr(Di h"2:*t\l 3
Dz.
1 2
-J K{X+!)
HL,.f~3
3~-~--
0C4K
"I
ENERGY
LOSS IN
HYDRAULIC
JUMP
FIG.7.15
NOTE: Data
from msrnoror.dum by C.R. Bun.y,Aprll 16,1939
''~"' l>&n
'"'"
~~ 'J"'-""rl ~~~~ 1:·~~ ~~-~~~
sur~ac:o
·'"'
(3!25 .112 Jll 137.98 [.11 138.0! 138.70 .100 1387
,..0
De= Ccl tical do pH• for flo w corn !aorrt-d,
.!?.! -
.100 .128 J26
20 .II 6
:lis
,,.,,
.113
and after ji.Sllp, for hydrqullc jumps in rectangular
Water
.146
:1~
l?l'-86
16~ 1:;;;;;; "~ >f
lov!ll! floor.
HL=Difterenceln energy levels at Up41rllam and dawnstroom ond$ of j~ST:p_
....
... I~
.....
I·''" ~7.2·
I
J33 [zan i.
, I 114 l>sz:
;::-_.~
.-
]2 .20
~""
32
...
~:rr i 3~~~ "•·
1<0.13
1 _,., 123:<: .131 [2M8 1.130 J31 iZO-"" .129 13021 .128 130.20 I .128 13031 .128 I30AO !71:11< :.127 [31.3 131. 131.9
·"
·'''I·'""'
~~
;.:
.1141362: .11413640 137.11 .12131.18 13 i>S9a .112 136:0' i l l ~ '36.81 : ·"' IY>.OO [57.4< [Ji2 '3i'<7 T!i :~m ~~~.-:til im9 .II IOZ70 13ii>< i3791 !5aJ3 .110 13331< 1310. .i1c 1'31!.:!< jj 1334: :iTo [3849 .I. 138M .108 l38iiii l.i09 i3a92 J()9 ~ J09 ~ [lw [31>.38 56.06
)I
l::·::r:Jii 16.3!)i 192 1"-48 .191 f16.t14 '.191 17.43 IME 176 118.48 I·' I 173 1&.33 .172 19.<: .I fi•Jl2 .166 ~016 H7 20.20 .167 120.34 121.34 I·'" ~·o 1-'"" i 21JO I .1az 121.25 21.9: 122.2 23.02 .154 123JI .103 .lti() 123.71 •23.8C .1:><: t3.1l9
l3l23 I .ll I 133.31 .
', . 136.10
I
110
' :24<
·1'! ~ li: ·"'"~.-, ~~~~ 1:~~~ 1.~~01 •••• "'. '\82 ii:;,: ... ...... i;~c~1 r:::~ r::::~ ~·~ ;:;; 1:~ i:~~~:r•• -,, r••·"'' .....-= '"" :- , •••• 8
!0.66 -368 la.oz .362 7.!8 .319 i '6.7.-
I
l12.bl .2Z7 i2.u .226 12.79
!o2~~ o,;J
.112
.10
:.o
'::~ ' .108
"
- = 0 !162 Ho
_, '----'-- __Lj .• >·!. ,,·-1 . -.''
-'-··: :.-L L1
0., y
"'
0
·--!-•
----!.
_,
0
(i)
•
• ~
"'
=0.083
0
00
_,
f.O
0., y
"' ·0.!1
_,
---
(D To@ condition
of flow as indicaTed in fiQvre 7.4
EXPLANATlON ~
WATER SURFACE
PRESSURE
®
F JG L.f1£ 7. I 6- FlOH OVER $l..BiV.ERGED CAMS-TYP!CAL PRESSURE ANJ SURFACE PROFILES.
It becomes suddenly small on either side. Due to beam action the floor slab over a length resists the uplift. As such the average intensity of effective uplift is Jess than maximum point intensity. b) Jet of water flowing over the downstream slope exerts a downward force, which counteracts the unbalanced head to some extent. In view of the above factors, the uplift pressure (unbalanced head) for design purposes are taken 2/3 of the calculated value.
7.9 PROTECTION WORKS
As discussed under para 7.3.3 "scour and silt factor", scour depth is related to silt factor of the bed material. The predominant factors which determine the maximum depth of scour downstream of diversion structure arc the discharge intensity and the turbulence (as indicated by I the effect of sides being negligible because of the wider section of the river. Other important factors are the flow pattern and the grade of bed material. \Vhcnever a rigid structure constricting the waterway is built across a natural channel, the velocity and discharge intensity being greater than the channel, when the water leaves the structure and flows over the bed of the natural channel, the depth of flow gradually increases downstream of the masonry or concrete floor. inducing a scour hole and then gradtqJly decreases till the stable depth is reached. Since the scour below the structure is affected by width to depth ratio and in case of overflow dam/weir, the waterway provided is longer compared to channels, the maximum scour downstream at various locations is as given in table 7 .2.
f
-;N
,~
t.l-.-
··
7.9.1
Upstream Block Protection
Beyond the upstream end of the impervious floor pervious protection comprising of cement concrete blocks of adequate size laid over loose stone should be provided. Cement concrete blocks should be of adequate size so as not to get dislodged. These can be of 1500 x 1500 x 900 mm for diversion dams/ weirs in alluvial reaches. The size of blocks can be bigger for such structure in boulder stage of the river.
,>{
,L~
The length of upstream block protection should be nearly equal to D, which can be determined as under D
~
XR - !High flood level - floor level) where X is the multiplier in Fig. 7.17 (a) & R is the scour depth as given in table 7.2 under para 7.3.3 without taking into account 20°/o concehtration in the value of q (discharge intensity)
7.9.2 Downstream Block Protection
Beyond the downstream endof the impervious floor as well pervious block protection should be provided. The blocks should he of adequate size laid over a suitably designed inverted filter for the base material in the river bed. The cement concrete blocks ·should be generally not smaller than 1500 x 1500 x 900 rum size to be laid with joints of 60 mm to 75 mm width packed with gravels. Heavier blocks are preferred since wide shallow blocks are apt to be carried away by high velocity (current of water). Deep blocks get wedged in n.nd resist disloeation. The length of downstream block protection should be approximately equal to 1.5 d. If the downstream length is such that construction of inverted filter with blocks becomes costly or unmanageable. part of calculated length may be provided with inverted filter with block protection and the remaining part only block protection should be provided. It is, however, desirable that length of block protection over filter should not be less than the depth of cutoff. 79.2.~
Ulhf!P IN()rk_inq 0'!+ 1)
n_0 ~-~lnw:>n('p ~0"' "')::C'?~~.".:!,~_;_,~ <:-hn,l!j J,p ~.'l(~_p ;~ +.!">P (~~<:;chp_~Q'C Qn: '.l!"~t
iength vn.!ul) of \ql, in (:.ornp\lt.ing t.hr: normai s<:OlJr h. Ior r.hf~ up~fTPrtm :lnrl (iownst.rnam prote<'.f.ion works;
101
-
.,,""'
FIG. 7.16
FLOW OVER SUBMERGED DAM -TYPICAL PRESSURE AND SURFACE PROFILES- CONTINUED
(13)
~~~; d, 1.7(; L
1[.;-=0.619
o.._,_~
LO
- ~- ~",. _L
1:-.- f
0
7"
~ ~t~ f::,.
- ~-f£ft ~£ --r'
\
~---
1--\-·
-·
,_
-
~~--
-
\/
0
1'1<1 .. <1
L
110 =2
@)
14
ON-...!
'-
!
I
'-
'
t"·t4m
I '~t'l--f--: 'r/
;1-n
ro~o.9~5
L
i I' 1'-N ' I ~n --+:- ' :~LL-1\ .I ~+- I • .r I -.: I
0
" '
I
I '
f__J.-· -- -- ...--I
--1- .-
I
,,[,
bii-- -= :;:-II /::::::; -1-j i- -~---' I J- I
++ I
['.I
--
/
i I
•
7
7 1.0
y
Ho
..
7.9.2.2 A toe wall of masonry or concrete extending to about 500 mm below the bottom of the filter should be provided at the end of the inverted filter to prevent it from getting disturbed. Arrangement is shown in Fig. 7.17(b) & 7.17(c). 7.9.3 Design of Graded Filter
The graded filter should roughly correspond to the design criteria.
D15 of the filter D85 of base material
= 5 or less
where Dl5 & D8.5 denote grain sizes and
D15 is the size such that 15 percent of the soil grains is smaller than that particular size.
D85 is the size such that 85 percent of the soil grain is smaller. than that particular size. The above percentages are by weight as determined by mechanical analysis. ii} The filter may be provided in two or more layers. The grain size curves of the filter layers and the base material should be roughly parallel.
7.9.4
Loose Stone Protection
7.9.4.1 Beyond the block protection on the upstream and downstream of a dam or weir located on per· meable foundation, loose boulder or stones shall be provided as a launching apron, to spread uniformly over scoured slopes.
Design Considerations for Stone Protection Work should be as under:a) Individual stone size should be heavy enough to resist displacement by highest calculated velocity at the end of the floor. b) Thickness of stone layer should be adequate to prevent escape of river bed material. c) The size of stone used should not be less than 300 mm size and no stone should weigh less than 40 kg. In case the velocity is too high or when somewhat smaller size stones arc used, the loose stone apron should be provided in the form of blocks of suitable size depending on cost. In boulder reaches of the river, cement concrete blocks are preferred. d) Volume of riprap work should be adequate to cover the slope of scour hole to calculated depth. e) The calculated volume should be placed in a limited length such that riprap after launching shall not move beyond the slope to be protected.
fl Individual stone size to be determined from the curve described hereunder. 7.9.4.2 The Corps of engineers, Civil Works Investigators Vicksburg USA have prepared design charts correlating the bottom velocity against stone, on the bed of channel and rivers. Fig. 7.18 shows the curve prepared by the United States, Bureau of Heclamation, correlating stable size of stone and its weight in flowing water with average bottom velocity against stone (bottom velocity is 0.7 of average velocity). The chart can be used as a guide for the selection of stone for the riprap.
103
--------..-;;~·;;-;_y~,~
_Jj.f''.-.!·L:=--------------.---,,--
'\SPACES TO !IE FILLED
?>
i\ITH GRAVEL
~f-----'cc...·5_:D:.___ _ _--l
·· ... · •... ,, 11 -u 11 v. i'·; ·..... ~:\'! .. , ... ·.,....., . . ··:·.~· .·:.·. ·-'.'"•"
>'
Ul I
....
I I
LOOSE STONE PRQ.TECTION_J GRADED FILTER ......_ ....... ·SHEET PILE .......
.....
XR" DtY D
I \
.....
' ....
I \
\ ....... ......_,
"'- ..... ·;~
• _, __._@_
LAUNCHING
FIG. 7.17 a
APRON
'','~'~ (LOOSE STONE i PROTECTION
ku/S
I
END OF IMPERVIOUS FLOOR
·>.···:_... ,· ... ,."'-··:..o·. . ·._. ; •··. ·, ·:.
GRADED FILTER
j
-SHEET
UPSTREAM PROTECTION WORKS ·~
••. ·i~
r
PILE
FIG.7.17b
D/S END OF IMPERVIOUS FLOOR OPEN JOINTED CONCRETE BLOCKS SPACES TO BE FILLED
STONE PROTECTION
TOE WALL '---~- GRADED FILTER
·SHEET PIUE
DOWN STREAM PROTECTION WORKS
PROTECTION
WORKS FIG. 717 c
7.9.4.3
Slope of Launched Apron
The slope of the launched apron is dependent on the grade and size of material in the river bed. Slope length should be calculated taking the slope of scour hole as 1.5:1 in boulder and gravel, 2:1 in gravel and coarse sand, 2.5:1 in fine sand and silt for downstream protections. For upstream protection, the slopes may be assumed as 1:1 for boulder and gravel, 1.5:1 for gravel and coarse sand, and 2:1 for sand and silt. 7.9.4.4 Thickness and Length of Material on Launched Slopes
Volume of material calculated to cover slope length should be increased by 25% to account for packing and losses. If D be the depth of scour hole and n is the slope (H:V), then horizontal distance of deepest scour is nD. The calculated volume is generally spread over a length equal to 1.5D. Model experiences indicate that the performance is better if it is laid in gradually increasing thickness starting from the floor end towards river ward end, where severity of attack and hence, possibility of loss of stone is greater. 7.9.5
Protection Around Divide Wall
The protection arrangement in the form of block protection and loose stone - riprap should be on the same design principles as indicated in the preceding paras. Factor of safety against scour should also be the same without taking into consideration the concentration while working out the discharge intensity, (para 7 .9.2.1 ).
7.10
FREE BOARD
Free board to be provided for various components should be as under:7.10.1
Free board in abutment
Top level should be fixed on the basis of 500-year flood with the provision that:a) There should be no overtopping by waves. b) There should be no overtopping due to surface roughness. c) In the bridge portion, the top level be fixed to offer sufficient clearance for floating material under the bndges dunng h1gh flood with 100-year frequency.
Determination of Wave Height
Wave height can be calculated by Molitor's formula:hw and
0.032 JFv + 0.763 - 0.271 0.032 FV for F
where F
=
1
W
for F
< 32 kms
> 32 kms
Fetch
-~ ~or
Fetch p_erpendicula~ to the abutments_ ~hould be taken as distance between abutments on two banks. calcu!atmg wave he1ght for pond cond1tton, maximum wind velocity as experienced in the past should e taken. In the absence of data, 80 km/hr for 500 year flood condition may be adopted. A..Jter!"!.:Jtively
~-Jrr-e ?o~.rd
of I
~o
LS m, s,houid be
;",ood or above the aiiJuxea water leveL
-~H·ov!ded
above the flood 1eve! for J _tn 500 vear .· ' ··
105
STREA M OF STILLING BASINS
13 5
~~~S~IZ~E~O~F~R~I~P~R~A~P~TO~B~E~U;S~E~D~DCM~T'N~~~~~-t~li=rJCJ:JC1 I --I~J,L\--\-~-+-t--1 -+-- -l-+-l-1---t--i-H+-H_lr-~-H_j-_t+-~t-_tt-:-+--rH--lf-1-t-tlt__ _ ~--~--~-~_j~~~,N~00TiE~_J-~
f-l-+---t--ITh• rlprap should bo compos•d of a f-J--t-~t--1 ws !I gradsd mixture but moat of the
'-1---f---\--t--1-1/+-+--irlt-11
atones should be of the size indicated r
120 f-l-+--t--1 by th• curv• -R ;prop ahould be plac•d \-l--~+--+-+f-l--4--1r--Jr-t--t--1 +-+-+--t--1 over a filter blanket or bedd.~ng of \--~'--+-1---1 graded grav€11 in a Jayar 1.5 t!ml!s I
1--i-+--t--1 105
l-+-t-t-t-lft-t-t-H-tl-- -19oo.2s
(or more) asthlck as the largest &tone diameter.
I
~l~tr-~',-=~~~11~=~~-l=~~-l--~+--+~Jt-+~-t-itl--1t_ti-;1t-t'c1fjt_j~~J=~;=~f=b2-1+-l-l~-t-t-t-t-t-t-llll7-r-_ ___ !-r~~--- ~-
1~-+-+-+--r-r-J,_I
1320.57
::1!
ttr=r~l--4-~J-r-+-+----~~+-=tt=.t1~~tJri=t1t-t-n-tii=tu ~ 9 o '~ - -\- J--1~-l--1-4--h-1-+-+~5sr-+--b/r-.'r-: +t-t-HIIt1::: _ 11-- ____ -1--++-+'- -\-.-1-++-+--'H--t--t-1 i1 <-+- 1 ·1~ -~,-1~~=--~-~CC~--1--++++--+--+-A 1 ~rt-rt-t-l!lll 1 ..... -- - ---1- ---+-v,·I+~+--~--~'
'
<3)
896.86
--1--
f=
~ 75 h '
I
!:
,
!
i .
1
.. '
•
1
_;__~-'-- _LJIS_.o --+L-1-~L'-''~-1--l---l--Hr+, -t-'H-t-1 _ t--1~' __ _I I sL-t--t-H-++H---t-l
i . ; :
+ -'- ..,
w t-o %1
·
r-f-~~i-
\ f \ ' •
. .•
- t-1 i ' I !
'
(')()
!
,
"
,
~ t±J~Tr~:=r=:~l-T··-,-L-'-l~~····~~ Ll i_:_l, . i-,--r--- ',~Az'"' I ' 1
'
'
.
;
g
~+-·-t-'---~-t·--r--
"'45
l i : '· I ! ~f---f--+·-f---t~+--f---t-! 1 ' - - -1--
-
1
~rr
1---t
30
___
:
--+- - - --f---- -t-1-Vf---i-, 7W-
--
4S->
1
~-
f--\--'1-/ ~
1j
---
_~--~-~--
1-- ---- --15 1- -- ...
1 ,
-~-
--- -----
v\ Y •
.,
F
--1~ -~
=-t,-j= j-~.---+-+--H-+-+--ir-2<~4.9<> I
1-F--- f·ZFF --1--1--lc--t--1---- --~
0
I
,
-1--
1 i
-r
-~1-+-++-+--t--i
Q
I,
'
- - . . . . ·····/' -....... - - -
I -- -
subJ&Cttochoogeasa
result of further tests '-4L78 or oporatingexperlencos F points ore prototype riprap Installations ,_20.63 which fallod.
s
3
4
0 .J
"'3 w
"'1"' 0
<(
0
"'~ 1:X:
"'
I4J
lit
fno;~:~a~~:n•satistactwy ' - 9.23
: ~~r ·~~.~ -J~t=~·2
CD
it -71L89
Curve _is tentative and
L
:1!c::
a::
----~~ ;~:~:;a,::~:.':":n•;ory ra
F
0
"'"'1""
w
Curve shows minimum
\
-=
..J
t-139.21
NOTES
•
" cs
U)
+-1-+--t--t--+-+-+++-t--l-;~aO.O
1
1/
652.92
1
--- i 1 1 • :
i
I
:
j
;L -r-T--t ,
_
I
I
10
5
.~::{ 6
1
BOTTOM VELOCITY IN METER PER SECOND CURVE TO DETERMINE MAXIMUM STONE SIZE IN RIPRAP MIXTURE FIG_ 7.18
I
I I
'
II
I I
!
•
7.10.2
Free Board in Stilling Basin
Free board is provided against overtopping by surges, splash, spray and wave action set up by the turbulence of the jump. The following empirical formula expressions provides values which have proved to be satisfactory Free board (in ftl ~ 0.1 (V 1 + dz) where
vl
~ velocity before formation of jump
d2 ~ depth below the jump 7.10.3
Free Board in Piers
For the bridge portion, height of piers will be the same as abutment. For the upstream, 1 m free board may be adopted above afflux elevation corresponding to 100-year flood. For the downstream, it would be based on surface roughness.
7.11
DESIGN PROCEDURE IN BRIEF
For the facility of the designer, various steps involved in the design of dam are summarised as under:
I. Data Required i) Design flood ii) Stage discharge rating curve at the site iii} Maximum water level
iv) River cross sections at the site, upstream and downstream
II. Parameters to be assumed i) Lacey's silt factor (f) iii Safe exit gradient iii) Afflux III. Following to be designed i) Pond level iii Shape of crest, glacis slope. coefficient of discharge iii) Water way, discharge coefficient, crest levels, afflux iv) Depth of sheet piles in relation to scour depth and exit gradient v} Level and length of downstream floor in relation to the energy dissipation arrangement
vi) Thickness of downstream apron (floor) with respect to a) uplift pressure b) hydraulic jump vii) Design of protection works their length, thickness etc. The procedure is illustrated in Fig. 7.19.
107
Appendix A The Gumbel Method
This method will give a fair assurance of yielding acceptable values, especially for estimates up to 25year floods, if available streamflow records covers at least' 20 years. For such short recoid, graphical linearization by plotting on Gumbel probability paper is adequate. However, for bigger floods like the 50year, 100-year or 200-year flood, corresponding theoretical values can be extrapolated by mathematical computation. Briefly, the Method involves the collection of streamflow records for a damsite under consideration from the annual publications of the "Surface Water Supply Bulletin" prepared by the Hydrology Division of the Bureau of ~ublic Works covering at least twenty (20) years, picking out the biggest flood discharge in each year (Annual Peak Flow), thence rearranging the data is descending magnitude, followed by computing the probability that a flood of specified magnitude will not occur in a given year and/or calculating the reduced variate and corresponding flood discharge for any selected return period, and finally plotting in the Gumbel probability paper. The following is a step-by-step procedure of performing flood frequency distribution analysis by Gumbel Method: A.
Graphical Linearization (Good up to 20-year flood only) (1) After compiling the annual or yearly peak discharges (designated as Q in the tabulation), rearrange them in descending magnitude and in another column designated as "m" (order number) number them from 1 toN (N being the number of years of record). (2) Calculate the probability, P, that an event will be equalled or exceeded in any one year as given by the formula p
_!]1_
~
(Record this in another column.)
N + 1
(3) In the next column, calculate the prol)_ability that the event will not occur in a given year, as given by the formula · ~
P,
1
~
P
(4) Plot the points on the Gumbel probability paper with P, ~ values as abscissas and Q ~ values as ordmates.
(5) Draw with a straight edge the mean line that best ~ fit through the plotted points.
B.
By Mathematical Computation using Statistical Principle (For 25-year, 50-year, 100-year and 200-year flood)
(6)
Sum up all the yearly flood discharges and take the mean, i.e., L;Q
Q~N (7) In another column place the heading. Q
Q value.
Q. This means that you have to subtract Q from each
Q) ~ values and sum up to obtain L;(Q ~ QJZ.
(8)
Square all the (Q
(9)
Calculate the standard deviation, D,, according to the equation.
Do
·'
~
j L:
(Q
N
~- Q)Z ~
1
108
DIVERSION
HYDRAULIC
FOR
t
DESIGN
DESIGN
r
SUBSOIL FLOW
DETERMINATION OF UPLIFT PRESSURE
DAM
I I
STRUCTURAL
t
FOR SURFACE FLOW
J, DETER Ml NATION OF EXIT GRADIENT
DESIGN
l
CREST LEVELS SPILLWAY AND UNDERSLUICES
WATERWAY AFFLUX AND DISCHARGE/ M.
+
DESIGN WITH EVAWATlON OF RESPECT TO EFFECT OF R E TROGPE SSION HYDRAUUC JUO'P
~
DESIGN WITH RESPECT TO SCOUR
f DETERMINimON OF DOWNSTREAM
DETERMINATION
OF
FLOOR LEVEL LENGTH OF DOWSTREAM
DETERMINATION OF UPLIFT PRESSURE
FLOOR
DETERMINATION
OF
MINIMUM
DEPTH OF UPSTREAM AND DOWNSTREAM SHEET
STEPWISE
DESIGN OF DOWNSTREAM PROTECTION WORKS
PILES
PROCEDURE FIG. 7.19
(10)
Compute the value of the reduced variate, "y", for the required return periods, T, (say T, 25 years, 50 years and 100 years)
according to the equation y
~
-
Ln
- 2.3
1) log /2.3 log T \ T,- 1
(11) By substituting computed values o! "y" from step 10, calculate the corresponding flood discharge Qr, for each return period, according to the relation:
y
~
-a'
D,
(Q, -
-
Q) +
c
where the factors a' and C are a function of N (see Table 2 ·- 1) Table 2 - 1
N
a'
c
10
0.970
0.500
15
1.021
0.513
20
1.063
0.524
25
1.092
0.531
30
1.112
0.536
35
1.129
0.540
40
1.141
0.544
50
1.161
0.549
100
1.206
0.560
1,000
1.269
0.574
1.283
0.577
(12) Plot the points on the Gumbel probability paper with the return periods, T., or reduced variates, y, as abscissas and the respective calculated flood discharges, Q., as ordinates.
··-~-:~
110
Numerical Example: Flood Frequency Distribution Computation by qumbel Method
Year
1951 1952
Annual
Peaks (cu. m./scc)
Q
m
60 46 29
83
1 2
1953 1954 1955 1956 1957
31 27
1958 1959 1960
36 57 83
60 57 46
5 18
3 4
36 31 29 27 18 5
p
m N + 1 0.091
5 6 7 8 9 10
0.637 0.727 0.818 0.909
1- p
0.909 0.818
0.182 0.273 0.363 0.455 0.546
P,
0.727 0.637 0.545 0.454 0.363 0.273 0.182 0.091
Plot the values of Q and P, on the Gumbel probability paper and draw a straight line thru the points plotted. To determine the magnitude of say, the floods, proceed as follows:
25-year, 50-year and 100-year flood and/or other bigger
(Q- Q)
(Q- Q)2
83
43,8
1920.0
60
20.8 17.8
435.0
Q
57 46
Total N
~ ~
Q~
6.8
319.0 46.0
36 31 29
3.2 8.2 - 10.2
27
- 12.2
104.0 149.0
18 5 392 10 392 10
- 21.2 - 34.2 (Q - Q)2 ~
450.0 1170.0 4667.0
-
39.2
111
10.2 63.8
j
~
Standard Deviation, Ds
(Q- QP N- 1
j 46967
~
520
For return period, T r
25
Reduced Variates, y
-Ln(Ln ~) T,- 1 - 2.3 log (2.3 log
22.8
25 ) 24
- 2.3 log (2.3 x 0.01703) - 2.3 (8.59829 - 10)
~
~
- 2.3 log 0.0392
- 2.3 ( -1.4067)
3.23 For return period, Tr
50:
Reduced Variates, y
50 - 2.3 log (2. 3 log - ) 49 - 2.3 (8.296 - 10)
~
3.92
100:
For return period, T r
- 2.3 log (2.3 log
y
- 2.3 (7.997 - 10)
100
99' ) ~
4.61
The other equation for reduced variate is:
a'
y
From Table 2-1, for N
10: 0.97 22.8
y
(Q,- Q) +
a'
c
~ ~
c
0.97 0.50
(Q, - 39.2) + 0.50
0.0425 Q, - 1.666 + 0.50 0.0425 Q, - 1.166
Finally, ForT,
~
25:
3.182
0.0425 Q25 - 1.66
Q25
3.182 0.0+ 1.166 425
~
1 0 2. 30 cu. m./sec.
112
ForT, Qso
For Tr QJOO
50: 3.92 + 1.166 0.0425
119.50 cu. m./sec.
100: 4.61 + 1.166 0.0425
~
135.91 cu. m./sec.
The plotted flood frequency curve on Gumbel probability paper is shown in Figure A·l.
113
:----·--Gil• '·~
'POBABILITY
·r , . ' . .. ·~: .,; :
o:: .. ,·I'.H. ,,
.. :
··.:.L.·
·-
'···.· ·,. '.' '
1
u· ;I
CJ (Jl
"
11
I
' :I I I '
!
..
I':
1
:····.·.·.,
'I''
,,
i'H•
· .. .
.
'" ....
'
L
'
.
. ' i, ... y
.
.....
------
' :
'
A'
·
:. , · 1'-.',._ r1.•
+.·.·
!li:i
·c
'
I
..
_
1
.I
iri :: I
,._·-····H!··t+~···"j- 1+~+
·
'·--.'.J..!--+J--.. ,..,...,¥V::_;..l_.f..;...''-4---·i-''''---+---C.,.'~ 'H+mic'-itlttti++'+'+1·tti . ' '/ ; . I: I I I H+i++'+'+ .. -.' _:_ ;A' '·'·' ' ::!.it:!!·
~ .· t:.'if'l,·'··!''.·,'.. ; ' '' -:-1'-H~•H+;+tti:+thct+•-,;-t-i-7-·•t ··---; r~f
:
:'·tl·i•·..
E • c':, ..· ,l. ,.:·, :·, ·-- ~=-~+·-'rcc-~·1--'-++-+---'• :.·~"-···'+· 'r' -:·. ·.· :[.~ I? ' · . ··. ..:• ;·, ': '· .' ---:·
· ·· · · · · · · · ·• · · · · · · · · ·
..
'lh
•.c..·
---'_+'",",-·,+,-.. '
'•
,.F
1
:I !TiTTT: ',.· ··,',• . . L,,·
. ... I
m
.. 1:
+
...... •. . .'.•·.·•_.··.t;,,·,, I ~ fL+t.l.r.·.H•··Ul\.. 'r!-l. ... • :::l·f.'l ···!'''!···•······:.·. ' I·! 1.11\ ~: ITtll·.
•... f
'
"
':
,.,
::·tr
·'. .·~ . + . . · · -· .. '
, , -. - - · · •. ·••·•,,·••,. c·.·-.·.',I , ,·.•. . . •.·.· ·-· ', . , .........
H+ 'itl~-7-'i-•-H+t+•·r- ·
Cl
::
trlil: ,'I ·.. ·• ' l!J i · ' HlfHrH ,.• • ,.... · · · · ·
·• • '
: :'
J:
~
': .·:
', , :'' I'
:
(l
[: ; . ' :
' I i !.'·.. Iii H: I
·
·+1.0<".,7+(-+~:.;.·~•H+:...~++H-1......;-++-'-++++__L_,+."H. ' ' ·'.,
z (l
c
:;:: "tJ
m :n
.
(Jl
m
1
~~#l~t~~~ r~:rt~ t :~-:;f_=H-:'=·,~· _··=:~::~:-·L~_+1-+ -+ "'-:•1:~ ',-,·-~:,+-':;'~_:;:L~. it.,·+~t~I~-:-"+i;f+.~ ~f.Hf fi:H~r ;!it.+ HHit1t-~t-·
tJt1h±H
i 1:::' ! 1 :Ti ti~ K rr~ ~+ . tPiTj; 1, : rt4i++L~.·.irQ .J[_! ' 1 ~ . r, ·'+::4·.~....., ·.+--+,-_F,--. ' ' 1 I I I I ~J. ; :1 f iTt!+.~IJJ..LLL dlfL!Jli TTl 1 .~ '. LLLL_I'L- .!·'..,:~.;.·.·.:r: l....: •-.~-1---.~-.,!_.;.. ,.---~--·! · +1H'l·tU-J%.:. .~.·1cL.·_ j.r' .• ,: __.: .:.:_..~ .L...LLLlJ_,: ', H 1 +H.-r-m·"~ 'ly,j;+: 'I,,;,; '•LflLLHl'il.:,.,._:u_:,.thl.' '"'r·+-··~i--ri+tL_C:~: '•, i 1 TTITI 11 1;LUi• 111 1111l 1 11U4ll LL:U : : : " ..1. 1: 1 '
(l
0
z
1
CJ
I
1
,!
" " " +.'- , :
!
•
1
Ill
Pr -.Oil G
-2.0
y~
'--
I
lilil
.10 .20 .30
Ill
.50
I'
ilililll !iT':;
.70
.80
:
-r-
;
I
.90
'
.95
.97
1
..t-H
~~-~
L,_
th-l+l+IHH+I--t+t++-H++-H
JiriJnL ~t 1 !-I! 1 l i ' ! : '1 -~'- iHH-t'fHHriH+tn 1 :'L!i :.LU!',I·II.,I'LI' '-+-~~T'.,•.-i,--llrl_~,_11llliT ·,rr ~T-·.::.T'-h-:~·~···.~,~:,. ··Hi'HfHttt+i ' .:',! TTTT ' r-+'+1+++ 1
: ;
. 1
.
-
1
:
1
!
:
!
! :
1
1
.'·,··It'.'
+i'
!+'
1
II
.98
.99
.995
.9975
.999
_.fL.o_________o_L.o__~g~e_m_.__LLo_________2.J.._o________3~._o_________4~._o______~_5L.o _________6L,.o ________~7.0y m 1! •8ABILITY, Pr • (I- N +I I
.. ---........
I
·
1
~c URE__ ~"".-~ I
FREQUENCY
REDUCED DISTRIBUTION
ANALYSIS
BY
VARIATE , y.- ln ( Ln GUMBEL
METHOD
Tr
-:r;:::[ I ---·- · - - - - · · · - - - - - - - - - '
.CHAPTER 8
STRUCTURAL DESIGN A. (SPILLWAY SECTION) Solid Gravity
8.1
GENERAL
In the structural design of the solid gravity overflow section of the dam, it is essential that all design loads are carefully chosen to represent as nearly as can be determined, to actual loads that will act on the structure, and that all the resistive forces used in design represent as accurate an evaluation as possible. The design can be made more rational by utilizing fully the data available from the investigations. A gravity dam as the name implies derives its stability from the force of gravity of the material in its section and thus should have sufficient weight to withstand the forces and the overturning moment carried by the water impounded behind it. The loads in turn are transferred to the foundations.
8.2
REQUIREMENT FOR STABILITY
The design shall satisfy the following requirements of stability. a) The section should be safe against sliding on any plane or combination qfplanes. b) The section should be safe against overturning. c) Unit stresses in the material of the structure (body of the dam) or pressure on the foundation should not exceed the permissible limit.
8.3 8.3.1
LOADS AND FORCES
The following forces shall be considered in the design:a) Dead load (and liveload, if any) b) Pressure of pond water and tail water c) Earth and silt pressure d) Uplift pressures e) Earthquake forces i) Horizontal Inertia Forces
iD Vertical Inertia Forces iii) Hydrodynamic Forces fl Reaction of the foundation 115
There would be no occasion for ice pressure. Wind pressure and wave pressure would be negligible compared to other forces and hence this need not be considered in the design of spillway section. Stresses due to variation of temperature being insignificant, are also ignored.
8.3.2
Forces Inducing Stability and Instability
a) Factors which promote instability are:
i) Pond level and tailwater loads iii Earthquake forces and silt pressure iii) Uplift pressure b) Factors which promote stability are: i) iii iii) iv)
Dead load Shear resistance of the dam and foundation Quality and strength of concrete Strength characteristics of foundation and abutments. These factors also vary. Most important are dimensions and weight.
Whereas, factors as dead load and static reservoir load can be calculated accurately, other factors such as earthquake, silt and uplift pressure, should be based on assumptions. In case of dams on permeable (soil) foundation concrete cutoff walls are generally provided. The cutoff, if properly located provides an additional ,factor against sliding.
8.3.3
Design Assumption
The following assumptions are made for calculation of stability:a) The base width determined by projecting the spillway slope to the foundation line will act as effective foundation width and all concrete downstream of this point is disregarded. b) Loads coming on any transverse section is carried entirely by this section without transfer of load to adjacent transfer section or to the abutment. c) Vertical stresses vary linearly from upstream face to downstream face.
d) The material in the foundation and in the body of the structure is homogeneous. e) The stresses in the foundation and the body of the structure are within elastic limits. f)
No movements are caused in the foundation due to transference of load.
g) The resistance offered by steel sheet piles or cutoffs against sliding and overturning are disregarded. 8.4
LOAD COMBINATIONS
Design should normally be tested for the following load combinations. Any other combination of loads which have reasonable probability of simultaneous occurrence depending upon peculiar loading conditions and/or special site conditions, should also be considered: Load Combination A
Dam completed but no water in pond and no tailwater.
Load Combination B
Water level on upstream at crest level. normal dry weather tailwater, full uplift, silt deposit up to crest level.
Load Combination C
Upstream water at afflux elevation, tailwater at retrogressed flood elevation, full uplift, silt on upstream at crest level.
116
·'*lNA\ii&iL&. 2
Load Combination C1
Same as C - with no silt deposit on upstream
Load Combination D
Combination A with earthquake.
Load Combination E
Combination B with earthquake.
8.5 DESIGN CRITERIA 8.5.1
Dead Load
This shall comprise of weight of concrete/masonry plus the weight of piers, bridges and other appurtenances, if any. For preliminary design unit weight of reinforce concrete shall be taken as 2400 kg./cu.m. and of masonry 2300 kg/cu.m. The total weight of dam acts vertically through the centre of gravity of the cross section.
8.5.2
Live Load
Liveload, in case a bridge is combined with diversion dam, should be taken as per standard practice in !fridge design.
8.5.3 Pressure of Pond Water, Tailwater and Flowing Water Upstream i)
Static Pressure
Unit weight of water shall be assumed as 1000 kg/cum. Static water pressure shall be assumed to act normal to the face of the section and to vary linearly from top to bottom. Tailwater pressure shall be based on water surface elevation as per retrogressed rating curve. ii) Dynamic pressure
The force of current intensity of pressure due to water current may be calculated by the formula:
where P V
=
intensity of pressure in kg./sq.m. due to water current.
=
velocity of the current in metre per sec. at the point where the pressure intensity is being calculated.
The value of v2 shall be assumed to vary linearly from zero at the point of deepest scour to the square of the maximum velocity at the free surface of water. The maximum velocity may be assumed to be .[2 times the maximum mean velocity of the current. 8.5.4 Earth and Silt Pressures 8.5.4.1
General
Gravity dams are subjected to earth pressures on the upstream and downstream sides where the foundation trench is so backfilled. In the low dams, earth pressure are insignificant and should be ignored. Silt deposited on the upstream of the dam, for purpose of design, is treated as a saturated cohesionless soil having full uplift and whose value of internal friction is not materially changed on account of submergence. The experiments carried out indicate that silt pressure and water pressure exist together in a submerged fill and that the silt pressure on the dam is reduced in the proportion that the weight of the fill gets reduced by submergence.
117
8.5.4.2
Criteria
For calculating forces due to silt, the following criteria is recommended:
a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid weighing 1360 kg .1m3 b) Vertical "silt and water" pressure with a combined density of 1925 kg/m 3 c) Increase of silt pressure due to earthquake being insignificant may be neglected.
' 8.5.5 Uplift Pressure 8.5.5.1 Uplift pressure under a concrete dam on pervious (soil) foundation arc related to seepage through permeable materials. In such type of foundation, little resistance is offered by the soil to retard water percolation. The forces developed on the dam and its sub·structurc by the seeping water can cause two types of failures. i) Uplifting of floor ii) Undermining of the sub-soil termed as "piping" The concept of this type of failure has been discussed in Chapter 5 (Dams on Pervious Foundation). The methods developed for the analysis of design of structure against the above two types of failures have been detailed in Chapter 5, para. 5.7. Uplift pressure at any point can be calculated by the procedure discussed therein taking into account the effect of the upstream and downstream cutoffs, interference of cutoffs (piles), thickness of floor and slope etc.
8.5.5.2
Effect of Earthquake
During an earthquake, the water pressure is changed by the hydrodynamic effect. However, the change is not considered effective in producing a corresponding increase or reduction in the uplift farce; Because of its transitory nature, it shall be assumed not to have any effect on the uplift.
8.5.6 8.5.6.1
Earthquake Forces General
Earthquake imparts acceleration to the dam which usually increases the effective loadings in the structure. In the stability analysis therefore because of the additional force, both horizontal and vertical earthquake loads should be applied in the direction which produces the least stable structure. In order to determine the total forces due to an earthquake, it is necessary to establish the earthquake intensity or acceleration. This is usually expressed in relation to the acceleration due to gravity. Seismic coefficient used in the, design of any structure, is dependent on many variable factors including the variation in the soil conditions. It is necessary to indicate broadly the seismic coefficients that could be adopted in different parts of the country, though, a rigorous analysis considering all the factors involved, has got to be made in the case of all important projects to arrive at suitable seismic coefficients for design.
Fig. 8.1 shows the seismic map of the Philippines showing the different zones and the different soils on the basis of which the country can be classified.
118
td
.•
II •
•
18°
14~------·
•
10•
s• ll
LEGEND : ZONE 2 3
D ~
llililllll
MAP OF THE PHILIPPINES SHOWING SEISMIC ZONES
FIG. 61
8.5.6.2 Inertia Forces For Philippine conditions and size of the structure, in tl;w analysis of low dams should be followed:
followiJ;~g
criteria
a) A horizontal acceleration of 0.15 g shall be assumed and vertical acceleration of 0.1 g unless modified by seismic observations. b) Vertical acceleration should be considered only in stability analysis and not in calculation of stresses. c) Inertia forces should be considered to act at the centre of gravity of the structure. d) The horizontal inertia force shall be assumed to act from upstream to downstream or downstream to upstream to get the worst condition in design. Similarly, vertical inertia force shall be assumed to act upwards or downwards. Also, horizontal inertia force and vertical inertia force shall be assumed to act simultaneously. e) Full weight of the mass and not buoyant weight should be used in calculation of inertia forces. 8.5.7 Hydrodynamic Forces Due to horizontal acceleration of the foundation and dam, there is an instantaneous hydro· dynamic pressure (or suction) exterted against the dam in addition to the hydro-static forces. The direction of hydro-dynamic force is opposite to the direction of earthquake acceleration. Based on the assumption that water is incompressible, the hydro-dynamic pressure at depth y below the reservoir surface may be calculated by the formula: Py ~ C o: h wh
hydrodynamic pressure in kg{m 2 at depth y
where Py
c
coefficient which varies with shape and depth. design horizontal seismic coefficient
o:h
w
unit weight of water in kg/m3
h
total depth of water in m.
The value of C for vertical face or constant upstream slopes may be obtained from the equation:-
c where
y
~
Cm ~
depth below surface maximum value of C obtained from figure 8.2
The value of C can also be obtained directly from figure 8.3 The approximate value of total horizontal shear and moment about the centre of gravity of a section are given by the relations:vh
0.726 Py
Mh
0.299 Py h2
120
"'·
'
I
J ..J
< u
...
...a;> ... ... %
;I
90 0
2
0
..."' ...u ...< ... 0
~t"---- . . .......,
A ..
v. '~ ~ ~
'-......
60 •
..........
0
f1 Roa. W.S.-·-.
'·
~
)...
...
.....
1 I 1
~:·:-,:~
..... TYPICAL ..... t'--.-,SECTION F" <1( ~ ~ 'N; ,--.....
--.t
.,
--~max.
,
-P. baoo E
TYPICAL PRESSURE 0 lAG RAM
i
I I
I : '~
..J
<
0.1
0 . 2.
0.3
0.4
PRESSURE
0.5
-r--
I
~
ZO"
::>
>
~I ~
I
/}'·':.:., :. ;~: •. :
--1'..:1
""<
0
:::;_
...~ f...
.,~
~
.<'N~-'f"
'
I
i : '~ 0.7
0.6
COEFFICIENT
I
I
I 0.8
0.9
C
BASE AND MAXIMUM PRESSURE COEFFICIENTS FOR CONSTANT SLOPING Fti:ES
FIG. 8. 2
0
'
)-...
0. I
.... !!':
g
0.2
"'"' ..,...,
0.3
~
ILO::
::> .,
.. a: 0
IL
-'0
.... m:>:
1-
'\ ~ ....... t-.- 1--. f\ ~ ~ ~ r:::-- 1'--.. \ [\ ~ ~ t' ~"-....
0.4
\
(.)ILl
~_J <(
- 1-
"'o
0.6
I-
ll
0.6
,.,j.:
4!30
\ ~
75°
~
"" """ 1\.- "
\ c60 ~
.
0.7
........
"<; '30
\
\
ILIQ.
z c
1\
\
0.5
"""
\
\
\ \
l ...
•· Res. W.S. ~.-- '.S:9
·oo
'
~
"'
\
.·. :·
--. ·.. :
0
0.2
0.3
0.4
-
;
[\
\
1\
0.1
I
""" !'\. \
1\ \
I 0
I
0 .··:
0.9 1.0
·,.
1-
l
h
't l
:_'_ --~ - - !_ -:
\ \ 1\
\
1
1-
0.5
0.6
0.7
0.6
PRESSURE . COEFFICIENT C I c--c -(2--J+ jY -(2--l -2mh h h h
(y
y
y]
wh.ere: Cm is maximum C value from figure
COEFFICIENTS FOR PRESSURE DISTRIBUTION FOR CONSTANT SLOPING FACES
FIG.8.3
:t
i
j
Mti\\t'M%! tAb
IU¥
I
2
0.9
where
8.5.8
vh
hydrodynamic shear in kg./m at any depth y
Mh
moment in kg. m due to hydro-dynamic force at any depth y.
Reaction of Foundation
8.5.8.1 Under stable conditions, the resultant of the horizontal and vertical loads on the dam shall be balanced by an equal and appositive force which constitutes the vertical reaction of the foundation. In the stability analysis the following shall be the assumptions. i) It will be assumed that resultant of all vertical and horizontal forces including uplift is balanced by an equal and opposite reaction at the foundation, consisting of the total vertical reaction and the total horizontal shear and friction. It will also be assumed that locations of these forces are such that summation of their moments is equal to zero. ii) Distribution of vertical reaction along the base shall be assumed as linear and calculated by eccentric loading formula. The foundation reaction shall be computed taking into account the uplift pressures and to this computed foundation reaction, uplift pressure should be added to determine the total upward force at any point. iii) Internal stresses and foundation pressures should be calculated both with and without uplift to determine the worst condition. ','k
-;~
8.5.8.2
General Procedure
\}
The general procedure in calculating the foundation reaction should be as follows: i) In fig. 8.4, by analysing one metre strip of the dam, moments of all the forces should be taken about the toe i.e. point D. ii) Take the summation of all overturning moments and resisting moments (M 0 & MR) iii) Compute net moments LM
~
LMR - LM 0
iv) Take the summation of all vertical forces T-V v)
Distance from the toe where resultant meets the base of the dam.
X the resultant of all forces should be within the middle third. vi) Foundation Reaction at the toe and at the heel fo (at the toe)~
fA(attheheel)~
I:V(1-6e) B B
z;Y( 1
+
6e)
B
B
where e is the eccentricity
8.6 SAFETY CRITERIA 8.6.1 Sliding Resistance a)
General
. Many of the loads on the dam are horizontal or have horizontal components which are resisted by fnctwnal or sheanng forces along horizontal or nearly horizontal planes in the body of the dam. In order
123
MAXIMUM
n~~---~-=! _t_
I
WATER SURFACE
a
b
---·--/, ..
i
'\
,
,'
'
'
>..~,.,.._-.,.,.,.-~
j_~--- ~--- --..·-·_.:.__,;;_;___=:_·_. ·_._· I
... •
.. ·..
=--"+'---'---'-'--"'
c
WATER
d
CONTRACTION JOINT
PRESSURES ACTING ON
_j'{!'!'_TER
AN
OVERFLOW CONCRETE DAM
S~f!F~ACE... ;
• . . . ". : ...·.' -
-~
..
I '
B
BASE
AND
'o
UPLIFT PRESSURES FIG. 8.4
t~
;u wa
*Ed •Rt-kl
9b
8.
f4 IQ
that the dam may not slide on any planes, the total forces tending to slide the dam should not exceed a certain ratio of the normal force on the planes. Sliding resistance is also a function of the cohesion inherent in the materials and at their contacts and the angle of internal friction of the materials at the surface of sliding. b)
Criteria for Design
i) If cohesion is absent or insignificant and friction is the only force resisting sliding the factor of safety against sliding shall be calculated as follows: F1 ~ (W- U)tanQ p where:
F1
factor of safety against sliding
W
total weight of the dam
U
total uplift force
0
coeff. of internal friction of the material
p
~
total horizontal force
The factor of safety thus calculated shall not be less than that specified below:Loading Combination
11
-~
;E
Factor of Safety Against Sliding
2.0 1.5 1.2
A.B.C,C 1 D ··:
E
Shear friction factor
'< 4 3 1.5 'I
ii) In case where cohesion, in addition to friction, resist sliding, the factor of safety against sliding shall be calculated as follows: (W -
U) tan p
0+
CA
where
Fz
il
shear friction factor
c
cohesion of the foundation material
A
area of base considered
The factor of safety calculated shall not be less than the value mentioned in the table above. iii) The value of C and 1/J may be assumed for the purpose of preliminary design on the basis of availabe data on similar or comparable materials. The values given in the table below may serve as a guide. For final design, these should be determined from actual investigation at the site. iv) In the stability analysis resistance offered by cutoff shall be ignored. v) Factor of safety against overturning should be more than 1.5 under normal condition and more than 1.1 under seismic condition.
125
J
vi) Jn case shear keys are required, the forces induced by the keys i.e. passive pressure etc. should be accounted for. ,
Table No. 8.1
Material
C (KN/m 2 or 100kg/cm2)
1. Hard boulder clay, hard fissured clay, hard weathered mudstones ....................... .
300
2. Very stiff bolder clay, , ...................... .
150-300
3. Stiff fissured clay, stiff weathered boulder clay
75-150
4. Firm normally consolidated clay, fluvioglacial and lake clay ............................... .
40-75
5. Soft normally consolidated clay (e.g. marine, river and estuarine clay) ..................... .
20-40
6. Medium uniform sand 0.58 0.62 0.72
a) relative density 38% b) relative density 50% c) relative density 72% 7. Coarse uniform sand
0.65 0.75
a) relative density 50% b) relative density 70% 8. Gravelly sand a) 20% gravel, maxim urn size 3/4" i) relative density 50% ii) relative density 70%
0.68 0.77
b) 20% gravel, maximum stze 1 1/z", R.D. 70%
0.78
c) 35% gravel, maximum size 3/4" R.D. 70%
0.81
d) 50% gravel, maximum size 3/4", R.D. 70%
0.85
e) 50% gravel, maximum size 3"' R.D. 70%
0.87
9. Crushed rock, maximum size 3" a) R.D. 50% b) R.D. 90%
8.6.2
0.85 0.98
Resistance Against Overturning
Before the structure overturns, other type of failure would occur, such as cracking of upstream material due to tension, crushing of toe material and sliding. The structure may therefore, be considered safe against overturning, if the following conditions are fulfilled; i) No tension on the upstream face
ii) Compressive stress on the downstream is within allowable limit in the foundation material. iii) Factor of safety is adequate against sliding.
126
8.6.3
Safety Against Foundation Failure
8.6.3.1
Design Criteria
The unit stress in the foundation must be kept within prescribed maximum values. The foundation should be investigated and maximum allowable stress established. i) Maximum vertical pressure on foundation material near the toe shall not exceed the allowable
bearing pressure of the soil.
iii The allowable bearing pressure in final design should be based on field soil investigation.
De termination of Bearing Pressure: Allowable bearing pressure
Q
5 - 4 N2 B + 16 (100 ;. N 21 D; where:
Q
Allowable bearing pressure in kg./sq. m.
N
Number of blows per 30 em. in the standard penetration test.
=
B
Smaller dimension of the well cross·section in metres
])
Depth of foundation below scour level in m.
For preliminary design the allo~;vablc bearing pressure may be assumed as follows:-
Material
Allowable Tons/sq. ft.
Bearing pressure kg./cm.Z
1. V cry dense sand and gravel (N > 50)
4.0
3.6
2. Dense sand & gravel· (N ~ 30 - 491
3.5
3.2
3. Medium dense sand and gravel (N ~ 10 - 29)
3.0
2.7
0.25 0.50
0.23 0.66
1.00
0.91
1.50
1.37
4. Silt and clays Soft (N
~
6)
Medium (N
~
5 to 101
~
11 to 20) Hard (N > 201
Soft (N
where: N
=
is the standard penetration value, for silt and clays.
iii) During operation and after construction of the dam, the foundation will remain submerged. The value of allowable bearing pressure mentioned above should be halved for use in design calculations.
iv) Under seismic condition allowable bearing pressure shall be increased by 25 per cent. In case of raft it can be increased by 50 per cent.
3.6.3.? !~the "::'!"" of N foro q~,.. s0il '-" !es~ the.:!"!. 1.0 v~lJ:?.~.:n:: r:1useC bv ee.::thoua_ke may cause liaue· factior.. ln ·cna·c (:a.st:, adeqtWH: comp;H:tion is to or, don(; r,o aehir:v(: suitai1i<: vallle of N.
127
8.6.4 Safety Against Failure of Materials of the Dam 8.6.4.1
Quality and Strength of Concrete Ma;>onry
The strength of concrete/masonry shall exceed the stresses anticipated m the structure by a safe margin. Tensile stresses at the upstream face of the dam shall not be permitted even under seismic conditions. Tensile stresses not exceeding 5 kg./cm 2 may be permitted on the downstream face.
8.6.4.2 Allowable stress under earthquake i) In all calculations involving earthquake forces, either the permissible stresses shall be increased by 33 1/30fu or the total forces or moments should be reduced to 75°/o. ii) The permissible increase in bearing pressure is 250Jo.
8.7 CONTRACTION JOINTS
A contraction joint is a formed vertical or inclined surface between masses of concrete/masonry placed at different times. If the concrete dam is appreciably more than about 15 metres in length it is necessary to divide the structure into blocks by providing transverse contraction joints. Contraction joints in diversion dam structure requires careful consideration. They must be water tight in spite of differential settlement and must be provided for contraction and expansion of concrete to prevent cracking.
8.7.1
Longitudinal Joint (Joint parallel to Dam axis)
No contraction joint shall be provided parallel to dam axis, except at the junction of the toe and the stilling basin: This longitudinal joint should be provided with double water stop of metal or of rubber. In addition, a drainage arrangement should be provided just upstream of the toe to create pressure relief. 8.7.2 Transverse (Joints perpendicular to dam axis)
Transverse contraction joints shall be provided at regular spaces. Locations may be selected to suit the phasing of contraction. In no ease should the spacing exceed 15 m to 20 m. According to latest practice transverse joints are not keyed .or grouted. If considered necessary to avoid seepage loss, waterstop. may be provided near the upstream face of the joint.
8.7.3 Transverse contraction joints form seepage planes along which the pond water can flow to the downstream. To check this seepage, watertop is to be provided horizontally from the upstream sheet pile to the longitudinal contraction joint to prevent the possibility of piping.
128
STRUCTURAL DESIGN B -SLUICEWAY
8.8
GENERAL
Sluiceway portion of the diversion structure consists of the following essential components. i)
Impervious floor cutoffs
ii) Abutments iii) Piers
iv) Divide wall
v) Energy dissipating arrangement vi) Cutoffs VIII Protections viii) Flank and flared out walls ix) Gates and gate bridge, hoist and hoist bridge, cte.
8.9
DESIGN CRITERIA
8.9.1
Cutoffs
The upstream and downstream cutoffs of the diversion structure maybe of the steel sheet piles or of masonry or reinforced cement concrete. The depth of the cutoff shall be fixed as per provisions given under Hydraulic Design.
8.9.2 Impervious Floor
I ~ .)~ ·.._·
·_J- ~._-
Generally there arc two types of floor, viz: a) Gravity type: where the uplift is balanced by the self weight of the floor only considering unit length of floor. b) Reinforced concrete raft: In this type, the uplift pressure is balanced by the weight of the floor, piers and other self imposed dead loads, considering the span as single unit. While the gravity type of floor can be of either plain concrete or masonry, the raft type should be of reinforced concrete only.
8.9.2.1
General Requirements
The floor shall be designed against the following forces: o.) Uplift pressure Uplift pressure at any point shall be calcnlated as per procedure detailed in Chapter 5 "Dams on Pervious Foundation", taking into account the upstream and downstream cutoffs, 10tcrmcdiatc cutoffs If any, interference of cutoffs (piles), thickness of floor and shape of the glacis. bl Hydraulic Jump The adequacy of the thickness of downstream glacis and downstream floor shall be checked under unbalanced head condition with the formation of hydraulic jump, as laid down under para 7.8 (ii). For this purpose, the gate operation chart and corresponding downstream water level may be made usc of.
8.92.2 a)
Design Criteria Gra.1Jity floor
The following should be the design requirements. il The thickness of floor should be at least 10% more than the thickness calculated to counteract the uplift pressure at that point under the worst combination of loads under different conditions considering submerged weight of floor. ii) The floor may be independent of piers and abutments {in which case water tight seals can be pro· virled) or rnonolithic with piers and abutments. iii) When the floor is of plain concrete, suitable temperature reinforcement shall be provided. b)
Reinforced Concrete Raft
i) The floor shall be designed considering as a raft as per the theory of beams on elastic foundations. Theoretically, the soil reaction is not uniform but for simplicity in calculation, it may be taken as uniform except for large spans on flexible foundations in· which case it should be evaluated by Heteyni's method of beam on elastic foundation. ii) For shorter spans the floor shall be designed as a continuous beam resting on homogeneous foundation. The abutment, if necessary can be made independent by providing a joint in the raft with water seals of acceptable standards. The raft shall be designed for the moments caused by the worst combinations of forces
a) Uplift b) Soil reaction c) Moments transferred from abutments d) Seismic forces iii) For the purpose of analysis, the entire width of the raft may be divided into different sections, dependmg on the loads and moments anticipated to act over the different sections, such as upstream section (including glacis), gate, downstream glacis and downstream floor sections. 8.9.3 Abutments 8.9.3.1
General
Abutments are vertical walls or inclined walls. Mostly they are kept vertical from the crest for a sufficient length, then flared or warped.
130
Abutments ar(~ gPIH~r:dly dcsi);Iwd :ts rcl31nmg '-'Valls separated from the main floor hy an ex~ pansion _joint. \Vil.h sells. In somp cases, if t.hP totZ~l \vaterway between the abutmcots docs no\. exceed -10-50 met.res, the abu!mi:n(s ;~n: !'onsi.r·u<"tcr.l monolithic with the main floor and the whole section is designed as
8.9.3.2 Top Width Top widl.h of !.he ahut.rncn\ in (':tch hlo<'k shall be fixed as per tlw requirements due to the loads and moments, minim urn width rpquir('d for b\oekouls of main gale, stop log grooves and bridge bearing etc. Minimum width of ahut.ment.s clear of bJ·ock outs should be about 600 mm. In the gate and road/ rail bridge blotk it. is g(~tH:rally kt~pl as l2SO t.o 1100 rnm.
8.9.3.3
Design Criterb Loads and Forces
Abutment. should he (ksigncd !.o withstand !.he following loads and forces a) Dead load
b) Live load due t.o t.hc moving t.r·al"fic
ovr~r
the bridge where, provided
c) Impacl and Braking effect d) Earth pr('-'>surc, livl' load surcharge and sat.urat.ion pressure~. e) Earthquake
i) Horizontal
forces:-··
inertia fore<·
ii) Dynamic increment. of earth pressures. Water lev(:\ on sluiceway sid(~ .will he governed by the river stage. Water level on the backfill side will be governed hy hydraulic gradient line from upstream to downstream prevailing, due U; the river st.1gc during the post. construction period, an·d subsoil water level during construction period. In order to find out the "vorst. effect, cakulations should he done under the foilowing conditions:-
t·omplekd, full backfill, sat.urat.ion level at. subsoil \Vater level or ;)ond level \Vhichever is higher, sluiceway side dry. "
i) Construction
ii) Design floor! co:;rlit.ion iii) Annual or norm{tl flood tondit.ion Un case annual flood data is not available, it. may be taken a.s 60°/o Lo 70°/o of cksign flood).
8.9.3.4 Safety Criteria i) The abulment scr:lion shall he checked for sarcty against allow
and sliding. Allowable bearing pressure should be determined by field and laboratory tests:-ii) Factor of safety againsl overturning and sliding should be as under:
F.O.S.
Loading Condition ----~-·~-~·---
Sliding
-1
I
1
j
l
!l :t
Overturning
I
I
under seismic condition
1.50
under normal condition
1.20
under seismic condition
i) coarse sandy soil
containing no silt or clay
ii) Coarse sandy soil containing silt
!
l
1.20
8.9.3.5 The value of friction shall be determined by field investigations, However, following values may serve as a guide.
1 I
Ij
1.50
under normal condition
8.9.4 8.9.4.1
0.55 0..15
Piers
General
In diversion dams constructed as reinforced cement concrete structure, the piers are constructed monolithic with the floor of the diversion structure. This is so in the case of floor designed as a raft, in the case of gravity type of floor, piers are generally constructed independent of the floor. Proper joint and sections arrangement between the gravity floor and all round should be provided. 8.9.4.2 Thickness of Pier
The thickness of the pier shall be fixed from consideration of (i) forces and moments transferred by the pier to the floor/foundation (ii) minimum thickness required at the block-outs for the main gate and stop log grooves and also (iii) the weight of the pier required for counteracting the uplift pressure. The thickness of the pier for reinforced cement concrete structures generally varies from 1.5 to 2.5 meters
8.9.4.3
i
Length of Pier
In the case of a raft type floor of the diversion structure, the piers shall generally be extended up to full width of the raft to avoid cantilever action of the raft at the ends. In the case of gravity type floor, the length of pier can, however, be restricted according to the minimum requirement from con· siderations of road/rail bridges, hoist bridge space required for housing instruments if any, main gate groove, stoplog grooves, space for storage of stoplogs, adequate length to prevent cross flows occurring which may cause damages to the floor and beyond. 8.9.4.4
Height of Pier
On the upstream side, the pier shall generally be constructed above the pond level with adequate free board. The height shall be fixed as per requirement of the weight of the pier in counteracting uplift pressure. On the downstream side, the piers shall generally be constructed above the high flood level up to about 3 m beyond the end of the bridges and instrumentation platform, if any, and thereafter the height could
I
132
be reduced according to low flood levels on the downstream side. In the portions where road/rail bridges are provided the height of the piers shall be fixed such that the bearings of the bridges are not affected by the high flood. Adequate allowance shall also be made so that the beams of the bridges are not hit by floating debris during high floods. In the main gate portion, the height of the pier shall be fixed so that during high flood, the bottom of the gate is at least 1 metre clear of the affluxed high flood level. In earthquake regions, however, the top level of the pier could be restricted to the top level of the abutments and steel trestles provided over the piers for housing the hoist bridges for operation of the gates and stoplogs. This arrangement would reduce the loads and moments due to inertia during earthquakes.
8.9.4.5 Design Criteria General
Pier shall be designed to withstand the following loads and forces: a) Dead loads b) live loads due to road/railway bridges.
c) Force due to the braking effects of live loads on road bridge. d) Dead and live loads of gates, stop logs counterweights and the hoist bridge. e) Force due to water current flowing obliquely.
fl Differential hydrostatic pressure with one side gate open and the other adjacent gate closed. g) Seismic forces. h) Hydrodynamic forces due to seismic conditions.
Wave pressure is insignificant compared to other forces and may be neglected. Forces due to tractive effort on road bridge is less than that due to braking effect and thus need not be considered. Where the height of pier is more than 12M, tractive force or braking effect should be considered.
8.9.5 Loading Combination While considering earthquake forces, the following points should be kept in view:-
il Earthquake need not be considered to act simultaneously with design flood. Earthquake may be considered to occur along with mean annual flood. In the absence of data on annual flood, the value of mean annual flood may be taken as 60% to 70% of design flood. iii Vertical inertia forces should be considered while testing stability of the structure. It need not be considered in calculating stresses. iii) While evaluating inertia forces, full weight of the mass should be taken, ignoring reduction due to buoyancy or uplift. Only in case of evaluation of dynamic increment or decrement of lateral earth pressure, buoyant weight should be adopted. iv) Horizontal earthquake forces are to be calculated for full dead load, but not for full live load. Fraction of live load for which earthquake force is to be calculated, is indicated below:For stability calculation -
l~ ~
Railway Bridge
50%
Road Bridge
25°/o
133
For stress calculatian 100%
Hailway Bridge
50°/o
Iload Bridgl'
For working out. live load or hridg(', seismic force due to live loarl shall he ignored when acting in the direction of traffic but shall iw LakPn int.o eon~idcration \vhen actin~ in a direction perpendicular to traffic. v) I\~rrnissible stresses maybe irwn~ased by ;-n 1;:-J 0/o or Uw forees/monwnts m;tyhe reduced to 75°/o.
Subj('d to the considerations inclie;~t.cd above, t.hP loadinJ-?: eornbinaLions which givP t.he \vorsl effect, should be used in t.he design.
8.9.6
Evaluation of Forces a) Dead Load
F'or preliminary design the following data maybe adopted:--·· 62.5 lbs/c.ft. (1000 kp;lm")
Unit weight of water Unit weight of dry eart-h
=
100 lhs!c.ft. (lGOO kg/m:3) J 20 lhs/c.ft (Hl20 1q;lm'1)
~
1:JG lbs/e.ft (21 GO kr;/m 1 )
Unit \Veight of <·ompact.ed ('Jrl h Unit wl'i~~ht. of s:lt.urat.Pd c:1rth Unit
w(~ight.
72.:51 lbs/c.ft (llGO kg/nr3)
of suhnwrg('d l'arth
Unit wPight of conerdt'
~
1:JO lbs/c.ft (2:100 kp;lm 1)
For
Gat.P Brid,t;(' ··- It. should he designed for the rated load of gates, hoists and hoisting equipmcnts. In abscnee of data, prclin1inary calculations maybe based on following assumptions il 100 Jbs/ft." 1-188 lZgim 2) -~ for opcratinr; platforms without sloplogs, for operating platforms \Vith sloplogs. Detailed design should be cheeked using actual loading ::tftcr the gates, hoists and hoisting cquipmenl.s have been designed. In road bridges, standard bridge loading should be used, c)
Earth Pressure
Earth pressure rnay be evaluated by the procedure detailed in para 8.9.7. The following assumptions should be made when working out the earth pres~ure. i) If the backfill is compacted. water load <.can be ncglecterl. If the backfill is not. compacted. hydro· static pressure up to maximum saturation level should be considered in addition to earth prc;surc which will r.onsisL of two parts:~
134
a) Pressure of submerged earth below saturation level b) Pressure of moist earth above saturation level ii) Weep holes, though provided, should b'e considered as inactive when analysis is done under nonscisrnic condition. d)
Inertia Forces
Unless otherwise specified in relevant earthquake code of the country, the seismic coefficient should he adopted as under:~ Horizontal seismic coefficient
-- 0.15 g 0.1 g
V erlical
The inertia force which is obtained by multiplying the full dead load (without reduction due to bouyancy) by the seismic coefficient maybe considered unchanged along the height of the structure. The inertia forces will be assumed to act at the center of gravity of the structure. The horizontal inertia force shall be assumed to act in any horizontal direction. For calculating stability, it shall be combined with verticil inertia force acting upward, and [or calculating foundation pressure it shall be combined with vertical inertia force acting downward. As discussed under para 8.5.6.2 for the design of spillway, vertical inertia force need not be considered while calculating concrete stresses. Vertical inertia force should be calctilated not only on concrete masonry, steel etc. but also on water load. c)
Hydrodynamic Forces
Method of evaluation of hydrodynamic forces due to earthquake has already been elaborated in para 8.5.7.
8.9.7 Effect of Earthquake on Earth Pressure 8.9.7.1
Active Pressure Without Surcharge
Active pressure due to earth[ill under seismic condition shall be calculated by the formula:--- This is illustrated in Fig. 8.5.A.
where:
Pa
active earth pressure in kg/n1eter length of the wall
w ~ unit weight o[ soil in kg/m3 h
c,
height o[ backfill (1 - cxv) cos2 cos
i\ cos 2
A cos ( J" + o: + ?t )
[
(r/J-1
horizontal seismic coefficient
o:v
vertical seismic coefficient
rl)
sin((/) + J ) sin((/) - i - A ) + --- ( cos (i -cx:l cos (! + o: + A )
The maximum of the two being the value for design. o: h
ex -
135
yJ
angle of internal frielion of soil
J
angle of friction belv·/een the wall and the carthfill
ex::
angle which earth face of wall makes with the vertical slope of earthfill
Point of /\pplication
From thP total pressure calculated as above. the static active pressure, calculated by putting '-< " = ,1, '"' 0, is subtraetcd. This gives the dynamic increment. The static component is assumed t.o act at h/:~ above the base. The dynamic increment. is assumed to act at mid-height. c< h
=
3.9.7.2
Passive Pressure Without Surcharge
Passive pressure due to earthfi!l may be calculated as follows: - This is illustrated in fig. 8.5B.
(l.oC
where A
COS I! CO!)
~-~--:-~(. ,····· COS fJ --
c
o-
vi cos2 ((/J + « - A I + I!A I [ 1 - ( sin(~ ·- J cos (i
-o<.
l
I sin. 1
Point of Application
Minimum of the two values being adopted in design.
n
From the static passive pressure obtairied by putting oe h = c<. v = 0, the dynamic passive pressure as obtained above is to be deducted to get the dynamic decrement. The static passive pressure will act at h/:J and the dynamic decrement shall be assumed to act at an elevation 0.66 h above the base.
8.9.7.3
Active Pressure Due to Uniform Surcharge
where: q
=
uniform surcharge per unit area of inclined earthfill.
Point of Application
Static component will act at mid·height and dynamic increment will act at 0.66 h.
136
jj
;&J.
1J££ill&k iii.
CU£1 JWi
DIRECTION Of HORIZONTA~,.,-- EARTHQUAKE ACCI!:LERATION
B ACTIVE
PRESSURE
EARTH
(FIG. 8. 5 A)
PASSIVE
PRI!:SSUI'lE
(FIG. 8. 58)
PRESSURE DUE TO EARTHQUAKE ON RETAINING WALLS !ABUTMENTS) FIG.8.5
r - - - - - - , - - - -··--- ., ' / /
''
'
/ /
/
h
'
'
Dl STRIBUTION OF THE RATIO WITH HEIGHT OF WALL
LATERAL DYNAMIC INCREMENT VERTICAL EFFECTIVE PRESSURE
FIG. 8.6
8.9.7.4
Passive Pressure Due to Uniform Surcharge
,:.;
1
•••••
Point of Application
Static component will act at mid·lwight and dynamic decrement at 0.66 h.
8.9.7.5
Effect of Earthquake on Subsoil Water in the Backfill
Dynamic increment in active earth pressure and dynamic decrement in passive earth pressure, in case of submerged backfill shall be calculated as indicated above with the following modifications:a) /
for submerged fill - 'h x
J for dry fill
where:
W s = saturated unit weight of soil in gm/c.c. c) For w use buoyant weight while calculating static pressure as well as dynamic pressure.
Hydrodynamic pressure on account of water, contained in earthfill shall not be considered separately as the effect of acceleration on water has been considered indirectly. In partially submerged backfill, the ratio of the lateral dynamic increment in active pressures to the vertical pressure at various depths along the height of wall may be taken as shown in figure 8.6.
··~.· -~ .. ••
The pressure distribution of dynamic increment in active pressures may be obtained by multiplying the vertical effective pressures by the coefficients in the figure, at corresponding depths. This procedure may also be used for determining the distribution of dynamic pressure increments. In the fig. 8.6, h
height of wall
h' ,.- height of submergence
•.:!'·• .•'}§
Ca
value of dynamic earth ptessure for saturated backfill
Ca'
value of dynamic earth pressure for submerged backfill
Ka
value of Caw hen« h ~
Ka'
value of Ca' when« h
~
«. v «:
~
il
v ~
il
~ ~
0 0
To find the value of dynamic increment in active pressure at any depth (d), calculate the vertical pressure at d. Also from the diagram find out the value of horizontal intercept (X). Then dynamic increment = vertical pressure x (X).
138
8.9.8 Hydrostatic Pressure
When one of the adjacent gate is jamncd and water is static on that side of the pier, while water is flowing on the other side, a hydrostatic pressure equivalent to velocity head will act on the pier. The worst effect will be when velocity is highest i.e. during design flood condition.
8.9.9
Force Due to Water Current
Piers intended to be parallel to the direction of the current shall be designed for a deviation of 20 degrees from the normal direction of the current. Intensity of pressure normal to the pier and acting on the area of the side elevation of the pier can be solved by taking the component of velocity of the current in a direction normal to the pier and applying the following formula:-
where:
v2
p
78
p
intensity of pressure in kg./sq. m.
V
~
component of the velocity at the surface (normal to the pier) in m/s
The value of V2 shall be assumed to vary linearly from zero at the junction of pier with floor to the square of maximum velocity at the free surface of water. For the purpose assume maximum velocity to be [2 times the maximum mean velocity of the current.
The value of P should be integrated to full height to get the total head on the pier. The effect of current shall in no case be taken as less than that of a static force due to a difference
of head of 0.25 metre between the opposite [aces of a pier at high flood leveL
8.9.10
Longitudinal Forces in Road Bridge Force Due to Braking Effect on Road Bridge
Unless otherwise specified by road bridge' code of the country, the braking effect shall be evaluated as follows:' a) In case of a single lane or a two.Jane bridge. Braking effect shall be twenty percent of the first train load plus ten percent of the loads of the succeeding train or part thereof. Considering loads in one lane only when the entire first train is not on the full span, the braking force shall be taken as equal to twenty percent of the loads actually on the span. b) In case of bridge having more than two lanes As in case {a) above, for the first two lanes plus five percent of the loads on the lanes in excess of two.
c) The loads shall not be increased on account of impact. d) The force due to braking effect shall be assumed to act along a line parallel to the roadway and 1.829 m above it. While transferring the force to the bearings, the change in the vertical reaction at the bearings should be taken into account.
139
.§@
&W
zp
tm
;c
8.9.10.2
Longitudinal Forces Due to Reactions in Bearings
The longitudinal force at any free bearing shall be limited to the sum of dead and live load reactions at the bearing, multiplied by the appropriate coefficient of friction as under:-
Roller bearing
0.03
Sliding bearings of hard copper alloy
0.15
Sliding bearing of
steel on cast iron or steel on steel
0.25
Longitudinal force at the fixed bearing shall be taken as the algebraic sum of the longitudinal forces, at the free bearings in the bridge unit under consideration and the force due to the braking effect on the wheels. 8.10 MISCELLANEOUS DETAILS 8.10.1
Longitudinal Joints
No longitudinal joints parallel to the dam axis should be provided in the sluiceway portions except under special circumstances. As the downstream toe of the sluiceway, where piers get discontinued, a longitudinal joint may be provided for convenience of construction. This joint shall however be protected by a double line of water stops. In addition, suitable arrangement for pressure release should be provided just upstream of the toe. The joint so provided should be continued up to the abutment and end of the pier of sluiceway. Vertical joint in the abutment shall also be backed by a suitable graded filter.
8.10.2
Transverse Contraction Joints
The transverse contraction joints form seepage planes along which reservoir water can flow. For providing such joints the procedure to be followed shall be the same as discussed under "Spillway".
8.10.3 Weep Holes
·.·.~
. Weepholes shall be provided in the abutments to facilitate drainage of the backfills. The spacing and Size of weep holes would depend upon the amount of drainage, type of soil in backfill and sub-base and the extent to which the structure should be protected against uplift. Graded filters at the back of the weep holes should be provided to prevent washing of the backfill materiaL 8.10.4
Backfill Material
Expansive soils should not be used for backfill behind abutments. Wherever possible free drainage materials should be used, because: '
il The higher angle of internal friction of the free-draining material will result in lower lateral earth pressures.
ii) Saturation pressure can be reduced or eliminated by adequately draining the backfilL
:;~
i, ·.• ...·.·1··
140
8.10.5
Backfill Drainage
Backfill should be maintained in a drained condition. There are many techniques available to achieve this objective such as: i) Preventing water from entering the backfill. ii) Installation of sub-surface drainage system. iii) Providing weep holes as discussed above.
141
STRUCTURAL DESIGN C -DIVIDE WALL (TRAINING WALL)
8.11
GENERAL
Location
8.11.1
Undersluices, river sluices and spillway of the diversion dam are separated by properly designed divide walls (lraininp; walls). Their function lay out and hydraulic requirements have been discussed in chapters 6 and 7. As described thereunder, the divide wall may extend both upstream and downstream to the end of the impervious floor. In some. cases, however, the length of downstream portion of the divide wall can be curtailed/omitted if so indicated by hydraulic model tests.
Position, Length and Height
8.11.2
Lengths and height and width of pocket to be determined as detailed out under para 6.4.
8.12
DESIGN CRITERIA
Divide wall shall be designed as a retaining wall against the following forces. a) Hydrostatic pressure due to difference in water level on the two sides created with one side gale open and other side gate closed. b) Force due to water current flowing obliquely. c) Wave pressure
d) Inertia force due to earthquake. e) Hydrodynamic force due to earthquake. f)
8.12.1
Earth pressure assuming pond silted up to spillway crest.
Load Combination
Load combination shall be the same as specified for the design of pier described under para 8.9.5 and para 8.12.5. This should also be tested when spillway portion is silted up.
8.12.2
Evaluation of Loads and Forces
Evaluation of loads and forces shall be due according to para 8.9.3.3.
••.·.;..(J
'~
142
l----1<~" rb•tructed
"
ere st
\ \ \
i I~ hw
I
PW
Unobstructed crest
------r-------pproxtmate pressure line
= 2/3 hw
StIll
ter level
l-----2.4hw
WIND
PRESSURE
AND WAVEDIAGRAM
FIG. 8. 7
8.12.3 Wave Pressure Wind blowing over the pond area causes a drag on the surface. The effect of the drag is to pull the top surface along the direction of wind and thus ripples and waves are formed. Wave pressure can be calculated as under:-
hw ·~ 0.032 1/VF + 0.76:1 - 0.271 Fli1 where: hw
height of wave in m.
F
fetch of reservoir in km.
V
velocity of wind on water surface in km./hr.
Maximum unit pressure Pw due to wave (in t/m 2l is given by the formula. (Fig. 8.7} Pw
~
2.4 whw. This occurs at 0.125'hw above the still water level.
The total pressure (in t} is given by the relation: Pw
2400 hw2 kgJm2 ~ 2000 h2w (approximately} represented by triangle ABC acting at 0.357 h2 w
8.12.4 Foundation Design The foundation of the divide wall should be extended below the scour depth. It can be limited to a depth of 1.25 H below the HFL at the nose portion in view of the protection of cement concrete blowing and riprap all round the nose. In case of masonry divide walls, it is necessary to provide well foundation, otherwise it can be of steel sheet piles where it is possible to drive the sheet piles to sufficient depths, taking them well below the deepest possible scour. The depth of the foundation may be decreased progressively towards the impervious floor of the dam up to a minimum depth of the main cutoff. The upstream and downstream walls shall be separated out from the main pier beyond the impervious floor of the dam/weir by joints ..Joints shall also be provided in the divide walls at places where there are changes from sheetpile/cutoff foundation to wall foundation.
.·.il ~]
8.12.5 The divide wall is likely to be subjected to a maximum differential pressure when the full discharge of the river is passing through the undersluices and no discharge is passing over the spillway. In this case there will be difference of water level on two sides. Upstream wall shall be designed to resist the differential head on account of the velocity of flow in the pocket due to the gates adjacent to the divide wall on either side being kept open and closed position. The downstream wall shall be designed to resist the moments due to the differential head caused by the closure of gate on the side and opening of gate on the other side. It shall be based on model studies. As it is difficult to assess the hydrostatic forces discussed under 8.12, it is customary to design the wall for a differential head of 2 m for preliminary designs. This can be combined with silt pressure where such a combination becomes critical.
I
144
CHAPTER 9
INTAKE WORK
9.1
GENERAL
The intake works also known as Head Hegulators or Head \Vorks is provided ::tl the head of the off taking canal, immediately upstrearn of a diversion dam. Its detailed funrtion, location and layout have heen discussed in Chapter 6. para G.G. As discussed therein, the int:d'\e work should he suitably aiigncd so as to reduce silt entry into the ('anal to a minimum and avoid back flow and formation of stagnant zone in the pocket.
9.2 LAYOUT
The axis of the work is thus normally aligned at <:~n angle 90° - 110° with the axis of diversion dam as shown in Fig. 9.1. Because of its inter-relation with the diversion worl\, the layout should be established with model studies, if considered necessary. Where possible, intake works should be located on the outside of the bend or the eoncavc bank of the river for effective sediment exclusion from the offtaking eanal. At the outside of a river bend, the top high velocity water flows outwards and is then defleeted towards th(~ head whereas the slow moving bed water is deflected across the bed to\vards the inner convex side of the curve causing deposition of sediment at that side. A canal taking off from this convex side will draw excessive charge of bed materials. In the case where a eanal is to be considered on each side of the river, the dam and head works structure should be located on a straight portion of the river. It is also preferable to locate the dam and intake work at a point along the river where the offtaking canal will be along natural ground above designed flood level. Such a location will require a higher dam, but will avoid the need for expensive ·protection work along the canal to protect it against damage from flood flows in the river. In most of the eases the intake works will be constructed on the lower reaches of the river on per· meable foundation comprising of sand, gravel or silt. Mostly, intake loeation and surface topography rather than geology would thus determine the site to be adopted. The engineer in charge should therefore review carefully the possible sites before any extensive foundation exploration \'>'ork is undertaken.
9.3 TYPES OF STRUCTURE
Type of Intake Works may be open type or pipeibox conduits. The form of the control structure for the intake works depends on the type of gate used which arc most distinguishing feature of this structure. The type of gate to be selected would depend on the flow condition of the river under which they arc to operate and the amount of water to be diverted. Barrel (Box) type may be adopted \vhen chance of silting up is remote and where open type trough structure is uneconomical.
145
DAN
AXIS
i
01'" CANAL INTAKE
i ALIGNMENT OF
INTAKE
DAN AXIS
.J9____ fi
BLOCK PROTECTION
l
j' r--1. - . /
"~
--,-
l
SHEET PILl!"
UPSTREAM INPflf;VIOU8 FL.O<m
! Plli:R
.SLOPE IIH 2
DO'ilNSTFEAM FLOOf\ DOWHST£'".!:1\N SH!:ET PILE
n==---·-· .
.
INVERTED FILTER TOE WALL
L
LOOSE APRON
CANAL
TYPICAL PLAN
OF
INTAKE
LAYOUT OF CANAL HEAD REGULATOR FIG.9.1
In the open type, the ~ontrol structure takes the shape of a rectangular flume subdivided into a number of bays by piers which are surmounted by an operating deck. On some of the larger installations where an operaLing road is involved, a bridge is provided in addition to the operating deck. This type of control structure is generally preferred because of its acccssability for repairing the gates and for the removal of debris and drift. At some locations submerged gates arc employed. The vertical-lift gates or the top-scaled gates are the customary submerged types. To close the remaining portion of the structure that is not closed by the gate panel walls, also termed as breast, walls arc used. These walls are designed as reinforced-concrete slabs supported against the piers or buttresses. At reservoirs where the intake structure is constructed through an earth embankment or a small earth dam, the so-called closed type of structure is used. This type is essentially a buried conduit consisting of a single or mtdtiplc·barrcl reinforced-concrete section which extends through the embankment. The flow through the structure is controlled by top-scaled radial gates or vertical-lift gates which arc located near either the center of the embankment or the upstream end of the conduit. To avoid having the conduit under full reservoir pressure, a condition which would increase the danger from leakage if cracks should develop in the \valls of the conduit, it is desirable to have the control gates located ncar the intake to the structure. The disadvantage of this location is that an access bridge would be required from the top of the embankrnent to the operating deck of the eontrol structure. Trashracks should always be provided at the intake to prevent trees and other clrift from getting lodged in the closed conduit.
9.4
9.4.1
HYDRAULIC DESIGN
Pond level
Pond level upstn~am of Lhe intake is generally obt11incd by adding the working head to the designed fuH supply level in the canal. The working head is the head required for passing the designed discharge into the canal and includes the head losses in the regulator.
' 9.4.2 Water way and Sill level
'
Sill level is fixed by subtracting from pond level the head over the sill required to pass the full supply discharge in the canal at. specified pond level. Considerations for fixing sill level (crest level) and water way, have been detailed in Chapter G, para 6.6.3. The hydraulics of intake 'vorks usually involves either one or both of two conditions of flow - i) open channel or (free flow) ii) full conduit or (pressure flow). Analysis of open channel flow either in open water way or in a part full conduit, is based on the principle of steady non"uniform flow conforming to the law of conservation of energy. Full pipe flow in closed conduit is based on pressure flow, which involves a study of hydraulic losses to determine the total heads needed to produce the required discharges.
il*
Hydraulic jump, basins and impact. block clissipators are normally employed to dissipate the energy of flow at the downstream end of the intake work. These devices arc designed on the basis of the law of conservation of momentum_
:!~
9.5 OPEN TYPE STRUCTURE ?:ii
>$
;q
A
'·~
DESIGN PROCEDURES
The following design procedure is generally adopted in determining various components of the (open type) intake work. i) Crest level and waterway and the number of piers and span width are first determined. ii)
A sloping glacis is provided and continued to the level for the formation of hydr<1ulic jump under different diScharge conditions. 147
iii) A length of horizontal floor beyond the glacis to satisfy requirements of stilling basin discussed in chapter 7.5 "Energy dissipation" is provided. At the end of the basin a vertical cutoff is provided. iv) The glacis and downstream floor is checked against uplift and exit gradient as per procedure ou tlincd in chapter 5.
i)'
Normally, under static condition, the worst condition for checking the floor would be, when the canal is dry and design flood is passing in the river. If the uplift pressures are too high, a reinforced concrete mat maybe necessary from economic consideration and in such a case the piers may have to be extended down the glacis to provide for the end support against bending of the slab.
-?) '.'.'.1'
v) At the time of the formation of hydraulic jump when part discharge is passing into the canal, the glacis should be checked for the unbalanced head for different discharge intensities. vi) For reducing pressure, the impervious floor upstream of undersluices is sometimes extended to the end of the intake work.
vii) In the case of gate controlled structure, when the high flood level is much higher than the pond level, provision of a breast wall i~ necessary, as discussed in para 9.3. The breast wall spans from pier to pier and is designed to support its own weight in addition to the water pressure acting against it. When fully closed, it shuts the opening from crest to bottom of breast wall. Without the breast wall it is sometimes uneconomical to provide gates right up to high flood level.
.'I'
;"f
.··
~X }; ·.·,···
Various parameters/components involved in the design should be determined as under:-
I j
9.5.1
Waterway
Flow in an open channel intake work shall be similar to that in open channel spillway discussed in the foregoing chapters. When the submerged slide gates are used, discharges through the control with the gates completely raised will be open crest flow.
The required head over the sill H, for passing a discharge Q, with an effective waterway L, shall be worked from the following formula: Q
CLc H c3 l2
where Q C
=
discharge in m:3/s,
a coefficient effective waterway in m; and
II,
required head over the crest for passing a discharged Q, in m.
. In the formula given above the exact value of C depends on many factors including the head over the stll, shape, wtdth of the stl!, tts hetght over the upstream floor and roughness of its surface. It is, therefore, recommended that for large mtakc structure. the value of C be determined by model studies where values l~ascd on prototype observations on similar structures arc not available. The value of C should be modifled for submerged conditions as outlined in Chapter 7.
After having decided the effective waterway, the total waterway between the abutments including piers shall be worked out from the following formula: L, ·~ L c + 2 (N K p + K.,\ I H.t +. W
148
I
where LL
Le
total =
N
w~terway,
effective waterway number of piers pier contraction coefficient
Ka
9.5.1.1
=
abutment contraction coefficient
II(,
head over erest,
W
total width of all piers.
Recommended values of KP are as follows:
a) For square nose piers with corners roun?ed with a radius equal to about 0.1 of the pier thickness:
KP
*
~
0.02
rR
-
O.lt
I
t
b) For rounded nose piers: KP
~
O.Ql
~-R
0.5t
j t
c) For pointed nose piers: Kr ~ 0.00
\ ):60°
9.5.1.2 Recommended values of Ka are as under: a) In square abutments with head walls at goo to the direction of flow:-
Ka
~
0.2
b) For rounded abutments with head walls at goo to the direction of flow: for 0.5 H 0 .> r > 0.15 H,,
K0
~
0.1
9.5.2 Width and shape of sill - Width of sill shall be kept according to the requirements of the gates, trash racks and stop logs subject to a minimum of 2/3 H, The edges of sill shall be rounded off with a radius equal to H 8 • The upstream face shall generally be kept vertical and the downstream sloped at 2:1 or flatter.
14g
9.5.3 Shape of Approaches and Other Component Parts
The shape of approaches and other component parts may prcf(~rabiy he fixed by means of model studies. However, for works of medium size, the eritcria given as under may be adopt.cd.
9.5.3.1
Upstream Transition
Joining the vertical abutment of slui(~eway with upstream verLical abutment of intake is made hy providing a circular curve of radius equal to three times the depth of flow over sill excludei roof or over intake sill, if there is no silt excluder. This is done for smooth entry of \Vater. The other upstream vertical abutment. of intake is joinC'd to vertical wing wall in similar manner. The transition from vertical \ving- wall to guide bank slope is done by means of warped retaining wall or broken back transition.
9.5.3.2
Downstream transitions
Wing walls shall normally lw kept vertical up to the end of t.lw Impervious floor beyond which they shall be flared from vertical to the actual .slope of the canal section. However, in order to obtain greater economy the wing walls may be kept. ycrtieal up to the t.cc of glacis beyond which they may be flared g-radually to l/2H: l Vat the end of impervious floor. In the remaining length, wing walls may be flared from 1/z:l to the actu~d shape of the canal section. The wing wall section shall be designed as retaining wall only up to the \Vater side sloPe of 1h:l beyond which it shall he assumed resting on compacted backfill and designed accordingly. Vertical joints should be provided al the end of impervious floor and on the self-supporting scdion.
9.5.4
Design Criteria from Surface
Flow cons£deraiion - In the case of intake work on permeable foundations, the following factors need to be determined. (In case of downstream non-erodible beds protective measures may not he necessary).
9.5.4.1
Depth of Upstream Cut·Off
On the upstream .side of the intake .work, cut·off shall be provided and taken to the same depth as the cutoff upstream of diversion work. 9.5.4.2
Stilling Basin Dimension and Appurtenances
Thes0 shall be provided on the same principles as adopted in case of sluiceway.
9.5.4.3 Thickness of Floor on Sloping Glacis with Reference to Hydraulic Jump - The hydraulic JUmp profile shall be plotted under different conditions of flow. Average heig-hl of the jump trough shall th~n b~ obtamed by deducting the levels of the jump profile from corresponding hydraulic gradient line. Th_Is will be taken as the unbalanced head for which safety of glacis rloor shall be ensured. As a rough g-erde the unbalanced head may be assumed to be 'h ld 2 - d 11. where d 1 and d 2 arc conjugate depths at the bcg1nmng and end of the hydraulic jump.
150
9.5.4.4
Downstream protection
At the end of concrete floor on the downstream an inverted filter 1.5 to 2 D long (D being the depth of scour below bed), consisting of 600 to 900 mm deep concrete blocks with open joints laid over 500 to 800 mm graded filter should be provided. Downstream of the inverted filter, loose apron 1.5 D long of boulders of stones in wire boulder crates shall be provided so as to ensure a minimum thickness of lm launched position. The individual stone size should be heavy enough to resist displacement by the highest calculated velocity at the end of the floor. Where the working head and velocity are too small, small size blocks or required stone size can be considered as rip rap. Upstream of the impervious floor, blocks and loose apron shall be provided which should be similar to that provided upstream of the dam.
9.5.5
Design Criteria from Sub·surface
Flow Considerations - The factors enumerated below should be considered. 9.5.5.1
Exit Gradient at the end of Impervious Floor
Value of exit gradient should be determined as per the procedure formulae given m Chapter 5. The factors of safety for exit gradient for different types of soils shall be as follows: Shingle
4 to 5
Coarse sand
5 to 6
Fine sand
6 to 7
9.5.5.2 Total Length of Impervious Floor As the length of downstream floor and downstream cut off are inter-related, total floor can be decreased by increasing the depth of downstream cut-off and vice versa but increase in depth of cut off would result in the concentration of uplift pressure, specially in the lower half of the floor. A balance between the two should be worked out on the basis of economic studies. Total floor length required shall be the sum of: a) horizontal floor in the downstream from the surface flow considerations; b) length required to accommodate sloping glacis and crest; c) about 3m upstream of the crest or length required from other considerations. Depth of downstream cut-off should be worked out for this floor length lo ensure safe exit gradient. If depth of downstream cut·off so calculated is excessive, it can be reduced by increasing upstream floor length. As a rough guide depth of downstream cut-off should nol be less than (cl/2 + 0.5), where d is the water depth corresponding to full supply discharge.
9.5.5.3
Floor Pressure
Uplift pressures at key points on the floor should be determined as per procedure discussed in
151
-----~~-~·~................ ~------
Chapter :), coiTl'sponding- t.o the condition t.h,1t there: is hi.Rh flood level in the river upstream of head regulator and no water in tlw canal downstream of the intake \Vork. Upstream of sill, only nornimal floor thickness of about l m. should lw provided.
Structural design
9.5.6
The componl'nl.s of the intake work being· similar to those of sluiceway their structural design shall lw done in Lhe s;1nw manner and as for sluiceway.
9.5.7
Hoist Bridge or Operating Platform
/\ bridge or working platform shall be provided for operation o[ g::J.Les and stop logs. of \Vorking platform should dqwnd on the travel of gates.
9.5.8
The length
Free Bourd
J\cl(:quatc· free bo:trd ;1s pPr n·quirPnlent disrussed in Chapter 7 should be provided while fixing height of abutments. which on the upsLn~am sick should be at the same level as for diversion dam.
A typical open type Intake \York is sho\vn in Ii'igurc 9.2A & 9.ZB.
9.6
BARREL TYPE STRUCTURE
9.6.1
General In this type of work. the hydr<1ulics usually involves the following conditions of flow i) Part full conduit (free Dow) iii Full conduit (pressure flow)
Analysis of parl.ial!y flo\ving flow is based on the principle of conversion of energy and that for the ful! condud. flm\· involve'S sl.udy of hydraulic losses to determine the total head needed to produce the requirpd disl·harges. Tlw siJ.(' of tlw water way would thus be fixed from the following considerations. a) Total head loss should remain within !.he permissible limits. b) VPlocit.y should lw mon· than flushing velocity and less than eroding velocity. c\ ?vlinimun1 dinH'nsinn of opening should In~ such as t.o p('rmit. clearance of debris inside the barre~ manually.
9.6.2
Evaluation of Losses
9.6.2.1
Transition losses
Loss at. inl<'L :1nd ouLll't due- to ch:tngP in flow section ·-
.,
This should hl' ctlcu!al.t'd :1s K (
v,)~
tg·
152
•
where
v
~
the higher of the velocities of flow before and after transition
=
lOwer of the velocities of flow
1
v
2
g ""' acceleration due to gravity K ~ a coefficient, its value depending on different types of transition, given in the table 9,1,
Table 9.1 Types of transition
Coefficient at inlet
Coefficient at outlet
0.1
0.2
0.2
0.3
0.3
0.4
0.3
0.5
oA
0.7
0.1
0.2
--~-----·---··--·--·---------
a) Open transition to closed conduit i) stream line warped to rectangular opening ii) straight warped to rectangular opening iii) straight warped with bottom corner fillets to pipe opening iv) broken back to rectangular opening v) broken back to pipe opening b) Closed transition square or rectangular to round (maximum angle with center line ~
7'h 0 )
If the submergence at outlet exceeds one sixth of the depth of opening, the head loss coefficient would be 1.0 at the outlet. 9.6.2.2
Friction Losses
a) Friction loss in Barrel (conduit) ·'_-~ ·~~
Friction loss should be calculated by Manning's Formula:y2 L n2 R 4/3 where:
V = average velocity in barrel in m/s
length of barre I
L n R
=
coefficient of rugosity of barrel surface hydraulic radius of barrel section.
Value of n for concrete surface may be selected from the table 9.2.
155
Table 9.2 Concrete with rough joints ................... .
0.016 to 0.017
Concrete with dry mix, rough' forms .. _........ .
0.015 to 0.016
Concrete with wet mix, steel forms ........... .
0.012 to 0.014
Concrete, very smooth ....................... .
0.011 to 0.012
For design calculations, n may be taken as 0.015 unless otherwise specified. b) Loss in pipe
L y2 f ·--!) 2g f
where
9.6.2.3
Darcy's friction factor - its relationship with Manning's n, is given in Fig. 9.2 C
=
D
Diameter of pipe in metres
V
Velocity in m/scc.
L
Length in metres
Bend Losses
Coefficient of loss at bend in circular section may be adopted from figure 9A·1 (Head Loss in pipe bends) • Head loss at bend in rectangular section may be calculated from the following formula:?
hb ~ F I :!:.. 2g where
G ) 180
hb = head loss due to bend in metre F
0.124 + 3.104 I Z~
S
width of the barrel in metre
)1!2
R
=
radius of bend along centre line of box in metre
G
~
angle of bend
g
9.6.2.4
X
acceleration of deviation in.dcgrces
Loss due to change in direction of flow while entering from approach channel toward the intake:
This can be calculated by the formula given in para 9.6.2.3 above.
156
'~----···->''·•' '''"'/!": •,__:,_-,,_.
;;x;~!_,~:'!-~\5:""-;-;.,~;'.'J>.':;;.;:,<.o'""<-
_;c:,i;;:~:i;&;{;:,;0":f)~;;:lfiiliL::-,-;;:--" ' "-'""' ;~:;,,,-. "·'"''' ,·,..... ~ .... Y·'.
,,,,__._,.
' , •. ,
· ,.,,<~;.,:,;;;
'
•>'.->
'-H"'~.'uo>•~·"'·~-"•'
---------------------"'·----------------~-------------"""""11
OPERATING SHED AND PLATFORM
"
EL 123.40
I:r==1::J 1=1"'" EL 12o,wj__
--
'
::
::
~G-1. PIPE 'RAILINGS
~EL.I22.00
EL .121.00
-,
[
~c-
>OO,HCOO~ FFLUX ;L,
(6 .S;t
M. THK. RICH CRETE
&
I I ..
I DRAIN
PIPE
GRADED
FILiER
.
~
.. -·
l
I 3oo
'~'
3oo
LEAN CONC
t:
350_W_o_
'
I
"·-4-4.30X 1,55 INTAKE STEEL GATE
8
I
2020
I I
410
1.50X 1.50X .90CONC.BLJ)CKS
GOKGS.BOULDER RIP RAP' 90 CM. THICK
I 21so
PART-IAL SE:CTION OF SLUICEWAY SHOWING FACE OF INLET INTAKE
LEL.121.00
!!Il==
J.
'I
r--"'
•
.
•,:._-
•
....
•
• -
.
'j
j
"'
y
REINFORCEMENT ARE NOT SHOWN, SECTION ARE OF CONCRETE OUTLINE ONLY.
/-
,,
'
·,
·'
.. "-·:·
NOTE:
..
-~
.'
~
'.
'-·
....
,..:--
----
.....___
"j:
j
y
·>
_,
0 Q
...
I
0 Q
lJ
lJ_
jLEL.I12.45
430
•.. P,
.. ·.
,
·.l
.'
*' .• )
't>
~-
.-v.
•\
I·
••
I 1100
430
I
SECTION
430
w•~N-'""'---·---
1ool l
• ••
• I>
•••
'
~
\
'
MAG·ASAWANG TUBIG RIVER IRRIG. PROJ.
lGJ
430
Is~\
PROVINCE OF ORIENTAL 114 I ND 0 R 0
-,
_________________
1-1
-----------------·-----,.,, 0
-~ ~'
I
LoJ 11
'' .
,.
·a
' t'
INTAKE WORKS
11
FIG,9.28
SECTIONS
SHEET
®
_,.,,_,,,,.,..•
~.----
. ---·..
···~··~
~~···""'"'''·" ""~"' .,"·'""""' ...~'=· ·-'·~''"·"-· ,0.,,.
..
,:.. ;.;;;;.i;;;.:;;~;:,;,;t.!£fi0Pri£''o::': ;·: .;,-;;,,;::.A-;;:._, ::;.).:.;.•::;~C·-·:c.:;~;; ,•:;,\'.:l
OPERATING SHED AND PLATFORM
'
EL. 123.40
~G.I. PIPE RAILINGS '122.00 EL 12o.wj_,
:;
::
[
J
tEL.I16.00
CREST
30CM. THK. RICH CONCRETE
:-
CONC+E I :4:8 DRAIN
PI f'E--------"
GRA OED
I
300
I
J
300
FILTER
350
-+-------
410
~
PART-IAL SE:CTIO N OF
LEL.121.00
....
''
.- .
,i
~:
.I ;;
'"'
,.'
'. ' ..,:.,. I
0
Q
• )
.,
''
l EL.I16.00
Ii
II_~
I· ...
:.
0
Ul"C" " ---~6J
Ij
. '.<.·.·"
\.,: -~· '-:,.' ·, .. ·., ·,' :~. '
·',.'
~
Q
y
',''
·-
430
430100
SECTION
"'·-·-----------
'
·..
..
~- ~·--·-·-····
'' '
~
. ,.,
\IOC
II
--~·'·
··i
;
.. "
_______,,,_'""""""'"""""''"'""".
-~-~~~------------·
''"'"'"'"'·"~~-;.;.=~~">if{,;.h,
~
/
OPERATING
SHED
a
PLATFORM
~ Il[l[ljljljljllJi,IJ Jl [ 1
1 1 1 [ [
THICK CONC. WEARING SURFACE DRAIN PIPE
EL.II2.00
..
I
1111
"(;! ''
IIIII•
DRAIN PIPE
1,
'"~' ·~· " '"'"
STO~E SPAWLS WITH 75 GAP FILLED W/GRAVEL TAIL WATER
E L .112.33 .
GRADED F I~ TER
"-- --t-;--BLQCK PROTECT[ Mo...---'
I
!''
1' 1'1 1 1' I' 11 111' 1'1 ' 1'1 ' '" r , ,·, ,." ,
I I
840 CONCRETE CLrr -OFF WALL OR SHEET PILE
OPEN - TYPE
INTAKE
MAG-ASAWANG TUBIG RIVER IRRIGATION PROJECT
WORKS
PROVINCE OF ORIENTAL
INTAKE
MINDORO
WORKS SHEET
SECTION
FIG.9.2A
OF !!
~;.
~;.."'=-".
~£~ 1
=~~·~
n;wrv""=-
"'"
~=,...,.,.._,.,,~o
'_,_., ·-e
'>''f",~~'~"-'"·'~'" ~·?''" <'o''C',',;>":_"'"'""'""{•,Wfo=,.,~~,.,.~,,~"""'"'·"""""~~~.'0'-"-"'~"'''""' --~~---~.~-,.~-~-"'-"'o'<'N~-·
··>~-~~~1
--·--·----· '
-.J';;;e~'"W''t'
;,.~==""""-~"""'U~'<~-=~''":I".!:l"'""•="~'·''"''''·'·''"'
.
:·' .
'
I
I I
I I I I OPERATING
I
SHED
a
PLATFORM
I I I
I
'---
BLOCK PROTECT! 0
I I
I OPEN -
I
TYPE
INTAKE
i
I ~-!
1i'
..~ ""'""''""'~,,...,,,"~"'~-
,,,..,, .,
-~It?
'10(!::1-~~M~~""~".l..'l"
It$'
"f
1H
~~~"""""'~ '<
-·-
'"'-"·'''~-----~
9.6.3 Contraction and expansion
To minimize head losses and to avoid cavitation tendencies along the conduit surface, contraction and expansion on transition to and from gate control sections in the pressure conduit should be gradual. For contractions, the maximum convergent an?le should not exceed that given below:tan
oC
1
= u
angle of the conduit wall surfaces with respect to its center line
where u
v&D
arbitrary parameter
fgD
the average velocities and the diameters at the beginning and end of the transition.
Expansion should be more gradual than contraction because of the danger of cavitation, where sharp change in the side wall occur. Normally tan o<: ~ 1/2 u. o<: should not exceed 10°.
9.6.4 Calculation of waterway
In order to minimize deposition of debris within the barrel, it is necessary to provide a m1mmum velocity of 1.5 m./sec. Size of the barrel should be approximately determined on the basis of required discharge and assumed velocity and then tested for head loss. If the calculated head loss is more than permissible, the size and velocity should be adjusted. The size of the gate opening may be approximately determined by using the submerged orifice flow as follows and then tested for head losses. Q ~ 0.75 A J2g H
where
H
available head in difference between the upstream and downstream water level
A
Area of opening, Q
~
Discharge
9.7 ENERGY DISSIPATION
To avoid undesirable disturbance from the jump and to maintain free flow conditions, ample free board should be provided in the closed conduit from the control gate to the outlet. Where sufficient head is available it would be desirable to have the water flow at super critical velocity through the conduit and to have the stilling pool located beyond the outlet to avoid objectionable vibration of the structure under the embankment. An example illustrating barrel type intake work is shown in Fig. 9.3 A & 9.3 b. 9.8 TRASHRACKS
Trashracks arc desirable at the entrance to some head works structures and arc essential when desilting works are involved or where a lot of detritus/debris is coming in the river. Racks usually arc constructed of flat steel bars which are set on edge and arc joined by bolts or welded to the edges of the cross bars. The welded type provides more space teeth to pass between the bars. The rakes can be made in panels for ease in handling. For the facility of clearing, these arc generally inclined at a slope of 1(H) to 4(V). Velocity of flow through the gross area should not exceed about 1 m/sec. Bar spacing is generally 25 mm to 100 mm for fine & 10 em to 30 em. for coarse racks. · On small Intake works, the racks are usually raked by hand but on larger works, mechanical rakes arc generally provided.
159
-'=·
NOMOGRAPH FOR DARCY's " 0.055
$
0.10
0.050
0.09
0.045
llfll
f "
----"nj' }(
-------"
D OJ1 11
•y
7
,,
30.0 It
20.0
0.08
·.'.·lk
0.040 0.07 0.035
0.06
10.0 9.0 -8.0 . 7.0
0.030
0.05
. 6.0
0.04 20
.. 0.03
0.02
= c = en
-
= = _en >u a:
Cl)
a: u
<(
...J
::E
:::::J <(
0.010 DARCY'S EQUATION FOR FRICTION LOSS IN CIRCULAR PIPES L V2 ht • t- D 2g
0.01
h ,I~ .h. f D 3 D 0.005
0.003
0.004
=Cl
= a:
3
0.5
LJJ
0.4
<(
Cl
y2 2g
RELATIONSHIP BETWEEN 11 11 MANNING'S n AND DARCyt$ II f II
!•~ • D I
0.2
116.5n2
1/3 0.4-
FLOW IN CIRCULAR CONDUITS REFERENCE: SMALL
1.0 a: 0.9 Cl 0.8 >0.7 :I: 0.6
LJJ
1-
::E
MANNING'S EQUATION FOR FRICTION LOSS IN PIPES
DAMS, P. 451
FIG.9.2C
•
<(
2.0
4
OF
Cl
10
z z z
DESIGN
4.0 3.0 :::::J
(!)
<(
._!£1
15 -
-
Cl
;~ •
-0.020
5.0
0. I
Appendix- 1
Loss of Heads in a Structure
Loss of Head in a structure may consist of the following:a) Loss at inlet transition and outlet transition. b) Loss inside the structure due to skin friction. c) Loss at bends due to change in direction of flow. d) Loss due to obstruction, e.g. in trash racck. Method of estimating the different types of losses are indicated in the following paragraphs: 1. Loss at inlet transition and outlet transitionv 2
This should be calculated ask ( -2~
-
V?
2
)
2g
velocity of flow before transition
where v2
velocity of flow after transition
g
acceleration due to gravity
k
a coefficient value of which is given in table 9.1 for different types of transition.
2. Loss inside the structure due to friction-
Friction loss shall be calculated by Manning's formula:-
where
v
=
average velocity in the structure in m/s
L
length of the structure in m
n
coefficient of rugosity of barrel surface
R
hydraulic radius of barrel section
Value of n for concrete surface may be selected from the table 9.2:-
For calculation of loss inside structure n may be taken as 0.013 unless otherwise specified. For determining depth of [low in a concrete lined channel, a value of n of about 0.018 should be assumed in order to account for air swell, wave action etc. For determining specific energies of flow needed for designing the dissipating device, a value of
n of about 0.008 should be assumed.
161
3. Loss at Bend Coefficient of Joss of head at bend in a circular conduit may be adopted from Figure 9A-1. Head loss at bend in rectangular section may be calculated from the following formula ~ y2 GF'•··-·2g 180 hh
head loss due to bend in meter
F
0.121 + 3.101 ( z~ )'I'
S
~
width of barrel in meter
H
radius of bend along central line of barrel in meter
g
acceleration due to gravity in m/sec.2
0 ~ angle of deviation in degrees
4. Loss in trashrack
Head loss in trashrack may be calculated from the formula:H ~ 1.32 ( where:
i;' )2 (sin A) (sec 1518 B)
in~hes
H
Head loss in trashrack in
T
thickness of trashrack in inches
V
velocity below trashrack in It/sec.
A
angle of inclination of the rack with horizontal
B
angle of approach
D
center to center spacing of trashrack bar in inches.
162 ._-_a
~
n 'L,~
.26
I I
Loh cb•tilcH•nt!. f•r dioe
'1
en~s
.24
'I,
Re
4-*-
.22 Y
. 20
,..,:X: )>
0
5 (/) (/)
·<
m (/)
,.., :X:
)>
0
.181~
r
.16
0
"U "U
.14
,z
.
.08
"
.06
0
(/)
tD
I
1 I I± I
1
•
I I+-+ i
i
':l 4
I
•
I
i
I'
1
I 1
I II II.J-..+ ~~ J.--1-:tJ I I
1
1
k'l ' I, /i. ._ . :
I
I
= . .+
'
it
,
1
I~
~~-~
.04
!
J~
L.J:j
(', 0
&(0
10'
15'
20'
I'
•.
· _I
i.
25'
u 30'
!
I
r±' t=ti'j!t ----r= -- - · · I
I
1 -
I
',
_
'I
y
"
-
: I I L ~~~ ' !,--r·r· ··;i ~. . I i I I I ~
I
-.;:;-~ I
I
c __
.,
I
!
I I i I l I I I I ITq_: -~,Q-~~m=ffftft r-r-t
...02
I
1
1
!Y/1 1 I I I ; I / IY i i i I I "1'- .I J I I I I l V ,---:-}" I I 1_...-Yi
.10
.0
opg11 . 1' ~··r. brndr i I I I
iSing~
l_ __ _
.12
z
...;
i /j ,
-E-
I
,..,
()
i
Y
~
:,
I
,..,..,0 .,.,
!'
[,'I
I II I I jI'-~f{',I I II
(/)
Ii
lin
(/)
,.., Ol
I I 1/
i
I.
F•~nol~s ruribe
I
()
2
J---t- I
I
s+>oolh ihte~i or
df
._
_ _' :_:::-_-
-j--
• -
75°
80'
+-
I I 35'
40'
45'
DEFLECTION
50'
55'
60'
65'
70°
8 5°
ANGLE
FIG
~i
:t1
90°
9A- I
CHAPTER 10
SILT EXCLUDER
10.1
GENERAL
River flowing in erodible v3.llcys C
10.2
LOCATION AND ALIGNMENT
The excluder tunnels are required to be placed in front of the intake structure and are aligned parallel to the intake. Change in the alignment if found necessary, should be in a smooth curve of radius 10 to 15 times the width of the individual tunnel. River approach also plays an important part and it should be kept straight to the mouth of the tunnel as far as possible.
10.3
DESIGN CRITERIA
Silt excluder comprises a number of rectangular tunnels running parallel to the axis of intake work and terminating close to the undersluices. The principle being to pass the slow moving bed water, con· taining heavy charge of the coarse grades of material through the tunnels whose tops act as platforms at canal sill level. These tunnels lead to pocket gates in the dam/weir which are so regulated that: i) the tunnel does not get clogged with sand ii) the bed water and top water are divided without causing eddies, which would throw the coarse grades into suspension.
10.3.1
Number and Size of Tunnels
The number of tunnels is determined by the available discharge for escapage. Number of tunnels is also fixed from following considerations:a) With the discharge available for flushing, the velocity through the tunnels shall exceed the self clearing velocity.
164
EL.221.34
I.
ExlstlnQ floor
EL. 217.68
PLAN OF TUNNt:.LS
H.f.L. EL. 224 62
EXISTING POND LEVEL EL. 223.17
R.C.SLAB
13M
2.13M
TOP
2.13 M.
EL. 221.34
2.l3M
2.13M
EL. 2
CROSS
SECTION OF TUNNELS
SILT
EXCLUDER
PLAN S. SECTION
FIG. 10.1
b) Size of tunnel should be such as to permit manual cleaning.
d Height of tunnel ~hould be more than the height of silt deposit in order to avoid choking. The number of tunnels usually provided arc 4 to 6. Size of tunnels depends upon the number of tunnels, self-clearing velocity of flow required to be provided which for the alluvial rivers be 3 m./sec. and 4.0 to 1.5 m.{scc. for the~ boulder stage river and the discharge available for escapage. Generally, 1.8 x 2.1 m. size excluder tunnels are adopted for river in alluvial stage and 1.8 x 2.4 m. size tunnels for river in boulder stage.
10.3.2
Spacing and Shape
The tunnc~l nearest to the head regulator has to be of the same length as that of regulator. The con· sccutive tunnels should he spaced at distances such that the mouth of the one nearer to the head regulator comes within the suction zone of the succeeding tunnel so that no dead zone is left between the two to permit sediment. to deposit. Generally, a distance of about 12 m ..may be adequate for head regulators of longer depth. The tunnels shall be suitably bell mouthed at the inlet to mm1m1ze entry losses and improve suction. Bell mouthing shall be done .within the thickness of divide wall and may be done on any suitable elliptical curve. 10.3.3
Roof and Bed of Tunnels
The roof slab of the tunnel should be kept flush with sill of the canal regulators and the bed kept at the upstream floor level of sluiceway. It is also helpful to draw graph of velocity distribution and the corresponding integrated graph of discharge derived therefrom. 10.3.4
Exit and Exit Channel
All the tunnels outfall into the stilling basin through one or two undcrsluice bays adjacent to the intake work. The tunnels should be suitable, throttled laterally or vertically or bothway so as to produce accelerating velocities in the tunnels; maximum being at the exit end so that sediment material once extracted docs not deposit anywhere in the tunnels. The exit channel to take the silt bearing water into the river should be properly maintained downstream of the excluder. A separate outfall channel is generally not required when the bed material is sand. If, however, boulder and gravels arc to be excluded and the main channel is away, a lined channel may be necessary, or a channel may have to be created with suitable training measures.
10.3.5 Transitions
Transitions to piers, in bell mouthing at top or sides, should preferably be elliptical, the major axis being in the direction of flow and two to three times the minor axis.
10.4
ESCAPAGE DISCHARGE AND MINIMUM WORKING HEAD
Eseapage discharge (flushing discharge) of 15 to 20 per cent of the canal discharge is generally reqmred. A m1mmum of 0.5 to 0.75 m. of working head is required for sediment excluders on sandy rivers and minimum of 1.0 to 1.25 m. is required for excluders on shingle or boulder beds.
1
l
166
l!lU
10.5 LOSSES IN TUNNELS
These shall comprise of friction losses and losses at the bends and transitions and shall be computed
by the following formula: a) Friction loss
where hr
head loss in m.
=
V
velocity in m/s
L
length of tunnel in m
N
rugosity coefficient depending on type of surface
R
hydraulic mean depth.
b) Loss due to bend
where
Hn
loss due to bend
F
0.124 + 3.134 (S/2r) 'lz
g
acceleration due to gravity
0
angle of deviation in degrees
S
width of tunnel in m
r ~ radius of bend along centre line of tunnel in m. c) Transitional loss due to change of velocity in contraction
vzo
v2
1 ) h" ~ 0.1 (-"- - - 2g 2g
where he
=
transitional loss due to change of velocity in contraction.
V and V 2 ~ the velocities before and after the transition. 1
I
g ~ acceleration due to gravity. 10.6 STRUCTURAL DESIGN
i) The tunnels should be stable against buoyancy and uplift ii) Roof of the. tunnel should be designed for water load and silt load. iii) Floor should be designed against uplift and foundation reaction.
167
!1:((;; .·; ; ·;
3
2&
2
iv) Walls should be designed against hydrostatic pressure assuming adjacent tunnel to be empty. Hydrodynamic effect and inertia forces to be taken into consideration.
j I
10.7 CONTROL STRUCTURES & MISCELLANEOUS FACTORS
1i
10.7.1
l
The excluder tunnels are operated by the undersluice gates. These should be regulated either for the tunnels to run full bore or to remain completely closed.
]j j
Il l
Control Structure
10.7.2 Miscellaneous Factors The following draw backs should be kept in veiw while designing a silt excluder.
l
i) In case the excluder is unfavorably located, tunnels do not flush the incoming debris and may get partly choked resulting in a large,percentage of bed material to be drawn by the canals.
l
ii) Higher discharge in the pocket brings in relatively more sand and thus counteract favorable curvature. In view of the above factors, it is recommended that before finalizing a theoretical design of work of this nature, hydraulic model tests be carried out to check the performance of the proposed design. 10.8 EFFICIENCY OF SILT EXCLUDER Efficiency of silt excluder is given by E
so- s,
~---..::.__
where
S0
total silt content that would enter the canal if there is no silt excluder provided.
S,
silt actually entering the canal
In practice efforts are made to exclude everything larger than 0.2 mm in diameter and efficiency is worked out on material of coarse grade. The same excluder may be expected to work effectively, where the proportion of coarse silt is greater. On the other hand, the coarser the heaviest grade of silt carried, the greater will be the slope and velocity and consequently less concentration of silt in the lower layers.
10.9
I
FIELD STUDY BASED ON MODEL EXPERIMENTS
A silt excluder along the barrage at Narora in U.P. Province, India, across river Ganga, was con structed in 1967. A brief description of the work along with model studies carried out at the Research institute and comparison of the results with the prototype performance of the excluder has been given in Appendix A. The study of field performance of the silt excluder highlights the fact that the excluder functions well and that the qualitative studies on hydraulic model do bear a relationship with the quantitative results expected on prototype.
168
Appendix A
ILLUSTRATION OF NARORA SILT-EXCLUDER
A-1
Introduction
A 924 m long barrage designed for a flood discharge of 14.150 m3/sec was constructed across River Ganga at Narora in U.P. State in India. A six tunnel excluder was also constructed to reduce the sediment entry into the offtaking canal from its right bank and sited just upstream of the barrage at an angle of 107° (Figure A-ll. The excluder covers extreme right under sluice bay of 15.3 m width infront of the head regulator. The mouth of the tunnel has been staggered by a distance of 10.8 m so as to cover the entire width of the head regulator. The size of the tunnels has been kept such that loss through each tunnel is the same. Top of the crest of head regulator is at R.L. 176.36 and the floor of the pocket is at R.L. 174.63. Thus, a clear depth of 1.43 m is available for excluder tunnels allowing 0.3 m as thickness for top scale. The water depth which is available for most of the time above this floor when pond level is maintained at R.L. 178.96, is 4.33 m. The excluder thus covers 1/3 water depth of heavy sediment concentration. Other salient features of the barrage are given in Table A - 1. TableA-1 1. Total length of Barrage
924.2 m
2. Spans for weir portion 54 No. of 12.2 m
658.4 m
3. Spans for sluiceway 7 No. of 15.2 m
106.7 m
4. 52 piers of 2.44 m for weir portion
126.8 m
5. 6 piers of 3.03 m for undersluices
18.2 m
6. 2 cross wall of 2.44 each
6.9 m
7. 1 divide wall between under sluice and weir bays
3.0 m
8. Fish ladder
6.2 m
9. Designed flood discharge
141.50 m3/sec.
10. Barrage upstream floor level R.L.
175.6 m
11. Barrage bay crest top R.L.
176.2 m
12. Under sluice upstream floor R.L.
174.6 m
Model studies were carried out at the Irrigation Research Station. Sand of size (Dsol 0.23 mm was injected into the model manually upstream of the dam axis. The efficiency of the excluder was determined by measuring the quantity of silt entering into the canal without excluder and with excluder with the following formula:Silt Collected without excluder-Silt Efficiency of Excluder ~ _ _ __:C_:o_ll_:e..:.c_te_:d_:_w_i_th__::E:.:x:.:c..:.lu:_d:_e:_r_:__ _ 0.01 x (Silt collected without excluder)
169
iliii'?¥
NARORA
SILT
12
0
6 !m
SCALE
SCALE
#'¥¥¥&
m
FOR PLAN
FLOW
I R=
I
DAM AXlS (BARR AGE)
R=12.42
10.82
I R = 9.22
FOR SECTION
i@!"
EXCLUDER
H0.668-H0.668----H0.6 68 -+-10. 66&-+ 10.6 68--1 0
fi%4
A 6
R =aDO!
~~
,~-----.\=:==2~3~d~~~ AXIS OF HEAD REGULATOR
O.S.FLOOR
R. L 175.25
' - - - - - - - - A PRO._.N_,__ _ _ _ _ _ _ _ _ __L__
P L
"'&PIER
•
. . SECTION
A N
• • __
;~:i;:::;ili
THROUGH TUNNEL NO. I
·~· ' 1'1Fill'~j;~~ll '~' ~,,:;;: SECTION
- '·~
ON
f
~'.'~;~~;t"' '""
A- A FIG.
A- I
~-
A-2
Efficiency of Excluder
As the effieierwy of a silt excluder is the percentage reduction in the quantity of sediment wh~ch would enter the canal, if there is no such structure, for determining efficiency the silt concentratiOn shall need to he observed as under:~ i)
in the channel upstream of the excluding tunnels
ii) in the canal downstream of the regulator; or i)
in the excluder out fa\\ or ::tt the exit of excluder tunnels
ii) in the downstream of the regulator
In the first case, the pcrr.entage efficiency would be:Percentage Efficiency
=
Ir - I,
x 1 00
(1)
I, where
Ir -= silt intensity in parts per thousand of water by volume Ic -= silt intensity in parts per thousand of water by volume in the canal downstream of canal regulator
In the second case, the silt intensity in the river pocket can be calculated from the formula:(2)
where Ie
Silt intensity passing through the excluder
Qr,
Discharge through the excluder
QC
Discharge in canal
From the equation (1) above, the efficiency of silt excluder can be determined. The function of the excluder being in removal of the bed sediment, its efficiency can also be directly evaluated with the formula.
Percentage efficiency
where
X
br
intensity of bed load in the river
be
bed load in the canal
100
(3)
A·3 Discussion on test results & Conclusions The efficiency of the excluder on the basis of formulae (1) & (2) which are based on total load carried by the river is not possible to be determined in view of taking observations in turbulent flow and thus Equation (3) based on bed load was used for working out the efficiency. The efficiency so worked out
•
171
I
I
I j
collected from field data, varies from 51 percent to 87 percent. The variation of efficiency percentage, discharge and bed load of the river have been plotted against time in Fig. A-2. It can be seen that the general average value of exclusion efficiency remains above 60 percent for most of the discharges. The maximum value of efficiency of the bed load exclusion works out to 87 percent against a corresponding figure of 91.6 percent (as per equation 3) obtained on model. This is an indication of the fact that results of model studies of silt exclusion are fairly representative of Prototype performance, and thus high light the fact that the qualitative studies on hydraulic model do bear- a relationship with the quantitative result~ expected on prototype.
j l
l
NARORA
J
SILT
A FIELD STUDY OF
EXCLUDER ITS
EFFICIENCY
II '
I'
...
l
l j' '
>
100
~
l
.
l
!
j
l1
J j
.."" ..'• • • ".' " ."
<.>
:I<JOO
.,""
...
60
1000
.. c;
40
'!:
c
"'<.>
.;
0
0
.
-' 0
20 0
r;; ""-' <.>
..,."".
..u,. ....... "' ".. r
DISCHARQE
<.>
8£0
LOAD
10
0
_j
I 1•
40
20
EI""I""ICI £HCY
0 $~7·71
14-T-71
19-7-71
24-7-71
Z9-7-71
TIME
VARIATION OF EFF!CIENC!ES 7BED LOAD AND DiSCHARGE WITH TIME
FlG.A-2
•_,_®
il
172
CHAPTER 11
RIVER TRAINING WORKS
11.1
GENERAL
Rivers in the alluvial planes generally flow over a flat country and meander widely within their permanent river banks. River training works are provided guide the river to flow exially through the dam/weir and to check out.flanking. Besides, these are reqmred to:
:o
minimize possible cross flows through the structur·e which may endanger the structure and protection works. ii) prevent flooding of the riverine lands upstream of the work due to the afflux caused by the construction of the obstruction (dam) in the rivBr. iii) provide favourable curvature of flow at the intake from the consideration of sediment entry into the canal. i)
Though the planning and design of training works is done largely by empirical methods, reliance has to be placed on the intuition and judgment of experienced engineers. Model experiments are extremely useful and valuable in the layout of these works.
11.2 TYPES OF TRAINING WORKS
River training works generally provided at diversion dams may be classified as below: a) Guide banks embankments b) Approach embankments c) Afflux embankments
,11.3 GUIDEBANKS
In a flat country in view of wide alluvial belt through which the river flows, it is necessary to narrow down and restrict its course and help the river flowing centrally through the dam constructed across it. Guide banks, as the name implies, are artificial embankments meant for guiding the river flow past the dam without causing any damage to the structure and other appurtenant works. The guidebanks usually consist of two heavily built embankments in the river in the form of bell mouth. The portion of the river between the normal river banks and the guidebanks is closed by simple embankments. Sometimes, if the river has high scour resistant marginal lines, it becomes economical to dispense with one of the guidebanks. Guidebanks are constructed in pairs generally symmetrical in flow. These are constructed both•upstream and downstream in the direction of flow on one flank or both flanks according to the requirements at site. Proper alignment of guidebanks induces favorable curvature of flow, which in turn helps in minimizing sediment entry into the canal system. Guidebanks should be
173
'""''
so designed that even if the river swings, a safe marginal distance is available between its extreme swing an9 approach embankments.
Classification
11.3.1
Guidebanks can further be classified: i) according to their form in plan and ii) according to their geometrical shape. The latter are named for the shape of their curved heads such as straight, elliptical guide banks with circular or multi radii curve heads. These are however applicable to wide flood plains and longer diversion works. For low dams, the former are generally adopted. 11.3.1.1
Form in Plan
Guide banks can either be divergent, convergent or parallel (Fig. 11.1) Divergent guidebanks exercise attracting influence on the flow. With such guidebanks, the approach embankment gets relatively less protection in worst possible embayment as compared to the equal bank length of parallel guidebanks. Divergent guidebanks would thus require a longer length compared to parallel guidebanks for the same degree of protection. Besides, divergent guidebanks induce oblique flow on to the diversion dam and give rise to tendency of shoal foundation because of larger waterway between curved heads . '~
.• ' M_.
.
The convergent guidebanks have a disadvantage of and tendency of shoaling along the bank. Sometimes of these draw backs, convergent guidebanks are rarely suitable. They provide uniform flow from their head to the
excessive attack and heavy scour at the head the end bays are rendered inactive. Because used. Parallel guidebanks are considered most axis of the dam.
11.3.1.2 Geometrical Shape
-i~.·
According to the shape of the curved head, the guidebanks can be classified as straight and elliptical as shown in Figure 11.2. In the case of elliptical guidebanks, the elliptical curve is up to the quadrant of the ellipse and followed by single or ~ometimes multi radii circular curves. Due to gradual change in curvature, the flow, in the elliptical shape hugs the guide banks all along its length as against separa· tion of flow occurring in case of straight banks. Elliptical guide banks are however more suitable in wide flood plain rivers. 11.3.2
Layout of Guidebank
The layout of the guidebanks should be such as to guide the flood smoothly through the work. This is however dependent on local topography, site of the structure and alignment of approach embankment. The exact shape of the guidebank is dictated by model studies.
11.3.3
Length of Guidebank
Length of the guide banks should be considered from the following requirements:i) The maximum obliquity of current should be limited to reasonable value. iii Approach embankment or both sides should be fully protected in the event of main channel of the river em baying considerably behind the training works.
··~,!1
Based on experiments, the length of guidebanks can be co-related with the length of the structure 4 between abutments (L).
'.·~ ;%\
The length of guidebanks on the upstream can thus be kept 1.0 L to 1.5 L. Obliquity of flow be
174
Wont
..... 0
b l
/
"'
I
I
~
•
'
'\
a::
sulca
bank
' '
{i
f
~
i
-"
''
c 0
"'
\ \
0
Divergent~~
.?
pouiblo lmbaymont
I
I
1/
I
'·
!
0::
/
~
--Diatance between the approach bank and the wont p011lble embayment.
a) DIVERGENT I PARALLEL GUIDE BANK
.... c
) ~
~
u::::
•i
...c
~ ~
0
.,
"'
~
~
a:
Shoal
•
JLAxlo
b) CONVERGENT
GUIDE
UPSTREAM
BANKS- FORM IN PLAN FIG. II. I
r
R•0.46f>w
...1
"'-
li2 0
-
L
I I
1I
~g~
0
0
AI STRAIGHT GUIDE BANK
5•To eo•
0.3 TO 0.!5R
Bl ELLIPTICAL GUIDE BANK
GEOMETRICAL SHAPE OF GUIDE BANKS FIG. II. 2
limited to 30° to protect the bank from severe river action. In case of wide alluvial belt, the length is generally determined by fitting meander loops between the guidebank and afflux embankment. The radius of worst possible loop should be ascertained from available data. In case no data is available, the 3 radius can be determined by dividing the radius of the river by 2.5 for discharges up to 5000 m /sec. an
11.4 APPROACH EMBANKMENTS
In the construction of diversion dams on alluvial rivers, the natural waterway is restricted for better flow conditions and economy through the diversion dam. The unbridged width can be blocked by con· structing embankments called as approach embankments shown in fig. 11.3. The approach embankments on sides between the guidebank and alluvial belt edge are generally aligned in line with the axis of the dam/weir up to a point beyond the range of worst anticipated loop.
11.5 AFFLUX EMBANKMENTS
Afflux embankments are the embankments extending from the abutments or the approach embank· ment as per such toporgraphy of the dam site and connected on the upstream to the ground above affluxed high flood level. On the downstream, it should be taken to a length necessary to protect the canal or approach embankment from high flood. Afflux embankments should be located outside the zone of em· bayment of the meandering river as indicated in fig. 11.3 showing typical layout of river training works. In case a road exits near the site of dam, the afflux embankment may be continued with the road lor economy. Where flood embankment already exist, same can be used as afflux embankments. The section may however be strengthened when necessary, The alignment of the embankments need to be reviewed from time to time when river flow conditions change.
11.6 DESIGN OF GUIDEBANKS
Guidebanks have to b~ perm~nent structures like the dams/weirs they protect; and warrant great care ~n de~1gn and executwn. Gmdebanks should be so designed that even if the river swings, a safe margmal distance should be available between its extreme swing and approach embankment.
177
••
- 22
..D&J
., -..." ...;
=II""" I/
s
(/')
0
:li!
e
5~
0
...; !i<
l5
xi ~ 31:
"'iii
z
0
"'
g
~
1LLJ
a:>
12
IL.
?
<(" K 0
a::
0
~
0:: 0..
)P=-
~
~-' ...
-'
>
~
!i
"'
0::
~
~ ~
!:(:I
...3
"'
0
~
0
~
!;i!E :;:o !11~
_J
_J
(.)
a.. >-
1-
•
.&LM¥
.Q
&;;;;.;,
11.6.1 11.6.1.1
Section Top Width
In Lh<~ straight portion of the guidehank, also known as "shank", the top width be kept about 6 metres aL formation level (the height of the embankment) to permit carriage and storage of material. At the noses, the width may be inereased suitably to enable the vehicles to take turn. On small banks lesser width ean be provided keeping- in view olh(~r means of carriage of material along the guidebanks.
11.6.1.2 Free Board A free hoard of 1 to 1.5 m. should be provided above the highest flood level (HFL) for 1 in 500-year flood or ahove the afflux water level and as per r:ritcria discussed in Chapter 7.
11.6.1.3 Side Slope & Protection Side slopes of guidchanks depend upon the material they are to he made of and the height. Side ·slope of 2:1 to 2:5:1 is considered adequate and slope 2:1 is generally adopted. Due to surge waves, current v
guide banks between high flood and low water level, therefore, requires protection. Stone pitching should normally be done all along the shank on the river side and continued round the back of the head. Turfing can he done on the rear side of the shank. Full allowance for settlement should also be made. The size cf stone required on the sloping surface may be worked out from the curves given in fig. 11.4. Generally, stone pitching on slopes is done manually. Pitching with stone weighing from 40 to 70 kg (0.3 to 0.4 ml for an average velocity up to 3.5 m/s should be done. Cement concrete blocks of depth equal to the thickness of pitching for higher velocity may be used, but round and smooth boulders should be avoided. Pattern grouting in blocks of required size can also be done, where necessary. Hand packing & careful gradation of stone between sand and the large stone are necessary to prevent the sand being sucked
out by high velocity flow.
11.6.2 Thickness Of Pitching The thickness of pitching is generally adopted as equal to the size of stone but not less than 0.25 m. For higher velocities where size of stone works out more than 0.4 meter, cement concrete blocks of 0.4 to
0.5 m thickness can be used. 11.6.3 Filters A graded filter of about 20 em to 30 em in thickness and generally satisfying criteria as described in chapter 7 - para 7.9.3 should be provided below the pitching to protect the embankment material from being sucked out.
Thickness of stone pitching can also be worked out with the help of formula:
where Q
Discharge in cumec.
T
Thickness of pitching in m
' This would however be applicable to rivers with high discharges. In this ease, the stone pitching should be patch grouted covering 25% to 30% of the area and in suitable number of panels. Hand packing
179
:s
I
I
I
I
I /;
2
II I f-
I~
·st NK SLOPE 1:1~
~
0.6
~LOPE 2:
BANK
1
0.4
0.2
LIJ
:::E
-
<{
-
I 1/
= -
3: I ----._ I
0
0 .I
f-
0 0.0 G fIll)
VI
I
I
I
E
// ::
i"/j_
v
I()
Ul
N
IJ..
"
0
1-
..
Ul
!
(!)
-=
1.0
w :1: i
-
!
Ii
i-
0.0
I
I 4
2
=
G
4
I=
I
G
10
VELOCITY, m /s
SIZE OF PITCHING STONE
Vs. VELOCITY
FIG.
;:~&Q¥4 . .29!¥12\
-~- Q ~- 1&1&1@4 4
i
i
:z:
f-
0.7
10
=
I II
ill/ : ; I/,
;z 0 1-
-
0.06
o.o
LIJ
-
BANK SLOF
w
w z
~X
I
=
-
-
0::
1-
-
-
I I II
f-
E
/=
./
0.8
f-
I
.&
Q£;)5J;l$
!\§;.Qk
2
11.4
and careful gradation of the stone, with smaller stuff, as quarry refuse between the sand and Jarg(~ stones should be used to prevent the sand from being sucked out by high velocity flow. VVith this arrangement, the thickness of pitching can be suitably reduced.
11.6.4 Loose stone Apron. (Launching Apron) The face of the guidebank which is protected up to the river bed level with stone pitching so that the sloping portion is not damaged, is alsO subjected to scour which would occur at the t.oc with consequent undermining and collapse of the stone pitching. To obviate such damage to the slopes, stone cover, in the form of apron is laid beyond the toe on the horizontal river bed so that scour would undermine the bed first, starting at its farthest end and works backwards towards the slope. The apron then launches to cover the face of the scour with stone forming a continuous carpet below the permanent slopes of the guidebanks. Adequate quantity of stone should thus be required to ensure complete protection. The quantity would depend on the apron thickness, depth of scour and slope of the launched apron.
11.6.4.1
Size of stone
The size of stone for the launching apron can be determined with the help of curve given in Fig. 7.18. In case required size of stones arc not economically available, cement concrete blocks or smaller size stones in wire crates may be used. The crates (gabions) '3hould be made out of 14 mm Gl wire with double knots and should be closely knit and securely tied.
11.6.4.2
Depth of Scour
The depth of scour at different locations of the guidcbank shall be different as discussed under para 7.3.3. Scour depth may be adopted as given below:Noses of guidcbanks
2.0 to 2.5 R
Transition from noses to straight
1.25 to 1.75
Straight reach
1.0 to 1.5 R
The status of flood for which the scour should be calculated, should be 100-year flood and the apron be designed accordingly.
11.6.4.3 Thickness of Launched Apron In launching, the apron will not form a uniform thickness as by hand packing. The thickness should be
25 to 50 percent more than thickness of pitching on slopes. 11.6.4.4 Slope of Launched Apron The materials of the armour on the slope of the shank and in apron act only as a "face wall". Since the guideb.~nk slopes arc to be stable by themselves, the slope should have the same angle as the "angle of repose of th~ mater~al of which It IS constructed. Model experiments have shown that an apron does not launch sat1sfactorrly unless the angle of repose of the underlying material is flatter than that of the protection work. Observatwns at guidebanks on various rivers in alluvial beds have shown that the actual slope of a launched apron ranges from 1:5 to 3:1. However, in most cases the average approximate to 2:1.
181
The face slope of the launched apron should not therefore be assumed steeper than 2:1 for loose boulders and stones and 1.5:1 for concrete blocks or stones in wire crates. 11.6.4.5 Shape and Size of Launching Apron
It has been observed that whatever be the type of apron section, a certain dispersion, which will be the maximum at the outer edge, is unavoidable where the apron launches. As a general practice, an adequate thickness of apron should be provided at the toe of the slope to ensure a strength after launching equal to that of the stone pitching on the slope face. Additional stone out of the total apron quantity worked out, should be provided for irregularity of launching and washing away of stones; this can be better distributed in the apron in triangular wedge shape with maximum thickness at the outer edge. It has been observed that shallow and wide aprons launch evenly if the scour takes place rapidly. If the scour is gradual, the effect of the width on the launching apron is marginal. As discussed under para 7.9 of chapter 7 under "Protection Works", depth of scour "D" should be worked out from the normal scour "R". A width of launching apron equal to 1.5D has been found adequate and should be provided in the shank portion as well as in the curved portion of the guidebank. A minimum thickness of 1.5T (where T is the thickness of slope stone) should be provided in the straight portion. For the curved head where the apron has to cover a wider area, the thickness should be gradually increased to 2.25 T. It would be desirable to extend the apron from the shank to the head in a length L 1 equal to one fourth of the radius of the curved head. To minimize the loss of stones, the apron should be placed at the lowest possible bed level. A complete design of the guidcbank has been illustrated in Fig. 11.5.
'
Volume of Stone i)
Slope Portion For a slope of 2:1 with F as free board
I
total length ~
..f5 (Y
+ F)
If T be the thickness and allowing 25% extra as losses
I Il i
Quantity of stone/unit length ~ {5 x 1.25 x T (Y + F) ii) Apron Portion a) Straight reach Volume of unit length b) Curved reach volume
f5 X
D
l
Il
1
I
11.7
1.25T ~ 2.25D
X
1.251'
'h (1.51' + 2.251') x 1.5D
2.81D
1
X
X
T
DESIGN OF APPROACH EMBANKMENTS
In the design of the approach embankment, same guidelines as applicable to the design of guidcbanks should be followed with the modification given hereunder: 11.7.1
Top Width
Top width_ of the approach embankment should be from 6 m to 9 m at the form>tt - 1'011 leve 1. ') , n sma 11 wor k s Iesser WI d t h be provided keeping in view other means of transport of material.
182
45° TO 60°
0.3 TO 0.5R
/
-----
'
' \\ '
'
R" 0.45 Pw Pw" 4.83
"\JO
,r
.E L.
T
-,-_J.-L.j--.x R "I D
s LEGEND:
']
;~ ~ /ill
f y D X R
" " " " " T s ""
+Y
1.5 T
FREE SOARD DEPTH OF WATER ABOVE LOW WATER DEPTH OF DEEPEST SCOUR MULTIPLIER NORMAL SCOUR DEPTH THICKNESS OFSLOPE STONE THICKNESS OF SOLING (FILTER)
D
SECTION X- X (NOT TO SCALE)
';~ 'di
~···H.
F L.
··--·-···· ____ j --..,------·,..
xR
' ' ' ' I ' ..............',
DEEPEST
- - -
SECTION (Nill
SCOURJL _ _J
---~
_
'-......_ '-....::_ _ -, _ _;;,.
D
__
Y-Y
TO SCALE)
DETAILS OF GUIDE BANK
FIG. II, 5
D-tY
11.7.2 Section The section of the approach embankment on canal side shall he designed for the condition when there is high flood on the upstream and no water on the downstream. On the other side, the embankment should be safe against differential head with affluxed level on the upstream and retrogressed level on the downstream. 11.7.3 Size of Stone In workinR out the size of stone, the velocity should he calculated with 40 per cent of the design discharge.
11.7.4 Thickness of Pitching Thickness shall he based on 40 per cent of the design discharge.
11.7.5 Filter Generally, no filter should be needed. In case, however, a wider and higher section is required, filter shall be provided for as in the case of guide banks.
11.7.6
I
l
I I
I
Launching Apron
Size of stone shall be same as for Ruidebanks. Depth of scour shall however be 0.5 D to 1.0 D, as indi· cated in Fig. 11.3. Thickness and slope should be the same as for guidebank as per para 11.6.4.3 and 11.6.4.4. A width of launching apron equal to 0.5 D should be provided from the abutment up to the distance covered by the curved head of the guidebank and beyond it where it is exposed, the width of the launching apron should be increased to 1.0 D.
11.8 DESIGN OF AFFLUX EMBANKMENT 11.8.1
Top Width
Top width and also other parameters should be same as for approach embankment.
11.8.2 Slope and Protection Works As afflux embankments are generally constructed away from the main channel of the river, these are not subjected to strong river currents. The provision of pitching of slopes and launching apron is not considered necessary. Reaches which are likely to be subjected to strong river currents, paving of slopes may be necessary, otherwise it is desirable to provide turfing only.
11.9 GENERAL BEHAVIOUR OF LAUNCHING APRONS Results of model tests and field observations show that for satisfactory launching, bed material should scour easily and evenly. With an apron laid on the river bed consisting of alternate layers of sand & clay, stones slide down as sand layers scour and clay layers subside & thus stones from clay layer are
184
washed away. Clay bands cannot thus be used as dependable foundation for aprons. Where unavoidable, heavy mainte;nance of such apron is required. 11.10
OTHER TRAINING WORKS
In the case of very wide rivers, it may be found that necessary extension of the guidebanks to provide protection for the entire length of the approach bank is not possible on consideration of economy.
In such cases, the approach banks across/along the river banks, have to be protected by additional protective works such as revetment, spurs, etc. If, howeveor, the groyne can he avoided by a moderate increase in the length of the guidebank, it should be done. If suitable points are available for the con· struction of training measures like groynes or pitched islands, the length of the guidebanks can be considerably reduced and security of diversion structure increased under post construction conditions.
185
CHAPTER 12
1
INSTRUMENTATION
l
l 12.1
\
GENERAL
Diversion dams and related works form key structures to the development of river basin development for irrigation as discussed in the earlier chapter. These structures are built to last for many decades. Du~ to the pondagc created upstream, there arc tremendous forces inherent, which the dam is expected to Withstand throughout its operation life. Effect of failure of such a dam and consequent negation of planned benefits of the project, make it imperative that means should be available in the dam for providing informa-
J
I l
tion on continued assurance of its safety.
l l
12.2 OBJECTS OF INSTRUMENTATION
1
The objectives of instrumentation are two fold. Firstly, the instruments installed at the damsite keep a constant watch over their performance in service and indicate the distress spots which call for remedial measures. Secondly, observations from the instruments form a cumulative record of the structural behaviour. The information obtained through measurements promotes the understanding of the influence of various parameters and other assumption in the design on the structural behavior and thus leads to formulation of more realistic design criteria . .
I
12.3
INSTRUMENTATION FOR STRUCTURES ON PERMEABLE FOUNDATIONS
In hydraulic structure on permeable foundations water stored percolates below the foundation of the structure. The pressure gradient acting along the direction of flow is a critical design parameter at the exit end of the structure. Design of the structure, as outlined in the previous chapters involves calculation of these pressures and gradient on the basis of certain assumptions. Actual observations of the pressure, during the operation therefore becomes important. For the observations of pressures, instruments (pressures pipes), which are installed in the structure serve the following purposes:
i) These act as tell tales watching the stability and to predict undesirable developments.
ii) To investigate if the actual pressures at various points on the structure are in conformity with those assumed for the purposes of design.
A systematic record of these observations, apart from its scientific value, will be as necessary for the maintenance of the structure as a record of usual sub-surface sounding and probings.
12.4
UPLIFT PRESSURE PIPES
In dams on permeable foundations, the pond water percolates below foundation due to the difference
186
·~-m.MLJ&
w
of head "H" (Fig. 12.1). At intermediate points along the bottom profile at A. B residual pressures d"ve\op, which act upwards and tend to lift the structure unless counter weight i.e. dead weight of the structure itself counteracts the same. The pressure gradient at the exit and at C is also a critical design parameter. At this, if the force of water is in excess of the effective weight, piping can occur. The measurement of these pressures at all such critical locations iS thus very important. 12.5 NUMBER AND LOCATION 12.5.1 The pressure points to be installed can be divided into three groups: i)
Along and beneath the horizontal floor.
iil Along the vertical cutoffs. iii) At different depths under the foundations. For very low dams where the foundation strata consists of boulders & gravels installation of pressure points at lower depths at (ii) & (iii) can be eliminated. This shall however depend on individual site.
The location of these points should however be pbnncd carefully taking into account the presence of stratification in sub·soil and other geological and design features specifically presence of clay bands. A typical arrangement of the pressure points under a structure is shown in Fig. 12.2. On a diversion dam, suitable locations under the horizontal floor are:-
i) Upstream and downstream aprons ii) Immediately upstream and downstream of vertical cutoffs. iii) Intermediate points at regular intervals.
Under the floor at the selected locations, uplift pressures are helpful from stand point of safety of the structure, whereas tapping points located along the faces of vertical cut offs at suitable depths help in the correct evaluation of the effect of the depth and spacing of cutoffs in addition to that of stratification of the sub·soil on uplift pressures. In the sub·soil the pressure points should be located at suitable depths and intervals under the pervious and impervious floors. Normal distribution of pressures ':lot effected by the vertical cut offs shall be indicated at these points. Pressure points along the downstream vertical cutoff on the downstream face, at C shown in figure 12.1. shall indicate the pressure required for computing the exit gradient. 12.5.2 Location Pressure tapping points should generally be located along the abutments of the structures and at a member of intermediate sections between the abutments, at -suitable intervals. Normally, at a minimum number of three intermediate sections, pressure tapping points should be provided. This shall however be dependent on the importance and magnitude of the structure. The pipes from the filter points (described in para 12.6) are led to the piers or abutment walls to enable water level readings to be taken throughout the year.
12.6 DESIGN OF PRESSURE PIPES 12.6.1 12.6.1.1
Filter Points Pressure tapping points consist of filter points made of brass at the base, of 50 mm inner dia·
187
,-~-'-~""'"' ---.--~-~---·""---~-~-~--··-·"-----··-·-- -~·~.:c.~~--~~------"'-"'-'"""-''-"~--~-"~--"""-'>"-''"""''" ·'·~------~'·""-'"'-"'-"4~,~-0>..C"""'~·-···="'~'-"~'''~·"""""-":."'"''"'"".,:"''-"·"'·'"--'~-"'''''""'-=;,~,~-'-.~''"~"""-·''"·'./-~~---~-, -··----~
UPSTREAM WATER LEVEL
j
STAND PIPE PEIZOMETER)
•
_j__ p
t
-r
~DOWNSTREAM
WATER LEVEL
A
DAM ON PERMEABLE FOUNDATION
FIG.
11
12. I
. "':ik
}.~!4%
'fj;!i&,&_.-i,:
iW'*4
;l.~;i;Oi\~·f
"'"""*''·><· •F0,' 0
'
NOT!:S: L OC A TICN OF TAPP!NG POINTS WOULD VARY WITH THE SHAPE a SIZE.
---i~ "'...
UPSTREAN
Al.L DIMENSIONS ARE IN MM.
k~
FIG. 12.2
meter and 100 em in length with 500 micron wire gauge strainer as shown ln fig. 12.2. These arc further connected by 40 mm G.!. pipes to suitable stand pipes located in the dam structure for measurement of water level. The filter point shall be fitted with a driving point at one end and a threaded blind pipe of 50 mm diameter and 75 mm length at the other end. 12.6.1.2 The filter points shall be laid horizontal where excavation permits or alternatively these <:an he
driven down to the desired levels. In case of foundation soil being too hard and difficulty experienced in driving to larger depth, the filter point along with blind pipe can be inserted in a bore hole of about 100 mm diameter. 12.6.2 Generally graded filter material around the filter points is not required since no flow through these points is expected. In case of very fine sub-soil material, however, graded filter can be provided to avoid choking up of the point. From economic considerations, where the above arrangement is costly porous tube piezometers, where porous ceramic tube acts as a filter tip, can be provided. 12.7 INSTALLATION
The filter points are laid horizontally or driven vertically and connected to pipe laid vertically in piers and abutments. Points directly under a pier or abutment wall shall be connected to the observations platform by a single vertical lengths of piping. But those away from piers and abutment walls shall be connected by horizontal lcnghts of piping. The horizontal lengths shall be placed well below the lowest pressure level that is likely to occur at the respectivP lJOints. Otherwise, no observations will be possible during certain water level conditions~when the observations stand pipe will be dry. The horizontal piping between the filter point and the observation point on the superstructure shall be slightly inclined downwards in the direction of the filter point to avoid any possible air lock. When more than one pipe are driven at the same place to different depths, they shall be spaced not closer than 30 em so as to avoid direct connection between any two filter points. During the installation of pressure pipes the properties of soil around the tip, should be observed, particularly when the tips are located in soil with different properties and permeabilities recorded. This may be of help in subsequent analysis and interpretation of observations. During erection, the ends of all pipes shall be kept closed by caps to avoid foreign matter finding its way into the pipes making observation of water level unreliable.
12.8 PRECAUTIONS
i) All vertical pipes shall be kept dead vertical and no link of any sort shall be allowed. Failure in this requirement may make it impossible to lower the bell sounder to the right place for observations. iii Each pressure~tapping point shall be given a distinct number and that number shall be marked on the filter point and on each length of connecting pipe. These distinctive numbers shall be stamped on the caps at the end of the stand pipes and on the masonry or concrete platform where these are located. 12.9
MAINTENANCE
i) Alter installation every year before the onset of monsoons, each pipe shall be tested to see that the fi_lter pornt rs not choked. If any choking has occurred remedial measures like using compressed air or water under pressure by jetting through the pipe, shall be taken; iii E ac 11 vertica . I stand pipe shall be provided with a screw cap to avoid bird nests and tampering. iii) All m1ssmg . . screw caps on the tops of stand pipes shall be replaced with their original numbers stamped. 190
if!Ld. iJ _
:a
iv) All the piping including the stand shall be coated with a good quality anti-corrosive paint, taking care that original pipe numbers are not obliterated.
12.10 OBSERVATIONS 12.10.1 i) ii)
The following observations should be made: Upstream and downstream water level should be read from water level gauges suitably fixed. The water levels in all the stand pip~s shall be read by means of a bell sounder lowered into the pipe by a steel tape or by electrical devices.
iii) The depth of sediment on the upstream and downstream floors and the soil characteristics of the sediment shall be observed. Depth of sediment can be measured by sounding. 12.10.2 Record of Observation The observation shall be recorded suitably in the Registers & Forms as per the format given in Appendix 12 A Following observations shall be recorded. a) Date of observations. b) Upstream and downstream water level. c) Total head H - the difference between upstream and downstream levels. d) Temperature of river water and temperature of water in selected pipes.
e) Depth of sediments on upstream and downstream floors. f)
Water levels in all pipes.
g) Residual pressure (P) level.
difference of water level in stand pipe and downstream river water
h) Velocity potential percentage,
1:5
(P/Hl x 100
While recording all the above observations and their analysis, pipes should be grouped by lines laid in a single section from upstream to downstream. For proper maintenance record, one page shall be earmarked for one line and sufficient pages shall be reserved for subsequent record of observations for at least one year. Record is generally prepared in the field in duplicate, i.e., one additional copy for the central office. 12.10.3 Time Lag When there are large fluctuations in the upstream and downstream water level in the river, during rising or falling floods or when river supply is ponded up, the results are likely· to be influenced by time lag. A rise in the upstream level will give relatively lower readings and when falling, higher readings are expected. With sufficient and regular observations, it shall be possible to make due allowance. A sudden rise in downstream level will give relatively lower pipe readings and vice versa. Similarly, in rising flood when upstream and downstream levels arc rising, the pipes will read relatively low and in falling flood, when upstream and downstream levels are falling, they will read high.
12.10.4 Frequency of Observations The frequency of observations shall depend on local requirements. In case of special problem, obscrva tions can be more frequently recorded.
191
I j
Generally, once a week at key points and once a fortnight for other points shall be enough for watching stability of structure. Daily observations shall be made during such periods when water fluctuations arc more, and specially during floods.
I '
12.11
1 l I l
.I
l
I
PRESENTATION & ANALYSIS OF DATA
For a given pressure point, 0 ie P/H values remain constant for any structure provided temperature of water and depth of sediment, scour on upstream and downstream do not alter. The function.ll should form the basis for plotting, for any variation in its normal value needs to be explained by temperature and sedimentation. Following graphs shall be prepared to have a quick idea about the behaviors of different pipes at any time of the year and bring to light the abnormality .
Date of observation should be plotted on X axis and variable on Y axis. Variable being: a) value of 0 b) river temperature
c) value of H d) downstream water level e) depths of sediment/scour at the pervious floors, upstream and downstream. These graphs should be kept plotted up to date for all key points to help ascertaining unfavourable developments.
192
:::WMSZZ::,
Z:.¥
till
·~
~';'$
"'--~'-\1;;'•.;-.--·
''
• .,,. ~-~,:-F\'-" -
tii-~;;:';;f;
•.-,••'A0i,f;.•:t::;f,··~·'-''' "''~"-~"·"-""'"''"',/,
d''·'''"-,,;;,;;,·-
•'«'"''
·APPENDIX
REGISTER
Of UPLIFT PRESSURE NamG
of
Nome
of hydrou11c structure :
s
K
I
l
OBSERVATIONS
River:
E T
(ALONG
PIPE
12A
PIPE
c
*
H
UNEI
i' •..
: !
Name of observer:
•
t,'
,[)ate
of observatIon :
Time
of o bservotion :
·<
Upstream water leveJ:
(m)
Downstream water level:
(m)
Head " H Snode temperature:
(m)
;
•c •c
River water temperature:
De p!ll of sed lment on upstream pervk>LtS tloor: Dep!ll of sediment on downstream pervk>us tlooc
Line Distance t ron Reduced level RedUC
PIPE
( II
NO. Upstroo m Eod of Bottom ot of Impervious Pipe floor ( 2)
( 3)
NOTE :lf Sketch to show: i}Foundation prot lie : II) Uplift pressure pipe
0
Bond ~ipos,
It
Any ( 4)
No.
(m) (m)
Toto! floor lenotn :
Reduced level Pipe water Depth of water Reduced le.,.. p io m of Top of pipe, Temperature In Pipa, of w:rter In (col.8- d/$ m. •c m. river W.L.) Pipe, m. ( 5)
( 6)
( 7)
( 8)
( 9)
0 ' ( P/H) DESIGNED (I 0)
X
100
REMARKS
OBSERVED ( II)
( 12)
with rumbers: .
~
~
r.·_j_ .
}I :~
CHAPTER 13
~
..Jg····
DAMS ON IMPERVIOUS FOUNDATIONS
13.1
GENERAL
To make water flowing in a stream or river available for irrigation use, it must be diverted by means of a diversion dam, intake works and other appurtenant works. Such diversion works are generally
l~ ~
'I~.
·.•.•
located in or close to the channel of an alluvial river. Though the location of a diversion dam is established by the requirements of project plan, it may be required to place dam on almost any type of foundation material. Very few sites for diversion dams. however, permit construction on rock foundation, since bed rock is usually at such depth that its use as a foundation is impracticable and uneconomical. At some locations, however, the project plan may so dictate as to found the dam on impermeable (rock) foundation. Considerations in the design of diversion dam founded on impervious material {rock) being similar to those in the design of gravity dam, its analysis has been discussed in this chapter as applicable to design of gravity dam.
13.2
DESIGN ANALYSIS
For the design of gravity dam, it is necessary to determine the forces which may be expected to effect the stability of the structure. The forces which should be considered are: i) The external water pressure. iii Internal water pressure (pore pressure or uplift) in the dam and foundation. iii) Silt pressure. iv) Earthquake forces. vi Weight of the structure. vi) Forces from gates or other appurtenant structures. Design Assumptions for calculation of stability, evaluation of load and forces promoting stability and inducing instability and load combination shall be the same as adopted for the spillway section (solid
gravity) on permeable foundation discussed in chapter 8. The forces to be evaluated are however, briefly described as under:a) Forces which induce instability are:i) Pond level and tail water loads. iii Earthquake and silt pressure. iii) Uplift pressures. b) Factors that promote stability are:i) Dead load.
194
•
ii) Shear resistance of dam and foundation.
iii) Quality and strength of concrete.
iv) Strength characteristics of the foundation and abutments.
13.3 SAFETY CRITERIA
Safety criteria against sliding and overturning shall be the same as for dams on permeable foundation discussed in Chapter 8 of the manual. In the evaluation of the forces and design analysis the following features for dams on rock foundation shall, however, need special consideration.
1. Uplift pressure. 2. Foundation and its treatment
3. Water way - crest length
4. Stilling basin 5. Protection Works
13.4 INTERNAL OR UPLIFT PRESSURE
Uplift forces occur as internal pressures in pores, cracks and seams within the body of the dam, at the contact between the dam and its foundation and within the foundation. It is recognized that there are two constituent clements in uplift pressure; the area factor or the percentage of the area on which uplift
acts and the intensity factor or the ratio which the actual intensity of uplift pressure bears to the intensity gradient extending from head water to tail water at various points.
Effective downstream drainage, whether natural or artificial, will generally limit the uplift at the toe of the dam to tailwater pressure. Drainage holes drilled subsequent to grouting in the foundation or other drainage arrangement, where maintained in good repair; are effective in giving a partial relief
to the uplift pressure intensities under and in the body of the dam. The degree of effectiveness of the system will depend upon the character of the foundation and the dependability of tho effective maintenance of the dr;oinage system.
13.4.1
Criteria for Design
The following criteria are recommended for calculating uplift forces: a) In case of highly jointed & broken foundation, the pressure distribution may be required to be based on electrical analogy. For low diversion dams, however, where there are no formed drains, it is safe to assume the straight line variation from head water to tail water pressures as a measure
of uplift. b) The uplift is assumed to act over 100 per cent of the area. c) It is assumed that uplift pressures are not affected by earthquakes. Uplift pressures can be reduced by forming drains through the concrete of the dam and by drilling drainage holes into the foundation rock. Such drains arc usually provided near the upstream wall of the dam·exercising proper care to ensure that direct piping from the reservoir will not result. Other methods
used to reduce the uplift at the contact of the dam with the foundation include construction of cutoff wall under the upstream face, construction of drainage channels (usually of sewer type) between the dam and the foundation. These methods usually are considered only additional safety factors in the design of low dams and should not be considered in reducing the design and stability requirements.
195
13.5
FOUNDATION
13.5.1
Structural Competency
The foundation should be investigated and the maximum allowable stress established though the structural competency of the foundation is not usually amenable to precise and complete evaluation. The data from exploration and tests should be supplemented by engineering and geological judgment. Where the quality of the foundation materials approach or falls below that of the concrete/masonry of the structure on it, the foundation competency becomes a controlling factor and should be appraised in full detail. 13.6
FOUNDATION TREATMENT
13.6.1
General
The rocks near the bed of the river are characterized by numerous structural distressed conditions which may be undesirable in dam foundation. It is of great importance that all important geologic condi· tions affecting the feasibility of the dam site are ascertained before commencement of construction.
In the case where geology exhibits extensive discontinuities such as faults, clay filled joints or bedding planes, strength is governed solely by the resistance against shearing along these discontinuities. In the case of soft, uncemented shales or slates having weak, near horizontal planes, the toe may not be considered strong enough to provide sufficient assurance against sliding. In such cases a deep heel trench below the upstream side of the dam, is provided to be filled with reinforced concrete securely anchored to the dam. 13.6.2 METHODS OF TREATMENT
Several methods of treatment of foundation, such as excavation of seams of decayed or weak rock by open excavation, mining of objectional material and back filling with concrete, or by drilling and grouting can be resorted to. For small dams foundation of rock, including hard shale, generally do not present any problem of bearing strength. Rock foundation, should however, be carefully investigated to determine their per· meability. If erosive leakage, excessive uplift, high water loss occur through crevices of faultp]anes, the foundation should be grouted. The determination of whether or not a foundation should be grouted is made by examining the site geology and by analyzing the water loss quantities of foundation exploration holes. A great deal of experience is required to make this decision since every foundation is unique. Normally in the case of storage dam and in its estimation, foundation grouting is provided. For detention dams or for extremely low diversion dams, grouting of rock foundation is not required. The treatment of foundation for such type of dams may only consist of low pressure blanket grouting, which can be divided into two parts. a) surface treatment b) sub surface operations.
13.6.2.1
Surface Preparation
Where surface rock is weathered and distressed, it would be necessary to remove over burden and shattered mass before laying concrete. 13.6.2.2 Subsurface operation where required. This consist of grouting & drainage treatment.
196
13.6.3 Procedure for Grouting 13.6.3.1 The principal objectives of foundation grouting arc to establish an effective barrier to ·seepage and to consolidate the rock under the structure.
13.6.3.2 a)
Low Pressure Grouti·ng The general plan for treating consists of low pressure blanket grouting. rl:hc _low pressure grouting aims at improving the foundation condition by providing a general co~sol!dat1_o~ of th_e sur face rock and to fill and seal major surface scams and crevices. The surface IS thus JOinted tnLo a monolithic mass.
b) High Pressure Grouting
High pressure grouting aims affecting a relatively water tight seal across leaky zones in the foundation. Procedures diff(.~r significanlly according to the conditions of the foundation and irnpor tance of the structure. c)
Depth of grout holes The required depth of the deepest holes depends upon nature of the rock in foundation. Actual assessment of depth can be made by water testing up to varying depth to determine extent of leakage. Generally, depth can be determined with the following formula:~ d
~
0.33 h + c
where d
depth of holes in metres.
h
height of dam in metres.
c
constant varying from 7.!) to 25.
In a hard dense foundation, the depth may vary from 30 to 40 per cent of the head and in a poorer foundation, the holes may reach deeper as much as 70 per cent of the head.
13.6.4 Drainage The final treatment of the foundation is to provide drainage. Suitable drainage measures, which should be maintained under constant repair, contribute significantly to the reduction of uplift. Drainage is accomplished by drilling one or more lines of holes immediately downstream of the glacis. Sizes, spacing and depth of holes can be assumed on the basis of judgment and characteristics of rock.
I
13.7 WATERWAY- CREST LENGTH
In a diversion dam, the water way and afflux are co-related and for an economically and hydraulically efficient design the combination between the tWo is made by trial and error. In the case of diversion dams on permeable foundations the waterway is fixed taldng into consideration the stable width of the river worked out from the Lacey equation. For dams on impervious foundations, major factor governing the crest length of the overflow weir is the cost. The most economical length for a given flood can be determined by comparing the cost of weirs of various lengths with the cost of other features effected by crest length. Some of the important features to be considered in making the cost comparisons can be public utility services affected by back water, like
197
roads, rails, telegraph, pipe lines, etc.; Cost of appurtenant works - sluiceway, head works, stilling basins, protection and river training works. Though it may not always be practical to use the economical crest length because of other controlling features, cost studies can he used as a guide. 13.8 STILLING BASIN - FLOOR THICKNESS
Thickness of floor for the stilling basin (downstream apron) shall be dependent on uplift pressures which in turn would he different from those worked out in case of dam or pervious foundation by Khosli Theory discussed in Chapter 5. In the case of basins of dams on impervious (rock) foundation, the floor (which is at a lower level than river channel) shall be subjected to an uplift equal to the tail water heador higher depending on the sur· face. During spillway,. running, the basin flow is subjected to higher head because of the slope of jump profile. The basin floor must be' heavy enough to withstand the unbalanced head. For design, the stilling basin floor is considered to be a free body in equilibrium with foundation reactions balancing active loads. Uplift forcc.s caused by hydrostatic head on the bottom of the slab are counter balanced by the weight of concrete and effective weight of water in the basin.
13.9
PROTECTION WORKS
In view of the non-erosive characteristics of the river channel, protection works as required on dams on permeable foundation, may not be required in case of those on impermeable {rock) foundations. Curtain walls downstream of the stilling basin may however be taken to suitable depth. Necessary rip rap material shall be provided consistent with the velocities both upstream and downstream of the dam.
198
CHAPTER 14
OPERATION AND MAINTENANCE
GENERAL
Every dam and the water retained structure presents a potential danger. A probable failure of the dam means not only the loss of structure and the water stored, but the stored water though in small measure, may also cause damage to life and property in the areas immediately downstream of the dam on its failure. The necessity for proper inspection and maintenance of dams and appurtenant structures is evident. The risks of damage to the works can be increased as much by neglect of proper and timely operation and maintenance as by inadequacies in design and construction. Diversion dams which are founded on sandy or gravelly stream bed materials have their stability ensured by a broad base with cutoff walls, be safeguarded by frequent inspections for evidence of piping or boils. An increase in the volume of seepage or piping would call for suitable remedial measures. Proper operation and maintenance of such dams is thus of great importance. It is necessary that the engineer incharge makes themselves familiar with the basis of design and manual of work. Operation and maintenance of diversion dam and appurtenant works can be divided into two categories:~
a) Hydro-mechanical installations b) Civil Works
14.2 HYDRO-MECHANICAL INSTALLATION
Main items under this category are generally gates and winches. 14.2.1
Operation of Gates
il All lift gates should be operated at suitable intervals to keep them in working order. In low supplies when opening are not desirable, raising of gates by 15 ems for a few minutes would be sufficient. Such gates which have not been moved for a long time should not be forcibly raised at once but should be lifted slowly by almost 3 em or so and left in that position for about 10 minutes till the silt deposited against the gates is softened and water leaks through. This is necessary to a void strain on the machinery. ii) The operation speed of the gates should be limited to the maximum speed indicated by the manufacturers. iii) The operation of undersluice gate should be based on model studies for optimum hydraulic efficiency, structural safety, and maximum silt exclusion.
I
iv) ?peration of ~eir shutter~, where provided, and their sequence of operation shall be based taking mto account rrver behaviour, shoal formation and scour. It should be decided by the engineer
199
in charge. Generally. the gates starting from the centre are opened first and then moving on either side. v) The gates are opened in installment of 30 em or so at a time. Gate opening should be increased to allow passage of boulders, etc. vi) The head gate of the intake work should be opened equally unless otherwise indicated on model
studies. vii) The openings of the silt excluders, where provided, should be closed slowly to avoid water hammer. viii) The operation of gates should be done in such a manner that the permissible difference in static head is not exceeded on either side of divide wall to endanger safety of the structure.
Maintenance
14.2.2
All machinery and gates at the works should be kept clean, tidy and in proper working order. Care should be exercised that it is properly handled in conformity with the manufacturers instructions.
14.2.2.1
Gates
For proper maintenance of gates, following instructions should be taken into consideration. i) The gates and counter balanced boxes should hang vertical and plumb. Any adjustment if required to keep them in such shape should be made from time to time. ii) For shutters, all the holding devices like chains/anchors holding them should be kept free from rust. iii) All the gate grooves, angles for gates/shutters should be maintained clear of debris, silt, etc. iv) The upstream face which would be coming in contact with water should be provided with a suitable and proper paint to make its life long. Other parts of the gates and shutters not in contact with water can be painted with a suitable paint which can be ready mix paint but be water and heat resistant. v) Machined surfaces, stainless surfaces, brass or bronze do not require any painting.
14.2.2.2
Gate groove and Seals
i) The grooves and their machined faces should be maintained clean and well lubricated. Before application of lubricants, these should be cleaned of all sticky deposits. ii) All seals just after construction and during closures should be tested for their efficiency. The verticality and horizontality of the seal seat and wall plate should be checked. Rubber seals should be suitably tested to press uniformly by inserting paper strips or other means. Seals should be adjusted or replaced as necessary. 14.2.2.3
Steel Wire Ropes
i) All steel wire ropes should be kept clean and well lubricated. Generally, at least once during a year, the ropes should be lubricated after proper cleaning. The lower portion of ropes under water can be wrapped with gunny (jute) bags after proper lubrication. ii) Inspection and maintenance of wire clamps should be done periodically. It is recommended that clamping devices should be tested at least once in three years.
200
14.2.2.4 Winches and Hoists
il All winches and lifting arrangement should be examined to ensure that all gears and axles are clean and properly lubricated. All bearings should be kept lubricated. The alignment of shafts should be checked and coupling bolts tightened. iii All grease cups be kept full of lubricants to protect the bearings against rust. The winches should be tested for workability periodically. These should be operated in right direction and for that properly marked directions and limits should be indicated. iii) For electrically operated hoists, all precautions necessary for ensuring safety and proper operation of motors, switching devices, upkeep of proper insulation of wirings be taken. The electrical and mechanical parts should be periodically inspected. iv) The decking should be properly maintained. Wooden planking where provided in the decking of hoist bridge, should be checked frequently. Loose nuts and bolts be tightened and worn out planks be replaced. v) All lift gates/fall shutters should be tested and the engineers in charge should submit a certificate to the competent authority before the advent of monsoons that all gates/falling shutters are in good operation cOndition.
14.2.2.5 Lighting and other Arrangement
il All the lighting arrangement and illumination where provided, should be checked daily during flood season and once in a week in slack season.
ii) For the road bridge, the bridge bearings should be cleaned and greased once in a year after the monsoon season. iii) Painting of super structure :;hould be done once in 2 years.
14.3 CIVIL WORK Inspection and Maintenance
Periodical inspection of diversion dams is necessary for ascertaining and detection of any damage and to ensure proper functioning. Further objectives are as follows:
a) to ensure adequacy of the structures to serve the purpose for which they are designed. b) to verify the condition and monitor behaviour. c) to investigate conditions which cause distress.
d) to study the extent of damage, if any, and planning of maintenance and repairs. Adequate inspection shall be carried out by competent personnel after an unusual event. All inspection observations should be compared with the design assumptions and results of model studies. Such inspections are generally carried out annually for all underwater works after monsoons. Besides, detailed inspection should be carried out in stages by drying various portions of the dam and inspecting them against crack or damage, at least once in every five years. All such repairs shall however be carried out before on-set of next monsoons.
Inspection and maintenance of the following components of the dam should be given special attention. i) Aprons ii) Impervious floors
201
iii) Sediment excluding devices iv) Intake work v) Instrumentation
l
vi) Other observations vii) River training works
I -i
14.3.1
Aprons
The sounding and probing of the upstream and downstream apron and the arc~tS just upstream and downstream shall be carried out immediately after the monsoon to assess the scour and launching of aprons. The block protection portion particularly on downstream shall be carefully examined against any settlement and effectiveness of the filter. Suitable measures shall be taken to protect against harmful retrogression.
14.3.2 a)
Impervious Floors
Upstream Floor
Upstream and downstream floors upstream floor is not much during lean lamps. In the boulder reaches joints repairs can be carried out under water, b)
I
shall be inspected after the monsoons. When water depth on the water flows, inspections can be done with the help of under water of the stone sets should be carefully inspected. Whereas minor for major repairs the areas may need to be isolated.
Downstream Floor i) Downstream floor of the stilling basin should be inspected carefully. Inspection can he properly
done during winter flows and necessary repairs should be carried out before the monsoons. ii) In case of permeable sandy foundation, for deep cisterns requiring cleaning and dewatering, inspection can be done by probing. In boulder reaches, however, dewatering and cleaning may be carried out in rotation once in three years.
:~ cl
iii) Deep cistern when being dewatered should be ensured against the maximum head for which it was designed. In the operation/regulation manual, this should be specified.
l
14.3.3
I Cj
A complete inspection of the silt excluder including the opening, roofs, tunnel mouths, etc. should be carried out during winter months with the help of divers and under water lamps. Whereas, m1nor repairs can be carried out under water, major repairs may require local isolation.
14.3.4
i
Sediment Excluder
Intake Work
A thorough inspection of the work should be carried out in winter months. The upstream floor can be inspected by probing and downstream cistern when dry or by local isolation. After 3 years or so the upstream floor shou1d be examined thoroughly by isolating the area. Necessary repairs should be carried out in time.
l l
14.3.5
Instrumentation Data
The data observed on the instruments (pressure observation pipes) em~edded for pressure, described
I j
202
Cj
I
·-:g
n_ __ s_ X. MlJiMKJJ?JM ...
iJ@i . _..JLL..
't
in Chapter 12, should be examined and performance report prepared every year. The observed pressures should be compared with the design uplift pressures with the help of graphs recommended m the format. If any additioanl uplift pressure pipe is required at the critical locations, this may be installed. The uplift pipes should be maintained in accordance with the recommendations discussed in the chapter on instrum~ntation taking special care that each pipe is; i) given a separate number
ii) mouths of all pipes are kept closed to avoid clogging with foreign matter iiil each pipe is tested frequently with water jetting. 14.3.6 Other Observations 14.3.6.1
Pressure Release Pipes
For checking efficiency of the drainage system provided downstream of the dam, pressure release pipes should be checked for the quantity and quality of the discharge. The sediment content in the affluent should be observed. It may be possible to take observations during dry season. Suitable remedial measures shall be taken when required. 14.3.6.2 Standing Wave Profile
Hydraulic jump profile shall be recorded with the help of strip gauges which require to be painted on the wing walls and the divide wall. The profile in the propotype should be compared with the design profile for various conditions of flow. 14.3.6.3 Sediment Observation
Water samples both upstream and downstream of the undersluices for sediment content should be taken. In addition, below the canal intake work the samples be collected to assess the suspended sediment. These observations should be made at least once a week during the monsoons to determine the efficiency of the sediment excluders. Such observation would be useful in ascertaining any modification in the regulation procedure. 14.3.6.4 Aggradation of Riverbed
With the construction of dam, river bed upstream is likely to get silted and aggradate resulting in increased afflux and consequently reduced free board. Gauges should be fixed upstream at intervals of 1000 m or so to record water level and to determine increase in the afflux elevation if any. If necessary. , training works may have to be raised suitably for restoring the design free board. 14.3.6.5 Retrogression
The retrogression expected in the river bed downstream of the dam should be observed with the help of gauges fixed downstream of the work at an interval of 10 m. or so. In case of excessive retrogression as provided in the design, remedial measures should be taken when required to ensure safety of the structure.
203
14.3.6.6
Settlement
For measurement of any settlement of the structure, because of presence of clay layer or other soft soil in the foundation, permanent observation points should be established by precise leveling. The levels should be observed in the winter months every year and remedial measures taken, if necessary.
14.3.7
River Training Works
i) A detailed survey of the river training works upstream and downstream of the dam shall be carried out every year. ii) On the upstream, the survey should extend about 1 meter above the design flood level on both the banks. On the downstream, the survey should extend to a length up to which changes with the construction of dam have occurred. iii) A number of permanent bench marks should be established on banks to facilitate survey. iv) Changes in river course should be examined and remedial measures should be taken, if necessary. v) Guidebanks, afflux embankment and approach embankments should be examined in winter months and necessary repairs to the embankment section, pitching and apron be carried out before monsoons. vi) Sufficient stock of stones and boulders should be made available in the close vicinity of the protection works for use in case of emergency. 14.4
REGULATION AND OPERATION
14.4.1
General requirements i) For efficient functioning of the system, it is necessary that adequate regulation staff should be
provided and their duties clearly specified. ii) Sufficient stock of material, tools and plants required to meet emergencies should be maintained at all sites of diversion dams_ A complete list of all such items be maintained and their availability periodically checked. ' iii) The gauge sites should be linked with telephone/telegraph lines to the diversion works. On major rivers, the installation of wireless transmitting stations, for speedy transmission of flood wirings should be considered. 14.4.2 The operation and regulation can be divided into three periods depending upon the flow in the river. i) Pre monsoon
ii) During monsoon iii) Post monsoon
14.4.2.1
Pre monsoon operation
During this period, river flow is low and thus no wastage of water would be desirable. In case of diversion dams where falling shutters are provided, these shall be so regulated to conserve all the a~ailable s~pplies to maintain Pond level. The releases through the Intake into the canals should be done wrth the help of gauges and corresponding discharge tables, which should be checked occasionally.
204
;;;
zan~
14.4.2.2 Monsoon Operation
i) A number of gauges shall be installed sufficiently upstream of the dam to ensure timely information for operation of gates at the dam site. ii) During floods, the recording of gauges and transmission of the same should be done more frequently so that the Engineer and staff in charge of the regulation are posted with the rising flood. iii) To help minimum entry of sediment into the canal, still pond regulation should be done as far as possible. In case of locations where it is not possible for canals to be closed for flushing of the sediment, still pond regulation can be adopted. The attempt should be to resort to still pond regulation as much as possible, depending on flow in the river. iv) The excluders should be kept open during floods taking into consideration safety of the structure. The under sluice gates under no circumstances be allowed to be overstopped. v) Sediment Change Observation
It is important to keep constant watch over the sediment content entering the head work, that being ejected out by the excluder and the sediment deposited in the canal. In this respect following action should be taken.
a) In low floods, sediment charge observation be done and frequency can be fixed depending on the silt charge. b) cross section of the canal be observed at~suitable intervals.
c) water surface slope in the head reach of the canal be kept under observation with the help of gauges.
d) ponding upstream of power station, if any in the canal reach be restricted to the required extent.
vi) Operation of weir shutters, where provided, should preferably be based on model studies. vii) Pond level be kept minimum to feed the canal with the required discharge by suitably opening the gates.
--~~
14.4.2.3 Post-monsoon operation
i) Sediment change observation & cross sections on the canal shall be continued till satisfactory results are obtained. iii Still or semi still pond operation, with sediment excluder operating depending on the surplus water, should be continued till water is reasonably clear.
iii) When canal is opened, it should be run with a low supply for few hours and discharge gradually increased. The rate of filling and lowering of the canal should be prescribed depending upon water table along the canal, type of drainage filter behind lining, etc. Generally, rate of filling is faster than lowering of supply in the canal, to avoid cracks in the lining. 14.5 HISTORY OF DIVERSION WORKS
A continuous history of the river behaviour and overall performance of the diversion dam Intake work and riv~r traini~g works should?~ maintained on all major works. In the history of work, in~pection ~cports. repairs earned out and additional measures adopted for river training works and any other important feature relevant t.o the over all performance of the structure, should be given.
205
0 h
c
CHAPTER 15
"s F t
DESIGN OF DIVERSION DAM AN ILLUSTRATIVE EXAMPLE
15.1
GENERAL
The concept of flow of water through the sub soil below the dams on permeable foundation and consequent attendant hydraulic gradients and uplift pressures generated have been discussed in the fore· going chapters. The design of such a dam thus involves besides the above forces, the determination of the most economical length and section of overflow portion (spillway), sluiceway portion, having the required capacity to pass the maximum allowable flood, maximum afflux elevation; energy dissipation arrange· ment, protection works against expected scour on the downstream and upstream, stability analysis and structural design of all the components of diversion works. Whereas in the preceding chapters the design principles & procedure of designs are covered, in this chapter, an illustrative design example based on the design criteria described in the previous chapters, has been presented.
15.2 15.2.1
ILLUSTRATIVE EXAMPLE MAG-ASAWANG TUBIG DIVERSION DAM LOCATION
The proposed diversion dam will be located on Mag-Asawang Tubig River approximately at the con· fluence of Ibolo and Aglubang Rivers. The dam would be an ogee type, about 2.00 metres high and is proposed to be of concrete and rubble masonry. The intake on the left bank would have capacities of 5.539 ems. during land soaking and 7.959 ems. during crop maintenance and that on the right bank would have a capacity of 33.97 ems. for crop maintenance and 40.998 ems. for land soaking. Based on the command area, the operating water surface of the dam would be at elevation 115.80 m. 15.2.2 GEOLOGY: Shallow boreholes were drilled along the dam line with a combined depth penetration of about 5.60 m. which indicated that bed rock exposure at the dam line is sparse. Gravels, boulders and coarse sands occurring in a highly variable penetration are preponderant in the river bed and in the flood plains. The material at depth is pervious and the relative proportion of sand mix and gravel is highly variable.
15.2.3
AFFLUX & CREST LEVELS
The existing H.F.L. corresponding to the discharge rating curve for a design flood of 5320 cumcc as determined, is E1.117.93 IUR) at the dam site. The usual value of allowable afflux as a result of con· struction of the dam is 1.5 to 2.0 m. in foot hills, i.e., steep reaches of the river with boulders and
206
1
0.60 m. to 1 m. in plains (para. 7.3.2 of the Manual). For the Mag·Asawang Tubig, an afflux of about 1 m. has been provided keeping in view location of the dam and also to avoid excessive pumping during construction. According to the cross sections of the river, the deepest river bed in the portion of the under sluices is El.113.00 m. In the ogee portion, the average level is at El.114.0. Accordingly, the crests of under sluices has been kept at El. 114.00 and for ogee at El. 116.00, allowing a free board of 0.2 m. over the Pond level of 115.80, adopted from consideration of irrigating the service area to be commanded from the proposed dam.
15.3 HYDRAULIC DESIGN 15.3.1
DESIGN DATA
1. Index map showing the river bank 2 km upstream and 1 km downstream. 2. Location of the dam sites shown on the map with river profile and cross sections of the riVer upstream and downstream. (Refer to Fig. 15·1 - 15.5). ·
3. Photographs of the dam site including aerial photo contact prints. 4. Design flood - 5320 cumecs (100.year frequency). 5. Stage discharge rating curve at the site (Fig. 15.6.)
6. Lacey's silt factor f 7. Safe exit gradient
~
~
4 1/5
8. Retrogression of bed gressed)
Fig. 15.6 -
showing tail water rating curve (Retrogressed &
Unretro-
15.3.2 Determination of water way, afflux elevation, downstream basin elevation and downstream floor length under different flow conditions of spillway and sluiceway will be made as detailed out here under: A.
WATERWAY
The total water way lor the dam is decided by trial and error, and the water way for sluiceway as per guidelines under para. 7.4.5. In the present case, based on the actual topography at site, a total length of 440.00 m. has been adopted in order to have suitable alignment of guidebanks. 1. Looseness factor
As per Lacey's formula p
where
P Q
4.83 =
Looseness factor
JQ
minimum st3.blc waterway in. meters maximum flood discharge
p ~ 4.83 J5320
352.29 m
440.00 335.00
1.2
5320 cu. m/sec. Thus,
actual waterway Lacey's waterway
say 335.0 m.
L·I···
207
MAG-ASAWANG
TUB~G
RIVER
IRRIGATION
PROJECT
0
v 0
,'"-
~\
\ \
\
\
DAM
::r:
(!)
NOT CRAWN TO
LOCATION
PLAN FIG. 15.1
SCALE
EL.I25.00
I~
"
l:i: .......
I' I~
lr
z EL.I20 .00
t--
"
1-
"t;}
"'
'" "
0
!"--.
U1
li<; I~
... 10
"'-
-.......
0
I;~; I~
!'--
u
EL.II5 .00
10 I"'
·b.
I
I
"' U1
"- 1'
lo
I(J EL .110.00
otooo
0+200
o+soo
otsoo
o+400
1+000
RIVER BED PROFILE OF RIVER "A' SCALE :
H • I : 8000 MTS. V = 1: 200 MTS.
FIG.
15.2
LJ3 0
8
0
......
2!l
0
!"---
.
·-
~~
..
"'
1'- f-.,
I;';
2
:- 1---
1\\
1-
"'U1
" ~
1\
;; I~
~
£ u
EL .!Ia.
,,....
' r-
... ·Jf
1\
:It
.._ I " I--.
<
c<4
'1li -i
;;,
c- - ·-
c
'\
0 1\
L
u
L
0+000
ot200
0+400
0-+800
1 +ooo
1+200
RIVER BED PROFILE OF RIVER "B" SCALE:
"'
H= I : 8000 MTS. V= I: 200 MTS.
FIG. 15.3
'
EL. 12 5.00
In
~
+
t-......... EL. 120.00
"'
1'-.
...
" )'-., 1'-
!
10
b
...
~
0
.. + XI{)
r--....
!
EL.II!I.OO
- ---
0 0
1-'
~
1\
~0
\
t'--
v
r---
i:"
'\. f'. 1\ !-"
EL. 110,00
' ' !
.I
'
EL 105.00 0+000
0+200
0+400
RIVER BED PROFILE SCALE
0+600
0+800
1+000
OF !BOLO RIVER :aooo
H •
I
V=
1:200
MTS. MTS.
FIG. 15.4
il
t 22
L L &bGWG-MJJAlJZ.'
i
!&% ; &
lt200
EL. I 2!5.00
UNO R Ul ES 10 H: 40. 0
I:!I
WID
H
EE
0
'"
EL. 120.00
e- I'-
" "'
\ ~
\
I
/C fE TL OlE
~ U/!
:i, u
AP ON
EL
V.
a
RE T
""
~~
L
Sl
U/
A RO
S Ul
E
"
Ot400
--
.. -·
1--
--- --
--
~
z
"' ~-
-
!'\
f'\
r-'
E
FC F
OtSOO
! •
I
I Ot200
II~()(
DA
0
~
ELEVi OF
AY'
EL.IIO.OO OtOOO
-~.
t-i
/
E L.ll!5.00
0
[-.
a: '--" ·-
-
«
0
I
I
Ot800
I +000
I t200
I +400
CROSS- SECTION@ DAM AXIS(FACING UPSTREAM) STATION: I t !500 SCALE: H = I: 8,000 mrs V I: 2 00 mts
=
!
FIG. 15.5
!
LOW STAGE - 100 CMS- 1.20 M HIGH STAGE - ~320 CMS .30M
EL.I20.00 EL.II9.00 EL. 118.00 EL.II7.00 EL.II6.00 EL.I15.00 EL. 114.00 E L.ll3.00 EL.II2.00 EL.III.OO
RETROGRESSION RETROGRESSION
LiU~R T 'OE RESS D Tt IL~ A E
R TF OE RES SED T IL NA E R TI~G Cl~VI
1000
2000
!RAT NE
3000
4000
CURV
5000
6000
7000
DISCHARGE (CMS) SCALE
TAILWATER
1:200
MTS.
RATING
CURVE
FfG.
15. G
This falls with in the range specified under para. 7.4.2. 2. Width of undersluice & agee portion For the area to be commanded from two canals, one on each side of the river, two sluiceways are provided to create suitable pond. Intake works for the left and right side canal for discharges 7.959 cumecs, and 40.998 cumecs respectively shall also be provided. Assuming water way for sluiceway as follows:
a) Right portion 33.00 m.
6 bays of 5.50 m. each
7.00 m.
5 piers of 1.40 m. each
40.00 m. b) Left portion 11.00 m.
2 bays of 5.50 m. each 1•
1.40
1 pier of 1.40 m. w~,
12.40 m.
W net ~ 33.00 + 11.00 ~ 44.00 m.
W gross ~ 40.00 + 12.40 ~ 52.40 m.
Effective length of sluice way due to piers:
For the calculation of discharge in the formula, Q ~
C L H e 3t2
L is the effective length taking into account end contractions for the piers and abutments as per para 7.3.7. I
where
and
L
L
~
L"- 2 (N kp
~
effective length
+ ka) H,
L'
net length of the crest
N
number of piers
kp
pier contraction coefficient
ka
abutment contraction coefficient
H,
total head on crest
I}~ 44.00 m. N
kp
6
.V1 (refer to page 7-23 for contraction coefficients) 119.20 - 114.00
5.20 m.
(Where U/S EL.
119.20 & afflux. El.
~
based from the succeeding computations)
213
118.92
L'
~
44 - 2 [ 6 (.01)
~
43.38 m.
8
3. Length - agee portion
Width o[ two training walls of 1 m. each ~ 2.00 m. Length a[ ogee ~ 440 - 52.40 - 2.00 ~- 385.60 m. (Abutment contraction coefficient has been ignored, therefore take the net length of the ogee as the effective length which is equal to 385.60 m) B.
DETERMINATION OF AFFLUX ELEVATION, DOWNSTREAM BASIN ELEVATION AND DOWNSTREAM FLOOR LENGTH
The following abbreviations have been adopted working out the discharges over spillway and sluiceway. h,
for various parameters in the calculations for
head due to velocity of approach energy elev. - afflux elev.
He
head producing flow energy elcv.- crest elev.
da
depth of approach afflux elev.- u/s apron elev.
h,
afflux elev. - crest elev.
h5
tail water elcv. - crest clev. discharge per meter run
q
va
5320 440 ~ 12.09
velocity of approach
qid,
I.
First Condition: High flood condition, spillway not silted up
a)
Spillway Portion In Sketch 1 Try Energy El.
~
119.22 m.
P = crest el. - u/s apron el.
P/he
116.00 - 114.00
2.00 m.
119.22 - 116.00
3.22 m.
2.00/3.22
~
0.62
From Fig. 7 .1; C0 ~ 3.835 (discharge coefficient for free flow condition) hd
U/S Energy level - tail water level 119.22 - 117.63
~
1.59
214
.,
i&
d ~ tailwater level - dis retrogressed bed level 117.63 - 112.80
a)
~
4.83 m.
SPILLWAY PORTION UiS Energy IEL . :_:,~:::.....:1:.:1::,:9•::.22:_-:---- - - - - - - - - - - - - - - - - - - - - - 1 Afflux
EL.I~
..\ ·()
118.92
p~'--~~~~~~--~--DiS T .E.L. ~ 11:,.:7c.:.:.9'-'4'----,---Tailwater EL. ~ 117.63 (R)
:::::.....•c.........:..---'---:-"-- -:/
. Crest EL.
UiS Apron EL.
~
~
116.00:
/
114.00 DiS Bed Level
~
112.80
SKETCH 1
b)
SLUICEWAY PORTION
UiS T.E.L. - 119.20 Afflux EL.
~
118.92
---~D~iS~T~.E~.L~~~~11~8~.0~5_ _ _ __
Tailwater EL.
EL. 114.00
-U: ,:i: ,:S: ,:2Ap::,:r::,:on~E:.:L~.-~~11::,:3::,:.0::,:0_~~
~
117.63 (R)
Crest level
~~-----~~~~~~~~~~--~ DiS Bed Level ~ 112.80
.
SKETCH 2
215
&3
SkWJ
&;g;;g;_a
2
1.59 3.22
hd + d ~ 1.59 + 4.83 ~ 6.42
.49
~ 1.99 :o--1.7
6 .4 2 3.22
the downstream floor position has little effect on the coefficient of discharge which is solely by tailwater submergence.
l
~ ~
with
~
0.49 &
1.99;
c
I
% correction ~ O.S% (from Fig. 7 .4) Where Cs is the correction for submergence or decrease in coefficient for flow over submerged weir.
C,
~
(1.00 - 0.005) C0
~
0.995
X
3.835
~
3.81
In this, the modification in the value of the coefficient can be read directly from Fig. 7.6.
0.975
When
i.e. C,
3.74
q
Adopting value of C, q ~
~
3.74
3.74 (3.22)312 1.811 11.93 cms./m.
Try afflux El.
~
118.92 m.
119.22 - 118.92
0.30 m.
118.92 - 114.00
4.92 m.·
v A ~ ~ ~ 2.42 m.lsec. hA ~ va 2 I 2g ~ (2.42J2 I 2 (9.80) ~ .299 Adopt UIS Total Energy level Q ogee ~
11.93 (385.60)
~
~
119.22 m. & afflux El.
4600.20 ems.
I. First Condition
High flood with dam not yet silted up b)
Sluiceway portion Determination of UIS Energy Elevation Refer to Sketch 2
216
118.92 m.
With afflux El. = 118.92 m. (from the spillway computation) h, = 117.63- 114.00 = 3.63 (where 117.63 is the retrogressed level, assuming a retrogression of 0.3 m.) h, = 118.92 - 114.00 = 4.92 m . .!!s = 3 ·63 = 0 738 h, 4.92 . Refer to Fig. 7.7
C, I C 0.70 .10 [
J
.038
.8x56J
0.738 0.80
y
J
.078
.778
.078
.10 .038
y y
= .30
X
= .856 - .30 = .826 =
c, I c
C, = .826 (C) = .826 (1.706) = 1.409 Try Energy Elevation = 119.39 m. ha
=
119.39 - 118.92
=
.47 m.
H, = 119.39 - 114.00 = 5.39 m. da = 118.92 - 113.00 = 5.92 m. q = 1.409 (5.39)312 = 17.63 cmslm ~
17.63 (43.38) . = 2 47 5.92 (52.40) / . ha = (2.47)2 119.60 = .31 < .47 m. not O.K., since Assumed ha should be equal to computed h•. Try Energy Elevation
=
119.20 m.
ha = 119.20 - 118.92 = .28m. He = 119.20 - 114.00 = 5.20 m. da = 118.92 - 113.00 = 5.92 m. q = 1.409 (5.20)312 = 16.71 cms./m. 16.71 (43.38) . ( .40) 5 92 52
=
2.34 mlsec.
ha = (2.34)2 119.60 = .279 "'- .28 O.K.
217
Adopt U/S Energy Elev. ~ 119.20 m. & Afflux El. ~
Q sluice
16.71 (43.38)
Q ogee
11.93 (385.60)
Q TOTAL
5325.08 >
C.
118.92 m. for sluiceway.
724.88 ems. ~
4600.20 ems.
5320 ems O.K.
LENGTH AND LEVEL OF BASIN FLOOR
a) Spillway portion
From the previous computation;
q ~ 11.93 cms./m. U/S Energy Elevation ~ 119.22 m Afflux Elevation ~ 118.92 m. Tailwater Elevation ~ 117.63 m. (Retrogressed) 1. Determination of downstream energy elevation
Depth of water ~ 117.63 - 112.80 ~ 4.83 m. (112.80 being the bed level) 11.93 4.83
~
2.47 m/sec.
(2.47)2 119.60 ~ .31 m.
hots
Downstream energy elevation
tailwater elev. + hots 117.63 + .31 117.94 m.
I 'j
2. Determination of basin length:
!
HL ~ difference in energy levels at upstream and downstream ends of jumps ~
119.22- 117.94
~
1.28 m.
Accounting for 20% concentration (as per para. 7.5.9) q,
~
1.20 (11.93)
~
q,
Discharge per meter length with 20% concentration
D,
critical depth for flow considered
j
(14.32)2 9.80
2.76 m. 1.28/2.76
.46
218
,) Jf
4U&Z
I J i I
14.32 cms/m.
where
3
r
!JMi
hkiiMkli&ilkLJ&ki£& SMJ£1 I .( £,;;;;;;;QJ£ Jil£2 . ' j j &
Refer to Fig. 715 for values of D2 I D 1 & D1 I De corresponding to computed values of HL I De where D ~ depth at upstream end of jump 1 D ~ depth at downstream end of jump 2 By means of interpolation, exact value~ corresponding to computed HL I D,. which cannot be found from the values listed in Fig. 7.15 can be determined, as the case shown hereunder.
1.40 .46
.10
D 1 1 D,
Dzl D1
HL I DC
.541]
J
3.09] .06 X1 3.35
L.5o
'J
X2 .26 .516
"]
.025
for Dz I D1 .
:1
4$ ;
.025
.10 .06
.26 Y1
.10 .06
Yz
'1
Y1
.06 (.26) 0.10
~
Yz
(.06) (.25) .10
~
.015
.16 X
1
Dzl D1
3.09 + .16
.541 - .015
D1 I D,
xz
.526
3.25 forHLIDc
~
.46;
;
D2 1D 1 ~ 3.25 & D 1 I D, ~ .526
D I D, ~ .526; D 1 ~ .526 (D,l ~ .526 (2.76) ~ 1.45 m. 1
Dzl D1 ~ 3.25; D2 ~ 3.75 (D 1) ~ 3.25 (1.45) ~ 4.71 m. V
1
V
2
(velocity of flow at the beginning of the jump) q, I D1 ~ 14.32 11.45 ~ 9.88 mlsec. (velocity of flow at the end of jump) q, I D2 ~ 14.32 14.71 ~ 3.04 mlsec. Froude's No. (F)
F ~ 2.62 <
~ 9.881
I
2.62
J-9-.8-(_1._4-5)
4.5 Design of Basin Type IV has been adopted as per para 7.5.4.1
Refer to Fig. 7.14
219
•
~
for F Lb
~
~
2.62 ; Lb/D2
4.84
length of basin
4.84 (D 2 l 4.84 (4.71) 22.80
3. Basin floor elevation Asper para. 7.5.4.3, item i, the elevation of the basin floor may be calculated as follows; a)
Basin level where hvl
U/S Total energy line - D 1 - hvl head due to velocity of flow at the beginning of the jump vl2 /2g
Basin level
119.22 - 1.45 - (9.88)2 I 2 (9.80) 112.79 m. ~
b) Basin level where hv 2
DiS Total energy line - Dz - Hvz head due to velocity of flow at the end of the jump
Basin level
117.94 - 4.71 - (3.04)2 /2 (9.80) 112.76 m.
B) Sluiceway Portion:
Determination of Length and Level of basin floor q = 16.71 cms/m. U/S Energy Elev. = 119.20 m. ' Afflux Elcv. = 118.92 m. Tailwater El.
=
117.63 m. (Retrogressed)
1. Computing for energy elevation at downstream; depth of water ~ 117.63 - 112.80 ~ 4.83 m. V D•S ~ 16.71 113.381 ~ 2.86 m/sec., where .
4.83 (52.40)
52.40 m.; is the gross width h 01 s ~ (2.86)2 /19.60 ~ .42 m. ~
DiS Energy Elevation
117.63 + .42
2. Determination of Basin Length
HL
~
119.20 - 118.05
~
1.15 m.
With 20% concentration;
q,
1.20 (16.71)
D ~ 3) c Hr, .' D, ~
120.051' _ 9.80 -
~
20.05 cms/m.
3· 4 D, m.
1.15/3.45 ~ .33
Referring to Fig. 7.15
220
118.05 m.
By interpolation: HL I D,
c-30] .10 0.33 .03 0.40
D2 I D 1
D1 ID,
2.81] ~ Y1 0.28 x 1 3.09
0.572] 2 x 2 · Y .031 0.541
for D1 I D,
for Dz I D1:
Y!
.031 Yz
.10 .03
.28 Y!
.10 .03
J
.03 (.28) .10
Yz
.03 (.031) .10 .009
2.81 + .08 D I D, ~ .563; 1 D I D ~ 2.89; 1 2 v ~ q, I D 1 ~ 1 v 2 ~ q, I D 2 ~
~
2.89 & x 2
~
D1
D,
~
.572 - .009
~
D 1 ~ .563 (D,l ~ .563 (3.45) ~ 1.94 m. D2 ~ 2.89 (D 1) ~ 2.89 (1.94) ~ 5.61 m. 20.05 I 1.94 ~ 10.34 mlsec. 20.05 I 5.61 ~ 3.57 mlsec.
Froude's No. (F)~ V 1
jiT5i
~ 10.34 f9.80 (1.94) ~ 2.37
From Fig. 7.14 For F ~ 2.36; LB I D2 ~ 4.6 L 8 ~ 3.50 D2 ~ 4.60 (5.61) ~ 25.81
3. Basin Floor Elevation; a) Basin Level ~ UIS EE - D 1 - hvl 119.20 - 1.94 - (10.34P I 19.60 111.81 m. b) Basin Level ~ DIS EE - Dz - hvz 111.79 m.
221
.563
Afflux El.
~
119.48
rD/S T.E.L. (
117.92 _ _
Tailwater El. ~ 117.63
116.00
rCrest El.
rU/S Apron El.
~
114.00 DiS Bed level ~ 112.80 (assumed)
Sketch 3
11. Second Condition: High Flood with dam silted a) Spillway Pprtion (s•;e Sketch 3) In this condition wherein the upstream of the dam has been silted up to crest, the basic coefficient of discharge should be taken as for broad-crested weir which is 1.706 in metric units. However, the effect of submergence should be calculated as per values listed under Fig. 7.7. Assume afflux El.
~
119.48 m.
Try U/S Energy Elevation 119.48 - 116.00
3.48 m. ·
117.63- 116.00
1.63 m.
119.60 - 116.00
3.60 m.
119.60 - 119.48
.12m.
1.63 3.48
.17
222
§iW
£ =.Jdii@)@lJ
r
~
119.60 m.
.. ~HL
UiS T.E.L.
119.84
Afflux EL.
119.48 DiS T.E.L
~
.::.1..:::18::.:..2::9:___ _ _ _ _ _ __
Tailwater EL. ~ 117.63 (Retrogressed)
.. EL. 114.00
UiS Apron EL. ~ 113.00
DiS Bed Level ~ 112.80 (Retrogressed)
b) Sluiceway Portion - Second Condition - High Flood with dam silted up to Spillway Crest
Sketch 4
I II
Refer to Fig. 7.7 for values of C,IC By interpolation:
l
I
C, I C
h, I h, .10
.40 .470
[
J
.937
II
.10 .07
~
.019 y
y
~
.013
X
~
.956 - .013
.47; C, I C
c,
yJ .019
X
.50
for h 5 I h,_
J
.956 .07
~
q ~
.943
~
c
c, (H)
~
.943
.943 ~
~
.943 (1.706)
1.609
312
1.609 (3.60fl12 10.99 cmslm. da
~
119.48 - 114.00 10.99 .4 5 8
~
~
5.48 m.
2 .01 m Isec. .
ha ~ (2.01J2 I 19.60 ~ .21
Try UIS Energy Elevations
~
> .12 m.
not O.K.
119.70 m.
ha
~
119.70 - 119.48
~
.22m.
He
~
119.70 - 116.00
~
3.70 m.
q ~ 1.609 (3.70f312 ~ 11.45 cms./m. 11.45 .4 5 8
~
2.089 mlsec.
ha ~ (2.089)2 I 19.60 ~ 0.223 "" 0.22 Adapt UIS Energy Elevation
~
119.70 m.
& Afflux Elevation
~
119.48 m.
O.K.
Q ogee ~ 11.45 (385.60) ~ 4415.12 ems.
224
b) Sluiceway Portion (See Sketch 4)
Length and Level of Downstream Floor 1. Computing Energy Elevation
Adopt afflux El. ~ 119.48, as worked out for spillway portion in silted up condition h, ~ 117.63 - 114.00
3.63 m.
h, ~ 119.48 - 114.00
5.48 m.
h, I h,
~
3.6315..48 ~ 0.660
Refer to Figure 7.7 for values of C, I C computed h, I h, based on formula Q ~ 1.706 L H3i2 are By interpolation:
C, I C
h, I h,
0.907
[0.60 0.660
X
0.856
0.70 By ratio & proportion: .10 .06
X ~
If h, I h,
~
.907 -
C, where C
c,
I
031
~
,876
.660; the interpolated value of
C, I c
I
y
,031
~
y
.052
~
~
~
.876
.876 (CI; 1.706 (coefficient of Discharge for free flow condition)
~
.876 (1.706)
~
1.49
After performing various trials; Try Energy Elevation
~
119.84 m,
hn
~
119.84 - 119.48
~
0.36 m.
H,
~
119.84 - 114.00
~
5.84 m.
~
6.48 m.
•
c, H312 1.49 (5.84)3/2 21.02 cms./m. da
119.48 - 113.00
225
q x W net of sluiceway da x W gross of sluiceway
va
2 1. 02 (43 ·38 ) ~ 2.685 mlsec. 6.48 (52.40) ha ~ v0 2 I 2g ~ (2.685)2 I 2 (9.80) 0.368 "' 0.36 m.
O.K.
Adopt Energy Elevation ~ 119.84 m. & Afflux Elevation ~ 119.48 m .. Q sluice ~ 21.02 (43.38) ~ 911.84 ems. ~
Q ogee
4415.12 ems 5326 ems
Q Total ~ 4415.12 + 911.84
>
5320
0 .K.
2. Basin Length
From the previous computations: q
~
21.02 cms.lm.
UIS Energy Elevation ~ 119.84
Afflux Elevation
~
119.48
Retrogressed Tailwater Elevation ~ 117.63 (assuming 0.30 m. retrogression at high stage flow) Computing for Energy Elevation at Downstream:-
depth of water ~ retrogressed tailwater El.Assumed retrogressed bed level ~
'i
~
117.63 - 112.80
Velocity at downstream
~
4.83 m.
q x W net of sluiceway da x W gross of sluiceway
21.02 (43.38) 4.83 (52.40) 3.60 mlsec.
l
Velocity head, ho 1s
(Vo/sl2 I 2g (3.60) 2 I 2 (9.80)
hors
.66 m.
DIS Energy Elevation
Tailwater Elevation + horo 117.63 + 0.66 118.29 m.
226
HL
difference in energy levels at upstream and downstream ends of jump 119.84 - 118.29
Accounting for 20°/o concentration;
q,
~
~
1.20 (21.021
critical depth D,
25.22 cmslm.
~
4.02 m.
3
HL I D, ~ 1.55 I 4.02 ~ .386 Refer to Fig. 7.15 for values of D2 I D 1 & D 1 I D, corresponding to computed value of HL I d, By interpolation: ).
,,'
HL I D,
l ,;
D2 1 D1
D1 I D,
2.81
0.572
[ 0.30 .10 [086
0.386
J Y! 128
Xj
0.541
' 3.09
0.40
By ratio & Proportions:
..J.Q_ .086
.28
Y!
.24
Xj
Y!
2.81 + .24
..JQ..
.031
.086
Y2
~
Yz
.027
xz
.572 - .027
3.05 ~ D2 I D1
~
.545
~
xz
D 1 I D,
227
J Yz
]""
D1 D2
depth at upstream end of jump depth at downstream end of jump
D,
~
critical depth for flow considered
D1 I D,
~
.545; D1
.545 ID,l
~
.545 (4.02)
~
2.19 m .
3.05; D2
3.05 ID 1)
~
3.05 12.19)
~
6.68 m.
'lc I Dl
25.22 /2.19
q, I D2
25.22 I 6.68
Dz/ Dl
VI Vz
~
Froude's No. (F) ~ V1
JgD 1
From Fig. 7 .14; for F ~ 2.486 L 8 I D2 where L 8 Ln
~
~
length of basin
~
4.80 IDzl
~
~
11.52 m/sec. 3.78 m/sec.
~ 11.52/J 9.8 (2.19) ~ 2.486
4.8
4.80 (6.68)
~
32.06 m.
3. Basin Level
Basin Level
U/S Energy El. - D1 - hvJ h . ; w ere vl2/2g 119.84 - 2.19 - (11.52? /19.60 110.88 m.
Basin Level
Did Energy El. - hv2 - vl/2g 118.29 - 6.68 - (3.78J2 /19,60 110.88 m.
Adopt basin floor level
~
110.88 m,
a) Spillway portion
I
Length and Level of Downstream floor 1. Computing for energy elevation at downstream:
From the previous computation;
q
~
11.45 cms/m.
U/S Energy Elevation
119.70 m.
afflux Elevation
119.48 m,
Tailwater Elev.ation
117.63 m. (Retrogressed)
depth of water
117.63 - 112.80
~
228
4.83 m.
11.45 4.83
~
2 .37 m Isec.
H 01s ~ (2.37)2 /19.60 ~ 0.29
m. 117.63 + .29
Downstream Energy Elevation
117.29 m. HL ~ 119.70 - 117.92 ~ 1.78 m.
2. Basin Length
Accounting for 20% concentration; q, ~ 1.20 (11.45) ~ 13.74 cms/m. 3
j (13.7 4)2 9.80
~ 2.68
m.
H1, I D, ~ 1.78 I 2.68 ~ .664 Refer to Fig. 7.15
By interpolation: HL I D, [ .10
.60] .644
D 1 / D,
D 2 / D1 3.60]
.064
J
.22
Y!
Xj
xz .477
3.82
.70
.494] Yz
J
.017
For D2 I D 1 ;
y1
Y!
~
y2
.14 3:60 + .14 D2/D 1
:1::. '
.10 .064
.22
.10 .064
~
~
.010
x2 ~ 0.494 - .010 ~ .484
3.74
x2 ~ D1 I D, ~ 0.484
3.74 .494 D, ~ .484 (2.68) ~ 1.30 m.
D1 I D,
.484 ; D1
Dz I D1
3.74; D2 ~ 3.74 D 1 ~ 3.74 (1.30) ~ 4.86 m.
v1
~
q,!D 1
~
13.74 /1.30
~
10.57 m/sec.
V2
~
q,/D 2
~
13.7 4 I 4.86
~
2.83 m/sec.
Froude's No. (F) ~ v 1 / JgD1
~
~ 10.57 I )9.8 (1.30) ~ 2.96
229
II
Refer to Fig. 7.14 For F
~
2.96; Ln/D2
~
5.1
Ln ~ Length of Basin ~ 5.1 (4.86) 3.
I
24.79 m.
Basin Level
Basin Level
~
U iS EE - D1 - hvl 119.70 - 1.30 - (10.57)2 i19.60 112.70 m.
Basin Level
DiS EE - Dz - hvz 117.92-4.86- (2.83)2 i19.60 112.65 m.
Adopt Basin floor level
~
112.65 m.
Ill) Third Condition-Medium Flood Condition {See Sketch 5) Water Level on Upstream is just 0.30 m. {one foot} above the ogee crest
I
The 0 gee Crest
I
a) Spillway Portion
Gwux
El.
~
i!
116.30
Urest El.
~
116.00 Tail water El.
i
=
115.00
I
114.00
l
I
DiS bed level
l
l
112.80 (assum!f:
Sketch 5
+
1.0 Determination of UiS Energy Elevation Afflux El.
~
Ii
116 + 0.30 {Crest level + 0.30 m.)
230
116.30
d,
116.30 - 114.00
Try Energy Elevation
~
~
2.30 m.
116.3009 0.0009
116.3009 - 116.30 p
116.00 - 114.00 ~
~
2.00
116.3009 - 116.00
~
He
actual head
.3009
Ho
design head (head during high flood condition, where US!EE
119.22 m.)
3.22 m.
And from Fig. 7.1, discharge coefficient for free flow condition, C0 ~ 3.835 Since the crest has been shaped for a head higher than the head consideration, as per para. 7.3.7.2, item b, and Fig. 7.2, variation of the coefficientas related to values of Hc/H 0 are shown.
-{:-~ 0
c
-c-~ 0
q
d,
v,
.3009 3.22 .82;
c
~
~
3.14 1.811
~
.82 (3.835)
.82 C0 (.3009/312
116.30 . 114.00
~
(0.13)2 19.60
~ .00086
.29 (385.60)
~
3.14
2.30 m .
0.13 m/sec.
~
~
.29 cms./m.
.29 2.30
Adopt U/S Energy El. Q ogee
.093; Refer to Fig. 7.2
;::J
.0009
O.K
116.3009 & Afflux El.
116.30
111.82 ems.
b) Sluiceway Portion
Afflux Elevation
116.30 (since water is just 0.30 m. above the crest level)
(See Sketch on the next page)
231
i
! 116.30 116.00
Crest El.
I
Ots T.E.L. ~
Tail water El.
l l
__
115.42 ~ ·n5.20
El. 114.00'-'--'-----,_
1f]_E~L~11~3~.o~o
__________
~
El. 112.80
Sketch 6 1.0
Determine U/S Energy Elevation
Since tailwater elevation is not yet known, assuming that it is free-flow condition and use the formula, Q
I I
Ii
q
1.706 LH 312 or 1.706 H 312
After various trials;
Try Energy El. ha
~
116.44 m. 0.14
116.44 - 116.30
2.44 m. 116.44 - 114.00 He 312 q ~ 1.706 (2.44) ~ 6.50 cms./m. da
116.30 - 113.00 6.50 (43.38) 3.30 (52.40)
~
~
3.30 m.
1.63 m/sec.
ha ~ (1.63)2 /19.60 ~ .136 ':":! .14 Q sluice Q ogee
~ ~
6.50 (43.38)
~
281.97 ems.
111.82 ems.
Q Total ~ 281.97 + 111.82
393.79 ems.
1.1. Determination of Tailwater Elevation
From the tailwater Rating Curve, for Q ~ 393.79 ems; corresponding Unretrogressed Tailwater El. ~ 115.95 m, & Retrogressed Tailwater El. ~ 115.20 m. These values can be verified by referring to the Tabulation of Discharges at various elevations, through interpolation.
232
+&&
Discharge (ems)
Elevation (m)
.50
["'~"J
[ 198.201 215.29 393.79
195.59
y
413.49
116.00 .50 -y
~
215.29 195.59
y
.50 (195.59) 215.29
=
.45
El. x = 115.50 + .45 = 115.95 m. Unretrogressed tailwater El. = 115.95; & Retrogressed Tail water El. = 115.20 m. As the tailwater elevation is higher than the crest elevation of the sluiceway, therefore effect of submergence should be accounted for: With afflux Elev. = 116.30 m. & Retrogressed Tailwater El.
=
115.20
h, = 115.20 - 114.00 = 1.20 m.
h, ~ 116.30 - 114.00
=
2.30 m.
h,lh, = 1.20 12.30 = .52 Refer to Fig. 7.7
C, I C
"l:::1"' 0.60
.J2Q_
.10
Ji2 X
y
=
;y
=
.006
.02 (.03) .10
= .937 - .006 = .931
for h, I h, = .52 ; C, I C = .931
•
C, = .931 (C) = .931 (1.706) = 1.59
Try UIS Energy El.
=
116.42 m.
233
I .. ···-- :····:::·.. .....:..::
......
-~-..-...._
__
.._ _________ ___ ........_ .................
.,....
~
Tabulation of Discharges at Different Elevations
Section 0 + 550
s
"'""
0.011
n
~
0.04
AREA
ACCUMULATED AREA
PERIMETER
ACCUMULATED PERIMETER
113.50
4.0
4.0
10
10
0.40
2.622
1.423
114.00
16.0
20.0
8
18
1.111
2.622
2.813
56.26
ELEVATION
~
~
R
~AlP
K (ls 1h) n
V (KR213)
Q
5.692
36
1.00
2.622
2.622
94.39
0.765
2.622
2.193
114.04
74
. 142
0.685
2.622
2.037
198.20
(i4
196
0.878
2.622
2.404
413.49
330
526
0.639
2.622
1.945
653.52
712.0
118
714
0.997
2.622
2.617
1863.30
412
1124.0
228
942
1.193
2.622
2.949
3314.68
118.00
492
1676.0
104
1046
1.545
2.622
3.504
5662.46
118.50
468
3084.0
22
1068
1.951
2.622
4.094
8531.90
114.50
16.0
36.0
18
115.00
16.0
52.0
32
115.50
45.30
97.30
116.00
74.70
172.0
116.50
164.0
336.0
117.00
376
117.50
Lowest bed level
Q100
~
113.0. M.
5320 ems.
Tw ELEV. Q500 ~
~
~
117.93 (Unretrogressed)
6910 ems.
TW ELEV.
~
118.22 (Unretrogressed)
Average Lowest bed level Retrogressed bed level
~ ~
114.00 M. 112.80 M.
·•
68
ha
~
116.42 - 116.30
He
0.12 m.
116.42 - 114.00 2.42 m. 2 1.59 (2.42)31 ~ 5.99 cms/m.
q
da ~ 116.30 - 113.00 ~ 3.30 m. 5.99 (43.38) 3.30 (52.40)
1.503 m/sec.
~
ha ~ (1.503)2 /19.60 ~.115m. ~ .12 Adopt U/S Energy El. ~ 116.42 and afflux El.
Q sluice
~
5.99 (43.38)
~
116.30 m.
259.85 ems.
Q ogee
111.82 ems.
Q total
111.82 + 259.85
~
371.67 ems.
2. Determination of length and level of the basin floor.
a) Spillway Portion
116.3009 m.
1. Upstream Energy El.
afflux El.
116.30 m.
Retrogressed Tailwater El.
115.20 m.
depth of water
115.20 - 112.80
Determine Downstream Energy Elevation q ~ 0.29 cms/m, (from the previous computation) VDIS
~
0 2
2 ~ 0 ~ 0.12 m/sec.
~ (0.12)2 /19.60 ~ .0007
h01 s
Downstream Energy El.
115.20 + .0007 115.2007 say 115.20 m.
2. Determine basin length
H 1,
116.30 - 115.20 1.10 m.
With 20% concentration; q0
~
1.20 (0.29)
~
0.35 cms./m.
235
2.40 m.
De
J
3
HL I De
~
L35J2 9.80
~
1.1010.23
~
0.23 m.
4.78
from Fig. 7.15;
4.7.01
.08
.12
X!
4.80
10.01
~
D2 I D1
.08 (.002) .10 .0016
9.89 + .10
~
for HL I De
~
~
.2634 D 0
D2
~
9.99 D 1
~
~
9.99 x2
4.78; Dz I D1
D1
V1
~ ~
0.351 .06
~
~
D 1!Dc
9.99 & D1 I De ~
.2634 (.23) 9.99 (.06)
~
~
~
.265 - .0016 =
0.2634
0.06 m.
0.61 m.
5.83 mlsec.
V 2 ~ 0.351.61 ~ 0.57 mlsec.
hvi ~ (5.83)2 119.60 ~ L73 m.
hv2 ~ (0,57)2 119.60 ~ .02 m. Froude's No. (F)~ V 1 F
lfil\
~ 5.83 IJ9.80 (.06) ~ 7.60
> 4.5; Adopt Design of stilling basin 11
from Fig. 7.12 A: for F ~ 7.60; L 8 I D2 ~ 4.20 Length of basin, L 8
=
.002
Y2
.10 x1
_j
.002
.08 (.12) .10
~
X2
.263
.10 .08
.10 .08
Y!
Y2
Y!
4.78
.10
.265l
9.89]
4.20 (D21 ~ 4.20 (.61) ~ 2.56 m.
236
= .2634
;~ )~
8.28
Froude's No. (F)
~ 9.81 ~
X
0.868
2.84
From Fig. 7.14 C for F
~
2.84;
5.1 (3.06)
3.90 (D2 l 15.60 M 3. Basin floor elevation
i) Basin level
~
U/S EE - D1 - hvl 116.42 - .87 - 3.48 112.08 m.
ii) Basin level
DiD EE - Dz - hvz 115.42 - 3.07 - .28 112.07
Hence adopt basin floor El.
·;ijl.·.~.
IV
112.07 m.
Fourth Condition Low flood conditions i.e. Water flowing in the sluiceway is just 1.00 m. above the sluiceway crest
U /s TEL ~ 115.03 Afflux El ~ 115.00
Dis TEL
EL 114.00
~
114.60
Tailwater EL
EL 113.00
DiS Bed level
~
113.07 ~
112.30
Sketch 7
Since w'ter is just 1.00 m. above the sluiceway crest and the afflux El. will pass through the ogee portion, its crest level being at El. 116.00 m. Assuming various values of Energy El. and checking with ha by trial;
238
115.00 m., no discharge
3. Basin floor elevation
i) Basin level 116.3009 - 0.06 - 1.73 114.51 m.
DID EE - Dz - hvz
ii) Basin level
115.20 - 0.61 - .02 114.57 m. ~
Hence adopt basin level
114.51 m.
b) Sluiceway Portion
•
1. Upstream & downstream energy elevation
Upstream Energy EJ.· ~
Afflux Elevation
~
116.42 m.
116.30 m. ~
Retrogressed Tailwater El.
115.20 m.
Determine downstream energy elevation;
Depth of water
q
~
115.20 - 112.80
~
2.40 m.
5.99 cms/m.
~
5.99 (43.38) 2.40 (52.40)
~
2.07 m/sec.
ho1s ~ (2.07)2 /19.60 ~ 0.22 m. Downstream Energy EJ.
~
115.20 + ,0.22
b) Basin length
HL
~
116.42 - 115.42
1.00 m.
With 20% concentration; qe
~
1.20 (5.99)
a ~
7.19 cms./m.
I (7.19P
~
1.74
1.00 /1.7 4
~
0.575
v
HL/De
~
9.80
Refer to Fig. 7.14; for HL iDe ~ 0.575; D2 JD1 ~ 3.53; and D1iDe ~
~
D1
.4990 De
D2
3.53 D1
V1
7.19/0.868
Vz
7.19/3.06 ~ 2.35 m.isec.
~
.499 (1.7 4)
3.53 (0.868) ~
~
0.868 M 3.06 M
8.28 m/sec.
237
.4990
~
115.42 m.
1.0 Determine U/S Energy Elevation Try U/S Energy Elevation ~ 115.03 m. ha ~ 115.03 - 115.00 ~ .03 m.
He
~
115.03 - 114.00
~
1.03 m.
q ~ CH312 q ~ 1.706 (1.03)312 (adopting a free flow condition)
1.78 cms/m. da ~ 115.00 - 113.00 ~ 2.00 m.
1.78 143.38) 2.00 (52.40)
.74 m/sec.
ha ~ (.7 4)2 /19.60 ~ .028 "'- .03 Adopt U/S T.E.L ~ 115.03 & afflux EL. ~ 115.00 Q sluice ~ 1.78 (43.38) ~ 77.22 ems. Determine Tail water Elevation (See Tabulation of Discharges at various elevations) By interpolation Discharge (ems)
Elevation (m)
56.26l
El. 114.00] y
.50
20.96
El. X
[
[
77.22
38.13
94.39
El 114.50
By ratio and proportion
.50
38.13 20.96
y y
El. x
~
.27
114.00 + .27
~
114.27 m.
for Q ~ 77.22; Tailwater El. ~ 114.27 m (Unretrogressed) Taking into account a retrogression of 1.20 m. (as per para. 7 .3.6)
239
Retrogressed Tailwater EL ~ 114.27 - 1.20 ~ 113.07 m. Compute for the downstream energy elevation; water depth d/s ~ 113.07 - 112.80 ~ 0.27 m. 1.78 (43.38) 0.27 (52.40) hD/S ~
~
5.46 m/sec.
(5.46)2 /19.60 ~ 1.52 m.
DiS Energy Elevation ~ 113.08 + 1.52
114.60 m. Determine length and level of basin floor
2.0 Basin Length
HL
U/S EE - DIS EE 115.03 - 114.60 .43 m.
I
I
With 20% concentration; qc
~
1.20 (1.78)
~
3j (2.14J2 9.80 H L I De
~
.43 I .78
~
2.14 cms/m.
~ 0 78 .
m
.
.551
Refer to Fig. 7.15;
.50] .051 .551
.10
i
I I!
[ .60
.25
.10 .051
Yt
.051 (.25) .10 .12
.10 .051
.022 Y2
Y2
.051 1.022) .10 .011
x1 ~ D !D ~ 3.35 + .12 ~ 3.47 x 2 ~ D1!D, ~ .516 - .011 ~ .505 2 1
for HL!Dc ~ .551; D2/D 1 ~ 3.47 & D1 I D, ~ .505 D D
1
2
V1
~
.505 D, ~ .505 (0.78) ~ .39 m.
~
3.47 D 1 ~ 3.47 (,39) ~ 1.35 m.
~
2.14 I .39 ~ 5.49 m/sec.
V2 - 2.14 /1.35
~ 1.59
m/sec.
5.49
Froude's No. (F)
~
~ J9.80 (.39)
2.80
From Fig. 7.14 C; for F ~ 2.80; L 8 /D2 ~ 5.00 L8
~ 5.00
(Dz)
~ 5.00 (1.35) ~ 6.75
m.
3. Basin floor level
a) Basin level ~ UIS EE - D1 - hvl 115.03 - .39 113.10
(5.49J2 19.60
m.
b) Basin Level ~ DID EE - Dz - hvz 114.60 - 1.35 -
(1.59JZ 19.60
113.12 m. Hence adopt basin level ~ 113,10 m.
15.3.3 GOVERNING VALUES
For governing values to be adopted in the qes!gn (Refer to '!'abies 15.1 & 15.2)
15.3.3.4 Sluiceway
Based on the various con
Length of basin iil Dl iii) D2 iv) Basin floor level i)
110.88 m,
241
say 32.00 m.
Basin appurtenances
D1 & D2 are values required in determining the dimensions of basin appurtenances such as chute blocks, basin blocks, and end sill. For convenience in the calculations of such dimensions, Figure 7.12, 7.13 & 7.14 can be used.
The floor level of 110.88 if adopted, is likely to involve quite an excavation. But para. 7.5.5.1 of. the manual states "that if the raising of the floor becomes obligatory due to site conditions, the same could be raised up to 15% of D2 . The basin in that case should, however, be supplemented by chute and basin blocks. Therefore, revised basin floor level
~
110.88 + 0.15 D2 ::::. 112.00m.
15.3.3.5 Spillway;
The governing condition is the same as fdr the sluiceway.
i) ii) iii) iv)
Length of basin D1 ~ 1.30 m. D 2 ~ 4.86 m. Basin floor level
~
24.80 m.
~
112.65 m.
Due to small difference between retrogressed bed level and governing floor level, the floor can be raised up to retrogressed bed level which is equal to El. 112.80 m. Summary of the results of parameters worked out for both sluiceway and spillway are tabulated as Table 15.1 and 15.2 respectively. D.
CREST SHAPE
Coordinates of the ogee profile adopted for the spillway crest are worked out by the formula discussed in Chapter 7 para. 7.4.3.
I I I
Downstream profile (Based on the U.S. Army Corps of Engineers) Upstream profile
Y
0.724 (X + 0.270 Hd)L 85 + 0.125 Hd - 0.4315 Hct0. 375 (X + 0.270 Hd)0.625 Hdo.ss
X & Y coordinates thus computed are shown in Fig.15.7 along with the shape of the crest. E.
DEPTH OF CUTOFF
a) Spillway Portion
Depth of upstream and downstream cutoff walls from scour considerations. i) Upstream Cutoff In the formula, R
1.35 (
~
)!13 (Refer para 7.3.3) from the previous computation
242
i I
I
I
EQUATION FOR OOWSTREAM PROFILE,
X
X
1.8.5
Xl~!52.0
H4
~ 85
0 0
OF
q'ORIGIN
e
0
~.
COORDINATES ~
8
~
y
8
"'
0
"'~
a,.,
0
"'o
0
,.C\
0
"',.;
8 .;
.,0
,.;
8
"'
• !50 .20
0. 0!509
• 0 10
.2!5
0.0769
• 0 I !5
.30
0. 1078
.3!5
0. 14 34
. 021 . 029
~0
0. 1836
. 037
.45
0.2283
. 046
.50
0. 2174
.056
.60
0. 3887
.. 078
.10
0. 5169
.104
.80
0. 6618
. 133
.90
0. 8229
. 165
1.00
I. 000
.201
1.20
I. 401
.262
1.40
I. 864
4.00
1.60
2. :566
.374 .480
1.80
2. 967
.600
4.!50
2.00
3. 60!5
.124
2.50 3.00
5. 447 1. 633
I. 09!5 1.534
3.50
I 0. I !5 I
4.00
12. 996
2.163
16. 160
3 . 2!50
5.00 5.50
19. 638 23.42!5
3 . 949 4 .710
y
• .126 Hd •. 36
I I
1·50
''. I
'
2.00 'I
2.50 I I I!
3.00
'
,'!
I
'I 'I'' i 1 I'
I I
3.50 ;
''I
I I
I
'' I
I I I'
' i I.
''
I.
''''
!5. 00
2.041
4.50
FOR UPSTREAM PROFILE, X • 270 Hd •. 78
1·00
Hd • AFFLUX. EL.- CREST •118.92 -116.00 • 2.92 lol.
c
R E
s
T
I
p R 0 F I L E FIG.
15.7
I I
I
tI I
TABLE 15.1 SLUICEWAY PORTION Unit
Summary of calculations at sluiceway under different flow conditions
911.84
259.85
77.22
cms/m
16.71
21.02
5.99
1.78
cms/m
20.05
25.22
7.19
2.14
ems
2) Discharge intensity, q 3) q with 20% concentration
119.20
119.84
116.42
118.92
119.48
116.30
115.03 115.00
117.63
117.63
115.20
113.07
114.00
114.00
114.00
114.00
5.92
6.48
3.30
2.00
m/sec . m
2.34
2.685
1.503
.74
.28
.36
.12
.03
m
5.20
5.84
2.42
1.03
m m m
118.05
118.29
115.42
114.60
l
113.00
113.00
113.00
113.00
112.80
112.80
112.80
112.80
m
4.83
4.83
2.40
.27
m/sec,
2.86
3.60
2.07
5.46
I I
m m m
4) Upstream Total Energy Line 5) Afflux Elevation 6) Tailwater Elevation (Retrogressed)
m m
7) Crest Elevation 8) Depth of water upstream, da
•
10) Upstream ha 11) He = head above crest h0 12) Downstream Total Energy Line 13) U/S Apron El. 14) DiS Apron El, (assumed) 15). Depth of water downstream, d 01s 16) V018 17) hws 18) Head Loss, EL 19) D,
(4) (3) (2) Low Flood Medium High flood with Dam Flood Water water level is just 1.00 is .30m. silted up above the m. above the to crest ogee crest sluice crest level
724.88
1) Total Discharge, Q
9) Upstream Va
(1) High flood with Dam not yet silted
m
.42
.66
.22
1.52
m
1.15
1.55
1.00
.43
m
3.45
4.02
1.74
.78
.57
.551
.33
20) El< I D, 21) D2/ D1 22) D1 I D,
2.89
.368 3.05
.563
23) D1
.545
.505
.501
1.94
2.19
24) Da 25) V1 26l v 2 27) hvl
m m/sec. m/sec.
5.61
6.68
.87 3.07
1.35
10.34
11.52
8.26
5.49
3.57
3.78
2.34
1.59
m
5.45
6.77
2.48
1.54
28) hy
m
.65
.73
.38
.13
2.37
2.486
2.83
2.80
m m
3l) Basin floor level
i
3.47
3.53
m
29) froude's No. (F) 30) Length of Basin; L 8
II lI
.39
25.81
32.06
15.60
6.75
111.79
110.88
112.07
113.10
I j '
! I _\
i
I )
:!
l
'
244
-;;
'
=
ilh. L
2£
33
kiU!i144
! &.- 9:11
.£J&_Q Ri
; £&
I2
I
@}.$
TABLE 15.2 SPILLWAY PORTION Summary of calculations at spillway under different flow conditions
1) 21 3) 4) 51
Total Discharge, Q Discharge Intensity, q q with 20% concentration Upstream Total Energy Line Afflux Elevation
Unit
ems
8) Depth of Water, upstream, da 9) Upstream va
(2) (3) (4) High flood Medium Low flood with Dam Flood Water water level silted up is .30m is just 1.00 to crest above the m. above the Level ogee crest sluice crest
4600.20
4415.12
cms/m
11.93 14.32
m
119.22 118.92
11.45 13.74 119.70
• cm·s/m
6) Tailwater Elevation (Retrogressed) 7) Crest Elevation
(1) High flood with Dam not yet silted
m m
117.63
m m m/sec.
116:00 4.92 2.42
116.00
.22. 3.70
119.48 117.63 5.48 2.089
111.82 .29 .35 116.3009 116.30 115.20 116.00 2.30 .13
101 U/S ha 11) He ~ head above crest
m
.30
m
3.22
12) Downstream Total Energy Line 131 U/S Apron El. 14) DiS Apron El. (assumed)
m
117.94 114.00
117.92
115.20
m
114.00
114.00
m m
112.80 4.83
112.80 4.83 2.37
112.80 2.40 .12
151 Depth of water downstream, d 16) Downstrea, vD/S 17), hD/S 18). Head Loss, H 1, 19)/ Critical depth, De 20)! HL I DC 21) Dz/ D1
DiS
m/sec.
m m m
2.47 .31 1.28 2.76 .46 3.25
.29 1.78 2.68 .664 3.74
.0009 .3009
.007 1.10 .23 4.78 9.99
'
22) D1 I De 23) D1 241 D2 251 v1 261 V2 27) 28) 29) 30)
hvl hv 2 Froude's No. (F)
Length of Basin, L 8 31) Basin floor level
.526
.484
.2634
m
1.45
1.30
.06
m
4.71 9.88
4.86
.61
m/sec. m/sec. m m
10.57
3.04 4.98 .47
2.83 5.70 .41
5.83 .57 1.73 .02
m
2.62 22.80
2.96 24.79
m
112.76
112.65
245
7.60 2.56 114.51
q ~ 11.93 cms./m (from Condition 1 - High Flood water with dam not yet silted) f
~
4.0
Accounting for 20% concentration qc
~
1.20 (11.93)
~
14.32 cms.im.i
1.35 [ (14.432)2 ]1i3
Depth of scour, R ~
5.02 m.
·On the upstream side; allow scour
~
xR
~
~
1.1 R
1.1 (5.02)
~
5.52 m.
Required depth of cutoff (D) ~ xR - (Afflux el. - UiS apron el.)
·'- ~
~,
-~
.
p
5.52 - (118.92 - 114.00) 0.60 m. However provide a minimum of 1.50 m. deep cutoff. ii) Downstream Cutoff On the downstream side; allow scour
~
1.25R
~
1.25 (5.02)
~
6.28 m.
Required depth of cutoff (D) ~ 1.25R - (tailwater el. - dis bed level) 6.28 - (117.63- 112.80) 1.45 m. Provide 2.60 m. dis cutoff at El. 110.20 m. b) Sluiceway Portion
Depth of upstream and downstream cutoff walls from scour consideration i)
Upstream Cutoff q ~ 21.02 cms./m. (refer to previous computations)
With 20% concentration 1.20 (21.02)
~
25.22 cms.im. 1i3
R
On the upstream side allow for R Depth of scour (D)
~
7.31 m. ~
1.1 (7.31)
8.04 - (119.48 - 113.00)
Provide 2.00 m. uis cutoff
I
246
~ ~
8.04 m. 1.56 m.
iil Downstream Cutoff On the downstream allow for 1.25 R ~ 1.25 (7.31) ~ 9.14 m.
= 9.14 - (117.63 - 112.80) = 4.31 m.
Depth of scour (D)
However, the dis depth of pile is restricted to 4.00 m. for facility of excavation, being difficult in boulder reach. F.
LENGTH OF WORK & EXIT GRADIENT (Para 7.3.4) a) Spiliway Portion
Safe exit gradient ~ 1/5 (refer to Table 7.2) Maximum statio head (H) ~ 116.00 - 112.80 = 3.20 m. ~
2.60 m.
2.60 ~ 3.20
0.1625
Depth of downstream cutoff (D)
G
E
~ H d
1
X
7f
.{?:
Hence 1
"!i\ 1
7[
[A
GE X d = l. ~ ~H5 ~
0.1625 ~
0.1625 (n) ( J?l)
1
1 [ 0.1625 (n)
j""l;.-;;:;;'l 1 +
·I
o<:2
~ ~
r
2 /l
Q
3.84
- 1
[2 (3.84) - 1] 2
oC
J 44.62
«
b d b
X
- 1
6.60 (2.10)
~
~
6.60
17.16 m
44.62
Figure 5.11 of chapter 5 of Design Manual can also be used in determining any value of oc with the given value of 1 without any detailed calculations.
7fJlf Computed value of total length of floor (B) is less than the computed vlaue of the stilling basin floor; the total length of spillway is then based on other considerations: i.e., from required length of downstream apron and from stability. • ~
Adopted floor length
31.50 m.
The floor length shall be provided as below: 19.20 m.
Downstream horizontal floor
6.10 m
Width of ogee Upstream floor Total
6.20 m. 31.50 m.
b) Sluiceway portion
Total floor length and exit gradient Safe exit gradient
~
1/5
~
Depth of d/s cut-off (d)
4.00 m.
116.00 - 112.00
Maximum Static Head (H)
4.00 m.
__ 1_
4.00 4.00
1f[X'
0.20
From Fig. 5.11, Chapter 5 for
1 7f rx
~
0.20;
.X:
~
3.93;
/\ ~ 2.53
""~ ~ ; b ~ oe d ~ d ~ 3.93 (4.00) ~ 15.72 m. Stilling Basin Length
~
24.10 m.
However, to accommodate the glacis, u/s floor, total length of flow as 39.60 m. has been adopted as per details given on the next page:
248
-.\ ' &£ -.
1Jiik@
s ;;;gg;g; TI_ .:
_a~z.
_.&.. LU¥it
atJ££2
;J£..
The floor length shall be provided as below: 24.10 m.
Downstream horizontal floor
Downstream glacis length with 1:3 slopes ~ 3 (114.00 - 112.00)
6.00 m. 3.00 m.
Crest Width Upstream glacis length with 1:3 slope ~ 3 (114.00 - 113.00)
3.00 m. 3.50 m. (minimum)
Balance shall be for U /S floor Total
39.60 m. 112.80 - 4.00
Elevation of downstream cutoff G.
~
PROTECTION WORKS
As per requirements discussed under para 7.9, beyond the upstream and downstream of impervious floors, cement concrete blocks along with loose boulder or stone aprons are provided to protect the work against scour. a) Spillway Portion
Refer para7.9.2.1, while working out D. no allowance for concentration should be made in the discharge per unit length (q) in computing the normal scour R for the upstream and downstream protection works;
R
1.35 (
02
f
)113
where q
f
11.93 cms./m. (based on the governing condition for spillway portion) ~
4.0
R
4.44 m.
1) Upstream Block Protection
The length of upstream block protection should be nearly equal to D, which can be determined as uilder:
D
~
xR- (High flood level - floor level)
where
~
R
~
scour depth
~
x
~
multiplier (refer to Fig. 7.17)
xR
~
1.5 R ~ mean design scour for upstream portion (Para 7.3.3.1)
4.44 m.
.~ ~
-~
{~
.· ·.·~
:~
249
D
1.5 (4.44) - (118.92 - 114.00) 6.66 - 4.92 1.74 m.
Length of upstream block protection "'- D ~ 1.74 m. say 1.80 m. Provide blocks of 1.5 m x 1.5 m. x 0.9 m. cement concrete blocks, at least 2 blocks So block protection length ~ 2 x 1.5 ~ 3.0 meters Toe Wall ~ 0.6 m. Total length provided ~ 3.0 +. 0.6 ~ 3.60 m.
2) Launching Apron (Upstream)
As per para 7.9.4; beyond the block protection on the upstream and downstream of the dam, loose boulder or stones shall be provided as launching apron. i) Size of Stone: Velocity (v) ~
d
where
d q
,I
vuts
~
afflux el. - upstream apron el. 118.92 - 114.00 ~ 4.92 m. 11.93 cms/m (from the previous computation) 11.93/4.92 ~ 2.42 m/sec.
Referring to Fig. 7.18 Chapter 7; for v ~ 2.42 m/sec., Size of stone ~ 26 em. ~ .26 m. diameter Corresponding weight of stone ~ 25 kg. The weight of the stone can also be calculated by finding the weight of a spherical stone with the required diameter in -neter and using density of 2641.04 kgim3 for boulders.
Weight of stone Ws
.J
i.
7T
r3 x 2641.04
4 3
7T (
~) 3
3
2
X
2641.04
24.30 kg. Size of stone should not, however, be less than 30 em., and no stone should weigh less than 40 kg. Use 40 kg., 30 em 0 Loose stone protection as launching apron.
250
l
4L
.....ZkM&.g;ggg;QQMQZ£ &
ti.
WSL
Required length of launching apron (L)
4.34 - 1.80 2.54 m. say 2.60 m.
Provide 40 kg., 30 em.
Oloose stone protection in a length of 2.60 m.
3) Downstream Block Protection
R
~
4.44 m. (from previous computation)
Anticipated scour depth, xR
2.0 R (para 7 .3.3.1) 2.0 (4.441 8.88 m.
2.0 R - (Tailwater el. - retrogressed bed level)
d
8.88 - (117 .63 - 112.80) 4.05 m.
•
Length of block protection "' 1.5 D (para 7.9.2r 1.5 (4.05) 6.08 m. say 6.50 m.
Provide 6 rows of 1.5 m x 1.5 m x .90m c.c. blocks with 2 sets of 60 em. thick toe wall of concrete extending to about 60 em. below the bottom of the blocks for protecting the filter. The filter three rows should be provided with 7.50 em. gap in between, filled with gravel on 60 em. thick graded filter. Total length beyond impervious floor: Block protection
~
1.50 x 6
9.00 m.
X
2
1.20 m.
Gap between blocks
~
Toe wall
~
.60
.075 x 4 Total
0.30 m. 10.50 m. O.K.
>
6.50 m
4) Downstream Launching Apron i) Size of Stone
v
d
~
q/d tailwater el. - dis retrogressed bed level
251
ii)
'I
Apron Thickness
.90 m. equal to the thickness of the cement concrete blocks used.
Thickness of launching apron It) iii)
I
Apron Length
Required length of launching apron; assuming the slope of scour hole as 1.5 : 1 at the upstream
Possible settling of upstream stone protection due to effect of scour
If D be the depth of scour hole and n is the slope (H:V) then horizontal distance of deepest scour is nD (para 7 .9.4 .4)
Horizontal distance of deepest scour
1Ji.
(1.74)
1
2.61 m. Quantity of launched apron
j3.25 D say 1.80 D cum/m
Volume (V)
~
1.80 (1.74)
~
3.13 cum/m
Accounting for an increase of 25% for packing and losses; Vrequired
~
1.25 (3.13)
~
3.91 cum/m
Required length of launching apron (L)
Volume/m-strip thickness 3.91 cum/m .90 m .4.34 m.
The required length of block protection is 1.80 m. only. Since block protection has been provided in a length of 3.60 m., deduct the excess amount from the computed length of loose stone protection.
252
I
117.63 - 112.80 4.83 m. 11.93/4.83 ~ 2.47 m/sec.
Vct/s
Since Vu;s
r--
vd/s;
therefore. adopt same size
Of stone as
for launching apron on the upstream.
ii) Apron Thickness
Depth of toe wall ~ 1.50 m. Since block protection thickness is equal to .90 m., provide .60 m. thick filter bed underneath . Launching apron thickness (t)
.90 + .60
1.50 m. iii) Apron Length
Assuming a slope of scour hole as 2:1 for dis portion
1-----_::_1.::::...5Do:.,__-----1,
_j_ t
T
From previous computation;
D
~
4.05 m.
Horizontal distance of deepest scour ~
2 T (4.05)
8.10 m.
J5IT
Volume (V)/m-strip
say 2.25 D
With 25% F.S.;
v
2.25 (4.05)
9.11 cum/m.
V
1.25 (9.11)
11.39 cum/m
Required length (L)
11.39 cum/m 1.5 m. 7.59 m. say 8.00 m
253
'
Since actual length of non-launching apron is equal to 10.50 m., and the required length is only 6.50 m., there is an excess of 4.00 m., subtract this excess from the computed length of launching apron. _ ,_\'_!11
-~
Required length of loose stone protection ~ 8.00 - 4.00 ~
Provide 40 kg., 30 em.
•.
, +~L
-.
4.00 m.
0 loose stone protection in a length of 4.00 m. to serve as the launching apron.
__
b) Sluiceway Portion i)
Upstream Block Protection q ~ 21.02 cms/m (based from the governing condition for sluiceway portion) f
~
4.0
R ~ 1.35 [ (21~2)2 )1/3 6.48 m. Anticipated scour depth ~ 1.5 R ~ 1.5 (6.48) ~ 9.72 m. Scour depth (D) ~ 1.5 R - (afflux el. - u/s apron el.) 9.72 - (119.48 - 113.00) 3.24 m.
Length of U /S block protection
D ~ ·3.24 m. say 3.50 m.
Provide 3 - rows of 1.5 m x 1.5 x .9 m. cc. blocks with 60 em. thick toe wall extending to about 60 em. below the bottom of the blocks. Total length provided
~
5.10 m.
(1.50 x 3) + .60
2) Upstream Launching Apron i) Size of Stone q
21.02 cms/m.
d
afflux el. - u/s apron el. 119.48 - 113.00
v
~
6.48 m.
q X Wnct d X W gross
254
where ~
W net
Wgro.c;s
43.38 m. =
52.40
m.
21.02 X 43.38 6.48 X 52.40
v
2.69 m/sec.
Refer to Fig. 7.18 for v
~
2.69 m/sec.
Size of stone ~ 31 em. ~ Corresponding weight of stone
41.20 kg.
Since the corresponding weight of stone is greater than the minimum weight of 40 kg.; adopt 60 kg. stone which is the maximum allowable. -· ~
Size of stone
35 em.
0
ii) Apron Thickness Thickness of launching apron (t) ~
1.50 m.
depth of toe wall
iii) Apron Length Slope of scour hole for u/s portion
1.5:1
1.5D
POSSIBLE SETTLING OF UPSTREAM STONE PROTECTION DUE TO EFFECT OF SCOUR
1.5 D
Horizontal distance of deepest scour
1.5 (3.24) 4.86 m. Volume 'V)
J3.25 D say 1.80 D 1.80 (3.24)
~
5.8iJ cum/m
With 25% F.S.;
v
1.25 (5.83)
7.29 cum/m
255
-
------ ~----------·----~------·-------
7.29 cum/m 1.50 m.
Required length (L)
4.86 m Actual length of non-launching apron provided Excess provide ~ 1.60 m. Required length of launching apron
~
5.10 m. against 3.50 m.
4.86 - 1.60 3.26 m. say 4.00 m.
Provide 60 kg., 35 em. ¢ loose stone protection in a length of 4.00 m. 3) Downstream Block Protection
R
~
6.48
Anticipated scour depth D
~
~
2.0 R
~
2.0 (6.48)
~
12.96 m,
2.0 R - (Tailwater el. - dis retrogressed bed level) 12.96 - (117 .63 - 112.80) 8.13 m.
Length of block protection 1.5 D
1.5 (8.13) 12.20 m.
Provide 8-rows of 1.5 m x 1.5 m x .9 m cc. blocks with 7.50 em. gap, filled with gravels on 60 em. thick graded reverse filter and a toe wall of concrete extending to about 60 em. below the bottom of the blocks. Total length provided; Block protection
1.50
Gap in between
X
.075
Toe Wall
.60
X
8 X
12.00 9
0.675 0.60
1
Total
13.275 m.
4) Downstream Launching Apron i) Size of stone
q
21.02 cms./m.
d
117.63 - 112.80
v
21.02 I 4.83
~
~
4.83
3.60 m/sec.
From Fig. 7.18, Chapter 7; for v
~
3.60 m/sec
Size of stone
~
52 em. fJ
256 . J
The maximum allowable weight of stone is 60 kg. only. When larger size of stones are dictated as the case herein, additional rows of cement concrete blocks may be used. Adopt 60 kg .. 35 em. 0 loose stone protection li) Apron
Thickness .
Required thickness of launching apron (t) ~ 1.50 m. Iii) Apron Length
Slope of scour hole for dis portion
2:1
1.5 D
Horizontal distance of deepest scour ~
f (8.13) ~ 16.26
Volume (V) ~ 2.25 D ~ 2.25 (8.13) ~ 18.29 cum/m With 25% F.S.; V - 1.25 (18.29)
~
22.86 cum/m.
•
Required length (L) ~ 2 2 ·86 1.50 ~
15.24
m.
Providing 4 rows of 1.5 m-x 1.5 m x .9m cc blocks and 60 em. thick toe wall; Additional length of blocks
~
(4
x 1.50)
+ ,60
6.60 m. Actual required leng·th of loose stone
15.24
~
6.60
8.64 m say 9.00 m. Provide 60 kg., 35 em.
¢ loose stone protection in a length of 9.00 m.
257
H.
TRAINING WALL a) Hydraulic Design 1) Top Level
Max. afflux
~
119.48
Keeping the top of the training wall on the upstream at the same level. Top of wall
~
119.40
21 Determination of u/s Cut-Off
EL. 119.40
6.40
1.75 R
EL. 113.00 EL. 111.00
Assuming scour q
~
R
~
1.75 R
~
5320/440 12.09 cms/m 13 1.35 (q2 I !)113 ~ 1.35 [ (12.049) 1 ]
Q/L
7.84 (For 1.25 R, Dis negative)
1.75 R 1.75 R
~
(119.40 - 113.00) + D
D
~
7.84 - 119.40 + 113
D
~
2.00 mts
Bottom Level
~
113.0 - 2.0
Adopt
4.48 m.
1.44 mts.
!I
111.00
1
3) Determination of u/s Cut-Off
At the noses; Assuming scour depth
~
l
2 R, (foundation materials being mostly sand and gravel)
258
EL 119 40
"'
EL. 116.00
.--
2R · 3.00 m.
EL. 113.00
LJ
D
L
EL. 110.40
where:
R ~ 1.35 (q2ll)!i3 ~ 1.35 [ (12.~9)2 ] 1/3 2R
~
8.96 mts.
8.76
~
119.40 - 113.00 + D
D
~
8.96 - 119.40 + 113
~
4.48
2.36 mts. say 2.60 mts.
4) Determination of DIS Cut-Off
Keeping the top of the wall at Ei. 118.00, i.e., above the retrogressed F.S.L., i.e., 117.93 scour depth ~ 2R
,-------
EL 118 00
EL. 117.93 (Unretrogressed)
5.13 m. 2R
'm ;'~'
EL. 112.80
LJ
L
259
D EL. 108.95
where: R
~
1.35 (q2 1 f)113
~
~
2R
~
8.96 mts.
D
~
8.96 - 117.93 + 112.80
1.35
~
UseD
2 (12 ·09 1 4
[
~
]113 ~
4.48 mts.
3.83 mts.
3.85 mts.
b) Protection Works 1 l Downstream Portion
Mean Scour
~
2.0 R
where R ~ 1.35 (q2 1 £)113 2.0 R
~
12.96 mts. ~ 9A3 ftlsec. (2.875 MISec.)
(21.02) (3.28) (44) 5.83 (52.40) ~
size of stone weight
~
270 lbs
~
D
~
I I
0
15" ~
122 kg.
use 65 kgs. or 14" 2.0 R
(21.02P 113 ] ~ 6.48 mts. 4
1.35 [
!.
0
117.93 - 112.80 + D 12.96 - 117.93 + 112.80
~
I
7.83 mts.
I I
L ~ {5(D) ~ 17.51 mts;
tl
~
(1.50
1.25
X
14 ( _
12 3 281
) ~ 0.66 say 0.70 mts.
Volume~
Lt1
~
1.50 D
17.51 (0.70) ~
~
1.50 (7 .83)
Volume 1.5 D Use concrete blocks
~
12.26 ~
11.75 12.26 11.75 ~ l.0 4
1.50 x 1.50 x 0.90
in a length of 1.5 D Upstream Portion
R
~
6.48 mts.
2.0 R
~
12.96 mts. 21.02 (3.28) 119.48 - 113.0
21.02 (3.28) 6.48
260
10.64 ft./sec. (3.24 MISec.)
Stone
~
Weight
15" 0 ~
270 # s
122 kgs.
use 65 kgs. or 14" 2.0 R
119.48 - 113.0 + D
D
12.96 - 119.48 + 113
L
}3.25D ~ 11.68 mts.
1.50 X 14 ) tl ~ 1.25 ( 12 (3.28) 11.68 (0.70)
Vol.
¢
Vol 1.5
6.48
0.67 "" 0.70 mts.
8.18 8 ·18 1.5 (6.48)
~ 0.84 mts.
Use concrete blocks 1.50 x 1.50 x 0.90 in a length of 1.0 D
15.4
STRUCTURAL DESIGN PRESSURE CALCULATIONS: (Spillway Portion)
a} Spillway Portion - For determining uplift pressures, according to Khosla's theory, it is essential to assume the floor thickness at the upstream & downstream cut-off.
The assumed length and floor thickness are indicated in the Fig. given below.
B H=3.20¥.
A
\
~·=":..:·':::'2:.:.·"'=-+-t--+--'1'-
:1·~
~
f-
140 170
EL.112.1!K)
t __
W~! IOOlJOO,I00"-1"''++-,,---'1
.-:-----~-:-----i--'",;J
\ +------·~---.......---'-------------~:..::""=·•cc•_":.:.·i------+------1----l ro I 10'11 •"" ~ 2•o 1- ,..,~~
~
Sketch H
261
Referring to para. 5.7 of the Manual b
~
total length of floor
~
31.50 m.
b'
distance between two piles
d
depth of pile on which the effect is to be determined
D ~ the depth of the pile line, the influence of which has to be determined on the neighboring pile of depth d (adopted as 1.5 m.)
C ~ correction to be applied as percentage of head 19
J~
X
d + D b
H ~ head acting ~ crest level - dis basin floor level
I '
In order to determine uplift pressures acting on the floor, the % pressure at upstream and downstream sheet piles are worked out. The pressure distribution from upstream sheet pile line to downstream sheet pile line is assumed to be linear. % Pressure at key points
i
i)
Upstream Pile Line 1.50 m.
114.00 - 112.50 31.50 m. 1
d b
~
1.50 31.50
~
048 .
Refer to Fig. 5.10, chapter 5 14% 20% % of pressure at the bottom of the sheet pile ~ 100 % of pressure at the boLtom of the floor ~ 100
0E 100 - 0E 100 -
'·
-1·
~
0o
0E
100 - 14 ~ 86%
~ 100 -
20 ~ 80~o
a) Correction for thickness The thickness of the floor at the location of the sheet piles are tentatively assumed for correcting the values of 0, and (1,2 in the upstream and 0E in the downstream. If t is the floor thickness at U/S sheet pile of depth d, correction due to floor thickness ~ (0 01 - 0c 1 l which is positive.
f
1
Assume 0.30 m. floor thickness UIS; f/Jc 1 correction for thickness ~
114.00 - 113.70 1.50 1.20(-r) 262
(86-80)
b) Correction due to mutual interference of sheet piles is worked out by the following formula:
c~ ¢c 1 correction
0c 1 corrected
[f.
19
d
X
~
D
due to interference of the 2.nd pile line; D
113.70 - 111.30
2.40 m.
d
113.70 - 112.50
1.20 m.
b'
1.40 m.
b
31.50 m.
c
19
fflo
(
0
80 + 1.20 + 2.84
1.20 +2.40 31.50
2.84 (+)
84.04%
ii) Intermediate Pile Line
b
31.50 m.
b1
1.70 m.
d2
114.00 - 111.30 m. (including floor thickness)
-,,
~
base ratio
;;
1.70 31.50
horizontal distance between U/S pile and intermediate pile I total floor length
.054
b dz
31.50 2.70
11.67
The value of 0c 2 can be read directly from Fig. 5.10 (Sheet pile not at end) for given values of "'- and base ratio~ . To find 0E 2 for the known vaue of oe. and base ratio~ read for base ratio 1 - ~ for that value of oe and subtract from 100. To get 0o 2 for ~- less than 0.50, read ¢; 0 for base ratio 1 oC and subtract from 100.
)}]. b
Qc C/JE2
.05 ;
11.67 1
oC ~
72.50%
~
bl
b
.95
0c 2
100 - 0c: 0c ~ 7% 100 - 7
~
93%
~D2
100 -
¢D2
100 - 20.50
00
~ 20.50% ~
79.50%
a) Correction for thickness If tz is the floor thickness at downstream sheet pile of depth d2, the correction ~ .,1;- (9E 2 which is negative. -
263
002 )
1::2 ~ 114.00 - 112.30 ~ 1.70 m.
70 (93 - 79.50) ~ 0E2 correction for thickness ~ 1. 2.70 8.50 m. ( - ) b) Correction for interference of UiS pile line, ~
113.70 - 112.50
~
1.20 m.
d
113.70 - 111.30
~
2.40 m.
c
19
D
+ 1.20 {Wo ( 2.4031.50 .
0E 2 corrected For
1.54 (-)
93 - 8.50 - ; .54 ~ 82.96%
0c 2 . '
a) Correction for thickness
d2
1.70 m. d2
1.70 ( ) 2 .70 79.50 - 72.50
~
4.411+)
2.70 m.
b) Correction for interference of DiS pile line
Il I l
~
112.30 - 110.20
2.10 m.
d
112.30 - 111.30
1.00 m.
b'
28.10 m.
b
31.50 m.
D
j
19 ~C2 ~
,
+
2.10 ) 51 ( ) ( 1.00 31.50 ~ . +
2.10 28.10
corrected " 72.50 + 4.41, + .51
~
77.42%
Ill} Downstream pile line
Assuming 1.00 m. thickness for dis floor;
I I
d3
~
112.80 - 110.20
b
~
31.50 m.
~
2.60 m.
28.80 m.
b1
1
,ia_
oC
b
2.60 31.50
~
083 .
Refer to Fig. 5.10 0E ~ 27% ¢E3
Oo
~ 18% ~
903 264
23 2 3 .Ji
.. ;£!1i.Wk11lik j ·~I d•
L2
.&
i&l4tLLL Jiktilmu& 2 EtC 2 i.
I! 1
112.80 - 111.80 (27 - 18) 2.60
a) Correction for thickness
3.46(-) b) Correction for interference of intermediate pile line; d ~ 111.80 - 111.30 ~ 0.50 m. d ~ 111.80 - 110.20 b'
=
28.10 m.
b
~
31.50 m.
c
~ 19
j 28.10 0.50
1.60 m.
( 1.60 + 0.50 ) ~ 0.17 (-) 31.50
c) Slope Correction (Refer to para 5.73) for slope 1 in 1.5
By interpolation (from Table 5.6) 0/o
Slope 1:
'JJ
1:.2
1
/z
1:
1~/z
of correction
1 6.5
1:
1 .50
J
4.7 y
y ~
'
'}'
.50 (4.7) 2.35
X
~
11.2 - 2.35
8.85%
for slope fo 1 : 1/z
c
~
8.85%
L'
~
28.30
actual correction
OE3
corrected
8.85
X
1. 65 -- .~r-2 ( + 28.30
27 - 3.46 - .17 + .52
~
23.89%
265
Thus the pressure calculated at key points are given below;
Intermediate Pile Line
Upstream Pile Line
fiEl 100%
0m
llc1
86%
84.04%
OE2 82.96%
0D2
Downstream Pile Line
flc2
79.50%
0E3
77.42%
23.89%
0os 18%
0cs 0
Pressure Calculations
b) Sluiceway Portion
By ratio &
proportion elevation at different points on the sloping glacis are determined.
t
OF CREST
€L.U2,00
I
23.10
5
.60 m.
b
Sketch I i) Upstream pile line
dl b
1 oC
113.00 - 111.00
2.00 m. (including floor thickness)
39.60 m. dl b
2.00 39.60
.051
266
From Fig. 5.10, chapter 5 ~D ~ 14%
20%
j\E
¢m
100 - 14
~
86%
¢cl
100 - 20
~
80%
Assume U/S floor thickness ~ 1.00 m. a) Corrections for thickness ~
t:gg
(86 - 80) ~ 3.00 ( + )
b) Correction due to interference of downstream pile line D
112.00 - 108.80 ~ 3.20 m.
d
112.00 - 111.00
b'
38.00 m.
~
1.00 m.
39.60 m.
b
~
c
~ 19
j 38.00 3.20
( 1.00 + 3.20 ) ~ 0 58 ( ) + 39.60 .
Since slope correction is only applicable in cases wherein intermediate piles are provided, therefore, this correction will not be treated in here: ~Cl Corrected ~ 80 + 3.00 + .58 ~ 83.58%
ii) Downstream pile line
Assuming 1.50 m. thickness; d2
~
112.00 - 108.80
b
~
39.60 m. d
1
~ b
DC
3.20 m.
3.20 ~ 39.60- ~ ·081
From Fig. 5.10 (chapter 5, Design Manual) ~E
26%
0E2
0D
18%
¢D2
a) Correction for thickness
l.OO 3.20
(26 - 18) ~ 2.50 (-)
267
b) Correction due to interference of U/S pile line
D
~
111.00 - 111.00
~
0
No interference
0Ez corrected ~ 26 - 2.50
23.50%
Downstream Pile Line
Upstream Pile Line
¢£1
001
9cz
100% 86% 83.58%
J.
~E2
¢oz
23.50%
18%
~C2 0
FLOOR THICKNESS
1. Hydraulic gradient & Jump Profile ~;, .·.·.·.;.1
The floor shall be designed for the following two conditions 1) Uplift pressure due to static head.~ 2) Uplift due to hydraulic jump. 2. Uplift Pressure under static head
~' l···
:_~_:
:i
In the first condition, the floor should be adequate to resist uplift and soil reaction. Based on the uplift pressure worked out in the foregoing para., the level of hydraulic gradient lines at key points under different flow condition a) during high flood; b) under normal flow operation with water at the crest level, worked out at key points for both spillway & sluiceway portion. These are tabulated in Table J-1. In view of the varying requirement of concrete sections as indicated in the sketches, the thickness of floor shall be determined at a number of points A, B, C, D, E, F, G, H, E1, Ez, D1 , Dz, etc. as shown in Sketches H & I and as such hydraulic gradient line (sub·soil pressure worked out at then points for spillway & sluiceway are given in table J·2, J-3 respectively. 3. Uplift due to Hydraulic jump.
When water overflows the crests, with the formation of hydraulic jump, high unbalanced pressure are developed in the Stilling Basin Trough. In determining the pressure in the jump limited Complete Water Surface profile of the standing wave is plotted with the help of Fig. 15.8 15.9. Knowing q and Er at different locations where Er is the specific energy in the formula.
. Corresponding values of Dare read from Fig. 15.8 and thus the water profile before jump forma· tlon can be plotted. In plotting water profile after jump F (Froude Number) is determined by
268
15
EIH~RGY OF FL0\!1 CURV~S
'
I \
~
l\
I~ ol.t 0
I
2
3
4 Di"-1'1"H
5
6
7
8
9
{0)-{m)
FIG. 15.8
10
rr;2,,.,
6
•F,~ 6
~
..
~
/_
-p
2
17/ 0
~ /7
0
~
-
,~ !-
/
~ ./~ /
.....
/ / lr'l' ''" -1~u~P
NO
•,2' eJ
!.....;.,~,.~
/ t
~
10
15
20
25
30
L 01
RELATION BETWEEN lliE LENGTH
a THE HEIGHT CF JWP
FIG. 15.9
TABLE J - 1 The level of the hydraulic gradient lines at keypoints under different flow conditions. Spillway Portion
Condition
Downstream water level
Upstream water
(Datum)
level
m.
m.
(1)
(2)
High
117.63
0Dl
iflc1
0E2
ifloz
1/Jcz
iflE3
0D3
0c3
m.
100%
86%
84.04%
82.96%
79.50%
77.42%
23.89%
18%
0
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
119.48
1.85
1.85
1.59
1.56
1.53
1.47
1.43
.44
.33
119.48
119.22
119.19
119.16
119.10
119.06
118.07
117.96
3.20
2.75
2.69
2.65
2.54
2.48
.76
.58
116.00
115.55
115.49
115.45
115.34
115.28
113.56
113.38
.. 112.80
DiS Pile Line
iPEl
Flood Normal
Height/Elevation of subsoil H.G. Line above datum Intermediate Pile Line U/S Pile Line
Head
116.00
3.20
Operation
117.63
112.80
Sluiceway Portion Condition
Downstream water level
Upstream water
Head
(Datum) (1)
(2)
(31
(4)
High
117.63
119.48
1.85
Flood
Normal Operation
112.00
116.00
Height/Elevation of subsoil H.G. line above datum Upstream Pile Line Downstream Pile Line
4.00
QlEl
Q)Dl
1/)Cl
I/JE2
0D2
0cz
100% (51
86% (6)
83.58% (7)
23.50% (8)
18% (9)
0 (10)
1.85
1.59
1.54
.43
.33
119.48
119.22
119.17
118.06
117.96
4.00
3.44
3.34
.94
.72
116.00
115.44
115.34
112.94
112.72
-
117.63
112.00
TABLEJ- 2 Spillway Portion
(1)
(2) Horizontal distance from U/S end of floor
%
(5) H.G. Line Elevation during high flood ~ T.W.E. + col. 4 ~ 117.63 + (4)
(6) Height of sub.soil ~ Hyd. grad. line ~ H x col. 3/100 where H ~ 13.20 m. IN or mal op.)
(7) Hyd. grad. lim elev. during normal condi tion ~ D,S floor level t (6) ~ 112.80 + (6)
m
El
-
100
1.85
119.48
3.20
116.00
Dl
-
86
1.59
119.22
2.75
115.55
C1
.30
84.04
1.56
119.19
2.09
115.49
E2
.30 + 1.40
~
1.70
82.96
1.53
119.16
2.65
115.45
79.50
1.47
119.10
2.54
115.34
2.40
77.42
1.43
119.06
2.48
115.28
70.18
1.30
118.93
2.24
115.04
1.30
118.93
2.24
115.04
68.66
1.27
118.90
2.20
115.00
8.00
66.75
1.23
118.85
2.13
114.93
9.00
64.84
1.20
118.83
2.07
114.87
10.00
62.94
1.16
118.79
2.01
114.81
61.03
1.12
118.75
1.95
114.75
1.70
J
C2 A
1.70 + .70
~
2.40 + 3.80
~
(
B
•
(4) Height of subsoil ~ Hyd. line ~ Hxcol 3/100 where H ~ 1.85 M. (During high flood)
Point
D2
I.
(3) Pressure Percentage
c D
6.20
6.20 6.20 + .80
'
70.18 ~
7.00 + 1.00
7.00
~
~
'
E
8.00 + 1.00
F
9.00 + 1.00
G
10 + 1.00
H
11.00 + 1.30
~
12.30
58.56
1.08
118.71
1.87
114.67
HI I
12.30 + 2.00
~
14.30
54.75
1.01
118.64
1.75
114.55
14.30 + 3.00
~
17.30
49.03
.90
118.53
1.57
114.37
J
17.30 + 5.00
~
22.30
39.51
.73
118.36
:j
1.26
114.06
K
22.30 + 5.00
~
27.30
29.99
.55
118.18
.96
'I'
113.76
L
27.30 + 2.00
~
29.30
26.18
.48
118.11
.83
•
Ea
29.30 + 1.20
~
30.50
23.89
.44
ll8.07
.76
113.56
'-l
Da
30.50 + .40
~
30.90
18.
.33
117.96
.58
113.38
Ca
30.90 + .60
~
31.50
0
-
117.63
-
112.80
J
i ;;
' ,'j
j
~ ~
11.00
.
,,I
1
272
.
113.63
'1&1'1'
. ..
>~<
TABLE J - 3 Sluiceway Portion (1)
(2)
(3)
Horizontal
Pressure
distance from
Percentage
(4) Height of Subsoil
~
Hydraulic Gradient Line Elev.
Hyd.
gradient line
~
U/S end of
High flood
Normal Opera-
floors
condition
ting condition
H
~
119.48 -
H
H x col. 3/100
~
(7)
(6)
(5)
116.00 -
117.63
112.00
1.85 m.
~
~
T.W.E. + 4
~
~
117.63 + col.4 + col. 5
DIS flood el.
High Flood
Normal Opera-
condition
ting Condition
(m)
(m)
Point
(m)
El
-
100
1.85
4.00
119.48
116.00
Dl
-
86
1.59
3.44
119.22
115.44
C1
.60
83.58
1.54
3.34
119.17
115.34
A
.60 + 2.90
~
3.50
78.99
1.46
3.16
119.09
115.16
B
3.50 + 3.00
~
6.50
74.25
1.37
2.97
119.00
114.97
c
6.50 + 3.00
~
9.50
69.50
1.29
2.78
118.92
114.78
D
9.50 + 2.00 11.50 + 3.00
~
E
~
11.50 13.50
66.34 63.18
1.22 1.17
2.65 2.52
118.85 118.80
F
13.50 + 2.00
~
15.50
60.02
1.11
2.40
118.74
114.65 114.52 114.40
Fl
15.50 + 1.00
~
16.50
58.44
1.08
2.33
118.71
114.33
Fz
16.50+1.00
17.50
56.86
1.05
2.27
118.68
114.27
F3
17.50 + 1.00
~
18.50
55.28
1.02
2.21
118.65
114.21
F4 G
18.50 + 1.00
~
19.50
53.70
.99
2.14
118.62
114.14
19.50 + 1.00
~
20.50
52.11
.96
2.08
118.59
114.08
H
20.50 + 5.00
~
25.50
44.21
.81
1.77
118.44
113.77
I
25.50 +. 5.00
~
30.50
36.30
.67
1.45
118.30
113.45
J
30.50 + 5.00
~
35.50
28.40
.52
1.13
118.15
113.13
Ez
35.50 + 3.10
~
38.60
23.50
.43
.94
118.06
112.94
Dz Cz
38.60 + .40
~
39.00
18
.33
.72
117.96
112.72
39.00 + .60
~
39.60
-
-
117.63
112.00
%
~
0
~
.
273
4.00 m.
..
ikii+V
if?it!W
' ''VW1
TABLE J - 4 SPILLWAY PORTION
A) Prejump profiles - High Flood Condition Upstream Total Energy Level ~ 119.70 m.
Distance from Upstream End
Elevation at point of consideration
1
2
C) 7.00 m.
116.00
Energy of flow (Ef) ~ U/S T.E.L. - 2
q
~
13.7 4 cms./m.
3
4 3.70 m
D) 8.00 m D1 ) 8.28 m
115.80 115.68
3.90 m 4.02 m
E) 9.00 m
115.28
4.42 m
F) 10.00 m
114.47
5.23 m
G) 11.00 m
113.39
6.31 m
H) 12.30 m
112.80
6.90
De~
Dl
Location of critical depth De
~
~
,,
5
3.08 m.
116.00 + 3.08
~
119.08
2.70 m 2.68 m
115.80 + 2.70 115.68 + 2.68
~
118.50 118.36
1.99 m 1.64 m
115.28 + 1.99 114.47 + 1.64
~
117.27 116.:l_l
1.40 m
113.39 + 1.40
~
114.79
1.30 m
112.80 + 1.30
~
114.10
El. 115.68; 1.28 m. from C of the crest
274
Elevation of Water surface
Depth of Water, (D) from Energy of flow curves (Fig. 15.8)
~ ~
TABLEJ- 5 SPILLWAY PORTION
b) Post jump profiles F F2
Point
Distance from the toe of glacis ~ X
~
~
DiS T.E.L. = 117.92 m
2.96
DiS Floor Level = 112.80 m.
8.76
X
y
Dl
Dl
y
Elevation of water surface
Hl
2.00
1.54
1.50
1.95
112.80 + 1.95
H2
3.00
2.30
1.81
2.35
112.80 + 2.35
·•
H3
4.00
3.08
2.10
2.73
112.80 + 2.73
I
5.00
0.84
2.30
2.99
112.80 + 2.99
J
10.00
1.69
3.15
4.10
112.80 + 4.10
K
15.00
11.53
3.38
4.39
112.80 + 4.39
L
17.00
13.08
3.43
4.46
112.80 + 4.46
c3
19.20
14.77
3.63
4.72
112.80 + 4.72
275
= = = = = = = =
114.75 115.15 115.53 115.79 116.90 117.19 117.26 117.52
TABLE J -6' SLUICEWAY PORTION a) Prejump Profiles During high flood condition Distance from Elevation at Upstream End point of consideration
Energy of flow (Efl UIS T.E.L. ~
Elevation of water surface
Depth of Water, (D) from energy of flow curves
119.15 m
Bl
6.50 m
114.00
5.84
5.15
114.00 + 5.15
~
Cl
9.50 m
114.00
5.84
4.10
114.00 + 4.10
~
118.10 m
C11 10.04 m
113.82
6.02
4.02
113.82 + 4.02
~
117.84 m
Dl 11.50 m
113.33
6.51
3.07
113.33 + 3.07
~
116.40 m
El 13.50 m
112.66
7.18
2.68
112.66 + 2.68
~
115.34 m
Fl 15.50 m
112.00
7.84
2.19
112.00 + 2.19
~
114.19 m
Location of Critical depth, D
~
De
Dl
~
~
El. 113.82,@ .54 m. away from the end of the sluice crest.
b) Post jump profiles
F2 F Point
~ ~
DIS T.E.L.
2.486 6.18
Distance from the toe of glacis ~ X
~
118.29
DIS Floor Level
X Dl
-
'y
~
112.00
y
Elevation of water surface in meter
Dl
Fl
1.00
.46
1.05
2.30
112.00 + 2.30 ~ 114.30
F2
2.00
.91
1.25
2.73
112.00 + 2.73 ~ 114.73
F3
3.00
1.37
1.43
3.13
112.00 + 3.13 ~ 115.13
F4
4.00
1.82
1.62
3.54
112.00 + 3.54 ~ 115.54
G
5.00
2.28
1.81
3.96
112.00 + 3.96 ~ 115.96 ~
H
10.00
4.57
2.26
4.96
112.00 + 4.96
I
15.00
6.85
2.57
5.63
112.00 + 5.63 ~ 117.63
276
116.96
F
9
jgD 13 Knowing F2, the relation between the abscissa ordinate of the profile can be read from Fig. 15.9. For spillway the values for prejump & post jump profiles are given in Table J4 & J-5, & for sluiceway in Table J -6.
4.
Floor Thickness
With the help of plots of H.G. Line and Water Surface profiles, the design uplift pressures can be evaluated. The ordinates measured from the hydraulic gradient line to water surface are the uplift pressures for which lhe floor is to be designed. The uplift pressure which will occur with the maximum pond level upstream and no flow downstream should also be determined (as shown in fig. 5.10 & 5.11). The requirement of floor thickness is worked out by taking the larger of the two uplift pressures and dividing it by the submerged density of floor material i.e. (G-1) . In this example, specific weight of concrete is taken as .144 pcf. and the specific weight of water as 62.40 pel. Submerged density of the floor material
144 - 62.40 62.40
~
1.30
a) Spillway portion
;I
1. Downstream floor 1
~
It would be clear from the drawing that the static head governs the thickness of the spillway floor up to 9.20 m. from the downstream floor end while beyond it, the dynamic condition governs the thickness because of the larger value of the ordinates after applying the requirement of 2/3 of the head. In view of the tendency of water into the jump trough rolling backwards, thus reducing the unbalanced head, the 2/3 value is considered adequate for working out the floor thickness. i) at point Ea
Unbalanced head
.54 m.
Static head
.76 m.
The static head is more than the unbalanced head. Floor thickness required ~
.76 ~ .58 m. 1.30
Taking into account a 10% factor of safety as per para 7.8.1 Floor thickness
~
1.10 (.58)
~
.63 m.
For construction purposes provide 65 em. floor thickness at 1.00 m. from downstream end of floor. ii) at point K
Unbalanced head
.99 m ..
Static head
.96 m.
277
Static head still governs, since design unbalanced head is 'l/3 of the actual unbalanced head, which is lesser Floor thickness required ~ ·9 6 ~ .73 m. 1.30 With 10.% F.S; t
~
1.10 (.73)
~
I I
.80 m.
Provide 80 em. floor thickness at 4.20 m. from dis end of floor
1··2 i
I
iii) at point J Unbalanced head ~ 1.46 m; 2/3 (1.46)
.97 m.
~ !}.13
Static head 1.26 m. governs Floor thickness required ~ }.26 ~ .97 m. 1.30 . With 10% F.S.; t
~
~
1.10 (.97)
1.07 m.
Provide 110 em. thickness at 9.20 m. from dis end of floor iv) at point I Unbalanced head ~ 2.7 4 m; 2i3 (2.7 4) Static head
~
1.82 m.
1.57 m.
Here, unbalanced head governs Floor thickness required ~ 1.82 ~ 1.40 m. 1.30 With 10% F. S.; t
~
1.10 (1.40)
~
1.54 m.
Provide 160 em. floor thickness at 14.20 m. from dis end of floor
v) at point Ha Unbalanced head Static head
~
~
3.04 m; 2i3 (3.04)
~
2.02 m. governs
1.63 m.
Floor thickness required
~
1.10 x
!:~~ ~
1.70 m.
Provide 170 cm.·floor thickness at 15.20 m. from dis end of floor vi) at point H 2 Unbalanced head Static head
~
~
3.45 m; 2/3 (3.45)
2.30 m. governs
1.69 m.
Floor thickness required
~ 1.10 x i:~~ ~ 1.94 m.
Provide 195 em. floor thickness at 16.20 m. from dis end of floor
278
1-
}
j j j j j j j j j j
'
j
.4$
...
ll .f3
ll .lt-4
1.411
1.11
.
'
·r·
j
Ctn2
j
n
j
II 2.00
Jo
i'
j j
Ita
j j j j
L ·O
000
~·o
40
[)
j j
fj
j j j j
J
j j j
"".so
2e.•o
ao<>
j
••
j j j j j j j j j j
: \y )
j j
----------------------------------------------~F~IG_..~~5~·-"------~,Jj j
-----.!!!!--..
='"'·""'=--~~·-..............------: j j
j j j
U/ S T. £ L.. • I ~I · lA !II
. 118.8!1
118.80
Ill .74
11!1.71
118.&8
.s.ee
118.!19
I
I
t'"""
~ ..
! . 48 4.:55
•5 .!12
nlao ...,....
2 .65
••.o• .a.]···
I TI:S
. 70
I
lUUI
I
..
118, 82
.!12
. 41 1 117.8ll
117.85
117. 80
.. --ll·;:.s·
T. f.L . • Wl.lt
111.
Jlr.w. f. L . • 117 . 81 Ill .
117. 8-ll
3 .08
11 .o8
, 2.21
2 .SI
2.14
I
2.08
l.l ll
c1 112 .u . 72 112.00
4
i
22!1
.14
n.. 112.00
f· 2:!0
II . 84
1.48
I
I
280
.4!1 II . IS
I
1!10
:oo
12!1
...
l e.L.
::c:Q_
200
200
100
100
!!>_0
300
1100
S IO
!100
:S CIO
J
J
-.U~d------;:~------b--_;____~-.t;;;--'1• 21.40
88.!10· 88.8!1
88.54
83 .18
eo.o2
!18 .44
:58 .88
11821
113.70
JUMP
AND
44.21
82 .11
38 . 80
DRAULIC
23.~
341 .30
Ill.
-
UPLIFT
PRESSURES
(SLUICE WAY)
I
FIG . 15 . 11 --------~ - ~------------------~----------------------------___;,...;..;._..;..;;.....;...__ _--1\
--- -....
--
ue.IO 19.08
ua.oo
•••
117.
II&. 1\e..OO
EL.II6.QQ (CREST LEVEL)
-==-
11:34. 11:3.34 _ _ _ _ _ _ _ __
~
" -~•--:r::-.;----'--1. . 17 H 81
2.44
~--....,p~....::.:.........u~o!!YJll cl
2.\8
II S ,00
D
El 100
cl
!)I
200
•oo •.
too
60
•oo
·-
l
I
I
.. a3.&S
••
100
78.88
74.215
ea. oo. 6&. 615
I
•.
·~'·'
I •
J ." ~
0' ..,,
-- ·-:-- ----
•• • • ·,
---
._.., .. --.-~ -·~--,.~-----
..
U/ S
T. E. L. . :
119. 70 M.
HYO.
118.83
118. 79
118.78
118. 71
118 . ~
118. 88
U8 .60
118 . 57
118.53
--
GRA DI ENT
L I NE
FOR
MAX.
118.36
U/ S
:I_
1.46 2.7-4
El.. 114.00
117. 63
II . li
II .iO
3 .04 3. ee
.w. £ L.. •
/
....:
2 .88
RMo\1. CONOITIONl.
11 8 .18
T. £1. . : 117 . 82
3 .45
3. 8i
4.81 4. 53
_j[ I
HYO.
114 . 43
114.37
I. 63
1.117
QRAOIENT
LINE
FOR
NO
A 1.-n!
1. 81
.87
1.6$
F
.: " ( - --
ll l5. 76
1.26 112 .80
11 3 38
I
.88
·"
'
.76
c~
·-------~~----~.'----+--~ 11 2.80
H;s
Hz
HI_
Ho
11 3 . 63 113. 58
·I 0 160 260
EL.. 110. 20~
450
...
'~
245
165
+ t
~
2 00
200
-100
-L
~
000 - ---------
I
I
,•
_1'0. 18
68.00
68.75
64.94
.,•
39.51
8 .37
62 .94
23
26.18
29 . 99 :J 0 1
....
4 20
6 1.03
56 .66
.116
52.114
54.75
!!10.94
'1&.03
520
31.~
HYDRAULIC
JUMP
100
.. .
AND
UPLIFT
PRESSURES
{SPILLWAY) FIG . '15.10
/
..
-··~--·------= --:;;;:=-s~jiji!'!QW!'£::!'•~~~~~~ ~~·.,..,. .-.
.. . . --~--~-~~·~ """="'""'"__,_._.._
....._.4.___ ..._...~-
-
-------~---------
---
-~--4·-·
-
___
.___,_..~
AFFLUX EL . .1 119.48M.J,
----c _. Z.&! M
1.04 EL. 114.00
r
~ +- ,.o 1"" 1
r-~--~~r~o~---t~----------~·~•~o, __________~~-----~•··~·~---
•
n.••
....,.
_10.18
79.!:10
68.00
66.70
82,96
l>G
100
140
uo I
I
vii) at point H 1 Unbalanced head
Static head
=
~
3.89 m; 2i3 (3.89)
~
2.59 m. governs
~-.
1.75 m.
Floor thickness required
~
1.10 x
i:;~ ~
2.19 m.
Provide 220 em. floor thickness at 17.20 m. from d/s end of floor
viii) at point Ho Unbalanced head ~ Static head
~
1.81 m.
Floor thickness required
•·.~·: '!
3.02 m. governs
4.53 m; 213 (4.53)
1.10 x
~:~~ ~ m~
Provide 255 em. floor thickness at 18.20
2.55 m from dis end of floor
ix) at point H Unbalanced head Static head
~
~
4.61 m; 2i3 (4.61)
3.07 m. governs
1.87 m. 1.10
Floor thickness required
X
3.07 1.30
2.60 m.
Provide 260 em. floor thickness at the toe of the glacis.
2) Upstream floor
The subsoil hydraulic gradient line is below the water level, i.e., all the unbalanced head acting on the floor is counterbalanced by the self weight of the water. Thus, only a nominal thickness of about 4·5 ft., i.e., 1.6 to 1.7 m. is provided.
bl Sluiceway Portion
a) Downstream floor i) at point E 2 Unbalanced head Static head
~
~
.43 m.
.94 m. governs
Floor thickness required
~
1.10 x
i~4 ~
0
.80 m.
Provide 80 em. floor thickness at 1.00 m. from dis end of floor. ii) at point
J
Unbalanced head
.52 m.
281
Static head
~
1.13 m.
governs
~ 1.10 x i:~~ ~ .95 m.
Floor thickness required
Provide 95 em. floor thickness at 4.10 m. from dis end of floor. iii) at point I Unbalanced heaC: Static head
~
~
.70 m.
1.45 m.
governs
)
Floor thickness required ~ 1.10 x 1. 45 ~ 1.22 m. 1.30 Provide 125 em. floor thickness at 9.10 from dis end of floor. iv) at point H Unbalanced head Static head
~
~
1.48 m.
1.77 m.
governs
Floor thickness required ~ 1.10 x 1.n 1.30
~ 1.50 m.
Provide 150 em. floor thickness at 14.10 m. from dis end of floor. v) at point G Unbalanced head ~ 2.63 m.; 2i3 (2.63) Static head
~
i ·I
l
1.75 m.
1.10
X
2.08 1.30
1.76 m.
Provide 180 em. floor thickness at 19.10 m. from dis end of floor. vi) at point F 4 Unbalan.ced head ~
l l
2.08 m.
Floor thickness required
Static head
I
~
3.08 m; 2/3 (3.08)
2.14 m.
Floor thickness required
governs
~ 1.10 x
~
i:;t
2.05 m.
l I
I
~ 1.81 m.
Provide 185 em. floor thickness at 20.10 m. from dis end of floor viii) at point F 3 Unbalanced head Static head
~
~
3.52; 2i3 (3.52)
2.34 m.
2.21 m.
Unbalanced head governs Floor thickness required ~ 1.10 x 21.30 ·34 ~ 1 .98 m . Provide 200 em. floor thickness at 21.10 m. from dis end of floor 282
,_:y
viii) at point F 2 3.95 m; 2/3 (3.95)
Unbalanced head Static head
~
• 2.63 m. governs
~·
2.27 m.
Floor thickness required
~ 1.10 x i:~~ ~ 2.22 m.
Provide 225 em. floor thickness at 22.10 m. from dis end of floor. ix) at point F 1 Unbalanced head Static head
~
~
4.41 m; 2/3 (4.41)
2.33 m.
Floor thickness required
~
~
2.94 governs
i:;6
1.10 x
~
2.49 m.
Provide 250 em. floor thickness at 23.10 m. from d/s end of floor. x) at point F Unbalanced head Static head
~
~
4.55 m; 2/3 (4.55)
~
3.03 m governs
2.40 m.
Floor thickness required
1.10
X
3.03 1.30
2.56 m.
Provide 260 em. floor thickness at the toe of the glacis b) Upstream floor· (See procedure for floor thickness as for spillway portion)
K. STABILITY ANALYSIS
As discussed in Chapter 8 of the Manual, in the structural design of the solid gravity over flow dam, the section of the dam shall satisfy the following requirements of stability. i) ii)
The section should be safe against sliding. The section should be safe against over turning.
iii) Unit stress in the material of the structure or pressure on the foundation, should not exceed the permissible limit. Testing the dam section as per sketch on the next page for the following combination of forces.
·~!:~
Combination A
Dam completed but no pond water & no tail water.
Combination C
Maximum flood with dam silted to crest level.
Combination E
Water level on upstream at crest level, tail water at retrogressed floor elevation, silt upstream & with earthquake. 283
. eo
4.50
3.30
2.00
0
Wz
1
----1
w.;
1- ________ "ti:_::2._,Eo=.L:.e.llc-c5".2"'e_Jll__
I I I 3
.•
El.l14.00
WI
2
~.47-1
I I I I I I
.20
I I
0
______ T _______ _"'0
El. 112.30
4
I I I
I El. 111'.3oJ}
iJI::L. 111.20
I 7
I
.:so+~
ol g!
0 N
El.110.20
~------------~6~.~9~5_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __l_ __l~1~.6~5_ _~8i_________________lj9 2.00
Dam completed but no water in pond and
Load Combination A:
no tailwater
Area of the section & C.G. AREA (sq, m) 1.70
X
~
4.5
y (From toe) m.
1.70 2.10 + -2-
7.65 ~
/z (.40) (.80)
1
.16
Ay
~
2.95
2.10 + 1.70 + 1.60 + ~
~
3.30 (.80)
~
(.72) (2.00)
A4
1/z
A5
2.00 (2.98)
As
1
A7
2.64
~
~
1
Ag
2.10 (2.00)
LA
27.728
~
~
~
~
2.10 + 2.98 + 2.98 2.10 +
3.84
1.65
/z (1.10) (1.65)
As
.72
5.96
/z (2.48) (3.10)
.50 (3.30)
2.10 +
3 0 :
4.20
3
5.53
-~2 ~
(.40) .884 9.90
3.75
5.32
3.83
--z:-
3.59
21.40
2.60 +
2.48 3
3.43
13.17
2.10 +
.50 2
~
2.35
2 1.00 + 3(1.10) 2.10 -2~ 1.05
.908
22.568 1
~
1.73
3.88 1.57 4.41 81.61
Y
8 61 ~ 27.728 1. ~ 2.94 m. from the toe
•
285
DAM WEIGHTS & MOMENTS
Taking 1 -metre strip
l 1
l
I
Moment@ Toe (Kg.m.)
Lever Arm (M)
Dam weights (Kg_)
wl
7.65 (2400)
~
18360
4.25 3.30 + 2.00 + .80 + -2-
W2
0.16 (2400)
~
384
5.30 +
-3-~
5.57
w3
2.64 (2400)
~
6336
5.30 +
-~0 ~
5.70
w4
0.72 (2400)
~
1728
3.30 +
w5
5.96 (2400)
~
14304
3.30 +
w6
3.84 (2400)
~
9216
.20 +
32
W7
1.65 (2400)
~
3960
3.:0
~ 1.65
Ws
0.908 (2400)
Wg
4.20 (2400)
LW
~
~
~
2179.2
10080
.80
32
(2.00)
2.00 2
(3.10)
2.00 +
1.65 3
2.00 2
1.00
~
~
~
8.35
153306 2138.88 36115.2
4.63
4.30 ~
~
2.27
8000.64 61507.2 20920.3 6534
~
2.55
5556.4 10080
LM ~ 304158.62 (
66547.2
•
286
Foundation Reactions:
304158.62 66589.88
X=
<x
10.60 ~ 3.53 3
L 3
f toe~
LW -s
~
4.57 m. from the toe
.. within the middle third
( + ~ 1
B
where
e
~
B/2 10.60 2
ftoe
=
fheel =
x - 4.57
~
66589.88 10.60
[
8877.86 Kg./m 2
<
LW
1 +
(1-
6e) B
6.6589.88 10.60
[
3686.26 Kg./m 2
<
If
0.73 6 (0.73) 10.60
32290 Kg./m 2
1 -
] Qb
6 (0.73) 10.60
]
32290 Kg./m 2
O.K.
287
•
§
At
rm
2EZ££LZZL
I
Load Combination C: Maximum flood with dam silted up to crest level Afflux El.
119.48 m.
~
Tailwa tcr El.
~
117.63 m.
A} Static & Uplift Pressure
Point
Net head
Pressure in Kg./m 2
3.48· 3.88
3.48
3480
3.88
3880
5.48 5.48 •
5480 5480
Pressure
Available Head, in m. ~ W.S. el. - elev. of point, ~ h
Percent~
age(%)
119.48 - 116.00 119.48 - 115.60
0 1 2
5 6 7 9 10 11 . 12
l
7.18 7.18
82.96 77.42
117.90 - 112.30
5.60
65.51
3.69
3690
115.77 - 111.20
4.57 5.57
62.37
2.85
62.37
3.47
2850 3470
3.90 1.30 1.40 1.99
58.56
2.28 1.30 1.40 1.99
2280 1300 1400 1990
115.77 114.10 114.10 114.20 117.27
8
~
5.48 5.48 5.96 5.56
119.48 119.48 119.48 119.48
3 4
~
-
-
114.00 114.00 112.30 112.30
110.20 110.20 112.80 112.80 115.28
~ ~ ~
~
5960 5560
B) Silt pressure
It is assumed that silt and water mixture is completely fluid, the resulting horizontal pressure is assumed to be equivalent to that of a fluid having a unit weight of 85 pel (1360 Kg./m3) and the vertical pressure with 120 pel (1925 Kg.Jm 3)
As the water thrust has already been considered, an additional thrust due to the fluid of density 22.60 pel (363 Kg./m3} for horizontal pressure and 57.60 pel (924 Kg./m3) for vertical pressure may be added. Thus, for silt pressure, h
116.00 - 114.00
~
2.00 m.
PH
363.0 x 2.00 ~ 726 Kg./m2
Pv
924 x 2.00 ~ 1848 Kg./m2
(See tabulation of summation of stabilizing and overturning Moments)
288
Summary:
356720.3 + 304158.62 - 347815.1 ~ 313063.82 Kg. m.
2: MR
2830.7 + 66589.88 ~ 69420.58 Kg.
Z:V
~
X=
~
21120.2 Kg.
~ 4.51
313063.82 69420.58
>
3
~ 3.53 O.K.
10 60
(within the middle third)
B
e
- 4 51 -X= 10.60 2 .
2
0.79 m.
Foundation Reactions: ftoe
=
LV B
9477.7 Kg./m2
69420.58 10.60
6e B
( 1 +
<
32290 Kg./m2
B
B
3620.5 Kg./m 2
<
1 + 6 (.79) 10.60
]
O.K.
69420.58 10.60
( 1 - ~)
L,V
[
32290 Kg./m2
1 -
[
6 (.79) 10.60
]
O.K. Safe
(W- U) tan 0
Factor of Safety Against sliding. F 1
p
where:
total external forces acting downward + Total weight of dam itself ~ 46914.5 + 66589.88
w
~
U
~
total uplift force
~
P
~
total horizontal force
113504.38 Kg .
•
44083.8 Kg. ~
21120.2 Kg.
Assuming a gravelly sand material. 20% gravel, maximum size is 3/4" and with a relative density of 70%, the corresponding value of tan fi ~ .77 from Table No. 8.1. F1
~
113504.38 - 44083.8) (0.77) 21120.2 2.53
>
2.00
289
Allowable Factor of Safety for this loading combination ~ 2.00 F1
>
F,
O.K.
Factor of Safety against overturning 660878.92 347815.1 1.90 > 1.5
O.K.
..
Safe
Load Combination E - Water level on upstream at crest level, dry weather tailwater, full uplift, silt deposit up to crest level with earthquake. A)
Static and Uplift Pressures
Available Head, in m. Point
~
W.S. el - of point
Pressure
Net Head
Pressure
Percentage
h in m.
(in Kg{m 2)
(%)
0 1 2
0 116.00 - 115.60 116.00 - 114.00
.40 2.00
3 4
2.00 116.00 - 112.30
3.70
5 6 7
3.70 116.00 - 112.30 116.00 - 111.20
8 9 10 11 12
116.00 116.00 116.00 116.00
-
0
~
82.96 77.42
3.70
65.51 62.37
4.80
110.20 110.20 112.80 112.80
~
5.80 5.80 3.20 3.20
116.00 - 115.28
~
.72
~
62.37 58.56
.40
400
2.00 2.00
2000
3.07
3070 2860
2.86 2.42
2000
2420
2.99
2990
3.62 3.40 3.20
3620 3400 3200
3.20
3200
.72
720
i
!r
I I I
from the previous computation, silt pressure values are:
PH ~ 726 Kg./m2 Pv ~ 1848 Kg./m2
290
I I
l
Taking the statical moment about the toe of the dam, the distance of the center of gravity from the toe will be X ~ _i_ ( pl + 2Pl) 3 Pl + Pz STABILITY ANALYSIS: - Moments due to external forces MOMENT ABOUT TOE !KG-ml LEVER ARM (m)
EXTERNAL FORCES !KG)
RIGHTING
Po-I
-
Water~
pl-2
~ 3880 + 5480 121 ~ 9360
Silt -"--->
pl-2
Water~
Pz-s
t
!
~
Silt
5.30 + -80 ~ 5.70 2
3480 + 3880 1.80) ~ 2944.0 2 .
726 12.00) -
~
)
6882.5
)
28588.5
)
99 1025 . +_lQ_~ 2
41328
)
4.74
~
24660
Pz_s - 1848 14.50)
~
8316
-
44366.4
1452
5480 14.501
~
16780.8 (
3.80 + 2.00 11123.40 + 1590.801 ~ 4.74 3 1918.80
2
6.10 +
4
-~0 ~ 8.35
205911 (
8.35
69438.6 (
2.10 + L70 11221.80 + 2246.80) ~ 2 _9 4 '""'3 2345,20
5480 ; 5960 IL7 0I ~ 9724
OVERTURNING
~
Ps-•
t
r,_5
t
Ps-6 "" 5560 +2 3690 16 .25) ~. 28906,3
3,65 + 6.25 ~ 6.78 2
195984.4
)
t
Ps-7y
3690 ; 2850 IL 651
2.00 + !.65 1584.25 + 1512.9013 29 . 2 1340.70 -
17751.2
)
--';
p6-71!
1 .oo + 1-10 1584.25 + !512.90 I ~ 1 57 1340.7Q . 3
5647.3
)
~
p7-8
~ 2850
1516.8
)
t
Ps-9
~ 3470 + 2280 121
5750
)
t
p!0-11
t
Pn-12y
~
! ..,__
t t
___,..
5960 ;
~
~
3690
~
5560 1.701 -
12850 11.10)
+ 3470 Ill
Pn-12 11 p12-0y
2
~
~
~
~
~
5395.5
~
3597
~
3160
~
5750
2
-
4032
).00 1711.35 + 1168.501 3 1295.1)0
1300 ; 1400 1.20) ~ 270
~1287 + 5331 ~ 0.15
2
553.50
40.5 (
1400 ; 1990 13.!0)
~
5254.5
.20 + 3.10 1407.95 + 57 41 ~ 2.39 2 694.95
12558.3 (
!400 ; 1990 (2.48)
~
4203,6
2 60 + 2.48,407.95 + 57 41 ~ 3 77 64~5 -
15847.5 (
3,30 + 2.00 1713.40 + 815.901 ~ 4 66 2 1121.35 .
25490.2 (
:;,08 + _,1g_ 1713.40 + 815.901 ~ fi.41 3 1121.35 -
10653.4
.
!990 + 3480 121 ~ 5470 ~
.
Fy Net - 46914.5 - 44083,8 Fy Net 2830.7
~ FH -
0.48
. 2~0 ~ 1.00
p12-0l! - !990 ; 3480 1.721 ~ !969.2 ~ ~
~
t
~
21120.2
;
t
s
~ Fy
~
291
Fy
~
~
46914,5 44083.8
c
~MR ~356720.3 ~ M ~ 347815.1 0
~
.)
··~ B.
STABILITY ANALYSIS:
MOMENT ABOUT TOE (KG·ml LEVER ARM (ml
EXTERNAL FORCES (KGI
RIGHTING
Po-I
Water~
pl-2
~
Silt-
pl-2
~
726 121
1
p2-3
~
2000 14.50)
~
9000
J
p2-3
~
1848 (4.501
~
8316
Water Silt
~
.
l
•
·.··1··
•
1 ~
1 ~
~·
..•.
1
-£{4
! l
y
y
....·.·.·.1·
)1.• '~
~
l -'
0 + 400 I 801 - 160 2 . -
l
-
~
P,_,
p6-7
Po-7 P,_g Pg_g
~
2400
2.00
3.80 +
932.8 (,
410 + 164 492
I
3
~
)
4.58
10992 )
450 2
6.10 +
8.35
~
75150
8.35
2000 + 3070 IL701 ~ 4309.5 2 3070 ; 2860 1.701 ~ 2075.5
.70
2
c.
69438.6 (,
629.35 + 820 2 .10 + 170 3 I 1039.35 9.90 +
J
6650.16
4.58
1452
)
2.89
~
12454.5 )
~
10.25
21273.8 J
~
6.78
11187.0)
2800 + 2420 16.251 2
~
16500
3.65 + 6.25 2
2420 + 2990 - - - 2 - - IL101
~
2975.5
2.00 +
~
2420 + 2990 IL651 2
~
4463.3
I 612.95 + 992.20 I 2. OO + L65 3 1109.05
~
2990 + 3620 ILOO) 2
~
3305
I 742.10 + 1225.90 I ~ 2.73 2.00 + 100 1355.05 2
9022.6)
2.00 /2
7020 )
~
~
3620 + 3400 (2) ~ 7020 2
~
pl0-11
3200 1.20)
=;
Pll-12v
~
pll-121-I""
Piz-ov
=
Piz-ou
=
F.vNET =
640
~
.20 2
11101 612.95 + 992.20 ~ 3.06 . 1109.05
1. 3
2.60 + I 147.60 + 1312 I 803.60'
121
~
720 1.72) 2
5146.8 lbs
720 ~
259.2
2
3.30 +
3
5.08 +
T
(2.00)
.72
~
~
~
~
4.10
4.63
5.32
l
LFv
~
24912 lbs
1
LFv
~
30058 lbs
1L
F.vNET ~ 30058 - 24912 ~ 5146 Kg
292
12497.1
J
64 (
3200 2+ 720 2.48 ~ 4860.8
2
2.80
0.10
~
3.10 I 147.60 + 1312 I 0.20 + -2803.60
720
~
9105.0 )
LOO
~
3200 2+ 720 13.101 ~ 6076
F.n - 9322 lbs
r
~
2
~
p4-5 ~
P,_,
400 + 2000 (2)
80x2 583 3 .
5.30 +
OVERTURNING
3.02
18349.5 ( 19929.3 ( 3333.6 ( 1378.94 (
t7MR 188576.74 I
r
Mo200885.16
Forces due to earthquake from the previous computation: 2.94 m. from the toe
Weight of dam ~ 66589.88 Kg. a) Inertia forces (para. 8.5.62) Fw ~ 0.15 WT ~ 0.15 (66589.88) ~ 9988.48 Kg. The effect of vertical earthquake is, however, not considered in this analysis and is neglected. Moment due to earhtquake, Me
Fw
(g
0
9988.48
X
2.94
29366.13 Kg. m.
J
b) Hydrodynamic forces (para 8.5.7) C<{h
wh
hydrodynamic pressure in Kg./m2
Where Py
at depth y <\'h
design horizontal seismic coefficient 0.15
W h
unit weight of water 1000 Kg./m3 ~
total depth of water in meter 2.00 m.
Where y ~ depth below surface
Cm
when
maximum value of C obtained from fig. 8.2
C
Cm
y
h, as the case hereunder
y
2.00 m. 2.00 2.00
Maximum value of C
~
1 .73
Unit weight of water 1000 Kg./m3 py
.73 (.15) (1000) (2.00) 219 kg./m2
Vh
Total horizontal shear
293
Vh
~
0.726 Py . h ~
0.726 (219) (2.00)
317.99 kg./m.
'
Mh ~ 0.299 py h2 0.299 (219) (2.00)2 261.92 kg.m/m. - strip SUMMARY:
From the previous computation: Due to dam weight: MR
~ 304158.62 1
F v ~ 66589.881 ~MR ~
304158.62 + 188576.74 ~ 492735.36 Kg. m.
~M 0 ~
200885.16 + 29366.13 + 261.92 ~ 230513.21 Kg. m.
~M ~ ~MR
~
V
~
~H ~
-
~M 0 ~
262222.15 Kg. m.
66589.88 - 5146
~
61,443.88 Kg. ~
9322 + 9988.48 + 317.99
~ 4,27 m. from the toe
262,222.15 61,443.88
X=
19628.47 Kg.
>
10,60 3
(within the middle third) O.K. e
~
. _ll_ 2
- x
~
10 6
· ~ - 4,27 ~ 1.03 m •
fgundation Reactions;
( 1- ~ ) B
!heel ~
6 (1.03)
61443.88 10.60
[
1-
2417.0 Kg./m2
<
32290 J(g./mz
61443.88 10.60
[
1
9176.11
<
+
10,60
6 (1.03) 10.60
32290 Kg./m 2
Factor of Safety against sliding,
] 0.1<;.
] O.K.
!'\ ~
(W -
where W
~
total weight of the <1am
U
~
total uplift force
P
~
total horizontal force
~
~ 66,58~.88
Rg.
5146 Kg. ~
19,628.47 Kg. 294
¥) tan 0
Assuming a gravelly sand material, 20% grave, maximum ~ 3/4" with a relative density of 70°,\J; Refer to Table no. 8.1 ~
Tan 0 ~
F 1
.77 2.4 1
(66,589.88 - 5146) (.77) 19,628.47
Allowable F .S. for this condition ~ 1.2 F1
>
allow F.S.
O.K.
Factor of Safety against overturning, F 0 492,735.36 230,513.21
L.
~
2.14
>
1.50 O.K.
DESIGN OF GUIDE BANKS a) Layout
Total Width ~ L ~ 440.0 m. As per fig. 11.2, for the elliptical guide banks, U /S Length, a ~ 1.0 L ~ (1.0) 440 ~ 440 m. DiS Length, 0.25 L ~ (440) ~ 110 m .. 0.40 L • Pw ~ 4.83
~
0.40 (440)
JQ ~ 4.83 J5320
~
176 m.
~ 352.29 m.
0.45 (352.29) ~ 158.53 say 160m.
Radius of curve R b
~
R
~
160.0 m.
For layout the coordinates calculated with the formula
y2
+
b2
~
1
295
y
y
X
1
440
80
381
5 10
440
90
364
439
100
343
15
438
105
332
20
436
110
320
25
435
115
306
30
432
120
291
35
429
125
275
40
426
130
257
45
422
135
236
50
418
140
213
55
413
145
186
60
408
150
153
65
402
155
109
70
396
160
0
b) Protection Works
Intensity of discharge d/s of sluiceway ~ 21.02 cms/m. scour depth varies from 1.0 R - 1.5 R Taking Mean depth of scour ~ 1.25 R Where:
R
~
1.35
8.10
(21.02)2 ]1/3 4.00
[
1.25 R ~
D
~
4.00
1.5 D
~
6.00 m.
v
~
X
~
6.48
8.10 mts.
(117.63 - 112.80) + D
21.02 (3.28) _ 4 83 size of stone weight
~
~ ~
4.35 m/sec. 13"
~
16.28 ft./sec.
0
110 lbs
~
50 kgs.
296
[5 D
~
L
~
8.94
. 1.25 ( 1.50 V
Lt1
~
X
12
8.94 (0.62)
v
~
1.5 D
13 3.28
~
X
~
0.62
5.54
5·54 6.00
~ 0 92 say 0.90 mts. .
Use protection works for upstream and downstream portion ~ 1.50 D ~ 6.00 mts.
Nose Protection
Depth of scour ~ 2 R D ~ 2R- (117.63- 112.8) R where
~
1.35
[
l16.71P 4
113
]
~ 5.56
q ~ 16.71 cms./m. D ~ 5.56 x 2 - 4.83 ~ 6.29 1.5 D ~ 1.5 x 6.29 = 10.50 m .
'J :1'·;·.·.·.
.
297
CHAPTER 16
DESIGN OF INTAKE WORK
1.
CANAL SECTION
Main Canal No. 1
DATA:
Qcm
~
33.97 ems.
(Discharge for crop maintenance)
QLS
~
40.998 ems.
(Discharge for land soaking)
Rugosity Coefficient n
0.015
~
Side slope (SS) ~ 1 1/z : 1 Water surface slopeS ~ 0.0002
where
b
~
2d
b
~
bed width
d ~ depth of water
1.1 For crop maintenance X ~
y ~
1.5 d
J(1.5d)2
+ d2
y
~
1.803 d
A
~
bd + (1.50 d) (d) ( 1/z) (2)
298
A~
2d2 + 1.5 d 2 3.5 d2
p
~
b + 2 (2.803 d)
p
~
5.606 d
R
~
AlP~
v
~
A~
-
3.5d2 5.606 d
Rzt3
1
0.624 d
s'h
n
1 1 0.015 (0.624 dJ2'3 (0.0002) /2
v ~ v
~ 0.688 d 213
Q
~
Av 33.97 ~ 3.5 d 2 (0.688 d213)
d ~
33.97 ) . (0. 3 50 688
b
~
2d
~
Using b
~
5.40
d
~
2.70
2 (2.70)
3/8 ~ 2.70 mts. ~
5.40 mts.
A ~ bd + 1.50 d2 A ~ 5.40 (2.70) + 1.5 (2.70J2 A
~
25.515 sqm
p
~
b + 3.606 d
p
~
5.40 + 3.606 (2.70)
P
~
15.14 m
R
~
AlP
v ~
v V
~
25.515 I 15.14
~
1.685
2.. Rzts sl/2 n
~
-
~
1.34 m/sec.
10.015
(1.685)213 (0.0002)112
Q ~ Av ~ 25.515 x 1.34
Q
~
34.19 ems.
299
1.2 For land soaking
QLS
~
40.998 ems.
Using b Try d
~
5.40 mts. 2.97 mts.
Check: A ~ bd + 1.5d 2
A
5.40 (2.97) + 1.50 (2.97P
A
29.27 sq. m.
p
b + 3.606 d
p
5.40 + 3.606 (2.97)
P
~
16.11 mts.
R
~
A!P
~
29.27 /16.11
1.82
_ 1_ (1 82J213 (0 0002) 112 0.015 . . V
~
1.405 m/sec. 29.27 (1.405)
Q
Av
Q
41.12 ems.
~
>
Free board
40.998 ems.
0.25 d + 0.30 0.25 (2.97) + 0.30 1.05 mts.
Canal Elements: Qcm
"7
33.97 ems
QLS
40.998 ems.
dcm
2.70 mts
dLs
2.97 mts
v
1.34 m/sec.
R
1.685 m
p
15.14 m.
b
~
5.40 m.
A~
25.515 sq. m.
de
3.05 m.
~
300
'it£
1&
&
• , 2 2. L Lti4-AU£WMIJJtJ§WJ&%t4
2 LZL.2 2 &%Q_£&4.JQJ!il4&.QL4 St&Z&.Zlt. LL
n % 0.015
s
% 0.0002
;
D% 4.00 m CB
112.33 mts.
TB
116.33 mts.
WSEL% 115.30 (LS) WS EL% 115.03 (em.) 2. DESIGN OF INTAKE STRUCTURES: A. Design Data:
Diversion Dam Pond level % 115.80 mts. Crest level % 116.00 mts. All. Elev. (Dam silted) % 119.48 mts. Sluiceway U/S apron Elev. % 113.00 mts. Sluiceway crest elev. % 114.00 mts. Canal Discharge % 40.998 (LS) 33.970 (em) %
Silt factor % 4.00 Exit Gradient % 1/5 1.15 (33.97)
Additional 15% for the discharge (em) of MC# 1 is needed for the silt ejector ems. Since it is smaller than QLS• LS governs.
39.07
B. SILL LEVEL OF INTAKE
It should be as much as possible above sluiceway crest. Consistent with economy (after carrying out trials). On this consideration sill level is fixed 0.45 m. (1.476 ft.) above sluiceway crest, i.e., at elev. 114.00 + 0.45 % 114.45 mts.
EL. 116.00 EL. 115.80
l
EL. 115.30 (LS) EL 115.03 (CM) - - - _EL
~
~4.451
EL. 113.00
ASSUMED VALUE
+
301
2
LKL&Ad&M4¥
L£&
C. WATERWAY Using Elev. 116.00, i.e., W.L. at crest level hs
~
115.30 - 114.45
~
0.85
he
~
116.00 - 114.45
~
1.55
hslhc
~
0.85 11.55
C, 1 C
~
0.937 -
c,
~
0.9226 (1.706)
Q
~
CLH3i2
L
~
~
0.548
0.048 (0.0 3) ~ 0.9226 0.10 ~
1.57
41 1.57 (1.55)3t2
13.53 mts.
Using Elev. 115.80 mts (pond level) hs
~
115.. 30 - 114.45
~
0.85
he
~
115.80 - 114.45
~
1.35
hslhc
~
0.85 11.35
C5 I C
c,
~ 0.907 ~
~
0.63
~:~~ (0.051) ~ 0.8917
1.706 (0.8917)
~
1.52
41 1.52 (1.35)312 Use LT
~
17.20 mts.
17.20 mts. Provide 4 - 4.30 mts with 3 - 1.00 mt. pier width total width
~
17.2 + 3 x 1
~
20.2 M
302
K4K. 2 .
g_ H.£%Mt&.G.J&0J .iLL
e
I •
A
•
2
&&
.£.
RATING CURVE OF MC #1
Velocity (V) ~ -1.486 x R2!3 X Sll2 n-
' 1
j
~
I I
WATER WATER LEVEL DEPTH
112.33 112.53 112.73 112.93 113.13 113.33 113.53 113.73 113.93 114.13 114.33 114.53 114.73 114.93 115.13 115.30
0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 2.97
AREA A
PERIMETER p
~
s
~
R
0.186 0.351 0.500 0.640 0.766 0.888 1.005 1.117 1.226 1.332 1.436 1.537 1.636 1.734 1.817
6.12 6.84 7.56 8.28 9.01 9.73 10.45 11.17 11.89 12.61 13.33 14.05 14.78 15.50 16.11
1.14 2.40 3.78 5.28 6.90 8.64 10.50 12.48 14.58 . 16.80 19.14 21.60 24.18 26.88 29.27
where n
.015 .0002
v~
R213
0.326 0.498 0.680 0.743 0.837 0.924 1.003 1.077 1.145 1.211 1.273 1.332 1.388 1.443 1.489
.943R 213
0.307 0.470 0.594 0.701 0.789 0.871 0.946 1.016 1.080 1.142 1.200 1.256 1.309 1.361 1.404
Q~AV
0.350 1.128 2.245 3.701 5.444 7.525 9.933 12.680 15.746 19.186 22.968 27.130 31.652 36.584 41.095
D. DETERMINATION OF BASIN LEVEL & LENGTH
This is checked for three conditions: a. Pond at crest level 116.00
and only onegate is partially opened say 0.10 MT Area of length
~
~
0.10 X 4.30
0.43 M.
Depth of water up to MD-Height of opening ~ 116.00 - 114.45 -
h
~
0.1°/2 ~ 1.50 mt. (0.10 being the total opening)
1.50 M or 4.92 ft.
Velocity~ cd j2 gh ; cd ~ 0.60
V
~
0.60 2(32.20) (4.92)
Q
~
AV
~
0.43 (3.26)
~
~
10.68 FPS
~
326 mps
1.40 ems.
303
-::::0::::
-= - -=-
-:ZSS.G
!ZL
22
•.· \.·.1·.' 'i-
From rating curve:
WL
Q
1.128 1.117
J
J
.272 112.73
y ]
X
1.40 [
.20
112.93
2.245
112.73 + 0.272 (0.20) 1.117
X
112.78 m.
X
.i,'!!il
?~
l rEL.116.00 rEL.112.78
r
D
EL.114.45
~~-1
Neglecting velocity head which is very small: ~
116.00 - 112.78 ~ 3.22 1.40/4.30 ~ 0.326 cms/m. 120 q ~ 1.20 (0.326) ~ 0.39 cms/m.
F q
~
De
~ 3po.39J2 ~ o.249 9.80
FiDe
~
3.22/0.249
~
12.93 ~
Refer Fig. 7.15 taking F HLiDe 12.90
.10
[
12.93
HL
•
Dz/ Dl
J
'""] 'J
.03
Xj
13.00
0.176
xz
.09
18.67
0.176
18.58 + 0.03 (0.09) 0.10
18.61 m.
D1/De ~ 0.176 D1
0.176 (0.249)
D2 Vl
18.61 (0.04) q/D 1
~
~ ~
0.04 0.7 4
0.39/0.04
~
9.75 M/sec.
304
J
EL.112.33
Froude No.
9.75
V1
~
J9.80
jqDl
X
15.57
0.04
As from fig. 7·16 A (For Basin II) LB/D2 ~ 3.90 (0.74) ~ 2.89 M. 112.04 mts.
Basin Level ~ 112.78 - 0.74 b) Water at crest level
and all gates opened UIS WE ~ 116.00 M. DIS WE ~ 115.50 M. hs ~ 115.50 - 114.45 ~ 1.05 he ~ 116.00 - 114.45 ~ 1.55 hslhc
1.0511.55 ~ 0.677
e81e
0.907 - 0.077 (0.051) 0.10
e8 q ~
~
0.86773
1.706 ro.86773l ~ 1.48
e5 HcL 50
~ (1.48) (1.55)1. 50
2.86 cmslm.
1.20 q ~ 1.20 (2.86) ~ 3.43 cmslm. (3.43)2 ~ 1.06 M. 9.80
De
116.00 - 115.00 ~ 0.50 ~
0.5011.06
0.4 72
D1 IDe
D2 I D1 0.400
J
.072
.10
[
3.09
0.472
0.541
'
~6
x2
Xj
0.516
3.35
0.50
D /Dc ~ 0.541 1
D1
J
~
0.523 (1.06)
0.072 (0.025) 0.10 ~
0.523
0.55
D 2/D 1 ~ 3.09 + 0.072 (0.26) 0.10 ~
3.2772
D2
3.2772 (0.55)
v1
q/Dl ~ 3.4310.55 ~ 6.24
1.80
(6.24)2 2(9.80)
1.99
305
J
.025
l
v2
~
1.91
~ y22 ~ (1.91J2 ~ 0.19
hv 2
2(9.80)
2g ~
Basin Level
113.51 M.
115.50 - 1.80 - 0.19 V1
Froude No.
jgDl 6.24 )9.80 (0.55) 2.69
~
3.80
LBiD 2 LB
~
6.84 M.
3.80 (1.80)
c) During maximum flood
Gates opened at a height required to flow the design discharge ~
Max. Affl. Elev.
119.48 M.
Intake Design Discharge
~
41 ems
'
EL. 119.48
W.E.
EL. 114.45 EL. 113.00
In partial gate opening, discharge can be calculated with the help of Submerged orifice formula.
Cd A /2gJl 119.48 - 115.30
Q
h
~
If x is the opening & Q
40.998 X
0.6
X
~
17.2
~
4.18 m.
40.998
J2
g
X
40.998 0.6 X 17.2 /19.62 40.998 93.458
~
6.18 X
9.18
0.439 m.
306
So gate opening
~
0.439 m. 40.998 17.2 X 0.439
Velocity through the opening
5.43 misec. 0.5
Loss of head at the entry
vz 2g
0.75 m. 119.84 m. T .E.L. just uis of gate 119.84 - 0.75 ~ 119.09 T.E.L. dis of gate Downstream Wl.L. ~ 115.30 Head Loss HL ~ 119.09 - 115.30 ~ 3.79 q ~ 40.998 ~ 2.38 17.2 HL ~ 3.79 q ~ Q!L ~ 40.998i17 .20 ~ 2.38 cmsim. 1.20 q ~ 1.20 (2.38) ~ 2.86 cmsim. D
~ 3F2.83'r' ~ 0.94
c
9.80
HLiDc ~ 3.79i0.94 ~ 4.03 Dz/Dl ~ 8.4 D1!Dc ~ 0.279 D1 0.279 (0.94) ~ 0.26 D2 8.4 (0.26) ~ 2.20 q/Dl 2.86i0 .26 11.0 q/D2 ~ 2.83i2.20 1.3 Froude No.
mo11.0w.26l
LBiDz ~ 3.4 LB ~ 3.4 x 2.2
~
~
6.89
7.4 m.
Basin Elev. 119.09 - 0.26 -
(11)2
2g
119.09 - 0.26 - 6.173 Basin Elev. 115.30 - 2.2 -
~
112.66 m.
1.32 ~ 113.0 m. 2g
E. DEPTH OF CUT OFF
Determination of uis and dis cut-off depth: q
~
1.20 (4l.Oi17.20)
2.86 cmsim.
R ~ 1.35 (q 2!f)113
1.71 m.
307
Bottom level UiS cut-off
~
116.0 - 1.71
114.29 m.
For DiS Portion: q ~ 1.20 (4l.Oi8) ~ 6.15 cmsim. R ~ 1.35 (6.152 i4)1i3 ~ 2.85 m. DiS cut off ~ 1.25 R ~ 1.25 x 2.85 "" 3.56 Bottom level 115.30 - 3.56 111.74 DiS d ~ 112.33 - 111.7 4
~
0.59 m.
As the UiS cut-off of intake is located adjacent to uis cut-off of sluiceway the bottom level should not be above the bottom level of sluiceway cut off i.e. it should be up to EL. 111.00 ~ 113.00 - 111.000 ~ 2.0 m. upstream depth of cut off.
F. TOTAL LENGTH OF FLOOR:
Maximum static head will act during design flood and when canal is dry. High flood level with Dam silted ~ 119.48 Canal bed level ~ 112.33 Maximum static head, H ~ 7.15 Exit gradient GE ~ 1i5 Using d ~ 4.00 m. 1 - GE 1fji\-
X
..!!_ ~
l.
H
5
X
~ 7.15
~ 7.85 (with exit gradient as 0.202) ot:. ~ J(2A- 1)2 - 1 ~ )(2(7.85)- 12- 1
j\
b/d b
~
oC
~
14.67 (4.0)
~
Adopt length of floor Basin elevation
14.67
58.70 m. 78.70 m. 112.00 m.
308
rEL.112.33
EL. 113.0:
'-----------~E~L~.1~1~2~.00~------=---__/
~ ~11.0
-r
1
rEL. 108.33
3:1
I~ '"
8.70
I
I
7.35
l
8.40
18.30
1
1
4.15 1
7.80
58.70
G. ANALYSIS FOR PERCENTAGE OF PRESSURE
l r"'"'
EL. 114.45 EL. 113.0
~\ I~ 4.0
I .
'I_________________________E=:L.::.·_::1.::.12_.o_::o___________
~
111.0 8.70
7.35
38.65 58.70
I .
309
EL.108.33
First Pile Line: depth of downstream cut-off 112.33 - 108.33 ~ 4.00 m. length of floor ~ 58.70 113.00 - 111.00 ~ 2.00 m.
D d b d1 1 -
dl ~ _bQ__ ~ 0.034 58.70
""' - b
From Fig. 5.10
l
l l
GlD 11% 0E ~ 17% i)JD
1
GlC 1
~
100 - 11
89%
~
100 - 17
83%
i) Correction for thickness
C ~ 113.00 - 112.00 (89 - 83) ~ 3.0% additive 2 ii) Correction for interference of 2nd pile with the first pile.
c ~
[~]
19Flr
Where:
I
112.00 - 108.33 112.00 - 111.00 b ~ 58.70 m.
I c
~ 19
3.67 m. 1.00 m.
j58.70 3.67 [3.67+1.0] ~ 58.70
Final value for 0c
1
~
83+0.38 +3
0.38% ( +) ~
86.38%
Second Pile Line d b
~
1 ~ o<:
112.33 - 108.33 58.70
_i_
~
b
4.00 58.70
~
4.00 m.
0.068
From Fig. 5.10
I
0E2 ~ i)JD
25%
GlD2 ~ i)JD
17%
iii) Correction for thickness
c
~ _1_ (25- 17)
4.0
2% (-)
310
&&Lb
Correction for interference of pile number 1 on pile number 2
~
C
19Jf.
[D~d]
where:
D d b
111.33 - 111.00 111.33 - 108.33 b ~ 58.70 m.
c
19
0.33 m. 3.00 m.
j 58.70 0.33 [0.33+3.0] ~ 58.70
0.08% (-)
I·
iv) Correction for slope: For sS
3:1 4.5% 58.70 m.
c L1
c
~
"'"'%~
4.50 (7.35) ~ 0.56% ( +) 58.70
Corrected (j)E
2
~
·I
58.70
23.48%
23.48%
25 - 0.08 - 2 + 0.56
H. FLOOR THICKNESS i) During maximum flood
EL. 119.48
I
EL. 115.28
EL. 114.45 EL. 113.0
-:. J
EL. 112.0
//1.(<,.){11/0\'
' 12.70
I I
4.35 I
8.4o
4.15
18.30
I
3.0
I
I
c
b
a
L-
38.65
58.70 h
119.48 - 115.28
311
4.2 m.
7.80
Points
a b c d
:II }!
Dist. from U/S end
28.45 46.75 50.90 58.70
Ofo
pressure
55.89 36.28 31.84 23.48
Effective Uplift Head 2.3 1.50 1.33 0.98
Thickness
1.77 1.15 1.02 0.76
d,'f!i
The depth of floor can be checked for other flow conditions as in the case of dam.
X---- X---·-- X - - - - - X - - - - - X - - - - X
NOTES:
1. Design details of the work are given in the appended Plates No.1 to 10 .
·•••• }
•••
•.
2. The design of Mag·Asawang Tubig Diversion Dam was modified on the availability of more field data such as grain size analysis of the bed material, latest survey, etc. and the appended drawings also accordingly modified.
~u~.·
•.··..
312
..~~-.---~"0:-::·~-~~~-::~----: ·. -~~
-
..
"7"
~./.--_:
....
--------
........
PLATE
. 0 +000)
"'o
<
us
\
L
"'"'
\
\I
\\
\ I
I I
-
I
'
l .
\
\
I
\
/
\ \
'\
'
S.!io-... _ _ .
. 2 000
SCALE
I
\ I
"E I'L ;.,, OF 0/S Gli!Q(
./
/
/
/
/
//:/
I
f
.,II'.
I
0• t.J
I
:-
' I
.,
I
I
I I
I
.."" /
I
I I
I I
/ I
__..// /'
/
/
/ I I
'
MAG·ASAWANG TUBIG RIVERlliRIGATON ffiDJE
I
DIVERSION GENERAL
I
'DE: SIGfo!EO:
DEVELOPMENT SCALE
M-- -- -------
ORAIIr'H: ~:.M...;~'!.!!'!,.._t;!'!..--
'
PLAN
WORKS
LAYOUTPLAN c~o: ---------"' EVJilW£0: ._- - - - - - - SU Mlln£0:.--- - - - - - -
RECaM!ii£NOEO
l. 2,000
APPROVED
AsST:- A0NjNI3nUT0RDW'PlO ___ _
A0WINI
SH(fT__OF.--
A
e.J,G[.f't<.J, _..
PLATE
2
NOTES: I. A.LL LEVELS A.f£ IN WETER S AND OIWENSIOHS IN
CENTIMETERS.
2..LINQTH AHD HEIGHT Of THE: TRAIMIMI Wii..LLI1 TH! ~f-T1J(I AHGU: 0' TH! M'CTAKE 111UJCTUUS A MD TH!: t..EHQ!f'~ AHD LAYOOT ~ THE QUID It BAHKI ARE TO BE T£8iED ott TtC NODI:L, I. NOD£L TEST IHCOLO H CARRIED OUT TO CN:ItOC THE WIDTH l'f'OVIMD 'LOW COMOITlOIII AND 'INIEitiY 1)4.1:$-tPATION IYITIM.
IMPORTANT NOTE ! WITH
THt: AVAILAIIUTY 0, WO,.. fiELO DATA
IIZE ANALYStS Of THE fOlJNOA.TIOH WATERIAL, ETC) AHD hiOM:L STUDIES, LEHGTH Of' -..T£RWAY
(GRAIN EL.II!.CO
CONNECTED 'WORKS
..
20
10
30 I
I ; 400 WTS.
SC,t,LE:
EL.112 .DO
EL.Il2.80
U1"11et.IC C1' ntl P"HU.Ir>t"UIU
NATIOfriAL IRftlGATION
ADMINSTR'AT10N
FHILIA"NE MEDIUM SCALE IRRIGATICN PR:lJE I"JtO.,..al: Clf c.aTAL WINDOtte.
~ASAWANG TUIMG RIVER IRRIGATION PROJ..ECT
DIVERSION GENERAL ~IKIA;t);_
GENERAL
PLAN
SCALE :
1:400
________
WMP DUWIII' ; _ _ _ _ _ _ _
WORKS PLAN -----RIIV1rrlfta.:_ _ _ _ _ _ - ·
c::K>:~:
IU!i:llll"TU,;_ _ _ _ _ _
ltECOIEIWEND£0
w.vco.C»iiULiift'
;;JS.JTc'f ~ ~
-··
......-.:
PLATE
1----
le
I
1 . ~.1 . ~•.80C:O.OC!It:TI: LOCK III 000 I'll ) 0\lt:R CW. QRADt:D fiLT£11
(IOOOPSI) 'til 7 ~ Clll. GAP IN IETWE[N I F.ILL ED WI . . A\IEL ) O H .t>O Clol. THI(. QJIAOEO REVt:IISE
I---__, )
l ty,.__--i ~---i
ltll70 fiLTEII
•o
s....£.CTION
11
SCALE : 120 117.'5
NOTES.
11
A •· A r:roo
1. ALL L;.VELS A RE IN t.IETERS ll Dl t.IENSION
I N CMS .
2 . "EFER ' RELEVANT SHEETS SHOWIN G L AY OUT PLAN
311.5
AN D SECT I 0 N S .
3 . Dt S GLA CIS
... OF IRIDGE
DETAIL "Aw ~ALl:
I : 40 MTS.
...
AGA I NST
~.
--==
-
APRON ARE T O BE PI\OTEC TED WIT
ABRAS IO N.
4. ENERGY DISSI~ATION . ARRI.ItG£M ENT CHECKED
El. . ll7.fS3(R!lJtooESSED ) 01 00
a
EXTRA HI\RDENING MATER IALS TO GUARD
TO B E
ON MODEL ST U DI ES
D E SI GN OF BR I DGE
I
I S HO T GIVEN W 1 kEPORT .
0
IZ3
ltCALir IIJI-
4
'!...Ill
: 100 -------~ I
- 1000 l'tl Coti~ITC '
Cot!CIIITI
III'\IIUC Of'Tl11! I'HILIPI'IIIU
NATIONAL IARIGATION
i
·~-+--
....
I",........'" Ill
uo
A OM ... I STRATI ON
PHILFP1NE MEDIUM SCALE I ~ I GAT!~ PROJI' 1'910W.C:C 07 ClttiiiTAL MII OOtiO
MAG-~NG Tuete ~A llilAI ••UlOfl PR OJEC
DIT ~L • A •
DIVERSIO N
10 0
~ -----+-----~eoo ~~---+------=to~o ~----+-
WORKS
-•no - - - ---- ---DllMIC 1!1,.1!_1.!'~--- ·~ ~~~ R ~~~~=-~~----
1000 I'll
s
E
c
T 1o N
S CALI : RII'IIA~
-
"s- a"
1: 100
PLATE
4
AXIS OF SUJICEWAY
NOTE: ALL il\MENSIONS
AHO LEVELS IH
METRES,
CONCRETE ~ l~m.xllSOt.'\. x Q.9Qf1111(10::0 pal) PLACED AS SHOWN w/ 7.ei0 C<''l. G» IN BET\!IEEH {FILLED w/ GR..eVEI...) C*l 60 csn. -~HK. GRACED
DETAIL "B" SCALE
ARE IH CENTIMETRES
1:2!5 M.
REVERSED ALTER.
ICOCN:. THK.
II I.
75CM.VRti.E) ALTER.-~--·-'7
i .
~~ J___g45~~tJ=~±~:~4=~--~-~3~ -~=~~m =:=·~~-·~·~-~~-le20Jii-:~---DE~~!:'~~"'
I I
.
\
l-<13~_::!?0_
___
SECTION "C-C" SCALE :
I :75
WITS.
WATER STOP
.
DETAIL
SCA!...£
"A"
•'
.·:: ~-- .
I
._::1 ' -. -,
t:z!5 MTS
FILTER
L50xi.!Ox.90CONCRETE SLOO<S
r--- OO::Opc!)W/ncm.ALLER ON
:~-
112.00
I
60 om.'I'HK.
A
GRADE
I
WATER~
DETAIL OF GRADED FILTER
220
DET. OF JOINT Ar TOE
Rfi"U3UC
NATIONAL
DETAIL OF DENTATED END SILL (Sluiceway)
Of THE
IRRIGATION
I
S
ADMINISTRATION
PHILIPPINE MEOIUM SCALE IRRIGATION PROJECT P'ft0VINC£0f'<JHENT4L NlMDORO
MAG-ASAW~G 11JBtG RIVER
IRRIGATION
OlVER SlON WORKS OGEE SECTION
8 APPURTENANT OCTAILS
I
·--~
PLATE 38!.W
3100
12~0
1830
1270
OF BRIDGE
1170
100
118.00
SOKG. BOJLDER THICK
30 CM. !5000 PSI CONCRETE
1~0
CM.
1.!50x 1.50x 0.90 BLOCKS WITH 7.0 CM GAP IN
1.50 xl.50x 0.90
CONCf~ETE
CONCRETE BLOCKS OVER
1 BET'WEEN ON A SO CM GRADED FILTER
SQ CM. THICK GRAD EO
FILTER 290
300
300
$00
949
13!50
SECTION "D- 0" SCALE
100
NOTES: l. ALL LEVELS ARE IN METER AND Dltr.IENSIONS IN CENTIMETER. 2. REFER RELEVANT SHEET FOR GENERAL LAYOUT PLAN AND SECTION.
3. DIS GLACIS AND APRON ARE TO BE PROTECTED WITH EXTRA HARDENING MATERIALS TO GUARD AGAINST ABRASION .
..
OF BR!DGE
1-------'•=oo_ _ ~t----~ 600
rrt~Tl
·
1
REPU8LIC
01" THE
PHlL!PPlHES
NATIONAL IRRIGATION
ADMINISTRATION
PHILIPPINE MEDIUM SCALE IRRIGATION PROJECT PROVIHCI!:OI'"OII:II!:ltTAL MlNOOII:O
MAG-ASAWANG TUBIG RIVER IRRIGATION PROJECT
DIVERSION RIGHT
a
WORKS
LEFT SLUICEWAY SHOWING TRAINING WALL
DUI$MI!:O : _. __ • ___ _
CHECK£0: - - - - - - - - - - ·
:~~:~~: :. ·_-:: :: :: :
60 KGS. BOULDER
:.: :
RECOMMENDED
,.
L~Oxi.~Ox0.90
CONCRETE BLOCKS WITH 7.0 ChLOAP IN BETWEEN ON A 60 CM. GRADED FILLER
SECTION "E -E" SCALE
1:100
WA...Co!cOirciuL.tAHT - ~Ci' ~" -ote,iit:iteKa-al'ici. iiidf. APPROVED - As"i-iiiwiiui.,..-MDWt..litO.-- _
'- ic*iiuiTitAToftaMttT-01'"_
f'MI(MO. - - -
PLATE
NOTE;
l REfER GATE DRAWINGS FOR DETAILS OF BLOCK OUT FOR GATE GROOVE AND STOP LOG GROOVE.
I
I
r-oFUE.:Ol£0
73S
sEc Tl oN • G scALE
1
125
G
1830
I
omn~~"::oo ~~:tn~~~
I
-&
:
~
j
I
~.-._,_r~ -;:::-·c:::··~·=··:;:::.-i~=:-~ L-;: ~-:"-~: -;: -~=~- -:<: ::
·~-
' " :I "'
0
Wldddu11;i
EL 1&1 00
•.
,. .
I
'I
'
:#
.
r;
i
-Lt
. --;:::-:::::··:::::"=,·= .. =···=-r·~-=--.-=·-=--=·:::=:-::2··::'1· ·:/ ··
.'7 -'.
;::::;.-.:z,·=--·=···:o:··
1
i
.
·.
•
.
.,
:;
::
11iill 11
! -----t-
0:o, ~·
----1
l----~, _. , '
I '. -+. .
I
EL .!16.33
8
r,
"'
II
! I
~~~uu~~~~mu~!_lw~wW~~n~rr~~~~---
-,-
i EL.II2.00
~ EL.112. 0
El..112.
Ill
Th'
SECTION
" GJ-
I',
111111
I
I
GJ"
I
I
I
'
i
I~
REf>(JBLIC
OF'
THE
NATIONAL IRRIGATION
PHILII'f>INES
AOMINIS':'"RAilON
fHILIPPINE MEDIUM SCALE IRRIGATION PROJECT
I I
PROVINCE OF ORIENTAL
,-1-
MINDORO
MAG·ASAWANG TUBIG RIVER IRRIGATION PROJECT
DIVER Sl 0 N W 0 R K S EL.IIG 33
~-=- RIGHT I.-rAKE ;}[SIGf./£0
"'"" "
-----'--------------~···-·
. ·--------
·_,_LUJ
STR'""'' >< CHECKED.------
i~:~;~~·
---
--- ~ - - -·
~
~~~-~~-~--~~-------~---
PLATE 'I
=·=~·=";±fl,,
r;:,,,;;::;::.
7
=9=
....
!
'
•
'
IJliL I
1------------------SECTIO~N SCALI: :
'F,)-
l: lOO
I
430
I o4:5~ '
F
IIITt.
I
I I --f r---~~
-~
II
T_l.. ' .
'
:' I
12!.00
\
\ I
\
-- -!' ~
'-1 2'
~
I
•
I
~~
~ \
I
J
I
.
eJ:l
I SECTIQN'~-F~·
. -'..
·.-:,:_:·.'
'. ~·
~·
<:',
',, ' '
I
I,!! I,
EL.Il3.Be
EL.!l4.4~-
/
/ //
/
ftl:f'UIIUC
ott
THE PHILIPPIP'I£!1
N~AL IRRIGATION
ADMINISTRATION
PHIUPF'I'JE MEDIUM S:::ALE IRRIGAllO'< PROJECT P'ltCMNCE r::JF OltiEIHAL WliiOOfi.O
MAO"'J\SA~NO TUBIG RIVER IRRIOAT ION
PROJECT
DIVIRSION Vltm&C!i
L EFT INTJJ(! I TRU;;C~T"U~·~·=-----J DESIGNi-t).:... _ _ _ _ _ CHEc o:_ ______ _ .,
0 0 fV"''fl
N" . NWI" FEVlEWED.:., _ _ _ _ ___ :.__ - - - SUBWITTE-9'----...:...
RECOM!.iEH
PLAN SCALE :
CJF
INTAKE #2 1: I 00
MTS.
-Aiir-:-A'DwiiiaTJtAi"OR
Aoliiiili'fttAToR
4~----------------------------------------==~----------------------------------------~~---L--------------------·-------------------------------------------------------------·------------t:~~~~D~W~·~·~O~-~~~-~~T==~~i·A2E~~:J
PLATE
OF BRIDGE
EL.l
EL.Il9.40
EL.
ne.oo
"" ",, "" ""
soo
000
SECTION SCALE:
ii[]
~
I •
:
il'' li jl .' .
•I
..
( ( (
I IHJJ .,
_
..
,..,___,
IiI
I':
1:1'0
I
~
"•
I
'2
' ,,'
;
I
I
I '
:'
'
I
II
II
II
II
II
II
I
)))
,7
7:'
i
I
I :
1:
~.
II
I
I
00"
II
I
0
~~
~3
I
D
'IIIIi
I
~~
: :~
!
11
0
I'.
i
Xo - X a
D D
:Jillllii
!
11
I
,... ...
.
( I
!, ALL LEVELS ARE IN METERS AND Olp.,jENS!ONS IN CM.
H ,,
EL.
I
NOTES:
ltf:MIIl.IC 01" THE
NATIONAL
IRRIGATION
ADMINISTRATION
PHILIPPINE MEDIUM SCALE IRRIGATION PROJECT l"ftOVIMCI:
•
1"1-rll.II"I"IN[J
01" Olti!MTAL
MIMOOfiiO
WAG·ASAWANG TUBlG RIVER IRRIGATION PROJECT
DIVERSION WORK POSITION OF PRESSURE PIPES
I
I
t>U'ISfot[t>: • • • • • • • • • • • pu-.·tt:_i!A_L..~~ •., . •••
IN
OGEE
CHt:CICt:P: - - - - - - - - - · - · ltf:VIf:WI:O: - - ___ • - - · · - · .UOYITTI:D __ • - - - - • ., ••
RECOMMENDED
PLAN OF OGEE SHOWING POSITION OF PRESSURE PIPES SCALE:
irA.OcOic~fANT ;;.o.ii:.;T~ APPROVED
1:120
..; ..
:.
;.<,
•·
OtC.te**'•*""-PIPi"·
PLATE
9
GENERAL NOTES FOR OBSERVATION PIPES!
"'''' ''
8
,,
'''''"'
I 0
'
I""'~
~I 'I
""
::
9
I
----·
RIPRAP
'"""
-~------
I
EACH STANO PIPE SHALL BE FITTED AT THE TOP WITH SCREW CAPS.
2.
ALL CONNj::CTIONS SHALL BE LEAKPRooF ,'\NO ALL PIPELINES SHALL BE COATED WiTH ANT!-CORROSIIJ.E PAINTS.
3.
ALL VERTICAL PIPES SHALL BE DEAD VERTICAL WITHOUT ANY BEND CON_TR iCTION.
4.
WHERE MORE Tl-lAN ONE PRESSURE ·poiNT IS LAID AT SOME DEPTH, THESE 3HALL NOT BE Sf.>ACEO CLOSt:H. THAN 30 CM.
0.
FOR PRECAUTION AND !NSTAL"-ATION PROCEDURE REFER TO RELE'/ANT MANUAL:
'
.-~--~-----~-~-~
I
)1!0 II
•:
' -l
60 KG. BOULDER
ISO[
I.
NOTES: I.
...
ALL DIMENSIONS ARE IN CM. ELEVATIONS IN METERS.
AND
---~----'-----
1-·
r-
1---
I c.i~.:---~---
.-
---------------------------
-.;. ____:L __ -~,---,----,--.-----T--
---··
"-~--~-_J_----1
-.--,-----,-I
;
•
- . ::.-
"""=-~-=---
I PLAN OF SLUICEWAY ~HOWING POSITION OF PRESSURE PIPES (LEFT SIDE) : 120
SCALE
I
r
...•, ,,"
,, ",,
,,,,,,
~
EL.11'!63(REToRGGRESSED) -~-
,,
"•',, .,'
= = ~~· = =
=
'•' ' ::
"
=
l:-
I' I'
,'
REP1.J8LIC OF THE Pl-l!liPI>IH[S NATIONAL IRRIGATION ADMINISTR:ATION
PHILJPPI'JE MEDIUM SCALE IRRIGATION PROJECT PROVINCE'OFQ~li(IH.AL
~INOORO
MAG-ASAWANG TUB!G RIVER IRRIGATION PROJECT
DIVERSION WORKS
I
~&{__
I
eq
330
360
4"0
\
- '--------+
360
I
I
300-L
300
F
i
300
GRADED FIL
600
+
500 500 --r-----.:c=___-----+ ____c:_::.::_______
PRESSURE OSSERVATIOt.l PIPES -LEFT SU;iCEWAY o€SI.wtrD: - - _- .• - - - --
500 !}OO 350 /60j_ ----'-'-.:______-+--_.::::;_:____1~::::_--.+::1,
DltAII .. :
SCALE:
~·v
Ccit..i'~A.T.\HT
l>!i~icT
_ vu 100
: - - - - JtEVIFWrD: ___ .••
:JU.WITTl't>'. ___ ·
--qECOfiYOIOI!D
WAf..CoS
SECTION
ICHI!!~Xf'D
_f!A~~~- ·- ____
W.-HMitPI-0-1¢: ofi_,-~ ~-Sioie-i: OUT.
APPROVED
ASsT. AOi.llHliTitAToii
"'*lfiliTftATO!i-
--------------------------------------------------------------------------------------~----------------------------------------------------------------------------------------------------------------------------------------r=========~o~w~o-~. -~-o~~ _~at~ __ ~
PLATE
10
.r DAN. AXIS ~--·v-oCM .
!SOCO PSI COS:: RETE WEARINJ SURFACE
1 40KGS. BOLLCER Rlf'RAP ' 60CM. THK ON A 30 CM
GRAVEL
_____ 15_00.
-~ _ -··---+---~--
y -Y
"
--~~--.
s
E
cT I0 N
SCALE:
I :715
II
-----
BED
~5.00________ j__3Zu____j_d_ed___ ~--~~-~--~-~~---· __ 4__~_0____ ----~~G'L __ --~4.00~.---'
~=~-=----~
-
.
--~:~'-!
~--------~--2nd ~-~----
STit.GE
POLYTHENE
CONCRETE PA-PER
STAGE CONCRETE
BBER
DETAIL OF TRANSVERSE
JOINT
SElL
DE"IAIL OF CONSTRUCTION JOINT
.----------jI IIEI"!.JtUC 01" THt f"""CUI",.!J<[t:
NA.riONAL IRRIG.t...TION
ADidiNISTRATION
_
~ !U PP1N;R~~~, ~:~L ~~~~~!ON ~-~~~1 IM¥l-ASM'AHG TUB10 RIVER lRRK3-ATlON Pf~O.t:CT
~
DIVERSION
WORKS
~~ er.~1 ~A~~~-~--=~-~--~~-~~:.1S_r~-~~~~L~~{:~ J~?J~!~r.::. rtr:VIr.\r-.n . . . . . . . . . __ .
p.:_":_~_,_!__ rY~~':!.~ _~; :~~~~~J-~t;::;~ ~~~--:-·
t
~i;:.CO.
oo•iu_caiir
,t..~tSTJ:iitMIII~M~
~ttO.--
-~~"~'"""'"''"·.,;&,i,ts·.lili!!.£!l§ll!91i!!i!i!l!li@=£~£~i£!1.1£t!ll£11!
Nia Design Manual For Diversion Dams.pdf
June 2020 9
Nia
May 2020 26
Design Procedure Manual For Bridges.pdf
June 2020 1
Design Manual
November 2019 14
Design Manual
May 2020 8
Nia Dour
November 2019 50More Documents from ""
Concept Of Orthogonality.docx
June 2020 5
Assignment Version 3.xlsx
June 2020 9
Nia Design Manual For Diversion Dams.pdf
June 2020 9
Alphabets
May 2020 18
Shapes
June 2020 20