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OOD AND AGRICULTURE ORGANIZATION IF THE UNITED NATIONS ROME

.

.

. .-:.*.:~A$A *,~,%-.

Already issued in this series: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 1. 12. 13. 14. 15. 16. 17. 18. 19. 20. 2 1.. 22. 23. 24. 25.

Irrigation practice and water management, 197 1 (F, S) * Irrigation canal lining, 1971 (F, S) Design criteria for basin irrigation systems, 1971 Village irrigation programmes - a new approach in water economy, 1971 (F) Automated irrigation, 197 1 (F, S) Drainage of heavy soils, 1971 (F) Salinity seminar, Baghdad, 1971 (F) Water and the environment, 197 1 (S) Drainage materials, 1972 (F) Integrated farm water management, 1971 (S) Planning methodology seminar, Bucharest, 1972 (F) Farm water management seminar, Manila, 1972 Water use seminar, Damascus, 1972 (F) Trickle irrigation, 1973 (F, S) Drainage machinery, 1973 (F) Drainage of salty soils, 1973 (F) Man's influence on the hydrological cycle, 1973 (F, S) Groundwater seminar, Granada, 1 973 (F, S) Mathematical models in hydrology, 1973 Water laws in Moslem countries, vol. 1, 1973 Groundwater models, 1973 Water for agriculture, 1973 (F, S) Simulation methods in water development, 1974 Crop water requirements, 1974 Effective rainfall, 1974

Inquiries concerning docun~entsissued in this series should be addressed to the Water Resources, Development and Management Service, Land and Water Development Division, Food and Agriculture Organization of the United Nations, Romei (*) Also issued in French (F) and/or Spanish (S)

irrigation and drainage paper

small hydraulic

structures

BY D. B. KRAATZ HYDRAULIC ENGINEER AND I. K. MAHAJAN SECRETARY, lCID

PREPARED WITH THE SUPPORT OF THE INTERNATIONAL COMMISSION ON IRRIGATION AND

Water Resources Development and Management Service Land and Water Development Division

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS ROME 1975

TABLE

OF

CONTENTS

VOLUME

I

Page PREFACE

1.

INTRODUCTION

2.

IRRIGATIONNETWORKOPERATION 2. 1 2.2 2.3 2.4 2.5 2.6

3.

General Considerations Entirely Manually Operated Systems Hydro-Mechanically Automated Flow Control Systems Electrically-Electronically Automated Flow Control Manual Versus Automated Control Water Distribution on the F a r m

INTAKE STRUCTURES Introduction Intakes of Small Canals (Punjab Type) Silt Selective Head Intake Constant-Head Orifice (CHO) Intake Neyrpic Orifice Module Intake Double Orifice Module Intake Open Intake Structure made of P r e - C a s t Reinforced Concrete (U. S.S. R.) Intake Structure Discharging into a Flume Channel (U. S. S. R. ) Pipe Regulator with Crossing made qf P r e - C a s t Reinforced Concrete (U. S. S. R. ) Intake Structure on Secondary Canals (Calombia) Gate Valve Intake (Czechoslovakia) Venturi Head Intake Square Head Intake Dupuis Canal Intake h (~ustralia) Intake with Stone ' ~ e s Weir Groyne Intake and Ancillary Works (Cyprus) King's Silt Vanes Gibbl s Groyne

33 38 55 63 81 90 91 107 110 119 128 134 138 142 151 158 162 166

Table of Contents Cont'd.

Page INTAKE STRUCTURES (Cont'd. ) 3. 19 3. 20 3.21 3.22 3.23

4.

-

FLOW DIVIDING STRUCTURES 4. 1 4. 2 4. 3 4.4 4.5

5.

Curved Wing with Silt Vane@ Silt Platforms Reverse Vanes Vortex Tube Sand T r a p Sloping Sill Sand Screer

Introduction Fixed Proportional Divisors Structures with Adjustable Splitter Proportional Distributors Division o r Diversion Boxes

OUTLETS OR FARM TURNOUTS Introduction Constant-Head Orifice F a r m Turnout (U. S. A. ) Neyrpic Orifice Module (France) Double Orifice Module o r Siphon Module Dethridge Meter (Australia) P l a s t i c Siphon Outlet fitted with an Intake Tube (Turkey) Open Flume Outlet (India and Pakistan) Adjustable Orifice Semi-Module (India and Pakistan) J a m r a o Type Orifice Semi-Module (Sind, Pakistan) Pipe Semi-Module (India and Pakistan) Fayoum Standard Weir F a r m Outlet (Arab Republic of Egypt) Scratchley Outlet (India and Pakistan) Pipe Outlet (India and Pakistan) F a r m Outlet (U. S. S. R. ) P r e - Cast F a r m Turnout (Turkey) Adjustable Weir F a r m Outlet (Malaysia) PVC Pipe Turnout (Republic of Korea) Pipe Outlet (Philippines) Gated Pipe Outlet ( F e r r a r a Type, Italy) Outlet Structures on the F a r m

Table of Contents Cont'd.

Page

LIST OF REFERENCES

NOTATIONS AND SYMBOLS

LIST

OF

FIGURES

Page

Figure

-

2- 1. 2- 2. 2-3.

Sketch of a typical irrigation system (Punjab)

7

Diagrammatic layout of an upstream controlled network.

15

Diagrammatic layout of a downstream controlled network.

17

2-4. Example of tampering. Brush and stone dam built by f a r m e r s across a minor irrigation canal to increase flow through a pipe outlet

3-1 (a) and (b).

-

28

Silt deposition at the intake to a secondary canal.

3-2. - Intake to small c a n a s (Punjab type) roof block.

-

-

details of pre-cast RC

-

3-3. Intake to small canals (Punjab type) standard sections for wing walls and abutments of CDO type fall (Punjab). 3-4. - Intake to small canals (Punjab type) minor canal at 60°.

-

3-5. 3-6.

-

intake structure for a

Silt selective head intake.

-

Diagram of a constant-head orifice intake o r turnout.

,

-

Constant-head orifice f a r m turnout with check-gate in foreground 3-7. East Ghor Canal Project, Jordan. 3- 8.

-

A single-barrel constant-head orifice turnout.

3- 9(a). 3-9(b). 3- 10. 3- 11.

-

-

-

Standard constant- head orifice intake o r turnout design guide. Constant head orifice intake o r turnout. Stilling baffles to reduce water surface fluctuations a t staff gauges.

Dimensions for a constant head orifice.

3-12. Various arrangements of Neyrpic orifice module with auxiliary equipment and structures. 3- 13. 3-14. plate. 3-15. 3- 16.

-

Perspective views of Figures 3- 12(a).to 3- 12(c).

-'

Neyrpic orifice module before and after installation of the fixed

-

Neyrpic distributor with compartments for 5, 10, 15 and 30 11s Upstream view of Neyrpic distributor type XX1300.

.

85

L i s t of F i g u r e s Cont'd. Figure

Page

-

3-17. P e r cent variations in discharges of modules types X and XX for variations of H(,,t) within pre-determined limits.

'

87

3- 18.

-

Double orifice module intake

89

3-19.

-

Open intake structure made of p r e - c a s t reinforced concrete

93

3- 20.

-

Intake structure discharing into a flume channel.

105

3-21.

-

Pipe regulator with crossing made of p r e - c a s t reinforced concrete.

11 1

3-22.

-

Intake structure on secondary canals.

117

Intake structure on secondary canals, construction details.

121

Intake structure on secondary canals, spillway and-wooden gate.

123

Intake structure on secondary canals, outlet to t e r t i a r y canal.

124

3-23. 3- 24. 3-25.

-

-

Gate valve intake, relationship between head, discharge and pipe 3- 26. diameter. 3-27.

.3- 28.

3-29,.

3-30. 3-31.

-

-

Gate valve intake, gate valve.

-

132

Venturi head intake. Square head intake. Dupuis canal intake. P r e - c a s t concrete inlet box for intake with stone mesh weir.

General view of the stone m e s h weir and inlet box. (Almost all 3-32. 12 f t 3 / s - i s being diverted.) the flow 3-33.

130

149

-

152

Another view showing stone m e s h basket construction and inlet box.

152

-

3-34. Stone mesh basket construction, stone m e s h apron, and location and general construction of inlet box with s c r e e n and screw type gate. (About 3 ft3/ s passing over the weir. )

154

3-35. - Dissipation structure and measuring weir a t the pipe outlet. (Discharge 10 ft3/s)

-

3-36. Model t e s t rating for measuring weir, capacity 30 f t 3 / s , 6 ft c r e s t suppressed weir. 3-37. 3-38. 3- 39. 3-40. 3-41. 3-42.

-

-

155

Intake structure with stone m e s h weir.

157

Groyne intake and ancillary works.

159

Groyne intake s t r u c t u r e (Cyprus).

161

King's silt vanes, layout plan of the vanes.

163

King's silt vanes, details of layout.

164

Gibb's Groyne, general layout.

167

L i s t of Figures Cont'd. Figure

Page

3-43.

-

Curved wing with silt vanes, general layout.

3-44.

-

Silt platform.

-

3-45. 3-46. 3-47. 3-48.

Silt platform with a guide wall. General layout of r e v e r s e vanes. Vortex tube. Sloping - sill sand screen.

-

Simple fixed proportional flow divisor on small irrigation canal in 4- 1. Cyprus. 4-2.

-

Simple fixed proportional flow divisor of low accuracy.

-

Fixed proportional divisor splitting a given flow into four s t r e a m s 4-3. of exact constant proportion: 4-4.

-

Dimensions of a fixed divisor splitting flow into two equal s t r e a m s .

4-5. - Fixed flow divisor m a t e r i a l s required. 4- 6. 4- 7. 4-8. 4-9.

-

-

Relationship between discharge capacity and

Unflumed divisor with a triangular sill. Divisor with triangular sill

-

-

Relation ship between Y2 and Hc .

H(c-b) Hc

Flow divisor with adjustable splitter, Argentina. Flow divisor with adjustable splitter, Spain.

4- 10.

-

Flow divisor with adjustable splitter, U. S. A.

4- 11.

-

Flow divisor with adjustable splitter, F r e n c h type.

4- 12.

-

Flow divisor ( ~ e ~ r ~ iOned c ) , Fodda network, Algeria.

4- 13.

-

Three-way distribution by m e a n s of two consecutive flow divisors.

4-18.

-

4- 19.

-

4- 14. 4- 15. 4- 16. 4- 17.

Three examples of proportional distributors. Proportional distributor (Punjab

- India).

Khosla's safe exit gradient curve. USBR Division boxes

- types 5 and 6.

Sketch of standard division box for e a r t h canals P l a n for a concrete rectangular division box

-

- Italy

U. S. A.

-

Distribution s t r u c t u r e with proportional weirs constructed f r o m 4-20. prefabricated p a r t s - E a s t Ghor Extension P r o j e c t - Jordan. 4-21.

-

Standard division structure using prefabricated parts.

List of Figures Cont'd. Figure

Page

4- 22.

-

Prefabricated division box

4-23.

-

Prefabricated division boxes

-

4-24.

-

- dimensions.

Concrete block division box.

Concrete block division box 4-25. Yemen.

-

4-26.

Taiwan.

227

-

People's Democratic Republic of 228

Typical timber division box.

229

-

Three-way timber division box 4-27. Conservation Service.

-

Standard timber division box 4-28. Canada.

-

4-29.

'5- 1.

-

standard design USDA Soil 229

Alberta Department of Agriculture, 230

Layout of furrow irrigation using an automatic irrigation diverter.

4- 30(a) and (b). 4- 31.

-

-

-

General views of irrigation diverters.

Sketch of diverter installation.

24 6

Dethridge m e t e r outlet details.

-

Large Dethridge m e t e r outlet.

5-3(b). - Section CC of Figure 5-3(a) and details of meter wheel of large Dethridge meter outlet. 5-4(a).

-

Small Dethridge m e t e r outlet.

-

5-4(b). Section CC of Figure 5-4(a) and details of meter wheel of small Dethridge meter outlet. 5-5.

-

Large Dethridge m e t e r outlet in operation with free outfall.

-

Typical setting of Dethridge meter just upstream of a regulator. 5-6. The supply level of the canal i s indicated by the bottom of the slot in the wall on the right hand side of the regulator. 5-7.

-

Channel design, f r e e board and setting of Dethridge m e t e r outlets.

5-8. Dethridge m e t e r . Gate calibration tables for LMO and SMO rivetted galvabond type gates. 5-9.

-

Plastic siphon outlet fitted with an intake tube.

5- 10. - q l a s t i c siphon fitted with an intake tube. discharge and depth of submergence. 5- 11. 5-12.

-

23 1 23 2

5-2. P r e - c a s t large m e t e r emplacements with wheels installed; note reinforcement to tie into cut-offs and pre-cast head wall. 5-3(a).

230

Relationship between

Plastic siphon fitted with an intake tube. Efficiency of siphon. Photographs of open flume f a r m outlet (Punjab type)

L i s t of Figures Cont'd. Figure 5- 13.

-

Page Open flume f a r m outlet (Punjab type).

5-14. Adjustable plate iron block for open flume outlets for B(t) 6 c m to 20 cm. 5-15. 5- 16.

-

Open flume outlet.

Arrangement of open flume outlet upstream of a fall.

5. 17. Open flume outlet. f a r m outlets. 5- 18. 5- 19. India)

-

.

5-23.

-

5-24.

-

5-20. 5-21. 5-22.

5-28.

-

5-29.

-

5-25. 5-26. 5- 27.

Details of roof block. 27 6

Plan of tail cluster of two, three and four 279

Open flume f a r m outlet.

Relationship of discharge to B(t) and H(crt).

283

General view of an AOSM outlet to a f a r m watercourse (Haryana, 290 P l a n of adjustable orifice semi-module.

29 1

Adjustable orifice s,emi-module.

Details and fixing of roof block.

29 3

Adjustable orifice semi-module.

Design graph ( B ( t ) = 7.5 cm).

30 1

Adjustable orifice semi-module.

Design graph ( B ( t ) = 6 cm).

303

Adjustable orifice semi-module.

Design graph ( B ( t ) = 9 cm).

305

Adjustable orifice semi-module.

Design graph ( B ( t ) = 12 cm).

307

Adjustable orifice semi-module.

Design graph ( B ( t ) = 15 cm).

309

I

J a m r a o type orifice semi-module.

312

Pipe semi-module.

3 15

Four examples.

Fayoum standard weir f a r m outlet; general structural design.

32 1

-

5-30. Fayoum standard weir f a r m outlet with a group of field outlet w e i r s o r "nasbas".

322

5-31. - Fayoum standard weir f a r m outlet. Relation of upstream c o r n e r s to width of weir.

323

5-32. - Fayoum standard weir f a r m outlet. Relationship between H(crt) and H(s) for all values of H(crt) egv o r of Q.

326

5-33. 5- 34.

-

Scratchley outlet. Submerged pipe outlet.

5-35. F a r m outlet (U. S. S. R. ) to a temporary feed ditch for discharges of up to 150 11s outlet submerged.

-

-

F a r m outlet (U. S. S. R. ) to a temporary feed ditch for discharges 5- 36. of up to 150 l / s f r e e outfall. 5-37. 5- 38.

-

331

-

342 343

P r e - c a s t f a r m turnout.

347

Rating curve of p r e - c a s t f a r m turnout.

349

L i s t of F i g u r e s Cont'd. Page

Figure 5-39. 5-40. 5-41.

-

Adjustable weir f a r m outlet. Adjustable weir f a r m outlet.

Q v e r s u s H(,,t).

Adjustable weir f a r m outlet.

Q v e r s u s J.

-

5-42. Sectional view of the PVC outlet installed through a canal embankment.

5-47.

-

5-48.

-

Discharge diagram for a PVC pipe turnout for D(p) = 107 rnrn.

5-49.

-

Pipe f a r m outlet with standard inlet.

5-50.

-

Prefabricated gated outlet ( F e r r a r a type).

5-43. 5-44. 5-45. 5-46.

5-51. 5-52. 5- 53.

-

5-54(a). 5-54(b).

View of the PVC pipe assembly. View of bell-mouth inlet with c r o s s b a r s . View of PVC pipe outlet closed. View of PVC pipe outlet open. PVC pipe turnout, for D(p) = 107 mm.

Prefabricated gated outlet ( F e r r a r a type). Prefabricated gated outlet ( F e r r a r a type). Prefabricated gated outlet ( F e r r a r a type).

-

Gated pipe outlet ( F e r r a r a type). Pipe stand in gated pipe outlet ( F e r r a r a type).

- Complete outlet stand (high type) for 400 m m p b e . 5-55(b) - Complete outlet stand (low type) for 400 m m pipe. 5-56. - Concrete pipe outlet f r o m p r i m a r y to secondary f a r m ditches. 5-57. - Concrete pipe outlet with tap at one end. 5-58. - Wooden outlet for furrow irrigation. 5-59. - Wooden outlet for basin o r border irrigation - maximum discharge 5-55(a)

'

around 85 l / s. 5-60.

-

Wooden outlet for basin o r border irrigation.

5- 61.

-

Snake River auto- s t a r t siphon.

5-62(a) and (b).

-

Outlet boxes for border irrigation.

3 60

PREFACE

This publication i s the result of a joint effort by the Food and Agriculture Organization of the United Nations (FAO) and the International Commission on Irrigation and Drainage (ICID) in producing a Handbook on small hydraulic structures and devices used in open- channel irrigation distribution systems.

There has been general recognition of

a need to review the abundant information and experience available on the subject and to condense and dovetail them into a comprehensive and practical Handbook.

Much basic

material for the Handbook has been generously provided by National Committees of the ICID and by F A 0 projects and contacts in Member Countries, while complementary data and information have been assembled from the extensive survey of the literature. The scope of the Handbook i s confined a s the title suggests to small structures used a t the f a r m level in fields, and in networks with small discharges at the intakes, such a s from small surface o r ground water resources.

Such. structures, having

capacities of l e s s than 1 cubic m e t r e per second, and, indeed, many of them having capacities of l e s s than 300 l i t r e s per second, account for more than 70 per cent of all the hydraulic structures installed in many irrigation networks. In the past these small structures have not always received the attention they deserve from planners and designers.

It .should be recognized that irrigation head

works, and other irrigation engineering works, however spectacular, would have little value without an efficient distribution system (requiring small structures) extending right down to the f a r m e r s ' fields.

The heavy investments normally involved in an

irrigation system can be justified, through conversion into cash benefits and the social welfare of the r u r a l population, only by paying full attention to the function and place of each of the small structures described in this Handbook.

xiv.

The Handbook i s published in three volumes.

1 to 5.

Volume 1 comprises Chapters

The types of small hydraulic structures available, and their importance for

efficient distribution

of irrigation water supplies a r e discussed in Chapter 1.

Chapter 2 discusses the operation of irrigation systems and how this governs the choice of the type of small hydraulic structure best suited to the purpose.

Chapters

3 to 5 deal with small intake structures, small flow-dividing structures, and outlets o r farm and field turnouts.

Volume 11 comprises Chapters 6 and 7.

Chapter 6

deals with small water-level and velocity control structures (i.e. checks or c r o s s regulators, falls o r drops, and chutes) and Chapter 7 with small hydraulic structures and devices useful for measuring flow in irrigation networks. Volume 111, which will be issued a t a later date, will cover small cross-drainage works, escapes and miscellaneous structures and will include a chapter on the detailed design of gate s.

F o r definitions of terms, reference should be made to the ICID Muitilingual Technical Dictionary on Irrigation and Drainage.

Units of measurement a r e generally

expressed in the units from which the formulae, designs, tables and graphs have been derived (and a r e thus best known in that system) but in certain cases it has been considered advantageous to convert English to Metric units for application in countries using only the Metric system.

Since the Handbook attempts to assemble and describe many types of small hydraulic structures which have proved successful in certairi countries, and which may be used elsewhere under similar conditions, i t i s hoped that i t will prove useful to.young engineers, technicians and extension workers involved in the remodelling of existing irrigation systems o r in the design of new projects.

It i s also hoped that

the Handbook will stimulate exchanges of ideas and information on techniques and designs which have often been evolved in isolation.

The present edition i s a provisional version; it i s intended that an updated version covering Volumes 1 to 111 will be printed in final form at a later date.

Any comments o r further contributions which readers might like to offer will be gratefully received and will be considered for incorporation in the next edition.

Acknowledgments a r e due to the many who have assisted in the production of this Handbook, some with systematic contributions, such a s the ICID National Committee s of:

.Arab Republic of Egypt

E duado r

Australia

Federal Republic of Germany

Bulgaria

Guyana

Canada

Hungary

Columbia

India

Czechoslovakia

Japan

Republic of Korea

Republic of China

Malaysia

S r i Lanka

Mexico

Turkey

Philippines

U.S.A. U. S. S. R.

and personnel of F A 0 and individual contacts who have rendered valuable information and advice, and to Mr. I. Constantinesco for his lucid editing of the manuscript.

Dated

Edouard Saouma Director Land and Water Development Division Food & Agriculture Organization of the United Nations

K. K. Fraviji Secretary General International Commission on Irrigation & Drainage

1. INTRODUCTION

Irrigation, with an adequate water supply, suitable soil, and good management, should ensure sustained high yields of c r o p s 2 e r unit a r e a of land.

When the water

supply i s l e s s than adequate o r costly, the a i m m u s t be to obtain the best possible yields p e r unit of water in combination with carefully selected agronomic and managerial practices.

The success of an irrigation project in meeting these requirements depends,

to a l a r g e extent, on the proper functioning of i t s water conveyance and distribution system.

P r o p e r functioning i s essentially identified with proper operation of the system

so that equitable and reliable apportionment of water among u s e r s and the conveyance of water with minimum l o s s e s can be ensured.

While operation i s dependent on good

organizational and institutional backing, i t s effectiveness i s basically dependent on awe11 planned, designed and constructed network f r o m the source of the water supply down to the f a r m e r ' s field. Engineering planning, design and construction of dams, b a r r a g e s , diversion weirs, main intake works, pumping stations and main canals a r e usually c a r r i e d out a t a high degree of efficiency.

Generally the operation of such headworks i s also efficient and

well organized and thus the amount of water l o s t f r o m the total supply i s usually small. Sometimes, however, secondary and t e r t i a r y canals and control s t r u c t u r e s a r e l e s s carefully made, while smaller canals and those a t the f a r m level and their s t r u c t u r e s a r e m o r e often badly made o r omitted entirely from engineering plans.

It m u s t not be

overlooked that besides headworks and l a r g e r canals, irrigation works involve the building of many small s t r u c t u r e s and s m a l l earthworks of unsophisticated design spread over extensive a r e a s of land.

Engineers have often neglected these "minor" works,

particularly those required a t the f a r m level; to contractors they do not mean much profit and they a r e dispersed and d.fficult to supervise; and l a s t but not least, authorities have sometimes appeared l e s s willing to invest in tens of thousands of such small scattered works than in l a r g e works having g r e a t e r prestige value.

This r e s u l t s

in many omissions of essential small structures, and failures o r unnecessary deficiencies in some irrigation systems.

The g r e a t impact of small s t r u c t u r e s on satisfactory operation and overall performance of gravity irrigation systems i s , however, apparent from their l a r g e number.

In gravity flow systems, 90 out of 100 s t r u c t u r e s usually have capacities of

l e s s than 1,000 l i t r e s p e r second.

The total number p e r unit a r e a depends largely on

the s i s e of holdings and fields, on the delivery pattern and on the topography, but ranges f r o m a few hundred to several thousand p e r thousand ha.

The total i r r i g a t e d a r e a of

the world a t present exceeds 200 million ha and potentials exist for doubling this a r e a . The number of small hydraulic s t r u c t u r e s already in existence exceeds 100 million, and the number that will need to be modified, replaced o r newly built every y e a r i s likely to run into millions.

In view of their g r e a t impact on the savinp, equitable delivery and reliable supply of water, small hydraulic s t r u c t u r e s m u s t be designed, built and operated with much the sarne completeness, efficiency and accuracy a s l a r g e ones. The factors governing the design and subsequent construction and operation of irrigation works a r e the water r e s o u r c e s available, the methods of water delivery to f a r m e r s , and the methods of water application practised by them.

Successful operation

r e q u i r e s adequate facilities for the control and measurement of flow at all strategic points along the whole network, including the f a r m and field levels.

Each small

hydraulic structure m u s t be efficient, simple in design, construction and operation, and m u s t be durable. The "largest" s t r u c t u r e s discussed in this publication a r e the intakes f r o m t e r t i a r y canals o r intakes f r o m small rivers'into complete irrigation systems, the head discharges of which do not exceed one cubic m e t r e per second.

Intakes a r e required

to control flow into a subsequent canal o r canal system; often they a r e combined with silt- excluding devices,

Intakes should be designed to control and regulate water with

minimum entrance l o s s e s and a s little disturbance a s possible.

Flow-dividing

s t r u c t u r e s o r proportional distributors a r e c l o s ~ l yrelated control structures, but a r e d i s c u s ~ e dseparately in this handbook. F o r the purpose of this manual an intake s t r u c t u r e is one for regulation of flow into an offtaking canal with a full supply capacity of not m o r e than 25 p e r cent of that of the parent canal.

When the offtaking canal o r canals leave l e s s than 75 p e r cent of the

discharge in the parent canal downstream, the structure effecting the division of the

flow into two, t h r e e o r m o r e portions i s considered a "divisor" o r "proportional distributort'.

The proportions may be fixed, o r adjusted by varying the control opening.

A divisor on a f a r m channel i s usually called a 'division box'.

An outlet (or f a r m turnout), the purpose of which i s to control water delivery to the f a r m , i s a structure. a t the head of a watercourse, f a r m supply ditch o r field channel which connects with a distribution canal ( e . g. a t e r t i a r y o r a quaternary).

The distri-

bution canal i s under the control of an operator of the authority in charge of the irrigation system. and the f a r m e r .

Thus, the f a r m outlet is the connecting link between the authority The fact that there a r e approximately a s many f a r m outlets a s t h e r e

a r e f a r m holdings, o r even a s many field outlets a s t h e r e a r e fields in an irrigation system, underlines the importance of this type of structure. A check o r c r o s s regulator i s a structure designed to r a i s e the water level in a channel.

This would be necedsary, for example, during periods of low discharge in the

parent channel when the check would r a i s e the water level sufficiently to feed an offtake canal, o r several in rotation.

A check may a l s o s e r v e to close temporarily and divert

the supply to the downstream end of a parent channel to allow time for r e p a i r s o r maintenance.

Checks a l s o help in temporarily absorbing fluctuations of water supply in

various sections of the canal system, in controlling flow velocities and in preventing breaches in the tail reaches.

F a l l s , measuring structures, bridges and other

s t r u c t u r e s can also be combined with checks o r c r o s s regulators.

At the f a r m level,

checks a r e required to divert water from one to another canal o r to serve a s a control for water delivery to the field. Whenever the slope of the land i s g r e a t e r than the grade required of the irrigation canal, the difference i s adjusted by constructing drops ( o r falls) o r chutes a t suitable intervals.

Generally, a water level control structure will be called a drop o r fall when

the reduction of the slope i s accomplished over a short'distance.

When water i s con-

veyed over longer distances and along slopes that a r e m o r e gentle, but still steep enough t o maintain high velocities, the s t r u c t u r e s used a r e known a s chutes. The measurement of irrigation water i s an essential element for i t s f a i r distribution and economical use.

Measurement s e r v e s to ensure the maintenance of proper

delivery schedules, to determine the amounts of water delivered and to single out anomalies in distribution.

Knowledge of the amount of water delivered facilitates proper

application to the field and, where applicable, provides a basis for water charges. i s also useful for estimating conveyance losses and detecting their origin.

It

A variety of

measuring methods, devices and structures has been developed to suit various conditions.

Measuring structures and devices a r e commonly combined with other

structures, such a s farm outlets, checks o r falls.

Some types have been standardised

-on a national scale in several countries; others a r e available commercially. Whenever an irrigation canal intercepts natural streams o r drainage channels in i t s fiassage, cross drainage works have to be constructed.

Cross drainage works may

be either syphons, aqueducts or level crossings, which pass the drainage water either under o r above o r s t the same level a s the irrigation canal.

Sometimes drainage and

irrigation water i s deliberately intermixed. Escapes act a s safety valves for dioposing of surplus water resulting from conditions such a s the follo+ng:

slack c r no demand for irrigation water due to

excessive rainfall on the comrrianded area; sudden closure of a canal due to a breach o r breaches; admittance of drain water into the canal through inlet and level crossings; temporary closure of a canal for urgent repairs; e r r o r s in regulation; and failure of P-PS -.

The regulation and distribution of flow through a hydraulic structure on an irrigation system i s , to a large extent, controlled by gates of various types.

The type

1

of gate required for a particular purpose depends on the dimensions of the gate, the water pressure, the mode of operation and on the availability and cost of local materials. The wide range of types of gates available has merited a special chapter in this handbook, to deal in detail with the design of gates which have proved successful in practice. In addition to the structures mentioned above, distribution systems may require bridges, road culverts, cart and cattle crossings; and other complementary structures. Automatic mechanical gate control has become a common asset in headworks and large conveyance canals, while in the terminal distribution system and on the farm such means of control a r e still quite r a r e . &ideal

completely automatic irrigation system i s one in which the water require-

ments of the plants a r e m e t by means of devices which m e a s u r e soil moisture and initiate a s e r i e s of operations to convey the necessary water through the network a t the right moment and a t the required r a t e and duration.

Technically, automation i s much

m o r e difficult to introduce in open flow networks than in p r e s s u r e pipe systems. Therefore, in new projects where labour and other conditions favour automatic delivery on demand, the trend i s toward's piped, systems, rather than open-channel systems. However, existing open-channel systems cannot be readily converted into closed pipe systems.

Yet social changes and changes in cultivation practices (multiple cropping,

use of high yielding varieties, etc.) may call for modification of traditional gravity flow networks. The problem h e r e is how to provide economically for the large capacities required, whilst minimising modification of existing engineering works.

This requires recon-

.

sidering existing safe limits with a view to increasing water levels and velocities, i. e. flow capacity and reduction of spill and other losses, especially in the terminal portion ,,

of a network. Auto-mechanisation of the control of irrigation water supply i s being tried a t the f a r m level in some countries with a view to economising on water deliveries to the field and to save labour. In new f a r m distribution networks it i s sometimes difficult to plan in advance the layout of the entire distribution system so that i t meets actual operational demands in respect of locations, dimensions and types of outlets and control structures.

Therefore,

i t i s sometimes advantageous to install temporary structqres a t f i r s t and then to replace them with permanent ones after gaining experience of operating the system.

.

F a r m irrigation structures that remain in place for m o r e than one irrigation season a r e considered permanent; those that a r e moved from place to place during each irrigation a r e considered portable o r temporary.

Provisional and temporary structures

m a y be m a d e of wood, metal o r plastic, while permanent ones a r e usually built of concrete o r masonry.

Water levels and discharges a r e regulated by wooden flashboards

o r wooden o r steel gates. In m'any gravity irrigation systems maintenance i s poor, resulting in reduced efficiency and the deterioration of the structures.

Where the reasons for poor mainten-

ance cannot be easily overcome, the only remedy i s the construction of simple, robust and "over -dimensioned1' structures

.

Where technically feasible, multifunctional

structures such a s drops, outlets, flow-dividing structures and measuring equipment, should be employed. The large number of structures of the same types and sizes used in l a r g e projects lends itself to standardization and centralized production.

In fact many small irrigation

structures have been standardized and a r e m a s s produced o r precast in quantity in central workshops.

The advantages a r e savings in costs; better co-ordination of

supply and transport; reduction in time of construction; better quality and uniformity of the final products; and quicker r e p a i r s o r replacements.

(It i s noteworthy also that

the off-demand periods of irrigation in many a r e a s coincide with unsuitable weather for in situ construction.) --

Furthermore, stock-piled precast components of structures can

be installed relatively quickly a t any convenient time.

Savings of 20 to 30 per cent in

precast a s compared with in situ construction have been cited from experience in the

There i s also a t r e n d t o be noted in the production of prefabricated p a r t s o r components which can be combined and assembled to form different structures, thus reducing both initial cost and repair o r replacement expenses. Despite i t s advantages prefabrication i s unrealistic in countries and a r e a s w h e ~ e labour-intensive construction would help to diminish under-employment and a t the same time result in a saving in costs. Successful irrigation ultimately depends to a large extent on the response of the f a r m e r , and thus the irrigation facilities made available to him must be designed and maintained with a view to meeting his requirements and aptitudes.

Wherever possible,

f a r m e r s and the staff operating the irrigation network should be encouraged to participate together in the selection of suitable structures, and in their operation and maintenance.

Such co-operation can prevent the'design and installation of structures

which may prove too complicated o r difficult to be mastered by people lacking the necessary technical training, and which would in practice be found to be functionally inadequate.

2.

2.1

IRRIGATION NETWORK OPERATION

GENERAL CONSIDERATIONS An irrigation system comprises all the physical and organizational facilities and services required to convey water for the irrigation of crops f r o m a source of supply to the f a r m e r s ' fields.

Systems may vary in size, scope and design from

one m e r e l y aiming at spreading the flood water of a r i v e r over adjacent a r e a s , o r conveying small flows f r o m a surface o r ground water source over limited distances and a r e a s to l a r g e networks ramifying over the land like the branches of a t r e e (Figure 2- 1)

FIGURE 2- 1.

-

Sketch of a typical irrigation system (punjab).

Networks for gravity flow irrigation may consist of open canals and hitches o r of low p r e s s u r e buried pipes, o r both, in the tail portion of the system. Buried pipes a r e not usually economical -in gravity flow irrigation systems where the pipe diameter has to be large enough for a flow exceeding 1000 l / s e c .

2.1.1

Need for Flow Control Irrigation is in effect synonymous with artificial flow control from the water source to the plant.

Artificial flow control i s achieved by the interaction

of physical facilities (e. g. hydraulic structures) and organized operation (e.g. an irrigation programme).

Planning and design ob a flow system should extend

from the f a = m e r s l fields towards the source of water supply.

The basic data for

the entire system a r e those related to the soil, the climate, the plant, the f a r m e r and his fields.

The adequacy of these data and their correct application will

ultimately determine the success of the project.

The distribution system must

allow the delivery of a definite quantity of water to each field and i t must be possible to vary the quantity with time.

In addition to irrigating efficiently, o r

m o r e precisely, and thus satisfying the crops' needs for water, the quantities delivered to each point must be so determined that they also satisfy certain economic and practical requirements.

This i s usually achieved by establishing

in advance a "programme" for the system a s a whole and for each branch canal a s well-1 1 The establishment of a programme shoufd be the joint concern of the

.

agronomist, the i'rrigation practices specialist, the f a r m e r and the engineer. The latter then h a s to take all the steps necessary to ensure that the operations can be smoothly and accurately carried out.

If the water cannot be distributed

strictly to programme the crops will suffer and a danger of exhausting the water resources o r of waterlogging, salinity, etc. could develop.

The need to save

water and to provide accurate metering facilities from which realistic unit costs of water can be computed must always be the primary consideration.

The amount

e

of water to be suppli d to each f a r m plot varies f r o m year to year, and even during the same year, depending on the crops grown and the needs of the plants during the various stages of their growth.

And, lastly, the effects of the weather

during the growing period have to be taken into account.

L'

Guidance a s to the synthesis of the different factors involved i s provided in references (77) and (2'3)

Since the very aim of irrigation i s to supply crops with the water they need for their growth at the right time, ideal distribution implies the ability to control the discharge from each canal a t any instant so that the canal o r i t s branches can satisfy'the net demand of the a r e a they serve.

In other words, the response to

demand must be a s accurate and immediate a s possible. At the same time a s the discharges to the water consumers ( i r r i g a t o r s ) a r e met the water level must be controlled, for two main purposes, depending on circumstances: to r a i s e the water level a s high a s economically possible with a view to increasing the a r e a under irrigation; and to control variations in the water level:

-

to a minimum level, to prevent canal deterioration o r to store water in the network

-

to a maximum level, i f there i s a danger of overflowing

-

to a predetermined level for crops that have to be completely submerged (such a s rice), the depth depepding on the stage of growth of the plants

-

to a r e s t r i c t e d range, a s an aid to discharge control at intakes, f a r m outlets, etc. The conveyance and distribution of water can be controlled by two principal

methods

-

upstream control and downstream control, and by combinations of both

these methods.

Upstream Control In an upstream controlled irrigation system water i s discharged from the supply in a predetermined amount at the head of the main irrigation canal.

The

amount of water to be discharged i s specified in a previously drawn up irrigation programme.

The subsequent distribution in the branch and various distributary

canals occurs according to recognized demands o r "water duties". Hydraulic structures for this system a r e designed to maintain a constant and predetermined upstream water level, the discharge capacity of the structure being set by a fixed c r e s t o r by manual o r automatic means using such devices a s stop-logs, slide gates and radial gates.

The volume of water in upstream con"

trolled canals remains practically constant during operation, whatever the flows

through the various sections may be. Upstream control of an irrigation system i s used where water r e s o u r c e s a r e limited, a t l e a s t during p a r t of the y e a r .

Where water i s delivered in

rotation, it may be applied in this way to entire sub-networks o r may be practised among single l a t e r a l canals such a s distributaries, minors, and watercourses o r among individual, f a r m s along a lateral, depending on the size of the whole system, the supply-demand situation and other factors ( s e e also Chapter 5).

Downstream Control If the supply available a t the head is always g r e a t e r than the demand, dischsrge control considerations become l e s s imperative, and each u s e r can be allowed to regulate his own supply, providing that the effect of the amount he draws i s transmitted step by step to the head of the system and causes the overall supply to the network to be adjustec?.to suit the cumulative demand.

In the

practical application of downstream control some form of automatic control equipment i s necessary.

2.2

(See 2 . 3 and 2 . 4 below. )

ENTIRELY MANUALLY OPERATED SYSTEMS Under entirely manually operated systems water supplies a r e released from upstream (upstream control) on a cropped a r e a basis, o r on a volumetric basis, o r according to special agreements.

The intake to the main canal and all

subsequent regulating and control s t r u c t u r e s a r e manually controlled ( o r a r e of fixed c r e s t o r fixed orifice type in which case, except for periodical inspection, no human interference i s required).

Large gates in these systems a r e often

operated by motors, but the motors have to be manually started and stopped. Manual operation i s the most wide- spread and traditional form of conveyance management and continues to be dominant in' all major irrigation a r e a s of the world.

Manual operation differs little f r o m one country to another except in

organizational aspects.

Within the scope of this Handbook manual control a s

practised in India s e r v e s a s an example. Example from India In the case of r e s e r v o i r storage ensuring adequate water supplies the water

is manually released ( f r o m upstream) a t the time and to the extent required by the recognized demand.

In the case of run-of-the-river supplies t h e r e a r e often

seasonal shortages of water and under these conditions the principle employed i s equitable distribution of the supply available.

Either each distributary system i s

run in rotation with full supply o r with a supply proportional to the total available supply in the system at the time.

(Usually there i s an acceptable period every

y e a r during which the whole network i s closed down for maintenance work; in Northern India this period i s three weeks). The staif responsible f o r the manual operation of a canal system i s headed by a "Divisional Engineer", in charge of the headworks; under him i s a "Sub-

'

Divisional Officer (Headworks)", who i s s u e s o r d e r s for the regulation of supplies in the canals under the instructions of a "Regulating Officer".

These o r d e r s a r e

based on the recorded knowledge of the water levels throughout the system of canals down to the head of the distributaries and of the "indents" (requests) for water supply received f r o m other officers responsible for the detailed d i s t r i bution of water in individual canals o r p a r t s of the system. E a r l y every morning gauges installed at various control points on a canal system a r e read by appointed gauge-readers and the data a r e communicated to a staff of signallers who transmit them by telegraph to the Regulating Office a t the headworks where they a r e tabulated on a printed form.

This tabulated f o r m i s

generally delivered to the Regulating Officer by 8.00 a . m . each day.

By this

time he will have received the indents for a reduction o r an i n c r e a s e of discharges of water a t different control points of the system.

He i s thus in a

position, (by correlating gauge readings and discharge indents) to i s s u e the o r d e r s n e c e s s a r y to bring about a redistribution of the water supply in accordance with the indents.

Twenty-four hours' notice i s usually required to

meet an indent. F o r the safety of the canal system, i t i s important that the amount of water in each.cana1 be within i t s safe capacity.

In o r d e r to allow for mis-regulation

and for e r r o r s and omission by gauge r e a d e r s and gate operators, for unforeseen rainfall and for preventing canal breaches, safety devices (such a s escapes) a r e provided in the system for the disposal of surplus water.

In the summer, when supplies from the run-of-the-river o r the storage

r e s e r v o i r a r e plentiful, all canals of a canal system a r e run a t full capacity throughout the season, except when t h e r e i s little o r no demand for irrigation water due to heavy rainfall, o r when t h e r e a r e major floods in the r i v e r from which supplies a r e being taken, (in which c a s e the canal h a s to be partially o r fully closed a t the head to p r e u m t excess silt entering Qntoit). During the winter, r i v e r supplies a r e limited and i t i s not possible to run all the canals a t the same time, and thus partial supplies a r e run in the main canal o r the branch canals but distributaries a r e run ' f a 1 in rotation.

For

efficient distribution i t is highly desirable that d i s t r i h t a r i e s and/or m i n o r s run either full o r remain closed; in Northern India it i s common practice to open them for 20 days and to close them for 10 days.

(This practice h a s been

developed in India to suit local conditions; there a r e .of course alternative ways of dealing with seasonal water shortages. ) Manual control by the Irrigation' Authority does aot usually r e a c h further down the system than the intake gate to a minor canal.

F r o m this canal a

number of f a r m outlets r e l e a s e the water "automatically" into f a r m water courses.

A water course may s e r v e an average of 15 f a r m s .

The water i s

shared according to an agreed r o s t e r on the basis of equitable distribution.

It

may be n e c e s s a r y for the Irrigation Authority to intervene in the internal rotation system when disputes a r i s e among u s e r s .

In India t h e r e a r e two

systems on which the r o s t e r distribution of water supplies i s planned: the "fixed-turn" system and the tffollow-on" system. Under the "fixed-turn" system, individual holdings take water for a definite period according to the size of the fields, so that the'tarn of each holding o c c u r s once a week on a fixed day at a fixed hour for a fixed time.

The t u r n s a r e so

computed that the total of their periods i s equal to one week.

The advantage

of

this system i s that every cultivator knows exactly when he i s responsible for handling the discharge from the watercourse.

On the other hand, should i t be

n e c e s s a r y to employ rotational running and different t i m e s of delivery because of a water shortage one o r m o r e of the cultivators may completely m i s s a turn, o r even two, and the crops may suffer in consequence. Under the "follow-on" system, the turn t i m e for each cultivator is calculated on the basis of s o many hours u s e of discharge per 100 a c r e s .

The

rate varies in different localitiee according to the crops grown and other relevant When one cultivator has finished his turn, he hands over the supply to

factore.

the next on the roster.

The obvious advantage of thie aystem i s that every

cultivator gets his turn in due course..

The cultivator who irrigates at the end

of one period of the channel rotation, resumes irrigation at the next running and thus finishes his allotted turn.

This requires careful assignment of hours to

each area. Under both these systems the roster l i s t s have to be prepared very careThe water should be turned into the fields nearest to the outlet, first to

fully.

the right and then to the left, but always proceeding down the watercourse. reaching a branch watercourse

On

irrigation should proceed down that branch and

be completed on all areas covered by it.

Then irrigation should s t a r t again from

the junction and proceed down the o ~ p o s i t ebranch, if any, in a similar manner. Finally, irrigation should then proceed down the main watercour se again.

This

method ensures that water i s utilized a s soon a s there i s water in the supply line. When a cultivator's turn a t the end of a rcain watercourse o r branch water

-

course i s over, the branch watercourse i s closed off and the water left in the channel may run into his field.

This 'balance' of water compensates the

cultivator for any shortfall in supply because of the distance of his field from the water source. eubetantial.

When the watercourse i s long the volume of water left in i t i s Even so, disputes can a r i s e among the cultivators at the extreme

end of a watercourse a s to their share in this 'balance' water.

These disputes

can be settled by letting the cultivators take turns at being last to receive water and by making due allowance for the amount of balance water received by adjusting the time.

The problem of irrigating high areas, too, may be eettled

by experimenting on the site, in the presence

01 all

the cultivators involved.

The

exact extra time of flow required for the cultivator concerned in such a r e a s during a turn would be accepted by all once they had seen the justice of the arrangements made.

In order to avoid hardship to any cultivator whose turn falls during the night, two sets of r o s t e r s a r e drawn up, each differing from the other by twelve hours.

Each of these sets i s operated by rotation in alternate years.

Systems varying slightly from these practices, in accordance with local

14.

needs, a r e followed in different p a r t s of India.

2.3

1/ HYDRO-MECHANICALLY AUTOMATED FLOW CONTROL SYSTEMS-Hydro-mechanically automated flow control systems a r e those in which various types of gates equipped with floats and w e i r s with long c r e s t s a r e used for automatic regulation of water levels in irrigation systems.

The m o s t

important of these a r e the NEYRPIC constant level upstream gate (AMIL) and the constant level downstream gates (AVIS) and (AVIO).

Technical descriptions of

these gates and the weirs a r e given in Chapter 6. 1.

Functional aspects a r e

discussed below.

Hydro-mechanical Devices for Automatic Control of Constant Upstream Water Level

2.3.1

Hydraulically-automated conscant upstream water level gates (AMIL), o r weirs of long c r e s t (such a s duckbill weirs o r diagonal weirs) divide the canals into successive reaches.

The constant upstream level gate, a s i t s name implies,

automatically maintains a constant water level upstream with only insignificant variations.

Similarly, a long c r e s t weir, (because of its cl'est length in relation

to discharge), automatically controls the upstream water level within n a r r c v limits, and m o r e o r l e s s independently of variations In discharge. The intervals between these automatic level control s t r u c t u r e s depend upon the grade of the canal and the number and size of offtakes along it.

Regulation

a t the main intake i s achieved by manual means and follows a previously established programme.

Subsequent distribution in the branch o r distributary

canals i s c a r r i e d out in accordance with recognized demands.

The demands may

be based on the a r e a served o r the actual demands of the f a r m e r s based on their c r o p water requirements. Figure 2-2 illustrates the function of this system; in a given canal reach the smallest level range between full and z e r o discharge o c c u r s immediately upstream of the gates o r weirs, while the highest level range o c c u r s downstream

It

This section i s largely based on reference (69)

Off take channel

I Upstream constant water level gate7

.

\

1

\ I

I

~ ~ ~ o c l u l

V II"

rI Off take channel Duckbill Weir (as alternative to I the upstream constant water e I level gate)7

I

Downstream constant water level gate

k:::echanneIs-l

FIGURE'2-2.

-- - . Module

'TI

Off take channel

-

Diagrammatic layout of an upstream controlled network.

of these control structures.

Consequently i t i s desirable to group offtakes in the

vicinity upstream of the control structures (Zone B).

Offtakes which for other

reasons have to be located near the downstream side of a control structure (Zone A) have to be equipped with a device for automatic constant downstream level regulation (e. g. an AVIO o r AVIS gate) in order to ensure that the discharge in the offtaking canal is independent of variations of water level in the parent canal. Alternatively, the distances between the constant upstream control gates in the parent canal may be reduced.

The optimum design to employ has to be arrived

a t by comparing the results, and construction costs, of using a given number of small level gates in the offtaking canals with the addition of one large gate in the parent canal. In this system, the shutters of the offtakes o r outlets a r e usually of semimodule type and manually operated.

Once set they give the desired discharge.

The most commonly used devices a r e the NEYRPIC distributors (described in Chapter 3). #

2.3.2

Hydro-mechanical Devices for Automatic Control of Constant Downstrealh I Water Level I

A downstream-controlled distribution system i s one in which the discharges

through it a r e controlled by the a s e r e along the canal.

Each individual demand

i s automatically transmitted back to the head of the system and causes the overall supply to the network to be adjusted to suit the cumulative demand. The step-bystep transmission of the demand i s ensured by constant downstream level gates, regularly spaced along the entire length of the network.

At a call for water in

a given canal section, the gate a t the upstream end of the canal opens to compensate for the falling level; this causes the section upstream of it to s t a r t emptying, whereupon the next gate lifts

- a process

repeated ali along the line.

Decreasing demand has the opposite effect; all the gates close one after the other, downstream to upstream.

heref fore

the m e r e act of setting a water

intake to a plot of land to the required flow i s enough to ensure that the c o r r e sponding amount i s supplied right through the entire network. Figure 2-3 is a diagram showing the layout of a downstream controlled system.

As can be seen, the offtakes equipped with shutters only a r e grouped

immediately downstream of the constant level gate (I in Zone A) while offtakes

located further down the reach and particularly those near the upstream side of the control gate (I1 in Zone B) a r e equipped with additiofral constant downstream level gates (111), in order to ensure that the discharge i s iddependent of level variations in the parent canal (level range "a").

Similar to the upstream control

system there i s an economical optimum between the length of the control reaches in the parent canal and the number of offtakes to be equipped with constant downstream level gates.

Shutters a r e of the same type a s in the upstream controlled

system, (see also Chapter 3).

---

T--pB Freeboard -

eve1 ~t zero discharge

---------i

Downstream .constant water

Offtake canals

Supply channel

c

Level at full supply discharge

-----

-

Downstream constant water level

tant water level ITI Off take canals

FIGURE 2- 3.

- Diagrammatic layout of a downstream controlled network.

Comparison between Upstream and

owns stream Control Systems

Wherever the consumers cannot be provided with unlimited water because of inadequate supplies o r for other reasons, u p s t r e a m control i s the only possible method

- a t l e a s t a s f a r a s the t e r m i n a l portion of

a network i s concerned, since

i t enables each u s e r ' s consumption to be fixed fairly in accordance with the water supplies available. However, the supply of water to the head of the s y s t e m and the sharing out of the water available among the u s e r s r e q u i r e s a l a r g e and highly experienced operating staff.

It i s practically impossible t o s e t all the individual discharges

in a network exactly equal to the sum of the discharge drawn off f r o m the supply canal plus leakage and evaporation.

Thus deficits in distribution, o r some

wastage due t o over-distribution, can hardly be avoided.

Since the lower

r e a c h e s a r e only supplied when the upper r e a c h e s a r e full, a certain amount of e x c e s s water h a s to be kept in r e s e r v e u p s t r e a m a s a safety m a r g i n .

This i s

obviously wasteful, causing a l o s s of a s much a s 100/o of the total water supplied, even assuming a carefully drawn up p r o g r a m m e and painstaking distribution (69).

A further drawback i s the unavoidable time lag in the t r a n s m i s s i o n of orders.

This a r i s e s f r o m the fact that the amount of water stored in each canal

section, and t h e r e f o r e the storage capacity of the network, i n c r e a s e s with canal discharge, ( s e e hatched section in F i g u r e 2-2).

If t h e amount of water supplied

a t the head i n c r e a s e s , the f i r s t section h a s to f i l l up to the level associated with the higher discharge before the f i r s t gate lifts; s i m i l a r l y , the second constant u p s t r e a m level gate does not open until the second r e a c h h a s found i t s new level, and s o on a l l the way down the line.

The aforementioned applies, with only

negligible variations, to long c r e s t e d w e i r s for u p s t r e a m control.

To i n c r e a s e

the discharge a t the tail of such a system, a c e r t a i n amount of water (which i s s t o r e d in the successive, u p s t r e a m , r e a c h e s ) h a s f i r s t t o be sent through the system; conversely, closure of the head gate does not have any effect on the flow a t the downstream end until some of the water in all the r e a c h e s h a s f i r s t run off. Thus, s e v e r a l h o u r s may sometimes be required for a given discharge s e t a t the headwork, to r e a c h the f a r m e r s , particularly where canals a r e long and conveyance velocity i s low. The main feature of downstream control i s that it provides a completely

automatic distribution system that responds immediately to every demand. Water i s saved because the quantities supplied a r e exactly the same a s those Another out-

drawn off, even allowing for leakage and exceptional demands.

standing feature i s that no water i s wasted i f consumption ceases completely; the discharge supplied to the head of the system, during the interval of time required for the closing o r d e r to be transmitted, i s m e r e l y added to the amount already stored in the system, where it remains available until a f r e s h demand is made. However, downstream control also h a s serious drawbacks.

It i s seldom

possible to get i r r i g a t o r s to limit their demand, and during a water shortage, therefore, the canal will gradually empty section by section from i t s upstream end downwards, and deprive the consumers depending on the upper reaches of water, whilst those a t the lower end a r e still drawing their full supply.

The

same situation would occur in the case of a breakdown in the system, such a s blockage of a gate, breakdown of a pump, o r a canal breach.

In such c a s e s all

automatic gates situated downstream of the point of accident would open, in o r d e r to m e e t the current demand. pletely emptied.

Consequently the network below would be com-

Apart from inconvenience to the consumers, serious damage to

the canal (e.g. uplift of linings) might occur. Downstream control generally requires fairly large canals, since their capacities have to be l a r g e enough to contain the volume of water corresponding .to the levels a t z e r o discharge which a r e of course above those for full discharge. Therefore, extensive e a r t h and canal lining works a r e required.

A comparison

between the costs of establishing downstream and upstream controlled s y s t e m s shows that the cost of the f o r m e r rapidly becomes prohibitive a s the slope of the ground i n c r e a s e s .

Slopes of m o r e than 30 centimetres per kilometre may be

considered a s a practical upper limit for downstream control (69). The specific limitations of either the upstream o r downstream control methods can be alleviated o r eliminated by combining them with each other o r by integrating them with other control m e a s u r e s discussed in the following Sections of this chapter.

2.3.4

Combined Hydro-Mechanical Control Systems There a r e two systems in u s e where upstream and downstream control a r e

combined; one may be described a s the "longitudinally combined system" and the other the "composite gate system". The longitudinally combined system In many instances i t i s advantageous to use different control systems in the lower and upper part of an irrigation network. Upstream control i s usually resorted to for the terminal portions of a network in view of the general necessity of keeping the offtakes under the supervision of the operating staff, to avoid exhausting the water resources and to enable the consumption to be checked.

The main supply system, however, (that i s to

say the main conveyance canals), may be equipped for downstream control, thus making i t very much easier to c a r r y out the irrigation programme by doing away with the necessity of going all the way to the head intake (sometimes situated a considerable distance away) whenever the discharge supplied to the network has to be changed.

Moreover, the system can be brought into operation much m o r e

quickly and no water will be wasted when setting the controls.

With such a

layout, the programme can be made very comprehensive and even allow for unexpected demand.

Finally, a system of this kind can usually be kept within

economic bounds since the main canals a r e often laid out along a contour line and the secondary canals run m o r e o r l e s s down the steepest slope. The composite gate system I

The composite gate system i s confined to the use of NEYRPIC hydromechanical gates of composite type in the main network and constant downstream level control gates, followed by shutters (distributors), in the offtakes o r outlets. When conditions a r e normal, (i.e. when the supply i s equal to the consumption) composite control gates behave in exactly the same way a s constant downstream level control gates and therefore have all their characteristics. When the water supply exceeds the consumption, the gates, which then control a constant level upstream, open to prevent overflowing.

Here the composite gate

s e r v e s exactly the same purpose a s emergency siphons o r escape structures in downstream controlled systems, without however wasting water.

When the supply

to the canal i s below the overall downstream demand, the gates close before the reaches upstream of them a r e completely exhausted so that some water i s always

kept in r e s e r v e throughout the system.

This means that the canals a r e no longer

in danger of running dry, nor do the upstream u s e r s suffer, while those downs t r e a m a r e still drawing their .full supply.

Thus the total r e s e r v e available can

be shared out fairly among all the u s e r s until f r e s h supplies a r e made available. A canal network thus equipped with composite control gates can be made to function a s a compensation reservoir, which can absorb supply-demand differences and these differences may be caused by: varying head supplies resulting from upstream hydro-power generation; the need to make use of a constant flow supply while demand varies during the course of an irrigation day; sudden r i s e s due to storm water entering into the system; sudden drops due to a canal breach o r other breakdown.

2.4

2.4.1

ELECTRICALLY-ELECTRONICALLY AUTOMATED FLOW CONTROL

Local Electric Control, Telemetering and Remote Electric Control Electrically operated gates can be used for any of the previously described control methods

- upstream,

either locally o r remotely.

downstream and combined.

They can be controlled

Control is usually based on electrical sensing of the

water level using floats o r electric probes.

Variations in water level in the

control section of the canal a r e conveyed to the gate by means of a transmitter, an amplifier and an integrator in accordance with the operational scheme adopted. Thus individual control gates can act automatically in much the same way a s hydro-mechanically operated gates.

Electrically controlled gates a r e usually

chosen for intakes and other structures on large canals, where a supply of electricity i s readily available and where gate dimensions a r e too l a r g e for hand operated o r hydraulically automated devices.

Usually the controls a r e set

to operate one gate while the other gates o r stop-logs have to be adjusted manually a s demand changes during the season.

Telemetering and remote control Electric sensing of water levels or flows and electric operation of gates lend themselves to telemetering and remote control, which provide the advantage of integrating single gate operations into the overall operation of the network a t a central point.

Telemetering o r remote metering i s a means by which water

level, flow, p r e s s u r e and other data can be m e a s u r e d remotely and communicated to a control point for display o r recording o r both. Remote control o r supervisory control in this context i s a means whereby an operator a t one location can control the function of devices a t a location remote f r o m himself with electrical o r electronic equipment, o r both.

The operator

simply controls the operation of the remote devices by activating push-buttons o r other kinds of switches'on a console.

Gates o r valves can be opened o r closed,

with indicator lights showing their position.

Pumps can be started o r stopped, f

with indicator lights showing whether they a r e working o r not.

A l a r m s may be

activated to a l e r t operators t o a high o r a low water level, a power failure, o r the l o s s of a communication o r control channel. An examplebf a remote o r supervisory control system i s the one installed on the Salt River P r o j e c t in the U. S. A . , which s e r v e s about 100,000 ha of i r r i g a t e d land.

The operating agency i s installing a supervisory electronic

control system by means of which i t will monitor and operate 174 canal gates and 54 of i t s 250 deep well pumps f r o m a central operating station (109).

The system

comprises and provides remote control and information facilities a s listed below:

-

stilling wells with water level t r a n s d u c e r s (to permit sensing of upstream and downstream water levels)

-

gate position transducers (to sense and indicate gate positions) gate controls (to open o r close a gate o r to s e t the gate a t the required opening) pump controls (to turn the pumps on o r off) valve controls (to adjust a valve from open to closed) a l a r m system to sense =ii

a l e r t the operator to high o r low

water, local power failures, and communication link failure

-

device for information retrieval data logging for special records

-

graphic display panel of the canal and lateral system

2.4.2

operations console with all means for control and warning all central control equipment for the whole system.

Comparison of Hydraulically and Electrically Controlled Systeme Among the advantages of hydraulic control a r e :

-

simplicity of installation (prefabricated devices .available) robustness of the devices (float equipped gates) automatic functioning not requiring skilled operators little maintenance no electric energy supply needed.

Unfavourable points a r e :

-

a greater amount of earthwork and lining i s required (concerns only downstream control)

-

the working parameter s of a hydraulic control gate cannot easily be modified once the gate has been installed, while a change in the working parameters of an electrically automated gate simply means modification in the transmitters

-

the hydraulically automated device cannot be operated by remote control, thus the many advantages of centralized operation (with o r without computer aid) cannot be obtained

-

susceptibility to blockage by debris, algae, etc.

As a consequence of the factors listed above i t i s preferable in most cases not to introduce either of the systems alone throughout an irrigation network but to control the supply canals electrically and the distribution canals hydraulically, o r manually.

accurate regulation (estimates of consumption, time of transit, etc. )

-

g r e a t e r flexibility a s to the choice of r e s e r v o i r s i t e s reduction in overall, size of canal reache s and s t r u c t u r e s a s compared to conventional downstream controlled systems (in which each reach i s calculated to cope with any demand o r non-acceptance, even the m o s t improbable). Dynamic regulation also affords a g r e a t e r safety margin in the event

of random accidents.

Any conveyance system i s m o r e o r l e s s susceptible to

accidents o r malfunctions at the gates o r faulty operation of recording devices o r other failures, which in conventional systems may result in wrong operation. A dynamic regulation system through a p r . o g r a m e f o r comparing data received

can recognize that the data f r o m a level recording'device i s wrong o r that the 1

device i s out of action and can o r d e r a temporary remedy. Even in case of failure during transmission the computer i s warned of the abnormal condition because of a built-in self-control system.

The

computer itself i s provided with an automatic protective system against power supply breakdowns, etc., and signals an a l a r m to the operator in c a s e of improper working of the control programme. With dynamic regulation, the flow at the head of each main branch of a canal is adjusted, for example, in line with the state of the water r e s e r v e s a t a given moment and in accordance with estimated consumption.

If the

probability of this estimate i s known a s a result of a preliminary statistical study, the amount of regulation can be determined in conformity with the standards chosen by the operator.

Fine adjustments to distribute the volume of

water in line with demand among the various reaches i s achieved by the automatic hydraulic devices on the basis of water levels, a s mentioned e a r l i e r . Thus the average water level in the canal will often be low.

In c a s e of an

accident this leaves a much g r e a t e r margin of manoeuvre before the accident causes over- spill.

2.5

MANUAL VERSUS AUTOMATED CONTROL Manual control essentially r e f e r s to visual level measurement, verbal

transmission of data and o r d e r s , and hand operation of gates, stop-logs o r valves in regulating s t r u c t u r e s along an irrigation network.

The dlrrnount of water

passed, for example, t!hrough a gate depends both on the extent to which the gate i s open and on the aapstx-eam and downstream water levels, so that, even i f stable upstream and downstream levels a r e assumed to exist after a setting h a s been made, the d i s c h g e is difficult to a s s e s s , since it h a s to be calculated, o r

determined, from tables.

F u r t h e r m o r e , the water levels never remain constant

because of &hesettings being c a r r i e d out on neighbouring offtakes and changes in a n d conditions.

The fact that the amount of water supplied to an individual

parcel of land thus depends on such an extensive group of regulating works and a

large number of control operations, all of which a r e likely to occur anywhere along many kilometres of canals, may c r e a t e a fairly complicated system. Finally, any change in the operation of a network requires a certain'time lag, and scetthgs a r e bound to be inaccurate because the control operations often have to be c a r r i e d out before the new conditions have been stabilized.

Control can be made

e a s i e r , and distribution improved, by in stalling regulators o r checks at suitable points along the main canal, which divide i t into successive reaches and reduce the amplitude of the water level variations above the offtakes; this a l s o partly solves the problem of controlling the levels.

Nevertheless the r e s u l t s obtained

in this way a r e still incomplete, and not only the head l o s s e s remain high, but the positions of the gates o r heights of the stop-logs frequently have to be adjusted, requiring a considerable amount of handling. In the terminal portions of manually operated sy atems distribution i s often c a r r i e d out in the "automatic" fashion described under Section 2. 2, i.e. proportional distribution through ungated outlets designed to provide equitable delivery of water over each a r e a .

This system functions reasonably well, a s

long a s operational conditions remain close to those f o r which the system was designed.

In practice, however, i t often fails because of poor supervision,

inadequate supply level, tampering ( s e e Figure 2-4) and lack of f a r m e r s ' confidence that they a r e receiving their fair s h a r e of the supply.

The system

also lacks flexibility, which m a y become desirable a t a l a t e r date, to enable, for example, changes in water delivery o r cropping pattern.

-

FIGURE 2-4. Example of tampering. B r u s h and stone dam built by f a r m e r s a c r o s s a minor irrigation canal to i n c r e a s e flow through a pipe outlet.

When considering improvement of existing s y s t e m s o r the design of new ones t h e r e a r e s e v e r a l possibilities to be examined before introducing automated control.

Such possibilities include organizational and supervisory improve-

m e n t s (for example, extending control of water delivery by an irrigation authority to the f a r m level).

F u r t h e r improvements which could be introduced

a r e : regular water m e a s u r e m e n t , r e c o r d keeping and evaluation of r e c o r d s for improved irrigation p r o g r a m m e s , equipping of f a r m outlets with gates, road a c c e s s along canals, etc. The introduction of automation into the conveyance and distribution of irrigation water i s not m e r e l y a replacement f o r human action but i s p r i m a r i l y a m e a n s of making water t r a n s p o r t m o r e efficient, m o r e reliable and m o r e flexible.

It i s c l e a r that, when water conveyance efficiency r e m a i n s a t a low

level, while demands i n c r e a s e steadily, o r when labour costs exceed a certain limit, i t i s advisable to consider whether and how to make u s e of automation in one f o r m o r another. The change f r o m manual t o automated control can be made gradually,

starting perhaps with the electro-mechanization of the head gate and proceeding, a s know-how in handling the equipment i n c r e a s e s , t o downstream regulating structures.

.Because of cost considerations, the u s e s of supervisory control o r

dynamic regulation s y s t e m s a r e r e s t r i c t e d to l a r g e complex water t r a n s p o r t s y s t e m s having a number of functions.

However, remote control and tele-

m e t e r i n g equipment may be considered f o r use in s m a l l e r s y s t e m s . Questions of considerable c u r r e n t i n t e r e s t with r e g a r d to automation a r e the modification of rigid rotational s y s t e m s to ones which a r e m o r e demand oriented, and conversions of non- stop delivery s y s t e m s into ones of daytime delivery only. The m a i n problem to be solved h e r e i s how to provide for the l a r g e r capacities required without, f o r economic r e a s o n s , having t o make m a j o r modifications of existing engineering works.

This r e q u i r e s reconsideration of existing safeguards

with a view to increasing water levels and velocities, i. e. capacity and reduction of spill and other l o s s e s , especially in the t e r m i n a l portions of the s y s t e m s .

In

other words, an elaboration of the entire physical a s well a s the organizational s y s t e m s i s required, and this can only be accomplished with some f o r m of automation o r remote control.

2.6

WATER DISTRIBUTION ON THE FARM

Conventional Surface Irrigation After water h a s entered the f a r m unit through the f a r m turnout, i t i s conveyed in a system of p r i m a r y and secondary channels ( o r ditches) to the head of the field to be i r r i g a t e d .

The system h a s to be laid out so that water can be made

available t o each p a r t of the f a r m a t the c o r r e c t r a t e and elevation for the selected method of water application.

The capacities of channels in s m a l l f a r m s

a r e usually designed to accept the total r a t e a t which water i s delivered.

In l a r g e

f a r m s the flow m a y be split into s e v e r a l s t r e a m s , especially during peak demand periods, to i r r i g a t e s e v e r a l a r e a s simultaneously.

Operation of the field channel

i s c h a r a c t e r i z e d by a period of flow of a few h o u r s only, whereas the main channel m a y be in operation f o r one o r two days during each irrigation period.

The

dimensions selected for the channels which deliver the water should be such that one man can handle the flow o r that two men can handle i t in the c a s e of long channels o r b o r d e r s .

The system should be sufficiently flexible to m e e t monthly

variations in demand and to allow f o r possible future changes, e. g. in cropping patterns and intensity of cropping.

The actual field application r e q u i r e s a depth

of water which depends on the s s i l m o i s t u r e content and on the depth of wetting required which for efficient irrigation m u s t be m u c h the s a m e a s the depth of the plant root zones a t the time of application.

The water i s applied f r o m the f a r m

channel(or d i t c q to the fields by f r e e surface outlets, pipe outlets, siphons o r spiles, depending mainly on the method of irrigation.

These s t r u c t u r e s a r e

discussed in Chapter 5.

In o r d e r to apply water efficiently to the fields the water level in the head channel should be 15 to 30 c m above the level of the ground to be i r r i g a t e d .

If

possible the channels should be nearly level ( l e s s than 10 c m p e r 100 m fall) so that the water can be backed up for a maximum distance, thus requiring a minimum of checks and labour to control the flow.

It should be kept in mind,

however, that the higher the water h a s to be dammed up in the head channel, the l a r g e r will be the wetted p e r i m e t e r , and the water l o s s e s in the channel, i f i t be unlined.

Accurate distribution of water to the various fields i s controlled by

various types of s t r u c t u r e s .

F o r efficient operation i t i s important that the

c o r r e c t type of s t r u c t u r e be selected and properly located. s t r u c t u r e s m a y be permanent, semi-permanent o r portable.

F a r m irrigation Although i t i s

desirable to u s e permanent s t r u c t u r e s f r o m the very beginning of the construction of f a r m irrigation s y s t e m s , i t i s sometimes hardly possible to avoid e r r o r s in location o r elevation, with the r e s u l t that the s t r u c t u r e s work ineffectively and have to be removed l a t e r on; thus i t i s e a s i e r to u s e ' s e m i - p e r m a n e n t s t r u c t u r e s such a s wooden division boxes, checks and outlets o r portable checks o r siphons. When i t i s known that no changes will be made in the elevation o r location of a s t r u c t u r e , both the original and replacement s t r u c t u r e s a r e usually made of concrete o r masonry. Operation of f a r m distribution s y s t e m s i s to a 1-arge extent conditioned by the skills of f a r m e r s , t h e i r experience, habits and traditional outlooks.

Thus

in the design of new irrigation s y s t e m s o r in the remodelling of existing s y s t e m s both social and technical a s p e c t s m u s t be considered.

During the planning and

designing of a new irrigation project i t i s difficult to f o r e s e e all future f a r m operations with accuracy, but the establishment of field t r i a l s o r pilot f a r m s a t the e a r l i e s t possible moment can do much towards providing the data n e c e s s a r y

f o r forward planning.

The various physical data required on irrigation p r a c t i c e s

can be accumulated f r o m these t r i a l s but the exact behaviour of the f a r m e r s will stiil be difficult to predict.

Therefore allowances f o r unpredictable behaviour o r

events should be incorporated in the designs of the supply and distribution s y s t e m s without, however, supplying water in e x c e s s of 'realistic field requirements.

Auto-Mechanization of s u r f a c e Irrigation Auto-mechanization of surface irrigation on the f a r m r e f e r s to the u s e of mechanical gates, s t r u c t u r e s , o r other devices and s y s t e m s that automatically divert water onto a f a r m field in the right amount and a t the right time t o m e e t c r o p demands; i t enables the f a r m e r to apply water m o r e efficiently and with a minimum of labour. B o r d e r and basin, irrigation s y s t e m s a r e particularly well suited f o r automation and have received the m o s t attention.

F u r r o w and corrugation

s y s t e m s a r e much m o r e difficult to automate; obtaining uniform water distribution for a l l furrows i s a problem.

~ b t o m a t e ds t r u c t u r e s operate a s

either water level control o r a s discharge control devices.

In either c a s e they

automatically terminate irrigation on one portion of the field o r f a r m and d i r e c t water to the other sections in sequence.

They can be portable o r permanent and

a r e used in both lined and unlined channels o r ditches. Mechanical irrigation s t r u c t u r e s , devices and s y s t e m s a r e normally classified a s semi-automatic o r automatic, depending upon their method of Some portions of a given system may be automatic while o t h e r s a r e

operation.

semi-automatic o r manual.

Semi-automatic s y s t e m s and equipment r e q u i r e

manual attention for each irrigation.

These normally u s e mechanical t i m e r s ,

such a s a l a r m clocks, o r e l e c t r i c o r hydraulic devices to t r i p the s t r u c t u r e s a t a p r e s e t time.

The i r r i g a t o r usually determines the need for irrigation and i t s

duration, and also manually r e s e t s o r r e t u r n s the devices to their initial position o r moves them f r o m one location to another p r i o r to an irrigation.

Automatic

s t r u c t u r e s , on the other hand, normally operate without attention f r o m the o p e r a t o r other than for periodic inspections.

The i r r i g a t o r frequently

determines when and for how long to i r r i g a t e , t u r n s water into the system, and/or s t a r t s programmed controllers before the automated portions of the s y s t e m function.

Fully automatic s y s t e m s sense the need for irrigation, introduce water

to the f a r m distribution channels, and complete the irrigation without operator intervention.

The need for irrigation i s customarily determined by soil moisture

sensors, such a s electrical resfstance blocks o r tensiometers; these activate electrical control apparatus when the soil moisture has fallen to a predetermined level.

The duration of irrigation i s usually controlled by programmed t i m e r s ,

o r soil o r surface water sensors.

Technical design of automatic control gates

will be discussed in Volume I11 of this Handbook. Auto-mechanization enables more efficient water conveyance and application to be achieved in addition to the saving of labour.

The higher

equipment costs and the greater emphasis that needs to be put on maintenance and operational skills compared to conventional surface irrigation suggests that the use of auto-mechanized equipment is feasible only where i t i s important to save labour a s well a s water.

3.

3.1

INTAKE STRUCTURES

INTRODUCTION Intake s t r u c t u r e s o r head r e g u l a t o r s a r e hydraulic devices built a t the head of an irrigation canal.

(Irrigation canals in t h i s context include main canals

and branch canals o r distributaries, m i n o r s and sub-divisions of t h e m . )

The

purpose of these devices i s to admit and regulate water f r o m a parent canal o r original source of supply, such a s a dam o r a r i v e r .

These s t r u c t u r e s may a l s o

s e r v e to m e t e r the amount of water flowing through them. The scope of this chapter i s based largely on p r a c t i c e and experience in India and i s limited to the intakes of s m a l l to medium sized distribution canals, whose source of supply may be a r i v e r o r a s t r e a m , the main canal, a branch canal o r a minor, and to the intakes of m i n o r s (and sub-divi sions of them) whose head discharges a r e one cubic m e t r e p e r second o r l e s s .

In c a s e s where the

discharges of the offtake canals a r e m o r e than 25% of the capacity of the parent canal, the control s t r u c t u r e s regulating the flow into two o r m o r e sub-canals a r e called "flow dividing s t r u c t u r e s " ( s e e Chapter 4); on the f a r m they a r e usually called "division boxes". The flowing water in the parent canal may be s i l t - f r e e o r charged with sediment, and the discharge may be constant, a t a l m o s t constant water level, o r i t m a y vary. F o r constant head discharge, the intake s t r u c t u r e m a y be a module of one kind o r another, and be operated manually or automatically (built-in o r remotely controlled). canal.

In this c a s e t h e r e is no need for a check s t r u c t u r e on the parent

But, when the parent canal r u n s periodically with low discharge, i t i s

essential to build a check o r c r o s s - r e g u l a t o r in the parent canal to r a i s e the water level sufficiently to feed the offtake canal up to i t s full demand, o r to r a i s e the water level for installing automatic hydraulically operated u p s t r e a m o r downs t r e a m constant level gates.

The c r o s s regulator o r check a l s o helps to a b s o r b

FIGURE 3- 1 (a) and ( b ) .

-

intake to a secondary canal.

Silt deposition at the

fluctuations in the various sections of the, canal system, to facilitate the provision of a bridge a c r o s s the canal at little additional cost, and to shut off o r reduce flow temporarily for repairs to breaches in the lower sections of the canal. When the water in the parent canal i s silt-free, the centreline of the offtake canal may be a t any angle with the centreline of the parent c a n d .

When the water

i s laden with silt an important function of.the intake i e to control the entry of silt into the offtake canal so that i t draws i t s fair share of the sediment charge, which should a s far a s possible, be carried in suspension to the fields.

This may

require either having a suitable offtake from the outside of a curve in the parent canal, o r a suitable alignment of the offtake to the centreline of the parent canal, or silt-excluding devices in the parent canal a t the head of the intakes.

Figure

3- 1 (a and b) illustrates the magnitude of the silt problem when an intake i s not In this case the silt content i s very high, while

properly located or designed. the intake

- a gated pipe - i s taking off laterally instead of being inclined towards

the direction of flow. This chapter provides descriptione of 15 types of intake structures and 7 types of devices for controlling the entry of silt into offtake canals a s listed below. The reference numbers correspond to the section numbers in the text and the Table of Contents. A.

INTAKES

3.2

Intakes of small canals (Punjab type).

3.3

Silt selective head intake.

3.4

Constant-head orifice intakefturnout.

3.5

Neyrpic orifice module intake.

3.6

Double orifice module intake.

3.7

P r e - c a s t R C C open regulator.

3.8

Intake discharging into a flume (U. S. S. R. ).

3.9

pipe regulator (with pre-cast R C C

,

crossing, U. S. S. R. )

.

3.10 Intake for secondary canals, combined with f a l l (Colombia). 3.1 1 Gate valve intake (Czechoslovakia). 3.12 Venturi head intake. '

3.13 Square head intake.

3.14

Dupui s canal intake.

3.15

Intake with stone-mesh weir.

3.16

Groyne intake and ancillary works (Cyprus).

B.

SILT CONTROL DEVICES

3. 17 King's silt vanes, 3.18

Gibbls groyne.

3.19

Curved wing with silt vanes

3.20

Silt platforms : ( a ) simple platform; (b) silt platform with a guide wall

3.21

Reverse vanes.

3.22

Vortex tube sand trap.

3.23

Sloping- sill sand screen.

Designers should study the designs particularly a s r e g a r d s relative costs, simplicity and e a s e of construction, and availability of labour and local m a t e r i a l s , with a view to selecting the structure best suited to their conditions.

General guidelines a s to which structures may be suitable under

given conditions of water supply a r e summarized below. (a)

Under constant water supply in the parent canal (at designed discharge) and f r e e from silt. Use one of the intake s t r u c t u r e s 3. 1 o r 3.3 to 3.8, o r 3. 10 to 3. 13.

All these s t r u c t u r e s may be used in combination

with falls o r bridges if required. (b)

Water supply in the parent canal lower than the designed discharge and f r e e f r o m silt. Use one of the intake s t r u c t u r e s 3. 1 o r 3. 3 to 3.8, o r 3. 10 to 3.13 with a check o r c r o s s regulator i n the parent canal, o r constant upstream o r downstream water level gates.

(c)

Water supply in the parent canal constant (at designed discharge) and charged with silt. Use one of the intake s t r u c t u r e s 3.1 3. 12 o r 3.13.

to 3 . 3 o r 3.6, 3.8,

Suitable silt-excluding devices may be used

i n conjunction with 3. 1, 3.3 and 3. 12, if necessary.

(d)

Water supply in the parent canal below the designed discharge and charged with silt. Use of the intake structures 3.3 o r 3.12 o r 3.13.

Structures 3. 1 to 3. 10 and 3. 13 have features which enable them to m e t e r the water let into the offtaking canals. Structure 3. 14 i s particularly suitable when there i s gravel sub- stratum a t the site, when the use of sheet pile protection of the weir i s precluded, and when, due to the porous nature of the strata, the conventional type of weir i s either impractical o r too expensive. Structure 3.15 i s used on large gravel bed r i v e r s with a bed width up to 600 ft. Among the silt-excluding devices, 3. 17 i s not suitable where a small offtaking canal i s situated between two large canal branches and when i t s bed i s a t a high level, and/or where the water level i s likely to surge over a considerable range.

When the offtaking canal has its bed a t a high level, the

device 3.20 i s preferable. 3. 18 i s used when both the offtaking and the supply canals have the same

sediment carrying capacity.

When the effect of this device i s not sufficient to

control the entry of silt into the offtaking canal, the device 3. 17 may be used in addition to enhance the performance of 3.18. The silt platforms (3.20) a r e suitable only where the parent canal i s deep. The device 3.20 (b) has the advantage that (on account of the slight heading caused by the curved wing) a small head of 3 to. 4.5 cm i s created a t the offtake which increases the velocity of the water and prevents silt being deposited in the head reach of the offtaking canal. When a canal divides into two canals, one of which sil!s

up very badly, and

there i s not enough room to accommodate vanes, the device 3.21 may be built to pass m o r e silt into the canal which does not silt. The device 3.22 i s suitable for small canals whose bed widths a r e l e s s than 3 metres.

It requires that some extra discharge be let into the offtaking canal

f o r the operation of the tube. The device 3.23 h a s been used in Egypt to c o r r e c t local asymmetry in the flow pattern a t the intake of distributary canals.

A. 3.2

INTAKES

INTAKES OF SMALL CANALS (PUNJAB TYPE)

3.2.1

General The intakes for minor and sub-minor canals developed in Punjab and Haryana a r e designed for proportional distribution of supplies.

The types in

common u s e a r e the "Open Flume" and the "Adjustable Proportional Flume

(APF)". Whichever of the two types a r e adopted the following conditions should be satisfied. (a)

F o r open flumes the setting of the c r e s t should be a t 0.9 of the full supply depth of the parent canal (Y i ) ; the c r e s t m u s t be above the level of the downstream canal bed; the width of the c r e s t a c r o s s the flow must be a t l e a s t 6 cm.

(b)

If the above conditions cannot be satisfied, an adjustable proportional flume should be used, where the c r e s t will be a t 0.75 of the full supply depth of the offtake canal (YZ), and s o that the-depth of the underside of the roof block below the full supply level in the parent canal (H(,,~)) ranges between 0.35 y 1 to 0.48 y 1.

The setting of the c r e s t must also

be above the level of the downstream canal bed. I n b o t h c a s e s , thedrowningratio shouldbe 0 . 8 o r l e s s .

These t y p e s o f

intakes, designed and set according to the conditions indicated above, will ensure proportional distribution over a wide range of flows. No control work i s required in the parent canal.

Even paving of the bed

section of the parent canal opposite the intake i s not needed.

However, when a

bridge i s required over the parent canal a t the site of the intake, a common wing wall i s provided between the intake and the bridge.

The intake structure itself

may have a bridge over it, i f required. These intake s t r u c t u r e s a r e automatic in action and need no manual control.

Very little maintenance i s required except for routine inspections.

The capacity

of the A P F can be i n c r e a s e d o r d e c r e a s e d , when needed, by simply adjusting the roof block.

Structural Design The Open F l u m e intake s t r u c t u r e consists of u p s t r e a m approach walls to the throat walls, u p s t r e a m curved glacis jbining the bed of the parent canal with the c r e s t and u p s t r e a m curtain wall, c r e s t , downstream glacis and downstream expansion, c i s t e r n , curtain wall and downstream protection. The Adjustable Proportional F l u m e (APF)

h a s the s a m e s t r u c t u r a l p a r t s

and details except that i t i s fitted with a roof block having i t s face s e t 5 c m f r o m the starting point of the parallel throat. The entire s t r u c t u r e (except the roof block, which i s made partly of reinforced concrete) i s in brick m a s o n r y . 3.2.2.1

Upstream approaches to the throat When t h e r e i s no bridge on the parent canal, the radius for the curve joining the u p s t r e a m side walls of the throat of the intake to the toe of the 0 . 5 : 1 o r 1 : 1 side slopes of the parent canal should be equal to 7 . 5

, The downstream Y1 side walls of the throat should be c a r r i e d straight into the parent canal to m e e t

i t s side slopes.

The depth of the u p s t r e a m curtain wall (including concrete grouting) should be equal to Y 3

1.

3.2.2.2

Crest The throat should s t a r t f r o m the point where the u p s t r e a m curve approach wing wall m e e t s the side walls of the intake tangentially. t h r o a t o r length of the c r e s t will be equal to 2H

(crt)

The length of the

(where H

(4i s the head

over the c r e s t ) . In the c a s e of the A P F a roof block of reinforced concrete ( F i g u r e 3-2) i s s e t with i t s face 5 c m f r o m the s t a r t of the throat.

It h a s a laminated curve a t

the bottom with a tilt of 1 : 7 . 5 in o r d e r to converge the water, (instead of a horizontal base which would diverge it). about 30 c m thick.

+

The reinforced roof block should be

rr~~ Bar 3mm

Bar No.1

Front

elevation Bar 3 m m Bar No. 2 Cross section on 0-0 S.W.G.(Standard wire gauge

Bar No.3 Detoils of precast R. C. Roof

I

I

F A O - I C I D INTAKE TO

SMALL CANALS (PUNJAB T Y P E )

DETAILS O F P R E C A S T R . C . R O O F BLOCK

Project, Region , Country lndio and Pokiston

Note: A// dinensions ore in centimetres.

I

Figure No. 3

-2

I

The c r e s t should be joined with the upstream bed level of the parent canal with a radius R(b,c) = 2.5 H(crt). Let

L(app) be the length of the upstream glacis

yapp)

Then where H(b.c)

3.2.2.3

,

J { 5 H(crt) -

=

H(b-c)

}

H(b-c)

is the height of the c r e s t above the upstream bed level.

Downstream glacis The downstream glacis should have a 2.5 : 1 slope and will join the c r e s t

with a curve of radius 0.60 m . Let

L(gl) be the length of the downstream glacis. Then

--

L(gl)

2. H ( c - ~ ~ )

where H(c-SB) i s the depth of the cistern floor below the c r e s t .

3.2.2.4

Cistern The depth of the cistern should be calculated thus:

q = -Q B(t)

.

0.67

(approximate discharge intensity a t point of standing wave)

Find Hc by the formula Hc

=

2

(

g

)

1.

F o r a given working head, h(wk) (difference between the full supply levels in the parent canal and the offtake canal), calculate the value

F r o m Table 3- 1 find the value of

Then E 2 can be worked out.

Hc -

L.

for the calculated value

TABLE

3- I

The depth of the c i s t e r n below the bed level of the offtake canal, will be E 2

-

y2, and in no c a s e should be l e s s than 7.5 c m .

The length of the c i s t e r n LSB should be equal to Y 2

owns stream

3.2.2.5

The r a d i u s of expansion,

-

h(wk).

expansion

The length of the downstream expansion,

3.2.2.6

+

L(exp),

R(exp),

Curtain wall and downstream protection The depth of the downstream c u r t a i n wall

-

2

subject to a minimum of 0.45 m .

2 The length of the bed protection of b r i c k b a t s

The thickness of the bed protection

The thickness of the walls of the t h r o a t , u p s t r e a m approach, and downstream expansion should be a s given in F i g u r e 3-3.

3.2.3

Design

3.2.3. 1

Open F l u m e

-3

Value of 'C' to be a s follows:

HSB,

\

Mosonry Note!

Coping to be of

wing

wolls concrete ( l : 3 : 6 ) with

cement

brick ballast. Thickness to be 15 c m . When

bed is

unprotected

When b e d

is

protected

Road level

Rood level

Rood level

Road level

-125-

Not less than 60 cm

Not less than 60 crn

Mosonry obutrnents with protected floor Note: Bearing slobs for the decking t o be of 15 cm thickness, cement concrete 1 : 3 : 6 with brick bollost. Rood level

Rood level

R.C. Slab Brick on edge 12 Cm

I

Floor

Not less than 30 crn Not less thon 60 cm

Not less thon 60 cm

/A// dimensions are in centimetres)

r

F A 0

-

ICID

INTAKE TO SMALL CANALS (PUNJAB TYPE)

STANDARD S E C T I O N S FOR WING W A L L S AND ABUTMENTS OF C.D.O. TYPE F A L L ( P U N J A B ) Country, Region, Project Punjob and Horyono (India 1 thon 45 cm

cm '

Figure No. 3 - 3

Q

Value of ' C 1 Intake angle 60°

3.2.3.2

Intake angle 45O

up to 0.56 m 3 / s

1.60

1.61

0.57 to 1 . 4 m 3 / s

1.61

1.63'

Adjustable ~ r o p o r t i o n a lF l u m e ( A P F )

Where

H(orf) =

the height of the opening o r orifice above the c r e s t ,

H(sOf)

=

the depth of the underside of the roof block below the full supply level i n the parent canal,

B(t)

=

width of throat.

The value of H(sof) should fall within the range of 0.375 y to 0.48 y

3.2.4

Numerical Example

3.2.4. 1

Design an intake s t r u c t u r e for a minor canal with the following data: P a r e n t Canal U p s t r e a m Downstream

Discharge

,

Bed level (above s e a level) F u l l supply depth F u l l supply level Bed width Angle of offtake Working head 100.36) (100.96

-

,

Minor Offtake

1

(i)

C r e s t l e v e l and t h r o a t width H(crt)

C r e s t level

--

0 . 9 ~ ~

-

0.9

---

. 0.96

0.864 m . 100.960

-

0.864

100.096

which i s higher than the bed l e v e l of the offtake channel

(ii)

--

1 . 60

Fluming ratio

-

0.39 2. 18

Length of c r e s t , L(crt)

=

( f r o m section 3. 2. 3.1).

ycrt)

Glacisandcistern

q, a f t e r allowing 67% f o r splay

working head,

?wi> h(wk) -

0.60

1

Now h(wk) I%

F r o m Table 3- 1, f o r h(wk) Hc E2 -

--

0.60 -

--

0.42 1.43

-

1.43

-

2. 181 1 (by interpolation)

Hc

Depth of c i s t e r n below bed level of the offtake canal

-

--

0.916

=

100.36

--

99.44

Length of downstream glacis

-

2.5

Length of c i s t e r n

-

Y2

--

0.50

--

1. l m .

F l o o r level of c i s t e r n

0.416m.

.

+

-

0.916

0.652

ywk)

+

+

(iii) Down s t r e a m expansion

o r , length of Glacis

0.50

c i s t e r n length

0.6

Radius of expansion

(iv)

LL(~XP) B2 - B(t)

t

B2

- B(t)

4

Downstream curtain wall below floor level of c i s t e r n L e t dwc2 Then

be depth of downstream curtain wall. dwc2

Minimum r e q u i r e d

(v)

=

-

0.5m.

U p s t r e a m curtain wall L e t dwc

1

Then

be depth of u p s t r e a m curtain wall. dwcl

-

-

Yl 3

Adopt 0.45 m minimum, i.e. 0.30 m m a s o n r y o v e r 0. 15m concrete.

(vi)

Downstream bed protection

Let

L(prot)2

be length of bed protection.

Then L(prot)2

=

Y2

-

0.50

-

1. l m .

h(wk)

+

+ 0.60

Thickness of bed protection of brick-bats

(vii) Upstream approach Setting of c r e s t above the u p s t r e a m bed level, H(b- c )

--

100.096

-

0.096

-

1oo.00

Radius joining the c r e s t with the u p s t r e a m bed,

-

R(b c)

-

2.

H(crt)

The design i s shown on F i g u r e 3-4.

3.3

SILT SELECTIVE HEAD INTAKE

3.3.1

General The design of a s i l t selective intake was evolved by the l a t e K. R. Sharma of the Punjab Irrigation Department in 1936 on the assumption that the concentration of s i l t in a s t r e a m in the lower l a y e r s i s g r e a t e r than that in the upper ones, and i f the lower l a y e r s were allowed to escape without interfering with the silt distribution, the remaining water would have l e s s s i l t p e r unit volume than the water u p s t r e a m of the intake.

Full-sized

glazed models w e r e made to t r a c e the s i l t laden s t r e a m - l i n e s and the following conclusions were a r r i v e d a t :

(a)

Under ideal conditions, the ability of the intake to conduct silt does not vary with the discharge of an offtaking canal so long a s the depth in the approach chamber i s not changed;

(b)

The ability to conduct s i l t v a r i e s according to some power ( h e r e 113) of the r a t i o of the depth in the approach channel to the depth in the

Plan

SILT S E L E C T I V E H E A D INTAKE

Note: All dimensions are in metres unless otherwise stated.

Project, Region , Country Punjob, India Figure No. 3

-5

parent canal. The structure s e r v e s to regulate, to reduce silt in the offtaking canal and to m e t e r the water flowing through i t .

3.2

Structural Desipn The structure consists of t h r e e p a r t s : the approach chamber; the regulator; and the flume.

Silt selection i s c a r r i e d out in the approach chamber

The discharge i s regulated upstream of the weir flume which m e t e r s the supply.

In addition, side and bed pitching i s provided in the parent a s well a s in the offtaking canals

.

A platform i s provided in front of 'the structure' in the parent canal and the bed of the platform i s 10% higher than the average depth of the parent canal. The length of the platform i s equal to three times the depth of the platform upstream and two t i m e s the depth of i t downstream. The floor of the approach chamber m u s t be higher than the canal bed.

The

slope from the platform to the approach chamber i s set a t 0.5 : 1 because a vertical wall would cause disturbance.

The width of the approach i s determined

by the formula

-

where B(ac)

=

width of approach chamber

C

=

a constant varying between 1 . 5 and 2.0

Q

=

discharge of offtake canal

Q1

=

discharge of parent canal

B1

=

width of parent canal

=

depth of parent canal

=

depth in approach chamber.

Y1 y(ac)

The radius of the upstream wing wall of the approach chamber i s 3 t i m e s the depth in the approach chamber and the wing wall i s flared a s shown in Figure 3-5.

The projection of the downstream wing wall i s given by

Q2 Y1 Q T ( B ~ +7 )

,

in m e t r e s and i t s radius i s two t i m e s the depth in the approach chamber. Grooves a r e provided for regulation by stop-logs o r vertical needles.

A

gauge chamber a t a distance of 2.5 H(crt) downstream of the grooves i s provided up to the c r e s t of the flume, where H(crt) = head over c r e s t of the flume. The width of the flume, B(t), should not be l e s s than

2

The length of the c r e s t i s 2.5 H(crt), and the vertical approach curve f r o m floor to c r e s t has a radius of 2H(crt). The c r e s t i s followed by a glacis a t 1:5 which r e s t s on a toe wall. The floor length i s calculated according to Blighls theory.

(Refer to

"square- head intake" for computation procedure. ) Bed and side pitching of 3 m beyond the glacis i s provided, which i s followed by a 3 m brick-bat protection in the bed and side brick pitching in the same length.

3.3.3

The bed pitching r e s t s on a toe wall.

Hydraulic Design (a) The required silt selective ability, r ( s c h ) l , i s determined f r o m the formula,

=' ratio of critical velocity ratios downstream of the intake of an (cv.1 The method of determining critical velocity offtake to that in the parent canal. where r

ratio, (cvr), i s given below.

r(s~h)l

r(sch)2

=

ratio of silt charge downstream of the intake to that in the parent canal

=

r a t i o of silt grade downstream of the intake to that in the parent canal

=

ratio of depth in parent canal to depth in offtake

\

r(sch)3

.

(cvr)

=

v

- ,

where

=

v

vc

A =

a r e a i n m2

.

vc

=

a A

'

=

Q

cO'64,' where

3 in m / s ;

discharge,

y = depth in m e t r e s ,

Y

and C i s a constant d e t e r m i n e d a s tabulated below in the Kennedy f o r m u l a . -

~

Type of s i l t

~~

Value of C

F o r s o i l s of Punjab and U t t a r P r a d e s h (UP) C o a r s e s i l t and sand

0.70 ( a s in South India)

Sandy l o a m

0.65 ( a s in T a m i l Nadu)

Light sandy s i l t of c o a r s e r variety

0.59 ( a s in B u r m a )

F i n e sandy s i l t

0.55 ( a s i n Punjab and UP)

V e r y fine s i l t

0.41 ( a s in Sind)

P e a gravel G r a v e l and P e a g r a v e l Boulders, e t c .

0.85 ) in r i v e r s o r head 1.83 ) r e a c h e s of c a n a l s 2.1 )

(b)

D e t e r m i n e y tat) f r o m the formula 1

7

I(sch)

(c)

( x f d ) 3 0 . 9 ~

-

-

t o d e t e r m i n e floor l e v e l of the approach c h a m b e r

Work out projection of the downstream wing wall by the

f o r m u l a given in 3 . 3 . 2,

i. e .

B(,p) (d)

Work out B

(a.1

=

C

.

0.9~1

Y(ac)

,

where

Work out the c r e s t H(crt) f r o m the formula

(e)

=

Q

1.71 B

2 (t) H(crt)

where B ( q =

B(ac) 2 4

Dimensions of wings, c r e s t , gauge placing and protection a r e

(f)

determined according to the formulae given under 3.3.2 S t r u c t u r a l Design.

3.3.4

Numerical Example See F i g u r e 3-5. Design a s i l t selective head intake i n accordance with the following data : P a r e n t canal

Offtake canal

Ql

=

8.5m3/s

Q2

= 0.8m3/s

B1

=

1.30 m

B2

= 0.60 m

Y1

=

9.5m

y2

= 3.00

0.5 : 1

( s s ) = 0.5 : 1

(ss) =

F u l l supply l e v e l = 10 1.30

F u l l supply level = 100.50

Adopt a value of C in the Kennedy formula equal to' 0.55.

3. 3.4. 1

Velocities and velocity r a t i o s

(cvr) =

0.644 0. 65

=

0.99

-

r(sch)3

depth .in parent canal depth in offtake canal

-- -1 . 3- 0.6

2.166

3.3.4.2

Silt selective ability of intake,

3.3.4.3

Depth i n approach chamber for 85% selection of silt in the parent canal

-

Y(ac)

0.85~

.

assuming

1.17

= say

Radius of u p s t r e a m wing

=

0.719 m 0.72

3y(,;)

=

3

say 2 . 2 fl

3.3.4.4

Projection of downstream winp B(sp)

--

0.955 m

say O . 9 6 m

.

0.72

=

2.16 m

3.3.4.5

Radius of downstream wing

=

where

3.3.4.6

R

-

( dsw)

291 r 2

=

angle of offtake

--

Width of approach B(ac)

K

lm5yl t 6

f

+ B(t)

60°

B

0 . 9 ~ t

Y(ac) where K

3.3.4.7

=

1.5

Width of flume,

B(t)

3.3.4.8

Discharge in offtaking canal,

1.17m; =

' 2

say 1 . 2 0 m 3 H (crt)

z

1.71B(t)

3

= 3.3.4.9

3.3.4.10

0.534m,

Length of c r e s t L(crt)

.

=

2.5

=

1.33 m

H(crt)

.= 2.5

.

0.53

Distance of gauge hole from the beginning of c r e s t

straight portion upstream

3.3.4.11

say 0.53 m

=

3 . 5 H(crt)

=

1.86 m ;

=)

2 . 5 H(.crt)

.

=

3.5

=

1.33 m

0.53-

Masonry and concrete Use 20 c m of masonry over 20 cm of concrete for flooring.

'

3.4

CONSTANT-HEAD ORIFICE (CHO) INTAKE 1/

3.4.1.

General The Constant-Head Orifice i s a combination of a regulating and m e a s u r i n g s t r u c t u r e that u s e s an adjustable submerged orifice for the m e a s u r e m e n t of the discharge.

It h a s been developed and widely adopted a s a delivery device to /

a s a f a r m turnout, by the United States Bureau of Reclamation. / s e in some other countries.

s m a l l canals o It i s a l s o in

P

The calibration t e s t s for the turnout have been conducted in the l a b o r a t o r i e s of the Bureau of Reclamation, Denver, Colorado, and t e s t s have been made a t the Colorado Agricultural Experiment Station t o investigate effects of changes in ups t r e a m and downstream water levels, sediment deposits, plugging of the orifice gate with weeds and debris, and approach flow conditions.

3.4.2

Structural C h a r a c t e r i s t i c s and Design The Constant-Head Orifice Intake o r Turnout ( F i g u r e s 3-6, 3-7 and 3-8) c o n s i s t s of a s h o r t entrance channel leading to a head wall containing one o r m o r e gate-controlled openings, a stilling basin, and a downstream head wall with one o r m o r e gate-controlled openings that r e l e a s e the flow into a delivery conduit. The conduit i s a p r e - c a s t concrete p r e s s u r e pipe (horizontal o r inclined a s required), the length of which depends on the width of the canal bank and whether t h e r e i s a road crossing o r not. Originally, the head differential a c r o s s the orifice, o r u p s t r e a m gate, was determined by reading staff gauges just u p s t r e a m and downstream f r o m the head wall.

Fluctuations of the water levels a t these gauges, particularly during l a r g e

flows, caused significant reading e r r o r s .

The staff gauges a r e now placed

in external stilling wells (Figure 3- 9 (b) ) u p s t r e a m and downstream f r o m

Essentially based on the information given in the USBR Water Measurement Manual, second edition.

FIGURE 3-6. - Diagram of a constant-head orifice intake o r turnout.

the orifice gate to i n c r e a s e the accuracy of head readings, and hence of the discharge measurement.

F o r existing structures, small wooden o r metal

baffle-type stilling devices a c r o s s the entrance passage and a c r o s s the stilling d

basin passage n e a r the staff gauges help to reduce reading e r r o r s . The whole structure i s of reinforced concrete and thus strong, but initial cost i s relatively high because two gates a r e necessary. have been effected by using a cheaper downstream gate.

Some savings

This gate need not be

wateltight since i t i s used for regulation only, the shut-off being accomplished by the upstream gate.

The structure containing the gates i s designed so that

i t i s essentially self-cleaning except when operating a t very low flows.

If

backwater is excessive, flow through the structure will be low.

3.4.2.1

Dimensions F o r accurate measurement, t h e r e must be a level floor in front of the orifice gate, of a length equal to o r g r e a t e r than the height of the orifice gate opening a t full capacity.

The minimum inside length of the measuring box

FIGURE 3 - 7 . - Constant-head o r i f i c e f a r m turnouts with E a s t Ghor Canal P r o j e c t , Jordan. check-gate i n foreground

-

FIGURE 3-8. turnout.

-

A s i n g l e - b a r r e l constant-head o r i f i c e

should be: a t l e a s t 2.25 t i m e s the orifice gate opening a t maximum capacity o r 1.75 t i m e s the wall opening, whichever i s the larger,for turnouts with maximum capacities up to approximately 300 l / s (10 ft3/ s) ; and 2.75 t i m e s the height of the orifice gate opening a t maximum capacity for s t r u c t u r e s with maximum capacities above approximately 300 11s (10 f t 3 / s ) ( s e e Figure 3-11).

The inlet

walls should be parallel unless e x t r a width i s needed a t the inlet cut-off to prevent i t acting a s a control, in which c a s e extra width may be obtained by flaring the walls, usually a t 8 : 1. The distance between the inlet cut-off and orifice gate should be a minimum of 1.5 times the difference in elevation between the invert a t the cut-off and a t the orifice gate.

In an earth canal., the top of the sloping inlet walls should

i n t e r s e c t the canal side slope a t o r a few inches above normal water level. The inlet walls a r e usually,sloped steeper than the canal side slope and set back into the bank so that they will not be out in the canal i f i t i s widened at the bottom during cleaning o r reshaping.

The following tabulation shows recommended

inlet wall slopes corresponding to various canal side slopes.

Canal side slope

Inlet wall slope

1.5: 1 s e t into the bank 12 to 24 inches depending on size of canal and local conditions

.

The dimensions of other p a r t s a r e given in Table 3- 2.

3.4.2.2

~ i d velocities e Full pipe velocity i s limited to about 1.07 m / s ( 3 . 5 f t / s ) i f the structure h a s no concrete outlet transition, but, i f i t has, the full pipe velocity should be about 1 . 5 m / s (5 f t / s ) . The top of the pipe a t the outlet should have a minimum submergence of hv(~)

Hydraulic P r o p e r t i e s The rate of flow i s m e a s u r e d by using the principle that a submerged orifice of a given size operating under a specific differential head will always p a s s a known quantity of water.

The upstream gate o r gates constitute the

orifice, the size of which can be increased o r decreased by opening o r closing the gates.

The head a c r o s s the orifice i s usually about 6 c m ( 0 . 2 ft) but may

be m o r e than 6 cm i f additional head is available.

This differential head i s

maintained by adjusting the downstream gate o r gates, and i s measured by staff gauges mounted inside the stilling walls, upstream and downstream from the orjfice gate head wall (Figure 3-9 (b)).

To set a given flow, a typical orifice

would be operated a s follows. The opening of the orifice for the desired discharge i s obtained from discharge Tables 3-3, 3-4, o r 3-5.

With the upstream gate o r gates set at

the opening, the downstream gate o r gates a r e adjusted until the differential head a c r o s s the orifice, a s measured by the staff gauges in the stilling wells, i s a t 6 cm ( 0 . 2 ft).

3.4. 3,. 1

The discharge will then be a t the d e s i r e d value.

Effects of upstream water depths s e e section 3.4.4.

3.4.3.2

Flow through the structure Flow through the structure i s controlled by the size of the orifice and the head a c r o s s the orifice.

f o s s will

The l o s s through the structure i s computed and this -

determine the maximum elevation to which delivery can be made.

If

the canal below a turnout i s dry and the turnout gate i s opened some manipulation of the gate may be required until the canal fills. gated structure. )

(This would be t r u e of any

f i t s i d e of this fining operation, tall-wtKtCr docs

nat cmtrol

the flow through a turnout.

3.4.3.3

Effects of sediment and weeds Sediment particles of the sizes normally found in field installations a r e swept through the orifice gate and the downstream gate during the normal course

A - 3 . 7 3 2 0 (E1.A-El B-H(p-b)p-LOL-

Note: Lip)* etc.... volues given in Table I . Minimum losses bosed on 30 feet length

Form turnout lype 4 4.55),(L3=L1 5 )

( L 2 = Ll+1.5qp-612+

+

FAO-

ICID

STANDARD CONSTANT H E A D ORIFICE INTAKE OR T U R N O U T DESIGN GUIDE 3

Q = 2 to 2 4 f t / s , yl = 4.0 f t to 6.5 f t ss = 2 : l Form turno:~

'rlP'e5

+ 5)) , / L + = L l+ L ( p ) z - k 4 5 8 - 2

fL2 ' L I + I S ~ - b ] r + 4 5 5 ) , / L 3 = L ,

*;142H(b-nls;

-

Project. Region , Country II S A Figure N o . 3 - 9 ( a )

offle for vortex , prevention ot upstreom

ownstreom staff

Stilling basin and onti -vortex- boffles extend ocrosr chonnel t o f i t tightly against side walls..

-

Stilling baffles to reduce water surface fluctuations at staff gauges

FIGURE

3 - 10.

-

.

:Gate

opening for max. Q, gote ieot trove^ Submergence

H(90) yfg,) = FUII

otttice. gote loot

:.. .:.. *:

=

E !L

.;: ....

?'..D:

.i

W. S.

W.S.

4)

*

Hoa

I

*

q+01 1

(ln'

b

must be equol to or greoter than for mox. O . 'hib; is equol to or greoter thon 'qgo;for good occurocy. 3 I 3 For 0 up t o 10 ft/s, LI must be ot leost 2~ qg0) or IT^^^,), whichever ir grcotcr . (i- minimum 1. 3 minimum. For 0 above 10 f t / s , LI :2 $

h(

FIGURE

3 - 11.

-

mon.

ye+)

Dimensions for' a constant head orifice.

. of operation.

The small amount of sediment which accumulates in the stilling

basin between the gates h a s little o r no effect upon constant-head orifice turnouts. On the other hand chokingby weeds that become lodged within the m e a s u r i n g orifice can be serious.

Moreover, choking can be difficult to detect

when sediment-laden water is flowing because the orifice cannot be seen. principal cause of choking i s the p r e s e n c e in the opening. turnout.

\ of waterlogged

The

weeds that get caught

These weeds may t r a p other p a r t i c l e s and eventually plug the

The m e a s u r i n g accuracy of constant-head orifice turnouts i s sensitive

t o the condition of the orifice and i s greatly reduced by the p r e s e n c e of even a few weeds.

Therefore, during regular operation, c a r e m u s t be taken to e n s u r e

that the orifice and the a r e a u p s t r e a m of i t a r e kept completely c l e a r of weeds and other d e b r i s .

Effects of approach flow conditions Usually the turnouts a r e placed a t 90

0

to the canal centreline.

when the water in the canal flows past the turnout, an disturbances occur a t the turnout entrance. disturbances affect the flow into the turnout.

A s a result,

eddy and r e l a t e d flow

This eddy and other flow The intensity of the disturbances

depends largely upon the velocity of the canal flow.

F o r small gate openings,

the discharge coefficient, CQ, for the turnout i n c r e a s e s f r o m a value of 0.64 a t a canal velocity of about 0 . 3 m / s ( 1 foot p e r second) to a value of 0. 69 for a canal velocity of about 0 . 9 m / s ( 3 feet p e r second).

On the other hand, with

l a r g e gate openings, increasing the canal velocity n e a r the turnout d e c r e a s e s the coefficient f r o m high values of about 0 . 7 4 a t 1.0 ft/ s to low values of about 0.63 a t 3 . 0 ft/s.

This appreciable, but inconsistent, effect upon the m e a s u r i n g

accuracy of constant-head orifice turnouts m u s t be recognized. mental effect i s g r e a t e s t a t the l a r g e r orifice openings.

The detbri-

Therefore, whenever

possible, installations should be designed so that relatively low flow velocities prevail a t the turnout, especially i f l a r g e r openings a r e to be used. Fortunately, the n o r m a l velocity distribution in canals provides relatively low velocities n e a r the banks.

3.4.4

Design The t e s t s c a r r i e d out show that for bhe general case, the r a t e of flow can be computed f r o m the formula

where

Q

=

discharge of the turnout in ft3/ s

A h

=

differential head on orifice gate = 0. 20 ft

A(go)

=

=Q

=

coefficient of discharge

g

=

acceleration due to gravity,

a r e a of orifice gate opening in ft 2

32.2 f t / s / s

The discharge coefficient, CQ, i s approximately 0. 65 for the n o r m a l operating condition where the head u p s t r e a m f r o m the turnout i s 2 . 5 o r m o r e *

t i m e s the maximum gate opening and no reinforcing s t r i p i s used a t the bottom of the gate.

If an angle-iron reinforcement i s used, the coefficient will be

i n c r e a s e d to approximately 0.72.

F o r convenience in u s e , discharge Tables

3-3 and 3-4 have been p r e p a r e d f r o m experimental data for the n o r m a l operation of single and double-barrel installations using 24 by 18 inch and 30 by 2 4 inch constant head orifice turnouts.

Tables 3 - 2 and 3 - 3 show the orifice gate

openings n e c e s s a r y to p a s s the d e s i r e d discharges. Studies show that when the depth of water u p s t r e a m of the orifice gate i s four o r m o r e t i m e s the height of the opening of the orifice, the coefficient of discharge, CQ, r e m a i n s essentially constant a t 0.65.

However, when the depth

of water u p s t r e a m i s l e s s than four t i m e s the orifice opening, the coefficient increases.

The r a t e of i n c r e a s e i s moderate a t submergence r a t i o s between

4 and 2.5, but i s rapid a t submergence r a t i o s below 2.5.

It i s i m p r a c t i c a l and

inaccurate to attempt to predict the coefficients for different installations having low submergence r a t i o s , and the p r a c t i c e of doing so i s not recommended. Instead, and i f possible, the s t r u c t u r e s should be installed so the water depth in front of the orifice gate be 2.5 t i m e s , and preferably four o r m o r e times, the maximum expected gate opening.

In some c a s e s , to place the s t r u c t u r e low

enough, i t may be n e c e s s a r y to slope the inlet channel downward ( F i g u r e 3 - 9 (b) ). An alternative design in which the inlet floor i s stepped abruptly downward i s a l s o used.

TABLE

3-3

Discharge of Constant-Head Orifice Turnout in ft3/ s . Capacity 20 ft3/s, Gate Size 30 by 24 Inches h = 0.20 ft Discharge ft31 s e c

Gate opening in ft 2 gates 1 gate

Discharge ft3/ s e c

0.02 0.04 0.06 0.08

0.04 0.08 0.12 0.16

10.25 10.50 10.75 11.00

0.10 0.12 0. 1 4 0. 16

0..20 0.24 0. 28 0. 32

11.25 11.50 11.75 12.00

0. 18 0.20 0.22 0.24

0.36 0.40 0.44 0.48

12.25 12.50 12.75 13.00

0.26 0. 28 0.30 0. 32

0.52 0.56 0.60 0. 64

13.25 13.50 13.75 14.00

,

Gate opening in ft 2 gates 1 gate

TABLE

3-4

D i s c h a r g e of ,Constant-Head O r i f i c e Turnout in ft Capacity 10 ft3, Gate Size 24 by 18 Inches

Discharge ft3/ s e c

Gate opening in ft 2 gates 1 gate

Discharge ft3/ s e c

3

/ s,

Gate opening in ft 1 gate 2 gates

C u r r e n t USBR d e s i g n s provide s t a n d a r d CHO turnouts f o r 2, 4, 6, 9, 12, 15, 18, 24 and 30 ft3/ s d e l i v e r i e s .

On the 2 'ft3/ s s i z e , with m i n i m u m canal

w a t e r s u r f a c e elevation and m a x i m u m recommended g a t e opening, the subm e r g e n c e r a t i o i s about 4 . 0 .

A s the turnout s i z e i n c r e a s e s , the m i n i m u m

s u b m e r g e n c e r a t i o d e c r e a s e s t o become about 2 . 0 for the 15 ft 3 / s and l a r g e r sizes.

Approximate d i s c h a r g e s b a s e d upon a coefficient of 0.70 a r e provided

in Table 3-5, on the understanding that if a c c u r a c i e s b e t t e r than about

+

7% a r e

required, careful field ratings of the turnout m u s t be made.

F o r discharges

l a r g e r than about 30 f t 3 / s special s t r u c t u r e s involving multiple gates and b a r r e l s m u s t be designed for the particular site and flow requirements.

TABLE

3-5

Discharges for Standard 2, 4, 6, 9, 12, 15, 18, 24 and 30 ft3/s Constant-Head Orifice Turnouts (CQ =Ua70)

2 f t 3 / s Turnout (width of orifice 1. 5 ft)

9 ft 3/ s Turnout (width of orifice

Discharge ft3/ s

Discharge ft3/ s

~

Orifice gate opening, ft

2. 5 ft) Orifice gate opening, ft

4 ft3/ s Turnout (width of orifice 1 . 5 ft) Discharge ft3/ s

Orifice gate opening, ft

12 ft3/ s Turnout (width of orifice 2. 5 ft) Discharge ft3/ s

6 ft 3 / s Turnout (width of orifice 2.0 f t ) Discharge ft3/ s

Orifice gate opening, ft

Orifice gate opening, ft

TABLE 3-5 (Cont'd.) 15 f t 3 / s 3 ft)

-

urnb but

Discharge ft3/ e

(width. of orifice ,Orifice gate opening, f t

24 ft3/ s Turnout (width of o r i f i c e 4 ft) Discharge ft3/ s 2

Orifice gate opening, ft 0.20

1

0.13

2

0.27

4

3

0.40

6

0.60

0.53

8

0.80

4

.

-

30 ft3/ s Turnout (width of o r i f i c e 4 ft)

-

Discharge ft3/ s

Discharge ft3/-B

Orifice gate opening, f t

i,

18 f t 3 / s Turnout (width of orifice 3. 5 ft)

-

0.40

O r i f i c e gate opening, f t

1

0.10

2

0.20

3

0.30

4

0.40

r S m ~ l l voriotion of levels 1

-----I* Porent conol

------------ -------------------

Intake

Offtoke channel

r L ~ r ~.variation e of levels

------ Parent conol --- --------------------------------__-Constant level maintained

Stilling basin

1 (b)

--Offtoke

conol

(el

I

(f )

,

tk

lntoke

Duckbill weir (g)

v

0Module

dischorqe

tk- lntoke

.

rConstont level maintained I

Diagonal weir

'

Transversal weir

Guord gote

Constant 0d)ustoble dischorqe --"'

1 I

--/r 1 I

Offtoke ronol

lntoke

Constont level m o i n t ~ i n e d ~ Parent conol-------

-

Porent chonnel

/

1

Longitudinal weir

.

(h) FA0

-

ICID J

adjustable dischorqe

Or Avio i f the chute is important

Off toke conol

VARIOUS ARRANGEMENTS OF NEYRPlC ORIFICE MODULE, WITH AUXILIARY EQUIPMENT AND STRUCTURES Project, Region ,Country France and North Africa

-

Figure No. 3 12

3.4.5

Numerical Example Designs can be worked out on the basis of the data and tables in the preceding paragraphs.

3.5 3.5.1

NEY RPIC ORIFICE MODULE INTAKE General The Neyrpic orifice module, invented by the Neyrpic Laboratories,

-

Grenoble (France), is used a s an intake for distribution canals a s well a s a f a r m outlet o r f a r m turnout.

It i s a metering device and i s suitable when water

i s supplied on a volumetric basis. In order that the module may draw the amount of water for which i t h a s been designed the water level in the parent canal should be m o r e o r l e s s constant.

If

there is only a small variation in the water level in the parent canal the intake is installed directly on the bank of the parent canal (Figure 3- 12 (a) ).

If the water

'

level in the parent canal fluctuates beyond tolerable limits for constant flow in the offtaking channel, a constant downstream level gate must be installed a t the head of the offtaking channel upstream of the module,

( s e e also section 6.15).

Figure 3- 12 (b) shows a combination used when the discharge of the intake i s small compared to the discharge of the parent canal.

The intake structure has

a protection gate and a constant downstream water level gate before the orifice module. Figure 3-12(c) h a s a constant upstream water level gate in the parent canal below the intake structure. controlled networks.

This arrangement i s systematically used in upstream

Figure 3- 12 (d) shows an arrangement where the module

intake i s set immediately below the constant downstream water level gate installed in the parent canal.

This arrangement i s systematically used in

downstream-controlled systems. Figures 3- 13 (a) to (c) shoy perspective views of the arrangements of Figures 3- 12 (b), (c) and (d).

FA0

- ICID

PERSPECTIVE VIEWS OF THE ARRANGEMENTS OF FIGURES I (a1 to I4c) I

Project , Region, Countrb France and North Africa Figure No. 3

- 13

.

F i g u r e s 3- 12 (e) to 3- 12 (h) show arrangements wherein the orifice module intake is located just above a diagonal weir, t r a n s v e r s a l weir, duckbill weir, and longitudinal weir, respectively.

The purpose of these regulating s t r u c t u r e s i s to

keep the water level in the parent canal on their upstream side nearly constant. The best arrangement to .choose depends on economic and practical considerations such a s canal grade, height of canals above the lands to be irrigated, and topographical features. F o r this type of intake, only structural and design details of the orifice module intake a r e given h e r e .

'Information about the constant upstream and

downstream water levels gates and the w e i r s a r e given ih Chapter 6.

3.5.2

Structural and Desipn Characteristics The module consists of a sill, which h a s an upstream slope of 60 0

downstream glacis slope of 12 baffle.

, upon

0

and a

which is placed a fixed metallic plate o r

The sill and the fixed plate o r baffle a r e enclosed between two vertical,

parallel walls, and this arrangement c r e a t e s an orifice which can be closed by a sliding plate o r shutter.

-

When the width of the orifice exceeds 1 m , a foot-

path, 50 c m wide, i s necessary in o r d e r to operate the sliding plate.

The

module functions only when the sliding plate i s r a i s e d completely.

A distributor usually includes a number of modules. connected together, each one of different width and allowing the passage of a pre-determined discharge, the volume of which i s indicated on the corresponding sliding plate ( s e e Table 3-7 for various combinations).

By combining the raising of different sliding plates,

the required discharge can be obtained.

Thus a set of t h r e e sliding plates

(Module type XX/60, Table 3-7), which allows respectively the passage of 10, 20 and 30 l / s , enables one of the following d i s c h a ~ g e sto be. diverted: 10, 20, 30, 40, 50 o r 60

11s. '

Thus, discharge regulation i s very simple.

There i s no gate opening to be

regulated, no regime to establish, no water levels to b e checked, no head discharge curve to be plotted.

-

F ~ G U R E 3-14. Neyrpic orifice module before and after installation of the fixed plate.

Figure 3-14 shows the module before and after'installation of the fixed plate, the sliding plate and the sill.

Dimensions for different types of the module

a r e shown in Table 3-6.

TABLE

3-6

(Dimensions in cm) Module No

H(b-bk) min

.

H(bs-bk) min

.

Ll

L Z B(rec)

L3

H(b-c) min

H(crt)

H(c-ft)

-

FIGURE 3- 15. Neyrpic distributor with conxpartments for 5, 10, 15 and 30 11s.

FIGURE 3-16. type XXl300.

-

Upstream view of Neyrpic distributor

Table 3-7 gives normal dimensions and capacities of module types X and XX Module type L i s used for discharges over 500 l / s and has compartments with capacities 50, 100, 200 and 400 11s.

Module type C i s used for discharges

g r e a t e r than 1,000 11s and has compartments with capacities of 100, 200, 400, 600 and 1,000 11s.

TABLE

3-7

--

Type of module

Discharge of compartment l/s

Clear opening of compartment cm

Total width of compartment, cm

Figure 3- 15 shows a module with four compartments : 5 11s, 10 11s, 15 11s and 30 11s and Figure 3- 16 a view of module type XX/300. Advantage s . The head l o s s through the module i s low, because of the hydraulic jump being formed oh the downstream slope of the sill. of operation.

Wear and tear i s almost nil.

The device h a s a wide range

Any discharge (in multiples of 5)

can be obtained by employing the minimum number of compartments. easy to tamper with the outlet.

It i s not

Operation i s simple ; all that is required i s to

-

Module X

-~O/o-sO/o Q +S0/o +loO/o 9 9.5 10 10.5 I I r h e c

Section of the module

20 -10°/o-50/o

0

21

221hec

+SO/O+lOO/~

FIGURE 3-17. - P e r cent variations in discharges of modules Types X and XX for variation of H within pre- determined limits.

(4

open o r shut the gates a s required; once set, the combination can be locked once and for all.

The amount of water withdrawn from the supply canal o r supplied to

u s e r s can be determined merely by noting the opening times of the gates. Disadvantages The main disadvantage

of the module i s that it i s relatively expensive.

requires manual labour to open and close the outlet.

It

TABLE

3-8

Some Data for Variations in Discharge for Different Types of Module (Dimensions in cm except a s otherwise indicated) Type of modul e

.

3.5.3

Discharge per 10 cm width of sill

H ( ~ r t ) m i n H(crt) nor QQ10% 5%

H(crt) max

d~ (4 h(1)

H

('"Imin

Design Formula The discharge of the outlet follows the formula for a weir with shooting flow conditions (and i s thus always independent of the downstream water level)

-

When the upstream water level r i s e s and i s above the lower tip of the fixed plate o r the baffle, the weir flow changes to orifice flow conditions. discharge falls off slightly a t f i r s t

- due to the vena

The

contracta effect downstream

of the baffle and because the reduced section i s not quite compensated by the increased flow velocity

-

before beginning to increase again, thus obeying the

characteristics of flow through orifices under pressure, (coefficient C tending to decrease a s the height of water increases).

Due to i t s momentary increase in

the initial stages of orifice flow, the discharge varies between very narrow limits (5% to 10%) over a comparatively wide range of upstream water levels.

Under

designed water level upstream, the module gives exactly the discharge indicated on the shutter (Figure 3- 17).

r C o n t r o l of locks Open locked 9ote

Closed locked 9ote

FIGURE 3- 18. intake.

-

Double orifice module

3.5.4

Numerical Example D e s i g n s c a n b e w o r k e d out, with given d a t a , f r o m T a b l e s 3-6, 3 - 7 a n d 3-8.

3.6

NEYRPIC DOUBLE ORIFICE MUDULE INTAKE

3. 6. 1

General T h e double o r i f i c e m o d u l e i s a n i m p r o v e m e n t o v e r t h e N e y r p i c o r i f i c e module

and

c a t e r s f o r g r e a t e r v a r i a t i o n s of d i s c h a r g e i n t h e supply c a n a l .

Its

v a r i o u s a r r a n g e m e n t s with c o n s t a n t u p s t r e a m a n d d o w n s t r e a m w a t e r l e v e l g a t e s a n d r e g u l a t i n g c h e c k s t r u c t u r e s a r e t h e s a m e a s given u n d e r s e c t i o n 3.5. 3. 6 . 2

Structural Characteristics The double o r i f i c e module (Figure 3-18) c o n s i s t s of an i n c l i n e d s i l l s i m i l a r t o t h a t of t h e Neyrpic s i n g l e o r i f i c e module and i s provided with two v e r t i c a l m e t a l l i c covers forming a siphon.

The c a l i b r a t e d openings of d i f f e r e n t

widths discharge, say, 10 l / s , 20 l / s , 30 l / s o r more, f o r a given head. The o t h e r s t r u c t u r a l p r o p e r t i e s and dimensions of t h i s module a r e s i m i l a r t o those of t h e Neyrpic s i n g l e o r i f i c e module except H C ~ - ~which ), i s given i n Table 3-9, where H(b-c) i s t h e height of t h e c r e s t above t h e upstream bed l e v e l . This double o r i f i c e module has t h e same disadvantage a s t h e s i n g l e o r i f i c e device, moreover t h e siphon may become obstructed by weeds. TABLE

3-9

C h a r a c t e r i s t i c s of Double O r i f i c e Module for Q = 2 5 p e r cent

Type No.

Unit discharge l/ s/dm

H

( c r t )m i n

H

dH J . H (crt) (max) (crt) mln Dimensions i n centimetres

H (b- c )

Hydraulic Characteristics A s i n t h e c a s e of t h e N e y r p i c o r i f i c e m o d u l e , t h e r e i s a shooting flow downs t r e a m of t h e s i l l a n d t h e d i s c h a r g e t h r o u g h t h e m o d u l e i s independent of t h e water l e v e l s i n the delivery canal. The o u t l e t works a s a semi-module a s long a s t h e upstream water l e v e l does not reach t h e bottom edge of t h e two inner m e t a l l i c covers.

T h e r e a f t e r , with

f u r t h e r r i s e i n upstream water l e v e l , i t works a s a semi-module o r i f i c e t i l l t h e siphon primes and flows through, impinging on t h e j e t , f u r t h e r reducing t h e discharge.

T h u s t h i s i m p r o v e m e n t a l l o w s a l a r g e r r a n g e of v a r i a t i o n i n t h e u p s t r e a m l e v e l s t o give n e a r l y c o n s t a n t d i s c h a r g e .

able

F o r e x a m p l e , f o r m o d u l e Type 2

+

3-9) t h e p e r m i s s i b l e r a n g e f o r Q = - 570 i s 11 c m w h e r e a s i t i s 8 c m f o r

t h e N e y r p i c m o d u l e Type XX.

T h i s p e r m i t s t h e p a s s i n g of 50°/0 m o r e d i s c h a r g e

f o r t h e s a m e width, a g r e a t a d v a n t a g e o v e r t h e N e y r p i c s i n g l e o r i f i c e module.

Design T h e h y d r a u l i c c h a r a c t e r i s t i c s of t h e m o d u l e a r e g i v e n i n T a b l e 3 - 9 .

Numerical Example D e s i g n s c a n b e w o r k e d o u t , i n a c c o r d a n c e with given d a t a , f r o m T a b l e 3-9 a n d t h e T a b l e s given u n d e r s e c t i o n 3 . 5 .

3.7

O P E N INTAKE STRUCTURE MADE O F P R E - C A S T REINFORCED C O N C R E T E ( u . S. S. R. ) General E a r l y type of i n t a k e s in s o u t h e r n U. S. S. R. w e r e built with a n i n l e t s i l l (which c o n s i s t e d of a n i n c l i n e d w a l l ) , a c o r e wall, a n d a r e c t a n g u l a r f l u m e on t h e downstream side.

T r a n s i t i o n to t h e t r a p e z o i d a l c r o s s s e c t i o n of t h e c a n a l on t h e

d o w n s t r e a m s i d e c o n s i s t e d of i n c l i n e d w a l l s .

LJB a s e d on i n f o r m a t i o n

T h e s e s t r u c t u r e s w e r e difficult

supplied by A. T . Koshkina, E . P . ' M a r t i n , A. V. Shatalova, D . D. A l e v a n d B . V . K a z a r i n o v (u. S. S. R . )

Cross

section A A

Cross section B B

Cement mortar

Cross section C C

FA 0

- ICID

OPEN INTAKE STRUCTURE MADE OF PRECAST REINFORCED CONCRETE

Project, Region, Country USSR

P Ion ( A l l dimensions ore in cm )

Figure No. 3 - 19

to i n s t a l l and r e q u i r e d l a r g e quantities of reinforced c o n c r e t e . The s t r u c t u r e u s e d now i s m a d e of p r e - c a s t r e i n f o r c e d concrete and i s simple to manufacture.

The p a r t s a r e standardized f o r a s s e m b l y and the

s t r u c t u r e r e q u i r e s low hydraulic d r o p s . operation.

The s t r u c t u r e does not s i l t during

Sediments deposited a t low d i s c h a r g e a r e removed during high

discharges.

S t r u c t u r a l C h a r a c t e r i s t i c s and Design The intake ( F i g u r e 3-19) c o n s i s t s of a ribbed bulkhead wall (diaphragm) and ribbed stiffening plates m a d e of reinforced concrete. installed in the canal p a r a l l e l to i t s a x i s of flow. gate i s volted to the bulkhead wall. sealed.

The side slope i s 1 . 2 5 : 1.

p l a t e s a r e filled with c e m e n t m o r t a r .

The bulkhead wall i s

A m e t a l l i c f r a m e with a plate

The wall and f r a m e of the gate a r e suitably The joints between the reinforced concrete At the end of the downstream i m p e r m e a b l e

apron t h e r e i s a rock-filled knife-edged support. Hydraulic s t r u c t u r e s need to be f i r m and durable and these r e q u i r e m e n t s have been m e t by the design, m a t e r i a l s used and careful supervision of the construction of t h i s device.

F u r t h e r m o r e the construction of this s t r u c t u r e allows

for modifications i f n e c e s s a r y ;

stiffening plates can be replaced, the basin length

i n c r e a s e d o r other p a r t s may be replaced. This s t r u c t u r e needs only periodical checks of i t s operation and condition. The jacks need lubrication and the m e t a l l i c p a r t s need protecting with an antic o r r o s i v e coating. The s t r u c t u r e can be improved by introducing automatic operation and by designing s e p a r a t e p a r t s and stiffening plates with fewer joints in t h e m s o that a s s e m b l y can be speeded up. The s t r u c t u r e is designed for discharges f r o m 0. 20 to 0.85 m3/ s . The minim u m acceptable canal depth i s 40 c m and the maximum depth, 80 c m .

Hydraulic

drops a r e adopted a t 5 to 20 cm. Embankment height above the s u r f a c e water level on the u p s t r e a m side of the s t r u c t u r e should be 25- 30 c m and on the downstream side 35-50 c m .

The d i s c h a r g e capacities of different types of the s t r u c t u r e

TABLE

3-10

3 Discharge, m / s

Type of structure RO-60

.

40

RO-60

.

60

h(k), c m

H' (crt)

RO-80

.

.

10

15

20

qgo), cm 0. 205

0.244

0.244

-

0 . 257

0.316

0.329

-

60

0 . 307

0.379

0.415

0.415

50

0.359

0.442

0.461

-

60

0.434

0.53s

0.538

0.588

70

0.503

0.625

0.704

0.718

0.571

0.712

0.810

0.852

40

60

50

RO-80

5

'

60

80

60

80

80 80

L i s t of P a r t s Type of structure

RO-60

.

40

T y p e of p a r t

AO- 60 P-60

.

120

AO- 60 RO-60

RO-80

.

60

. 60 .

80

Quantity

(No>

1, 170

1

110

13

1 , 170

1

P-60

.

120

110

7

P-60

.

180

325

4

2,435

1

325

11

2,435

1

325

13

AO-80 P-120

RO-80

Weight of one p a r t kg

.

180

AO-80 P-120

.

180

Total quantity (No) 14

12

12

14

TABLE 3- 10

(Cont'd. )

Design Dimensions

Type of construction

H(c-~) cm

B~~ cm

L~~ cm

H~~ cm

L~~ cm

B~~ cm

L~~ cm

RO- 60

. 40

15

60

120

150

390

60

290

PO-60

. 60

20

60

120

150

390

60

310

RO-80. 60

25

120

180

210

540

120

430

RO-80 . 8 0

30

120

180

210

540

120

460

Volume of m a i n works Type of s t r u c t u r e Material

Unit

Concrete

m3

Reinforcement

kg

TZRM 100

m

0.31

0.43

0.77

0.91

Sandfilling

m3

1.0

1.4

2.5

2.9

Rockfilling

m

0.5

0.9

1.1

2.1

Metal construction s

kg

73.1

72.5

38.4

93.1

Name

Reinforced concrete parts Cement

RO-60

.

1.04 52.5

40

RO-60

. 60

1. 30 57.2

R O - 8 0 . 60

2.40 107.9

RO-80

. 80

2.66 113.6

a r e given i n Table 3- 10.

The intake o p e r a t e s n o r m a l l y a t a l l h e a d s given i n that

Table.

Design Calculating f o r m u l a

3

7

where Q C(ds~) +(lat) CQ B(so)

H

(crt)

=

proposed d i s c h a r g e of the r e g u l a t o r ( m3 / s ) ;

=

d i s p e r s i o n coefficient;

=

coefficient of l a t e r a l contraction;

=

d i s c h a r g e coefficient equal to 0.40, obtained f r o m l a b o r a t o r y data;

=

width of gate opening of the r e g u l a t o r ;

=

head over c r e s t with approach velocity 3

where

"(~PP)

=

velocity of approach

The d i s p e r s i o n coefficient i s

i function of the r a t i o

of s u b m e r g e n c e ( s e e F i g u r e 3-10).

H(crt)

w h e r e H( s ) = depth

Values of d i s p e r s i o n coefficient a r e given in

Table 3- 11. TABLE

3-11

Values of D i s p e r s i o n Coefficient,

C(dsp)

Coefficient of l a t e r a l contraction

i s d e t e r m i n e d by using the following

formula :

where C

C

( shp) ( shh)

=

coefficient of shape of spillway v e r t i c a l r i b s ;

=

1 for the given s t r u c t u r e .

Toe basin depth in t h e s e s t r u c t u r e s i s chosen in a c c o r d a n c e with the r e q u i r e m e n t of hydraulic jump s u b m e r g e n c e with a s u b m e r g e n c e coefficient, C (js) =

1.2.

Total b a s i n length i s d e t e r m i n e d f r o m the f o r m u l a

where

H(c-s~) H

=

=

4H

=

height of the s i l l f r o m the d o w n s t r e a m f l o o r

= ( r e ~ i ) ~

( r e ~ i ) ~

the length of hydraulic jump;

the second r e c i p r o c a l depth.

The second r e c i p r o c a l depth H(reci)2 i s d e t e r m i n e d i n the following sequence. C r i t i c a l depth i s d e t e r m i n e d by the f o r m u l a where

q

=

q B(go)

=

H

C

=

d i s c h a r g e p e r unit w i d t h ;

g

Incident energy

rH( go)

=

total height of u p s t r e a m energy line over downstream floor on apron

=l

-E

EI

,

SB 1

i s defined by using Table 3-12 in t e r m s of SB1

SB 1

TABLE

r~

q

(no)

E' SB 1

A E' SB 1

E SB 1

3-12

rH(go) E' SB 1

2SB 1

3 2 ) E' SB 1

Magnitude of the gate opening i s d e t e r m i n e d f r o m the formula r

H

(go)

-

E

~

H(go) E ~1 l SB 1

Depth in the contracted c r o s s section i s determined f r o m the following formula

where C

+(vert)

=

coefficient of jet v e r t i c a l c o n t r a c t i o n magnitudes of which a r e given in Table 3- 1 3 in t e r m s of magnitudes

TABLE

3-13

The second r e c i p r o c a l depth i s d e t e r m i n e d by using the following formula:

C

where

3.7.4

1 ' 0. 17 f 0 . 8 3 C"

1

=

Numerical Examples

Data

3.7.4.1

The t o t a l B(go) = 0 . 8 m ,

head

sill

over

hydraulic drop,

(crt) = 0.70 m,

is

h(wk) = 0. 13 m .

gate opening,

It i s r e q u i r e d to check

d i s c h a r g e of the s t r u c t u r e and conditions of t h e h y d r a u l i c jump s u b m e r g e n c e on the d o w n s t r e a m side.

h(wk)

=

0.13 m ,

=

0.57 m

F o r value % H

C

(dsp)

=

0.91,

s u b m e r g e n c e depth,

= (crt)

-

-

0.57 0.70

f r o m Table 3-11

-

H ( s ) = 0.7

0.814

-

0. 13

f r o m f o r m u l a (1) 3

q

.

=

0.91

=

0.62 m 3 / s

.

0.825

0.40

.

Z Jm. 0.7

0.80

The basin length i s d e t e r m i n e d f r o m f o r m u l a (3). F o r s t r u c t u r e type RO l o ) , H(c-SB) = 0. 3 m . ,

-

80

. 80 (chosen in a c c o r d a n c e with

F o r t h i s value LSB

1

Table 3-

= 1.25 / 0. 7 (0. 3 t 0.45

.

0. 7)

To d e t e r m i n e the hydraulic jump length, L ( j ) , v a r i o u s d i s c h a r g e Q , , , and r e c i p r o c a l depths a r e magnitudes a r e taken, namely, Q,

Q $

calculated f o r t h e s e d i s c h a r g e s a s well a s the length of the d o w n s t r e a m apron. F r o m the data obtained the m a x i m a l value i s accepted a s the length of s t r u c t u r a l apron downstream. An example for computing hydraulic jump length for

' 2

(0. 31 m 3 / a )

i s given below. Unit d i s c h a r g e ,

q

=

c r i t i c a l depth,

Hc

--

-O m-3 '

-

q

=

;/

0. 8

3

0.388m3/ s

*j

The magnitude of the gate opening i s d e t e r m i n e d f r o m f o r m u l a (4) together with Table 3- 12,

i. e .

H(go)

=

ESB 1 =

1.0

.

0.157

0.157

=

0.157 m.

The depth i n contracted c r o s s section i s d e t e r m i n e d f r o m f o r m u l a (5) together with Table 3- 13.

C' i s d e t e r m i n e d f r o m the following formula:

The second r e c i p r o c a l depth i s d e t e r m i n e d f r o m f o r m u l a (6), i. e .

The hydraulic jump length i s t h e n :

=

L(j)

.

4

0.51

=

2.04 m

The total d o w n s t r e a m apron length i s d e t e r m i n e d by using f o r m u l a (3) :

L ( ~ ~ =)

=

L ( ~ ~ +l )L(j)

0. 82

+

2.04

=

2.86 m

The difference i n the d o w n s t r e a m depth and the second r e c i p r o c a l depth i s then calculated: Y2

-

=

H(j)Z

( 0 . 7 0 - 0 . 13) - 0 . 5 1

- O . OE m

The s u b m e r g e n c e coefficient of the hydraulic jump, H(s)

(js)

=

-

(0.7

H ( 12 j

- 0.13) 0.52

=

1.12.

Hence, t h e j u m p i s

s u b m e r g e d with an adequate safety f a c t o r .

Choosing an open intake s t r u c t u r e

3.7.4.2

A s s u m e that the canal d i s c h a r g e i s 0. 35 m 3 / s , the w a t e r depth in the canal i s 65 c m , and the canal width i s 60 c m . F r o m Table 3-10 i t i s seen that a s t r u c t u r e of RO -60

.

60 type with

H(crt) = 60 c m and B (go) = 60 c m will convey a d i s c h a r g e o f 0 . 3 1 m 3 / s f o r F o r conh(wk) = 5 c m and a d i s c h a r g e of 0.38 m 3 / s for h (wk) = 10 cm. 3 veyance of a d i s c h a r g e of 0 . 3 5 m / s i t i s n e c e s s a r y to d e t e r m i n e h(wk) by interpolation, r e s u l t i n g in H(crt)

=

bottom (65

60 c m .

- 60

=

h(wk)

=

8 cm.

With a canal depth of 65 c m ,

Hence the s t r u c t u r e should be 5 c m higher than the canal 5 cm).

Gouging device DRS-60

Cross section A-A

Flume

-

FAO- lClD INTAK STRUCTURE DISCHARGING

INTO A FLUME CHANNEL Cross section 13-13 Cross section

0-8

Project, Region , Country USSR

Cross section C-C

Figure No. 3 - 20

3.8

INTAKE STRUCTURE DISCHARGING INTO A FLUME CHANNEL

3.8. 1

(u. S. S. R. )

General The types of intake s t r u c t u r e s d e s c r i b e d h e r e i n a r e f o r delivering water f r o m unlined and lined canals into a flume i r r i g a t i o n canal.

The d i s c h a r g e

through the intake into a flume 60 c m in depth i s 400 1/ s , and into a flume 80 crn in depth, 900 l f s . S t r u c t u r a l C h a r a c t e r i s t i c s and Design The intake s t r u c t u r e ( F i g u r e 3-20) c o n s i s t s of an entrance s t r u c t u r e and a pipeline.

The entrance ' s t r u c t u r e i s a well with an adjacent u p s t r e a m p a r t com-

p r i s i n g a reinforced concrete pipe.

T h e r e i s an opening in the pipe for embedding

the lower end of the water gauge device. with a DRS

The top p a r t of the water gauge device

- 60 water gauge i s s e c u r e d to a well.

A m e t a l slide gate with a

s c r e w jack i s mounted in the well The pipeline c o n s i s t s of a number of reinforced c o n c r e t e bell-and- spigot pipes.

A c r o s s i n g , 7 m wide, i s provided above the pipeline.

The pipe and the

flume a r e connected by m e a n s of a diaphragm which h a s cut-outs corresponding to the pipe and the flume outer p e r i p h e r y . The intake s t r u c t u r a l p a r t s a r e p r e f a b r i c a t e d and mounted and adjusted according t o special i n s t r u c t i o n s . The design, the m a t e r i a l s used and careful construction provide f o r the r e q u i r e d strength and life of the device. The s t r u c t u r e r e q u i r e s periodical inspection to s e e that i t i s operating c o r r e c t l y and to check on the state of the construction and to c a r r y out running r e p a i r s if any defects be found.

It i s n e c e s s a r y to l u b r i c a t e the jacks

periodically and to p r o t e c t the p a r t s by a suitable' a n t i - c o r r o s i o n compound. 3.8. 3

D e sign Calculating f o r m u l a

where

Ax(p)

=

sectional a r e a of a pipe, rnZ

h(,k)

=

hydraulic d r o p o r working head, m

Q

=

3 designed d i s c h a r g e , m / s

c~

=

d i s c h a r g e coefficient

c(f-OL)

=

1

=

outlet r e s i s t a n c e coefficient ;

0.4

=

e n t r a n c e r e s i s t a n c e coefficient, accepted according to l a b o r a t o r y t e s t data;

0.26

=

well r e s i s t a n c e coefficient, accepted according to l a b o r a t o r y t e s t data;

=

pipe length r e s i s t a n c e coefficient with coefficient of roughness N = 0.012;

C ( f - ~ ~ ) =

C(f- shaft)

9f-p)

=

-

.?bdcz ~

( ~ )

=

pipe length;

C

=

72

R ( ~ )

=

4

g

=

9.81

L(~)

=

Chezy f r i c t i o n coefficient;

=

hydraulic r a d i u s ;

=

gravity a c c e l e r a t i o n

The r e q u i r e d head a t the pipe e n t r a n c e i s calculated i n a c c o r d a n c e with n o r m a l water gauge operation, when the pipe upper edge i s 20-25 c m l o w e r than the rated water level. T a b l e s of d i s c h a r g e capacity, dimensions, and l i s t s of construction m a t e r i a l s and p a r t s a r e given in Table 3-14.

3.8.4

N u m e r i c a l Example Data Design d i s c h a r g e

=

F l u m e depth

-

Pipeline diameter Length of pipe

D(p)

700 l / s Y2

=

0.7m3/s

=

80 c m

=

80 c m

-,

12 m

TABLE

3-14

D i s c h a r g e Capacity Type of . Structure

h(wk), cm

5 --

VKLV VKLV

-

Q, m 3 / s 3 Q, m / S

60 80

0.2

0. 28

0.34

0.40

-

-

0. 36

0.51

0.63

0 . 72

0. 8 1

0.89

Design Dimensions Type of Structure

D

HWL cm

( P)

Y2 cm

HTS cm

60

60

60

105

165

120

56

1000

VKLV - 80

80

80

130

185

140

44

1000

VKLV

-

BWL cm

d(toe) cm

L cm (~)

Volume of Main Works Type of Structure Type - 60 Type

-

80

Reinforced c o n c r e t e d e t a i l s Concrete Reinforcement m3 kg

Cement m3

Gravel Filling m3

Metal Construction kg

3.05

207

0.11

1.33

145

4. 38

2 62

0. 1 4

1.38

167

L i s t of P a r t s Type of Structure

Weight of p a r t kg

No. of p a r t s

8

660

6

SH - 60

2,200

1

Type of p a r t TR -

VKLV- 60

P

-

1

220

1

D

-

1

7 25

1

125

4

TR - 10

1,140

6

SH - 8 0

2,500

1

S

VKLV- 8 0

P

-

1

220

1

D

-

2

875

1

125

4

S

Total number

It i s n e c e s s a r y t o calculate the working head h(,k). Solution According to the f o r m u l a given under 3 . 8 . 3

=

3.9

3.9.1

0 . 1 8 7 m o r 19 c m .

P I P E REGULATOR WITH CROSSING MADE O F PRE-CAST REINFORCED CONCRETE ( U . S . S . R . ) General P i p e r e g u l a t o r s a r e u s e d f o r the d i v e r s i o n of water to s m a l l c a n a l s .

As

c o m p a r e d with e a r l y types in u s e , the l a t e s t s t r u c t u r e p r o v i d e s b e t t e r dissipation of energy on the d o w n s t r e a m side and a l s o h a s only one joint in the pipeline. C

S t r u c t u r a l C h a r a c t e r i s t i c s and Design The pipe regulator

( F i g u r e 3-21) c o n s i s t s of t h r e e m a i n units: i n l e t sill,

pipeline and downstream a p r o n . The inlet s i l l c o n s i s t s of an inclined wall which i s supported by bottom and portal decks.

,

i ' ~ a s e d on a note by A. T. K o s h k k a , E. P. M a r t i n , A. V. Shatalova, D. D. Aliev and B. V . Kazarinov (U. S. S. R. ).

Stilling bosh a damper

Cross

section A - A

FAO-ICID Plon

In the drawing (Plan) alternative applications of pipe regulator are shown at the top AS on impound structure, at the bottonAs on outlet.

-

Note: All dimensions are in centirnetres.

MADE O f PRECAST R.C.C.

Project, ~ e ~ i o Country n, USSR Figure -No. 3 - 21

1

The pipeline consists of two pipe lengths with t h e i r bells laid on a levelled e a r t h foundation.

The joint i s sealed with tow o r m i n e r a l wool, impregnated

with bitumen, and then covered with cement.

The outlet pipe on the downstream

side extends f r o m the sloping wall a s shown.

This pipe i s followed by a damper

of semi-cylindrical shape with a ring diaphragm a t the end. Slope stabilization a t the inlet sill and the downstream apron i s secured by p r e - c a s t reinforced plates; the side slope i s 1 : 25: 1.

In front of the inlet sill

t h e r e i s a well and a t the end of the downstream apron t h e r e i s a rock-filled knife-edged support.

The height of the embankment above water level on the

u p s t r e a m side of the s t r u c t u r e i s 25-30 cm and on the downstream side 30-35 c m . The design, m a t e r i a l s used and careful supervision of construction e n s u r e that this type of s t r u c t u r e h a s the n e c e s s a r y strength and durability. The s t r u c t u r e r e q u i r e s periodical checking for i t s c o r r e c t operation and condition and for carrying our running r e p a i r s if required.

Lubrication of

the jacks and protection of the metallic p a r t s with an anti-corrosive coating i s n e c e s s a r y periodically. This type of s t r u c t u r e conveys discharges f r o m 0.30 to 0.90 m 3/ s . working heads permissible range f r o m 10 to 30 cm, and the water depth in front of the gate i s taken at 125 and 150 cm.

See Table 3-15 for discharge

capacities, dimensions, construction m a t e r i a l s and l i s t of p a r t s .

Design Calculating formula Discharge capacity (in m 3 / s ) i s determined f r o m the formula

The

TABLE

3-15

Discharge Capacity m3/s T y p e of Structure

h(wk)* c m

D(P),

L i s t of P a r t s 10

15

20

25

30

cm

T R - 60- 125

60

0 . 29

0.36

0.42

0.47

0.51

T R - 8 0 - 150

80

0.53

0.66

0.76

0.85

0.93

T y p e of structure

Design Dimensions Type. of Structure

D(p) H ( c r t ) L~~ L ( p r o t ) 'IN cm cm cm cm cm

B cm

E ; Bi ~ ~ cm

T R - 60- 125

T y p e of part

Weight of one p a r t kg

Number of parts

T R - 60

1,350

2

ON- 60

450

1

N

- 60

352

2

D

- 60

418

1

G T - 60

433

1

120

110

1

. 180

325

4

310

4

100

4

2,575

2

80

598

1

N - 80

455

2

-

80

580

1

GT-80

928

1

325

12

SH- 1 2 0

138

16

S

100

4

P-60 TR-60-125

60

125

210

320

90

60

115

TR-80-150

80

150

240

580

110

120

150

.

P-120 SH

- 180

S

Total number

20

V o l u m e of m a i n w o r k s TR-80 N Reinforced

Concrete

m TR-60-150

D

-

P- 1 2 0

.

180

39

where

c~ C~ C(f-IN)

C(f-OL)

9f-p)

h

( wk)

L

(P)

C

J C(t-IN)

+

C(f-OL)

+

C(t-p)

=

discharge coefficient

-

0.5

=

i n l e t .friction coefficient, accepted according to laboratory data;

-

1

=

outlet friction coefficient accepted according to laboratory data;

-- 'gL(p)

=

friction coefficient along the pipe for roughness coefficient N = 0.01 2;

C2 R ( ~ > =

hydraulic d r o p o r working head, m ;

=

pipe length, m ;

-

Chezy coefficient;

-

g

1

-

4

hydraulic m e a n radius, m ;

=

9.81

-

pipe c r o s s - sectional a r e a .

*X(P)

=

gravity acceleration, m / s 2 ;

Protection length on the downstream side i s determined f r o m the following formula :

where =

velocity in the pipe, m / s;

=

velocity allowed f a r wash-out, accepted for medium loam, a s equal to 0.8 m / s . In the c a s e fine and sandy loam soils in the foundation of i s a s s u m e d to be equal to structures v (flu) 0.45 m / s .

"(P)

"(flu)

3.9.4

Numerical Example Data h(wk)

-

0.17 m;

pipe diameter = 0 . 8 m,

L(p) = 12.5 m

( s t r u c t u r e : pipe regulator, TR- 80- 150 type). Bed soil : medium loam.

C

=

72

What i s the discharge of the structure, and basin length on the downstream side ? Solution Determine discharge f r o m the formula

where:

hence :

Q

=

0.758

.

0.503J 2

.

9.81

. 0.17

0.696 m3/s

=

Assume average velocity, allowed for wash-out, in the canal v(flu) = 0.8 m / s (for medium loam), then determine the basin length :

-

L ( ~ m t )-

2.2

. V(p> "( flu)

D(~> =

2.2

.0.8

'

0.8

=

3,l m

Section D-D

Compacted fill of selected material Note: All dimensions are in metres.

F A 0- ICID

INTAKE STRUCTURE ON SECONDARY CANALS

-

PLAN SECTIONS-DIMENSIONS

3.10

INTAKE STRUCTURE ON SECONDARY CANALS (COLOMBIA)

General The s t r u c t u r e d e s c r i b e d h e r e i n m e e t s the r e q u i r e m e n t s f o r w a t e r l e v e l control f o r the d i v e r s i o n of w a t e r into t e r t i a r y c a n a l s , providing a t the s a m e t i m e the n e c e s s a r y d r o p i n the secondary canal. T h e s e s t r u c t u r e s have been designed a t the Instituto Colombiano de l a R e f o r m a A g r a r i a , INCORA, to m e e t the r e q u i r e m e n t s of t h e Bolivar No. 1 I r r i g a t i o n and D r a i n a g e P r o j e c t , located a t the Bolivar D e p a r t m e n t , in t h e n o r t h of Colombia. t h i s type.

The P r o j e c t ' s plan f o r e s e e s the construction of 120 s t r u c t u r e s of

Up t o now 40 have been built, of which 90% a r e i n operation.

T h i s intake s t r u c t u r e ( F i g u r e 3-22) i s not yet c o n s i d e r e d sufficiently. t e s t e d b e c a u s e development of i r r i g a t i o n in Colombia i s r e c e n t .

The s t r u c t u r e i s e a s y

to c o n s t r u c t f r o m i t s p r e - c a s t c o n c r e t e p a r t s .

10.2

Structural Characteristics The s t r u c t u r e i s designed f o r flexibility and u s e under different topographi c a l conditions and a t different p l a c e s , both on the p r o j e c t a r e a and within the canal s y s t e m .

F o r example, some dimensions m a y be v a r i e d s o that the canal

design engineer can f i t the s t r u c t u r e according to r e q u i r e m e n t s a t different s i t e s . The fixed dimensions of the s t r u c t u r e depend upon the m a x i m u m design flow, and upon the longitudinal fall of the secondary canal. B a s e d on t h e s e p a r a m e t e r s , s t r u c t u r e s have been designed and c o n s t r u c t e d which allow the d i v e r s i o n of flows between 100 l / s and 750 l / s on e a r t h secondary c a n a l s whose c a p a c i t i e s a r e between 500 11s and 3,000 11s.

O b s e r v a t i o n s on the

prototype under different operating conditions, p a r t i c u l a r l y a s to flow and heading u p of w a t e r , have not indicated i m p o r t a n t deficiencies i n the hydraulic operation of t h e s t r u c t u r e . The depth of the intake s t r u c t u r e on secondary c a n a l s i s s m a l l , between 0.80 m and 1.40 m , and the side w a l l s have been designed t o r e s i s t both the e a r t h

Note: The broken lines show, the wall perimeter.

vCentre line

E

Section 1-1

Element Iembedded\ in concrete slab-

Section F-F

/

I

0'05'

I

L

H

Qrnbedment of the element Gote 2

Gote I

SectionI D-D

d 4 --

F

Reinforcement detail Section G-G

Plon of bottom slob A and B type structures

Cotwolk- Longitudinol section

2cm mortar finish for rmll crest

Note: For structures A and D LfcotW) = 2'20 rn For structures B and E

2 cm mortar joint

4cotwj

-

E

= 1.80 m

Spillway reinforcement detoil

Wall detoil

* Groded g rnoxirrmm

6 5+0 4 . + - d b

FA0

sand layer Section J-J

Weep holes detoil

Plon of bottom slob

I

Note :

- ICID

INTAKE STRUCTURE ON SECONDARY

I

CANALS

$steel bars anchored to the bottom slob by Ineons of 3 0 cm souore hooks.

CONSTRUCTION D E T A I L S

Project, Region , Country Colombia

Note: All dimensions ore in metres unless otherwise specified.

I

Figure No. 3 - 23

I

load and the m a x i m u m w a t e r p r e s s u r e .

To e n s u r e adequate strength of the p r e -

c a s t concrete side w a l l s and head walls, s t r u c t u r a l s t e e l b a r s a r e embedded in the joints of the blocks, anchoring t h e m to the slab foundation ( F i g u r e 3- 23). This reinforced c o n c r e t e slab h a s cut-off walls and weep holes t o reduce the uplift p r e s s u r e .

At the inlet and outlet of the s t r u c t u r e , t h e r e a r e r e v e t m e n t s

consisting of 50 c m x 50 c m x 6 c m p r e - c a s t s l a b s which a r e placed o v e r a thin l a y e r of concrete.

This lining i s provided with a cut-off apron a s indicated on

F i g u r e 3-22. F o r economic r e a s o n s the idea of constructing a transition a t the inlet of the s t r u c t u r e h a s been discarded, especially a s the hydraulic head l o s s e s a r e compensated by the operation of the weir g a t e s .

Wooden g a t e s f o r the regulation

of the diverted d i s c h a r g e include pins on the f r a m e , allowing total o r p a r t i a l opening of the intake ( F i g u r e 3-24).

FIGURE 3- 24. - Intake s t r u c t u r e on secondary canals, spillway and wooden gate.

To c a l i b r a t e the s t r u c t u r e and t o m e a s u r e the d i v e r t e d flow, a r o d and m e a s u r i n g weir a r e located a t the outlet of the intake ( F i g u r e 3-25).

FIGURE 3-25. - Intake s t r u c t u r e on secondary c a n a l s , outlet to t e r t i a r y canal.

3.10.3

Design F o r m u l a e

3.10.3.1

Design of control w e i r on secondary canal The w e i r length i s chosen i n a c c o r d a n c e with the canal width, and i t s

depth i s chosen according to the heading up needed f o r m i n i m u m flow. 3.10.3.2

Wall height Taking the design m a x i m u m flow,

H(crt)

i s calculated by m e a n s

of the f o r m u l a :

where:

-

flow in r n 3 / s ;

-

discharge c o e f f i c i e n t

-

width of w e i r n o r m a l to flow;

=

depth of u p s t r e a m w a t e r s u r f a c e l e v e l o v e r the w e i r .

The wall height of the s t r u c t u r e i s obtained by adding to H ( c r t ) the weir height over the floor s l a b and an adequate f r e e - b o a r d . 3.10.3.3

Length of the stilling basin To d e t e r m i n e the length of the stilling pool the following f o r m u l a i s

used:

where:

=

L~~ H(j)Z

length of the stilling basin t h e o r e t i c a l water depth downstream of the hydraulic

=

jump H(j) 1

=

t h e o r e t i c a l water depth u p s t r e a m of the hydraulic jump.

The coefficient '5' i s a s s u m e d a s the stilling basin i s of a trapezoidal section; however, t h e r e i s not yet sufficient experimental evidence to prove t h i s . N e v e r t h e l e s s , dimensions chosen according to t h i s f o r m u l a have s o f a r proved t o be s a t i s f a c t o r y on the s t r u c t u r e s a l r e a d y built. Water depths, H ( j ) l and H ( j ) 2 a r e calculated by the conventional methods, applying B e r n o u l l i ' s equation. 3.10.3.4

Dimensions of t e r t i a r y intake The d i a m e t e r of the c o n c r e t e pipe i s d e t e r m i n e d a s s u m i n g that the

m a x i m u m velocity of the w a t e r through the pipe i s 1 m / s, f o r the m a x i m u m design flow of the t e r t i a r y canal. Intake l o s s e s a r e determined according to the following formula:

where: h ( e . ent)

=

l o s s of head a t e n t r a n c e

h ( l f)

.

=

l o s s of head through the pipe

h((. OL)

=

l o s s of head a t outlet

(HR)

=

intake headloss

C ( ~ ~ ) C(f)

"(PI 3. 1 0 . 4

=

inlet coefficient

=

f r i c t i o n coefficient

=

velocity through pipe

1. 5

'= =

0. 023

Numerical Example A design example f o r a secondary canal having a m a x i m u m capacity of 1, 000 l / s with a n offtake,of 250 l/ s i s shown below. secondary canal i s 1 m .

The bed width of the

Drop = 1.4 m .

Control weir design for the secondary canal

3. 1 0 . 4 . 1

A C r e a g e r section i s designed f o r a w e i r width of 1 m and a height of 0 . 4 0 m above the u p s t r e a m bed l e v e l o r floor slab. Height of the w a l l s

3. 1 0 . 4 . 2

Data:

we obtain H

Q

=

1 m3/s

CQ

=

2.00

B(t)

=

1.00 m

=

0.63 m

(4

Applying an approximate f r e e - b o a r d of 3070 of the depth of the w a t e r , we obtain:

T h i s height of the s t r u c t u r e ' s walls i s a l s o applied to the canal design u p s t r e a m s o that heading up does not cause overflow. 3. 10.4. 3

Length of the stilling b a s i n F o r a d r o p of 1 . 4 0 m in the s t r u c t u r e and w a t e r depth in the canal

u p s t r e a m of 1. 03 m , the approximate velocity u p s t r e a m of the weir will be:

and t h e t o t a l energy w i l l be:

H

'

~

=

~

1.40 +

1 . 0 3 + 0.05

=

2.48 m

Applying B e r n o u l l i ' s e q u a t i o n :

where

v(j)

and H ( j )

a r e c o n d i t i o n s b e f o r e t h e h y d r a u l i c jump, i n t r o d u c i n g

a c o e f f i c i e n t f o r t h e f r i c t i o n l o s s e s over t h e h y d r a u l i c jump, e q u a t i o n (4) becomes : '

- 1

On t h e o t h e r hand

Q

B~

=

1 m3/s

From (5) and (6) t h e v a l u e of H ( j )

=

0.186 i s found.

=

v(jJl

H(jIl

Therefore: v

(jIl

=

5.38m,/s

The d e p t h of water over t h e h y d r a u l i c jump i s d e f i n e d by:

where :

v(j)l

F(j)l being F( j)

=

(gH(j),) i / 2

3.98

=

t h e Fraude' s number.

from where

H(j)2

=

0.96

and t h e l e n g t h o f t h e s t i l l i n g b a s i n L~~

3.10.4.4

=

s(0.96-0.19)

3.85m.

=

Tertiary canal intake design Tertiary canal discharge

=

0 . 2 5 m3/ s

Velocity i n the pipe, v ( ~ )

=

l.OOm/s

L e n g t h of t h e p i p e , L(p)

=

10.00 m

Hydraulic a r e a =

D i a m e t e r of the s e l e c t e d pipe

0.62 m ;

(Area

= 0.30 m2)

Intake head l o s s e s a r e equal to:

J

=

(HR)

h([. ent)

+

h

(l. ent)

h

ce.

+ f)

h

(l. OL)

=

1 . 5 &) ( e n t r a n c e l o s s e s ) 2g

=

0.023

(friction l o s s e s )

D ( ~ ) 2g h

J

(C. out) (HR)

=

1.0

=

(1.5

(outlet l o s s e s )

2g

+

0.023

10 0

t.1

)

2g

= :146 , .

T h i s shows t h a t the w a t e r l e v e l u p s t r e a m in the secondary canal should b e 0.146 m above the l e v e l on the t e r t i a r y canal i n o r d e r t o d i v e r t 0.25 m 3 1 s . The design i s shownon F i g u r e s 3-22 and 3-23.

3.11.1

General The gate valve i n t a k e s d e s c r i b e d i n t h i s section s e r v e to feed w a t e r f r o m higher o r d e r to l o w e r o r d e r c a n a l s .

Water l e v e l i s controlled by a canal s e a l -

gate mounted on a pipe p a s s i n g through the canal embankment.

3.11.2

Structural Characteristics

3.11.2.1

P a r t s of the s t r u c t u r e The s t r u c t u r e c o n s i s t s of fixed and v a r i a b l e p a r t s a s d e s c r i b e d

below.

-

Fixed p a r t s

( a ) an inlet p a r t including s i l l , side wings, c o n c r e t e paving of inlet bottom with pitching, inlet walls; (b) canal seal-gate with lifting m e c h a n i s m ; ( c ) outlet wall; (d)

s t r e a m bed protection downstream of the outlet including pitching and sill.

Variable p a r t s

-

( a ) earthwork;

(b) length of piping in relation to type and height of s t r u c t u r e varying f r o m 125 c m

- 2m; 175 c m - 3 m and 250 c m -

5 m.

3.11.2.2

Types of s t r u c t u r e T h e r e a r e five types, according to the inside d i a m e t e r of the pipe,

height of the inlet wall and kind of lift a s shown in Table 3-16.

F o r example,

Type RSZ 501250 denotes that the inside d i a m e t e r of the pipe i s 50 c m and the height of the inlet wall i s 250 c m .

TABLE

3-16

Inside d i a m e t e r of pipe, c m

Lifting mechanism

30

unshielded

RS 1 RS 1

301 175

shielded

RS2

301 175

unshielded

RS 1

501175

shielded

RS 2'

501 250

50

3.11.2.3

Type and height of inlet wall, c m

301 125

Instructions f o r u s e Unshielded lifting mechanism-s should not be used in places exposed

t o the danger of i n t e r f e r e n c e o r damage by unauthorized p e r s o n s .

In such c a s e s

FIGURE 3- 26. - Gate valve intake, relationship between head, discharge and pipe diameter.

shielded lifting m e c h a n i s m s should be u s e d .

The u s e of shielded m e c h a n i s m s i s

l i m i t e d by the m i n i m u m height of the inlet wall a s r e q u i r e d by the design. The height of the s t r u c t u r e should be chosen so that the top of the bank of the canal i s n e i t h e r higher than the inlet wall by m o r e than 25 c m nor f a l l s s h o r t of i t by m o r e than 50 cm; otherwise i t will be n e c e s s a r y to a r r a n g e a c c e s s to the lifting m e c h a n i s m . 3.11.2.4

Building m a t e r i a l s r e q u i r e d Table 3- 17 gives the type of s t r u c t u r e , volume of concrete, paving

c o n c r e t e , and type of seal-gate.

TABLE

3-17

' V o l u m e of Main S t r u c t u r e s

Type of structure

3. 1 1 . 3

Concrete m3

Form work m2

Paving concrete m2

Canal Seal- a t e JS 300 J S 500 pieces pieces

Weight tons

Design F i g u r e 3- 2 6 gives, for a 4 m pipe length, the relationship between head, d i s c h a r g e and the pipe d i a m e t e r .

3.11.4

N u m e r i c a l Example Design a gate valve s t r u c t u r e f o r a d i s c h a r g e of 500 11s and a head of

60 c m .

The height of the inlet wall i s 175 c m .

~ectionol'view 1-1

Sectionol view 2-2

1

Sectional view 3-3

Czechoslovakia Note: All dimensions ore in centirnetres.

Figure No. 3 - 27

F r o m F i g u r e 3-26, the r e q u i r e d d i a m e t e r of the pipe f o r 500 11s d i s c h a r g e and f o r the head of 60 c m i s 50 cm. See Table 3-16, u s e type RS1 501175. The dimensions of the v a r i o u s p a r t s a r e given in F i g u r e 3-27 and o t h e r d e t a i l s n e c e s s a r y m a y .be found f r o m t h i s figure f o r design p u r p o s e s .

3.12

VENTURI HEAD INTAKE

General The design of t h i s venturi head intake was evolved in the 1920's during the construction of the S a r d a Canal in Uttar P r a d e s h , India, to effect economy by providing a flumed t h r o a t with suitable wing wall connections to r e s t o r e the full bed width of the offtake canal. The c h a r a c t e r i s t i c s of this s t r u c t u r e a r e a s follows. (a)

The h e a d l o s s i s

H(crtl 9

o r l e s s and the d i s c h a r g e i s a little over the

t h e o r e t i c a l value due t o the s t r e a m l i n e d approach. (b)

The venturi head may be designed for any angle of offtake f r o m 60

0

to 90°

and f o r any bed width of the offtake canal up to 7 ? 5 m . (c)

The design i s such that the e x c e s s energy of the w a t e r i s dissipated by the

formation of a hydraulic jump. (d)

The s t r u c t u r e does not m e a s u r e d i s c h a r g e c o r r e c t l y and i s not successful

in controlling the entry of s i l t into.the offtake canal. A bridge i s generally provided over the s t r u c t u r e .

3.12.2

S t r u c t u r a l Design The s t r u c t u r e ( F i g u r e 3-28) c o n s i s t s of u p s t r e a m wings and side pitching, t h r o a t , downstream wings, and downstream bed and side protection. The u p s t r e a m face wing i s curved and i s uarpedas shown on F i g u r e 3-28. The sill i s rounded off a t the top.

The r a d i u s of the wing wall i s generally kept

T h e r e a f t e r i t i s extended f u r t h e r to m e e t the 0. 5 : 1 slope.

at 5H(crt).

The length of the curved s i l l i s and i t s radius i s kept a t H 2 2 L ( a ~ ~ )H(b-c) where i s the length of the curved s i l l o r approach, (~PP) 2H(b- c) i s the difference between the s i l l level and the bed of the p a r e n t canal. and H (b-c) The downstream wing wall in the p a r e n t canal i s rounded off and turned a t right angles, and i t i s extended 0. 5 to 1 . 0 m to m e e t the side pitching. The width of the t h r o a t should not be l e s s than one t h i r d the bed width of the offtake canal nor l e s s than the width determined f r o m the f o r m u l a -

B(t) -

-3 1

.

where B(t) = t h r o a t width, and H

2 ~ (crt)

~

(crt)

i s the head over the

sill. Where t h e r e i s a c r o s s regulator o r a l a r g e d r o p in the p a r e n t canal downs t r e a m of the venturi hkad, the t h r o a t width m a y be d e c r e a s e d to 0. 25 of the bed

-:

Q .

width o r to

-

whichever i s g r e a t e r , subject to a minimum of 0. 60 m .

1 . 4 ~ ' (crt) The d r o p in the water s u r f a c e should be a s s u m e d a t a t l e a s t

8

to be on the

safe side. The t h r o a t should be s e t back by ( 1 . 4 B(t) t 0. 6 ) m f r o m the full supply line in the p a r e n t canal.

The side slope of the p a r e n t canal should be 0 . 5 : 1.

The length of the t h r o a t should be 2.5 H ( c r t ) . The grooves f o r the stop-logs o r gate should be s e t a s f a r down the t h r o a t a s possible to avoid swirling.

In n a r r o w flumes the grooves should preferably

be fixed below the flume. The floor should extend up to the s t a r t of the downstream wing walls. If the bed of the offtake canal i s below the th.roat sill, the drop, i f l e s s than , 0 . 1 5 m, should be negotiated by a g l a c i s a t a slope of 1 : 10 and a minimum floor length of 1. 5 m should be provided below the glacis.

If the d r o p exceeds 0.15 m ,

a v e r t i c a l d r o p immediately below the t h r o a t should be provided.

The side w a l l s of the flume should be splayed out f r o m the end of the t h r o a t a t 1 : 10 f o r a length of 4 . 5 m o r until the width of the flume b e c o m e s two-thirds of t h e bed width of the offtake channel.

A f t e r w a r d s the splay should be i n c r e a s e d

to 1 : 3 until the full bed width i s attained and should be stepped down a t 1 : 1. The side w a l l s should be v e r t i c a l .

The floor end should r e s t on a toe wall.

Bed and side pitchings should be provided in the offtake canal a f t e r the wing w a l l s u p to a d i s t a n c e o f ' 3 m , and t h i s should be followed by side pitching only of equal length.

3.12.3

Hydraulic Design The t h e o r e t i c a l m a x i m u m d i s c h a r g e of a n open v e n t u r i flume i n which hydraulic jump i s obtained, and the velocity of a p p r o a c h i s ignored, i s given by

=

where

B(t)

and

H(crt) =

3.12.4

t h r o a t width a s d e t e r m i n e d f r o m the d a t a given above the depth of w a t e r u p s t r e a m of the t h r o a t m e a s u r e d f r o m the sill.

Numerical Example Design a venturi head intake f o r the following d a t a : P a r e n t canal

y1

=

Offtake canal

1.2 m

B e d elkvation

=

200

T h e r e i s no c r o s s r e g u l a t o r o r d r o p i n the p a r e n t canal d o w n s t r e a m of the v e n t u r i head. Design F u l l supply l e v e l i n the p a r e n t c a n a l i s 201.20.

Allow f o r a d r o p i n head

of 0. 15 m and k e e p the full supply i n the offtake canal a t 201.20

-

0. 15 = 20 1 . 0 5 .

The bed of offtake canal

- 0.65

201.05

=

=

200.4

The width of the t h r o a t B(t) should b e not l e s s than the lower of the following values (a)

-31

-

bed width of offtake canal,

3.5 3

i. e .

=

1.1 67 m

The following v a l u e s of H(crt) corresponding to B(t) a r e obtained:

Sill l e v e l (201.20

- H(,,t))

200. 32

200.42

200.49

The bed l e v e l of the offtake canal i s a t 200.4; r u l e d out.

200.55

t h e r e f o r e the f i r s t value i s

Adopt the t h i r d combination of B(t) and H(crt),

H(crt) = 0.71.

The sill l e v e l will be (200.49

t h a t of the offtake canal.

- 200.40)

B(t) = 1 . 4 ;

i.e.

= 0 . 0 9 m higher than

The d r o p ( 0 . 09 m ) i s l e s s than 0. 15 m; a g l a c i s a t

1 : 10 slope will be provided to negotiate the d o w n s t r e a m bed level. Setback of t h r o a t

=

(1.4B(q

=

2.56 m

+

0.60)

1.4.

=

1.4

+

0.60

f r o m the supply level of the p a r e n t channel. Length of t h r o a t

=

Lengthofcurvedsill

= f i ~ l . ~ (

Radius of s i l l

-

2 . 5 H(,,t.)

(4

2 (app) + H(b-c)

2.5

=

0

2 (b-c)

.

. 7

--

0.71 1

)

=

1.77m

=~

1.041-11

+

1 . 0 4 ~ (0.49)' 2

.

0.49

F l o o r t h i c k n e s s and length F o r the w o r s t condition the head will be 201.20 0.80

m a x i m u m floor t h i c k n e s s will be

5

.

4

=

- 200.40 = 0 . 8 m .

0.64 m ,

say 0. 65 m .

t h i c k n e s s i s r e d u c e d according to the hydraulic g r a d i e n t l i n e . gradient of 1:9 m a y be adopted.

The length of floor will be 9

The The

A hydraulic

.

0.8

=

7.2 m .

D o w n s t r e a m wing w a l l s After the t h r o a t length (i.e . 1. 77 m ) , the wing w a l l s m u s t be splayed a t 1 : 10 to attain t w o - t h i r d s width of the offtake.

- 1.4) 0 5

(3.5 3

splayed a t 1 : 3.

= 1.755 m .

.

10

=

4.65 m .

The length will be

A f t e r w a r d s the wings m u s t be

The length of the splayed wings will be ( 3 . 5

- 2. 33) 0 . 5

.3

The 1 : 3 splayed wings m u s t be stepped down f r o m the section

inter-sected by a 45O angle f r o m the end of the toe wall of the d o w n s t r e a m floor. P r o v i d e a 20 c m thick and 3 m long d r y pitching i n the bed and the s i d e s and then provide side d r y pitching 3 m long. F i g u r e 3- 2 8 gives the d e t a i l s of the s t r u c t u r e .

The thickness of the walls

of the t h r o a t , the u p s t r e a m approach, and the d o w n s t r e a m wings should be a s given in F i g u r e 3-3 of section 3. 2.

3.13

SQUARE-HEAD INTAKE

General The s q u a r e - h e a d r e g u l a t o r i s a simple intake s t r u c t u r e provided a t the h e a d s of secondary and t e r t i a r y c a n a l s t o d r a w w a t e r supplies f r o m a m a i n o r b r a n c h o r secondary canal, t h e l a t t e r being called the p a r e n t canal and the f o r m e r the offtaking canal. canal.

The s t r u c t u r e i s usually sited a t r i g h t angles to the p a r e n t

The s t r u c t u r e i s not a m e t e r and i t i s not s i l t - selective.

m e a n t t o regulate w a t e r supplies into the offtaking canal.

It i s p r i m a r i l y

Regulation i s effected

by m e a n s of the i n s e r t i o n of stop-logs o r a sliding gate in the grooves provided on the u p s t r e a m side i n ~ t h eabutments.

A bridge i s provided over the s t r u c t u r e when the width of the controlling

section i s m o r e than 0.60 m o r when the canal bank i s m e a n t to c a r r y vehicular traffic.

S t r u c t u r a l Design The s t r u c t u r e c o n s i s t s of u p s t r e a m wing walls, u p s t r e a m bed protection, s i l l , abutments, downstream wing walls, side and floor protection. The thickness of abutments for n o r m a l loading conditions should be a s given in F i g u r e 3 - 3 of section 3 . 2 . The wing walls on the u p s t r e a m and downstream s i d e s a r e l a i d out s t r a i g h t to connect the banks.

The c u r v e s of the wing walls a r e s h a r p with little con-

sideration to streamlining. The floor of the controlling section i s designed on Bligh's theory f o r the w o r s t condition when the p a r e n t canal i s running a t full supply level and the offtake canal i s d r y . The length of floor =

C H ( F s - ~ ) where C i s Bligh's coefficient, and

H ( ~ ~ - ibs )the difference between the full supply level i n the p a r e n t canal and the bed l e v e l of the offtaking canal.

The recommended values of C a r e a s given

hereunder. F o r fine m i c a c e o u s sand in North Indian r i v e r s ,

C = 15

F o r c o a r s e grained sand in North Indian r i v e r s ,

C = 12

F o r sand mixed with boulders and gravel and l o a m soil

C = 5to9

The floor thickness i s designed according to the hydraulic gradient l i n e . The head m e a s u r e d a t a point f r o m the hydraulic gradient line i s the n e t head working a t that point and a suitable floor thickness i s then provided.

F o r con-

c r e t e floor s the thickness i s d e t e r m i n e d by dividing the design head by a factor of

1. 25.

This factor i s the submerged r e l a t i v e density of c o n c r e t e .

At the end of

the floor a toe wall i s provided. The downstream bed pitching i s l a i d a t a slope of 1 : 10 and 3 m beyond the end of the floor, and side pitching i s provided to p r o t e c t the e a r t h s i d e s f r o m

embayment.

A toe wall i s provided a t the end of the bed pitching.

The side

pitching a l s o r e s t s on the toe wall.

Hydraulic Design

A depth equal to the full supply depth of the offtake canal i s maintained d o w n s t r e a m of the grooves. The i n s e r t i o n of stop-logs c r e a t e s o v e r - s h o t flow conditions.

The

d i s c h a r g e f o r m u l a , (neglecting velocity of approach), f o r o v e r - shot flow i s 1

w h e r e B(t) = width of the sill,

H

i s the hydraulic drop, H( i s the (dr) difference between the offtake canal w a t e r l e v e l and the stop-log c r e s t . When a gate i s o p e r a t e d i n the groove, the flow i s u n d e r - s h o t , and the d i s c h a r g e i s e s t i m a t e d by t h e f o r m u l a

w h e r e A i s the a r e a of the opening The velocity in the controlling section should be about 1 m / s .

3. 13.4

N u m e r i c a l Example

'

Design a s q u a r e - h e a d intake f o r an offtake canal, the difference between the full supply l e v e l s of the p a r e n t and the offtake canals being 0. 2 m . regulation of the w a t e r supply i n the offtake i s effected by a gate.

The

The o t h e r

data a r e a s follows:

P a r e n t canal Discharge Q

Offtake canal =

17 m3/ s

y1 =

1.8m

B1 =

120 m

Discharge Q Y2

=

0.90 m 3 / s

=

0.54 m

B2 =

Design The width' of the opening i s d e t e r m i n e d by the f o r m u l a

3.0 m

IOcm r l 0 c m grooves for qote and stop logs3

2-I, -15

cm brick pitching crn R.C.C. slob Side brick pitching-

-20 cm brick pitching 0.30,

I

I

Longitudinal section

-

Porent channel

Half foundation plan Note : Sections of mosonry wolis ore odopted os given in Figure No.2

SQUARE

HEAD

INTAKE

of Intakes of Smoll Conols (Punjob Type).

All dimensions ore i n metres unless otherwise specified.

India

-

Figure No. 3 2 9

Velocity i n the contr'olling section will be acceptable.

= 1.2m/swhichis

Oe9 0.54

.

1.4

F i g u r e 3-29 shows the s t r u c t u r e and hydraulic d e t a i l s of the above example. When the regulation i s done by stop-logs,

the flow f o r m u l a given in 3. 13. 3

i s u s e d to d e t e r m i n e the s i l l l e v e l . A s s u m e the R. L . of the bed of the p a r e n t canal to be 100. l e v e l of the p a r e n t canal will be 100 offtaking canal will be 1 0 1 . 8

- 0.2

+

1. 8 = 101.8.

=

The full supply

The full supply l e v e l of the

101.6.

The bed l e v e l of the offtaking canal will be 101.6

-

0 . 5 4 = 101.06.

The sill l e v e l will a l s o be 101.06. The length of the f l o o r , a s s u m i n g a hydraulic gradient of 1 : 6, will b e (101.8

- 101.06) 6

= 0.74

.

6 = 4.44 m ,

say 4 . 5 m .

The thickness of the floor just d o w n s t r e a m of the gate grooves, which i s subject to m a x i m u m head, will be

H ( ~ ~ - b )= 25

0.74

=

0.60m.

The thickness of the floor beyond half i t s length should be reduced to 0 . 3 0 m .

3.14

3. 14. 1

D U P U I S CANAL INTAKE

General The design of the Dupuis canal intake w a s evolved by C. E. Dupuis in 1903 in Egypt to satisfy the condition that the d i s c h a r g e of a n intake on a secondary

T

Longitudinal section

Plan

Note: All dimensions ore in metres.

FAO-

ICID

DUPUIS CANAL INTAKE Project, Region, Country ARE Figure No. 3

i

- 30

canal ( d i s t r i b u t a r y ) should be proportional to the a r e a i t s e r v e s . of this s t r u c t u r e i s now a common p r a c t i c e in Egypt.

The installation

These intakes a r e

generally spaced a t 200 to 300 m a p a r t .

3. 14.2

S t r u c t u r a l Design The Dupuis canal intake ( F i g u r e 3-30) consists of a m e t a l o r earthenware pipe of a given diameter, with m a s o n r y heads a t entry and exit. F r o m the two top c o r n e r s of the canal bank through which the pipe p a s s e s , l i n e s of 1 . 5 : 1 slope a r e drawn t o m e e t the points on the b a s e on which the pipe F r o m those b a s e points lines of 1 : 1 a r e projected 0. 25 m above the full

rests.

supply line on the u p s t r e a m side and about a m e t r e o r so above the bed of the offtake canal on the downstream side.

The two points s o obtained become

respectively the top levels of the u p s t r e a m and downstream head walls.

The

u p s t r e a m face of the u p s t r e a m headwall i s obtained by drawing a line a t a slope of 0 . 5 : 1.

The downstream face of the downstream headwall i s generally kept

vertical ( F i g u r e 3-30). The top widths of the headwalls a r e s e t a t 25 cm. The side slopes at the u p s t r e a m face a r e rounded for smooth entry of water.

Hydraulic Design The diameter of the pipe, discharge, and the a r e a i r r i g a t e d i s determined f r o m Table 3-18 a s given by Dupuis on the b a s i s of L(p) = 10 m a and H(dr)

= 0.25 m,

where L

(P) =

length of pipe;

H

( d r ) = hydraulic drop.

Table 3-19 gives the values of correction a s a percentage when the H

(dr)

i s s m a l l e r o r l a r g e r than 0.25 m . Table 3-20 gives values of a r e a i r r i g a t e d f r o m pipe d i a m e t e r s ranging f r o m 10 cm t o 120 cm f o r a pipe length of 5 t o 100 and a head of0.25 m. original values of Dupuis a r e reproduced in the f i r s t column of the table.

The

TABLE 3-18

i!one

Velocity (appro4 m/ s

A r e a of pipe cm2

Diameter of pipe, cm

Discharge of pipe rn3/ s

Area irrigated in 7 days a t 3 350 m p e r ,

feddan = 4200.83 m 2 = 1.038 a c r e s TABLE

3-19

P e r c e n t a g e Differences f o r Headings-up Below o r Above 25 C e n t i m e t r e s

Cm.

0

1

2

3

4

5

6

7

8

9

TABLE Giving the D i a m e t e r and Lengths of

-

10

-4

k

s .Y' r3 !& Fed.

Diam. in cm

Lengths of pipes i n m e t r e s and a r e a s e r v e d i n F e d d a n s a s s u m i n g :

5 Fed.

10

15

Fed.

Fed.

20

25

30

35

Fed.

Fed.

Fed.

Fed.

45

40 Fed.

Fed.

10

10.0

16

13

11

9

9

8

7

7

7

20

12.5

25

22

19

17

15

14

13

12

12

30

15.0

41

34

30

27

24

23

21

20

19

45

17. 5

58

49

43

39

36

33

31

30

28

65

20.0

78

67

59

54

50

46

44

41

39

85

22.5

101

88

78

72

66

62

58

55

53

110

25.0

126

111

100

92

85

80

75

71

68

135

27.5

157

138

125

115

107

101

95

90

86

165

30.0

188

167

152

140

131

123

117

111

106

235

35.0

262

236

217

201

188

177

168

167

154

315

40.0

348

316

292

272

256

242

231

221

212

405

45.0

447

406

380

356

337

320

305

293

28 1

510

50.0

555

513

479

451

427

407

389

374

360

625

55.0

680

630

590

558

530

506

488

465

45 0

760

60.0

817

760

715

678

647

618

593

578

551

900

65.0

960

900

847

840

772

740

710

687

663

1,060

70.0

1,120

1,051

998

952

909

873

840

813

785

1,220

75.0

1,290

1,220

1,158

1,105

1,060

1,018

983

950

920

1,400

80.0

1,478

1,398

1,330

1,273

1,222

1,177

1,137

1,100

1,066

1,590

85.0

1 , 675

1,590

1 , 515

1,452

1 , 397

1 , 347

1 , 303

1, 262

1, 223

1 , 790

90.0

1,880

1,790

1 , 7 1 2 1 , 644

1,585

1,530

1,480

1,435

1, 395

2,000

95.0

2, 100

2,000

1,922

1,846

1,785

1,723

1 , 670

1, 618

1,575

2,230

100.0

2, 330

2, 230

2, 140

2,060

1,990

1,925

1,865

1,813

1,760

105.0

2,575

2,465

2, 370

2, 285

2, 205

2,140

2,080

2, 020

1,970

110.0

2,840

2,720

2,620

2,530

2,455

2,370

2,305

2,240

2,180

115.0

3, 100

2,980

2,870

2,770

2, 690

2, 610

2,540

2,470

2,410

120.0

3 , 390

3,260

3,150

3,050

2,960

2,870

2,790

2, 720

2, 650

I r r i g a t i o n Pipes and Areas Served Water Duty .50 m 3 / F e d . and Heading-up 25 cm.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

Fed.

,

0

I

"

Covers 4 - 3 x 1 - 7 i x 2 Rein.-Arc Ref.- No. 610

Note: Details of sliding gate and lifting frame not shown.

operoting spindle

Exposed section to be scabbled

Plon Section

A-A

Port section of gote connection

Notes :

I . Concrete to hove a minimum compressive strength

of 3 5 0 0 psi ot 28 days. Concrete works shall conform with specification C- S- 5 9 2 . All fillets 2 inches.

F A 0 - ICID

PRECAST CONCRETE INLET BOX

FOR INTAKE WITH STONE MESH WEIR Project, Region ,Country Australia

-

~ i ~ u rNo. e 3 31

3. 1 4 . 4

Numerical Example Design a Dupuis intake f o r a d i s c h a r g e of 0 . 4 4 m 3 / s and a h e a d of 0.25 m . The length of pipe i s 10 m .

The data for the p a r e n t canal and the offtake canal

are: P a r e n t canal Q

=

10m3/s

Offtake canal Q

=

3 0.44m / s

y2

=

1.0 m

B2

=

2.0 m

F r o m Table 3- 18, the r e q u i r e d pipe d i a m e t e r corresponding t o the d i s c h a r g e of 0 . 4 4 m 3 / s i s 60 c m

The a r e a which will be i r r i g a t e d by t h i s

d i s c h a r g e with a w a t e r duty of 50 rn3 p e r day p e r feddan will be 760 feddans

(320 ha) The design layout i s shown i n F i g u r e 3-30

3.15

3.15.1

INTAKE WITH STONE-MESH DIVERSION WEIR (AUSTRALIA)

General The s t o n e - m e s h diversion weir d i v e r t s flows i n s m a l l , rapidly flowing streams.

It i s p a r t i c u l a r l y suitable where the s u b - s t r a t u m of the s i t e i s of

g r a v e l and p r e c l u d e s the u s e of sheet pile w e i r s and w h e r e a conventional type of w e i r would be i m p r a c t i c a l o r too expensive due to the p o r o u s n a t u r e of the ground.

3.15.2

S t r u c t u r a l Design F i g u r e 3-32 shows a g e n e r a l view of the w e i r and the intake when a l m o s t a l l the flow i s being diverted. The intake of the s t o n e - m e s h d i v e r s i o n w e i r c o n s i s t s of a p r e - c a s t c o n c r e t e inlet box with a s c r e e n f r a m e ( m e s h s i z e 1$" x 1$") and a c o n c r e t e pipe 2 f t long with a sliding gate u p s t r e a m in the inlet box ( F i g u r e s 3-31 to

FIGURE 3-32. - General view of the stone m e s h weir and inlet box. (Almost a l l the flow - 12 f t 3 / s - i s being d i v e r t e d . )

FIGURE 3-33. - Another view showing stone m e s h basket construction and inlet box.

An outlet pipe 4 ft long joins the 2 ft pipe and opens into the weir structure.

3-33).

The weir s t r u c t u r e contains a r e c o r d e r well of reinforced concrete 30 inches in d i a m e t e r , with i t s c e n t r e 3 ft f r o m the c r e s t .

T h e r e i s a stilling basin 8 ft long

on the downstream side and bed protection of stone, and a transition section converging to the designed bed width of the offtake canal. 3- 37.

F o r details s e e F i g u r e

F i g u r e 3-35 shows the m e a s u r i n g weir and the dissipation s t r u c t u r e a t

the end of the pipeline.

The weir apron i s of stone encased in w i r e b a s k e t s

( F i g u r e s 3-33 and 3-34). The s t r u c t u r e i s of low cost, u t i l i z e s l o c a l m a t e r i a l s and does not r e q u i r e skilled labour f o r i t s construction.

The w i r e s of the b a s k e t s a r e welded a t e v e r y

point of contact and hence i f one section b r e a k s the whole s t r u c t u r e i s not threatened.

The weir of the inlet s t r u c t u r e cannot be easily damaged and the

control gate on the inlet box i s padlocked. The weir can be e a s i l y r a i s e d o r widened by adding m o r e b a s k e t s . The total cost of the s t r u c t u r e i s about A$ 3, 000. i n s p e c t the w e i r s once a week.

The s u p e r v i s o r should

Maintenance involves removing l e a v e s and s m a l l

b r a n c h e s f r o m the entrance to the inlet box.

Design F i g u r e 3-36 i s a g r a p h of a rating c u r v e for the m e a s u r i n g weir of the intake s t r u c t u r e used for design p u r p o s e s .

It h a s been developed f r o m model

t e s t s f o r a 6 ft c r e s t su$pressed w e i r .

Numerical Example Design an intake s t r u c t u r e with a s t o n e - m e s h weir in the p a r e n t canal ( o r s t r e a m ) f o r a d i s c h a r g e of 30 f t 3 / s .

The u p s t r e a m w a t e r l e v e l above the

m e a s u r i n g weir i s 1380.25. Design Refer to the rating curve, F i g u r e 3-36. o v e r the c r e s t of the w e i r will be 1 . 0 5 ft. 1380.25

- 1 . 0 5 = 1379.20.

F o r a d i s c h a r g e of 30 ft3/s the head

T h e r e f o r e , the level of the c r e s t i s

Other details and dimensions a r e shown in F i g u r e

FIGURE 3-34. - Stone m e s h basket construction, stone m e s h a p r o n , and location and g e n e r a l construction of inlet box with s c r e e n and screw-type gate. (About 3 ft3/ s passing over the weir. )

FIGURE 3 - 35. - Dissipation s t r u c t u r e and m e a s u r i n g weir a t (~ischar~ 10e ft3/ s . ) the pipe outlet.

FIGURE 3- 36. - Model t e s t rating for measuring weir capacity 30 ft3/ s

6 ft c r e s t suppressed weir.

3.16

GROYNE INTAKE AND ANCILLARY WORKS (CYPRUS)

3. 16. 1

General The groyne type of intake i s in u s e i n Cyprus on r i v e r s with g r a v e l beds and widths up to about 183 m (600 ft).

In such c a s e s i t i s uneconomical to build a

w e i r a c r o s s the full width of the r i v e r . developed to o v e r c o m e t h i s difficulty.

These intake s t r u c t u r e s have been They have only a m a s o n r y groyne

extending out into the r i v e r bed. Such intakes a r e not, of c o u r s e , intended to d r a w off the flood d i s c h a r g e of the r i v e r , which may amount to a s much a s 10,000 ft 3 / s o r m o r e , and t h e intake d i s c h a r g e i s not m o r e than 10 to 12 ft 3 / s .

S t r u c t u r a l C h a r a c t e r i s t i c s and Design

3. 16. 2

The s t r u c t u r e ( F i g u r e 3-38) c o n s i s t s of a groyne o r weir about 38 ft long n e a r the head of the intake, and an intake with side-walls of m a s o n r y and a 6" bed of reinforced c o n c r e t e , a diaphragm and a screw-wheel gate a t the intake head. The groyne a c r o s s the r i v e r bed i s a weir type with a c r e s t width of 2 ft, a downstream g l a c i s with a slope of 1 : 3 and a downstream apron.

Near the

intake head t h e r e i s a shutter gate in the groyne f o r washing out any gravel, soil o r sediment that m a y accumulate on the u p s t r e a m side of the weir.

The channel

below the intake gate i s of rectangular c r o s s section with side walls in m a s o n r y and coping of concrete on the top. than the other side wall.

The side wall facing the r i v e r i s 2 ft higher

This i s to prevent the d i s c h a r g e passing over the

channel when the r i v e r i s in flood. An overflow spillway i s usually placed in the channel a s n e a r to the intake a s possible but f a r enough downstream to be above high flood level.

This i s

e s s e n t i a l under Cyprus conditions where heavy floods would frequently s u b m e r g e the intake works and overflow the upper p a r t of the channel.

The spillway itself

i s simply a weir with i t s c r e s t a t the full supply level of the i r r i g a t i o n canal.

The

spill channel h a s to be protected against erosion f r o m both the overflow w a t e r and

0

Rubble mosonry woll: Stones os supplied from quorry, bedded in lime mortor 1 : 2 . Slope foced with stones roughly squored with joints in cement mortor I : 3

Concrete lip; dio. steel ties ot 5 feet centres

Section

Notes: Sections between exponsion joints to be completed olternotely. Foundotion in lime concrete to be cost in l i f t s 1 6 deep ; lime concrete usuolly prop. 1:4, bottom lifts of weir foundotions ond cutoffs moy in some cases be cost in cement concrete 1:3:6. Focing of wolls with block- in- c o u r s i mosonry. oll interior work in k b b l e ; mosonry courses I$ .deep. eoch seporote section to be worked up in complete courses. Allow one week for settlement of lime concrete foundotions ; one week before cement plostering ond rendering.

C-D

Alternative positi of extension woll

r t h or lined offtoke channel continue beyond the spillwoy Upper foce of cutoff

erflow spillwoy to be constructed uitoble site above flood woter level

Verticol joint down to bed rock Lower foce of cutoff

Cutoff round this s i d e 1

)/ Screw gote,

r

Expansion joint

I

Level River

F A O -

ICID

L ~ e c o n dcloss concrete (1:6) Downstreom toe of

Section

Project, Region , Country

A-B

Cyprus

I

Figure

No. 3 - 38

I

f r o m high floods in the r i v e r .

The i r r i g a t i o n canal between the intake and the

overflow spillway i s built to withstand s u b m e r s i o n during heavy floods.

(The

d e t a i l s a r e shown in F i g u r e 3- 38. ) Two points d e s e r v e special attention in the construction of intakes of t h i s type.

The f i r s t i s to found the abutment of the groyne on solid r o c k o r a t l e a s t on

v e r y f i r m ground s o that t h e r e be no possibility of e r o s i o n in, o r about, the head of the channel and that the s t r u c t u r e be s e c u r e l y anchored. the depth of the foundation and the width of the b a s e .

The other point i s

The depth should be well

below the region of n o r m a l scour in a shifting channel, p a r t i c u l a r l y a t the o u t e r end.

The width.of the b a s e should be sufficient to prevent tilting of the

structure.

It i s c u s t o m a r y in Cyprus to take the foundation down to about 6 ft

below the level of the lowest p a r t of the r i v e r channel.

FIGURE 3-39.

- Photograph of groyne intake s t r c c t u r e

(Cyprus).

F i g u r e 3-39 .shows a slightly modified design with a s m a l l e r d i s c h a r g e capacity.

The intake gate i s replaced by an o r i f i c e and a double shut-off gate a t

a distance of about 3 m downstream. flushing and d i s c h a r g e control.

This s e r v e s the dual purpose of sediment

B.

3.17

3. 17. 1

SILT CONTROL DEVICES

KING'S SILT VANES

General In 1933, H. W. King designed a device with c u r v e d vanes on the channel bed which would prevent h e a v i e r s i l t entering an offtake.

This w o r k s on the principle

that the w a t e r n e a r the bed of the p a r e n t canal o r channel contains a r e l a t i v e l y high s i l t c h a r g e which should, t h e r e f o r e , be deflected away without d i s t u r b a n c e , a t a n angle of about 30

3. 17. 2

0

f r o m the direction of the flow.

S t r u c t u r a l Design The layout plan of the vanes i s shown in F i g u r e 3-40. The dimensions of the s i l t vanes a r e given in Table 3- 21.

See F i g u r e 3-40

f o r X I , X 2 and R . The length and position of the longest vane and the vane spacing a r e thus d e t e r m i n e d .

TABLE

3-21

Width of offtake channel = B2 in m e t r e s

0.6

1.8

2.4

F o r strong effect

Value of X i Value of X2 Value of R

1.5 1.8 1.2 1.2 0.9 0.6 9.0 10.5 12.0

1.8 1.5 12.0

Cheaper design ( l e s s effect)

Value of X i Value of X2 Value of R

0.9 0.6 7.5

1.2 0.6 9.0

1.2 1.2 9.0

1.5 1.2 9.0

1.5 1.5 9.0

1.8 1.8 1.5 1.8 9.0 10.5

Minimum dimen sions recommended

Value of X i Value of X2 Value of R

0.9 0.6 7. 5

0.9 0.6 7.5

1.2 0.9 9.0

1.2 1.2 9.0

1.5 1.5 9.0

1.5 1.5 9.0

1.2

3.0

3.6

4.8

2.1 2.4 1 . 8 2.4 13.5 13.5

2.4 2.7 15.0

1.8 1.8 10.5

(iv) (v)

(vi) (vii)

Parent channel

From upstream edge of offtake A , draw a line AV at right angles t o centre line of parent channel of length x (Table II. Through V draw a line at 2 to1 with centre line of parent channel. Draw a line ST, 0 - 3 m away from toe of the side pitching which cuts the line U V M ( 2 t o I) at G. Draw an arc of radius R (~able3-zbangentialto lines STand UVM. From centre 0, with radius R = OQ, draw an arc QP of such length thatathe inclination of OP to OQ is 2 to I. All curres must end at the'lines OQ and OP. From downstream edge of offtake, draw a line NKW at 2 to1 with centre line of the parent channel. From P toy o f f PZ along the line UVM setting off K Z equal to 1-5 m . QPZ is the posirion of the longest vane.

=/

L

QG = GP

Of ftake

FIGURE 3 - 4 0 .

-

channel

King's silt vanes, layout plan of the vanes

The height of the vanes i s one-third to one q u a r t e r of the depth of the parent canal. The thickness of the masonry vanes i s 12 c m for a height up to 0.36 m and for g r e a t e r heights the thickness i s 24 cm.

However for efficiency the thinner

the better. The width of channels between the vanes i s normally 1$ t i m e s the height of the vanes. The u p s t r e a m ends of the vanes beyond the line OQ (Figure 3-40) m u s t be finished off to a slope of 1 vertical to 3 horizontal in a V-shape to act a s cutw a t e r s (Figure 3-41). The downstream ends of the vanes should be vertical. The channels between the vanes and the vanes themselves should be plastered. The bed of the parent canal covered by the vanes, and for a distance of 15 m to 30 m u p s t r e a m of the vanes, m u s t be smoothly pitched and i t should be 15 c m higher than the n o r m a l silted bed level.

The upstream 4.5 m of the pitching

should be built a t a slope of, 1 in 10. The side slope of the parent canal on the side of the offtake m u s t be pitched up to the length of the pitched floor. It should be noted that King's silt vanes a r e not suitable in the following situations: where the offtake canal discharge i s m o r e than one-third of that of the parent canal; where the offtake canal i s very small and takes off f r o m a deep parent canal; where the parent canal does not have adequate width; and where violent approach with a strong 'draw' towards the intake head exists.

3.17.3.

Numerical Example Design King's silt vanes for an intake head with the following data:

P a r e n t canal

Offtake canal

5

1

6

3

1.2

0. 6

Design The values of X i , X2, and R f r o m Table 3-21 for the c a s e of recommended minimumvalues are,

XI

Heightofvanes =

= 1.5 m ,

1 1 - t o - of 3 4

yl

Thickness of vanes in m a s o n r y

X2 = 1 . 5 m ,

and

= 0.4to 0.3m; =

Width of channel between vanes =

R = 9.0m.

adopt 0 . 3 m

12 cm 3 -

2 -

0. 3 = 0.45 m ; number of

vanes F o r other layout details,

3.18

3.18. 1

s e e F i g u r e 3-41

GIBB'S GROYNE

General F e a t u r e s The Gibb's groyne wall i s u s e d in c a s e s where the offtaking canal, on account of i t s gradient, h a s the s a m e s i l t c a r r y i n g capacity a s the p a r e n t canal. The Gibb's groyne e n s u r e s m o r e o r l e s s proportional s i l t distribution to the offtaking canal. The device i s a curved v e r t i c a l wall ( F i g u r e 3-42) constructed in the p a r e n t canal. f r e e board.

I t s height f r o m the canal bed i s equal t o the full supply depth plus It s t a r t s f r o m the downstream end of the offtake abutment and

continues u p s t r e a m to e i t h e r the opposite point of the offtake end o r t h r e e q u a r t e r s of the offtake width. I t i s not n e c e s s a r y that i t s u p s t r e a m end be tangential to the c e n t r a l line of flow of the p a r e n t canal n o r should the downstream end be tangential to the downs t r e a m abutment of the offtake channel.

Parent channel

FIGURE 3-42.

-

Gibb's Groyne,

general layout.

It i s not necessary to pitch the bed and side of the parent canal. Normally the Gibbls groyne would project far enough to enclose enough of the discharge of the parent canal to run the offtake canal full when the parent canal i s running a t i t s lowest supply level.

The amount of projection would also

depend upon the actual velocities existing near the offtake head.

Numerical Example

P a r e n t canal

Offtake canal

3 QB m I s

5.0

1.0

BB

6.0

3.0

-

V,

mls

1.O

v,

near the offtake

0.80

Consider 10% e x t r a discharge to fill the offtake approach.

The width t o be enclosed in the p a r e n t canal f o r drawing 1 m 3 / s in the off-

.

t a k e will b e

1.1

(T)

.

6

.

1 0.8

( -)

=

1.65 m.

F i g u r e 3-42 shows the

details.

CURVED WING WITH SILT VANES

3.19

, When the effect of a c u r v e d wing (Gibb's groyne) alone i s not

satisfactory

t o control the e n t r y of s i l t into the offtake canal, King's vanes m a y be added to The curved wing in t h i s c a s e i s

enhance the p e r f o r m a n c e of the curved wing. t e r m i n a t e d a t the 2 : 1 l i n e of the longest vane.

The design of King's vanes h a s been explained in section 3.17.

F i g u r e 3-43,

however, shows the a r r a n g e m e n t using King's vanes and a curved wing.

Portnt chonnel

Groyne

channel

FIGURE 3-43. - Curved wing with s i l t vanes,

g e n e r a l layout.

3.20

SILT PLATFORMS

3.20.1

General A silt platform consists of a horizontal slab, usually of reinforced concrete supported on p i e r s a t a suitable level to exclude bottom water heavily loaded with silt o r debris.

The device i s suitable only in deep parent canals.

There a r e two variations of this device: (a)

a simple platform;

(b)

a platform with a curved extension of the downstream wing wall.

Design of a Simple P l a t f o r m Level of platform

3. 20.2. 1

F o r practical considerations, the height of the tunnels should be 0. 6 m to 0 . 9 m so that the stray debris may p a s s through the tunnels without choking them. Other considerations to be borne in mind a r e that a s much a s possible of the bottom water should be excluded and t h e r e should be enough top water to fill the offtake. 3.20.2.2

Width of platform The platform should be wide enough to take enough water over i t

to f i l l the offtake with 2570 to spare. The downstream edge of the platform should preferably. be a t an angle of 60

0

to the centreline of the parent canal and a s well to the u p s t r e a m edge. The u p s t r e a m edge of the platform a t the edge of the parent canal may

be 1 . 5 m u p s t r e a m of the u p s t r e a m edge of the offtake.

A width equal to the depth of the parent canal will do, provided i t i s not possible to a r r a n g e for a g r e a t e r width. Mean side velocity (near the offtake) of the upper water flow in the

0-3

15 m t o 30 rn brick pitching

0-23 rn thick guide wall

--------------------Side pitching

Plon

Of ftake

channel

FA0

- lClD

SILT P L A T F O R M CUM- GUIDE W A L L Project, Region, Country S e c t ~ o n A-A Note. All dimensions are in metres

India Figure No. 3-45

p a r e n t canal should be equal to that i n the offtake canal. The p i e r supporting the s l a b should be sloped a t 1 v e r t i c a l to 3 horizontal a t i t s u p s t r e a m end and i t should be of cut w a t e r shape. p a r t should begin 0. 3 m away f r o m the edge of the p l a t f o r m . tunnels and the side should be pitched.

The sloping

The floor under the

The length of the pitched floor m a y r a n g e

f r o m 15 m to 30 m . 3.20.2.3

N u m e r i c a l example Design a s i l t platform f o r a n offtaking canal f r o m the following

data. (a)

Mean o b s e r v e d velocity i n the p a r e n t canal when running with m i n i m u m discharge

0.6 m / s

(b)

F u l l supply d i s c h a r g e of the offtaking channel

(c)

F u l l supply depth of the offtaking canal

1 m3/s

0. 6 m .

Solution The s i l t p l a t f o r m should allow to p a s s o v e r i t a d i s c h a r g e of 1.0 m3/ s

+

25 p e r cent = 1. 25 m 3 / s .

Width of the p l a t f o r m placed a t 0. 6 m

below the w a t e r l e v e l with m i n i m u m d i s c h a r g e in the p a r e n t canal 1. 25

0 . 6 . 0.6

=

3.47 m ,

of 23 c m t h i c k n e s s .

say 3 . 5 m .

=

The p l a t f o r m to be supported on p i e r s

P r o v i d e bed pitching and side pitching e a c h 15 m long.

F i g u r e 3-44 shows the design and d e t a i l s of the device.

Design of a Silt P l a t f o r m with a Guide Wall

3.20.3 3.20.3.1

General Ieatures A guide wall i s sometimes added t o improve t h e flow p a t t e r n (see

Figure 3-45).

The curved wing wall i s b u i l t over t h e s i l t platform.

The

design p r i n c i p l e f o r a r r i v i n g a t t h e level of t h e platform i s t h e same as explained above f o r t h e s i l t platform. T h e r e should b e enough w a t e r passing o v e r t h e platform, a t the l o w e s t supply in the p a r e n t canal to enable the offtake t o r u n full with 10% to

spare. The d o w n s t r e a m edge of the p l a t f o r m i s g e n e r a l l y

.

curved t o

the c e n t r e l i n e of the p a r e n t canal and the u p s t r e a m edge i s t e r m i n a t e d a t the u p s t r e a m edge of the offtake. The design of the p i e r s r e m a i n s the s a m e a s f o r the s i l t p l a t f o r m . The u p s t r e a m edge of the curved wing wall i s sloped a t 1 : 1. 3. 20. 3 . 2

N u m e r i c a l example .Design a s i l t p l a t f o r m with a c u r v e d wing f o r an offtake canal f r o m

t h e following data: (a)

Observed m e a n velocity with m i n i m u m d i s c h a r g e in the p a r e n t canal n e a r the offtake = 0. 60 m / s

(b)

3 F u l l supply d i s o h a r g e of the offtake canal = 1 m / s

(c)

F u l l supply depth of the offtake channel = 0. 6 m .

Solution The s i l t p l a t f o r m should p a s s

1

+

1070 = 1. 10 m 3 / s .

The width of

the p l a t f o r m , placed a t 0 . 6 m below the full supply of the p a r e n t canal will be 1.10 0 . 6 . 0.6

=

3. 05 m ,

say 3. 0 m .

P r o v i d e a p i e r t o support the p l a t f o r m .

P r o v i d e bed and side pitching o v e r a length of 15 m .

See d e t a i l s in

F i g u r e 3-45.

3.2 1

R E V E R S E VANES When a canal bifurcates i n t o two sub-canals or channels, one of which s i l t s very badly, reverse vanes may be b u i l t t o pass more s i l t i n t o the canal which does not s i l t up. The principle of design of the vanes r e m a i n s the s a m e , a s previously d e s c r i b e d , but they a r e r e v e r s e d i n direction. See F i g u r e 3-46 f o r the g e n e r a l layout.

FIGURE 3-46.

3.22

3.22.1

VORTEX

-

General layout of r e v e r s e vanes.

TUBE SAND TRAP

General F e a t u r e s F o r small canals (under 3 m bottom width) the vortex tube sand t r a p , developed by the Colorado Agricultural Experiment Station, i s essentially used to remove bed load m a t e r i a l that i s moving a t o r near -the bed and i s c o a r s e , i. e. the size i s g r e a t e r than 0.50 m m . The device consists of a slotted nitch in the floor of a channel placed a t an angle with the direction of flow.

The vortex tube i s located between the'intake

head and a measuring structure in the offtake canal.

The e x t r a amount of water

n e c e s s a r y for the operation of the tube i s returned to the parent canal through a collection chamber. The velocity of the water over the tube m u s t be great enough to supply the energy n e c e s s a r y to cause rotation of the water in the tube. of 1 . 2 to 1.8 m / s i s required.

Usually a velocity

In operation, sand drops into the tube, the

rotation k e e p s i t agitated and the longitudinal flow in the tube t r a n s p o r t s i t to the outlet.

The outlet end should be equipped with a slide gate for control of the

flow since i t does not need to be fully open to function.

Design C h a r a c t e r i s t i c s The canal section where the tube i s installed should be the s a m e a s the canal width but with the bottom r a i s e d .

The F r o u d e n u m b e r of the flow a c r o s s

the canal section containing the tube should approximate to 0.8. Where the flow d e s i g n flow should be selected that will e x i s t for a g r e a t e r

v a r i e s widely, portion of t i m e .

The length of the r a i s e d platform i s 2.5 t i m e s the width of the

canal. The flow removed usually r a n g e s f r o m 5% to 1570 of the total d i s c h a r g e . The width of opening should usually be in the range of 0. 15 m to 0 . 3 m The r a t i o of length of tube to width of opening

L(tube) :

B(tube) should

not exceed 20 with the maximum length of tube being approximately 4. 5 m . S e v e r a l successful field installations have the values 11 to 15 f o r this r a t i o . The tube angle oc

should be 45O.

Straight tubes o p e r a t e a s well a s t a p e r e d ones. The elevation of the l i p s of the tubes can be the s a m e r a t h e r than the downstream l i p being lower. The shape of the tube i s not p a r t i c u l a r l y important. c o m m e r c i a l l y fabricated pipe i s suitable.

A tube m a d e f r o m a

Normally the tube i s shaped f r o m

about 16 gauge galvanized s t e e l . The r e q u i r e d a r e a of the tube can be approximated by the formula

*(tube)

-

0.06 -

fi

B(tube)

B(tube) = Tube opening; The depth ih the tube,

L(tube)'

where

A(tube) = A r e a of tube,

L(tube) = Length of the tube. d(tube)

is

2 3

-

of the tube opening in

practice. When the bed load i s excessive, two p a r a l l e l vortex tubes may be installed.

3.22.3

N u m e r i c a l Example Design a v o r t e x tube f o r an unlined canal with the following data: Offtake canal

f o r rect'angular section,

1

=

velocity

3.0

.

=

0.6

0.56 m / s

Solution Length of tube,

%

20,

=

= 3 . G

L(tube)

> 0.15;

and 0. 3 . 2 B(tube)

B(tube) L(tube)

=

B(tube) A r e a of tube

=

16.96

4.2411-1

adopt B(tube)

= 0.25 m

say 17

0. 25

Depth in the tube

-32

=

O 6 B(tube) L(tube)

=

A(tube)

a-

.

0.25

=

0.17 m

.The s i d e s of the tube t o be rounded t o i n c r e a s e the a r e a to 0 . 0 4 5 m 2 a s the rectangular a r e a = 0 . 2 5 Length of platform

. 0.17 =

F o r broad c r e s t e d weir

=

0 . 0 4 2 5 m2.

2.5

.3

Q

=

=

7.5 m

1.7 B (crt)

C r e s t will be 0 . 6 0

-

F r o u d e .Number

=

= 0.263 m

0.337

v

above the bed

0.989

-

.

J( 9 . 8 1

=

0.544.

=

0.80

0.337

The value of F r o u d e Number i s l e s s than 0.8. Adopt H(crt)

= 0. 26

F r o u d e Number

v

=

1. 28

-

J g ~(crt)

J

9.81

.

0.26

T h i s value i s s a t i s f a c t o r y . The c r e s t will be 0 . 6

- 0 . 2 6 = 0 . 3 4 m above the bed of the canal.

The

u p s t r e a m and d o w n s t r e a m a p p r o a c h e s will be rounded a s shown i n F i g u r e 3-47.

3.23

SLOPING-SILL SAND SCREEN

3. 23. 1

General F e a t u r e s The sloping- s i l l sand s c r e e n was evolved in Egypt by engineer Abdel Azim I s m a i l who o b s e r v e d the l o c a l a s y m m e t r y i n the flow p a t t e r n a t the intake of d i s t r i b u t a r y c a n a l s .

The device c o m p r i s e s a curved wall placed i n

the p a r e n t canal opposite the intake of a n offtake canal.

The s c r e e n s l o p e s u p

f r o m the u p s t r e a m end t o the downstream end. The length of the s c r e e n i s determined on the b a s i s that: (i) The 1 1 depth of w a t e r o v e r the s c r e e n s i l l will be - to - of the depth in the p a r e n t 3 4 canal; (ii) the velocity o v e r the s i l l will be about the s a m e a s in the offtake Once the length h a s been d e t e r m i n e d i t i s possible to a s c e r t a i n the

canal. height.

The difference between the s i l l elevations (in m e t r e s ) a t e a c h end of the s c r e e n i s d e t e r m i n e d f r o m two f o r m u l a e , one of which i s t h e o r e t i c a l , and the

o t h e r i s e m p i r i c a l ; t h e a v e r a g e of the two i s adopted f o r design. The t h e o r e t i c a l f o r m u l a i s a s follows :

2v 2 g

=

,

0< 2

tan

w h e r e v i s the velocity in the offtake canal,

a i s the angle of offtake m e a s u r e d

f r o m the c e n t r e l i n e of the p a r e n t canal and e x p r e s s e d in r a d i a n s . The e m p i r i c a l f o r m u l a t a k e s the following f o r m :

ac. '32 35 fr

A$ =

where B 2

;

=

bed width of the offtake

canal i n m e t r e s . The sill i s m a d e of r e c t a n g u l a r section and i s 23 c m thick.

3 . 23. 2

N u m e r i c a l Example Data Offtake canal

P a r e n t canal

Design A s s u m e a depth between between 0 . 5 and 0.375 m. Length of s c r e e n length of s c r e e n

1

1

3 and - of the p a r e n t canal over the sill, i. e . 4

Adopt 0 . 4 m .

. depth o v e r

. 0.4 . 0.5

screen

=

F r o m theoretical formula,A2

=

Length of s c r e e n

= 1 or

= 1. 1 0.2

. .

. velocity

5m

To find value of b /3 2 v2

g

tan

a

- = 2

2

0 5 2 981

= 0.051 m

Average value of A

'

=

0.047 m

Average height of s c r e e n

=

=

4.7 cm,

0.6 - 0.4

=

say 5 cm.

0.2 m

=

20 c m .

The u p s t r e a m end will be 1 7 . 5 c m above the bottom of the offtake and the downstream end will be 22. 5 c m above the bottom of the offtake. d e t a i l s s e e F i g u r e 3-48.

For

4.

4.1

FLOW DIVIDING STRUCTURES

INTRODUCTION Many s t r u c t u r e s f o r flow division have been developed t o suit a wide v a r i e t y of conditions but all of them a r e u s e d i n i r r i g a t i o n networks to divide the flow of a channel into two o r m o r e p a r t s . flow.

E a c h p a r t i s a defined proportion of the total

Thus, flow dividing s t r u c t u r e s differ distinctly f r o m intakes and outlets in

that the l a t t e r a r e designed t o d r a w off a definite fraction of the flow in the p a r e n t channel, but the exact proportion of t h i s fraction t o the total flow o r to the r e m a i n d e r in the p a r e n t channel i s generally i m m a t e r i a l .

A flow dividing

s t r u c t u r e r e q u i r e s a control section in both the offtake channel and in the p a r e n t channel.

N e v e r t h e l e s s , not all flow dividing s t r u c t u r e s a r e built t o give exactly

proportional division.

To distinguish t h e m f u r t h e r f r o m intake s t r u c t u r e s , a

s t r u c t u r e which d i v e r t s m o r e than 2570 of the flow of the p a r e n t channel through i t i s a l s o r e g a r d e d a s a flow dividing s t r u c t u r e . Another m a i n distinction can be m a d e between s t r u c t u r e s which a r e designed to provide a permanently fixed proportional division of flow and those which a r e equipped with a movable s p l i t t e r .

The splitter allows the proportion of flow

between the resulting s t r e a m s t o be changed according to r e q u i r e m e n t s .

Such

changes m a y be r e q u i r e d to m e e t seasonal o r t e m p o r a r y v a r i a t i o n s in demand

in

the a r e a s supplied by the branch c a n a l s . The s t r u c t u r e s which provide division of flow may be sub-divided i n t o : t h o s e which a r e designed to give s t r i c t l y a c c u r a t e proportions, (using a control section a c r o s s the supply channel

- causing shooting flow o r f r e e fall); and those

which do not provide a s t r i c t l y proportional division, ( a t l e a s t not over the whole r a n g e f r o m z e r o flow to full design d i s c h a r g e ) .

A significant f e a t u r e of the

f o r m e r category i s that the flow i s divided by thin plated (splitting). walls.

The

l a t t e r category includes mainly division o r diversion boxes which have the additional function of alternating the flow between different offtake channels, using slide g a t e s o r f l a s h b o a r d s . It should be noted that m o s t flow dividing s t r u c t u r e s offer excellent con-

ditions f o r adaptation t o w a t e r m e a s u r e m e n t because of t h e i r capacity to g e n e r a t e shooting flow o r f r e e overfall. Flow dividing s t r u c t u r e s can be t e s t e d f o r a c c u r a c y by m e a s u r i n g the flows f r o m e a c h side.

Such t e s t s should be m a d e frequently and o v e r a wide range of

conditions a s relationships frequently change with the amount of w a t e r being diverted.

4.2

FIXED PROPORTIONAL DIVISORS The m a i n f e a t u r e of fixed proportional d i v i s o r s i s that the permanent splitting of the' flow into two o r m o r e p a r t s takes place in a control section where a s t a t e of s u p e r c r i t i c a l flow, i . e . shooting flow, o r f r e e fall, i s generated.

This

c a l l s f o r some head l o s s in the s t r u c t u r e by letting the flow p a s s a s i l l o r flumed section o r by creating a drop, but i s not r e q u i r e d f o r the splitting of a flow into two exactly equal proportions provided that: the dimensions a r e s y m m e t r i c a l ; that the flow section in the s t r u c t u r e i s of uniform roughness; that t h e r e i s a straight canal alignment of 5 to 10 m u p s t r e a m of the divisor; and, finally, that no backwater effect i s c r e a t e d in either of the offtake channels.

By installing a

subsequent 1 : 1 divisor, the flow will be split into the proportions of 2 : 1 : 1 o r , by rejoining two of the s t r e a m s , a proportion of 3 : 1 may be obtained.

FIGURE 4- 1. - Simple fixed proportional flow divisor on small i r r i g a t i o n canal in Cyprus.

FIGURE 4- 2 . - Simple fixed proportional flow divisor of low a c c u r a c y .

FIGURE 4-3. - F i x e d proportional divisor splitting a given flow into four s t r e a m s of exact constant proportion. ( 2 7 )

r

Section A-A

F A O - ICID

Note: All dimensions given above ate to be multiplied by Hc

D I M E N S l O N S O F FIXED DIVISOR SPLITTING FLOW I N T O T W O E Q U A L STREAMS Project, Region, Country Morocco

k

Figure No. 4 - 4

F i g u r e 4- 1 i l l u s t r a t e s a s m a l l fixed divisor c o r r e c t l y designed t o provide proportional division, while F i g u r e 4- 2 i l l u s t r a t e s a solution of poor a c c u r a c y because of insufficient velocity (no control section) and unsuitable splitting wall. F i g u r e 41 3 shows a divisor of the f r e e fall type, dividing any given supply up to m a x i m u m c a r r y i n g capacity into four s t r e a m s of exact constant proportion.

Divisor for Splitting Flow into Two Equal S t r e a m s General f e a t u r e s A fixed flow divisor, developed in F r a n c e and used in Morocco and other countries in North A f r i c a , i s shown in F i g u r e 4-4.

It i s eminently

suitable f o r dividing flow into two equal p a r t s a t the tail end of a distributing canal. This divisor c o n s i s t s of: an u p s t r e a m approach in an e a r t h e n channel (to i n c r e a s e the width of the channel section gradually to the width of the control section of the s t r u c t u r e ) ; u p s t r e a m head walls;

side walls of the control section;

a r a i s e d sill; a thin s t e e l sheet to divide the flow; downstream side walls and wing walls in the two channels; an u p s t r e a m and a downstream floor with a cutoff a t both ends.

The s t r u c t u r e may be built entirely in concrete o r in b r i c k

masonry. This type of flow divisor h a s two useful f e a t u r e s .

F i r s t l y , the

r a i s e d sill g e n e r a t e s a state of shooting flow on i t s downstream side, so that the division of the water into two equal p a r t s i s quite independent of the l e v e l s in the offtake channels, and thus the d i s c h a r g e passing into each of the offtakes i s not affected by control operations in either of them.

Secondly, the change back f r o m

shooting to tranquil flow in the offtake channels shows up a s a hydraulic jump, thanks to which, p a r t of the kinetic energy of the water i s regained, and the head l o s s e s a s s o c i a t e d with a flow divisor a r e t h e r e f o r e kept down to a minimum. Thickness of walls and floor The thickness of walls (i.e . the head walls, side walls, downstream side walls and wing walls, except the dividing wall of the two offtakes) should be a s given in Table 4- 1 .

TABLE 4-1 Design of Fixed Flow Divisor

Q in 11s

10 t o 80

-

Thickness of Walls

Thickness in c m Concrete Masonry 15

30

The minimum thickness of the dividing wall should be 10 c m for c o n c r e t e and 20 c m f o r b r i c k m a s o n r y constructions.

6 m m thick.

The steel plate should be

The thickness of the floor should be a s given in Table 4-2.

TABLE 4-2 Design of Fixed Flow Divisor Q in l / s

- Thickness of F l o o r

Thickness in c m Concrete Masonry

The depth of the cut-offs should be twice the thickness of f l o o r s for the corresponding d i s c h a r g e s . The relationship between d i s c h a r g e capacity and quantity of m a t e r i a l s i s given in F i g u r e 4-5

300

(D

\

200

0,

P 0

C

0 ul

a loo

0 0

2 3 4 5 6 7 3 Volume of ordinary concrete or of masonry in ril

I

8

- Relationship between discharge capacity and m a t e r i a l s requirement for fixed flow divisor.

FIGURE 4-5.

Design The formula generally used f o r calculating the discharge

where

Q =

discharge of the parent channel

C =

coefficient

B(t) H(c,t)

=

=

width of the controlling section ( a c r o s s axis of flow) head over the sill.

C v a r i e s f r o m 0. 38 to 0.41 (0.38 for a s h a r p upstream end of the sill and 0.41 f o r a rounded u p s t r e a m end ( 5 to 10 cm radius). The length of the sill, L(,,t),

should be equal to

B(t) should be taken a s equal to

10 Hc.

3 . 5 Hc.

F o r formation of hydraulic jump, the working head, 0 . 4 H(crt)

be

or

h(wk), should

0 . 6 Hc.

A working head equal to Hc i s advisable. The dimensions of Hc,

H(b-c),

y2,

y(b-bk) a r e given i n

Table 4-3.

TABLE

Discharge in l / s

H

4-3

H

C

(b- c)

..............................

qb-bk)

Y1

cm

...........................

F i g u r e 4-4 shows dimensions f o r the v a r i o u s p a r t s which should b e multiplied by Hc a t t h e maximum discharge. 4.2.1.3

N u m e r i c a l example Design a fixed flow divisor

1: 1 i n a c c o r d a n c e with the following

data : d i s c h a r g e of supply channel,

Q

depth of water i n supply channel, y F r o m Table 4-3, H

(b-c)

=

0.25m

Hc for Q

=

333 11s

=

0.49 m

=

333 11s i s 0 . 1 6 m.

Then,

The thickness of the right and left walls = 25 c m for concrete o r 30 cm for m a s o n r y construction. The thickness of the floor o r apron should be 15 cm for concrete and 20 cm for masonry construction. The depth of the cut-offs should be 30 c m for concrete and 40 c m for m a s o n r y construction. Other dimensions should be a s given in F i g u r e 4-4 but multiplied

Divisor for Splittin? Flow into Two Unequal S t r e a m s

4.2.2

General f e a t u r e s With a broad rectangular sill, a s in the s t r u c t u r e previously described, the position of the control s e c t i 0 n . i ~not defined with the precision required for the division of flow into unequal s t r e a m s of given proportions.

In

this section a s t r u c t u r e i s described wherein the sill i s triangular, with gentle In this divisor, the depth in the control

slopes u p s t r e a m and downstream.

section i s just equal to the c r i t i c a l depth, i r r e s p e c t i v e of the discharge, and i s m e a s u r e d exactly f r o m the top of the triangle.

The absolute minimum value of

the apex of the c r e s t above the bed level of the offtake i s equal to that of the hydraulic jump when i t f o r m s a t a distance of 2 Hc f r o m the apex of the c r e s t or sill. The m a s o n r y floor approach -up to the beginning of the 'upstream

glacis i s equal to 2 . 5 y l

where y l

i s the water depth in the parent channel.

The length of the u p s t r e a m wing walls

where

y

=

(FB) =

Yl

+

{

YI

+

(FBI)

depth of water u p s t r e a m of the s t r u c t u r e , f r e e board,

(ss)

(ss)

=

side slope of the canal section.

The depth of the u p s t r e a m and d o w n s t r e a m cut-off s i s equal to twi,ce the t h i c k n e s s of the floor. The u p s t r e a m g l a c i s h a s a slope of 1 : 4 to 1 : 5. The top of the t r i a n g u l a r s i l l i s slightly c u r v e d i n o r d e r t o avoid cont r a c t i o n of flow. The r a d i u s of the c u r v e i s equal t o 3 H c (Hc i s the c r i t i c a l depth). The length of t h e rounded c r e s t of the s i l l i s equal t o 1.176 Hc.

The

length of the downstream g l a c i s (slope 1 : 5) from t h e centre of t h e t r i a n g u l a r

ort ti on

of the s i l l i s equal t o 5 HcCmb)+ 0.59 Hc (where H(,-b)

i s the height of

t h e centre of t h e t r i a n g u l a r portion of t h e s i l l above t h e downstream bed level of t h e offtaking channel). The beds of t h e parent channels and offtake channels a r e a t t h e same l e v e l s , and t h e water depth i n t h e parent channel may d i f f e r somewhat from t h e designed depth, but t h i s may be gradually compensated f o r i n t h e reach immediately above the s t r u c t u r e .

Similarly, t h e bed level of the other offtake immediately below

t h e s t r u c t u r e i s t h e same a s t h e parent channel and any modification i n water l e v e l , i f needed, may be effected lower down. The f o r m and shape of the s t r u c t u r e below the downstream g l a c i s i s a s shown in F i g u r e 4- 6. The thickness of w a l i s ( u p s t r e a m wing walls, side walls of the control section, d o w n s t r e a m side walls, wing walls and dividing wall), and the f l o o r , depends on the type of soil a t the s i t e and the depth a t which the floor i s situated. A s a rough guide, the t h i c k n e s s e s of t h e s e p a r t s for different d i s c h a r g e s in the p a r e n t channel m a y b e derived f r o m Table 4-4.

TABLE

4-4

Divisor with a T r i a n g u l a r Sill - Thickness of Walls Thickness Concrete Masonry 10 29 59 103 162 239 283 333 and above up to 1,000

-

Plan

I

Section A-A

FA0

Note: All dimensions ore in metres unless otherwise specified.

-

ICID

UNFLUMED DIVISOR WITH 'A TRIANGULAR SILL

Project, Region , Country Fr once

-

Figure No. 4 6 I

'

The t h i c k n e s s of the dividing wall i s 10 c m f o r c o n c r e t e and 20 c m f o r b r i c k m a s o n r y construction. The t h i c k n e s s of the floor should be a s given i n Table 4-5.

TABLE

4-5

D i v i s o r with a T r i a n g u l a r Sill

-

Thickness of F l o o r

Thickness Concrete Masonry cm cm 10 29 59 103 162 239 283 333 and above u p t o 1 , 0 0 0

The depth of the downstream cut-offs should be twice the t h i c k n e s s of the floor f o r the corresponding d i s c h a r g e . The t h i c k n e s s of the s t e e l plate dividing the flow i s 6 m m and i t extends u p to a distance equal to 2 Hc d o w n s t r e a m of the apex of the t r i a n g u l a r s i l l . Desi~n

4.2.2.2

Calculate q ( d i s c h a r g e p e r m e t r e width of the control section) which i s

-

whereQisthedis~hargeinm~/softheparentchannelandB(~) B(t) i s the width of the control section i n m e t r e s .

equal t o

Calculate Hc f o r q H

(c-b)

m a y be calculated a s follows:

The value of

m a y be r e a d f r o m F i g u r e 4 - 7 f o r a known value Hc

Hc being known,

w h e r e ,H

(4i s height

H(c-b)

can then be calculated.

o v e r the c r e s t and C i s a coefficient d e t e r m i n e d f r o m :

and w h e r e H(c-b) i s t h e height of the c r e s t above the d o w n s t r e a m bed. Water depth on the u p s t r e a m side will then be:

Head l o s s through the s t r u c t u r e

4.2.2.3

=

Y1

- Y2

N u m e r i c a l example Design a divisor with a t r i a n g u l a r s i l l i n a c c o r d a n c e with t h e

following data: D i s c h a r g e of the p a r e n t channel, Q1 = 600 11s = 0. 60 m 3 / s D i s c h a r g e of offtake A,

Q2

= 400 11s = 0 . 4 0 m 3 / s

D i s c h a r g e of offtake B,

Q3

= 200 11s = 0 . 2 0 m 3 / s

B1

=

2.5 m

B2

=

1.85m

=

If the discharge of the parent channel

then

Hc (corresponding to q = 0 . 2 4 m 3 1 s )

0.60 m 3 / s

0.24 . 0.24 ( 9.8 1

=

.

FIGURE 4-7 - Divisor with triangular s i l l . Relationship between Y2 a n d H ( ~ - ~ )

.

-

From Figure 4-7,

y2 = for Hc

2 . 2 2 and X j

=

2Hc

1

-

W a t e r depth u p s t r e a m ,

Head l o s s =

Y 1 - Y2

Y1

=

H

=

0.61t0.27

=

0.88m

=

0.88

=

0.48

(c-b)

-

t H

( cr t )

0.40

The design and o t h e r d i m e n s i o n s of the s t r u c t u r e a r e shown i n F i g u r e 4-6.

4.3 4.3.1

STRUCTURES WITH ADJUSTABLE S P L I T T E R General S t r u c t u r e s with an a d j u s t a b l e s p l i t t e r usually c o n s i s t of a hinged g a t e m a d e of s h e e t m e t a l which can be moved a c r o s s the flow section of the p a r e n t channel and fixed i n any d e s i r e d position with the h e l p of an a r c h b a r o r s c r e w b a r .

The

flow h a s t o be m a d e s u p e r c r i t i c a l f o r a c c u r a t e proportioning by the introduction of a c o n t r o l c r e s t o r fall i n bed l e v e l , a s a l r e a d y d i s c u s s e d i n section 4. 2.

Figures

4- 8 , 4- 9 and 4- 10 show e x a m p l e s of d e s i g n s f r o m A r g e n t i n a , Spain and the U . S. A.

FIGURE 4 - 8 . - Flow divisor with adjustable s p l i t t e r , Argentina.

-

FIGURE 4-9. Flow divisor with adjustable splitter, hksn'egros Canal, Spain (72).

FIGURE. 4- 10.

4.3.2

-

Flow divisor with adjustable splitter

(u. S. A. )

F r e n c h Divisor The F r e n c h divisor shown in F i g u r e s 4- 11, 4- 12 and 4- 13 was developed by the Neyrpic Laboratories, Grenoble, and i s used in Southern Europe and North Africa.

It i s suitable for canals with gentle slopes.

The structure consists of a concrete approach equal to the width of the parent channel, a flumed section with upstream splayed wall, the c r e s t , the downs t r e a m sloping glacis, divide wall, channel sections of offtake channels in concrete with concrete floor and a dividing blade o r gate.

The steel dividing

blade i s adjustable and calibrations have been made so that the flow may be varied proportionally between the two channels.

The whole structure i s made of

prefabricated p a r t s . The structure e n s u r e s a hypercritical regime and the formation of a hydraulic jump immediately downstream of the c r e s t .

TheMivision of flow i s

therefore independent of the water levels in the offtake channels and any control operations in them.

Minimum working heads a r e given in Table 4- 6.

Water surface level in off take channel

Section

P Ion

Adjustable blade

FA

0 - ICID

FLOW DIVISOR WITH ADJUSTABLE SPLITTER - FRENCH TYPE Project, Region , Country Southern Europe ond North Africa Perspective

view Figure No. 4

- ll

FIGURE 4- 12. - F l o w d i v i s o r ( N e y r p i c ) , Oned F o d d a n e t w o r k , A l g e r i a . 1 . F l a p hinge. 2. C o n t r o l g e a r . 3 . F l a p positioning. 4. Movable flap. 5. H y d r a u l i c jump. ( 6 9 )

FIGURE 4-13. - T h r e e - w a y distribution by m e a n s of two consecutive flow d i v i s o r s . (69)

The design f o r m u l a u s e d f o r the s t r u c t u r e i s a s follows:

where: d i s c h a r g e of the p a r e n t channel in m 3 / s

Q

=

RGA

=

radiusofthebladeinmetres (seeFigure4-11)

=

height of u p s t r e a m w a t e r l e v e l o v e r c r e s t in m

H(crt)

Minimum l o s s of head

=

0 . 4 H(crt).

Table 4 - 6 gives d a t a f o r s i x different types of s t r u c t u r e .

.

4-6

TABLE

Design of S t r u c t u r e s with Adjustable Splitter

-

Minimum Working Heads - -

Type Number of S t r u c t u r e

4.4

Adjustable Blade Radius Height in c m in c m

Maximum Discharge Q in l / s

Maximum Head C r eOver st Hicrt) In cm

Head L o s s f o r Maximum Q h(wk) in c m

6

60

35

54

17.5

7.0

8

80

45

110

23.0

9.0

10

100

54

193

29.0

11.5

12

120

65

305

35.0

13.5

16

160

84

625

46.5

18. 0

20

200

104

1,093

58.0

23. 0

PROPORTIONAL DISTRIBUTORS Under t h i s heading s t r u c t u r e s a r e d e s c r i b e d which can be c o n s i d e r e d a c r o s s between flow d i v i s o r s , i n t a k e s and outlets.

A typical f e a t u r e of proportional

d i s t r i b u t o r s i s that the flow i s not divided into f r a c t i o n s by thin plated d i v i s o r s but i s diverted f r o m the p a r e n t channel into the offtakes by m e a n s of individual

r

/

/'\

/

"I-

o 0

a LL

V)

4 5

E

03

0 I3 v, W

a 2

0

a

*

L C

.

C

* -

ii

0 3

2

0

=

I

3 c *0 .Y2 *

a0 .:0

"

=Fp o -. z U

C

e-

.%

a

openings, which, however, a r e grouped to f o r m a single s t r u c t u r e .

Each

opening o r offtake i s constructed a s a flume o r f r e e overfall weir and i s dimensioned so a s to p a s s a given fraction of the total flow.

In o t h e r words, the

controlling section (flume section, elevated floor, o r weir c r e s t ) i s not in the supply channel, a s in the d i v i s o r s d i s c u s s e d in section 4. 2, but i s in the individual offtakes.

This a r r a n g e m e n t r e q u i r e s a c c u r a t e calibration by model

t e s t s o r field rating and g r e a t a c c u r a c y in construction. Representative of t h i s category i s the proportional d i s t r i b u t o r developed in the Punjab and Haryana (India) and d e s c r i b e d in 4.4.1 below.

A s t r u c t u r e of

s i m i l a r type and function i s the "Nasba-Weir" which i s in u s e in Egypt.

The

l a t t e r i s d e s c r i b e d in Chapter 5 a s i t i s considered basically an outlet s t r u c t u r e . A s can be seen f r o m the d e s c r i p t i o n s , t h e s e types of s t r u c t u r e s have comparatively high m a t e r i a l and labour r e q u i r e m e n t s , which m a k e

them uncompetitive with flow d i v i s o r s especially f o r capacities below 1 m 3 / s .

It

should be noted f u r t h e r that they a r e a l s o l e s s efficient in drawing proportional s i l t load than flow d i v i s o r s .

Their distinct advantage i s g r e a t r o b u s t n e s s , which

m a k e s t h e m suitable f a r a r e a s where tampering i s a problem.

Punjab- Type P r o p o r t i o n a l Distributor T h r e e examples of Punjab-Type proportional d i s t r i b u t o r s a r e i l l u s t r a t e d in F i g u r e 4- 14.

These s t r u c t u r e s can distribute water supplies proportionally

without regulation, including to a l a r g e extent sediment in the w a t e r .

Example

( a ) ( F i g u r e 4-14) c o n s i s t s of a flume in the p a r e n t channel and a left-hand offtake s e t a t 60';

example (b) h a s a flume in the p a r e n t channel and a right-hand

offtake s e t a t 45O; the t h i r d example ( c ) h a s a flume in the p a r e n t channel and two offtakes, one on the right s e t a t 45O and the other on the left s e t a t 60°. F u l l details of construction, based on example ( c ) , a r e given in F i g u r e 4- 15.

4.4.1.1

P a r e n t channel flume

A point to note regarding the p a r e n t channel flume i s that a single s e t of stop-log grooves should be provided on the u p s t r e a m side for emergency

closures.

These grooves should have a c l e a r a n c e of 0 . 1 5 m f r o m the face of the

u p s t r e a m parapet of the bridge.

Another point i s that the segment of the a r c

forming the v e r t i c a l walls in the included angles should be of radii varying f r o m :

where H ( c r t ) i s the head over the c r e s t ,

(and this will depend on the throat

widths). When the fall in the supply channel i s combined with a bridge, the minimum r a d i i should be 2.55 m and 2.1 m for 60° and 45O offtakes respectively. This will e n s u r e a 6 m r a d i u s for the centreline of the r o a d leading f r o m bridge to bridge.

Both ends of the a r c s of the walls m u s t be tangential to the side walls

of both the p a r e n t channel and flume and the offtakes. The length of the approach m u s t correspond to 3.15 H

(crt)'

and the

t h r o a t should s t a r t f r o m the end of the c i r c u l a r walls.

Offtake flumes

4.4.1.2

All offtakes should be designed to take off a t 60

0

o r 45

0

.

The c r e s t of a l l the offtakes and the flume in the parent channel should be a t the s a m e level, and a t l e a s t 0. 15 m above the downstream bed level of the highest channel,

This m e a n s the fluming of the p a r e n t channel may have

to v a r y f r o m 60% to 757'0; but such variations should not be m a d e if they can be avoided.

To e n s u r e modularity, the submergence ratio should not be g r e a t e r

than 0.8. The length of throat (L( ) will be 2 H(crt). t) The u p s t r e a m side approach should be joined with the bed in a curve and f l a r e d out to m e e t the side slopes a t 1 : 1 o r 0.5 : 1.

The r a d i i of these

c u r v e s will depend on the angle of the offtake and the distance of the throat f r o m the toe of the side slopes in the bed. The depth of the c i s t e r n should be calculated f r o m :

where

q(t)

=

d i s c h a r g e p e r m e t r e of t h r o a t width,

Q

=

total d i s c h a r g e ,

B(t)

=

width of throat.

The value of Hc i s given by the f o r m u l a

F o r a given working head,

(difference between the full supply h(,k) l e v e l i n the p a r e n t channel and the offtaking channel) calculate the value

%&)

.

Hc E2

- f o r the

F r o m Table 3-1 i n Chapter 3, find out the value of

Hc (wk)

calculated value of

.

Then E 2 can be worked out.

Hc The depth of the c i s t e r n below the bed level of the offtake channel, HSB , will be E2

- y 2 , and in no c a s e should be l e s s than 7 . 5 c m .

The length of the c i s t e r n should be y2

+

h

(wk)'

The depth of the c u r t a i n walls should be equal to (-

(

y2 3 +

0.3) m

respectively.

y1 3

+

0. 3 ) m and

a t the u p s t r e a m and downstream ends of the m a s o n r y work The depth of the downstream c u r t a i n wall should be t e s t e d for

m a x i m u m p e r m i s s i b l e safe value of the exit gradient a s given below :-, Type of m a t e r i a l

Safe value of exit gradient

Shingle C o a r s e sand F i n e sand

0. 17 to 0.14

F o r Punjab s o i l s the safe value of the exit gradient i s taken a s 0.30.

L* ,

where L* i s the length from 2 curtain wall to curtain wall and dwc2 i s the depth of the downstream curtain To calculate

the value of

wall, see diagram of Khoslals safe exit gradient curve below.

FIGURE 4-16.

-

Khosla's safe &it gradient curve.

F o r the calculated value of

Then, the exit gradient,

. a,

H* -

s ( ~ -~ ~ ~ ) dwc2

read the value of 1

V

K

1

~0-

The m i n i m u m thickness of the f l o o r s should b e 0. 12 m in b r i c k m a s o n r y and 0 . 1 5 m in c o n c r e t e . T h e r e should be a single s e t of stop-log grooves on the u p s t r e a m side.

A regulating b r i d g e should be provided only when the flume i s not combined with a r o a d bridge and the t h r o a t width i s g r e a t e r than 0. 6 m .

A r a d i u s c u r v e of 0 . 6 m should join the c r e s t with the downstream glacis.

The g l a c i s should have a slope of 2 . 5 : 1. The length of the downstream expansion,

L

(exp)'

will be equal

to:

o r the length f r o m the downstream edge of the c r e s t to the end of the c i s t e r n , whichever i s the g r e a t e r . The r a d i u s of the expansion walls,

R(exp),

will be equal

to:

where

B2

=

qt) =

bed width of the offtake channel, width of throat.

T h e r e i s no bed protection in the p a r e n t channel other than a curtain wall on which the u p s t r e a m curved g l a c i s r e s t s .

The side protection below the downstream expansion should be equal to 3LSB (where LSB = the length of the stilling basin o r c i s t e r n ) and should consist of d r y b r i c k pitching, 20 c m thick, supported on a toe wall of depth equal

t o 0 . 5 y 2 ( y 2 = w a t e r depth in the offtake channel) subject to a minimum 27 c m d e e p m a s o n r y wall o v e r 15 c m thick c o n c r e t e . pitching.

It i s p r e f e r a b l e to l a y roughened

The bed protection should c o n s i s t of b r i c k - b a t s with thickness a s

follows : 3 u p to 0 . 7 m / S 0.7 to 1 . 4 m 3 / s

15 c m 23 c m

Thick b r i c k - b a t protection in the bed should extend up to a length

+

H( d r ) beyond the downstream end of the side expansion and 2 should be hand-packed and not simply dumped.

equal to y

Standard sections of wing walls and abutments should be a s given in F i g u r e 3- 3 i n Chapter 3.

Desivn f o r m u l a

4.4.1.3

T h e design f o r m u l a u s e d i s :

The value of C should be a s follows: Value of C

Q Angle of offtake 60°

m3/ s

Angle of offtake 45O

F o r full d e t a i l s of the design s e e F i g u r e 4- 15.

4.4.2

USBR-Type P r o p o r t i o n a l Division S t r u c t u r e -1/ The s t r u c t u r e d e s c r i b e d h e r e i n i s a l s o kn0.m a s a division box.

It has

been standardized and i s commonly u s e d by the USBR for the division of flows of

-11 B a s e d on information f r o m the U. S. A. National ICID Committee and on the USBR design manual.

Extend cutoff ond winqwolls vertically ond horizontolly with, unreinforced concrete os directed. Concrete plonks

Detoil of connection to concrete lining

edoes

af

Typical directions

Reinf. not shown

S e c t i o n A-A

Design delivery W.S.

DIVISION 6 min. pool depth

Section

B-B

BOXES

up to say 350 l / s into two o r t h r e e take-off s t r e a m s . channels may be open o r piped. i n F i g u r e 4-17.

Supply and/or take-off

A typical design for open channel flow i s shown

Reference (107) provides standard designs for 9 other types of

division boxes, including piped and open channels

.

The proportional division of

a given flow i s accomplished by weir s t r u c t u r e s a t each offtake.

The w e i r s may

be of the fixed c r e s t type to ensure a permanent proportion between the outlets o r they may be fitted with slide gates o r stop-logs for variation of flow proportions. It i s often desirable to use the weir for water measurement, in which c a s e i t s design m u s t follow the standard weir conditions described in the USBR Water Measurement Manual, r e f e r e n c e (81).

In any case, i t i s recommended that a

staff gauge be provided on the wall of the structure to m e a s u r e the head on the weirs. The water m u s t be delivered to the structure a t an elevation which will provide enough head for the w e i r s to furnish the required flow at the design delivery water surface.

When the water i s delivered through a pipe, the

velocity in the pipe should not exceed 0 . 5 m / s to avoid excessive turbulence ahead of the weirs.

This division s t r u c t u r e i s not recommended for systems

with heavy silt loads.

4.5

DIVISION OR DIVERSION BOXES Division s t r u c t u r e s o r boxes regulate the flow f r o m one canal to another, o r to s e v e r a l others.

They usually consist of a box with vertical walls in which

controllable openings a r e provided.

Metal o r wooden slide gates o r stop-logs

a r e usually installed to regulate the division of flow a t all t i m e s and to shut off flow in any branch when desired.

The walls can be either of concrete ( p r e - c a s t

o r in situ), o r of masonry o r wood. The width of each outlet i s generally proportional to the division of water to be made.

In lined canals, a full gate opening a t the intake to the box i s made

covering approximately the same a r e a a s the canal flow section since the canal i s designed to c a r r y water a t relatively high velocities.

In e a r t h canals, gate

openings can be dimensioned by assuming a velocity of about 1 . 5 m / s in the opening section.

Reinforced concrete transitions a r e provided below the gates

Scole

0

I

2

I

I

I

FA0

I Section A-B

I

I

I I

3 metres I

- ICID

FOR E A R T H C A N A L S Project, Region, Country I t oly

Isometric view

621k4P Sectional elevation A-A Sectional elevation C-C

Sectional elevation B-B Flow_

A = Height of side wolls B =Width of flow pottern C Depth of wall below slob D = Depth of woll including slob Note: Reinforcement to be $dio. reinforcinp bors on 12"cc loced ~n centre of concwire mesh rnoy be rete slob 6*1t?No.!O used in ploce of dio. reinforcinp bors

Sheet rnetol recpmmended

Detail of gate slot

FIGURE 4-19. box

-

U.S.A.

-

-

P l a n f o r a c o n c r e t e r e c t a n g u l a r division

on l a r g e r s t r u c t u r e s .

Hydraulic l o s s e s through gate openings a r e seldom con-

trolling f a c t o r s in designing division boxes.

When the gates a r e operated at full

openings, entrance l o s s e s a r e simply transition l o s s e s .

When operated a t

partial openings, available heads a r e not fully utilized so that i n c r e a s e d l o s s e s due to gate contractions a r e not important. * 4.5.1

In Situ Concrete Division Boxes In situ concrete i s the m o s t commonly used m a t e r i a l for permanent division boxes for any capacity f r o m 10 11s upwards.

A standard type of concrete

s t r u c t u r e used in Italy since 1970 i s shown in Figure 4- 18. capacity of 150 11s each and a r e unlined.

The structure i s c a s t in place using

concrete with 200 kg of cement per m 3 of aggregate. with cement p l a s t e r .

The canals have a

The surfaces a r e finished

The walls have a thickness of 25 c m while the bottom

plate i s 10 cm. An example of another design, developed by the US Soil Conservation Service, i s shown in Figure 4-19.

4.5.2

Prefabricated Concrete Division Boxes The l a r g e number of division boxes of identical size required in medium-tol a r g e projects m a k e s t h e m suitable for prefabrication. F i g u r e 4-20 (a) and (b) shows a standard division box constructed with p r e fabricated p a r t s .

This type i s exclusively used in the E a s t Ghor Extension

P r o j e c t in Jordan, where a total of some 225 s t r u c t u r e s have been installed. Outlet openings a r e standardized to a full capacity of 30 11s each, serving an i r r i g a t e d a r e a of 30 donurns ( 3 ha).

The structure p e r m i t s quite flexible

operation and safety against tampering. Figure 4-21 shows a standard design adopted for a project in the San F r a n c i s c o Valley ( ~ r a z i l ) . The s t r u c t u r e i s composed of t h r e e types of p r e fabricated p a r t s .

These together with a variable gate opening can be used for

a wide range of design flows.

The p a r t s a r e placed in an in situ concrete bed

and t h e i r vertical sides a r e held together by steel clamps.

The joints can be

left unsealed if the backfill i s impermeable m a t e r i a l o r can be sealed by plastic

FIGURE 4 - 2 0 ( a ) and (b). - Distribution structure with proportional weirs constructed f r o m prefabricated p a r t s P r o j e c t , Jordan.

- E a s t Ghor Extension

o r rubber s t r i p s .

F o r a c c e s s to the gates, wooden planks a r e placed over the

(The fixing of the gate will be de'scribed in Volume 111. )

structure.

The division box shown in F i g u r e s 4 - 2 2 and 4 - 2 3 s e r v e s to rotate the irrigation flow between f a r m ditches in a rotational irrigation system for paddy r i c e a s practised in Taiwan.

A f a r m ditch usually i r r i g a t e s some 50 ha of paddy

land. The division box i s constructed a t the junction point where an irrigation sub-lateral branches into two o r t h r e e f a r m ditches.

The division box i s not

used to divide water c a r r i e d in by the sub-lateral between f a r m ditches.

It i s

used to turn the whole flow of water alternatively into one of the ditches according to a p r e s e t irrigation schedule. There a r e t h r e e types of design in use.

The f i r s t i s a rigid concrete box;

the second i s a brick laid box; the third i s the design shown, a box made of p r e fabricated concrete p a r t s . Taiwan.

In fact, the f i r s t type i s m o r e extensively used in

However, the third type i s m o r e economical, i s easy to construct and

i s durable.

It i s therefore recommended for wider application.

Viewed on the plan, the division box i s a rectangle with four sides open for inserting flashboards ( s e e Figure 4-23).

In the two diagonal directions of the

rectangle, four baffle walls made of prefabricated concrete a r e placed with the head enlarged to accommodate flashboard grooves and with the tail portion projected into e a r t h ditch banks for stabilization.

The lower portion of the walls,

fastened together by connecting plates, i s buried underneath the bottom of the ditches a s cut-offs. The e a r t h ditch banks into which the baffle walls of this structure a r e projected for stabilization may deteriorate after some time.

This will endanger

the stability of the s t r u c t u r e so the earthen banks should be examined f r o m time to time and be repaired i f n e c e s s a r y . Hydraulically, this division box i s a very simple structure, since t h e r e i s no actual division of irrigation flow involved.

The whole flow coming into the

box goes out of one of the openings into the f a r m ditch required during rotational irrigation.

The head l o s s for a flow through the box i s usually no

m o r e than 3 cm.

It i s not recommended that this type of division box be used i f

t h e r e i s a heavy silt load in the irrigation water.

4.5.3

Masonry Division Boxes An example of a design of division box made of concrete blocks i s shown in F i g u r e 4- 24. The structure should be placed a t l e a s t 30 cm below the surface of the ground on a 10 to 15 cm concrete foundation

-

the floor of the s t r u c t u r e .

The

f i r s t course of 20 cm x 20 cm x 40 c m block can be laid in a bed of m o r t a r on top of the foundation o r be placed in the f r e s h concrete of the foundation. case, c a r e should be taken to level the blocks.

In either

Partition woll

Plon of

portition woll

Plon - section o f cutoff plote

Perspective drawing of precost division box

Vertical section of

Verticol section of

The thickness of wosher is 3mm ond the outside diometer is 3 centirnetres Set-in screw retaining plote is determined by designer occording to the hydrou-

Every port con be costed In Iron model with re~nforced concrete ond then

F~xedby

1 3mm

screw

15

9 mm

screw hole Quontities required for each cutoff wall PREFABRICATED

Construction o f precast division box

A -A Detoils of groove connection

0.40

0.013

Lenplhlrn)

0.56 0.31

0.3IePumliIii Weight(*)

0.249 0004

2

I

O.M 2

0.56 I

0.258 0.124

DIVISION B O X E S

DIMENSIONS 0.715

All dimensionr o n in mrtrrr

FIGURE 4-24.

-

Concrete block division box,

The s i z e s of the openings gauge the amount o f , w a t e r flowing through the structure.

Although dimensions of the openings a r e sho-wn irl the drawings,

adjustments may be n e c e s s a r y , depending on the g r a d e of the canal o r ditch and the amount of water to be handled.

The water flow i s controlled by removable

gates m a d e of 2-inch thick t i m b e r . When the blocks have been laid, a coping should be placed on the top c o u r s e and on the c r e s t of the openings.

This can be done with a coping block o r the

c o r e s of the block can be filled with concrete. The ditch i n v e r t downstream of the box openings should be protected by rubble o r concrete lining f o r a length of a t l e a s t 80 cm. F i g u r e 4-25 shows another example of a design using concrete blocks and wooden gates.

FIGURE 4-25. - Concrete block division box Democratic Republic of Yemen.

- People's

Timber Division Boxes Wooden division boxes a r e used in f a r m ditch s y s t e m s in a r e a s where the construction m a t e r i a l , which can be s c r a p timber, i s cheap.

E a s y and cheap

construction and t r a n s p o r t h a s to be weighed against the s h o r t life expectancy of the s t r u c t u r e . exceed 200 11s.

The maximum discharge for wooden s t r u c t u r e s does n o r m a l l y not F i g u r e s 4-26 to 4-28 show t h r e e examples of suitable designs

f o r t h e s e t i m b e r division boxes.

FIGURE 4 - 2 6 .

-

Typical timber division box (13)

1

BILL OF MATERIAL SIZE

L-F

l x4

15'-4"

5.0 26.2

0-F

1" x 4"

39'-4"

2"x 6"

34'-0"

34.0

2"xl2"

70'- 0"

140.0

Total Nails 16-d

.

205 2 3.5 Ibs

FIGURE 4 - 2 7 . - Three way timber division box - standard design USDA Soil Conservation S e r v i c e . (Note: 1 ' = 30.5 c m ; 1" = 2.5 c m )

i x 4"

stop

FIGURE 4- 28. - Standard t i m b e r division box Alberta Department of Agriculture, Canada.

4.5.5

-

Irrigation D i v e r t e r The i r r i g a t i o n d i v e r t e r i s an automatic hydraulic device developed by F r e e m a n ( 3 7 ) to divert flows f r o m an i r r i g a t i o n canal to a branch canal o r field ( s e e F i g u r e 4-29).

-

Layout of furrow i r r i g a t i o n using an FIGURE 4-29. automatic i r r i g a t i o n d i v e r t e r .

FIGURE 4- 30 ( a ) and f b ) . diverters (37)

-

General views of i r r i g a t i o n

General views of d i v e r t e r s a r e shown in F i g u r e 4-30 ( a ) and (b). r e s e m b l e s a hollow "Y",

The unit

with w a t e r entering into the s t e m of the Y and d i s c h a r g -

ing f r o m both of the two branches.

One branch supplies the field to be i r r i g a t e d

o r field ditch and the other the downslope supply ditch to the next d i v e r t e r . floor of the device i s inclined upwards towards the downstream end.

The

This h a s

been found n e c e s s a r y in o r d e r t o d i v e r t flow into one outlet without leakage into

the other outlet.

Two control vents a r e located on top of the device.

operation one vent i s open and the other i s closed.

In

If the right vent i s open, the

water flows to the left branch and vice v e r s a . The functioning of the vents depends on the suction of the s t r e a m in the venturi-type inlet.

The device can be manually-operated by plugging one vent a t

a t i m e with a simple rubber o r plastic plug. a t v e r y little e x t r a cost.

The device can a l s o be automated

A "dip tube" level s e n s o r i s s e t into the distribution

canal so that the bottom of the sensor i s a t the d e s i r e d water level in the canal. (See F i g u r e s 4 - 2 9 and 4-30. ) The dip tube c o n s i s t s of a v e r t i c a l length of light pipe connected by a light hose to the supply canal control vent. by a s m a l l orifice.

The vent on the turnout side i s r e s t r i c t e d

When the d i v e r t e r s t a r t s flowing much m o r e a i r e n t e r s

through the dip tube s e n s o r hose and into the control vent on the supply canal side than through the s m a l l orifice.

The water flow i s thus d i r e c t e d into the

distribution canal until the end of the dip tube sensor i s submerged.

This stops

the a i r flow into the supply side control vent, and the a i r flow through the orifice then switches the d i s c h a r g e into the downstream supply canal to the next d i v e r t e r in the line, where the sequence i s repeated.

In many applications i t may be

n e c e s s a r y to add a bowl under the dip tube s e n s o r (Figu;e 4-28 ( a ) ) in o r d e r to prevent r e t r i g g e r i n g of flow into the s a m e turnout before all other turnouts have been serviced. The device i s still in an experimental stage.

F i e l d t e s t s c a r r i e d out on

sugar cane plantations in Hawaii indicate that i t provides improvements over the traditional methods of surface i r r i g a t i o n on sloping lands.

5.

5.1

OUTLETS OR FARM TURNOUTS

INTRODUCTION An outlet o r a f a r m turnout i s a structure at the head of a watercourse, a f a r m irrigation canal, o r a f a r m o r field l a t e r a l , which connects i t with a supply canal.

The supply (or distribution) canal i s usually under the control of an

irrigation authority.

The authority may be a Government department, a public,

o r semi-public organization such a s a d i s t r i c t o r an irrigation association.

Thus

the f a r m outlet i s the connecting link between the canal operator representing the authority and the f a r m e r o r u s e r .

It follows that the design and operation of the

outlet m u s t satisfy the needs of both p a r t i e s a s f a r a s possible.

F o r example, the

f a r m e r will want to be satisfied that he receives h i s due s h a r e o f t h e water a t the right time and the operator will want to be satisfied that the outlet s t r u c t u r e s e x e r c i s e effective control over the distribution of the supplies. There a r e m o r e f a r m outlets than other s t r u c t u r e s in an irrigation system and therefore they have a decisive influence on the functioning of a system.

(In

the U. S.A. in 1960 t h e r e were some 160,000 f a r m outlets on irrigation systems. The number of outlets on irrigation systems in a single Province in India (the punjab) in 1947 was over 41,000.

In the U . S. S. R. a t present m o r e than half of

the total number of s t r u c t u r e s on irrigation s y s t e m s a r e outlets. ) F a c t o r s Influencing the Design of F a r m Outlets 5.1.1.1

Quantity of water to be delivered In the design of irrigation distribution systems, the basic factor to b e considered i s the quantity of water to be delivered a t the f a r m outlet.

(This

quantity i s based on the a r e a of land to be i r r i g a t e d and a number of other factors, i n t e r alia, the type of soil, water requirements of the crops, effective rainfall, depth to the water table, et c e t e r a . )

The design of the outlets will also depend on

the method of water delivery adopted ( s e e 5.1 .l. 3).

Sources and nature of water The main s o u r c e s of water for irrigation a r e : r i v e r s , dams, wells, and natural springs.

Water f r o m the t h r e e l a t t e r sources i s generally f r e e of

silt and in these c a s e s the f a r m outlets can be designed for proportional d i s t r i bution i f required ( e . g. when the supply in the distributing canal fluctuates). Water f r o m r i v e r s i s usually charged with sediment, and this h a s to be taken into account in the design of the outlets so that the passage of silt i s r e s t r i c t e d , but a t the s a m e time silting up of the downstream r e a c h of the r i v e r i s avoided.

5. 1.1. 3

Methods of water delivery There a r e t h r e e methods for delivering water to f a r m s , known a s

continuous, rotational and demand respectively. In the continuous method each f a r m receives i t s rightful1 s h a r e of the supply in relation to the a r e a of each holding in an uninterrupted flow.

That i s ,

water i s always available, although i t may not always be n e c e s s a r y to use i t ; and so i t s actual u s e may be, and often i s , intermittent.

The method and canal

. system i s easy to operate but disadvantages a r e the tendency to waste time and water, particularly in sandy soils due to percolation, and the possibility of waterlogging.

Where f a r m s o r f a r m units vary greatly in size, the size of the

outlets may have to vary f r o m very small to very l a r g e s t r u c t u r e s , thus making i t difficult i f not impossible to standardize them. With the rotational method water i s in effect moved f r o m one a r e a to another en bloc, and each u s e r receives a fixed amount of water a t defined intervals of time.

This kind of rotation can be applied between just two o r m o r e

f a r m e r s , between two o r m o r e groups of f a r m e r s , o r between defined portions of an entire irrigation scheme.

By using l a r g e heads water can be moved rapidly

over the surface of the ground, thus minimizing l o s s e s through percolation and promoting good irrigation efficiencies. When water supplies a r e adequate t h e r e i s no particular problem, but when the supply i s insufficient to m e e t the full demand, the water available h a s to be distributed according to the principle of equitable sharing ( s e e Chapter 2 2.2. 1).

A f a r m outlet may s e r v e just one f a r m e r o r a group.

-

In some countries,

e . g. India and Pakistan, the entire discharge f r o m one outlet i s taken by different f a r m e r s in turn, the duration of the turn being fixed in proportion to the i r r i g a b l e a r e a in each c a s e .

Under these conditions a m o r e standardized f o r m of outlet

can be employed. The demand delivery method i s particularly favoured by f a r m e r s because the water i s delivered a t the f a r m outlets i n the quantity and a t the time requested.

It p e r m i t s a f a r m e r to draw any flow of water he may d e s i r e , limited

only by the capacity of the canal system.

At the same time i t encourages

economical use when water charges a r e based on the volume delivered.

On the

other hand i t i s not economically feasible to construct a canal system of sufficient capacity to satisfy the full demands of all f a r m e r s at the same time, and the method i s not practicable either in the c a s e of l a r g e canals drawing their supplies The extent to which f a r m outlets can be

f r o m uncontrolled r i v e r sources.

standardized for use in the demand method i s only limited. In many c a s e s , practical considerations call for the u s e of a combination of two o r all t h r e e of the delivery methods within the same irrigation project a t various t i m e s o r in various locations. of high seasonal runoff,

F o r example, during periods

the main canals can run a t full capacity and supply

f a r m e r s with water ( i f they require i t ) by the continuous flow method.

At other

t i m e s , where r e s e r v e s of water in storage a r e being drawn, o r where the supply in the r i v e r s i s insufficient for the demand, the project can be switched to rotational delivery. 5.1.1.4

Methods of a s s e s s i n g water charges In some countries ( e . g. A r a b Republic of Egypt, S r i Lanka, Thailand)

f a r m e r s a r e not charged for the irrigation water they receive, but in most other countries they have to pay for it.

However, where charges a r e made, the

s y s t e m s of a s s e s s i n g them vary in various p a r t s of the world. There a r e four main methods of a s s e s s i n g water charges o r water r a t e s for irrigation water based on: (i)

r a t e of flow, which entails metering the r a t e of flow and

maintaining the n e c e s s a r y r e c o r d s ; (ii)

volume of water delivered, entailing a volumetric measuring

device o r a combined r a t e of flow and time measuring device; (iii) a r e a , and type of, crops i r r i g a t e d ; control and record-keeping i s necessary; (iv)

each irrigation over a given a r e a , e. g. where irrigation i s only

supplementary, o r where the s a m e c r o p i s grown over l a r g e a r e a s y e a r after y e a r , such a s r i c e for example; again control and adequate r e c o r d s a r e n e c e s s a r y . In the c a s e of methods (i) and (ii) the f a r m outlets have to be accurate measuring

devices.

In the case of methods (iii) and (iv) the m e a s u r e -

ment of discharge i s not essential, but the f a r m outlets should be designed to ensure equitable distribution of water. Operation of the canal system

5.1.1.5

When water supplies a r e plentiful throughout the c r o p season the entire canal system runs a t full capacity; under these conditions the design of f a r m outlets i s a simple m a t t e r .

When the supplies a r e insufficient to m e e t the

demand their distribution h a s to be controlled and rationed out.

This can be done

in the various ways described below. The various distributing canals can be run continuously carrying their s h a r e of the water supplies available.

In this c a s e the f a r m outlets in the canals

will have to either ( a ) draw whatever water i s available o r (b) run in rotation. case (a) the outlets should be able to draw a proportion of the discharge.

In

However

i f the supply i s laden with silt, the outlets will not draw their f a i r s h a r e of i t and

the problem of silting a t the head reaches of the distributing canal may a r i s e .

In

c a s e (b) the outlet needs a check structure in the supply canal to feed it.

F'urther-

m o r e each outlet m u s t be fitted with a manually operated shutting device.

There

may a l s o be a silting problem in the distributing canal due to heading up of water. Desiderata for the Design of a F a r m Outlet As f a r back a s 1906, Kennedy s e t forth desiderata for the efficient design of a f a r m outlet, in Punjab Irrigation P a p e r No. 12, quoted below. "(a)

To keep the discharge automatically constant a s adjusted, and indicated, however much (within working l i m i t s ) the water levels may vary in the distributary channel, o r in the watercourse, o r in both a t once.

(b)

To allow of slight variations in the d i s c h a r g e s a s adjusted, so a s to avoid the need of constantly removing and replacing the outlet, whenever the d i s c h a r g e m u s t be somewhat a l t e r e d .

-

(c)

To work with high 'heads' a s well a s low

(d)

To be f r e e f r o m derangement by silt o r weeds.

(e)

To be light, portable, easily removed and replaced elsewhere.

(f)

To be cheap and durable, with no complicated mechanism.

(g)

To be all closed in and immune f r o m outside i n t e r f e r e n c e o r derangement

down to t h r e e inches o r so.

in working. (h)

To be capable of being opened o r closed off entirely by the cultivators f r o m outside.

(i)

To indicate f r o m outside when the working head i s insufficient to give the full discharge, and t h e r e f o r e a l s o the n e c e s s i t y for c l e a r a n c e of the watercourse.

(j)

If s o d e s i r e d and adjusted, to work a s a module, only within c e r t a i n l i m i t s of level in the feeder, above and below these l i m i t s to give proportionately i n c r e a s e d o r d e c r e a s e d d i s c h a r g e s .

( T h i s i s with special

r e f e r e n c e to f a r m e r s ' canals, where each m a n i s entitled to a proportion of the whole available supply.) (k)

Floods-1/ in the distributary to be passed off by i n c r e a s e d d i s c h a r g e s through the outlets, so a s to avoid damage.

(1)

When the distributary supply i s v e r y low and inadequate, i t will be m o r e o r l e s s proportionally distributed to all outlets, those with v e r y high command not being allowed to d r a w off all the water t h e r e i s .

(m)

D i s c h a r g e s to be provided for may be anything between half and four cusecs-2 / with possible duplication above the l a t t e r figures. Of c o u r s e i t i s not possible to satisfy a l l the conditions enumerated by

Kennedy in any one type of f a r m outlet, even in India, for which the d e s i d e r a t a

- i . e . excess water.

cubic f e e t p e r second.

were propounded.

However, i t i s highly desirable that every outlet be strong and

so designed that i t cannot easily be tampered with.

The cost of construction

should be low, using local m a t e r i a l s whenever available, and the aim should be to standardize a s much a s possible.

In a r e a s of only little slope the f a r m outlet

should work efficiently with a small working head, (because the g r e a t e r the l o s s of head the higher the water level required in the supply canal for command and, consequently, the higher the cost of the entire distribution s y s t e m ) .

Where

supplies a r e delivered on a volumetric b a s i s , the outlet should have metering facilities and should preferably include a recording device.

Finally, where

supplies a r e charged with sediment, the f a r m outlet should draw i t s f a i r s h a r e of the silt, without being liable to blockage by silt o r weeds. Throughout the i r r i g a t e d a r e a s of the world engineers, f a r m e r s and o t h e r s have invented o r designed various kinds of f a r m outlets for particular conditions. Some of the ideas never came to fruition but a l a r g e number of them have proved satisfactory and have stood the t e s t of t i m e .

F o r example the Adjustable Orifice

Semi-Module, developed in the Punjab befpre 1947 and the J a m r a o IIfrpe Orifice Semi-Module, developed in Sind even though developed independently, a r e intkrchangeable.

Thus, abundant experience h a s been accumulated on which to base

the choice of the best type of outlet to suit local condition?.

However, this does

not preclude the possibility of improving existing outlets o r evolving new types which m a y be superior structurally, hydraulically and economically.

In fact a

g r e a t deal of r e s e a r c h r e m a i n s to be done on this subject.

Classification and Selection of F a r m Outlets

,

18 types of f a r m outlets a r e covered in this chapter a s l i s t e d below. In addition, examples of simple outlets f o r u s e a t the f a r m level a r e described

i n the l a s t Section.

The reference number allocated to each s t r u c t u r e corresponds

to the Section number in the text. 5.4) a r e described in Chapter 3

-

The f i r s t t h r e e s t r u c t u r e s listed (5. 2, 5. 3 and Intake Structures.

5.2

Constant-Head Orifice F a r m Turnout (U. S . A . )

5.3

Orifice Module (France)

5.4

Double Orifice Module o r Syphon Module Outlet ( F r a n c e )

5.5

Dethridge Meter (Australia)

5.6

P l a s t i c Syphon Outlet F i t t e d With anIntake Tube (Turkey)

5.7

Open F l u m e F a r m Outlet (India and Pakistan)

5.8

Adjustable Orifice Semi- Module (India and ~ a k i s t a n ) J a m r a o Type Orifice Semi-Module (Sind, Pakistan) P i p e Semi-Module (India and P a k i s t a n ) Fayoum Standard Weir F a r m Outlet ( A r a b Republic of Egypt) Scratchley Outlet (India and P a k i s t a n ) P i p e Outlet (India and Pakistan) F a r m Outlet to a T e m p o r a r y F e e d Ditch (U. S. S. R. ) P r e - C a s t F a r m Turnout (Turkey) Adjustable Weir F a r m Outlet (Malaysia) PVC P i p e Turnout (Republic of Korea) P i p e Outlet with Standard Inlet (Philippines) Gated P i p e Outlet ( F e r r a r a Type) Outlet S t r u c t u r e s on the F a r m Outlets may be divided broadly into the following t h r e e c l a s s e s :

A

B

(i)

Modules o r modular f a r m outlets

(ii)

Totalizer type m e t e r f a r m outlets

Semi-modules

-

- 5. 2, 5. 3 and 5.4 - 5.5

5.6, 5. 7, 5 . 8 , 5 . 9 , 5. 10, 5. 17 and a l s o 5. 11, 5. 12, 5. 13, 5. 14, 5. 16 and 5.18 when tliey have a f r e e fall.

C

Non-modular f a r m outlets

- 5. 12 and 5. 15, 5. 13, 5. 14, 5.16,

5.18 and 5.19 under submerged conditions. The advantages, .disadvantages and limitations of each of the outlets listed a r e d i s c u s s e d in detail under the relevant Section headings.

General guidelines

on selection of c l a s s e s and types a r e given below.

Selection of c l a s s e s of f a r m outlets A.

Modules

In a module outlet the discharge i s , within reasonable working l i m i t s , independent of the water l e v e l in the supply canal and the w a t e r c o u r s e o r field lateral.

This c l a s s of outlet m a y be r e g a r d e d a s the b e s t type of f a r m outlet f r o m

the f a r m e r ' s viewpoint.

However, modules cannot a b s o r b fluctuations of water

supplies in the parent canal and, therefore, the parent canal could either flood o r become dry in the tail r e a c h .

Thus, modules should be limited to: branch canals

o r d i s t r i b u t a r i e s and m i n o r s in which the supply v a r i e s only within predetermined limits; outlets located above control points where water levels can be maintained; canals in which additional water i s delivered to certain selected outlets for leaching o r for other purposes. When water i s supplied on a volumetric basis modules a r e ideal. Under any of the c a s e s mentioned above constant-head orifice turnouts may be found adequate, whether the water supply i s charged with sediment o r not. suitable types may be P a r s h a l l flume outlets and m e t e r - g a t e s .

Other

When the water

supply i s silt f r e e the constant-head orifice turnout, the Neyrpic orifice module, the double orifice module, and the Dethridge m e t e r may be employed. In c a s e s where the water i s not being supplied on a volumetric basis, but may be a t a l a t e r date, i t may be convenient to r e s o r t to p r e - c a s t f a r m outlets ( 5 . 15) which can l a t e r be converted into constant-head orifice turnouts. B.

Semi-modules

The discharge of a semi-module outlet i s independent of the water levels in the watercourse o r field l a t e r a l , but dependent on the water levels in the supply canal, so long a s a minimum working head i s available for the device. These types of modules a r e not useful for supplying water to f a r m e r s on a volumetric b a s i s unless they be accompanied by an auxiliary device, such a s a notch weir, a venturi flume, a P a r s h a l l flume o r an open flow-meter attachment on the downstream side.

The usual use of semi-modules i s to distribute, m o r e

o r l e s s equitably, upstream variations in the supply canal within their range of operation.

The plastic syphon outlet (5.6) fitted with an intake tube can be used

advantageously in small canals.

The free-fall outlet to a t e m p o r a r y feed ditch

(5. 14), the adjustable weir f a r m outlet (5. 16), the PVC pipe turnout (5. 17) and the pipe outlet with standard inlets (5. 18), may all be used where a shut-off gate i s included in the outlet. When the-water supply to the outlets i s f r e e of silt and a shut-off gate i s not n e c e s s a r y , the following outlets a r e open to choice. (i)

Open flume outlets

-

a t tail c l u s t e r s , and in tail r e a c h e s with

-

setting of the c r e s t a t 0 . 9 y

1

f o r proportional discharge.

- in head r e a c h e s with setting of

Adjustable orifice semi-module

(ii) the c r e s t a t 0 . 6 y

1

f o r proportional discharge.

(iii) J a m r a o type orifice semi-module

of the c r e s t a t 0 . 9 6 y (iv)

1

-

in head r e a c h e s with setting

f o r proportional discharge.

Scratchley outlet

- if i t i s not d e s i r a b l e (because of cost) to

i n s t a l l any other type of semi-module. (v)

-

P i p e semi-module

when the banks of the supply canal a r e very

wide; the setting of the module will be a s indicated in ( i ) , (ii), (iii), and (iv) above f o r the respective conditions. (vi)

Fayoum standard weir outlet

-

i t s setting h a s been standardized,

and i t m a y be used successfully on all distributing canals. (vii) P i p e outlet

- in view of i t s low cost, a pipe outlet may be used

on all distributing channels with c e n t r e of the pipe s e t a t 0. 3 y

1'

When the water supply to the f a r m outlets i s charged with silt, i t i s e s s e n t i a l to u s e semi-modules which can d r a w a proportional s h a r e of the silt. In this c a s e proportional

distribution

of the water i s neither n e c e s s a r y n o r

feasible and the following types of outlets may be used. (i)

Open flume outlet

- with setting of the c r e s t a t o r n e a r the bed

of the distributing canal provided the width of the c r e s t i s not l e s s that 6 c m and the n e c e s s a r y working head i s available.

If the working head available i s not

sufficient, a combined pipe-open flume outlet may be used which p e r m i t s a higher setting of the open flume outlet beyond the pipe. in lower r e a c h e s of distributing canals.

This type i s eminently suitable

It should be used a t tail c l u s t e r s and

above control points (within 300 m e t r e s u p s t r e a m ) . (ii)

module

Adjustable orifice semi-module and J a m r a o type o r i f i c e s e m i -

- because of t h e i r low flexibility, t h e s e a r e eminently suitable for

installation in the head r e a c h e s of a distributing s y s t e m , with their settings a t o r n e a r the bed level of the supply canal, provided the n e c e s s a r y head i s available. If sufficient head i s not available, an open flume fitted with a roof block having a s i m i l a r setting m a y be used.

(iii) P i p e semi-module

-

In c a s e s where the c r e s t of the outlet

cannot be placed a t o r n e a r the bed level, a'pipe semi-module of the lowest possible flexibility may be used. (iv)

As f a r a s possible, the s a m e type of f a r m outlets with the s a m e

head over .the c r e s t s should be used between two control points on a distributing canal. (v)

Pipe outlets with their u p s t r e a m end a t o r n e a r the bed may be

used, but their coefficient of discharge i s not constant and f a r m e r s m a y be tempted to i n c r e a s e t h e i r flow by heading up the water in the watercourse thus partially submerging the outlets.

C.

Non-modular f a r m outlets

The discharge of non-modular outlets depends on the difference of water levels in the supply canal and the watercourse o r f a r m l a t e r a l .

The water

level in the watercourse below the outlet v a r i e s considerably, depending on: whether high o r low a r e a s a r e being i r r i g a t e d a t any given time; and where silting o c c u r s , the extent of silt clearance in the f a r m l a t e r a l .

Where silting i s

a dominant feature, the canals fitted with non-modular outlets a r e always liable to flooding a t the tail of the canal when f a r m e r s in the head r e a c h do not clear silt so that they draw their full s h a r e of water during periods of slack demand.

On

the other hand, water i s always in short supply at the tail end during periods of keen demand, when f a r m e r s in the upper r e a c h e s tend to do the opposite, to c l e a r their watercourses too much. Non-modular outlets should, therefore, be avoided a s f a r a s possible. Their use i s justified only when the working head available i s s o small that a semimodular outlet cannot be used.

Selection of types of f a r m outlets A s indicated above outlets have been divided into t h r e e main c l a s s e s namely, modules, semi-modules and non-modular outlets and the general circumstances in which each c l a s s may be used i s described in 5.1.3.1. selection of the particular type of outlet depends on factors such as:

-

cost

-

available working head

The

-

-

e a s e of adjustment

-

ability to withdraw s i l t immunity f r o m tampering. It should a l s o be noted that modules with moving p a r t s c o m p r i s e m o r e

o r l e s s complicated m e c h a n i s m s with the resulting possibility of the moving p a r t s becoming jammed. In conclusion, the information and d a t a on f a r m outlets p r e s e n t e d i n this chapter will, i t i s hoped, be of help to the designer in making the b e s t possible selection of the type o r types of f a r m outlets he should adopt f o r distributing canals to suit m o s t conditions and r e q u i r e m e n t s .

No c l a i m i s m a d e

a s t o the completeness of t h i s chapter and i t i s hoped to m a k e up any deficiency in the r e v i s e d edition of t h i s handbook.

5.2

CONSTANT-HEAD ORIFICE FARM TURNOUT (U. S. A. ) See Section 3 . 4 of Chapter 3.

5.3

O R L F I C E MODULE (FRANCE) See Section 3.5 of Chapter 3 .

5.4

DOUBLE ORIFICE MODULE OR SIPHON MODULE See Section 3. 6 of Chapter 3.

5.5.1

Background The Dethridge Meter i s a self-integrating m e a s u r i n g s t r u c t u r e used to deliver w a t e r to f a r m e r s and to check the volume of w a t e r supplied f o r application

"B a s e d on information

supplied by the A u s t r a l i a n National Committee, ICID.

Cyclometer f ~ x e dto the wheel Wheel Cylinder ond vones of 14 gouge hot dip golvonised

Welght of wheels: Lorge meter outlet not golvonised--187 Lorge meter outlet golvonised--..-I94 Smoll meter outlet not golvonised.10 1 Smoll meter outlet golvonised----.I07

Old Type Counter

mild steel.

"

I,

'/e

diameter hi. S. Spokes

,

,,

4, ,#

Diameter of cylinder 3 (2-%) Depth of vones rodlolly 10 (7%) Outside dio,me/!er ,of,,wheel to tlps

Anthony beorrng

,

of vones 5 - 0 (4;0),, , , Width of wheel 2 - 6 (1-8) Cleoronces between wheel ond emplocernent :

I/

I#

Cost Iron counter housing for wire connection type counter

Concrete In emplocernent IS 4 thlck (except where fl\!eted) ond relnfgrced by o g r ~ dof v4 bors spoced 4 both woys, for both lorge ond smoll meters

Note: D/mensions shown ore for both Meters lLorge and Smolll Those in brockets ore for Smoll Meter Outlets.

FA0 - l C l D *

DETHRIDGE METER OUTLET D E T A I L S

Region, Country Austrolio

Project,

Figure No. 5-1

of water c h a r g e s . The device i s widely used in the State of Victoria and other States in A u s t r a l i a and to some extent in the U . S. A. and in Asian countries.

Approximately

15,500 l a r g e m e t e r s and 7, 000 s m a l l m e t e r s a r e in operation in Victoria. The Dethridge Meter was invented in 1910 by the Commissioner a t that t i m e , the late J . S. Dethridge, of the State R i v e r s and Water Supply Commission, Victoria, and adopted by the Commission after t e s t s for t h r e e y e a r s under field conditions.

The original Dethridge Meter was a simple m e a s u r i n g device giving

a positive m e a s u r e m e n t of volume discharged, and recording i t directly in a c r e -

2

3. 5 p e r cent for f r e e outfall conditions over a range 3 of d i s c h a r g e s f r o m 42 11s t o 140 11s ( 1 . 5 f t / s to 5 f t 3 / s ) . A s m a l l m e t e r f o r capacities f r o m 14 l / s to 70 11s ( 0 . 5 ft 3/ s to 2. 5 ft 3 / s ) was developed in the feet, with an a c c u r a c y of

1920's. The construction of the Dethridge M e t e r h a s remained basically the s a m e over the y e a r s , i. e. concrete emplacement with m i n o r variations to head wall and transition shape.

Laboratory and field t e s t s have r e s u l t e d in a standard setting

of the wheel in relation to channel flow level and mechanical improvements for the wheel and fittings, e . g. wheel with a m i l d s t e e l plate d r u m , vanes and w a t e r pipe axle, originally with t i m b e r spokes and rivetted, now all steel and welded; t i m b e r bearing blocks now replaced by a m o r e robust sealed unit;

s t e e l gate in a t i m b e r

f r a m e , now replaced by neoprene guides s e t in concrete.

S t r u c t u r a l Design The g e n e r a l f o r m and main dimensions of the two standard s i z e s of m e t e r a r e shown in F i g u r e 5- 1.

The wheel i s m a d e up of a cylinder of 14 gauge

( 2 . 0 3 m m o r 0.08inch thickness) mild steel sheet, bearing eight e x t e r n a l vanes of the s a m e m a t e r i a l , and internally braced by t h r e e c r o s s e d p a i r s of s t e e l spokes placed a t the middle and both ends of the cylinder. inch) d i a m e t e r galvanized pipe welded t o the spokes.

The axle i s a 2.54 c m (one The cylinder, vane

attachments and spokes a r e fabricated by welding. The vanes a r e "V" shaped a s shown ( F i g u r e 5- l ) , with the apex of the "V" leading in the direction of rotation.

At the b a s e of each vane, and a t the apex of

the "V", t h e r e i s an a i r vent to facilitate the filling and emptying of adjacent comp a r t m e n t s a s they e n t e r and leave the s t r e a m of water passing under the wheel. The outer c o r n e r s of the vanes a r e chamfered to suit the fillets a t the junction of the walls and floor of the concrete emplacement. The co.mplete wheel unit i s galvanized for protection against corrosion.

The

wheel unit i s supported accurately in the emplacement with the ends of the axle r e s t i n g on ball bearings in a Delrin r a c e fixed on the walls of the flume.

(~imbew

bearing blocks w e r e originally u s e d . ) A pendant actuated sealed cyclometer unit i s rivetted to the wheel cylinder and r o t a t e s with it.

The older m e t e r s have a c a s t i r o n housing attached to the

flume wall f o r the revolution counter, which i s connected to the end of the axle by a flexible w i r e link. The shape of the flume can b e s t be d e s c r i b e d by considering i t in t h r e e parts.

U p s t r e a m of the wheel it i s of simple rectangular section, with level

floor in the vicinity of the wheel.

While the walls r e m a i n plane and p a r a l l e l , the

floor i s indented to accommodate an a r c of approximately 70 circumference.

0

of the wheel's

And immediately downstream of the wheel the walls a r e splayed

outward and the floor i s sloped up to a l i p a t sufficient height to e n s u r e drowning of the passage swept by the vanes under the wheel. At the entry to the flume a cut-off wall extends to either side into the canal bank, and downward below the n a t u r a l s u r f a c e of the ground to prevent seepage around the s t r u c t u r e .

The galvanized steel sluice gate to control discharge

through the m e t e r i s fitted in neoprene guide s t r i p s s e t in r e c e s s e s so that they a r e flush with the walls and floor a t the entry f r o m the channel. The flume and cut-off wall a r e constructed of reinforced concrete, although other m a t e r i a l s may be employed.

In e a r l y t i m e s of cement shortage, t i m b e r

was used, but proved unsatisfactory because of leakage between the t i m b e r s and distortion. All concrete i s of high quality and 10 c m ( 4 inch) thickness (except where filleted), and reinforced with a grid of 6 m m (0. 25 inch) d i a m e t e r b a r s spaced 10 c m ( 4 inches) a p a r t both ways.

The s t r u c t u r e may be either c a s t in-situ o r

assembled f r o m p r e - c a s t units.

The p r e - c a s t units provide a m o r e accurate

emplacement with better concrete quality.

(See Figure 5- 2)

FIGURE 5-2. - P r e - c a s t l a r g e m e t e r emplacement with wheels installed; note reinforcement to tie into cut-offs and p r e - c a s t head wall.

R i p - r a p protection i s placed on the bed and b a t t e r s of the f a r m ditch immediately downstream of the m e t e r to prevent scouring. F o r l a r g e canals, where continuous a c c e s s i s required along the canal bank, a pipe outlet i s installed through the bank and the m e t e r i s e r e c t e d outside the bank a t the end of the pipe. F i g u r e s 5-3 ( a ) , 5-3 (b), 5-4 ( a ) and 5-4 (b) show details of both l a r g e and small m e t e r s . F i g u r e 5-5 shows a l a r g e Meter Outlet in operation with f r e e outfall.

250.

Groa* to be formed i n wall 1 - 0 above I10 10 bndi~otemoiimum ollowoble toilrmter leve

-----

y level 10 be painted block

Earth platform level wlth l o p 01 side wall en both s~dcs

C d i o . bolh G o n g . set 5 s bl block-OUI 01 8 centres for Anthony brorinqr. For lumber 9"blocks 3bmdto bolts o n

Anthony bearing

G01.s

-

Go1vonosed, mqether w t h 0 x 1 ~

L A R G E DETHRIDGE METER OUTLET

R81hg & hrge meter cutlet The odopled rolinq is 2 9 . 0 4 l ? l r e v o ~ l l ~rquivolrnl n l o 1,500 r e v ~ l u t i o n r= 1 ocre-foot based on colibrotion Ierls with free ~ l f f o l lOM, the end st11 H i i r loilwolerr incrcosc the volume porred The cyclometers clod d~rccltyin o c r l I t I r p m I orrr-lootl doy opproxlmolely Ckor0nc.s between the wheel ond cmplocement should be occurolrly mointo~nedand the wheel .hod4 (DI be 0110wed to turn ol less thon 3 r p.m.

.

Australia

161

&l:block

{long for Anthony beorlngs, 1l"long for tlmber blocks

out

.-

Wlngwall shape when occers culverl reaulred or road crosslnq

,

7-10

er 6 feet transition

, , - .-

--

--

S e c t ~ o n C C ( s e e Flgure 3 (01

P

Upstreorn View o f D r u m

Sect~on D D

N

be~ngcons~dered

Developmsnt o f Vane

Meter Wheel Dctoil (See note 7 ond B on Ftqure 3(01

P o s ~ t ~ oonf Wheal In E m p i o c e r n e n t T ~ m b e rAxle Block [See note 4 on F~gure3 ( 0 ) Block made In halves from seasoned R G or other durable tlmber dressad to dlmens~onsshown Slot ond houslnp bolt hdes rrqulred on one set only

FA0

-ICID

SECTION CC OF FIGURE 3(0) AND

DETAILS OF METER WHEEL '/:dm

ecentres ln centre of plonk

~i 1

T

?g dto Q e c e n t r e r

/ 2 1 - - ~ ~ + j

Section EE Precoct R C Foot Plonk Timber plank may be used as olternot~ve

OF LARGE DETHRIDGE METER OUTLET

, Reg~on,Country Austrol~o

Project

Flgure No 5-3 (b)

Groove to be formed in w l l 9 obove f l w r I l o indicote moximum ollowoblc toilwoter level

Top of gote nom~nollylevel with tap of heodwoll Gotes ow~loble ore 1-07 2I6: $-9"or 3'-0"high. Select gate r i l e neorest to required height. See notes 2 ond 3.

C--------I Groove to be pointed

I

block with chlorlnoted rubber pain

Half

Notes

Sect~on

A-A

4,

If the derlgned rnlnlmum free board lo the lop of bonks 1% less than < / F B ~ ~ = / 11 11 0s equal to or greater thDn 2 ' then /FB,,/ = 12" The helght of the heodwaliond gote lo ttxed from Ihe D 0 L of the channel The moilmum h e q N ot gote su8toble for hond lhfttng IS 3'-0'where larger gate s necessary, type 'C' lhfttng gear should be used Anthony bearongs ore adopted as stondord Concrete tor emplacement should be of mcsmum compnssse strength 3,500 p n I ot 2 8 doys for durobll~ty Backfoll should be well compacted around the heodwoll ond sfde walls Protectton of metal work on currenl use Goter Galvobond plate Wheels- Golvon~rcd, together w~th oxle All bolts galvonlsed Feld tests are on hond for cold potnl ~ p p l t c o l l ~ non r gates e g,Galvofrotd woth Mvconox re01 coat ond for olumlnlum wheels

/

Half S e c t i o n 8 - 8

Plan

FA0

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ICID

-

SMALL DETHRIDGE METER OUTLET

Rofmg of smo// meter out/et The odopled rotlng IS I 2 45 f?/revolutnon cquwalent to 3 500 revclvttont = I ocre-foot bosed on collbrattan tests w ~ l hfree outfoll owr the end 91; Hfgher to#lwatcrs lncreose the volume possed The cyclometers read directly I n ocre-fecl/r p m = 3 ocrt-tcet/doy opproxlmately Clearances between the wheel ond emplacement should be accutotely mo~nto8ned ond Ihe wheel should not be ollowed to turn ot less than 3 r p m

P r o j e c t , Region, Country Austrolia F i g u r e No 5 - 4 ( 0 )

Section C-C See Figure 3 ( 0 )

t

Dovies Shephord pendant weight counter

fi-

a+

On@ vona rhown-

\6ntcrnol diameter stondord rned~urnplpe (not golvonlsed. not vornished, not oil painfed) -

I

dio. M.S.apokes welded l o drum

Sectlon D - D

Upstreom V ~ e w o f Drum

Side E l e v o t l o n ,vote use of o/um~n~um wheels of whfed or nveled construmon IS &IW consd8nd

7-0

rodlur Oevelopment of Vans

Meter Wheel D e t o ~ l Sa@ note 7 ond 8 on F ~ g u r e4 (0)

POSI~IO~of Wheel ln Emplocemenl

'/;; C S

x racers for cover plate of C I housing (on counter $~de only I

bolts ~n centre of blocks

Block mode In halves from seasoned R G or other durable ttmber dressed to dlmens~ons shown Slot and hous~ngbolt holes nqulred ~n one set only

FA0

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ICID

SECTION C-C OF FIGURE 5 4 (a) AND DETAILS OF METER WHEEL OF SMALL DETHRlDGE METER OUTLET

woter plpe d x 2 % " x l i Tlmber Axle Block Sea note 4 on F ~ g u r e4 (0) Scale ? ? ? 9 Inches

Project, Region, Country Australto Flgure 5-4 (b)

FIGURE 5-5. f r e e outfall.

-

L a r g e Dethridge m e t e r outlet in operation with

The s t r u c t u r e i s robust in construction and reasonably r e s i s t a n t to illegal tampering. to counters.

If n e c e s s a r y , gates can be padlocked in position and lead s e a l s fixed The counter h a s a ratchet arrangement to prevent winding back if

the wheel i s rotated in the opposite direction (important against tampering).

One

vane i s painted so that the canal attendant can see f r o m a distance whether the wheel i s turning correctly.

The m e t e r will p a s s a f a i r amount of floating debris

in the water without damage to, o r stoppage of, the wheel. Once installed correctly in an accurate emplacement the m e t e r does not require adjustment.

Regulation of discharge through the m e t e r i s c a r r i e d out by

simple adjustment of the gate opening.

Hydraulic Characteristics It i s important that the m e t e r be installed a t the c o r r e c t level in relation to the designed discharge level of the water in the canal, so a s to make the b e s t use of the generally limited head available while at the same time ensuring sufficient head and yet avoiding drowning of the wheel by water that may back up f r o m the f a r m ditch.

The standard setting for the l a r g e m e t e r , a s shown in F i g u r e 5-6,

i s to have the floor of the flume, a t entry, 38 c m (15 inches) below supply level in the canal.

F o r the small m e t e r this depth i s 30.5 c m (12 inches).

The supply level of the canal i s fixed by the designed upstream level of each canal regulator and i s taken a s a horizontal pool a t this level extending upstream f r o m the regulator to the next regulator.

This level i s fixed in relation to the

land to be supplied. The commanded land i s defined a s the land 15 c m ( 6 inches) o r m o r e below the supply level making allowances where n e c e s s a r y for f a r m canal grades and structures.

This allows about 7.5 c m ( 3 inches) drop through the m e t e r to the

maximum allowable level in the f a r m e r ' s ditch and 7.5 cm ( 3 inches) depth of m e t e r over the highest ground.

As the canal running level i s above the

horizontal supply level, m e t e r s , set some distance upstream of a regulator, have a water depth over the floor of the flume in excess of the standard 37.5 cm (15 inches) o r 30.5 cm (12 inches) respectively and the gate and head wall levels m u s t be raised accordingly to provide the required f r e e board.

As f a r a s possible,

regulators a r e located so that m e t e r s a r e within a short distance upstream. If t h e r e w e r e no necessity for clearances between the wheel and the flume, the m e t e r would give an exact positive measurement of the water passing through it, a s each revolution of the wheel would pass an invariable quantity of the water between the vanes and the cylinder swept through the emplacement.

With the

provision of the n e c e s s a r y clearance ( s e e section 5.5.4) however, leakage occurs through the clearance space at a r a t e dependent not only on the r a t e of rotation of the wheel but also on other factors, such a s the difference in water levels immediately upstream and downstream of the wheel, and the depth of submergence. The quantity of water passed p e r revolution of the wheel does, therefore, vary to some extent under operating conditions.

F o r this reason, the clearance i s kept

a s small a s practical.

5.5.3.1

Relationship between revolutions and discharge F o r the conversion of revolutions to acre-feet, constant ratios a r e assumed: 1,500 revolutions per acre-foot for the l a r g e wheel and 3,500 revolutions p e r acre-foot for the small wheel.

The dimensions of the wheels

have in fact been designed to provide these simple ratios, and the e r r o r i s not

m o r e than 5% over the r a n g e of n o r m a l operating conditions f o r e i t h e r m e t e r . This d e g r e e of a c c u r a c y i s considered quite reasonable f o r the m e a s u r e m e n t of w a t e r d e l i v e r i e s for i r r i g a t i o n . S i m i l a r l y revolution counters a r e g e a r e d i n the r a t i o of 15 : 1 f o r the l a r g e m e t e r , and 35: 1 f o r the s m a l l m e t e r , and s o the dial r e a d s d i r e c t l y in a c r e - f e e t . The counters a s manufactured r e a d to two d e c i m a l places, but i r r i g a t i o n r e c o r d s a r e rounded off to one d e c i m a l place. 3 A d i s c h a r g e r a t e of one acre-foot p e r day (14 l / s o r 0 . 5 f t / s ) c o r r e s p o n d s 1 closely to one revolution p e r minute of the l a r g e r wheel o r 27 revolutions p e r minute of the s m a l l wheel.

These relationships provide a v e r y convenient guide

to the canal o p e r a t o r o r water-bailiff i n setting a m e t e r gate to give whatever discharge r a t e i s required. Design r a t i n g s (a)

Ratings and settings adopted i n relation to the supply canal l e v e l s for

the two m e t e r s a r e a s indicated below: Ratings

L a r g e M e t e r Outlet

Small Meter Outlet

(i) F r e e overfall

820 l / revolution o r 29.04 ft3

349 l / revolution o r 12.45 f t 3

(ii) Tail w a t e r a s i n (iii)

860 l / revolution o r 30.38 ft3

( i i i ) Tail Water Above downstream sill level

17.8 c m o r 7 inches

1 1 3 . 3 c m o r 5 3 inches

Above u p s t r e a m sill l e v e l

3 0 . 5 c m o r 12 i n c h e s

22.9 c m o r 9 i n c h e s

38.1 c m o r 15 inches

30.5 c m o r 12 i n c h e s

Bottom

1 0.64 c m o r q inch

1 0. 64 c m o r 2 inch

Side

3 0 . 9 5 c m o r 8 inch

1 0.64 crn o r 7 inch

(iv) Designed supply level Above u p s t r e a m sill l e v e l (v) C l e a r a n c e between the M e t e r wheel and the Qutlet S t r u c t u r e

f-

5.5.4

Installation and Operation Canal regulators a r e located to regulate supply levels for the m e t e r outlets a s shown in F i g u r e s 5-6 and 5 - 7 .

FIGURE 5-6. - Typical setting of Dethridge m e t e r just u p s t r e a m of a regulator. The supply level of the canal i s indicated by the bottom of the slot in the wall on the right hand side of the regulator.

Generally one m e t e r i s provided for each f a r m , a small one for a r e a s up to 40 a c r e s and a l a r g e one for l a r g e r f a r m s .

If the f a r m a r e a commanded by

gravity and suitable for irrigation exceeds 120 h a (300 a c r e s ) , a second l a r g e m e t e r may be provided.

These general r u l e s may be modified, i f n e c e s s a r y ,

because of particular f a r m layouts and topography. The canal attendant regulates flow to the f a r m through the m e t e r a t required intervals during the irrigation season to provide flows a s requested by the irrigator.

Regular readings of the counter a r e taken to keep check on the total

volume supplied. The flow i s regulated by adjusting the gate to the appropriate opening a s indicated in F i g u r e 5-8.

In actual operation the flow i s readily checked by

Earth free board annel regulator

-

L!7f

-L

L.M.0 Floor level ( 1 ) Bed

I

t

Min. drop 0 . 2 0

---

+

3 maximum stripping of drop bars

T ypicol Longitudinal Sect ion D.D L . Designed discharge level Flow profile for maximum discharge with roughness and grade selected S L. -Supply level for setting meter outlets and spur offtakes /I

(I 1

Capacity in ft3/ s

0 - 30 30-490

>

*

490

For SMO setting of floor i s 12 below supply level

Earth Minjmum* crest w ~ d t h~nfeet

bank Minimum free board in feet

-'1

6" 6"

2'-

0"

1'-

6

8 12

Structures Minimum concrete Prestressed slab free board bridges

**

1

] 9" 12"

il

The required crest width may be larger than the minimum t o meet percolation gradient requirements or construction equipment requirements. If access is required along the bank a

.

t

FA0 - ICID

minimum crest width of 12 is required.

CHANNEL DESIGN, FREE BOARD

* * ~ e a s u r e d from D. D. L. a t abutments t o top o f prestressed slab.

AND SETTING OF DETHRIDGE METER OUTLETS P r o j e c t , Region, Country Aus trolio F i g u r e No. i

5-7

counting the revolutions p e r minute of the.whee1 and reference to the table i s not necessary. Accurate ratings have been obtained in the laboratory to give graphs for volume passed per revolution of the wheel over a range of discharges and for different canal levels and tailwater levels. c a r r i e d out for non-standard clearances.

Other s e r i e s of t e s t s have been These ratings a r e not required for

operations o r normal design purposes because average ratings have been adopted for the counter gearing.

Detailed ratings a r e , however, available for special

investigations. 5.5.4.1

Costs and quantities of m a t e r i a l s Direct labour and m a t e r i a l costs of the installed p r e - c a s t outlet (excluding fittings) a r e in Australia approximately: small m e t e r outlet

$ A 350

l a r g e m e t e r outlet

$A 450

These costs vary depending on site conditions and a c c e s s .

The costs

of fittings, (wheel, gate, guides, bearings, cyclometer) a r e $ A . 43 for a small m e t e r and $ A . 63 for a l a r g e m e t e r and a r e included in the above cost. Approximate concrete volumes for p r e - c a s t units a r e : small m e t e r outlet l a r g e meter outlet Maintenance The main item of regular maintenance for the old type of m e t e r was recoating the wheel with t a r , for protection against corrosion.

This was done

each year in the winter months, when t h e r e was no irrigation.

This h a s now been

eliminated by using galvanized metal parts.

When the canal operator takes a

reading of the counter o r adjusts the discharge, he should check that the wheel, the bearings and counter a r e operating correctly. should be treated with a cold zinc-rich epoxy paint.

Any damage to the galvanizing The f a r m e r ' s ditch m u s t be

checked to ensure that i t i s clean of weeds which would increase the water level and cause high tailwater levels on the m e t e r .

-(Large Meter Outlet)

12 Inches tallwater

Dfscharge In ft3/s Approximate No of r p m of wheel given in brackets

Marks on precast from upstream water depth

(Small Meter Outlet

angle"^"

9 Inches tallwater

angle“^"

FA0

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ICID

GATE CALIBRATION TABLES FOR L M.O. AND S.M.O. RIVETTED GALVABOND TYPE GATES

R e g i o n , Country A U S r0li0 ~

Project, Precast frame wlth gate locked In closed pos~t~onUpstream vtew

angle"^"

~ n g l e ~ ~ *

F ~ g u r eNo. 5 - 8

Numerical Example Design a Dethridge Meter Outlet under f r e e overfall conditions for standard ratings and setting to deliver during p a r t of t h e irrigation season 2.5 ft 3/ s and in the other p a r t of the season 3 . 8 ft 3 / s . Refer to the Tables on Figure 5-8, which show that for standard ratings and settings ( s e e 5 . 5 . 3 . 2), the l a r g e m e t e r outlet will give a discharge of 2.5 f t3 / s 5 3 with a gate opening of 5g inch and a discharge of 3.8 ft / s with the gate fully open. Other dimensions and details a r e a s given in 5.5.3 and 5.5.4. F u r t h e r R e s e a r c h to Improve the Structure Experiments have been c a r r i e d out for some time to t r y and find an a r r a n g e ment with better accuracy a t low discharge. the wheel t o seal the clearances.

T e s t s have included rubber flaps on

A modified emplacement i s at present under

t e s t with the downstream section of the flume the same width a s the upstream section, instead of flared, and the floor horizontal.

A gate i s installed on the

downstream end to control the flow of water and to ensure filling of the volume between the vanes and drum.

A somewhat similar modified arrangement i s used in New South Wales. T r i a l aluminium wheels, rivetted and welded, *have been installed but a r e much m o r e expensive than galvanized steel.

Fibreglass wheels have been t r i e d

in New South Wales. Summary The Dethridge m e t e r outlet can operate with a small head l o s s , a s low a s 7.6 c m ( o r 3 inches) for discharges up to 112 11s ( o r 4 ft 31 s ) for a l a r g e m e t e r . A head l o s s of a t l e a s t 17 cm ( o r 7 inches) i s required for best accuracy. The device m e a s u r e s by positive action. integrating.

The measurement i s self-

Accuracy of the measurement i s around 5% which i s well within

reasonable l i m i t s for irrigation deliveries. The accuracy drops off sharply with high tailwater levels.

The water level

in the f a r m e r ' s ditch must be controlled to keep i t a t l e a s t 7.5 c m ( 3 inches) below

the m e t e r supply level.

Even a t the 3 inch control level the e r r o r i n c r e a s e s to

6. 7% a t 42 11s ( 1 . 5 f t 3 / s ) . The a c c u r a c y a l s o d r o p s off sharply f o r d i s c h a r g e s lower than the minimum indicated above.

F o r t h i s r e a s o n the s m a l l m e t e r i s not suitable f o r m e a s u r i n g

flows to a r e a s under 2 h a ( 5 a c r e s ) Considerable c a r e should be taken to provide a c c u r a t e setting of the wheel and c l e a r a n c e between the wheel and emplacement. most critical.

The bottom c l e a r a n c e i s the

A c l e a r a n c e of 2.54 c m (1 inch) instead of the standard 0. 64 c m

(0.25 inch) would i n c r e a s e the volume p a s s e d by m o r e than 10%. F o r f r e e outfall conditions, a "Pelton Wheel" situation o c c u r s if the canal level i s m o r e than 7 6 . 2 c m (30 inches) above the u p s t r e a m floor.

A jet flow

develops under the wheel without completely filling the swept volume beneath the vanes and d r u m such that the actual volume p a s s e d can be a s much a s 10% l e s s than the r a t e d volume. This outlet i s not suitable f o r canals c a r r y i n g s i l t charged water a s i t cannot d r a w i t s s h a r e of the silt.

5.6

11 PLASTIC S I P H O N O U T L E T F I T T E D W I T H A N I N T A K E TUBE-

5.6.1

General A P l a s t i c Siphon Outlet fitted with an intake tube h a s been evolved in Turkey for delivering w a t e r to f a r m l a t e r a l s f r o m elevated flumed t e r t i a r y canals running under shooting flow conditions.

Elevated flume i r r i g a t i o n s y s t e m s a r e being

increasingly used in Turkey because of the reduced construction t i m e n e c e s s a r y (through prefabrication) and other economic advantages. Delivery of the water to individual f a r m s i s effected by m e a n s of plastic siphons.

/'

P l a s t i c siphons enable water to be delivered f r o m any point along

Based on a note p r e p a r e d by Onden Bilen, Turkey, and supplied by the Turkish National Committee, ICID.

7c'J;

b

I

lntoke tube --!yoter

level

---'-is;

20 19 18

17 . 16 15

-

\" 14 Y

6 13 12

. 7

8

9

95

II

FAO-ICID 10

PLASTIC SIPHON FITTED wrrH AN INTAKE TUBE RELATIONSHIP BETWEEN DISCHARGE AND DEPTH OF SUBMERGENCE

9 8. 4

yb,

, Depth

5

6

of submergence

, cm

Project ,Region, Country Turkey Figure No. 5-10

canals and thus they a r e adaptable to a wide range of f a r m layouts. Economically, relatively s t e e p slopes and high water velocities in elevated flume type t e r t i a r i e s a r e d e s i r a b l e ; on the o t h e r hand, this c r e a t e s difficulty in withdrawing water under shooting flow conditions. To overcome t h i s problem, v2 the dynamic head (Z) developed by the velocity of flow in an elevated flume, h a s g been utilized by connecting intake tubes to the siphons.

Structure The outlet c o n s i s t s of a plastic siphon with an intake tube 1 2 c m long and 9. 3 c m inside d i a m e t e r ( F i g u r e 5-9). The siphon i s fixed to the s i d e s of the elevated flume by m e a n s of a s t e e l ring ( 2 m m thick) and m i l d s t e e l plates (5 m m thick) a s shown in F i g u r e 5-9. The m e a n s f o r fixing the siphon i s easily adjusted f o r different depths of submergence.

The siphon h a s a f r e e flow and d i s c h a r g e s either directly into the

f a r m l a t e r a l o r into a s m a l l m a s o n r y o r concrete receptacle f r o m where the water flows on to the f a r m l a t e r a l .

With this device i t i s possible t o withdraw

about 18 11s of water under shooting flow conditions.

Hydraulics The angle between the siphon inlet and the intake tube h a s been selected a t 90°.

The discharge capacity of the siphon d e c r e a s e s with the i n c r e a s e of this

angle. F i g u r e 5-10 i s a graph, developed f r o m experiments, which shows the relation between discharge and depth of submergence.

F i g u r e 5- 1 1 i s a graph

showing the relationship between the length of the intake tube and the efficiency of the siphon ( r a t i o between the actual and theoretical d i s c h a r g e s ) f o r depths of submergence f r o m 5 cm to 9 c m .

The graphs a r e valid for an intake tube length

of 12 c m and siphon of inside d i a m e t e r of 9 . 3 cm.

I t will be seen f r o m F i g u r e

5- 11 that with an intake tube of 12 c m in length and a depth of submergence of

5 c m (corresponding to

= D

of complete submergence

0.552)

gives an efficiency of 98%.

In the c a s e

(t)

(

= 1.00 ) efficiency i s 66%. D(t)

It i s advisable

to u s e a submergence depth between 5 c m and 8 c m . In p r a c t i c e the w a t e r velocity under shooting flow conditions in t e r t i a r y c a n a l s v a r i e s between 2 . 5 m / s to 3 . 5 m / s , and the g r a p h s in F i g u r e s 5- 10 and 5- 11 give s a t i s f a c t o r y r e s u l t s and a r e valid f o r t h i s velocity range. Design The s u b m e r g e n c e depth for a given d i s c h a r g e to be p a s s e d f o r a tube with a n inside d i a m e t e r of 9. 3 c m i s found f r o m F i g u r e 5- 11.

F o r t h i s submergence

depth and the adopted tube length, the efficiency of the outlet, a s explained in 5.6. 3, i s found.

Thjs efficiency should be a s high a s possible.

F o r example: let

Q of the outlet be

15 11s

d i a m e t e r of inlet tube b e 9. 3 c m length of inlet tube

12 c m

F r o m F i g u r e 5- 10, submergence depth f o r a d i s c h a r g e of 15 l/ s = 5.5 c m . The efficiency for this design i s about 9 7 . 5 %.

5.7

1/ OPEN FLUME OUTLET (INDIA AND PAKISTAN) -

5.7.1

General The open flume outlet i s widely u s e d with s u c c e s s in Punjab, Haryana and some other s t a t e s of India, and the provinces of Sind and Punjab of P a k i s t a n . (See F i g u r e 5-12) The e a r l i e r types of outlets developed in India

- Kennedy's s i l l outlet,

Kennedy's gauge outlet, the Harvey outlet, the Harvey Stoddard i r r i g a t i o n outlet

-

have been superseded either due to t h e i r not having been immune to

B a s e d on a note p r e p a r e d by A. D. Choudhry, Chief Engineer, I r r i g a t i o n Works, Haryana (India), and K. C. Gupta, Executive Engineer, C e n t r a l Designs I r r i g a t i o n Works, Haryana (India).

tampering o r due to improved designs.

The open flume outlet, a development of

the idea underlying the Harvey outlet, was f i r s t introduced by Crump in the Punjab, and underwent improvements and modifications f r o m time to t i m e .

The

two types which finally emerged a r e : the open flume a s used in the Punjab, and the J a m r a o type open flumeL1 a s used in the Province of Sind, Pakistan.

FIGURE 5- 12. (Punjab type)

5.7.2

-

Photogr aphs of open flume f a r m outlet

Structural and Design C h a r a c t e r i s t i c s The open flume outlet ( a typical design i s shown in F i g u r e 5-13) i s essentially a smooth weir with a throat constricted sufficiently to ensure a velocity above the critical and long enough to ensure that the controlling section

?-I P a r t i c u l a r s

not furnished in this Handbook. The J a m r a o type open flume outlet r e q u i r e s m o r e working head than the Punjab type open flume outlet.

A flared

r e m a i n s within the parallel throat a t all discharges up to the maximum.

u p s t r e a m head wall and a gradually expanding flume, i s provided a t the outfall to obtain the maximum recovery of head.

The entire s t r u c t u r e i s built in brick

masonry; but to prevent tampering and also to help in the construction of a p r e c i s e section of the throat o r gullet, an i r o n b a s e plate of about 6 m m thickness and about 30 c m long should be fitted in the centre of the controlling section in the gullet.

Cast i r o n plates, 300 m m x 300 m m x 10 m m , should also be provided on

the side of the gullet in conjunction with the base plate a s shown in F i g u r e 5-14. In c a s e of need to widen o r n a r r o w the controlling section a t a l a t e r stage, check plates can be adjusted by the required width by m e a n s of sliding bolts. steel plates may also be used, after welding to size.

Mild

The thickness of the base

plate and the check plate should be 5 to 6 m m when mild steel plates a r e used and 10 to 12 m m when with c a s t i r o n plates. The length of the throat should be equal to 2H(crt) and the controlling section s t a r t s a t a distance of 2H canal.

( 4f r o m the

toe of the side slope of the supply

In o r d e r to obtain the maximum recovery of head, the downstream side

walls a r e splayed at 1 : 10 for a length of 1 . 5 m (5 ft), having a width at the end equal to 30 c m ( 1 ft) plus bed width of the watercourse o r f a r m canal. downstream end t h e r e a r e wing walls on both sides.

At the

The s t r u c t u r e i s fitted

with a gauge on the u p s t r e a m s.ide to read H(,,t). The slope of the downstream glacis depends on the bed level of the watercourse o r f a r m canal. The f r e e board on the supply canal water surface up to the downstream end of the throat i s 15 cm ( 6 inches).

Downstream side walls o r wing walls a r e a t

l e a s t 15 c m ( 6 inches) above the water surface level in the watercourse o r f a r m canal. The s t r u c t u r e i s very durable and has a very long serviceable life.

It i s

immune f r o m tampering when cast iron o r steel f r a m e and side check plates a r e fitted in t h e throat.

It can be adjusted, when required, by dismantling one side

wall, then either raising o r lowering the c r e s t level o r reducing o r increasing the width of the throat and rebuilding the side wall at the required distance.

Gullet of outlet Check plates

*

f Base plate

-----in base plate

I

I

10-

L-375

Plan A - A

Sectional

Base plates to check plates to be 10 mm to 12 mm thick when of cost iron and 5 mm to 6 mm thick when of mild steel plates

qt, varies

5

min. 6 cm7

Face of masonry wing walls

L- shape check plate to be of cast iron or mild steel plates welded to shope

bond with mortar

FA0

Section B-I3 Showing instollation of check plates on base plate

f A// ohensions ore in centimetresl

'

- ICID

ADJUSTABLE PLATE IRON BLOCK FOR OPEN FLUME OUTLETS FOR 6 cm TO 2 0 cm

ej)

Project, Region, Country lndio ond Pokiston F i g u r e No.

5-14

The s t r u c t u r e o p e r a t e s without any control.

The gauge at the outlet i s

r e a d once a month when the canal supervisor c a r r i e s out inspection of the outlet.

5.7.3

Hydraulic C h a r a c t e r i s t i c s

5.7.3.1

Accuracy In this outlet discharge can be calculated f r o m the design formula

( s e e 5 . 7 . 4 ) so long a s

steady standing wave f o r m s downstream and the water

surface level in the supply canal does not touch the bottom of the roof block, i f the outlet i s fitted with one. 5.7.3.2

Flexibility Flexibility (Fl) (the ratio which the r a t e of change of discharge of

outlet b e a r s to the r a t e of change of discharge of the supply canal) of the open flume outlet i s given by: dQ 2

Q1 i s the discharge of the supply i s the depth a t full supply level in the supply canal.

Where Q2 i s the discharge of the outlet, canal, and y

1

It will thus be seen that proportionality in discharge can be secured by fixing the c r e s t of the outlet at 0 . 9 of the depth of the supply canal.

If the

c r e s t be higher than this, the outlet becomes m o r e flexible, i. e. hyperproportional, and i f lower, i t tends towards rigidity.

With a fall in the full

supply level in the supply canal, the flexibility would i n c r e a s e and with a r i s e in the water level i t would d e c r e a s e To d e c r e a s e the defect of high flexibility, a roof block ( F i g u r e 5-15) i s fitted in the gullet of an open flume, a t the vena contracta, c l e a r of the water surface in the gullet when the outlet i s drawing i t s full

supply discharge.

F.S.L.

I

(A// dimensions ore in centimetres)

Length ocross the oxis of flow = Bit) + 45 cm

.

.

*-I. I

I

30

Roof Block

FIGURE 5-15. - Open flume outlet. Details of roof block.

This clearance i s generally set a t 1.,5 cm in the head reach and 3 cm in the tail reach of the distributing canals.

The roof block should be fixed a t a distance

equal to H (crt) below the upstream end of the throat and the bottom of the roof

block should be a t a height of 0.75 H(crt) above t h e c r e s t p l u s t h e clearance of 1.5 t o 3 cm. The roof block should have a square edge a t t h e bottom and it may be of b r i c k masonry o r reinforced concrete, t h e height up t o t h e t o p of t h e s i d e w a l l s and length along t h e flow varying from 1 2 cm t o 23 cm. This device enables t h e open flume t o s t a r t working a s an o r i f i c e a s soon a s t h e supply l e v e l i n t h e F e n t canal i s above t h e bottom of t h e roof block.

Silt-drawing capacity The higher the c r e s t of the outlet compared with the bed level of the supply canal, the l e s s i s i t s silt-drawing capacity.

h

practice, the width of the

throat of the outlet i s limited to a minimum of 6 c m ( 0 . 2 ft) and, because of this, i t often becomes necessary to r a i s e the c r e s t of the outlet much above the bed

FIGURE 5- 16. - Arrangement of open flume outlet upstream of a fall.

level of the supply canal.

It i s apparent (except in small canals) that i t i s seldom

possible to place the c r e s t of an open flume outlet with a normal discharge of l e s s than 56 11s ( 2 f t 3 / s ) a t the bed level of the canal. 5.7.3.4

Range of operation This outlet can work a s a semi-module for all heads over the

minimum modular head and for all discharges generally required for outlets. The open flume outlet with i t s c r e s t s e t a t 0.9 y

(provided B 1' (t) i s not l e s s than 6 cm), can be used with advantage for proportional distribution of water when the supply canals have to be run below the full supply level.

The

working head r e q u i r e d for modularity under both full and minimum supply conditions should be derived f r o m the expression:

where h

i s the minimum working head corresponding to Qmi,

(wk)min

i s the depth of water in the supply canal

y1

Qmin i s the lowest l i m i t of discharge a t which the supply canal i s to be run. For

Qmin ---

=

0.55

Ql

Thus, an open flume outlet with i t s c r e s t a t 0 . 9 y not l e s s than 0 . 4 2 y

1

and having a working head of 1 will draw proportional discharge within the l i m i t s of 55% to

100% supply in the parent canal. Open flumes a r e recommended for u s e within 300 m (1,000 ft) u p s t r e a m of control points ( F i g u r e 5- 16), a t tail c l u s t e r s ( F i g u r e 5- 17) and where adjustable semi-orifice module outlets cannot be designed with their c r e s t a t o r n e a r bed level of the supply canal.

Where banks a r e wide, i t is used in

combination with a pipe outlet. 5.7.3.5

Tail c l u s t e r s When the discharge of a secondary, t e r t i a r y o r q u a r t e r n a r y canal

diminishes t o below 150 l / s , it i s d e s i r a b l e t o c o n s t r u c t a l l the t a i l outlets in the f o r m of a c l u s t e r f o r equal distribution of w a t e r .

F i g u r e 5-17 shows t h r e e

s t a n d a r d d e s i g n s f o r open flume outlets i n t a i l c l u s t e r s . The c r e s t s of the o u t e r f l u m e s i n a t a i l c l u s t e r (three-way and fourway) a r e s o m e t i m e s built 0.06 c m lower than t h e i n n e r flume.

To compensate

f o r velocity of approach the s t a n d a r d 30 c m (one ft) gauge should i n such c a s e s With t h i s a r r a n g e -

be fixed with i t s z e r o a t the c r e s t l e v e l of the i n n e r flume.

m e n t the widths of the flume above the full

supply level a r e m a d e proportional

t o t h e designed width of the flume below the full supply l e v e l i n o r d e r to d i s t r i b u t e benefits of any e x c e s s w a t e r reaching the t a i l of the canal proportionately t o a l l the outlets.

5.7.4

Design F o r m u l a The design f o r m u l a u s e d for a n open flume outlet i s :

where

Q

B(t) H(crt)

C

B(t) ( c m )

=

design d i s c h a r g e of the outlet i n 11s o r f t3, 1 s ;

=

width of the t h r o a t in c m o r f t ( t h r o a t width l e s s than 6 c m o r 0.20 ft m u s t not be adopted);

=

height of the designed full supply l e v e l in the supply canal above the c r e s t l e v e l of the outlet in c m o r ft; and

=

a coefficient having the following values f o r different widths of t h r o a t ( i n m e t r i c and B r i t i s h u n i t s ) .

C

B(t) (ft)

C

9.0

0.0160

0.20 to 0.29

2.90

9 . 1 $0 12.0

0.0163

0.30 to 0.39

2.95

Over

0.0166

0.40 and over

3.00

6

to

12.0

The m i n i m u m modular head adopted i s 0 . 2 n e c e s s a r y t o a s s u m e a p a r t i c u l a r value of B(t) o r H(,rt),

When designing, it i s then calculate the

other, and w o r k out the m i n i m u m modular h e a d , and then see i f the l a t t e r i s l e s s

I

FA0

-

ICID

1

To find B(t) o r H

than the available working head.

(crt)

f o r a given d i s c h a r g e

and a working head, F i g u r e 5- 18 m a y be used.

5.7.5

N u m e r i c a l Examples Example 1

-

Design an open flume outlet, with a d i s c h a r g e of 50 l i t r e s

p e r second, for a canal with a full supply depth of 100 cm.

The working head

available f o r the outlet i s 15 c m .

Since h

(minimum working head) for an open flume outlet i s (~k)min

0. 2 H( c r t ) , With

the m a x i m u m H

H(crt.

= 75 c m ,

(crt)

f o r the outlet can be 75 c m .

and Q = 50 l / s ,

the width B

(9

will b e 4 . 8 cm,

i. e . l e s s than 6 cm.

. .

Adopt the m i n i m u m value of B

Then 50

f o r which,

=

h(wk)min

0.016.

6 .

(t>

H

= 6 cm 2

(4

( r e f e r to f o r m u l a under 5. 7.4).

= 1 2 . 9 c m against 15 c m available.

suitable but setting of the outlet i s

64.7 100

=

The design i s

0.647.

This i s not too high but the outlet will not take i t s f a i r s h a r e of silt. Should i t be n e c e s s a r y t o conduct s i l t effectively i t i s possible, though costly, to build a combined pipe and open flume outlet.

(See Section 5. 10)

To s a v e m a t h e m a t i c a l calculation, r e f e r e n c e m a y be m a d e t o the d i a g r a m in F i g u r e 5- 18. B(,) i s l e s s than 6 c m . 64.73 cm.

It will be seen that f o r For B

(t>

H(,,.)

= 75 c m the value of

= 6 c m and Q = 50 1 / s , HtCrt) i s

Example 2

-

A s s u m e the outlet in Example 1 i s located just above o r within

a s h o r t distance of a fall o r d r o p in the canal which h a s a depth over c r e s t under Let the available working head of the outlet be

full supply condition of 50 c m . 70 c m .

Design a suitable outlet.

Since the outlet i s above o r close to a control point in the canal,

H(crt) of the This will open flume outlet should be the s a m e a s that of the fall, viz: 50 c m . e n s u r e proportional distribution. Thus

and Then

Q

=

50l/s

C

=

0.0160

=

8.84cm.

B(t)

This value of B(t) m a y a l s o be r e a d f r o m F i g u r e 5- 18. The minimum working head r e q u i r e d f o r this s i z e i s 10 c m , which i s much l e s s than the available working head. If proportionality i s not n e c e s s a r y , a m o r e rigid outlet can be obtained by designing an o r i f i c e semi-module. Example 3

-

Design a t h r e e way tail c l u s t e r open flume outlet in

accordance with the following data:

= 12 c m

Tail right outlet

:

Q = 50 l / s and h

Tail c e n t r e outlet

:

Q = 80 l / s and h (wk) = 30 c m

Tail left outlet

:

Q = 30 l / s and h (wk) =

(wk)

15 c m

The full supply depth of the distributing canal a t the tail i s 40 c m , and full supply depth i s 100 m. An examination of the available working heads of the t h r e e outlets shows

that if H

(crt)

be 30 c m , a l l the outlets will work modularly.

and

=

h(wk)min

Then f o r Tail r i g h t outlet Q

6 cm

3 2

=

C B ( t ) H(ct)

50

=

CB(t)

( C

=

0.0166)

Adopt c r e s t l e v e l

=

99.64

Tail c e n t r e outlet 80

=

CB(t)

C r e s t level

=

99.70 ( a x i s a s s u m e d p a r a l l e l to flow in supply canal)

or

.

3 30 2

-3

.

30

2

2

Tail left outlet 30

=

Adopt c r e s t level The s a m e values of B

(t)

99.64

can be found f o r the given data f r o m .

F i g u r e 5- 18. In c a s e of t h r e e and four-way c l u s t e r s ( F i g u r e 5-17) the c r e s t of the c e n t r a l outlet with i t s a x i s p a r a l l e l t o the flow i n the supply canal will be 0.060 m higher than the other two o r t h r e e side outlets. Example 4

-

Design an open flume outlet for proportional distribution in a

supply canal, the n o r m a l supply of which i s 5570 of the full supply. Data:

As h

(wk)

i s m o r e than 0 . 4 2 y

i. e . 5 0 . 4 c m , the open flume will d r a w

proportional d i s c h a r g e f r o m 55% to 100% of the supply in the m a i n canal.

H( crt > B(t)

Since B(tlmin = 6 cm, the value of R(crt)

is 60.3 cm.

The open flume outlet r e q u i r e s only small working heads.

It i s very

suifable for proportional distributors, and f o r outlets within 300 m of a control point, in tail r e a c h c l u s t e r s and even in head r e a c h e s of a supply canal when the working head available i s only small.

The s t r u c t u r e r e q u i r e s no manual control.

The water surface level in the supply canal above the c r e s t of the outlet i s r e a d and the corresponding discharge obtained.

A disadvantage i s that the outlet i s not provided with any gate arrangement and i t i s not possible to shut i t when the supply canal i s running. The open flume outlet h a s been developed for s y s t e m s in which distributing canals a r e run a t full supply level with little fluctuation of discharge in the supply canal.

These.systems a r e usually not equipped with check s t r u c t u r e s , so that

in the c a s e of low discharges, the outlets would not draw t h e i r design discharge. In many c a s e s the open flume outlet h a s to be either deep and n a r r o w (in which c a s e i t i s easily blocked) o r shallow and wide (in which c a s e i t i s hyperproportional and also f a i l s to draw i t s f a i r s h a r e of silt).

To overcome the

defect of high flexibility, a roof block i s fitted in the gullet of an open flume, a t the vena contracta, c l e a r of the water surface in the gullet when the outlet i s drawing i t s full supply discharge. This outlet can be tampered with by placing a thin wooden plank, fitting the throat, half way between the c r e s t and the water level.

This i n c r e a s e s the

discharge if the outlet i s working a s a f r e e fall one by about 16%.

5.8

AD JUSTABLE ORIFICE SEMI-MODULE (INDIA AND PAKISTAN)

5.8.1

I'

Gen e r a1 Adjustable orifice semi-module outlets a r e widely used in Punjab and Haryana and other p a r t s of India and in Pakistan. Thqre a r e various f o r m s of these outlets but the e a r l i e s t of therri i s the one

.

introduced by E S. Crurnp in 1922 and called the "Adjustable Proportional Module" (APM).

Crumpl s design aimed at fixing the c r e s t at a setting of 0 . 6 of the supply

depth in the parent canal, which ensured exact proportionality.

However,

according to past experience in Punjab with this APM canals fitted with i t silted up The problem was that i t could not draw i t s fair s h a r e of silt, an essential

badly.

requirement for those Punjab irrigation systems which draw their supplies f r o m rivers.

Thus the APM h a s now been replaced in India and Pakistan by the

Adjustable Orifice Semi-Module (AOSM) which i s neither proportional nor fully modular, but e n s u r e s f a i r distribution of silt.

(See Figure 5-19).

Structural and Design Characteristics Structurally, the AOSM ( F i g u r e 5-20) m a y be regarded a s a long throated flume with a roof block capable of vertical adjustment in the u p s t r e a m end of the parallel throat,

It differs from the open flume outlet a s r e g a r d s the length of

the throat and a l s o in that the upstream water level in the supply canal i s m o r e than one third 5.8.2.1

above the bottom of the roof block. Upstream and downstream approaches The u p s t r e a m face wall o r u p s t r e a m wing wall on th,e supply canal i s

curved and flared, the curvature ending 7 . 5 cm upstream of the s t a r t of the c r e s t . The downstream face wall, o r downstream wing wall on the supply canal, i s s e t forward inside the canal by a distance, which i s generally equal to:

L'

Based on a note prepared by A. D. Choudhry, Chief Engineer, Irrigation Works, Haryana, and K. C. Gupta, Executive Engineer, Central,Designs, Irrigation Works, Haryana.

where Ql,

Q2

=

B1 and y

d i s c h a r g e of outlet, 1

a r e respectively the discharge, bed width and depth of the

supply canal section just u p s t r e a m of the outlet.

Setting forward should only be

done when the bed width of the canal i s reduced below the outlet, keeping the downstream wing wall a t the downstream end and the u p s t r e a m wing wall a t the u p s t r e a m toe slope.

II G U ~ Z 5 - 19. - General view of an AOSM outlet t o a f a r m w a t e r c o u r s e (Haryana, India).

The length of the p a r a l l e l t h r o a t i s 60 c m ( 2 ft) f o r a l l c a s e s .

There

i s no horizontal c r e s t portion of the t h r o a t and a g l a c i s sloping a t 1 in 15 s t a r t s right f r o m the u p s t r e a m end of the p a r a l l e l s i d e s of the throat.

l r

pr4/

+23

Front elevation r B

Bar No.l

Bar 3 mm

I I ! Bar No.2 -&,I+

7

4

Cross section on 0 - 8

Detoils of precast R.C. Roof

S W.G. (Standard wire gauge1

Fixing of precast R C . Roof block

G x Lonqitudinol section through outlet

I

F A 0 ADJUSTABLE

Note

AN dimensions ore

III

I C I D

I

ORIFICE

SEMI - MODULE

centfmefres

DETAILS AND FIXING O F ROOF BLOCK

r I

-

-.

P r o j e c t , R e g i o n , .Country lndio and Pokiston Figure No. 5-21

I I

Roof block The roof block m a y be of c a s t i r o n but i t i s now generally of reinforced cement ( s e e F i g u r e s 5-20 and 5-21).

The face of the roof block i s

s e t 5 c m f r o m the starting point of the p a r a l l e l throat.

It has a l a m n i s c a t e curve

a t the bottom with a tilt of 1 in 7 . 5 in o r d e r t o converge the water instead of a horizontal b a s e which would diverge i t .

The c a s t i r o n roof block i s 30 cm thick.

The p a r a l l e l t h r o a t h a s a c a s t i r o n bed and check plates. 5.8.2. 3

Side walls The side walls downstream of the throat a r e given a splay of 1 in 10,

i. e . up to 150 c m ( 5 ft), a f t e r which they a r e straight up to a length depending on

the bank width ( ~ i ~ u 5-20) r e 5.8.2.4

Susceptibility to tampering The s t r u c t u r e of the outlet i s v e r y strong and h a s a long serviceable

life.

However, c a s e s of tampering with the outlet a r e not infrequent.

The roof

block i s s o m e t i m e s r a i s e d bodily and refixed but the tampering i s easily detected.

A wooden plank i s s o m e t i m e s i n s e r t e d a t the downstream side of the roof block and covered with e a r t h and g r a s s , thus forming an air-tight roof in continuation of the roof block.

This i n c r e a s e s the discharge due to i m p e r f e c t aeration of the

jet.

Hydraulic P r o p e r t i e s Flexibility This type of outlet i s instantaneously proportional when the bottom of the roof block i s a t 0 . 3 of the full supply depth of the supply canal. in the full supply level the flexibility, which i s equal to

2

With a r i s e

,

i s reduced

10H(crt) and the outlet becomes sub-proportional.

Similarly with a fall in the full supply

level the flexibility i s i n c r e a s e d and the outlet becomes hyper - proportional. When the outlet i s s e t n e a r bed level, with any r i s e in the full supply level, the value of

3 H

f a l l s and the outlet tends t o move f u r t h e r f r o m

(4

proportionality in the direction of rigidity.

A fall in the full supply level

similarly i n c r e a s e s the flexibility and the outlet moves towards proportionality. With the outlet s e t a t bed level, the flexibility remains constant at 0. 3 .

This outlet draws a t bed level about 1470 and below bed level at 12/ 10th setting) about 29% m o r e silt than i t would draw a t 6110th setting when i t i s proportional. 5.8.3.3

Adjustability The outlet i s easily adjustable, a t a small cost, either by raising o r

lowering the roof block o r by dismantling one side wall.

5.8. 3 . 4

Range of operation The outlet can work semi-modularly for all heads and.with all working

heads above the minimum modular head and for all discharges f r o m 28 l / s to 150 11s ( 1 f t 3 / s to 5 ft 3/ s ) - i. e . the discharge range generally required for outlets. 5.8.3.5

Suitability This outlet i s eminently suitable in head reaches of distributing

canals.

In the c a s e of distributing canals carrying silt, setting a t bed levels i s

considered the best for silt-conduction into the outlet. canals receiving water f r e e of silt, a setting a t 0. 6 y ality

.

1

In the c a s e of distributing i s the b e s t for proportion-

The outlet can also be designed to draw proportional discharge f r o m a distributing canal in which the discharge fluctuates from a certain minimum to full supply discharge. Design Formula

where Q2 o r

(According to Grump)

Q

=

the discharge of the outlet in 11s

C

=

0.0403

B

(t)

H(orf)

H( sof)

=

width of the throat in cm

=

height of the orifice in c m

=

the depression head o r height of the full supply level in the supply canal above the bottom of the roof block = H ( 4- H( o r f )

Also, minimum modular head = h

(wk)min

=

0.82 H(

sof)

-

0.5 B ( q

It should be noted that recent r e s e a r c h c a r r i e d out on the adjustable orifice semi-module in Pakistan h a s shown that the coefficient of discharge v a r i e s with the throat width of the structure, i. e. with the ratios

y1 -

and

Y1 .

H(crt)

B(t)

Consequently the s t r u c t u r a l shape of the outlet differs slightly f r o m the design F o r further information r e f e r e n c e may be made to the

presented h e r e .

proceedings of the 7th NESA Irrigation P r a c t i c e s Seminar, Lahore, Pakistan,

To design an orifice semi-module, i t i s n e c e s s a r y f i r s t to m a k e a t r i a l

H

(orf)

a r e f i r s t assumed; the value of (crt) H(orf) h a s to be l e s s than half H a r e then calculated.

Suitable value of B(t) and H

calculation. and h

(wkImin

to ensure the orifice flowing full.

(4

If h(wk)min, a s calculated, i s l e s s than the

available working head, the p r o c e s s h a s to be repeated with modified values of H( c r t ) Or B(t)' To save this "trial and e r r o r " arithmetical work, diagrams have been prepared ( F i g u r e s 5-22 to 5-26) in which the relationships between outlet discharges and minimum working heads for different values of H( c r t ) and H(orf) a r e shown.

There a r e s e p a r a t e d i a g r a m s for B(t) = 6 cm, 7.5 cm,

9 cm, 1 2 cm and 15 cm, which a r e the m o s t commonly adopted. 5.8.5

Numerical Examples Example 1 Design an orifice semi-module having a discharge of 60 l / s on a canal with full supply depth of 90 cm. Given Data

Q

=

60 l / s

y1

=

90cm

Available working head i s 54 c m .

If i t i s d e s i r e d that the o r i f i c e semi-module should be proportional, then H(crt) = 0 . 6 ~= ~ 0 . 6

.

90

=

54 cm.

An examination of the c u r v e s of F i g u r e 5-22 shows that for B(t) = 6 c m and Q = 60 l / s ,

Similarly with B(t) = 7 . 5

H(crt) m u s t be m o r e than 54 c m .

c m and B(t) = 9 . 0 c m vide F i g u r e s 5-23 and 5-24. ( F i g u r e 5-25) the intersection of H = 21 c m ,

H(,,f)

(4=

F o r B(t) = '12 c m

54 c m and Q = 60 l / s gives

H(sof)l = 5 4 - 21 = 3 3 c m , and h(wk)min

= 20cm.

The setting being 0 . 6 y l , the outlet i s l i k e l y t o draw i t s fair share of silt.

F o r canals c a r r y i n g c l e a r water the setting a t 0.6 y

1

i s ideal f o r

proportionality. F o r rigidity and b e t t e r s i l t drawing capacity, the outlet should be s e t a t bed level, viz: H(crt) = 90 c m . F r o m F i g u r e 5-25, with B (t) = 12 c m for a discharge of 60 l / s,

H(orf) should be 14.2 c m , but the minimum modular head i s

a little m o r e than the available working head of 54 cm. I t i s , t h e r e f o r e , n e c e s s a r y t o reduce H( s o f ) l . 5-23 shows that with B(t) = 7 . 5 c m and H

( 4=

60 l / s ,

H(orf) should be 24.5 c m and h

(wk)min

An inspection of F i g u r e

90 c m f o r a d i s c h a r g e of

= 50 c m which i s l e s s than the

available working head. Another suitable s i z e i s H( c r t ) = 90 c m , B(t) = 6 c m ( F i g u r e 5- 22) with h(wk)min

H(,,*)

= 3 3 . 0 c m and

= 45 c m .

Should the working head be v e r y low and no suitable s i z e of o r i f i c e s e m i module with a reasonably deep setting can be designed, it would be n e c e s s a r y to r e s o r t to another type of outlet such a s a combined pipe and semi-module. Example 2 An o r i f i c e semi-module i s found t o be working non-modularly during an inspection of the outlet. found to be 30 c m .

The actual working head on m e a s u r e m e n t i s

How can the outlet be adjusted to give i t s design d i s c h a r g e ?

Given Data Q = 60 l / s ,

H(crt) = 90 cm,

H(orf) = 29.5 c m and B(t) = 6 cm.

Figure 5- 22 shows that for the given data, only 30 cm actually available.

h(wk)min

should bl 47 c m against

The f i r s t immediate remedy i s to c l e a r s i l t from

the watercourse if possible, and to i n c r e a s e the available head to 47 cm.

If

this i s not possible, then the roof block may be r a i s e d so that the orifice working under submerged conditions gives the requisite discharge, which can be calculated roughly by adopting C = 0. 0354 in the formula:

If neither of the above two alternatives a r e possible, i t i s essential to provide a temporary outlet to supplement the discharge of the existing outlet. The final remedy l i e s in redesigning the outlet with H(crt) = 75 cm and H(,,f)

= 36.5 c m which gives h(wk)min = 29.0 cm.

This would involve

raising the c r e s t of the outlet by dismantling one side wall.

Summary The outlet i s easily adjustable a t a nominal cost.

It i s generally immune

to tampering due to i t s c a s t iron o r reinforced concrete roof block. The discharge of the outlet i s independent of the water level in the watercourse provided a standing wave f o r m s . The outlet i s eminently suitable (provided a minimum working head i s available) in head reaches of distributing canals, for drawing i t s fair share of silt. It i s automatic in operation.

The gauge a t the outlet i s r e a d once a month

during routine inspection by the canal supervisor. This outlet requires m o r e working head than the open flume outlet and i s not suitable for use a t t a i l s o r immediately upstream of control points.

5.9

JAMRAO TYPE ORIFICE SEMI-MODULE (SIND, PAKISTAN)

Gener a1 The J a m r a o Type Orifice Semi-Module ( F i g u r e 5-27) h a s been widely u s e d on t h e J a m r a o Canal in Sind in Pakistan. It h a s proved to be

The outlet was evolved in Sind by Kirkpatrick in 1925.

a successful m a s o n r y replacement f o r the Kennedy Gauge Outlet.

(See 5.1.2. )

Structural C h a r a c t e r i s t i c s The u p s t r e a m approach of the outlet i s only 0 . 6 m (2 ft) long and i s shaped like a truncated square pyramid with a convergence of 1 to 4.

The control i s a

The downstream flume, 3.0 m (10 ft) long, h a s a horizontal floor with the side walls a t a splay of B where B(t) i s 300 The horizontal floor i s then sloped down to m e e t the bed of the waterin c m . square orifice in an angle-iron f r a m e .

4

course. Kirkpatrick h a s stated that the coefficient of discharge for this converging orifice i s a s nearly constant a s that of the elongated bell-mouth and that h i s design gives the best r e s u l t s a s r e g a r d s the maximum recovery of head. The m o s t essential feature of this semi-module i s the introduction of baffles i n the downstream f l u a , which work a s a roof sloping gradually upwards, and through their presence recover considerable head. discharges under f r e e atmospheric conditions. (i. e. lower edge of the baffles) i s 1 in 15.

At the s a m e t i m e the outlet

The optimum slope for the roof

The optimum number of baffles i s

t

nine, of which the f i r s t six a r e equally spaced, and the l a s t t h r e e somewhat spread out.

To protect the baffles and the angle-iron f r a m e f r o m outside

interference, an expanded m e t a l sheet i s fixed a s shown in F i g u r e 5-27. The outlet works proportionately when the centre of the orifice below the full supply level,

H(c,t)

, i s a t 0. 3 of the depth in the parent canal.

The outlet i s very durable and h a s a long serviceable life.

It i s immune

f r o m tampering because of the expanded metal protection and angle-iron f r a m e of

the controlling section.

It i s automatic in operation and there a r e no mainten-

ance problems.

Longitudinal secrion

FIGURE 5-27.

5.9.3

-

J a m r a o type orifice semi-module.

Hydraulic Characteristics This orifice type module with

H(cnt) l e s s than 0.45 m (1.5 ft) i s

susceptible to upstream variation but a s i t works proportionately at 0 . 3 y

of

the full supply level, i t can be used on canals with depths g r e a t e r than 1.37 m

(4.5 ft) so a s to give H(cnt) of 0.45 m ( 1 , 5 ft) a t least. This module r e q u i r e s m o r e working head than an open flume type for the s a m e H(crt) a s H

( cnt)

in this device.

a g r e a t e r working head i s available.

Consequently i t can be employed where This generally happens a t heads of l a r g e

distributaries, and so these semi-modules a r e used in head reaches of l a r g e canals. The baffles introduced in this type of device help recovery of considerable head.

F o r example, when the centre of the orifice below full supply level i s

0.90 m the working head without baffles i s 0.52 m , but with 9 baffles for a 12 c m square orifice the working head i s only 0.27 m .

This reduction i s due to the

fact that water between the baffles automatically applies the c o r r e c t p r e s s u r e to secure a roof which suppresses the standing wave. The capacity to draw silt will depend on the setting, and in this r e s p e c t i t may be considered similar to the Adjustable Orifice Semi-Module except that the upstream approaches of the AOSM a r e likely to be m o r e conducive to silt induction than the truncated square pyramid set back in the bank.

5.9.4

Design Formula The discharge formula applicable to the J a m r a o Type Orifice Module was determined experimentally and i s

Numerical Example Design a J a m r a o Type Orifice Semi-Module with a discharge of 60 l / s , on The available working head i s 20 cm.

a canal with a full supply depth of 200 cm.

F o r proportionality H

(4

=

0.3

.

200

'

=

60 cm.

By t r i a l and e r r o r L ' i t i s found that for H = 60 cm, and an orifice of (cnt) 14 c m x 14 cm, the discharge i s 60. 37 l / s , which i s satisfactory.

h(wk)

h(wk)

. .

5.10.1

for 6 baffles

=

! f d -

-

60 3.8

for 9 baffles

--

% 4.55

=

13cm

3.8

-

16cm

u s e six baffles.

General The Pipe Semi-Module i s widely used in Haryana and Punjab (India) and in Pakistan.

The outlet may be regarded a s a development of the Stoddard-

Harvey improved irrigation outlet.

This outlet i s eminently suitable when the

supply canal h a s wide banks and/or i s in high filling because an open flume o r an orifice semi-module built in such a bank would be much m o r e expensive. This type of device i s a l s o used in lined canals.

F u r t h e r , the outlet i s suitable

for drawing i t s share of silt when i t i s not possible to achieve a deep setting a s required by an open flume o r an orifice semi-module.

The lead-in pipe i s s e t

a t o r n e a r the bed level and it opens into a tank on the downstream side to which an open flume o r an orifice semi-module (Punjab o r J a m r a o type) o r a Scratchley outlet (with f r e e flow conditions) i s fitted. 5.10.2

Structural Characteristics The outlet (Figure 5-28) consists of a lead-in pipe from the supply canal which discharges into a tank on the outer side of the bank of the supply canal. The upstream end of the lead-in pipe can be placed a t any suitable level in the supply canal depending upon the desired silt-draw.

The downstream end of the

L ' ~ a b l e s a r e available in (105) and other references. ? / ~ a s e don a contribution by A . D . Chaudhry, Chief Engineer Irrigation Works, Haryana, and K. C. Gupta, Executive Engineer Central Designs, Irrigation Works, Haryana (India).

Pipe -cum

- Open

Flume

Bank

Pipe-cum- A . O . S . M .

Pipe

- cum - Jomroo

Type Orifice - Semi - Module

PIPE SEMI- MODULE FOUR EXAMPLES

Pipe -cum

-

Free Fall Scrotchley Outlet

lead-in pipe m a y be horizontal o r given an upward slope of about 1 to 12 to reduce the depth of the tank.

The tank is'approximately 60 c m square for a

30 11s discharge, 80 cm square for 60 11s and 100 c m square for 90 11s discharge.

In the downstream wall of the tank a semi-module i s fitted, four examples of which a r e shown in F i g u r e 5-28.

5.10.3

Hydraulic C h a r a c t e r i s t i c s Flexibility The flexibility of this outlet depends on the flexibility of the semimodule incorporated with i t except that i t s flexibility will be modified slightly by the action of the pipe between the supply canal and the tank.

The flexibility may

be expressed thus: P i p e with open flume

1 ° h 3

P i p e with Jamrao-type orifice semi-module

(4

+ lo 9

H

(crt)

YI

>

Pipe with adjustable orifice semi-module

In actual practice, to obtain maximum possible rigidity, a pipe with an open flume i s used when the working head available i s smal1,and a pipe with an orifice semi-module when the working head i s somewhat g r e a t e r . 5.10.3.2

Efficiency The efficiency of this outlet depends on the type of semi-module fixed

to i t and on the l o s s of working head through the pipe.

As the amount of silt

induction into the outlet depends only on the position of the pipe with r e s p e c t to the bed, the c r e s t of the semi-module can be placed a t any level so that the l o s s in head through the pipe can be m o r e than compensated for by a higher setting of the outlet.

The position of the pipe does not affect either the discharge o r the

proportionality and the pipe can be r a i s e d o r lowered depending on the silt-draw requirements .

5.10.4

Design Formula The size of the lead-in pipe i s fixed so a s to achieve a minimum l o s s of head, subject of course to obtaining sufficient velocity in the pipe t~ convey silt from the supply canal. Various sizes of lead-in pipe for various discharges a r e given below.

Pipe size

Discharge (11s)

Width (cm)

Height (cm)

Let h ( o be %as of head Then h(&) (or

'6(wk)l)

=

926

Q2 A~ X

where

A

X

Q

=

cross section of the pipe in cm 2

=

discharge in 11s.

F r o m this the water level in the tank can be determined a s the FSL in the supply canal minus the l o s s of head through the pipe.

The semi-modular outlet

(open flume, orifice semi-module, Scratchley etc.) can now be designed, the type depending on the head available and other conditions.

5.10.5

Numerical Example Design a pipe semi-module for the following data:

Assume any standard size of pipe, say 30 cm diameter.

Then

. . If h

h

=

(wk)1

926

Q~ -

2 *x

.

926

=

42'

.

0.7854'

=

3 cm

900'

The working head l e f t for the semi-module i s 17 c m . (wk)2

85 c m .

= 17 c m ,

) for (4

the maximum head (H

F o r Q = 42 1 / s ,

= 6 cm,

the open flume outlet can be

H(crt) = 57 cm;

h(wk)min f o r t h i s

s i z e i s 12 c m against 17 c m available. In the above example, instead of an open flume, an o r i f i c e semi-module could a l s o be designed with the following dimensions:

J a m r a o type o r i f i c e semi-module

AOSM

5.10.6

Summary The o u t l e t i s v e r y durable a n d h a s a l o n g s e r v i c e a b l e l i f e .

Ithas ahigh

degree of immunity f r o m i n t e r f e r e n c e due t o the certainty of e a r l y detection, and it h a s a wide range of modularity.

The working head r e q u i r e d i s low,

particularly with an open flume outlet attached.

It i s l e s s costly than the open

flume o r o r i f i c e semi-module outlets built in the bank of wide supply canals. It can be conveniently adjusted while the supply canal i s running.

This type of

outlet can be used with advantage if it i s r e q u i r e d t o work under conditions of high supply.

The pipe can be placed with i t s sill above the low supply level and

since the head o v e r the c r e s t i s m e a s u r e d in the tank, the design of the outlet i s simple.

No water will e n t e r the tank until the water level in the supply canal

r i s e s above the low supply level.

5.11

FAYOUM STANDARD WEIR

5.11.1

FARM OUTLET (ARAB REPUBLIC OF EGYPT)

1-I

General The Fayourn Standard Weir F a r m Outlet d e l i v e r s water f r o m distributing i r r i g a t i o n canals to an i r r i g a t o r o r a group of i r r i g a t o r s .

It i s a simple a c c u r a t e

device f o r both m e a s u r i n g and controlling water and i s widely u s e d in the Fayoum province of the A r a b Republic of Egypt, where the slope of the i r r i g a t e d a r e a i s s t e e p enough to p e r m i t the u s e of weir outlets. I t can be used e i t h e r a s a c l e a r overfall weir: 21 o r a s a submerged weir-3 / The i r r i g a t i o n s y s t e m inithe Fayoum

.

p r o v i n c e i s of g r e a t antiquity and the w e i r s on it a r e , a s originally constructed, of rough m a s o n r y ; they have now reached a m o r e o r l e s s standard section a s described in 5. 11. 2.

.

Much r e s e a r c h work h a s been c a r r i e d out in the past on this w e i r , taking into consideration the following p a r a m e t e r s :

-

width of w e i r ,

-

height of w e i r ,

H(bmc)

-

length of c r e s t ,

L(crt)

-

upstream corners

-

velocity of approach,

-

aeration.

B(t)

v

(~ P P )

F o r example, 500 experiments w e r e c a r r i e d out on the weir in the Delta B a r r a g e experimental tank by AD. Butcher in 1920 and m o r e investigations have been m a d e subsequently on a model a t the Hydraulic R e s e a r c h and E x p e r i m e n t

L/

B a s e d on information supplied by A. A. Eldarwish, Inspector General of I r r i g a t i o n & D i r e c t o r General Hydraulic R e s e a r c h & Experiment Station, Delta B a r r a g e , and M. Kotb Nadar, Deputy ~ i r e c t o r ,HRES A r a b Republic of Egypt.

.2/ A weir

i s described a s a c l e a r overfall when the downstream w a t e r level i s below the c r e s t of the w e i r .

A weir i s d e s c r i b e d a s submerged when the downstream water level is above the c r e s t of the weir.

Station in recent years. has been deduced.

As a result of these experiments, the discharge formula

It has been found that the average variation of an individual

experiment from the corresponding deduced formula i s below three per cent. practical purposes, this deviation i s acceptable.

For

Further, the experimental

results have led to the following conclusions:

-

by correctly adjusting the radius of the corners to the width of the weir a s e r i e s of weirs of widths from 0.01 m upwards may be constructed, for which the discharge per m e t r e width i s the same for all widths.

-

the discharge can be determined accurately from the upstream gauge with a probable e r r o r of l e s s than bne per cent.

-

the velocity of approach may be neglected in practice. the downstream conditions (wing walls, etc.) do not affect discharge, provided that the downstream slope of the weir i s about half to one.

5.11.2

Structural Design The standard Fayoum type weir (Figure 5-29) i s essentially a very simple construction of rough masonry.

It consists of a masonry wall 0.5 m thick a t the

c r e s t , vertical on the upstream face, and with a 1: 0.5 slope on the downstream side.

The c r e s t i s usually of dressed stone and the width of the weir i s defined

by blocks of masonry on the c r e s t itself. with a smooth surface a r e used.

Sometimes pre-cast concrete blocks

In this way the width of the weir can be

adjusted to a very high degree of accuracy.

The standard height has been

adopted a t 0.65 m which, with a full supply depth 0.5 m dver the weir, will give a depth of 1.15 m in the canal,. which i s believed to be about the mean depth of Fayoum small canals.

Variations from this depth will inevitably occur, but

this will not result in e r r o r s of more than 2%. The total discharge of any outlet i s usually known from the cultivated a r e a i t serves and the crop water duty.

According to the depth of water in the canal

and land levels, the water head (over the c r e s t level) can be easily determined. As the water head i s fixed and the discharge i s known, the width of the weir (i. e. 4

the defined width between the blocks) can be calculated either from the formulae

( (1) or (2) ) given under 5. 11.4, or i t can be taken directly from Table 5-1.

Section A-A

-

FIGURE 5-29. Fayoum standard weir f a r m outlet; general structural design.

The field outlet weirs feeding several f a r m ditches a r e usually situated in groups, and called "nasbas" (Figure 5-30); the crest' of all the w e i r s in the group a r e a t the same level and the width of each weir i s varied to give the required discharge. The flowing water, after passing the weir c r e s t , i s conveyed t o the irrigated land through open field laterals.

If t h e r e a r e banks, the flow may

f i r s t pass through a culvert, such a s a pipe ok a brickwork arch.

5. 11.3

Hydraulic Characteristics

. The structure can be used either a s a controlling device o r for water distribution.

Section A - A

Plan

FIGURE 5 - 3 0 . - Fayoum standard weir farm outlet with a group 6f field outlet weirs or "nasbas".

It has been demonstrated that the discharge per rnetre width can be the same for all widths of weir provided the radius of the upstream corner i s correctly adjusted to the weir width.

Weirs so designed and with their c r e s t s

0 . 6 5 m above the upstream bed a r e r e f e r r e d to a s standard weirs.

The dis-

charges resulting from various depths of water over the weir a r e shown in Table 5-1.

Dischorge I per cent obove rtondord

-----*-/c--Discharge stondord7

4~~

I

/

'

-/-t----1-

I

/ ~ ~ i s c h o r g eI per cent below standard

0

0-5

1.0 1- 5 Width of the weir in rn

2-5

2.0

3.0

FIGURE 5-31. - Fayourn standard weir f a r m outlet. Relation of upstream corners to width of weir.

The correct relation of the radius of the upstream c o r n e r s to the width of the weir i s shown in Figure 5-31.

It can be seen from Figure 5-31 that the

radius of the corners necessary to give standard discharge d e c r e a s e s with the width of the weir until a width of about 0 . 2 m i s reached. radius of the c o r n e r s again i n c r e a s e s rapidly.

Thereafter, the

This increase in the radius for

emall widthe i s probably , h e to the increasing action of friction on the sides of

the w e i r .

This friction b e c o m e s relatively m o r e i m p o r t a n t when the w e i r i s u

n a r r o w , and i t i s probable that below a width of 0 . 2 m the d i s c h a r g e depends l a r g e l y on the d e g r e e of roughness of the m a t e r i a l used.

Above a width of 3 m

the action of the c o r n e r s will b e relatively s m a l l and a r a d i u s of 0.25 m will probably be c o r r e c t f o r a l l w i d e r w e i r s .

This r a d i u s i s t h a t a d o p t e d f o r a l l

l a r g e w e i r s i n Fayourn. 5. 1 1 . 3 . 1

Accuracy The Fayoum w e i r type i s considered t o be a v e r y a c c u r a t e field

outlet. 5. 1 1 . 3 . 2

Silting A s the standard w e i r height i s 0. 65 m above the bed level, i t i s

possible that silting m a y o c c u r .

However, because a l l c r e s t l e v e l s a r e the s a m e

in the "nasba", the d i s c h a r g e passing through each outlet will not be affected a t all .

5. 1 1 . 3 . 3

Range of operation The Fayoum weir can work for a l l heads s t a r t i n g f r o m 0 . 0 1 m to

1.00 m , and for a l l widths f r o m 0.01 m to 10 m .

Design F o r m u l a e 5. 1 1 . 4 . 1

C l e a r overfall weir In general, the method used to d e r i v e the standard f o r m u l a i s t o plot

the logarithm of the d i s c h a r g e against the logarithm of the depth of w a t e r on the w e i r f o r the standard types of w e i r and t o d r a w a m e a n c u r v e through the resulting points. This c u r v e i s found to be b e s t p r e s e n t e d by a s t r a i g h t line between H ( c r t ) = 0 m a n d H(

4

H(,,.)

= 0. 14 m and a slightly curved line above

0 . 1 4 m , the corresponding f o r m u l a being : From H

(crt)

= 0 m to 0 . 1 4 m I

From H

(crt)

= 0.14 t o 1 . 0 0 m

q = 1.956H

(

4

+

0.014

(2)

=

w h e r e H(crt) 9

=

depth of u p s t r e a m water l e v e l on the w e i r , and

=

discharge p e r

m3/s per metre width.

The two f o r m u l a e o v e r l a p i n the neighbourhood o ~ . H ( , , ~ ) = 0.14 m and the f i g u r e s in Table 5- 1 a r e calculated d i r e c t f r o m t h e m . 5. 1 1 . 4 . 2

Submerged w e i r (i) D e p r e s s i o n head.

The l e v e l of the w a t e r u p s t r e a m of the w e i r

m e a s u r e d f r o m the weir c r e s t l e v e l i s called the d e p r e s s i o n head (symbol H(,rt)).

The t e r m i s usually applied only to a weir working a s a c l e a r overfall

and f o r any p a r t i c u l a r width of w e i r the d i s c h a r g e Q depends only on H(crt), but i t h a s been u s e d h e r e a l s o for a submerged w e i r to bring out the equivalent head f o r a s u b m e r g e d w e i r f o r different d e g r e e s of submergence in t e r m s of H(crt). (ii) Depth of s u b m e r g e n c e . When a weir i s s u b m e r g e d the depth of

w a t e r on the u p s t r e a m side i s r e f e r r e d to simply a s the " u p s t r e a m depth" (symbol H(crt)) and the depth of w a t e r on the downstream side, m e a s u r e d beyond the region of disturbance, a s the "depth of submergence", (symbol H(,)). (iii) P e r c e n t a g e submergence.

It i s often convenient t o e x p r e s s the

depth of submergence 'H(,)' a s a percentage of the u p s t r e a m depth 'H(crt)' and t o call i t briefly the percentage submergence 'r(,)'. Studies on 1 : 12.5 s c a l e m o d e l s f o r submerged conditions have been conducted a t the Delta B a r r a g e .

The w e i r s w e r e a l l of standard shape without

wing walls but v a r i e d in width f r o m 0. 20 m (2.50 m ) to 0. 01 m (0. 125 m ) and w e r e t e s t e d a t a l l depths f r o m 0.024 m ( 0 . 3 0 m). to 0.08 m ( 1 .OO m).-1/ In p r a c t i c e , two c a s e s of the u s e of submerged w e i r s m u s t be distinguished: ( a ) a submerged w e i r fed f r o m a l a r g e pool (unaffected by changes i n the d i s c h a r g e o v e r the w e i r ) ; (

(b) a s u b m e r g e d w e i r s e t in a canal ( s o that the whole d i s c h a r g e h a s to p a s s o v e r the weir).

The f i g u r e s i n b r a c k e t s would be the corresponding full- s i z e dimensions. ?

0

0.10

0.20

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 H/,I in m

FA0

-

1.10

lClD

THE FAYOUM STANDARD WEIR FARM OUTLET RELATIONSHIP BETWEEN

ys,FOR,ALL VALUES

H,CrIl AND

OF &r&qr

OR OF Q

Project, Region, Country Fayourn Province, A.R. E. Figure No. 5-32 b

TABLE

5-1

Discharge of Standard Weirs per Metre Width 1.54 H(crt) = 0.00 to 0. 14 m. Discharge = 1.652 H(4

H(crt) = 0.14 to 1.00 m . p p

(

4

+ 0.014

--

H(c,t) Discharge m

Discharge = 1.9555 H

m3/ s

H(c,t) Discharge m

m3/ s

H(,,.) m

Discharge 3 m /S

H(c,t) m

Discharge m3/ s

In c a s e ( a ) the effect of gradually submerging a clear overfall weir will be to momentarily reduce discharge: but ultimately to r a i s e the upstream level, the discharge returning to i t s original value.

In c a s e (b) the effect of submergence will be to permanently reduce discharge, the upstream level remaining constant.

In o r d e r to ascertain what

reduction in the discharge of a clear overfall weir will be caused by a definite degree of submergence r e f e r e n c e may be made to Figure 5-32 o r to Table 5-2. TABLE

Depth a s clear overfall

5-2

Reduction in discharge 5%

10%

15%

20%

Percentage submergence causing reduction in discharge

Numerical Examples Example 1

-

clear overfall weir

Design a field outlet (clear overfall weir) with a discharge of 0.25 3 The available working head of m /s on a canal with a full supply depth of 90 cm. the outlet i s 25 cm. F o r m u l a (2) of section 5.11.4 i s applicable

=

0.194 m 3/ s p e r m e t r e width

F r o m Table 5-1,

q for H

(4

A s the total d i s c h a r g e the b r e a d t h of the weir should be

Example 2

-

=

0.25

=

0.1941 m 3 / s p e r m e t r e width

=

0.25 m 3 /s

=

0. 25 0.194

can be r e a d off

submerged w e i r

( a ) What will b e the r e q u i r e d width of a submerged w e i r c a r r y i n g a d i s c h a r g e of 4 . 0 0 m 3 / s having u p s t r e a m and downstream w a t e r depth of 0 . 9 3 m and 0.73 m r e s p e c t i v e l y ? F r o m F i g u r e 5-32 the d e p r e s s i o n head H

CJL

(crt)eqv

can be d e t e r m i n e d

f o r the u p s t r e a m and downstream water depths. F o r H(

)

H(crt)eqv

=

0.93 m and H(,)

=

0.80m

=

0.73 m

(Figure5-32)

T h i s gives a n actual d i s c h a r g e of 1. 346 m 3 / s p e r m e t r e width. A s the total d i s c h a r g e

=

4.00 m 3 / s

the width of this submerged w e i r

-

4.00 1.346

(b) What will b e the reduction i n the d i s c h a r g e of a w e i r 1.00 m wide having an u p s t r e a m head of 0.50 m , if i t i s submerged t o a depth of 0.40 m ? F r o m F i g u r e 5-32, a s

H(,,.) =

0 . 5 m and H(s) = 0.4 m ,

H(crt)eqv = 0.45 which gives a d i s c h a r g e of 0.509 m 3 / s p e r m e t r e width. F o r a c l e a r o v e r flow w e i r of H ( ( T a b l e 5-1)

=

)

= 0 . 5 m , the d i s c h a r g e

0. 608 m 3 / s p e r m e t r e width.

The reduction percentage i n d i s c h a r g e = =

16.3 p e r cent.

The s a m e reduction percentage can be taken for H(crt)eq, Table 5-2

a s 1 6 . 3 p e r cent.

= 0.45 m from

( c ) A weir of 1.00 m width with an u p s t r e a m depth of 0.60 m i s What will be the new weir.width if it i-s s u b m e r g e d with H(s) = 0 . 4 6 m so a s to give the s a m e d i s c h a r g e ?

working f r e e l y .

A c l e a r overfall w e i r of 1.00 m width under a head of 0.60 m gives a d i s c h a r g e (Q) of 0.826 m 3 / s .

A submerged w e i r of 1.00 m width having H

(4=

0.60 m and

H S = 0 . 4 6 m gives a d i s c h a r g e of 0.705 d / s ( f r o m F i g u r e 5-32). To keep the d i s c h a r g e the s a m e a s the c l e a r overfall weir (0.826 m 3 / s ) t h e submerged w e i r should be i n c r e a s e d i n i t s width by A B(t) which gives a d i s c h a r g e of: 0.826

5.12

5.12.1

-

0.705

=

0.121 m 3 / s .

A B

-

0.121 0.705

i. e .

the i n c r e a s e in width,

i. e .

the new width of the w e i r should b e :

SCRATCHLEY OUTLET (INDIA AND

(t)

0.172 m

PAKISTAN^1/

General The Scratchley Outlet i s u s e d in Punjab and Haryana i n India and in P a k i s t a n when working h e a d s available a r e s m a l l . This non-modular type of outlet differs f r o m the pipe outlet only a t its downstream end.

I t s advantage o v e r the pipe outlet i s i t s e a s y adjustment and i t

h a s a m o r e o r l e s s constant coefficient o f discharge. 5.12.2

S t r u c t u r a l and Design C h a r a c t e r i s t i c s The Scratchley Outlet ( F i g u r e 5- 33) c o n s i s t s of an inlet pipe (or b a r r e l )

L/ B a s e d on a contribution by A.D.

Chaudhry. Chief Engineer, I r r i g a t i o n Works. Haryana, and K. C. Gupta, Executive Engineer, C e n t r a l Designs I r r i g a t i o n Works, Haryana (India).

. insertion of

stop-logs t o close the outlet

)e!Jt?l of the ~OtarCOurSe

Plan

c-wr,-I Note: Dimensions ore in cm.

Section

FIGURE 5 - 3 3 .

-

A-A

Scratchley outlet.

which opens into a small tank o r cistern a t the outer side of the bank of the distributing canal, a t the outer end of which i s fixed a c a s t iron, stone o r concrete orifice of the c o r r e c t dimensions for the required discharge of the outlet.

This

orifice i s kept submerged in the tail water to function a s a non-modular outlet.

If

the orifice i s s e t c l e a r of the tail water, i t will function a s a semi-module due to f r e e flow conditions. The s i z e of the c r o s s section of the pipe ( o r b a r r e l ) should be l a r g e enough to pass the required discharge a t a nominal head loss.

The recommended size of

the c r o s s section of the orifice and the corresponding c r o s s section of the b a r r e l a r e a s given in Table 5- 3. TABLE

C r o s s section of orifice (cm2)

5-3

C r o s s section of b a r r e l Breadth (cm)

Height (cm)

A standard pipe instead of a b a r r e l , approximately equal in a r e a of c r o s s

section to the b a r r e l a s given in Table 5-3 may a l s o be used. The dimension of the cistern o r tank for outlet discharges below 30 l / s i s generally 60 c m by 60 cm.

F o r discharges between 30 11s and 60 l / s , the cistern

i s generally 80 c m by 80 c m and for discharge over 60 11s the cistern i s 100 c m by 100 c m . The sill of the b a r r e l i s generally placed a t the bed of the supply canal and J

the orifice a t bed level of the f a r m watercourse, unless i t can work a s a f r e e fall, when the sill of the orifice i s placed higher than the.water level in the watercourse. Stone blocks a r e used on the side of the orifice to discourage tampering.

The

s t r u c t u r e o p e r a t e s automatically. The size of the orifice can be modified i f required with the channel running. The cost of alteration i s small. be dismantled and rebuilt. )

(It i s only the downstream end-wall which h a s to

The tolerances in the capacity of the b a r r e l a r e

l a r g e enough to allow for a small change in the designed discharge.

The coefficient of discharge i s the same for all orifices, provided the length of the orifice along the axis of flow i s f r o m 1 . 5 to 3 t i m e s the l e a s t of the dimensions of width ( a c r o s s the axis of flow) o r height of the orifice. The outlet r e q u i r e s only a small working head although a little m o r e , say

2- 3 cm, than the direct pipe outlet, a s a small amount of head i s l o s t in the leadin pipe. The working head of the outlet can be m e a s u r e d much m o r e easily than in the case of those outlets where the supply water level and the delivery water level a r e somewhat a p a r t ; h e r e , i t i s the difference in water levels on either side of the sam'e wall. Silt entry into the outlet can be better controlled by placing the upstream end of the inlet pipe a t , above, o r below the bed of the supply canal. The main disadvantage of the Scratchley outlet i s that i t i s not immune f r o m tampering.

The discharge can be i n c r e a s e d by: lowering the water level in the

watercourse; rounding the edges of the orifice; and making holes in the cistern wall. When the orifice i s s e t for f r e e flow conditions, a comparatively l a r g e working head i s required.

F a r m e r s could thus r a i s e the water level in the

watercourse and render the orifice partially submerged. some i n c r e a s e in the discharge. detected in this type of outlet. 5.12.3

Design Formula

This would r e s u l t in

However, tampering can be fairly easily

where

Q

=

d i s c h a r g e of the outlet i n l / s

Axorf

=

c r o s s - s e c t i o n a l a r e a of the o r i f i c e ,

=

working head, i. e . the difference i n w a t e r l e v e l s of the c i s t e r n and the w a t e r c o u r s e , i n c m

=

coefficient of d i s c h a r g e

=

l o s s of head through b a r r e l o r pipe

h(wk)

C

h@l

in c m

0.0354

=

(In working out

where .A, i s the c r o s s sectional a r e a of b a r r e l o r pipe. hg) 5. 1 2 . 4

, it

L

would b e b e t t e r ,to u s e t h i s f o r m u l a d i r e c t . )

N u m e r i c a l Example Design a Scratchley outlet in accordance with the following data:

A s s u m e the water l e v e l i n the supply canal Then the water l e v e l in the w a t e r c o u r s e

=

200.00 m

=

200.00

=

199.85m

-

0.15

A s an approximation, a s s u m e i n the f i r s t instance a lo'ss of head of 3 c m through the b a r r e l . 12 c m .

The working head available f o r the o r i f i c e will then be

F o r the drowned condition,

The s i z e of the o r i f i c e i s 18. 5 c m x 15 c m and accordingly, vide Table 5- 3, the c r o s s section of the b a r r e l will b e 3 0 . 5 c m x 30.5 c m . F o r a b a r r e l s i z e a s worked out above, to calculate the exact differences of w a t e r l e v e l s in the distributing canal and the c i s t e r n u s e the formula:

where Ax i s a r e a of b a r r e l o r pipe which gives h

(wk)

=

1 . 2 cm.

The water s u r f a c e l e v e l i n the c i s t e r n

The working head for the outlet

- 0.012

=

200.00

=

199.988 m

=

199.988-199.850

=

0.138 m

=

13.8 cm

Now adopt t h i s value of h(wk) in the f o r m u l a

and r e c a l c u l a t e Ax(orf) which c o m e s to 259 c m 2 . ,

5.13

.

The s i z e of the o r i f i c e should be 17 c m x 15.25 c m .

11 P I P E OUTLET (INDIA AND PAKISTAN) -

5. 13. 1

General The pipe outlet i s the s i m p l e s t and oldest known type of outlet.

Originally

the pipes w e r e of earthenware but w e r e gradually replaced, in m o s t p l a c e s , by rectangular wooden and m a s o n r y b a r r e l s .

C a s t iron, s t e e l and concrete pipes w e r e

p r o g r e s s i v e l y introduced a t l a t e r s t a g e s .

This outlet can be built a s a f r e e fall type if sufficient head i s available. I t i s generally u s e d w h e r e the s i l t c h a r g e i n the supply canal i s low and the canal i s running i n high filling.

I'

When the head available i s v e r y s m a l l , the submerged

B a s e d on a contribution by A.D. Choudhry, Chief Engineer, I r r i g a t i o n Works J a r y a n a , and K. C. Gupta, Executive Engineer, C e n t r a l Designs, I r r i g a t i o n Works, Haryana (India).

pipe outlet i s the obvious choice.

5. 13.2

Structural and Design Characteristics The pipe outlet consists of an upstream headwall, a pipe and a downstream headwall.

In a submerged pipe outlet (Figure 5-34) the upstream end i s a little

above, o r sometimes below, the bed level, depending upon the desired silt draw. The other end of the pipe opens into the f a r m watercourse below the water surface level.

The pipe can be placed horizontally o r sloping slightly up a t 1 in 12 down-

stream.

Both ends of the pipe a r e built into masonry to prevent tampering and to

guard against any leakage along the outer side of the pipe.

FIGURE 5-34.

-

Submerged pipe outlet.

The f r e e fall pipe outlet may have a horizontal pipe with i t s downstream end above the water surface level in the watercourse but in that case i t cannot, usually, draw i t s fair s h a r e of silt.

In an attempt to fix the pipe a t the bed level of the

supply canal and yet obtain f r e e fall conditions (where levels permit), some pipes have been laid with their upstream ends a t bed level of supply canals and sloping upwards through the banks s o that the downstream lips reach a height of 15 cm above the highest water level in the watercourse.

There a r e practical limits to

the amount of slope that can be given and i t should not generally be m o r e than 1 in 12 (based on experience on the Western Yarnuna Canal).

Hydraulic Characteristics 5. 13. 3. 1

Flow through pipe outlets The conditions of flow through a pipe apply equally to pipe

outlets. 5.13. 3.2

Flexibility The flexibility of a pipe outlet depends on the ratio which h(wk) o r

H(cnt) b e a r s to the full supply depth in the supply c a n a l .

In Punjab and

Haryana (India), wherever pipe outlets a r e used, i t has been the general practice to place them a t the bed level of the supply canal.

This setting i s given to enable

the outlets to draw their f a i r s h a r e of silt from the supply canal. 5. 13.3.3

Silt drawing capacity No r e s u l t s of comprehensive experiments on the silt drawing capacity

of pipe outlets appear to have been reported, but such experiments would be useful.

On the other hand, according to experience so far, canals fitted with pipe

outlets at bed level seldom give any silt trouble. 5. 1 3 . 3 . 4

Efficiency Pipe outlets, working under non-modular conditions, suffer f r o m all

the defects inherent in the non-modular type.

They a r e important, however, in

that they can pass the required discharge with a very small working head, (even only 2.5 cm, with which no semi-module can function). 5. 13.3.5

Adjustability While appreciable adjustment of the design discharge would require

dismantling and reconstruction of the structure; o r p a r t of it, small changes in discharge can be effected by lowering, o r raising, the bed level a t the pipe outlet, and this would change the working head. 5.13.4 5.13.4. 1

Design Formulae Submerged pipe outlet

1

'

Where

Q

-

discharge of the outlet in 11s;

A,

-

c r o s s sectional a r e a of the pipe in cm2;

ywk)

--

g

-

difference in water surface levels in the supply channel and the water course in cm; .

acceleration due to gravity in c m / s 2

.

The value of C for ordinary cast i r o n pipe of 15 c m internal diameter h a s been found to be nearly 0.00074.

A simpler formula for the outlet i s

Where

5.13.4.2

--

C

0.028 and 0.033 respectively for long ( m o r e than 600 cm) and short ( l e s s than 600 cm) pipe.

F r e e fall pipe outlet

Where

Q Ax

C H

(4

--

discharge of the outlet in 11s;

-

c r o s s sectional (internal) a r e a of the pipe;

--

coefficient of discharge

-

head of upstream water surface over the centre of the pipe.

=

0.0276

The discharge i s sensibly c o r r e c t so long a s H

i s greater

(4

than 2 D(p) (inside diameter of the pipe) and i s approximately c o r r e c t for H(cnt) = D(p)

but i s appreciably different f r o m that given by the formula for

H

5. 13.5

( cnt)

l e s s than D

(P)'

Numerical Example

Example 1 Design an outlet for a discharge of 3 4 11s on a distributing canal having a full supply depth of 90 cm and with an available working head of 6 cm.

The available working head of 6 cm i s low and the outlet must be submerged.

or

D (P)

--

23.3 cm.

This may be rounded to the nearest standard size of pipe available.

Example 2 If in Example 1 the available working head i s 75 cm, a wide choice would be possible for the type of the outlet to use.

If, however, a

pipe outlet i s considered desirable, either f r o m the point of view of cost o r i f the outlet i s required only temporarily, i t would be desirable t o install a f r e e fall outlet so that i t might work a s a semi-module.

Assuming water level to be 200.00 in the distributing canal, the water level in the watercourse would be 200.00 a s s u m e a pipe of 15 c m diameter,

- 0.75

= 199.25.

As a t r i a l ,

then:

48.6 cm.

The centre of the 15 cm diameter pipe should thus be a t a level of 200.00

-

0.485 = 199.515 and the outlet would be semi-modular, since the water

surface level in the watercourse would be 199.25,

(which i s well below the bottom

level of the outlet, i. e. 199.36).

Summary The pipe outlet i s the simplest and the cheapest type of outlet. function with very small working heads, even a s small a s 2.5 cm.

It can Under

submerged conditions the discharge of the pipe outlet depends on the downstream water level (i. e. the level in the watercourse).

The discharge can be

increased by lowering the water level in the watercourse.

On the other hand,

if the watercourse should silt up, the working head will reduce, resulting in a d e c r e a s e in discharge.

In a f r e e fall outlet, i r r i g a t o r s may be tempted to r a i s e

the water level in the watercourse to make the outlet partially submerged and thus obtain an increased discharge. i s not con stant.

The discharge coefficient of the pipe outlet

5.14

5.14.1

FARMOUTLET ( U . S . S . R . ) - 1/

General The outlet described h e r e i s used to deliver water to a temporary feedditch, up to a discharge of 150 l/ s and with a working head of up to 60 cm. The early f a r m outlets of this type were controlled by wooden flap gates. The main shortcomings of these early outlets were their short life and frequent failures.

The wooden gates havenow been r e p l a c e d b y steel discs.

These

outlets a r e .the smallest s t r u c t u r e s on irrigation systems but they a r e also the m o s t numerous, accounting for m o r e than half of the total number of s t r u c t u r e s in a system.

5. 14. 2

Structural Characteristics The main p a r t s of the f a r m outlet a r e : a pipe; a disc gate; and, for drops over 20 cm, a damper o r a stilling basin. In the submerged f a r m outlets,types VT- 300 and VT-400 (Figure 5- 35),

horizontal pipes ape used for working heads up to 20 cm. In the f r e e fall f a r m outlets, types VTP-300 and VTP-400 (Figure 5-36), inclined pipes a r e used up to working heads over 20 cm but not exceeding 60 cm. In both c a s e s the pipes a r e of low p r e s s u r e asbestos cement, 30 c m and 40 c m in diameter. Regulation of the water supply i s accomplished by the manually operated disc gate.

5. 14. 3

Hydraulics The discharge capacity of the f a r m outlet for design purposes i s determined by using the following formula.

Based on information provided by A. T. Koshkina, E. P. Martin, A. V. Shatalova, D.D. Aliera and B. V. Kazarinov (U. S. S. R. )

I

bO-d

Cross

Section I - I

-bIPqd-,

Hydroulic

chorocteristics

+

cast-in-situ con crel'e

Free outfall

h k )

&PI H(crf/

I

stilling basin-

an mm Cm

aJ

.-a n

-E

I

- . s- I 5

Discharge

:;

," E g g 2 ' .i 5 g g.8

50

H m ~ ~ YcrfJAx@k H c f j ~ vcf,x/ YCI~J cm m/s cm m2 crn m k cm

Cross Section 2 - 2

~-zoo-+,~oo

&{IN/ & f t ~

d cm

details

0.25

11.94

I

I

-

-

I

-

-

I

QiJ=

$;'386 mq

Volume of moin works

r

--q

&IN)

m2 cm

Lt0

Detoil

Cross Section 3 - 3

150

100

oncrete volume Detoil reinforce- Number of details according structures' 'YWs for detail lment w e i ~ h t

Tvoe o f detoil

Asbestos cement pipe 4i,, = 400 cm

I

f/s

e0wI

m2

L i s t of

4

VTP 9 0 0 100 ( 150 1 6 0 cm 400 36 47

Hydroulic chorocteristics with inlet unsubmerged

k l l 6 ~&+-104 r

3

-

VTP - 300 50 20 300 27

Type of structure Dischorge 11s

I

I

Llrl.r IVUIIIC

Reinforced concrete details Pipes

Dl )w? = 300 mm Dr$out =

400

..-.-wurerlu~ ? - I

Concrete Reinforcement Asbestos cement

mm

Disc valves

(All dimensions are in cm)

FIGURE 5-35. - F a r m outlet ( U . S. S. R. ) to a t e m p o r a r y feed ditch for d i s c h a r g e s of up to 150 l / s - outlet submerged.

grovlty

I.. . 1. T- v ~ eof unit

5 kq

4

ptper

Steel

structure VTP-300 VTP-400 0.25 0.25 11.94 11 - 94

kg

674 -

4 - -.

8.59

I

Cross Section 1-1

Hydroulic

chorocteristics Outlet VT-300

Type of structure Discharge l / s h(~k)- an D@l- rnm

-

%,tl

submerged VT

50 7 300

mm

100 9

- 400

1

150

1

20

400

49

59

i

Hydraulic porometers with inlet submerged t

O

1

a 0 a:

P

VT-300

1

Discharge

..

a,

50 8;F E. o g h ~ ~ k l Y s d 4 4 c r t ) 4 w k ) , -.= z* cm rn/s cm cm cm 29110067 7

0-75 13

1

0s

100

150

' 'q q)bltl )brt) m/scrn crn crn N s c m cm - - - - - - - -

49

V V

Y Ys s~ ~ ff h h% % rr // )) 'IWXI 'IWXI

vV

hfwklL-- 2 0 cm List of detoils Type of detail

structures' types detail

L = 4 0 0 cm

Volume Cross Section

3-3

of moin

Material

Norne

Cross Section 2 - 2

P~pes '

&,=300mm

qD,= 400mm -29'

DISC valves

2

-386

cm

( A l l dimensions ore in c m )

- Farm outlet ( U . S. S. R. ) to a temporary feed ditch for discharges of up to 150 11s free outfall.

FIGURE 5 - 3 6 .

-

works

Asbestos qrovlty PlPes

Steel

Un~t

rn

kg

.Type of structure VT-300 VT-400 - 4-4 6.74

-

-

8.59

With submerged outlet

where

C

1

= 1

=

=

s

C(Hr)

750

-

=

0.65

=

C(f-IN)

(f-P)

l o c a l hydraulic r e s i s t a n c e coefficient

i n which ( s s )

C(f-~)

+

= +

+C

= 0.976

0.833

90° frictional r e s i s t a n c e coefficient

with C1I =

8

N2 (

-4

=

C"

l

) 7

=

0.0612

4P)

with L(p) = D(p)=

4 m

0.389m

N = coefficient of roughness

=

0.015 (for a s b e s t o s cement pipes).

The depth of the w a t e r s u r f a c e above the bottom of the pipe a t the inlet should be:

( h(wk)

with

+ ,

H(sof)2 )

=

0.20m,

With f r e e flow outlet The depth a t the inlet,

where

Vc(1~)

=

H c ( ~ ~ =) =v D ( ~ )

H(crt)a i s defined a s :

c r i t i c a l i n l e t velocity, c r i t i c a l inlet depth,

=

coefficientofvelocity

=

pipe d i a m e t e r .

=

0.85,

The c r i t i c a l inlet depth (for a round c r o s s section) i s defined by A . M .

Latishenkovls method, a s Hc(IN) Hc(cir)

=

--

Hc(cir)~

critical depth for circular c r o s s section with D(p) = 1 m and discharge q =

Q D2. 5

(PI Critical velocity v C(IN)

Q

--

The value of the critical depth Hc(cir) depends upon q and according to the following table:

The design discharges a r e co-ordinated with the discharges of the temporary feed-ditches and a r e assumed a t 50 and 150 11s. The height of the embankment above the water surface on the downstream side i s 20 c m and on the upstream side 30 cm o r more. Depending upon the conditions of operation of the disc gates, the upper inlet edge of the pipe should not be submerged by m o r e than 20 cm.

5. 14.4

Numerical Example A f a r m outlet i s required to discharge 90 l / s through a pipe, of 400 m m outside diameter, 5. 7 m m thick and 4 m long, under submerged conditions. Find out the working head and design the outlet. Let inner diameter of the pipe be Let sectional a r e a be

A,

The coefficient of discharge C

D

=

0.389 m

=;

0.119m2

=

0.65

The design of the outlet will be a s for type VT-300 detailed in F i g u r e 5-35.

5.15

1/ PRE-CAST FARM TURNOUT (TURKEY) -

5. 15. 1

General The P r e - c a s t F a r m Turnout described h e r e i n i s widely used i n some p a r t s of Turkey to deliver water to f a r m l a t e r a l s f r o m distributing canals. of the turnout i s adapted f r o m that of a constant head orifice.

The design

All p a r t s of the

device a r e p r e - c a s t units, and the whole s t r u c t u r e i s very strong and durable.

5. 1 5 . 2

S t r u c t u r a l and Hydraulic C h a r a c t e r i s t i c s A s shown in F i g u r e s 5- 37 and 5-38, the turnout consists of the following parts:

a p r e - c a s t concrete pipe of 30 c m d i a m e t e r provided with a gate a t the

inlet; in front of this pipe t h e r e i s an approach box.

The wing walls on both

sides, the headwall and the floor of the box a r e a s s e m b l e d a s one p r e - c a s t reinforced concrete unit.

Th'e reinforcing s t e e l b a r s a r e of 6 m m d i a m e t e r and

the spacing between them m u s t not exceed 20 c m ; t h e r e i s a p r e - c a s t downstream head wall; t h e r e a r e p r e - c a s t check blocks in the supply canal, downstream of the turnout, provided with grooves for insertion of stop-logs to r a i s e the water level; p r e - c a s t lining i s provided in the supply canal on t h e s i d e s and on the bed;

t h e r e i s c o a r s e gravel protection in the f a r m l a t e r a l just below the turnout

structure. The quantity of cement in the p r e - c a s t concrete m i x i s 400 kg p e r cubic m e t r e of concrete. At p r e s e n t , the turnout i s simply a pipe outlet operating under submerged conditions.

However, i t can be c ~ n v e r t e dto a constant head o r i f i c e should the

Based on a note p r e p a r e d by Htiseyin K ~ m t l r c t i o g l u (Turkey).

Sect~onD D

Detail of central plate 3 number

Front elevation

Check block

The details of entrance

, The detoll of downstream head wall of the turnout

Note

- 1\11 the

Section H-H

d~mens~ons are In cm

F A 0-ICID

PRECAST FARM Section B-B

TURNOUT Project, Region, Country Turkey

Sectlon F-F

Sectlon A-A

Figure 5-37

r

-

Bottom of farmer's ditch

1.r

Cross section of precosl form turnout

30 r

25, I 0

. .

Note: Dimensions are in centimetres.

P

.-CC

b

F A O - ICID

Q,

t

RATING CURVE OF PRECAST FARM TURNOUT

0

10

20

30

40

Discharge , O , 1 /s T h e roting curve

50

60

70

80

Project, Region , Country Turkey Figure No. 5-38

need a r i s e by installing a gate in the guides provided for the purpose. The turnout works satisfactorily for all discharges up to 60 11s.

5. 15.3

Design F o r m u l a See F i g u r e 5-38 which gives discharges for different openings of the gate.

5. 1 5 . 4

Numerical Example Design a p r e - c a s t turnout for a discharge of Q Difference of surface water levels in the supply canal and the f a r m l a t e r a l ,

=

h(wk) =

.

45 11s 10 c m

F r o m F i g u r e 5-38, for a working head of 10 c m and a discharge of 45 11s the gate opening H(go) = 19 cm.

Other dimensions a r e given in Figure 5- 37.

General With the advent of double cropping in Malaysia, suitable conditions have to be developed for proper water management, which c a l l s for better water control systems. The adjustable weir f a r m outlet, meant for delivering water f r o m a distributing canal to a field canal serving a group of i r r i g a t o r s , h a s been developed by the Design Branch of the Malaysian Drainage and Irrigation Department to overcome some of the field problems inherent in the e a r l i e r types of outlets.

These e a r l i e r outlets ranged f r o m simple orifices to orifices

incorporating some rudimentary f o r m s of regulating valves; they were largely u s e d on irrigation systems drawing water supplies f r o m r i v e r s for irrigating one crop.

Basically they suffered f r o m inadequate control in the supply, l a r g e head

l o s s e s and wastages due to tampering.

<J

L1

Based on information supplied by the Malaysian National Committee, ICID.

Structural Characteristics The outlet i s strong and durable.

I t c o n s i s t s ( s e e F i g u r e 5-39) of an inlet

pipe opening into a well-chamber (inside dimensions 5 ft by 5 ft) a t the downs t r e a m end of which i s an opening with i t s top 2 ft f r o m the floor of the wellchamber.

This opening i s 2 ft wide ( a c r o s s the flow) and 3 ft high. The 1 3 adjustable w e i r , consisting of m i l d s t e e l plate, - inch thick, 7- inches wide 4 4 and 2 ft 10 inches high, s l i d e s up and down by m e a n s of a screw-down device. A gauge i s fixed to the weir t o enable the head over the c r e s t t o be r e a d .

On the downstream side of the weir t h e r e i s a s m a l l c i s t e r n , 5 f t long a c r o s s the flow and 2 ft wide in the direction of the flow.

I t i s roofed by p r e -

c a s t s l a b s which s e r v e a s a platform to o p e r a t e the screw-down device.

At the

end of the c i s t e r n t h e r e i s an opening of the s a m e section a s the c r o s s section of the field canal.

5. 1 6 . 3

Hydraulic C h a r a c t e r i s t i c s A s r e g a r d s hydr&lic p r o p e r t i e s of the outlet, i t i s simply a thin weir s t r u c t u r e which h a s been calibrated in the l a b b r a t o r y . formulae a r e required.

No calculations using

The operational p r o c e d u r e of this outlet h a s been

simplified by preparing c h a r t s , a s r e f e r r e d to in 5 . 1 6 . 4 below.

The o p e r a t o r

m e r e l y s e t s the head above the weir c r e s t a s r e q u i r e d f o r any p a r t i c u l a r discharge, (which i s r e a d off directly f r o m F i g u r e 5-40), by moving the weir up and down by the screw-down device.

( F o r details of this gate s e e Vol. 111

of this handbook. ) Additional s e t s of graphs a r e being p r e p a r e d for varying depths of chamber w a t e r lever t o r e n d e r the operation of the outlet m o r e v e r s a t i l e . This outlet i s not suitable f o r withdrawal of s i l t f r o m the supply canal. The outlet generally o p e r a t e s f o r a discharge of 85 l / s ( 3 ft 3/ s ) but a g r e a t e r capacity could be achieved if sufficient head w e r e available.

The

difficulty would be in taking the readings because of the unsteady condition of flow prevailing in the well-chamber f o r d i s c h a r g e s over 85 11s ( 3 f t 3 / s ) .

5. 1 6 . 4

Design P r o c e d u r e The outlet h a s been c a l i b r a t e d i n the Hydraulics Laboratory, with a constant w a t e r depth of 4 f t in the c h a m b e r .

Two g r a p h s ( F i g u r e s 5-40 and 5-41) and one

table (given on F i g u r e 5-40) have been developed f o r operating t h i s outlet. F i g u r e 5-40 shows the relationship between d i s c h a r g e through the weir against the head o v e r t h e weir f o r two conditions of flow, i. e. ( a ) f r e e flow, and (b) dowhstream level 4 i n c h e s below u p s t r e a m l e v e l (submerged).

F i g u r e 5-41

gives the head l o s s e s i n the inlet pipe for varying discharges:

The Table on

F i g u r e 5- 40 shows the m i n i m u m differential h e a d r e q u i r e d f o r v a r i o u s d i s c h a r g e s f o r maintaining m i n i m u m f r e e flow condition.

5.16.5

N u m e r i c a l Examples Example 1 if

D e t e r m i n e the head o v e r the w e i r H(crt),

-

(i)

the d i s c h a r g e Q r e q u i r e d i s 3 f t 5 / s ;

(ii)

F S L ~ ' in the i r r i g a t i o n canal i s 20.00;

(iii)

w a t e r l e v e l r e q u i r e d in the field canal i s 18.00. F r o m F i g u r e 5-41,

head l o s s through delivery pipe =.

J

1.4 inches (0. 12 ft)

F r o m the Table on F i g u r e 5-40, for f r e e flow condition the differential head =

. .

Total head r e q u i r e d f o r f r e e flow condition

Head available i s 20.00

..

5 . 8 inches (0.48 ft)

-

+

=

(1.4

=

7 . 2 i n c h e s ( 0 . 6 ft)

18.00

-

5.8)

2.0 ft.

D i s c h a r g e o v e r weir i s i n a f r e e flow condition F r o m F i g u r e 5-40, H(crt)

=

-

0.62 ft.

Example 2 D e t e r m i n e t h e head over the w e i r H(crt),

FSL,

f e e t above s e a level.

if

(i)

the discharge Q required i s 3 f t 3 / s ;

(ii)

FSL in the irrigation canal i s 18.45;

(iii) water level required in the field canal i s 18.00.

..

=

Head available h(wk)

0.45

=

(18.45

-

18.00)ft

F r o m Example 1 total head required f o r the f r e e flow condition

..

=

0.6ft

=

1 . 4 inches (0.12 ft)

Flow i s submerged F r o m Figure 5-41,

J

Differential head between chamber and field canal water level i s 0.45

-

0.12

=

0.33ft (4inches)

Hence f r o m Figure 5-40 (submerged condition)

5.17

5. 17.1

11 PVC P I P E TURNOUT (REPUBLIC O F KOREA) -

General An e a r l i e r f a r m turnout designed in the Republic of Korea consisted of a pair of wing walls and an inlet floor, a concrete conduit and a steel o r wooden side gate.

The construction of each turnout involved a significant amount of

concrete and mechanical work a t considerable cost in m a t e r i a l s and time.

Also,

the maintenande of the slide gate h a s been a problem owing to the turnouts being scattered a l l along the irrigation canals f a r f r o m villages.

The Agricultural

Engineering Research Center, one of the prominent laboratories in Korea, has developed the PVC pipe turnout to overcome the disadvantages of the older type of structure.

It was developed in 1969 and h a s been recommended for u s e on

new irrigation projects in the country.

L1 Based on information

supplied by U. C. Yeo,

Republic of Korea.

Structural Design The turnout consists of a bell-mouth inlet, a pipe and a specially designed outlet with a screwed stopper (Figure 5-42).

The inlet bell-mouth h a s two hori-

zontal b a r s to prevent weeds, debris, etc. blocking the pipe, and any m a t e r i a l collecting t h e r e can be easily removed.

- Sectional view of the P V C outlet installed through a canal embankment. Note lining of the bed in the field canal below the downstream end of the pipe and in the parent canal.

F I G U R E 5-42.

The PVC pipe h a s a length equal to the bottom width of the canal embankment plus 20 cm.

The bell-mouth inlet and the outlet device a r e connected to

each end of the pipe with synthetic bond.

F I G U R E 5-43.

-

View of the P V C pipe assembly.

FIGURE 5-44.

- View of bell-mouth inlet with c r o s s b a r s .

F i g u r e 5-43 i s a view of the assembled PVC pipe a s s e m b l y .

F i g u r e 5-44

shows the bell-mouth inlet, and F i g u r e s 5-45 and 5-46 show the outlet closed and open respectively.

This type of outlet can be used for d i s c h a r g e s a s shown in

Table 5-4 for different lengths and d i a m e t e r s of pipe and working heads.

FIGURE 5-45.

-

View of PVC pipe outlet closed.

FIGURE 5-46.

- View of P V C pipe outlet open

As the pipe assembly i s made of light weight m a t e r i a l i t i s easy to t r a n s p o r t 1 1 and to install. Mass production i s possible and i t s cost i s low (;T to -i; of the cost of existing f a r m turnouts).

The l o s s of head i s low and the discharge f r o m the

outlet can be controlled easily by the screwed stopper device. susceptible to damage by heat but i s not inflammable.

The outlet i s

It would be weak to

mechanical impact a t t e m p e r a t u r e s below freezing point. Discharge through the outlet i s accurate provided the head and the length and diameter of the pipe a r e carefully designed.

5. 17.3

Hydraulics and Design F o r m u l a In the design of this outlet the standard pipe flow formula i s u s e d :

where

Q

=

discharge of the outlet in m 3 / s ;

*x

=

internal c r o s s sectional a r e a of P V C pipe in rn2;

h(wk)

=

difference between designed full supply level in the supply canal and water level of the field channel in m ;

g

=

acceleration due to gravity in m / s 2 ;

C

=

coefficient derived f r o m the following formula:

where

C(f-IN)

C(f-p)

N D(~)

L(~)

R(H)

=

l o s s coefficient of inlet, screen ( b a r s ) and outlet p a r t (by t e s t i t h a s been found that the coefficient = 1. 135);

=

l o s s coefficient for friction in pipe having a value of

=

8gN2 R(H)'/~ L(p)/D(p) roughness c o e f f i c i e n t = 0.01 o f t h i s m a t e r i a l

= =

internal diameter of pipe; length of pipe;

=

hydraulic radius.

Figure 5-47 i s a standard design drawing for a PVC pipe turnout with D(

P) =

107 m m .

Table 5-4 shows velocity and discharge of a PVC turnout for D(p) =

107 mm. Figure 5-48 i s a discharge diagram for a PVC turnout with D(p) = 107 mm.

-

FIGURE 5-48. Discharge diagram for a PVC pipe turnout for D(p) = 107 m m . 5.17.4

Numerical Example

@

The difference in the designed full supply level in the supply canal above the water level in the field canal i s 0.5 m . pipe i s 107 m m and i t s length i s 6 m . We have

h

(wk)

--

The internal diameter of the P V C

Calculate the discharge of the outlet. 0.5m

II

p,

A

JV

(-

I

. . I . . 4 d

d d d d

NCOmCO sat-r0 0 0 0

. . . .

0 0 0 0

m m m d o a m a

2 g x

. . . .

N N

m m o o

d d

a ~ m m m m

I L

N N N N

m m o a

N N N N

m 4 t - N ????

0

0

.I 1

?gat-: NNN'N'l

E

g

+

( Y t - m C O ; FI COmm*'

0

. . . . . . . . . . . . uG

N N N N

mmv*;L2

N t - N P 1 O

I

. . . . ; u

N

0 0 0 0 1

E

a1 - 4 ~ 1 - 4 --.IP-~N; m c o m a mcoNa, N ~ M v ~ ~ m m ' o

~0 ~ 0 - 4

N N N N

0

O

N N N N

Nt-Nt--INN N N N N 0 0 0 0

C

C

Nt-Ntm m o o 0 0 0 0

O

d H d d

4 4 N N 0 0 0 0

N CO m mmd'o

a

. . .

d d N ?

N N N N

mm-t* m a 9

I---------'

t-mCO,*

. . . .

. . . .

9

a,

&

M

0

a,

*

-

Q

k

N m m N NNIS

a

I-mm'a m**'m

NNNIN

I

00010 . . . I .

~

. . . . 0 0 0 0

N N N N 0 0 0 1 0 0 0 0 0

J

ddd;d

a,

w

N C Q * d

NCO*O Gar-CO N N N N 0 0 0 0

. . . .

-

. . . . . . . . . . . .

at-cOcO

d d d d

d N N N 0 0 0 0

d

- 4 m m

0 0 0 0

a m m m COmmo

N N N N 0 0 0 0

0 0 0 , o

d

d H d N 0 0 0 0

0 0 0 0

ar-t-:co

4

0 0 0 0

?**In

- m o d N N m m

d

0 0 0 0

O H N N

N N ' N ' N ' N'N'N'IN'

d

0 0 0 0

N N N N

COamm mat-t-

0 0 0 0

. . . . . . . . . . . . . . . . . . . .

r-df*m m m o o a s o r m o ~ N da o I ~ m m w m a a t O O , d d 1

d d d d

0 0 0 0

mt-'ofi 000.0

0 0 0 0

COmldN O O 1 d d

oo]oo

0 0 0 0

0 0 0

.

.

.

.

.

.

. I .

d'mat-

N N N N 0 0 0 0

m't- a

~

N P * O

~

m m * * 0CQmm

m m m m

. . . .

P- N 0 tNcOaN O O d N

0 0 0 0

NNN,N N N N N 0 0 0 1 0 0 0 0 0

I

O O 0 ' 0

I

CO m l N 0 m a ~ r - a

t-

CQ

. I .

*IN

d

0 0 0 0

0 0 0 0

. . . .

4 N m m m m m m 0 0 0 0

~

F i d d d

d d d d

4

z

0

E

~ o m m - d * ~ '') m a d ' * a a m m 6 . 1

dddd

t-COmo N N N m

N N N N N N N N N NImm -----I r - -C-Q- -m-h-l -o r-d'Nd

.

CO a m1d 4 N mud' N N N N 0 0 010

0 0 O ( 0 F a m m l ~o r - m e a m - 0 N - ~ I C O *mm1a

N

m o m 0

d N N 0

m o m 0

t-CQmo O d N m * m a 0 0 . . . . m. m. m. m. m. m. m. m. N

a,

~ 0 m - u 9 m ~ - 1 ~ *- H P - ~ O t - m o t - m ~ ~ ~ m mTd.'m .a 2NNN *mml\D . . . . . . . . . . . . O. d d d . . . . . . . . .I:

0 0 0 0

4

d O m m O O d N

dd;dd

- - --I

,omr-CO -

.

d- 1 4 4 ,

d

N N N N 0 010 0

d

I D O d N O

mt-mm m m m d m

d dl6 d 4

m

. . . . t- 9 ' N

~

N N N N 0 0 0 0

.

M O N O

. m. m. *. m.a t.- P. C. . m ' 0 . mN. Nm. mm. m* . m

0 0 0 0

.

. . . .

.

1 0 0 0 0

.

; o o o o r

I

.

N

L O P - C O OO O ~ N o ~ V I P -t - t - m m at-CQm O d N m

I d c o d * I O a o N I d d m * l d d d d

I

-

. . . I . N N N ~ NN 4

.

N N N N ~

.

m o m o

.

m o m 0

0 0 0 0

a

m o m o

0 0 0 0

;P-+CQN m ~ m ; O N m m I

m o m 0

0 0 0 0

m a a t - I-COCOm m o o 4 d N N m m * * m . . . . . . . . . . . . . . . . . . . . . . . . m. * .* m. . O d d +

m o m o

0 0 0 0

Now

- % =

R ( ~ ) -

P(w)

n -

0.107' 4 0.107

=

0.02675 m

The difference f r o m Table 5-4 i s 0.0004 m 3 / s and i s i m m a t e r i a l f o r practical purposes. The d i s c h a r g e can be found e a s i l y f r o m the d i s c h a r g e d i a g r a m , F i g u r e 5-48 ( p r e p a r e d by hydraulic t e s t ) . If t h i s d i s c h a r g e i s l e s s than the design d i s c h a r g e , u s e a l a r g e r d i a m e t e r pipe o r two pipes i n p a r a l l e l .

5.18 5.18.1

PIPE OUTLET (PHILIPPINES)- 11 General A P i p e F a r m Outlet with a standard inlet was introduced in the Philippines

B a s e d on information supplied by the Philippines National Committee, ICID

.

i n 1969 to be u s e d i n a l l National I r r i g a t i o n S y s t e m s i n the country. With the water management p r o g r a m m e being undertaken by the National Irrigation Administration (NIA), i t b e c a m e n e c e s s a r y to gate a l l i r r i g a t i o n t u r n outs f o r p r o p e r control of w a t e r .

At the s t a r t of the p r o g r a m m e , the NIA needed

t o provide about 20, 000 gated turnouts.

The existing turnout headwalls i n the

Philippines w e r e designed t o c a r r y concrete pipe conduits.

These headwalls

c o s t about $ 22.00 f o r a 12-inch d i a m e t e r pipe, while the corresponding standard hand-operated s t e e l g a t e s c o s t about $ 22.00, o r a total of $ 44. 00 p e r complete unit.

F o r the 20,000 gated turnouts, the total outlay would be $ 880,000, an

amount which'the operation and maintenance budget of the NIA could not finance. T h e r e f o r e , due t o the urgency of instafling gated turnouts and the n e c e s s i t y for'adjusting the elevations of s o m e of the existing ones, a p r e - c a s t headwall h a s been designed, which i s supported by the pipe conduit i n s t e a d of vice v e r s a a s i n the c a s e of t h e e a r l i e r s t r u c t u r e s .

It i s light and can be c a r r i e d by hand to the

s i t e even during the rainy season.

It can be installed i n a few h o u r s without sub-

stantially affecting the water supply i n the canal.

If the elevation of a n outlet

s t r u c t u r e h a s to be adjusted, it i s e a s y to accomplish and a t l i t t l e cost.

The

p r e - c a s t headwall for a 12 inch d i a m e t e r outlet c o s t s only $2.00 installed while the corresponding s t e e l gate c o s t s only $3.00 o r $ 5 . 00 p e r unit.

5.18.2

S t r u c t u r a l and Hydraulic C h a r a c t e r i s t i c s The outlet s t r u c t u r e con-sists of a p r e f a b r i c a t e d headwall and a gate ( F i g u r e 5-49) and a pipe.

The g a t e s a r e designed f o r manual operation.

the gate i s controlled a t the s t e m of the headwall.

The opening of

The original design did not

f o r e s e e that f a r m e r s would u s e the s t e m for tethering t h e i r w a t e r buffaloes; the r e v i s e d design t a k e s t h i s into account. The s t r u c t u r e i s on the whole durable although t h e r e a r e s o m e r e s e r v a t i o n s about the c o n c r e t e s t e m . by unskilled labour.

All components a r e p r e f a b r i c a t e d and e a s y to a s s e m b l e

Quality control and m a s s production i s possible using l o c a l

labour and minimum supervision .

Installation can be done a t any t i m e of the

year. -The m a x i m u m head recommended f o r t h i s outlet i s 20 c m .

The diameter of the pipe i s usually dictated by the standard commercial pipe available. adopted.

The nearest l a r g e r diameter pipe to the design diameter i s usually The intake of water i s controlled by the pre-determined and permanent

calibration of the gate stem.

The degree of accuracy i s on the low side.

The capacity of the turnout i s from 20 to 90 l / s , which will serve from 10 to 30 ha of land.

5. 18. 3

Design Formula The usual pipe formula i s used, namely:

where

Q

Ax g

design discharge;

=

internal c r o s s sectional a r e a of concrete pipe;

=

acceleration due to gravity;

=

head of water - the difference in water level a t the inlet and outlet of the structure;

=

coefficient related to transition, contraction, friction and other losses, which may be determined from actual discharge measurements through a typical headwall.

8

h(wk)

C

=

On a fairly well finished headwall, a coefficient of 0.6 i s a conservative value.

5.18.4

Numerical Example A farm having an irrigable a r e a of 15 ha requires water from an irrigation canal.

The water duty a t maximum demand i s about 3 l / s / h a .

The difference

in the water surface of the supply canal and the farm canal at maximum designed

capacity should not exceed 0.05 m .

Q

=

Design the size of outlet required.

0.003 x 15 = 0.045 m 3 / s , maximum design discharge of f a r m canal

Use s t a n d a r d s i z e of c o n c r e t e pipe with d i a m e t e r of 31 cm, say, 12 inches. The crown of the outlet pipe m u s t be s e t a t l e a s t 0.05 m below the w a t e r s u r f a c e elevation.

5.19

5.19.1

GATED P I P E OUTLET (FERRARA TYPE, ITALY)

Application and S t r u c t u r a l F e a t u r e s This Gated P i p e Outlet h a s been developed by the t'Consorzio della Grande Bonificazione F e r r a r e s e t ' in F e r r a r a , Italy, and i s widely used i n the i r r i g a t i o n d i s t r i c t s of the P o Delta in n o r t h e r n Italy, p a r t i c u l a r l y for r i c e cultivation.

FIGURE 5-50.

-

P r e f a b r i c a t e d gated outlet ( F e r r a r a type).

As F i g u r e s 5-50 to 5-55 illustrate, the s t r u c t u r e consists of a covered conc r e t e pipe, connecting the distribution canal with the f a r m watercourse o r basin to be i r r i g a t e d .

At a convenient place along the pipe (normally n e a r the d o w n s t r e a r

end) t h e r e i s a stand which houses a flow regulating gate. As the figures show, the stand consists of a concrete block fitted to the pipe and a covering slab o r box.

If

required the stand can be extended by putting a second block on top of the bottom one.

All p a r t s of the stand a r e prefabricated.

The metallic concrete moulds a r e

hired to contractors by the "Consorzio" for fabrication of the concrete p a r t s a s required.

The n o r m a l sheet m e t a l slide gate i s operated f r o m above, a s can be

seen f r o m the F i g u r e s .

The F i g u r e s also show that the stand i s equipped with an

anti-tampering.locking mechanism.

FIGURE 5-51.

- P r e f a b r i c a t e d gated outlet ( F e r r a r a type)

FIGURE 5-52.

-

P r e f a b r i c a t e d gated outlet ( F e r r a r a type).

FIGURE 5-53.

-

P r e f a b r i c a t e d gated outlet ( F e r r a r a type).

Field ditch or basin

Longitudinal section

Open,

Ti

-Ti--

~inished

Plan

F A 0

- ICID

Note: Unless otherwise stated, oll dimensions ore in centirnetres.

GATED

PIPE O U T L E T

(FERRARA T Y P E )

Project, Region, Country Italy Figure No. 5-54

(a)

Plon

of

stond

FAO-ICID

I

PlPE STAND IN GATED PlPE OUTLET ( F E R R A R A TYPE)

Sect~on A-B Project, Region, Country I1 oly Note: All dimensions ore In centimetres. Figure No. 5-54 (b )

I

Note: All dimensions ore in millimetres.

Note All d~rnenstons ore In rnill~rnetres

600 x 400 steel gate FAO-ICID

COMPLETE

OUTLET FOR

4

S T A N D (LOW T Y P E )

4 0 0 m m PIPE

Project , R e g ~ o n Country , lfoly F ~ g u r eN o 5-55 (b) 1

Materials required for a 10 m long outlet a r e : stand

(low type)

pipe

-

0.25 m 3 concrete; 25 kg reinforcement steel; 53 kg steel for the gate.

-

9 m prefabricated concrete pipe; 2.3 m in situ concrete; 69 kg reinforcement steel.

3

The total cost for prefabrication and installation of a 10 m outlet amounted to approximately $ 450 in 1970, of which the gated stand accounted for approximately

$ 170. This outlet i s a simple and reliable structure adaptable to any bank width. It i s particularly suited to a r e a s where the working head of outlets has to be kept small, i. e. where outlets have to work under submerged conditions. of the s t r u c t u r e i s simple and tampering i s not possible. for instantaneous discharge measurements.

Operation

The gate can be used

Outlet gates for smaller pipe

diameters a r e being developed.

5.19.2

Hydraulic P r o p e r t i e s and Operation The outlet i s basically non-modular because the downstream end i s usually submerged.

The discharge for a given opening i s thus a function of the difference

in levels between the water surface in the supply canal and the f a r m o r field watercourse.

The relationships between working head, gate opening and discharge

have been determined experimentally for the standard 400 m m diameter outlet under submerged conditions and a r e shown in Table 5-5.

In Table 5-5 the gate opening i s converted into the number of revolutions of the screw used to lift the gate.

This enables the table to be used directly for

determining the discharge for any given head.

The head i s r e a d f r o m staff

gauges permanently installed at both ends of the outlet.

The table shown i s a

sample extract f r o m the original table which ranges from 3 to 32 screw revolutions and correspondingly f r o m 2.5 to 407 l i t r e s per second. this outlet i s not available.

A discharge formula for

TABLE

5-5

D i s c h a r g e Table f o r 400 m m D i a m e t e r Gated P i p e Outlet (Submerged Condition)

"Number of s c r e w revolutions ?wk)

14

15

16

17

20 18 19 D i s c h a r g e in l / s

21

22

23

24

/

%

GATE

GUIDE DETAIL

..

SECTIONAL

ELEVATION

G o k guide,

,

I Drill fhoieg (r

------

Grout joint or direct*d

------

L

I ' Additlonol pope as noadad - not part d structure described 1

-.(

END ELEVATION

PLAN

VIEW N O R Y ~ LTO HEADWALL

SIDE ELEVATION

DIMENSIONS

I

PIPE

a

wins ocmra top min.

'fo.c.

4"0. C mar.

QUANTITIES FOR DIFFERENT CAPACITIES AS INDICATED

IHEADWAUI

GATE

I

QUANTITIES

FIGURE 5-5.6. - Concrete pipe outlet from primary to secondary farm ditches. ( 1 3 )

.

5.20

OUTLET STRUCTURES ON THE FARM On the f a r m outlet s t r u c t u r e s and devices a r e used to divert water f r o m a p r i m a r y into a secondary watercourse o r ditch o r f r o m a head ditch onto the field. Structures for these purposes a r e usually small with capacities f r o m a few l i t r e s p e r second up t o the maximum discharge delivered to the f a r m . The type and capacity 'of field outlets depends primarily on the method of irrigation. ditch.

In basin irrigation, outlets a r e spaced along each side of the supply

Each outlet may s e r v e one o r s e v e r a l successive basins inter connected

with control gates.

Capacities of outlets m a y be a s high a s 500 l i t r e s p e r second.

Outlets should be provided with slide gates o r flashboards in o r d e r to allow control of flow and basin water level; this i s of paramount importance in r i c e irrigation.

.A

commonly used permanent concrete pipe outlet discharging f r o m p r i m a r y

ditches into field ditches i s shown in Figure 5-56. permanent field outlet a s well.

This type can be used a s a

Another type of concrete pipe outlet with a t a p a t

one end i s shown in Figure 5-57. Table 5-6 gives the approximate discharge of these types of outlets. Reference may a l s o be made to the pipe outlets discussed in previous Sections of this chapter.

TABLE

5-6

Flow through Concrete Pipe Field Outlet in L i t r e s p e r Second

Diameter of pipe (cm)

P r e s s u r e head 5

10

15

- cm 20

25

PERSPECTIVE

FIGURE 5-57. end.

Figures .5-58,

-

SECTION OF TAP

Concrete pipe outlet with tap at one

5-59 and 5 - 6 0

show commonly used wooden outlet boxes.

For the selection of a suitable width for such outlets reference may be made to Table 5-7.

FIGURE 5-58.

-

Wooden outlet for furrow irrigation ( 1 3)

FIGURE 5-59. - Wooden outlet for basin or border irrigation maximum discharge around 85 l / s ( 1 3 ) .

-

FIGURE 5-60. irrigation.

- Wooden outlet for basin o r border

TABLE

5-7

Approximate D i s c h a r g e of Small Wooden F i e l d Outlets Depth of w a t e r o v e r the s i l l a t the intake cm

"

D i s c h a r g e p e r 10 c m width of sill-1/ l i t r e s p e r second

Valid in the range of 30 t o 100 c m width.

The m o s t popular type of outlet device in f u r r o w i r r i g a t i o n i s the siphon.

Siphons

a r e usually p r e - f o r m e d f r o m aluminium o r plastic pipe, but a r e s o m e t i m e s made of flexible m a t e r i a l s such a s butyl r u b b e r o r corrugated plastic.

They have the

advantage s of e a s y installation and r e m o v a l without disturbing the ditch bank and 1

The flow can be regulated by changing

portability' r e d u c e s the n u m b e r required.

the p r e s s u r e head o r varying the s i z e o r number of siphons.

Commercially

available siphons have capacities f r o m 4 l / s up t o s e v e r a l hundred 11s.

The l a r g e

ones a r e s o m e t i m e s u s e a a s field outlets o r for diversion of flow f r o m lined into unlined ditches.

L a r g e ones r e q u i r e a device f o r priming.

TABLE 5-8 Flow through Small Siphons in L i t r e s p e r Second Diameter of syphon cm

I 1

P r e s s u r e head 5

7.5

10

12.5

-

cm 15

17.5

20

The flows through v a r i o u s s i z e s of siphons when operated under different p r e s s u r e h e a d s a r e g i v e n i n Table 5-7.

The p r e s s u r e h e a d i s t h e d i f f e r e n c e i n

elevation between the water s u r f a c e in the f a r m ditch and either the c e n t r e of the outlet if i t i s f r e e flowing o r the water s u r f a c e above the outlet if i t i s submerged. A disadvantage of the siphon i s that i t m a y become deprimed during operation due t o falling water level o r blockage by t r a s h o r s i l t .

Recent r e s e a r c h

c a r r i e d out by the USDA to r e m e d y this problem h a s r e s u l t e d in the Snake River auto- s t a r t siphon a s shown in F i g u r e 5- 61.

This siphon i s equipped with a cup on

each end that holds the water o v e r the ends of the siphon so that a i r cannot e n t e r the tube when the water supply level r e c e d e s .

The cups hold enough water to

maintain the water level above the tube ends for 10 to 14 days; thus no repriming i s required during this period.

Both

CURS

must be at approximately the same

elevation.

The minimum recommended cup diameter i s 2.25 times the tube

diameter.

The length of the cup from bottom to lip equals 1.41 (E)

=

E

where (inches),

d

nominal 10 day evaporation (inches), D

= tube diameter (inches), S

=

=

-

(D

- d - S)

diameter of cup

distance between bottom of cup

and tube end (inches).

R A W SCREEN f INLET END

FIGURE 5-61..

-

CLAWS

- TO

Snake River auto-start eiphon (89).

In border irrigation the same type of outlets a s used for basin irrigation can be installed a s well a s siphons.

Outlet capacity depends on the width of the

check between two borders, the slopes and the' soil, and may range from a few l i t r e s per second up to 300 11s.

Structures may be temporary (portable) or

permanently installed and a r e usually equipped with flashboards or slide gates. FrequentIy used materials a r e wood and concrete.

The bottom of the outlets

should be placed a t a lower elevation than the surface of the border so that the water will discharge into a pool at the downstream end.

Dii~mmmotioSution Through Open TIP~ Box

FIGURE 5-62 ( a ) and (b). irrigation. ( 1 3 , 65)

-

Outlet boxes for border

LIST O F REFERENCES

USDA Soil Conservation S e r v i c e - National Engineering Handbook. I r r i g a t i o n , Chapter 3, Planning F a r m I r r i g a t i o n S y s t e m s . Canada.

Water Rights B r a n c h of the Department of Lands and F o r e s t s . P r a c t i c a l Information on Irrigation for B r i t i s h Columbia Water U s e r s .

S i r A. Gibb & P a r t n e r s . irrigation and Drainage P r o j e c t in the Balikh Basin. G e n e r a l ~ d m i n i s t r a t i o nf o r the Development of the E u p h r a t e s Basin. Edward, G. Y. Mechanics of F i e l d I r r i g a t i o n . Scheduling, utilizing Bouyoucos 1951 blocks. Agric. Eng. 32: pp 148- 151, 154. Pruit.

Irrigation Scheduling Guide. A g r . Eng. 37: pp 180- 181.

A m e r i c a n Society of Agricultural E n g i n e e r s . Managing Irrigation Water on the F a r m . T r a n s a c t i o n s 8: pp 433-436. F e r g u s o n , D. S. I r r i g a t i o n & Drainage (with p a r t i c u l a r r e f e r e n c e s t o efficiency 1968 of land and water u s e ) . Water for P e a c e , Vol 7 . U. S. Government P r i n t i n g Office: pp 252-261. M e r r i a m , J. L. I r r i g a t i o n System Evaluation and Improvement. 1968 S t r e e t , San L u i s , Obispo, California 93401.

1415 Monterey

M. B a k e r Jr. INC. R o c h e s t e r , Pennsylvania and H a r z a Engineering Company. 1955 J o r d a n Valley P r o j e c t , M a s t e r P l a n Report, Vol VII. Recommended Irrigation Features . Nath, B. 19 69

C r i t e r i a for Fixing Outlets and t h e i r Commands of P e r e n n i a l I r r i g a t i o n S y s t e m s in Northern India. 5th I r r i g a t i o n P r a c t i c e s Seminar NESA Region, New Delhi.

L a m b a , S. S. and Murthy, A. N. Determination of Command A r e a of Outlets. 5th I r r i g a t i o n P r a c t i c e s S e m i n a r NESA Region, New Delhi. 1969 Cummins, J. 1959 U. S. A. 1951

Check S t r u c t u r e s in I r r i g a t i o n Channels.

USDA B u r e a u of Reclamation, Washington.

ICID Annual Bulletin.

I r r i g a t i o n A d v i s e r s ' Guide.

Robinson, E . P . Water Right S t r u c t u r e s f o r F a r m Channels. Victoria State 19 64 R i v e r s and Water Supply Commission, Melbourne. P o r t l a n d Cement A s s o c . , Chicago. 1952

I r r i g a t i o n with Concrete P i p e .

L i s t of References Cont'd.

-

16.

Proceedings of The ASCE ( J R 4 ) 1967

17.

Butler, S. S. Irrigation Systems of the T i g r i s and Euphrates Valleys. P r o c . ASCE, 1960 ( J R ~ )No 86

18.

Second Regional Irrigation p r a c t i c e s Leadership Seminar, Teheran. 1958

Rectangular Cut-throat Flow ~ e a s u r i Flume. n~

Rolley, P. Ameliorations Agricoles. Irrigations. 480 p. 1953 ~ a i l l i ' e r e & Fils. 19, Rue Hautefeuille, P a r i s .

L i b r a i r i e J. -B.

Lauritzen, C. W. Butyl for the Collection, Storage and Conveyance of Water. 1967 Bull. 465 Utah State University, Logan, U. S. A. r Colorado State University Experiment Station - F a r m Irrigation Structures. 1966 Bulletin No.496-S. F o r t Collins. . Houk, I. E. Irrigation Engineering. 1951 OEEC, Greece 1954

-

F a r m Irrigation.

Vol I and 11. John Wiley & Sons Inc. New York

Report on a Training Course.

Boardman, J. A Comparative Study of Irrigation Canal Outlets. Southampton, Department of Civil Engineering. 1966 Mahbub, S. J. and Gulhati, N. D. Irrigation Outlets. 1951 K a s h m e r e Gate, Delhi.

184 p.

University of

Atma Ram & Sons,

USDA Bureau of Reclamation, Reclamation Manual - Canals and Related Structures. Design Supplement No 3 to P a r t 2 Engineering Design of Volume X, 1952 Design and Construction. USDA Soil Conservation Service, National Engineering Handbook - Irrigation. 1967 Chapter 3, Section 15. Planning F a r m Irrigation Systems. Washington. Neyrpic Grenoble

-

~ k a l i s a t i o n sNeyrpic dans l e domaine de l'irrigation. R. J. 580.

29.

Rouse, H. Engineering Hydraulics. Proceedings of the Fourth Hydraulic 1949 Conference, Iowa, Institute of Hydraulic Research, Iowa, June 12-15. John Wiley & Sons Inc. New York.

30.

Priyani, V. B. Irrigation Engineering. Revised Edition. 1964 Tulsi Sadan, 'Statron Road, Anand, India.

31.

Cantor, L. M. A World Geography of Irrigation. Olives & Boyd, Tweeddale Court, ~ Street, London. Edinburgh and 3 9 Welbeck

Charotar Book Stall

L i s t of R e f e r e n c e s Cont'd. Ozal, K. 1965

P r i n c i p l e s and Methods of Capacity Determination of I r r i g a t i o n Distribution Systems. Miihendislik Fakiiltesi, Ankara, Yayin, Publication No 22. Ankara.

Robinson A. R. e t al. Distribution Control and M e a s u r e m e n t of I r r i g a t i o n Water on 1963 the F a r m . USDA Miscellaneous Publication No 926. Thomas, C. W. World P r a c t i c e s i n Water M e a s u r e m e n t a t Turnouts. 1960 I r r i g . & Drainage Division. ASCE. 86 (IR2: pp 29-54).

Journ.

Associazione Nationale della Bonifiche delle I r r i g a z i o n i e dg Migliamenti fondiari 1958 Manufatti Idraulici N o r m a l i p e r l a Bonifica e l ' i r r i g a z i o n e . Tipographia Tip. Roma. P e t e r k a , A. J. Water M e a s u r e m e n t P r o c e d u r e s . I r r i g a t i o n O p e r a t o r s ' Workshop 19 64 USBR Division of I r r i g a t i o n Operations Office of Chief Engineer, Denver, Col. F r e e m a n , P. A. F l u i d i c s in Water Management. 0 1971

A g r i c u l t u r a l Engineering, June.

H a r z a Engineering Company International. Etude d'un P r o g r a m m e d 1 ~ r n e / n a g e m e n t Hydro-Agricole d e s T e r r e s Rizicultivables d e l a ~ a s s e - ~ u i n e / eRapport . 19 69 F i n a l Vol IV, Juillet. H e r sey-Sparling M e t e r Company. 1964

Sparling I r r i g a t i o n M e t e r s .

Bulletin 500.

R a y a r a t n a m , V. N. Distribut?on S y s t e m s to S e r v e S m a l l Holdings. 3 r d Regional 1960 I r r i g a t i o n P r a c t i c e s L e a d e r ship Seminar, L a h o r e . F A 0 l i b r a r y : 631.6(063)/N27/ 1960. Deloye, M. and Rebour, H. L 1 i r r i g a t i o n en grande culture. ~ r e / ~ a r a t i oe tn gali is at ion d'un p r o j e c t d'irrigation k l a f e r m e . 286 p. P a r i s . 1958 USDA :Soil Conservation S e r v i c e - Conservation Irrigation in Humid A r e a s . 1957 A g r i c u l t u r e Handbook No 107. Washington. USDI/USDA 1959 FA0 1963

-

I r r i g a t i o n on Western F a r m s .

Agric. Inf. Bull. No 199.

- R e p o r t t o the Government of t h e U.A. R. .for Improvement of I r r i g .

Neyrpic 1951

Practices.

-

By Doneen, L.D.

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NO TATIONS AND SYMBOLS-11

A -

Area A r e a of c r o s s section A r e a of critical section

Breadth o r width (usually a c r o s s the axis of flow) Bed width of canal upstream a f a l l , syphon, aqueduct, etc., and in parent channel in c a s e of intakes and outlets

B1

"

Bed width of canal downstream a fall, syphon, aqueduct, etc. Bed width in the offtake channel below the intake o r watercourse below the outlet Width of throat o r controlling section o r width of w e i r c r e s t acrQss the axis of flow

B

(t)

Width of inlet i

-11 F o r

Width a t outlet end

B

Width of gate opening o r sluice opening

B

Width of stilling basin, cistern, etc.

B

(OUT) (go) (bas) Or

(SB)

terminology and definitions reference should be made to the Multilingual Technical Dictionary on Irrigation and Drainage published by the ICID in 1967.

c_ coefficient of d i s c h a r g e Coefficient of roughness Coefficient of submergence Coefficient of submergence of hydraulic jump Coefficient in Chezy's f o r m u l a Coefficient, approach velocity

Depth of canal Designed depth of canal (if distinguished) Depth of canal u p s t r e a m of f a l l s , proportional d i s t r i b u t o r s o r d i v i s o r s , syphons, aqueducts, etc. , and in p a r e n t channels of outlets and offtake channels 0 Depth of canal downstream of f a l l s , etc. and depth of offtake channels 'below intakes and of w a t e r c o u r s e s below outlets Depth of stilling basin Diameter D i a m e t e r of pipe Discharge D i s c h a r g e intensity o r d i s c h a r g e p e r unit width D i s c h a r g e i n the p a r e n t canal D i s c h a r g e of offtake channels o r outlets S m a l l i n c r e m e n t in d i s c h a r g e D i s t a n c e s and spacings

Efficiencies

Flexibility F r e e board Froude number

Height over hardings Head over c r e s t , etc. Working head Head due to velocity of approach Head l o s s Height of gate opening Height of c r e s t above upstream bed level Height of c r e s t above bottom level of stilling basin. Height of c r e s t above downstream bed level

H (C-SB) H (c-b)

of u p s t r e a m water level above .Height soffits oT orifices, pipes, etc. Height of orifice above c r e s t o r bottom level of control section

H (orf)

Hydraulic d r o p

H (dr)

Depth of flow a t the beginning of hydraulic jump o r supercritical sequent depth Depth of flow a t the end of hydraulic jump o r subcritical sequent depth Critical depth corresponding to minimum energy

Length Length of crest along the axis of flow Length of glacis Length of stilling basin Length of pipe Length of jump

Proportionality

Radius Hydraulic radius Ratio

Sensitivity Shear stress Slope (longitudinal) Side slope

Thickness

Velocity Critical velocity von Karman's constant

Weights Specific weight of fluid