River System & Flow Routing Of North East Region Of Bangladesh_mohammad Ali

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STUDY OF RIVER SYSTEM AND FLOW ROUTING OF <-

NORTH EAST REGION OF BANGLADESH

MOHAMMAD ALl (0097310228) MD. MAKSlJDlJL AMIN (0097310230)

February,

DEPARTMENT

2005

OF CIVIL AND ENVIRONMENTAL

ENGINEERING

SHAH JALAL UNIVERSITY OF SCIENCE AND TECHNOLOGY, SYLHET, BANGLADESH

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STUDY OF RIVER SYSTEM AND FLOW ROUTING OF -

NORTH EAST REGION OF BANGLADESH

A thesis By MOHAMMAD ALl Reg.No.:0097310228 MD. MAKSUDUL AMIN Re~. No. : 0097310230

. February,

2005

Submitted to: The Department of Civil and Environmental

Engineering, Shah Jalal University of

Science and Technology, Sylhet, Bangladesh in partial fulfillment of the requirements for the degree of Bachelor of Science in Civil & Environmental Engineering.

\iI r3il5fls

DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING SHAH JALAL UNIVERSITY

OF SCIENCE AND TECHNOLOGY, BAN(;LA()I~SH

SYLHET,

STUDY OF RIVER SYSTEM AND FLOW ROUTING OF NORTH EAST REGION OF BANGLADESH

An undergraduate thesis submitted to The Department of Civil and Environmental Engineering, Shah Jalal University of Science and Technology, Sylhct, Bangladesh in partial fulfillment of the requirements for the degree of Bachelor of Science in Civil & Environmental Engineering.

Approved as to style and content by:

/

g.() 2. Z'rJCJ§"

Supervisor

Rezaul Kabir Chowdhury Lecturer Dept. of Civil and Environmental Engineering. Shah lalal University of Science & Technology. Sylhet, Bangladesh.

~t1--,l)1

z,/ 5'

Md. Misbah Uddin Lecturer Dept. of Civil and Environmental Engineering. Shah lalal University of Science & Technology. Sylhet, Bangladesh.

External Examiner

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DEDICA TED TO OUR PARENTS

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...

DECLARA TJ()N

We here by declared that the study submitted herewith was performed by us as a study in partial fulfillment of the requirements fiJr the degree of Bachelor of Science in Civil and Environmental engineering from Shah .la\a\ lJni\'ersit~ of Science and Technology (Sl'ST). Sylhet, Bangladesh. This thesis contains no material. which has been accepted for the award of any other degree from any other institution. Further to the best of our knowledge and belief the thesis work contains no martial previously published or written by another person. except ".here specific references are made.

FEBRUARY 2005

MOHAMMAD ALl

Md. t-1aJ<MlrlM.( ~Y\ MD. MAKSUDUL AMIN

ACKNOWLEDGEMENT ,

For conducting this study, we would like to express our heartfelt gratitude to those who definitely deserved that. First of aiL we would like to thank Almighty Allah for giving us the ability to complete the work. We arc extremely indebted to our supervisor Rezaul Kabir Chowdhury, Lecturer, Civil and Environmental I':ngincering Dcpartment. Shah Jalal

.

University of Science and Technology under whose enicient guidance and supervision. this thesis work has been completed. Without his cautious supervision and guidance. it would have been impossible for us to conduct this study. We are also grateful to Md. Misbah Uddin. Lecturer. Md. Aktarul Islam Chowdhury. Assistant Professor, Head. Dcpt. of Civil and Environmcntal Engineering. Dr. Jahir Bin Alam. Assistant Professor, Dr. Mushtaq Ahamcd. Assistant Professor. Muhammad Azizul Haque. Assistant Professor, Md. Shahjahan Kaisar Alam SarkaI'.Assistant Profcssor. Raquibul Alam. Lecturer, Saidur Rhaman Chowdhury, Lecturer, Ribth Sharmin. Lecturer. Department of Civil and Environmental Engineering, Shah lalal Uni\crsity of Science and Technology for providing us necessary document, information and not to mention with wise guidance. We would express our deep gratitude to our friends whose support and encouragement help us to enrich our report. Especially. wc would like to thank all of our respected teachers for their cooperation. We finally thanks to CEGIS. WARPO. BWDB and IWM for utilizing their library. data and valuable suggestions.

II

ABSTRACT

Bangladesh is a deltaic country located at the lower part of the basins of the three greatest rivers of the world the Ganges, the Bhramaputra and the Meghna. Due to geographical position, of Bangladesh is suffering repeatedly extensive damage by flood of the three huge

rivers and the small or middle scale rivers in eastern and northern mountain areas. Each veal' a large amount of economic and lives losses occurred. rhis type of divesting flood occurs at monsoon season. But at present flash flood is great concern for this country, especially in the hilly areas like as North- East region. Flash flood occurs in very short time and cause a g~eat damage of the mature crops of ollr border areas. So it is important to forecast flash l1ood.But there is no reliable forecasting system of l1ash 1100din present world. That is why for the aim of forecasting flash flood of the North-East region. our study is the first step to completion of this long analytical way. Study of the river system and hydrological characteristics is important, which is done by our study. It is important to know the water i!""',

level and the rate of discharge with respect to time and distance to forecast the flash flood, which can also be calculated by a computer based programme is developed. In this research work we have taken two stations namely Dulura and Muslimpur of .Thalukhaliriwr and Lubachara and kanairghat of Lubachara river as a case study. Saint Venant equations lor . . . . oh clO clQ. . d lstnbuted fl00d routmg has been used to dctcrmme - , -=-, of two rivers. 01 ax al

111

TABLE OF CONTENTS

Page No.

DECLARATION ACKNOWLEDGEMENT

II

ABSTRACT

1Il

TABLE OF CONTENTS

IV

LIST OF FIGURES

VlIl

LIST OF TABLES

IX

APPENDIX

XI

Chapter One: Introduction ] .] Background ].].]

General (monsoon) Flooo

] .] .2

Flash Flood

].2 Objective of thc Study

2

].3 Methodology

,)

..,

Chapter Two: Literature Review

2.] Introduction

5

2.2 Types of floods

6

2.3 The factors for causing floods in Bangladesh

7

Chapter Three: Overview of Bangladesh 3.1 Background

9

3.1.1

Geographical Location

9

3.].2

Area and Boundaries

9

3.1.3

Physiography

10

3.1.4

Rivers

10 IV

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3.2 Definition or the seasons in Bangladesh

10

3.3 Climate

11

3.3.1

Atmospheric pressure and winds

12

3.3.2

Temperature

13

3.3.3

Humidity

13

3.3.4

Clouds

13

3.3.5

Rainfall

14

3.3.6

Climatic stations

15 15

3.4 River and Drainage System 3.4.1 Brahmaputra-Jamuna River System,

16

3.4.2 Ganges-Padma River System,

17

3.4.3 Surma-Meghna River System,

17

3.4.4 Chittagong Region Rivcr Systcm.

18 19

3.5 Floodplain

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.

3.5.1

Old Himalayan Piedmont Plain

19

3.5.2

Tista Floodplain

20

3.5.3

Old Brahmaputra Floodplain

20

3.5.4

Jamuna (Young Brahmaputra) Floodplain

20

3.5.5

Haor Basin

21

3.5.6

Surma-Kushiyara Floodplain

21

3.5.7

Middle Meghna Floodplain

22

3.5.8

Lowcr Meghna Floodplain

22

3.5.9

Old Meghna Estuarine Floodplain

22

3.5.10 Young Meghna Estuarine Floodplain

23

3.5.11 Ganges River Floodplain

23

3.5.12 Ganges Tidal Floodplain

24

3.5.13 Sundarbans

24

3.5.] 4 Lower Atrai Basin

r-)

3.5.] 5 Arial Beel 3.5.] 6 Gopalganj-Khulna Peat Basin

25 .,-)

3.5.17 Chittagong Coastal Plain

26

3.5.18 Northern and Eastern Piedmont Plains

26

v

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Chapter Four: Study Site 27

4.1 The North-East Region 4.2 Topography of The Northeast Region and Adjacent rributary Areas 4.2.1 Indo-Burman Ranges 4.2.2

Shillong Plateau

4.2.3

Tura Range

4.2.4

Madhapur Tract

28 28 29 30 30

4.3 Climate

.31

4.3.1

The Monsoon

31

4.3.2

South-West Monsoon (Wet Season)

31

4.3.3

North-East Monsoon (Dry Season)

4.3.4

Inter Monsoon Transitions (Pre-and POst-llh1l1S001l seasolls)

"'') :>-

32

4.4 North-East Region Plain

33

4.5 Surma-Meghna River System

35

4.6 Regional River System

36

4.6. I

Barak system

4.6.2

Kushiyara system

4.6.3

Kangsha-Baulaieystem

4.6.4

Meghna system

4.6.5

Old Brahmaputra- Lakhya system

37

4.6.6

Surma system

37

36 36 37

38

4.6.6.1 Lubha

39

4.6.6.2 Sarigowain

39

4.6.6.3 Piyain 4.6.6.4 Umium

40

4.6.6.5 Dhalai

40

4.6.6.61halukhaIi

41

4.6.6.71adukata

41

VI

Chapter Five: Flood Routing 5.I Introduction

43

5.2 Lumped System Routing

43

5.3 Level Pool Routing

46

5.4 Distributed Flow Routing

49

5.4.I

Saint-Venant Equations

50

5.4.2

Continuity Equation

50

5.4.3

Momentum Equation

5.4.4

Momentum

53

5.4.5

Net Momentum Outflow

53

5.4.6

Momentum Storage

54 55

5.5 Finite-Difference Approximations 5.5.1 Finite Differences

56

5.5.2

Explicit Scheme

58

5.5.3

Implicit Scheme

59

5.6 Dynamic Wave Routing

59

5.6. I

Dynamic Stage-Discharge Relationships

60

5.6.2

Implicit Dynamic Wave Model

6 -'

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Chapter Six: Computer Programme and Application 6. I Introduction

66

6.2 Initial input

67

6.3 Output

68

6.4 Programme Execution

68

6.5 Application

69

6.5. I Case Study I

69

6.5.2 Case Study 2

70

VB

Chapter Seven: Conclusion and Recommendation 7.1 Recommendation

71

7.2 Limitation

7l

7.3 Concluding Remark

71

References

1'2

VIII

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List of Fieu res No. of Figures

Description

Page ~o.

Fig 5.2.1

Relationships between discharge and storage

45

Fig 5.2.2

Conceptual interpretation of the time or flood movement.

46

Fig. 5.3.1

Change of storage during a routing period /11

.n

Fig 5.3.2

Development of the storage-out 110\\ runction for \eye! pool muting

~8

on the basis of storage-elevation and elevation outflow curves

Fig. 5.4.2(a)

Elevation View

Fig. 5.4.2(b)

Plan View

51

Fig. 5.4.2(c)

Cross Section

51

Fig. 5.5

The grid on the x-I plane used for numerical solution of the Saint-

56

Venant

Fig 5.6.I.a

.

equations by finite differcnces

Loop rating curves. The ulli rorm Ilow rating curve does not rellect

61

backwater effects, whereas the looped curve docs

,

,-

.51

Fig 5.6.1.b

Loop stage-discharge relation for Red River. Alexandria. Louisiana

61

Fig 5.6.1.c

Aspects of flow in natural rivers

62

Fig 5.6.1.d

Loop rating curve with significant backwater effects.

63

Fig 5.6.2

The x-I solution plane.

64

Fig 6.1

Flow chart of the program execution

68

Fig. 6.2

Interface of the Software.

68

Fig. 6.4.1

The two station of .lhalukhali river.

69

Fig. 6.4.2

The two station of Lubachara river

70

IX

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List of Tables .........

Table No.

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,r-

Description

Page No. 11

Table 3.2

Seasons of Bangladesh

Table 6.5(a)

Monsoon water level or .lhalllkhaliriver.

69

Table 6.5b)

Pre-Monsoon water Icwl of Jhalllkhali river.

69

Table 6.5(c)

Monsoon water level or LlIbachara river

70

Table 6.5(d)

Pre-Monsoon water level of Lubachara river

x

.70

Appendix

...........

Page No.

No. of Figures

Description

Figure: 2.3(a)

The flood affected areas.

Figure: 2.3 (b)

Intensity of Flood of Bangladesh. 1954-1999.

Figure: 3.1(a)

Location and Boundary of Bangladesh.

III

Figure: 3.2 (b)

Physiography of Bangladesh.

IV

Figure: 3.1.4

Main river of Bangladesh.

\'

Figure: 3.3

The Climatic Condition of Bangladesh.

VI

Figure: 3.3.5

Mean Annual Rainfall of Bangladesh

\'11

11

\"111

Figure: 3.4

The Brahmaputra. .Ganges and Meghna basin.

Figure: 3.5

The Flood Prone area of Bangladesh.

IX

Figure: 3.4.1

The Brahmaputra-Jamuna

x

Figure 3.4.2

The Ganges-Padma System.

XI

Figure: 4.1

The North East Region of Bangladesh.

Xll

Figure: 4.2

Regional River System of North East Region.

Xlll

1-

System.

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XI

"'"--

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Chapter One INTRODUCTION

Introductiun

~-

Chapter

INTRODUCTION

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1

1.1 Background Floods are the most common and widespread of all natural disasters-except fire. Most communities in the United States can experience some kind of flooding after spring rains. heavy thunderstorms, or winter snow thaws. Floods can be slow. or fast rising but generally

-

develop over a period of days. From the ancient time, flood has been viewed as a natural calamity caused mainly by

.........

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overflowing of banks of river due to excessive rainl~\11in a river basin or in the upstream of river basin. The flood normally moves from the upstream to the dOVv'nstream of a river in the form of solitary wave. Therefore, it has an advancing rront. a pick & the recession limb. As a flood wave moves down the river channel. the depth or water increases gradually at a-station & water spreads over the section of the station so long the peak of the flood reaches at the station. Flood engulfs first the lowering areas. then the dwelling houses & buildings. afterwards the roads, highways. railways. runways etc. depending on land features. The advancing front may come at a place very quickly or may come slowly. Floods may be categorized as:

. .

General (monsoon) flood Flash flood

There are also some other 1<)J'JllS or Ilood stich as tides. \vayes. tidal bores. back water flow, & flood from cyclone typhoon & hurricane.

1.1.1 General (monsoon) Flood General floods normally advance quite slowly contrary to flash floods. So, people get enough time for moving out from the flooded area to safe places with their movable properties. animals & family members. As such flood advances gradually. it attains peak height slowly &

Inlroducl ion

also recedes from a place quite slowly. Therefore the devastating eflCct of general flood does

-

not depend on advance, rather on the peak height & on the duration of flood. If we try to examine the causes of damage in general floods, it can be observed that the damage is due to pollution. rotting, flushing away, collapsing, falling of trees & structures. decomposition of dead animals, bacterial & microbiological activities of both pathogens & non-pathogens. 1.1.2 Flash Flood Flash floods usually result li'om intense storms dropping large amounts of rain within a brief period. Flash floods occur with little or no warning and can reach full peak in only a few minutes in hilly rivers. It does not give enough time for evacuation or transfer of valuable properties & lives to safe places. As a result most of the damage are done by adyancing front

of such flood. Once the initial damage is done. the rate of damage slows down. Flash floods usually have short duration, supereritical velocity & vcry high dcgrading crfects. Here are five examples of flash floods.

]. Inadequate urban drainage systems transform sm..allintense rainstorms into killer catastrophes such as the flash floods in Dallas. Texas in May. 1995 and in Fort Collins. Colorado in July, 1997. 2. Severe stalled thunderstorms over steep mountain watersheds havc calamitous results as in the Big Thompson Canyon in Colorado in July 1976 whcn 145 peoplt; were killed.

3. In the eastern United States. flash floods oftcn result li'om hurricane landfalls. 4. Areas flooded by ice jams often are not in the designated flood plains but are rather upstream of bridges or other obstacles that allow the ice to accumulate.

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5. Many aging dams in the United States have officials focusing attention on the threat of flash floods resulting from dam breaks.

1.2 Objective of the Study I. Study the river system of North I~astregion. 2. To develop a computer programme to compute the rate of change of discharge and water level of river. 3. Application of computer programme to Jhalukhali and Lubachara river of North East regIon.

")

Introduction

. -, 1.3 Methodology

The basic continuity and momentum equation derived for one-dimensional gradually \"aried flow are follow (Chow, 1988)

Finite Difference Equation The conservation form of the Saint-Venant equation is used because this form pro\"idc the versatility required to simulate a wide range of flows from gradual long duration tlood waves in rivers to abrupt waves similar to those caused by a dam failure. The equations are developed as follows: Continuity Equation

(~~ + o( A

where.

;(Au ) _

(I

=

()

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I .:1. a

(is time and !\ is cross sectional area, II is lateral inflow per unit length along the

rIver.

Momentum equation

oQ+ o(p{/ / A) + ~A ot

ox

ox _ ( oy

,).() + ,I.,' ./" + ,\,

) _ lilfl'x +H'IN

= ()

1.3.b

where, x = longitudinal distance along the channel or river (kill) (

= time

(hr.)

A = cross-section area of flow (m2) q = lateral inflow per unit length along the channel (m~ s) h

= water

Vx =

surface elevation

(m)

velocity oflateral flow in the direction of channel Ilow (m/s)

,\j= friction slope Se = eddy loss slope B = width of the channel at the water surface (m) Wf= wind shear force (N) fJ = momentum correction factor (1.0 for straight prismatic channel and 1.33 for river valleys with floodplains)

~ = acceleration due to gravity (m/s2) The above basic equations are hyperbolic partial difTcrL'ntialand Ill' analytical solutions cxist. The weighted four point finite difTerence approxim;ltions given by equations 5.6.2.a and 3

Introduction

5.6.2.c are used for dynamic routing with the Saint-Venant equations. The spatial dcriyatiyes aQ and ah are estimatcd bctwecn adjacent time lines according to 5.6.2.b ax ax .

aQ -

--

1)'+1 _(J/tl 0 ~,+I -,

ax ah -= ax here 0.5

$

Vx,

(J' -()' + (I - 0) -- It1 - ,

... I ..J.c

Vx,

h 1+1 - h,+1 hi - h' e ,+1 , +(I-O)-..!2I---, Vx, \7x,

1.3.d

0 $ 1.0. This scheme has a second-order accuracy when e = 0.5 and a

first-ordcr accuracy whenO = 1.0 and the time derivati\'es are estimated using 5.6.:!.a.

aQ

Q1'+I + Q1+1 /+I - Q' - Q' 1 ,+1

at

2V'1

-=

-

...............................

...

I._"f

The continuity and momentum equations are considered at each of the N-I rectangular grids show in fig.5.6.2.a, between the upstream boundary at i = I and the downstream boundary at i=N. this yields 2N-2 cquations. There arc two ul1knO\\I1at each of the N grid point (Q. h). so there are two unknown in all. The two additional equations required to complete the solution are supplied by the upstream and down stream boundary condition. The upstream boundary condition is usually specified by as a know stage hydrograph, a known discharge hydrograph or known relationship between stage and discharge.

4

Chapter Two LITERATURE REVIEW

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Lllerwure

J\ev/ew

Chapter 2 LITERATURE REVIEW

2.1 Introduction Flood relatively high flow of water that overtops the natural or artificial banks in any or the reaches of a stream. When banks are overtopped. \\ ater spreads over the floodplain and generally causes problems for inhabitants. crops and vegetation. Since floodplain is a desirable location for man and his activities. it is important to control floods so that the damage does not exceed an acceptable level. Floods are more or less a recurring phenomenon in Bangladesh and often have heen within tolerable limits. But occasionally they become devastating. Each year in Bangladesh

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about 26,000 sq km, 18% of the country is flooded. Ihiring severe floods. the affected area may exceed 55% of the total area of the country. In an average year. 844.000 million cubic metre of water flows into the country during the humid period (May to October) through the three main rivers the Ganges, the Brahmaputra-Jamuna and the Meghna (figure: 2.1). This volume is 95% of the total annual inflow. By comparison only about 187.000 million eu m of stream flow is generated by rainfall inside the country during the same period. In Bangladesh, the definition of /lood appears differently. During the ram)' season when the water flow exceeds the holding capacity or river, canals (kIwIs). beels. haors. lowlying areas it inundates the whole area causing damage to crops. homesteads. roads and other properties. In the Bangladesh context there is a relation between inundation and cropping. The country has two district seasons, dry season from November to May and wet season from June to October. Over 80% of the rainfall occurs during the monsoon. or rainy season when flood invariably occurs. Long periods of steady rainl~11Itill' several days are common during

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Literature Review

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monsoon season, but sometimes local high intensity rainfalls of short duration occur. Flash Flood occurs on the small rivers of steep gradient in high intensity rainfall areas. While Bangladesh, including its North-East region, is mostly located on the low-lying, relatively featureless deltaic plains of the Ganges or Brahmaputra or Meghna rivers system, more or less high land exists to the west, cast and north of the country. ThesL'highlands exert a potent influence on the climate, weather and hydrology of Bangladesh. 2.2 Types of floods

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Floods in Bangladesh can be divided into three categories: I. Monsoon flood - seasonal, increases slowly and decreases slowly, inundates vast areas and causes huge losses to life and property: 2. Flash flood - water increases and decreases suddenly, generally happens in the valleys of the hilly areas; and 3. Tidal flood - short duration, height is generally 3m to 6m, blocks inland flood drainage. The combined annual flood wave li.om the Ganges, Brahmaputra and l\kghna rin~rs passes through a single outlet, the Lower Meghna tide levels in the Bay of Bengal, rL'oucingthe slope and discharge capacity of the Lower Meghna. The crfects of these high river water levels extend over most of the country and are the main determinant of the drainage condition and capacity. The discharge from minor rivers is reduced and surface drainage by gravity is limited to land above the prevailing flood level. Flooding caused by this drainage congestion

- ....

exists nearly everywhere except in the highland and hilly areas in the northern and eastern parts of the country. In the northwest region an embankment protects the right floodplains of the Tista and

- ....

the Brahmaputra. In the north there are large areas of shallow flooding interspersed with more deeply flooded pockets in meander scars and old Ilood basins. In the south a highland area separates the Ganges from the deep flood basin in chalan beel. Nearly all the monsoon drainage of the northwest region east of the atrai river and south of the Tista river passes through this flood basin to the Brahmaputra. In the northeast region floodplains can be divided into three distinct areas

- the

Brahmaputra

and Padma len floodplain:

the old Brahmaputra

river valley separated from the Brahmaputra by the 1\1adhupur tract: and the Mcghna rivcr basin.

6

Literature Review

The Meghna basin is dominated hy the great Sylhet depression when~ the Surma and

--'-

Kushiyara rivers join to form the Meghna. Iligh \\ ateI' levels in Meghna are controlled downstream by the watcr Icvels of the Padma during the flood season. It tills rapidly with floodwater early in the monsoon and remains full until the Lower Meghna l~llls in the postmonsoon period. Drainage rates of this basin arc low.

-

Hill catchment draining into the northeast and southeast regions is characterised by flash floods that are mostly of short duration but unpredictable in frequency and intensity.

-

Several floods may occur in the flashy rivers in any water year. Throughout

most of the south-central

and south\vest regions. flooding is mainly

associated with tidal influences, storm surges and poor drainage. The northern half of the southcentral region is the principal floodplain of thc Padma and Lower Meghna. while the southern half is the main network of Estuary channels which distribute ahout 4000 of the Lower Meghna flow to the sea. The drainage systcm of the southwest region consists mainly of silted former distributaries

'--

--

of the Ganges connected

to thc sea by a largely I'v10ribund Delta.

Consequently there is extensive shallow nooding.

2.3 The factors for causing floods in Bangladesh a. gcneral low topography of the countr) with major rivcrs draining through Bangladesh including a congcstcd rivcr network system. b. rainfall in the upstream country or in the mainland. c. snow-melt in the Himalayas and glacial displacement (natural). d. river siltation/lateral river contraction/landslides.

-

......

e. synchronisation of m<~jorrivcr pcaks and influcnccs of one river on thc other.

f. human intervention of thc cnvironmcnt. g. tidal and wind effects on slowing dO\vnthc rivcr outflow (backwater effect). h. construction of barrages and protective works along the hanks of the ri\.er some are very close to both the banks

- in the

upper reaches thus making the

passage of water flow downstream increasingly narrower and resulting 111 greater acceleration of water /low downstream prcsently than before. I. deforestation in the upper reaches of thc rivcrs is not only leading acceleration of water flow downstrcam but also lead deposition of loads in the river beds. resulting in reduced channel flow and consequcnt overland runoff water and J.

tectonic anomalies (earthquakc) those change in river now/morphology.

7

Literature Review

History of floods in this country is perhaps inseparable from the history of this land. In every century, the Bengal Delta witnessed the visit of nearly half a dOl.en 1100ds.almost equal to the magnitude and intensity of those in 1987. 1988 and 19l)8 and as many with lesser magnitude. Figure: 2.3(a) shows the flood affected areas. The

monsoon

phenomenon

Mahabharat[Mahabharata]

has

been

mentioned

in the

holy

Ramayafl

and other Vedic books. In the book Artha-Shastra

.%astra] written during the reign of Chandragupta

and

[Artha-

Maurya (321-296 BC) by his minister

Kautilaya. there is mention of the amount of rain at di ITerent places indicating that they had knowledge of rainfall measurements.

The astronomer barahmihir IBarahamihira]

AD) used to predict rain. Astronomers

Arya Bhatta and Brahmagupta

(505-587

also studied the

monsoon. Kalidasa, the famous Sanskrit poet composed poems on monsoon clouds in his Meghdut and Ritusamahara. However, during the ancient times a lady named Khona [Khana] made most of the predictions on meteorology and agrometeorology.

Even to this day the

farmers of Bangladesh remember her verses. The Arabs used the knowledge of the changing pattern of monsoon winds very profitahly for trade with India. The term 'monsoon' is derived

from the Arabic word 'Mausam' meaning SEASON. The lirst comprehensive report of Professor PC Mahalanabish on floods in Bengal between 1870 and 1922 shows that moderate floods have occurred once in two years on an average. while severe floods have occurred once in 6-7

years on an average. Flooding in Bangladesh is a recurring phenomenon. Recurrent floods between 1787

-

and 1830 changed the old course of the Brahmaputra. After a major flood in northern Bengal in 1922, a Flood Committee was formed and a report was published in 1921 on the north Bengal floods between 1870 and 1922. Statistical analysis of available records rcvealed that severe floods can occur every 7 years. and catastrophic floods every 33-50 years. Figure:

-

-

-

2.3(b) Shows the intensity of flood, 1954-1999.

Chapter Three '-

-

-.

-

-

OVERVIEW OF BANGLADESH

Overview u.lBangladesh

Chapter 3

()VERVIEW

OF BANGLADESH

3.1 Background Bangladesh is a deltaic country located at the lower part of the basins of the threel!.reatest rivers of the world -the Ganges, the Bhramaputra and the Meglma. The Ilo{)d plain of these rivers and their tributaries and distributaries covers about 80% of the country. Due to flat topography of the flood plain one-fifth to one-third of the country is annually flooded by

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overflowing

rivers during monsoon (June-September).

In the northeast hill streams. !lash

flood occurs during the pre-monsoon months of April and May causes damage to thy-season crops just before or at the time harvesting and also to t(n\l1S and infrastructures. 6000 of the total runoff is produced by heavy rainfall in the short duration in the three Indian catchmentsthe Meghnalaya, the Barak and the Tripura river catchments.

3.1.1 Geographical location In South Asia, between 20°34' to 26°38' north latitude and 88°01' to 92°41' east longitude. Maximum extension is about 440 km in E-W direction and 760 km in NNW-SSE direction. 3.1.2 Area and Boundaries ~-

Area: 147,570 sq km. Boundaries: West Bengal (India) on the west; West Bengal. Assam and Meghalaya (all the Indian states) on the north; Indian states of Assam. Tripura and Mizoram together with Myanmar on the east; and Bay Of Bengal on the south. The total length of the land border is about 4,246 km, of which 93.9% is shared with India and the rest 6% with Myanmar. Limit of territorial water is 12 nautical miles (22.22 km) and the area of the high seas extending to 200 nautical miles (370.40 km) measured fi'om the baselines constitutes the

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Exclusive Economic Zone (EEZ). Figure: 3.1(a) shmvs It)cationand boundary of Bangladesh. <)

-Overview ufBang/ade.l'h

3.1.3 Physiography:

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Configuration of a land surface including its relief and contours, the distribution of mountains and valleys, the patterns of rivers, and all other featurcs, natural and artificial, that produce the landscape. Although Bangladesh is a small country, it has considerable topographic diversity. It has three distinctive features: 1. a broad alluvial plain subject to frequent flooding, 2. a slightly elevated relatively older plain, and

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3. a small hill region drained by flashy rivers. On the south, a highly irregular dcltaic coastline of about 600 km fissured by many estuarine rivers and channels flowing into the Bay of Bengal. The alluvial plain is part of the larger plain of Bengal, which is sometimes called the I.owcr Gangetic Plain. Elevations of the

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plains are less than 10m above the Sea Lcvel: elcvation furthers decline to a near sea level in the coastal south. The hilly areas of the southeastcrn region of Chittagong, the northeastern hills of

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Sylhet and highlands in the north and northwest are of low elevations. The Chittagong Hills constitute the only significant hill system in the country. It rises steeply to narrow ridgelines (average 36m wide), with elevation ranges bctwecn 600 and 900m abovc mcan sea level. In between the hilly ridges lie the valleys that gencrally run north to south. West of the Chittagong hills is a narrow, wet coastal plain lying parallel to the shorclinl'. Figure: 3.~ (b) shows the physiography of Bangladesh.

3.1.4 Rivers

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Total rivers including tributaries and distributaries are about 700 under three mighty river systems: Ganges-Padma River System, Brahmaputra-Jamuna Rivcr System and SurrnaMeghna River System. Rivers of the southeastern hilly region are considered as the Chittagong Region River System. Principal rivers arc: (ianges, Padma. Brahmaputra. Jamuna, Surma, Kushiyara, Meghna, Karnafuli, Old Brahmaputra, Arial Khan. Buriganga, Shitalakshya, Tista, Atrai, Gorai, Madhumati, Kobadak, Rupsa-Pashur. Feni. Figure: 3.1.4 shows the main river of Bangladesh.

3.2 Definition of the seasons in Banghldesh In Bangladesh the water years is defined as beginning on I st April and ending on 31 st March,

and it is divided into four more or less distinct season:

10

Overview a/Bangladesh -

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April and J\1ay -----

Pre-monsoon --

------

Monsoon --.-

--

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Post-monsoon -.

f---

Dry season

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.June through Septemher ----1 (ktober and Nowmber

,

I

I

I I

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Decemher through March Source: NfIC '. June 1995

3.3 Climate The average condition of the atmosphere near the earth's surl~lce over a long period of time. taking into account temperature, precipitation, humidity, wind, cloud. barometric pressure. etc. Geographical

location and physical settings govern the climate of any country. Bangladesh

extends from 20034'N to 26°38'N latitude and from 88°0 l'E to 92°41'[ longitude. Except the hilly southeast, most of the country is a low-lying plainland. It is surrounded by the Assam

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Hills in the east, the Meghalaya Plateau in the north, the lofty Himalayas lying farther to the north. To its south lies the Bay of Bengal. and to the west lie the plainland of West Bengal and the vast tract of the Gangetic Plain. Bangladesh is located in the tropical monsoon region and its climate is characterized by high temperature, heavy rainfall, onen excessive humidity, and fairly marked seasonal variations. The most striking feature of its climate is the reversal of the wind circulation between summer and winter, which is an integral part of the circulation system of the South Asian subcontinent. From the climatic point of view, three distinct seasons can be recognised in Bangladesh - the cool dry season from November through February, the pre-monsoon hot season from March through May. and the rain)' monsoon season which lasts from June through October. The month of March may also be considered as the spring season. and the period from mid-October through mid-November may be called the autumn season. The dry season begins first in the west-central part of the country by mid-December, where its duration is about four months, and it advances toward east and south. reaching the eastern and southern margins of the country by mid-March where its duration is about one month. The pre-monsoon hot season is characterised by high temperatures amI the occurrence of Thunderstorms. April is the hottest month when mean temperatures range from 2TC in the east and south to 31°C in the west-central part of the country. In the western part. summer temperature

sometimes

reaches up to 40°(', II

Alter the month of April. the temperature

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Overview ufBangladesh

dampens due to increased cloud cover. The pre-monsoon season is the transition period when the northerly or northwesterly winds of the winter season gradually changes to the southerly

or southwesterlywindsof the summermonsoonor rain)' season(June-September).Duringthe early part of this season, the winds arc neither strong nor persistent. Howe\'er. with the progressIOn of this season wind speed increases, and the wind direction becomes more persistent. During the early part of the pre-monsoon season, a narrow zone of aIr mass discontinuity lies across the country that extends from the southwestern part to the northeastern part. This narrow zone of discontinuity lies between the hot dry air coming from the upper Gangetic plain and the warm moist air coming from the Bay of Bengal. As this season progresses,

this discontinuity

weakens and retreats toward northwest. and finally

disappears by the end of the season, making room for the onset of the summer monsoon. The

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rainy season, which coincides with the summer monsoon, is characterised

by southerly or

southwesterly winds, very high humidity, heavy rainfall, and long consecutin~ days of rainfall which arc separated by short spells of dry days. Rainfall in this season is caused by the tropical depressions that enter the country from the Ba) of Bengal.

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Average maximum and minimum winter temperatures are 29°(' and 11°(' respectively: average maximum

and minimum summer temperatures are 34°(' and 210(' respectively.

Annual rainfall 1,194 mm to 3,454 mm. Highest humidity 80% to 100% (August-September), lowest 36% (February-March). Figure: 3.3 shows the climatic condition of Bangladesh.

3.3.1 Atmospheric Pressure and Winds These arc characterised by seasonal reversals between summer and ""'inter in Bangladesh. During the winter season, a centre of high pressure lies over the northwestern part of India. A stream of cold air flows eastward from this high pressure and enters the country through its northeast corner by changing its course clockwise, almost right-angle. This wind is the part of the winter monsoon circulation of the South Asian subcontinent. During this season, wind inside the country generally have a northerly component (I1O\vingfrom north or northwest). On the other hand, during the summer season, a centre of low pressure develops o\'cr the west-central part of India because of intense surt~lCeheat. As a result. a stream of warm and moist air from the Bay of Bengal flows toward the above-mentioned low pressure through Bangladesh (similar flow prevails from the Arabian Sea toward India). This wind is the part of the summer monsoon circulation of the sub-continent. So, the prevailing wind direction in Bangladesh during the summer season has generally a southerly component (flowing from the 12

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. Overview of Bangladesh

south, southwest or southeast). Ilowever, wind directions during the transition seasons (in spring and autumn) are variable. Generally, winds arc stronger in summer (8-16 km/hr) than in winter (3-6 km/hr). The mean pressure is 1,020 millibars in January and 1,005 millibars during March through September. 3.3.2 Temperature January is the coldest month in Bangladesh. Ill)\vever. the cold winter air that moves into the country from the northwestern part of India loses much of its intensity hy the time it reaches the northwestern corner of the country. J\verage temperatures in January vary from about 17°C in the northwestern and northeastern parts to 20°-21°C in the coastal areas. In late December and early January, minimum temperature in the extreme northwestern and northeastern parts of the country reaches within 4 to 7 degrees of freezing point. As the winter season progresses into the pre-monsoon hot season, temperature rises, reaching the maximum in April, which is the middle or the pre-monsoon hot season. Average temperatures in April vary from about 27°C in the northeast to 30°C in the extreme west central part of the country. In some places in Rajshahi and Kushtia districts the maximum temperature in summer season rises up to 40°C or more. After April, temperature decreases slightly during the summer months, which coincides with the rainy season. Widespread cloud covers causes dampening of temperature during the later part of the pre-monsoon season. Avcrage temperatures in July vary from about 27°C in the southeast to 29°(' in the northwestern part of the country.

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3.3.3 Humidity March and April are the least humid months over most of the western part of the country. The lowest average relative humidity (57%) has been recorded in Dinajpur in the month of March. The least humid months in the eastern areas are January to March. Here the lowest monthly average of 58.5% has been recorded at I3rahmanbaria in March. The relative humidity is everywhere over 80% during June through September. The average relative humidity for the whole year ranges from 7X.1IYoat Cox's Bazar to 70.50/0at Pabna. 3.3.4 Clouds In Bangladesh,

the cloud cover has two opposing seasonal patterns, coinciding with the

winter monsoon and the summer monsoon. As a result of the flow of cold-dry winds from the northwestern part of India during the winter season, the cloud cover is at a minimum. On an average, the cloud cover in this seilson is about 10'1()almost all over the country. With the

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Overview ofBungluJesh

progression of the season, the cloud cover increases. reaching 50-60% by the end of the premonsoon hot season. During the summer monsoon season. which is also the rainy season. the

cloud cover is very widespread.In the monthsor .luly and August.which is the middleof the rainy season, the cloud cover varies from 75 to <)O(Yo all over the country. However. it is more extensive in the southern and eastern parts (90();;))than in the northwestern part (75%). After the withdrawal of the summer monsoon. the cloud cover decreases rapidly. dropping to 25% in the northern and western parts, and 40-50% in the southern and eastern parts.

3.3.5 Rainfall

The single most dominant element of the climate of Bangladesh is the rainfall. Because of the country's

location in the tropical monsoon region. the amount of rainfall is very high.

However. there is a distinct seasonal pattern in the annual cycle of rainfall. \vhich is much more pronounced than the annual cycle or temperatlll'e. The winter season is very dry. and accounts for only 2%-4% of the total annual rainf~dl. Rainf~11lduring this season varies from

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less than 2 cm in the west and south to slightly over 4 cm in the northeast. The amount is slightly enhanced in the northeastern part due to the additional uplifting of moist air provided by the Meghalaya Plateau. As the winter season progresses into the pre-monsoon hot season. rainfall increases due to intense surl~lce heat and the influx of moisture from the Bay of

Bengal. Rainfall during this season accounts It)t. IO%-~5%of the total annual rainl~111 which is caused by the thunderstorms or Nor'wester (locally called Kalhllislwkhi [KalbaishakhiJ). The amount of rainfall in this season varies from about 20 em in the west central part to slightly over 80 cm in the northeast. The additional uplifting (by the Meghalaya Plateau) of ,..L,

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the moist air causes higher amount of rainfall in the northeast. Rainfall during the rainy season is caused by the tropical depressions that enter the country from the Bay of Bengal. These account for 70% of the annual total in the eastern part. SO(Yo in the southwest. and slightly over 85% in the northwestern part of Bangladesh. The amount of rainfall in this season varies from 100 cm in the west central part to over 200 cm in the south and northeast. Average rainy days during the season vary from 60 in the west-central part to 95 days in the southeastern and over 100 days in the northeastern part. Geographic distribution of annual rainfall shows a variation from 150 cm in the west-central part of the country to more than 400 cm in the northeastern and southeastern parts. The maximum amount of rainlall has been recorded in the northern part of Sylhet district and in the southeastern part of the country (Cox's Bazar and Bandarban districts). Figure: 3.3.5 shows the mean annual rainf~lll of Bangladesh.

14

Overview v.lBanglade.l'h

-3.3.6 Climatic Stations

Bangladesh Meteorological Departmcnt is rcsponsihle for observation. recording and archiving of climatic data for various stations in the country. Climatic stations arc scatten:d

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around the country - to record the diversc geographic conditions of the country. The major climatic stations from which long-term climatic data are available are - Barisal. Bhola. Bogra. Chittagong, Comilla, Cox's Bazar. Dhaka. Dinajpur. Faridpur. Feni. Ilatiya. Ishwardi. Jessore. Khepupara, Khulna. Kutubdia, Madaripur. Maijdi Court, Mymensingh. Patuakhali. Rajshahi. Rangamati, Rangpur. Saidpur, Sandwip. Sitakunda. Sreemangal. Sylhet and Teknaf.

3.4 River and Drainage System The rivers of Bangladesh are very extensivc and distinguish both the physiography of the country and the life of the people. Bangladesh is called a land of rivers as it has about 700 rivers including tributaries. The rivers are not, however. evenly distributed. For instance. they increase in numbers and size from the northwest of the n0l1hern region to the southeast of the '"""'-

southern region. The total length of all rivcrs. strcams. creeks and channels is about 24.140 km. In terms of catchment size. river length and volume of discharge. some of these rivers are amongst the largest on the earth. Usually the rivers flow south and serve as the main source of water for irrigation and as the principal arteries of commercial transportation. The rivers also provide sweetwater fish. an important source of protein. A large segment of population is thus engaged in the fishing sector. On the other hand. widespread riverbank erosion and regular

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flooding of the major rivers cause enormous hardship and destruction of resources hindering development. But it is also true to say that the river system brings a huge volume of new silt to

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replenish the natural fertility of the agricultural land. Moreover. the enormous volume of sediments that the rivers carry to the Bay of Bengal each year (approximately 2.4 billion tons) builds new land along the sea front. kecping hope alive for future extension of Settlement. Finally, during the monsoon, rivers also drain excess discharge to the Bay. Thus this great river system is the country's principal resource as well as its greatest hazard. Figure: 3.4 shows the Brahmaputra,

Ganges and Meghna basin. The system can be divided into four major

networks: (I) Brahmaputra-Jamuna

river systcm.

(2) Ganges-Padma river system. (3) Surma-Meghna river system. and (4) ChiUagong region rivcr systcm.

15

Overview of Bangladesh

The lirst three river systems together cover a drainage basin of about 1.T2 million sq km. although only 7% of this vast basin lies within Bangladesh. rhe combined annual discharge passing through the system into the Bay of Bengal reaches up to 1.174 billion cu m. ~'10st of the rivers are characterised by line sandy bottoms. /lat slopes. substantial meandering. banks susceptible to erosion, and channel shifting.

3.4.1 Brahmaputra-Jamuna The Brahmaputra-Jamuna

System

river is about 2XOkm long and extends II'om northern Bangladesh

to its confluence with the ganges. Before entering Ibngladesh.

the brahmaputra has a length

of 2,850 km and a catchment area of about 583.000 sq km. The river originates in Tibet as the Yarlung Zangbo Jiang and passes through Arunachal Pradesh of India as Brahmaputra (son of Brahma). Along this route, the river receives water from five major tributaries. of which Dihang and Luhit are prominent. Bangladesh,

At the point where Brahmaputra

it is called the jamuna. The Brahmaputra-Jamuna

meets the tista in

throughout its broad valley

section in Assam and in Bangladesh is l~lIll0USfor its braided nature. shining sub channels. and for the formation of chars (island/sandbars) within the channel. The recorded highest peak flow of Brahmaputra-Jamuna

is 98.000 cumec in 1988: the

maximum velocity ranges 11'om3-4 m/sec with a depth of 21-22m. The average discharge of the river is about 20,000 cumec with average annual silt load of 1.370 tons/sq km. The average slope of the Jamuna is about I: 11.400; however. the local gradient differs quite considerably from the average picture. Within

Bangladesh,

the

Brahmaputra-Jamuna

receives

four

major

right-bank

tributaries - the Budkumar, Dharla, Tista and fIurasagar. The first three arc flashy. rising in steep catchment on the southern side of the Ilimalayan

system between Darjeeling and

Bhutan. The Tista is one of the most important rivers of the northern region. Before 1787 it was the principal water source for the Karatoya, Atrai and Jamuneshwari. A devastating flood

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of 1787 brought in a vast amount of sand wave through the Tista and choked the mouth of the Atrai; as a result the Tista burst into the course of the Ghaghat river. The Tista has kept this course ever since. The present channel within Bangladesh is about 2XO km long. and varies between 280 to 550 m in width. It joins the Brahmaputra just south of Chilmari upazila. The Dharla and Dudhkumar

/low parallel to Tista. The Dharla is a last flowing river in the

monsoon but with the fall of water level it becomes braided. The Dudhkumar is a small river and flows southeast to join the Brahmaputra.

The combined discharge of the Atrai and

Karatoya passes through the Hurasagar to the Jamlllu

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Overview ofBung/uJesh

The old Brahmaputra and the Dhaleshwari are the important lelt bank distributaries of the Jamuna. Prior to the 1787 Assam flam\' the Brahmaputra was the main channel: since then the river has shifted its course southward along the Jhenai and Konai rivers to form the broad. braided Jamuna channel. The old course. named the Old Brahmaputra is now essentially a high-flow spill channel, active only during the monsoon. Taking off at Bahadurabad. the Old Brahmaputra flows southeast, passes by Jamalpur and Mymensingh towns and joins the Meghna at Bhairab Bazar. Its average gradient is 4.76 cm/km. Along its southeasterly journey. Dhaleshwari bifurcates at least twice. Two of its important branches arc the Kaliganga and Buriganga. The Ohaleshwari eventually meets the Shitalakshya at Narayanganj. Figure: 3.4.1 Shows the Brahmaputra-Jamuna System.

3.4.2 Ganges-Padma System This system is part of the greater Ganges system. The (ianges has a total length of about 2.600 km and a catchment arca or approximately 907.000 sq km. Within Bangladesh. Ganges is divided into two sections - lirst, the Ganges. 25X km long. starting li'om the western border with India to its confluence with Jamuna at Goalandaghat. some 72 km west of Dhaka. The

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second is the Padma, 126 km long, running from Goalandaghat confluence to Chandpur where it joins the Meghna. The Padma-Ganges is the central part of the deltaic river system with hundreds of rivers. The total drainage area of Ganges is about 9900400sq km of which only 38,880 sq km lie in Bangladesh. The recorded highest /low of Ganges was 76.000 cumec in 1981. and the mi.lximwn velocity ranging from 4-5 m/sec with depth varying IhHn 20m to 21m. The average discharge

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of the river is about 35,000 cumec with an approximate annual silt load of 492 tons/sq km. The average gradient for the reach between Allahabad to Benaras is I: I0.500, from Farakka (India) to Rampur-Boalia in Rajshahi (Bangladesh) is I: 18.700. from Rampur-Boalia through Hardinge Bridge to Goalandaghat is I :28.000. The slope flattens to 1:37.700 for a distance of 125 km from Goalandaghat to Chandpur. Within Bangladesh. the Mahananda tributary meets the Ganges at Godagari in Rajshahi and the distributary Baral takes off at Charghat on the leftbank. The important distributaries taking olT on the right-bank arc the Mathabhanga. GoraiMadhumati, Kumar, and Arial khan. Figure 3.4.2 Shows the Ganges-Padma System.

3.4.3 The Surma-Meghna system The Meghnais the longest(669 km) river in Bangladesh.It drains one of the heaviestrainfall areas (eg. about 1,000cm at Cherapunjiin Meghalaya)of the world. The river originatesin 17 ----

Overview {dBanglade.l'h

the hills of Shillong and Meghalaya of India. The main source is the Barak river. which has a considerable

catchment

area in the ridge and valley terrain of the Naga-Manipur

hills

bordering Myanmar. The Barak-Meghna has a length or 950 km of which ~40 km lie within Bangladesh. On reaching the border with Bangladesh at Amalshid in Sylhet district. the Barak bifurcates to form the steep and highly flashy rivers Surma and kushiyara. The Surma. flowing on the north of the Sylhet basin, receives tributaries I,'om Ihe Khasia and Jaintia hills of Shillong. Some of the important tributaries of these 1\\0 rivers are Luba. Kulia. shari-goyain. Chalti-nadi, Chengar-khal, piyain. Bogapani. Jadhukala. Someshwari and kangsa. The Surma meets the Meghna at Kuliarchar upazila of Kishoreganj district. The Kushiyara receives left bank tributaries from the Tripura hills. the principal one being the Manu. Unlike the Surma. the tributaries of the Kushiyara are less violent, although prone to producing flash floods. due in part to the lesser elevations and rainl~lll of Tripura hills. Between the Surma and Kushiyara. there lil's a complex basin an:a comprisl.'d of depressions

or haors, meandering flood channds.

and abandoned ri\'er Ct)urscs. This area

remains deeply flooded in the wet season. The two rivers rejoin at Markuli and flow via Bhairab as the Meghna to join Padma at Chandpur. The major tributaries of any size outside the Sylhet basin are the Gumti and khowai rivers, which rise in Tripura. Other hilly streams

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from Meghalaya and Assam join the Meghna. The total drainage area of the ~1cghna up to Bhairab Bazar is about 802,000 sq km, of which 36.~()0 sq km lie in Bangladesh. The peak flow of the Meghna is 19,800 eu m/sec. and the maximum velocity range from I-~ tn'sec with depth varying from 33m to 44m. The average discharge of the river is about 6.500 cu m/sec. It has a steep slope while flowing in the Indian hilly part. At nood stages. the slope of the Meghna downstream

at Bhairab Bazar is only I :88,000. In terms of drainage pattern. the

Meghna exhibits a meandering channel. and at some places it reflects an anastomosing pattern.

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3.4.4 The Chittagong Region System The rivers of Chittagong and Chittagong hill tracts arc not connected to the other riwr systems of the country. The main river or this region is kamaruli. It flows through the region of Chittagong and the Chittagong Hills. It cuts across the hills and runs rapidly downhill to the west and southwest and finally to the Bay of Bengal. Chittagong port is located on the bank of Karnafuli. The river has been dammed upstream at Kaptai to create a water reservoir for hydroelectric power generation. Other important rivcrs or the region are the Feni. ~Iuhuri. Sangu, Matamuhuri, Bakkhali, and Nal'. 18

Overview (?fBan~/aJesh

The four mighty river systems fhming through Bangladesh drain an area of somc 1.5 million sq km. During the wet season the rivers of Bangladesh flow to their maximum leycl. at about 140.000 cumec, and during the dry period, the flow diminishes to 7.000 cumsec. All the estuaries on the Bay of Bengal are known for their many estuarine islands.

3.5 Floodplain Relatively smooth valley floors adjacent to and fi.mned by alleviating ri\'ers which are subject to overflow. In the context of physiographic. Bangladesh may be classitied into three distinct regions. viz (A) Floodplain, (B) Terrace, and (C) Hill areas, Each having distinguishing

characteristics

of its own. A significant part of Bangladesh is

covered by floodplain formed by different rivers of the country. It is a very important type of landscape in the country in the context of agriculture and culture. Most of the fertile cultivable lands belong to this physiographic influenced

by the landscape.

region and the culture of the country is vcry much

Figure: 3.5 shows the flood prone area of Bangladesh.

Floodplains of Bangladesh have been divided into 18 sub-units: (i) Old Himalayan Piedmont Plain; (ii) Tista Floodplain: (iii) Old Brahmaputra Floodplain: (iv) Jamuna

(Young

Brahmaputra)

Floodplain;

(\)

lIaor

Basin: (vi) Surma-Kushiyara

Floodplain; (vii) Meghna Floodplain: (a) Middle Meghna Floodplain. (b) Lower Meghna Floodplain,

(c) Old Meghna

Estuarine

Floodplain.

and (d) Young Meghna

Estuarine

Floodplain; (viii) Ganges River Floodplain; (ix) Ganges Tidal Floodplain: (x) the Sundarbans: (xi) Lower Atrai Basin; (xii) Arial Beel; (xiii) Gopalgal~j-Khulna Peat Basin: (xiv) Chittagong Coastal Plain; and (xv) Northern and Eastern Piedmont Plain. 3.5.1 Old Himalayan Piedmont Plain Comprises gently sloping land at the fi.)othills \\ith epllll\ial and alluvial sediments derived from the hills deposited by rivers or streams. !\ portion of the Old Ilimalayan Piedmont Plain stretches into Bangladesh at the northwestern corner or the country. which occupies the whole of Thakurgaon, and major parts of Panchagarh and Dinajpur districts. This region is covered by Piedmont sands and gravels which were deposited as alluvial fans of the Mahananda and Karatoya Himalayas.

rivers and their distributaries

issuing from the Terai area at the foot of the

The piedmont deposits may possibly be as old as late Pleistocene or early

Holocene, but they are younger than the Madhupur ('lay. The drainage pattern is that of a 19

Overview of Bangladesh

braided river, with broad, smooth, but irregular ridges crossed by numerous broad. shallow channels which frequently branch out and are again reconnected. The Tista abandoned this landscape a long time ago, since when the area appears to have been uplifted so that small .....-

rivers crossing the plain are now entrenched up to about 6m deep (in the north: less in the south) below the main level of the plain. This plain gcntly slopes south from about 96m dO\\l1 to 33m above MSL (mean sea level).

3.5.2 Tista Floodplain A big sub-region stretching between the Old Himalayan Piedmont Plain in the west and the right bank of the N-S flowing Brahmaputra in the east. An elongated outlier representing the floodplain of the ancient Tista extends up to Sherpur upazila of Bogra district in the south. Most of the land is shallowly /looded during the monsoon.

rhere is a shallow depression

along the Ghaghat river, where nooding is of medium depth. The big river courses of the Tista, Dharla and Dudhkumar cut through the plain. The active floodplain of these rivers. with their sandbanks and diyarus, is usually less than six kilometres wide.

3.5.3 Old Brahmaputra Floodplain A remarkable change in the course of the Brahmaputra took place in 1787. In that year. the river shifted from a course around the eastern edge to the western side of the Madhupur Tract. This new portion of the Brahmaputra is named the .lamuna. The old course (Old Brahmaputra) between Bahadurabad and Bhairab shrank through silting into a small seasonal channel only two kilometres broad. The old river had already built up fair high levees on either side over which the present river rarely spills. The Old Brahmaputra floodplain stretching from the southwestern corner of the Garo Hills along the eastcrn rim of the tv1adhupurTract down to the Meghna river exhibits a gentle morphology composed of broad ridges and depressions. The latter are usually flooded to a depth of more than one metrc. whereas the ridges are subject to shallow flooding only in the monsoon.

3.5.4 Jamuna (Young Brahmaputra) Floodplain An alternative name used for the mighty Brahmaputra river, because the Jamuna channel is comparatively new and this course must be clearly distinguished from that of the older one. Before 1787, the Brahmaputra's course swung cast to follow the course of the present Old Brahmaputra. In that year, apparently, a severe /lood had the effect of turning the course southward along the .lenai and Konai rivers to ItJl'lllthe broad. braided .lamuna channel. The 20

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Overview of Bangladesh

change in course seems to have been completed by 1X30. Due to the upliftment of the two large Pleistocene blocks of the l3arind and Madhupur. the zone of subsidence between them was turned in to a rift valley and became the new course of the Brahmaputra as the great Jamuna. Both the left and right banks of the river arc included in this sub-region. The Brahmaputra-Jamuna

floodplain

again could

floodplain, the Jamuna-Ohaleshwari

be subdivided

into the Bangali-Karatoya

floodplain, and diyaras and chars.

The right bank of the Jamuna was once a part 0 f the Tista floodplain, and now through the Bangali distributary of the Jamuna is a part of the bigger floodplain. Several distributaries of the Jamuna flow through the left bank noodplain. of which the Ohaleswari is by far the largest; this floodplain is sub-classed as the Jamuna-Dhaleshwari

floodplain. The southern

part of this sub-region was once a part of the Ganges floodplain. Along the BrahmaputraJamuna, as along the Ganges, there arc many diyaras and chars. In fact there are more of them along this channel than in any other river in Bangladesh. There is a continuous line of chars from where this river enters Bangladesh to the ofT-take point of the Ohaleshwari. Both banks are punctuated by a profusion of diyaras. The soil and topography of chars and diyaras vary considerably. Some of the largest ones have point bars and swales. The elevation betv.ieen the lowest and the highest points of these accretions may be as much as 5m. The diffen:ncc between them and the higher levees on either bank call be up to 6m. Some of the ridges are shallowly flooded but most of the ridges and all the basins of this floodplain region are flooded more than 0.91m deep for about four months (mid-June to mid-October) during the

monsoon.

3.5.5 Haor Basin A large, gentle depressional

feature. is bounded by the Old Brahmaputra floodplain in the

west, the Shillong Plateau's foothills in the north and by the Sylhet high plain in the cast. Its greatest length, both E- Wand

N-S, is just over 113 kl11.numerous lakes (Beels) and large

swamps (Haors) cover this saucer-shaped area of about 7.250 sq km. The sinking of this large area into its present saucer-shape seems to be intimately connected with the up liftment of the

.

Madhupur Tract. Local tradition has it that the land sank 9 to 12m in the last 200 years. This area is still undergoing persistent subsidence. It is regularly nooded during the monsoon.

3.5.6 Surma-Kushiyara Floodplain Comprises the floodplain of the rivers draining frol11the eastern border towards the Sylhet Basin (Haor Basin). Some small hill and piedmont areas near the Sylhet hills, too small to 21 --

Overview u/Bangladesh

". map separately, are included within its boundaries. Elsewhere. the relief generally is smooth. comprising broad ridges and basins, but it is locally irregular alongside river channels. The soils are mainly heavy SILTson the ridges and clays in the basins. This area is subject to flash floods in the pre-monsoon, monsoon and post-monsoon seasons, so the extent and depth of flooding can vary greatly within a few days. Normal /looding is mainly shallow on the ridges and deep in the basins, with flood depths tending to the Haor Basin. The basin centres (haors) stay wet in the dry season. 3.5.7 Middle Meghna Floodplain The main channel of the Meghna upstream from its junction with the Dhaleshwari and Ganges rivers to Bhairab Bazar is known as the Middle Meghna. The floodplain of this river occupies a low-lying landscape of broad islands and many broad meandering channels which formed part of the Brahmaputra before it abandoned this channel when it changed course into the Jamuna two centuries ago. The Meghna sediments are mainly silty and clayey and only thinly

-

bury the former Brahmaputra char deposits. and sandy Brahmaputra sediments occur at the surface on some ridges in the north. Seasonal flooding from the Meghna is mainly deep. Basin sites are submerged early and drain latc.

3.5.8 Lower Meghna Floodplain Southward from the junction of the Meghna and Ganges rivers. the sediments on the left bank of the lower Meghna comprise mixed alluvium from the Ganges, Jamuna and Meghna rivers. These deposits are predominantly silty. Close to the riverbank the deposits are slightly '-.

calcareous because of the inclusion of Gangetic material. Further inland. the sediments are not calcareous and many have been deposited before the Ganges shifted from the Arial Khan channel into its present lower Meghna channel around 1840. This t100dplain area has a very slightly irregular ridge and basin relief, but large area mounds are used for settlement and cultivation. Seasonal flooding was tormerly moderately deep, fluctuating in depth twice daily with the tides in the south, but flooding is mainly shallow and 'by rainwater within the area protected and drained by the Chandpur irrigation project. 3.5.9 Old Meghna Estuarine Floodphlin The landscape in this extensive unit is quite different from that on river and tidal floodplains. The relief is almost level, with little difference in elevation between ridges and basins. Natural rivers and streams are far apart in the southern part, drainage is provided by a network of man22

Overview u.lBangladesh

made canals (khals). The sediments are predominantly deep and silty. out a shallo\\ clay layer in some basin centres overlies them. Seasonal flooding is mainly deep. but it is shallow in the

---

southeast. Some basin centres stay wet throughout the dry season. Virtually every\vhere. this flooding is by rainwater ponded on the land when external rivers are flowing at high levels:

--

the exceptions are the narrow floodplains alongside small rivers (such as the Gumti) which cross the unit from adjoining hill and piedmont areas.

3.5.1 0 Young Meghna Estuarine This sub-region

Floodplain

occupies almost level land within and adjoining the I'v1cghna estuary. It

includes both island and mainland areas. New deposition and erosion are constantly taking place on the margins. continuously altering the shape of the land areas. The sediments are deep silts. which are finally stratified and slightly calcareous. In many. but not in all parts. the soil surface becomes saline to varying degrees in the dry season. Seasonal flooding is mainly shallow, but fluctuates tidally, mainly by rainwater or non-saline rin'r water. Flol)ding by salt water occurs mainly on the land margins and during l'~ceptional high tides in the monsoon: also when storm surges associated with tropical cyclones occur.

3.5.11 Ganges River Floodplain Comprises the active floodplain of the Ganges and the adjoining meander floodplain. The latter mainly comprises a smooth landscape or ridges. basins and old channels. The relief is

-

locally irregular alongside the present and former ri\'er courses. especially in the west. comprising a rapidly alternating series or linear low ridges and depressions. The Ganges channel is constantly shifting within its active floodplain. eroding and depositing large areas of new char land in each flood season. but it is less braided than that of the BrahmaputraJamuna. Ganges alluvium is calcareous when deposited. but most basin clays and some older ridge soils have been decalcified and acidified in their upper layers: lime is found only in the subsoil or substratum of such soils. Clay soils predominate in basins and on the middle parts of most ridges, with loamy soils (and occasionally sands) occurring mainly on ridge crests. Seasonal flooding is mainly shallow in the west and north. with the highest ridge crests remaining above normal flood levels, but flood depths increase towards the east and the south. Flooding occurs mainly because of accumulated rainwater and the raised groundwater taole. except on the active Ganges floodplain and close to distributary channels which cross the meander floodplain. 23

Overview ofBang/aJe.l'h

Because of the small scale or the map, the Mahananda floodplain in the northwest and some detached areas of the Old Meghna Estuarine Floodplain in the southeast have been included within this unit. The Mahananda floodplain comprises all irregular landscapes of mixed Tista and Ganges sediments. The Cllt-offparts of the Meghna floodplain have a smooth relief and predominantly silty soils, which are deepl;. flooded (by rainwater) in the monsoon season. The unit covers most oj" Rajshahi, Natore, Pahna; the whole of K.ushtia, Rajbari. Faridpur, Meherpur, Chuadanga, Jhenaidaha, Magma; parts of Manikganj, Narayanganj, Munshiganj, Shariatpur, Madaripur, Barisal. Gopalganj, Narail, Khulna, Bagerhat. Satkhira; and most of Jessore districts. This physiographic unit is almost triangular in shape and -

bounded by the Ganges tidal floodplain on the south. This unit on its southern end traps the Gopalganj-Khulna Beels. 3.5.12 Ganges Tidal Floodplain The boundary between this unit and the (,anges river floodplain is traditional. The tidal landscape has a low ridge and basin relief crossed by innumerable tidal rivers and creeks. Local differences in elevation generally are less than Im compared with 2-3m on the Ganges

--

floodplain. The sediments are mainly non-calcareous clays but are silty and slightly calcareous on riverbanks and in a transitional zone in the cast adjoining the lower t"feghna. This unit covers most of Satkhira, Khulna, Bagerhat, Pirojpur, Barisal. Patuakhali, 8hola and the whok of the Jhalokati and Barguna districts, but excludes Khllina Sundarhans in the southwest.

-

The rivers carry fresh water throughout the year in the northeast and cast. but saltwater penetrates increasingly further inland towards the west. mainly in the dry season. In the northeast. there is moderately deep flooding in the monsoon, mainly by rainwater ponded on the land when Ganges distributaries and the lower Meghna are at high flood levels. Elsewhere, there is mainly shallow flooding at high tide, either throughout the year or only in the monsoon. except in the extensive areas where tidal flooding is prevented by embankments.

-

Within embankments, there is seasonal flooding with accumulated rainwater. The soils are non-saline throughout the year over substantial areas in the north and cast. but they become saline to varying degrees in the dry season in the southwest. 3.5.13 Sundarbans South of the Ganges tidal floodplain, there is a broad belt of land, barely above sea level with an elevation of only 0.91 m. This very low land of some 4JG7 sq km area, contains the Sundarbans

forest and the Sundarbans reclaimed estates (cultivated 24

land) - classified as

-Overview of Bangladesh

Sundarbans unit. There are two possible causes for the existence of such a large very lo\\! estuarine area

- insufficient deposition by the Ganges distributaries or subsidence. The main

distributaries of the Ganges never flowed through this region. and the small ones that did last a few centuries at the most. The building up or this estuarine area is consequently not complete. On the other hand, it is possible that subsidence has played a major part in depressing this

--

area. There is much evidence of this, such as large ruins in the heart of the swampy estuarine areas, eg at Shekertek and Bedkashi, and the presence of human artifacts and tree stumps. buried in the alluvium many feet below sea level. There is also an isolated part of the Sundarbans (Chakaria Sundarbans) at the mouth oCthe Matamuhuri rin~r near Cox's Bazar. 3.5.14 Lower A trai Basin

A small physiographic unit that occupies a low-lying area where mixed sediments from the Atrai and Ganges rivers and from the Barind Tract Q\'erlie the down-warped southern edge of the Barind Tract. The landscape north of the Atrai river is mainly smooth. but floodplain ridges and extensive basins occur to the south of the river. Heavy clay soils are predominant. but loamy soils occur on ridges in the south and west. Drainage from this unit is blocked when high river levels in the Jamuna burden the exit through the Hurasagar channel. Seasonal ~-

flooding was formerly deep. and extensive areas in (,halan beel used to remain wct throughout the year. The construction of polder projects since the 1960s has improved drainage to some extent. However, deep flooding can still occur within polders as well as outside \vhen there is

-

heavy rainfall locally and when l1ash floods flow down the Atrai or off the adjoining Barind Tract, causing natural or man-made breaches of embankments. 3.5.15 Arial Beel A large depression lying between the Ganges and Dhalcshwari rivers in the south of Dhaka region. Heavy clays occupy almost the whole landscape. Despite the proximity to the two

-----

major river channels, the deep seasonal l100ding is predominantly by accumulated rainwater which is unable to drain into rivers when they are running at high levels. Much of this unit remains wet though the dry season.

3.5.16 Gopalganj-Khulna Peat Basin This basin occupies a number of low-lying areas between the Ganges floodplain and the Ganges tidal floodplain. The two major heels or the area are Baghia and Chanda. Thick deposits of peat occupy perennially wet basins. but they are covered by clay around the edges 25

1. Overview of Bangladesh

T i

and by calcareous silty sediments alongside Ganges distributaries crossing the area. This is the

. '-

largest peat stock basin of Bangladesh. The basins are deeply flooded by c\car rainwater during the monsoon. In the basin close to Khulna. the floodwater is somewhat brackish. Subsidence is still going on in this physiographic unit area.

3.5.17 Chittagong Coastal Plain The plain along the coast extends from the Feni river to the Matamuhuri delta. a distance of 121 km. It comprises gently sloping piedmont plains near the hills. river floodplains alongside the Feni, Kamafuli, Ilaida and other rivers. tidal floodplains along the lower courses of these rivers, a small area of young estuarine floodplain in the north. adjoining the sub-region Young Meghna Estuarine Floodplain, and sandy beach ridges adjoining the coast in the south. Sediments near the hills are mainly silty. locally sandy. with clays more extensive in floodplain basins. The whole of the mainland area is subjected to flash floods. Flooding is mainly shallow and fluctuates in depth with the tide (e:\cept where this is prevented by river or

-

coastal embankments). The average daily rise in the tide is about t\vo metres. Some soils on tidal and estuarine floodplains become saline in the dry season. 3.5.18 Northern

and Eastern Piedmont Plains

Include the generally sloping piedmont plains border the northern and eastern hills. Similar piedmont plains adjoining the hills in Chittagong region have been included in the Chittagong coastal plain. These plains, which comprise coalesced alluvial fans. mainly have silty or sandy deposits near the hills, grading into the basin adjoining neighboring floodplains. The whole area is subject to flash floods during the rainy season. On the higher parts. flooding is mainly intermittent and shallow; but it is moderately deep or deep in the basin. The sub-region covers most or parts of Nalitabari, Tahirpur, BishwamvarpuL Dowarabazar. Companiganj (Sylhet). Gowainghat, Madhabpur, Habiganj Sadar. Chunarughat. SreemangaL Kamalganj and Kulaura upazilas of the country.

26

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Chapter Four --,",-

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STUDY SITE

Study Site

.>-

Chapter 4 STUDY SITE

4.1 The North-East Region The northeast region is defined as the arl:a east of the old Brahmaputra or Lakhya rivCf channel, and north of the upper Meghna river channel and the Titas river basin. It comprises an area of 24,265 km2, and constitutes 17% of the country and 20% of its deltaic sector. It can be divided conveniently into two distinct sub regions, the larger Meghna sub region in the east comprising 4,004 km2 or 16.5% of thl: region. Although the two sub region experience essentially the same climate and are similar geologically, they differ hydrologically. The Meghna sub region receives many /lash-flood from the adjacent Indian states of Tripura which lies south of the region, and Meghalya which lies to the north: it also receives the substantial outflow of the Barak river basin which lies to the east and occupies parts of the Indian states of Assam, Mizoram and Manipur. In contrast the old Brahmaputra sub region mainly receives flood waters spilling into it from the Brahmaputra river. Figure: 4.1 shows the north east region of Bangladesh.

-

Characteristically, the northeast region is flood affected during the wet season. and affected by soil moisture deficits in the dry season. Wet season flooding involves inundation of much of the region particularly in the central part of the Meghna sub region \",here the depth of inundation ranges up to about 7 meters in the lowest-lying areas. The land of the Northeast region and its adjacent tributary areas (45.574 km2) plays an important role in determining the spatial distributions of rainfall, evapotranspiration, surface and ground waters within the region. This role is asserted mainly by the varied topography of the land, but the underlying geological materials and thc overlying vegetativc covers also

27

-,

.'.;tudy Site

influence these distributions. The properties of the land, especially its topography, are in turn functions of the extremely complex geophysical or geological history of the region. The process, which have led to the present form of the region and its adjacent tributary areas are complex and still active. The activity manifests itself in two ways of concern to water resource development and management: a. it is widely believed, though yet to be convincingly proven, that the north east region is slowly subsiding, the subsidence being at a maximum along the region's northern border; b. it is definitely known that the northeast region is seismically active, earthquakes of recent decades having produced some unusual phenomena at the surface, in the form of surficial mounds of sediment squeezed up from subsurface fissures.

4.2 Topography of the Northeast Region and Adjacent Tributary Areas The north-east region and its adjacent tributary areas constitute the river basin of the upper Meghna river. Within this river basin are five topographically and geologically very distinct areas a. the northern Indo-Burma ranges lying to the southeast of the north-east region but including the region's Tripura border area- a strip of land some 30 km wide along the region's southeastern border; b. the southern slopes of the Shillong plateau

lying north of the north-east region, but

towards the northeast; I

--'-

c.

the Tura range lying of the north-east region, but northwest;

d. the Madhapur Tract lying to the southwest of the north-east region; e. the north-east region plain comprised of the north-east region itself, except for its Tripura border area.

4.2.1 Indo-Burman Ranges The Indo-Burma ranges consist of a series of long, narrow north-south oriented anticlinal ridges. The most westerly of these ridges runs along the longitude of Comilla (91°E), and the rest occur at intervals of about 15 km eastwards at least to the longitude of Kohima (9~oE). Maximum elevations of the ridges occur approximately along latitude 23° 30° N, and they increase eastwards from about 80 m near Comilla to arollnd 2700 m ncar Kohima. The ridges plunge northwards from the latitude of their greatest elevation, and disappear beneath the 28

Study Site

~Holocene sediments of the north-east region at a distance of about 30 km northwest of the region's border with Tripura. They are known from seismic investigations of continue north wards beneath both the north-cast region and the CachaI' plain. to the foot of the foot of the Shilong plateau; some geologists believe that the ridges arc deflected cast\\'ards in this area. but geophysical mapping does not support this beliet". The ridges are heavily eroded and almost knife-edge appearance in many places. particularly.towards the east; the erosion products have filled the intervening synclinal valleys to a large extent and, as a result, the valleys are typically wide and flat-bottomed. From the

-

latitude of maximum elevation of the ridges (23°30° N) the valleys west of Tipaimukh fall and open northwards, and all the rivers draining these valleys now northv,:ardsinto either the north cast region or the CachaI' plain; cast of Tipaimukh. however the rivers predominantly flow southwards, apparently due to the intrusion of the Mount Javpo volcano (now extinct) at the end of the Pliocene and the consequent uplifting of the ridges towards the north-east. The rivers flowing between the ranges often cut. always westward. from synclinal valley to the next; river capture has, no doubt, been involved in this cutting but it has occurred where crossfaulting provides the rivers with exploitable zones of weakness in bedrock. The existence of significant cross- faults in the ranges is also suggested buy other major topographical or hydrological features, notaboy Ifaillbm.

Kawadighr I laoI'and Hakaluki Ilaor.

4.2.2 Shillong Plateau

-

The shillongplateau risesto a heightof 1975 m above sea level. and present a very steep face towards the south. This face extends southwards from the edge of the plateau surface to the

.......-

Dauki Fault at its foot; thus most of the 2000 m rise from the plain of the north-east region to the top surface of the Shillong plateu occurs in a very short distance. typically in the order of 15 km. the face is draped with cretaceous limestone dipping at 55° to the south; many springs emerge from this limestone and sustain high waterf~llls.some of which can easily be seen

.----.-

from the Bangladesh side of the border. One of those. in the Umium valley. has been developed recently by India as a small hydropower project. The southern face of the shillong plateau has been deeply incised by a number of fivers, most notably the Jadukata which drains much of the southwestern portion of the

.-

plateau. The valleys of these rivers. together with the intervening ridges. comprise topographic "traps" in which moisture laden air from the south is forced to rise very rapidly before passing northwards over the plateau. Not surprisingly. therefore. some of the world's heaviest rainfalls occur in these valleys; Cherrapunji, on the ridge between the Umium and Dhalai (N) rivers 29

Study Site

-has long claimed the world's records for rainfalls or durations of 3 hours and more and eYen higher rainfalls are now claimed to occur at Mawsynram, somc 30 km west of Cherrapunji on the ridge between the Umium and lhalukhali rivers. Corresponding to the steep land slopes on the southern face of the Shillong plakau are steep river bed slopes. The steepness of the dendritic (convergent) river networks. coupled with the high rainfall, results in the generation of Flash Floods in all the valleys draining from the plateau into Bangladesh. 4.2.3 Tura Range The Tura range consists of Eocene-age sediments. predominantly the Tura sandstone. which have been folded into a prominent anticline. The range runs west-northwest to east-southeast along the south-southwestern edge of the Shillong plateau. As s result of erosion the range is now quite narrow, sharp- crested, and has very steep slopes. Its maximum elevation (1413m) occurs just east of the town of Tura in west Meghalaya; from there the crest of the range descends gradually towards the mouth or the Jadukata river where it terminates at the Dauki faults. To the south-southwest the range gives way to the Neogene plains of \Vestl\kghalaya: these plains feature inselberg-like low hills which arc possibly the eroded n:mnants of lesser

---'-

folds in the Eocene sediments running parallel to the TlIra range anticline. The Tura range is cut through by only one river. the Somes\vari. \\'hich flows along the north-northwestern foot of the ranger for a considerable distance before passing through the range at the Someswari gorge and on southwards into Bangladesh.

--'-

4.2.4 Madhapur Tract The Madhapur Tract lies to the southwest or the Northest Region between the Jamuna {present

-

Barahmpurta) and Old Brahmputra rivers. It consists or an extensisve 94105 km2)tabular slab of Holocene-age clay; although it sis nowhere higher than 25 m PWD. it always remains above seasonal flood levels, although some ponding on its surface occurs locally. Possible as recently as 200 years ago, Torsional movement on the inferred Dubri Falult. which underlies the present course of the Jal11una(Brahamputra). resulted in the Madhapur Tract and its outliers being tilted slightly hl\vards the north and the Barind Tract being tilted slightly towards then south (Morgan and Mclntyre, 1959). Associated echdon faulting is seen in the Madhapur Tract, more particularly along its western side. About 1790 a major earthquake caused a major change in the course of the Brahmaputra River. Formerly it had flowed around the western end of Meghalya. as it does 30

Study Site

today, but is had then followed the course of the present Old Brahmaputra to meet the L'pper Meghna at Bhairab Bazar in the process, it eroded away the eastward extension of the Madhapur tract and replaced it with the extensive alluvial fan deposits which are seen today

-

emanating

from the vicinity

Subsequent

to this earthquake the main flow of the Brahmaputra

weakened

of Bahadurabad

southeastwards

towards

Bhairab

Bazar.

exploited the zone of

bedrock over the Dubri fault to establish the present course of the Jamuna

Brahmaputra, and it eroded away the Holocene clays whieh lormerly joined the barind and

-

Madhapur tracts. As a result, the former river course 't'om the alluvial fan deposits to the east of the old Brahmaputra. Abandonment of the lower course of the old Brahmaputra. between

-

Toke and Bhairab bazaar, was almost total; today flO\.voccurs in this lower course only during the highest floods, and all other flows coming down the old Brahmaputra

pass into the

Lakhya, which closely follows the eastern edge of the Madhapur tract.

4.3 Climate

-

Climate

plays

the

main role in determining

temporal distributions of

rainfall,

evapotranspiration, surface and ground waters. The north-cast region is located entirely to the """"'-

north of the tropic of Cancer, hence its monsoon climate is described as sub-tropical. The subtropical monsoon climate tends to have more sharply ddined seasons than the tropical one. 4.3.1 The Monsoon

-

The sub-tropical monsoon climate of the north-cast region is characterized by a t\vice-yearly reversal of air movement over the region. For about I'(Hlrmonths in winter December through

-

March air flows from the north-east, while I'()rmonths in summer (.June through September) it flows from the southwest. These air flows are called monsoon, that of winter being the "northeast monsoon" while that of summer is the "southwest monsoon". Agricultural activity is closely kinked to the monsoon periods, ruhi Crops (mainly bora rice) being cultivated with irrigation during the dry north-east monsoon, while kllltr;l(almost

exclusively ails and oman

rice) are grown during the southwest monsoon when rainf~lll is abundant.

4.3.2 South-West Monsoon (Wet Season) The southwest monsoon brings moist air into the north-cast region from the Bay of Bengal

-

along a circular route over the Chittagong region so that this air actually approaches the region from the southeast. Rainfall in this season is abundant and it is often referred to as "the monsoon" meaning the rainy season. Typicallu, raint~11Iincrease north-eastwards 11

across the

S'tudy Site -.....

regIOn and reaches a maximum on the southward-being

slopes of the Shillong plateau in

Meghalaya, Cherrapunji, on these slopes, is well known as the wettest place on Earth. its annual rainfall often exceeding 12 meteres. The distribution of the annual rainfall over the region and adjacent tributary areas in India strongly reflects the interaction of the southwest monsoon with the region topography, particularly the Shillong plateau, Across the north-east

-

region rainfall during the southwest monsoon ranges from around 1500 mm(about 6~% of annual total) in the southwest to around 4100 mm (ahout 74<%)in the north-east at the border with Meghalaya. floods occur frequently and thc central part of the region is always flooded to a depth of several metres.

4.3.3 North-East Monsoon (Dry Season)

-

The north-east monsoon brings dry air directly into the region from China. Dry season rainfall ranges from around 80 mm (4
-

--

4.3.4 Inter Monsoon Transitions (Pre-And Post-Monsoon Seasons) A reversal of the monsoons takes about two months. The first reversal occurs in April-May when the change of regional wind directions is from northeast to southwest viva north-east, and the second oeeurs in October-November when the change is from southwest to north-east via southeast. These periods of changing wind direction are called the pre-monsoon and post-

........

-

monsoon seasons.

The pre-monsoon season is characterized by increasing rainfall as the spring reversal progresses, the rainfall ranging from around 490 mm (24%) in the southwest to around 1290 mm (18%) in the North-East, and by flash Hoods of increasing frequency.

.-

The post monsoon season is characterized by decreasing and more sporadic rainfalL the rainfall ranging from around 170 mm (WYo)in the southwest to around 320 mm (6%) in the north-east, and by the draining of flood water which has accumulated during the monsoon

season.

4.4 North-East

Region Plain

The north-east region except for its Tripura border area consists of a deltaic plain which is basically a triangle in plan. The apices of this triangular area may be identified with: a. the bifurcation, just north of Bahaduranad in the northwest. of the Brahmaputra river into the Jamuna and old Brahmaputra rivers,

32 ----

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Study Site

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b. the bifurcation, at Amalshid in the north-east . of the Barak river into the Kushiyara and Surma rivers. c. the confluence, near Satnal in the south. Old Brahmaputra or Lakhya or Ohaleshwari and Meghna rivers. The topography of this plain, all or which lies at elevations below approximately 25 m PWD. is characterized by low relief and by deltaic morphological features. The surface geology consists exclusively of alluvial and swamp sediments or late Holocene age. Throughout the plain the topography consists of a three- dimensional alternation of: I. River channels 2. Natural river levees along the river channel banks 3. Inter- riverine depressions. known as haors. which occupy most of the area

4.5 Surma-Meghna River System One of the three major river systems of Bangladesh. It is the longest river (669 km) system in the country. It also drains one of the world's heaviest rainfall areas (eg about 1.000 cm at Cherapunji, Meghalaya, India). East of Brahmaputra-Jamuna river system is Surma-Meghna River System. The surma originates in the hills or Shillong and Mcghalaya of India. The main source is Barak river. which has a considerable catchment in the ridge and valley terrain of Naga-Manipur hills bordering Myanmar. Barak-Meghna has a length of 950 km of \\hieh 3..W km lies within Bangladesh. On reaching the border \\ith Bangladesh at Amalshid in Sylhet district, Barak bifurcates to form the steep and highly flashy rivers Surma and kushiyara. Surma flows west and then southwest to Sylhet town. From there it flows northwest .....

and west to Sunamganj town. Then it maintains a course southwest and then south to Markuli to meet Kushiyara. The joint course flows upto Bhairab Bazar as the Kalni. Flowing north of the Sylhet basin, Surma receives tributaries from Khasi and Jaintia Iii lis of Shillong plateau. East to west they are Lubha. Hari. Goyain Gang. Piyain. Bogapani. Jadukata. Shomeshwari. Kangsa and Mogra. Surma bifurcates south of Mohanganj soon alter it receives Kangsa and further south the Mogra. The western channel is known as Ohanu in its upper course. Boulai in the middle and Ghorautra lower down. It joins Kalni near Bhairab Bazar of Kishoreganj district and the name Meghna is given to the course from this confluence to the Bay of Bengal. Meghna receives old Brahmaputra on its right-bank at Bhairab Bazar and on the way to the Bay it carries the water of Padma from Chandpur. Kushiyara receives left bank tributaries from Tripura hills. the principal ones being Manu, north of Maulvi Bazar town and bifurcates into northern channel. the Bibiyana and a ..,.., _L)

; . I...

Study Site

-.....

southern one, which resumes the original name, Barak. Bibiyana changes its name to Kalni lower down its course and joins Surma near Ajmiriganj. Barak reCl~i\'l~s Gopla and Khowai from Tripura Hills and falls into Surma at Madna. Unlike Surma, the tributaries of Kushiyara are less violent although prone to producing flash floods in part due to lesser elevations and rainfall of Tripura Hills. Between Surma and Kushiyara, there lies a complex basin area comprised of depressions (haors). Most of the Surma system falls in the Haor basin, where the line of drainage is not clear or well defined. In the piedmont tract from Durgapur to Jaintiapur, the network of streams and channels overflows in the rain;. season and creates vast sheets of water which connect the haors with the rivers. Meghna has two distinct parts. l Jpper Meghna from Bhairab Bazar to Shaitnol is comparatively a small river. Lower Meghna below Shaitnot is one of the largest rivers in the world, because it is the mouth of Ganges-Padma and Brahmaputra-Jamuna rivers. It is a tidal reach carrying almost the entire fluvial discharge of Ganges, Brahmaputra and Upper Meghna river. The net discharge through this river varies from 10,000 cumec in the dry season to 160,000 cumec in the wet season. A little above the confluence, Meghna has a railway bridge over it. The width of the river there is three quarters ora kilometre. Several small channels branch out li'om Meghna, meander through the low land bordering the marginal Tippera surface, ICdby a number of hill streams and rejoin the main river downstream. The most important of these offshoots is Titas. which takes off south of .

Chatalpar and after meandering through two long-bends, extending over 240 km rejoins the Meghna through two channels in Nabinagar upazila. It receives the Howrah hill stream near

Akhaura. Brahmanbaria and Akhaura arc both on the banks of this river. Other offshoots of the Meghna are Pagli, Katalia, Ohanagoda, Matlab and lldhamdi. Meghna and these ofTshoots receive the waters of a number of streams li'om Tripura lIills including Ciumti, lIo\\ rah, Kagni, Senai Buri, Hari, Mangal, Kakri, Pagli, Kurulia, Balujuri, Sonaichhari, Handachhora, Jangalia and Ourduria. All of these are liable to flash floods, but Gumti. Kakri and Howrah are the major ones. They have silted their beds to the extent that they now flow above the mean level of the land when brimful. Embankments have been built to contain them. Every other year one or the other of these streams overflow and cause considerable damage to cmps. livestock and houses. The tectonic evolution indicates that the Meghna-Old Brahmaputra drainage post-dates the Ganges drainage when the main channel of the Ganges was the sole drainage beside Calcutta. As a consequence, the delta of the old Brahmaputra-Meghna

34

river system covers a

Study Site

very small area compared to the flanges delta. Addition of the water of the Padma in recent

---

years has not been able to make any significant contribution in enlarging the delta. The present deltaic Meghna, being the combination of Padma and Meghna. is the largest river in Bangladesh. From the beginning of the delta small islands create two main channels. The larger eastern channel and the smaller western channel measured five to eight kilometres and about two kilometres in width respectively. Ncar Muladi. Shafipur is an offshoot from the western bank. Further south, Meghna is divides into three channels. which arc. from west to cast. IIsha, Shahbazpur and Bamni. The IIsha channel. 5-(d km wide. separates Bhola from the Barisal mainland. The Shahbazpur channel, 5-8 km wide. flows bet\veen Bhola and RamgatiHatiya islands. The Bamni, which used to flow between the islands of Ramgati. and Char Lakkhi and Noakhali mainland forming the main outlet of the Meghna. does not seem to exist now. The estuary of Meghna may be considered to he IIsha and Shahhazpur. which together have a width of 32 km at the sea front. Gumti falls into Meghna at Daudkandi. Another tributary It'om Tippera SurElce is dakatia. The main source of this river was Kakrai. but the little reni cuts back and captured this upper portion. Dakatia now has its source in Chauddagram khal (canal). which connects it with Little Feni. Dakatia sends out a channel southwards, which forms the Noakhali khal. The

-

main channel meanders westward to Shakherhat, from where the old course goes south to join Meghna at Raipur, and the new and stronger channel passes through Chandpur khal to join

~-

west of Chandpur town. For three-fourths or the year tidal currents feed the Dakatia from Meghna. Little Feni follows a very tortuous course southward. and I~lllsinto Meghna estuary. southeast of Companiganj and a few kilometres It.om Big Feni estuary. Little Feni is a tidal river; in the rainy season its flow is around 15.000cusec.

4.6 Regional River System The principal rivers of the northeast region and their more significant tributaries are shown in Figure: 4.2 The principal rivers arc: 0

Barak

0

Kushiyara

0

Surma

0

Kangsha

0

Baulai

0

Old Brahmaputra

35

Study Site ,-1.....

o

Lakhya

o

Meghna

The some part of the region, particularly the topographic depression in the north central area.

-

changes in river locations and inter connections are fairly frequent. Changes in channel locations and river connections within the historical period are discussed in the separate

specialist study on river sedimentation and 11l0rrholog~. For hydrological purposes it is convenient to deal \\ith these ri\'Crs and their tributaries in terms of the following component systems or the river network.

-

-

-

o

Barak system

o

Kushiyara system

o

Surma system

o

Kangsha-Baulai system

o

Meghna system

o

Old Brahmaputra-Lakhya

system

4.6.1 Barak System The Barak is the principal headwater tributary to the Meghna system. It enters the northeast region at Amalshid where it bifurcates. From Amalshid about two-thirds of the average flow of the Barak passes into the Kushiyara. And the other third into the Surma. The Barak drains a substantial area in the Indo-Burma ranges. within which a number of I~l\'orabledam :-;ites exist. If India develops these dam sites the now regimcs or the Kushiyara and Surma would change significantly.

4.6.2 Kushiyara System The Kushiyara constitutes the prince;link between the Barak and the Meghna. In addition to carrying two- third of the Barak's flow, it collects all outflows from Tripura and the SurmaKushyara flood plain. The lowerpart or the Kushiyara is known as the Kalni.

4.6.3 Kangsha-Baulai System The Kangsha collects substantias outflows from western Meghalaya and delivers them into the Baulai which constitutes an extension of the Surma and also collects the flow of the Mogra. There are many past and present channel linkages between the kangsha and its tributaries and between the Kangshsa and the Mogra. Hence it is advisable to consider the Kangsha. Mogra

-

-

- ... Study Site

and Baulai as one system. The Baulai delivers substantially more water into the Meghna than

-

the Kuahiyara.

4.6.4 Meghna System Outflows from the Kushiyara and Kangsha-Baulai systems converge at Dilalpur to from the

-

Meghna. Downstream of Dilalpur the Meghna seems to cut through some hard material. possibly the Madhapur tract in the subsurface, which appears to cause some congestion of the

~-

drainage. A large lake forms upstream in thc monsoon season. Downstream of Bhairab bazaar the hydraulic gradient increases.

4.6.5 Old Brahmaputra-

Lakhya System

Most of the flow in fihe Old Brahmaputra originates as spill from the Brahmaputra just upstream of Bahadurabad. The Old Brahmaputra bifurcates at toke where most of the flow passes to the Lakhya and thence via the Dhaleshwari to the lower Meghna near Santa!. The rest continyues in the Old Brahmaputea channel to join the Meghna near Bhairab bazaar.

4.6.6 Surma System The Surma originates from thc bifurcation of the Barak at Amalshid from where it follows a westerly course until it joins thc Baulai just north of Sukdevpur. Flow in the Surma has been measured by the BWDB in its upper reaches only, since the lower reaches are flooded O\-ef in the wet season. Water balance studies indicate that the Surma's contribution to the Baulai amounts to 2214 m3/s, or 69.7 km3/year.The tributaries total 35.2km3/year. or 20.3% of the total water supply to the Meghna sub region; this is equivalent to a mean annual flow of 1116m/s. the inflows occur mainly in the monsoon season partly as Flash Floods with peak flows ranging up to 30 or more times average flows. Base flows in these rivers are \'ery small by the end of the dry season, and may dry up compktdy.

The Surma also collects 35% of the

Barak inflow. Discharge in the Surma been measured by the BWDB at Kanairghat and Sylhet. The 22 years of record for Kanairghat indicate a mean annual discharge of 549 m3/s, and a range of daily discharges from 2.2m/s to 2730m3/s. The Surma, the third largest river of the Meghna sub-region, rcceives on average 35%

of the Barak flow and also collects inflows from the eastern 7540 km2 or 56°~, of the trinity area in Meghslya. This area occupies most of the southern slopes of the Shillong plateau.

37

Study Site

-

which rises to a maximum elevation of 2575 m and records the ,,'orld's

greatest annual

rainfall. The Surma has seven significant tributaries originating in f\1eghalaya and entering from the north; in downstream (East to West) order these are:

- .-

0

Luba

0

Sarigowain

0

Piyain

0

Dhalai

0

Umium

0

Jhalukhali

0

Jadukhata

The Jadukata also drains the eastern part of the Tura range. As seen from Banlaresh all these catchment appear to be covered by secondary forest. Figure: 4.2 Shows Norh- East region

rIvers.

--

4.6.6.1 Lubha With India, the Lubha has a catchmcnt area of 771 kl11~the catchment is yery steep, rising in a distance of 35 km from an elevation of 10 111at the border to an elevation of 1627 m on the

plateau. The Lubha and its tributaries arc deeply incised into the southern slope of the plateau, and the entire outflow from the catchment passes through at the border of 109.3 m3/s. or 3.4 kInJIyear. Within Banglaresh, the Lubha follows a southcrly curse for 7 km to its confluence with the Surma at Lubhachara. Just within the border a spill channel takes off from the right bank. but it is thought to be rarely active. About ~ km upstream of Lubhachara a small tributary enters at he right bank; when the Lubhais in flood. back flow into this tributary causes spill onto agricultural lands west of the Lubha. Under normal flow conditions Lubha water passes down the Surma through Kanairghat, but hydraulic conditions at the Lubha/ Surma contllience are more complicated when one river is in high flood and the other is not. When the Lubhais in high flood and the Surma is not, some Lubha water flows up the Surma towards Amalshid and may even enter the Kushiyara. When the Surma is in high flood and the Lubha is not. Surma water backs up the Lubha at least as far as Lubhachara and possibly as far as the Indo- Bangladesh border. Discharges in the Lubha have been measured by the BWDB at Lubhachara. Unfortunately, since the current meter measurements "'ere made in the period 1971-80. and 38

Study Site

~the water level observations in the period 19X2-90. it is not possible to process the data into

--

mean

discharges

since rating

curves

cannot

be established.

The 212 current

meter

measurements available for Lubhachara indicate a mean annual discharge of 124.2 n//s. and a maximum discharge of 800 m3Is.

4.6.6.2 Sarigowain With in India, the Sarigowain has a catchment area or 840 km2 the catchment is very steep. rising in a distance of 35 km from an elevation or 10m at the border to an elevation of I-W5 m on the plateau. In the lower half or the catchment the Sarigowain and distributaries are deeply incised into the southern slope of the plateau, and the entrire outflow from the catchment passes through a gorge on the Indian side of the border. Water balance studies indicate a mean annual outflow at the border of 131 m3Is, or 4.1 km3/yrar. Within Bangladesh, the Sarigowain follows a southwesterly course for about 60 km. through Sarighat, Gowainghat and Salutukar. to its confluence with the Surma about 10 km upstream of Chhatak. At the border. 7 km upstream or Sarighat. a spill channel takes off from the right bank of the Sarigowain, and rollows a westerly course before re-joining the Sarigowain just upstream of Gowainghal. At Gowainghat the Sarigowain is joined by the Jaflong spill channel of the Piyain. Discharges of the Sarigowain have been measured by the BWDB at Sarighat and Salutikar. The 26 years of record available for Sarighat indicate a

meanannualdischargeof 130m3/s,and a rangein dailydischargefrom2.5 mJ/sto 1730m:; s. 4.6.6.3 Piyain Within India, the piyain has catchment area 0 1003 km '. the catchment is very steep rising in a distance of 40 km from an elevation of 10 mat he border to an elevation of 1945 m on the plateau. In the lower half of the catchment the Piyain and its tributaries are deeply incised into the southern slope of the plateau, and the entire outflow from the catchment passes through a gorge on the Indian side of the border. Water balance studies indicate a mean annual outflow

at the borderof 190.5m3/sor 6.0 kmJ/year. -

Within Banglaresh. the Piyain rollows a southwesterly course for 35 km. through Ratnerbhanga and Companiganj, to its confluence with the Surma at chhatak. At the border. 7 km upstream of Ratnerbhaga, a significant spill channel takes off from the left bank of the Piyain and follows a southerly course, through Jaflong. to join the Sarigowain at G owainghat. At Companiganj the Piyain is jopined by the Dhalai. and at Ambari it is joined by the main channel. 39

.....

Study.,.jite ......

4.6.6.4 Umium

-

Within India, the Umium has a catchment area of 431 km. the catchmcnt is very steep. rising in a distance of 50 km from an elevation of 10m at the b04der to a maximum elevation of 1965 m on the Plaeau. Except in their highest reaches. the Umium and its tributaries are deeply incised into the southern slope of the plateau. and the entire outflo\\' passes through a

,....-

gorge on the Indian side of the horder. From the Gorge the llmium follows a southeasterly course for about 8 km across a t~lirlyextensive allU\'ial area before it enters Bangladesh at Chelasonapur. Within this alluvial area the Umium birurcates twice. so that the outflow enters Bangladesh through a main channel and two spill channel takes off westwards towards the border. The second bifurcation occurs in the center or the alluvial area. from where a major sill channel, the Nawagang, takes off southwestwards to enter Bangladesh at Urugoan. Water balance studies indicate a mean annual outflow at the border of 90A m/s. or 2.9 km year. Cherrapunji, site of the world's largest annual rainf~lli.is located on the castern watershed of the catchment, while Mawsynram with comparahk raint~lll is located on its western watershed.

-..........

Within Bangladesh, the Umium turns south at Chelasonapur and maintains a southerlu course for 10 km to its confluence with the Surma at Chhatak. At Urugoan the Nawaganga also turns south, maintaining this course for 8 km its confluence with the Surma at Oohalia. Between Urugaan and Oohalia the Nawagang is joined by the first spill channel and sc\'eral streams coming off the lower slopes of the plateau. The BWOB opened gauging stations on the lJmium at Chclasonapur and on the Nawagang at Urugoan in 1990. 4.6.6.5 Dhalai Within India, the Ohalai has a catchment area of 340 km2.The catchment is very steep. rising in a distance of 30 km from an elevation of 10m at the border to a maximum elevation of 1892 m on the plateau. Throughout the catchment the Ohalai and its tributaries are dceply incised into the southern slope of the plateau. and the entire outflow passcs through a gorge on

--

the Indian side of the. From the gorge the Dhalai follows a southwesterly course for about 2 km before it enters Bangladesh at Islampur. Water balance studies indicate a mean annual outflow at the border of 83.2 mis, or 2.6 km/year. Cherrapunji, site of the world's greatest annual rainfall, located on the western watershed of the catchment. Within Bangladesh. the

Ohalai follows a southerlycourse for about 10 km from Islampur to its confluence with the

40

Study Site

Piyain at Comaniganj. The BWDB opened a gauging station on the Dhalai at Islampur in 1990.

4.6.6.6 Jhalukhali Within India, the ]halukhali has a catchment area of 591 knr~ . The catchment is wry steep.

rising in a distance of 40 km from an elevation of I() m at the hordcr to a maximum elevation of 1885 m on the plateau. Throughout the catchmcnt the .lhalukhali and its trihutaries are deeply incised into the southern slope of the plateau. and the entire outflow passes through a gorge on the Indian side of the border. From the gorge the Jhalukhali follows a southerly course for about 5 km across an extensive alluvial area before it enters Bangladesh at Dulura. Within this area the .lhalukhali bifurcates twice, so that the outflow enters Bangladesh through a main channel and two spill channels. The first hifurcation occurs ahout 2 km inside India. and the second at the harder. Water halance studies indicate a mean annual outflow at the border of 142.9 m3/s, or 4.5 km3/year. Within Bangladesh, the .lhalukhali follows a southerly course for 10 km from Dulura. through Muslimpur, to its confluence with the Surma at Sunamganj. The two spill channels follow more westerly courses to join the jadukata near Tahirpur and Satepur; their £1o\\.sare

finally discharged into the Surma though the .ladukat. at Durlabpur near Jamalganj. The BWDB opened a gauging station on the .lhalukhali at l'vluslimpurin 1988. and the IWM opened one at Dulura in 1990.

4.6.6.7 Jadukata Within India, the ]adukata has a catchment area of 2399 km:!. This large catchment drains most of the southwestern slope of the plateau. The catchment is very steep. rising in a distance

of 45 km from an elevation of 10m at the border to a maximum elevation of 1925 m on the plateau. The lower reach of the ]adukata in India follows a southeasterly course through a narrow valley separating the southwestern slope of the Shillong plateau from the eastern end of the Tura range. Its upper reach, and its tributaries. all originate on the southwestern slope of the plateau and follow southwesterly courses to their confluence with the lower reach. At the border the ]adukata turns south and enters Bangladesh at Lorergarh. Water balance studies

-

indicate a mean annual outflow at the border 01'365.7 nr~/s or 11.5 km3/year. Within Bangladesh,

the Jadukata

follows a southerly

course for 25 km to its

confluence with the Surma at Durlabpur. near Jamalganj: the lower stretch of this reach. s\.)uth of Tahirpur, is called the Rakti. Between Lorergarh and Durlabpur theJakukata bifurcates 41

Study Site

three times. The first bifurcation occurs I km south of Lorergarh where the Patni takes off westwards for 10 km before turning south to its confluence with the Bauaai west of Tahirpur.the second bifurcation occurs 10 km south or Lorergarh where the Baulai takes off westwards, through Tahirpuf. The third hifurcation occurs 15 in south of Lorergarh where the Nawa takes off west wards through.

-

-

-

42

--

-

Chapter Five

~----

FLOOD ROUTING

-

-

-

--

Flood ROlllin~

~-

Chapter

5

FLOOD ROUTING

-

5.1 Introduction Flow routi/1R is a procedure to determine the time and magnitude or flow (i.e.. the flow hydrograph) at a point on a watercourse rrom known or assumed hydrographs at one or more

--

points upstream. If the flow is a /lood. the procedure is speei tically known as )lood routing. In a broad sense, flow routing may be considered as an analysis to trace the flow through a hydrologic system, given the input. The difference bct\veen lumped and distributed system routing is that in a lumped system modeL the flow is calculated as a function of time alone at

-

a particular location. while in a distributed system routing the flow is calculated as a function of space and time throughout the system. Routing hy lumped system methods is sometimes

called hydrologic routing, and routing by distributed systems methods is sometimes referred to as hydraulic routing

5.2 Lumped System Routing

-

For a hydrologic system, input 1(1). output Q(I) and storage S(I) arc related by the continuity equation: d\' -

dt

= / (t) -

(I( t) .,

. . . . . . . . . . . . . . .,.,.5.:2. a

If the inflow hydrograph, /(1). is known. Eq. (5.2.a) cannot be solved directly to obtain the outflow hydrograph.

Q(t) because both Q and S are unknown. A second relationship. or

storage function is needed to relate S. I, and Q: coupling the storage function with the continuity equation provides a solvable combination or two equations and two unkno\\'I1s. In .1" '-t .' -

-

Flood ROll/inK

general, the storage function may he written as an arbitrary function of I. <J.and their tim~ derivatives as shown by

,)' = f(f,

-

£11_,£12;

£II £It

(j. dQ. dt d2~ dt-

)

5.2.b

These two equations can he solved by difTerentiating a inearized form of Eq. (5.2.b). substituting

the result for d)'jdt into Eq. (5.2.a). then integrating the resulting diffen:ntial

equation to obtain Q(t} as a function of /(1).1!ere a finite difference solution method is applied to the two equations. The time horizon is divided into finite intervals. and the continuity equation (5.2.a) is solved recursively from one time point to the next using the storage function (5.2.b) to account for the value of storage at cach time point. The specific form of the storage function to be employed in this procedure depends on the nature of the system being analyzed. In this section. three particular systems are analyzed. First, reservoir routing hy the level fJoolm('tf1od, in which storage is a nonlinear function of Q only: 8=.I(Q}...

-

...

,

...(5.2.c)

and the function RQ} is determined hy relating reservoir storage and outflow to reservoir water level. Second, storage is linearly related to I and Q in the Afuskingun7method for flow routing in channels. Finally. several lincar rcs('J"mirmodels arc analyzed in ,vhich (5.2.b) becomes a linear function of Q and its time derivatives.

"

The relationship between the outflow and the storage of a hydrologic system has an important influence on flow routing. This relationship may be either invariable or variable. as shown in Fig. 5.2.1. An invariable storage function has the form of Eq. (S.2.c) and applies to a reservoir with a horizontal water surface. Such reservoirs have a pool that is wide and deep compared with its length in the direction of /low. The velocity of /low in the reservoir is very low. The invariable storage relationship requires that there be a fixed discharge from the

-

reservoir for a given water surface elevation. which means that the reservoir outlet ,,'orks must be either uncontrolled. or controlled by gates held at a fixed position. If the control gate position is changed, the discharge and water surface elevation change at the dam. and the effect propagates upstream in the reservoir to create a sloping water surface temporarily. until a new equilibrium water surface elevation is established throughout the reservoir. When a reservoir has a horizontal water surface, its storage is a function of its ,vater surface elevation, or depth in the pool. Likewise. the outflow discharge is a function of the water surface elevation, or head on the outlet works. By combining these two functions. the 44

--Flood Rouling

-reservoir storage and discharge can be relatcd to produce an invariable. single-valued storage function. S =f(Q), shown in Fig. 5.2.1(a). For such reservoirs. the peak outnow occurs when the outflow hydrograph intersects the inflow hydrograph. Because the maximum storage occurs.

..

.

()

p

Qj I I

I

. s

() '-

n ((J)

Invanable

I

()

.\

(I

relationship

(h)

Vanahle

.

relall0nship

--

Figure: 5.2.1 Relationships between discharge and storage (Chow. 1988)

-

whendS/ dt = ! - Q = 0, and the storage and outflow are related by S =f(Q). This is indicated in Fig. 5.2. I(a) where the points denoting the maximum storage. R, and maximum outflow, P. coincide. J\ variable storage-outflow relationship applies to long. narrow reservoirs. and to open channels or streams, where the water surttlee prolile may be signilicantly curved due to backwater effects. The amount of storage due to backwater depends on the time rate of change of flow through the system. As shown in Fig. 5.2.I(b). the resulting relationship

-

between the discharge and the system storage is no longer a single-valued function but exhibits a curve usually in the form of a single or twisted loop. depending on the storage characteristics of the system. Because of the retarding effect due to bacbvater. the peak outflow usually occurs later than the lime when the inflow and ollllltm hydrographs intersect. as indicated in Fig. 5.2.I(b). where the points Rand P do not coincide. If the backwater ctTect 45

;., Flood Routing

is not very significant, the loop shown in Fig. 5.2.1(b) may be replaced by an awrage cun"c shown by the dashed line. Thus level pool routing methods can also be applied in an approximate way to routing with a variable discharge-storage relations. n. The preceding discussion indicates that the crfect of storage is to redistribute the hydrograph by shifting the centroid of the inflow hydrograph to the position of that of the outflow hydrograph in a time of redistrihution. In very long channels the entire flood wave also travels a considerable distance and the centroid of its hydrograph may then be shifted by a time period longer than the time of redistribution. This additional time may be considered as time (~rtranslation. As shown in Fig. 5.2.2. the total/ill1(,0(1100d 1I1(}\'C1Il('111 between the centroids of the outflow and inflow hydrographs is equal to the sum of the time of redistribution and the time of translation. The process of redistribution modities the shape of the hydrograph, while translation changes its position.

!

. 1

-

+1

1.1+1 \.

TIOJCof 1100(!movemcnl

Timeof redistribution

Q

/ /

.

/ / /

.

I-"-~."+j I.-} -~ Timeof translation

·r

Figure: 5.2.2 Conceptual interpretation of the time of flood movement. (Chow. 1988)

5.3 Level Pool Routing Level pool routing is a procedure for calculating the outflow hydrograph from a rescn"oir with

a horizontal

characteristics.

water

surface,

given

its inflO\\' hydrograph

and storage-outflow

A number of procedures have been proposed for this purpose (e.g.. Chow.

46

-. Flood Routing

1951. 1(59) and with the advance or computeri:1ation !,!,raphicalprocedures are heing replaced by tabular or functional methods so that the computat ional procedure can be automated. The time horizon is broken into intervals of duration 11/ indexed by j. that is t = O. !J./. 211r. . . .

.j 111.(j

+

I) /).1... .. and the continuityequation(5.2.1) is integratedover each time interval.

as shown in Fig. 5.3.1. For the 7-th time interval:

f

,,, £1.\'

=

I

f

f,

"I).\/

l+I)i\t

l(t)d/-

li\/

U(l)d/

5.~.a

\1

The inflow values at the beginning and end of the j-th time interval are II and /1' '.

respectively, and the corresponding values of the outflow are

QI

and Q

>I

. Ikre. both

inflow and outflow are flow rates measured as sample data. rather than inflow being pulse data and outflow being sample data as was the case for the unit hydrograph. If the variation of inflow and outflow over the interval is

t "

.

I

J

I I I I I I I I I I I J

--

t-

j6.'

Time:

(j + 1)6., I

I

I I 1 1 I I I I I I _4.-._ I

I I I 1 I I I I I I

I I I

Tinw

FIGURE 5.3.1 Change of storage during a routing period 11/.(Chow. 1988) approximately

linear, the change in storage over the interval. ,~/

5.~. can be found by

rewriting (5.3.a) as

/+/

0+0

~

-~

1:11- ~'-

2

_~I~ 1:11

5.3.b

2

The values of 1;and 1;+/ are known because they are prespecified. The values of Q} and 5} are known at the j-th time interval from calculation during the previous time interval. Hence. Eq.

47 --

Flood Routing

(5.3.b) contains two unknowns. (jlll and ,\'1;I. which are isolated hy multiplying (:,J.b) through by 2//).1, and rearranging thc result to produce: 281+1 ~ -.

(

~+QI+I

)=

(11 +11+1)+

281

(

..,

!y"t -QI

5.-'.c

)

In order to calcul'ate the outflow, QII I. li'om Eq. (5.3.c). a sloragc-olil/lmr!lIncl ion relating 2S/ /),/ + Q and Q is needed. The method f()r dcveloping this function using ekvation-stl)rag~ and elevation-outflow

relationships is shown in Fig. 5.3.2. The relationship between water

surface elevation and reservoir storage can he derivcd by planimetering topographic maps or from field surveys. The elcvation-discharge

relation is derived from hydraulic equations

relating head and discharge, such as those shown in 0..111"",

f ()utnn.."

-

I

"I

It,) f

o

-------.-

,

V I

~ .~__

r

.' II

('

./'.-

.

~--~--_.---------

1,.,/

-"

i

I I ,I ,~

/,/"

"A'su to, 'IirtuL'('

"'''''': ",

,,\ Ar t

.

SCur;,).",,'

'-

t~urlh"" funt,:,,()n

(l t

s I I III I

\)4,lluc'r

"'UI'~ln.:

c°Ic'",'I"1I1

Figure: 5.3.1 Development of the storage-out flow function for level pool routing on the basis of storage-elevation and elevation outflow curves. (Chow. 1988) Development of the storage-outflow function for level pool routing on the basis of storageelevation and elevation-outflow curvcs. and outlet works. The valuc of /),/ is taken as the time interval of the inflow hydrograph. For a given valuc of water surface e\cvation. the values of storage i)' and discharge

Q are determined

Iparts

(a) and

(11)

of Fig. 5.3.:21. thcn the

value of 2S1 /),1 + Q is calculated and plotted on thc horizontal axis of a graph with the value of the outflow Q on the vertical axis [part (c) of Fig. 5.3.2].

In routing the flow through time intervalj. all tcrms on the right side of Eq. (5.3.c) are known, and so the value of 2Sj+I//),/ +Q .j+I can be computed. The corresponding value of Q .1+I , can be determined from the storage-outflow function 2S/ /),1 + Q versus Q. either graphically 48

;

r Flood

ROliling

...-

or by linear interpolation of tabular values. To set up the data required for the next time interval, the value of2Sj+l/ I':..t- Q j+ds calculated by

( 2~;.c _Q,., )= (

2~;., + Q,.,

)_

2Q,.,

H..

HH

.H5.3.d

The computation is then repeated ('or subsequent routing periods.

5.4 Distributed Flow Routing The flow of water through the soil and stream channels of a watershed is a distributed process because the flow rate, velocity, and depth vary in space throughout the watershed. Estimates of the flow rate or water level at important locations in the channel system can be obtained using a distributed .flow routing model. This type of model is based on partial differential equations (the Saint-Venant equations for one-dimensional now) that allow the tlo\\"rate and water level to be computed as functions of space and time. rather than of time alone as in the lumped models described. The computation of flood water level is needed because this level delineate- the flood plain and determines the required height of structures such as bridges and levees: the computation of flood flow rate is also important; first. because the flow rate determines the water level, and second, because the design of a flood storage structure such as a detention pond or reservoir requires an estimate of its inflow hydrograph. The alternativc to using a distributed flow routing model is to use a lumped model to calculate the tlow rat~ at the desired location, then compute the corresponding water level by assuming steady nonuniform flow along the channel at the site. rhe advantage of a distributed flow routing model over this alternative is that the distributed model computes the flow rate and water level simultaneously instead of separately, so that the model more closely approximates the actual unsteady nonuniform nature of flow propagation in a channel. Distributed flow routing models can be used to describe the transformation of storn1 rainfall into runoff over a watershed to produce a now hydrograph for the watershed outlet and then to take this hydrograph as input at the upstream end of a river or pipc system and route it to the downstream end. Distributed models can also be used for routing low flows. such as irrigation water deliveries through a canal or river system. The true flow process in either of these applications varies in all three-space dimensions: for example. the velocity in a river varies along the river, across it, and also from the water surface to the river bed. However for many practical purposes. the spatial variation in velocity across the channel and with respect to the depth can be ignored, so that thc now process can he approximatcd as 49

Flood ROliling

varying in only one space dimension along the flow channel. or in the direction of flo\\. The Saint-Venant equation, first developed by Barre de Saint-Venant in 1871. describe onedimensional unsteady open channclfloVv.Which is applicable in this case.

5.4.1 Saint -Venant Equations The following assumptions arc necessary fix derivation ufthe Saint-Venant equations: 1. The flow is one-dimensional; depth and velocity vary only in the longitudinal direction of the channel. This implies that the velocity is constant and the \\'ater surface is horizontal across any section perpendicular to the longitudinal axis. 2. Flow is assumed to vary gradually along the channel so that hydrostatic pressure prevails and vertical accelerations can be neglected «(,hO\\. 1959). 3. The longitudinal axis of the channel is approximated as a straight line. 4. The bottom slope of the channel is small and the channel bed is fixed; that is. the effects of scour and deposition are negligible. 5. Resistance

coefficients

for steady

uniform

turbulent

flow are applicable

so that

relationships such as Manning's equation can be used to describe resistance effects. 6. The fluid is incompressible and of constant density throughout the 110\\'.

5.4.2 Continuity Equation The continuity I.!quulion for an unsteady variable-density l1o\\' through a wntrol volume can

be written as in Lq. (4.2.a):

o= ~ dl

fff,aN+ ffpV.dA (. ,.

..A.2.a

(..\

consider an element control volume of length dy in a channel. Fig. 5.4.2. sho\\'s three vie\\'s of the control vol ume: (a) an elevation view from the side (b) a plan view from above, and (c) a channel cross section. the inflow to the control volume is the sum of the flow Q entering the control volume at the up stream and of the channel and the lateral inflow q entering the control \'olumc as a distributed flow along the side of thc channel. So the dimension of if are those of tlow per unit length of channel, so the rate of lateral inflow is (I dv: and the mass inflow rate is

50

Flood Routing ..

-

()

(a) I :!c\'ation view. ;. .1. (),III)llI

<.r== f~"

(h) Plan vie\\'.

ConlTol Volume

R

---

y

-

h

(c) Cross section.

~\\\'\'\''\j,\\'\j,\\\.w,.W>.W~-h

,

I \I'

l~

Datum

_J

Fig: 5.4.2 An elemental reach of channel for derivation of the Saint-Venent equations.

ffpV.dA = -p(Q + qdx)

5.4.2.b

m/('I

This is negative because in/lows are considered to be negative in the Renynolds transport

theorem. The mass outflows from the control volume is

ffpV.dA =

-p

lI1/el

where

aQ is the

ax

ax ) (Q+ aQdX

5.4.2.c

rate of change of channel /low with distance. The volume of the channel

element is Adx, where A is the average cross-sectional area. so the rate of chance of mass

stored within the control volume is

d

. .2 .d

..........

dt I!I pd\1 = a(pAdx) at.

54

where the partial derivative is used because the control volume is defined to be fixed in size (through the water level may vary within it). The net outflow of mass from the control

volume is found by substituting eqs.(5.4.2.c) to (5.4.2.d) into (5.4.2.a)

o(pAdx) _ ( 01

pQ+qdx

) +p

of)

51 - ---

-

_

( Q+ oX ) -O

-

0)

?,

4._.c

Flood Routing

Assuming the fluid density p is constant. ().4.2.e) is simplified by dividing through by plY and rearranging to produce the conservation from of the continuity equation.

aQ

-

ax

vA

_ - q - 0 ....................

+--

at

5.4.2.f

which is applicable at a channel cross section. This equation is valid for a prismatic or a nonprismatic channel; a prismatic channel is one in \\hich the cross sectional shape does not vary along the channel and the bed slope is constant. For some methods of solving the Saint-Venant equations. thc nonconservation from of the continuity equation is used, in which the average flow velocity V is a dep~ndent variable, instead of Q. This form of the continuity equation can be derived for a unit width of flow within the channel, neglecting lateral inflow. as follow. For a unit width of flow A = y x l=yand Q =VA=Vy. Substituting into (5.4.2.t)

a(Vy) + -ay = O ax

_

).4.2.L!

at

~

or

V -oy+ y- ovoy + - =0

ox

,

...).4._.h

ox 01

5.4.3 Momentum Equation Newton's second law is written in the form ofReynolu's transport as in

L

F

=~ dl

HI VpdV C.I'.

+

H Vpl'

.dll

5.4.3.a

C..I'.

This states that the sum of the forces applied is equal to the rate of change of momentum stored within the control volumc plus the nct outflow of momcntum across the control

surface. This equation, in the form

L F = 0, was applied to steady uniform flow in an open

channel. Here, unsteady nonuniform flow is considcrd.

FORCES: There are livc forccs acting on the control \olul1lc.

LF =

F):

+ F> + F:. + F". -t I;~,

5.4..1.b

where,

Fg=the gravity force along the channcl due to thc weight of the water in the control volume. Fj= the friction force along the bottom and sides of the control volume Fe = the contraction or expansion force produced by abrupt changes in the channel cross section 52

Flood Routing

F".= the wind shear force on the water surl~lCe Fp=the unbalanced pressure force The sum of the five forces in Eq. S.4.3.b can be expressed

L F = pgAS"dx - pgAS'1 dx - pgAS',.d, - WI Bpd, - IXA ~y dr

5.4.3.e

. ('x

5.4.4 Momentum The two momentum terms on the right-hand side of hi 5.4.3.a represent the rate of change of storage of momentum in the control volume. and the net outflow of momentum across the control surface. respectively.

5.4.5 Net Momentum Outflow The mass inflow rate to the control volume is - p((J + (I(LY).representing both stream inflow and lateral inflow. The corresponding

momentum is computed by multiplying

the mass

inflow rates by their respective velocities and a momcntum correction factor fJ

ffVpV.dA = - p{fJVQ + fJvxqdx)

inlet where pfJVQ

..5A.5.a

is the momentum entering from the upstream end of the channel. and

p(fJv xqdx) is the momentum entering main channel with the lateral inflow. which has a velocity Vx in the x direction. The term Ii is known as the momentum cod'ticicnt

or

Boussinesq coefficient. fJ is given by

fJ = ~

V2A

ffv2dA

SA.S.b

where v is the velocity through a small clement of area dA in the channel cross section. The value of fJ ranges from 1.01 for straight prismatic channels to 1.33. Momentum leaving the control volume is

JfpV.dA = {OVQ+ iJ(~:Q)l

545C

The net outflow of momentum across the control surlilce is the sum of

J!VpV .dA = - p(fJVQ + fJv,qdx ) + p[fJVQ + a(-:Q)]

= - p[fJv,q - a(-:Q)}tx

...

54.S.d

Flood Rouling

5.4.6 Momentum Storage ..

The time rate of change of momentum stored in the control volume is found by using the fact that the volume of the clement channel is Ad\'. so its momentum is (','Idrl'.

or f'Qth

and

then

d

- HfVpdV = p dt

(

I'

no

-=- dx at

5.4.6.a

After substituting the force terms and thc momcntum tcrms from (S.4.3.e) and (SA.S.d) into the momentum equation (S.4.6.a). it reads pgASodx - fJKASf dx - PKAS,.dx - Wf Bf'tl.r - pgA

= -p fJv, -

[

here.

,)' f

==

a(

::

_

~ ao f3 Q £Ix+ p-=-th ax )] ell

.

).4.6.b

friction slope

So = bottom slope

S, = eddy loss slope Dividing through by pdx, replacing V with ~FA. and rearranging the consenation form of the momentum equation:

aQ + O(j3Q2/ A) at

ax

+ (TA :J~

(")

( ax"

_S

.

+S +S f

,f',J

- (X/ I' +U' f B

=0

.. ..5.4.6.c

The depth throughy in Eq. (5.4.6.b) can bc replaced by the water surface elevation h. using [see Fig. 5.4.2.(a)] h= y + Z

5.4.6.d

where z is the elevation of the channel bottom abovc a datum such as mean sea level. The derivative of Eq. (5.4.6.d) with respect to the longitudinal distance x alon the channel is

But azl = -8 lax

o'

-

Dh=-'-+oy Dz_ ax ax Ax

).4.6.c

ah _- ay- s 0 ax ax

5.4.6.f

so

The momentum equation can now be expressed in terms of h by using (5.4.6.f) in (5.4.6.c) aQ + a(j3Q2 / A) + gA ah + Sf + S.. - j3qv, + 11/,B at ax ( ox J

54 -

-

=0

...5.4.6.g

Flood Routing ..

The Saint-Venant equations, (5.4.2.1) for continuity and (5.4.6.g) for momentum, are the governing equations for one dimensional, unsteady now in and open channel. The use of the terms Sj and 5;1"in (5.4.6.g) which represents the rate of energy loss as the flow passes through the channel,

illustrates the close relationship

between energy and momentum

considerations in describing the flow Strelk off shO\nxl that the momentum equation for the Saint- Venant equations can also be derived from energy principles, rather than by using Newton's second law as presented here. The non conservation form of the momentum equation can be derived in a similar manner to the non conservation lorm of thc continuity equation. Ncglecting eddy losses, wind shear effect, and lateral inflow, the non conservation form of the momentum equation for a unit width in the flow is

av

av

-+ at

V -+ ax

01'

g

~( ax

So + S r . J

o

.,5.4.6.h

5.5 Finite-Difference Approximations The Saint- Venant equations for distributed routing arc not amenable to analytical solution except in a few special simple cases. They are partial differcntial equations that in general, must be solved using numcrical methods. Methods Ii)!' solving partial differential equations may be classified as direct numerical method,' lIlld c/1uraclerislic melhods. In direct methods, finite-difference

equations are formulated from the original partial differential equations for

continuity and momentum. Solutions for the flow rate and \vater surface elevation are then obtained for incremental times and distances along the stream or river. In characteristic methods, the partial differential equations arc lirst transformcd to a characteristic form. and the characteristic equations are solvcd analytically, as shmvn prcviously for the kinematic wave, or by using a finite-difference representation. In numerical methods for solving partial difll:rcntial equations, the calculations are performed on a grid placed over the x-I plane. The x-t grid is a network of points defined by taking distance increments of length L~Xand time increments of duration ~/. As shown in fig. . the distance points are denoted by index i and the time points by index j. A lime line is a line parallel to the x axis through all the distance points at a given value of time. Numerical schemes transform the governing partial differential equations into a set of algebraic finite - difference equations, which may be liner or nonlinear. The finite- difference equations represent the spatial and temporal derivatives in terms of unknown variables on

55

Flood Routing

i

both the current time line,

+ I. and the preceding time line. i. where all the "alues arc

known from previous computation. Time

(

--LTm1c Iinc , ) + I

~

I. } +

t

7

1.J

/. I

Timr line .

I

I

() ()

(/

1)/\\

/\\

(/

t

1),\\

I

Distance

Figure: 5.5 The grid on the x-I plane used for numerical solution of the Saint- Venant equations by finite differences

The solution of the Saint - Venant equations advance from one time line to the next. 5.5.1 Finite Differences Finite- difference approximations can be derived !()r a function [((x) as slww in Fig 9.5.2. A Taylor series expansion of u(x) at x + L\x produces ux+Lix (

where u'(x)=a~x,

_ ( ) +Lixu (x )+-~\ ) -ux 2 /

u"(x)=a2~x1

I

u "(.\. )+ 1 ~\ .J u '" (.\. ) + ()

.~

" 5._.l.a

and so on. The Taylor series expansion at

x - Lix is

A central-difference approximation uses the difTerencedefined by subtracting (5.5.1.b) from (5.5.] .a)

u(x + LU)- u(x - ~) = 2,ixu'(x) + O(~.1)

..5.5.1.c

Flood Routing

where 0 (L1x3)represents a residual containing the third and higher order terms. Soh"ing for

u'(x) assuming 0 (L1x3):::::: 0 results in

u'(x) ~ u(x + Ill)2-IIIu(x - ~.x)

.. .... ... .. ... .. .... .. ..5.5 .l.d

which has an error of approximation of order Lix2. This approximation error. due to dropping the higher order terms. is also referred to as a truncatinn error. A forward difference approximation is defined by subtracting u(x)

u (x + L1x) - u(x)

= ;}.xu'(x)

+ o(Llx '

from (5.5.I.a)

) . . . . . . . . . . . . . . . . . . . . .5.5.

I.e

Assuming second and higher order terms are negligible. solving for u'(x) gives

u,(x)", u(x +

~- u(x)

.5.5.1.1

which has an error of approximation of order i1x The backward-difference approximation uses the difference defined by subtracting (5.5.1.b) from

u(x). u(x) - u(x

+ Llx) =

LlxU,(X)

+ O(i1x ,)

..5.5.I.g

so that solving for u'(x) gives u'(x)::::::u(x + L1x)- u(x) ;}.x

..5.5.I.h

A finite - difference method may employ either an explicit scheme or an Implicit scheme for solution. The main difference between the two is that in the explicit method. the unknown values are solved sequentially along a time line from one distance point to the next. while in the implicit method the unknown values 0\1 a given time line area all determined

-

simultaneously.

The explicit method is simpler but can be unstable. which means that small

values of L1x and i1t are required for convergencc of numerical procedure. The explicit method is convenient because results arc given at the grid points. and it can treat slightly varying channel geometry from section to section. but it less efficient than the implicit method and hence not suitahle for routing flood flows nver a long time period. The implicit method is mathematically complicated. but with the use of computers this is not a serious problem once the method is programmed. The method is stable for large computation steps with little loss of accuracy and hence works much faster than the explicit method. The implicit method can also handle channel geometry varying significantly from one channel cross section to thc next. 57

Flood ROll/inK

5.5.2 Explicit Scheme The finite-difference representation is shown by the mesh of points on the time- distance plan shown in fig.5.5.1. Assuming that at time I (time line;). the hydraulic quantities u are known. the problem is to determine the unknown quantity at point ( i. j+ I) at time f + /).f . that is U,101 .

The simplest determines the partial derivati\es

at point ( i. j+ I) in terms of the

quantities at adjacent points ( i -I. j). ( i.j) and ( i + 1.j) using

auI at

--

1+1

UI+1

I

-

ul,

I1t

and 5.5.2.b A forward-difference

scheme is used for the time derivative and a central-difference

scheme

is used for the spatial derivative. Note that the spatial derivative is written using known terms on time line j. Implicit schemes on the other hand use finite - difference approximations for both the temporal and spatial derivatives in terms of the unknown time linej+ I. The discretization of the x-I plane a grid for the integration of the tinite- difference equations introduces numerical errors into the computation. A finite-difference equations introduces numerical errors into the computation. i\ finite-difference scheme is stable,if such errors are not amplified during successive computation depends on the relative grid size. A necessary but insufficient condition for stability of an explicit scheme is the Courant condition (Courant and Friedrichs, 1948 ). For the kinematic v,'ave equations. the Courant condition is /).f ~

fu:

.5.5.2.c

--.!

ck

where ck is the kinematic wave celerity. For the dynamic wave equations.

c,

is replcaced by

V + Cd in (9.5.11). The Courant condition requires that the time step be less than the time for a wave to travel the distance &,. If I1t is so large that the Courant condition is not satisfied. then there is, in effect, an accumulation or piling up of water. The Courant condition docs not apply to the implicit scheme.

58

Flood Routing

,

For computational

purposes in an explicit scheme, L1."X" is specified and kept fixed

throughout the computations, while I1t is determined at each time step. To do this. a !':"t,just meeting the Courant condition is computed at each grid point i on the time line j. and the smallest

I1t,

is used. Because the explicit method is unstable unless I1f is small. it is

sometimes advisable to determine the minimum I1f, at a time line .i then reduce it by some percentage.

The Courant condition does not guarantee stability, and therefore is only a

guideline.

5.5.3 Implicit Scheme: Implicit schemes use finite-dilTerence approximations for both the temporal and spatial derivative in the terms of the dependent variable on the unknown time line. As a simple example the space and time derivatives can be written for the unkno\vn point (i~ 1.j->-l) as

au 1+1

U ,+1

~=

1+1

ax

-

U /II

Ax

,

s-

.., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..._ .)..J.a

and -au,~tll

at

_

U,'t~ I - U" '~

I1f

..... . .. . . . . . . . . . . . . . . . . . . . .. . ...:'.:'. ~.b

This scheme is used for the kinematic wave model. In next a more complex implicit scheme, referred to as a weighted 4-point implicit scheme, is used for the dynamic full dynamic wave model.

5.6 Dynamic Wave Routing The propagation of flow in space and time through a river or a network of rivers is a complex problem. The desire to build and live along rivers creates the necessity tor accurate calculation of water levels and flow rates and provides the impetus to develop complex flow routing models, such as dynamic wave models. Another impetus for developing dynamic wave models is the need for more accurate hydrologic simulation. in particular, simulation of flow in urban watersheds and storm drainage systems. The dynamic wave model can also be used for routing low flows through rivers or irrigation channels to provide better control of water distribution. The propagation of /low along a ri\'l~rchannel or an urban drainage system is an unsteady non uniform flow, unsteady because it varies in time. non uniform because flow properties such as water surface elevation, velocity, and discharge are not constant along the channel. 59

-Flood Routing

One-dimensional

..-

distributed routing methods have been classified in this section as

kinematic wave routing, diffusion wave routing, and dynamic wave routing. Kinematic waves govern the flow when the inertial and pressure forces arc not important. that is. when the gravitational

force of the flow is balanced

by the frictional

demonstrated

that the kinematic wave approximation

resistance

force. Before

is useful for applications where the

channel slopes are steep and backwater effects arc negligible. When pressure forces become

important but inertial forces remain unimportant a dirfusion wave model is applicable. Both the kinematic

wave model and the di ffusion \\iave model arc helpful

in describing

downstream wave propagation when the channel slope is greater than about 0.5 ftlmi (0.0 I percent

and there are no waves propagating upstream due to disturbances such as tides.

tributary

inflows, or reservoir operations.

important,

such

as in mild-sloped

When both inertial and pressure forces are

rivers,

and backwater

effects

from

downstream

disturbances are not negligible, then both the inertial force and pressure force terms in the momentum

equation are needed. Under these circumstances

the dynamic wave routing

method is required, which involves numerical solution of the full Saint- Venant equations. Dynamic routing was first used by Stoker (I (53) and by Isaacson. Stoker. and Troesch (1954. 1956) in their pioneering investigation of /lood routing for the Ohio River. This chapter describes the theoretical development of dynamic wave routing modc1s using implicit jiniledifference approximations to solve the Saint- Venant equations.

5.6.1 Dynamic Stage-Discharge Relationships The momentum equation is written in the conservation larm (from (5.4.6.c)J as

aQ a(pQ2/ & +

fu

A)+.g

T

C!Y

A( fu'

_')0 ('

+.

\',

('

I +.)('

)

- 1'), )(/',.

IU , '>_ ) +Y/ ,/ f ~ - (

. .;; f. _ .\). I .<1

Uniform flow occurs when the bed slope So is equal to the friction slope Sf and all other terms are negligible, so that the relationship between discharge. or flow rate. and stage height. or water surface elevation, is a single-valued function derived from Manning's equation. as shown by the uniform flow rating curve in Fig. 5.6.I.a. When other terms in the momentum equation are not negligible. the stage-discharge relationship f<.mnsa loop as shown by the outer curve in Fig. 5.6.I.a, because the depth or stage is not just a function of discharge. but also a function of a variable energy slope. For a given stage. the discharge is usually higher on the rising limb of a flood hydrograph than on the recession limb. As the discharge rises and falls, the rating curve may even exhibit multiple loops as shown in Fig. 5.6.1.b for the Red River (Fread, 1973c). The rating curve for uniform flow is typical of lumped or hydrologic routing methods in which ----

Flood Routing

LOC1p rating (Dynmnic diffusion wavC'

-

Figurc:5.6.1.a

curve and modcl~)

Loop rating curves. The uni form 110wrating eurve docs not reflect back water effects, whereas the looped curve docs (Chow.1988) AO

--

70

.g

~

'~

60

_

M'

:'10

j\,~

1'-

: "~ll

,~' 4/1

,

J

1"

_1

.oil 1

N',

~~.

1-_

11.1 "11

40

'"

100

l.!lI

Figure :5.6.1.b Loop stage-discharge relation for Red River. Alexandria. Louisiana (May 5-

-

-

June 17, 1964. S'ource:Fread. J973) S=f(Q}

while the loop rating curve is typical of distributed or hydraulic routing methods. Flow propagation in natural rivers is complicated by several factors: junctions and

tributaries, variations in cross section, variations in resistance as a function both of flow depth and of location along the river. inundated areas. and meandering of the river. The interaction between the main channel and the /lood plain or inundated valley is one of the most important factors affecting /lood propagation. During the rising pan of a flood v,'ave. water flows into the flood plain or valley from the main channel. and during the falling flood. water flows from the inundated valley back into the main channel. The effect of the valley storage is to decrease the discharge during the falling flood. Also some losses occur in the valley due

-

to infiltration and evaporation. The flood plain has an effeet on the wave celerity because the Ilood wave progr,'sses more slowly in the inundated valley than in. the main channel of a river. This ditTerence in wave celerities disperses the flood wave and causes /low from the main channel to the !lood plain during the rising flood by creating a transverse water surface slope away from the channel. During the falling flood, the transverse slope is inward from the inundated ,'alley into the main channel and water then moves from the /lood plain back into the main channel [see Fig. 5.6.1(a) and (b)]. 61

Flood Routing

Because the longitudinal axes of the main channel and the flood plain \ alley are rardy parallel. the situation described above is even more complicated in a -.-.-

....--

(a) Transverse slop during rising flood

(b) Transverse slope during l~ll1ingflood

-

-

.

////////1///////////////////////////////////////////////1/

---

.

._- - ------

p-------

-.-

..

---

(c) Main channel parallel to valley

-

(d) Meandering main channel Figure: 5.6.1.c Aspects of f10w in natural rivers

meandering river. For a large flood, the axis of the flow becomes parallel to the valley axis [Fig. 5.6.1 (c) and (d)]. The valley water slope and valley water velocity (if depths are sufficient) can be greater than in the main channel. which has a longer flow path than the valley. This situation makes it difficult for flow to go from the main channel to the flood plain valley during the rising flood and vice versa during the tailing flood. Flood \\lave 62

Flood Rowing

propagation is more complex when the flow is varying rapidly. The description is also more complicated for a branching river systcm with tributaries and thc possibility of flood peaks from different tributaries coinciding. Also with tributarics. the effccts on flood propagation of backwater at the junctions must be considered.

When backwater effects exist, the loop rating curve may consist of a serics of loops. each corresponding to a different feature controlling watcr level in the channel (see Fig. 5.6.I.d). Backwater effects of reservoirs. channel junctions. narrowing of the natural river channel. and bridges can demonstrate this characteristic.

Figure:

5.6.I.d

loop

rating

cunT

with

signi ticant backwater effects. Back\vater effects arc duc

to downstream

reservoirs.

channel

junctions. highway crossings. narrowing of the river section. These produce a series of rating Dill' to back watt:'r effects

curves with each corresponding to a given backwater level. The backwater effects cause a variable cnergy slope that can he modeled using the rul! dynamic wavc modcl.

5.6.2 Implicit Dynamic Wave Model

Implicit finite-differencemethods advance the solution of the Saint-Venant equation from one time line to the next simultaneously for all points along the time line. A system of

algebraicequationsis generatedby applying the Saint-Venant equations simultaneously to all the unknown values on a time linc. Implicit mcthods were developcd because of the limitation on the time step size required for numerical stability of explicit methods. For

example. an explicit method might require a time step of one minute for stability. \\'hile an

implicitmethodappliedto the sameproblcmcoulduse a time step of one hour or longer. The implicit finite-difference scheme uses a weighted tour-point method between adjacent time lines at a point M. as shown in Fig. 5.6.2.a. If a given variable describing the flow. such as flow rate or water surface level. is denoted by u. the time derivative of u is

approximatedby the averageof the finite difference values at distance points i and i + I. The value at the ith distance point is (u/+I -II,' )/ /).( . and that at the (i + I)th distance point is (U/+~I-

u'~J)/6.t . so the approximation is

63

-'Flood Rouling

Iii

au

a;

/Ii

/

I

11, +11'11 -II, -11"1 --2M ......................

~

5.6.2.a

for the point M locatcd midway hetween the i th and (i I I) th distance point in Fig. 5.6.2.a

-

-, A slightly different approach is adopted to estimate the spatial derivative 011 and the

ax

variable u. For the spatial derivative, the difference terms at the.i th and {j + l)th time lines are calculated: (U,'+I- u,' )/ /;0.1,and (1<~1-1< 1)/ /;0.1 respectively: thcn a weighting factor B is applied to define thc spatial derivative as :;

/+1

~~OIlI+I

/+ I

-II,

ax

I

+(I-O)!!."CII,

Ax

I

5.6.2.b

/;o.x

t

Time /

i + I. J + 1

--

M

c.

'" "" c =

j

i.n

.E E "

];+ l.j

Node

5

iI'

-LI

L

2

4

.1

(i

-

1)

(/+1)(;+2)

I -~(N-3)(N-2)(N-I)

N

Distance .\

Inilial condition

time line

Figure: 5.6.2 The x-I solution plane. The linite-dit1erencc forms of the SaintVenant equations are solved at a discrete number of points (values of the independent

variables x and t) arranged to form the rectangular grid sho\\'n.

Lines parallel to the time axis reprcsent locations along thc channel. and thosc parallel to the distance axis rcpresent times. (J\licr Fread. 1974a).

and an average value for u is calculated similarly as /+1

u=Ou;

.1+1

+U'+I +(1-B)~I~ 2

/,

2

5.6.2.c

Flood Routing

The value of 0 = 11.1'locatcs point M vertically in the box in Fig. 5.6.2.a. A scheme usinc: I1.t

~

8=0.5 is called a box scheme. When 0 = 0, the point M is located on the j th time line and the scheme isJully explicit, while a valuc of 8= I is used in a.fidly implicit scheme with M lying in the (i + I)th time line. Implicit schemes arc those with () in the range 0.5 to 1.0: Fread (,] 973a. ] 974a) recommends a value or 0.55 to 0.6. A major difference bctwecn thc cxplicit and implicit mcthods is that implicit mcthods are conditionally

stablc for all time stcps. whcreas c\.plicit mcthods are numcrically stable

only for time steps less than a critical valuc dctermined by the Courant condition. Fread (.] 973a, I 974a) has shown that the weighted four-point scheme is unconditionally

linearly

stable for any time step if 0.5 ~ 8 ~ 1.0. This scheme has a second-order accuracy when 8 = 0.5 and a first-order accuracy when 8 = 1.0.

-.

Chapter

Six

COMPUTER

PROGRAMME AND APPLICATION

---

Computer Programme and Application

Chapter 6

COMPUTER PROGRAMME AND APPLICATION

6.1 Introduction In this step a programmehas been developed by Visual Basic. which is user friendlyto calculate time and space derivative of flow rate and water level ( 8Q . 8Q . ch ) for solving 8x af 8x Saint Venant equation based on weighted f()lII"point implicit finite difTerence approximatil)!1. ;'If)

()

~=o-~:='

ax

'+I

-oQ of

Q1

,"

( ) ,.1

() /

)

+(I_O)__III-=-L,

\lx,

Vx,

( )/+J 0' 0' +_111 --I --- -_It I

2\lf,

Where, h = height of water level of river t = time

Q = discharge e = a weighting factor

66

6.1.1

611 . .-

Computer Programme and Application

Time t

---

\-i + I. ) + I (j + 1)

M i + 1. J

-

-Llx

-

~

Node

t

- i-

L

3

2

4

(i - I)

(1+1)(1+2)

-~ (N-3)(N-2)(N-I)

N

Distance

\

Initial condition lime line

The continuity and momentum equations are considered at each of the N-l rectangular grids show in fig.5.6.2.a, between the upstream houndary at i -=I and the downstream boundary at i=N. this yields 2N-2 equations.

There are two unknown at each of the N grid

point (Q, h), so there are two unknown in all. The two additional equations required to complete the solution are supplied by the upstream and down stream boundary condition. The upstream boundary condition is usually specified by as a know stage hydrograph. a known discharge hydro graph or known relationship between stage and discharge. In the program one can put down and up stream water level. discharge and disfance of that two stations. For the immediate next grid data to solve the equation interpolation method is applied, which is based on the formula

1"1= I, -

(x, - X'+IXI, - IN)

Value of

x, -XN

i (distance of the grid) and.i (time interval) are increase as they run in a

different loop and asking for data, commuting all the equations and finally show the output.

6.2 Initial input 1. Known water level (h) and discharge (Q) of two station of same time. 2. Distance (d) between the two stations.

6.3 Output 1. Waterlevelof unknowndistance. 2. Rateof changeof dischargeand waterlevel. 67

-

Computer Programme and Application

6.4 Programme Execution Known h, Q and d of two station of same time

Figure: 6.1 Flow chart of thc program execution

Input Distance Between Two Station

Input

Input Distance Between Two Statton

J

Upstream Station

at 6

YourRequired

Distance

Downstram Station at 6

Output

I

Station at 6

DownStream Station at 6

Output

Output Waterlevel of ReqtJired

dh dx

Upstream

Distance

1-

EXit

dQ dx

_

dQ dt

=

I

Figure: 6.2 Interface of the Softwarc. 6.5 Application Here two river of North Easte region of Bangladesh name Jhalukhali and Lubachra have been applied in the programme for find out the (i) rate of change of discharge and water level (ii) water level of any distance.

Computer I'roKramme and Applicatio/1

6.5.1 Case Study 1 In this case of Jhalukhali river upstream station is Dulura ( station ID-333A) and next nearest station 14.7 km distance down stream station is Muslimpur (station 10-333, fig.6.4.I). Both of

the station is of BWOB. BWOB takes data of water level daily and

3 hour basis and

discharge daily basis. They started to take 3 hour basis data from the last four year. In the case of 3 hour basis data they start day with 6 am and end at ()pm.

figure:

6.4.1 Shows the two station of .Ihalukhali riwr.

Table 6.4(a) Monsoon water level of .Thalukhali river Date

Time

5T 10

WL

15/07/1992 15/07/1992 15/07/1992 15/07/1992 15/07/1992

6:00:00 9:00:00 12:00:00 15:00:00 18:00:00

333 333 333-333 333

8.69 9.38 9.39 -9.22 . 8.62

!-

5T 10 333A 333A 333A --333A 333A

WL

._--

11.94 12.7 12.13 11.55 11

ah Output: - = -0.22679

ax

Table 6.4(b) Pre-Monsoon water level of .Thalukhaliriver Date 15/04/1992 15/04/1992 15/04/1992 15/04/1992 15/04/1992

Time 6:00:00 9:00:00 12:00:00 15:00:00 18:00:00

5T 10 333A 333A 333A 333A 333A

WL 8.87 8.87 .8.87 _. 8.86 8.86 --

5T 10 333 333 333 333 333

WL 6.18 6.18 6.18 6.18 6.18

ah Output:- = -0.1855] ax 6.5.2 Case Study 2 In this case of Lubachara river

upstream station is I.ubachara (SW326) and next nearest

station 10 km distance down stream station is Kanaighat (SW266) (tig.6.4.2). Bothof the 69 ---

Computer Programme

and Applicatio/1

station is of BWDB. BWDB takes data of water level daily and 3 hour basis and discharge daily basis. They started to take 3 hour basis data from the last four year. In the case of 3 hour basis data they start day with 6 am and end at 6 pm.

~ l:d~~: ",;;)-*

-,

-

Figure: 6.4.2 Shows the t\VOstation of Lubachara river.

Table 6.5 (c) Pre-Monsoon water level of Lubachara river -_. - -

--

Output:

ah ax

Date 15-4-1999 15-4-1999 15-4-1999 15-4-1999

Time

ST 10

WL

0.25 0.375 0.5 0.625

SW326 SW326 SW326 SW326

14.02 14.00 13.98 13.97

15-4-1999

-0.75

-

- SW326

1395

ST 10 SW266 SW266 SW266 SW266 ----.

WL 13.75 13.73 13.71 _ 13.69_

--SW266

13.66

-0.027 Table 6.5 (d) Monsoon water level of Lubachara river

-

Date

Time

15-4-1999 15-4-1999

Output:

15-4-1999 15-4-1999

6:00 9:00 12:00 15:00

15-4-1999

18:00

ah = _ 0.14]

ST

.10 ._. --_.- WL

SW266 SW266 --SW266 SW266 SW266

99

ax

70

-

ST 10 SW326

4.59 4.59 4.59 .4.59

SW326 ---SW326 SW326

4.59

SW326

WL

6.01 6.01 6.01 6.01 6.01

I

-

Chapter Seven CONCLUSION AND RECOMMENDATION -

-

Conclusion and Recommendation

--

Chapter 7 CONCLUSION AND RECOMMENDATION

7.1 Recommendation I. Need more station for the effectively prcdiction or flood and spccially /lash Ilot,d in

north East region. 2. Need to use modern technology for updating the water level and rainfall data.

-'--

3. To further develop our program for the forecast of flash flood and lead time. 4. Frequently check the cross section of the flashy river. 5. Need easy access of global hydrological data. 7.2 Limitation 1. Lack of literature and research on !lash flood.

-

2. Lack of available data of river characteristic of 110rtheast region. 3. Lack of hydrological data in short duration gap. 4. It is not possible to collect the upstream (Indian Catchment) data.

7.3 Concluding Remark Initially our aim was to calculate the lead time or Ilash Ilood which is the majl)r portion of forecasting of flash flood, which is destructive ror the RoM ero/).\'in the north cast region. As

-

there is no suitable system to forecast flash flood in the present world and \ve feel that it is a long term supervision work with more analytical job. The forecasting system depends on not only hydrological, geological and topographical parameter of the regional area but also depend on the global parameter. Which is not possiblc to collect the rclatiyc datn from other neighbor country, for the lack of government collahoration and legislation. 71 --

--

-

--

Conclusion and Recommendatiol1

-As the initial part of the forecasting of flash flood which occur in pre-monsoon season (March to May) in the north east region of Bangladesh, we havc study the hydrological. topographical, and river system of the north east region as well as Bangladesh. Our analysis part is highly related with flood routing, which is helpl'ul to the further study of the forecasting of flash flood. Our study is comprise with a computer programme which calculate the rate of change water level and discharge and water level or any distance or thc down stream. which is a major portion to find out the lead time of flash flood. And finally we hope that it is possible to go ahead from this point to reach the goal that is forecasting of flash flood.

-

72

.

REFERENCES

-

-

References ....

REFERENCES .....

1. Abdul Wazed, Bangladesher Nadil1wla (Rivers ornangladesh,

in BangIa). Dhaka. 1991.

2. Bangladesh Bureau of Statistics (BBS), 1998 Statistical Ycar Book of Bangladesh. BBS. Dhaka, 1999.

3. BWDB, Morphological Features (?f'theAlajor RiI'ers (!fBlIl1gladesh- Part 1. Bangladesh Water Development Board, Dhaka, 1988.

4. Chow. Maidment, Mays, Applied Hydrology, 1988.

5. Encyclopedia of Bangladesh, Bal1glapidia. 2004.

6. F.B Khan, Geology (?I'Bangladesh. University Press Limited. Dhaka. 1991.

7. Haroun Er Rashid, Geography ofBangl(/(/esh. llnivcrsity Press Ltd. Dhaka. 1991.

8. Hugh Brammer, The Geography of the Soils (~f'Bal1gladesh,University Press Limited: Dhaka, 1996.

9. Internet, htlp://www..fema.go\'e/hazard/flood\',

10. Internet, The Flash Flood Laboratory, http://ww\V.cira.colostate.cdu/ftlab/

I I. JHE Garrett, Bengal District Gazel/eers: Nadia. Calcutta. 1910.

12. Khondker, M., G. Wilson and A. Klinting 1998 "Application of Neural Networks in Real Time Flash Flood Forecasting." International Conference in Copenhagen. Denmark. 13. K.N Mukherjee. Applied lIydrolog,y, 1995

72

References

14. MA Islam, "The Ganges-Brahmaputra

River Delta". Journal ojl 'ni1'l!r.0f.\'(~I5.'hellil!ld

(JeoloKical S'ociely (I), 1978.

--

15. MAIl Khan. 'Environmental aspects of surface water development projects in Bangladesh' in AA Rahman et al ed, £I1V;"Ol1mel1t and Developmcnt in Bangladesh. Vol 2. University Press Ltd, Dhaka, 1990.

16. Ming-Hsi Hsua,. Jin-Cheng Fua, Wen-Cheng Liub. Flood routing wilh real-lime \lage correU

ion methodfr)/'

flash floo((jiJrI!ClIsl

ing ;n Ihe 1(lI1s/llIi R ;,'er. 1(1;\1'(111.1'LS F V 1F R.

Journal of Hyrology.

17. Mashfi Saleh in and Abul Fazal m. Saleh. Alor/'/lOlogy and Hydrology (?Ithl! Crcllla Sylhet Basin, IFCDR, BUET.

18. Nile. North l.:ast Regional Watcr Managcmcnt

Project (FAP-6). !'vlarch 1995.

19. Report on North East Region. F'AP-().

20. Research Paper, fnvestiKation qjFlash Flood Ml!chan;sm, 1997

21. Schmittner, K.E. and P. Giresse 1996 "Modelling and Application of the Geomorphic and Environmental Controls on Flash Flood Flow." Geomorphology. Vol. 16.

-

--

73

,...0 "

APPENDIX

--

-

-

--

-- --

40

(1974) o 40

26' 40

80 km

( 19MM) o 40

26' 80 km

24"

24~ 'DHAKA 8,/

23'

23'

.

.

,KHULNA 'BARISAL

o F

BAY

BAY 21'

21'

91.

89'

91'

89'

92. E

92" E

FLOOD AFFECTED AREAS (1955) 40 r.--,

0 T

40 " ,

80km ,

24'

23'

23'

21' Source:

SOurce.

91'

89' I

Figure: 2.3(a) The flood affected areas.

----

MUJJag(,.,'em f)iS(J..'!er

I»jfJrmaIJO" an(}

,'oJarmgt'mt',,1

,89'

RW\'(J/(

Moniwrirtg d)M8).

(M1M)

[)ivi$;QlI

J()9<~

,91'

Appendix

06(, '.M ""'.J6 )66

I tt6 .

I

.

i66~ ~M

~

M

)fi6r

.",116

tilt)

~

. _116

__ fij6

__:1%

...

,

r1l""

t 116

._ :~6

--

116

~,,;

I ub

_ .~6

1~J."r

.

'l"J..6

I. :4."

I

r

1.6

:1..6.

'.'8]1

ut

,-16

~

1\'%

.% ""It,

.

;:'16 t'96

. "''''1''''''

''''Y'-~

'."

". ,ill..,1'

'.M'!IIIIISIW~W I~'

_ :<)6, %

.

.'.«'''''0>m~'--)Jm~

Hill!;'" ~

.

~ ~

iZ

~

~

i

:E

q

~

~ ~'

~ ~ .~

"

T

~

~ ~

;

."

m~ bs u~ papt}(}i .::f1l$;;uy 11

#<

:;;;:

«

t

'96[

_ '*~6' ~...o

~ h6 :;;;;

Appendix

6 .';

&..~!

p

',QL~I

BANGLADESH

l'

\

-,.

A.o;'Sam &...:)iA."

/) hi"..>.lj't')L,

'1-.. -,' H ;}. INDIA

M(.

. Jr', ',DIA'

II

'v' . MYI'f~1 "} K "

/4

'r'~:i 'l~u,A

~ .:

~, E'.!

~.::

\,

\

22

c "

C',',+-

\ .

6<.1'"

A,,~'!" Sl)ilport C,J;>'J1JI C'v'

. C~.:to'j(':

\,

";I!\,

:'r",,; ,'."

l"1e 'OA' ~MI fk..;;"~ ~r"1 BA!lipLALii.€H : ~'

T

f.I,ut.ji'

M

H"'-r-c,udJiS

R .~a\

I

\ '.'A,

"" <1 A',{k

I'll

Figure: 3.1(a) Location and Boundary of Bangladesh. 111 - --

\ ,j'."', \..1

I

SOUTH ASIA

'"

,~

2

[) ,,~!On 9o;,,~,!!>ry [) strlr.! Bou".oary

\ \

t~.\

'I. Ih ',l" \,'1 111.\ '...a.. 0, :! 'tin!

Appendix

90'

'89'

188'

192' E

/ WestBengal ", ~(t. '.) (INDIA) \~ .\ } \ ;t .,J.1 .,=,-. ""-

26' "N

"

(~ t...

50

100km

f.~,

, Assam

{ (INDIA) .! "

",

,

"'''~.:Il\

I

.'

(

"1,..\..___ :'~ ~"'''I~,

,

.,

16a

I.~. 1& '.

,~..",

Meghalaya

I.I"-~~!a

;t

, ,I "

/

o

50

\0;;'

": .J 'J;,"~'?~, "

PHYSIOGRAPHY

(INDIA) -

,If r 15

2

;

_."

,

,":

/:~..."'"

"...

\ ~ , :...""; If.

168

25'

L~-\ }~ t~'''~~..1}

5

16a

2

: Assam

6

2

<~8

I~"7b (INDIA)

2

., \",

11

i t.. ._,~?,.:15 >

7c

,...:.:;:.....

,

..\,...............

8

15 :::15 ~ ~

t

~:" I tf"..:.

J. 24'

,J ~l'..J. I '"', ~..

\,

..,...... ";J

," ,... c", ,.. '" I ~ ".. '" \. I 'I

Mizoram (INDIA)

17b !, 8

, ,..'

23'

Tnpura (INDIA)

,

\,

WestBengal (INDIA)

24

': !

, < ; '.i

\

.. ~"."" , .." I : "'.1 ~;17b J4

,.

..

..'

"' ,;:i

~ ,"

7c

13

13

I

7b

13

.r , \..

i

7c

i

14

17a

I

, <

7d

9 9

'I .'

,I " ",,

.\. '.

\ '

I

\\

rt

,

. ~.

fl'>

7d

I,'

,\

23

.,

,

7d

14

".

7d

,

11,1 u.,

22

27 9 'v

'7b

,.. G

0

BAY Physiographic

a

F

14'

Units 7c Oid MeghnaEstuarineFloodptain 14 J~ YoungMegltnaEstuarineFloodplain 1~

;j

Jamuna (Young Btahtnaputra) Floodplain

'8

Ganges River Floodplain

4 5 6 7a ib

Old BrahmaputraFloodplain Haor Basin Surma. KushiyaraFloodplwn MiddleMeghnafloodplain LowerMeghnaFIoodptain

9 10 11 [j'!' ::IT'

GangesTidal Floodplain Sondarban LowerAlrai Basin Aria! Bee! Gopalganj, KhulnaBaets

89"

17a

, 17br'

1. Old HimalayanPiedmontPlain 2 TlStaFloodplain

88'

Eo

16a

_

Chittagong CoaS181 Plain Northern and Eastern Piedmont Piain Barind Tract

161> Madhupur Tract Tippera Surface

1'78 The Low HPI Ra~6 17b The High Hill Ranges

90'

191'

Source: ModifiedFromSRDI,1997; Rashid, 1991;Reimann. 1993

Figure: 3.2 (b) Physiography of Bangladesh.

IV

,,

"'..\ '

:

\ \ , ,.1 '

MYANMAR 21

", \

\

\

Appendix

t

88'

92' E

(

." Y:'. ..' oY:'''

-< +';,

!f

I))....

;;..

aQt.

Wesl Bengal (INDIA)

OlJ....

MAINRIVERSOF BANGLADESH

?';

o

40 26' N

;

"" ..,

... o' '",..

40

=

80km

26'

....,

" ..o ->. '" 'C 0 ... .. " .. :>

E. OJ'

Meghalaya (INDIA)

.~ .. 3 c " ~

t:>

ot>

I-

.., "0,

... oj '3 '" '? "

<:

Assam (INDIA)

", "

.RAJSHAHi

G'> vIt) a1

i! ..

'"

"

24'

Tripura (INDIA)

Mizoram (INDIA) WestBengal (INDIA)

22"

KUTuaDIA CHANNEL

'v

(;

B

I-

A

y

o

88'

MAHESHKHAl' CHANNEL

Arakan (MYANMAR)

F

'~

90' I

92'

Figure: 3,1.4 Main river of Bangladesh. v

21'

Appendix

92<E

91

88' ..J

29

CLIMATE

¥

Pa'1ct1?9,ntf

o

4G

C

,

\ \

:I

, ,

80km __J

40

I

,

Rangpuf

,,

25 N

~

,,

'\

,

,

o " Tangail ,

G

24

18"

... A

,

DHAKA

,

\

Ralbari i

150/

l'

27"

200 , ~

;~::~

V~m.m~

.

Jesson:t

F

23

1

29' 20' 20a .#<

A \

28 flAY

ICTnpt.,"'famreI "C) January Apf\1 July 11

()F

r

Climatic

A C D

Rnmfall (em) ArmuaRalnfal1 B9'

, , , "

.........

G

Sub-regions South-eastern zone NQrth-oasterl1l0ne Northern part of the rtOTthern regloo NortrHvestemZQne Western zone Sooth-westernzone Sooth.centralzone 90' 91.

""\

92" Smut('

Figure: 3.3 The Climatic Condition of Bangladesh. VI

----

300

27'

\

Ra~i\ld, Ha.nma 1..1.1')'II

Appendix

90'

88'

92' E

MEAN ANNUAL RAINFALL o

50

':,

~

50 r

100ltm j

Jntemahonal Boundary Oistncl Boundary Jsohyte (in mm)

/.

~ ,': .. t'"

\ ,.. ~

~....

75

'.

I :.. '\

, .J Assam

".

'-', \ R:\JSHi ~.....

MaviviBazarl ,' ,.

(INDIA)

.'\~

'1'-'"

.t. I-~. . "..Ji 1500 J".1

(.~-'-."

.I'..I

24

r'

,

,'1

Tnpura (INDIA)

;:

\.

'.1'. .,I ,41.

" Comllla t

Jhenatdaha <.

-

." (".I

" ~

I;~ .. \

\ Mlzoram 't (INDIA) " "

~ " '. Khagrachhan\'

-\,;

J-..) I

West Bengal (. (INDIA) \

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Figure: 3.3.5 Mean Annual Rainfall of Bangladesh.

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84'

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Appendix

Il.

88'

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A.

FLOODPRONEAREA :113'

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Figure: 3.5 The Flood Prone area of Bangladesh. IX

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MYANMAR

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Appendix

89"30'

SH>30'

90"30'E

BRAHMAPUTRA .. JAMUNA26 RIVERSYSTEM 30' West Bengal (INOtAI

26" 00'

25' iCY

West Bengal

(INDIA)

25' 00'

24 J'I1' '.J

I

24" 00'

West Bengal

(INDIA)

9100'

89'00' Figure: 3.4.1 The Brahmaputra-Jamuna System.

x

--

-

Appendix

90" E

89"

25°

GANGESPADMARIVERSYSTEM 20

o

20

40km

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24° N

23°

24°

West Bengal (INDIA)

J/

- 22"

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BAY

OF BENGAL 90°

89°

Figure 3.4.2 The Ganges-Padma System. Xl

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91 '50'

92'00'

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Boy of B(tn~J~.1

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Appendix

~< 4>.

'tf c:

Assam (INDIA)

Meghalaya (INDIA)

o Assam (INDIA)

./

~ ~' .~

~ .~ ':'1 ,Q)

Tripura (INDIA)

Figure: 4.2 Regional River System of North East Region. Xlll

-

-

- -

- ---

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-

--