Impact Assessment of Predicted Sea Level Rise and Storm Surge on Coastal Embankments of Some Selected Polders Using Mathematical Model

7/29/2019 Impact Assessment of Predicted Sea Level Rise and Storm Surge on Coastal Embankments of Some Selected Pold…

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IMPACT ASSESS

STORM SURGE

SELECTED PO

DEPARTME

BANGLADESH UNI

- 1 -

ENT OF PREDICTED SEA LEVEL

ON COASTAL EMBANKMENTS O

DERS USING MATHEMATICAL

AHSAN AZHAR SHOPAN

T OF WATER RESOURCES ENGINEERI

VERSITY OF ENGINEERING AND TECH

DHAKA

FEBRUARY, 2011

ISE AND

SOME

ODEL

G

NOLOGY



I

DECLARATION

I, do, hereby, declare that the research work reported in this thesis has been performed

by the author under the supervision of Dr. M. Mirjahan Miah, Professor, Department

of Water Resources Engineering, Bangladesh University of Engineering and

Technology. Neither this thesis nor any part of it has been submitted or is

concurrently submitted elsewhere for any purpose. To the best use of my knowledge

and competence the thesis contains no material previously published partially or in

full or written by any other person except when due references are made in the text of

the thesis.

February, 2011

Signature of the Supervisor Signature of the Author

_______________________ _______________________

Prof. Dr. M. Mirjahan Miah Ahsan Azhar Shopan



II

ACKNOWLEDGEMENT

The author wishes to express his deepest gratitude to the almighty Allah for special

blessings leading to the writing of this thesis work. The author would like to express

his profound gratitude and indebtness to his superviser Dr. M. Mirjahan Miah,

Professor, Department of Water Resources Engineering, BUET, Dhaka for his

untiring encouragement, generous support, valuable advice and constant guidance and

inspiration at every stage of the work. The study would have been difficult to

comprehend as well as to achieve without his guidance.

The author is grateful to Professor Dr. M. A. Matin, Head, Department of Water

Resources Engineering, BUET for necessary help of the thesis work.

The author is indebted to Md. Mobassarul Hasan, Associate Specialist and Ms.

Farhana Akhter Kamal, Junior Specialist, Institute of Water Modelling (IWM), Dhaka

for their kind co-operation to provide necessary data for this thesis work.

The author is grateful to Md. Nasiruddin, Sub-Divisional Engineer and Mir Shahinur

Rahman, Sectional Officer, BWDB, Satkhira Zone to for their kind co-operation

during the field visit.

The author would like to specially thank Tonmoy Sarker and Md. Rakibul Hassan

Khan for their kind and valuable help in the operation of HEC-RAS model.

Finally the author gratefully acknowledges the help, encouragement and co-operation

received from his family and friends.



III

ABSTRACT

Polder no. 15, 7/1 and 7/2 are situated in Shyamnagar thana, Satkhira in SW zone of

Bangaldesh. The embankments of these polders have been overtopped and breached

during cyclonic storm surge Aila on May 25, 2009.

The present study assesses the performance of the embankments of these polders

during cyclone Aila and the adequacy of the existing embankment height against the

simultaneous impact of sea level rise and storm surge using mathematical model.

Data has been collected from different secondary sources. Embankment data has been

collected from BWDB, Satkhira Zone. Bathymetry data and water level and discharge

data has been collected from IWM. Other data used in this study hs been collected

from other different sources.

In this study the performance of coastal embankments of polder no. 15, 7/1 and 7/2

during the cyclone Aila has been assessed through field visit. It has been found that

these embankments constructed during the 1960s have now been unable to protect the

polders from the impact of storm surge. Climate change and associated sea level rise

will make the embankments more inadequate to resist storm surges.

From mathematical modeling it has been found that embankment height of the polders

selected for this study are not sufficient to resist the simultaneous impact of SLR and

cyclonic storm surge associated with high tide. In this study SIDR has been selected

as cyclonic storm as it is the most devastating than other historical cyclonic storm and

also because the original wind speed of SIDR (225 kmph) corresponds to that of a

cyclone with a return period of 20 years (223 kmph).

According to the 4th IPCC Report if temperature is increased by 4oC wind speed will

be increased by 10%. Thus considering temperature would increase by 4oC in the year

2100 the wind speed of cyclone SIDR has been considered as 248 kmph in

mathematical modeling. The cyclone has been considered to travel along the track of

AILA with 1.5 m SLR at high tide.



IV

In case of polder no. 15 surge height has been found to be 4.36 m above the crest level

of existing embankment where as in polder no. 7/1 it would be 4.42 m and in polder

no. 7/2 it would be 4.55 m. So it is essential to raise the embankment height with

proper redesign to protect the people and their livelihood inside the polders.

Finally the crest level for the selected polders has been calculated. In case of polder

no. 15 crest level is 9.64 mPWD. Again in case of polder no. 7/1 crest level is 9.8

mPWD and in case of polder no. 7/2 crest level is 9.93 mPWD. In all cases the

suggested crest level is much higher than the existing one and all the suggested height

are for the year 2100 considering 1.5 m sea level rise and normal high tide.

It is extremely important to reconsider the existing coastal infrastructure based on this

and redesign existing coastal embankments to rehabilitate these structures and make it

climate resilient. The risk of sea level rise and cyclonic storm surge are to be

considered for future planning and design of coastal structure. Proper adaptation

measures both structural and non-structural are to be planned in order to find a climate

resilient coastal environment for coastal people and their livelihood security.



V

Table of Contents

Page No.

Declaration I

Acknowledgement II

Abstract III

Table of Contents V

List of Figures VII

List of Tables X

Abbreviations XI

Chapter 1: Introduction

1.1. General

1.2. Background

1.3. Objectives of the Study

1

4

4

Chapter 2: Literature Review

2.1 General

2.2 Climate Change and Sea Level Rise

2.3 Tropical Cyclones and Storm Surges in Bangladesh

2.4 Coastal Embankment Related Studies

5

5

10

19

Chapter 3: Study Area

3.1 Study Area3.2 Polder 15

3.3 Polder 7/1

3.4 Polder 7/2

2122

24

26



VI

Chapter 4: Description of Models

4.1 General

4.2 HEC-RAS

4.3 MIKE-21

29

29

38

Chapter 5: Methodology

5.1 Data Collection

5.2 Field Visit

5.3 HEC-RAS 1D Hydrodynamic Modeling

48

51

52

Chapter 6: Results and Discussion

6.1 Performance of coastal embankments of the selected polders during

cyclone AILA

6.2 Adequacy of the existing embankment height

62

70

Chapter 7: Conclusions and Recommendations

7.1 Conclusions

7.2 Recommendations

81

82

References 83

Appendix-A: Streamline Co-ordinates 88

Appendix-B: Cross-Section Data 94

Appendix-C: Flow Hydrographs and Stage Hydrographs 109



VII

List of Figures

Figure 1.1: Potential impact of sea level rise on Bangladesh 1

Figure 1.2: Coastal Zone of Bangladesh 2

Figure 1.3: Coastal Zone of Bangladesh with polders 3

Figure 2.1: Causes of sea level rise 5

Figure 2.2: Tracks of cyclones 12

Figure 2.3: Number of cyclonic storms landed on Bangladesh coast in

different decades

13

Figure 2.4: Monthly variation of Cyclone during 1960-2007 13

Figure 2.5: Yearly variation of wind speed 14

Figure 2.6: Yearly variation of surge height 14

Figure 2.7: Satellite Image of Cyclone Aila 15

Figure 2.8: Track of Cyclone Aila 16

Figure 3.1: Location of the Study Area on Polder Map 21

Figure 3.2: Typical cross-section of the embankment of Polder 15 22

Figure 3.3: Index Map of Polder 15 23

Figure 3.4: Typical cross-section of the embankment of Polder 7/1 24

Figure 3.5: Index Map of Polder 7/1 25

Figure 3.6: Typical cross-section of the embankment of Polder 7/2 26

Figure 3.7: Index Map of Polder 7/2 27

Figure 3.8: Study Area on Google Earth 28

Figure 4.1: Schematic channel cross-section 31

Figure 4.2: Different Components of HEC-RAS 33

Figure 4.3: Model domain of three nested model 41

Figure 4.4: Calibration against discharge data at North Hatiya during 2006 45

Figure 4.5: Calibration against water level data at Charchenga during 2006 45

Figure 4.6: Validation against discharge data at West Shahbazpur Channel

during December 2007

46

Figure 4.7: Validation against Discharge data at East Shahbazpur Channel

during December 2007

46

Figure 4.8: South-West Region Model 47



VIII

Figure 5.1: Shape file of river reaches surrounding the study area 50

Figure 5.2: Point theme of study reach 50

Figure 5.3: Attribute table of point theme of study reach 51

Figure 5.4: Steps of HEC-RAS 1D Hydrodynamic Modeling 52

Figure 5.5: Entering streamline co-ordinates 53

Figure 5.6: Editing junction data 54

Figure 5.7: Schematized river system 54

Figure 5.8: Entering cross-section data 55

Figure 5.9: Interpolating cross-section data 56

Figure 5.10: View on Geometric Data Editor after interpolating cross-section

data

56

Figure 5.11: Calibration against water level during 2009 57

Figure 5.12: Different steps of entering boundary conditions and initial flows 58

Figure 5.13: Locations of application of boundary conditions 59

Figure 5.14: Performing Unsteady Flow Simulation 60

Figure 6.1: Illustration of damages to Polder no. 15 due to cyclone AILA 64

Figure 6.2: Illustration of damages to Polder no. 7/1 due to cyclone AILA 65

Figure 6.3: Illustration of damages to Polder no. 7/2 due to cyclone AILA 66

Figure 6.4: Images from the study area during field visit 67

Figure 6.5: Stage Hydrograph at a cross-section around Polder 15 duringSIDR along the track of AILA with 1.5 m SLR and at high tide

71

Figure 6.6: Maximum water level at a cross-section around Polder 15 during

SIDR along the track of AILA with 1.5 m SLR and at high tide

72

Figure 6.7: Detailed output table at a cross-section around Polder 15 during


73

Figure 6.8: Stage Hydrograph at a cross-section around Polder 7/1 during


74

Figure 6.9: Maximum water level at a cross-section around Polder 7/1 during


75

Figure 6.10: Detailed output table at a cross-section around Polder 7/1 during


76

Figure 6.11: Stage Hydrograph at a cross-section around Polder 7/2 during


77



IX

Figure 6.12: Maximum water level at a cross-section around Polder 7/2 during


78

Figure 6.13: Detailed output table at a cross-section around Polder 7/2 during


79



X

List of Tables

Table 2.1: Change in MSL in different tidal water level stations 7

Table 2.2: Trend of tidal level in three coastal stations of Bangladesh 8

Table 2.3: List of most vulnerable countries to cyclone 10

Table 2.4: List of major cyclones from 1960 to 2007 11

Table 2.5: Monthly variation of cyclone from the year 1960 to 2007 14

Table 2.6: Damages to Infrastructure by Cyclone Aila 16

Table 2.7: Cyclone Wind Speeds 17

Table 2.8: Design Surge Heights 17

Table 2.9: Embankments under CEP 20

Table 4.1: Model grid specification 42

Table 4.2: Relative grid positions 42

Table 4.3: Land-water data used for generating the bathymetry 42

Table 4.4: Manning number distribution 43

Table 4.5: Tidal constituents for Vishakhapatnam and Gwa Bay 44

Table 5.1: Survey period of collected cross-section data 48

Table 5.2: Scenario for model simulation 57

Table 6.1: Damages to the selected polders due to cyclone Aila 62

Table 6.2: Maximum water level around individual polders 70

Table 6.3: Design crest level of the embankments of selected polders 80



XI

Abbreviations

BBS Bangladesh Bureau of Statistics

BMD Bangladesh Meteorological Department

BoB Bay of Bengal

BoBM Bay of Bengal Model

BUET Bangladesh University of Engineering and Technology

BWDB Bangladesh Water Development Board

CEGIS Center for Environmental and Geographic Information Services

CEP Coastal Embankment Project

CERP Coastal Embankment Rehabilitation Project

CPP Coastal Protection Project

CZPo Coastal Zone Policy

DMB Disaster Management Bureau

EPWAPDA East Pakistan Water and Power Development Authority

FAP Flood Action Plan

HEC-RAS Hydrologic Engineering Center-River Analysis System

IPCC Intergovernmental Panel on Climate Change

IUCN International Union for Conservation of Nature

IWM Institute of Water Modelling

JTWC Joint Typhoon Warning Center

MSL Mean Sea Level

NAPA National Adaptation Program of Action

OECD Organization for Economic Co-operation and Development

PWD Public Work Datum

RSMC Regional Specialized Meteorological Center

SLR Sea Level Rise

SMRC SAARC Meteorogical Research Centre

SWMC Surface Water Modelling CenterUNDP United Nations Development Programme

UNEP United Nations Environment Programme

WARPO Water Resources Planning Organization

WMO World Meteorological Organization



1

CHAPTER 1

INTRODUCTION

1.1 General

Climate change is an important issue nowadays. Due to various human activities,

carbon dioxide (CO2) and other greenhouse gases are accumulated in the earth’s

atmosphere. The ultimate result is global warming, i.e. climate change. Rising

temperature in the atmosphere causes sea level rise. Sea level rise affects low lying

coastal areas and deltas of the world. Bangladesh is highly vulnerable to sea level rise,

as it is a densely populated low-lying coastal country of extremely gentle slope

comprising broad and narrow ridges and depressions (Brammer et al., 1993). The

World Bank (2000) estimates a 10 cm, 25 cm and 1 m rise in sea level by 2020, 2050

and 2100 respectively. This rise would inundate 2%, 4% and 17.5% of the total land

mass of the country. Milliman et al. (1989) reported a sea level rise in Bangladesh of

1.0 cm per year. According to the Synthesis Report of Copenhagen Summit on March

2009 maximum sea level rise (SLR) will be 0.5-1.5 m by the year 2100. Figure 1.1

illustrates the potential impact of 1.0 m and 1.5 m sea level rise on Bangladesh.

Figure: Potential Impact of Sea Level Rise on Bangladesh

Land submerged -22,000 sq. km.

People affected-18 million

1.5 meter sea level rise

BAY OF

BENGAL

Actual sea levelLand submerged -17,000 sq. km.

People affected-15 million

BAY OF

BENGAL1 meter sea level rise

Land submerged – 17,000 sq. km.People affected – 15 million

1.5 meter sea level rise

Land submerged – 22,000 sq. km.

People affected – 18 million

Figure 1.1: Potential impact of sea level rise on Bangladesh

1 meter sea level rise



2

The southern part of Bangladesh is connected to the Bay of Bengal through a 710 km

of coastline (CZPo, 2005). The coastal area of Bangladesh consists of 19 districts

comprising 147 upazilas (CZPo, 2005). Total area of the coastal zone is 47,201 sq. km

which is 32 % of the total landmass of the country (Islam, 2004, p. xvii). A total of 48

upazilas in 12 districts that are exposed to the sea and/or lower estuaries are defined

as the exposed coast and the remaining 99 upazilas of the coastal districts are termed

as interior coast. The total population living in the coastal zone is 35.1 million, i.e. 28

% of the total population of the country (BBS, 2003). Population density in the

exposed coast is 482 persons per sq km as opposed to 1,012 for the interior coast.

Average population density of the zone is 743 per sq km, compared with the national

average of 839.

Figure 1.2: Coastal Zone of Bangladesh (Source: Islam, 2004)

The coastal region is marked by morphological dynamic river network, sandy beaches

and estuarine system. The interaction of huge fresh water and sediment load coming

from the upstream and saline waterfront penetrating inland from the sea are the main

features of the coastal area. . Almost one third of the landmass lies within the 4 m

contour line. These vast areas of low-lying land are threatened by different levels of

sea level rise.



There are approximately

12,80,479 ha of land are

2001). Polders are low-l

that forms an artificial h

water other than through

from the mid-sixties to

Development Board (B

Development Authority

flooding and saline wa

without any consideratio

the normal tide only but

of all these polders vari

coastal embankment is a

Figure 1.3: Coa

3

123 polders in the coastal area. These polders

a under 11 coastal districts comprising 58 upa

ing tract of land enclosed by embankments kn

drological entity meaning it has no connectio

manually operated devices. These polders wer

he mid seventies of the 20th

century by Bang

DB) and its predecessor, East Pakistan Wat

(EPWAPDA) to protect the land from tidal

ter intrusion in order to increase agricultur

of safety against cyclonic surges. These polde

cannot resist storm surge height. Height of the

es between 3 m to 7 m (IWM, 2005). The t

out 5017 km (BWDB 2001).

tal Zone of Bangladesh with polders (Source: I

re protecting

zilas (BWDB,

own as dikes

with outside

e constructed

ladesh Water

r and Power

and monsoon

l production

rs can protect

embankment

tal length of

M)



4

1.2 Background

Bangladesh is widely recognized to be one of the most vulnerable countries of the

world to climate change and associated sea level rise. The country also experiences

frequent natural disasters such as tropical cyclones, storm surges and floods, which

cause loss of life, damage to infrastructure and economic assets. One of the most

devastating cyclones that recently hit Bangladesh is AILA. Cyclone AILA hit the

south-western coast of Bangladesh on May 25, 2009. A storm surge of 3 m (10 ft)

impacted western regions of Bangladesh, submerging numerous villages. Several

rivers broke through embankments, causing widespread inland flooding

(www.wikipedia.com). Torrential rains from AILA resulted in at least 190 fatalities

from flooding ( DMB, 2009).

According to IPCC’s Fourth Assessment Report (IPCC, 2007) the frequency and

intensity of such severe cyclones will increase by the year 2100 due to climate

change. Moreover, IWM and CEGIS (2007) have studied that about 13 polders will

be overtopped by the year 2080 due to sea level rise. Damages by cyclone AILA

indicates to the fact that the embankments of the polders which are designed to protect

the polders from tidal flooding only require design modification to protect the polders

from the effect of sea level rise and cyclonic storm surges.

This study will assess the susceptibility of the country’s coastal infrastructure to the

effects of sea level rise and cyclonic storm surges. Mathematical model will be

applied to assess the impact of sea level rise with cyclonic storm surge on coastal

embankments of some selected polders. Focus will be on the embankment height.

1.3 Objectives of the study

The objectives of the study have been described as follows:

• To assess the performance of coastal embankments of some selected polders

during cyclone AILA.

• To assess the adequacy of existing embankment height of selected polders

against the simultaneous impact of predicted sea level rise and storm surge

using mathematical model.

• To propose modification of crest level of the coastal embankments.



5

CHAPTER 2

LITERATURE REVIEW

2.1 General

The coastal region of Bangladesh is prone to multi-hazard threats such as cyclones,

storm surges, floods, earthquakes, tsunamis, and above all, climate change.

Bangladesh has been also identified as one amongst 27 countries, which are the most

vulnerable to the impacts of global warming induced accelerated sea level rise.

Several studies have been carried out all over the world to see the impact of climate

change, sea level rise and climate change induced cyclonic surge in the near future

and the possible adaptations against it. In this chapter some previous studies have

been reviewed which are related to the present study.

2.2 Climate Change and Sea Level Rise

The dominant factor for climate change is the increase in concentration of various

gases (CO2, CH4, N2O etc.) in the atmosphere. Rising temperature expands the ocean

volume in two ways. Firstly, it melts the mass volume of ice in the polar region and

secondly, it causes thermal expansion of the oceans. The human factor mainly

responsible for global warming and sea level rise is the burning of fossil fuels.

Deforestation is another human activity, responsible for decreasing the CO2 sink.

Figure 2.1: Causes of sea level rise (Source: IPCC, 2001)



6

The Intergovernmental Panel on Climate Change (IPCC)’s periodic assessments of

the causes, impacts and possible response strategies to climate change are the most

comprehensive and up-to-date reports available on this subject. IPCC, established

under World Meteorological Organization (WMO) and United Nations Environment

Program (UNEP), has published four assessment reports till now (2010) namely the

First Assessment Report (FAR, 1990), the Second Assessment Report (SAR, 1995),

the Third Assessment Report (TAR, 2001) and the Fourth Assessment Report (AR4,

2007).

In the First Assessment Report, IPCC estimated that global temperature would

increase by 1oC by the year 2050 and 2

oC by the year 2100 and sea level rise would

be 107 cm by the year 2100 considering the year 1990 as baseline (IPCC, 1990).

In the Second Assessment Report, it is projected that the average surface temperature

would increase by 1oC to 3.5

oC and the sea level would rise by 15 to 95 cm

respectively over the period 1990 to 2100 (IPCC, 1995).

In the Third Assessment Report, the average surface temperature is projected to

increase by 1.4oC to 5.8

oC over the period 1990 to 2100 and the sea level is projected

to rise by 9 to 88 cm over the same period (IPCC, 2001). National Adaptation

Program of Action (NAPA, 2005) has predicted the sea level rise for Bangladesh

based on IPCC’s Third Assessment Report (TAR, 2001) and stated that SLR for the

year 2030, 2050 and 2100 would be 14 cm, 32 cm and 88 cm respectively with

respect to the year 1990.

In the Fourth Assessment Report, it has been projected that global sea level rise and

temperature increase between 2090 and 2100 would be 18 to 59 cm and 1.1oC to

6.4oC respectively with respect to the year 1980-1999. The report has also shown that

the frequency and intensity of severe storm, rainfall intensity and drought will

increase dramatically (IPCC, 2007).

Again, according to the Synthesis Report of Copenhagen Summit on March 2009

maximum sea level rise will be 0.5-1.5 m by the year 2100 (Richardson et al, 2009).



7

2.2.1 Global Sea Level Rise

Global sea level has risen by 10 to 25 cm over the past 100 years and much of the rise

may be related to the increase in global mean temperature (IPCC, 2007). A study by

Church et al. (2004) found a sea level rise of 1.8±0.3 mm per year over a 51 year

period (1950-2000). Church & White (2006) estimated a sea level rise from January

1870 to December 2004 of 195 mm, a 20th century rate of sea level rise of 1.7±0.3

mm per year and a significant acceleration of sea level rise of 0.013±0.006 mm per

year. With this constant rate of acceleration, sea level rise from 1990 to 2100 would

range from 280 to 340 mm.

2.2.2 Sea Level Rise in Bangladesh

Matin (2008) in his report published for IUCN analyzed water level data of 13 coastal

stations of BWDB covering the whole coastal area from west to east using the

measured data of last 21 years to observe the sea level rise trend. Change of MSL for

all stations has been presented in Table 2.1. It has been found that 9 out of 13 stations

show increasing tendency of MSL. So it is an indication of sea level rise in the coastal

area of Bangladesh.

Table 2.1: Change in MSL in different tidal water level stations

River Station ID Station Change of MSL (mm/yr)

Ichamati (Western border) SW130 Kaikhali 10.50

Pussur SW244 Mongla 10.90

Alipur Khal Doratana SW1 Bagerhat 6.20

Baleswa SW107.2 Rayenda 3.70

Bishkhali SW39 Patharghata 15.70

Buriswar SW20 Amtali 4.5

Lower Meghna SW279 Tajumuddin 30.00

Hatiya SW321 Hatiya -1.80

Feni SW87 Sonapur 49.80

Sangu SW248 Dohazari -2.10

Matamuhuri SW204 Chiringa 20.70

Kutubdia Channel SW176 Lemsikhali -8.20

Moheskhali Channel SW200 Saflapur -6.20



8

Singh et al. (2000) analyzed the tidal level in Hiron Point, Char Changa and Cox’s

Bazar based on the tidal gauge record of the period 1977-1998. All three stations

showed increasing tendency of tidal level. Table 2.2 represents the trend of tidal

levels in these stations.

Table 2.2: Trend of tidal level in three coastal stations of Bangladesh

Tidal

Station

Region Latitude

(N)

Longitude

(E)

Datum

(m)

Trend

(mm/yr)

Hiron Point Western 21o48’ 89

o28’ 3.784 4.0

Char Changa Central 22o08’ 91

o06’ 4.996 6.0

Cox’s Bazar Eastern 21o26’ 91

o59’ 4.836 7.8

(Source: Singh et al., 2000, SMRC Report No. 3)

Two estimates for potential future sea level rise in Bangladesh prior to the projection

of NAPA (2005) are 0.3-1.5 m and 0.3-0.5 m for the year 2050 considering the year

1990 as baseline. Besides ice melting and thermal expansion, area specific land

subsidence and uplifting is a contributory factor to the sea level rise in Bangladesh.

The Ganges and the Brahmaputra deliver approximately 1.6 billion tons of sediment

annually to the face of Bangladesh (Broadus, 1993). These sediments compensate for

the natural compaction and subsidence of the delta and keep its size relatively stable.

Sediment replenishment is considered to balance the subsidence of the delta that

results in a net sea level rise (Agrawala et al., 2003, p. 15).

Studies from OECD (2003) shows that, due to a rise of 100 cm in the sea level,

Bangladesh would face a catastrophic situation, including permanent inundation of

about 15-18% of its low-lying coastal areas, loss of different species in the

Sundarbans, displacements of over 10 million people, loss of vulnerable agricultural

land and extension of salinity intrusion further inland. Hasan (2008) (Ref. no. 15) in

his study estimated that due to 27 cm sea level rise brackish water area will be

increased by 6% and due to 62 cm sea level rise, brackish water area will be increased

by 9%. It is also estimated that for the base condition (2005), about 6.0 million people

are already exposed to high salinity (>5 ppt), which will be increased to 13.6 million

by the year 2050 and 14.8 million by the year 2080.



9

BUET (2008) carried out project on preparation of Look–up Table and generation of

PRECIS scenarios for Bangladesh. In this study a regional climate modeling system

called PRECIS (Providing Regional Climate for Impact Studies) is used to generate

projection for rainfall and temperature using SAES A2 emission scenario. Projected

annual rainfall obtained are 6.93, 6.84 and 7.17 mm/d in 2030, 2050 and 2070

respectively whereas the baseline (1961-1990) rainfall is 6.78 mm/d. The important

notice is that in Bangladesh monsoon and post monsoon rainfall will increase whereas

dry season rainfall will be remaining closer to historical amount. Monthly average

maximum temperature will change from -1.2 to +4.7oC in 2030, from -1.2 to +2.5

oC

in 2050 and from -1.20 to +3.0oC in 2070. On the other hand monthly average

minimum temperature will increase in all period and vary from 0.3 to 2.4oC in 2030.

A Look–up Table has been prepared to obtain the project rainfall and temperature

(maximum and minimum), firstly in reference to the observed data during baseline

period 1961-1990. Using the prepared Look-up table PRECIS generated scenarios

was validated for 1989, 1990, 2000 and 2001. Validation was found significant at

99% level. Through validation a projection factor for each parameter is obtained

which is employed in preparing projected rainfall and temperature.

IWM (2005) has made a detailed impact assessment of sea level rise on inundation,

drainage congestion, salinity intrusion and change of surge height in the coastal zone

of Bangladesh. The potential effect of climate change were studied for different sea

level rise i.e. 14 cm, 32 cm and 88 cm for the projected years 2030, 2050 and 2100.

Mathematical models of the Bay of Bengal have been used to transfer the sea level

rise in the deep sea along the southwest region river and Meghna Estuary. The study

shows that about 11% more area (4107 km) will be inundated due to 88 cm sea level

rise in addition to the existing (year 2000) inundation area under the same upstream

flow. The 5 ppt saline front moves landward remarkably for sea level rise of 88 cm.

About 84% of the Sundarbans area becomes deeply inundated due to 32 cm sea level

rise and for 88 cm sea level rise Sundarbans will be completely lost. Due to 32 cm sea

level rise surge height increase in the range of 5% to 15% in the eastern coast. It also

observes that 10% increase in wind speed of 1991 cyclone along with 32 cm sea level

rise would produce 7.8 m to 9.5 m high storm near Kutubdia- Cox’s Bazar coast.



10

2.3 Tropical Cyclones and Storm Surges in Bangladesh

The coastal area of Bangladesh is always vulnerable to cyclone-induced storm surge.

There are mainly three reasons behind it. The first one is that the continental shelf is

long and shallow and the funnel shape of the coast tends to concentrate and amplify

the surge in the northern part of the Bay. Secondly the coastal zone is low-lying with

62% of the land have an elevation of up to 3 meters and 86% up to 5 meters from the

mean sea level (IWM, 2009). The third reason is that the coastal area is densely

populated (Total Population: 35.1 million, Average population density: 743 per sq

km; Source: BBS, 2003). UNDP has identified Bangladesh to be the most vulnerable

country in the world to tropical cyclones.

Table 2.3: List of most vulnerable countries to cyclone

Rank Country Deaths/100000 people exposed to cyclone

1 Bangladesh 32.1

2 India 20.2

3 Philippines 8.3

4 Honduras 7.3

5 Vietnam 5.5

6 China 2.8

(Source: UNDP, 2004)

About one-tenth of the global total cyclones forming in different regions of the tropics

occur in the Bay of Bengal. About one-sixth of the tropical storms generated in the

Bay of Bengal usually hit the Bangladesh coast. Historical record shows that more

than 15 severe cyclones are generated in the Bay of Bengal in every ten years. During

1960-2007 about 18 severe cyclones hit the coast of Bangladesh (IWM, 2009). During

the period 1582 to 1997 there were 82 cyclones that devastated the coastline of

Bangladesh (Jacobson et al. 2006). These cyclones originated mainly in Indian Ocean

or Bay of Bengal generally form in the months just before and after the monsoon and

typically move to north to northeastern direction towards the land (SMRC, 1998). It is

seen that the eastern coast experiences maximum inundation between 4-6 m and

western coast experiences inundation within the range of 3-5 m (IWM, 2009).



11

Table 2.4 represents the cyclones and associated maximum wind speeds and surge

heights since 1960. Figure 2.2 illustrates the tracks of the cyclones.

Table 2.4: List of major cyclones from 1960 to 2007

Year Time Storm surge (m) Wind (kmph) Zone

1960 9-Oct 6 129 Meghna estuary

1960 30-Oct 4.2 210 Chittagong coast

1961 9-May 9 145 Meghna estuary

1961 27-May 7 145 Chittagong-Noakhali coast

1962 26-Oct 5.8 200 Feni-Chittagong Coast

1963 28-May 5 203 Noakhali-Cox's Bazar Coast

1963 5-Jun 3.1 135 Sunderban

1963 25-Oct 2.2 105 Teknaf

1965 10-May 4 161 Barisal-Chittagong coast

1965 31-May 7.1 162 Chittagong Coast

1965 14-Dec 4 210 Cox's Bazar-Teknaf coast

1966 10-Oct 7 145 Chittagong and Sandwip

1967 11-Oct 3 160 Sunderban-Noakhali coast

1967 23-Oct 2 130 Chittagong-Cox's Bazar coast

1970 5-May 2.3 148 Chittagong-Teknaf coast

1970 12-Nov 9 222 Khulna-Chittagong coast

1971 16-Nov 1 110 Sunderban coast

1973 29-Nov 3.5 165 Chittagong coast1973 9-Dec 7.5 122 Sundarban-Patuakhali coast

1974 28-Nov 5 161

Cox's Bazar-Chittagong-

offshore

1983 9-Nov 3 136 Chittagong-Teknaf coast

1985 25-May 4.5 154 Noakhali-Cox's Bazar coast

1988 29-Nov 3.5 160 Sundarban

1990 18-Dec 2 115 Cox's Bazar coast

1991 29-Apr 7.5 225 Patuakhali-Cox's Bazar coast

1991 May 1.9 90

Cox's Bazar-Chittagong-

offshore1994 April 4 210 Cox's Bazar

1997 May 3 220 Chittagong-Cox's Bazar coast

1997 Sep 3 150 Chittagong-Cox's Bazar coast

1998 Nov 2.4 100 Khulna, Barishal & Patuakhali

2007 15-Nov 8 225 Khulna, Barishal & Patuakhali

(Source: BMD)



12

Figure 2.2: Tracks of cyclones (Source: CEGIS)



13

A decade wise and month wise distribution of cyclones are presented in Figure 2.3

and Figure 2.4 respectively. These figures illustrate that 1960s decade can be

recognized for receiving maximum number of cyclones in the Bay of Bengal and that

pre-monsoon and post-monsoon cyclones are in appearance.

Figure 2.3: Number of cyclonic storms landed on Bangladesh coast in different

decades (Source: BMD)

Figure 2.4: Monthly variation of cyclone during 1960-2007(Source: BMD)

Monthly Variation of Cyclone(1990-2000)Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Monthly Variation of Cyclone (1960-2007)



14

Table 2.5 represents the monthly variation in number of cyclones to hit the

Bangladesh coast from the year 1960 to 2007.

Table 2.5: Monthly variation of cyclone from the year 1960 to 2007

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Cyclone(s) 0 0 0 2 9 1 0 0 1 7 8 3

(Source: BMD)

A yearly variation of wind speed and surge height of cyclones to hit the coast of

Bangladesh during the period 1960-2007 are presented in Figure 2.5 and Figure 2.6

respectively.

Figure 2.5: Yearly variation of wind speed (Source: BMD)

Figure 2.6: Yearly variation of surge height (Source: BMD)

Yearly Variation of Surge Height

0

1

2345

678

910

1 9 6 0

1 9 6 1

1 9 6 2

1 9 6 3

1 9 6 5

1 9 6 5

1 9 6 7

1 9 6 9

1 9 7 0

1 9 7 3

1 9 7 4

1 9 8 5

1 9 9 0

1 9 9 2

1 9 9 7

1 9 9 8

Year

S u r g e H e i g h t ( m )

Yearly Variation of Wind Speed

0

50

100

150

200

250

1950 1960 1970 1980 1990 2000 2010

Year

Wind

Speed

(km/h)



15

2.3.1 Cyclone AILA

Cyclone AILA (RSMC Designation: BOB02, JTWC Designation: 02B) is one of the

historical tropical cyclones to hit the coast of Bangladesh. It was the second tropical

cyclone to form within the Northern Indian Ocean during 2009. The disturbance that

was to become Cyclone AILA formed on 21 May 2009 about 950 kilometers (590

miles) to the south of Kolkata, India. Over the next couple of days the disturbance

slowly developed before a Tropical Cyclone Formation Alert was issued by JTWC

early on 23 May 2009 and being designated as a depression by RSMC New Delhi as

Depression BOB 02. Depression BOB02 continued to intensify slowly until early the

next day when it was upgraded to a Deep Depression by RSMC New Delhi, and

designated as Tropical Cyclone 02B by the JTWC. Later that day, RSMC New Delhi

reported that the deep depression had intensified into a Cyclonic storm and had been

named as AILA whilst located about 350 kilometers (220 miles) to the southeast of

Sagar Island. AILA became a severe cyclonic storm at 12 pm local time on May 25

and made landfall at its peak intensity between 2 pm and 3 pm on that day. Highest

wind speed of Cyclone AILA was 120 km/hr (1-minute sustained) and lowest

pressure was 96.8 kPa. (www.wikipedia.org). Figure 2.7 and Figure 2.8 represent the

satellite image and the track of Cyclone Aila respectively.

Figure 2.7: Satellite Image of Cyclone AILA (Source: Wikipedia)



Figure 2.8

More than 400,000 peop

severe flooding in coas

completely submerged i

infrastructure by Cyclon

Table 2.6: Damages to In

Road (km)

Full Parti

2233 6621

(Source: DMB, 2009)

16

: Track of Cyclone AILA (Source: Wikipedia)

le were reportedly isolated and at least 190 pe

al regions of Bangladesh. Numerous village

n floodwaters or destroyed. A summary of

AILA has been presented in Table 2.6.

frastructure by Cyclone AILA

Embankment (km) Bridg

l

1742.53

ple killed by

were either

damages to

/Culvert

157



17

2.3.2 Past Studies on Tropical Cyclone and Storm Surges

The "Multipurpose Cyclone Shelter Programme" (MCSP) made a very thorough

analysis of various aspects of the generation of cyclone surges and its penetration in-

land, and introduced a number of important statistical considerations to take into

account when studying surge heights for engineering purposes. Ultimately it was

decided to base the design surge heights on an empirical formula by Reid that relates

surge height to the cyclone wind speed and the length of the continental shelf. The

yearly maximum wind speed (anywhere in the Bay) was analyzed statistically using

Reid's formula resulting in a relationship between return period and wind speed as

showed in Table 2.7.

Table 2.7: Cyclone Wind Speeds

Return Period

(years)5 10 20 25 50 100

Wind Speed (km/h) 165 195 223 233 261 289

(Source: BMD)

The High Risk Area (HRA) was defined as any area having experienced flooding of

one meter or more. The design flood depth within the area was subsequently

determined by linear interpolation between the one meter on the landward boundary

and the height of the surge plus spring tide at the coast are shown in Table 2.3.

Table 2.8: Design Surge Heights

Return Period 20 50 100

Wind Speed (m/s) 223 261 289

Region Surge Height (m) Length of shelf (km)

Teknaf-Cox's Bazar 2.7 3.7 4.5 140Cox's Bazar-Chittagong 4.3 5.8 7 230

Chittagong-Bhola 4.8 6.5 7.8 260

Bhola-Barguna 3.8 5.1 6.2 200

Barguna-Khulna 3.1 4.3 5.2 160

(Source: Banglapedia)



18

Shahadat (2000) used a numerical 2-dimensional hydrodynamic model to simulate

1876 cyclonic surge. “Bakerganj cyclone”, which crossed the outskirts of the then

Bakerganj area in Bangladesh on November 1, 1876, has been classified as greatest

natural catastrophe ever experienced in Bangladesh. The maximum surge residual,

obtained from the model, is close to the recorded information of BMD. The

sensitivity analyses for four different conditions prove that during cyclone time,

wind speed determine the surge height while cyclonic pressure is far less important.

Ali (1997) in his study studied the interaction among river discharge, storm surges

and tides in the Meghna river estuary in Bangladesh using a two-dimensional

vertically integrated numerical model of the northern Bay of Bengal. River discharge

and tidal flow across the river mouth act both positively and negatively depending on

the tidal phase, positively during high tide and negatively during low tide. This is also

true for the combination of all three forces .On the other hand, in the most of the

crisis, river discharge acts in opposition to the storm surge. Under certain conditions

and on rare occasions thy acts positively. The interactions between river discharge

and storm surge, depend on their relative magnitudes. In respect of total elevation in

the estuarial region, river discharge tends to increase the surge height. However, away

from estuary, the effect of river discharge is hardly discernible.

As-Salek (2001) in his study found that in general the tide-surge interaction in the

Meghna estuary shows a progressive wave nature of the local tide. If the peak of the

maximum surge coincides with the tidal peak near the landfall, the surge propagates

toward the north faster than when the surge peak coincides with the tidal trough.

Cyclone that make landfall before the arrival of the tidal peak produce higher but

shorter duration surges than those that make landfall after the arrival of the tidal peak.

If the rainfall time of the cyclone is kept fixed, the surge peaks are found to arrive

earlier with the increase in the propagation speed of the cyclone and with the decrease

of radius of the maximum cyclone wind. The arrival time of maximum peak surge in

the northernmost estuary may be delayed by about 3-4 hr the arrival time in the

southern landfall point.



19

Hasan and Navera (2009) estimated the potential impacts of climate change induced

cyclonic storm surge on selected islands of Bangladesh namely Sandwip and Hatiya.

The study estimates that SIDR is the most devastating cyclone considering wind

speed. It also shows that the wind speed is increasing gradually from historical wind

speed data. Two different local models were made for two islands and SIDR track

considering high tide was used to assess the impact on those islands. Again three

different tracks were taken for these two islands one is to the north, one is to the

middle and one is to the south. It was found that if the cyclone crosses to the north of

island it creates most devastation. Model result shows that if SIDR type cyclone

comes with SLR 0.59 m and 1.0 m during high tide maximum surge height will be 6.5

m and 6.82 m for Sandwip Island and 9.2 m and 9.5 m for Hatiya Island respectively.

According to the 4th

IPCC report, if temperature is increased by 4oC wind speed will

be increased by 10%. If the same cyclone comes with 10% increased wind speed

during high tide with 59 cm SLR then surge height may increase by 0.9 m for both the

islands. Adaptation measures such as mangrove afforestation, cyclone shelter and

embankment height were also analyzed in this study. From the model result it has

been found that mangrove afforestation of 400 m and 600 m width will reduce surge

height by 15 cm and 20 cm where as it reduces current speed to one-third.

2.4 Coastal Embankment Related Studies

In 1960, the then East Pakistan Water and power Development Board (EPWAPDA)

launched a project named `Coastal Embankment Project' to increase crop production

through protection of agricultural lands from saline water inundation during normal

high tides. This project covered 13765 sq. km area of which 71% were agricultural

lands. The project extended from the Haribhanga River near India border to the border

Makam south tip of Bangladesh stretching along the 710 km shoreline.

Three types of embankments exist in the coastal area of Bangladesh. Larger

embankments are situated along the sides of Bay of Bengal and major rivers where

high waves and current are expected. Interior embankments are along the bank of

rivers where moderate waves and current persist. Marginal embankments are provided

along the bank of small rivers and streams of minor waves and current.



20

The embankments have dimensional variations in height, width, slopes, according to

their purposes. For the selection of embankment height, annual maximum water levels

between 1960 and 1968 were analyzed, and a 20 year return period maximum water

level during monsoon was used.

Table 2.9: Embankments under CEP

Embankment

Type

Sea side

slope

Country

Side slope

Crest

Width

(m)

Crest

Level (m)

Set back

distance

(m)

Sea dike 1: 7 1:2 4.27 x + 1.52 76

Interior dike 1 : 3 1:2 4.27 x + 0.91 53

Mar inal dike 1:2 1:2 2.44 x + 1.52 38

x is the design flood level(Source: BWDB)

Materials from borrow-pits along the sides of the embankments were used for

embankment construction. Sands and organic materials were avoided. A blanket of

cohesive earth over the embankment was provided to encourage good Turfing and to

prevent surface erosion where the embankment materials are not of good condition.

The inner slopes and crest levels of all types of embankments were protected from

erosion, due to rainfall, with the use of grass mat. Depending on the purpose, there is

a variation in the sea or river facing slope protection work from wave and current

erosion.

CPP-II (1992) proposed for repair and rehabilitation of the sea facing embankment of

23 selected polders and also proposed for redesign of those embankments considering

possible overtopping by a 20 year return period cyclonic storm. Second Coastal

Embankment Rehabilitation Project (BWDB, 2001) prepared a development for the

entire Coastal Embankment System and a prioritized 5-year investment programme

and also prepared detailed design for a number of polders. The performance of the

existing coastal polders against storm surge was assessed through hydraulic modeling

study. Foreshore bathymetry has also been generated for Sitakundu area based on the

two dimensional BoBM results under this project. Under this project, approximately

5000 km length of coastal embankments was surveyed.



21

CHAPTER 3

STUDY AREA

3.1 Study Area

In this study, three polders situated in Satkhira district at the western coast of

Bangladesh have been selected as study area. These are Polder no. 15, 7/1 and 7/2. All

of these polders have been damaged more or less during the cyclone AILA on 25

May, 2009. These polders are home to about 250,000 people. Shrimp culture is

currently the third most important source of foreign exchanges for Bangladesh. The

Embankments constructed around these polders individually are mostly earthen

embankments made of silty clay or loamy clay which is the general soil type of the

western coastal region of Bangladesh. Figure 3.1 represents the location of the study

area on polder map.

Figure 3.1: Location of the Study Area on Polder Map (Source: IWM)



3.2 Polder 15

• Location: Union-Gab

Thana-Shy

District-Sat

• Population: 40000 (a

• Gross protected area:

• Embankment Type: I

• Embankment length:

• Regulator(s): 5

• Flushing Inlet: 0

• Drainage Channel: 1

Typical cross-section of t

in Figure 3.2.

1.45 mPWD

Figure 3.2: Ty

22

ura,

mnagar,

khira.

prox.)

3441 ha

nterior Dyke

27.34 km

.5 km

• Embankment Cross-section

o Design crest level: 4.

o Crest width: 4.27 m (

o Riverside Slope: 1:3

o Countryside Slope: 1

o Mean Riverside Gro

1.45 m PWD

o Mean Countryside

1.0 m PWD

(So

he embankment of Polder 15 has been illustrate

1.

ical cross-section of the embankment of Polder

eatures:

27 mPWD.

14 ft )

:2

nd Level:

round Level:

urce: BWDB)

d with details

mPWD

15



The index map of Polde

seen that Polder 15 is su

Arpangasia.

Figure 3.3: In

23

r 15 has been presented in Figure 3.3. From t

rrounded by several rivers such as Kholpetua,

dex Map of Polder 15 (Source: BWDB, Satkhir

e figure it is

Kobadak and

)



3.3 Polder 7/1

• Location: Union-Pad

Thana-Shy

District-Sat

• Population: 30000 (a

• Gross protected area:


• Embankment length:

• Regulator(s): 9



Typical cross-section of

details in Figure 3.4.

1.65 mPWD

Figure 3.4: Ty

24

apukur,

mnagar,

khira.

prox.)

3887 ha

nterior Dyke

34.21 km

.5 km

• Embankment Cross-sec

Features:

o Design crest level:

o Crest width: 4.27 m

o Riverside Slope: 1:3

o Countryside Slope:

o Mean Riverside Gro

1.65 mPWD

o Mean Countryside

1.1 mPWD

(So

the embankment of Polder 7/1 has been ill

1.

ical cross-section of the embankment of Polder

ion

.27 mPWD

(14 ft )

1:2

und Level:

round Level:

urce: BWDB)

ustrated with

1 mPWD

7/1




seen that Polder 7/1 is su

Figure 3.5: In

25

7/1 has been presented in Figure 3.5. From t

rounded by Kholpetua and Kobadak rivers.

dex Map of Polder 7/1 (Source: BWDB, Satkhir

e Figure it is

a)



3.4 Polder 7/2

• Location: Union-Prot

Thana-Ass

District-Sat

• Population: 125000 (

• Gross Protected Area


• Embankment Length:

• Regulator(s): 16



Typical cross-section of

details in Figure 3.6.

1.8 mPWD

Figure 3.6: Ty

26

apnogor,

sunia,

khira.

pprox.)

: 10486 ha

nterior Dyke

59.59 km

.1 km

• Embankment Cross-Sect

Features:

o Design Crest Level:

o Crest Width: 4.27 m

o River-side Slope: 1:3

o Country-side Slope:

o Mean River-side Gro

1.8 mPWD

o Mean Country-side

1.2 mPWD

(So

the embankment of Polder 7/2 has been ill

1.

ical cross-section of the embankment of polder

ion

.27 mPWD.

(14 ft )

1:2

und Level:

round Level:

urce: BWDB)

ustrated with

mPWD

7/2




seen that Polder 7/2 is su

Figure 3.7: In

27

7/2 has been presented in Figure 3.7. From t

rounded by Morirchap, Kholpetua and Kobada

dex Map of Polder 7/2 (Source: BWDB, Satkhir

e figure it is

rivers.

a)



28

A satellite image of the study area collected from Google Earth has been presented in

Figure 3.8. The area within the red box shows the location of the study area in the

image.

Figure 3.8: Study Area on Google Earth

7/2

7/1

15



29

CHAPTER 4

DESCRIPTION OF MODELS

4.1 General

The mathematical or numerical models used in this study have been discussed in this

chapter. A mathematical 1-dimensional model named HEC-RAS has been primarily

used for simulation in this study. Values to be used as boundary conditions in HEC-

RAS for simulation of the scenario considered in this study has been obtained from a

numerical 2-dimensional model named MIKE-21. The simulation of MIKE-21 model

has been performed by IWM (Institute of Water Modelling). Description of HEC-

RAS has been adapted from HEC-RAS 4.0 Manual (2009), Sarker (2009) and Khan

(2009) and descriptions of MIKE-21 and BoBM have been adapted from Hasan

(2008) (Ref. no. 33).

4.2 HEC-RAS

HEC-RAS is a computer program that models the hydraulics of water flow through

natural rivers and other channels developed by Hydrologic Engineering Centre, US

army corps of engineers. The model is originally developed for 1D open channel

hydrodynamic analysis. It can simulate both steady and unsteady flows and includeselaborate treatment for complex channel cross-sections and structures like bridges,

culverts and dams. HEC-RAS is an integrated system of software, designed for

interactive use in a multi-tasking environment. The system is comprised of a graphical

user interface (GUI), separate analysis components, data storage and management

capabilities, graphics and reporting facilities. The HEC-RAS system contains four

one-dimensional river analysis components for:

1. Steady flow water surface profile computations

2. Unsteady flow simulation

3. Movable boundary sediment transport computations; and

4. Water quality analysis



4.2.1 Governing Equati

HEC-RAS uses the dyna

of the continuity and mo

equations for sediment

Preissmann four-point

sectional geometry and n

Saint-Venant Equation:

The continuity equation

The dynamic or moment

Where,

A = the cross-sectional ar

h = depth of flow at the s

z = elevation of surface a

v = mean velocity at the

Q = discharge at the secti

b = width of the top of th

x = position of the sectio

t = time

g = acceleration due to gr

ρ = mass density of the fl

30

ns in HEC-RAS

ic wave theory of the Saint-Venant equations

entum equation to describe flow movement an

ransport modeling. The flow is also solved

cheme, with elaborate considerations of co

merous types of hydraulic structures.

ൌ 0 ……………………….

m equation

ൌ ሺ െ ሻ ……………………….

ea of the section

ction

bove the datum at the section

ection

on

section

measured from the upstream end

avity

uid

which consist

d equilibrium

by using the

mplex cross-

.Equation 4.1

.Equation 4.2



Using the Chezy expressi

ൌଶ

ଶ

Where C is the Chezy C

ൌ

Mass Conservation for

b∂η

∂t

In which x is the local

elevation to a fixed hori

integrated flow, b(x,t) is

per unit length. The syste

Fig

31

on, j, can be written as

……………………….

nd m is the hydraulic mean radius given by

ൌ ………………………..

ater:

……………………….

osition, t is the local time, η(x,t) is the local

zontal datum, Q(x,t) is the local discharge or

the local surface width and q(x,t) is the local

m has been illustrated in Figure 4.1.

re 4.1: Schematic channel cross-section

.Equation 4.3

Equation 4.4

.Equation 4.5

water surface

ross-section-

lateral inflow



Momentum Conservatio

ଶ

ൌ െ

Where, g is the gravitatio

P(x,t) is the local wetted

friction model, either

In which f, C and n ar

respectively. R = A/P is t

4.2.2 Components of H

HECRAS consists of 13

a. Geometric data edito

b.

Steady flow datac. Steady flow analysis

d. Unsteady flow data

e. Unsteady flow analys

f. Quasi Unsteady flow

g. Sediment Data

h. Sediment analysis

i. Hydraulic design fun

j. Water Quality Data

k. Water Quality analys

l. Printing and Viewing

m. Help

32

for Water:

െ

……………………….

nal accelelaration, A(x,t) is the local flow cross

erimeter. The boundary shear (x,t) is estimat

……………………….

the Darcy-Wiesbach, Chezy and Manning fr

he local hydraulic radius.

CRAS

ollowing components:

is

data

tions

is

results

.Equation 4.6

-section and

ed from a

.Equation 4.7

iction factors



Different components of

Figure 4.2: Diff

33

HEC-RAS have been shown in Figure 4.2.

rent Components of HEC-RAS (HEC-RAS Ma

ual)



34

4.2.3 Overview of Program Capabilities

HEC-RAS is designed to perform one-dimensional hydraulic calculations for a full

network of natural and constructed channels. The following is a description of the

major capabilities of HEC-RAS.

User Interface

The user interacts with HEC-RAS through a graphical user interface (GUI). The main

focus in the design of the interface was to make it easy to use the software, while still

maintaining a high level of efficiency for the user. The interface provides for the

following functions:

o File management

o Data entry and editing

o River analysis

o Tabulation and graphical displays of input and output data

o Reporting facilities

o On-line help

River Analysis Components

Steady Flow Water Surface Profiles

This component of the modeling system is intended for calculating water surface

profiles for steady gradually varied flow. The system can handle a full network of

channels, a dendritic system, or a single river reach. The steady flow component is

capable of modeling subcritical, supercritical and mixed flow regime water surface

profiles.

The basic computational procedure is based on the solution of the one-dimensional

energy equation. Energy losses are evaluated by friction (Manning’s equation) and

contraction/expansion (coefficient multiplied by the change in velocity head). The

momentum equation is utilized in situations where the water surface profile is rapidly

varied. These situations include mixed flow regime calculations (i.e., hydraulic

jumps), hydraulics of bridges and evaluating profiles at river confluences (stream

junctions).



35

The effects of various obstructions such as bridges, culverts, dams, weirs and other

structures in the flood plain may be considered in the computations. The steady flow

system is designed for application in flood plain management and flood insurance

studies to evaluate floodway encroachments. Also, capabilities are available for

assessing the change in water surface profiles due to channel modifications and

levees.

Special features of the steady flow component include:

o Multiple plan analyses

o Multiple profile computations

o Multiple bridge and/or culvert opening analysis

o Bridge scour analysis

o Split flow optimization; and

o Stable channel design and analysis

Unsteady Flow Simulation

This component of the HEC-RAS modeling system is capable of simulating one-

dimensional unsteady flow through a full network of open channels. The unsteady

flow equation solver was adapted from Dr. Robert L. Baraku’s UNET model. The

unsteady flow component was developed primarily for subcritical flow regime

calculations. However, with the release of version 3.1, the model can now perform

mixed flow regime (subcritical, supercritical, hydraulic jumps and draw downs)

calculations in the unsteady flow computations module.

The hydraulic calculations for cross-sections, bridges, culverts and other hydraulic

structures that were developed for the steady flow component were incorporated into

the unsteady flow module.

Special features of the unsteady flow component include:

o Dam break analysis

o Levee breaching and overtopping

o Pumping stations

o Navigation dam operations; and

o Pressurized pipe systems



36

Sediment Transport/Movable Boundary Computations

This component of the modeling system is intended for the simulation of one-

dimensional sediment transport/movable boundary calculations resulting from scour

and deposition over moderate time periods (typically years, although applications to

single flood events are possible).

The sediment transport potential is computed by grain size fraction, thereby allowing

the simulation of hydraulic sorting and armoring. Major features include the ability o

model a full network of streams, channel dredging, various levee and encroachment

alternatives, and the use of several different equations for the computation of

sediment transport.

The model is designed to simulate long-term trends of scour and deposition in a

stream channel that might result from modifying the frequency and duration of the

water discharge and stage, or modifying the channel geometry. This system can be

used to:

o Evaluate deposition in reservoirs

o Design channel contractions required to maintain navigation depths

o Predict the influence of dredging on the rate of deposition

o Estimate maximum possible scour during large flood events; and

o Evaluate sedimentation in fixed channels

Water Quality Analysis

This component of the modeling system is intended to allow the user to perform

riverine water quality analyses. The current version of HEC-RAS can perform

detailed temperature analysis and transport of a limited number of water quality

constituents (Algae, Dissolved Oxygen, Carbonaceuos Biological Oxygen Demand,

Dissolved Organic Phosphorus, Dissolved Ammonium Nitrate, Dissolved Nitrite

Nitrogen and Dissolved Organic Nitrogen).



37

Data Storage and Management

Data storage is accomplished through the use of “flat” files (ASCII and binary). User

input data are stored in fat files under separate categories of project, plan, geometry,

steady flow, unsteady flow, quasi steady flow, sediment flow and water quality

information. Output data is predominantly stored in separate binary files. Data can be

transferred between HEC-RAS and other programs by utilizing the HEC-DSS.

Data management is accomplished through the user interface. The modeler is

requested to enter a single filename for the project being developed. Once the project

filename is entered, all other files are automatically created and named by the

interface needed. The interface provides for renaming, moving and deletion of files on

a project-by-project basis.

Graphics and Reporting

Graphics include X-Y plots of the river system schematic, cross-sections, profiles,

rating curves, hydrographs and many other hydraulic variables. A three-dimensional

plot of multiple cross-sections is also provided. Tabular output is available. Users can

select from pre-defined tables or develop their own customized tables. All graphical

and tabular output can be displayed on the screen, sent directly o a printer (or plotter),

or passed through the Windows Clipboard to other software, such as a word-processor

or spreadsheet.

How to perform an analysis

o Starting a New Project

o Entering Geometric Data

o Entering Steady Flow Data

o Performing the Hydraulic Calculations

o Viewing Results

o Printing Graphics and Tables

o Exiting the Program



38

Performing a Steady Flow Analysis

o Entering and Editing Steady Flow Data

o Performing Steady Flow Calculations

Performing an Unsteady Flow Analysis

o Entering and Editing Unsteady Flow Data

o Performing Unsteady Flow Calculations

o Calibration of Unsteady Flow Models

o Model Accuracy, Stability and Sensitivity

Viewing Results

The current version of the program allows the user to view cross sections, water

surface profiles, general profiles, rating curves, hydrographs, X-Y-Z perspective plots,

detailed tabular output at a single location and summary tabular output at many cross

sections. Users also have the ability to develop their own output tables.

Contents

o Cross Sections, Profiles and Rating Curves

o X-Y-Z Perspective Plots

o Tabular Output

o Viewing Results From the River System Schematic

o Stage and Flow Hydrographs

o Viewing Computational Level Output for Unsteady Flow

o Viewing Ice Information

o Viewing Data Contained in an HEC-DSS File

o Exporting Results to HEC-DSS

4.3 MIKE-21

MIKE-21 is a numerical 2-dimensional model developed by DHI Water and

Environment. MIKE-21 has a number of modules for different purpose and each

module has different sets of equations. To provide the boundary conditions to be used

in simulation for the scenario considered in this study, hydrodynamic and cyclone

module of MIKE-21 have been used by IWM. The governing equations for these

modules have been described in the next section.



39

4.3.1 Governing Equations in Hydrodynamic Module

The governing equations used in MIKE-21 in solving hydraulic problems in coastal

areas are:

Conservation of Mass Equation:

0=∂

∂+

∂

∂+

∂

∂

y

q

x

p

t

ε

……………..Equation 4.8

Conservation of Momentum Equation:

The momentum equation in the x-direction is given by:

01

2 2

22

2

=∂

∂+−Ω−

∂

∂−

++

∂

∂+

∂

∂

+∂

∂

+∂

∂

x

phWW C q

y

h p

h

q p f

xgh

y

h pq

x

h p

t

p a

xw

a xy

ρ ρ

ρ τ

ρ

ε

….……………Equation 4.9

The momentum equation in the y-direction is given by:

01

2 2

22

2

=∂

∂+−Ω+

∂

∂−

++

∂

∂+

∂

∂

+∂

∂

+∂

∂

y

phWW C p

x

hq

h

q p f

ygh

x

h pq

y

hq

t

q a

yw

a yx

ρ ρ

ρ τ

ρ

ε

...……………Equation 4.10

Where, p and q flux in x and y directions respectively (m

3 /s/m).

t time (s), x and y (m) are Cartesian Co –ordinate (s).

h water depth (m).

g acceleration due to gravity (9.81 m2 /s) .

ε sea surface elevation (m).

ρ w & ρ a air and water density respectively (kg /m3),

C w wind friction factor = 0.0008 + 0.000065W in accordance with

Wu (1982).

W wind speed (m/s).

Ω Coriolis parameter

Pa atmospheric pressure (kg/m/s2).



40

4.3.2 Governing Equations in Cyclone Module

The Cyclone module of MIKE-21 has been used to generate the pressure and wind

distributions all over the Bay of Bengal. Cyclone Model is normally described by

relatively few parameters relate to pressure field, which is imposed to the water

surface and a wind field which is acting as a drag force on the water body through a

wind shear stress description. The wind fields consist of a rotational and a

translational component.

At a distance R from the centre of the cyclone the rotational wind speed V r is given as

))1(7exp()( 7

mm

mr R

R

R

RV V −= for R< Rm ............................... Equation 4.11

))1(exp( mmr R

R

C V V −= for R> Rm ............................... Equation 4.12

Where R and Rm are in km and C is given as

05.00025.0 +=m

RC ............................... Equation 4.13

C determines the shape of wind distribution for R> Rm.

And translational component V t is given as

)cos(5.0 θ −−= f t V V ............................... Equation 4.14

Where, θ is the angle between the radial arm and the line of maximum winds. The

total wind speed is t r V V V +=

And finally, the pressure at particular location is given as

)exp()( R

RPPPP m

cnc−−+= ............................... Equation 4.15

Where, P is pressure at radius R, Pc is central pressure, Pn is neutral pressure.



41

Two different hydrodynamic models have been used by IWM for the study. Brief

descriptions of the models have been given in the next section.

4.3.3 The Bay of Bengal Model

A Bay of Bengal Model (BoBM) has been used to simulate the cyclonic events foreach scenario considered in the study. Water levels obtained for each scenario at the

mouth of selected rivers downstream to the study area are then used as downstream

boundary conditions in the other MIKE-21 based hydrodynamic model, South-West

Region Model. BoBM is a 2D one layer MIKE-21 based hydrodynamic model. The

model covers the northern part of the Bay of Bengal from latitude 17000

/ 00

// to the

coast of Bangladesh. The total model has been updated up to 2008.

Size of the model (Model Grid)

The base model is a three way nested model and the resolutions are 5400 m, 1800 m

and 600 m. The Meghna Estuary is resolved on a 600 m grid. Extent of models has

been shown in the Figure 4.3 and information on the grid is given in Table 4.1 and

Table 4.2.

Figure 4.3: Model domain of three nested model (Source: IWM )

India

Vishakhapatnam Gwa Bay

ChandpurIndia

India

Bay of Bengal

Coarse Model

Intermediate Model

Fine Model



42

Table 4.1: Model grid specification

Model Origin

(degree)

Grid Spacing

(m)Grid Numbers

Length and

Width

Coarse grid Lon = 84.64

Lat = 18.91

Dx = 5400

Dy = 5400

180×93 972km ×

502km

Intermediate

grid

Lon = 86.75

Lat = 20.81

Dx = 1800

Dy = 1800

321×156 578km ×

281km

Fine grid Lon = 89.9712

Lat = 21.3393

Dx = 600

Dy = 600

396×357 238km ×

214km

Table 4.2: Relative grid positions

Model In Fine Grid In Intermediate Grid In Coarse Grid

Coarse grid - - (0,0)

Intermediate grid - (0,0) (42,38)

Fine grid (0,0) (174,31) (100,48)

Bathymetry Generation

Bathymetry is the bottom topography of a river or a sea. As the model area is a huge

one, it is not possible to survey all the area at a time. To make the bathymetry, bed

level data has been collected from different sources of different time. Table 4.3 shows

the source of bed level data utilized for generation of bathymetry of the model.

Table 4.3: Land-water data used for generating the bathymetry

Model

component

Type of Data

Bathymetry Source Land Source Land-water

boundary

Coarse grid Admiralty

maps

CSPS - - Admiralty maps

Intermediate

grid

Admiralty

maps

CSPS - - Admiralty maps



43

Model

component

Type of Data

Bathymetry Bathymetry Bathymetry

Fine grid Admiralty

maps

FAP 5B

Survey (1997)

MES II survey

(1998, 99)

BIWTA maps

MES I

MES I

MES II

MES II

DEM (1952-

64)

FINMAP

(1990)

KJDRP

(1997)

CSPS

CSPS

IWM

LANDSAT

1996

LANDSAT

1998

In addition polder levels and alignment surveyed under 2nd

CERP have been used.

Bed Resistance

In shallow areas bed friction is important and can effectively be used to adjust the

amplitude of tides. Bed friction is defined by the Manning number, M (m1/3

/s). The

map of the Manning used during the calibration period has been shown in Table 4.4.

Table 4.4: Manning number distribution

Areas with depths Manning Number

Less than –1000 60

-1000 to –200 70

-200 to –50 80

-50 to –20 90

-20 to –3 100

-3 to –1 90

-1 to 1 80

Greater than 1 25



44

Eddy Viscosity

The eddy viscosity E has been specified in the model as a time-varying function of the

local gradients in the velocity field. This formulation is based on the so-called

Smagorinsky concept, which yields:

∂

∂+

∂

∂+

∂

∂+

∂

∂∆=

y

V

x

U

y

U

x

U C E

s2

12

22

In the model only the value of Cs has to be specified and the recommended value is

0.2 to 1.0. In this research work 0.3 has been used.

Boundary Generation

The model has a wide, deep and open ocean boundary in the south situated along the

line extending from Vishakhapatnam in India to Gwa Bay in Myanmar (Figure 4.3).

Predictions on water levels have been made for these two extreme points along this

boundary based on tidal constituents. The water level along the open boundary was

obtained by interpolating the two predicted water levels. Tidal constituents that were

used to predict water level for these two stations have been shown in the Table 4.5.

Table 4.5 Tidal constituents for Vishakhapatnam and Gwa Bay

Tidal

constituents

Vishakhapatnam Gwa Bay

Phase

(Degree)

Magnitude

(m)

Phase

(Degree)

Magnitude

(m)

M2 239 0.48 266 0.69

S2 274 0.21 304 0.32

K1 336 0.11 330 0.14

O1 320 0.04 338 0.05

f 4 - - 180 0.193

(Source: Admiralty Tide Table)

To the north the model has a narrow, shallow, open boundary in the Meghna River.

The boundary is located at Chandpur and observed water level at Chandpur has been

used as north boundary of the model.



45

Calibration and Validation of Bay of Bengal model

The existing BoBM model has been calibrated and validated against the measured

water level and discharge data. The model has been calibrated against water level at

Charchenga and discharge at North Hatiya during 2006. Calibration plots have been

shown in the Figure 4.4 and Figure 4.5.

Figure 4.4: Calibration against discharge data at North Hatiya during 2006

Figure 4.5: Calibration against water level data at Charchenga during 2006

Location of water level

Measured

Discharge Location

Measured

Simulated

Simulated



46

The model has been validated against discharge at West Shahbazpur Channel and at

East Shahbazpur Channel during December 2007. The validation plots have been

shown in the Figure 4.6 and Figure 4.7.

Figure 4.6: Validation against discharge data at West Shahbazpur Channel during

December 2007

Figure 4.7: Validation against Discharge data at East Shahbazpur Channel duringDecember 2007

Discharge Location

Measured

Discharge Location

Measured

Simulated

Simulated



4.3.2 The South-West R

The South-West Region

and downstream bounda

each scenario under cons

based hydrodynamic mo

Model at the mouth of th

as downstream boundar

Model. Normal discharg

as upstream boundary

South-West Region Mod

Figure 4.

47

egion Model

Model has been used in this study to obtain

y conditions to be used in HEC-RAS for the

ideration of the study. The model is a 2D one la

del. The water levels obtained from the simul

e selected rivers downstream to the study area

conditions in the simulation of the South-

s at the upstream locations of the model area h

onditions for simulation. Figure 4.8 present

l.

8: South-West Region Model (Source: IWM)

oth upstream

simulation of

yer MIKE-21

ation of BoB

as been used

West Region

ve been used

the map of



48

CHAPTER 5

METHODOLOGY

5.1 Data Collection

In this study geometric data such as cross-sections and hydrometric data such as

discharge and water level have been collected from IWM. A shape file of the major

rivers of Bangladesh has been collected from the Department of Water Resources

Engineering, BUET in order to generate the streamline co-ordinates required for the

schematization of river system surrounding the study area in the HEC-RAS based

numerical modeling. A brief description of all the data used in this study is presented

in the following section.

5.1.1 Geometric Data

Cross-section data of rivers surrounding the study area such as Kobadak, Kholpetua,

and Morirchap reaches as well as cross-section data of other rivers that are connected

to the rivers surrounding the study area such as Betna, Galghasia, Kalagachi and

Arpangasia have been collected from IWM. These data include bank to bank width

during the high tide and elevation in meter with respect to PWD datum. Plot of the

cross section data have been provided in Appendix-B. The survey period of collected

cross-section data has been presented in Table 5.1.

Table 5.1: Survey period of collected cross-section data

River Survey Period Survey Organization

Kobadak 2003 IWM

Kholpetua 1990-1992 SWMC

Galghasia 1990-1992 SWMC

Morirchap 1998 SWMC

Betna 1998 SWMC

Arpangasia 2000 SWMC

Kalagachi 2000 SWMC



49

5.1.2 Discharge and Water Level Data

Discharge and water level data have been collected from IWM which are generated

specially for this study. These data are generated using two 2-D hydrodynamic

models based on MIKE-21 named as the Bay of Bengal Model and the South-West

Region Model. The discharge data of Kobadak, Betna and Galghasia and the water

level data of Arpangasia River near the Kobadak-Kholpetua-Arpangasia junction,

Kalagachi River near the Kholpetua-Kalagachi junction and Morirchap River near

Morirchap-Kholpetua junction have been used as boundary conditions. Both the

discharge data and water level data began from May 24, 2009 at 9.00 am to May 26,

2009 at 9.00 am and include reading at an interval of 30 minutes. Plotting of

discharge data (flow hydrograph) and water level data (stage hydrograph) have been

provided in Appendix-C.

5.1.3 Generation of Streamline Co-ordinates

Since the co-ordinates of the river streamline were not available so it has been

generated. A shape file of the major rivers of Bangladesh has been opened in

ArcView GIS 3.3 and then zoomed up to the study area of this study. The river

reaches surrounding the study area such as Kobadak, Kholpetua, Morirchap, Betna,

Galghasia, Kalagachi and Arpangasia have been selected using select feature tool and

a new shape file has been created. Thus the shape file of river reaches surrounding the

study area has been obtained. A new point theme has been created and points have

been drawn over a river streamline in the new shape file by selecting point from

toolbar. All the points have been selected using select feature tool. The co-ordinates

(based on Bangladesh Transverse Mercator (BTM) projection) will be font in the

attributed table of the point theme file. New point themes have been created again and

the process has been repeated for each individual river reaches of the new shape file.

The co-ordinates then have been exported to an excel file. Co-ordinates of the river

reaches have been provided in Appendix-A.

The shape file of river reaches surrounding the study area has been presented in

Figure 5.1. Figure 5.2 and Figure 5.3 represent examples of point theme of study

reach and attribute table of point theme of study reach.



Figure 5.1: Sha

50

e file of the river system surrounding the study

igure 5.2: Point theme of study reach

area



Figure 5.3

5.2 Field Visit

A field visit was cond

embankments of the sel

AILA and to observe t

damages to the polders d

Satkhira BWDB staffs an

field visit. These are:

1. Observation of th

The existing condition of

breaches were observed.

2. Group discussion

A group discussion with

real situation in the selec

height above the emban

AILA situation.

3. Interview with Sa

Further information on d

the causes of failure o

BWDB, Satkhira zone.

51

: Attribute table of point theme of study reach

cted in June, 2010 to assess the performan

ected polders (Polder no. 15, 7/1 and 7/2) d

e existing condition of the embankments. In

ue to cyclone AILA was gathered from the loc

d field engineers. Three main steps were follo

existing condition of the embankments of sele

the embankments such as crest level, crest wid

with the local people.

the local people was held in order to be acquai

ed polders during cyclone AILA such as the es

ment crest level and area of inundation as wel

tkhira BWDB engineers.

amages to the polders due to cyclone AILA an

the embankments were collected from the

e of coastal

ring cyclone

formation on

al people and

ed during the

cted polders.

th, slopes and

nted with the

timated surge

l as the post-

opinions on

engineers of



52

5.3 HEC-RAS 1D Hydrodynamic Modeling

After collecting and generating all the necessary data a 1D Hydrodynamic Model has

been developed using HEC-RAS. The steps of HEC-RAS 1D Hydrodynamic

Modeling is shown as a flow chart in Figure 5.4.

Figure 5.4: Steps of HEC-RAS 1D Hydrodynamic Modeling

Model Output and Result Analysis

Fix the Simulation Period and

Run the Hydrodynamic Model

(Unsteady Flow Simulation)

Entering and Interpolating Cross-Section Data

Schematization of River System

(Kobadak-Morirchap-Khalpetua River System)

and Editing the Junction Data

Entering Boundary Conditions and Initial Flow data

(Flow Hydrographs at upstream boundaries and

Stage Hydrographs at downstream boundaries)



5.3.1 Schematization of

Schematization of river

Khalpetua River Syste

through Reach Invert L

junctions have been co

Morirchap Junction, M

and Kobadak-Khalpetua

data or negligible poten

connected to the river s

represent examples of e

respectively. The Sche

presented in Figure 5.7.

Fig

53

River System

system surrounding the study area (Kobada

) has been done by entering the streamline

ine Tables under GIS Tools in Geometric D

nsidered. These are Kobadak-Morirchap Jun

rirchap-Khalpetua Junction, Galghasia-Khalp

Arpangasia Junction. Due to unavailability of

ial to affect the model result, other river rea

ystem has not been considered. Figure 5.5 a

tering streamline co-ordinates and editing of

atized river system surrounding the study a

re 5.5: Entering streamline co-ordinates

k-Morirchap-

co-ordinates

ata Editor. 5

ction, Betna-

tua Junction

cross-section

ches actually

d Figure 5.6

junction data

rea has been



54

Figure 5.6: Editing junction data

igure 5.7: Schematized river system



55

5.3.2 Entering and Interpolating Cross-Section Data

The cross-section station, elevation, downstream reach lengths (left bank, right bank

and channel chainage), manning’s n values, main channel bank stations and

contraction or expansion co-efficient have been entered within Geometric Data

Editor. Cross-sections’ numbers are ordered from downstream to upstream within a

reach. In a particular cross-section data, elevation has been given with respect to

PWD datum in meter, manning’s n values has been considered 0.21~0.51 and

contraction and expansion co-efficient have been considered as .01 and .03

respectively. The cross-section data have been interpolated at an interval of 200 m

through XS Interpolation under Tools in Geometric Data Editor. Figure 5.8 and

Figure 5.9 represent examples of entering and interpolating cross-section data. The

view on Geometric Data Editor after interpolation has been presented in Figure 5.10.

Figure 5.8: Entering cross-section data



Fig

Figure 5.10: View on Ge

56

re 5.9: Interpolating cross-section data

metric Data Editor after interpolating cross-se tion data



57

5.3.3 Calibration of the Model

The model has been calibrated against the measured water level at Kobadak during

2009. Calibration plot has been shown in figure 5.11.

Figure 5.11: Calibration against water level at Kobadak during 2009

5.3.4 Development of Scenario

The scenario for this study has been developed considering a cyclonic event with the

magnitudes of cyclone SIDR along the track of cyclone AILA, maximum predicted

sea level rise (1.5 m) by the year 2100 according to the Copenhagen Summit, March

2009 and high tide. The wind speed of cyclone SIDR has been originally estimated as

225 kmph which is equivalent to that of a cyclone with a return period of 20 years.

But according to the 4th

IPCC Report if temperature is increased by 2oC wind speed

will be increased by 5% and if temperature is increased by 4oC wind speed will be

increased by 10%. Thus considering temperature would increase by 4oC in the year

2100 the wind speed of cyclone SIDR has been considered as 248 kmph in the

simulation of MIKE-21 based BoB Model by IWM for this study. The option has

been furnished in table 5.2.

Table 5.2: Scenario for model simulation

Scenario

Track of AILA + SIDR + 1.5 m SLR + High Tide

-2

-1

0

1

2

3

4

5/23/2009 19:125/24/2009 0:005/24/2009 4:485/24/2009 9:365/24/2009 14:245/24/2009 19:125/25/2009 0:00

Measured

Simulated W a t e r L e v e l m P W D

Time



5.3.5 Entering Boundar

Stage hydrographs at 3 p

boundary conditions as i

scenario through Unstea

entering boundary condit

application of boundary

Figure 5.12: Differe

58

y Conditions for Individual Scenarios

oints and flow hydrographs at 3 points have b

put data along with initial flows at 12 points f

y Flow Data Editor. Figure 5.12 represents dif

ions and initial flows. Figure 5.13 illustrates th

onditions.

t steps of entering boundary conditions and init

en entered as

r the selected

erent steps of

e locations of

ial flows



Figure 5.13:

Flow Hydrograph

Flow Hydrograph

Stage Hydrograph

Stage Hydrograph

59

Locations of application of boundary condition

Flow

Stage

s

ydrograph

ydrograph



60

5.3.6 Performing Unsteady Flow Simulation

After entering boundary conditions and initial flows for a particular scenario,

unsteady flow simulation has been performed for 48-hr simulation period starting

from 24 May 2009 at 09:00 AM to 26 May 2009 at 09:00 AM. Instruction of the

programs to run has been provided. The programs were Geometry Preprocessor,

Unsteady Flow Simulation and Post Processor. All time intervals under Computation

Settings have been set to 1 hour. Then the Compute button has been clicked to run the

model. Figure 5.13 represents an example of performing unsteady flow simulation.

Figure 5.14: Performing Unsteady Flow Simulation



61

5.3.7 Model Output and Result Analysis

After the computation the HEC-RAS model provided output as stage hydrograph,

water surface profile and detail output table at individual cross-sections. The result

has been analyzed to obtain the maximum water surface elevation around any

individual polder for a particular scenario that has been then compared with the

embankment crest level of that particular polder to assess the adequacy of the existing

embankment height. In case of finding inadequacy of the existing embankment

height, modification of crest level of the embankment of the polder has been proposed

by adding free board as recommended by BWDB under CEP.



62

CHAPTER 6

RESULTS AND DISCUSSION

6.1 Performance of embankments of the selected polders during cyclone AILA

During the field visit the existing condition of the embankments of the selected

polders was found to very poor. In many parts of the embankments the crest width has

been decreased to about 1 m only instead of 4.27 m. The riverside slopes of the

embankments were out of proper shape due to severe erosion as well due to the

impact of cyclone AILA. The country side slopes were rather in better shape. The

height of the embankments has been proved to be inadequate to resist storm surges

like AILA as shown in Table 6.1. The area of inundation indicates to the inadequateheight of the existing embankments and the number of breaches proves the weakness

of the embankments to resist storm surge and to survive high velocity flow.

Table 6.1: Damages to the selected polders due to cyclone Aila

Polder

No.

Area

Inundated

Breach

Points

Regulator(s)

Damaged

Estimated Surge Height Above

Embankment Crest Level

15 100% 10 1 1.5 m

7/1 70% 5 0 1.0 m

7/2 20% 2 1 0.0-1.0 m

Some of the breaches in the embankments have been repaired shortly after the

cyclone Aila. In other cases ring embankments were constructed around the breaches

inward the country side of the breached embankments. The ring embankments were

typically built with the crest level at 4.27 mPWD, crest width of 2.5 m and a slope of

1:2 on both sides of the embankment.



63

According to the engineers of BWDB, Satkhira Zone, such damages to the polders

during cyclone AILA occurred due to the following reasons:

• AILA took place during high tide. This made the overtopping easier and

overtopping caused the breaching of the embankments.

• The embankments were never built to protect the polder against storm surges.

These polders can protect the normal tide only.

• The total length of the embankment of a polder should be re-sectioned in

every 4 years. But very few parts of the embankments were re-sectioned since

2000.

• The workmanship during earlier re-sectioning works might have not beendone properly.

• Numerous boring of pipes through the embankments for shrimp culture caused

undermining and erosion of the embankments.

Again according to the Satkhira BWDB engineers an overall design modification of

the embankments of the polders considering climate change and sea level rise in

future is very necessary and should be implemented as soon as possible. These

embankments have been constructed during the 1960s which served at a highly

satisfactory level for many years but recent climate change conditions and sea level

rise have made these embankments inadequate to protect the polders from storm

surges. It has been recommended by the engineers that the height of the embankments

should be raised by atleast a meter and the crest width should be doubled.

Figure 6.1, Figure 6.2, and Figure 6.3 illustrate the breach points of the embankments

and area of inundation of individual polders due to cyclone AILA. From the figures it

has been found that the eastern side of the polders experienced more damages than the

western side. Images from the study area taken during field visit have been presented

in figure 6.4.



64

Figure 6.1: Illustration of damages to Polder no. 15 due to cyclone AILA

Breach Point of Embankment

Inundated Area



Figure 6.2: Illustra

Breach Point

Inundated Are

65

ion of damages to Polder no. 7/1 due to cyclon

f Embankment

a

AILA



Figure 6.3: Illustra

66

ion of damages to Polder no. 7/2 due to cyclon

Breach Point of

Inundated Area

AILA

Embankment



67

Damaged Embankment

amaged Permanent Protection Work



68

Embankment Repairing Work

Re-sectioned Embankment



Figure 6.

69

Temporary Protection Work

Damaged Ring Embankment

: Images from the study area during field visit



70

6.2 Adequacy of the existing embankment height

The adequacy of the existing embankment height of the selected polders has been

assessed by analyzing the results of the HEC-RAS based 1-D hydrodynamic

modeling. Maximum water level found from model output around individual polders

for the scenario considered in this study has been presented in Table 6.2.

Table 6.2: Maximum water level around individual polders

Scenario Track of AILA + SIDR + 1.5 m SLR + High Tide

Polder no. Maximum Water Level

(mPWD)

Embankment Crest Level

(mPWD)

Remarks

15 8.63 4.27 Inadequate

7/1 8.69 4.27 Inadequate

7/2 8.82 4.27 Inadequate

From Table 6.2 it has been found that embankments of all the three polders will be

overtopped for the considered scenario and thus the embankment height of each of the

selected polders are inadequate against the scenario considering simultaneous impact

of predicted sea level rise and storm surge at high tide.

Figure 6.5, Figure 6.6 and Figure 6.7 present the Stage Hydrograph, maximum water

level and detailed output table at a cross-section respectively around Polder 15 due to

SIDR along the track of AILA at high tide with 1.5 m SLR.

Figure 6.8, Figure 6.9 and Figure 6.10 present the Stage Hydrograph, maximum water

level and detailed output table at a cross-section respectively around Polder 7/1 due to

SIDR along the track of AILA at high tide with 1.5 m SLR.

Figure 6.11, Figure 6.12 and Figure 6.13 present the Stage Hydrograph, maximum

water level and detailed output table at a cross-section respectively around Polder 7/2

due to SIDR along the track of AILA at high tide with 1.5 m SLR.



Figure 6.5: Stage Hydrograph at a cross-s

71

ction around Polder 15 during SIDR along the track of AILA wit



72

Figure 6.6: Maximum water level at a cross-section around Polder 15 during SIDR along the track of AILA w



73

Figure 6.7: Detailed output table at a cross-section around Polder 15 during SIDR along the track of AILA wi



Figure 6.8: Stage Hydrograph at a cross-se

74

ction around Polder 7/1 during SIDR along the track of AILA wit



75

Figure 6.9: Maximum water level at a cross-section around Polder 7/1 during SIDR along the track of AILA w



76

Figure 6.10: Detailed output table at a cross-section around Polder 7/1 during SIDR along the track of AILA w



77

Figure 6.11: Stage Hydrograph at a cross-section around Polder 7/2 during SIDR along the track of AILA wi



78

Figure 6.12: Maximum water level at a cross-section around Polder 7/2 during SIDR along the track of AILA w



79

Figure 6.13: Detailed output table at a cross-section around Polder 7/2 during SIDR along the track of AILA w



80

The criteria for fixing the crest level of an interior dyke is as followed:

Design crest level of dyke = Design water level + Free board

The design water level corresponding to 20 years return period has been calculated

from the model. According to the Table 2.9 a free board of 0.91 m should be provided

for an interior dyke. As all the embankments considered in this study are interior

dykes the design crest level of the embankments of the selected polders should be as

presented in Table 6.3.

Table 6.3: Design crest level of the embankments of selected polders

Polder No. Design Water Level

(mPWD)

Free Board

(m)

Design Crest Level

(mPWD)

15 8.63 0.91 9.54

7/1 8.69 0.91 9.60

7/2 8.82 0.91 9.73



81

CHAPTER 7

CONCLUSIONS AND RECOMMENDATIONS

7.1 Conclusions

The coastal zone of Bangladesh is very much vulnerable to cyclonic storm surge

because of geographical setting and sea level rise due to global warming will be the

additional threat to this area. According to 4th

IPCC report both intensity and

frequency of storm surge will be increased in the Bay of Bengal in the coming future.

Considering all of these, Bangladesh coastal area will be under severe threat if no

remedial measures are taken immediately.

In this study the performance of coastal embankments of polder no. 15, 7/1 and 7/2

during the cyclone Aila has been assessed through field visit. It has been found that

these embankments constructed during the 1960s have now been unable to protect the

polders from the impact of storm surge. Climate change and associated sea level rise

will make the embankments more inadequate to resist storm surges.

From mathematical modeling it has been found that embankment height of the polders

selected for this study are not sufficient to resist the simultaneous impact of SLR and

cyclonic storm surge associated with high tide. In this study SIDR has been selected

as cyclonic storm as it is the most devastating than other historical cyclonic storm and

also because the original wind speed of SIDR (225 kmph) corresponds to that of a

cyclone with a return period of 20 years (223 kmph).

According to the 4th

IPCC Report if temperature is increased by 4oC wind speed will

be increased by 10%. Thus considering temperature would increase by 4o

C in the year

2100 the wind speed of cyclone SIDR has been considered as 248 kmph in

mathematical modeling. The cyclone has been considered to travel along the track of

AILA with 1.5 m SLR at high tide.



82

In case of polder no. 15 surge height has been found to be 4.36 m above the crest level

of existing embankment where as in polder no. 7/1 it would be 4.42 m and in polder

no. 7/2 it would be 4.55 m. So it is essential to raise the embankment height with

proper redesign to protect the people and their livelihood inside the polders.

Finally the crest level for the selected polders has been calculated. In case of polder

no. 15 crest level is 9.64 mPWD. Again in case of polder no. 7/1 crest level is 9.8

mPWD and in case of polder no. 7/2 crest level is 9.93 mPWD. In all cases the

suggested crest level is much higher than the existing one and all the suggested height

are for the year 2100 considering 1.5 m sea level rise and normal high tide.

It is extremely important to reconsider the existing coastal infrastructure based on this

and redesign existing coastal embankments to rehabilitate these structures and make it

climate resilient. The risk of sea level rise and cyclonic storm surge are to be

considered for future planning and design of coastal structure. Proper adaptation

measures both structural and non-structural are to be planned in order to find a climate

resilient coastal environment for coastal people and their livelihood security.

7.2 Recommendations

Similar studies can be carried out in the other polders of the coastal zone and the

results can be compared with the results of this research work.

Only the extreme climate change scenario has been considered in this study. Inclusion

of more scenarios will enhance the quality of the study.

Change in bathymetry by the year 2100 has not been considered in this study.

Analysis based on change in bathymetry by the selected year will provide more

accurate results.

Bathymetry data of 1990-2003 has been used in this study. Inclusion of more recent

bathymetry will improve the reliability of the model results of the present study.



83

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88

Appendix-A

Streamline Co-ordinates



89

Co-ordinates of Kholpetua River

X_COORD Y_COORD X_COORD Y_COORD X_COORD Y_COORD

416640.04964 494424.77869 416852.51691 488208.74907 418633.97325 482096.22914

416667.28903 494310.37324 416841.62115 487898.21998 418557.70295 482014.51096

416754.45509 494103.35385 416836.17327 487571.34726 418437.84962 481911.00127416819.82964 493912.67809 416830.72539 487304.40120 418323.44416 481840.17884

416885.20418 493672.97143 416836.17327 487162.75636 418214.48659 481747.56491

416917.89145 493422.36901 416841.62115 487037.45514 418089.18538 481644.05521

416917.89145 493422.36901 416868.86054 486901.25818 418012.91508 481529.64976

416912.44357 493220.79749 416906.99570 486748.71757 417974.77993 481431.58794

416928.78721 493024.67386 416972.37024 486585.28121 417942.09265 481175.53764

416928.78721 492883.02901 417032.29691 486449.08424 417936.64477 480990.30976

416917.89145 492648.77023 417086.77569 486334.67879 417931.19690 480772.39462

416890.65206 492523.46902 417108.56721 486242.06485 417909.40538 480581.71886

416885.20418 492381.82417 417130.35872 486127.65940 417871.27023 480401.93886

416874.30842 492256.52296 417119.46297 486007.80607 417822.23932 480189.47159

416868.86054 492136.66963 417103.11933 485882.50486 417800.44781 480004.24372

416830.72539 492027.71205 417086.77569 485773.54728 417784.10417 479846.25523

416721.76782 491902.41084 417059.53630 485610.11092 417794.99993 479710.05826

416580.12297 491766.21387 417064.98418 485517.49698 417833.13508 479595.65281

416443.92600 491608.22539 417184.83751 485332.26910 417903.95750 479486.69524

416340.41631 491455.68479 417304.69084 485201.52002 418023.81083 479350.49827

416269.59389 491335.83146 417348.27387 485108.90608 418209.03871 479203.40554

416204.21934 491243.21752 417380.96114 484929.12608 418475.98477 479050.86494

416149.74055 491145.15570 417402.75266 484787.48123 418770.17022 478920.11585

416117.05328 491025.30237 417429.99205 484651.28426 419015.32476 478805.71040416127.94904 490900.00116 417473.57508 484449.71275 419222.34415 478642.27404

416187.87570 490791.04358 417506.26236 484362.54669 419282.27082 478538.76434

416302.28116 490698.42965 417615.21993 484231.79760 419298.61445 478386.22374

416531.09206 490518.64965 417713.28175 484139.18366 419255.03142 478260.92252

416759.90297 490371.55692 417794.99993 484046.56973 419173.31324 478130.17344

416950.57873 490240.80783 417909.40538 483932.16427 419042.56415 478026.66374

417010.50539 490137.29814 418105.52901 483746.93640 418857.33628 477868.67526

417015.95327 490050.13208 418279.86113 483567.15640 418655.76477 477710.68677

416999.60963 489919.38299 418443.29750 483398.27216 418421.50598 477552.69829

416950.57873 489679.67632 418590.39022 483256.62731 418241.72598 477449.18860

416857.96479 489412.73027 418786.51385 483060.50368 417958.43629 477340.23102

416787.14236 489194.81512 418819.20113 482978.78550 417745.96902 477280.30435

416765.35085 489025.93088 418830.09688 482837.14065 417549.84539 477225.82557

416776.24661 488873.39027 418835.54476 482717.28732 417348.27387 477144.10739

416803.48600 488699.05815 418835.54476 482613.77762 417184.83751 477051.49345

416830.72539 488573.75694 418813.75325 482455.78914 417070.43206 476942.53587

416852.51691 488464.79937 418764.72234 482319.59217 416999.60963 476817.23466

416874.30842 488350.39392 418721.13931 482210.63460 416923.33933 476670.14194



90


416852.51691 476512.15345 418939.05446 470029.17778 424092.74773 463524.41058

417549.84539 477225.82557 419031.66840 469876.63717 423874.83258 463420.90089

417348.27387 477144.10739 419107.93870 469756.78384 423673.26107 463317.39119

417184.83751 477051.49345 419173.31324 469631.48263 423493.48107 463241.12089

417070.43206 476942.53587 419211.44840 469468.04627 423259.22229 463137.61119416999.60963 476817.23466 419331.30173 469032.21597 423155.71259 463072.23665

416923.33933 476670.14194 419429.36354 468639.96870 423073.99441 462968.72695

416852.51691 476512.15345 419478.39445 468394.81416 422954.14108 462783.49908

416879.75630 476272.44679 419532.87324 468274.96083 422839.73562 462565.58393

416956.02660 475831.16861 419609.14354 468116.97235 422801.60047 462369.46029

417059.53630 475324.51589 419723.54899 467817.33902 422763.46532 462233.26333

417130.35872 474817.86317 419805.26718 467583.08023 422752.56956 462064.37909

417163.04600 474327.55409 419886.98536 467327.02993 422763.46532 461906.39060

417190.28539 473957.09833 419985.04717 466989.26145 422796.15259 461753.85000

417222.97266 473793.66197 420066.76535 466733.21115 422807.04835 461579.51788

417321.03448 473564.85107 420094.00475 466613.35782 422834.28774 461437.87303

417495.36660 473401.41471 420137.58778 466428.12994 422850.63138 461268.98879

417745.96902 473297.90501 420208.41020 466232.00631 422845.18350 461165.47910

418007.46720 473276.11349 420306.47202 466046.77843 422796.15259 460931.22031

418296.20477 473314.24865 420382.74232 465932.37298 422676.29926 460506.28577

418623.07749 473412.31046 420475.35626 465812.51965 422589.13320 460195.75668

418917.26294 473526.71592 420600.65747 465681.77056 422489.44305 459895.21807

419178.76112 473657.46500 420824.02050 465501.99056 422452.93623 459743.58275

419434.81142 473728.28743 421085.51868 465349.44995 422338.53078 459324.09609

419908.77687 473744.63106 421357.91261 465186.01359 422262.26048 459029.91064

420072.21323 473657.46500 421581.27564 465049.81663 422213.22957 458670.35064420251.99323 473352.38380 421837.32594 464929.96329 422196.88593 458272.65550

420202.96232 472992.82380 422066.13685 464826.45360 422267.70836 457973.02217

420066.76535 472660.50320 422256.81260 464766.52693 422436.59260 457646.14944

419914.22475 472437.14017 422665.40350 464668.46511 422610.92472 457411.89066

419652.72657 472175.64199 423014.06774 464646.67360 422710.25190 457306.38341

419358.54112 471979.51836 423389.97137 464668.46511 422856.07926 457210.31915

419091.59506 471821.52988 423760.42713 464701.15239 423155.71259 457046.88278

418846.44052 471636.30200 424114.53924 464755.63117 425187.77135 456937.92521

418628.52537 471434.73049 424446.85985 464695.70451 425596.36226 456921.58157

418590.39022 471309.42928 424708.35802 464608.53845 426168.38952 456883.44642

418546.80719 471129.64928 424844.55499 464477.78936 426696.83376 456861.65491

418508.67204 470988.00443 424871.79439 464281.66573 427263.41315 456818.07188

418530.46355 470840.91171 424871.79439 464080.09421 427726.48284 456790.83248

418650.31689 470601.20504 424724.70166 463905.76209 428178.65677 456725.45794

418753.82658 470394.18565 424517.68227 463775.01300 428412.91556 456709.11430

418851.88840 470181.71838 424316.11076 463633.36816 428663.51798 456665.53127



91

Co-ordinates of Kobadak River


428663.51798 456665.53127 429709.51069 464515.92451 423640.57380 469566.10809

428794.26707 456921.58157 429513.38706 464613.98633 423455.34592 469778.57536

429028.52585 457319.27672 429355.39857 464673.91299 423275.56592 470029.17778429213.75373 457586.22278 429213.75373 464701.15239 423172.05623 470247.09293

429339.05494 457853.16883 429012.18221 464695.70451 423166.60835 470519.48686

429453.46039 458147.35429 428739.78828 464663.01724 423188.39986 470710.16262

429453.46039 458370.71731 428380.22828 464564.95542 423226.53501 470862.70322

429426.22100 458626.76761 428058.80344 464494.13300 423362.73198 471015.24383

429197.41009 458910.05731 427829.99253 464445.10209 423537.06410 471102.40988

428995.83858 459106.18094 427568.49435 464368.83179 423765.87501 471080.61837

428837.85010 459264.16942 427252.51739 464319.80088 423967.44652 471026.13958

428668.96586 459405.81427 427056.39375 464292.56148 424179.91379 470955.31716

428516.42525 459542.01124 426882.06163 464287.11360 424419.62045 470879.04686

428260.37495 459754.47851 426713.17739 464292.56148 424621.19197 470791.88080

428037.01192 459977.84154 426571.53255 464379.72754 424784.62833 470710.16262

427846.33617 460195.75668 426462.57497 464510.47663 424926.27317 470666.57959

427726.48284 460337.40153 426386.30467 464717.49602 425111.50105 470622.99656

427628.42102 460468.15062 426310.03437 464946.30693 425274.93741 470672.02747

427535.80708 460609.79547 426250.10770 465098.84753 425416.58226 470759.19353

427481.32829 460778.67971 426195.62892 465251.38814 425536.43559 470873.59898

427410.50587 460936.66819 426130.25437 465458.40753 425569.12286 471075.17049

427426.84951 461165.47910 426119.35861 465627.29177 425547.33135 471353.01231

427475.88042 461361.60273 426103.01498 465779.83237 425492.85256 471576.37533

427573.94223 461579.51788 426097.56710 465970.50813 425427.47802 471826.97775427759.17011 461764.74576 426070.32771 466166.63176 425334.86408 472077.58018

428031.56404 461955.42151 425977.71377 466444.47358 425269.48953 472279.15169

428293.06222 462102.51424 425912.33922 466662.38873 425209.56287 472464.37957

428543.66464 462255.05484 425836.06892 466874.85600 425106.05317 472753.11714

428734.34040 462374.90817 425727.11135 467060.08387 425002.54348 472992.82380

428946.80767 462489.31363 425618.15377 467261.65539 424942.61681 473287.00925

429202.85797 462658.19787 425465.61317 467479.57053 424888.13802 473581.19470

429420.77312 462794.39483 425296.72893 467637.55902 424860.89863 473831.79712

429589.65736 462919.69605 425127.84469 467811.89114 424855.45075 474049.71227

429747.64584 463055.89301 424969.85620 467953.53598 424866.34651 474224.04439

429911.08220 463257.46453 424790.07621 468089.73295 424741.04530 474311.21045

430025.48766 463410.00513 424653.87924 468264.06507 424539.47378 474431.06378

430101.75796 463611.57664 424523.13015 468389.36628 424359.69379 474496.43833

430145.34099 463775.01300 424392.38106 468574.59416 424179.91379 474518.22984

430167.13250 463933.00149 424218.04894 468732.58264 423945.65500 474447.40742

430139.89311 464107.33361 424076.40409 468955.94567 423793.11440 474392.92863

430036.38341 464281.66573 423940.20713 469179.30870 423596.99077 474256.73166

429905.63433 464406.96694 423793.11440 469375.43233 423395.41925 474125.98258



92


423123.02532 473984.33773 422638.16411 482085.33339 425563.67498 486231.16909

422926.90168 473869.93228 422752.56956 482335.93581 425351.20771 486329.23091

422779.80896 473826.34925 422850.63138 482488.47641 425165.97984 486410.94909

422621.82047 473771.87046 423019.51562 482608.32974 424969.85620 486509.01091

422393.00957 473777.31834 423221.08713 482646.46490 424768.28469 486623.41636422218.67745 473842.69288 423406.31501 482641.01702 424588.50469 486726.92606

422104.27200 473973.44197 423640.57380 482624.67338 424414.17257 486852.22727

422000.76230 474125.98258 423847.59319 482624.67338 424228.94470 486966.63272

421968.07503 474376.58500 424027.37318 482722.73520 424136.33076 487091.93393

422055.24109 474676.21833 424076.40409 482815.34914 424021.92531 487228.13090

422213.22957 474888.68560 424098.19561 482929.75459 423891.17622 487402.46302

422409.35320 475024.88256 424087.29985 483055.05580 423793.11440 487527.76423

422616.37260 475193.76680 424021.92531 483196.70065 423700.50046 487702.09635

422790.70471 475286.38074 423940.20713 483371.03277 423591.54289 487865.53271

423008.61986 475449.81710 423863.93682 483572.60428 423488.03319 488072.55210

423204.74350 475591.46195 423825.80167 483725.14488 423417.21077 488246.88422

423444.45016 475765.79407 423793.11440 483915.82064 423357.28410 488470.24725

423602.43864 475940.12619 423782.21864 484084.70488 423286.46168 488682.71452

423716.84410 476141.69770 423853.04107 484210.00609 423231.98289 488900.62967

423809.45804 476365.06073 423940.20713 484346.20306 423204.74350 489096.75330

423863.93682 476642.90254 424136.33076 484444.26487 423188.39986 489271.08542

423836.69743 477007.91042 424316.11076 484520.53518 423177.50410 489489.00057

423793.11440 477263.96072 424550.36954 484569.56608 423182.95198 489739.60299

423700.50046 477503.66738 424751.94105 484585.90972 423204.74350 490039.23632

423575.19925 477716.13465 424980.75196 484645.83639 423182.95198 490311.63026

423406.31501 477917.70617 425236.80226 484678.52366 423193.84774 490605.81571

423286.46168 478064.79889 425471.06105 484738.45032 423172.05623 490998.06298

423090.33804 478260.92252 425754.35074 484809.27275 423112.12956 491275.90479

422937.79744 478451.59828 425955.92225 484869.19941 423019.51562 491640.91266

422801.60047 478642.27404 426168.38952 484934.57396 422932.34956 491847.93206

422676.29926 478865.63706 426375.40891 485016.29214 422812.49623 492060.39933

422556.44593 479094.44797 426522.50164 485098.01032 422649.05987 492245.62720

422409.35320 479366.84190 426626.01133 485174.28062 422442.04048 492447.19871

422305.84351 479628.34008 426696.83376 485266.89456 422311.29139 492588.84356

422224.12533 479895.28614 426734.96891 485381.30001 422169.64654 492752.27992

422180.54230 480162.23220 426740.41679 485512.04910 422038.89745 492921.16416422169.64654 480538.13583 426702.28164 485648.24607 421940.83563 493030.12174

422213.22957 480837.76916 426571.53255 485773.54728 421848.22170 493215.34961

422289.49987 481088.37158 426418.99194 485866.16122 421793.74291 493373.33810

422322.18714 481317.18249 426168.38952 485964.22304 421761.05564 493580.35749

422393.00957 481507.85824 425961.37013 486051.38910 421750.15988 493711.10658

422491.07139 481753.01278 425776.14225 486127.65940 421744.71200 493858.19930



93

X_COORD Y_COORD X_COORD Y_COORD Galghasia River

421722.92049 493988.94839 417942.09265 495367.26171

421830.50343 494042.57696 417784.10417 495405.39686 X_COORD Y_COORD

421917.66949 494107.95150 417631.56357 495421.74050 416852.51691 476512.15345

422010.28342 494222.35696 417457.23145 495448.97989 416613.10788 476659.37902

422097.44948 494325.86665 417315.58660 495432.63625 416471.46304 476741.09720422211.85494 494565.57332 --- --- 416302.57880 476822.81539

422277.22948 494761.69695 416640.04964 494424.77869 416122.79880 476915.42932

422353.49978 494946.92483 416565.33007 494425.36458 415943.01880 476986.25175

422418.87433 495110.36119 416512.33311 494416.99664 415763.23880 477062.52205

422495.14463 495252.00603 416464.91477 494389.10350 415594.35456 477122.44871

422571.41493 495350.06785 416423.07506 494361.21036 415436.36608 477182.37538

416370.07810 494313.79202 415283.82548 477231.40629

416305.92388 494258.00574 415169.42002 477258.64568

Morirchap River 416252.92691 494213.37672

416188.77269 494143.64387

X_COORD Y_COORD 416119.03985 494079.48965 Kalagachi River

421722.92049 493988.94839 416068.83220 494023.70338

421586.72352 494136.04112 416015.83523 493959.54916 X_COORD Y_COORD

421406.94352 494299.47748 415979.57415 493906.55219 422710.25190 457306.38341

421199.92413 494446.57021 422383.05368 456825.20955

421047.38352 494533.73627 422277.19543 456642.36348

420922.08231 494593.66293 Betna River 422161.71370 456478.76436

420780.43747 494659.03748 422036.60849 456334.41220

420633.34474 494718.96414 X_COORD Y_COORD 421911.50329 456151.56613

420486.25202 494757.09929 416926.93889 496037.42381 421786.39808 455987.96702

420311.91990 494784.33869 416983.59683 495980.76587 421642.04592 455833.99138420159.37929 494784.33869 417031.53817 495928.46624 421507.31724 455718.50966

420017.73445 494757.09929 417075.12120 495880.52491 421343.71813 455631.89836

419870.64172 494735.30778 417118.70423 495797.71715

419712.65324 494675.38111 417166.64556 495701.83448

419521.97748 494571.87142 417218.94519 495605.95182 Arpangasia River

419413.01991 494522.84051 417262.52822 495514.42746

419293.16658 494452.01808 417315.58660 495432.63625 X_COORD Y_COORD

419200.55264 494419.33081 417173.94175 495274.64777 428663.51798 456665.53127

419097.04294 494424.77869 417032.29691 495040.38899 428936.64172 456459.51741

418971.74173 494457.46596 416917.89145 494866.05687 429100.24083 456257.42439

418873.67991 494555.52778 416814.38176 494697.17263 429254.21647 456055.33136

418742.93082 494675.38111 416710.87206 494555.52778 429427.43906 455853.23834

418606.73386 494838.81747 416640.04964 494424.77869 429542.92079 455651.14532

418492.32840 494975.01444 429648.77904 455410.55838

418399.71447 495111.21141 429735.39033 455198.84188

418268.96538 495236.51262 429802.75467 454996.74886

418110.97689 495301.88716



94

Appendix-B

Cross-Section Data



95

Cross-Section: Kholpetua 0.00

Cross-Section: Kholpetua 8500.00



96

ross-Section: Kholpetua 21000.00




97





98





99


ross-Section: Kobadak 189000.00



100





101





102





103





104

Cross-Section: Morirchap 0.00

ross-Section: Morirchap 5000.00



105

ross-Section: Morirchap 7000.00

Cross-Section: Betna 86750.00



106





107

ross-Section: Galghasia 20500.00

Cross-Section: Arpangasia 0.00



108

Arpangasia 2400.00

Cross-Section: Kalagachi 0.00



109

Appendix-C

Flow Hydrographs and Stage Hydrographs



Flo

Flo

Fl

St

110

Hydrograph at Galghasia 15625.00

Hydrograph at Kobadak 185755.00

ow Hydrograph at Betna 867500.00

ge Hydropgraph at Arpangasia 0.00



Stage Hydrograph at Kalagachi 0.00

Documents

Impact Assessment of Predicted Sea Level Rise and Storm Surge on Coastal Embankments of Some Selected Polders Using Mathematical Model