Technical Assistance Consultant’s Report

Project Number: 44167-012 December 2013

Bangladesh: Main River Flood and Bank Erosion Risk Management Program (Financed by the Japan Fund for Poverty Reduction)

Prepared by Northwest Hydraulic Consultants, Canada

In association with Resource Planning and Management Consultants Ltd., Bangladesh

For Bangladesh Water Development Board

This consultant’s report does not necessarily reflect the views of ADB or the Government concerned, and ADB and the Government cannot be held liable for its contents. (For project preparatory technical assistance: All the views expressed herein may not be incorporated into the proposed project’s design.

Government of the People's Republic of Bangladesh

Bangladesh Water Development Board

Project Preparatory Technical Assistance No. 8054 BAN

Main River Flood and Bank Erosion Risk Management Program

Final Report, Annex D Hydrology and Flood Modelling

September 2013

In association with

Resource Planning & Management

Consultants Ltd.

Asian Development Bank

Funded by the Japan Fund for Poverty Reduction

Government of the People’s Republic of Bangladesh Bangladesh Water Development Board

Project Preparatory Technical Assistance 8054 BAN Main River Flood and Bank Erosion Risk Management Program

Final Report, Annex D Hydrology and Flood Modelling

September 2013

PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program

Document Status

Title: Hydrology and Flood Modelling, Annex D

Principal Author: Dave McLean Contributions: Vanessa O’Connor IWM model results Final version: September 2013

Document Development

Draft Final 15 June, 2013

Revision for Final R13, 28 July 2013 Formatting, spell check, font change, justification

R13, 30 July 2013 Formatting, spell check, font change, justification

R14, 30 July 2013 Page setup and Print

R15, 22SEP2013 Format cover page, header and footer

R17 30SEP2013 Updated after comments from ADB

R18 28 Feb 2014 Reprinted R17

Review by : ko, 1 August, 2013

ADB, Natsuko Totsuka 9 September, 2013

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MAIN REPORT

ANNEXES

Annex A Priority Sub-reach Selection & Sub-reach Descriptions Annex A1 Priority Sub-reach Selection Annex A2 Sub-reach Description Annex B Background Data Annex B1 National Water Resources Database Annex B2 Socio-economic Data Annex B3 Surveys and Field Visits Annex C Institutional and Financial Assessment Annex D Hydrology and Flood Modelling Annex E River and Charland Morphology and River Engineering Annex F Design Issues Annex F1 Geotechnical Investigations Annex F2 Technical Designs Annex G Economic Feasibility Annex G1 Project Cost Annex G2 Economic Assessment Annex H Implementation and Procurement Planning Annex I Social Gender Equity Strategy & Action Plan Annex J Environmental Impact Assessment Annex K Involuntary Resettlement Annex K1 Resettlement Framework Annex K2 Resettlement Plan

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Table of Content

1 Introduction ...... 1 1.1 Purpose ...... 1 1.2 Scope of Work ...... 1 1.3 Project Location ...... 1 1.4 Outline of Report ...... 3 2 Physical Setting ...... 4 2.1 Physiography ...... 4 2.2 Floodplain Topography ...... 5 2.3 Climate ...... 5 3 Flood Hydrology ...... 7 3.1 Hydrological Data ...... 7 3.1.1 Available Information ...... 7 3.2 Runoff Pattern...... 7 3.3 Flood Magnitude and Frequency ...... 9 3.4 Flooding and Drainage ...... 12 3.4.1 Factors Affecting Flood Inundation ...... 12 3.4.2 Relation Between Flooding and Embankment Breaching ...... 13 3.4.3 Flooding Extent ...... 13 3.5 Flood Damages ...... 16 3.6 Climate Change ...... 20 4 Flood Model Development ...... 22 4.1 Method of Approach ...... 22 4.2 Regional Models and Model Boundaries...... 23 4.3 Model Limitations and Accuracy ...... 24 4.3.1 Limitations of 1D Modelling ...... 24 4.3.2 Limitations of Boundary Conditions ...... 24 4.3.3 Limitations of Floodplain Topography ...... 27 4.4 Inflow Conditions ...... 29 4.5 Model Validation ...... 30 5 JRB-1 Embankment Project ...... 36 5.1 Project Area ...... 36 5.1.1 General Features ...... 36 5.1.2 Proposed JRB-1 Project ...... 36

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5.2 JRB-1 Without Project Flood Conditions ...... 38 5.3 Scenario 1, Tranche-1 and 2 JRB-1: Closing the BRE and restoring the Hurashagar FCD Project 41 5.4 Scenario 2 JRB-1: Tranche-2 Securing the BRE (Effect of Embankment Breach at Enayetpur) . 45 6 JLB-2 and PLB-1Projects ...... 48 6.1 Project Area ...... 48 6.1.1 Physical Features ...... 48 6.1.2 Project Description ...... 48 6.2 “Without Project” Scenarios ...... 51 6.3 Effect of Upgrading Short Section of Embankment ...... 55 6.4 JLB-2 & PLB-1 Alternative 1: Continuous Embankment and Distributary Closures ...... 57 6.5 Alternative 2 - Polders ...... 61 7 Requirements for Improved Flood Modelling ...... 66 7.1 General Requirements ...... 66 7.2 Improved Hydraulic Modelling ...... 66 7.2.1 1D Versus 2D Modelling ...... 66 7.2.2 Model Selection ...... 67 7.2.3 Model Input Requirements ...... 67 7.2.4 Model Development Approach ...... 67 8 Conclusions ...... 69 9 References ...... 70

List of Tables

Table 3-1: Frequency of peak discharges on main rivers ...... 10 Table 3-2: Frequency of peak water levels on main rivers and distributary branches ...... 11 Table 3-3: Observed maximum water levels and discharges in 1988 to 2007 ...... 11 Table 3-5: Inventory of RADARSAT images used in study ...... 14 Table 3-6: Projected change in surface air temperature for South Asia (IPCC, 2007b, Table 10.5) ...... 20 Table 3-7: Projected change in precipitation for South Asia (from IPCC, 2007b, Table 10.5) ...... 21 Table 3-8: Simulated change in maximum discharge with increased precipitation (IWM, 2008) ...... 21 Table 5-1: Land types in JRB-1 ...... 36 Table 5-2: Initial flood depth estimates using IWM results inside JRB-1 embankments (without project) ...... 38 Table 5-3: Adjusted flooded areas inside JRB-1 embankments (without project) ...... 41 Table 5-4: Adjusted flooded areas inside JRB-1 embankments (with project condition) ...... 41 Table 5-5: Effect of JRB-1 project on flooded areas and flood depths ...... 44 Table 5-6: Adjusted flooded areas inside JRB-1 embankments (with breach scenario) ...... 47 Table 5-7: Effect of breach at JRB-1 (breach – with project) ...... 47 Table 6-1 : Land types in combined JLB-2 and PLB-1 floodplain areas ...... 48

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Table 6-2: Initial flood depth distribution, IWM results for JLB-2 and PLB-1 (WO project) ...... 52 Table 6-3: Adjusted flood depth distribution for JLB-2 and PLB-1 (WO project) ...... 52 Table 6-4: Adjusted flood depth distribution for JLB-2 (WO project) ...... 53 Table 6-5: Adjusted flood depth distribution for PLB-1 (WO project) ...... 53 Table 6-6: Effect of short embankment on flooded areas, unadjusted IWM results, 2003 flood condition ...... 56 Table 6-7: Initial flood depth distribution, IWM results for JLB-2 and PLB-1 full embankment (with project) ...... 57 Table 6-8: Adjusted flood depth distribution for JLB-2 and PLB-1 full embankment (with project) ...... 57 Table 6-9: Effect of Alternative 1on flooded areas for combined JLB-2 and PLB-1 sub-projects ...... 60 Table 6-10: Effect of Alternative 1 on flooded areas for JLB-2 sub-project ...... 61 Table 6-11: Effect of Alternative 1 on flooded areas for PLB-1 sub-project ...... 61 Table 6-12: Alternative 2 flood areas with RADARSAT adjustment, JLB-2+PLB-1 combined ...... 63 Table 6-13: Alternative 2 flood areas with RADARSAT adjustment,JLB2 only ...... 63 Table 6-14: Alternative 2 floodedareas with RADARSAT adjustment, PLB-1 only ...... 63 Table 6-15: Effect of polders on flooded areas and flood depths by land type ...... 64 Table 6-16: Effect of polders on flooded areas and flood depths by land type, JLB-2 only ...... 64 Table 6-17: Effect of polders on flooded areas and flood depths by land type, PLB-1 only ...... 64

List of Figures

Figure 1-1: Sub-project areas in Main River Flood and Bank Erosion Risk Management Project ...... 2 Figure 2-1 Main river systems in Bangladesh ...... 4 Figure 2-2: Topography in the project area ...... 5 Figure 2-3: Total rainfall from June 1 during six different flood events ...... 6 Figure 3-1: Pattern of runoff on Jamuna River, Ganges River and Upper Meghna River ...... 9 Figure 3-2: Frequency analysis of peak discharges, Jamuna River at Bahadurabad and Ganges River at Hardinge Bridge ...... 10 Figure 3-3: Variation in flood levels relative to the 2 year flood at 5 stations on Jamuna and Padma River ...... 12 Figure 3-4: Flood extent in project area, Sept 17, 1998 based on RADARSAT imagery (CEGIS) ...... 15 Figure 3-5: Relation between flood inundation and river stage, determined by interpretation of RADARSAT imagery 1998 to 2007 ...... 16 Figure 3-6: Extent of flood inundation in sub-project areas determined by RADARSAT imagery ...... 17 Figure 3-7: Estimated damages in the JRB-1 region from various floods ...... 18 Figure 3-8: Estimated damages in JRB-1 from various floods ...... 19 Figure 4-1: Hydrological regions ...... 23 Figure 4-2: North west regional model ...... 25 Figure 4-3: North Central regional model ...... 26 Figure 4-4: Stage-discharge rating curve at Bahadurabad on Jamuna River and Mawa on Padma River 27 Figure 4-6: Discharges on Jamuna River at Bahadurabad in 1998 and 2003 ...... 29 Figure 4-7: Observed water levels at BWDB gauges in 1998, 2003 and 2007 ...... 30 Figure 4-8: Comparison of simulated and observed water levels in 2007 on Jamuna River at Sirajganj (162500) ...... 31 Figure 4-9: Comparison of simulated and observed water levels in 2007 on Jamuna River at Aricha (235650) ...... 32

Page ix PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program

Figure 4-10: Comparison of simulated and observed water levels in 2007 on Padma River at Baruria (12000) ...... 32 Figure 4-11: Comparison of simulated and observed water levels in 2007 on Kaliganga River at Taraghat ...... 33 Figure 4-12:Comparison of simulated and observed water levels in 1998 on Jamuna River at Sirajganj. 33 Figure 4-13: Comparison of simulated and observed water levels in 1998 on Jamuna River at Mathura 34 Figure 4-14: Comparison of simulated and observed water levels in 1998 on Padma River at Baruria (12000) ...... 34 Figure 4-15: Comparison of simulated and observed water level in 1998 on Kaliganga River at Taraghat ...... 35 Figure 5-1: Sub-project JRB-1 land elevation and infrastructure ...... 37 Figure 5-2: Land area-elevation relation in JRB-1 ...... 38 Figure 5-3: JRB-1 Flood depths, IWM results, without project, 2003 flood condition ...... 39 Figure 5-4: JRB-1 Flood depths adjusted using RADARSAT imagery without project,2003 flood condition ...... 40 Figure 5-5: Flood depths using initial IWM analysis, “with project”,2003 flood condition ...... 42 Figure 5-6: Flood depths adjusted using RADARSAT, with project, 2003 flood condition ...... 43 Figure 5-7: Effect of project on water levels adjusted using RADARSAT inside JRB-1, 2003 flood condition ...... 44 Figure 5-8: F0 +F1 land area versus flood return period for without project and with project conditions at JRB-1 ...... 45 Figure 5-9: Effect of embankment breach on flood depth inside project area ...... 46 Figure 6-1: JLB-2 project area ...... 49 Figure 6-2: PLB-1 project area ...... 50 Figure 6-3: Land area elevation relation in JLB-2 and PLB-1 ...... 50 Figure 6-4: Assumed “with project” scenarios at JLB-2 and PLB-1 ...... 51 Figure 6-5: Base flood depth map by IWM for JLB-2 and PLB-1 WO project (2003 flood) ...... 54 Figure 6-6: Distribution of flood depths, adjusted using RADARSAT imagery, without project, 2003 flood ...... 55 Figure 6-7: Shortembankment, 2003 flood condition ...... 56 Figure 6-8: Alternative 1, IWM results, 2003 flood condition ...... 58 Figure 6-9: Alternative 1, RADARSAT adjusted, 2003 flood condition ...... 59 Figure 6-10: Change in water levels (WP-WO) due to Alternative 1, 2003 flood condition ...... 60 Figure 6-11: Flood depths with Alternative 2, 2003 flood condition ...... 62 Figure 6-12: Impact of Alternative 2 on flood depths, 2003 flood condition ...... 65 Figure 7-1: Proposed model extents for floodplain models ...... 68

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1 Introduction

1.1 Purpose This report summarizes the hydrological conditions in the 13 sub-reaches along the main rivers in central Bangladesh and provides a detailed understanding about inundation patterns in three priority sub-reaches that were identified during the project’s pre-feasibility assessment phase (NHC, 2012). The three priority projects are:

• JRB-1 on the right bank of Jamuna River • JLB-2. on the left bank of Jamuna River • PLB-1 on the left bank of the Padma River

The projects involve river bank stabilization measures and other flood mitigation works, including embankments. Hydraulic modelling was carried out to estimate the magnitude and extent of the flooding in these project areas under “without project” and “with project” conditions. This analysis was used to estimate the project benefits that accrue from improved flood protection.

1.2 Scope of Work NHC carried out a range of hydrological and hydraulic investigations, including a review of the available hydrological records, flood frequency analysis of water level and discharge and interpretation of flooding extent using RADARSAT flood imagery. The RADARSAT information was provided to NHC by CEGIS. Potential climate change impacts were assessed by reviewing the available scientific literature and past studies.

NHC engaged the Institute for Water Modelling (IWM) to carry out the hydraulic modelling. IWM already has developed hydraulic models that extend over the project area and is the only competent national organization in Bangladesh that has conducted the required type of large-scale modelling. NHC specialists supervised the modelling activities to ensure the results met the required accuracy and quality.

1.3 Project Location Figure 1-1 shows the boundaries of the 14 sub-projects that were investigated in the PPTA. The project area includes portions of the Jamuna River, Ganges River and Padma River. Three of these sub-projects were studied at a feasibility level for the Tranche 1 financing:

• JRB-1 on the right bank of Jamuna River from Jamuna Bridge to HurarshagarRiver. • JLB-2 on the left bank of Jamuna River from Dhaleswariofftake. • PLB-1on left bank of Padma River.

While all three have different river conditions the floodplains show some similarities. With respect to riverbank erosion, JRB-1 is located at the consolidated Jamuna right (west) bank, JLB-2 at the younger depositions of the Jamuna left (east) bank, and both are characterized by a braided, dynamically changing river system. PLB-1 boarders a single confluence channel that later splits and carries the combined flow of Jamuna and Ganges. The banks are of similar depositional material as the JLB-2 site. The floodplains all get deeply flooded during high floods mostly from the Jamuna/Padma but also from tributaries. JLB-2 and PLB-1 are largely influenced by Jamuna flooding, in parts contributed through

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Jamuna distributaries (Dhaleswari system) due to the existing but somewhat limited flood protection from an embankment/road alignment along the Padma left bank.

Figure 1-1: Sub-project areas in Main River Flood and Bank Erosion Risk Management Project

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1.4 Outline of Report In addition to this brief Introduction, this annex contains eight chapters. Chapter 2, Physical Setting, describes the physiography, floodplain topography and climate of the region. Chapter 3, Flood Hydrology, describes the available data in the region and characterizes the historic runoff patterns, magnitude and frequency of flooding, flood damages and flood extent using previous RADARSAT imagery. Chapter 4, Flood Model Development outlines the development of IWM’s North Central and Northwest regional flood models, including the calibration, validation and limitations of the models. Chapter 5, JRB-1 Embankment Project, assesses the effect of a flood embankment in JRB-1.Chapter 6, JLB-2 and PLB-1 Projects, summarizes the effect of two different alternatives (long embankment and polders) on flooding conditions in the area of JLB-2 and PLB-1. Chapter 7, Requirements for Improved Flood Modelling, describes the additional field surveys, hydrometric data and improved modelling methodology that should be carried out for a detailed assessment of the floodplain hydrology in the three sub-projects. Chapter 8, Conclusions summarizes the key findings of this study.

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2 Physical Setting

2.1 Physiography The greater part of Bangladesh consists of deltaic land that has been laid down over a period of some five to six million years by the two large river systems of the Ganges and the Brahmaputra, both originating principally in the Himalayan mountain region of India, Nepal, Bhutan and China. The combined Ganges-Brahmaputra watershed covers an area of 1,621,000 km2. The Brahmaputra River is called the Jamuna River in Bangladesh and has a catchment area of 536,000 km2. Figure 2-1 shows the country of Bangladesh divided by the major river systems.

Figure 2-1 Main river systems in Bangladesh

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2.2 Floodplain Topography The floodplain in the project area consists mainly of low-lying alluvial sediments, with the elevation ranging between 2 m to 12 m PWD (Figure 2-2). The floodplains have a very gently undulating relief comprising broad and narrow ridges and depressions. Differences in elevations range from 1-3 m.

Figure 2-2: Topography in the project area The floodplain has been mapped using an Agro- Ecological land classification system (MPO, 1990). Most of the project area consists of stable floodplains, which include higher elevation ridges and lower level flood basins. In general, the highest lands consist of natural levees adjacent to the river banks and the flood basins are further back from the active channels.

2.3 Climate Bangladesh has a sub-tropical monsoon dominated climate that is characterized by a hot and dry pre- monsoon season from March to May, a rainy monsoon season from June through October and a cool dry winter season from November through February. The rainy season coincides with the south west monsoon, with tropical depressions moving inland from the Bay of Bengal. From March to May, violent thunderstorms (referred to as “northwesters”) are common. Cyclones occur in the months of October to December (post-monsoon) and March to May (pre-monsoon). These events can generate very large storm surges and are responsible for significant flood damage to the coastal regions of the country. Table 2-1shows the typical rainfall pattern in the study area. The mean annual rainfall in the project area is approximately 1,800 mm/year (FAP-3, 1992).

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Table 2-1: Seasonal rainfall pattern in Bangladesh Season Period % of Annual Precipitation Pre-Monsoon April-May 16.6 Monsoon June –September 72.2 Post-Monsoon October-November 8.4 Dry Season December-March 2.8

Figure 2-3 shows the cumulative rainfall pattern from June 1 for six different years. The measurements were made at Tangail (Station 2). This plot shows that during typical years the rainfall in the monsoon season averages between 800 to 1200 mm. During a very rainy year, up to 2000 mm of rain can occur in the monsoon season.

2,000

1,600

1,200

800 Commulative (mm) rainfall

400

- 31-May 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct

1998 2000 2001 2002 2003 2004 2007

Figure 2-3: Total rainfall from June 1 during six different flood events

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3 Flood Hydrology

3.1 Hydrological Data Available Information Figure 3-1 shows the hydrometric stations in the study area. Daily data from these stations was provided in spreadsheet format to the project by CEGIS. Published discharge records on the Jamuna River at Bahadurabad (46.9L), Ganges River at Hardinge Bridge (90) and Padma River at Baruria Transit (91.9R) and Mawa (93.5) extend back to 1965. These stations also include water level measurements and typically have 40 to 45 years of records, accounting for missing data in some years (such as 1971).

Key mainstream water level stations are located at Sirajganj (49), Mathura (50.3) on Jamuna River, Aricha (50.6) and Bhagyakul (93.4L) on Padma River and Chandpur (277) on the Meghna River.

There are a number of water level stations on the floodplain distributary channels. Key stations include Bagharbari (151) on the Atrai River, Jugini (186) on Old Dhaleswari River, Porabari (50), Tilli (68) and Taraghat (137A)on Dhaleswari River and Chowdhury Char on the Arial Khan.

3.2 Runoff Pattern The seasonal flow pattern through the year is of the monsoon type, with maxima in August-October and minima in February-March. The year-to-year variation is moderate. The rise and fall of the main rivers is not strongly related to the local precipitation pattern, since the vast majority of the river’s runoff is generated outside the country. The Jamuna River has an annual average discharge of 9,600 m3/s at Bahadurabad Transit. The flow varies from a low of 8,000 m3/s to a maximum of 100,000 m3/s. Bankfull discharge is around 48,000 m3/s. The river typically peaks in July to August.

The Ganges River has a longterm mean flow of about 8000 m3/s or about 40% of the Jamuna. The discharge during flood times reaches up to 80,000 m3/s. The river typically peaks later than the Jamuna, in August – September (Figure 3-1). The Ganges has the lowest water yield, particularly in the dry season, with flows dropping below 650 m3/s. The Padma River drains the combined Ganges-Jamuna River and has an average discharge of around 30,000 m3/s. The discharge at Mawa varies from a minimum of 10,000 m3/s up to 120,000 m3/s. The most severe floods occur when the Jamuna and Ganges Rivers peak together (such as in 1988). Substantial overland flow occurs along the Padma River to the southern coastal and as such counters salinity intrusion, but also leads to reducing discharges downstream. The Padma River is weakly tidal during the dry season.

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Figure 3.1: Hydrometric stations in project area (key gauging stations outside of the map are Bahadurabad for the Jamuna and Hardinge Bridge for the Ganges)

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140000 1965 Ganges-Hardinge Bridge 120000 Jamuna R. at Bahadurabad 100000 Padma

80000

60000

40000 Discharge (m3/s)

20000

0 1-Apr 1-May 31-May 30-Jun 30-Jul 29-Aug 28-Sep 28-Oct 27-Nov 27-Dec

180000 1998 160000 Ganges Jamuna 140000 Padma 120000 100000 80000 60000 Discharge (m3/s) 40000 20000 0 1-Apr 1-May 31-May 30-Jun 30-Jul 29-Aug 28-Sep 28-Oct 27-Nov 27-Dec

Figure 3-1: Pattern of runoff on Jamuna River, Ganges River and Upper Meghna River

3.3 Flood Magnitude and Frequency Bangladesh is one of the most flood prone countries in the world. In the 19th century, six major floods were recorded in 1842, 1858, 1871, 1875, 1885 and 1892 (Halcrow, 1994, FAP2, 1992). Eighteen major floods occurred in the 20th century with at least seven during the last 50 years affecting between 35 to 75% of the land area (BWDB,2011). In recent years, major flooding occurred in 1987, 1988, and 1998, 2004 and 2007.

The catastrophic flood of 1987 occurred throughout July and August and affected 57,300 km2 of land, (about 40% of the total area of the country. The seriously affected regions were on the western side of the Brahmaputra, the area below the confluence of the Ganges and the Brahmaputra and considerable areas north of Khulna. The flood of 1988 was also of catastrophic consequence and occurred throughout August and September. The waters inundated about 82,000 km2 of land and lasted 15 to 20 days. In 1998, over 60% of the total area of the country was flooded. The 2004 flood was very similar to the 1988 and 1998 floods with two thirds of the country under water (BWDB, 2011). It has been reported that the frequency, magnitude, and duration of floods have increased over time, possibly due to climate change. However, statistical tests on discharge time series from 1965 to 2012 showed only weak evidence of a change in the average flood discharge during this period.

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000000 Discharge (m3/s) Discharge

100000

Jamuna

Ganges

10000 1.003 1.05 1.25 2 5 10 20 50 100 200 500 Return period (years) Figure 3-2: Frequency analysis of peak discharges, Jamuna River at Bahadurabad and Ganges River at Hardinge Bridge There are reasonably reliable and long (> 40 years) records available on the main rivers which provides a basis for estimating the frequency of flood discharges. The Gumbel and Log-Pearson III distributions fit the annual maximum discharges on the Jamuna, Ganges and Padma Rivers (Figure 3-2). The 95% confidence limits on the 100-year flood at Bahadurabad range from 115,300 to 94,900 m3/s. The 95% confidence limits on the 2-year flood range from 70,200 to 63,400 m3/s. Table 3-1 summarizes the adopted flood discharge statistics on the Padma, Ganges and Jamuna Rivers.

Table 3-1: Frequency of peak discharges on main rivers Return Period Padma at Baruria Transit Ganges at Hardinge Bridge Jamuna at Bahadurabad m3/s m3/s m3/s 2-year 94,000 49,700 66,000 10-year 122,000 64,600 85,100 20-year 132,000 69,600 91,400 50-year 143,000 75,700 99,100 100-year 152,000 80,000 104,600

In terms of the published discharge records for Jamuna River, the 1998 flood is the flood of record, with a return period of nearly 100 years. However, in terms of published water levels at Bahadurabad, the 1988 flood was slightly higher than 1998.

Flood water level statistics were generated by CEGIS using historical water level data at all long-term stations on the main rivers and major distributaries. The data were fit using a Gumbel probability distribution to generate flood statistics (Table 3-2). There is less than 2 m of variation between a 2-year and 100-year water level on the mainstream rivers. Figure 3-3 shows the flood levels plotted relative to the 2-year level at five different stations. The overall trend for the stations is very similar, although the

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variation is slightly smaller on the Padma than on the Jamuna River. This makes it relatively easy to interpolate flood levels to other sites along the project area.

Table 3-2: Frequency of peak water levels on main rivers and distributary branches River Station ID Name 2-yr 10-yr 20-yr 50-yr 100-yr Jamuna 46.9L Bahadurabad 19.7 20.3 20.6 20.9 21.1 49 Sirajganj 13.9 14.5 14.8 15.1 15.4 50.3 Mathura 10.1 11.0 11.3 11.7 12.0 50.6 Aricha 9.5 10.2 10.5 10.9 11.1 Ganges 91.2 Mohendrapur 10.54 11.33 11.63 12.02 12.31 Padma 91.9L Baruria Transit 8.32 9.06 9.34 9.71 9.98 93.5L Mawa 6.07 6.62 6.84 7.11 7.32 Meghna 277 Chandpur 4.75 5.2 5.36 5.57 5.73

River Station ID Name 2-yr 10-yr 20-yr 50-yr 100-yr Old Dhaleswari 186 Jugini 12.2 13.2 13.5 14.0 14.3 Jamuna 50 Porabari 12.0 12.9 13.2 13.7 14.0 Dhalewari 68 Tilli 9.1 10.5 11.0 11.7 12.2 Kaliganga 137A Taraghat 8.7 9.8 10.2 10.7 11.1 Atrai 151 Baghabari 10.78 11.75 12.12 12.6 12.95 Arial Khan 4A Chowdhury Char 6.4 7.2 7.5 7.9 8.1

Table 3-3 summarizes the observed maximum water levels and discharges in 1988, 1998, 2003, 2004 and 2007. Table 3-4 summarizes the approximate return period of these flood events. There is considerable variation in the estimated return periods of the flood events from year to year. The return period of the 1988 and 1998 events was probably greater than 50 years on the Jamuna River, at least in terms of the reported discharges. The peak water level at Bahadurabad was at least 0.30 m greater in 1988 than in 1998; however, the published maximum discharge in 1998 was substantially greater than in 1988.Floods in 2004 and 2007 had return periods of greater than 20 years on the Jamuna River but less than 10 years on most of the distributary channels on the floodplain. The 2003 flood represents a moderate, average flood condition, having a return period of 2 years on the Jamuna River and 1.5 years on the Ganges River (based on published discharge records). The return period of the peak water levels in 2003 ranged from 3 to 10 years and was typically about 5 years on the main rivers. Ideally, the return period of the discharge and water level should be very close. However, ongoing morphological changes can introduce additional complications and variability along the river (as illustrated in Figure 4-4).

Table 3-3: Observed maximum water levels and discharges in 1988 to 2007 River Station ID Name 1988 1998 2000 2002 2003 2004 2007 Jamuna 49 Sirajganj 15.11 14.76 14.03 14.38 14.33 14.81 14.95 Jamuna 50.3 Mathura 11.35 10.61 10.44 10.75 11.25 11.90 Jamuna 50.6 Aricha 10.57 10.75 9.74 10.18 10.06 10.31 10.67 Ganges 91.2 Mohendrapur 11.59 11.90 10.69 11.05 10.97 11.18 Padma 91.9L Baruria Transit 9.35 9.58 8.52 8.65 8.71 9.88 9.30 Old Dhaleswari 186 Jugini 13.43 13.28 12.38 13.16 12.49 13.79 Jamuna 50 Porabari 13.14 12.88 12.62 11.88 12.61 13.10 12.62 Dhalewari 68 Tilli 10.84 11.36 9.39 9.70 Kaliganga 137A Taraghat 10.37 9.84 8.99 9.13 9.92 9.43 Atrai 151 Baghabari 12.32 12.10 11.31 11.45 12.20 11.87 11.58 Discharge (m3/s) Jamuna 46.9L Bahadurabad 98300 103129 83792 68385 65287 68351 93700 Ganges SW90 Hardinge Bridge 71800 74278 42687 44425 42687 Padma Baruria Transit 132000 141935 84205 125818 132000

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Table 3-4: Approximate return period of recent floods River Station ID Name 1988 1998 2000 2002 2003 2004 2007 Jamuna 49 Sirajganj 50 20 3 7 6 22 35 Jamuna 50.3 Mathura 30 5 4 7 8 60 Jamuna 50.6 Aricha 20 40 5 10 5 7 Ganges 91.2 Mohendrapur 18 40 3 5 5 7 Padma 91.9L Baruria Transit 20 30 4 5 5 7 19 Old Dhaleswari 186 Jugini 20 12 4 10 7 30 Jamuna 50 Porabari 15 10 5 7 15 7 Dhalewari 68 Tilli 15 30 3 4 Kaliganga 137A Taraghat 30 15 4 5 17 6 Atrai 151 Baghabari 25 20 5 6 25 12 7 Discharge (m3/s) Jamuna 46.9L Bahadurabad 40 80 9 3 2.0 3 25 Ganges SW90 Hardinge Bridge 25 40 1.5 1.5 1.5 Padma Baruria Transit 20 30 10

Figure 3-3: Variation in flood levels relative to the 2 year flood at 5 stations on Jamuna and Padma River

3.4 Flooding and Drainage Factors Affecting Flood Inundation Flooding can occur from three sources: direct overbank spill from the main rivers, from the internal regional rivers and distributary channels and direct rainfall accumulation. The significance of each contribution varies from year to year and within the flood season. Floodplain inundation and drainage is governed by the interactions and inter-connections between the main rivers, distributaries, khals and beels. Excess rainwater accumulates first in beels until they are filled. The extent of inundation

Page 12 September 2013 Hydrology and Flood Modelling increases until the small khals which link the depressions begin to flow. The khals form an interlinking network within the internal drainage system and they are the means by which water is transferred between the distributaries and the floodplain. During the monsoon season, the accumulated rainfall excess together with overspill from boundary and distributaries remains on the floodplain. The main rivers and distributaries have natural levees which are elevated above the surrounding floodplain and low-lying flood basins. These natural “highlands” restrict the inter-change of water over the floodplain, which can delay or restrict drainage. The situation is aggravated when the main rivers remain high for prolonged periods, as this delays drainage and could cause prolonged local flooding.

Flooding from the main rivers can occur either directly or through breaches in existing embankment lines. Avulsions and river widening cause special issues: some river courses, such as the Jamuna are relatively young, not allowing the formation of a fully built natural levee. In addition, a river widening process, specifically along Jamuna and Padma Rivers, destroys the natural levees more quickly than they form, and as such, increases the overall flood risk.

Infrastructure alters the flood flows over the flood plain. The floodplain was used by people already thousands of years ago and people settled on high ground or build their homes on localized mounds of earth. However, these elevated home steads had little impacted on local flood patterns, especially as the population density was low. Roads and rail embankments have substantial impact. Initially rail embankments were built, which cut across the natural flow paths. It is reported (Gales, 1917) that the railway line crossing the Ganges at Hardinge Bridge and continuing to the north was also intended to prevent a connection of Ganges and Jamuna through the Baral and Chalan Beel. Nowadays, road embankments form major divides and lead to a “compartmentalization”. Compartmentalization was seen as one major solution to the flood problems during the times of the Flood Action Plan. While major highways have been raised in general above the 1988 flood level, more and more rural roads also impact on flow patterns albeit at lower, often not clearly established levels.

Relation Between Flooding and Embankment Breaching Failure of embankments by breaching and erosion has been a recurring problem along the main rivers. The most critical structure in the project area is the Brahmaputra Right Bank Embankment (BRE). The BRE was constructed in the 1960s to provide flood protection to about 230,000 ha lying on the western side of the Brahmaputra-Jamuna and Tista rivers. The embankment extended 217 km from Kaunia in Rangpur district at the northern end up to Beraupazila in Pabna district at the southern end. Since its construction, the BREhas been under constant threat of riverbank erosion and on-going erosion has led to breaches with attendant crop loss, damage to buildings and infrastructure and successive costly retirement of the embankment. CEGIS reported the embankment breached at three locations in 2006 and six locations in 2005. Approximately 3,400 m of the embankment was eroded in 2006, 8,300 m in 2005, 5,100 m in 2004 and 2,500 m in 2003. In many places, the embankment was re-located further away from the riverbank, and in some places, embankment was relocated many times.

Flooding Extent RADARSAT imagery was analysed by CEGIS to estimate the spatial extent of flooding throughout the monsoon season in 1998, 2000, 2001, 2002, 2003, 2004 and 2007. Between three to five images were typically acquired for each year (Table 3-5) .The images show the progressive increase in flooding and inundation in each sub-project during the monsoon season and the recession towards the end. Figure 3-4shows the flood extent for JLB-2 for August 26, 1998.

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Table 3-5: Inventory of RADARSAT images used in study Date of WL at WL at WL at Precip Inundation (% of gross area) Imagery Sirajganj Aricha Mathura (mm) at JRB-1 at JLB-2 at PLB-1 26-Aug-98 14.19 10.19 11.00 2011 72.0 71.3 60.6 10-Sep-98 14.62 10.75 11.78 2219 71.9 73.2 62.1 17-Sep-98 13.17 9.28 9.84 2219 69 62.5 48.5 20-Jul-00 13.16 8.72 9.65 320 38.3 15.1 15.5 13-Aug-00 13.46 9.51 10.15 669 45 34.1 36.3 6-Sep-00 13.04 8.98 9.7 990 34.2 14.3 13.2 30-Sep-00 12.77 8.73 9.44 1445 39.6 19 19.3 25-Oct-00 9.74 5.75 6.55 1485 5.3 2.2 3.8 21-Jun-01 11.08 6.43 7.32 593 10.7 1.8 2.1 15-Jul-01 11.26 7.38 7.98 718 13.5 2 3.2 8-Aug-01 13.03 9.36 9.88 868 40 27.8 16.4 1-Sep-01 13.33 9.57 10.08 1008 45.4 24.3 11.4 25-Sep-01 12.28 8.3 8.77 1355 29.1 10.4 7.7 16-Jun-02 11.23 6.45 6.91 165 10.1 0.5 1.3 10-Jul-02 13.02 8.76 8.98 575 36.7 11.1 9 3-Aug-02 13.74 9.9 10.12 857 49.1 44.5 31 27-Aug-02 13.11 9.21 9.37 1095 43 24.1 11.5 20-Sep-02 11.48 8.24 8.19 1154 19.4 4.6 5.9 14-Oct-02 11.44 7.38 7.63 1337 20.4 5.8 5.6 7-Nov-02 9.4 5.21 5.46 1400 6.4 2.4 3.2 11-Jun-03 10.61 5.83 6.44 155 2.7 0.6 0.7 5-Jul-03 13.73 9.23 10.04 425 47.4 33 23.2 29-Jul-03 12.81 8.78 9.34 583 47.4 33 23.2 22-Aug-03 12.95 9.01 9.33 791 41.2 22.9 16.6 15-Sep-03 13.14 9.44 9.84 946 42 31.5 24.7 9-Oct-03 12.9 8.8 9.21 1176 37.8 22.7 16.6 12-May-04 9.5 5.28 0.1 1.6 0.8 0.8 5-Jun-04 11.03 6.18 7.12 137 0.1 0.1 0.1 29-Jun-04 13.75 8.62 9.75 508 41.5 13.7 7.4 23-Jul-04 14.81 10.26 11.21 813 58.6 58.6 45.3 16-Aug-04 12.88 8.19 8.51 966 35.6 24.0 25.2 9-Sep-04 13.25 8.42 8.72 1040 42.5 23.5 17.8 3-Aug-07 14.88 10.67 11.90 916 55.5 58.3 45.6

The flooded inundation extent in the project area is governed primarily by the river stage and only weakly dependent on the accumulated rainfall (P in mm). Figure 3-5shows the relation between river stage and percent of area flooded for four sub-projects (JRB-1, JRB-2, JLB-2,and PLB-1). The water level is expressed relative to the 2-year annual flood peak (WL-WL2yr). At JRB-1, when the water level was 1 m below the 2-year flood level, about 35% of the project area was flooded. At a 2-year flood level, just under 50% of the area was flooded.

At JLB-2 and PLB-1, approximately 10% of the project area was flooded when the water level was 1 m below the 2-year flood level. The extent of inundation increased to 40% at the 2-year flood level. This indicates that considerable flooding was occurring through open khals and distributary channels before bankfull conditions were exceeded (bankfull conditions commonly correspond to a 1.5 to 2-year flood).

Page 14 September 2013 Hydrology and Flood Modelling

Figure 3-4: Flood extent in project area, Sept 17, 1998 based on RADARSAT imagery (CEGIS)

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Inundation at JRB1 Inundation at JLB-2 Using Mathura as Reference Flood Level Using Aricha as Reference Level 2 2 1 1 0 0 -1 -1

WL2yr (m) -2

- -2 WL2yr (m)

-3 -

WL -3 y = 0.0857x - 4.0234 -4 WL y = 0.8112ln(x) - 3.0249 R² = 0.8835 -4 R² = 0.7918 -5 -5 0 10 20 30 40 50 60 70 80 -6 0 10 20 30 40 50 60 70 80 Inundation (%) Inundation (%)

Inundation at JRB-2 Inundation at PLB-1 Using Mathura as Reference Level Using Aricha as Reference Level 3 2 2 1 1 0 0 -1 -1 -2 WL2yr (m) WL2yr (m) -

-2 - y = 3.1656ln(x) - 9.8317 -3 y = 0.879ln(x) - 3.0881

WL -3 R² = 0.6186 WL R² = 0.7376 -4 -4 -5 -5 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Inundation (%) Inundation (%)

Figure 3-5: Relation between flood inundation and river stage, determined by interpretation of RADARSAT imagery 1998 to 2007

Statistics on the maximum areal extent and per cent of land flooded were generated in each of the 13 sub-project areas (Figure 3-6). During a severe flood such as 1998, 70% of the total sub-project area was flooded on the right bank of the Jamuna River (JRB-1). Other areas subject to significant flooding included the left bank of the Jamuna (JLB-2) and Padma River left bank (PLB-1 and PLB-2). The areas reported by CEGIS represent total areas in each sub-project, not adjusted to represent the net cultivable land area.

3.5 Flood Damages Estimates of annual flood damages were compiled by the study team for the region surrounding the JRB-1 region. Figure 3-7 shows estimates of crop damage, damage to houses and roads in the region surrounding JRB-1 (Sirajganj district) for floods in 1988, 2000, 2002, 2003, 2004, 2005 and 20071. Figure 3-8 shows comparable results for the JRB-1 project area. The year to year variation in damages reflects the effect of varying flood magnitude (Table 3-4) as well as other effects due to local embankment breaching and ponding. These results show that damages to crops and houses from an extreme event such as 1988 were 5 to 10 times greater than for a moderate flood event such as 2003.

1 A review of data 1998 indicated it was incomplete and likely erroneous. Therefore it was not included in the subsequent analysis.

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60,000

50,000

40,000

Flooded Area 1998(ha) 30,000 Flooded Area 2001(ha) Flooded Area 2004 (ha) 20,000 Flooded Area 2007(ha)

10,000

0

Percentage Sub-reach Flooding

70%

60%

50%

40%

30%

20%

10%

0%

1998 flooded (% of total) 2001 flooded (% of total) 2004 flooded (% of total) 2007 flooded (% of total)

Figure 3-6: Extent of flood inundation in sub-project areas determined by RADARSAT imagery

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Figure 3-7: Estimated damages in the JRB-1 region from various floods

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Figure 3-8: Estimated damages in JRB-1 from various floods (1998 data erroneous and excluded from analysis)

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3.6 Climate Change It should be recognized that all aspects of climate change impact projections are subject to considerable uncertainty. The scientific research community has concluded with a high degree of confidence that globally averaged air temperatures will increase in the future (IPCC, 2007a), however the magnitude of change is quite uncertain and depends on a large number of factors such as future economy activity, technological change, and population growth, which in turn affect the emission of greenhouse gases, and, ultimately, change in air temperature. While increases in air temperature are projected with confidence, there is much lower confidence in projections of change to other meteorological parameters such as precipitation.

Quantitative assessment of climate change impacts on flood flows relies on translating uncertain change in air temperature and rainfall derived from Global Climate Models (GCMs) to change in discharges. The steps involved in this process are to take meteorological data output from a coarse resolution GCM, adjust (or downscale) those data to be representative of regional or local conditions, and to then transform the downscaled data to runoff and river discharge using a hydrological model. The process involves a long chain of assumptions and simplifications, and a corresponding propagation of, and increase in, uncertainty.

It should also be recognized that climate change impacts may be masked by either or both natural variability and other human activity. For example, current projections of change in rainfall are small compared with natural variability (meaning that detection or confirmation of climate change impacts to rainfall will be extremely difficult).

All climate change impact assessments are based on an assumed greenhouse gas emission scenario. For current purposes, we have mostly relied on published data derived from AR4 for the A1F1 emission scenario (IPCC, 2007a). The A1 family of emission scenarios assumes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and rapid introduction of new and more efficient technologies. The A1F1 scenario assumes fossil-intensive energy sources and results in the highest greenhouse gas emissions of any of the current set of IPCC emission scenarios. Under A1F1, the globally averaged CO2 concentration is projected to increase from the current value of about 380 ppm to about 570 ppm by 2050 and 980 ppm by 2100. Under A1F1, the global mean annual temperature is projected to increase by about 4°C by 2100 (IPCC-TGICA, 2007). Climate change impacts to hydro-meteorological parameters on the main rivers of Bangladesh would be higher under the A1F1 scenario than under any other current IPCC emission scenario. IPCC projections of change in area-averaged seasonal surface air temperatures over South Asia under the A1F1 emissions scenario are summarized in Table 3-6.

Table 3-6: Projected change in surface air temperature for South Asia (IPCC, 2007b, Table 10.5) Surface Air Temperature Change (°C ) Relative to 1961-1990 for A1F1 Scenario Region Season 2010-2039 2040-2069 2070-2099 DJF 1.2 3.2 5.4 South Asia MAM 1.2 3.0 5.2 (5N-30N; 65E-100E) JJA 0.5 1.3 3.1 SON 0.8 2.4 4.2

Projections of climate change impacts on precipitation show considerable variability and are highly uncertain. IPCC projections of change in area-averaged seasonal precipitation over South Asia under the A1F1 emissions scenario are summarized in Table 3-7.

Page 20 September 2013 Hydrology and Flood Modelling

Table 3-7: Projected change in precipitation for South Asia (from IPCC, 2007b, Table 10.5) Precipitation Change (%) Relative to 1961-1990 for A1F1 Scenario Region Season 2010-2039 2040-2069 2070-2099 DJF -3 0 -16 South Asia MAM 7 26 31 (5N-30N; 65E-100E) JJA 5 13 26 SON 1 8 26

Under climate change, the precipitation regime is expected to exhibit more extreme wet and dry periods and greater year-to-year variability as opposed to a simple shift in seasonal precipitation amounts. Note also that while AR4 (IPCC, 2007b) cites research projecting increases in the occurrence of intense precipitation events in South Asia, there is no consensus on the magnitude of such increases. In the absence of consensus, changes in seasonal amounts are often assumed to translate into equivalent changes in extreme precipitation amounts. AR4 (IPCC, 2007b) cites research projecting increases in the occurrence of intense precipitation events in South Asia. There is, however, no consensus on the magnitude of such increases, and in the absence of consensus, changes in seasonal amounts are often assumed to translate into equivalent changes in extreme precipitation amounts. Relying on this assumption, the Institute of Water Modeling (IWM, 2008) undertook a study of the impacts of climate change on monsoon flooding in Bangladesh assuming a 13% increase in precipitation over the Ganges-Brahmaputra-Meghna (GBM) basin to estimate flood impacts by 2040 under the A1F1 emissions scenario. The approach adopted by IWM was to develop a MIKE BASIN model of the GBM catchment, calibrated to measured discharges of the Ganges River at Hardinge Bridge and the Brahmaputra River at Bahadurabad using daily time series of satellite-measured rainfall (as opposed to conventional ground-measured rainfall) as input. MIKE BASIN model simulations were performed for the period 2004 through 2007. The high flow period of 2004 was defined as a “moderately large” flood,and 2005 as an “average” flood. MIKE BASIN simulations for those two years were repeated with rainfall increased by 13% and with the resulting simulated flows then used as input to a detailed hydrodynamic model to estimate the impacts of climate change on flood levels and inundation in selected districts of Bangladesh. Results from IWM (2008) on change in maximum discharge due to increased precipitation are summarized in Table 3-8.

Table 3-8: Simulated change in maximum discharge with increased precipitation (IWM, 2008) Station Flood Maximum Discharge (m3/s) Event Observed With 13% increase in % increase in maximum discharge precipitation Bahadurabad 2004 85,921 99,036 15 Bahadurabad 2005 67,060 71,064 6 HardingeBridge 2005 44,278 54,234 22

More recent studies (Dobler et al, 2011, Ghosh and Dutta, 2012) show different results, but also suggest that increasing runoff may occur in the monsoon season under assumed increases in temperature and precipitation. Therefore, the potential benefits of providing flood mitigation are expected to increase in the future as the magnitude and duration of peak discharges increase. However, the uncertainties associated with estimating changes to peak discharges appear to be so large that it is not possible to provide quantitative estimates at this time.

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4 Flood Model Development

4.1 Method of Approach The Institute of Water Modelling (IWM) was retained by NHC to carry out the hydraulic modelling. NHC carried out independent quality control on the work and reviewed key components such as selection of flood years for the simulations, assessment of model accuracy, representation of projects and GIS analysis of flood depths and flood extent. IWM provided NHC with Mike11 digital results files from the models in the model domain of the project area, but did not provide actual model input data or some details of the model formulation. IWM also provided digital GIS files of flood depths and floodplain topography to assist in the project evaluation.

The project area includes portions of IWM’s NorthWest and NorthCentral Regional Mike11 models. The models include Mike11 to represent channel hydrodynamics and NAM to represent local rainfall accumulation in interior flood basins.

The existing North West and North Central Regional Models were run for three monsoon season flood hydrographs:

• 1998, representing an extreme flood year (return period > 50 years). • 2003: representing an average flood year (return period between 2-5 years). • 2007: representing a large flood year (approximately 20 years).

The hydraulic modelling analysis was based only on the existing model with available channel topography and hydrometric information. No new surveys or data was collected for model calibration or validation. Additional surveys of the river and the floodplain and additional model development should be carried out during the detailed design phase.

The floodplain of the Mike11 models was represented by a network of distributary channels and floodplain conveyance channels. Water levels were computed at each node in the network. The levels were then interpolated over the extent of the floodplain on a regular grid using a digital elevation model (DEM). The ground elevation was also estimated at each grid point using IWM’s 300 m DEM. The water depth at each grid point was computed by subtracting the ground elevation from the water surface elevation. Flooded areas were estimated from the model using the following flood depth categories:

• F0: < 0.3 m • F1: 0.3 m to 0.6 m • F2: 0.6 m to 1.8 m • F3: 1.8 m to 3.6 m • F4: > 3.6 m

These categories are similar to the MPO (1987) flood depth classes that were developed for assessing flooding and agriculture. However, MPO’s values were derived from interviews with local people, rather than flood modelling, and specifically represent the flood depths over 3 day’s duration for a flood that occurs every other year (2 year return period). Flood depth maps and digital flood depth data (300 m grid) were supplied by IWM. These results did not account for different land use (settlements, agricultural land, ponds and permanent water bodies). Also the results included some areas outside of the proposed embankment boundaries. Therefore, a GIS analysis was required to analyze the raw

Page 22 September 2013 Hydrology and Flood Modelling results in order to use them for assessing the project impacts on land-use and project benefits. Furthermore, a careful review of the predicted flooding extent with actual observed flooding extent using RADARSAT imagery indicated the initial model results tended to over-estimate the flooding extent. This issue was not due to over-estimation of water levels by the models but was more related to the inherent difficulty of interpolating flooding patterns using a relatively coarse (300 m gridded digital elevation model) representation of the floodplain (further discussion of this is described in Section 4.3). The available digital elevation model of the floodplain cannot represent the complex pattern of roads, embankments and settlements that dissect the floodplain into a number of compartments. In effect, the model represents potential flooding if these features were not present. This problem is greatest for low and average flood years. At more extreme floods, the predicted flood extent tends to be more representative since these local embankments and compartments will be overtopped. Therefore, it was decided to use the RADARSAT imagery to refine the model flood depth predictions. This analysis involved mapping the observed flooding extent in GIS from the RADARSAT images, superimposing these extents on the land-use maps and flood depth maps and then re- calculating the flood depth statistics in the project boundaries. This hybrid approach, using both hydraulic modelling and remote sensing information is considered to provide more reliable results than either method could provide alone.

Simulations were made first fro “without project” (WO) conditions and then repeated for “with project” (WP) conditions. Several different WP scenarios were simulated. NHC developed two embankment scenario at JRB-1 and three embankment scenarios at JLB-2 and PLB-2. The effect of the projects on reducing flooding extent and flood depths was determined by comparing the WP and WO water levels in the project area.

4.2 Regional Models and Model Boundaries Figure 4-1 shows the hydrological regions of Bangladesh as well as the extent of IWM’s regional flood models. The North Central (NC) and North West (NW) models were used for the present analysis. The models use the one dimensional Mike11 hydrodynamic model developed by the Danish Hydraulics Institute (DHI) to simulate water levels and discharges along a branched network of river and floodplain cross sections. Input data at the upstream boundaries consists of discharge hydrographs at each major river or tributary. A hydrograph of water levels is provided at the downstream boundary. Rainfall was input at specified locations and was processed using the NAM rainfall-runoff program to estimate local discharge inflows.

Figure 4-1: Hydrological regions

Page 23 PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program

Figure 4-2 shows the extent of the North West Regional model, including the main river channels and tributaries. The NW regional model represents over240 km of the Jamuna River. The model starts about 48 km upstream of the Teesta River confluence and extends down to its confluence with the Ganges River. Approximately 130 km of the lower Ganges River is represented. The right bank floodplain and major right bank tributaries are also modelled, including the Teesta River, Atrai River and Karatoya River.

Figure 4-3 shows the extent of the North Central Regional model, including the main river channels and tributaries. The NC regional model represents about 260 km of the Jamuna and Padma River. The model includes the Old Brahmaputra River and Lakhya River as well as the major left bank distributaries including the Old Dhaleswari River, Dhaleswari River and GhiorKhal. The left bank floodplain and its distributary channels are also represented.

The models were first developed in the 1990’s as part of the Flood Action Plan (FAP). The cross sections in the main channels have been periodically updated. It was reported that most date to around 2004.

4.3 Model Limitations and Accuracy Limitations of 1D Modelling A one-dimensional analysis assumes one principal flow direction in the channel’s stream-wise direction. Secondary flows in the vertical or cross-stream directions are ignored. The assumption of one dimensional flow is not serious limitation in confined, relatively straight channels. However, the flow paths and streamlines on floodplains can be complex and may vary appreciably over the flood hydrograph. In this case the model schematic may not be appropriate for all flow conditions. Other common problems involve representing floodplain conveyance correctly, since local features such as depressions or basins may be ineffective in many flow conditions if the water movement is restricted by roads or embankments.

Limitations of Boundary Conditions The published discharges are estimated by BWDB from observed water levels and periodic measurements of discharge that are used to establish a stage-discharge rating curves. Figure 4-4 shows rating curves at Bahadurabad on Jamuna River and Mawa on the Padma River. The scatter on the Jamuna River rating curves is approximately ±5000 m3/s. Furthermore, the highest measured discharges since 2006 were typically only about 60% of the published peak discharge that occurred in this time period. Therefore, the rating curves had to be extrapolated to estimate the peak discharges. Both of these factors introduce considerable uncertainty in the published discharge records. It is likely that peak discharges are accurate to within ± 10 to 15%.

Page 24 September 2013 Hydrology and Flood Modelling

Figure 4-2: North west regional model

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Figure 4-3: North Central regional model

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22 Jamuna River at Bahadurabad (46.9) 21

20

19 2006 18 2007 17 2008 16 2010

Water Level (m PWD) (m Level Water 15 2011

14

13

12 0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000 3 Discharge (m /s)

Padma River at Mawa (93.5L) 8

7

6 1994 1995 1996 5 1997 1998 1999 4 2000 2001 2003 3 2004 Water Level (m PWD) (m Level Water 2005 2006 2 2008

1

0 0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 Discharge (m3/s)

Figure 4-4: Stage-discharge rating curve at Bahadurabad on Jamuna River and Mawa on Padma River

Limitations of Floodplain Topography Estimating flood depths on the floodplain requires reliable predictions of the water levels and reliable information on the floodplain topography. Unfortunately, the floodplain topography that is available in the project area is based mainly on surveys from the early 1960’s. This topography is represented in a digital elevation model (DEM) using a 300 m grid. The resolution of the grid is too low to represent many important floodplain features such as roads, settlement platforms or embankments which can affect flooding extent and flooding patterns. Furthermore, significant topographic changes have occurred in many areas of the floodplain due to sedimentation or erosion. Terrestrial surveys were made at a few locations within JRB-1, JLB-2 and PLB-1 to check the reliability of the DEM. Figure 4-5 shows the results of this comparison. The DEM represents the overall trend of the surface reasonably well in most locations but is up to 4 m too low in local areas, probably due to sedimentation effects. The DEM also does not capture any local features such as roads or embankments which compartmentalize the floodplain and can have a major impact on flooding extent. Based on these comparisons, the 300 m grid is inadequate for accurate representation of actual floodplain inundation extents and flood depths.

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In order to overcome this limitation, we incorporated additional information on flooding extent using RADARSAT imagery collected during the period 1998 to 2007 (refer to section 3.5.3 and table 3.5).

Figure 4-5: Comparison of DEM and terrestrial surveys of floodplain topography

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4.4 Inflow Conditions Three monsoon season flood hydrographs were simulated in the models:

• 1998, representing an extreme flood year (return period > 50 years). • 2003: representing an average flood year (return period between 2-5 years) • 2007: representing a large flood year (approximately 20 years).

Figure 4-6 shows published daily discharges on the Jamuna River at Bahadurabad and Ganges River at Hardinge Bridge during the monsoon season. There were missing data during the monsoon period in all years, which required interpolating values using comparisons with water levels recorded at other nearby gauges.

120000 1998 Ganges 100000 Jamuna 80000

60000

40000 Discharge (m3/s)

20000

0 1-Apr 1-May 31-May 30-Jun 30-Jul 29-Aug 28-Sep 28-Oct 27-Nov 27-Dec

120000 2003 Ganges 100000 Jamuna 80000

60000

40000 Discharge (m3/s)

20000

0 1-Apr 1-May 31-May 30-Jun 30-Jul 29-Aug 28-Sep 28-Oct 27-Nov 27-Dec

Figure 4-6: Discharges on Jamuna River at Bahadurabad in 1998 and 2003 Figure 4-7 shows the water levels recorded at six BWDB gauging stations in 1998, 2003 and 2007. Although 1998 had much higher discharges recorded at Bahadurabad, this did not always translate into higher water levels along the main river and on the floodplain. For example, in 2007 the peak water level at Sirajganj exceeded the 1998 flood peak. In 2003 the peak water level at Baghabari exceeded the peak level in 1998.

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16 12 Tilli (68) Sirajganj (49) 1998 15 11 14 2003 10 13 2007 9 12

11 8 1998 Water Level (m PWD) (m Level Water Water Level (m PWD) (m Level Water 10 2003 7 9 2007 6 8 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov

13 11 Taraghat (137A) Baghabari (151) 1998 12 10 2003 11

9 2007 10

9 8 1998

Water Level (m PWD) (m Level Water 8 Water Level (m PWD) (m Level Water 7 2003 7 2007 6 6 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov

11 Aricha (50.6) 14 Porabari (50) 1998 13 10 12 2003 11 9 2007 10

8 9 1998 Water Level (m PWD) (m Level Water Water Level (m PWD) (m Level Water 8 7 2003 7 2007 6 6 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov 20-Jun 10-Jul 30-Jul 19-Aug 8-Sep 28-Sep 18-Oct 7-Nov

Figure 4-7: Observed water levels at BWDB gauges in 1998, 2003 and 2007

4.5 Model Validation The regional models have been developed over the last 25 years and have been updated and refined periodically as new information comes available. During this period the main rivers have undergone substantial changes in planform geometry and cross section properties. After a review of the available data it was decided that the 2007 flood year was suitable for validating the model simulations. This hydrograph was chosen because the main river cross sections in the model are believed to be reasonably representative of 2007 conditions. A second run was made using the 1998 inflow conditions. The 1998 flood simulation was representative of an extreme flood. Comparing simulated and observed water levels in 1998 is problematic because the channel characteristics in 1998 may have been substantially different than in 2007 and there are doubts about the correlation of discharge and water level at Bahadurabad. The differences between and observed water levels in 1998 provide some insight on the expected variability in water levels induced by these morphological variations.

Figure 4-8and Figure 4-9 show simulated and observed water levels on the Jamuna River at Sirajganj (gauge 49) and Aricha (gauge 50.3).The location of the hydrometric stations is shown on Figure 3-1.The models under-predicted the rising and falling limbs of the hydrogaph by up to 1 m to but matched the flood peak to within 0.3 m. Figure 4-10 and Figure 4-11 show the same on the Padma River at Baruria

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(gauge 91.9L) and Kaliganga River at Taraghat (gauge 137A). On the Padma River, the model over- predicted the observed water levels systematically by up to 0.5 m and by about 0.3 m at the flood peak. Therefore, although the rising and falling limbs are not simulated very closely, the peak levels matched reasonably well. Figure 4-12 and Figure 4-13 show the simulated and observed water levels in 1998 on the Jamuna River at Sirajganj and Aricha. The simulated water levels exceeded the observed levels by up to 1 m. There is some uncertainty in the published discharges in 1998-although the magnitude of the discharge at Bahadurabad is the highest on record, the peak water level was not as extreme.

Figure 4-14 shows the simulated and observed water levels in 1998 on the Padma River at Baruria. The simulated water levels exceeded the observed levels by up to 0.6 m. Figure 4-15 shows the same on the Kaliganga River at Taraghat. The simulated water levels exceeded the observed levels by up to 1 m during the rising limb of the flood and by about 0.3 m at the peak of the flood.

These results illustrate that the overall model accuracy is expected to be in the range of ±0.3 to ±0.5 m.

Figure 4-8: Comparison of simulated and observed water levels in 2007 on Jamuna River at Sirajganj (162500)

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Figure 4-9: Comparison of simulated and observed water levels in 2007 on Jamuna River at Aricha (235650)

Figure 4-10: Comparison of simulated and observed water levels in 2007 on Padma River at Baruria (12000)

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Figure 4-11: Comparison of simulated and observed water levels in 2007 on Kaliganga River at Taraghat

Figure 4-12:Comparison of simulated and observed water levels in 1998 on Jamuna River at Sirajganj

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Figure 4-13: Comparison of simulated and observed water levels in 1998 on Jamuna River at Mathura

Figure 4-14: Comparison of simulated and observed water levels in 1998 on Padma River at Baruria (12000)

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Figure 4-15: Comparison of simulated and observed water level in 1998 on Kaliganga River at Taraghat

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5 JRB-1 Embankment Project

5.1 Project Area General Features Figure 5-1shows the JRB-1 project area. The project area is defined by the Jamuna River on the east, the Hurashagar (Baral) River on the south and by the upazila boundary on the west. The total gross area, based on the upazila boundary is 58,209 ha. For the flood depth assessment this gross project area was adjusted to eliminate river bank and channel areas lying outside of the existing and proposed embankments or lying on the south side of the Hurashagar River. The adjusted project area inside the embankments was estimated to be 41,067 ha. The project area consists of a mixture of land types, including settlements, raised village platforms, roads, treed areas, ponds, water bodies and agricultural land. Table 5-1 shows the approximate breakdown of the different land types inside the embanked area (data supplied by CEGIS).

Table 5-1: Land types in JRB-1 Land Type Area (ha) Per Cent of Total (%) Gross Project Area 58,209 Adjusted Project Area protected by Embankment (APA) 41,067 100 Settlement Areas 8,855 22.0 Ponds, streams, other non-agricultural land 2,213 5.5 Net Cultivable Area (NCA) 30,000 73

Figure 5-2 shows the distribution of land area by elevation in JRB-1. The DEM indicates 50% of the land is below elevation 9 m and virtually all of the land is below elevation 12 m.

Proposed JRB-1 Project This project incorporates three BWDB priority projects: (i) riverbank protection along the right bank of the bifurcating channel from upstream of Enayetpur towards Kaijuri, (ii) riverbank protection upstream of the Hurasagar, and (iii) reconstruction of the BRE and Hurashagar FCD embankment from Kaijuri to Shahjadpur. During Tranche-1, the main work will include:

• Reconstruction of the BRE for some 12.5 km along the Jamuna incorporating a road and rehabilitation of the embankment along Hurashagar/Baral (9.5 km). • 1 km of riverbank protection downstream of the existing protection towards the Hurashagar/Baral. • Different regulators for water management (including rehabilitation of existing structures). • Provision for the adaptation of existing work and for immediate stabilization of the Enayetpur spur.

This work will reinstate the original BRE and allow starting reaping the benefits from the Hurashagar FCD project that was destroyed in the 1990’s. Flood maps from 1987 when the Hurashagar FCD project was still functioning showed considerably less flooding than in 1998 after the project was not functioning.

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Figure 5-1: Sub-project JRB-1 land elevation and infrastructure

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14 JRB1 12

10

8

6 Elevation (m) 4

2

0 0% 20% 40% 60% 80% 100% Per Cent Less Than

Figure 5-2: Land area-elevation relation in JRB-1

5.2 JRB-1 Without Project Flood Conditions Figure 5-3 shows the predicted flood inundation for the average 2003 inflow conditions, based on the IWM model predictions. The model results included portions of the floodplain and chars lying outside of the adjusted project boundaries described in Section 5.1.1. The model indicates most of the southern project area that is not embanked is flooded to a depth of > 0.9 m during average flood years. The shallowest flooding is located in the northern section of the project area which is protected by the existing embankment. The model results indicated 74% of the project area inside the existing and proposed embankments would be flooded at 2003 discharge conditions. When settlement areas were superimposed on the flood depth map it was found that virtually all of the agricultural land was flooded, most of the flood-free land corresponded to settlements, roads and raised platforms. Table 5-2 summarizes the estimated flood areas for each of the MPO flood depth classes based on the initial IWM results.

Table 5-2: Initial flood depth estimates using IWM results inside JRB-1 embankments (without project) Areas (in ha) 1998 2003 2007

Adjusted project area (APA) 41,067 41,067 41,067 Settlement area 8,855 8,855 8,855 Other non-agricultural land 2,213 2,213 2,213 Net cultivable area (NCA) 30,000 30,000 30,000 F0 land (including flood free) 1 5 0 F1 land 12 103 20 F2 land 31 2,315 107 F3 land 2,696 8,985 7,047 F4 land 27,260 18,592 22,826 Total flooded cultivable area 30,000 30,000 30,000 Per cent of NCA flooded (%) 100% 100% 100% Per cent of APA flooded (%) 74% 74% 74%

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The model indicates most of the flooded areas are F3 and F4 land types.

Figure 5-3: JRB-1 Flood depths, IWM results, without project, 2003 flood condition

Comparison of RADARSAT imagery from 1998 and 2007 showed the IWM model over-predicted the extent of inundation. The flood-free areas delineated from the RADARSAT imagery were overlain on the IWM generated flood depths and the flood depth statistics were then re-calculated as outlined in Section 4.1. The map shows noticeably less inundation in the northern portion of the project which is presently protected by the BRE.

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RADARSAT imagery from July 5, 2003 showed about 47% of the gross project area was inundated. The water level on this date was slightly lower (0.3 to 0.6 m) than the flood peak that occurred in 2003. Imagery from July 23, 2004 was taken at water levels that coincided closely to the peak level in 2003. This image indicated about 58.6% of the project area was inundated. The generalized flood inundation relations in Figure 3-7 indicate that approximately 60% of the gross area in JRB-1 would be inundated under 2003 flood conditions. To provide conditions that more closely resembled a 2-year flood (51%), the July 5, 2003 imagery was used to adjust the 2003 IWM model results. Figure 5-4 shows the adjusted flood depth map for 2003. The RADARSAT imagery from August 7 2007 coincided closely to the peak of the 2007 flood. Imagery from September 17, 1998 was used for adjusting the 1998 flood model results.

Figure 5-4: JRB-1 Flood depths adjusted using RADARSAT imagery without project,2003 flood condition Table 5-3 summarizes the flooded areas by land type for without project conditions adjusted using the available RADARSAT imagery.

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Table 5-3: Adjusted flooded areas inside JRB-1 embankments (without project) Areas (in ha) 1998 2003 2007

Adjusted project area (APA) 41,067 41,067 41,067 Settlement area 8,855 8,855 8,855 Other non-agricultural land 2,213 2,213 2,213 Net cultivable area (NCA) 30,000 30,000 30,000 F0 land (including flood free) 2,737 12,673 8,164 F1 land 36 117 135 F2 land 2,196 2,313 2,880 F3 land 15,516 12,024 12,105 F4 land 10,359 3,825 7,461 Total flooded cultivable area 28,107 18,288 22,581 Per cent of NCA flooded (%) 91% 59% 73% Per cent of APA flooded (%) 74% 50% 60%

5.3 Scenario 1, Tranche-1 and 2 JRB-1: Closing the BRE and restoring the Hurashagar FCD Project Figure 5-5 shows the predicted extent of flooding using the IWM results with the new embankment for 2003 flood conditions. The RADARSAT imagery was also applied to the “with project” IWM flood depths. Figure 5-6 shows the adjusted flood depth map for the 2003 flood condition. Table 5-4 summarizes the distribution of areas inside the JRB-1 project by flood depth and land type (based on the RADARSAT adjusted flood extents). With the new embankment, the total flooded area (% of APA) for 1998, 2003 and 2007 flood scenarios were 61%, 35% and 48%% respectively. The corresponding values for the without project condition are 74%, 50% and 60%.

Table 5-4: Adjusted flooded areas inside JRB-1 embankments (with project condition) Areas (in ha) 1998 2003 2007

Adjusted project area (APA) 41,067 41,067 41,067 Settlement area 8,855 8,855 8,855 Other non-agricultural land 2,213 2,213 2,213 Net cultivable area (NCA) 30,000 30,000 30,000 F0 land (including flood free) 9,073 19,315 13,771 F1 land 2,322 1,116 2,187 F2 land 4,968 3,177 4,554 F3 land 10,242 5,319 6,921 F4 land 4,374 2,178 3,492 Total flooded cultivable area 22,824 12,051 17,568 Per cent of NCA flooded (%) 74% 39% 57% Per cent of APA flooded (%) 61% 35% 48%

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Figure 5-5: Flood depths using initial IWM analysis, “with project”,2003 flood condition

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Figure 5-6: Flood depths adjusted using RADARSAT, with project, 2003 flood condition

Figure 5-7 shows contours of water level differences (with project – without project) over the project area. Extending the embankment produces a large increase in the flood-free area immediately behind the embankment, particularly in the southern half of the project where flood depths were reduced by up to 3 m. The water level reduction extended beyond the western boundaries of the JRB-1 project area (the west side of the project is a political boundary and does not correspond to a hydrological or topographic feature).

Table 5-5 summarizes the estimated changes in cultivable land areas due to the embankment inside JRB- 1 for the 1998, 2003 and 2007 flood simulations.

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Figure 5-7: Effect of project on water levels adjusted using RADARSAT inside JRB-1, 2003 flood condition

Table 5-5: Effect of JRB-1 project on flooded areas and flood depths Change in area (ha) 1998 2003 2007 F0 land (including flood free) +6,336 +6,642 +5,607 F1 land +2,286 +999 +2,052 F2 land +2,772 +864 +1,674 F3 land -5,274 -6,705 -5,184 F4 land -5,985 -1,647 -3,969 Total flooded cultivable area -5,283 -6,237 -5,013 Per cent of NCA flooded (%) -17% -20% -17% Per cent of APA flooded (%) -13% -15% -12%

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Figure 5-8 shows a generalize relation between shallow flooding area (F0+F1) and flood return period for both without project and with project conditions.

25000

20000

F0+F1 Area With Project

15000

F0+F1 Area Without Project

10000 Area in ha

5000

0 1.003 1.05 1.25 2 5 10 20 50 100 200 500 Return Period (years)

Figure 5-8: F0 +F1 land area versus flood return period for without project and with project conditions at JRB-1

5.4 Scenario 2 JRB-1: Tranche-2 Securing the BRE (Effect of Embankment Breach at Enayetpur) Bank erosion has caused the existing embankment to fail on numerous occasions in the past. IWM simulated a 500 m long breach of the embankment near Enayetpur; the only location that remains at the risk from erosion at JRB-1.The remaining section of the existing and proposed new embankment were assumed to remain intact. The breach simulation was intended primarily for illustrative purposes; other breach configurations and geometries would produce different results. Figure 5-9 shows the inundation extent with the breach for a 2003 flood condition, based on IWM’s unadjusted flood maps, (without adjustment for settlement areas, boundaries, RADARSAT correction). This figure should be only compared with Figure 5-3 and Figure 5-5 (since these are also unadjusted maps). Comparing the breach results to the “without project” results (Figure 5-3), shows the hypothetical breach significantly increased the deeply flooded area in the southern portion of the project (worsening flood conditions compared to the WO Project scenario). The area of shallow flooding (F0 land) was greatly reduced by the breach but still remained slightly greater than in the WO project condition. Comparing the breach to the “with project” condition, the breach increased the F3 and F4 land and correspondingly reduced the F0 and F1 land. The breach greatly reduced the effectiveness of the project.

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Figure 5-9: Effect of embankment breach on flood depth inside project area

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A preliminary quantitative estimate of the effects of a breach on agricultural land-types was made by adjusting the initial IWM results for settlement areas and roads, excluding areas outside of the embankment project boundaries and attempting to incorporate the RADARSAT corrections that were applied in Section 5.3. Applying the same RADARSAT correction to the breach scenario is not correct since the water levels in some areas of the breach scenario were greater than in the “without project” condition. Table 5-6 summarizes the estimated land area types. The results are expected to over-predict F0 and F1 land areas.

Table 5-6: Adjusted flooded areas inside JRB-1 embankments (with breach scenario) Areas (in ha) 2003 2007

Adjusted project area (APA) 41,067 41,067 Settlement area 8,855 8,855 Other non-agricultural land 2,213 2,213 Net cultivable area (NCA) 30,000 30,000 F0 land (including flood free) 12,654 8,145 F1 land 198 612 F2 land 1,881 3,177 F3 land 6,858 7,164 F4 land 7,857 10,197 Total flooded cultivable area 16,803 21,150 Per cent of NCA flooded (%) 57 72 Per cent of APA flooded (%) 46 57

Table 5-7 summarizes the change in land areas (breach condition – with project condition) based on the GIS adjustments. These results show that the breach decreased the F0 and F1 land and increased the F3 and F4 land types.

Table 5-7: Effect of breach at JRB-1 (breach – with project) Change in area (ha) 2003 2007 F0 land (including flood free) -6,661 -5,626 F1 land -918 -1,575 F2 land -1,296 -1,377 F3 land +1,539 +243 F4 land +5,679 +6,705 Total flooded cultivable area +4,752 +3,582 Per cent of NCA flooded (%) +18.3% +15.4% Per cent of APA flooded (%) +12% +9%

Another way to interpret these results is to compare them to the project effects summarized in Table 5- 5. A lower bound estimate of the breach’s impact is that it eliminated all of the project’s gain in F0 and F1 land and all of the project’s reduction in F3 and F4 land.

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6 JLB-2 and PLB-1Projects

6.1 Project Area Physical Features JLB-2 and PLB-1 are similar in floodplain characteristic and flooding and cover the neighbouring area on the left bank of lower Jamuna and upper Padma. Figure 6-1 and Figure 6-2 show the boundaries of sub- project JLB-2 and PLB-1, respectively. Sub-project JLB-2 is located on the left bank of the Jamuna River, while PLB-1 is located on the left bank of the Padma River. The land-side of the projectsfollow political boundaries rather than topographic or hydrological features (although some portions follow distributary channels). JLB-2 starts near the Dhaleswariofftake and extends southwards and includesother distributary channels such as GhiorKhal and Kaliganga River. The western boundary extends across the width of the Jamuna River and includes even floodplain land adjacent to JRB-1. The gross area of the JLB-2 boundary is 119,226 ha. Figure 6-3 shows the distribution of floodplain land by elevation in JLB-2. Approximately 50% of the land is below elevation 7.2 m PWD and 90% is below elevation 9.1 m.

Sub-project PLB-1 includes a portion of the Dhaleswari River, starting in the north near Manikganj and extending down to near Harirampur. The western boundary extends into the middle of the Jamuna River. The gross area is 66,895 ha. Approximately 50% of the floodplain is below elevation 5.0 m PWD and 90% is below elevation 7 m PWD (Figure 6-3).

Areas of the sub-projects that include the Jamuna/Padma River channels and chars have been excluded from the analysis of flood depths on the floodplain. This adjustment decreases the combined area of JLB-2 and PLB-1 from 186,121 ha to 135,552 ha. Table 6-1 summarizes the distribution of land types in the adjusted project area (APA).

Table 6-1 : Land types in combined JLB-2 and PLB-1 floodplain areas Land Type Area (ha) Per Cent of Total Gross Project Area 186,121 Adjusted Project Area (APA) 135,552 100% Settlement Area 32,836 24% Ponds, streams, etc 3,778 3% Net Cultivable Area (NCA) 98,908 73%

Project Description The JLB-2 project area includes three BWDB priority flood control projects: (i) riverbank protection along vulnerable reaches with a view to work towards more systematic riverbank stabilization, (ii) strengthening of the existing 12 km long embankment from Aricha to Zionpur via Zaffarganj and extension to the Dhaleswari, (iii) provision of defined Dhaleswari and GhiorKhalofftake for dry and flood season flow including navigation.

The PLB-1 project area includes two BWDB priority projects: (i) riverbank protection along the left bank upstream and downstream of Harirampur to protect the large meander bend from forming again, and (ii) reconstruction of the Dhaka Southwest Project embankment from Paturia to Harirampur.

Two different flood mitigation alternatives were simulated in the flood models (Figure 6-4):

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• Alternative 1: continuous embankment along left bank, with control structures installed on three main distributaries (Dhaleswari River and GhiorKhal). • Alternative 2: two polders between Dhaleswari and GhiorKhal channels, with regulators installed at key points. This project is based on current BWDB plans. The total area of the two polders is 65,113 ha.

Figure 6-1: JLB-2 project area

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Figure 6-2: PLB-1 project area

Figure 6-3: Land area elevation relation in JLB-2 and PLB-1

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The control structures shown on the continuous embankment project in Alternative 1 would need to pass flow during low and moderate discharges, but would restrict the amount of flow during flood conditions. The structures were represented schematically as weirs in the models. Design of such structures would be a major undertaking and their impact on sedimentation at the distributary offtakes and downstream impacts to the distribution of flow in these channels would have to be assessed carefully during the design of future investment tranches. This analysis was outside the scope of this present modelling study.

Alternative 1: continuous embankment with Alternative 2: two polders with control structures at distributary offtakes regulators

Figure 6-4: Assumed “with project” scenarios at JLB-2 and PLB-1

The polder scheme is based on a BWDB proposal and would not require closure structures on the main distributary channels. However, regulators would still be required at major khals to allow for interior drainage. The regulators varied from 2 vent to 10 vent structures (1.5 m by 1.8 m or 2.5 m by 3.0 m) and were represented in the model as gated culverts.

The impact and effectiveness of individual flood embankment projects on the left bank of the Jamuna/Padma River cannot be assessed in isolation from one another, since the flooding extent at a project will depend on whether flooding can occur upstream or downstream from it. For example, the extent of flooding in PLB-1 will depend on whether embankments at JLB-2 are in-place.

6.2 “Without Project” Scenarios The existing left bank embankment along the Padma and Jamuna River is discontinuous and variable in height and integrity. It also contains a number of uncontrolled openings due to unregulated khals, culverts as well as low spots and breaches. Therefore, the initial without project scenario assumed the left bank was not embanked. The unadjusted results from the IWM model were used to represent this

Page 51 PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program scenario. Subsequently, the RADARSAT imagery was used to represent the partial confinement effect from the existing embankment.

Initial base results were provided by IWM Figure 6-5 shows the predicted flooding extent for a 2003 flood condition. Table 6-2 summarizes the initial estimate of the distribution of flooding by land type from this analysis. These results indicate that virtually all of the land was inundated during all three flood conditions and F0 land accounted for only about 1% of the total in each year, while most land consists of F2 and F3 land under a moderate flood (2003) and F3 land during more severe floods. These results represent the potential flooding that could occur if all existing local embankments and roads did not affect the movement of water on the floodplain.

Table 6-2: Initial flood depth distribution, IWM results for JLB-2 and PLB-1 (WO project) Areas (in ha) 1998 2003 2007 Area % of Area % of Area % of (ha) Total (ha) Total (ha) Total F0 land (including 810 1% 1,265 1% 1,071 1% flood free) F1 land 3,402 3% 12,649 10% 7,200 6% F2 land 19,071 15% 50,598 40% 35,631 28% F3 land 90,333 71% 53,127 42% 69,713 55% F4 land 12,879 10% 8,855 7% 12,879 10% Total flooded 126,495 100% 126,494 100% 126,494 100% area

A review of the RADARSAT imagery indicated that less than 50 % of the area was actually inundated in the 2003 flood and about 70% was inundated in 1998. The flood extent was limited by existing local embankments and roads which could not be fully represented in IWM’s 300 m digital elevation model of the floodplain. The RADARSAT imagery was used to adjust the distribution of flood depths to represent the existing state of the floodplain. Figure 6-6 shows the adjusted flood extent and distribution of flood depths. Table 6-3 summarizes the adjusted areas by land type.

Table 6-3: Adjusted flood depth distribution for JLB-2 and PLB-1 (WO project) Areas (in ha) 1998 2003 2007 Adjusted project area (APA) 135,552 135,552 135,552 Settlement area 32,836 32,836 32,836 Other non-agricultural land 3,778 3,778 3,778 Net cultivable area (NCA) 98,908 98,908 98,908 F0 land (including flood free) 25313 49082 30335 F1 land 297 1323 1323 F2 land 5661 11,079 12,438 F3 land 49,788 35,532 48,906 F4 land 19,917 4608 7758 Total flooded cultivable area 75,699 52,686 70,632 Per cent of NCA flooded (%) 75% 52% 70% Per cent of APA flooded (%) 59% 42% 55%

These results indicate the flooded cultivable land in the combined JLB-2 and PLB-1 area was 75,402 ha, 51,363 ha and 69,309 ha under 1998, 2003 and 2007 flood conditions respectively. By comparison, the

Page 52 September 2013 Hydrology and Flood Modelling initial model results (Table 6-2) (assuming no confinement from the existing local embankment) indicated 126,494 ha of land was inundated for the three flood scenarios. The difference between the results in Table 6-3 and Table 6-2 is a measure of the partial effectiveness of the existing local embankment. If the existing local embankment was not protected and was allowed to fail by bank erosion then the flood conditions in the project area would be approximately as represented in Table 6- 2. The additional flooded area due to the loss of the existing embankment was estimated as follows:

• 1998 flood: 51,092 ha. • 2003 flood: 75,131 ha. • 2007 flood: 57,185 ha.

Table 6-4and Table 6-5 summarize the flooded conditions in each sub-project.

Table 6-4: Adjusted flood depth distribution for JLB-2 (WO project) Areas (in ha) 1998 2003 2007

Adjusted project area (APA) 82,927 82,927 82,927 Settlement area 18,491 18,491 18,491 Other non-agricultural land 2696 2696 2696 Net cultivable area (NCA) 61,740 61,740 61,740 F0 land (including flood free) 12,667 27,364 16,141 F1 land 171 855 954 F2 land 2655 7668 7767 F3 land 36,279 24,669 33,750 F4 land 12,033 3474 5193 Total flooded cultivable area 51,174 36,783 47,844 Per cent of NCA flooded (%) 80% 57% 75% Per cent of APA flooded (%) 65% 48% 61%

Table 6-5: Adjusted flood depth distribution for PLB-1 (WO project) Areas (in ha) 1998 2003 2007

Adjusted project area (APA) 52,070 52,070 52,070 Settlement area 14,345 14,345 14,345 Other non-agricultural land 1082 1082 1082 Net cultivable area (NCA) 36,643 36,643 36,643 F0 land (including flood free) 12,646 21,718 14,194 F1 land 126 468 369 F2 land 3006 3411 4671 F3 land 13,509 10,863 15,156 F4 land 7884 1134 2565 Total flooded cultivable area 24,525 15,903 22,788 Per cent of NCA flooded (%) 66% 42% 62% Per cent of APA flooded (%) 49% 33% 46%

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Figure 6-5: Base flood depth map by IWM for JLB-2 and PLB-1 WO project (2003 flood)

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Figure 6-6: Distribution of flood depths, adjusted using RADARSAT imagery, without project, 2003 flood

6.3 Effect of Upgrading Short Section of Embankment A simulation was performed to demonstrate the limitations of raising and upgrading a relatively short section of embankment (approximately 42 km from Zionpur to Dohar). This new section was not tied-in to high ground at its upstream and downstream ends. No overtopping was allowed in this embanked section. However, inundation was allowed to occur in the embanked reach from upstream spills or by backwater flooding. The flooding extent is shown in Figure 6-7 and is very similar to the “without project” condition shown in Figure 6-5. Table 6-6 summarizes the unadjusted IWM model results for the 2003 flood condition (without using RADARSAT imagery or land-use adjustment to represent only

Page 55 PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program cultivable areas). The land area values for the initial unadjusted “without project” conditions are also shown for comparison.

Table 6-6: Effect of short embankment on flooded areas, unadjusted IWM results, 2003 flood condition Area (ha) Without Project (WO) With Project (WP) Difference WP-WO F0 land 1992 5,345 +3,353 F1 land 11295 11,673 +378 F2 land 40261 39,987 -274 F3 land 67397 64,512 -2,885 F4 land 5550 4977 -573

The 42 km embankment produced a small benefit by increasing the F0 land by 3353 ha or about 2.5% of the gross area of JLB-2+PLB-1. The results confirm that short, isolated sections of embankment will not produce significant flood mitigation benefits.

Figure 6-7: Shortembankment, 2003 flood condition

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6.4 JLB-2 & PLB-1 Alternative 1: Continuous Embankment and Distributary Closures This alternative assumes a continuous embankment is installed along the left bank of the river along the extent of PLB-1 and JLB-2 with control structures near the offtake of GhiorKhal and on the Dhaleswari River (Figure 6-4). The area remains open to inflow between Jamuna Bridge and Dhaleswari. In this area BWDB plans to build the Pungliofftake under the Buriganga restoration projects (also refer to Annex A, JLB-1). Figure 6-8 shows the estimated flooding extent and depth of flooding for 2003 flood conditions as produced by IWM. Table 6-7 summarizes the initial unadjusted estimates of flooded areas based on the IWM model results).

Table 6-7: Initial flood depth distribution, IWM results for JLB-2 and PLB-1 full embankment (with project) Areas (in ha) 1998 2003 2007 Area (ha) % of Total Area (ha) % of Total Area (ha) % of Total F0 land 54,188 43% 96,425 76% 85,661 68% F1 land 26,649 21% 14,670 12% 16,947 13% F2 land 24,885 20% 10,485 8% 15,354 12% F3 land 18,846 15% 4,572 4% 7,884 6% F4 land 1,926 2% 342 0% 648 1% Total flooded area 126,495 100% 126,494 100% 126,494 100%

The digital flood depth data were processed in GIS to account for the RADARSAT correction, boundary adjustments, land-use, and settlement patterns (Figure 6-9). Table 6-8 summarizes the final adjusted land area values.

Table 6-8: Adjusted flood depth distribution for JLB-2 and PLB-1 full embankment (with project) Areas (in ha) 1998 2003 2007 Adjusted project area (APA) 135,552 135,552 135,552 Settlement area 32,836 32,836 32,836 Other non-agricultural land 3,778 3,778 3,778 Net cultivable area (NCA) 98,908 98,908 98,908 F0 land 51,350 81,356 69,197 F1 land 15,138 8910 11,376 F2 land 17,172 6039 11,106 F3 land 12,555 2331 5094 F4 land 1215 234 522 Total flooded cultivable area 53,271 22,302 34,434 Per cent of NCA flooded (%) 55% 23% 35% Per cent of APA flooded (%) 42% 19% 28%

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Figure 6-8: Alternative 1, IWM results, 2003 flood condition

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Figure 6-9: Alternative 1, RADARSAT adjusted, 2003 flood condition Table 6-9 summarizes the effect of the proposed continuous embankments on land types. Figure 6-10 shows the effect of the project expressed in terms of water level differences (WP-WO).

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Figure 6-10: Change in water levels (WP-WO) due to Alternative 1, 2003 flood condition

Table 6-9: Effect of Alternative 1on flooded areas for combined JLB-2 and PLB-1 sub-projects Change in area due to project (ha) 1998 2003 2007 F0 land (including flood free) +26,037 +32,274 +38,862 F1 land +14,841 +7587 +10,053 F2 land +11,511 -5040 -1332 F3 land -37,233 -33,201 -43,812 F4 land -18,702 -4374 -7236 Change in total flooded NCA -22,428 -30,384 -36,198

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The values in Table 6-9 are for the combined JLB-2 + PLB-1 sub-projects since effective flood protection will depend on completion of both JLB-2 and PLB-1 embankments. In other words, the effectiveness of the embankments along PLB-2 depends on JLB-2 embankments being in-place concurrently and vice versa. However, the changes in land-types were also summarized separately for each sub-project as shown in Table 6-10 (JLB-2) and Table 6-11 (PLB-1).

Table 6-10: Effect of Alternative 1 on flooded areas for JLB-2 sub-project Change in area due to project (ha) 1998 2003 2007 F0 land (including flood free) +18,621 +23,994 +27,711 F1 land +10,575 +5328 +6813 F2 land +8739 -4347 -441 F3 land -29,187 -24,003 -32,031 F4 land -11,970 -3465 -5184 Change in total flooded NCA -16,902 -23,373 -26,766 Note: assumes sub-project PLB-1 is implemented concurrently

Table 6-11: Effect of Alternative 1 on flooded areas for PLB-1 sub-project Change in area due to project (ha) 1998 2003 2007 F0 land (including flood free) +7416 +8280 +11,151 F1 land +4266 +2259 +3240 F2 land +2772 -693 -891 F3 land -8046 -9198 -11,781 F4 land -6732 -909 -2052 Change in total flooded NCA -5526 -7011 -9432 Note: assumes sub-project JLB-2 is implementedconcurrently

6.5 Alternative 2 - Polders The two polders encompass a total area of 65,113 ha; the north polder is entirely inside JLB-2, while the south polder extends into PLB-1.Figure 6-11shows the flooding extent under 2003 flood conditions with the polders constructed.

Table 6-12 summarizes the “with project” flooded areas inside the combined JLB-2 and PLB-1 sub- project boundaries. Table 6-13 and Table 6-14 summarize the flooded areas inside JLB-2 and PLB-1 respectively.

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Figure 6-11: Flood depths with Alternative 2, 2003 flood condition

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Table 6-12: Alternative 2 flood areas with RADARSAT adjustment, JLB-2+PLB-1 combined Areas (in ha) 1998 2003 2007

Area of JLB-2+PLB-1 135,522 135,522 135,522 Settlement area 32,836 32,836 32,836 Other non-agricultural land 3778 3778 3778 Net cultivable area (NCA) 98,908 98,908 98,908 F0 land 30,326 71,735 42,413 F1 land 7263 7713 8703 F2 land 21,573 10,017 22,050 F3 land 35,127 11,133 26,118 F4 land 6651 684 1656 Total flooded cultivable area 73,098 34,902 61,308 Per cent of NCA flooded (%) 72% 34% 61% Per cent of APA flooded (%) 57% 29% 48%

Table 6-13: Alternative 2 flood areas with RADARSAT adjustment,JLB2 only Areas (in ha) 1998 2003 2007

Area of JLB-2 82,927 82,927 82,927 Settlement area 18,491 18,491 18,491 Other non-agricultural land 2696 2696 2696 Net cultivable area (NCA) 61,740 61,740 61,740 F0 land 17,635 47,614 27,967 F1 land 6822 5760 7542 F2 land 16,677 5571 15,066 F3 land 20,016 4482 12,663 F4 land 2358 117 234 Total flooded cultivable area 48,348 20,520 38,151 Per cent of NCA flooded (%) 76% 32% 60% Per cent of APA flooded (%) 62% 28% 49%

Table 6-14: Alternative 2 floodedareas with RADARSAT adjustment, PLB-1 only Areas (in ha) 1998 2003 2007

Area of PLB-1 52,070 52,070 52,070 Settlement area 14,345 14,345 14,345 Other non-agricultural land 1082 1082 1082 Net cultivable area (NCA) 36,643 36,643 36,643 F0 land 12,691 24,121 14,446 F1 land 441 1953 1161 F2 land 4896 4446 6984 F3 land 15,111 6651 13,455 F4 land 4293 567 1422 Total flooded cultivable area 24,750 14,382 23,157 Per cent of NCA flooded (%) 66% 38% 62% Per cent of APA flooded (%) 50% 30% 47%

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Figure 6-12 shows the effect of the polders (WP-W0) on the water depths in JLB-2 and PLB-1 for 2003 flood conditions. The plot shows that water depths inside the polders was reduced by up to 3 m, while the water depths were increased slightly along GhiorKhal due to confinement effects. The water depths on un-embanked portions of the floodplain east and south of the polders were reduced by up to 1 m. This shows the polders partially blocked the spills across the floodplain originating from the Dhaleswari channels.

Table 6-15 summarizes the effect of the polders on flooded areas in the combined JLB-2 and PLB-1 sub- project boundaries. Under a 2003 flood condition, the polder scheme increases the total F0 land by +22,653 ha and increases the F1 land by +6,390 ha.

Table 6-15: Effect of polders on flooded areas and flood depths by land type Change in area due to project (ha) 1998 2003 2007

F0 land (including flood free) +5013 +22,653 +12,078 F1 land +6966 +6390 +7380 F2 land +15,912 -1062 +9612 F3 land -14,661 -24,399 -22,788 F4 land -13,266 -3924 -6102 Total flooded cultivable area -2601 -17,784 -9324 Per cent of NCA flooded (%) -3% -17% -9% Per cent of APA flooded (%) -2% -13% -7%

Table 6-16 and Table 6-17 break down the results for JLB-2 and PLB-1 separately.

Table 6-16: Effect of polders on flooded areas and flood depths by land type, JLB-2 only Change in area due to project (ha) 1998 2003 2007

F0 land (including flood free) +4968 +20,250 +11,826 F1 land +2439 +4473 +2466 F2 land +6651 +4905 +6588 F3 land 14,022 -2097 +7299 F4 land -16,263 -20,187 -21,087 Total flooded cultivable area -2826 -16,263 -9693 Per cent of NCA flooded (%) -4% -25% -15% Per cent of APA flooded (%) -3% -20% -12%

Table 6-17: Effect of polders on flooded areas and flood depths by land type, PLB-1 only Change in area due to project (ha) 1998 2003 2007

F0 land (including flood free) +45 +2403 +252 F1 land +315 +1485 +792 F2 land +1890 +1035 +2313 F3 land +1602 -4212 -1701 F4 land -3591 -567 -1143 Total flooded cultivable area +225 -1521 +369 Per cent of NCA flooded (%) 0% -4% 0% Per cent of APA flooded (%) 0% -3% 1%

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Figure 6-12: Impact of Alternative 2 on flood depths, 2003 flood condition

The area protected by the polders is approximately two thirds of the area potentially protected by the long embankment scheme (Alternative 1). The PPTA team carried the long embankment (Alternative 1) forward to the economic feasibility as it provides larger benefits.

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7 Requirements for Improved Flood Modelling

7.1 General Requirements Improved estimates of flooding extent and flood depth patterns on the floodplain will require additional work in several different fields:

1. Updated, higher resolution topographic surveys of the floodplain, including all roads, embankments, settlement platforms and other topographic features that affect water movement and ponding. An updated digital elevation model (DEM) is a main requirement-the resolution of the grid should be in the order of 50 to 100 m (the present DEM has a 300 m grid spacing). 2. Additional hydrometric measurements on the floodplain (both water levels and discharges on principle distributaries and spill channels). 3. Additional information on flooding patterns and flood extent using RADARSAT imagery coinciding with the improved hydrometric measurement (see point 2, above) for model calibration and validation. 4. Better representation of floodplain hydrodynamics possibly using a 2D model to represent the floodplain flows.

7.2 Improved Hydraulic Modelling More detailed modelling of the flood prone areas is required to refine the impacts assessment and to design flood control structures or flood management activities to reduce flooding requires. The following sections provide details on the proposed modelling approach for the next project phase.

1D Versus 2D Modelling In terms of floodplain modelling, one of the crucial requirements is to properly represent the anticipated flow paths of water in river channels and on the floodplain over the duration of the monsoon season. A one-dimensional analysis assumes one principal flow direction, usually in the stream wise direction of a channel or river. Any variations in water depth or velocity in directions perpendicular to the main axis are neglected. When flows are below bankfull elevations or when confined narrow floodplains are inundated, this approach is usually valid. In river-floodplain systems such as the ones in Bangladesh, complex flooding patterns usually occur on the floodplain with flow velocities on floodplains being much less than those in the main channel and not following the main stream wise direction. Roads, embankments, raised settlement areas and other features can have a significant influence on the propagation of flood inundation on the floodplain. Two-dimensional modelling can incorporate these floodplain features and simulate the water depths and flow velocities (magnitude and direction) on the floodplain during a flood event.

The existing one-dimensional regional models provide a good preliminary estimate of flood levels and general water depths on the floodplain. However, the model is unable to simulate the impacts of roads and other raised features on the flood development and a fair amount of interpretation and adjustment of the one-dimensional results is required.

The disadvantage of two-dimensional modelling is the requirement for more detailed topographic information to represent the floodplain and river channels. A two-dimensional model approach is recommended, although it is recognized that some simplification of the floodplain topography will be required as a compromise to ensure reasonable run time and data processing.

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Another possible approach is to use a combination of 1D and 2D modelling, with the 1D model used to represent the main rivers and the 2D model used to represent the floodplain. The channels and floodplains are linked using a series of weirs that allow flow to be exchanged back and forth between the channels and floodplain. The MIKE-FLOOD model by DHI is an example of this approach. In systems with sinuous rivers that are not confined by embankments, it is difficult to establish the linksbetween the top of bank and the floodplain. Furthermore, in our experience the model development is more complex and time requirements for the model runs may become very long compared to other approaches.

A simplified two-dimensional model approach is recommended for the future floodplain modelling.

Model Selection A two-dimensional hydrodynamic model of the river channel and floodplain system will be developed for each of the North Central and North West regions. The models will extend over a reach of the main rivers and include the impacted floodplain area as shown in Figure 7-1.

Nowadays there are different suitable models available, some of which are free of cost. To this end a combined model approach could be applied: The free-of-cost TELEMAC-2D modelling software could be used for the two floodplain models, with boundary conditions established from IWM’s existing MIKE- 11 model of the Jamuna and Padma Rivers. (Other commercial 2D models such as MIKE21 by DHI should also be able to perform this analysis, however, do not come free of cost.) The TELEMAC system is developed by Electricité de France (EDF) and is used extensively by design offices and state organizations around the world. TELEMAC is open source software of high accuracy; relatively short run-times, and no licensing costs. TELEMAC-2D is used to simulate free-surface flows in two dimensions of horizontal space. At each point of the mesh, the program calculates the depth of water and the two velocity components. TELEMAC-2D solves the Saint-Venant equations using the finite-element method and can perform transient simulations where conditions are changing over time. It uses a computation mesh of triangular elements for better representation of irregular features and provides greater flexibility during mesh generation through the use of elements of varying sizes.

Model Input Requirements The following data sets will be required for model development: • Recent floodplain topography (LiDAR or detailed surveys) to generate new DEM • Profile elevations of major roads • Locations and geometry of roads bridges • Survey data of flood protection embankments • Water level and discharge measurements at significant locations • Ground elevations and delineation of settlement areas • Delineation of agricultural areas • Boundary conditions from IWM models

Model Development Approach The project areas are too large to allow detailed modelling and accurate representation of all features. The modelling approach will attempt to capture the most important features governing the flooding. The areas in JRB-1 and JLB-2 identified during the feasibility work as being significantly impacted by the proposed projects will be modelled in more detail than other areas falling within the model extents shown in Figure 7-1. The floodplain element edge lengths in the detailed areas are expected to be in

Page 67 PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program the 50 m to 200 m range. The size will be determined based on the resolution of the DEM and computational limitations. Elements outside of the detailed area will be larger. Roads will be modelled as thin weir features and assumed to not overtop unless field observations or surveyed road profiles indicate otherwise. Major bridges will be modelled using the bridge subroutine available in TELEMAC- 2D which enables modelling of structures that are smaller than the element size. The floodplain tributaries conveying a significant flow will be modelled using quadrangular elements aligned with the flow direction. Bathymetry for these tributaries will be interpolated from the available cross-section data unless bathymetric survey points are available.

Figure 7-1: Proposed model extents for floodplain models

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8 Conclusions The effect of several flood mitigation alternatives and future flooding scenarios was investigated using a hybrid approach, consisting of hydrodynamic modelling using IWM’s regional flood models supplemented by interpretation of RADARSAT imagery analyzed by CEGIS. This combined approach is believed to overcome some of the limitations imposed by the out-dated topographic information that is used to represent the floodplain topography. Unadjusted model results were found to often over- predict the flooding extent and flood depth since the only available digital elevation model of the floodplain cannot represent the effect of local embankments and roads on flooding extent.

The proposed new embankment in sub-project JRB-1 will increase the F0 and F1 land by 6642 ha and 999 ha respectively, during an average (2003) flood year. Upgrading river bank protection in JRB-1 near Eneyetpur will prevent breaching of the embankment, and will protect 11,772 ha of F0 land from becoming deeply inundated.

The existing local embankment along the left bank of the Jamuna/Padma River provides some protection against overtopping during moderate and low flood years, but has many openings and is not expected to function effectively during extreme floods. Loss of this embankment due to erosion or breaching will significantly worsen flooding in sub-projects JLB-2 and PLB-1.

Raising and upgrading a relatively short (approximately 42 km from Zionpur to Dohar) section of the embankment provided only minor flood protection to the project area. The total increase in F0 land was estimated to be 3239 ha for average (2003) flood conditions. This shows that relatively localized embankments can provide only limited flood protection.

The effectiveness of a continuous embankment extending along the left bank from near Zionpur to the Dhaleswari River offtake was assessed. Three control structures were represented at key distributary offtakes (GhiorKhal and Dhaleswari River). This embankment significantly reduced flooding inside both JLB-2 and PLB-1 sub-projects. F0 land was increased by 32,274 ha during a moderate (2003) flood condition. However, additional detailed design studies need to be undertaken to ensure reliable, effective control structures could be operated on the distributary channels and to assess the impact of these structures on sedimentation processes, water quality and quantity in the distributary channels.

The effect of constructing two polders on the left bank was also assessed. The scheme was similar to a previous proposal by BWDB. Most of the flood mitigation benefits occurred inside sub-project JLB-2. The project increased the total F0 land area by 22,653 ha under average flood conditions and as such provides less flood benefit than the long, continuous embankment.

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9 References BWDB, 1994: River Training Studies of the Brahmaputra River, Final Report, Main Report, Sir William Halcrow& Partners Ltd. report to Bangladesh Water Development Board, Government of Bangladesh.

Dobler, A., Yaoming, M., Sharma, N., Kienberger, S. Awrens, B., 2011. Regional climate projections in two alpine river basins: upper Danube and Upper Brahmaputra. Adv. Sci. Res. 7, 2011.

Robert Richard Gales, 1917: The Harding Bridge over the Lower Ganges at Sara, Institution of Civil Engineers, London, Paper No. 42000, November

Ghosh, S. and S. Dutta, 2012. Impact of climate change on flood characteristics in the Brahmaputra basin using a macro-scale distributed hydrological model. Jour. Earth Syst. Sci. 121, No. 3, pg. 637-657.

FAP 2, 1991: FAP2 North West Regional Study, Inception Report, March 1991, Government of the People’s Republic of Bangladesh.

FAP 3, 1990: North Central Regional Study Phase 1: Reconnaissance, R.1.d. Annexes and Maps, June 1990, 360pg.

FAP 3, 1992: FAP 3 North Central Regional Study, Regional Water Resources Development Plan-Final Report, November 1992, Government of the People’s Republic of Bangladesh, 99pg.

IPCC, 2007a: Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.Cambridge University Press, Cambridge, UK and New York, NY, USA.

IPCC, 2007b: Asia. In: Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.Cambridge University Press, Cambridge, UK.

IPCC-TGICA, 2007: General Guidelines on the Use of Scenario Data for Climate Impact and Adaptation Assessment. Version 2.Prepared by T.R. Carter on behalf of the Intergovernmental Panel on Climate Change, Task Group on Data and Scenario Support for Impact and Climate Assessment.

IWM, 2008: Impact Assessment of Climate Change and Sea Level Rise on Monsoon Flooding. Dhaka, Bangladesh.

MoEF, 2008: Bangladesh Climate Change Strategy and Action Plan 2008. Ministry of Environment and Forests, Government of the People's Republic of Bangladesh, Dhaka, Bangladesh.

MPO, 1987: Master Plan Organization, Definition of Land Types, Dhaka.

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