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 Page iv September 2013 Hydrology and Flood Modelling 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 Page v PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program Page vi September 2013 Hydrology and Flood Modelling 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 Page vii PPTA 8054-BAN: Main River Flood and Bank Erosion Risk Management Program 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 ..................................................................................................