MODELLING OF THE SURFACE WATER SALINITY IN THE SOUTHWEST REGION OF BANGLADESH MOHAMMAD ZIAUR RAHMAN DEPARTMENT OF WATER RESOURCES ENGINEERING BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY DHAKA-1000, BANGLADESH FEBRUARY 2015 MODELLING OF THE SURFACE WATER SALINITY IN THE SOUTHWEST REGION OF BANGLADESH MOHAMMAD ZIAUR RAHMAN DEPARTMENT OF WATER RESOURCES ENGINEERING BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY DHAKA-1000, BANGLADESH FEBRUARY 2015 i MODELLING OF THE SURFACE WATER SALINITY IN THE SOUTHWEST REGION OF BANGLADESH Submitted by Mohammad Ziaur Rahman (Roll No. 0412162005 P) In partial fulfillment of the requirement for the degree of MASTER OF SCIENCE IN WATER RESOURCES ENGINEERING DEPARTMENT OF WATER RESOURCES ENGINEERING BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY Dhaka-1000, Bangladesh February 2015 ii CERTIFICATION OF APPROVAL The thesis titled “Modelling of the Surface Water Salinity in the Southwest Region of Bangladesh” submitted by Mohammad Ziaur Rahman, Roll No. 0412162005 P, Session: April 2012, has been accepted as satisfactory in partial fulfillment of the requirement for the degree of Master of Science in Water Resources Engineering on 11th February, 2015. ........................................... Chairman Dr. Umme Kulsum Navera (Supervisor) Professor Department of WRE, BUET, Dhaka. ........................................... Member Dr. Md. Sabbir Mostafa Khan (Ex-Officio) Head Department of WRE, BUET, Dhaka. …………………………….. Member Dr. Md. Mirjahan Miah Professor Department of WRE, BUET, Dhaka. …………………………….. Member Mr. Abu Saleh Khan (External) Deputy Executive Director (Operation) Institute of Water Modelling House# 496, Road# 32 New DOHS, Mohakhali, Dhaka-1206. iii CANDIDATE’S DECLARATION It is hereby declared that this thesis or any part of it has not been submitted elsewhere for the award of any degree or diploma. Signature of the Candidate Mohammad Ziaur Rahman iv TABLE OF CONTENTS Page No. TABLE OF CONTENTS v LIST OF FIGURES viii LIST OF TABLES xiv LIST OF SYMBOLS xvi LIST OF ABBREVIATIONS xvii ACKNOWLEDGEMENT xviii ABSTRACT xix CHAPTER ONE INTRODUCTION 1.1 Background of the Study 1 1.2 Problems and Scope of the Study 4 1.3 Objectives of the Study 5 1.4 Organization of the Thesis 6 CHAPTER TWO LITERATURE REVIEW 2.1 General 8 2.2 Major River System of Bangladesh 8 2.3 Salinity Intrusion in Bangladesh 14 2.4 Study of Surface Water Salinity in Bangladesh 16 2.5 Study of Surface Water Salinity around the World 21 2.6 Study of Surface Water Salinity in Mathematical Modelling 24 2.6.1 Study in Bangladesh 24 2.6.2 Study around the world 28 2.7 Summary 29 CHAPTER THREE THEORY AND METHODOLOGY 3.1 General 30 3.2 Theory of Salinity 30 3.2.1 Salinity 30 v Page No. 3.2.2 Causes of Salinity 30 3.2.3 Composition and Measurement of Salinity of Sea Water 31 3.2.4 Salinity Distribution in the Ocean 33 3.2.5 Freshwater-Saltwater Interactions 35 3.2.6 Effect of Salinity 38 3.2.7 Governing Equations of Hydrodynamics & Model 39 3.3 Mathematical Modelling 43 3.3.1 MIKE 11 47 3.3.2 MIKE 21FM 53 3.4 Methodology 54 3.5 Summary 56 CHAPTER FOUR STUDY AREA & MATHEMATICAL MODEL SETUP 4.1 General 57 4.2 Study Area 57 4.2.1 River Systems of Southwest region of Bangladesh 57 4.2.2 Meteorology 59 4.2.3 Hydrology 63 4.2.4 Upstream Withdrawl and Impacts on Bangladesh 64 4.2.5 Activities in GDA 71 4.2.6 Flow Augmentation without Structural Intervention 80 4.3 Data Collection 84 4.3.1 Data Analysis 92 4.4 Mathematical Model Setup 107 4.4.1 Model Development 107 4.4.2 Salinity Modelling 117 4.5 Summary 124 CHAPTER FIVE RESULTS AND DISCUSSIONS 5.1 General 125 5.2 Description of the Flow Scenarios 125 5.3 Model Calibration 127 vi Page No. 5.4 Model Validation 139 5.5 Analysis of Different Flow Scenario 144 5.5.1 Temporal and Spatial Variation of Salinity 144 5.5.2 Salinity Intrusion Line for different flow Scenarios 157 5.5.3 Comparison of Salinity Intrusion Line 157 5.5.4 Changes in Salinity for different flow Scenarios 170 5.5.5 Estimation of Spatial area due to different Scenario 174 5.5.6 Effect of Trans-boundary Flow on Crop Production 179 5.5.7 Sundarban Salinity Map 179 5.5.8 Establishment of Hydrograph under Increased Flow Scenario 189 CHAPTER SIX CONCLUSIONS AND RECOMMENDATIONS 6.1 General 194 6.2 Conclusions 194 6.3 Recommendations 196 REFERENCES 197 vii LIST OF FIGURES Figure No. Title Page No. Figure 1.1: Map Showing River System of Southwest Region of Bangladesh (Source: IWM, 2012) 3 Figure 2.1: Major River System of Bangladesh (Source: IWM, 2012) 9 Figure 3.1: Annual mean of the sea surface salinity distribution (Source: World Ocean Atlas, 2005) 33 Figure 3.2: Annual mean of the sea surface temperature (Source: World Ocean Atlas, 2005) 34 Figure 3.3: Relationships between surface salinity and Evaporation minus Precipitation (E-P) patterns (Source: World Ocean Atlas, 2005) 34 Figure 3.4: The Ghyben-Herzberg relation 36 Figure 3.5: Classification of Mathematical Modelling 44 Figure 3.6: Mathematical Background of HD Modelling 50 Figure 3.7: Reach section with h- and q-grid points, on which the Saint Veannt Equations are solved (Source: DHI, 2014) 51 Figure 3.8: Centered 6-point Abbot Scheme (Source: DHI, 2014) 52 Figure 3.9: Model Hierarchy of Salinity Model 53 Figure 3.10: Flow diagram of Methodology to assess the Scenario & impact of Salinity Intrussion of Southwest Region of Bangladesh 56 Figure 4.1: Map Showing Study Area 60 Figure 4.2: Annual Rainfall Distribution in the Southwest Hydrological Regions of Bangladesh (Source: BMD, 2008) 61 Figure 4.3: The Hydrlogical Regions of Bangladesh 65 Figure 4.4: Ganges River Discharge at Hardinge Bridge (Source: BWDB, 2009) 66 Figure 4.5: Observed minimum flow of the Ganges River at Hardinge Bridge (Source: BWDB, 2012) 67 Figure 4.6: Maximum, Minimum and Average Water Level of the Ganges at Hardringe Bridge from 1909 to 2009 (Source: BWDB, 2009) 67 viii Figure No. Title Page No. Figure 4.7: Superimposed Daily Average Flow (m3/s) curve of the Ganges for the three periods (1973-74, 1975-96, 1997- 2010) (Source: BWDB, 2010) 69 Figure 4.8: Superimposed Daily Average Flow (m3/s) curve (dry season) for the three periods (1973-74, 1975-96, 1997- 2010) (Source: BWDB, 2010) 69 Figure 4.9: Layout of the proposed Ganges Barrage (Source: BWDB, 2012) 75 Figure 4.10: Schematic Diagram of Diversion System (Source: BWDB, 2012) 78 Figure 4.11: Link Channel of the Ganges Diversion System (Source: BWDB, 2012) 79 Figure 4.12: Schematic Diagram of the Ganges River Distributaries 81 Figure 4.13: Location of Discharge and Water Level measurement Stations (Source: IWM, 2012) 86 Figure 4.14: Location of Salinity measurement Stations (Source: IWM, 2012) 88 Figure 4.15: Seasonal variation of Tidal Rivers (Source: IWM, 2014) 93 Figure 4.16: Maximum, minimum and average water level of Ganges at Hardinge Bridge from 1912 to 2012 (Source: BWDB) 95 Figure 4.17: Maximum, minimum and average water level of Gorai at GRB from 1960 to 2009 (Source: BWDB/IWM) 95 Figure 4.18: Pussur River Water Level at Hiron Point (1984 - 2013) (Source: IWM) 96 Figure 4.19: Lower Meghna River Water Level at Chitol Khali (2011- 12) (Source: IWM) 96 Figure 4.20: Upper Meghna River Water Level at Bhairab Bazar (2011-14) (Source: IWM) 96 Figure 4.21: Minimum flow of Ganges at Hardinge Bridge from 1934 to 2012 (Source: BWDB) 98 Figure 4.22: Monthly Maximum, Minimum and average flow variation of Ganges at Hardinge Bridge (Source: BWDB) 98 Figure 4.23: Monthly variation of discharge in Gorai Railway Bridge during 1964-2013 (Source: IWM, 2014) 99 Figure 4.24: Monthly variation of water level in Gorai Railway Bridge during 1964-2013 (Source: IWM, 2014) 99 ix Figure No. Title Page No. Figure 4.25: Annual flow volume in the Ganges and the Gorai River (Source: IWM, 2014) 100 Figure 4.26: Seasonal variation of salinity in the three peripheral rivers from West to East 101 Figure 4.27: Salinity variation with upstream freshwater flow during dry season 102 Figure 4.28: Observed monthly average discharge in Ganges River from 2011-2014 103 Figure 4.29: Observed monthly average discharge in Gorai River from 2011-2014 104 Figure 4.30: Flow diversion through the Gorai during monsoon (top) and dry (bottom) period in different years 104 Figure 4.31: Monthly Salinity variation with upstream freshwater flow 105 Figure 4.32: Observed salinity at different location for with and without dredging 106 Figure 4.33: Catchment Boundaries of Rainfall Runoff Model (Source: IWM, 2012) 109 Figure 4.34: Map showing Existing Southwest Regional Model Domain and Boundary (Source: IWM, 2012) 110 Figure 4.35: Map showing Development of Extended Southwest Regional Model Network 112 Figure 4.36: Map showing Developed Model Network and Boundary 113 Figure 4.37: Methodology of the salinity modelling 117 Figure 4.38: Bathymetry of newly developed Bay of Bengal model (Source: IWM) 121 Figure 4.39: Water level variation at the south boundary 122 Figure 4.40: Manning map of newly developed Bay of Bengal Model (Source: IWM) 123 Figure 5.1: Gorai River Hydrograph at different flow scenario 127 Figure 5.2: Calibration of Southwest Regional Hydrodynamic Model against Water level (2012) 130 Figure 5.3: Calibration of Southwest Regional Hydrodynamic Model against Water level (2012) 131 Figure 5.4: Calibration of Southwest Regional Hydrodynamic Model against Flow (2012) 132 x Figure No.