ASSESSMENT OF IMPACT OF DIFFERENT IMPLEMENTATION SCHEMES FOR THE PROPOSED SANDWIP-URIR CHAR-NOAKHALI CROSS-DAM ON REGIONAL HYDRO MORPHOLOGICAL REGIME
MD. MONOWAR-UL HAQ
INSTITUTE OF WATER AND FLOOD MANAGEMENT BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
MARCH 2019
ASSESSMENT OF IMPACT OF DIFFERENT IMPLEMENTATION SCHEMES FOR THE PROPOSED SANDWIP-URIR CHAR-NOAKHALI CROSS-DAM ON REGIONAL HYDRO MORPHOLOGICAL REGIME
A Thesis Submitted
By
MD. MONOWAR-UL HAQ
In partial fulfillment of the requirement for the degree of
MASTER OF SCIENCE IN WATER RESOURCES DEVELOPMENT
INSTITUTE OF WATER AND FLOOD MANAGEMENT BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
MARCH 2019
Dedicated to
My Beloved Family
iii
TABLE OF CONTENTS
TABLE OF CONTENTS ...... iv
LIST OF FIGURES ...... vii
LIST OF TABLES ...... x
LIST OF NOTATIONS ...... xi
LIST OF ABBREVIATIONS ...... xii
ACKNOWLEDGEMENT ...... xiii
ABSTRACT ...... xiv
CHAPTER 1 INTRODUCTION ...... 1
1.1 Study Background ...... 1
1.2 Objectives ...... 2
1.3 Scope of the Study...... 3
1.4 Outline of Thesis ...... 4
CHAPTER 2 LITERATURE REVIEW ...... 5
2.1 Flow and Sediment Dynamics...... 5
2.2 Residual Flow ...... 6
2.3 Tidal Asymmetry...... 7
2.4 Historical Development of the Meghna Estuary ...... 8
2.5 Sediment Morphology in the Meghna Estuary ...... 13
2.6 Sediment Influx in the Meghna Estuary ...... 13
2.7 Sediment Dynamics in the Meghna Estuary ...... 16
2.8 Land Reclamation in the Meghna Estuary ...... 19
2.9 Proposed Sandwip-Urir Char-Noakhali Cross-dam ...... 23
2.10 Summarization ...... 26
CHAPTER 3 METHODOLOGY ...... 28
3.1 Study Approach ...... 28
iv 3.2 Hydrodynamic Modeling Exercise...... 28
3.3 Model Description ...... 31
3.4 Model Domain...... 32
3.5 Bathymetry Data ...... 33
3.6 Boundary Conditions...... 35
3.7 Asymmetry Measurements ...... 36
3.7.1 Wave Asymmetry Properties ...... 37
3.7.2 Flood/ Ebb Dominance ...... 38
3.7.3 Slack Water Durations ...... 39
3.8 Tidal Asymmetry in Dam Locations ...... 39
CHAPTER 4 RESULTS AND DISCUSSION ...... 42
4.1 Model Calibration and Validation ...... 42
4.2 Simulation Cases ...... 46
4.3 Flow Regime ...... 49
4.4 Residual Flow Patterns ...... 58
4.5 Wave Asymmetry ...... 61
4.5.1 Asymmetry 1 ...... 63
4.5.2 Asymmetry 2 ...... 63
4.6 Flood/ Ebb Dominance...... 65
4.6.1 Vertical (WL) Dominance ...... 67
4.6.2 Horizontal (Velocity) Dominance...... 67
4.7 Slack Water Duration ...... 69
4.7.1 Slack Before Flood (SBF) ...... 71
4.7.2 Slack Before Ebb (SBE) ...... 71
4.8 Sediment Dynamics...... 72
4.9 Discussion ...... 85
CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS ...... 88
v 5.1 Conclusions ...... 88
5.2 Recommendations for Future Studies ...... 90
REFERENCES ...... 92
vi LIST OF FIGURES
Figure 2.1. Temporal development of the Meghna Estuary (Sarker et al., 2011)...... 9 Figure 2.2. Map indicative of land gains within Noakhali Region. Left map is based on Rennel’s survey and right map is based on survey done during LRP (Barua, 1997)...... 12 Figure 2.3. Historical imagery showing land gains in the Meghna Estuary...... 12 Figure 2.4. Trend lines of sediment flux along the Ganges and Brahmaputra during different time periods using secondary data, BWDB and FAP data (Rahman et al., 2018)...... 16 Figure 2.5. Direction and relative importance of river and tidal flows in influencing sediment discharge (Sarker et al., 2011; based on Sokolewicz, & Louters, 2007)...... 19 Figure 2.6. Placement and land gains in the upper estuarine regions due to these historical cross-dams (source: BWDB)...... 22 Figure 2.7. Map of the Meghna Estuary from the MES (MoWR, 2001)...... 23 Figure 3.2. Bathymetry of the model computation domain...... 34 Figure 3.3. Bathymetry modifications to incorporate cross-dams into the model domain...... 35 Figure 3.4. Average monthly variation of water and suspended sediment discharge of the combined Ganges–Brahmaputra–Meghna River system (Islam et al., 2002)...... 36 Figure 3.5. Schematic diagram of tidal water level profile illustrating the definitions of wave asymmetry (Hussain et al., 2013)...... 38 Figure 3.6. Schematic diagram of tidal velocity profile illustrating the definitions of tide dominance asymmetry and asymmetry due to slack water durations (Hussain and Tajima, 2019)...... 39 Figure 3.7. Plot of measured water level, velocity and turbidity along the northern channel of Urir Char Island (Hussain et al., 2014a)...... 40 Figure 4.2. Model validation with 2012-2013 data: comparison between observed and calculated tidal water levels (mPWD) at two locations (A and B) shown in figure 3.7, And north and east velocity components (m/s) at location B (Hussain and Tajima, 2019)...... 45 Figure 4.3. Initial alignment for proposed Sandwip-Urir Char-Noakhali cross-dam (EDP, 2010)...... 47 Figure 4.4. Cross-dam cases in model simulation...... 48 Figure 4.6. Simulated tidal velocity distribution during peak ebb velocity in location L1, contour shows velocity magnitude in ms-1...... 51
vii Figure 4.7. Simulated tidal velocity distribution during slack before flood (SBF) in location L1, contour shows velocity magnitude in ms-1...... 52 Figure 4.8. Simulated tidal velocity distribution during slack before ebb (SBE) in location L1, contour shows velocity magnitude in ms-1...... 53 Figure 4.9. Comparison of simulated tidal velocity distribution in the study area during peak flood velocity at location L1 for the different cross-dam scenarios, contours show velocity magnitude in ms-1...... 56 Figure 4.10. Comparison of simulated tidal velocity distribution during peak ebb velocity at location L1 for the different cross-dam scenarios, contours show velocity magnitude in ms-1...... 57 Figure 4.11. Residual flow in the study area with no cross-dams installed...... 58 Figure 4.12. Comparison of simulated residual flow in study area for different cross-dam cases...... 60
Figure 4.13. Comparison of As1 and As2 for study area without and cross-dams implemented...... 61 Figure 4.14. Comparison of asymmetry 1 values among the different dam cases in the study area...... 62 Figure 4.15. Comparison of asymmetry 2 values among the different dam cases in the study area...... 64 Figure 4.16. Comparison of vertical vs horizontal dominance for study area...... 65 Figure 4.17. Comparison of vertical (water level) dominance in the study area for all dam cases...... 66 Figure 4.18. Comparison of horizontal (velocity) dominance in the study area for all dam cases...... 68 Figure 4.19. Comparison of “Slack Before Flood” (SBF) and “Slack Before Ebb” (SBE) durations in study area for A (no-dam), measurement scale in hours...... 69 Figure 4.20. Comparison of SBF of “Low Water Slack (LWS)” values among the different scenarios in the study area, measurement scale in hours...... 70 Figure 4.21. Comparison of SBE of “High Water Slack (LWS)” values among the different cross-dam cases in the study area, measurement scale in hours...... 72 Figure 4.22. Comparison of bed level changes in all four locations for each and every cross- dam case...... 74 Figure 4.23. Bed level changes in the study area in meters during three different chronological instances (16 days, 29 days and 47 days) for all cross-dam cases...... 76
viii Figure 4.24. Variation of depth in location L4 vs water levels over computation period showing effect of alternating spring-neap cycles on depth undulations...... 77 Figure 4.25. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in A (no-dam)...... 79 Figure 4.27. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in C (dam 2)...... 81 Figure 4.29. Comparison of bed level changes in all four locations in three different chronological time instances...... 84
ix LIST OF TABLES
Table 2.1. The time required for land development at different locations in the Meghna Estuary (Sarker et al., 2011)...... 20 Table 4.1. Error statistics from the comparison of observed and modeled tidal water level data at five locations along the Bangladesh coast shown in Figure 3.1 (Hussain and Tajima, 2019)...... 42 Table 4.2. Error statistics for the comparison of observed and modelled tidal water level data and tidal velocity data at two locations around Urir Char shown in Figure 3.7 (Hussain and Tajima, 2019)...... 44 Table 4.3. Bed level change values in meters in all locations for all cross-dam scenario for three selected time instances...... 83
x
LIST OF NOTATIONS
As1 & As2 Vertical tidal asymmetry components tc Time taken by wave profile from zero up-crossing to maximum (crest) water level during the tidal cycle
Tc Time taken by wave profile from minimum (trough) water level to maximum (crest) water level to the time of a full tide cycle
ηc Maximum (crest) water level during the tidal cycle
ηt Minimum (trough) water level during the tidal cycle
Uf Flood velocity
Ue Ebb velocity
xi
LIST OF ABBREVIATIONS
ALOS-PALSAR Advanced Land Observing Satellite's Phased Array L-band Synthetic Aperture Radar BCM Billion Cubic Meters BDP2100 Bangladesh Delta Plan 2100 BWDB Bangladesh Water Development Board CDSP Char Development and Settlement Project CEGIS Center for Environmental and Geographic Information Services EDP Estuarine Development Programme FAP Flood Action Plan GBM Ganges-Brahmaputra-Meghna GEBCO General Bathymetric Chart of the Ocean HWS High Water Slack IWM Institute of Water Management LRP Land Reclamation Project LWS Low Water Slack MES Meghna Estuary Studies MoWR Ministry of Water Resources NRMSD Normalized Root Mean Square Deviation ppm parts per million PWD Public Works Department RMSD Root Mean Square Deviation SBE Slack Before Ebb SBF Slack Before Flood SSC Suspended Sediment Concentration SVRS Sample Vital Registration System TWL Tidal Water Level
xii
ACKNOWLEDGEMENT
As is with the inevitable closure of all things good, this particular trial, though one of my most exciting and enlightening of experiences, is finally coming to fruition through this thesis effort. All praise goes to the Almighty for keeping me able throughout this time period.
Sincere and heartfelt gratitude to my supervisor Dr. Mohammed Abed Hossain, Professor, Institute of Water and Flood Management (IWFM), Bangladesh University of Engineering and Technology (BUET) who took me under his wing right from the get-go of my Masters initiation and has guided me through each and every stage of this effort. Granting me freedom in taking my own paths, guiding me through my mistakes with constructive criticism and constantly encouraging me to correct my errors has extremely benefitted me in completing this work.
Special gratitude to Dr. Mohammad Asad Hussain, Professor, IWFM for his constant guidance during my modeling exercises. This study was made possible with the coastal circulation model partially developed by Prof. Asad and amidst busy schedules, he has always made time for technical counsel sessions with me concerning various aspects whatsoever regarding calibrations, test simulations and result analyses. Sincere thanks my respected teachers at IWFM for their unreserved guidance and valuable teaching that has extended my knowledge base and helped me to reach this point. I acknowledge the kind cooperation I received from all officials and staff of IWFM during my M.Sc.
I would like to thank Mr. Malik Fida Abdullah Khan, Executive Director, Center for Environmental and Geographic Information Services (CEGIS) for his help and guidance. As a professional of CEGIS, I have learnt a lot under his mentorship and it has helped in maintaining balance between work and study. Many thanks to my respected colleagues at CEGIS for their support and guidance. My sincere gratitude and respect to Mr. Motaher Hossain, Chief Engineer, Design Section, Bangladesh Water Development Board (BWDB) for taking the time to serve as my external examiner and providing valuable advice for improving my work.
Last but not least, my eternal gratitude to my family members who’s constant, unrelenting guidance and encouragement has really spurred me on, even during the toughest of ordeals. I draw inspiration from my Mother and Father, who are the fixed point when comes to guidance in every aspect of my life. My wife Dr. Nuzhat Naurin Amin, who was involved from the beginning of my work is a constant source of support and has always reinforced my strive to better myself in every aspect.
Md. Monowar-ul Haq March, 2019
xiii ABSTRACT
The Meghna estuary houses one of the most dynamic and morphologically active coastal regions in the world. Land formation through natural accretion takes place every year at a steady but slow and diminishing rate. This combined with the decreasing rate of sediment influx poses a challenge in formulating long-term land reclamation strategies. The proposed Sandwip-Urir Char-Noakhali cross-dam region is dominated by tidal and wave forcing with minimal effect of river discharge and seasonal wind variations. Morpho-dynamics in the region is influenced by residual current and tidal asymmetry features.
A two-dimensional hydrodynamic model was applied to assess the impact of the Sandwip- Urir Char cross-dam on the different physical factors that contribute to the erosion-accretion processes. The model used global bathymetry data that has been refined for the shallower near to coast portions with data from MES studies. The model input included harmonic tidal constituent as downstream seaward boundary data and river flow and sediment discharge as upstream boundary data representing forcing mimicking that of a high monsoon discharge. Calibration and validation of model was done using observed water level data from the MES studies. The model was simulated for 17 days with a high-resolution grid for flow analyses and with a coarser grid resolution for 47 days for sediment analyses.
A two cross-dam setup was used with four different dam implementation cases. Results show strong upward residual current through the Sandwip channel circulating Urir Char from north and south ends and exiting through Jahajir Char-Sandwip and Jahajir Char-Noakhali. Natural accretion takes place in the southern tips of Jahajir Char and Sandwip. Impact from implementation of both cross-dams alter this flow and sediment regime and induces accelerated sediment deposition in the northern, western and southern sides of Urir Char. Flow analyses reveal the region to be highly flood dominated and asymmetry values as well as slack water durations indicate accelerated sediment transport and deposition in the vicinity of dams surrounding Urir Char and the northern ends of Jahajir Char and Sandwip. The proposed cross-dams will yield best results if used in conjunction. For a single cross-dam setup, the dam from Urir Char to Noakhali is to be used and to be built first in case of a two- dam setup. The proposed cross-dams will facilitate land formation through accelerated accretion in key locations and thus aid in land reclamation in the region.
xiv 1. CHAPTER 1 INTRODUCTION
1.1 Study Background
Bangladesh, the largest delta in the world is located at the tail-end of the Ganges, Brahmaputra and Meghna (GBM) river basins. It is the lower most riparian country in the GBM basins and out of a total catchment area of 1.72 million sq. km of the GBM basins, only around 7% basin area falls within the Bangladesh territory (Sharma et al., 2010). Annual flow contribution from the GBM is close to 1,000 BCM (Billion Cubic Meters), 85% of which enters the country between June and October (Mac Kirby et al., 2014). This is an important fact, as this also implies that bulk of the often reported 1 billion MT annual sediment flow (Sarker & Thorne, 2006; Sarker et al., 2011; Sarker et al., 2013; Akter et al., 2015) enters the country in this period. About one-third of this sediment load is deposited on the riverine floodplains and the rest two-thirds or roughly 700 million tons reach the Meghna estuary where they become subject to complex interaction of different morphological factors and hydrodynamic influences (Hussain et al., 2013).
Bangladesh, with its 144,570 km2 of landmass houses approximately, 161 million people with a very high population density of around 1,090 per km2 (SVRS, 2016). Housing this enormous population is a task in itself; and while we are a nation that has achieved food security, overall agriculture landmass is on the decline. With a still ominous 1.04% population increase rate, this creates stress on the economy as well as the biodiversity as forest areas has decreased to less than 10%. As a dynamic and still very much active delta (Kuehl et al., 1997; Allison, 1998; Goodbred & Kuehl, 2000a, 2000b; Kuehl et al., 2005; Brammer, 2014), the annual sediment flow coming to the Meghna Estuary is responsible for land formation along the southern coastal region. However, current annual land accretion is at a steady natural rate of 0.9 km2 of every year (EDP, 2010). This number can be boosted up significantly by structural “land-reclamation” interventions such as cross-dams.
Cross-dams built in the 1960s in Noakhali and Hatiya succeeded in accreting considerable lands which later led to studies such as Land Reclamation Project (LRP) which envisaged a gain of 30,000ha of land through a cross-dam in the Sandwip-Urir Char-Noakhali area (LRP,
1 1982; GED, 2018). Meghna Estuary Study (MES) proposed a Master Plan (1998) and a Development Plan (2001) (MoWR, 1998, 2001) to aid accretion process through interventions, later supplemented by the Estuarine Development Programme (EDP) (EDP, 2009). A total of 18 cross-dams including the Sandwip-Urir Char-Noakhali cross-dam were recommended. The development plan under MES is under active consideration for the investment plan formulated as part of the Bangladesh Delta Plan 2100 (GED, 2018b).
Implementation of long-term development strategies such as cross-dams in the Meghna Estuary region requires recognition of the distinguished characteristics of different physical factors that contribute to the erosion-accretion processes. In addition to physical factors such as water and sediment discharge through the river, wind, waves and tides etc., morphological changes and sedimentation process in the Meghna Estuary strongly depend on tide induced residual currents. Tidal asymmetry plays a vital role in varying circulations in between the developed chars in the estuary. It is therefore, of importance to understand these tidal behaviors in assessing and enforcing accelerated land formation strategies. The present work aims to identify and evaluate these forces at work in the Sandwip-Urir Char region of the Estuary via mathematical modeling and assess the future possibilities of land reclamation through establishment of cross-dams and based on tidal circulations and sediment patterns.
1.2 Objectives
The study is undertaken with the aim to assess the effects of proposed Sandwip-Urir Char- Noakhali cross-dam with alternative implementation scenarios on the hydro-morphological processes and provide suggestions for effective investment planning for land reclamation in the area.
Objectives: Setup a calibrated and validated hydro-dynamic tidal flow model for the Bay of Bengal with observed data for generating boundary data for a localized model to be used for the proposed Sandwip-Urir Char-Noakhali cross-dam; and Assess the effect of cross-dam through model application with phased spatial and chronological implementation on the tidal flow circulation, velocity and sediment dynamics.
2 Study Outcome: The study results will help the decision makers to assess the long-term impacts of cross dams in Meghna Estuary and decide on location and critical timing of implementation for maximizing benefit.
1.3 Scope of the Study
. This study aims at assessing the impact of proposed Sandwip-Urir Char-Noakhali cross-dam on surrounding flow regime and sediment dynamics through application of hydro-dynamic modeling for the Meghna Estuary with focus on the proposed cross- dam locations;
. The flow analysis would be limited to assessment of characteristics such as flow patterns, residual flow paths, tidal asymmetry features etc. for the study area;
. The study also analyses sediment dynamics resulting from cross-dam implementation and compares results with flow parameters in assessing overall impact of dam construction in the region with limited simulation time owing to limited sediment data;
. The study is also limited by the data constraints stretching from bathymetry, flow and sediment data;
. The model was not simulated for long periods of time thus long-term impacts pertaining to the overall effectiveness of cross-dam implementations were not analyzed; and
. The model was run for short periods of time due to data constraints especially in terms of sediment flow data and thus it provides an indication as to the effects of cross-dam on land accretion based on changes in flow regime and fluctuations in short-term sediment dynamics.
3 1.4 Outline of Thesis
The thesis has been divided into five chapters to delineate how the study has been done.
Chapter 1 provides a brief background of the study, study objectives, possible outcomes and scope of the study; Chapter 2 provides a thorough literature review on the study problem, historical sediment influx into the Meghna Estuary and provides theoretical backgrounds to the study analyses components; Chapter 3 provides brief description of the study area and provides the methodology of the study including model setup considerations, boundary conditions, assumptions etc.; Chapter 4 discusses model calibration and validations and presents the detailed results of the model simulations, based on which, discussion is provided regarding result interpretation; and Chapter 5 provides conclusions drawn from analysis of model results and gives recommendations based on difficulties encountered during study and its limitations.
4 2. CHAPTER 2 LITERATURE REVIEW
2.1 Flow and Sediment Dynamics
Interaction between physical processes as well as tidal action influence fluvial transport and alters the complex morpho-dynamics of an estuary. Essential elements that differentiate estuaries with upstream deltaic tidal flats from those without river input include constant freshwater supply, a fluvial sediment supply, and distributary channels that carry discharge after the tide has receded (Ralston et al., 2012). Morphological change in an estuary is constantly brought about by seasonal variations in river discharge, bed load and suspended sediment flux, wave conditions and tidal circulations. This action is however, reciprocated as the altered morphology also exerts stress on these forcing factors. Estuarine morphology is to a large extent, determined through residual sediment transport (Dronkers, 1986a), which again is influenced by the flow velocities and inter-linked higher-frequency tide harmonics (Groen, 1967; Van De Kreeke and Robaczewska, 1993).
Estuaries form the routes through which sediment is transported from their source through river channels and onto seas and oceans. For well-mixed estuaries such as the Meghna Estuary, tide induced re-suspension and transport is the primary forcing factor influencing sediment dynamics (Allen et al., 1980; Dronkers, 1986b; Nichols, 1984). Sediment transport occurs in primarily two processes, bed load and suspended load. Bed load usually consists of grains rolling, sliding and jumping in frequent or continuous contact with the bed, whereas suspended load consists of grains in suspension above the bed for extended periods of time. That part of the suspended load that is not found in the bed is referred to as wash load.
Tidal asymmetry is one of the more significant factors that impart residual sediment transport in estuaries (Wang et al., 1999; Hussain et al., 2014a). Tides become increasingly asymmetric when the astronomic (ocean) tide propagate into shallower estuaries. This is caused by the generation of high frequency harmonic constituents under the influence of nonlinear processes induced by a shallower basin morphology (Blanton et al., 2002; Friedrichs and Aubrey, 1988). Several studies (Bolle et al., 2010; Wang et al., 1999; Hussain et al., 2014a) have made distinctions between vertical tides and horizontal tides for
5 asymmetry analyses, where vertical tidal asymmetry refers to the tidal water level (TWL) variation and horizontal tidal asymmetry refers to the variation in horizontal tidal flow velocity.
2.2 Residual Flow
Residual flow in an estuary represents the resultant net flow at the end of a single tidal cycle that results from opposing river and tidal flows. Physical processes influencing residual flow in well-mixed estuaries include upstream river discharge, wind and tidally induced horizontal circulation. As sediment transport occurs to a large extent through both residual flow and tidal asymmetry, influence of residual flow on asymmetry is an important factor in assessing sediment dynamics. If residual flow in an estuary is dominated by upstream river discharge, then it also becomes the dominating factor in residual transport as river flow reduces influences tidal flow and thus subjugate tidal asymmetry into secondary characteristics (Wang et al., 1999). Sediment transport in the tide-dominated Meghna Estuary however, is highly influenced by asymmetry of tide.
The flow distribution in the Meghna Estuary is determined by the combined action of tides and fresh water flow. The flow in the western part (upper and mid-estuary, Tetulia and the Shabhazpur Channel) is dominated by the fresh water flow from the Lower Meghna. The eastern part of the estuary is mainly influenced by the tide and much less by the fresh water flow from the river system. Most of the fresh water discharge is conveyed to the Bay of Bengal through the Shabhazpur Channel, while the Hatia Channel is mainly a tidal channel. A prominent counter-clockwise residual circulation is present around Sandwip (Jakobsen et al., 2002; Hussain et al., 2014a). Flow is generally very strong and turbulent with current velocities up to 4 ms-1 have been observed in the Sandwip Channel during spring tide and in the upper reach of the Lower Meghna during high monsoon (MoWR, 1998).
Wang et al. (1999) has distinguished two types of residual circulation, the geometry induced or “horizontal” circulation and the bathymetry induced “vertical” circulation. Wang also stated that a general principle for the bathymetry-induced circulation is that in the relatively deep parts the residual flow tends to be in the ebb-direction and in relatively shallower parts, it tends to be in the flood direction. Residual flow in the flood direction as well as tidal pumping due to high velocity flood-tide flow in the shallow Meghna Estuary are responsible
6 for sediment trapping through a process of mixing involving continuous erosion, deposition and exchange of sediment in between flood-ebb cycles (MoWR, 1998). Trapping is an important factor for sediment deposition in an estuary.
In the Meghna Estuary, sediment transport occurs with the cycling of fine sediment through the turbidity maximum and somewhat coarser sediment cycling around the flood-ebb channel systems with individual particles spending considerable time moving within the system before finally depositing or passing through to the sea. Measurements done in the MES (MoWR, 1998) indicate that a substantial amount of sediment is being transported through the estuary rather than depositing in the estuary. This would indicate that trapping capacity of Meghna Estuary is on the lower side which, as a well-mixed estuary, is in accordance with Dyer (1978).
2.3 Tidal Asymmetry
Wang et al. (1999) defined tidal asymmetry in his work on the Western Scheldt Estuary. Tide usually follows a sinusoidal curve pattern whilst propagating forward which is more or less symmetric in deep ocean regions. In case of the Meghna Estuary, as the tide from the deep Indian Ocean enters into the shallower Bay of Bengal waters, external factors act on it that change its symmetry position with the horizontal/ vertical axes. As a result, both the tidal water level and velocity curve gets distorted and this alters the smooth propagation of tide into shallow waters. Tidal asymmetry refers to this distortion of the tidal wave that makes flood and ebb periods unequal to each other. Shorter rise periods than fall periods are denoted as ‘flood-dominant’ tide and longer rise periods followed by shorter fall periods is termed ‘ebb-dominant’ tide.
This study takes a slightly different approach in assessing tidal asymmetry in that it considers asymmetry from a morpho-dynamic standpoint and deals with the residual transport of fluvial and marine sediment. As a tide dominated estuary, tidal action is an important forcing factor in the Meghna Estuary and thus morphological development is strongly influenced by residual sediment transport. Tidal asymmetry is one of the most important factors which cause residual sediment transport in estuaries (Dronkers, 1986a and Wang et al., 1999).
7 While analyzing the residual sediment transport characteristics of the Western Scheldt estuary in the Netherlands, Wang et al. (1999) made a distinction between asymmetry of the ‘horizontal tide’ and ‘vertical tide’ in order to relate tidal asymmetry with morphological development. The authors defined vertical tidal asymmetry as the inequality between flood and ebb periods in terms of water level rise and fall. The horizontal tide refers to tidal flow velocity and it is considered asymmetric if it generates residual sediment transport. The horizontal tidal asymmetry again has been divided into two types: asymmetry between maximum flood and ebb velocity and asymmetry between flood and ebb slack water durations.
If the maximum flood velocity exceeds the maximum ebb velocity it is termed as flood dominant and vice versa. This type of asymmetry, when flood dominant, tends to induce residual bed load and suspended load transport of coarse sediments in flood direction, as the sediment transport non-linearly responds to velocity. On the other hand, a situation when the duration of slack water before ebb (SBE) exceeds the slack water before flood (SBF) is termed as flood dominant and vice versa. This type of asymmetry, when flood dominant, will favor a residual sediment transport of fine sediments in the flood direction (Hussain et al., 2014a). Based on theoretical considerations supplemented by filed observations, Dronkers (1986a) also showed that the asymmetry of the flood and ebb limbs of the tidal velocity curve, and in particular the length of the high-water slack period as compared to the low water slack, controls the net sediment budget in an estuary.
2.4 Historical Development of the Meghna Estuary
Owing to active delta characteristics, numerous islands have formed in and around the mouth of the Meghna Estuary. Due to the active hydro-morphology regime, the sediment laden river flow distribution processes change very rapidly as the sizes, shapes and locations of these islands have been changing over time, but these islands in-turn, also play a key role in distributing the flow and sediment in the estuary. Bhola is the largest among the islands in the region and is a separate district. The Tetulia Channel flows through the western side of this island and the Shahbazpur Channel through the eastern side. To the east of Shahbazpur Channel is Hatiya. This island has gone through substantial erosion and accretion cycles in recent decades. The Hatiya Channel flows through the eastern side and carries flow eastwards towards the Sandwip Island. This marks the easternmost portion of the estuary and houses the
8 focused area of this particular study. To the north of Sandwip is the small island of Urir Char. The cross-dam in question is to link the island of Sandwip to Noakhali mainland though Urir Char. A more recent advent in the vicinity is the emergence and gradual development of Jahajir Char to the south-southwest of Urir Char.
Historical development of the Meghna Estuary is intriguing in that morphological changes to the estuary advanced rapidly compared to geologic timescale and more so, following a nonlinear pattern. The survey of Rennels in the 1770s (Figure 2.1) showed that the now Padma (Ganges) and the Meghna entered the estuary at two different locations on the northwest and northeast sides respectively of the then Bhola Island (Sarker et al., 2011). The Padma followed a course close to the present-day course of the Tetulia Channel and the Meghna flowed eastwards along the northeast side of Bhola Island. At that time the Brahmaputra flowed on the eastern side of the Madhupur Tracts and joined the Meghna somewhere close to Bhairab, about 80 km north of Chandpur.
Figure 2.1. Temporal development of the Meghna Estuary (Sarker et al., 2011).
9 The Brahmaputra joined with the Ganges near Aricha in the early nineteenth century, moving southeast as Padma and joining with Meghna near Chandpur. During the mid-1900s, the combined flow branched into the three channels the Meghna, Shahbazpur and Tetulia channels. The shoreline at Noakhali retreated to the north by several km over the period of 170 years from 1776 to 1943, but the magnitude of shoreline changes was not that significant, considering the long period of time. However, the shapes, sizes and locations of the islands of Bhola and Hatiya changed a lot. Land accretion during this period (1776-1943) was mainly limited to the southwestern part of the Meghna Estuary.
Sarker et al. (2011) observed sharp morphological changes to the estuary from 1943 to 1973. The course of the Meghna River, at the east of Bhola Island, was abandoned, resulting in the shifting of flow to the Shahbazpur Channel. The Hatiya Channel was separated from this channel at the north of Hatiya Island. A very large land mass accreted to the south of Noakhali and Bhola. The change in 30 years from 1943 to 1973 was enormous in comparison and the extent of land accretion was also very high. The study also observed that during the following decades the development of the Meghna Estuary followed a similar trend, but the rate of change slowed down significantly.
Active morpho-dynamics results in the constant land formation and deformation in the Meghna Estuary owing to the complex interactions between large river discharge, enormous sediment load, tidal forces and estuarine circulation, a unique feat unlike anywhere else in the world (de Wilde, 2011). Erosion and accretion in the region were estimated during the Meghna Estuary Studies using satellite images for the period 1973-2000. Total accretion and erosion during this period were estimated to be 1731.68 km2 and 863.66 km2 respectively, which gives a net accretion of 508.2 km2 area at a rate of 18.8 km2/year (MoWR, 2001). Thus, the coastlines along the estuary are in a perpetual process of formation and deformation, an unprecedented and unique phenomenon.
The Ganges and the Brahmaputra, although ultimately flowing as a single channel into the Bay, exhibit somewhat differing hydrological characteristics and flow regimes (de Wilde, 2011). Flow hydrograph in the Brahmaputra rises towards peak one month earlier than the Ganges, whereas the recession in the Ganges starts earlier than the Brahmaputra. Peak flow in the Ganges occurs in August while in the Brahmaputra it is as early as July. Measurements taken during the Meghna Estuary Study (MES) project suggest that in an extreme year,
10 discharge through the estuary can be as much as fifteen times during the monsoon season compared to that during the dry season, and flow through the estuary reveals a certain level of skewness in differing seasons (MoWR, 1998). MES reports that on an average year the discharge through the estuary is about 20,000 m3/s during the dry season whereas it exceeds 100,000 m3/s during the monsoon. The estuary remains calm during the dry season (October- March) with mild winds flowing from north and a low river discharge persists, whereas the monsoon (June-September) reveals a more vigorous state with strong south-west winds blowing, much higher river discharge (de Wilde, 2011).
Meghna Estuary exhibits a well-mixed system with no visible stratification in any measurement campaigns during the MES studies (MoWR, 2001). Salinity in the estuary varies over a very wide range from the monsoon to the dry season. Very high discharges in the Lower Meghna River pushes the salinity far into the bay. During the dry season fresh water flow in the river reduces drastically (Sarker et al., 2011), resulting in salinity intrusion into the estuary. Salinity up to 1 p.p.t. (parts per thousand) may intrude up to the northern end of Shahbazpur Channel. Due to propagation of south easterly tides and being at a distance from the lower Meghna mouth, salinity in the eastern parts of the estuary may reach up to 10– 20 p.p.t. Despite this stark contrast in salinity from west to east within the Meghna Estuary, it never reaches the salinity level of sea water (~34.5 p.p.t.) (Sokolewicz & Louters, 2007).
Figure 2.2, adopted from Barua (1997) illustrates the land development within Meghna Estuary, in particular the uplifting of the Noakhali Region. This can be further depicted from the more recent image from Figure 2.3. It illustrates the clear gains in land in Noakhali as well as changes in the Bhola and Hatiya region and development of Sandwip, Urir Char and Jahajir Char Islands.
11
Figure 2.2. Map indicative of land gains within Noakhali Region. Left map is based on Rennel’s survey and right map is based on survey done during LRP (Barua, 1997).
Figure 2.3. Historical imagery showing land gains in the Meghna Estuary.
12 2.5 Sediment Morphology in the Meghna Estuary
Sediment is always in process of rework and re-deposition in a tide dominated estuary such as the Meghna Estuary whereas deposition occurs only by river flushing in the river dominated deltas (Hori & Saito, 2007). Annual river flux into the Meghna Estuary from the GBM system is approximately 1 x 1012 m3, which is distributed by a number of channels to the estuary. The Tetulia Channel carries about 15% of the monsoon flow, and the Hatiya Channel about 10%, and the rest is carried by the Shahbazpur Channel (MoWR, 2001). However, the ratio of the distribution of fresh water varies with the season and also over a period of decades, depending on the channel developing processes.
Field measurement data on sediment transport and morphological change in the Meghna estuary is quite scarce. According to MoWR (2001), beds of the channels in the estuary consist of fine sand and silt (25–50%), the representative grain size of which varies from 0.016 to 0.25 mm. However, in the suspended sediment the fraction of fine sand is very negligible, indicating that sand particles move as bed load.
Sokolewicz and Louters (2007) reported that the bed of the Lower Meghna River consists of silt and fine sand with a median bed material grain size varying from 16 to 200 μm. Also, more than 70% of the bed-materials have a median grain size smaller than 75 μm. Maximum depth average sediment concentration varies between 0.5 and 9 g/l during the dry season. Sediment concentration becomes about 2-5 times higher during spring tide compared to neap tide. Highest sediment concentrations were found in the areas Urir Char-Char Balua and Manpura-North Hatia. All the measurements were made during dry season and they predicted that sediment concentration would increase significantly during the monsoon with the increase of river discharge.
2.6 Sediment Influx in the Meghna Estuary
Annual sediment flux of the Ganges and the Brahmaputra varies significantly as was observed by Rahman et al. (2018). The study found that the sediment loading for Ganges varied from 262–680 MT/year (million tons per year) and for Brahmaputra 387 to 1157 MT/year, from studies in the literature that have been published over a span of 60 years (1958-2006). This provides a total estimate of sediment flux through the two major rivers to
13 range from 655 to 1850 MT/year with an average value of above 1250 MT/year, which is at the lower end of the range of sediment flux often cited in delta planning documents (1100- 2400 MT/year). Sediment flow in the two major rivers in the past few decades have been heavily influenced by the Assam earthquake which occurred in 1950, providing a massive sediment load to the rivers (Brammer, 2004; Sarker and Thorne, 2006). A chronological review of literature indicates that this influx is gradually decreasing.
Historically, sediment load to the Meghna Estuary has always been dominated by influx from the Ganges and the Brahmaputra rivers. Past studies of sediment flux for the Ganges and Brahmaputra measured at Hardinge Bridge and Bahadurabad stations respectively, show substantial variability, a trait that can be attributed to both measuring techniques and analysis periods. Holeman (1968) provided initial estimations on sediment loading in the Ganges and Brahmaputra rivers based on erosion rates in the two river basins. He suggested that annual sediment flow for the Ganges and Brahmaputra were 1600 MT and 800 MT respectively, which yields a combined sediment loading of 2.4 BT/year to the Meghna Estuary.
Coleman (1969) provided the first plausible estimates based on measurements done in the 1960s for the Ganges and Brahmaputra. According to these measurements, suspended sediment concentrations (SSC) for the Ganges was 190-1600 mg/l and for the Brahmaputra was 220-1400 mg/l. This gave an estimated combined load of 1100 MT/ year to the Meghna Estuary which is half of that estimated by Holeman (1968) and is an estimate still widely used in various planning documents by the government. Millman and Meade (1983) made estimations based on sediment rating curves from combined sediment samples taken in the Padma River during 1966-97 and found the influx to be 1670 MT/year.
Among the more recent studies, Hossain (1992) estimated sediment flux to vary between 400-850 MT/year for Ganges and 350-600 MT/year for Brahmaputra with a combined loading of 750-1450 MT/year based on FAP 24 study measurements in 1980s. Islam (1999) made estimations based on sediment data collected during 1979-1995 and 1989-1994 for the Ganges and Brahmaputra respectively and calculated influx to be approximately 1137 MT/year. Wasson (2003) presented an approximate sediment budget of the GBM system where he mentioned influx through the Ganges and Brahmaputra to be 440 MT/year and 540 MT/year for the Ganges and Brahmaputra respectively. Out of this total 980 MT/year, 21% deposited in the sub-aqueous delta and another 29% in the Bengal Fan. Rice (2007) made
14 somewhat contrasting estimations based on recent field measurement data collected during monsoon 2006. Her estimations show sediment influx to be 262 MT/year and 387 MT/year for Ganges and Brahmaputra respectively, indicating decreasing sediment inflow to the estuary.
The recent comprehensive analysis done by Rahman et al. (2018) lays the foundation for the modern-day sediment budget estimations in Meghna Estuary, with estimations based on a combination of extensive literature review and extrapolation of measured BWDB gage station data for Ganges and Brahmaputra. The study discarded sediment data for Meghna river influx due to both severe scarcity of data as well as its relevant significance (1% of total sediment loading) in total sediment flow to Meghna Estuary. Qualitative analysis of historically estimated sediment flux data was done via chronological documentation of published literature during 1958-2010. Quantitative analysis was performed with available long-term measured sediment transport rates in the Ganges and Brahmaputra.
First order of analyses by Rahman et al. (2018) involved thorough literature review, through which sediment data were extracted, organized corresponding to respective measurement periods and finally plotted on a chronological scale. The next order involved analysis of raw data from BWDB where yearly SSC values were derived via summation of available data sets using trapezoidal method (Sastry, 2006). Years with most available data have been considered and variability of transport processes have been addressed in terms of Q vs Qs graphs. Statistical testing of data has been conducted to examine whether the trends of change of sediment flux is significant or not. Trend analysis was done using non-parametric Mann- Kendall, t-test and F-tests. Figure 2.4 presents plots obtained from these statistical analyses.
Analyzing the time series of BWDB sediment concentration data for a period of 1960~2008 Rahman et al. (2018) found that the sediment flux through for the Ganges and Brahmaputra varies between 150-590 and 135-615 MT/year respectively. This provides an average total flux of 750 MT/year, ranging between 300–1200 MT/year. Based on analyses, average sediment flux for Ganges has been estimated for 2015 with a decreasing trend of -4.0 MT/year to be 220 MT/year at a range of 125-500 MT/year. Similarly, average sediment flux for Brahmaputra has been estimated at a rate of -6.0 MT/year to be 250 MT/year with a range of 175-800 MT/year. This gives a total estimated sediment flux for 2015 to the Meghna
15 Estuary of approximately 500 MT/year, and indication of gradual decrease that can be attributed to both natural means and anthropogenic causes.
(a) The regression for Ganges using all the (b) The regression for Brahmaputra using all scattered data is Sediment FluxAll data = − the scattered data is Sediment FluxAll data = 4.0 × year + 8341 with R2 = 0.13, while − 6.0173 × year + 12360 with R2 = 0.20, for the datasets ranging between 10 while for the datasets ranging between 10 percentile 90 percentile data this percentile 90 percentile data this regression is Sediment Fluxpercentile = − 3.7 regression is Sediment Fluxpercentile = − × year + 7747 with R2 = 0.19 4.2607 × year + 8859.5 with R2 = 0.18
Figure 2.4. Trend lines of sediment flux along the Ganges and Brahmaputra during different time periods using secondary data, BWDB and FAP data (Rahman et al., 2018).
2.7 Sediment Dynamics in the Meghna Estuary
A substantial percentage of the sediment flux entering the Meghna Estuary gets diverted towards the western coast by longshore currents and subsequently most of these sediments are conveyed into the deep ocean through the Swatch of No Ground (Hussain et al., 2013). Approximately, one-fifth to one-third of the initial sediment influx of 1.1 billion tons are ultimately retained within the Meghna Estuary. This sediment plays an important role in providing the material for land formation in the central part of the coast of Bangladesh (de Wilde, 2011; Sokolewicz and Louters, 2007).
16 High tide energy results in tide-dominated deltas like the Meghna Estuary, where distributary channels with linear river mouth bars are present (Fookes et al., 2007). Characteristics of tide and tidal range, waves and estuary planforms primarily govern the sediment distribution process in the estuary (Palinkas et al., 2006; Bird, 2008). Tide in the estuary is semidiurnal in nature and arrives from the south reaching the coast of Hiron Point (south of Khulna) and Cox’s Bazar at around the same time (de Wilde, 2011). The extensive shallow area in front of the large delta causes refraction and distortion of tidal waves.
Depending on tidal range, Meghna Estuary has been divided into three zones: micro-tidal (0~2m): from Tetulia river through to Chandpur, meso-tidal (2~4m): from west coast of Bangladesh to north Hatiya through south Bhola and south and west coast of Hatiya; and macro tidal (4~6m): from east coast of Hatiya to south and west coast of Sandwip. Apart from this the area from the Sandwip channel towards north coast of Urir Char is considered hyper-tidal with tidal range more than 6m (Carvajal et al., 2011, de Wilde, 2011). According to Azam et al., (2000), the incoming tides from the southern part of the Bay of Bengal are important for the flow and sediment transport in the Meghna Estuary and they contribute to the residual circulation significantly.
Barua (1990) and Barua et al. (1994) explained the suspended sediment dynamics along different channels of the Meghna Estuary and also residual transport along the estuary with measured flow and suspended sediment concentration (SSC) data obtained during the Land Reclamation Project during the 1980s. Barua (1990) showed that in the Meghna Estuary the aspect ratio of a channel: ratio between mean water depth and width of a channel, is lower in flood dominant channels compared to ebb dominant channels and this feature is consistent with the hypothesis of Dyer (1978). Locations of the flood dominant channels also corresponds to the hypothesis of Dyer (1978), who claimed that in the northern hemisphere flood flow occurs along the northeastern side of the channel.
Residual circulation plays an important role for the distribution of sediments entering the estuary. Jacobsen et al. (2002) performed numerical simulations through two-dimensional MIKE 21 model during the ‘Meghna Estuary Study’ (MoWR, 1998) and obtained a counter- clockwise residual circulation with a northward flow in the Sandwip Channel and a southward flow in the Tetulia River and in the area from Hatiya to Sandwip. It was found that
17 residual currents were not significantly influenced by wind stress. The fine fractions of the sediment dominate the sediment redistribution process (Sokolewicz & Louters, 2007).
Hussain et al. (2012, 2009a and 2009b) applied a 3D numerical model to investigate the seasonal variation of residual circulations in the Meghna Estuary considering four different seasons following Chowdhury et al., (1997). It was observed that throughout the year a counterclockwise circulation exists in the Meghna Estuary almost at all the depths. Average values of wind stress at different seasons of the year did not alter this circulation pattern significantly. Numerical experiments suggested that tidal current along with Coriolis Effect is the primary forcing factor behind the counterclockwise residual current in the Meghna Estuary. The results showed that residual circulations at different seasons were only influenced by wind stress when averages of maximum wind stress were applied as boundary conditions.
A significant portion of the river-borne sediment enters the estuary during the monsoon, a major part of which, especially the finer fraction, takes temporary residence in the zone of the turbidity maximum, close to the lower limit of the Shahbazpur Channel (Sokolewicz & Louters, 2007). This turbidity maximum generally occurs in the low salinity zone and shifts its location with the changes in flood discharge (Grabemann et al., 1995) and usually exhibits high sediment concentrations of about 2000 ppm (MoWR, 2001). The maximum flow velocity of 4ms-1 was observed in the northeastern part of the estuary in the Sandwip Channel (MoWR, 2001) with a maximum sediment concentration of about 9000 ppm. Higher flow velocity during flood tide in the shallow estuary brings sediment to the landward inter-tidal areas to settle, a process known as tidal pumping.
Dry season sediment supply is insignificant, but concentrations in the northeastern tide dominated part remain close to that of the monsoon (IWM, 2009, 2010). The monsoon sediment was moved temporarily by the river flow and was forced to remain close to the southern boundary of the estuary (downstream of the Shahbazpur Channel), having been brought to the north-eastern side by tidal circulation and tidal pumping process. This temporary storage of sediment during the monsoon in the zone of the turbidity maximum is the main source of sediment redistribution during the dry season (Sarker et al., 2011). Recent measurement shows that the values of maximum flow velocity and sediment concentration
18 are very similar in the monsoon and the dry seasons (IWM, 2009), although the riverine sediment input during the dry season is negligible during this period.
Figure 2.5. Direction and relative importance of river and tidal flows in influencing sediment discharge (Sarker et al., 2011; based on Sokolewicz, & Louters, 2007).
Relative strength of alternate flood and ebb tide patterns determines the locations for sedimentation build up (Bird, 2008). Based on the MoWR (2001) and Sokolewicz & Louters (2007), the relative importance of river and tidal flow with respect to sediment discharge drawn by Sarker et al. (2011) has been presented in Figure 2.5. It shows the tentative tidal meeting points and subsequent sedimentation prone locations in the northeastern part of the estuary, near the Sandwip and Urir Char cross-dam region. Tidal circulation process disperses fresh water and river-borne sediment into this northeastern part of the estuary.
2.8 Land Reclamation in the Meghna Estuary
Substantial annual influx from the GBM system from respective upper catchments deep within in India, China, Nepal and Bhutan to the downstream delta region of the Meghna
19 Estuary directly results in the still active delta building taking place within the estuary area (Sarker et al. 2011). Long-term sediment budget for the delta predicted by Goodbred & Kuehl (2000a) states that, one third of the sediment carried by these rivers is deposited on the flood plain and tidal plain, and one third is trapped in the sub-aqueous delta, causing vertical accretion and lateral progression of the sub-aqueous delta. Goodbred & Kuehl (2000a) were unable to assess the destination of the remaining sediment and concluded that it was probably transported to the deep ocean floor.
Estuarine land formation process is usually perceived when a bar (composed of fine sand, silt and clay) emerges during low tide. Initial elevation of this bar is very close to, but slightly higher than, the average low tide water level. As time progresses and more net accretion takes place, the elevation increases and the coverage of the bar surfaces changes and gradually forms land. The fastest land development as observed within the Shahbazpur Channel from time-series satellite imagery is 8 years. In some parts of the Tetulia Channel, it was found to be as much as 25 years (Sarker et al., 2011). The average time required for land development at different locations is shown in Table 2.1, with the mean tidal range of the location concerned. The time required for land development is not dependent on the tidal range, although higher tidal range demands higher amount of sediment for the same magnitude of land development. The land development process is relatively rapid where both riverine and marine processes are active.
Table 2.1. The time required for land development at different locations in the Meghna Estuary (Sarker et al., 2011). Locations Dominated Mean Tidal Time Required for land Process Range (m) development (year) Outfall of Tetulia Marine 2 22 Channel Shahbazpur Channel Mixed Energy 2 12 South of Noakhali Mixed Energy 3 12 Urir Char Marine 6 16
To pursue a controlled active delta development for overall coastal rehabilitation and land reclamation purposes in the Meghna Estuary, the Government of Bangladesh (GoB) during
20 the past several decades, has initiated several studies namely, the Land Reclamation Project (LRP, 1982), Meghna Estuary Studies (MoWR, 1998, 2001), Coastal Embankment Rehabilitation Project (Saari & Rachman, 2000), Char Development and Settlement Projects (CEGIS, 2009), Integrated Coastal Zone Management Projects, Estuary Development Program (EDP, 2010) etc. Aside from development and rehabilitation of polders for further strengthening the coastal zone, another substantial output from these studies came in the form of facilitated land formation utilizing the net accretion characteristics of the estuary.
An assortment of additional cross-dams planned for the region have later been proposed under the development plan of Meghna Estuary Study (MES), later supplemented by the Estuarine Development Programme (EDP). In an effort to reclaim land in the Meghna Estuary vicinity through enforcing the facilitated land accretion process, the LRP conducted the first of the comprehensive studies in the region, later followed by the MES studies, EDP etc. The LRP proposed and later facilitated the construction of the cross-dams Meghna Cross- dam – 1, Meghna Cross-dam – 2 and the Muhuri Cross-dam. Quantitative land gains from establishment of these dams are 300 km2 (up till 1985), 600 km2 (up till 1985), and 100 km2 respectively. Figure 2.6 illustrates the placement and subsequent land gains in the upper estuarine regions due to these historical cross-dams.
21
Figure 2.6. Placement and land gains in the upper estuarine regions due to these historical cross-dams (source: BWDB).
22 2.9 Proposed Sandwip-Urir Char-Noakhali Cross-dam
Combined erosion and accretion under the influence of river discharge with heavy sediment load, wind waves and tides act on the Sandwip-Urir Char region. The area falls under the macro-tidal zone with tidal range varying from 4m to 6m or even higher. The LRP conducted a pre-feasibility study in 1984 for protection of the Sandwip and Hatiya islands from erosion. The study recommended construction of a cross-dam between Sandwip and Noakhali via Urir Char for elimination of erosion and acceleration of land accretion in the vicinity. This later led to the “Feasibility Study on the Sandwip Cross-dam Development Scheme” on March 1987. The study concluded that the dam would reduce erosion as well as facilitate land formation in the area adjacent to northern Sandwip surrounding Urir Char and southern coast of Noakhali.
Figure 2.7. Map of the Meghna Estuary from the MES (MoWR, 2001).
23 Estimated land gains from implementation of this dam were envisaged approximately 18,000 ha in the then near future with the possibility of an additional 18,000 ha within 30 years. The feasibility study did not find any significant adverse impact from sedimentation due to project implementation. Following the conclusion of the LRP studies in early 1990s, land reclamation was taken up by mainly two studies namely, the Char Development and Settlement Project (CDSP) and the Meghna Estuary (MES) Studies. The CDSP was initiated to carry out the land based activities of the LRP whereas the MES focused more on the hydrological and morphological aspects of the estuary.
The Meghna Estuary Study Phase II (MES II) investigated the flow and sediment dynamics around the Urir Char and Sandwip region. Figure 2.7 shows the interventions map which illustrates the proposed Sandwip-Urir Char-Noakhali cross-dams. The possible impact of this dam on the ambient flow and sediment regime has been studied with the help of the hydro- dynamic model of the estuary. In regards to the LRP assessments, the MES takes a more cautious approach towards the cross-dam implementation in that it hypothesized the resultant increase in flow velocity and tidal range in the vicinity. The study recommends additional surveys and studies to ascertain future hydro-morphological changes in the area, including development of the inter-tidal zones in the alignment of the cross dams.
Following the MES, the Ministry of Water Resources (MoWR) formed a task force through Bangladesh Water Development Board (BWDB) in 2002 to review all previous studies/ reports on coastal interventions in Bangladesh in devising an effective method of harnessing the natural land accretion process. The taskforce report of 2003 recommended the establishment of 19 cross-dams as priority for land reclamation in the estuary and has laid out a 7-year implementation plan. The Sandwip-Urir Char-Noakhali cross-dam was at no. 4 in the serial of priority dams. It extended from Sandwip to Companiganj Upazila of Noakhali mainland and consisted of two enclosure segments: (i) 5 km wide Sandwip-Urir Char segment and (ii) 4 km wide Urir Char-Noakhali segment. The cumulative width of 9 km has, in recent times, reduced due to natural silting up of the channels. The taskforce envisaged a 4- year plan, 2 years for planning, feasibility and design and 2 years for implementation and had estimated an 18,000 ha land gain in the first 15 years and an additional 30,000 ha in the next 30 years.
24 The Estuary Development Program (EDP, 2007) kept the Sandwip-Urir Char-Noakhali cross- dam within the second year of implementation of its pilot cross-dams initiative. It emphasized the necessity of reviewing the previous studies, owing to the on-going morphological changes and socio-economic state. In recent times it has been observed that the northern channel of Urir Char adjacent to Noakhali is gradually silting up naturally. The MES study predicted that the Urir Char is to become part of Noakhali mainland via natural accretion. In this instance, establishment of the cross-dam would benefit the region and overall socio-economic condition as not only will it accelerate land formation, but also greatly reduce erosion in key locations in the area.
The Institute of Water Modeling (IWM) carried out a detailed mathematical modeling study of the Sandwip-Urir Char-Noakhali cross-dams as part of CDSP-III (IWM, 2009). The study preliminarily devised eight cross-dam scenarios, of which three options were assessed via screening. These options included a setup with one dam between Urir Char and Char Clark (Noakhali), a setup with one dam between Urir Char-Char Clark (Noakhali) and one dam between Urir Char-Sandwip, a setup with one dam between Urir Char-Char Saikat and one dam between Urir Char-Sandwip. The study analyzed flow patterns, assessed velocity profiles, residual sediment transport and provided with hypothetical land gains for defined periods of time. From the study, it was found via multi-criterion analysis that out the three options, the first scenario with a one dam setup between Urir Char-Char Clark (Noakhali) was the most viable setup for implementation. The study also hinted the benefit of a two dam setup in the long run.
Urir Char and Sandwip islands are located in a very dynamic zone in the northeastern part of the estuary and is thus subject to constant morphological alterations. Hussain et al. (2014) made a study observing the recent coastline changes for the region using PALSAR and Landsat images. It has been observed that the annual rate of accretion of Urir Char has decreased from 5.84 km2/year between 2007~2010 to 1.05 km2/year between 2010~2013. They also found that Sandwip had been eroding at a higher rate of 3.15 km2/year between 2010~2013 compared to 0.34 km2/year between 2007~2010. Cumulative area increase of 120 km2 had been calculated for Urir Char, Sandwip and Jahajir Char 2007 to 2013, which can be attributed to the drastic expansion of Jahajir Char Island in the recent decade.
25 The Bangladesh Delta Plan 2100 (GED, 2018a) puts emphasis on the construction of cross- dams within Meghna Estuary region bounds as well as in the Noakhali-Urir Char region for not only land reclamation but also as flood risk protection measures. BDP2100 also realizes that accretion and subsequent optimum use of land would significantly contribute to the regional economy and wellbeing of the local people. Sub-strategy CZ 3.2 of BDP2100 deals with accelerating land reclamation process in and around the Meghna Estuary which also includes the Urir Char-Noakhali cross-dam within its investment plan.
BDP2100 (GED, 2018a) also stressed on accelerating the natural accretion processes with cross-dams and other infrastructures in the highly dynamic environment of the coastal regions, which would offer new lands which could be developed in a preferred way. It stated that these new lands should be used for creating necessary infrastructures for industrial and other uses. An ample attention would however, be required for mitigation or adaptation to wind or storm surge hazards by elevating platforms as well as the fresh water availability issues.
2.10 Summarization
The Meghna Estuary is located at the end of one of the most morphologically active deltas in the world. The prevailing net accretion rate can prove to be a major asset if harnessed to full extent for Bangladesh, a country on the constant strive to become a developed nation by 2041. With a small area coverage and burdened with increasing population and depleting natural resources, the need for a way out has never been more sought after. And it has been sought after in recent decades with exploratory land reclamation schemes, some of which have yielded substantial land expansions in the greater Noakhali region. But this is still a vastly untapped phenomenon and one that has potential to not only result in land gains for housing, agriculture forestry etc., but also in combating the combined effects of land subsidence and futuristic climate change induced sea-level rise and subsequent salinity intrusion.
Implementation of some of the cross-dams proposed during the likes of the LRP, MES and EDP have shown promise. The Sandwip and Urir Char location in the northeastern portion the Meghna Estuary is a prime location for cross-dam construction and joining these two islands with mainland Noakhali via means of blocking the flow channels in-between, will
26 yield two-fold results in the form of blockage of flow passage thus facilitate accelerated accretion as well as reduce erosion in the southern Noakhali coast through merging the islands with the mainland. Altering the natural morphological regime will inflict some adverse effects on surrounding flow and sedimentation pattern and that is why these effects have been suggested to be further explored before potential cross-dam construction by both MES and EDP.
The mass-effect of the Assam earthquake has somewhat subsided over the recent decades and the so-far reassuring annual sediment influx mentioned in literature is quickly turning into myth, pointing to the glaring possibility that we might run out of sediment to fuel our hopes of strategic land reclamation and lateral expansion of the coastal region. The need for implementation of cross-dams such as the proposed Sandwip-Urir Char dam is thus of high order and has to be done so with “all cards dealt” in terms of weighing in the positive impacts with the negatives and if need be, nullifying them. In order to utilize the decreasing sediment flux in the Meghna Estuary, decisions have to be taken at the highest echelon of delta planning and this study seeks to aid in this regard and supplement the exploratory research of the effects of cross-dam construction through numerical modeling of hypothetical dam implementation and dam alignment scenarios.
27 3. CHAPTER 3 METHODOLOGY
3.1 Study Approach
This study assesses the impacts of cross-dam on the ambient flow and sediment morpho- dynamics in the north eastern Meghna Estuary region of Urir Char-Sandwip. It applies mathematical modeling in determining tidal asymmetry properties of flow and upon analyses, comparison is made with residual flow, sediment dynamics, depth and bed level changes in the regions closest to the cross-dam alignments. From analyses and comparisons, conclusion is drawn on the effect these dam(s) have on flow and sediment regime in the region and suggestions are made regarding suitable dam alignments and chronological sequencing of dam implementation.
To assess the flow and sediment regime in and around the study area, a two-dimensional hydrodynamic model in horizontal direction based on non-linear shallow water equations is applied to perform the required numerical experiments. The model will simulate the physical processes that influence, impact and alter the tidal flow and morpho-dynamics in the Meghna Estuary. Whereas the model will simulate processes for the entire Meghna Estuary as this will allow effective use of tidal harmonic constituent boundary conditions in the southern end of the model domain and will enable consideration of flow and sediment influx from lower Meghna River, focus will be concentrated to the portion regarding the cross-dam locations near Urir Char and Sandwip.
3.2 Hydrodynamic Modeling Exercise
The modeling exercise is to determine the impact of proposed Sandwip-Urir Char-Noakhali cross-dam on regional flow and sediment regime. Two separate models have been setup and simulated for the purpose. In both cases the underlying model code is the same but one instance has been run for flow analyses and the other for sediment analyses. The flow model has been simulated for a period of 17 days with 16 days perceived as model stabilization time. This is done so because for flow analyses, data from one tidal cycle measuring to 12.5 hours is used which is taken from the last day of simulation period. Also, studies done by
28 Hussain et al., (2013) as well as Hussain and Tajima, (2019) have revealed that there is minimal impact on flow and asymmetry parameters due to the alternating spring and neap cycles. As such, this does not require prolonged simulation periods, runtime it is kept to 17 days to reduce computation times.
The numerical model simulations for flow calculations does not include the effect of river discharge. Wang et al. (1999) stated that for a well-mixed tide dominated estuary, river discharge is negligible in terms of impact on asymmetry and other flow properties. Studies conducted for the Meghan Estuary (Hussain et al., 2009a, 2009b; Hussain et al., 2013; Hussain et al., 2014a; Hussain et al., 2019; Jakobsen et al., 2002) also reveal that, for the location in question i.e. the northeastern corner of the estuary near the Urir Char-Sandwip region, river discharge from the Lower Meghna has little to no impact on tidal flow properties. Actually, the residual flow in the Meghna Estuary especially at the eastern part is highly influenced by the tidal forcing and Coriolis force (Hussain et al., 2009b).
Even though during monsoon the estuary receives significant fresh water with high sediment concentration from the Meghna River and experiences relatively strong south wind, the primary factor that controls the anti-clockwise residual circulation pattern in the estuary remains as tidal forcing. Apparently during other three seasons (post-/ pre monsoon, and winter) wind stress and fresh water discharge become less significant and tidal forcing persists as the single most important factor which contributes to the sediment transport and morphological changes in these parts of the estuary. Only along some of the distributary channels, the residual flow during high discharge period is different from the one from the low discharge periods.
Four cases are considered for the modeling exercise as will be detailed out in the next chapter. To simulate the proposed Sandwip-Urir Char-Noakhali cross-dam, two separate dams are considered in the study at varying alignments. The four cases considered the two dams in combinations. The dam between Urir Char and Noakhali was termed “Dam 1” and the dam between Sandwip and Urir Char was termed “Dam 2”. In brief, the first case (A) has no cross-dams in it (dam 0). It is as the situation stands presently. The second case (B) has the cross-dam between Urir Char and mainland Noakhali (dam 1). The original alignment of this dam considered in previous studies has been slightly shifted to better accommodate ambient flow patterns as will be stated in chapter 4. The third case (C) has only the cross-dam
29 between Sandwip and Urir Char (dam 2). The fourth and final case (D) has both dams implemented and it represents the ultimate end case of this scheme (dam 1+2). The purpose of this exercise is to assess the impact of both cross-dams on the ambient flow regime both separately and in combination and to compare each case with the other as well as with the “no dam” situation.
Impact of cross-dams on flow will be studies through observing resultant flow patterns from model simulations. A particular point in the northern Urir Char channel will be selected as the primary point of interest and flow patterns for both maximum flood and ebb-tide flows in that particular location will be observed in all the study area. This point will be denoted as L1 and will later be one of four locations where sediment results will be assessed. The point in question in the northern Urir Char channel will be selected for the following reasons. Firstly, as the ultimate end result of this dam implementation scheme is the merging of mainland Noakhali with these coastal islands and this point marks the preliminary location for merging. Secondly, this location has also been marked as a point of interest during government studies as the northern mainland side if Noakhali is eroding and the southern, northern tip of Urir Char is accreting. The preliminary purpose of these dams is to put a stop to this phenomenon. Thirdly, this is a common location where all cross-dam scenarios are very likely to have a substantial impact upon both in terms of flow and sediment parameters. Finally, Hussain et al., (2014a) conducted analyses based on measured field data on the same location.
Residual flow patterns were analyzed for all dam scenarios and as a significant portion of sediment flow in Meghna Estuary is due to residual flow (Jakobsen et al., 2002; Hussain et al., 2013), these will facilitate in chalking out sediment deposition locations pending dam implementation. Tidal asymmetry is the prime cause for residual sediment transport in tide dominated estuaries such as the Meghna Estuary (Dronkers, 1986a; Wang et al., 1999).
Vertical wave asymmetry parameters (As1 and As2) will be compared for all scenarios. Horizontal asymmetry will be assessed through determination of tide dominance for dam scenarios to assess transport and deposition of coarse sediment both in bed and in suspension. Slack durations for both flood and ebb-tide will also be studied to assess transport of fine sediment in the area.
The sediment model has been setup with a coarse grid resolution. This has been done to reduce computation times as the sediment model is simulated for 47 days. With a 17 day
30 “model stabilization” time this gives 30 days for assessing sediment transport patterns and this also enables the study of effect of a spring-neap tide cycle upon sediment transport. Four locations have been selected to analyze sediment output from model results as will be explained in the following chapter. Bed level change for these locations will be studied to assess impact of different cross-dam cases on the sediment accretion/ erosion phenomenon on these points. Bed level change for all four cases will be studied and compared with one another to determine the most “effective” alignment of cross-dams and also to predict a chronological sequencing of dam implementation among the two proposed dams.
3.3 Model Description
Horizontal two-dimensional (2DH) depth-average tidal model, based on nonlinear shallow water equations is used to calculate the tidal propagations inside the computation domain. The present model has been successfully applied by Hussain et al. (2013, 2014a) and Taguchi et al. (2013) for the Meghna Estuary area. The numerical model extensively utilizes ‘shallow water equations’ for tidal and storm surge simulations as waves in the Bay have much longer wave length compared water depth (Gill 1982). The model accounts for Coriolis Effect for computations of large-scale tidal currents and is discretized based on a spherical coordinate system. This model adopts an alternating direction implicit (ADI) scheme for computational stability and employs a spherical coordinate system to ensure accuracy during wide-area calculations. The time-step for calculation is 5 seconds for flow model and 10 seconds for sediment dynamics model. The governing equations of the flow model are provided in equations (1) to (3);
UD VD (1) 0 t x y
U U U (2) U V xxx gRMF fV 0 t x y x
V V V (3) U V gRMF fU 0 t x y yyy y
where, F cx (4) x D
31
F cy (5) y D
and U U M x x yx y (6)
V V (7) M y x yx y (8) w HU
Here, Fx and Fy accounts for friction forces; Mx and My accounts for horizontal dispersion due to waves; τcx and τcy are bottom shear stress; is TWL; is density of sea water; f is Coriolis Coefficient and is horizontal dispersion coefficient.
The calibrated and validated numerical model is run for a duration of 17 days for flow calculations and 47 days for sediment calculations under the influence of tidal forcing from the southern boundary to analyze the tidal propagation and tidal asymmetry at the eastern part of Meghna Estuary. Results for tide cycles from the latter 1-2 days are considered for residual flow and tidal asymmetry calculations so that the initial instability in TWL and tidal velocity can be ignored. Wetting-and-drying boundary conditions are applied along the land boundary to account for the large tidal range and shallow mudflats around the offshore islands and also along some of the mainland areas.
3.4 Model Domain
The model domain captures the full extent of the Meghna Estuary area. The northern boundary is at the start of lower Meghna near Chandpur, eastern boundary covers the whole western coast along Chattogram division, the southern boundary is along the south of Kutubdia island and is on the shallower part of the continental shelf within Bay of Bengal and the western boundary lies west of the Tetulia channel, in the Sundarbans (Figure 3.1). The spatial resolution of the calculation domain is 15 arc-seconds (about 463m) for the flow calculation model and 30 arc-second (about 926m) for the sediment calculation model. The flow calculation model has 300 grid points along the north-south direction and 500 grid
32 points along east-west and the sediment model has 150 grid points along the north-south direction and 250 grid points along east-west, both of which gives a domain size of 138.9 km by 231.5 km.
Figure 3.1. The computation domain, the Meghna Estuary showing major islands and channels. Green circles show the measurement locations during Meghna Estuary Study project.
3.5 Bathymetry Data
Successful execution of a modeling exercise for evaluating tidal flow regime and subsequent morpho-dynamics of shallow coast requires backing from good quality bathymetry data. The 0 0 0 bathymetry coverage is from 23.25 N to the north and 21.5 N to the south and 90 E to the west to 920 E to the east. GEBCO (General Bathymetric Chart of the Oceans) 30 arc-second free source data (Ward, 2000) is used as the primary bathymetry data for the modeling exercise which is updated by Hussain and Tajima (2019) using bathymetry data measured during MES 1997. Modifications were made to the combined bathymetry data for abnormally high depth values, especially in the northern channel of Urir Char. Coastlines data rectified by (Hussain et al., 2014b) using ALOS-PALSAR (Advanced Land Observing Satellite’s
33 Phased Array L-band Synthetic Aperture Radar) satellite images, was used as part of bathymetry. This bathymetry has been successfully employed by Taguchi et al. (2013) and Hussain et al. (2019) for tidal model validation and investigating seasonal variation of coastline changes in the Urir Char and surrounding areas and impact of bathymetry changes on tidal characteristics of eastern Meghna Estuary. Modifications in the bathymetry have been made to incorporate cross-dam alignments into the model domain for both Urir Char- Noakhali and Sandwip-Urir Char cross-dams. Figure 3.2 shows the bathymetry data used in the modeling exercise highlighting the entire model domain and Figure 3.3 presents differing bathymetry for the Urir Char region pending different cross-dam alignments.
Figure 3.2. Bathymetry of the model computation domain.
34
Figure 3.3. Bathymetry modifications to incorporate cross-dams into the model domain.
3.6 Boundary Conditions
When it comes to models dealing with ocean tides, small extent domains are at risk of generating errors due to exclusion of certain tidal frequencies, local effects of the tide- generating forces and tidal responses of the solid Earth sphere. Availability of coastal tide gauged data renders predictions using tidal models, especially in shallow waters, inferior to harmonic analysis values (Flather and Williams, 2004). For this study, the offshore ocean TWL at the southern boundary of the model domain has been obtained using the NAO.99b model (Matsumoto et al., 2000). NAO.99b calculates the pure ocean tide with respect to the ocean floor based on five years of TOPEX/POSEIDON altimetry data and the long-period ocean tide map of Takanezawa et al. (2001).
The NAO.99b represents short-period tide values from 16 major constituents and 33 minor constituents and long-period tide values from seven constituents and has a spatial resolution of 0.5 degrees. It has been observed that there is about a two-hour phase difference between water levels at the eastern and western boundaries of the computation domain which is consistent with the findings of Cravajal et al. (2011). Ocean tide approaches towards the
35 northeast direction in an oblique manner and simultaneously reaches the coast of Hiron Point (south of Khulna) and Cox’s Bazar (Figure 3.1), also consistent with Carvajal et al. (2011). This water level data shows a 0.5m difference in spring tidal range between eastern and western end of the computational domain with the eastern end being higher, whereas tidal ranges are nearly identical along these coasts during the neap tide.
For the sediment simulations, river discharge is included in the model u/s boundary along with sediment discharge. Values for river discharge and sediment discharge used for sediment transport simulations are taken in line with Islam et al. (2002) presented in Figure 3.4. These flow and sediment discharge values are adopted so that river input can mimic conditions of a maximum discharge scenario during monsoon, ensuring maximum sediment input to the study location.
Figure 3.4. Average monthly variation of water and suspended sediment discharge of the combined Ganges–Brahmaputra–Meghna River system (Islam et al., 2002).
3.7 Asymmetry Measurements
Tidal asymmetry is the distortion of tidal waves from its symmetric sinusoidal curve patters as it propagates onto shallow waters from deep sea. Two types of asymmetry of tides are measured in the study for assessing flow characteristics namely, vertical and horizontal asymmetry. Vertical asymmetry is associated with the distortion of the water level curve
36 whereby a short rise duration as opposed to a longer fall duration would impart a flood dominance and vice-versa. Horizontal asymmetry is associated with two aspects, tidal velocity and slack water periods. Flood dominance is assumed when flood tide velocity is greater in magnitude than the ebb tide velocity or is slack before flood duration is less than slack before ebb duration and vice-versa. These definitions form the basis of the flow analyses performed in this study. Associated parameters are defined below.
3.7.1 Wave Asymmetry Properties
Wave asymmetry properties measured in the study have been used by Hussain et al. (2013) in studying impact of morphology change on tidal characteristics in the Meghna Estuary. These denote vertical asymmetry where asymmetry of tide is measured in respect to rise and propagation durations of the water level curve. Two types of wave asymmetry are considered.
They are termed as asymmetry 1 (As1) and 2 (As2) and are defined as below from Figure 3.5:
Here, both values are ratios of duration and hence are unit less values. As can be seen from the figure, As1 is the ratio of the time taken (tc) by wave profile from zero up-crossing to maximum (crest) water (ηc) level during the tidal cycle to the time of a full tide cycle and As2 is the ratio of the time taken (Tc) by wave profile from minimum (trough) water level (ηt) to maximum (crest) water level (ηc) to the time of a full tide cycle. In both instances, T is the wave period i.e. period of a full tidal cycle.
37
Figure 3.5. Schematic diagram of tidal water level profile illustrating the definitions of wave asymmetry (Hussain et al., 2013).
3.7.2 Flood/ Ebb Dominance
Tide dominance asymmetry have been defined by Wang et al. (1999) and used by Hussain and Tajima (2019) in determining effect of bathymetry change on tidal characteristics of the Meghna Estuary. These are horizontal tidal asymmetry properties and is measured using velocity profile of tide. As can be seen from Figure 3.6, flood velocity is denoted by Uf and ebb velocity is denoted by Ue. Dominance asymmetry is taken as the ratio of flood velocity to ebb velocity:
If Uf /Ue value is greater than 1 then tide is taken as flood dominant and if value is less than
1, it is ebb dominant. This also translates to the fact that, when Uf > Ue it represents a horizontal tidal asymmetry with flood dominance while the opposite represents the one with
38 ebb dominance. Tide dominance asymmetry is significant for coarse sediment transport of both suspended and bed load in the direction of dominant flow (Wang et al., 1999).
Figure 3.6. Schematic diagram of tidal velocity profile illustrating the definitions of tide dominance asymmetry and asymmetry due to slack water durations (Hussain and Tajima, 2019).
3.7.3 Slack Water Durations
As can be seen from Figure 3.6, both before rise of flood and ebb velocities is a period of slack water. These are termed slack before flood (SBF) or low-water slack (LWS) and slack before ebb (SBE) of high-water slack (HWS), respectively. From definitions, if SBE>SBF (or HWS>LWS) then tide is flood dominant and if SBF>SBE (or, LWS>HWS) then tide is ebb dominant. As time-duration and spatial locations of slack water determine the settling of suspended sediment, the asymmetry related to HWS and LWS is considered to be more important for fine sediment transport (Wang et al., 1999).
3.8 Tidal Asymmetry in Dam Locations
Hussain et al. (2014a) conducted a modeling and field exercise on the tidal characteristics and sediment regime around the Urir Char and Sandwip region. The authors took field measurements for tidal water level, velocity as well as turbidity values and found that strong vertical as well as horizontal tidal asymmetry existed around the highly accreting Urir Char
39 Island. Along the northern channel of Urir Char, flood velocity exceeds ebb velocity which induced residual transport of coarse sediments towards land. Also, duration of slack water before flood is much longer than that before ebb indicating residual transport of fine sediments. A plot comparing these data against each other for a full tidal cycle measured at location B (inset map) in the northern channel of Urir Char during monsoon season is presented in Figure 3.7. In this figure, the continuous black line represents the tidal water level, the red and blue lines represent the northern and eastern velocity components respectively. The dotted black line represents the measured turbidity values.
Figure 3.7. Plot of measured water level, velocity and turbidity along the northern channel of Urir Char Island (Hussain et al., 2014a).
The tidal water level curve in Figure 3.7 clearly shows that the flood-tide has a shorter period of water level rise and a prolonged duration of water level fall thus representing a forward leaning profile. This signifies the existence of vertical flood dominance in the northern channel of Urir Char. Also, from the fact that the red and blue northern and eastern velocity curves are on the negative side, it can be deduced that flow during flood-tide is in the opposite south-western direction. This indicates that primarily tidal water enters the measurement location from the eastern side through the Sandwip Channel as there is very shallow water areas along the western channel of Urir Char Island. As the flood velocity
40 magnitude also exceeds the ebb velocity magnitude, there exists horizontal flood dominance along the channel and it will induce a residual bed load and suspended load transport of coarse sediments to the flood direction (Dronkers, 1986a and Wang et al., 1999) i.e. southwestward direction.
From the plot it can also be deduced that the western flow has a higher velocity compared to the southern flow. During ebb-tide the flow is towards the north-east direction as can be seen from the positive nature of the curves with the northern velocity magnitude being slightly higher than the eastern. From the figure, it can also be seen that the duration of Slack tide Before Ebb (SBE) is significantly shorter than the duration of Slack tide Before Flood (SBF), which favors the residual transport of fine suspended sediments in the ebb direction (Wang et al., 1999). Two turbidity peaks can be observed at both sides with the middle sharp fall occurring during SBE. This means there is less suspended sediments during this time, which again picks up during ebb velocity acceleration. The physical phenomenon explained by Figure 3.7 is important as these sediment dynamics are going to be explored further in the following Chapter.
41 4. CHAPTER 4 RESULTS AND DISCUSSION
4.1 Model Calibration and Validation
The hydro-dynamic model used to assess impact of cross-dam on regional flow and sediment regime has been calibrated and validated by Hussain and Tajima (2019) and applied in determining effect on bathymetry changes on tidal characteristics in the estuary. The model has been validated with 1997 datasets from MES. TWL comparisons at five locations of Meghna Estuary (MES1, MES2, MES4, MES5 and MES6) are presented in Figure 4.1. Datum of TWL is meter Public Works Department (mPWD) which is 0.46m below the Mean Sea Level (MSL). Simulated TWLs show quite good agreement with the observed data except at locations MES1, MES2 and MES6. Model underestimates values for neap tides at these locations.
Table 4.1 shows the error statistics from the comparison of observed and modelled TWL data at the above mentioned five locations along the Bangladesh coast. The root means square deviation (RMSD) and normalized root mean square deviation (NRMSD) ranges from 0.291m~0.385m and 0.039~0.147, respectively. The table shows high correlation coefficient of 0.9 or above at four locations (MES1, MES4, MES5 and MES6) while at location MES2 the correlation coefficient is 0.79. Small values of RMSD and NRMSD as well as high correlation coefficient at all locations suggest that the model is efficient in predicting the tidal amplitude and phase at the study area.
Table 4.1. Error statistics from the comparison of observed and modeled tidal water level data at five locations along the Bangladesh coast shown in Figure 3.1 (Hussain and Tajima, 2019).
Location Duration RMSD1 NRMSD2 CORREL3 MES1 1 Jan 0:00 ~ 31 Jan 0:00; 1997 0.3019 0.1002 0.8999 MES2 1 Jan 0:00 ~ 31 Jan 0:00; 1997 0.2946 0.1470 0.7858 MES4 1 Jan 0:00 ~ 31 Jan 0:00; 1997 0.2910 0.0388 0.9889 MES5 1 Jan 0:00 ~ 31 Jan 0:00; 1997 0.3062 0.0798 0.9551 MES6 1 Jan 0:00 ~ 31 Jan 0:00; 1997 0.3850 0.0792 0.9534
42 1RMSD: Root Mean Square Deviation 2NRMSD: Normalized Root Mean Square Deviation 3CORREL: Correlation Coefficient
Figure 4.1. Model with 1997 data: comparison between observed and calculated tidal water levels (mPWD) at five locations (MES1, MES2, MES4, ME5 and MES 6) of Figure 3.2 (Hussain and Tajima, 2019).
43 The model is also validated in the northern channel of Urir Char (Hussain and Tajima, 2019) using data collected by Hussain et al. (2014a) at locations A and B (inset map of Figure 3.7) during December 2012 and TWL and tidal velocity component data at location B during July 2013. Figure 4.2 shows these comparisons. Comparison between observed and simulated TWLs and also north and east velocity components at location B during July 2013 are presented in the bottom three panels of Figure 4.2. Table 4.2 shows the error statistics for the comparison of observed and modelled TWL data and tidal velocity data at locations A and B around Urir Char. For TWL comparison, at location A the RMSD and NRMSD are 0.37m and 0.08, respectively and correlation coefficient is found to be 0.98. While, at location B at two different seasons, RMSD and NRMSD ranges from 0.24m~0.25m and 0.07~0.09, respectively and correlation coefficient 0.97~0.98.
Small values of RMSD and NRMSD as well as high correlation coefficient in both locations indicate that the model is effective in predicting the tidal amplitude and phase at the study area. The simulated values represent the depth averaged velocity components. So, some discrepancy between the observed and simulated velocity components, especially during high water level conditions, are expected. The strong correlation coefficients, 0.93 for north components and 0.96 for south components, between observed and simulated velocity components suggest that the model can simulate the velocity phase quite accurately.
Table 4.2. Error statistics for the comparison of observed and modelled tidal water level data and tidal velocity data at two locations around Urir Char shown in Figure 3.7 (Hussain and Tajima, 2019).
Location and parameter Duration RMSD NRMSD CORREL TWL at A 6 Dec 14:15 ~ 7 Dec 12:30, 2012 0.3741 0.0761 0.9770 TWL at B 6 Dec 20:30 ~ 7 Dec 17:15, 2012 0.2515 0.0715 0.9848 TWL at B 7 Jul 20:00 ~ 8 Jul 9:45, 2013 0.2452 0.0898 0.9734 North Velocity (u) at B 7 Jul 20:00 ~ 8 Jul 9:45, 2013 0.1042 0.1059 0.9327 South Velocity (v) at B 7 Jul 20:00 ~ 8 Jul 9:45, 2013 0.1002 0.0784 0.9638
44
Figure 4.2. Model validation with 2012-2013 data: comparison between observed and calculated tidal water levels (mPWD) at two locations (A and B) shown in figure 3.7, And north and east velocity components (m/s) at location B (Hussain and Tajima, 2019).
45 4.2 Simulation Cases
The Land Reclamation Project stated the necessity of joining Sandwip with mainland Noakhali via establishment of cross-dam. However, as this connection has to be made keeping the then newly developed Urir Char island in the middle, the initial alignment included two dams in total, one from mainland Noakhali to Urir Char and the other from Sandwip to Urir Char. Initial plans revealed that this dam, although will connect Sandwip with mainland via Urir Char, was to be a continuous structure spanning for 23 km as can be seen from Figure 4.3. This study differs in certain aspects and diverts the analogy conceived during these studies regarding cross-dam alignment.
Whereas, constructing a singular prolonged dam would yield certain benefits, it will however obstruct natural tidal floodplain regions. So, two separate dam structures are used in the modeling study. The cross-dam scenarios considered in the study differ somewhat from IWM (2009) in that a separate case is considered with only the dam between Sandwip-Urir Char has been considered. Other than this, the cases considered includes a dam between Urir Char- Mainland Noakhali (Char Clark) and a two dam setup with dams between Urir Char- Mainland Noakhali (Char Clark) and between Sandwip-Urir Char.
As can be seen from Figure 4.3, dam alignment connecting Sandwip to Urir Char is from the northern tip of Sandwip to south-southeastern tip of Urir Char and the dam connecting Urir Char to the mainland connects the northwestern tip of the former to the southeastern tip of the latter. The Urir Char-mainland cross-dam alignment in the model setup differs in this case because as can be seen from the flow pattern in Figure 4.5, there is a tidal meeting point in the southwestern corner of Urir Char and hence this place has been used as the alignment for the Urir Char-Noakhali cross-dam.
Four different cases have been simulated in the 2DH model. They have been named from A to D. The Urir Char-Noakhali dam is named “Dam 1” and the Sandwip-Urir Char dam is named “Dam 2”. A represents the base condition of the region where there are no cross-dams (A no-dam). The flow circulation pattern in this scenario is to mimic that of the existing flow regime. B has the Urir Char-Noakhali cross dam extending from the western coast of Urir Char southwest towards the mainland (B dam 1). C has the Sandwip-Urir Char cross-dam extending from the northern tip of Sandwip to south-southeastern tip of Urir Char (C dam 2).
46 D has both the cross-dam alignments (D dam 1+2). The goal here is to assess dam influences on flow and sediment regime for one alignment with the other and determine the effectiveness of these dam in facilitated accretion and probable sequencing of dam construction so as to minimize detrimental effects. The four cross-dam cases have been illustrated in Figure 4.4.
Figure 4.3. Initial alignment for proposed Sandwip-Urir Char-Noakhali cross-dam (EDP, 2010).
47
Figure 4.4. Cross-dam cases in model simulation.
48 4.3 Flow Regime
The flow model has been simulated for a period of 16 days with a time-step of 5s and flow results have been extracted from the last 12.5 hours so that it can avert any initial calculation fluctuations during the model stabilization phase. In the following section, flow results have been presented at instances of maximum velocity stages during both flood and ebb tides as well as during slack before flood (SBF, or low water slack (LWS)) and slack before ebb (SBE, or high water slack (HWS)). Flow directions are plotted as arrowheads and flow magnitude is presented as the underlying contour. Flow patterns along the major channels in the study domain namely, the Sandwip channel, the northern channel of Urir Char, the western channel between Sandwip and Jahajir Char and the westernmost channel between Jahajir Char and mainland Noakhali has been assessed to study the existing flow patterns and the impact of differing cross-dam alignments.
Figure 4.5. Simulated tidal velocity distribution during peak flood velocity in location L1, contour shows velocity magnitude in ms-1.
49 The simulated tidal velocity distribution and magnitude during peak flood velocity for location L1 (Figure 4.4) in the study area have been shown in Figure 4.5. Vertical and Horizontal scales on all spatial plots represent model grid values. The lighter color in the contour map represents higher magnitudes of velocity. During the build of flood waters, all current in the aforementioned three channels are in the northern direction. The bulk of this flow enters through the easternmost Sandwip channel as it dominates the flow in the channel around Urir Char, both from northeast and southeast sides. Strong currents in excess of 2 ms-1 enters from the eastern side and creates a counter-clockwise flow around the northern Urir Char channel which as was also observed by Hussain et al. (2014a); Jakobsen et al. (2002) and Barua (1990). This flow circles around the channel and on the south western side, meets up with northern flowing current from the south. This creates a tidal meeting point in the southwest corner of Urir Char and is thus the prime location for the placement of the Urir Char portion of the proposed Sandwip-Urir Char-Noakhali dam scheme, connecting the island with the mainland.
The flow through Sandwip channel is superior in both volume and magnitude as can be seen from its dominance in entering the proximity of Urir Char from both the island’s northern and southern channels. Northward current enters the study area from both the middle channel between Sandwip and Jahajir Char which from this point onwards will be termed “Jahajir Char Channel” and also the channel between Jahajir Char and Noakhali which from this point onwards will be termed “Noakhali Channel”. Both these flows get influenced into slightly diverting northwestwards by the strong southern portion of the Sandwip channel flow and this resultant flow enters the Urir Char channel from the southern side, meeting with the north to south flowing counter-clockwise current and creating the tidal meet point.
Flow velocity is on the high side as can be seen from Figure 4.5 and this can reach up to 3 ms-1 as flow enters the shallow and constricted northern and southern portions of the Urir Char channels. It is interesting to note the existence of zero flow zones in three location which are the in the western portion of the Urir Char channel and the northern portion of both Jahajir Char and Noakhali channels. These mark the tidal meeting points and are potential sediment deposition locations. It is also to be noted that there is a large zero flow region near the southeastern coast of Jahajir Char island.
50 Sediment dynamics in the particular point of interest (point L1 in Figure 4.4 and point B in inset map of Figure 3.7, on the northern Urir Char channel) can be explained by this flow cycle. Hussain et al. (2014a) observed a two-hour phase difference between peak turbidity values in that particular location through field sampling in 2012. Large tidal currents in the northern Urir Char channel will cause quicker pickup of local sediments at location L1. Small currents in the western Urir Char channel in-combination with the strong currents all along the northern channel, will continue to supply sediments from the eastern side towards location L1 and create a peak in the turbidity values with such a long phase lag. As such there will be natural accretion in this area. Similar situation was also observed by Barua (1990) along the eastern coast of Hatiya. Figure 4.5 also reveals that high flood current traveling through the northern Urir Char channel towards southwest direction has a larger western flow component as can be noticed also from Figure 3.7. High velocity magnitude of flow is also observed at the southern part of the Urir Char Island which is due to the distribution of the enormous volume of flood tide flow coming from the Sandwip channel.
Figure 4.6. Simulated tidal velocity distribution during peak ebb velocity in location L1, contour shows velocity magnitude in ms-1.
51 Simulated tidal velocity distribution and corresponding velocity magnitude contour during peak ebb velocity period at location L1, as presented in Figure 4.6 shows the opposite scenario where the flow convergence of the tidal meet points reverses into flow divergence resulting prolonged zero flow regions along western side of Urir Char channel, entire Noakhali channel and northern part of Jahajir Char channel. Clock-wise flow in present around Urir Char and all the eastern, western and southern channels retract flow from the study region. During this time, mudflats around the northern and southern parts of Urir Char start to get exposed, which increases up to peak ebb water levels. Peak velocities can reach up to around 2 ms-1 in the northern and southern narrow flow channels of Urir Char. It is evident from Figure 4.5 and Figure 4.6 that flood velocity exceeds ebb velocity, indicating a horizontal dominance, especially surrounding Urir Char. This will induce a residual bed load and suspended load transport of course sediments towards the flood direction. (Dronkers, 1986; Wang et al., 1999).
Figure 4.7. Simulated tidal velocity distribution during slack before flood (SBF) in location L1, contour shows velocity magnitude in ms-1.
52 Figure 4.7 illustrates the study area with the simulated tidal velocity distribution and corresponding velocity magnitude contour during slack before flood (SBF) in location L1. In this instance, almost the entire north and western Urir Char channel has near zero flow. This can be linked with the prolonged SBF periods as seen in Figure 3.7. Zero flow zones can also be seen in extended portions in Noakhali channel and northern portion of Jahajir Char channel. These are the locations where intertidal mudflats are exposed the most during ebb- tide.
Figure 4.8. Simulated tidal velocity distribution during slack before ebb (SBE) in location L1, contour shows velocity magnitude in ms-1.
Simulated tidal velocity distributions in the study region during the slack before ebb (SBE) at point L1 is presented in Figure 4.8. Dark portions indicate the presence of very small tidal velocity at the northern channel of Urir Char. In Figure 3.7, during SBE in the northern Urir Char channel, there is a sudden sharp fall of turbidity, which can be attributed to the presence of this slack water in that location. This is quickly followed by a sudden sharp rise in
53 sediment concentration at the end of SBE due to the rundown of tidal water from the intertidal mudflats carry sediment in suspension. The second turbidity peak is also followed by a sharp fall as suspended sediment concentration sharply reduces when the supply of sediment is cut-off during full exposure of intertidal mudflats.
Figure 4.9 shows a comparison between flow current and velocity magnitude in the study area for peak flood velocity in location L1. As seen before, without any cross-dams installed (A no-dam), flow enters the region from Sandwip channel to the east, Jahajir Char channel from the south and Noakhali channel from the western side. The dominating flow volume of the Sandwip channel imparts a counter-clockwise flow around Urir Char that meets up with flow from the southern channels along the south western part of the island and creates a tidal meet point. Tidal meet points also exist in the northern portion of Jahajir Char and Noakhali channels. High flood velocity exists in the north and south narrow channels surrounding Urir Char.
With cross-dam placement between Urir Char and mainland Noakhali (B dam 1), flow patterns start to change. Overall flow magnitude in the mid portion of Sandwip channel starts to decrease as the counter-clockwise flow around northern Urir Char gets blocked by the dam in place, creating a stagnant of somewhat backflow situation in the narrow channel, extending effect further southeast towards the Sandwip channel. The zero-flow portion of tidal meeting point from A (no-dam) increases several folds especially in the southern side of the cross- dam and merges with the northern tidal meet point of Noakhali channel, extending further southwards. Strong tidal currents still persist both in the northern and southern channels of Urir Char. This high velocity will pick up local sediments and will deposit them in the northern and southern vicinity of the cross-dam thus further accelerating the pre-existent natural accretion process.
C (dam 2) has the cross-dam that connects Sandwip with Urir Char. This however creates a unique situation in the area. From observation, it can be seen that there exists zero flow along the entire eastern coast of Sandwip up to the cross-dam side and south east corner of Urir Char as well. This is due to the fact that flow from the Sandwip channel cannot pass through the southern circular channel of Urir Char, instead hits the south eastern corner of the island. This creates the stagnant water locations in the area. The zero-flow region in north Noakhali channel decreases from the southern side as flow can pass through this channel relatively
54 unhindered as there is no Urir Char-Noakhali dam. This zero-flow zone gets pushed eastward as north flow from Jahajir Char channel meets with circular flow of Urir Channel and flow from Noakhali channel.
When both cross-dams are in place (D dam 1+2) the zero flow regions in Sandwip channel decreases as current altogether decreases in the Sandwip channel due to placement of both dams. Flow velocity in Jahajir Char channel increases in both C and D as placement of Sandwip-Urir Char dam blocks flow from Sandwip channel, allowing flow in the middle Jahajir Char channel to move about unobstructed. For similar reason flow velocity decreases in the southern circular channel of Urir Char as well.
Figure 4.10 shows the comparison between flow current and velocity magnitude in the study area for peak ebb velocity in location L1. In both case A (no-dam) and case B (dam 1), similar flow patterns exist as there is prominent ebb flow recession in the Sandwip channel. Zero-flow exist along the western portion of Urir Char, entirety of Noakhali channel and bulk of Jahajir Char channel. The highest velocity exists in the southern circular channel of Urir Char and as it retracts flow via Sandwip channel, there is comparatively higher velocity prevalent in Sandwip channel.
In the case of case C (dam 2) and case D (dam 1+2), different situation exists. In C, almost the entire area exhibits zero flow as the presence of only the Sandwip-Urir Char dam block flow retraction through the Sandwip channel and as flow retraction from Jahajir Char channel is not possible, the zero-flow zone extends all around Urir Char and Jahajir Char channel. In D, the unprecedented zero-flow region of C gets reduced due to the placement of Urir Char- Noakhali dam in conjunction with the Sandwip-Urir Char dam. The placement of Urir Char- Noakhali dam blocks retraction of southern Urir Char channel flow via the circular northern channel. This flow now moving through the Jahajir Char channel thus increases velocity in that particular channel.
55
Figure 4.9. Comparison of simulated tidal velocity distribution in the study area during peak flood velocity at location L1 for the different cross- dam scenarios, contours show velocity magnitude in ms-1.
56
Figure 4.10. Comparison of simulated tidal velocity distribution during peak ebb velocity at location L1 for the different cross-dam scenarios, contours show velocity magnitude in ms-1.
57 4.4 Residual Flow Patterns
Figure 4.11 shows the residual flow patterns in existence in the study area under natural conditions without the effects of any dams. The blue lines indicate the residual current and the underlying green arrows show the magnitude of current in that particular direction. There is a strong northern residual current flowing through the Sandwip channel that flows all the way through the northern Urir Char channel creating counterclockwise flow as well as the southern Urir Char channel. The net residual flow through Jahajir Char channel and Noakhali channel is from north to south direction. Flow velocities are high in the northern part of Sandwip channel due to a reduction in depth and in both the north and southern portion of Urir Char channel due to constriction of flow. There are some turbulence present in the southern portion of Sandwip channel, in the flow diversion portion in the southern Urir Char channel and at the tidal meet point at the west of Urir Char. Thus, flow enters though the eastern part and exits through the middle and western part.
Figure 4.11. Residual flow in the study area with no cross-dams installed.
58 A comparison among the residual flow patterns among the four cross-dam scenarios are presented in Figure 4.12. It is observed that these natural patterns start to change with the implementation of cross-dams in the vicinity. For B (dam 1), the dam in the west of Urir Char cuts off the counterclockwise flow coming in from the Sandwip channel thus creating a breakage of flow and turbid zone in the northeast corner of Urir Char. Flow magnitude in Noakhali channel is a bit more unevenly distributed as the southern Urir Char flow from Sandwip channel gets diverted into Noakhali channel. Flow velocity intensifies in the southern channel of Urir Char as this includes flow that would otherwise have circled the northern Urir Char channel. Flow magnitude also increases along the mid portion of Jahajir Char channel.
In C (dam 2), flow comes northwestward through the Sandwip channel and circles around Urir Char. Flow magnitude decreases in the Sandwip channel in comparison to B (dam 1) as flow passage is unhindered across Urir Char. Flow towards the southern channel of Urir Char is however obstructed and this flow mostly adds to the circling flow around Urir Char and increases flow magnitude. This flow circulates around Urir Char to the western part of the study area and is directed southwards via both Noakhali and Jahajir Char channels. As flow magnitude and volume is increased in this instance, smooth transition of flow to the two southern channels is not entirely possible as there is turbulence visible in the southern Urir Char channel and at the northern end of both Noakhali and Jahajir Char channels.
The overall flow regime from C (dam 2) is somewhat stabilized in D (dam 1+2) with the implementation of both dams. Although flow through the northern side of Urir Char is again obstructed and as it cannot pass through the southern Urir Char side as well, it reverts back south through Sandwip channel. As a result, two phenomena are observed, firstly there are turbulence zones in the mid portion of Sandwip channel and secondly, the southwards flow line that was barely visible in the first three scenarios have shifted significantly northwards. Flow patterns in both the other two southern channels have also shifted completely. As there is no north to south flow coming around Urir Char, both Noakhali and Jahajir Char channels have north-going flow with low magnitude. Flow partially gets diverted southwards near the southern triangular shaped region of Jahajir Char channel, creating turbulence near the southeast coast of Jahajir Char island.
59
Figure 4.12. Comparison of simulated residual flow in study area for different cross-dam cases.
60 4.5 Wave Asymmetry
As wave propagates forward, the distortion of tidal waves is measured in several ways. As described in section 3.7 (Figure 3.5), asymmetry 1 is the ratio of time taken by wave profile from zero-up-crossing to reach crest water level to the time of a total flood-ebb cycle; and asymmetry 2 (Figure 3.6) is the ratio of time taken by wave profile from trough water level up to maximum water level to the time of a total flood-ebb cycle. Lower values indicate higher asymmetry.
Figure 4.13 presents side-by-side comparison between asymmetry 1 and 2 values for the study region under natural conditions. Vertical and Horizontal scales on all these plots represent grid spacing values. As tidal wave propagates from the southern deep sea and enters the estuary region, both asymmetry 1 and 2 is increasing. As can be seen from the figure, asymmetry 1 values are relatively higher in the entire southwestern portion near Jahajir Char and Hatiya islands. Asymmetry is relatively lower in the Sandwip channel owing to its larger geometric section and deeper bathymetry. But as flow from Sandwip channel enters the narrow northern Urir Char channel, flow velocity increases and along with this, asymmetry values increase. Western portion of Urir Char and north half of Noakhali channel has the high asymmetry values in the region with highest values observed in the vicinity of the tidal meet point to the west of Urir Char.
Figure 4.13. Comparison of As1 and As2 for study area without and cross-dams implemented.
61
Figure 4.14. Comparison of asymmetry 1 values among the different dam cases in the study area.
62 4.5.1 Asymmetry 1
Figure 4.14 shows distribution of asymmetry 1 values in the study area under different cross- dam scenarios. It can be seen that in B (dam 1), asymmetry increases in the Noakhali channel as flow from Sandwip channel gets almost fully diverted through the southern narrow channel near Urir Char. Asymmetry increase along the northern portions of Sandwip channel and Jahajir Char channels as well due to this flow diversion. In C (dam 2), asymmetry along the entire length of Noakhali channel and mid-portion of Jahajir Char channel decreases due to the fact that as flow is cutoff from entering through the southern Urir Char channel via dam implementation, this flow almost entirely circulates through the northern Urir Char channel and this increased flow volume and magnitude lowers the asymmetry values in the immediate further reaches. The western portion of Urir Char still has relatively very high asymmetry due to continuing circular flow pattern through narrow and shallow channel. In D (dam 1+2), overall asymmetry values increase along both Noakhali channel and Jahajir Char channels.
4.5.2 Asymmetry 2
Asymmetry 2 values under different cross-dam scenarios are presented in Figure 4.15. For B (dam 1), asymmetry increases further southward along the Noakhali channel. However, asymmetry decreases somewhat along the northern circular reach of Urir Char as flow current is reduced in the northeastern side of Urir Char due to dam implementation. As a result, the highest asymmetry portion in A (no-dam), which is the same as the portion just in the northern vicinity of the dam in B (dam 1), is reduced. In C, the flow channel in the north of Urir Char is opened up and the one south is closed off by dam. This translates into relatively higher asymmetry 2 values in the west of Urir Char. Asymmetry however, reduces towards the south of Noakhali channel and at the mid portion of Jahajir Char channel. Asymmetry also increases along the north eastern corner of Urir Char due to increased velocity of unhindered flow. With both dams incorporated (D dam 1+2), asymmetry 2 decreases along the northeast of Urir Char and increases along south of Noakhali channel. At close proximity to the north of the Urir Char-Noakhali dam, asymmetry is higher compared to scenario B. Maximum asymmetry prevails in between the two cross-dams. High asymmetry values indicate sediment transport and deposition along dominated flow direction in the region thus indicating accretion possibilities in the vicinity of the cross dams.
63
Figure 4.15. Comparison of asymmetry 2 values among the different dam cases in the study area.
64 4.6 Flood/ Ebb Dominance
An estuarine region is flood dominated if the duration of flood tide is longer than that of ebb tide and vice-versa. Flood dominancy tend to induce transport of suspended sediment in the flood direction and same happens towards ebb dominated regions (Wang et al., 1999). Figure 4.16 presents the comparison of vertical (water level) and horizontal (velocity) dominance for the study area. The figure on the left shows the presence of strong flood dominance in the western part of Urir Char and the entirety of Noakhali channel. Also, the region surrounding the three islands and partly the east coast of Hatiya is flood dominated. This is in line with (Wang et al., 1999) that shallow potions are flood dominated and deeper portions are ebb dominated. Vertical dominance is the ratio of flood to ebb water level and horizontal dominance is the ratio of flood to ebb velocity. Values greater than 1 in both cases thus indicate flood dominance and vice-versa.
Horizontal dominance on the right-hand side shows similar patterns as strong flood dominance exists in the western side of Urir Char and along the Noakhali channel. The area surrounding the three islands are mostly flood dominated with the southern deeper zones tilting a bit more towards ebb dominance. In comparison with Figure 4.13-4.15, the portions with relatively high asymmetry have prevalent flood dominance. It is interesting to note from the figure below that the southernmost area in the extent, beyond the ebb dominated zone is again flood dominated.
Figure 4.16. Comparison of vertical vs horizontal dominance for study area.
65
Figure 4.17. Comparison of vertical (water level) dominance in the study area for all dam cases.
66 4.6.1 Vertical (WL) Dominance
The vertical asymmetry in terms of water level dominance in the study area has been portrayed in Figure 4.17. Western portions of Urir Char and the extent of Noakhali channel are highly flood dominated, with this dominance extending south halfway up to the mid-east coast of Hatiya. With the dam between Urir Char and mainland in place (B dam 1), the flood dominated area remains almost identical with intensity further propagating southwards. The Jahajir Char channel is converting fully to flood dominance. Placement of cross-dam between Sandwip and Urir Char only (C dam 2), shows more plausible impact on the natural setting (A no-dam). Here the high flood dominance extends from the western and southern Urir Char channels to almost the extent of Jahajir Char channel. Scenario D (dam 1+2) shows similar patterns as C but with increasing intensity of flood dominance much like between A and B. It is interesting that chronologically from scenarios A to D, flood dominance intensity gradually decreases in the Sandwip channel, indicating a shift in flood dominance from eastern to western-southwestern portion of the study area.
4.6.2 Horizontal (Velocity) Dominance
Figure 4.18 shows the horizontal asymmetry in terms of velocity dominance in the study area for all situations. Horizontal flood dominance is prevalent in the northern portion of the region with high values in the west of Urir Char and southern extent of Noakhali channel. There are small pockets of ebb dominance in between, especially in the northeast corner of Urir Char. Similar traits with water level dominance is that, horizontal flood dominance intensity also gradually decreases in the Sandwip channel and it increases in the south western side of the three islands as before, with similar increase in flood dominated area in Jahajir Char channel.
67
Figure 4.18. Comparison of horizontal (velocity) dominance in the study area for all dam cases.
68 4.7 Slack Water Duration
Slack durations represent the second type of horizontal asymmetry. Figure 4.19 shows the comparison of “Slack Before Flood” (SBF) and “Slack Before Ebb” (SBE) durations in study area for A (no-dam). It is observed that, flood slack durations are very low for almost the entire study area with the exception of some very little portions in west of Urir Char and south of Jahajir Char. Ebb slack durations are however, much higher in the northern channel of Urir Char channel and in the southern portion of Noakhali channel. Prolonged ebb slack durations also exist in the south coast of Jahajir Char island and northern portion of Jahajir char channel. As duration of slack before ebb (SBE) values are higher for the aforementioned locations than slack before flood (SBF) values, these locations are flood dominant which is consistent with the findings presented in the previous sections. Temporal duration and spatial locations of slack water determine the settling of suspended sediment, the asymmetry related to SBF and SBE is considered to be more important for fine sediment transport (Wang et al., 1999).
Figure 4.19. Comparison of “Slack Before Flood” (SBF) and “Slack Before Ebb” (SBE) durations in study area for A (no-dam), measurement scale in hours.
69
Figure 4.20. Comparison of SBF of “Low Water Slack (LWS)” values among the different scenarios in the study area, measurement scale in hours.
70 4.7.1 Slack Before Flood (SBF)
Slack before flood or “Low Water Slack (LWS)” value distribution within the study area are presented in Figure 4.20. LWS durations are low for most of the study area and this holds more or less true for all cross-dam scenarios. However, high slack duration area observed in scenarios B (dam 1) and D (dam 1+2) near the immediate vicinity of the Urir Char-Noakhali dam. There is also a slight increase in sack duration closest to the Sandwip-Urir Char dam location. These short slack durations for flood tide are consistent with the findings of Hussain and Tajima, (2019).
4.7.2 Slack Before Ebb (SBE)
Figure 4.21 presents the slack before ebb or “High Water Slack (HWS)” value distribution within the study area. HWS values show a starker contrast within the study area in comparison to flood slack values. High slack values are observed in the northern channel of Urir Char and in the southern part of Noakhali channel. High slack values are also observed in the northern part of Jahajir Char channel and at the southern coast of Jahajir Char island. This hold mostly true for B (dam 1) as there are no plausible temporal of spatial variation in HWS values. However, differences are observed in scenarios C (dam 2) and D (dam 1+2) i.e., when the Sandwip-Urir Char cross-dam is implemented. As can be seen from both C and D, the placement of this particular dam results in the slack durations reducing in the northern Jahajir Char channel and the spatial increase in durations in south coast of Jahajir Char island. Also, high slack durations in the northern and western Urir Char channel shifts to the west as values decrease in the northern channel. But overall slack durations increase throughout Sandwip channel. High slack duration in the southern part of Noakhali channel shifts to the north of the channel.
71
Figure 4.21. Comparison of SBE of “High Water Slack (LWS)” values among the different cross-dam cases in the study area, measurement scale in hours.
4.8 Sediment Dynamics
Sediment transport and deposition is a very unpredictable phenomenon for the Meghna Estuary. Discussion up till now has been focused on flow and tidal characteristics and how they will affect sediment transport within the study area. But the actual implications of these
72 flow mechanics lie in the aftermath which is the resultant erosion and accretion process that takes place within the cross-dam area as impact of dam implementation. Variations in depth and temporal bed-level changes are looked at for assessing sediment patterns.
Figure 4.22 illustrates the variations in bed levels at all four selected locations within the study area (L1, L2, L3 and L4) during simulation periods in meters. It can be seen from the figure that depth profile in locations L1 and L4 (marked in blue and red respectively) follow regular accretion patterns both in natural conditions as well as in cases with dams installed. L3 (marked in orange) follows an uneven pattern although overall accretion is taking place in that location. L2 (yellow) however, exhibits a clear overall erosion pattern.
Natural flow patterns in the region shows that the study location is flood dominated and residual flow comes through the eastern Sandwip channel and encompasses Urir Char both through the northern and southern channels. As per this flow circulation, both L1 and L4 are located in the outer periphery on the dam 1+2 bound region whereas, L2 and L3 are located in the inner bound (Figure 4.4). As can be seen from Figure 4.12, residual flow in Jahajir Char channel and Noakhali channel directly comes from the divergence of flow through the Sandwip channel.
For B (dam 1), flow from Sandwip channel through the northern Urir Char channel gets obstructed. This results in backwater effect and deposition of sediment. This also results in greater flow current going through the southern Urir Char channel. As this flow enters the channel through L4 location, deposition takes place, but as this flow traverses forward, gaining velocity, near L3 this deposition decreases and by the time it reaches L2 with high velocity, erosion takes place in that location. For C (dam 2), this is reversed with south Urir Char channel closed and north channel opened, resulting again in erosion L2 due to increased velocity. For D (dam 1+2), both channels are closed and flow through Sandwip channel is halted resulting in erosion in L1 and L4. Residual flow in Jahajir Char and Noakhali channels reverse (Figure 4.12) resulting in slow accretion in L3 and a substantial reduction in erosion in L2 (Figure 4.29) due to flow losing velocity.
73
Figure 4.22. Comparison of bed level changes in all four locations for each and every cross-dam case.
74 Since none of the locations exhibit a full regular erosion or accretion pattern, as sould be the case in real case, spatial bed level changes are taken in three instances. Although four locations across four cross-dam scenarios show differing accretion/ erosion patterns, some resemblence of similarity can be seen and as such, bed level changes are taken after 16 days and 29 days along with the simulation end time of 47 days. These spatial bed level snapshots for all locations across all cases are illustrated in Figure 4.23.
For A (no-dam) it can be immediately seen that, bulk of the natural accretion is more concentrated in the southern tips of Jahajir Char and Sandwip. These acretion, while could be beneficial in the long run, does not have signiicant impact on the merging of Urir Char or Jahajir Char with mainland Noakhali which is the imminent goal of land reclamation schemes. It can also be seen that comparitively, more erosion is taing place along the Noakhali channel and Jahajir Char channels, a phenomenon that has to be reversed for the purpose of accelerated facilitated land gains in the region.
Gradually, with B (dam 1), accretion patterns reduce in the southern part of the southern islands and it is more concentrated along northeast corner of Sadwip channels and in the Jahajir Char and Noakhali channels and in the vicinity of dam locations. Option C (dam 2) increases the erosion phenomenon in between dam locations (in locations L2) but this erosion is decreased in D (dam 1+2) as both northern and southern channels of Urir Char are blocked by dam installments. Overall erosion and accretion patterns suggest that implementation of both dams spark the start of facilitated accretion in regions adjacent to mainland and in between the islands that would, in the long run, accelerate land formation in the region.
Sedimentation pattern is affected by the spring neap cycle as can be seen from Figure 4.22 where depth variation for all dam cases are shown in location L4. This location experiences gradual, unhindered accretion characteristics all throughout the simulation periods and with all dam cases. It can be seen that this accretion pattern is high during spring, and somewhat dampened during neap tide periods. But for prolong periods, this does not do much to alter patterns or impact sedimentation processes in the region.
75
Figure 4.23. Bed level changes in the study area in meters during three different chronological instances (16 days, 29 days and 47 days) for all cross-dam cases.
76
Figure 4.24. Variation of depth in location L4 vs water levels over computation period showing effect of alternating spring-neap cycles on depth undulations.
77 Figure 4.25 to 4.28 shows snaps of variations in depth with respect to changes in water level, velocity magnitude and sediment concentrations for a tidal cycle for all locations across all cross-dam scenarios. As can be seen, these follow the overall patterns presented in Figure 20 with L1, L2, L3 showing accretion and L2 showing erosion. However, for D (dam 1+2) erosion is substantially reduced in L2 as found in Figure 4.28 which is due to changes in residual flow pattern as explained previously.
In these figures, depth measured in meters and is shown in black, water level is measured in meters and is shown in blue, velocity magnitude is measured in ms-1 and is shown in red and finally, sediment concentration is measured in mg/l and is shown in orange. It can be seen that accretion and erosion is marked as increase and decreases in depth curve which occurs just as tidal flow transitions from ebb to flood, which follows peaks in both flood velocity and sediment concentrations. Velocity and sediment concentration share common peaks periods and, in all cases, exhibit a phase lag of around 1.5 to 2 hours. These results are in accordance with the plot generated by Hussain et al. (2014a) presented as Figure 3.7.
From flow analyses, it has been found that all four location lie in flood dominated regions and this can also be seen from the fact in these figures that minimal to no change in depth occurs within the ebb portion of the tidal cycle. Also, it can be seen that both flood water level and velocity peaks are higher than corresponding ebb peaks which would imply that flood to ebb ratios for both water level and velocity would yield values in excess of 1 which establishes flood dominance asymmetry in the region in terms of vertical As1, As2 and dominance. From these figures, it can be seen that slack period before flood (LWS) exceeds slack before ebb (HWS) for L2, L3, L4 locations which implies horizontal ebb dominance and the reverse for L1 (flood dominance) in terms of slack durations as was found from flow analyses.
These observations are similar to ones presented in Figure 3.7 where flood-tide exhibits shorter rise and a prolonged fall duration, signifying existence of vertical flood dominance. Similarly, magnitude of flood velocity exceeds that of ebb velocity thus there exists horizontal flood dominance along the channel and it will induce a residual bed load and suspended load transport of coarse sediments to the flood direction (Dronkers, 1986a and Wang et al., 1999) i.e. southwestward direction.
78
Figure 4.25. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in A (no-dam).
79
Figure 4.26. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in B (dam 1).
80
Figure 4.27. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in C (dam 2).
81
Figure 4.28. Comparison of changes in depth with water level, velocity and sediment concentration for all locations in D (dam 1+2).
82 Based on above discussions and selected 16 days, 29 days and 47 days’ simulation time, bed level change values at all four locations for all cross-dam cases are presented in Table 4.3 and the corresponding bar plot is illustrated in Figure 4.29. It can be seen from the figure that, the three cases A, B, and C all induce accretion in L1, L3, L4 and erosion in L2 for reasons explained previously. Only D (dam 1+2) has reduced erosion in L2 to minimal and at times induce accretion as can be seen from the 29 days image. Ignoring the accretion value at L3 for the 16-day image as it is still early time in the simulation period, it can be deduced that at the end of the simulation period, land gains will be maximum from the combination of cross- dams 1 and 2. L4 shows higher accretion in B (dam 1) but firstly, that is not much higher than the case D (dam 1+2) and secondly, accretion increases are far higher in L1, L2 and L3 for D as compared to B or any other case.
Table 4.3. Bed level change values in meters in all locations for all cross-dam scenario for three selected time instances.
Time Location A (dam 0) B (dam 1) C (dam 2) D (dam 1+2) L1 8.40E-06 6.09E-06 1.30E-05 1.09E-05 16 L2 -5.95E-06 3.00E-09 -9.50E-06 -2.00E-09 days L3 4.48E-06 6.19E-05 4.21E-06 3.05E-06 L4 1.06E-05 1.58E-05 1.23E-05 1.35E-05 L1 1.44E-05 1.17E-05 2.21E-05 2.14E-05 29 L2 -9.18E-06 -3.81E-06 -1.49E-05 1.36E-06 days L3 1.20E-06 6.51E-06 6.02E-06 5.07E-06 L4 1.97E-05 2.81E-05 2.32E-05 2.28E-05 L1 1.69E-05 1.88E-05 2.30E-05 2.29E-05 47 L2 -1.20E-05 -1.77E-05 -2.31E-05 -7.98E-07 days L3 8.17E-06 6.93E-06 6.95E-06 6.37E-06 L4 2.05E-05 2.82E-05 2.12E-05 2.45E-05
83
Figure 4.29. Comparison of bed level changes in all four locations in three different chronological time instances.
84 4.9 Discussion
Residual flow in the estuarine portion of the study area dictates long term sediment dynamics (Jakobsen et al., 2002, Hussain et al., 2013). Flow in the Sandwip-Urir Char-Noakhali region is an isolated phenomenon free from influence of flow from the Meghna Estuary (Haque et al., 2016). Northward residual flow exists in the Sandwip channel, inducing sediment flow in a counter-clockwise manner around the northern and western channels of Urir Char. As a result, sediment deposition takes place in these regions. Sediment deposition takes place in the southern part of Jahajir Char and Sandwip islands for “no-dam” case where there are turbulent waters resulting from divergence in residual flow. The southward flow in Noakhali and Jahajir Char channel carries a portion of the sediment away from the Urir Char region, which would otherwise have deposited in the vicinity resulting from cross-dam implementations.
Dam implementations start to alter flow regimes and subsequently sediment dynamics as well. The dam between Urir Char and mainland Noakhali creates a turbulence in residual flow at the northeastern corner of Urir Char which results in a break in flow as flow now passes through the south channel of Urir Char. This could result in the deposition of sediment flowing towards the northern portion of Urir Char in the northern vicinity of the dam. Strong presence of asymmetry in the cross-dam cases imparts deposition within Noakhali channel and reverses the deposition characteristics from southern Sandwip and Jahajir Char islands to the northern cross-dam locations and northeast Sandwip channel to the east of Urir Char.
With just the dam between Sandwip and Urir Char in place, residual flow patterns become rather erratic and residual flow magnitudes lower overall in the region. Flow cannot pass from Sandwip channel through the southern Urir Char channel and passes entirely through the northern channel and once it circles and comes out the western side of Urir Char, this flow although of high velocity, creates turbulence in the eastern portion of the dam and along Noakhali channel. This flow as it staggers in between northern Sandwip and southern Urir Char in its passage towards the northern Urir Char channel, deposits sediments in the vicinity of dam 2. The flow encircles Urir Char and sediment deposition takes place at the northern portion of the island. As this flow exits the Urir Char channel, it erodes the southwestern location of Urir Char and is diverged outwards through Noakhali and Jahajir Char channels.
85 In case of D (dam 1+2), when both dams are in place, residual flow patterns change altogether. Flow from Sandwip is now entirely cutoff from circling around Urir Char and as such the strong flow force driving the southwards flow in Jahajir Char and Noakhali channels is now non-existent. As a result, residual flow direction reverses completely for these two channels and overall flow magnitude in the channels decrease. This drive sediment flow up towards the southern vicinity of the Urir Char-Noakhali cross-dam and this in conjunction with higher SBE values south of the dam and north of Noakhali channel will impart transport and deposition of suspended fine sediments in the region. This northwards flow induces deposition in between the two implemented dams and diminishes the otherwise eroding nature of the south vicinity of dam 1. As flow passage through Sandwip channel is reduced by both dams, flow now starts receding in Sandwip channel from a more northern position than before with any one or no cross-dam scenarios. As southern flow is in the ebb direction, this is also supported from the horizontal velocity plots as it indicated the upwards shifting of ebb dominance along the Sandwip channel for the both dam scenario (Figure 4.18).
Both the wave asymmetry values (1 & 2) show strong asymmetry along the western Urir Char and northern portion of Noakhali channel. Asymmetry values are also high in the northern Urir Char channel, northern tip of Sandwip channel (As1) and the full extent of Jahajir Char and Noakhali channels. This indicates of greater sediment depositions in those regions. Strong horizontal and vertical flood dominance is prevalent in the northern and western parts of Urir Char channels as well as in the Noakhali channel. Horizontal flood dominance would induce a coarse sediment transport of both suspended and bed load in the flow direction. Thus, natural accretion is to slowly take place in these regions.
Slack water duration analyses reveal a mixture of flood and ebb dominated portions in the vicinity of the dams. High water slack periods are prolonged in comparison to low water slacks in the northern and western Urir Char channel as well as in Noakhali channel, indicating the presence of flood dominance in the regions surrounding Urir Char to the north and west of the island, along the Noakhali channel. Low water slacks are prolonged in the southern Urir Char channel and in Jahajir Char channel. With both cross-dams implemented, this asymmetry increases to connect and engulf the northern Noakhali channel and western region of Urir Char. This will induce transport and settling of both coarse and fine sediment along these regions.
86 Sediment dynamics show accelerated accretion patterns in the vicinity of dams with cross- dam cases. No-dam case revealed erratic sedimentation patterns in the southern portion of Sandwip and Jahajir Char islands. Implementation of cross-dams realign these accretion locations towards the vicinity of dam locations. Differing dam alignments induce different magnitude of accretion in the close region with erosion found in a few locations. Implementation and functioning of both dams at the same time however, would alleviate this situation and further facilitate accelerated ate.
Thus, implementation of the proposed Sandwip-Urir Char-Noakhali cross-dams would facilitate accretion in the region and aid in land reclamation purposes. If a single dam is to be built instead of both, dam 1 which is between Urir Char and mainland Noakhali is to be built as building only the dam 2 between Sandwip and Urir Char will be counter-productive due to somewhat negative alterations in flow and sediment dynamics. Implementation and operation of both dams will yield the best result as it will enforce maximum accretion in the region. In this case however, dam 1 has to be built first as this will block the northern Urir Char channel and to a certain extent, achieve expected land reclamation purposes, although at a prolonged duration. This will yield certain benefits in terms of the timing of construction of dam 2 and will allow observation of impact of dam in regional flow and sediment dynamics.
The study done by IWM (2009) for the same cross-dams setup yield similar results in that dam alignments are somewhat same with the exception of dam 1 alignment slightly varying. Such data-driven and data-reliant models, although effective for a certain period of time, has limitations as it cannot fully depict all aspects of flow and sediment dynamics. This study lacks in measured data in comparison to the aforementioned exercised for which, extensive separate surveys have been conducted.
Having said that, it does provide indication as to specific accretion locations to consider and target post-dam construction. It also considers a wider range of flow and tidal characteristics in the form of tidal asymmetry and thus possess solid basis for presumptions made. It thus paves way for more detailed modeling studies that can be concentrated around specific dam locations and backed by long-term field measurements to further prove and support the assumptions made by this study and also as previous studies such as IWM (2009) made in the region are outdated due to the dynamic nature of Meghna Estuary and coastal Bangladesh.
87 5. CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions
Formulation of any long-term development strategies in the Meghna Estuary such as land reclamation schemes require substantial knowledge and information regarding the constant processes of erosion and accretion through complex interactions among large river discharge, varying sediment load, strong tidal forces and estuarine circulations. Out of the 18 cross- dams recommended by govt. studies, the Sandwip-Urir Char-Noakhali region is better suited as a zone for facilitated accretion and land formation. The present study applied 2D depth- averaged hydrodynamic model based on non-linear shallow water wave equations to assess the factors that affect sediment dynamics through analyses of impact on residual flow patterns and tidal asymmetry features due to implementation of the proposed Sandwip-Urir Char-Noakhali cross-dams. The study analyzed these forces at work in the Sandwip-Urir Char region of the Estuary and assessed future possibilities of land reclamation through cross-dam establishment based on tidal circulations and sediment patterns. Conclusions drawn from the study results are as follows.
. Morphological changes and sedimentation process in the Sandwip-Urir Char region is strongly influenced by tide induced residual currents in addition to factors such as wind, waves, sediment discharge through the river etc. Northward residual flow in the Sandwip channel induces sediment transport through both northern and southern Urir Char channels and after interim depositions, this sediment-laden flow exits the region through Jahajir Char and Noakhali channels. This outward flow, under no-dam situation, accretes substantial amount in the southern tip of both Sandwip and Jahajir Char islands. This scattered deposition yield little result in the long run as actual “land reclamation”, if it were to happen, would take considerably long time than intended for by policy makers to have any tangible impact for socioeconomic development objectives.
. Implementation of cross-dams induce substantial alterations in residual flow patterns in the area, whereby implementation of both cross-dams results in the reversal of flow
88 in the Jahajir Char and Noakhali channels. This shifts the sedimentation focus away from the southern parts of Sandwip and Urir Char and results in accelerated sediment deposition in the northern and southern Urir Char channels, near the vicinity of the cross-dams that have now effectively blocked flow passage through these two channels. This is more so the desired case in comparison to the previous situation as this instills establishment of accreted land to the mainland which accelerates the land reclamation process.
. These findings are well supplemented by analyses of tidal asymmetry parameters
where both wave asymmetry values (As1 and As2) indicate strong distortion of tidal curves in and around the cross-dam locations that extends into both Noakhali and Jahajir Char channels thus indicating sediment transport and deposition. Strong horizontal and vertical flood dominance is prevalent in the northern and western parts of Urir Char channels as well as in the Noakhali channel. Horizontal flood dominance would induce a coarse sediment transport of both suspended and bed load in the direction of flow. Thus, natural accretion is to slowly take place in these regions.
. Slack water duration analyses reveal a mixture of flood and ebb dominated portions in the vicinity of the dams. High water slack periods are prolonged in comparison to low water slacks in the northern and western Urir Char channel as well as in Noakhali channel, indicating the presence of flood dominance in the regions surrounding Urir Char to the north and west of the island, along the Noakhali channel. Low water slacks are prolonged in the southern Urir Char channel and in Jahajir Char channel. With both cross-dams implemented, this asymmetry increases to connect and engulf the northern Noakhali channel and western region of Urir Char. This will induce transport and settling of both coarse and fine sediment along these regions.
. Analysis of sediment dynamics reveal accelerated accretion patterns in the vicinity of dams with cross-dam cases. “No-dam” case revealed erratic sedimentation patterns in the southern portion of Sandwip and Jahajir Char islands. Implementation of cross- dams realign these accretion locations towards the vicinity of dam locations. Differing individual dam alignments induce different magnitude of accretion in the close region with erosion found in a few locations. Implementation and functioning of both dams
89 at the same time however, alleviated this situation and further facilitates accretion at an accelerated fashion.
Finally, it can be said that implementation of the proposed Sandwip-Urir Char-Noakhali cross-dams would indeed further facilitate accretion in the Sandwip- Urir Char region and accelerate land reclamation. A single dam setup is to involve implementation of only the Urir Char-Noakhali cross-dam as it will induce a more stable overall impact in the existing flow and sediment regime. However, a two cross-dam setup will yield the best overall result and it will enforce maximum accretion in the region. Sequencing of dams in this case will involve building the Urir Char-Noakhali dam first and Sandwip-Urir Char dam afterwards.
5.2 Recommendations for Future Studies
This study on the impact of proposed cross-dam alignments on the overall flow and sediment regime of northeastern Meghna Estuary near Sandwip-Urir Char region, applied mathematical modeling based on secondary hydro-morphological data and generates assumptions based on such.
Since Urir Char-Noakhali cross-dam is already under the planning process as part of the BDP2100, it should take under consideration implementation of the Sandwip-Urir Char dam in conjunction. The combined Sandwip-Urir Char-Noakhali cross-dam was one of the significant recommendations of the very first Land Reclamation Projects. This 2-dam setup is thus recommended to be taken under consideration for present as well as future planning processes as a combined implementation would greatly benefit land reclamation purposes for the region.
Modeling sediment dynamics in the coastal region has some inherent difficulties due to data insufficiencies and multi-dimensional implications of interventions. Based on this, some future study recommendations are made.
. Cross-dam implementation to harness land formation through accelerated accretion require significant structural interventions. Such interventions imparts significant impacts of structural and non-structural nature. Interventions such as theses affects the bio-diversity of the area including plants, mammals, reptiles, amphibians, and bird
90 species. It also changes the e-flow value as well as e-flow requirements for channels that are now subject to altered flow and sediment regimes due to dam implementations. The impact of changed flow and sediment dynamics, due to cross- dams implementation, on ecosystem functions, was outside the scope of the current study and thus deserves attention and can be considered for future studies.
. Further hydro-dynamic studies will have to be carried out in the region using up-to- dated bathymetry, flow and sediment data. These studies will also have to take into consideration the short and long-term impacts of storm surge upon the flow and sediment regime prevalent in the region to best assess the effects of cross-dam implementation.
91 REFERENCES
Ahmed, S. and de Wilde, K. [2011] “Setting the Stage,” Moving coastlines: Emergence and use of land in the Ganges-Brahmaputra-Meghna Estuary, ed. de Wilde, K., Dhaka: University Press Limited, 1-19.
Akter, J., Sarker, M. H., Popescu, I., and Roelvink, D. [2015] “Evolution of the Bengal Delta and its prevailing processes”. Journal of Coastal Research: Volume 32, Issue 5: pp. 1212 – 1226. doi: http://dx.doi.org/10.2112/JCOASTRES-D-14-00232.1.
Ali, A., Mynett, A. E. & Azam, A. H. [2007] “Sediment dynamics in the Meghna Estuary, Bangladesh: A model study,” J. Waterw. Port, Coast. Ocean Eng. 133(4), 255–263.
Allen, G.P., Salomon, J.C., Bassoulet, P., Du Pemhoat, Y. and De Grandpre, C. [1980] “Effects of tides on mixing and suspended sediment transport in macrotidal estuaries,” Sediment Geology 26, 69-90.
Allison, M. A. [1998] “Historical Changes in the Ganges-Brahmaputra Delta Front,” Journal of Coastal Research, Vol. 14, No.4, 1998 pp. 1269-1275.
Barua, D. K. [1990] “Suspended sediment movement in the estuary of the Ganges- Brahmaputra-Meghna river system,” Marine Geology 91, 243-253.
Barua, D. K., Kuehl, S. A., Miller, R. L. and Moore, W. S. [1994] “Suspended sediment distribution and residual transport in the coastal ocean off the Ganges-Brahmaputra river mouth,” Marine Geology 120, 41-61.
Barua, D. K. [1997] “The Active Delta of the Ganges-Brahmaputra Rivers: Dynamics of Its Present Formations,” Marine Geodesy, 20:1-12, 1997
BBS [2017] “Bangladesh Sample Vital Statistics 2016,” www.bbs.gov.bd Ministry of Planning, ISBN-978-984-519-094.
92 Bird, E. [2008] “Coastal Geomorphology: An Introduction,” John Wiley & Sons Ltd, UK.
Blanton, J. O., Lin, G. and Elston, S. A. [2002] “Tidal asymmetry in shallow estuaries and tidal creeks,” Continental Shelf Research 22, 1731-1743.
Bolle, A. Wang, Z. B. Amos, C. and Ronde, J. D. [2010] “The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt,” Continental Shelf Research 30, 871-882.
Boon, J. [2007] Secrets of the Tide: Tide and Tidal Current Analysis and Applications, Storm surges and Sea Level Trends, Chichester, UK: Horwood Publishing, 123p.
Brammer, H. [2004] “Can Bangladesh Be Protected from Flood,” Dhaka, Bangladesh: The University Press, 262p.
Brammer, H. [2014] “Bangladesh’s dynamic coastal regions and sea-level rise,” Climate Risk Management, 1, pp. 51–62.
Carvajal, M. I. F., Khan, Z. H. and Rahman, M. M. [2011] “Land formation and erosion in the estuary,” Moving coastlines: Emergence and use of land in the Ganges-Brahmaputra- Meghna Estuary, ed. de Wilde, K., Dhaka: University Press Limited, 21-37.
CEGIS [2009] “Monitoring planform developments in the EDP area using remote sensing,” Prepared for BWDB, Dhaka, Bangladesh, 1-79.
Chowdhury, J.U., Rahman, M.R. and Salehin, M. [1997] Flood control in a floodplain country, Experiences of Bangladesh, Report prepared for ISESCO, Morocco, 32.
Coleman, J. M. [1969] “Brahmaputra River: Channel processes and sedimentation,” Sedimentary Geology 3 (2/3), Special Issue, 129-239. de Wilde, K. (ed) [2011] “Moving Coastlines: Emergence and Use of Land in the Ganges- Brahmaputra-Meghna Estuary,” University Press Limited, Dhaka.
93 Dronkers, J. [1986a] “Tidal asymmetry and estuarine morphology,” Netherlands Journal of Sea Research 20(2/3), 117-131.
Dronkers, J. [1986b] “Tide induced residual transport of fine sediment,” Physics of Shallow Estuaries and Bays ed. J. van dee Kreeke, New York: Springer, 228-244.
Dyer, K.R. [1978] “The balance of suspended sediment in the Gironde and Thames estuaries,” Estuarine Transport Processes, ed. B. Kjerfve, Columbia: University of South Carolina Press, 135-145.
EDP [2009] "Estuary Development Programme", Inception Report, Submitted to Ministry of Water Resources, Bangladesh Water Development Board by DHV-HASKONING.
EDP [2010] "Estuary Development Programme", Final Report, Submitted to Ministry of Water Resources, Bangladesh Water Development Board by Institute of Water Modeling.
Flather, R. A. [1994] “A storm surge prediction model for the northern Bay of Bengal with application to the cyclone disaster in April 1991,” Journal of Physical Oceanography, 24, 172–190.
Fookes, P. G., Lee, E. M. & Griffiths, J. S. [2007] “Engineering Geomorphology: Theory and Practice,” Whittles Publishing, CRC Press, Taylor and Francis Group, Scotland, UK.
Friedrichs, C. T. and Aubrey, D. G. [1988] “Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis,” Estuarine Coastal and Shelf Science 27, 521-545.
GED [2018a] “Bangladesh Delta Plan 2100 Volume 1: Strategy. Prepared by General Economics Division (GED), Planning Commission, Government of Bangladesh.
GED [2018b] “Bangladesh Delta Plan 2100: Investment plan for land reclamation in the Meghna Estuary.” Formulation Project. Prepared by General Economics Division (GED), Planning Commission, Government of Bangladesh.
94 Gill, A. E. [1982] “Atmosphere-ocean dynamics”. Academic Press, London, 662 pp.
Goodbred Jr. S. L., Kuehl, S. A. [2000a] “Enormous Ganges–Brahmaputra sediment load during strengthened early Holocene monsoon,” Geology 28, 1083– 1086.
Goodbred Jr. S. L., Kuehl, S. A. [2000b] “The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges–Brahmaputra delta,” Sedimentary Geology 133, 227–248.
Grabemann, I, Kappenberg, J & Krause, G. [1995] “Aperiodic variations of the turbidity maxima of two German coastal plain estuaries,” Netherlands J. Aquatic Ecology 29(3–4), 217–227.
Groen, P. [1967] “On the residual transport of suspended load matter by an alternating tidal current,” Netherlands Journal of Sea Research 3, 564–674.
Haque, A., Sumaiya and Rahman, M. [2016] “Flow Distribution and Sediment Transport Mechanism in the Estuarine Systems of Ganges-Brahmaputra-Meghna Delta,” International Journal of Environmental Science and Development, Vol. 7, No. 1, January 2016
Holeman, J. N. [1968] “The sediment yield of major rivers of the world,” Water Resour. Res. 4: 737–747. https://doi.org/10.1029/WR004i004p00737.
Hori, K. and Saito, Y. [2007] “An early Holocene sea-level jump and delta initiation,” Geophysical Research Letters, Vol. 34, L18401, doi:10.1029/2007GL031029, 2007.
Hossain, M. M. [1992] “Total sediment load in the lower Ganges and Jamuna,” J. IEB 20, 1– 8.
Hussain, M. A., Hossain, M. A., and Haque, A. [2009a] “Numerical investigation of residual currents in the Meghna Estuary of Bangladesh,” Proceedings of Third International Conference on Estuary and Coast, Sendai, Japan, 1, 142-149.
95 Hussain, M. A., Hossain, M. A., and Haque, A. [2009b] “Seasonal variation of residual currents in the Meghna Estuary of Bangladesh,” Proceedings of Coastal Dynamics 2009, Impact of Human Activities on Dynamic Coastal Processes, eds. Masaru Mizuguchi and Shinji Sato, Tokyo, Japan, World Scientific, Paper ID 30, 1-11.
Hussain, M. A., Hossain, M. A., and Haque, A. [2012] “Hydro-meteorological impact on residual currents and salinity distribution at the Meghna Estuary of Bangladesh,” Coastal environments: Focus on Asian Regions, ed. Subramanian, V., Jointly published by Springer and Capital Publishing Company, 88-105.
Hussain, M. A., Tajima, Y., Taguchi, Y. and Gunasekara, K. [2013] “Tidal Characteristics Affected by Dynamic Morphology Change in the Meghna Estuary”, Proceedings of the 7th International Conference on Asian and Pacific Coasts (APAC), September 24-26, Bali, Indonesia.
Hussain, M. A., Tajima, Y., Hossain, M. A., Rana, S. [2014a] “Asymmetry of tide and suspended sediment concentrations observed at the north-eastern part of the Meghna Estuary,” Coastal Engineering, doi: 10.9753/icce.v34.sediment.77.
Hussain, M. A., Tajima, Y. Gunasekara, K., Rana, S., and Hasan, R. [2014b] “Recent coastline changes at the eastern part of the Meghna Estuary using PALSAR and Landsat images,” Proceedings of the 7th IGRSM International Conference and Exhibition on Remote Sensing and GIS, Kuala Lumpur, Malaysia, IOP Conf. Series: Earth and Environmental Science 20, Paper 012047 doi:10.1088/1755-1315/20/1/012047.
Hussain, M. A. and Tajima, Y. [2019] “Impact of Dynamic Bathymetry Change on Tidal Characteristics around the Eastern Part of Meghna Estuary” (under review)
Islam, M. R., Begum, S. F., Yamaguchi, Y. and Ogawa, K. [1999] “The Ganges and Brahmaputra rivers in Bangladesh: basin denudation and sedimentation,” Hydrol. Process. 13: 2907–2923. https://doi.org/10.1002/(SICI)1099 -1085(19991215)13: 17b2907:: AIDHYP906N3.0.CO;2-E.
96 Islam, M. R. Begum, S. F. Yamaguchi, Y. and Ogawa, K. [2002] “Distribution of suspended sediment in the coastal sea off the Ganges-Brahmaputra River mouth: observation from TM data,” Journal of Marine Systems 32, 307-321.
IWM [2009] “Survey and modelling study of Sandwip-Urir Char-Noakhlai cross-dams,” Final Report, Volume 1: Main Report. Prepared for CDSP – III, BWDB, Dhaka, Bangladesh.
IWM [2010] “Updating of hydrodynamic & morphological models to investigate land accretion & erosion in the Estuary Development Program (EDP) Area,” Draft Final Report. Prepared for Estuary Development Programme, BWDB, Dhaka, Bangladesh.
Jakobsen, F., Azam, M. H. and Kabir, M. M. [2002] “Residual flow in the Meghna Estuary on the coastline of Bangladesh,” Estuarine, Coastal and Shelf Science, 55, 587-597.
Kirby, M., Ahmad, M., Mainuddin, M., Palash, W., Qadir, E. and Shahnewaz, S., M. [2014] “Bangladesh Integrated Water Resources Assessment Supplementary Report: Approximate Regional Water Balances,” Water for a Healthy Country Flagship Report Series ISSN: 1835- 095X.
Kuehl, S. A., Levy, B. M., Moore, W. S. and Allison, M. A. [1997] “Subaqueous delta of the Ganges-Brahmaputra river system,” Marine Geology 144 (1997) 81-96.
Kuehl, S. A., Allison, M. A., Goodbred, S. L. and Kudrass, H. [2005] “The Ganges– Brahmaputra Delta,” Gosian, L. and Bhattacharya, J. (eds.), River Deltas: Concepts, Models, and Examples. Tulsa, Oklahoma. SEPM, Special Publication 83, pp. 413–434.
LRP [1982] "Sediment Concentrations and Salinities Between Bhola Island and Chattogram Coast," Land Reclamation Project, Dhaka, Bangladesh, Tech. Rep., 7.
Matsumoto, K., Takanezawa, T. and Ooe, M. [2000] “Ocean tide models developed by assimilating TOPEX/POSEIDON altimeter data into hydrodynamical model: A global model and a regional model around Japan,” Journal of Oceanography 56, 567-581.
97 Milliman, J. D., Meade, R. H. [1983] “World-wide delivery of river sediment to the oceans,” J. Geol. 91:1–21. https://doi.org/10.1086/628741.
Milliman, J. D. [1991] “Flux and fate of fluvial sediment and water in coastal seas,” Ocean Margin Processes in Global Change. eds. Mantoura, R.F.C., Martin, J.-M. & Wollast, R. Chichester: John Willey and Sons Limited, 69-89.
MoWR [1998] "Hydro-morphological dynamics of the Meghna estuary." Draft Master Plan Report. Prepared by DHV Consultants, Bangladesh Water Development Board, Bangladesh.
MoWR [2001] "Hydro-morphological dynamics of the Meghna estuary." Draft Development Plan Report. Prepared by DHV Consultants, Bangladesh Water Development Board, Bangladesh.
Nichols, M.M. [1984] “Effects of fine sediment resuspension in estuaries,” Estuarine Cohesive Sediment Dynamics, ed. A.J. Mehta, New York: Springer, 5-42.
Palinkas, C. M., Nittrouer, C. A. & Walsh, J. P. [2006] “Inner-shelf sedimentation in the Gulf of Papua, New Guinea: a mud-rich shallow shelf setting,” J. Coastal Res. 22(4), 760–772.
Rahman, M., Dustegir, M., Karim, R., and others [2018] “Recent sediment flux to the Ganges-Brahmaputra-Meghna delta system,” Science of the Total Environment, 643 (2018) 1054–1064 https://doi.org/10.1016/j.scitotenv.2018.06.147.
Ralston, D. K., Geyer, W. R., Traykovski, P. A. and Nidzieko, N. J. [2012] “Effects of estuarine and fluvial processes on sediment transport over deltaic tidal flats,” Continental Shelf Research, doi:10.1016/j.csr.2012.02.004.
Rice, S. K. [2007] “Suspended Sediment Transport in the Ganges-Brahmaputra River System, Bangladesh,”.
Sarker, M. H. and Thorne, C. R. [2006] “Morphological response of the Brahmaputra-Padma- Lower Meghna river system to the Assam Earthquake of 1950,” Braided Rivers: Process,
98 Deposits, Ecology and Management (ed. by G. H. S. Smith, J. L. Best, C. S. Bristow, & G. E. Petts), 289–310. Special Publication 36 of the IAS, Blackwell Publishing, UK.
Sarker, M. H., Akter, J., Ferdous, M. R., and Noor, F. [2011] “Sediment dispersal processes and management in coping with climate change in the Meghna Estuary, Bangladesh,” Proceedings of the Workshop on Sediment Problems and Sediment Management in Asian River Basins (Hyderabad, India, IAHS), Publication 349, pp. 203–218.
Sarker, M. H., Akter, J., and Rahman, M.M. [2013] “Century-scale dynamics of the Bengal Delta and future development,” Proceedings of the International Conference on Water and Flood Management (Dhaka, Bangladesh), pp. 91–104.
Sastry, S. S. [2006] “Introductory Methods of Numerical Analysis,” Chapter 5. PHI Learning Pvt. Ltd., New Delhi, pp. 187–239.
Sharma, B. R., Amarasinghe, U. A., Cai, X., Condappa, D. and others [2010] “The Indus and the Ganges: river basins under extreme pressure,” Water International Vol. 35, No. 5, September 2010, 493–521.
Sokolewicz, M. and Louters, T. [2007] “Hydro-morphological processes in the Meghna Estuary, Bangladesh,” Pre-conference volume of the 1st International Conference on Water & Flood Management, Dhaka, Bangladesh, 317-324.
Taguchi, Y. Tajima, Y. and Hussain. M. A. [2013] “Monitoring and Investigation of Dynamic Morphological Change in Meghna Estuary,” Journal of JSCE: Hydraulic, Coastal and Environmental Engineering, 69 (2), 636-640, (in Japanese).
Takanezawa, T., Matsumoto, K., Ooe, M. and Naito, I. [2001] “Effects of the Long-period Ocean Tide on Earth Rotation, Gravity and Crustal Deformation Predicted by Global Barotropic Model – periods from Mtm to Sa-,” J. Geod. Soc. Jpn., 47,545-550.
Van Maren, D.S. Hoekstra, P., and Hoitink, A. J. F. [2004] “Tidal flow asymmetry in the diurnal regime: bed load transport and morphologic changes around the Red River Delta,” Ocean Dynamics, 54, 424-434.
99
Wang, Z. B. Jeuken, M. C. J. L. and de Vriend, H. J. [1999] Tidal asymmetry and residual sediment transport in estuaries: A literature study and application to the Western Scheldt, WL|Delft Hydraulics, Report Z2749, 1-21.
Ward, R. [2010] “General Bathymetric Chart of the Oceans,” Hydro International, 14(5). http://www.hydro-international.com/issues/articles/id1218-General_Bathymetric_Charts_of_ the_Ocean.html
Wasson, R. J. [2003] “A sediment budget for the Ganga-Brahmaputra catchment,” Curr. Sci. https://doi.org/10.2307/24107666.
100