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Numerical Simulation: Hydro-Morphology of Meghna Estuary

Md. Misbah Uddin1*, Dr. Jahir Bin Alam1, Zahirul Haque Khan2, Dr. G. M. Jahid Hasan1 and Rubayat Alam2 1Department of Civil and Environmental Engineering, Shahjalal University of Science and Technology, Sylhet, 2Coast, Port and Estuary Division, Institute of Water Modelling, , Bangladesh *Corresponding author: [email protected]

ABSTRACT The Meghna Estuary is formed by the combined flows from three great rivers in the south-east Asia; the , the Brahmaputra and the Meghna. This estuary experiences dynamic hydro-morphological changes due to very high discharge of water and sediment from these three mighty rivers. Erosion and accretion occur simultaneously at very high rates; such that, the flow and sediment discharge through the estuary are the third highest and the highest, respectively, in the world. On the other hand, the tidal current itself is also strongly affected by the dynamic morphology changes. It is thus essential to understand the hydraulic behavior and interactive features of tides and morphology change in the estuary. With this end in view a two dimensional general model of Bay of was developed under cyclone shelter preparatory study (csps) undertaken by Bangladesh Government and the model was updated later during other projects. The present article focuses on development of model, its set-up, boundary conditions and few calibration results of this two dimensional hydrodynamic and morphology model. The model applications clearly show the variation of the flow structure; their speed and direction separately for monsoon and dry season around the model area which covers the northern part of more specifically the Meghna Estuary.

Keywords: 2D hydrodynamic model, morphology model, model calibration, water level, discharge, current speed, residual flow, sediment transport

1. INTRODUCTION The Meghna Estuary is a very dynamic estuarine and coastal system. Here, one of the world’s greatest rivers, the Lower finds its way to the Bay of Bengal (figure1). The Lower Meghna conveys the combined flows of the Brahmaputra (called in Bangladesh Jamuna), the Ganges, and the Upper Meghna. The sediment discharge from the Lower Meghna River is the highest (Coleman, 1969) and the water discharge, the third highest, of all river systems in the world (Milliman, 1991). The knowledge about the physical processes and morpho-dynamic behavior of the Lower Meghna Estuary is still fairly limited. A complicated interplay between the forces of the river, tide and the waves creates a complex pattern of sediment displacement in the estuary. Large quantities of sediment are transferred continuously towards the shallow coastal region of Bengal (Sindhu, 2012). Since almost no sediment is exchanged with the deeper parts of Bay of Bengal, the overall sediment budget is determined by the process of continuous redistribution of sediment in the river system upstream (Koen, 2011). The hydrodynamic factors that are playing dominant role in morphological development along the coast line of Bangladesh are; enormous volume of river water flow, sediment transport, strong tidal and wind actions, wave, salinity and cyclonic storm surge (Uddin et al., 2014).The displacement of sediment is a part of a continuous process of the estuarine-landscape striving to achieve dynamic equilibrium between the physical shape (morphology), and the continuously changing river discharge conditions and tidal flows.

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A mathematical model usually describes a system by a set of variables and a set of equations that establish relationships between the variables. Mathematical models have advancement to a point where they are considered to be effective tools for simulating natural phenomena in coastal regions. This article focuses on the development of Bay of Bengal Model which is being used to enhance the knowledge of the complex hydrodynamic and morphological processes in the Meghna estuary. This two dimensional Bay of Bengal model was developed under Cyclone Shelter Preparatory Study (CSPS, 1998) in 1996 and 1998. The model has been updated and calibrated during different projects undertaken by different government and non-government agencies. The mud transport module is coupled to the hydrodynamic module in MIKE 21 Flow Model FM version of MIKE ZERO environment and they run in parallel.

Figure 1: Model domain enclosed by shoreline of the Bay of Bengal

2. APPROACH AND METHODOLOGY 2.1 Approach A crucial element in the development of long-term plans for human interference in estuarine or coastal waters is the prediction of the impact on the bathymetry and coastlines. In recent years, substantial efforts have been put in to the numerical modelling of hydraulic and morphological changes in complex situations in the fluvial and coastal areas of Bangladesh. For estuaries and coastal waters, however, this has not yet resulted in to sufficiently reliable predictions at mid and long-term. On the other hand, statistical and empirical techniques applied to field data are useful to extrapolate existing trends, but they can hardly be used as sole predictor if these trends are changing due to human interference catastrophic events, sea level rise etc. So numerical model has been chosen as an effective tool to in the present study to represent the realistic picture of the Bay of Bengal coast and more particularly to simulate the hydrdynamic and morphological condition of the Meghna Estuary.

2.2 Methodology 2.2.1 Data collection and analysis To increase the understanding of the processes that govern the morphological development of the Meghna Estuary, an extensive data collection has been carried out by different agencies of Bangladesh Government during different projects executed by various local and foreign authorities. Some of these projects are Meghna Estuary Study, MES I and MES II (SWMC 2001), Land Reclamation Project (LRP), Estuary Development Programme (EDP, 2010), Char Development and Settlement Project (CSPS 1998) etc. Measurements including amongst others: bed and water levels, current velocities and discharge. Bed and suspended sediment samples were collected and analyzed. Bathymetric surveys were carried out to obtain information on changes of tidal channels and to set up a 2D numerical model of the area.

International Journal of Ocean and Climate Systems Md. Misbah Uddin, Dr. Jahir Bin Alam, Zahirul Haque Khan, 175 Dr. G. M. Jahid Hasan and Rubayat Alam

Bathymetric data The bathymetry data over the entire Meghna Estuary was surveyed and the data has been analysed. BWDB has checked the consistency of Bench Mark values that were used during bathymetric survey and corrected the bathymetry accordingly. MIKE-C Map which is global data base for water depth or water-land boundaries was used to generate land level data in the deep Bay of Bengal. Bathymetry used in the Bay of Bengal model is shallow around the estuary, and depth of the seabed increases gradually to reach nearly 3000 meters in the deep sea area. The bathymetry over central Bay of Bengal is relatively flat. The seafloor gradient decreases gradually from North to south. In the northern Bay of Bengal there exist some deep valleys. One such deep valley is known as the “Swatch of No Ground”. The deepest portion of the valley is recorded around 1340 m with an overall relief of 400-450 m from the surrounding mean seafloor depth of 1000 m. The width of the Swatch of no ground valley is about 14 km (Sarma et al., 2000).

Water Level Data Water level data are used to estimate the variation of water depth over the year, tidal characteristics and also to calibrate the hydrodynamic model. Water level observations have been carried out with pressure gauge over 24 hours at half an hour interval. All water level data are referred to Public Works Department (PWD) datum. The measured water level data at various locations have been plotted and considerable inconsistencies have been found. Bench mark error as well as processing error may have caused these inconsistencies in water level data. The consistent water level data have been used for calibration of the model.

Discharge Data Discharge measurement was carried out for 13 hours with one hour interval in the Lower Meghna River (at Kaliganj), Hatiya Channel, East Shahbazpur Channel, West Shahbazpur Channel, the Tetulia River and Sandwip Channels. These data have been utilised for calibration.

Sediment Data Suspended sediment concentration measurement is needed for estimating sediment transport and to calibrate the morphological model. Suspended sediment concentration has been measured at various locations in the estuary. The samples have been taken every hour for the full tidal cycle of 13 hours and three samples collected from surface, 0.2, 0.6 and 0.8 depths each time. Analysing all the measured data, only those data have been utilized for calibration that have reasonable profile over depth.

Fresh water sources used in Model In addition to upstream boundary in the lower Meghna River, there are so many small rivers which are discharging fresh water to the estuary. These rivers were also included in the model as a source of fresh water and since no field discharge measurement are available for these sources; logical constant discharge values were given for these fresh water sources. Such constant discharge input was given at eleven inlet points in the model domain.

2.2.2 Mesh Generation and Bathymetry Development Objective of mesh generation is to divide the whole model area in to a number of individual triangular flexible cells to perform the computation. To generate mesh, shoreline is essential. Shoreline can be extracted from satellite images using ARC View/ARC GIS or from MIKE-C Map. In case of BoB model shoreline is extracted from satellite images. The resolution of a numerical model always seems to be at the limit of computational ability. Therefore, it is often desirable to manage computational effort so that the near field region around the problem is emphasized, while far field regions are given less emphasis. The MIKE 21 FM model can do this by allowing variation in mesh size. Area included inside the shoreline represents a polygon (figure 1). In case of BoB model one polygon should not be used. If coarser resolution is used for whole area then the estuary cannot be

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represented properly. Again if finer resolution is used to represent the Meghna estuary then the total number of computational points will be increased and simulation time will be higher. To avoid this situation and to represent the Meghna estuary properly, total model area is divided into four polygons of different resolution as shown in figure 2. The resolution varies from 5400m x 5400m and 1800m x 1800m in the deep Bay of Bengal, through 600m x 600m in the Meghna Estuary and 200m x 200m in the areas of special interest.

Figure 2: Different polygon and their mesh resolution in model domain

2.2.3 Boundary conditions of the model There are two open boundaries in the model, one is in the Lower Meghna River at Chandpur, which is northern boundary and another one is in the Bay of Bengal that is southern boundary as shown in figure 3. Observed water level at Chandpur station has been used in the northern open boundary. Predicted or generated water level using Global Tide Model of two stations namely Vishakhapatnam (India) and Gwa Bay (Myanmar) was used for south open boundary. For sediment transport model, in the upstream boundary on the Lower Meghna River, suspended concentration of 350 mg/l has been applied. Zero sediment concentration has been assumed at the south boundary. Initial sediment concentration in the Meghna Estuary is also based on the field measurement.

Figure 3: Bathymetry and open boundaries of BoB Model

International Journal of Ocean and Climate Systems Md. Misbah Uddin, Dr. Jahir Bin Alam, Zahirul Haque Khan, 177 Dr. G. M. Jahid Hasan and Rubayat Alam

2.2.4 Numerical Modelling A good description of hydraulic phenomena that occur in the estuary is of immense value to understand the complex morphologic dynamics of this area. A well-calibrated 2D numerical model with a sufficient resolution is able to provide this information. Data on recent bathymetry and hydrometrics of the estuary have been utilized for setting-up and calibrating the Bay of Bengal model and establishing baseline conditions. The cohesive sediment transport module is coupled to the hydrodynamic module and they run in parallel. The governing equation for sediment transport is solved on the same mesh and applies information on water levels and currents from the hydrodynamic module to calculate the sediment transport. Model simulations were used to study the dominant hydraulic and morphological conditions that shape the estuary. To determine the dynamic behavior of the estuary, simulations were carried out for characteristic conditions (neap tide/spring tide and monsoon/dry period).

3. MODEL RESULTS 3.1 Calibration of Hydrodynamic model The two-dimensional hydrodynamic model of the Bay of Bengal have been calibrated against water level and discharge data at different locations comparing the model result with field measurement to make the model performance to a satisfactory level. The model is based on the surveyed data of 2009- 2010 and calibrated with water level and discharge measurements of 2009-2010 and found good agreement between the simulated data and observed data. Hydrodynamic simulation has been made for at least one month for both the dry and monsoon seasons in order to establish the hydraulic characteristics for low and high discharges. Location of calibration points for water level and discharge data for hydrodynamic model have been presented in figure 4. Calibration plots against water level in wet season are shown in figure 5 and calibration plots against water level in dry season are shown in figure 6. Discharge calibration for monsoon and dry season are shown in figure 7 and 8 respectively. Most of these calibration plots show good amplitude and phase agreement between the model results and observed data except little variation in dry season.

Figure 4: Calibration locations for water level and discharge for BoB hydrodynamic model and different channels in Meghna Estuary

The calibration plots of water level in dry season shows little mismatch with the field measured water level in figure 6(a) and (b) at location G4 and G5. During dry season upland fresh water flow into the Bay through the Lower Meghna River is very much lower than that of monsoon season. Since dry season flow is much lower than monsoon, the exact scenario is not replicated in the model result due to the lack of stability of model. The situation may be improved for dry season calibration by improving the boundary condition. However a model performance is evaluated by its calibration against discharge and figure 7 and 8 which are representing discharge calibration, showing good agreement with the observed data.

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Figure 5 (a): Calibration result at Sandwip for Bay of Bengal on water level for Monsoon

Figure 5 (b): Calibration result at Char Lengta for Bay of Bengal on water level for Monsoon

Figure 6(a): Calibration result on water level at point G4 of Bay of Bengal for dry season

International Journal of Ocean and Climate Systems Md. Misbah Uddin, Dr. Jahir Bin Alam, Zahirul Haque Khan, 179 Dr. G. M. Jahid Hasan and Rubayat Alam

Figure 6(b): Calibration result on water level at point G5 of Bay of Bengal for dry season

Figure 7(a): Calibration result on discharge at point Sandwip-west of Bay of Bengal for wet season

Figure 7(b): Calibration result on discharge at Hatiya channel of Bay of Bengal for wet season

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Figure 8(a): Calibration result on discharge at point D4 of Bay of Bengal for dry season

Figure 8(b): Calibration result on discharge at point D3 at Kalapani of Bay of Bengal for dry season

3.2 Calibration of Morphology model Sediment transport process form a crucial control in the estuarine process and specifically in the evolution of the Meghna Estuary. Morphological model is an effective tool for prediction of change in morphology and to understand the sediment distribution pattern in any study area. Sediment budget and erosion-accretion process in an estuary can be estimated with application of morphology model. Keeping this view in mind, mud transport model or sediment transport model was also calibrated against observed sediment concentration to establish the model performance to a satisfactory level. Calibration plots for the morphology model against observed data at two locations are shown in figure 9 (a) and 9 (b) and considered as indicating acceptable agreement. For suggesting a morphology model to be used for practical projects or to be used as a tool for policy makers, the model should be calibrated with more field data at more observation stations .

International Journal of Ocean and Climate Systems Md. Misbah Uddin, Dr. Jahir Bin Alam, Zahirul Haque Khan, 181 Dr. G. M. Jahid Hasan and Rubayat Alam

Figure 9(a): Calibration result on suspended sediment concentration at point S-J-1 of Bay of Bengal Model

Figure 9(b): Calibration result on suspended sediment concentration at point S-C-1 of Bay of Bengal Model

4. DISCUSSION During dry season upland fresh water flow into the Bay through the Lower Meghna River is very much lower than that of monsoon season. Tidal action becomes stronger and dominates water flow pattern in the estuary. The maximum depth average current speeds of about 2 to 3.2 m/s are found mainly in the West Shabhazpur Channel, north Hatiya channel and north of Urirchar i.e. in the Bamni channel during monsoon. Lowest velocities of about 0.25-1.0 m/s are found in the upper part of the lower Meghna River during dry period, where the tidal action is less dominant. 2D map of maximum depth-averaged current speed for dry period and monsoon are shown in figure 10 (a) and 10 (b). The estuary becomes morphologically very dynamic during monsoon. The upland flow is enormous during monsoon and the mean high water is considerably higher than during dry season. The

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distribution of flow and water level in the different channels of the estuary are governed by river discharge, the tide and the wind speed. Most of the accretion and erosion occurs during monsoon. The residual flow has been established based on the simulation results of one month covering spring and neap tides both for dry and monsoon season.

Figure 10 (a): 2D map of maximum current speed for dry period (February-March 2010)

Figure 10 (b): 2D map of maximum current speed for monsoon period (August-2009)

International Journal of Ocean and Climate Systems Md. Misbah Uddin, Dr. Jahir Bin Alam, Zahirul Haque Khan, 183 Dr. G. M. Jahid Hasan and Rubayat Alam

Simulation result shows a net water flow out of the Meghna estuary through West Shabhazpur channel and easterly flow outside the estuary during dry season. A prominent anti-clockwise circulation is prevailing around the Sandwip Island, which is mainly forced by tide. In the Sandwip channel, the residual anti-clockwise circulation during monsoon is similar to the circulation during dry season, which implies the area is dominated by tide both in dry and monsoon seasons. The net anti-clockwise circulation traps the sediment in this area. The net flow in between Sandwip and Hatiya Island is influenced by the river discharge. The residual flow and mean current speed is shown in Figure 11 (a) and Figure 11 (b) for dry and wet season respectively. In these igures contour maps are indicating mean current speed and vectors are showing direction of net current or residual flow.

Figure 11(a): Residual Flow Pattern in the Meghna Estuary in Wet season (August, 2009)

Figure 11(b): Residual Flow Pattern in the Meghna Estuary in Dry season (February, 2010)

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5. CONCLUSION A prominent anti-clockwise circulation is prevailing around the Sandwip Island, which is mainly forced by tide. In the Sandwip channel, the residual anti-clockwise circulation during monsoon is similar to the circulation during dry season, which implies the area is dominated by tide both in dry and monsoon seasons. The net anti-clockwise circulation traps the sediment in this area. During the initial development of BoB model, observed water level has been used in the upstream boundary and predicted tide has been used in the downstream boundary. To obtain optimum result, an attempt may be taken to extend the upstream boundary towards 20 km up where rating curve is available for generating discharge and hence the present boundary stations may be used as a calibration point. Use of discharge data at upstream boundary will reduce the boundary effect and improve the model performance. Furthermore, a sensitivity analysis can also be made to assess the sensitiveness of different parameters such as roughness, critical shear stress for erosion and deposition, time step and viscosity. In addition, with more measurement stations data available, the BoBM can be calibrated and validated more effectively to increase its robustness and prediction accuracy. Sediment transport calculations are influenced by significant uncertainties, and cohesive sediment transport modelling is still an empirical science. At the same time the required information is often scattered and limited. Even though the model performance can be improved by calibrating the model with more field data and more observation stations. Since, collection of sediment concentration data is very much costly and tough job, an approach of generating sediment concentration data by using satellite images can be adopted to calibrate the morphology model to a satisfactory level.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the support and contributions of the Institute of Water Modelling (IWM), Dhaka. The works presented in this article is based on the work, carried out during “Estuary Development Programme” project execution in IWM. Authors also acknowledge the financial support from Faruque-rashid waqf estate.

REFERENCES: Coleman, J.M. (1969), Brahmaputra River: Channel process and sedimentation, Sediment Geol., 3(196), pp.129-239. CSPS (1998), Cyclone Shelter Preparatory Study, Stage I: Feasibility Phase. Final Report. Mathematical Modelling of Cyclone-Surge and Related Flooding. European Commission, Directorate General External Economic Relations, Technical Unit for Asia Centre, 1998. EDP (2010), Estuary Development Programme, Final Report, submitted to Ministry of Water Resources, Bangladesh Water Development Board by Institute of Water Modelling, in December, 2010. Koen, W.D., (2011), Moving Coastlines, Emergence and Use of Land in the Ganges-Brahmaputra- Meghna Estuary, The University Press, 2p. Milliman, J.D. (1991), Flux and fate of fluvial sediment and water in coastal seas. In: Mantoura, R.F.C., Martin, J.-M. & Wollast, R. (eds). Ocean Margin Processes in Global Change. John Willey and Sons Ltd., Chichester, pp. 69-89. Sindhu, M. (2012), Numerical Modelling of Tides and Storm Surges in the Bay of Bengal, Goa University, Ph.D. thesis. S.W.M.C., (2001), Two-dimensional model of the Meghna Estuary, Second update report, April 2001, submitted to MES by Surface Water Modelling Center. Sarma, K. V. L. N. S., Ramana, M. V., Subrahmanyam, V., Krishna1, K. S., Ramprasad, T. and Desa, M. (2000) “Morphological Features of the Bay of Bengal”, Journal of Indian Geophysical Union, Vol. 4, No.2, pp. 185-190. Uddin, M., Alam, J.B., Khan, Z.H., Hasan, G.M.J. and Rahman, T. 2014. Two Dimensional Hydrodynamic Modelling of Northern Bay of Bengal Coastal Waters. Computational Water, Energy and Environmental Engineering, 3,140-151. http://dx.doi.org/10.4236/cweee.2014.34015

International Journal of Ocean and Climate Systems