Advanced numerical modelling for understanding the complex morphological behaviour in Meghna estuary

Coastal zones are generally extremely important for the functioning and well being of the people of many nations. The Bangladesh coast is endowed with the largest mangrove forest in the world, the only active delta-forming estuary among several other estuaries along the cost of the Bengal basin and the longest sandy beach in the world. The coastal morphology in Bangladesh especially inside the Meghna Estuary is characterized by a vast network ofrivers; an enormous discharge of river water heavily laden with sediments: alarge number of distributary channels; numerous off-shore sand/mud bars and temporary islands; strong tidal and wind action; tropical storms and associated wave surges. The estuary forms a complex network of braided tidal channels with strong tidal streams in many places. The entire Meghna Estuary as well as a part ofthe upstream river system is tidally influenced all the year. The area is characterized by a highly dynamic morphology, with flow channels shifting their course, and with intermittent erosion and accretion of banks and tidal flats. There is a moderate net accretion at around 10 km2 per year. There are frequent episodes of cyclonic storms and tidal surges along with highly dynamic morphology. This has made development planners and policy makers aware of the need for and importance of comprehensive plans for the coastal zones of Bangladesh. On the other hand, large areas are also accreted that are relatively rich in nutrients and form the habitat of highly productive ecosystems. Numerical modelling is such a tool that can help enormously for better understanding the processes involved with the river system. The objective of this study is to better understand the complex hydromorphodynamic processes involved in the Meghna Estuary using numerical simulation modelling and analysing the sensitivity of different parameters causing erosion and accretion in particular areas of the estuary. Testing the impact of some cross dams around Sandwip for land reclamation is also explored. A depth averaged two-dimensional morphological model has been setup using the Delft3D modelling system. Firstly, the model has been calibrated against water levels, discharges, bed level changes and sediment concentrations. Secondly, the calibrated model has been applied to simulate the hydraulic and morphological conditions without and with some cross dams in the interested areas (South of Noakhali mainland and west of Hatia and Urirchar). Flow and sediment transport patterns in the whole estuary as well as in the interested area during monsoon and dry periods have been analysed. In addition, transport of multiple sediment fractions (sand and mud) also have been simulated. This study is a first attempt to simulate the morphological processes in the complex estuary of the Meghna. It has been found from the study that numerical modelling of complex rivers and estuaries helps to obtain understanding of hydraulic and morphological processes by analysing the temporal and spatial distribution of flow and sediment transport. A well calibrated numerical model can help planners and policy makers to develop short term and long term plans for particular sites