The coastal embankment system has been gradually built during the last 40 years. The embankments were originally designed to increase agricultural production by preventing salt water intrusion not to protect against cyclonic storms. The alignment of the embankments did not consider the changing conditions in bathymetry of the sea and thalweg migration of the rivers and therefore many embankments are located under tidal water level and have severe toe and slope erosion problems during the monsoon season. The crest level and embankment cross sections have not optimized the protection of hinterland and the embankment itself and therefore the embankments typically only provide protection for the cyclones with 5-12 year return periods and the designed crest level of the sea facing coastal polder equal to the sum of normal maximum recorded water stage plus 1.50m. In this study to estimate the design crest level and side slope for sea facing embankment have been established based on maximum storm surge level, wave run-up for cyclonic wave, freeboard allowing 5 l/m/s overtopping ,potential climate change impact and land subsidence. Statistical analysis of surge level and wave run-up is carried out using Extreme Value Analysis (EVA) in MIKE Zero.

: -The Meghna Estuary region of Bangladesh is a unique environment where the constant process of land formation and erosion takes place due to the complex interactions between large river discharge, enormous sediment load, strong tidal forces, wind actions, wave, salinity & cyclonic storm surge and estuarine circulation. These hydrodynamic factors and their interactions shape the morphology of the Meghna Estuary. During dry season upland fresh water flow into the Bay through the estuary is very much lower than that of monsoon season. Tidal action becomes stronger and dominates water flow pattern in the Meghna estuary. The distribution of flow and water level in the different channels of the Meghna estuary are governed by river discharge, the tide and the wind speed. Velocity is higher during ebb tide than that of flood tide due to combined effect of upstream flow and downward tidal current. Velocity in monsoon is much higher compare to dry period because upstream discharge is higher in monsoon. Most of the accretion and erosion mainly occurs during monsoon and post monsoon period. In pre-monsoon and post monsoon, wave height is less compared to monsoon as wind speed is less during these periods. Along the left Bank of this estuary, significant wave is considerable. As a result, tremendous whirl action due to tidal effects and whirl action due to enormous drainage flow during monsoon, wave action, tidal surge of the lower Meghna causes breaching of the major parts of the embankments of polder 59/2 and adjacent area. The main purpose of this research paper is to understand the hydrodynamic features of the Meghna estuary.

The Meghna Estuary is formed by the combined flows from three great rivers in the southeast Asia; the Ganges, 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 Bengal 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 setup , 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 Bay of Bengal more specifically the Meghna Estuary.