Characteristics of Hydrodynamic Processes in the Meghna Estuary due to Dynamic Whirl Action

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 combined flow of the Ganges-Brahmaputra-Meghna (GBM) is drained through the Lower Meghna River into the Bay of Bengal via the estuary. Among the big rivers in the world, the combined flow of the Ganges-Brahmaputra-Meghna Rivers ranks third in terms of river flow and first in terms of total sediment discharge (1). There are distinct differences between Brahmaputra and Ganges river regimes (2). The Brahmaputra rises on average one month earlier than the Ganges, while flow recession in the Ganges river starts earlier that the Brahmaputra. The peak flow in the Brahmaputra occurs during July while in the Ganges River it is delayed until August. Discharge of river water into the Meghna estuary at different seasons is highly skewed. Some of the measurements done during the Meghna Estuary Study project suggest that in an extreme year, discharge through the estuary can be as much as fifteen times during the monsoon season compared to that during the dry season (3). It reported that in an average year the discharge through the estuary is about 20,000 m3/s during the dry season whereas it is 100,000 m3/s during the monsoon (4). In recent year IWM has measured discharge in the Lower Meghna under a project named as "Detailed Feasibility Study in Connection with Protection of Ramgati and Kamal Nagar Upazila and Adjacent Areas under Laxmipur District from the continuing Erosion of the Meghna River". The discharge through this river varies from about 10,000m3/s in dry season to 160,000m3/s in the wet season .In general, dry season (October -March) is the calm period in the estuary with weak wind from north and low river discharge. In contrast, monsoon season (June-September) is more vigorous with strong south-westerly wind and large river discharge and also higher mean water level (5). The interactions between the river discharge and tidal volume moving through the channels during the premonsoon and post-monsoon period in Lower Meghna Estuary, the estuary can be divided into 3 sub-units, the Tetulia and the west Shahbazpur channel can be termed as 'fluvial (in the sense that river outflow dominates over the tidal inflow)'; the east Shahbazpur and the west Hatiya channels can be termed as 'fluvio-tidal', and the east Hatiya and the Sandwip channels can be termed as 'tidal'. Monpura Island and Char Faizuddin divides the Shahbazpur channel into east and west. Char Nurul Islam divides the Hatiya channel into east and west (6).The 'fluvial' and the 'fluvio-tidal' sub-units as described above act as a tidal river with very high discharges in the