Residual Flow in the Meghna Estuary on the Coastline of Bangladesh

The Meghna Estuary is a coastal plain estuary on the coast of Bangladesh. The important driving forces for the flow in the Estuary are the bathymetry, tides and the outflow from the Meghna River. A numerical model covering the northern half of the Bay of Bengal including the Meghna Estuary was set up with the objective to increase our understanding of the hydrographic features and morphological dynamics in the Estuary, especially in the case where man-made physical interventions are constructed. The simulations revealed a counter-clockwise residual circulation with a northward net flow in the Sandwip Channel of 10 000 m3 s 1 during the dry season and 15 000 m3 s 1 during the wet season. The residual flow is forced by tides together with the bathymetry. During the dry season the flow is approximately equal to the river discharge and during the wet season it is approximately one-third to one-sixth of the river discharge. The residual circulation to some extent traps the river water inside the Meghna Estuary and is one of the reasons for the relatively low salinity in the estuary even during the dry season. It is also believed to be important for the morphological development. Finally, a suggested intervention north of Sandwip shows to stop the residual circulation in the Estuary, for which reason it was advised not to construct such an intervention without further detailed investigations.