Suspended sediment movement in the estuary of the Ganges-Brahmaputra-Meghna river system

Abstract

Topographical, flow volume and suspended sediment concentration (SSC) measurements are presented in the large area of a turbidity maximum in the estuary of the Ganges-Brahmaputra-Meghna river system in Bangladesh. The zone of turbidity maximum is indicated by the satellite imagery taken during the low river discharge period. Topographical and flow volume measurements, the aspect ratio and the flow ratio, indicate that the system is divided into flood- and ebb-dominated channels. Suspended sediment movement in the estuary appears to be associated with the horizontal stratification of flow and with the residual movement induced by the asymmetry of the tidal wave. The flow ratio at different sections indicates that net import of suspended sediment takes place through flood channels while the net export occurs through ebb channels. Despite this difference in the direction of net movement of sediments, measurements show that the SSC, both in the flood channel and in the ebb channel, is dependent on the tidal range: the spring tidal SSC is about twice that of the neap tide, and locations with higher tidal range show higher magnitudes of SSC. The response of sediments to the varying energy environment of tidal range and tidal current, however, is not instantaneous; the SSC shows a time lag in its variation with the varying energy condition. The amplification and asymmetry of the tidal wave in the flood channels, and the slack-water asymmetry measured in a flood-dominant location, indicate that a possible landward residual transport of suspended sediment due to this factor is more pronounced through flood channels.