Suspended sediment dynamics in Cochin estuary, West Coast, India

Abstract

An observational study was conducted in Cochin estuary to examine the relative impacts of tidal and river runoff forcing upon suspended sediment dynamics at different time scales. Time series measurements of salinity and suspended sediment concentration (SSC) were conducted at spring and neap periods of wet and dry seasons. These data were complemented by measurements of tides, currents, vertical profiling of hydrography and turbidity at the main inlet of the estuary. The results showed that SSC was maximum at the main inlet and decreased towards upstream for both seasons. The intra-tidal variations of water column stability suggested that prominent stratification occurred at the main inlet as a result of torrential rain during spring tides of wet season. During neap tides of wet season, the runoff was relatively lower and intense turbulent mixing occurred during the ebb. During dry season, well-mixed (spring) and partially stratified (neap) conditions were noticed. Suspended sediment fluxes were also analyzed using the decomposition method to identify mechanisms of turbidity maintenance at the inlet. The fluxes were dominated by an advective component during both tidal phases of wet season and dry-neap period. In contrast, the tidal pumping was the main contributing process for residual fluxes during the dry-spring period. The residual sediment transport at the inlet was directed outward irrespective of tidal ranges and seasons due to the strong ebb currents with longer duration. These findings are significant for the development and protection of harbors situated close to the inlet in estuaries and for the efficient implementation of an integrated coastal management system.