Vertical profile of suspended sediment concentration in the turbidity maximum zone of the partially stratified Changjiang Estuary

In order to study the vertical profile of suspended sediment concentration (SSC) and its temporal variation in the partially stratified estuaries, the profile of SSC, as well as the profiles of current and salinity were measured over a neap-spring cycle for 16 tidal cycles in April 2012 in the turbidity maximum zone (TMZ) of the North Passage in the Changjiang Estuary. The observations revealed that the TMZ was characterized by high SSCs, strong current velocities and remarkable saltwater intrusion. Both salinity and SSCs could yield strong density stratification which would exert important influence on the shape of SSC profile by damping sediment diffusion. The vertical profiles of SSCs mainly exhibited three typical types, i.e., two-layer structure profile, exponential profile and linear profile, and had significant flood-ebb and neap-spring variation patterns. In a tidal cycle, the two-layer structure profile mainly occurred during the strong stratification periods, and the exponential and linear profiles mainly occurred in the weak stratification periods. About 60% observed SSC profiles belonged to the two-layer structure profile, and 40% observed SSC profiles belonged to the exponential and linear profiles. The formation of the two-layer structure profiles during the latter half of floods and early half of ebbs was attributed to the bottom lateral currents, because it could drive the higher SSC and higher salinity in the deep channel to the south shoal through the bottom water layer. Two new empirical equations for the SSC profiles are proposed. They can predict the linear and exponential profiles accurately, and predict the two-layer structure profile reasonably. Both the exponential and linear SSC profiles had constant diffusion coefficient in the water column, and they can be delineated by a unified equation. Additionally, the bottom lateral currents directed to the south flank during 87% of the survey period, and could enhance the SSC, salinity and water exchanges between the channel and the shoal.