Effects of Sediment-Induced Density Gradients on the Estuarine Turbidity Maximum in the Yangtze Estuary

An estuarine turbidity maximum (ETM) is a region of elevated suspended sediment concentration (SSC) resulting from residual transport mechanisms driven by river flow, tides, and salinity-induced density gradients (SalDG). However, in energetic and highly turbid environments such as the Yangtze Estuary, SedDG may also substantially contribute to the formation and maintenance of the ETM. Since this mechanism is relatively poorly understood, we develop a three-dimensional model to explore the effect of SedDG on tidal dynamics and sediment transport. By running sensitivity simulations considering SalDG and/or SedDG, we conclude that the longitudinal SedDG leads to degeneration and landward movement of the ETM. Moreover, two effects of the vertical SedDG are identified to be responsible for sediment trapping: One by enhancing the vertical sediment concentration gradients, and another by additionally affecting hydrodynamics including the water levels, velocities and salinities. The longitudinal and vertical SedDG leads to seasonal and spring-neap variations of upstream migration of the salt wedge: Vertical SedDG is more pronounced at neap tides in the wet season due to stronger stratification effects, whereas longitudinal SedDG is more pronounced at intermediate tides in the dry season due to weaker mixing and limited deposition. These findings imply that the SedDG contributes substantially to channel siltation and salt intrusion in highly turbid systems, and need to be accounted for when numerically modeling such phenomena. Plain Language Summary In highly turbid estuaries, estuarine turbidity maximum (ETM) with high suspended sediment concentrations (SSC) influences the water quality and ecological system. These high SSCs interact with salinity, and the salinity and sediment transport influence siltation of navigation channels, freshwater resources and so on. In this study, we employ a three-dimensional numerical model to investigate the effect of sediment-induced density gradients (SedDG) on sediment transport and salt intrusion. The effects of the longitudinal and vertical SedDG weaken and promote the ETM (respectively), and influence salt intrusion. These findings are important for the management of sediment and freshwater resources in turbid estuaries.