Fortnightly variation in sediment-induced stratification in a macro-tidal estuary and its impacts on sediment movement

Understanding the evolution of density stratification in estuaries is essential due to its significant impact on hydrodynamics and material transport processes. In this study, the Environmental Fluid Dynamics Code (EFDC) is employed to investigate the influence of suspended sediment on density stratification, as well as the resulting feedback effects on sediment dynamics and bed changes. To quantitatively assess the role of sediment-induced stratification, two model scenarios are conducted—one accounting for sediment's contribution to water density and the other excluding it. The Sanmen Bay in Zhejiang Province, China, where stratification is strongly influenced by the vertical distribution of sediment, is selected as the study area. During the neap tide, density stratification is governed by vertical salinity gradient, and the influence of sediment is negligible, as the vertical suspended sediment concentration (SSC) gradient is less than 0.01 kg/m4. While during the spring tide, the vertical SSC gradient increases significantly (approximately 0.1–0.2 kg/m4), leading to pronounced sediment-induced stratification, particularly in the bottom layer. When sediment-induced stratification is taken into account, both the magnitude and spatial extent of net sediment deposition increase, primarily due to a reduction in bottom shear stress. The decrease in bottom friction further leads to diminished turbulent mixing, which suppresses the upward diffusion of sediment. Consequently, reductions in both shear stress and turbulent diffusion contribute to a decrease in SSC due to sediment-induced stratification. The results demonstrate the importance of sediment-induced stratification in regulating sediment dynamics in high-turbidity circumstance.