Positive Feedback Between the Tidal Flat Variations and Sediment Dynamics: An Example Study in the Macro-Tidal Turbid Hangzhou Bay

Tidal flat variations (curvature) in macro-tidal turbid bays highly correlate with sediment dynamics. Interactions between tidal flats and sediment dynamics in macro-tidal turbid estuaries, like Hangzhou Bay, China, have been studied through a fully validated 3D sediment model by considering high-turbidity, flocculation, and water-sediment density coupling. Numerical results show that the circulation and sediment flux in the curvature of Andong tidal flat have similar magnitudes in the along-estuarine and lateral directions, demonstrating the important impact of the geometry on hydrodynamics and sediment transport processes. Sediment moves toward the southern/northern bank near the bottom/surface level in curvature. Tidally averaged net sediment flux is southward in the curvature. Tidal flat reclamation changes the curvature of the tidal channel, and modulates the circulation and sediment transport, and then feeds back to the tidal flat evolution subsequently. Bottom friction, nonlinear advection, and centrifugal force are dominant forces for inducing the lateral circulation, followed by the Coriolis force and sediment-induced baroclinic process. Lateral circulation combined with the suspended sediment concentration (SSC) profile controls the distribution of instantaneous and tidally averaged net sediment fluxes in the curvature. The sediment flux positively correlates with the curvature of channel bends. The increased curvature of channel bends enhances the southward sediment fluxes mainly by increasing the effects of the centrifugal force, nonlinear advection, and Coriolis force on the lateral circulation and SSC. This study revealed a two-way feedback system between tidal flat and sediment dynamics, which contributes to the impact of anthropogenic activities on geomorphological evolution of similar estuaries worldwide.