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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. Plain Language Summary: In macro‐tidal turbid estuaries like Hangzhou Bay, tidal flat reclamation changes the curvatures of tidal channels, and modulates the currents and sediment transport, and feeds back to the tidal flat evolution subsequently. The sediment flux positively correlates with the curvature of tidal channel bends, as shown by numerical sensitivity tests. Bottom friction, nonlinear advection, and centrifugal force dominate the lateral circulation. The lateral circulation combined with the suspended sediment concentration (SSC) profile control the distribution of instantaneous and tidally averaged sediment fluxes in the curvature. The increased curvature of channel bends due to reclamation activities changes the lateral circulation mainly by modulating the effect of the centrifugal force and nonlinear advection. The lateral sediment fluxes are then modulated by lateral circulation and SSC. This study addresses a positive feedback process between the tidal flat variation and sediment dynamics in macro‐tidal turbid estuaries. Key Points: Tidal channel curvature and its impact on lateral sediment flux in macro‐tidal Hangzhou BayPositive feedback between tidal flat reclamation and sediment deposition at the channel bendCentrifugal force and nonlinear advection are dominant forcings driving lateral circulation and sediment flux at the bend [ABSTRACT FROM AUTHOR]

Surface suspended sediment concentration (SSC) retrieved from the satellite data was used to study the multidecadal (1984–2015) dynamics of the surface turbidity maximum zone (TMZ) in Hangzhou Bay (HZB), China. A correlation algorithm was calibrated to estimate the surface SSC using the remote sensing data. The averaged deviation of the retrieved SSC from the field data was 15.6%. The impacts of tidal amplitude and phase on remote sensing data were specifically considered and accurately removed. Five periods were organized to analyze the multidecadal variability of surface SSC: (a) 1984–1988, (b) 1990–1994, (c) 1997–2001, (d) 2002–2006, and (e) 2013–2015. Results showed that the areas of low turbidity zone (<200 mg L–1) increased from period a to e, indicated by the trend ratio (Ts) of 2.58%. The area of TMZ (>700 mg L–1) decreased significantly from 2205 km2 in the period a to 647 km2 in period e, with Ts = −4.57%. The distribution pattern of the SSC was more skewed in period e (Sk = 2.03) than that in the period a (Sk = 0.63). Tidal flat reclamation around HZB contributed to the multidecadal reduction of the surface SSC from 1984 to 2015, through changing the hydrodynamics and reducing sediment source. The decreased sediment load from the Changjiang River contributed to the decline in the surface SSC in HZB, by reducing the sediment source. Flooding of the Qiantang River during wet seasons played an important role in increasing the surface SSC at the bay head, as the deposited sediment during wet seasons reduced the water depth and increased the resuspension. This study helps the estuarine morphology and environment management worldwide. [ABSTRACT FROM AUTHOR]