Hydrodynamic and suspended sediment transport responses to reclamation in Yushan Island, Hangzhou Bay

Land reclamation reshapes nearshore hydrodynamics and sediment transport by altering coastline morphology. In this study, the reclamation project at Yushan Island in Hangzhou Bay was investigated. A two-dimensional hydrodynamic-sediment coupling model was built using MIKE21 and validated, from which tidal-current fields, bed shear stress, suspended sediment concentration (SSC) distributions, and seabed scour-siltation characteristics before and after project implementation were systematically analyzed. Results show that the blocking effect of artificial dikes reduced flow velocities by 0.2–0.5 m/s and bed shear stress by 0.2–0.4 N/m² on the frontal and lee sides, suppressing sediment resuspension and decreasing nearshore SSC by 0.02–0.06 kg/m³. At dike corners, flow deflection increased velocity and bed shear stress by 0.1–0.3 m/s and 0.1–0.3 N/m², respectively, causing increased SSC. The reclamation-induced scour-siltation variations showed pronounced local characteristics. In addition, the enhanced siltation was primarily concentrated within approximately 2 km of the artificial shoreline, and annual siltation rates increased by 0–0.6 m/a, whereas outer regions showed attenuated changes. Deep troughs on the north and south sides of Dayushan Island maintained high flow velocities and bed shear stress, showing limited morphological changes and overall stable channel-shoal patterns. Therefore, land reclamation altered nearshore current structures and the sediment resuspension-deposition balance, resulting in adjusted scour-siltation patterns, with the magnitude of these effects gradually decreasing with increasing distance from the artificial shoreline.