Hydrodynamic recovery and morphodynamic response in enclosed bays

: Opening an enclosed bay for tidal intake and drainage can restore water exchange, but its effectiveness depends on the tidal intake and drainage pathways and may alter the long-established water–sediment balance in the adjacent sea area. In this study, a two-dimensional hydrodynamic–sediment numerical model was established for Xinglin Bay and the Xiamen western coast to investigate water exchange and erosion–deposition evolution under different tidal regulation schemes. The results show that the tidal intake and drainage pathway is a key factor controlling water exchange efficiency within the bay. When both tidal intake and drainage occur through the main lake area, water tends to flow back and forth near the sluice, and the average water exchange rate reaches only about 15% after two tidal cycles. Separating the intake and drainage pathways forms a directional transport channel and increases the average water exchange rate to 41%–45%. The scale of sluice regulation is a key factor affecting erosion and deposition in the adjacent sea area. Under regular water level regulation, local erosion and deposition occur on the outer shoal-flat section, while slight erosion occurs in the deep-channel section. Under full-tidal-range water level regulation, widespread erosion occurs on the shoal-flat section, supplying sediment to the deep channel and causing transient deposition in the channel. As the shoal-flat section gradually reaches a new erosion–deposition equilibrium, the deep-channel section may shift back to erosion. This study provides insight into water exchange and morphodynamic responses during tidal restoration of enclosed bays.