Impact of different sluice operation modes on salt intrusion and fresh water in a branching estuary

Sluice construction in estuaries can mitigate salt intrusion and secure freshwater resources, but it alters hydrodynamics and salinity distribution, especially in multi-branch systems like the Yangtze River Estuary. The most outstanding characteristic of salt intrusion in the Yangtze River estuary is salt spillover into the South Branch from the North Branch, which affects the water intake of the reservoirs in the South Branch. This study evaluates how a sluice in the lower North Branch and its operation modes affect salt intrusion and flow dynamics. Results indicate that closing the sluice during flood tides twice daily and opening it during ebb tides effectively blocks salt intrusion in the North Branch and eliminates salt spillover, benefiting the Dongfengxiasha and Chenhang Reservoirs in the South Branch. However, this mode increases the North Branch’s water diversion ratio (the ratio of river discharge into the North Branch to the total river discharge) from −3.88% to 36.34% driven by barotropic pressure gradients force, creating an artificial ‘water pump’ effect. As more freshwater diverts to the North Branch, reduced runoff in the South Branch intensifies salt intrusion in the North Channel, threatening the Qingcaosha Reservoir intake. Alternative operations–such as closing the sluice only during night flood tides once daily or only during spring tide nights–significantly weaken the pumping effect, reducing the water diversion ratio to 14.39% and 4.27%, respectively. These modes better support Qingcaosha Reservoir’s water supply. This study elucidates the mechanisms of hydrodynamics and salt intrusion change of different sluice operation modes by water flux and momentum analysis, and provides the optimal operation mode for fresh water utilization in branching estuaries.