Abstract: The most prominent feature of saltwater intrusion in the Changjiang Estuary is the saltwater spillover from the North Branch into the South Branch during the dry season; the spillover is the only source of saltwater for the Dongfengxisha, Taicang, and Chenhang Reservoirs as well as the main source of saltwater for the Qingcaosha Reservoir. A three dimensional numerical saltwater intrusion model, validated on the Changjiang Estuary, was applied to simulate and analyze salinity variation both during and after construction of the sluice at the upper reaches of the North Branch; the model considered tide, climatic river discharge, and wind in January and February. The simulated results showed that the South Branch is occupied by freshwater, the phenomenon of saltwater spillover from the North Branch vanishes and the salinity in the upper reaches of the North Branch decreases substantially after building the sluice. The salinity at the water intakes of the Dongfengxisha, Taicang, and Chenhang Reservoirs approaches 0. The salinity at the water intake of the Qingcaosha Reservoir decreases significantly and is less than 0.45 and there is freshwater at all time. The operation of the sluice in the numerical experiments is adopted in two ways. One is open during ebb current, closed during flood current in the daytime, and open during flood current in the nighttime. The other is open during ebb current and closed during flood current in the daytime and nighttime. The salinity variations in the South Branch with the two operational schemes are nearly identical. This can be attributed to the fact that the saltwater intrusion in the upper reaches of the North Branch is substantially reduced by the former sluice operational scheme, and a freshwater area with salinity approaching 0.45 appears. Even though the sluice is open during day flood current, the water entering the South Branch is freshwater and the total seaward residual current in the South Branch is enhanced. Considering the numerical simulation results and operation costs, the former sluice operational scheme is recommended. The water intake time is significantly improved, not only for the Dongfengxisha, Chenhang, and Qingcaosha Reservoirs in Shanghai, but also for the Taicang Reservoir in Jiangsu; moreover, security in water supply is ensured for both places.

Key words: Changjiang Estuary, saltwater intrusion, water sources, building sluice, numerical simulation