The Effect of Salinity Intrusion on Dongfengxisha Reservoir in Chongming Island, China

Island is an important part of sea system. The construction of a reservoir to avoid salt water and store fresh water could help to solve the problem of scarce freshwater resources in the development of island. Located in the Yangtze Estuary, Chongming Island is the third largest one in China. The water supply mode has been transformed from the decentralized water supply to centralized water supply since the completion of Dongfenxisha Reservoir in 2014. The spilling over saltwater is a major threat to the safety of Dongfengxisha water source under natural conditions. Considering the effect of the northerly wind, a two-dimensional tidal-salinity mathematical model of the Yangtze Estuary is established to study the relationship between the salinity and the runoff process of the water intake in Dongfengxisha Reservoir. The salt water around the Dongfengxisha Reservoir is mainly spilled over from the North Branch. Saline water intrusion in the Yangtze Estuary is the most severe in January, with the maximum salinity of 2.31‰. Because of its short duration, the water sources safety can be guaranteed. Building a tidal gate in the North Branch is an effective way to improve the safety of Dongfengxisha Water Source and only closing it in the spring tide would already achieve a good results. If there are serious adverse factors for saline intrusion, such as small runoff and sustained northerly wind, the safety of water sources could also be ensured by prolonging the closing time. This research can also provide reference for the construction of water sources in other islands.