The effects of river regime changes in the Changjiang Estuary on hydrodynamics and salinity intrusion in the past 60 years Ⅲ.Saltwater intrusion

The 3D numerical model of the Changjiang Estuary built in the series paper Ⅱ was used to simulate the saltwater intrusion in 1950s, 1970s and 2012, and the changes of saltwater intrusion under different river regimes and their reasons were analyzed quantitatively in this paper. In the North Branch, the saltwater intrusion were controlled by the water diversion ratio and tidal range. In 1950s and 2012, saltwater spilled over from the North Branch to the South Branch and the magnitude was greater during spring tide than during neap tide, and was greater in 2012 than in 1950s. This phenomenon didn't appear in 1970s. In 1950s, saltwater intrusion was severe in the North Branch. In 1970s, it decreased greatly, and a large amount of fresh water appeared in the upper and middle reaches. In 2012, the saltwater intrusion was very severe and the whole North Branch was occupied by saltwater, sharp salinity front appeared in the upper reaches. In the South Branch, fresh water occupied the most area in 1950s and 1970s, and in 2012 saltwater with salinity greater than 0.45 appeared in the upper reaches because saltwater spilled over from the North Branch. In the North and South Channel, saltwater intrusion in the 1950s was the most severe. During the spring tide, the net water diversion ratio in the North Channel was higher than that that in the South Channel by 21.6%, which caused the salinity in the North Channel was lower than that in the South Channel, resulting in the saltwater from the open sea intruded mainly through the South Channel, and returned over from the South Channel to the North Channel. In 1970s, the mainstream of the South Branch turned to the South Channel, which caused the net water diversion ratio in the South Channel increased, and became higher than that in the North Channel by 10.4%, causing the salinity in the South Channel was lower than that in the North Channel; the saltwater intrusion was weakest in the South and North Channel. In 2012, the mainstream of the South Branch turned to the North Channel again and the net water diversion ratio in the North Channel was higher than that in the South Channel by 10.4%, resulting in the saltwater intrusion in the South Channel greater than that in the North Channel. Comparing with the situation in 1950s, the saltwater intrusion in 1970s was lower in the North and South Channel. Comparing with the situation in 1970s, the saltwater intrusion in 2012 was lower in the North Channel and was higher in the South Channel. During the neap tide, the net water diversion in the South Channel was lower than that that during the spring tide in 1950s, causing the saltwater intruded more farer and the salinity in the upper reaches was higher; in 1970s, the salinity intrusion in the North Channel decreased because of the reduction of the net water diversion ratio; in 2012, the saltwater that spilled over from the North Branch in the spring tide arrived the North Channel, which caused the salt flux in the neap tide being greater than that in the spring tide. The salinity was more stratified in vertical because of the lower tidal dynamics in the three periods, and the changes of saltwater intrusion was almost same as that during the spring tide.