Impact Of The North Branch Regime Change On The Hydrodynamics And Saltwater Intrusion In The Yangtze River Estuary:A Numerical Study

For the multiple bifurcations,Different types of saltwater intrusion is a natural phenomenon in the Yangtze River estuary,and the most important feature is the saltwater spilling over from the North Branch(NB)to the South Branch(SSO),Which restricts the utilization of freshwater resources in the South Branch(SB),is a serious threat to the safety of water supply in Shanghai.In recent years,with the deposition and narrowing of the NB,the hydrodynamics and saltwater intrusion in the NB have changed greatly.Simulating the dynamic process of the SSO under different river regime changes in the NB has a great significance to reveal the dynamic mechanism of SSO,and also has important application value for the utilization of freshwater resources in the SB.In this paper,the remote sensing technology is used to analyze the shoreline changes of the NB since 1974,and the three-dimensional numerical model ECOM-si is used to accurately simulate the change process of SSO in different river regimes of the NB since 1974.Combined with observation data,the influence of new sand bodies in the branch mouth of the NB and SB and deposition in the lower part of the NB on hydrodynamic and saltwater intrusion after 2007 is simulated.The main conclusions are as follows.Remote sensing technology was used to analyze the shoreline changes of the NB of the Yangtze River estuary.Before the 1990s,the main changes on both sides of the NB were the merging of Yonglong sand and the reclamation of Weijiao Sand near Haimen Port.After the 1990s,human activities were more frequent,and the shoreline of the NB changed greatly,mainly including the formation of Xinyue Sand in the upper section and the formation of silted beaches outside Chongtou,as well as the reclamation of Xunsha,Xinlongsha and Xincunsha in the middle reaches,and the reclamation of Beihu and Chongming East Beach in the lower reaches.Make the shoreline of the NB move outward greatly,the plane shape of the NB gradually narrows to the bell-shaped river regime with uniform narrow width in the upper and middle reaches.Since 1974,the reclamation area of the NB is about 483 km~2.The reclamation project directly narrows the river width and silts the channel,which has a great impact on the stability of the NB.Using the average river discharge at the Datong hydrological station and the average wind condition in January and February as the drought season hydrology and climatic conditions.The changes of hydrodynamic and SSO in the NB in 1974,2007and 2016 were numerically simulated.The simulation results show that the change of river regime from 1974 to 2007 led to a decrease of 28.2%of the tidal rise and 31.9%of the tidal fall during the dry season tide near Lianxingang,and a decrease of 44.2%of the tidal rise and 23.8%of the tidal fall during the neap tide.The change of river regime during 2007-2016 led to a decrease of 20.6%in the flood tide and 23.2%in the ebb tide,and a decrease of 26.2%in the ebb tide and 23.7%in the ebb tide.From the time series of salinity variation at the intakes of reservoirs in the SB,the aspect of the variation of the net transection water flux and water diversion ratio,unit width residual water flux and salt flux in the NB,the surface and bottom salinity distribution,it can be seen that the change of regime of NB leads to the obvious weakening of SSO,and the salinity at the intake of Dongfengxisha,Chenhang and Qingcaosha Reservoirs decreases,which is conducive to the utilization of freshwater resources in the SB.Based on the observation data at the upper reaches of the NB in May 2020 and the measured water depth data in 2007 and 2016,the influence of new sand body in the bifurcation of the NB and the SB and deposition in the lower reaches of the NB on the hydrodynamics and saltwater intrusion was analyzed.The data of field observation near the new sand body indicated that the ebb current had an absolute advantage.During the spring tide,the time of ebb tide is much longer than that of flood tide.The flood current from the SB into the NB in large quantities through the main channel.The new sand body blocked the entrance of flood current from the SB into the NB,changing the local hydrodynamics.The numerical simulation results show that the new sand body caused the net water volume spllied over from the NB into the SB in spring-neap period increase by 24.9%,significantly enhances the saltwater intrusion if the new sand body continues to deposit by 0.85 m,the saltwater intrusion will not increase.The deposition in the lower reaches of the NB resulted in the tidal prisms cross the section at Lianxingang during flood tide and ebb tide decrease by 15.2%and 16.4%in spring tide,and decrease by 21.2%and 19.0%during neap tide,respectively.During spring tide,the high tide level dropped,the low tide level rised,resulting in saltwater intrusion weakened.