Prediction Accuracy Improvement Of The Saltwater Intrusion In The Changjiang Estuary And Its Response To The Human Activities

Saltwater intrusion in large bifurcated estuaries have particular and complex patterns.Saltwater spillover(SSO)from the North Branch into South Branch is a special and important phenomenon of saltwater intrusion in the Changjiang Estuary.The SSO is an important factor for restricting the water intake safety of the reservoirs in the Changjiang Estuary.It is a serious threat to the safety of water supply in Shanghai.At the same time,under the influence of human activities in the river basin and estuarine,the pattern of saltwater intrusion in the Changjiang Estuary is changing constantly.It is of great scientific significance and application value for the utilization of water resources in the Changjing Estuary and the establishment of new water sources in the South Branch to ensure the simulation accuracy of the saltwater intrusion in the Changjiang estuary,especially the SSO.In this work,field data analysis,numerical simulation and dynamic analysis are used to study the dynamic mechanism of the saltwater intrusion and bottom drag coefficient in the extremely shallow water of the North Branch,and the impacts of the South-to-North Water Diversion Project and the major reclamation projects on the saltwater intrusion and freshwater resource in the Changjiang Estuary,and the unsuitable take water time for the longest duration in the lower Biandan shoal.The following main conclusions are obtained.1.Field observations with pedestal observed system were conducted in the North Branch from February 19 to March 1,2017.Eight measurement stations along the North Branch were located by four boats.The high-accuracy water velocity and salinity data were collected.The mechanisms of the saltwater intrusion in different reaches of the North Branch were revealed based on the flux mechanism decomposition of the unit width residual water flux and salt flux measured at the measured sites.2.Using the measured data mentioned above,aiming at the problem that there exists larger error of saltwater spillover from the North Branch in the saltwater intrusion prediction system,the improved Chézy-Manning formula was used to correct the bottom drag coefficient.The results show that the flood velocity and saltwater in the upper and lower reaches of the North Branch and the spillover from the North Branch become larger during spring and middle tide,and the model validation results are more accurate.Using the improved Chézy-Manning formula to solve the bottom drag coefficient,the simulation accuracy of the saltwater spillover from the North Branch is improved greatly.It solved the problem that the simulated saltwater spillover from the North Branch is too small during spring tide and too large during neap tide to improve the prediction accuracy of the saltwater intrusion prediction system in the Changjiang Estuary.3.Using the 3D numerical model of saltwater intrusion in the Changjiang Estuary,the effects of the South-to-North Water Diversion Project on the saltwater intrusion and freshwater resource in the Changjiang Estuary during the dry season were studied from both short-term and long-term perspectives.The results indicated that in the case of the short-term water transfer schemes of the eastern and middle route,the saltwater intrusion is enhanced with the water transfer of 1000 m~3/s,especially in the river mouth sandbar area of the North Channel,North Passage and South Passage,and in the upper reaches of the North Branch,where salinity rise distinctly and there is a larger area where the salinity rise is greater than 0.5.The intensity of saltwater spillover from the North Branch into the South Branch increased,the freshwater area in the South Branch decreased,and the isohalines in the river mouth sandbar area moved upstream,which indicate that the saltwater intrusion from the open sea is enhanced.During the neap-spring tide period in mid-late February,the continuous unsuitable take water times from Dongfengxisha Reservoir,Chenhang Reservoir and Qingcaosha Reservoir were 9.17,5.22 and 4.85 days,respectively.The continuous unsuitable take water times at the water sources increases with the decreased river discharge.In the case of the long-term water transfer schemes with water transfer of 1600 m~3/s,the saltwater intrusion is further intensified and continuous unsuitable take water times at the water sources become longer with further decreased river discharge.4.The effects of the four major reclamation projects(the Xincun Shoal reclamation project,the Qingcao Shoal reclamation project,the Eastern Hengsha Shoal reclamation project,and the Nanhui Shoal reclamation project)on saltwater intrusion pattern and water resource acquirement in the Changjiang Estuary were simulated.From the aspect of the variation of the net transection water flux and water diversion ratio,unit width residual water flux and salt flux,the surface and bottom salinity distribution in the bifurcated channels,the variation of saltwater intrusion in the North Branch,South Branch,North Channel,South Channel,North Passage and South Passage during the spring tide and neap tide induced by a single reclamation project and four reclamation projects were analyzed in detail.The combined effects of the four reclamation projects completion have reduced the intensity of saltwater spillover from the North Branch into the South Branch and decreased the salinity at the water intakes of the Dongfengxisha,Chenhang and Qingcaosha reservoirs,which are favorable for the water intakes of the three reservoirs in the Changjiang Estuary.5.The longest continuous unsuitable take water times at the lower Biandan Shoal was simulated and analyzed under the extremely low river discharge.The results show that the longest continuous suitable take water times at the lower Biandan shoal is greatly longer than that at the Qingcaosha Reservoir in downstream and Dongfengxisha Reservoir in upstream,meaning that the lower Biandan Shoal is an excellent reserve water source in the Changjiang Estuary.