| Saltwater intrusion is the common phenomenon in the tidal estuary.It not only has influence on the structure of estuarine circulation,characteristics of sediment transport,but also restrict the development and utilization of freshwater resource.The Yangtze River Estuary,as the main source of drinking water for Shanghai,has built three reservoirs: Qingcaosha Reservoir,Chenhang Reservoir and Dongfengxisha Reservoir.It is urgent to study the saltwater intrusion in the Yangtze River Estuary and measures to ensure the safety of water intake for the three reservoirs.At first,this dissertation used field observation data to analyze the spatial variation characteristics of saltwater intrusion and their transport mechanism.Second,using the 3D saltwater intrusion numerical model,the impacts of the river regime changes on saltwater intrusion and freshwater resource over the past decade in the Yangtze River Estuary were studied.Finally,considering the critical river discharge for the Chenhang Reservoir and the mitigation of the saltwater spillover from the North Branch by building sluice in the North Branch,the measures to ensure the safety utilization of freshwater resource were studied and proposed.The following main conclusions are obtained.(1)By analyzing a great mass of data of water velocity,direction and salinity in the Yangtze River Estuary,the characteristics of salinity distribution in the main channels of the Yangtze River Estuary and their transport mechanism were obtained.The spatial distribution of salinity was analyzed along the longitudinal,latitudinal and vertical direction.The analysis of longitudinal variations covers the four main channels,namely the North Branch,the South Branch-North Channel,the North Passage and the South Passage.The analysis of latitudinal variations included three parts: the middle section of the North Branch-middle section of the North Channel,the entrance of the North Branch – the entrance of the south Passage,and the entrance of the North Channel-the outside of the South Passage.The analysis of vertical variations is based on the vertical profile of salinity in each station.The transport mechanism and spatial characteristics of saltwater intrusion during different tide in the Yangtze River Estuary were revealed in detail.(2)The 3D estuarine and coastal numerical model ECOM-si was used to simulate and annlyze the impacts of the river regime changes from 2007 to 2017 on the saltwater intrusion and water resources in the Yangtze River Estuary.According to the variation of the net transection water flux and salt flux,water diversion ratio,residual unit width water flux,residual unit width salt flux,the surface and bottom salinity distribution,Lyuand the effects of the river regime changes on the saltwater intrusion in each channel were discussed.The results showed that the river regime changes in past decade reduced the intensity of saltwater spillover from the North Branch into the South Branch and slightly decreased the salinity in the water intakes of the three reservoirs,which means that the river regime changes in recent ten years can reduce the saltwater intrusion in Yangtze River Estuary and benefit to take water from the freshwater resources.(3)By simulating the saltwater intrusion under various river discharges in Datong hydrological gauging station,the relationship between the salinity at intake of Chenhang Reservoir and river discharge was analyzed,and the critical river discharge was obtained,which corresponds to the longest water supply day of the reservoir.The longest water residence time of the Chenhang reservoir is 7 days if no water is diverted from the Baogang reservoir.The simulation results show that the critical river discharge is 9500 m3/s,which is equivalent to the longest continuous unsuitable take water time of 7 days for the Chenhang reservoir due to saltwater intrusion.The longest water residence time of the Chenhang reservoir is 10 days if no water is diverted from the Baogang reservoir.The simulation results show that the critical river discharge is 8100 m3/s,which corresponds to the longest continuous unsuitable take water time of 10 days for the Chenhang reservoir.(4)The saltwater intrusion under different operation plans of a building sluice at the upper reaches of the North Branch was simulated and analyzed.The simulation results indicate that the operation plan of the sluice closing during flood current in night and open in day can eliminate the phenomenon of the saltwater spillover from the North Branch into the South Branch.The water intakes of Dongfengxisha and Chenhang reservoir are not threatened by saltwater intrusion,and the salinity at the water intake of Qingcaosha reservoir is less than 0.45.This indicates that building sluice at the upper reaches of the North Branch is an effective measure to reduce saltwater intrusion and significantly extend the water intake time of the reservoirs. |
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