Numerical Study On The Tidal Current Limit And Tidal Limit In The Changjiang River And The Response Of Saltwater Intrusion In The Changjiang Estuary To The River Discharge Change

A3D numerical model with high resolution was built to investigate the Changjiang River’s tidal current limit and tidal limit under various guarantee rates of river discharge in dry and flood seasons, as well as the saltwater intrusion in response to the changes of river discharge at the Datong and the Three Gorges Reservoir. The model domain covers the reaches of the Changjiang River from the Three Gorges Reservoir to the river month and its adjacent seas. The model exhibits the interactions among the tide, runoff, wind stress and topography. The model was validated by the measured water level, current velocity/direction and salinity in the Changjiang Estuary, as well as the water level in the lower reaches of the Changjiang River, and the results were consistent well with the observed data.According to the direction of the cross-sectional water flux along the river, the quantitative relationship between the tidal current limit and river discharge was analyzed. In flood season (July), with the river discharge increasing from34551to69765m3/s, the Changjiang River’s tidal current limit moves from the Taiping Island (144.5km upstream from the Xuliujing) to the Changqingsha Island (52km upstream from the Xuliujing). The relationship between the tidal current limit and river discharge is y=0.0643x2-9.3493x+392.07, in which the x (unit:×103m3/s) indicates the river discharge, and y (unit:km) represents the distance of the tidal current limit away from the Xuliujing (upstream positive). In dry season (January), with the river discharge varies between7850and16840m3/s, and the tidal current limit locates between the Taiyang Island (70km upstream from the Wuhu Station) and Yizheng channel (near the Zhengjiang Station). The relationship between the tidal current limit and river discharge is y=0.0894x2-22.104x+579.29.Based on the changes of the tidal range along the river, the relationship between the tidal limit and river discharge in flood and dry seasons in various guarantee rates were explored. In flood season (July), with the river discharge increasing from34551 to69765m3/s, the tidal limit moves from the Chongwen Island (upstream of the Datong Station) to the Wuhu Station with a range of about120km. The relationship between the tidal limit and river discharge is y=0.0571x2-9.5582x+775.94. In dry season (January), with the river discharge increasing from7850to16842m3/s, the tidal limit moves from the Gupai Island (656km upstream of the Jiujiang Station) to Anqing Station with a range of about82km. The relationship between the location of the tidal limit in dry season and river discharge is y=0.3428x2-17.9x+777.55.Under the constant river discharge of12000m3/s at the Datong Station and increased by3000m3/s for1day during the middle tide after the neap tide, the spring tide, the middle tide after the spring tide and the neap tide, respectively, the response time (the salinity difference reaches the peak) at the Qingcaosha Reservoir, Chenhang Reservoir and Dongfengxisha Reservoir are about6.0,5.0and3.5days. When the river discharge increases during the middle tide after the neap tide, the durations for taking the drinkable water from the Changjiang River at the Qingcaosha Reservoir, Chenhang Reservoir and the Dongfengxisha Reservoir are about211.5,274.2and160.4hours, respectively; during the spring tide, the durations in those three reservoirs are212.0,277.5and159.3hours, respectively; during the middle tide coming after the spring tide, the durations in those three reservoirs are226.5,289.2and165.8hours, respectively; during the neap tide, the durations in those three reservoirs are209.3,280.8and161.1hours, respectively. Additionally, the response time when the salinities begin to vary and duration at those reservoirs were also studied during different tidal types.In the extremely dry season that the discharge of Changjiang River is7000m3/s, the response of saltwater intrusion and the freshwater usage to the increasing of river discharge at the Three Gorges Reservoir under different tidal types and different discharges for different durations were studied. With the increasing of river discharge by5000m3/s at the Three Gorges Reservoir during the spring tide, the middle tide and the neap tide for5days, respectively, the durations for the Qingcaosha Reservoir taking the drinkable water are9.2,0.0and0.0hours, and the average response time for the maximum salinity variation at this reservoir is17.5days; the durations for the Chenhang Reservoir taking the drinkable water are112.2,172.0and167.3hours, and the average response time for the maximum salinity variation at this reservoir is16.5days; the duration for the Dongfengxisha Reservoir taking the drinkable water is86.3,44.2, and46.8hours, and the average response time for the maximum salinity variation at this reservoir is16.0hours. With the increasing of river discharge by3000,5000and7000m3/s during the spring tide, the durations for taking the drinkable water are0.0,9.2and18.7hours at the Qingcaosha Reservoir;65.8,112.2and182.0hours at the Chenhang Reservoir;44.0,86.3and99.0hours at the Dongfengxisha Reservoir. With the increasing of river discharge by5000m3/s for3,5and7days, respectively, the durations for taking the drinkable water are0.8,9.2and16.8hours at the Qingcaosha Reservoir;86.5,112.2and120.7hours at the Chenhang Reservoir;69.2,86.3and89.9hours at the Dongfengxisha Reservoir. Additionally, when the river discharge increases at the Three Gorges Reservoir under different tidal types and different river discharges for different durations, the response time when the salinities begin to vary and their durations at the reservoirs were also studied.