The Influence Of The Major Projects And Climate Change On Water Diversion Ratio And Salt Fluxes In The Changjiang River Estuary

The Yangtze River Delta region is one of the largest population and the most economically developed regions in China,therefor the demand for water is always unbalanced.For the multiple bifurcations,the Changjiang Estuary is characterized by the saltwater spilling over from the North Branch(NB)to the South Branch(SSO),which is the main factor affecting the reservoirs of the South Branch(SB).Meanwhile,the influences of major projects and climate change on hydrodynamic,saltwater intrusion and biological environments in estuary have been proved inextricably linked with local economic and habitat.Base on a semi-implicit estuarine,coastal and ocean model(ECOM-si)to simulate and quantitative analyze the influence of different major projects and climate change in Changjiang river estuary.The three-dimensional hydrodynamic numerical model ECOM-si,which has been used and improved by the instructor group for a long time.The latest research shows that using the Chézy formula which varies with water depth to calculate the frictional drag coefficient of the NB is significantly improves the SSO.Based on this research methods,this thesis simulates and analyzed the influences of the Three Gorges Project and the South-to-North Water Diversion Project which stand for major projects,and the variation of river discharge and sea level rise by climate change.The hydrodynamic process,saltwater intrusion and freshwater resources of each case are represented by statistically analyze and accurately calculate the water diversion ratio(WDR)and salt fluxes(SF).Besides,the drought period is simulated for notable change.The main conclusions are summarized as follows:1.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 for the ECOM-si.The result shows that the water fluxes enter the SB through the NB accounts for 3.9%of the river discharge,and the SF is 11.52 t/s during spring tide.Then the SSO disappears during neap tide,and the WDR accounts for only 1.2%.Means the salt transports upstream along main channel in the NB during spring tide,then it goes downstream on both the SB and NB during neap tide.The total SSO flux is 132 m~3/s during a complete spring-neap tidal cycle(CTC),and the WDR is 1.1%,SF is 6.10 t/s.The SB is bifurcated by Changxing Island and Hengsha Island into North Channel(NC)and South Channel(SC).The WDR during spring tide,neap tide and CTC are 75.9%,45.8%,60.2%,and the SF are 5.84,3.03and5.66 t/s,respectively.It evidences that most of the river water flows into the sea through the NC,and the the landward salt flux at the NC are from the SSO and intrusion from the SC.The salt exchange is distinct between the SC and NC.Saltwater transports upstream along the South Passage(SP)into the SC turns downstream in the NC.The same salt tansport between the SP and North Passage(NP)is more obviously.The WDR during spring tide,neap tide and CTC in the NP are66.6%,78.9%,75.7%,and the SF are 10.81,45.17 and 25.91 t/s,respectively.The more salt transports downstream by the NP for there strong salinity front in the SP which blocks the incoming river water flow.And a few saltwater transports into the NP from the NC.2.The Three Gorges Reservoir Project releases freshwater persistently during drought season can increase the river discharge to 13162 m~3/s,resulting in weakening the saltwater intrusion.During CTC,the SF in the NB is decreased by 1.32 t/s.The WDR in the SC rises by only 0.1%,and the landward SF in the SC and NC are decreased by 0.23 and 1.32 t/s.The SF is more sensitive than the WDR in the NC.The WDR in the NP is increased by 0.4%,but the SF is decreased by 3.19 t/s.The salinities at the water intakes of the three reservoirs in the SB decreased in different degree,and the duration of the salinities are lower than 0.45 psu(the drinking water standard)in Dongfengxisha Reservoir,Chenhang Reservoir and Qingcaosha Reservoir are increased by 4,4.6 and 13 days in February differently.The South-to-North Water Diversion Project decreases river discharge by 1000m3/s and enhances the SSO,and salinity increases obviously in the estuary.According to the WRD between the SC and NC does not change,the WDR between the South and NC depends on the river regime can be concluded.The SF in the NP rises by 1.52 t/s,which indicates that the reduced river discharge enhances the saltwater intrusion in the NP,and the salinity front moves upstream with a strengthened baroclinic force in the SP.The salinities at the water intakes of the three reservoirs all increased,the water intake times are all reduced.3.The results show that saltwater intrusion in Changjiang estuary can be weakened as the improved tendency of river discharge caused by climate change in the future.When the discharge increased to 12350 m~3/s,the SF in the NB,NC and NP are decreased by 0.27,0.77 and 1.43 t/s respectively,and the WDR in the NB and NP both reduced by 0.1%,and hardly changed in the NC.Even the salinities at the water intakes of the three reservoirs are increased by 0.5,2 and 6.5 days in Feabruary,respectively.The effect of sea-level rise includes improving tidal strength and deeper the channel.Thence the vertical flow exchange and the vertical stratification are stronger.At the upper reaches of the NB,the landward SF is increased by 0.66 t/s.The WDR is decreased by 0.3%,but the SF increased by 0.24 t/s in the NC.Especially,the salinities in the northern outlet of the NC show a totally converse variation with other areas.The salinity decreased around many tidal flat,because the nonlinear interaction become weaker with sea level rise.The salinities at the water intakes of the three reservoirs slightly increased after the sea-level rise,the water intake times in Fabruary are decreased by 0.5,1.5 and 4 days,respectively.