Research On Influence Mechanism Of Reservoir Operation To Water Temperature Characteristic Of Xiangxi Bay In Three Gorges Reservoir

After impounding of Three Gorges Reservoir (TGR), the mainstream of TGR andthe tributaries flow velocity decreased and eutrophication is increasing due to the risingwater level, causing serious algal bloom occurred part of the bay of tributaries in springand summer, some scholars believe that one of the main cause of the eutrophication is thewater flow become slow down, the viewpoint of improving the water flow conditionsthrough reservoir operation, restraining euthrophication has been approved by manyexperts and researchers, but they haven’t clearly put forward the ecological reservoirreoperation demand with prevention and control of tributaries algal bloom. Based onCE-QUAL-W2vertical two-dimensional water temperature and water flow model,building the Three Gorges Reservoir-Xiangxi Bay water temperature and water flowmodel, analyzed the influence of stratified density current to water temperaturecharacteristics of Xiangxi Bay,and set the different regulating methodsfor analyzing theshort-term fluctuations of water level on the influence of stratified density current diveddepthand the mixing depth of Xiangxi Bay. Main research work and conclusions are asfollows:(1) A two-dimensional, laterally averaged hydrodynamic and water temperaturemodel of Three Gorges Reservoir-Xiangxi Bay was established based on CE-QUAL-W2,which simulated water temperature andwater flow of mainstream of TGR and thetributaries successfully, the simulated value and observed value without big difference,the model could simulated hydrodynamic and stratified density current characteristics inthe Xiangxi Bay (XXB), it can basically reflect the plunging depth of the main streamwater and calculated vertical velocity distribution and their measured values agree well.Therefore, the model is adequate fo hydrodynamic simulation in TGR and XXB.Whichconfirmed the model of Three Gorges Reservoir-Xiangxi Bay is applicable, and also canmake up for the poor field monitoring, indoor experiment and situ experiment. Also canprovide reference for the same type bay and reservoir,and for further study on the othertributaries the mechanism of algal bloom and ecological control methods offer theplatform of numerical research supporting.(2) Based on model output data in March2010-February2011about watertemperature, water flow and water density of Three Gorges Reservoir-Xiangxi Bay,analyzed the influence of stratified density current to water temperature characteristics of Xiangxi Bay, the main conclusions are as follows:①In routine operation period of Three Gorges Reservoir, Xiangxi Bay is alwaysexisting significantly the phenomenon of stratified density current. In down period offlood period, the flow of upstream is mainly from the surface layer to the bottom layertransformed and mainstream water is mainly from the bottom layer to middle layer tosurface layer transformed. In flood period, the flow of upstream is mainly from thebottom layer form flow into bay and mainstream water is mainly present the middle layerform, and the middle layer can transformed the middle layer in continue an overflowingperiod. At the end of flood period and beginning impoundment, the flow of upstream isstill from the bottom layer forms flow into bay and mainstream water is mainly presentthe surface layer form, while the end of impoundment the mainstream water wouldtransform to the middle layer flow into Xiangxi Bay. In low water period, the flow ofupstream is still from the bottom layer forms flow into bay and mainstream water ismainly from the middle and lower layer to bottom layer transformed.②The vertical spatial and temporal distribution of Xiangxi Bay’s watertemperature has a significantly difference. Due to the influence of the flow of upstreamwater temperature, the water temperature vertical stratification is stronger. While thedownstream the influence of flowing back water and water level fluctuation, the watertemperature vertical stratification is weak. It is because of the difference watertemperature between mainstream water and bay water, bay water and the flow ofupstream, making the mainstream water have the kinds of forms diving into bay, such asthe bottom layer, middle-lower layer, middle layer, middle-up layer and surface layer.While the flow of upstream will flow from the surface layer into Xiangxi Bay in springand winter and will flow from the bottom layer into Xiangxi Bay in summer and autumn,and broken the original water temperature structure of Xiangxi Bay.③The water surface mixing depth of Xiangxi Bay has presently the features ofshallow in spring and summer and the features of deep in autumn and winter on the time,and has presently the features of low in upstream and the features of high in downstreamon the space. The surface water temperature of bay increases quickly, while the bottomwater temperature has a relative lag in spring. The bigger water temperature differencebetween surface and bottom lead to greatly reduce the surface mixed depth. In summerthe surface water temperature is highly and easily with low temperature water at thebottom forming the large water temperature difference, the surface mixing depth reaches the lowest in all the year. Light intensity is not big in winter and due to impoundment ofThree Gorges Reservoir the mainstream water flow from the surface into Xiangxi Bay,making the surface mixing depth increased gradually.④The water body vertical stability coefficient of Xiangxi Bay changes with theseasons difference is bigger. In spring as air temperature rise rapidly, the surface watertemperature of bay warming up faster, but the bottom water temperature of bay has arelative lag, water temperature difference between the surface layer and bottom layer isgradually increasing, leading tothe water body’s vertical stability coefficient increasesgradually. In summer, the water body vertical stability coefficient increases significantly,the water temperature stratified area was increased obviously. In upstream the watertemperature has presently a stronger stratified state and in downstream has presently aweak stratified state. In autumn as the impoundment of Three Gorges Reservoir, thewater temperature stratified state was gradually broken, the bay’s water body verticalstability coefficient significantly decreased. In winter, the estuary of Xiangxi Bay’s waterbody has presently a mixed type and in upstream has presently a weak stratified state. Onthe whole, the water body vertical stability coefficient of Xiangxi Bay increasesgradually from downstream to upstream, the flow water temperature of upstream relativeto bay’s water temperature is lower for most of the year, mainly flowing from the bottominto the Xiangxi Bay, leading to water temperature difference between the surface andbottom and water body has presently a tronger stratified state. While the downstream ofbay due to the long-term frequently exchange with the mainstream water, the water bodyhas presently a mixed type or weak stratified state in a long-term.⑤Based on the output datas of CE-QUAL-W2model, it finds that the stratifieddensity current can increase the vertical mixing degree of water flow, breaking theoriginal water temperature vertical stratification structure of Xiangxi Bay and makingpresent a unique form of distribution. The diving depth change of stratified densitycurrent is the main reason for short-term change of the surface mixing depth. And theshort impoundment of Three Gorges Reservoir can make the middle or middle-up layersrising to the surface layer, leading to the mixing depth rapidly changed.(3) When the initial water level is155m, water level risein1.19to1.78m andcontinue impounding3days, the flow backward water inmiddle-upstream of the baywillrise from the middle-uplayer to the surface layer and the water surface mixing depthincrease significantly which the growth range in0.92to6.15m. When the initial water level is145m, water level rise in0.80to2.40m and continue impounding2days, theflow backward water in middle-upstream of the bay will rise from the middle layer to thesurface layer and the water surface mixing depth increase significantly which the growthrange in0.64to8.24m.(4) In short-term of impoundment scheduling water conditions, with differentwater level daily fluctuation and the days of impounding can’t change the forms of thelong-standing existing stratified density current, but it can change the hydraulic elements,such as the diving into thickness and distance of stratified density current, and the biggerwater level daily fluctuation cause the greater thickness and the longer distance.Therefore,it is possible to use the stratified density current broken the water temperaturestratification and increase water surface mixing depth ofXiangxi Bay, achieving theprevention and control the feasibility of Xiangxi Bay’s algal bloom.