Research On The Changes Of Water Temperature And Hydrodynamic Characteristics Of The Main Stream Of The Three Gorges Reservoir And Its Tributary Under The Background Of The Operation Of Xiangjiaba And Xiluodu Cascade Reservoir

After the impoundment of the Three Gorges Reservoir(TGR),algal blooms in the side arm tributaries is frequently.The change of the water temperature and hydrodynamic characteristics of the reservoir area is a major reason.The operation of cascade reservoir of the Xiangjiaba and Xiluodu in the lower reaches of Jinsha River changes the flow of the river on downstream,water temperature,water and sand balance and the nutrient more significantly,causing the cumulative effects on the water environment.The changes of inflow and water temperature of inflow will affect the water temperature and hydrodynamic characteristics of the TGR,and then have an further influence for the reservoir water quality and algal blooms in the side arm tributaries.There is less research on the changes of water temperature and hydrodynamic characteristics of the TGR under the influence of cascade reservoir in the lower reaches of Jinsha River.Based on the long-term field monitoring data,this paper analyzes the changes of the inflow conditions of the TGR before and after the construction of Xiangjiaba and Xiluodu Cascade Reservoirs in the lower reaches of the Jinsha River,and based on CE-QUAL-W2,establishes the hydrodynamic model of the main Stream of the TGR and Xiangxi River.According to the change of inflow condition and the result of model calculation,different working conditions are designed,to analyze the characteristics of water temperature and velocity distribution of main section of main stream,and its response to the change of inflow condition after the construction of cascade reservoir.Taking Xiangxi River,a key tributary of the TGR,as an example,based on the long-term field monitoring data and the result of model calculation,to analyze the influence on the water temperature stratification structure and the depth of density current intruded to Xiangxi Bay from the mainstream of the TGR of the construction of cascade reservoir,and the influence on the algal blooms in Xiangxi Bay of the variation were analyzed.The main conclusions are as follows:(1)After the construction of Xiangjiaba and Xiluodu Cascade Reservoirs,the inflow of the TGR increases in the dry season and decreases in the wet season,the extreme value decreases and the flow process is more smooth.It has a certain homogenization effect.The water storage of the TGR has a certain homogenization effect on the water temperature.After the construction of Xiangjiaba and Xiluodu Cascade Reservoirs,the average minimum water temperature is further increased and the extreme value variation is further reduced.The variation of water temperature is lagging behind,the cooling process tends to be gentle after September,and the effect of water temperature homogenization is obvious.After the impoundment of the Xiangjiaba,Xiluodu Cascade Reservoir,the sediment concentration of discharge water is reduced,the sediment of inflow of the TGR decreases obviously,shows that the Cascade Reservoirs has an obvious effect on the sediment retention.(2)The hydrodynamic model of the main stream of the TGR and Xiangxi River based on CE-QUAL-W2 can accurately simulate the distribution characteristics and variation law of the water temperature and hydrodynamic force of the TGR and the tributary of the Xiangxi River,as well as the depth,distance and thickness of density current intruded to Xiangxi Bay from the mainstream of TGR,and meet the requirement of precision.It can be used to simulate the water temperature and hydrodynamic characteristics of the main stream of the TGR and Xiangxi River as a tool for relevant research and analysis.(3)After the construction of Xiangjiaba and Xiluodu Cascade Reservoirs,the water temperature of the main stream inflow of the TGR is homogenized,and the extreme value of the water temperature of the surface layer of the reservoir decreases in a year,which is less and lagging than that of the inlet flow,and the amplitude of the temperature difference between the the head region and the tail area of the TGR is reduced by about 1?.The head region water temperature of the main stream is lower than that of the tail of the reservoir during temperature raising period of March to June,and have the opposite result during the cooling period of October to February,and the water temperature of the main stream along the course tends to be the same temperature during the same temperature period of July to September.The vertical delamination of the water temperature at the head region of the reservoir decreases after the variation of the inlet water temperature,and there is also a vertical stratification of the calefacient water temperature during the obvious period of the vertical stratification of the water temperature in the main stream.When the flow rate increases,the water temperature of the main stream decreases relatively,and the change of the inflow flow rate has little effect on the water temperature of the main stream.The velocity of the vertical line along the main stream is basically the distribution of inverse C-shaped and semi-U shape,and the velocity in flood season is obviously higher than that in other periods.The variation of inlet water temperature has little effect on the vertical velocity along the section of the main stream,and the variation of the inflow flow rate has a great effect on the velocity of the vertical line along the section of the main stream.As the inflow rate increases,the velocity of the main stream increases,and the distribution pattern changes little.(4)After the change of the inlet water temperature of the main stream,the extreme value of the water temperature in estuary of the Xiangxi River decreases,the water temperature delamination weakens,the vertical calescence of the water temperature along the Xiangxi Bay is basically consistent with the change of the main stream,and the variation degree of the water temperature along the course from the estuary to the upper reaches of the Xiangxi Bay is weakened.The water temperature the surface changes small influenced by air temperature,and the vertical calescence is smaller in the middle and lower layer than in the upper layer.After the change of the water temperature in the main stream,the number of times of density current of surface and the upper layers intruded to Xiangxi Bay from the mainstream of TGR increased,and the number of times of density current of middle and lower layers intruded to Xiangxi Bay from the mainstream of TGR were reduced accordingly,this variation may reduce the risk of the algal blooms in Xiangxi Bay.Due to the increase of inflow rate during flood period,the depth of density current intruded to Xiangxi Bay from the mainstream of TGR varied,which can decrease the risk of the algal blooms in Xiangxi Bay.