The effect of the front retaining wall on the released water temperature of Dongqing reservoir
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Abstract:
Water,environment,and ecology problems brought by the construction of hydropower plants are getting more and more attention.Water temperature is one of the most important influencing factors for water environment assessment.Water temperature plays an important role in the physical,chemical and biological processes in aquatic systems,and is a fundamental element of water environment research,with a very wide range of influence.Large reservoirs after storage,easy to form water temperature stratification,the temperature difference between the surface and bottom layer can be as high as 20 ℃.Most of the completed reservoirs have low intake locations and are not easy to change,resulting in low-temperature water released,which in turn causes serious environmental impacts on the reservoir area and downstream.The most direct harm of low-temperature water released is to reduce crop yields,delay the spawning period of fish.Water temperature stratification causes stratification of water quality parameters,resulting in water quality deterioration,which harms water ecology.Reservoir low-temperature water released has become a hot issue in the current research.In this regard,it is necessary to adopt a series of engineering measures to improve the negative impact caused by the low-temperature water released. Taking Dongqing reservoir as an example,a three-dimensional water temperature-hydrodynamic mathematical model was constructed to simulate the temperature of water released from the reservoir and the water temperature distribution in front of the dam under different retaining wall elevation,and to analyze the variation law between the front retaining wall elevation and the temperature of water released and water temperature in front of the dam. The results show that the improvement effect of the released temperature is closely related to the retaining wall elevation.When the retaining wall was lower than the intake,the change of the retaining wall elevation has almost no effect on the released temperature.When the retaining wall was higher than the intake by a certain height,the higher the retaining wall elevation is,and higher the released temperature.When the retaining wall elevation is 480 m,the maximum increase of the released temperature in April is 1.11 ℃.The increase of the released temperature is linearly related to the difference of the vertical water temperature in front of the dam corresponding to the location of the retaining wall and the water intake.The vertical water temperature distribution in front of the dam is also affected by the retaining wall,when the elevation of the retaining wall is higher than the water intake elevation by a certain height,as the elevation of the retaining wall increases,the thickness of the thermocline increases and the position rises.The relationship between the improvement effect of the released temperature and the height of the retaining wall is more complicated,and the location of the faster growth of the released temperature increase occurs at the retaining wall elevation of 471 m and 475 m. The front retaining wall can be used to improve the negative impact on the environment caused by the low temperature of water released.When the retaining wall elevation is lower than the intake elevation,the change of retaining wall elevation has almost no effect on the temperature of the water released from the reservoir.When the retaining wall elevation is higher than the water intake elevation by a certain height,the retaining wall elevation determines the temperature of water released,with the increase of the retaining wall elevation,the temperature of the water released from March to June increases,the higher the retaining wall elevation,the more obvious the improvement effect.When the retaining wall elevation is lower than the water intake elevation,the water temperature distribution in front of the dam is not affected by the retaining wall.When the top elevation of the former retaining wall is higher than the water intake elevation by a certain height,with the increase of the retaining wall elevation,more surface high temperature water is released,and the temperature leap layer position rises,and the thickness increases.The increase of the retaining wall elevation weakens the vertical heat exchange of the bottom water body in front of the dam.When the retaining wall elevation is higher than the water intake elevation by a certain height,the degree of improvement of the water temperature released by the retaining wall is related to the difference in the vertical water temperature between the retaining wall and the water intake elevation,the larger the difference,the greater the degree of improvement of the lower released water temperature,and there is a linear correlation.Analyzing the encrypted calculation scheme,there are two extreme points for the growth rate of the lower released water temperature increase,and the growth rate of the increase is greatest at the height of the retaining wall 475 m and 471 m.Considering the engineering feasibility,the top elevation of the retaining wall is set to 471 m as the optimal choice.
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