Water Environment Response Characteristics Of A Stratified Reservoir And Effect Of Mitigation Measures

It is of great significance to study the thermal stratification characteristics of reservoirs and its effect on water environment to improve water environment quality and alleviate the environmental problems caused by water conservancy projects.Taking Daheiting Reservoir as an example,this study adopts the method combining field monitoring,laboratory analysis and numerical simulation.Based on the data of field monitoring and laboratory analysis,the evolution process of thermal stratification and the response characteristics of water quality indexes such as water temperature and dissolved oxygen in Daheiting reservoir are analyzed.A two-dimensional hydrodynamic water quality model was established to study the parameter sensitivity of water temperature distribution and thermal stratification structure stability and the response characteristics of thermal stratification to climate change.According to the practical requirements of water environment management in reservoirs,a kind of aeration device was designed to alleviate the thermal stratification effect and improve the water environment quality.Based on the water quality model,the aeration situation was simulated and the key parameters of the aeration system were optimized.The main research results and conclusions were as follows:(1)The periodic evolution process of thermal stratification and the response characteristics of water quality in Daheiting reservoir were investigated.The Daheiting reservoir presents a single cycle mixed mode throughout the year.The periodic dynamic stratification process of the reservoir during the year is expressed as "mixing period-formation period-stratification period-extinction period",and the evolution of thermal stratification provides the basis and conditions for the distribution of various water quality indicators.Dissolved oxygen,pH,COD,ammonia nitrogen,iron,manganese,total nitrogen,total phosphorus and chlorophyll-a showed uneven distribution of space and time,in which the concentrations of iron,manganese,total nitrogen and total phosphorus showed overall The stratification period in summer is higher than in other seasons,and the spatial distribution of the bottom layer is higher than the surface layer.(2)The reasons for the formation of anoxic zone and the driving effect of thermal stratification on the formation of anoxic zone in the hypolimnion were analyzed.The hypoxia at the bottom of the water volume is mainly affected by the oxygen consumption of the bottom water volume,the oxygen consumption of the sedimentary material and the thermal stratification barrier.The correlation analysis of the vertical water temperature and dissolved oxygen concentration in Daheiting reservoir showed that the water temperature and dissolved oxygen concentration are highly correlated,and the thermal stratification provides conditions for the formation and evolution of hypoxia in the hypolimnion.Affected by the large flow from July to August,the anoxic zone of the reservoir appeared in June and September in 2018,with a range of about 2km in front of the dam and a depth of about 2m above the bottom of the reservoir.(3)A two-dimensional hydrodynamic water quality model was established,based on which sensitivity analysis of water temperature parameters and response characteristics of thermal stratification to future climate change were carried out.This paper uses the CE-QUAL-W2 model to establish a two-dimensional hydrodynamic water quality model of the Daheiting reservoir based on the 2018 measured data of the reservoir.The parameters of the model are calibrated,and the calibrated error results are within a reasonable range.It shows that the model has good applicability to Daheiting reservoir.Based on the water temperature module of the CE-QUAL-W2 model,the influence of water temperature parameters such as solar radiation absorption rate,air temperature,wind shading coefficient,storage water temperature,extinction coefficient and outlet location on the thermal stratification structure of the reservoir was studied.The stability of thermal stratification of the reservoir was evaluated using the APE potential energy index.The results show that the influence of water temperature parameters on the stability of thermal stratification structure is mainly manifested in the summer stratification period,when the reservoir is under the conditions of low solar radiation,low air temperature,high wind disturbance,low inlet water temperature,low extinction coefficient and bottom water.Low thermal stratification stability,can alleviate delamination effects.The overall response of the reservoir under future climate changes(temperature increase and wind speed decrease)is that the thermal stratification stability is enhanced and the thermal stratification effect is intensified.Based on the water temperature model,a multi-factor combination simulation of climate change factors and the operation mode of the reservoir The results show that the trend of increasing thermal stratification can be alleviated by changing the operation mode of the reservoir.(4)A mixed oxygenated aeration system was proposed.The effect of aeration on water environment was analyzed,and the aeration parameters were optimized.The mixed aeration system proposed in this study can realize the local aeration of water by artificially controlling the aeration position,inlet volume and inlet gas type.In the summer of 2019,seven aeration tests were carried out on the reservoir.Taking vertical water temperature and dissolved oxygen concentration as evaluation indexes,the improvement effect of different aeration schemes on the water environment in the reservoir area was evaluated.The experimental results show that a cylindrical oxygen-increasing region with a radius of 6m and a height of 5m is formed by aeration near the aeration point,and the average oxygenation rate is 3%-12.5%.Aeration reduces the temperature difference between the upper and lower water volume and weakens the stability of thermal stratification.Based on CE-QUAL-W2 model,the aeration model was established in this paper,and the aeration test of reservoir was successfully simulated.The simulation results show that both the aeration depth and the air intake increase the aeration effect.Considering the actual air intake and cost of the equipment,the optimal design of aeration parameters is as follows:aeration depth of 22m,air intake of 2 m3/h-3m3/h,can achieve an average oxygen increase rate of 8%-15%in cylindrical waters about 7m above the aeration point and 6m in the horizontal direction.Compared with the parameters before optimization,the thickness of the oxygenated water increased by 1m-2m and the oxygenated water rate increased by 3%-5%.