Vertical Distribution Characteristics Of Water Temperature In Front Of The Dam Of Xiaowan Hydropower Plant,the Lancang River

In recent years,the number of large dams in southwestern China has increased significantly,and the environmental impact of its engineering operations has received much attention from the engineering community.The vertical water temperature in front of the dam is one of the important aspects.Prototype observation and research of ultradeep water in giant reservoirs is also a research difficulty.Xiaowan Reservoir serves as the leading reservoir in the “two reservoirs and eight levels” program of the Lancang River Basin.The Xiaowan Dam is currently the second highest arch dam in the world,and there are few reports on the distribution of vertical ultra-deep water temperature in front of the dam..In order to investigate whether the vertical water temperature in front of the dam of Xiaowan Hydropower plant has a stable seasonal stratification and its variation with time on the seasonal,monthly,and daily scales,the influence of the diurnal variation process of temperature on the water temperature structure is further explored.In this study,Germany’s advanced ultra-deep water temperature prototype observation system was introduced.Twenty water temperature measuring units were installed along the depth of the water,with a water depth of 200 meters.The distance between the measurement units increased in proper order from top to bottom,in front of the dam of Xiaowan Hydropower plant.The water temperature has conducted continuous highdensity water temperature data for a long period of nearly one year(August 2016 to June 2017).Using the methods of historical data collection and statistical analysis,the vertical water temperature distribution characteristics of Xiaowan Hydropower plant were studied.The main results and conclusions of the study are as follows:(1)The α-β discriminant method,Norton Density Foster’s number method and reservoir width-depth ratio discriminant method were combined with the data of Xiaowan reservoir environmental impact assessment and acceptance report in 2006 to identify the water temperature structure type of Xiaowan reservoir,and the discrimination result was typical.Layered reservoir.(2)Vertical water temperature structure distribution characteristics of the Xiaowan Hydropower plant: The characteristics of the lake are obvious,and the vertical water temperature stratifies.From the measured data,the temperature of the water in front of the dam of the Xiaowan Hydropower plant has a clear phenomenon of water temperature stratification throughout the year.The vertical water temperature can basically be divided into 3 layers,the water depth 0-40 m is relatively high temperature water layer,the water temperature is 18.40°C~19.17°C;the water depth is 40m~120m is the transitional layer(warm water layer),the water temperature is 14.77°C~18.40°C Below 120 m is a constant low temperature water layer,and the water temperature is 14.16°C~14.81°C.(3)The average monthly water temperature at the surface of 1m is the largest in the year,with a temperature difference of 8.2°C throughout the year.The water temperature of the bottom 200 m is basically stable,and the temperature difference is below 1°C throughout the year.The highest and lowest temperatures of the monthly average water temperature in the surface layer 1m and the bottom layer 200 m were observed in August and February respectively.The temperature difference at the bottom of the table has gradually increased since February,peaking in August and gradually decreasing thereafter.The maximum and minimum vertical monthly average water temperature difference between the reservoir table and the bottom of the reservoir(1 to 200 m)also appeared in August(12.96 °C)and February(4.45 °C),respectively.(4)The change trend of thermocline depth is from August to October(substantially unchanged)→ from October to January(rise)→ from January to April(decline)→ April to June(basic constant).The maximum thermocline depth appeared in winter 2017(January),which is 35.65 meters.The average thickness of the thermocline in each month varies greatly,with the maximum and minimum values appearing in November(48.5m)and February(18.7m),respectively,with a difference of nearly 30 meters.The highest and lowest values of thermocline intensity occurred in May(0.36)and November(0.07),respectively.(5)The vertical temperature in front of the dam of the Xiao wan Hydropower plant has a good correlation with the surface water temperature of 1 meter.The minimum value of monthly mean water temperature at the surface of 1 meter is two months behind the minimum value of temperature.The Pearson coefficient of correlation analysis between temperature and surface water temperature of 5 meters is above 0.85,and the correlation between temperature and water temperature weakens as the water depth deepens.