Research On The Effect Of South-to-north Water Diversion Project On Flow Field And Water Quality Of Downstream Nansi Lake

The East Route of South-to-North Water Diversion Project is a national strategic project aimed at alleviating the water shortage in the east of Huanghuaihai Plain and Shandong area and optimizing the allocation of water resources.It is an important part of the overall layout of the South-to-North Water Diversion Project.The Downstream Nansi Lake is an important storage Lake in the East Route of South to North Water Diversion Project,it is very important to quantitatively study its hydrodynamic conditions,water exchange capacity and water quality,it has scientific value and practical significance for the water environment protection of the Nansi lakes and management and protection of the East Route of South to North Water Diversion Project.This research took the Downstream Nansi Lake as study area,Mike21 software was used for the numerical simulation of hydrodynamic and water quality,and evaluated water quality based on the water quality monitoring data.This research took the water diversion project of the East Route of South to North Water Diversion from 2018 to 2019 as an example,the main research contents and results are as follows:（1）Established the hydrodynamic model of Downstream Nansi Lake and simulated the change of hydrodynamic characteristics.The average velocity during the water diversion period was about three times of that before the water diversion,the area of backwater section reduced by 65%,the area of waters which velocity of flow was higher than 0.01m/s increased by 89%.The flow direction after water diversion was more complicated than that before water diversion,there were many circulations in the wide water area,which were helpful for water exchange and pollutant diffusion,thus improved the lake water environment.The hydrodynamic conditions of Downstream Nansi Lake were improved significantly during the water diversion period.（2）Established the water quality model of Downstream Nansi Lake and simulated the transport and diffusion of TN,TP and NH3-N during the water diversion period.The concentration of three pollutants decreased in different degrees during the water diversion period,the concentration of NH3-N decreased most significantly.As a result,the water quality has been significantly improved.The diffusion rate of pollutants in wide water area was slow due to the low velocity of flow.The better hydrodynamic conditions of the channel made the pollutant diffusion faster.The complex topography around Wei Shan island and reed area restricts the flow movement and easily led to the retention of pollutants.（3）Established the water exchange model of Downstream Nansi Lake.Evaluation of water exchange capacity in different areas of the Downstream Nansi Lake in terms of water exchange rate and water renewal time.At the end of the water diversion period,the average water exchange rate of the Downstream Nansi Lake was 72%,the water exchange capacity of channel was the highest and the water exchange capacity of reed area was the lowest.The average renewal time of the Downstream Nansi Lake was 89 days,the average renewal time of channel was 38 days.About 67% of the Downstream Nansi Lake have been renewed by the South to North Water Diversion Project.（4）Evaluated water quality of the Downstream Nansi Lake based on the water quality monitoring data.The concentrations of COD_Mn,NH3-N and TP decreased significantly during the water diversion period.Comprehensive water quality identification index method,Nemerow index method and grey correlation analysis method were used to evaluate water quality,the results showed that the water quality level was improved during the water diversion period.