Hydrodynamic And Water Quality Simulation Of The River-lake Connection Project And Its Effect Evaluation In Datong Lake Basin

Datong Lake is the core area of Dongting Lake's important wetland protection circle.In recent decades,the ecological and environmental problems in the Datong Lake basin have become increasingly prominent.Based on the water quality concentration data of Datong Lake in the past seven years,the change trend of the comprehensive nutrient status of Datong Lake is calculated and analyzed,and the pollution load and the amount of pollution from different pollution sources in the basin are estimated using the pollutant discharge coefficient method.The MIKE21 numerical simulation software was used to carry out the hydrodynamic-water quality numerical simulation of the current river-lake connection project in the Datong Lake Basin in different periods,and a comprehensive index evaluation system suitable for the Datong Lake Basin was established.Finally,the structural connectivity index is introduced to obtain the best optimization plan for the northwest part of the Datong Lake Basin.The results of this research have important reference value and practical significance for the implementation of reasonable river-lake connection projects in the Datong Lake Basin,reconstruction of hydrodynamic status and improvement of the water ecological environment.The main results of this study are as follows:(1)From 2014 to 2020,the comprehensive nutritional status of Datong Lake is 38.6,43.7,51.4,53.7,67.057.2554.0,and the water quality and nutritional status are amazing and have a trend of further improvement.The current comprehensive nutritional status of the lake is eutrophication.In terms of administrative divisions,etc.,Congjiang City has the largest amount of total nitrogen and total phosphorus pollution load,accounting for 41.50%and 41.56%of the pollution generated in the basin respectively.The pollution of total nitrogen and total phosphorus in Datong Lake constitutes a huge proportion,accounting for 72.61%and 63.06%of the total output respectively.Based on the total phosphorus target of Datong Lake in 2025,the water environment capacity is 930.38 t/a and 93.04 t/a,and the reduction is 1388.7 t/a and 115.77 t/a,respectively.(2)Based on the planned river-lake connection project in the Datong Lake basin,a hydrodynamic-water quality indicator system for the river-lake connection project suitable for the Datong Lake basin has been constructed,including the proportion of stagnant area,water exchange period,water quality concentration improvement rate and concentration change index,etc.Seven main indicators,combined with the distribution of hydrodynamic flow field and the spatial distribution of total nitrogen and total phosphorus,analyze and evaluate a total of 14 working conditions during the flood season and non-flood season.The best scheduling plan for improving the water environment of Datong Lake is to control the diversion flow at 40 m3/s and the outlet water level at 25.88 m between 0-60 d in the early diversion period during nonflood seasons;the outlet water level will be controlled between 60 d and stable diversion.Reduced to 25.48 m,the diversion flow remains unchanged.During the flood season,between 0-60 d in the early stage of diversion,the diversion flow of the two water intakes is controlled at 20 m3/s,the outlet water level is controlled at 27.08 m,and then the outlet water level is reduced to 26.28 m.(3)The water volume of the connected lakes of Datong Lake is huge.Therefore,the river and lake project in the northwest of Datong Lake has been optimized.The optimized river network of the last three schemes has increased by 3.28%,5.90%and 10.82%.Water system connectivity increased by 6.62%and 1.15%,but the three-water system circumstance and water system connectivity of the first and scheme decreased by 1.47%,21.32%,0.46%,and 4.85%,respectively.For the improvement of power and water quality,Option 2 can be set as the recommended solution for the improvement of rivers and lakes in the Datong Lake Basin.