Water Quality Simulation Of Urban Shallow Lakes Based On Hydrodynamics

Water nourishes all things and is the source of life.The importance of water to all things in nature is self-evident.As the core component of ecological water environment,lake is an important part of ecological water cycle.In China,the problem of water environment pollution of small lakes and urban artificial lakes is serious,which needs to be controlled urgently.As a typical urban artificial shallow lake,Qixi Lake Park in Xi ’an,Shaanxi Province presents a closed or semi-closed water characteristic state,and the water velocity in the lake is slow and the water regeneration time is long.In addition,surface runoff,man-made activities of tourists in the lake caused pollution and other problems,affecting the water environment of Qixi Lake.Through the investigation and exploration of the existing problems in Qixi Lake Basin,the MIKE21 model was used to simulate the water renewal time and water quality.Guided by the enhancement of hydrodynamic conditions,the design conditions were designed to explore the change of water regeneration time.In order to provide suggestions for the protection of water environment in the lake area,the water quality of the lake area was simulated with the guidance of the source control of fresh water.The research results are as follows:(1)The measured data and collected hydrometeorological data were collated and analyzed,and the single factor water quality assessment method and comprehensive nutrition index method were applied to evaluate the water quality of Qixi Lake.Single factor evaluation method showed that the high nitrogen content was the main factor that caused the deterioration of water quality in Qixi Lake.The comprehensive nutritional index method showed that the comprehensive nutritional state index value of the lake area was 66,showing moderate eutrophication.(2)The hydrological and water quality data collected were used to construct the hydrodynamic water regeneration time model and water quality model in line with the characteristics of Qixi Lake Basin,and then calibrated and verified.(3)According to the hydrodynamic simulation results of Qixi Lake under various working conditions,the average velocity of the lake is in the range of 0.002m/s～0.0064m/s.The comparison results of average flow velocity in each working condition are as follows:working condition 5<working condition 1<working condition 2<working condition 3<working condition 4.From the simulation results of water regeneration time in the lake area,it can be seen that conditions 2 to 5 are improved.Working condition 4:Pushing machine has the best effect,and the water body renewal time of the lake area can be reduced by 30 days at most,and the water body renewal time of 99%of the lake area is less than 30 days.In view of the improvement of water body renewal time in the west of the Lake District,the addition of inlet condition in the fifth working condition had the most significant improvement on the water body renewal time in the west of the Lake District,which decreased from the maximum of 65 days to 3-10 days.(4)According to the simulation results of water quality in the lake area,under the current situation of the lake area,COD concentration is in the range of class Ⅳ water,TN concentration belongs to poor class Ⅴ water,and TP concentration belongs to class Ⅳ and Ⅴ water as a whole.Centering on the criterion of source control of running water,working condition 2 of the push machine has a certain effect on the overall improvement of water quality concentration in the lake area.COD has been generally improved to class Ⅲ water quality,TN still belongs to poor class Ⅴwater,and TP belongs to class Ⅳ and Ⅴ water.Source control condition 3 has a significant effect on the reduction of water quality concentration in the lake area.COD concentration further decreases,TN concentration ranges from 4～4.6mg/L to 1.07～1.21mg/L,and TN concentration improves from inferior V class water to IV class water.TP concentration decreased from 0.084～0.1 lmg/L to 0.035～0.045mg/L,and TP concentration improved from Ⅳ and Ⅴ class water to Ⅲ class water.Source control measures directly affect the overall water quality of Qixi Lake,and source control can effectively improve the lake water quality.In working condition 4,the inlet discharge of the lake area was further increased,and the concentrations of COD and TN were improved to a certain extent,and the TN concentration was further close to that of class Ⅲ water.