Research On Flow Field Characteristics And Water Quality Prediction Of Wuliangsu Lake Based On Numerical Simulation

In recent years,with the rapid growth of population,industrial and agricultural production activities and urbanization,the lakes in China have shown a trend of continuous shrinkage,a severe weakening of function and increasing pollution.Therefore,the management and protection of the lakes’ environment become more and more indispensable.Wuliangsu Lake,as an important link between Inner Mongolia River Area and the Yellow River since the 1990 s,the water pollution was serious.Despite that the overall water quality has been significantly improved after several measures,the water environment of Wuliangsu Lake still worths further studying.However,real-time monitoring of the water environment of large shallow lakes can be complex,numerical simulation is an effective method to monitor lakes’ water environment.In this study,the Kriging interpolation method is applied to interpolate the water quality of Wuliang Lake in 2019 with Wuliangsu Lake as the study area,and the spatial and temporal distribution characteristics of pollutants are analyzed.The Wuliangsu Lake locates in a windy area,and the main driving forces are wind field and throughput flow.Therefore,this study uses MIKE21 software to construct a two-dimensional hydrodynamic model of Wuliangsu Lake,and compares the differences between the flow patterns in the lake area under the effect of two wind fields,SSW(south-southwest direction)and ENE(east-northeast direction),and the effect of throughput flow on the flow field in the lake area.The main impact areas,impact magnitudes and flow response mechanisms of the constant wind field and throughput flow on the Wuliangsu Lake are analyzed to provide theoretical support for the study of the migration,transformation and distribution patterns of pollutants in the Wuliangsu Lake.The water quality model of Wuliangsu Lake predicts and analyzes the nutrient concentration in the lake area in 2020,so that we can propose reasonable prevention,control measures as well as countermeasures in advance.Moreover,the water quality model can also provide some technical reference for the actual engineering of Wuliangsu Lake.The results of the study are stated below.Each pollutant in the Wuliangsu Lake shows different spatial and temporal distribution patterns.The distribution of total nitrogen and total phosphorus showed a trend of gradually decreasing from north to south.the distribution of CODcr concentration in July and October was high in the south and low in the north,and the distribution of CODcr was exactly opposite during the freezing period.The spatial and seasonal distribution of BOD5 is not significantly specific.The average chlorophyll concentration showed that the freezing period > summer > autumn > spring.The nitrogen and phosphorus nutrients in the ice body migrated to the water body during this period,and the phytoplankton also migrated to the water body,and the volume of the water body was reduced due to the freezing,which finally led to the higher chlorophyll-a concentration during the freezing period in the Wuliangsu Lake.Under the windless conditions in the Wuliangsu Lake,the flow velocity in the lake area is small,and a sizeable stagnant area appears.The single throughput flow cannot meet the hydrodynamic conditions of the Wuliangsu Lake.In two different wind fields,SSW and ENE,the three distinct areas have similar circulation locations and morphological structures but the circulation direction is opposite,and the shallow water area is more likely to be influenced.the flow direction is the same as the wind direction,and the deep water area generates the compensation flow in the opposite direction;wind-generated flow is influenced by the throughput flow,and the strength of the circulation decreases with the increase of the flow;both the effect of wind-generated flow and throughput flow affect the distribution of pollutants in the lake area,and the effect of wind-generated flow and throughput flow should be considered when studying the spatial and temporal distribution of nutrients in the Wuliangsu Lake.The relative errors between the simulated and measured values of the water quality model are mostly less than 20%,which indicates that the simulation effect of the water quality model is good.According to the water quality model prediction of April 3,2020,and August 4,2020,lake pollutant distribution can be found.Compared to April,the water quality had been greatly improved in August.The pollutant concentration in the water near the main drainage is relatively severe,and most of the pollutant concentrations are gradually decreasing from the north to the south;the pollutant concentration in the confined area formed by the reed obstruction is often higher than that in the reed area,so improving the hydrodynamic conditions is conducive to increasing the diffusion rate of pollutants,which plays an important role in improving water quality.