Hydrodynamics Method To Reduce Lake Endogenous Release Research

With the rapid development of China's economy,environmental problems have become increasingly prominent,which water pollution has brought great losses to our people's lives and production.The problems of eutrophication of lakes in our country have been urgently needed.Such problems have aroused great attention from the government and relevant departments and research units.Controlling the content of nutrients such as nitrogen and phosphorus in shallow lake is the key to control the cyanobacteria bloom in lake.Taihu Lake is the third largest freshwater lake in China,In recent years,algal blooms caused by eutrophication of lakes have caused great impacts on the daily life of local residents.With the introduction of relevant national policies to control industrial,agricultural and domestic sewage discharged directly into the Taihu Lake and other large shallow lakes,external input nutrients will gradually decrease over time,but algal blooms still occur from time to time.For large shallow lakes,such as Taihu Lake,the impact of dynamic endogenous pollution on lake water quality is very large due to shallow water,violent storm disturbance,frequent resuspension of sediment.Scientifically controlling the release of endogenous pollutants from shallow lakes has been paid more and more attention in algae bloom disaster control while phenomenon of algal blooms still occurs after controlling extrinsic input in eutrophication control of Chinese and foreign lakes.Therefore,it is important to study the technology of controlling the secondary release of nitrogen and phosphorus into Taihu Lake,while preventing pollution around Taihu Lake into Taihu Lake effectively,which has important theoretical and engineering significance.Based on the relationship between lake sediments resuspension and lake hydrodynamic action,a method of placing geocells at the bottom of Taihu Lake is put forward.The aim of reducing the release of endogenous pollutants in Taihu Lake is achieved by reducing the velocity of bottom water and reducing the resuspension of lake sediments.In this paper,the Delft3D model is used to simulate wind-driven current in the bottom of Taihu Lake,according to a large number of actual observation data and previous research results.The southeast and northwest winds are taken as typical wind direction,which wind speed is 4m/s,6m/s and 8m/s.According to the research of critical wind speed by endogenous release,it is found that the velocity of the bottom layer in Taihu Lake is larger than that in the center of the lake and the velocity of bottom water in narrow water area is larger than that in broad water area.When the critical wind speed is 6m/s,the water velocity reaches the critical value of the bottom mud starting velocity in the west bank,northeast corner,east Taihu Lake and Man Shan Lake.The comparison of the research results with the existing results has verified the correctness of the numerical simulation.Numerical calculation studies lowering water velocity in bottom layer by placing geocells at the bottom of Taihu Lake.A 5 ×1×1.5m tank model is established by using Delft3D model.The geocell is placed on the bottom of the tank model,and the water head reduction and the flow velocity reduction are calculated after the geocell.The equivalent roughness of the geocell placed in Taihu Lake is approximately 0.03 by the Manning formula.On the basis of studying the flow distribution of the Taihu Lake,this paper picks up four regions with high water flow speed to place geocells,namely the west bank,northeast comer,east Taihu Lake and Man Shan Lake.The numerical simulation is carried out to calculate the change of the flow field after the geocell is placed.The results show that the velocity of the lake bottom is lower than that of the bottom mud starting even under the wind speed of 8m/s,which can effectively reduce the endogenous release of Taihu Lake.At the same time,by studying different geocell placing methods,we find that when using strip geocell placing,the flow velocity of east Taihu Lake and west bank are decreased obviously,and the other areas have bad effect of reducing flow velocity in the four regions.The geocell is placed on the bottom of the lake,and the new method of reducing the flow velocity of the lake bottom by using the geocell to block the water flow is simulated and analyzed.The results of this study provide a scientific basis for the further study on the effect of geocell on flow velocity,and thus reduce the resuspension of bottom mud in Taihu Lake and the release of effective pollutants from Taihu Lake.