Research On Numerical Simulation Of Water Quality Improvement In Yangshapao Lake

Yangshapao Lake is located in the Zhenlai county of Jinlin province. The lake had an average water depth of 1.5 m and the most of reservoir area of 36km2, It is typical plain lake. Yangshapao Lake is used as anti-regulating reservoir in the project of water diversion from Nenjiang River to the Baicheng, it is Urban Domestic Water Source and agricultural irrigation water Source, but fluorine content seriously exceeds the standard is prominent environmental problems in the project, so project means is taken to improve Water Environment and Ecological in the Yangshapao Lake. Hydrodynamics and water quality modeling technique, which is an importantmeans for understanding the status and trends of lake environment, has been extensively applied in the protection and management of shallow lake water environment. So numerical simulation method is used to prognosticate and analyze hydrodynamics and water quality change after water transfer in the Yangshapao Lake, analyzing lake eco-environment change after water transfer in the Yangshapao Lake.Water ecological environment is worse in the Yangshapao Lake, inflow rivers is zero flow in most tine. salinization and alkalization is quite serious in the Yangshapao of Peripheral Areas. So this cause by water environmental quality deterioration of Yangshapao Lake, water self-purification capacity is very poor. Experimental analysis result show that: main over standard factor: pH,TN,TP,CODmn,F-,As, especially fluorine content seriously exceeds the standard. Cohesive Soil lay in the most of Yangshapao Lake foor. Experimental analysis result shows that: fluorine content is relatively large in the soluble salt of sediment, it will be main pollution source after water transfer.In the Yangshapao Lake, its depth is small, short and show flow, velocity gradient is mini and velocity change is weak in vertical. the sublacustrine terrain is relative smooth, flowing state is singleness, it is easily affected by inflow and outflow, and is supplied by Erlongtaohe River what is seasonal river, but water quality of Erlongtaohe River is worse, so the river is truncated. Yangshapao Lake's water quality measured data form 2006 to 2007 and reservoir design Data have been used in this paper, different water quality Improvement Scheme is designed. Base on hydrological condition, Establishing Yangshapao's hydrodynamics and water quality conceptual model, and according to conceptual model, hydrodynamics and water quality mathematical model is established. The software of Delft-3D is used to solve mathematical model, modeling coming five Years of hydrodynamics and water quality temporal and spatial variation in different scheme the structure of flow field and the distribution rule of pollutant under different abstraction have been studied emphatically, determining time which is need when water quality of mixed water reach to standards for drinking water. The main contents of the research are as follows:Base on incompressible fluid, shallow water, Bossiness assumption Navier-Stokes equation, Establishing hydrodynamics and water quality mathematical model of Yangshapao Lake. The arterial canal for conducting water is stream entering lake, as flow boundary and material input boundary; at the downstream, the arterial canal for conducting water of Wujiazi irrigation district is the outlet of the lake and urban water supply pipe inlake, as flow boundary and material output boundary.In the process of model parameters selection, the main influencing parameters of hydrodynamic model are Manning coefficient n: 0.015 and Eddy viscosity VH: 10; For water quality model, as the selected indexes are F-, CODmn and NH4+, the main influencing parameters are diffusion coefficient: 0.022.The finite difference approach is used to solve the numeric model, using orthogonal curvilinear grid to disperse space. ADI algorithm is used to solve the model. Contrast with finite difference method, the ADI algorithm is precise, flexible, efficient, stable, etc. Hydrodynamic simulation results show that: the hydrodynamic field (flow field) is mainly controlled by the location of inlet and outlet, flux and the topography of lake bed. The locations of the inlet and outlet control the main flow direction, while the topography of lake bed and the flux control the partial direction and velocity of the flow; The flow of reservoir has a common property with lake current, that is the elevation difference is relatively small between upstream and downstream, and the flow velocity is relatively small, generally 10-3 magnitude. Only at the narrow cross section of the inlet and oulet, the flow velocity is relatively large. Average flow velocity is from 0.003⁰.004. Dead angle is in the northwest of Yangshapao Lake, the flow rate is extremelly small, generally 10-4 magnitude, the period of water cycle is long, and the metabolism is slow. In some partial area of the reservoir occurs circulation flow, which is mainly influenced by wind and topography. Water moving under the influence of inertia, and influenced by topography and wind, circulation flow occurs at corners.The results of water quality simulation show that: Hydrodynamic field control the distribution variation of the concentration field, the variation of water quality movement in the reservoir is also influenced by diffusion, inlet concentration. Forecasting and analyzing water quality change of Yangshapao Lake in six Water Regulation schemes. Time that COD_mn content reach to standards for drinking water in Yangshapao Lake is 2 year; it takes five years to make fluorine content reach to standards for drinking water. Water transfer effect is evident for water quality improved of Yangshapao Lake, making water quality of Yangshapao Lake reach to standards for drinking water in next year summer, water quality, but water transfer stop in winter every year, it make fluorine content significantly increase for sediment to water body diffusion, and the diffusion is in the long time scale, and then, water quality of Yangshapao Lake exceeds the standard in winter for 3 year. It take 4 years make water quality of Yangshapao Lake reach to standards for drinking water.