Research On Mathematical Model Of Water Environment Based On Unstructured Grid

With the accelerating process of industrialization in China,the production activities of human being in the coastal areas of the rivers are becoming more frequent,which causes the constantly increasing risk of environmental pollution in coastal waters.The shore boundaries of natural rivers are generally complex.Thus,how to use mathematical model to improve the simulation accuracy at the river boundary is an urgent research problem to be solved.Based on unstructured grid and finite volume method,this paper constructed a mathematical model of water environment suitable for complex bank boundary,and applied it to study the water environment characteristics of Wanjiazhai Reservoir.The main research work and conclusions are as follows:(1)Aiming at the complex characteristics of natural river shore boundary,the water environment mathematical model is constructed based on finite volume method and unstructured grid,and the reliability of the mathematical model is verified.The results show that the average relative error between the calculated values and the measured values is only 1.08%.The COD and NH3-N calculated values in front of the dam are basically consistent with the measured concentration values,and the maximum deviation of the average logarithmic ratio is 0.297,which shows a strong consistency in terms of data.Both the hydrodynamic model and the water quality model have good reliability and meet the model precision for numerical analysis of reservoir water environment.(2)An unstructured grid water environment mathematical model was used to simulate the hydrodynamic process and analyze the spatio-temporal variation of pollutant concentration in Wanjiazhai Reservoir area under the present situation,and the water environment capacity in the region was calculated under the setting design condition.The results show that the Wanjiazhai reservoir section of the Yellow River has typical hydrodynamic characteristics of the channel type reservoir.Under the condition that there is no external load in the reservoir,the variation process of pollutant concentration in front of dam is closely related to the change process of pollutant concentration in and out of upstream inflow boundary.The maximum critical discharge concentrations of COD continuous pollution sources were 426 mg/L,310 mg/L and 252mg/L in low-water periods,flat-water period and high-water period respectively.The water environment capacity of COD are 14.4t,15.9t and 18.1t in low-water periods,flat-water period and high-water period respectively.YWhat5s more,The distance required for COD decaying from third to second standard concentration was 45.84 km,58.51 km and 64.70 km in low-water periods,flat-water period and high-water period respectively.(3)Under the different kinds of sudden pollution accidents,the transport characteristics and spatio-temporal changes of pollutants in the Wanjiazhai Reservoir area were studied,and the suggestions of pollution emergency warning were put forward.In the sudden pollution leakage accident in which the pollutants were non-degradable,the pollutant input at the upstream boundary was completely blocked after 24 hours,and the pollution area in the waters of Wanjiazhai Reservoir reached 67.5 km.,The water areas within 30km range before the dam gradually began to be affected by pollutants.Therefore,it is suggested that the emergency response time should be set for 24 hours after the accident occurs.In the sudden pollution leakage accident in which the pollutants were degradable,the transport speed and influence range of the pollution zone in summer is obviously greater than that in winter under the same hydrological year condition,but the concentration degradation rate is less than that in winter.In summer,The pollution zone under given conditions can be transport to 5.17km in front of the dam while It can be transport to 17.5km in front of the dam in winter on the promise that the COD concentration meets the the water quality standard.If the pollution accident occurs,the emergency warning distance is at least 24.83 km and the minimum emergency warning time is 6 hours in the summer most adverse condition.