Study On Numerical Simulation Of Water Environment In Chan-ba Ecological District

As a country without plentiful water resources,China has an uneven spatial-temporal distribution of water resources.The inter-annual variation of precipitation is explicit,and the precipitation is not abundant,particularly in arid and semi-arid regions of northwest China.The contradiction between utilization and exploitation of water resources and ecological environment is increasingly prominent.As an important region of political,economic and cultural development,Northern China suffers severe problems of water shortages.Influenced by continuous drought and lager consumption of water resources,surface runoff decreases definitely and various rivers run dry.As a complicated nonlinear system,rivers play important roles in connecting eco-environmental functions in the basin.River plays a significant role in regional sustainable development,and supply many ecology function namely irrigation,Drinking Water supply,fisheries,transportation and entertainment etc.The Chanhe River and Bahe River in Chan-Ba Ecological District Xi’an China was selected as the study area and some research that the characteristics of water environment in Chan-Ba Ecological District were analyzed on the foundation of monitored data was conducted.Based on the hydrological and meteorological data,the two-dimensional hydrodynamic model was developed to study the hydrodynamic characteristics in Chanhe and Bahe Rivers.The hydrodynamic equation,advection-dispersion equation and differential equations of distinct state variables’ biochemical processes were coupled to form water quality model,and the finite volume method was employed to spatially discrete water quality model to simulate the concentration field of various pollutants.Based on the results of simulation,the evaluation of water quality on the foundation of BP neural network approach was conducted.Characteristics of various pollutants’concentration field and water quality were forecasted in different hydrological years.The main results were as follows:Characteristics of water environment in Chan-Ba Ecological District were analyzed systematically.The eleven point sources,which were seven domestic sewage point sources and four industrial wastewater point sources separately,were discharged into Chanhe and Bahe Rivers in the region of Chan-Ba Ecological District.The total discharge flows of domestic sewage and industrial wastewater point sources were 1.2135 m3/s and 1.2656 m3/s respectively.The maximum emission concentrations for Chemical Oxygen Demand(COD)and Ammonia Nitrogen(NH3-N)were 486 mg/L and 27.5 mg/L separately.The main contaminant in this study area was Total Nitrogen,whose maximum concentration was more than Grade V by 73 percent in monitored time.Additionally,spatial-temporal average concentration of TN is 2.5 mg/L,which was more than Grade V by 75 percent.The characteristics of flow field in study area at different hydrological year were analyzed.The results from calculation of the hydrodynamic model showed that the flow velocity in the studied area varied from 0 to 0.0667 m/s.After the water body in Bahe and Chanhe intersected in confluence area and passed through No.4 dam of Bahe River,the flow velocity would increase definitely.Moreover,the flow direction of the water body in Bahe River in confluence area turned to left bank and drove the water body in Chanhe River to flow to the same direction.The flow velocity in confluence area would significantly increase in high flow years.The water-quality model performance was analyzed,and it was utilized to simulate the distribution characteristic of different pollutants.Additionally,the model calibrated by measured data can predict concentration of distinct contaminants accurately.The TP,NH3-N and COD concentration in the water body in Chanhe River was more than them in Bahe River before the water body flowed into confluence area.The TN concentration in calculated zone except zone A exceeded the standard of Grade V(GB3 83 8-2002)on the condition from scenario 1 to scenario 4.After the water body in Bahe River intersected with water body in Chanhe River and passed through No.4 dam of Bahe River,the COD and NH3-N concentration definitely increased because of the influence of point sources.The NH3-N concentration in the water body of downstream of No.4 Dam would exceed the standard of Grade V(GB3838-2002)in low flow years.The disturbance area of NH3-N,TP and COD of the Chan River in the confluence area is controlled by the flow velocity in the confluence region,and the disturbance area is smaller when the flow velocity is larger.The type of pollution in the downstream of No.4 dam of Bahe River would be changed from single pollution of TN to mixed pollution of TN,NH3-N and COD in low flow years。The evaluation of water quality was conducted based on the results of calculation of model and BP neural network.The water quality of Bahe River met the target water quality requirement of "Shaanxi water function zoning" and "Xi’an water environment plan" before entering the confluence area,and the water quality in Chanhe River only met the target water quality requirement of "Shaanxi water function zoning" on the condition of scenario 1.After the water body in Bahe River intersected with the water body in Chanhe and passed through No.4 dam of Bahe River,the water quality would be reduced to the standard of Grade V,which only met the target water quality requirement of "Shaanxi water function zoning".The water quality in Bahe River would meet the target water quality requirement of "Shaanxi water function zoning" and "Xi’an water environment plan",and the water quality in Chanhe River would only meet the target water quality requirement of“Shaanxi water function zoning" in high flow years(scenario 2).The water quality in Chanhe River would only meet the target water quality requirement of "Shaanxi water function zoning",the water quality in Bahe River would meet the target water quality requirement of "Shaanxi water function zoning" and "Xi’an water environment plan" before flowing into the confluence area,and the water qulity woud be reduced to the standard of Grade IV after passing through No.4 dam of Bahe River both in normal and low flow years.