Ecological Restoration And Numerical Simulation Of Water Environment In Bahe River Basin

As the main river crossing Xi’an City,Bahe River plays a key role in the development of Xi’an.This study takes the Bahe River Basin as the object,evaluates the water quality and ecological disturbance assessment of the Bahe River Basin,and deeply analyzes the existing problems in the Bahe River Basin.Based on the water quality evaluation results,the Lantian and the ecological sections are selected for hydrodynamic-water quality simulation.Based on the above research results,the river ecological restoration suggestions and countermeasures are proposed for the problems of different river sections in the Bahe River Basin.The specific research results are as follows:(1)The measured data and the collected data were summarized,and the water quality evaluation of the Bahe River was carried out by using the single factor evaluation method and the gray correlation evaluation method.The single factor evaluation method shows that the nitrogen content is the main factor leading to the deterioration of the Bahe River water quality.The results of the grey correlation degree evaluation are as follows:the water quality at the source and upstream sections of the Bahe Ri-ver is good,but the water pollution is gradually increasing with the progress of the water flow.After passing through the Chan-Ba intersection,the water quality of the main channel of the Bahe River reached the V standard due to the inflow of the Grade V water from the Chanhe River.From the outlet of the fifth sewage treatment plant to the mouth of the Bahe River,the water quality is restored to Grade IV water.(2)According to the results of ecological interference assessment,the values of water quality compliance rate,biodiversit,y index and buffer zone vegetation width are larger,which are 0.125,0.123 and 0.122 respectively,which indicates that human activities have strong interference with the above three indicators.According to the water quality evaluation results:After the water body flows through the Lantian section and the Chan-Ba ecological section,the water quality changes significantly.Therefore,the river sections of the two major cities in the Bahe River Basin are selected for simulation study.Based on the hydrodynamic model,a hydrodynamic-water quality coupling model consistent with the characteristics of the Bahe River Basin was constructed,and the applicability of the model was verified by comparing the measured values with the simulated values.(3)The MIKE21 hydrodynamic model is used to analyze the flow field characteristics of the study area under different hydrological years.The calculation results explain that:the overall flow rate of the study area in the Lantian section of the Bahe River is basically 0-0.1198m/s under the 20%guarantee rate of the wet year,the 50%guarantee rate of the flat water year and the 95%guarantee rate of the dry year is 0-0.1006 m/s,0-0.070 m/s,wherein the flow rate increases in some areas.In the area where the Wangchuan River meets,due to the disturbance of the topography of the Bahe River,some of the water bodies are shifted to the left and merge with the water body of the Wangchuan River,and then flow to the Lantian No.1 Dam.Under the condition of 20%guaranteed rate hydraulic calculation in high flow Year,the velocity of the Chan-Ba ecoregion is basically between 0-0.073m/s;the velocity distribution under the 50%guarantee rate of the plain water year is basically consistent with the wet years.The flow velocity in most regions is between 0-0.073m/s.Under the hydraulic calculation condition of 95%guarantee rate in dry season,the flow velocity shows a significant downward trend.Due to the decrease of the amount of water coming from the upstream of the main channel of the Bahe River,the main flow velocity is reduced to 0.Between-0.030m/s,the water in the study area flows from south to north along the river course,while the hydraulic conditions in the Chanhe area are poor.(4)Introducing the ECO Lab water quality module on the basis of the hydrodynamic model,which can reflect the real situation of the water flow and the change of state variables in the main rivers of the Bahe River.It can be seen from the simulation results that when the water bodies of the Wangchuan River and the Chanhe River are connected to the main channel of the Bahe River,the size of the disturbed area of the water body of the Bahe River is mainly controlled by the water flow velocity of the two rivers before the convergence and the state variables.The difference in concentration,the flow rate in the junction area changes significantly,resulting in a change in the disturbance area.(5)The linear fitting results of the upstream flow of the Bahe River and the state variables show that:There is a significant linear positive correlation between DO,COD,NO3 and flow Q.There is a significant negative linear correlation between NH4,Chla and flow Q.There is no significant linear correlation between NO2 and flow Q.(6)According to the water quality assessment and river ecological disturbance assessment results,the river ecological restoration suggestions and countermeasures are proposed for the ecologically damaged river section of the Bahe River Basin.In order to build a perfect watershed ecosystem,under the premise of improving water quality,the river channel will be built according to the requirements of ecological management,and the ecological space of biological migration and reproduction will be restored through the joint stocking of aquatic animals and plants,which will be active for water quality improvement and ecological management.