Study On Change Rule And Simulation Of Water Quality In Urban Rivers With Rainfall Sources

The rainwater-type urban rivers in southern China have short-flowing sources and small water environment capacity.Under the background of regional economic and social development,they often bear excessive pollution load and exhibit obvious characteristics of“micro-capacity and heavy load”.As a result,the river’s water quality has declined and the ecological deterioration has brought great impact on the environment along the river and the downstream urban area,residents’living and sustainable development.This paper selects the Pingshan River Basin in Shenzhen（the area is 129.4km²）as a typical research object.It studies the variation law of urban river water quality and the law of non-point source pollution,which can provide strong data support for water environment problems.（1）Selecting three sections of Pingshan River Biling,Honghuatan and Shangyang,using principal component analysis method to analyze the temporal and spatial variation of 24 conventional pollution indicators of Pingshan River water quality,and obtain the water quality of Pingshan River Biling from 2011 to 2015.The best,the water quality of the Honghuatan and Shangyang sections is long inferior to the V-type water,and there are certain seasonal changes.In space,there are some differences in the pollutants in the three sections,but NH₄⁺-N is a common pollutant.In terms of time,arsenic and ammonia nitrogen have always been the main pollutants,and fecal coliforms are also a common pollutant.In general,the water quality of the Pingshan River has improved.（2）Using the combination of literature research and site survey to systematically analyze the source,characteristics and influencing factors of urban non-point source pollution in China,analyze and summarize the characteristic values of non-point source pollutants in cities and towns at home and abroad,and carry out Pingshan River during heavy rain.The sampling and investigation of the pollution field in the basin has provided the parameters for the surface pollution simulation of Pingshan River.（3）Based on field investigation and experimental research,the entire Pingshan River Basin is divided into 145 sub-catchment areas,and the structure of the main drainage pipe network is generalized according to the sewage pipe network plan.The total number of drainage pipe network topology points is 766.757 pipelines were constructed,and the SWMM water quality model of Pingshan River with good simulation effect was constructed by parameter identification,calibration and verification.（4）The pollution of three pollutants such as COD,NH₄⁺-N and TP in Pingshan River Basin with different rainfall intensities was simulated by using the model.The conclusions show that:Under the same rain type and different rainfall intensity,the rainfall scouring intensity is different.The greater the intensity,the corresponding increase in the concentration of the contaminant,but the amplitude becomes lower.The rate of increase in the concentration of pollutants in the initial stage of precipitation gradually slows down and suddenly becomes larger before reaching the peak.After the maximum value,the velocity decreases with a gradually decreasing acceleration.The maximum concentration of pollutants occurs in an hour after a precipitation occurs,but the rainfall time varies from site to site.When the intensity of precipitation increases,the time at which the concentration of pollutants reaches a maximum will also be advanced.（5）Using the model to simulate the pollution status of COD,NH₄⁺-N and TP in Pingshan River Basin under different urbanization rainfall scenarios,the conclusions show that:Under the same rainfall situation,the concentration of each pollutant is roughly the same under the three urbanization degrees before,during and after the development of the city,but the concentration of pollutants in the three scenarios is different.Before the development<now<after development,As the degree of urbanization increases,the peak concentration of pollutants increases.The concentration of pollutants peaked at a higher degree of urbanization was slightly earlier than that of urbanization.It can be seen that under the same rainfall conditions,with the increase of urbanization degree,the surface hardening area increases,the runoff coefficient increases,the pollutant scouring intensity increases,and the pollutant concentration increases greatly.