Simulation And Analysis Of Nitrogen Pollution Control Of Surface Runoff In Sponge City Based On Swmm

As the second largest non-point source pollution next to agricultural non-point source pollution,urban surface runoff carries a large number of dissolved and solid pollutants into the receiving water body in the process of formation,and the input of nitrogen pollutants is the main reason for the deterioration of water quality of receiving water bodies.In order to deal with the problem of surface runoff pollution,many cities in China have been accelerating the development and construction of sponge cities with low impact.In order to alleviate the negative impact of urban development.The water network of Ningbo city is densely distributed,but the eutrophication of water body is serious.Since 2017,Ningbo municipal government has carried out a three-year pilot project of sponge city construction in Yaojiang-Cicheng area of Jiangbei District,using sponge facilities such as road permeable pavement,ecological tree pool,rainwater garden,biological detention facilities,etc.to form a rainwater management pattern of runoff control,pollutant control and rainwater resource utilization in the pilot project.In order to enhance the integrity and systematization of sponge city construction,and evaluate the control effect of low impact development on urban rainwater runoff non-point source pollution,Cicheng-Yaojiang area,the pilot area of Ningbo sponge City,is selected as the research area in this study.Based on Arc GIS,SWMM,SWAT and inp PINS software,and through field investigation,sampling analysis and laboratory modeling The SWMM model of the study area is constructed to study the control effect of rainwater runoff and nitrogen pollutants before and after the low impact development of each catchment area in the pilot area under three measured rainfall events,and the characteristics of the surface runoff nitrogen pollutant load caused by the low impact development of sponge city under the design rainfall conditions are simulated and analyzed.The main research results are as follows:（1）Under three rainfall events,low impact development and urban reconstruction in the pilot area have good control effects on surface runoff and nitrogen pollution.The reduction rate of runoff is 27.62%-41.55%,the reduction rate of runoff peak is 20.91%-41.88%,which can delay the peak occurrence time by 10-30 min;the reduction rate of nitrogen pollutants will decrease with the increase of rainfall intensity,among which the East catchment area of airport road has the best control effect on nitrogen pollution,and the average reduction rates of ammonia nitrogen（NH₃-N）,nitrate nitrogen（NO₃-N）and total nitrogen（TN）are 40.30%,40.39%and 41.93%,respectively.（2）After the development of sponge city with low impact,the runoff velocity and peak value are positively correlated with the rainfall intensity under the four design rainfall intensities.With the increase of R value（relative position of rain peak）,the time of peak value of runoff is also pushed back and the peak value of runoff is also increasing.The change process of nitrogen pollutant emission per unit time period（reporting time step 10 min）of the total discharge outlet is consistent with the law of surface runoff.（3）In the early stage of rainfall,the concentration of nitrogen pollution increased rapidly,showing obvious initial effect.After the peak appears,the change of nitrogen pollution concentration and quality shows the law of rapid decrease with the increase of rainfall intensity.Therefore,after low impact development,the initial effect of nitrogen pollutants is still a problem to be concerned.（4）By comparing the nitrogen pollution load of each catchment area after low impact development,the unit area load of NH₃-N and TN in the catchment area of Cichengxinchengdong,sunjiacaozihe and Yaojiang new area ranks the top three.The three types of cultivated land accounted for（66.6%,58.9%,53.23%）respectively,and cultivated land became the main output source of NH₃-N and TN.The load of NO₃-N in the old city area with high density is significantly higher than that of other catchment areas.It points out the direction for further improvement and efficient governance of sponge city construction.