The Influence And Optimization Of LID Facilities On Urban Runoff Pollution Control Based On SWMM

With the continuous improvement of urbanization level,urban construction has changed the original natural attributes such as topography,geology,water system,climate,vegetation and hydrology,so that the impervious area of land has increased sharply,and the total runoff coefficient has changed significantly before and after development,resulting in urban waterlogging and water non-point source pollution,which brings great problems to traditional stormwater management.Based on the SWMM（Storm Water Management Model）model,this study takes Chaoyang New Town in Nanchang as the research object.According to the functional area of the city,Chaoyang New Town is divided into three categories:logistics area,residential area and public building area.The variation law of rainwater runoff pollutants under different land types,the control effect of low impact development（LID）facilities on runoff pollutants and the optimization of LID facilities are studied.The following conclusions are drawn:（1）Through the field investigation and monitoring of rainfall and runoff,the current situation of runoff pollution in different urban functional zones of Chaoyang New Town was studied.Taking the COD_Cr index of the public construction area as an example,the runoff pollution under different rainfall return periods was analyzed.It was found that the degree of runoff pollution was closely related to the rainfall intensity at each moment before the rainfall washed most of the pollutants off the ground.The greater the rainfall intensity,the more serious the COD_Cr pollution of the initial runoff rainwater.Due to the strong scouring effect,the COD_Crconcentration of the runoff rainwater in the later stage of rainfall will be lower than that of the low rainfall intensity runoff rainwater.Taking the first monitored rainfall as an example,the runoff pollution in different functional areas was analyzed.It was found that the runoff pollution in the public construction area was generally light,the SS and TP pollution in the logistics area was more serious than other underlying surfaces,and the COD_Cr pollution in the residential area was the most serious.（2）The modified Morris screening method was used to analyze the sensitivity of the parameters of the SWMM model.The sensitivity of the 24 parameters of the model established in this study to the total runoff volume,runoff coefficient,peak flow,peak time and the total amount of pollutants SS in the study area,SS peak concentration,SS peak time,a total of 7indicators.（3）This paper studies the contribution of LID facilities to urban runoff pollution control.Through the study of LID facilities and runoff pollution control effects,according to the underlying surface conditions of Chaoyang New Town in Nanchang City,the selection order of four LID facilities designed according to the parameters selected during the research and analysis is as follows:There is a selection order for runoff control effects:green roof>permeable pavement>bioretention pool>grass planting ditch.There is a selection order for SS control effect:P green roof>bioretention pool>grass planting ditch in rainfall recurrence period;when the rainfall return period P>3.2 years,permeable pavement>bioretention pool>grass swale>green roof.There is a selection order for COD_Cr control effect:P green roof>biological retention pool>grass planting ditch in rainfall recurrence period;when the rainfall return period P>4 years,permeable pavement>bioretention pool>grassed swale>green roof.There is a selection order for TN control effect:permeable pavement>green roof>bioretention pool>grass planting ditch.There is a selection order for TP control effect:bioretention pool>grass planting ditch>permeable pavement>green roof.The control effect of combined LID measures on runoff and pollution will not fluctuate greatly with the change of rainfall return period,and the effect is stable.In the practical application of sponge city construction,it is more recommended to use combined LID facilities to control runoff and pollution.（4）In this paper,the optimal configuration scheme of LID facilities is studied,and the research scheme of LID system is determined according to the proportion of LID facilities layout area to their respective underlying surface.Under the design rainfall condition of rainfall return period P=2 years,based on AHP analytic hierarchy process,the optimal scheme of logistics area is 90%of the layout ratio,the optimal scheme of residential area is 90%of the layout ratio,and the optimal scheme of public building area is 60%of the layout ratio.（5）This paper analyzes the comprehensive benefits of the optimization scheme,including economic benefits,environmental benefits and social benefits.In the analysis of economic benefits,the benefits of rainwater resource reuse,saving sewage treatment and pipe network operation costs and alleviating water shortage are comprehensively considered.In the analysis of environmental benefits,the benefits of urban non-point source pollution control and urban rainstorm waterlogging disaster prevention and control are comprehensively considered.When analyzing social benefits,we mainly consider the educational significance of landscape and demonstration effects,creating jobs to alleviate employment problems and saving water resources.However,social benefits are difficult to quantify and monetize.Although they are intangible,they are actually present and mainly qualitative analysis.In a word,the comprehensive benefits brought by the logistics area every year are at least 26.36 million yuan,the residential area is at least 41.98 million yuan,and the public construction area is at least50.85 million yuan.Low impact development and construction play a positive role in promoting the virtuous circle of urban water system and the sustainable development of society.