The Influence Of Sub-catchment Size Delineation On Urban Non-point Source Pollutants Load Estimation Based On SWMM

With the rapid urbanization in China, the growth of impervious surface directly lead to the increased stormwater runoff, and many contaminants runoff into the receiving water bodies through the city drainage system.Urban stormwater runoff pollution has become one of the main sources of pollution in the city.Therefore,quantitative calculation for urban non-point source pollution load is of great significance to urban planning decisions and non-point source pollution management and control.In this paper, the urban non-point source pollution load from a large urban area was estimated based on SWMM. To determine how to select the subcatchment size for SWMM modeling, five SWMM model were established based on Changzhou drainage plans: high-resolution SWMM model, high-middle resolution SWMM model, middle resolution SWMM model, middle-low resolution SWMM model, low resolution SWMM model. Respectively, sensitivity analysis of different modeling parameters were carried out. Three types of rainfall were considered:events rainfall, the single month of rainfall, multi-month (rainy season) continuous rainfall. According to the analysis and discussion of the calculation results, the suitable subcatchment size was obtained for different research. The results show that:as the subcatchment size changed, SWMM modeling parameters would not change accordingly. In the event rainfall, subcatchment size delineation great affect the water quantity and quality simulation results. In single month continuous rain or multi-month continuous rainfall, water quantity and quality simulations results were less affected by subcatchment size.On this basis, the impact.of the changes in urban land use and rainfall change on the urban non-point source pollution load in Zhonglou and Tianning District of Changzhou were further studied. The results show that urban non-point source pollution production and emissions showed a linear growth trend as the urban imprevious land area increae in the proportion of the total area. When the percentage of impervious land area increased by 30%, the discharge load of TSS, COD, TN and TP increased 60%、54%、48% and 46% respectively. The urban non-point source pollution emissions was positively correlated with changes in rainfall. When rainfall is reduced by 20%,the discharge load of TSS, COD, TN and TP reduced 8%、7%、7%、10%、when 20% increase in rainfall, the discharge load of TSS, COD, TN and TP increased 9%、9%、10%、11%, respectively. Two suggestions were put forward: first, it needs to control the growth of urban impervious land-use area in urban construction and development, in order to ensure a certain previous surface area ratios. Secondly it needs to consider the influence of rainfall change on the urban non-point source pollution load, while making urban development planning decisions.