Urban Catchment Rainfall Runoff Pollution Characteristics And Control Simulation

With the rapid development of urbanization, large area of earth surface with good water permeable was covered by buildings, asphalt and other impermeable material gradually. At the same time, urban hydrologic cycle changed, which caused urban waterlogging and serious water environment deterioration frequently. LID(Low Impact Development)- a new concept for urban stormwater management, reduces the runoff volume and pollutants by filtration, infiltration and storage etc. In order to manage storm water by LID effectively, large basic research should be carried out, such as non-point source pollution status, characteristics of urban stormwater runoff, influencing factors of LID controlling effeciency, including how the rainfall characteristics effect on LID. SWMM model(Storm Water Management Model) is recognized as the most important tool for urban stormwater runoff and pollution simulation study. In this research, Qianhu campus of Nanchang University was chosed as field test area, pollution status and characteristics of urban rainfall runoff of three typical underlying surfaces(roads, parking and roofs) were studied. SWMM model was used to simulate the LID(rain gardens, green roofs and permeable paving) control and management effection on rainfall runoff and runoff pollution load. Runoff peak flow, total flow, time-to-peak, pollutants(TSS, COD, TN and TP) were selected as SWMM model output variable parameters respectively. Rain garden was a relatively new urban LID, which pollutants removal efficiencies influened by rainfall characteristics(rainfall repetation and rainfall pattern) was also analyzed in the paper.This study provides basic data as reference for LID promotion and application. The main conclusions are as follows:(1) Urban runoff pollution was more serious and had a stronger randomcity in Nanchang city catchment. Also, the first flush effect of runoff pollution had different degrees on individual storms. According to the classificationⅤ of Surface Water Environment Quality Standard(GB 3838-2002), Rainfall runoff pollutants had 1.08~1.76 times more CODcr and 1.33~1.66 times more TN.Comparing with the secondary standard of Pollutants Emission Standard of Urban Wastewater Treatment Firm(GB 18918-2002), TSS concentration of runoff was 1.08~2.95 times. Storm runoff pollution had a stronger randomicity; the variabilities of EMC had wide range too. Variance coefficient of pollutant concentration was between 65.3%~133.3%, but EMC was between 20.0%~150.0%. The pollutants concentration was left-skewed distribution, and most values were small. Rain intensity relates to storm events. Pollutants concentration was relative higher at the beginning of rainfall, and then dropped to a certain lower level after a period of time.(2) LID(rain gardens, green roofs and permeable paving) had a good control and management effect on rainfall runoff and runoff pollution. Rainfall runoff peak flow and total flow could be reduced by 50.56%~61.42% and 59.78%~63.95% respectively, the peak flow delayed within 5~8 minutes. The removal efficiencies of TSS, COD, TN and TP reached 49.08%~63.84%, 47.48%~66.34%, 35.53%~70.67% and 31.43%~72.86% respectively.(3) Rainfall repetition period had significant effect on rainfall runoff and pollution control and management by rain gardens. Rainfall runoff peak flow, total flow and pollution load, all showed a positively correlation with rainfall repetition period. Conversely, the control and management effect on rainfall runoff and pollution by rain gardens showed a negatively correlation with rainfall repetition period. The removal efficiency of TSS by rain gardens was greatest at the same rainfall repetition period, and the order was TSS> COD> TN> TP.(4) Rainfall pattern had weak effect on rainfall runoff and pollution control and management by rain gardens. With the crest factor of rainfall pattern changing, rainfall runoff total flow and pollution load had a small change, and rainfall runoff peak flow appeared later because the crest factor increased; furthermore, rainfall runoff peak flow changed depending on the areas of rain gardens. Rainfall runoff peak flow reduced with rainfall pattern crest factor increasing by rain gardens. Rainfall runoff total flow and peak-flow-time were affected by rainfall pattern crest factor. With the crest factor of rainfall pattern increased, the removal efficiency of rainfall runoff peak flow and pollution load by rain gardens decreased; the removal efficiency of rainfall runoff total flow and the effect of delay peak-flow-time by rain gardens were little differences at the different rainfall pattern.