Study On Sediment Erosion Model Of Drainage Pipeline And Calculation Method Of Initial Rainwater Based On SWMM

As the surface pollutant is washed into drainage pipeline by the rainfall runoff and with the settlement of particulate pollutants,the sediment will be inevitably caused.It will decrease the flow capacity of pipeline,cause the pipe blockage,and even produce acid substances that corrode the pipeline in anoxic or anaerobic environment,which will hinder the normal operation of the whole drainage system.More seriously,the sewer sediment will be resuspended under the erosion of rainfall runoff.It will discharge into the receiving water which is caused impact pollution.As soon as it rains,the drain will be black and the water will stink.In order to study the pollution characteristics and control measures of rainfall runoff caused by sediment as the main factor,this paper take a diverting storm sewer system in Guangzhou as an example.A pollution scour model of catchment surface is constructed based on SWMM,and a pollution scour model of sewer sediment is structured based on the formula of current carrying capacity.Parameter calibration and model verification are carried out by using the measured data.By simulating and calculating the current situation of sediment pollution,the impact of sewer sediment on the runoff pollution first flush effect and the main influencing factors of the first flush effect are analyzed.This paper puts forward and analyzes the pollution control measures for intercepting initial rainfall along the storm sewer system and at the end in the presence of sediment.It deduces the initial rainfall confluence regulation,studies the calculation method of the initial runoff and the closure facility volume.The results show that:（1）The SWMM model software can simulate the change of surface pollution by building the pollutant accumulation and flushing module.The erosion law of sediment pollution basically conforms to=₁2^-1 index model,which can be used to calculate sediment pollution load.The coupling of the two models can better simulate hydrological,hydraulic and water quality information such as rainfall generation,surface runoff,transportation system confluence and pollutant scouring.The errors between the simulation results and the measured data are within the allowable range which show the two models are accurate and effective.（2）In the study area,the simulation results show that the maximum suspended matter concentration of outfall produced by single rainfall runoff is 583.07mg/L,and the maximum mass of suspended matter is 1392.11kg.There are 1111.74kg of suspended matter composed of sediment which is the main source of rainfall runoff pollution in this area.Therefore,the sediment pollution in this area is relatively serious,and it will cause serious pollution if it is directly discharged into the receiving water.（3）The existence of sediment will weaken the first flush effect.It can decrease the pollutant contained in 40%of the initial runoff by about 10.74%,and reduce the efficiency of initial rainfall control measures.The impervious rate,slope and impervious Manning coefficient in the catchment area can affect the first flush effect and be used as the design parameter of pollution control.The first flush coefficient b is negatively correlated with the impervious rate and slope,and positively correlated with the impervious Manning coefficient.But FF₄₀ and MFF_n-max have no significant correlation with the influencing factors.（4）According to the two conditions of the maximum pollutant scouring intensity and effluent quality standard,the highest initial runoff volume of six rainfall influenced by sediment is 8.3mm and 10.1mm respectively.It need control higher initial rainfall than 3～5mm surface runoff.The volume of closure facilities can be selected based on the initial rainfall collection rate and closure facility utilization rate.The distributed closure facilities along the storm sewer system have higher environmental benefits than the centralized closure facilities at the end,which can intercept 78.78%of the suspended pollutants.But it also have the disadvantages of low utilization rate,inconvenient management and high cost.