Study On Measurement And Influence Factors Of Runoff Coefficient Of Pervious Surface

Large impervious area is one of the reasons for the increasingly serious problem of urban waterlogging.With the promotion of sponge city construction,the permeable pavement gradually replaces the existing hard pavement.As an important parameter of the permeable pavement,the value of the runoff coefficient concerns the realization of the waterlogging control and runoff control objectives.However,the design value of the runoff coefficient of the permeable pavement is generally determined by the specifications without the consideration of the rainfall parameters and the environmental conditions of the application area.As a result,there might be deviation between the design value and the actual value.This study aims to solve the problem of determine the value of the runoff coefficient of the permeable pavement.The main research content includes the determination of influencing factors,the design of artificial rainfall devices,the orthogonal test of various factors and the analysis of results.After collecting rainfall data from some areas of Fujian Province,three factors were determined,including the rainfall intensity,rainfall pattern and rainfall duration.The rainfall intensity was defined as 1.0 mm/min,1.5 mm/min and 2.5 mm/min.The rainfall pattern was classified into three types: the front peak type,the middle peak type,and the rear peak type.The rainfall duration was determined as 45 minutes,60 minutes,and 90 minutes.According to the test results,the value of the runoff coefficient was obtained,and the influence of each influencing factor on the runoff coefficient was analyzed.At the same time,the accuracy of the test results was verified by SWMM simulation software,and the runoff coefficients of different permeable pavements were simulated.The results and conclusions are as follows:(1)Design a artificial rainfall devices,which can adjust the rainfall height from0.5 m to 2.0 m,and the rainfall intensity from 0.6 mm/min to 10.0 mm/min.After testing,the rainfall area,rain intensity and uniformity of the device can meet the test requirements;(2)Through orthogonal test,the runoff coefficient ranges from 0.01 to 0.60 under different rainfall intensity,rainfall pattern and rainfall duration.The test results show that each factor has an effect on the runoff coefficient.The runoff coefficient is positively correlated with the rainfall intensity;The later the rain peak appears,the larger the runoff coefficient;the longer the rainfall time,and the runoff coefficient increases with time;(3)The exponential equation is used to fit the relationship between the three influencing factors and the runoff coefficient,through which the test conclusions can be reflected well.A functional relationship between the three factors and the runoff coefficient is obtained with the fitting degree of 0.94 under the influence of three influencing factors on the runoff coefficient;(4)The runoff coefficient under the design condition ranges from 0.32 to 0.66 when applying the fitting formula and the rainfall data of Fujian Province into the calculation,larger than the standard existing value of 0.08 to 0.45.Therefore,in actual projects,various factors and the local environment should be taken into consideration by determining the runoff coefficients;(5)Establish a model of the test device using SWMM simulation software.The simulation conditions are set to be the same as the experimental conditions.The simulation results are consistent with the test results with the maximum difference of0.02,and the influence of various factors on the runoff coefficient is consistent with the test.However,due to software limitations,the influence of rainfall pattern on runoff coefficient is difficult to simulate.At the same time,the runoff coefficients of different permeable pavements are also simulated under the same rainfall conditions by using the software.The result show that the ranking of runoff coefficients of different permeable pavement from small to large are pervious concrete,permeable asphalt,sand-based permeable brick,and ordinary permeable brick.It indicates that the value of the runoff coefficient is also related to the infiltration coefficient of the permeable pavement.The smaller the infiltration coefficient,the smaller the runoff coefficient.