Study On The Factors Of Rainfall-runoff Coefficient

The process of urbanization is speeding up, which leads to the percentage ofhardened roads increased in the city. In addition, extreme weather happens frequentlyin recent years, they make local flooding in the city seriously. The runoff coefficientis an important parameter in the storm water network design. The runoff coefficient inthe “Code for design of outdoor wastewater engineering” now, which is determined bythe experience and don’t take the surface slope, the situation of geographicalenvironment around and other factors into consideration, is mainly estimated byground cover types. Therefore, it is necessary to study on the runoff coefficientaccording to the actual situation of Changsha, with the surface slope and soilproperties taken into consideration.The parameters of the simulated rainfall were determined by analyzing therainfall characteristics in Changsha. Typical soil（red soil）in Changsha was selectedas the underlying space of green soil with the surface slope of3°,7°,13°,an dimpervious tile pavement (water proof brick), concrete pavement which can easilycause urban waterlogging were taken as the types of the underlying surface.46simulated rainfall experiments were carried out to study the influence of the rainfall,slope, the peak rain location of the rainfalls, surface materials on runoff coefficient.Analyzing the experimental data by mathematical model, we can obtained thatthe mathematical relations between runoff coefficient and rainfall is linear, the runoffcoefficient increases with the increase of rainfall. Rainfall has a big impact on therunoff time, there is exponential relationship between the two.The steeper the slope, the shorter the runoff time, and the two of the above wereseen as a exponential relationship. If slope improved the same, torrential rain runoffcoefficient would increase the maximum, and heavy rainstorm, rainstorm heavy rainfollows.The initial runoff time of rainstorm, heavy rainstorm, torrential rain is forward torainfall peak. Therefore, the peak position has no effect on the initial runoff time ofthe three storm types. The initial runoff time of mid-peak rain is less than front-peak,mid-peak after rain is slightly smaller than the post-rain. The later the position ofrainfall’s peak, the bigger the runoff coefficient.Impervious brick road and concrete pavement produced large flow and the soil filtration is little. Consequently, the stage of the soil moisture has little effect onrunoff coefficient. The runoff coefficient of impervious brick road is smaller than thatof concrete pavement.