Geographic Variability of Rainfall Erosivity Estimation and Impact on Construction Site Erosion Control Design

The Revised Universal Soil Loss Equation (RUSLE) is used often by erosion control planners to estimate the soil loss from urban construction sites when sizing sediment ponds and determining the soil loss under vegetative mats. This project used the existing, complete-year rainfall record for 27 sites in the state of Pennsylvania to compare the USDA isoerodent maps to the annual rainfall erosivity, R, values calculated using the USEPA equations for the National Resource Conservation Service Type II rainfall. The USDA and USEPA maps showed a general trend of increasing median annual R from west-to-east and north-to-south. A trend analysis relating the median R values calculated during this project to geographic location had similar, large-scale geographic trends as the USDA and USEPA maps. However, the R values more closely followed a combination of the annual rainfall pattern and topography (the Appalachian mountains bisect the state). Two case studies of the impacts of these calculations were developed to show the impact of using different values of R on the design of sediment ponds and predicting vegetation establishment. The results of these scenarios indicate that the source of data to predict R can affect the frequency and cost of sediment pond maintenance and may under-predict the protection level required of a vegetated erosion control mat.