Estimating the Infiltration Area for Concentrated Stormwater Spreading over Grassed and Other Slopes

Abstract

This research developed a new approach for calculating the area over which water spreads after being released from a confined conduit onto a sloped planar surface with defined roughness. In particular, the goal was to predict how stormwater would spread onto a sloped grass lawn after being discharged from a disconnected gutter downspout or through a parking lot curb cut. The need for this stems primarily from regulators increasingly requiring developers to infiltrate more of the runoff created by site development, but designers not having good tools for estimating the infiltration area associated with such “overflow” practices. The model is largely based on Manning’s equation applied at multiple cross-sectional areas of flow downslope, with additional modifications allowing the water to spread laterally. The model results were compared to laboratory experiments of water spreading across a roughened painted surface and two different artificial turfs. The new model predicted the wetting area with average absolute errors of 6.0% and 5.9% for a fine-bladed artificial turf and a coarse-bladed artificial turf, respectively. In addition, while validating the modeled flow spreading across a range of roughnesses, the model had an absolute error of 5.2% for a rough painted surface meant to represent unfinished concrete.