Impervious surface impacts to runoff and sediment discharge under laboratory rainfall simulation

Urbanization of watersheds previously managed for agricultural uses results in hydrologic changes associated with increased flooding and erosion. Few studies have been conducted to quantify these effects under controlled conditions and standard rainfall simulation methodologies have not been previously established. In this study, a laboratory rainfall simulation procedure was developed and utilized to evaluate hydrologic and sheet erosional responses to various configurations of impervious surface cover at the small scale. Runoff and sediment losses from a sloped (5%) cascade of soil boxes having 50% impervious cover located at the top of the slope or at the bottom of the slope, or having 0% impervious cover were measured. Results indicate that the 50% upslope impervious treatment generated sediment at 3–5 times the rate of the 50% downslope impervious treatment. Upslope impervious cover resulted in initially lower water runoff rates than channel development, but this effect narrowed or reversed with continued rainfall. These results suggest that upslope impervious surfaces may represent a larger total on-site erosion risk than equivalent impervious surfaces located at lower positions along the slope, especially under high antecedent soil moisture and/or high intensity rainfall.