Do upslope impervious surfaces impact the run-on/runoff relationship?

Authors: Pappas, Elizabeth; Huang, Chi Hua; Bonta, James - Jim

Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2010
Publication Date: 9/15/2010
Citation: Pappas, E.A., Huang, C., Bonta, J.V. 2010. Do upslope impervious surfaces impact the run-on/runoff relationship? Journal Hydrologic Engineering. 16(4):345-350.

Interpretive Summary: When land is developed, surfaces such as rooftops and roads, which are impervious to water, replace natural soil that water can pass into and through. The amount of water that runs off of an impervious surface will be greater than the amount of water that runs off natural soil. However, the water that does run off of a natural soil carries some amount of soil or sediment with it because the soil is erodible. A road or rooftop should not be erobidle, so the runoff it produces is clear. Runoff from impervious or soil surfaces has to go somewhere. Unless it is generated near a storm drain or water body, runoff runs on to a downslope surface, which may or may not be erodible. A simple example of this is rain that falls on the roof of a house runs off and is discharged onto the lawn, where it is then considered run-on. In this study, we performed rainfall simulations on sloped soil boxes. The upslope 2 m were either impervious or soil surfaces, or half and half. Runoff from these surfaces ran onto a 2-m erodible soil downslope (and became runon). Water and sediment were measured in both the runon from the upslope surfaces and the runoff from the downslope surfaces. We found that the equations scientists use to predict runoff characteristics based on runon characteristics do not work very well when runon is generated on a largely impervious surface. For instance, the equations used to predict whether soil will erode or accumulate in a given area need to be modified to account for any upslope impervious areas. This research will provide the basis for customizing existing numerical models for developed landscapes and will aid developers and local authorities in land use planning.

Technical Abstract: Development of watersheds previously managed for agricultural uses for commercial and residential uses results in the replacement of pervious soil surfaces with impervious surfaces. Characteristics of runoff generated on new upslope impervious surfaces may differ from runoff generated on the predevelopment soil surface in terms of quantity, time of concentration, and sediment load. This may cause changes to the erosion regime on downslope soil surfaces. We used rainfall simulation having increasingly higher rainfall rates (20, 30, 40 mm hr-1) to generate runoff from 0.6 m2 boxes (impervious, pervious-soil). Boxes were either treated with an impervious surface (I), or filled with soil (S) 0.2 m deep, and were connected together in series of four boxes to produce different arrangements of impervious and pervious soil surfaces (0, 25, 50% impervious), and under two different antecedent soil moisture conditions. Results indicate that previously established numerical models predicting runoff characteristics as a function of runon characteristics generate very good correlations where there is 0% imperviousness, but these become insignificant as imperviousness increases. Imperviousness significantly influenced sediment regime such that previously established soil erosion predictive models cannot be simply applied to areas where runoff production occurs on surfaces having an impervious component.