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Impervious Surface Analysis: Facts Sheet
[07/22/08]

Background on Impervious Surface Analysis

Impervious surfaces (IS) are areas of the land hardened by such structures as houses, patios, driveways, and transportation infrastructure. The percentage of the landscape that is covered by IS increases as urbanization expands outward from the center of towns and cities. Increased imperviousness alters the hydrology within a watershed, with significant consequences on water quality and aquatic and riparian habitat. Effects that have been associated with increases in impervious area include the following1:

• Changes in stream flows. Greater stormwater volumes traveling over the surface and being delivered too rapidly to streams leads to increased stream flashiness and a reduction in summer base flows, sometimes causing perennial streams to become intermittent or to dry up completely. As a result, urbanized watersheds are prone to more frequent and bigger floods.
• Alterations to the stream channel. The rapid runoff associated with increased stormwater velocity and volume quickly erodes and incises the stream channel and banks. Channels widen and straighten and become disconnected from their floodplain. Pools, riffles, and sandbars are simplified or washed away, eliminating critical habitat for fish, waterfowl, and other species of animals and plants.
• Water quality degradation. Concentrations of pollutants in streams increase with increases in impervious area. Common urban pollutants include pesticides, bacteria, nutrients such as phosphorus and nitrogen, heavy metals and other contaminants.

The Office of Environmental Health Hazard Assessment (OEHHA) has been working on the development of a set of impervious surface coefficients for the past 4 years. Combined with spatial data of the aerial extent of land use categories (LUC), these coefficients can be used to calculate the amount of imperviousness for a variety of spatial scales: in a watershed, in a city or county, or for a specific development project. This type of analysis can be useful in stormwater management to as part of a runoff analysis, in land use planning to identify areas of higher or lower imperviousness and opportunities to reduce impervious cover, and in watershed assessment as an indicator of watershed health.

This project began with a preliminary set of coefficients for the Sacramento metropolitan area as part of the analysis of watershed stressors. Through our work with the California Water and Land Use Partnership (CA WALUP), it became apparent that there was a need for such coefficients for the state in general and for a variety of applications. With financial support from the US EPA and the State Water Resources Control Board, we developed a set of coefficients that reflect the development style in California. We increased the number of parcels that were analyzed in the Sacramento region, and collected new data from a large metropolitan city in southern California, Irvine, and a mid-sized coastal community, Santa Cruz. The factsheet describes in detail how the coefficients were developed and how they might be applied in a variety of settings.

Work on this project is continuing. The method for calculating residential coefficients is being refined to make it more useful for stormwater engineers. We are also writing a User’s Guide that will contain details on all the information in the factsheet as well as worked examples and a spreadsheet. This Guide will be available in Fall, 2008.

Special thanks are extended to Katie Yancey, currently with the City of West Sacramento, Lead GIS Analyst, and Jeff Sturman, Tetra-Tech, GIS Analyst, and Angela DePalma-Dow, OEHHA student intern who prepared the factsheet. Questions can be addressed to Barbara Washburn, Ecotoxicology Program, OEHHA; bwashburn@oehha.ca.gov.

Follow this link to download the updated fact sheet: Impervious Surface Coefficients a pdf file.