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Development of a Set of Impervious Surface Coefficients: A Tool for Watershed Analysis [01/30/07]

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.

As part of our work to develop useful tools and guidance to advance the science and practice of ecological risk assessment, especially watershed assessment, the Ecotoxicology Program at the Office of Environmental Health Hazard Assessment (OEHHA) has been working to develop impervious surface coefficients (ISC). Our involvement is part of a large community of research being conducted on the subject of estimating impervious surfaces. We work with the California Water and Land Use Partnership (CA WALUP), an affiliate of the National NEMO2 Network, on this and other related projects. We have recently completed a preliminary set of ISC for use in the six counties in the Sacramento metropolitan region. California-based research is rare and the literature generally fails to account for California land use trends, typically characterized by higher density development than elsewhere in the nation. These coefficients were created for the 19 General Land Use Categories (GLUC) in the Sacramento region using a Geographic Information System (GIS). The coefficients were developed by manually digitizing over 800 randomly selected sites representing all 19 GLUCs. This sample size is sufficient to provide 90 percent confidence that the ISC is plus or minus between 2-5 percent of the actual number, depending on the GLUC. We are in the process of validating these numbers for the entire Sacramento area as well as for the entire state. Since the coefficients were developed based on density (number of dwellings/acre or foot area ratio3), they should be applicable to any region of the state. We are currently working with CA WALUP to validate their use in other communities around the state.

Applications of Impervious Surface Analysis

IS analysis is useful in two primary ways: as a watershed indicator and as a tool for natural resource-based land use planning.

Watershed Indicators: Because of the positive correlation between percent IS and the quality of the aquatic ecosystem, imperviousness has been proposed for use as an environmental indicator. The US Environmental Protection Agency, US Geological Survey, and many environmental indicator projects throughout the country have identified the value of analyzing imperviousness as a watershed stressor. Its value as an indicator is linked to the fact that it is correlated with a variety of aquatic ecosystem conditions, it measures cumulative impacts, and it can be measured using a variety of methods. One of the limitations of using IC as an indicator at present is that this type of data is not regularly collected. Currently calculating these values can be quite costly. As described in detail in the accompanying fact sheet, once the ISC we have developed are fully validated, imperviousness can be calculated either using spreadsheet software or a GIS for a very modest effort.

Natural Resource-based Land Use Planning: In this approach to land use planning, important natural resources are inventoried prior to determining where to site infrastructure, housing, and commercial development within a community. By considering natural resources at the beginning of the planning process, instead of the end, aquatic resources, open space, and recreational areas can be protected and flood risk minimized. By making initial estimates of total IC, land use can be modified or the location changed to reduce imperviousness that drains to sensitive areas. If imperviousness exceeds protective amounts, infiltration and design practices that disconnect impervious areas can be employed to reduce these impacts and/or infiltration methods can be incorporated into the design to contain as much runoff as possible at its source.

Conclusions

We have developed methods for determining current percent imperviousness in a watershed and estimated build out percent imperviousness. We have prepared a fact sheet, “Estimating Watershed Imperviousness for Current and Build-Out Conditions” that describes our methods and how they can be applied. We welcome your comments using the feedback form provided.

Follow this link to download the updated fact sheet: Estimating Watershed Imperviousness for Current and Build-Out Conditions as a pdf file. The impervious surface coefficients and explanatory fact sheet were developed by Katie Yancey, OEHHA GIS student analyst.

Follow this link to download our survey form as a pdf file

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¹Drawn from the Heinz Center Environmental indicators Report, posted at: http://www.heinzctr.org/ecosystems/urban_technotes/urban_ttl_imperv_area.shtml

²The National NEMO (NonPoint Source Education for Municipal Officials) Network is a network of programs in over 30 states whose goal is to provide educational programs and tools for local land use decision makers on the relationship between land use and natural resource protection. For more information: www.nemonet.uconn.edu.

³Foot-Area Ratio (FAR) is the equivalent of dwelling density for commercial areas. It reflects the size of the footprint of the building relative to the area of the parcel.