Coastal Services Center

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Impervious Surface Indicator Information

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In the Scenarios | What are the Benefits of Less Paving? | What Can I Do? | References and Resources

Related Indicator Information Open Space Vegetated Buffers Runoff Clearing Costs

In the Scenarios

Impervious surface refers to anything that prevents water from soaking into the ground. Common examples include roofs, driveways, sidewalks, streets, and parking lots. Because roads are wider, large lots are spread across the site, and there is significantly less green space, the conventional scenario has more impervious surface compared to the conservation and new urbanist scenarios. The amount of impervious surface coverage in each of the scenarios was estimated based on published assumptions about the average imperviousness of different land use and density categories. For example, commercial land uses were assumed to be 85 percent impervious while forested areas were considered 1 percent impervious.

For details on how impervious surface area is calculated for each scenario, refer to the Indicator Methods section.

What Are the Benefits of Less Paving?

Because impervious surfaces do not allow water to soak into the ground, the amount and distribution of these surfaces can negatively impact an area's hydrology. When impervious surfaces cover areas where water naturally seeps into underground water sources, or aquifers, they reduce the amount of water available for wells and springs. Pollutants from the air and leaked by cars accumulate on impervious surfaces. During storms, excess water that could not seep into the ground, flows across impervious surfaces and sweeps these pollutants into surrounding waters and farther downstream. As impervious surfaces increase, so do stormwater runoff volumes, pollutant levels in runoff, and the velocity of stormwater flows (Schueler 1994).

While this project considered the imperviousness of specific site designs, it is important to consider imperviousness within a broader, watershed-level perspective as well. The amount of impervious cover in a watershed can be correlated with stream and watershed health. Research has shown that watershed health begins to decline when impervious surfaces coverage exceeds 10 percent and becomes severely impaired if this number climbs beyond 30 percent of the total watershed area (Arnold and Gibbons 1996).

Another unintended consequence of impervious surfaces is the urban heat island effect. Surfaces like roofs, parking lots, and roads absorb heat during the day and release it at night when outside temperatures fall. This causes the average temperature to be higher in areas where there are many impervious surfaces. The heat island effect can make summer days 6-to-8 degrees Fahrenheit hotter in a city than in surrounding areas. In practical terms, the heat island effect means higher energy bills. To combat this effect, reduce impervious surfaces, preserve or plant trees, have a reflective or green roof, and where pavement is necessary make it reflective. By applying mitigation measures such as these, a Lawrence Berkeley National Laboratory study measured direct energy savings at 10 to 40 percent for the residential and commercial buildings tested (Heat Island Group. 1999, 2000. EETD, LBNL, UC).

Site design and regional planning to reduce and compact impervious surface can help alleviate its negative effects. Reducing impervious surface often requires innovative techniques, materials, or devices. Simple design components can help too; for example, a grass parking area can help control temporary flooding, help filter pollutants out of water soaking into the ground, help reduce the heat island effect, and cost less than paving.

The benefits of reducing impervious surfaces are clear, yet challenges associated with the techniques for doing so remain. A few are shown in the table below.

What Can I Do?

Developers or citizens can do a lot, although local governments may be best suited to affect change in their jurisdictions. Local jurisdictions can modify existing development regulations and consider implementing some of the modifications listed below in new developments (Wells 2000). In some cases, the modifications may offer cost savings. For instance, narrower streets mean less paving material and fewer man hours to get the job done.

• Allow reduced residential street widths
• Relax parking requirements and encourage cooperative parking arrangements
• Eliminate paved sidewalks or narrow them to four feet in width
• Encourage use of alternative paving materials for sidewalks, parking lots, and roads
• Encourage cluster development and allow taller buildings
• Avoid clear-cutting lots where possible
• Encourage green roofs
• Preserve existing vegetation and plant more vegetation to absorb extra runoff
• Provide public transit to reduce the traffic demand for widened or new roads
• Encourage infill development in existing built areas

For jurisdictions that are at or near build-out, new development may be relatively infrequent and many areas may be already overwhelmed by imperviousness. In these instances, retrofitting and implementing stormwater best management practices (BMPs) are alternatives.

Stormwater BMPs can be organized into four categories: filtration, infiltration, ponds, and wetlands (SC NEMO). Filtration BMPs harness the natural capacity of plants to slow and filter runoff, increasing infiltration and reducing pollutant loads. Examples include vegetated buffers along streams and grassed swales along roadways. Infiltration devices such as porous pavement allow some water to soak into the ground, thereby reducing overall runoff volume. Stormwater ponds offer a collection point for runoff and allow captured water to infiltrate or gradually be released. Pollutant loads settle to the bottom of these ponds where vegetation helps absorb some of the pollutants. Where space is sufficient, man-made wetlands can reduce volume of runoff and pollutant loads as well as facilitate infiltration.

References and Resources

American Rivers provides this downloadable resource, Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought, published in August 2002. American Rivers is a nonprofit dedicated to protecting and restoring rivers nationwide.

Arnold, C.L., and C.J. Gibbons. 1996. "Impervious Surface Coverage: The Emergence of a Key Environmental Indicator." Journal of the American Planning Association. Volume 62, Number 2. Pages 243 to 258.

The Environmental Protection Agency provides information and strategies for reducing the urban heat island effect caused by impervious surfaces, such as roads, roofs, and parking lots, in urban areas.

Heat Island Group. 1999. Environmental Energy Technologies Division Lawrence Berkeley National Laboratory, University of California. "Learning About Urban Heat Islands."

Heat Island Group. 2000. Environmental Energy Technologies Division Lawrence Berkeley National Laboratory, University of California. "Vegetation," "Ways To Save Energy," and "Energy Use Overview Conclusions."

National Education for Municipal Officials (NEMO) provides case studies about pervious constructing materials and pervious landscape elements. Another resource is NEMO's downloadable technical papers on impervious reduction. NEMO is a University of Connecticut educational program for local land use officials that addresses the relationship of land use to natural resource protection.

The NOAA Coastal Services Center provides the impervious surface analysis tool (ISAT) free for download.

The Georgia Planning Act Minimum Standards outlined here set the minimum legal limits for impervious surface setbacks. Provided by the Office of Public Service and Outreach, Institute of Ecology, University of Georgia. The Institute of Ecology's mission is to provide expertise in the development of policies and practices to protect Georgia's natural resources, which are critical to the state's economic well-being.

Schueler, Thomas R. 1994. "The Importance Of Imperviousness." Watershed Protection Techniques. Volume 1, Number 3. Pages 100 to 111.

— 2000. "Use of Open Space Design to Protect Watersheds." The Practice of Watershed Protection. Pages 299 to 302.

— 2000. "Why Stormwater Matters." The Practice of Watershed Protection. Pages 365 to 370.

South Carolina National Education for Municipal Officials (SC NEMO) provides several presentations about impervious surfaces: best management practices, better site design, zoning options, and others. These were presented at the Taming Stormwater Workshop and are available for download.

The Stormwater Manager's Resource Center (SMRC) provides various resources on stormwater management including ways to monitor and assess impervious cover and a glossary of related terms. SMRC is maintained by the Center for Watershed Protection.

Towson University Department of Geography and Environmental Planning provides a literature review and other resources on imperviousness.

The University of Nebraska – Lincoln Excellence in Water Quality Program provides information about the importance of water resources and water safety issues through its on-line library. Another useful feature allows users to submit questions about water quality issues to a librarian free of charge.

The Urban Water Resources Research Council of the American Society of Civil Engineers provides the International Stormwater Best Management Practices Database, which contains the results of over 200 best management practices.

The Watershed Science Institute (WSI) provides area-wide planning tools such as a guide for assessing conditions of riparian-wetland corridors, a model planning guide, and indicators for assessing watershed conditions. The WSI is a program of the U.S. Department of Agriculture's National Resources Conservation Service which improves watershed processes and functions by adapting technology to further effective analysis of watershed-based resource issues.

Wells, Cedar. 2000. "Skinny Street and One-Sided Sidewalks: A Strategy for Not Paving Paradise." The Practice of Watershed Protection. Pages 296 to 298.

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