The reduction of impervious surfaces is a principle of conservation design that yields multiple benefits when implemented. Better Site Design defines impervious surface as "any surface in the urban landscape that cannot effectively absorb or infiltrate rainfall (NIPC, 2003)." The progression of new development typically results in the conversion of natural land to impervious surface cover. Common impervious surfaces include sidewalks, parking lots, building footprints, roads, swimming pools, roof tops, garages, and patios. All of these surfaces can be designed or retrofitted to redirect stormwater runoff away from the sewers opting for absorption directly into the ground or into a holding mechanism-natural or man-made. By keeping excessive stormwater runoff out of the sewer system and closer to the point of origin, water quality is increased because the stormwater accumulates fewer pollutants on its shortened path. Additional benefits include recharged soils, reduced flooding and reduced sewer overuse and related maintenance costs (NIPC, 2003).
Reducing impervious surface areas also has a substantial impact on the natural landscape. Increased impervious cover and the subsequent increased stormwater runoff can negatively impact stream functions, cause stream bank erosion, degrade stream habitats, increase pollutant loads in streams, deplete the surrounding wetlands and prairies, and lower the diversity of native fish species, insects, and fresh water organisms (Ibid).
The design of new development and its amenities should consider the reduction of impervious cover in the early stages of project conception. For example, interior roads within suburban developments can be narrowed and curbs reduced. Setbacks can also be reduced, and houses clustered, shortening roads and driveways, decreasing the amount of concrete poured and the cost to the developer for supplies and construction time. This also decreases the amount of water infrastructure needed to carry the stormwater runoff to the sewers. Based on 1997 costs, the Northeastern Illinois Planning Commission's Reducing the Impacts of Urban Runoff calculated an average savings of $910 per residence for reducing street, sidewalk, and driveway width in a new residential development. A particularly effective—and far-reaching—means of reducing impervious surfaces is for municipalities to design flexible ordinances that allow for the opportunity to utilize these conservation design techniques.
Green roofs, landscaped parking lots, and permeable pavement/pavers are a few practices that can also help reduce impervious cover and runoff. These are strategies especially beneficial to urban environments, where impervious surface is denser and less green space is available. It should be noted that these practices should take into consideration the accessibility (ex: sidewalks) and topography of a site and maybe not be appropriate for all situations.
Reducing impervious surfaces can also decrease the heat island effect found in many urban areas (The Nature Conservancy and Chicago Wilderness). According to the Environmental Protection Agency (EPA), the heat island effect can cause the air and surface temperatures to be 2-10 degrees warmer than surrounding rural areas, which can add to the effects caused by global warming.
The city of Warrenville isusing permeable pavers to rebuild a one-mile stretch of Warrenville Rd. The $4.7 million dollar project is expected to be completed this fall. Mayor David Brummel states that "While the road is more expensive than a traditional surface, it reduces the possibility of flooding and prevents the "freeze-thaw cycle" that causes potholes." For more information click here.
Imagine that….in 2040, every roof top is a green roof….
New technology is being used to help monitor the degree of imperviousness on an individual site and even at the city level. This can be seen in the following e xample:
Innovation in Mapping: Impervious Surface Mapping Using Satellite Remote Sensing
The Metropolitan Council of Minnesota is working with the University of Minnesota's Remote Sensing and Geospatial Analysis Laboratory to use satellite imagery to generate Imperious Surface Area maps. By integrating aerial photography and spectral-radiometric responses of Landsat TM imagery satellite data, the Metropolitan Council of Minnesota can calculate the imperious surface area percentage of a location as well as the degree of imperviousness ranging from 0%-100%. An example map of the Twin Cities is posted at http://rsl.gis.umn.edu/impervious.html