Impervious areas such as road and parking pavement, building surfaces, and walkways/driveways significantly increase stormwater runoff volumes, which in turn causes flooding and streambank erosion. Impervious surfaces also facilitate the wash-off and transport of pollutants like oil, grease and sediment into downstream rivers, lakes and wetlands. This fact sheet identifies methods and design standards used to achieve a reduction in the total runoff volume from impervious surfaces and gives examples of municipal ordinances that foster the reduction of impervious surfaces.
Reduced imperviousness results in smaller stormwater discharges which enhances flood control, reduces erosion and increases infiltration. Any reduction in runoff volume translates into reduced pollutant loads to downstream waters. Reduced runoff can also reduce the size and cost of stormwater management systems. Increased greenspace can facilitate recreational and community activities that enhance the quality of life of residents/employees.
Managing the extent of impervious area of buildings, roads and parking pavements occurs through the site planning and design process. Example methods to reduce imperviousness include but are not limited to, narrower road sections, alternative road layouts, reduced application of sidewalks and on-street parking, cul-de-sac design, parking lot design, house setbacks, structure/building impervious area limits and driveway designs. These methods are a component of design methodologies such as low impact development, design with nature, sustainable development and conservation design, and could become a part of standard building codes.
This strategy relies on several techniques to reduce the total area of rooftops, parking lots, streets, sidewalks and other types of impervious cover created at a development site. The basic approach is to reduce each type of impervious cover by downsizing the required minimum geometry specified in current local codes, keeping in mind