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 Impervious area can also be effectively removed by routing runoff flow to an area that will absorb the water, such as a yard, swale or bioretention area. Below are several techniques that can be used to reduce imperviousness. The City of Inver Grove Heights, MN, has implemented several of these techniques in its ordinance for the Northwest Area.
Narrower streets Many communities require residential streets that are much wider than needed to support travel lanes, on-street parking, and emergency access. Some communities currently require residential streets as wide as 32 to 40 feet, which provide two parking lanes and two moving lanes.
Local experience has shown that residential streets can have pavement widths as narrow as 22 to 26 feet, and still accommodate all access and parking needs (ITE, 1997). Even narrower access streets or shared driveways can be used when only a handful of homes are served. The City of Inver Grove Heights Northwest Area requires a 28 foot paved local public street in addition to a sidewalk or trail on one side of the street. Local private streets have a 24-foot width requirement. Narrower streets help reduce traffic speeds in residential neighborhoods which, in turn, improve pedestrian safety.
Local public works, police and fire departments might object to narrower streets. Referring to the documents in the Additional Resources section can help identify how to address some of their concerns.
Slimmer sidewalks Many communities require sidewalks that are excessively wide or are located adjacent to the street where the pedestrians are at risk from vehicles. A better site design technique modifies the width and location of sidewalks to promote safer pedestrian mobility. Impervious cover is reduced when sidewalks are reduced in width and located away from the street. Sidewalks can also be disconnected so they drain to lawns or landscaping instead of the gutter and storm drain system, or they can be constructed with permeable concrete, asphalt or blocks.
Smaller cul-de-sacs Impervious cover can be reduced by minimizing the diameter of residential street cul-de-sacs and/or incorporating landscaped islands. Many communities require cul-de-sacs that have a greater diameter than needed to allow emergency and large vehicles to adequately turn around. Alternatives to the traditional 80 foot diameter cul-de-sac include 60 foot diameter cul-de-sacs, hammerhead turnarounds and loop roads. The Northwest Area zoning ordinance requires an outside roadway radius of 35 feet and a street property line (right-of-way) of 50 feet.
In addition, the inside of the turnaround can be landscaped as a bioretention area to further reduce impervious cover and improve stormwater treatment. Trees and vegetation planted in landscaped islands can be used to intercept rain water and treat stormwater runoff from surrounding pavement. Each of these alternative turnaround options produces a more attractive and safe environment for residents.