Parking Lots

As more and more people own cars, more parking lots become necessary. Unfortunately, parking lots can adversely affect the environment as well as detract from community character. Paved parking lots are typically designed to collect and concentrate large areas of stormwater runoff, which can impact a receiving streams hydrography as well as water quality. Paved parking lots can generate heat, raising the surrounding areas air temperature and the temperature of the first flush of stormwater, creating significant ecological impacts. In Washington, the City of Olympia's Public Works Department found that parking lots account for 53% of imperviousness on a commercial site and 15% of multi-family sites. These figures are typical of most communities. Therefore, careful attention to their design will go a long way toward protecting your community's water resources.

Landscaped islands used as bioretention areas in a Connecticut mall parking lot. These kinds of practices help slow and treat runoff coming off of large parking lots.

Tips for Reducing Runoff from Parking Lots

Planning/Regulatory Techniques
Reducing Imperviousness
Reducing Impacts

For more information download NEMO's Technical Paper on Parking Lots (PDF - 41k).

Planning/Regulatory Techniques

In your Plan of Conservation and Development (POCD) include a discussion of parking impacts on community character and water resource health. Pay particular attention to municipal lots (public works garages, fire departments and schools) and associated housekeeping/pollution prevention.
Consider including an "impervious cover build out analysis," showing the location and amount of imperviousness that will be generated if the community develops according to present zoning. With this information, you can make recommendations regarding the location, size and design of future parking facilities emphasizing their potential environmental impact.
Many parking standards are based on peak utilization periods, such as the week before Christmas. As a result, many spaces are not utilized during most of the year. A municipality can conduct a parking utilization study to determine whether present zoning requirements for parking are excessive. The Litchfield Hills Council of Elected Officials and the Northwest Council of Governments jointly sponsored a 2-phased parking study, conducted by Fitzgerald and Halliday Inc., that investigated parking utilization, determined actual parking needs by land use, and developed strategies to decrease impervious surface. Phase I (PDF - 776Kb) is a detailed parking assessment of the two planning regions. Phase II (PDF - 1.5Mb) suggests specific model regulations for alternative parking requirements.
Review and revise, if necessary, zoning site plan requirements to include a stormwater management plan. Encourage alternative design for parking that is protective of water resources. Incorporate shared parking incentives. All parking areas, other than those associated with single-family detached residential units, should require special permits and be subject to site plan review.
The zoning regulation's "statement of intent" should describe why landscaping is required in parking areas. In addition to landscaping's role of improving lot appearance and safety, mention its value regarding water quality protection and stormwater management.
Landscaping regulations typically require a certain percentage of a parking area be vegetated. Stormwater management plans can incorporate these areas! For example, storm water runoff can be directed to sunken vegetated islands (typically raised and curbed) that can filter and infiltrate storm water (raingardens or bio-retention areas).

Reducing Imperviousness

Based on a parking utilization study, paved areas can be sized to accommodate expected daily use, which will likely require less paved area. Overflow areas can then be increased and designed with porous paving materials allowing for the infiltration of water, as well as for the reduction of heat island effects.
Pave only the travelway and utilize porous paving materials in parking spaces. The porous parking spaces can also be designed to treat runoff from the travelway.
Porous alternatives to asphalt or concrete can be utilized for an entire parking area that will see seasonal heavy use or will have regular maintenance year round.

Pervious pavements, such as the grass- and gravelpave lot above, reduce the impact of parking lot runoff and are good to use in overflow parking lots.

Reducing Stormwater Impacts

Curbing can be removed from the perimeter of paved areas, allowing sheet flow of stormwater over vegetated areas or through vegetated swales that can filter and infiltrate the runoff.
Stormwater runoff can be directed to sunken vegetated islands (typically raised and curbed) that can filter and infiltrate storm water (raingardens or bioretention areas).
Perforated pipe can be utilized between catch basins to encourage infiltration. Stormwater leaching fields can be designed beneath paved areas.
Stormwater can be directed to an on-site infiltration basin or through a series of wet ponds that will filter out pollutants and can attenuate hydrologic impacts (stormwater "treatment train").

Pervious pavers used for an overflow parking area at a local medical office facility.

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