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Minimizing Impervious Areas

General

Minimizing impervious areas provides more areas for stormwater to infiltrate, reducing the amount of stormwater that leaves a site. In addition, less impervious area reduces the urban heat island effect and reduces construction and maintenance costs.

The amount of impervious area can be reduced in a variety of ways. Streets can be laid out differently and designed narrower, vegetation can be incorporated into street design, or driveways may be designed narrower, constructed with alternative materials, or shared with neighbors. However, all of these methods require that the developer, contractor, and the designer work together to plan and implement these practices.

Advantages

  • Produces less runoff
  • Cost-effective
  • Narrower streets reduce speeds
  • Improve aesthetics

Disadvantages

  • Options may be limited by local ordinances
  • Proper soils are required for infiltration practices

Design

Street Design

One of the easiest ways to reduce the amount of impervious surface is to design and build narrower streets. Many communities require widths of 32 to 40 feet to provide 2 lanes of traffic and parking on both sides of the street. However, if parking is restricted to one side of the street, streets can be designed with widths as narrow as 22 feet without sacrificing emergency vehicle access or safe traffic flow.

Streets may also be laid out differently. A typical grid system results in approximately 20,800 lineal feet of impervious surface, while alternative layouts that utilize cul-de-sacs, such as loops and lollipops, result in as little as 15,300 lineal feet of impervious surface when applied over the same area. Cul-de-sacs may be designed with the smallest practical radius, generally 40 feet, as this radius will accommodate most emergency vehicles. Cul-de-sacs may also incorporate a vegetated center that is designed to collect runoff from the surrounding pavement and function as a rain garden, while grassed swales may be used in addition to, or as an alternative to traditional curb and gutter (refer to Rain Gardens, pg. I.R-1; or Grassed Swale, pg. I.G-2).

However, streets should be designed according to the individual needs of the development. Population density should be taken into consideration when designing street width, as it affects both traffic volume and the number of parking spaces that are required. In addition, many communities have ordinances requiring minimum street widths or certain types of street layout that must be followed. As a result, planners should check all local zoning ordinances for such requirements before proceeding.

Source: Prince George's County, Maryland

Driveway Design

Rethinking driveway design is another way to minimize the amount of impervious area that is created by development. Driveways may be designed with shorter lengths, constructed with alternative materials, or shared with neighbors and should be designed to drain into the yard rather than into the street. However, in some instances, the setbacks required by local ordinances may require developers to obtain a variance before implementing these practices.

Many driveways are built with additional parking capacity that largely goes unused, resulting in unnecessary impervious area. Driveways that meet the minimum setback requirements or those that are tapered to one lane as they meet the street are two options that are available. On street parking, if available, may be utilized to provide additional parking as needed.

Driveways may also be constructed using alternative materials and methods. Vegetation may be incorporated into driveways, between the driving lanes, to allow runoff to infiltrate. In these types of driveways, grass is typically planted in strips no more than 3½ feet wide and is bordered by pavement, at least 1 foot wide, that is used for vehicle traffic. Pervious pavement systems (refer to Pervious Pavement, pg. I.P-2) or pavement blocks may also be used.

These methods allow water to infiltrate through them while providing the benefits of traditional pavement systems. They are generally recommended for overflow parking areas or in front of a third garage stall, however, as frequent use may require maintenance more often than other forms of pavement.

Residents may also share driveways as a way to reduce impervious areas. This option is especially applicable for longer driveways, which may branch off into individual driveways. In addition to reducing the amount of runoff that leaves their property, residents will enjoy cost savings in maintenance and snow removal.

Source: Adapted from Valley Branch Watershed District

Construction

  • Care should be taken to avoid unnecessary compaction of soils- deep till as necessary
  • Vegetation should be selected carefully. Refer to Native Plants, Permanent Seeding, Temporary Seeding and Tree Planting for additional information.
  • Pavement will settle after installation and care should be taken to ensure that it does not settle below the adjacent soil surface, as this may connect impervious areas.