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Protecting our Waters -- Subdivision Design

Downspouts | Rain Gardens | Drainage Systems | Street Widths and Configuration
Common Driveways and Alley Access | Conservation Designs | Phased Development

Aerial view of a conservation subdivision designMany communities have realized that a departure from the typical subdivision design is a better choice to help protect the quality of our waterways. However, the majority of the recommendations to improve subdivision design require variances from typical ordinances. Community ordinances and regulations must be integrated to work efficiently and enhance a smooth process. Municipalities should review their community regulations to ensure rules are consistent with state and federal legislation as well as current development practices.

The Center for Watershed Protection has developed a Codes and Ordinances Worksheet (COW) to help communities evaluate their current development rules. This tool is best used by a local-site planning roundtable made up of representatives from government, developers, and environmental groups. The COW consists of 66 questions that correspond to the principles of better site design and points are awarded if the local development rule agrees with the COW principle.

Subdivision design refers to the development of a large parcel of land, often for residential use, which is divided into several individual parcels. The trend in subdivision design has been toward offering large lots, often near 1 acre. Many new subdivisions are also designed with sidewalks, wide paved driveways, and wide streets and cul-de-sacs with curb and gutter. Many existing subdivisions have sidewalks and paved driveways, and some have alleys. Many of the items identified above are required by local ordinances. All of them cause an increase in stormwater runoff and an increase in pollutants reaching our waterways.

Soil erosion from land that has been cleared of vegetation can be a problem in new subdivisions. The Department of Commerce, WDNR, and local ordinances require the establishment of vegetative cover a certain number of days after construction has been completed. The enforcement of the Uniform Dwelling Code (UDC) and commercial erosion contol requirements is typically left to the local building inspection department, who may not have adequate staff to track compliance.

Some of these issues associated with subdivision design can be minimized by allowing variances from existing local ordinances for such things as reduced street widths and exclusion of sidewalks. In some cases, new ordinances must be developed to address construction techniques and erosion control. The items below describe ordinance elements that can lead to stormwater-friendly design.

Downspouts -- Back to Top
It has been shown that rooftops are a major source of zinc in urban stormwater, yet some municipalities require downspouts be directly connected to the storm sewer. This allows both pollutants and increased water volume to be emptied directly into natural waterways. A 1,000 square foot rooftop will contribute almost 350 gallons of runoff from only 1 inch of rain. This additional volume and pollutant load has been shown to have a detrimental effect on fish and wildlife habitat. (1) In areas with combined sewers (sewers carrying both sanitary sewerage and stormwater), this increase in volume can lead to overflows which allow the release of untreated, diluted sewerage into natural waterways. Allowing downspouts to discharge onto grass areas will help reduce the amount of runoff by allowing some infiltration, and the soil and vegetation will remove some of the pollutants.

Rain gardens -- Back to Top
Rain gardens are another means to reduce runoff from rooftops. For more information on rain gardens follow the link here.

Image of a grassy swale between roadsDrainage Systems --
Subdivisions use either grass drainage swales or storm sewer systems to convey runoff away from properties. Use of grass swales instead of storm sewers as the drainage system in lower density residential areas has both water quality and quantity benefits. Road runoff is allowed to sheet flow over the grass swale, which will remove some of the pollutants. Swales also provide some infiltration if soil conditions permit. Bioretention systems can also be constructed in the swales to increase runoff and pollutant removal.

Image of a grassy swale next to a parking lot.Grass swales also provide a place for snow storage in winter months. An added benefit is that as the snow melts, the pollutants and salt carried from the street are contained in the ditches rather than going directly into the storm sewer as the snow melts. One disadvantage is that the chloride in road salt moves through the soil fairly rapidly and plants in the swale can be adversely affected by salt. A community should use salt judiciously in areas of swale drainage.

Grass swales may not be feasible in medium- to high-density residential areas or in commercial areas. These areas tend to have driveways spaced closely together, which would require closely spaced culverts. In addition, there may not be enough space to construct swales in these areas without encroaching on buildings or parking lots. It is generally recommended that curbs and gutters be used in these situations.

Street Widths and Configuration -- Back to TopIt has been estimated that streets account for 40 to 50 percent of the impervious cover in residential areas.
It has been estimated that streets account for 40 to 50 percent of the impervious cover in residential areas. (1) As discussed in the section on roads and parking lots, roads generate the highest pollutant load in urban areas. Implementing the following methods to reduce pavement area can lead to a 5 to 20 percent overall reduction in impervious area, with similar reductions in runoff volume and pollutants. (2)

Many communities require residential streets to be 32 to 40 feet wide to allow for two driving lanes and up to two parking lanes. Often more on-street parking is provided than needed considering most vehicles are parked in driveways. It has been shown that residential streets can be as narrow as 22 to 26 feet wide and still allow safe passage of emergency vehicles. Other design features such a mountable curbs can be used to address these concerns. On-street parking can still be provided on one side of the street even with the narrower width. (2)

Traditional cul-de-sacs, which typically have a paved radius of 40 feet or more, are a significant source of pavement and runoff. The use of a landscaped center island is suggested to reduce the amount of impervious cover. Loop roads can also be used to reduce impervious cover. (3)

Requirements for street width and cul-de-sac radii for safe passage of emergency vehicles vary by municipality as they are closely related to the size of vehicles used. How a vehicle will fit within the existing and proposed roadway widths and layout must be considered when purchasing a new emergency or maintenance vehicle.

Common Driveways and Alley Access -- Back to Top
The common driveway concept can be applied to traditional subdivision layouts. Adjoining lots can be configured so garages are located along a common lot line, and one driveway can be constructed to serve both properties. When this concept is applied to an entire subdivision, there can be a significant reduction in the amount of impervious area and runoff. (4)

Many existing higher density residential areas were constructed with alleys for garage access rather than using driveways. A typical residential lot in these areas is much longer than it is wide, with garages located at the rear of the property. By eliminating the long driveway, construction of alleys typically reduced the amount of impervious area. If planned densities in newly developing areas would be high enough to benefit from alley access, it could result in a decrease in runoff.

Conservation Designs -- Back to Top
Conservation design, also known as cluster development or open-space design, is a flexible planning tool that allows communities to meet both their development and conservation goals. It is a development concept that concentrates homes in a small portion of a site in an effort to preserve the existing natural resource areas. The open areas remaining in the development become common areas that can be used for recreational or resource protection purposes. For more information on open-space designs follow the link here.

Phased Development -- Back to Top

Phasing is a technique used to reduce off site-erosion during construction. In phasing construction in new subdivisions, a master development plan is created to show the ultimate parcelization, utility, and roadway layout. The site is then divided into distinct portions and only one portion is cleared at a time, with roadways being constructed only in the current phase of the work. This limits the amount of time soil is exposed and reduces runoff and soil erosion. It is important to note that a site may need to “balance”, which means that any material removed (cut) during grading is used elsewhere on the site (fill). Use of phased development may be challenging if each phase has to balance.

Subsequent construction phases can begin once a milestone, such as a percentage of lots sold or constructed upon, is reached in the previous phase. (5) This approach may add cost due to the need to remobilize the construction equipment several times instead of only once. The effectiveness of phased development depends on the topography and the size of the site being developed.

Source for text in oval:
Storm Center, Fact Sheet, "Better Site Design, Narrower Streets"

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Streets and Roads | Buffers | Detention and Infiltration Basins | Street Trees
Erosion Control & Land Clearing | Illicit Discharges | Impact Fees | Ag. Nonpoint Source Pollution
Subdivision Design | Open-Space Design | Overlay Districts | Stormwater Control

The Milwaukee River Basin Partnership is a voluntary coalition of businesses, non-profit groups, public agencies, educational institutions, organizations, and individuals committed to restoring and sustaining the ecosystem of the Milwaukee River Basin while ensuring its economic viability. To learn more visit,
Photos by Jim Ritchie, Wisconsin DNR, Dave Schilling, Southeast Wisconsin Regional Planning Commission

This page was created on March 26, 2003.
This page was last updated on August 26, 2003.