Decisions about lot layout, building density, location of public rights-of-way, protection of sensitive areas, and preservation of open space all have an impact on the quality and quantity of stormwater runoff. Good stormwater management does not begin with site disturbance and construction. To encourage early consideration of these issues, the Dane County Erosion Control and Stormwater Management Ordinance includes a voluntary Preliminary Review consultation that takes place before land is platted and the final design is set.

When using site-planning techniques to control stormwater, designers should keep local zoning, land division and building codes in mind. Many communities have adopted site design or land division criteria to serve a variety of land use goals that may or may not directly relate to stormwater runoff. Examples include:

• preserving neighborhood or rural character
• protecting specific natural or scenic resources
• promoting smooth traffic flow
• allowing for future land division
• ensuring adequate pedestrian, bicycle or emergency vehicle access

Usually, such goals complement or reinforce good design for stormwater control. However, in some cases, such as choosing between grid-pattern or cul-de-sac street layouts, the designer may need to strike a balance between competing land use goals. For example, in a community seeking to promote traditional neighborhood design, engineered stormwater basins may be preferable to a curvilinear street layout.

Many techniques can be employed during the site planning and design stage of development to reduce the volume of runoff, thus reducing the need for structural practices to store and treat stormwater. Design and location of stable outlets for site runoff is also important to consider at this time, to avoid causing problems for downstream neighbors. Consider implementing the following techniques to minimize the volume of stormwater runoff.

Identify and Avoid Sensitive Areas Local variations in topography, soil types, vegetation and hydrology can have a significant influence on the nature and amount of stormwater runoff. The first step in site planning for stormwater management should be identification and mapping of areas that:

• contain features that could be adversely impacted by stormwater runoff (such as wetlands, floodplains, lakes, streams, and shallow fractured bedrock);
• in their natural state, contribute to infiltration, soil and water retention, groundwater recharge or temperature control (such as highly pervious soils, native grasslands, woodlands or hydric soils);
• provide natural drainage ways for surface water runoff (such as intermittent or perennial streams, natural or artificial drainage ways); or
• could be a source of sedimentation, channelized flow or erosion if disturbed (such as steep slopes or easily eroded soils).

Development should be designed and construction operations planned to avoid disturbing these areas wherever possible. Federal, state or local regulations protect some natural features, such as wetlands or navigable waterways. Changes in volume and direction of stormwater flow resulting from development or other stormwater practices should be carefully designed and controlled to avoid secondary impacts to natural areas. For example, increased runoff volume can erode streambeds and banks or damage natural wetlands without careful consideration early in the planning process.

Working around sensitive areas should be incorporated as part of the preliminary design, which not only avoids these areas but also highlights them as natural amenities that add value to the development. These sensitive areas complement the functions and values provided by the countywide network of open space corridors.

Minimize Impervious Surfaces Imperviousness is the primary source of runoff. Hence, the single most effective means of reducing runoff volume is by minimizing the site’s impervious surface area.

Streets and Roads

• Road length Minimize subdivision roadway length by using a roadway layout with the least pavement length suitable for the site’s topography and other planning goals.
• Road width Work within local zoning requirements and planned unit development provisions to minimize road width by narrowing road sections and/or reducing on-street parking. On-street parking may be restricted to one side of the street or eliminated altogether. Pavement and right-of-way width must still meet minimum standards described in local land division and zoning ordinances, and should allow for safe vehicular travel and emergency vehicle access.
• Design road patterns to match landforms In rolling terrain, for example, local streets should branch from collector streets and end in short loops or cul-de-sacs, where consistent with other local ordinances and land use goals. Some local ordinances and plans seek to create traditional grid

Lot Layout

• Rooftops Reduce the impervious rooftop area by minimizing the building footprint of houses or utilizing green roof technology. Use vertical space rather than horizontal house layouts.
• Driveways Where permitted under local driveway, zoning or land division ordinances, reduce impervious driveway area by using shared driveways, limiting driveway width, using pervious pavement, and using reduced building setbacks.
• Parking lots For commercial sites, reduce overall impervious area by providing compact car spaces, eliminating excessive or unnecessary spaces, utilizing shared parking, minimizing stall dimensions, incorporating efficient parking lands, and using pervious materials in spillover parking areas.

Site and Lot Vegetation

• Pre-development vegetation. Maintain as much predevelopment vegetation as possible. Vegetation prevents erosion and absorbs water and, therefore, reduces runoff volume.
• Swales. Use shallow grassed roadside swales, boulevards and sunken parking lot islands with check dams instead of curb and gutter storm drain systems to handle runoff, wherever possible.
• Natural buffers and drainage ways. Maintain natural buffers between development sites and water bodies. Buffers slow runoff, remove sediment and enhance infiltration. Natural depressions and channels should be maintained to slow, store, and infiltrate water.

Preserve and Reproduce Pre-Development Hydrologic Conditions *Utilize natural drainage flow paths. Dane County strongly recommends the use of grass waterways, vegetated drainage channels and/or water quality swales along street right-of-ways or back lots to channel runoff without abrupt changes in the direction of flow. *Restore soil permeability. Use practices such as deep tilling, chisel plowing and incorporating organic matter into the upper soil layer to restore soil infiltration capacity on heavily disturbed sites. When soil is compacted its capacity to infiltrate water is greatly diminished. On heavily disturbed sites where practices are used to restore soil permeability, the county may waive the requirement to lower the soil permeability class rating in hydrologic calculations. Compaction mitigation is required when disturbed green space is included in recharge calculations. *Minimize directly connected impervious area. Downspouts and driveways should be directed to pervious areas, where feasible. This reduces the directly connected impervious area, promotes infiltration and reduces the velocity of runoff water. Other strategies for minimizing connected impervious area include directing sheet flow through vegetated areas and locating impervious areas so they drain to vegetated buffers or other pervious areas. *Use bioretention and other practices to increase infiltration. Bioretention devices are engineered practices that use natural processes, including microbial soil processes, infiltration, and evapotranspiration to improve stormwater quality. Rain gardens, often very attractive, are one type of practice commonly designed for residential lots to soak up rainwater from roofs, driveways, and lawns. Rock trenches or rock beds can also be used as a conduit to more permeable layers with higher percolation rates.

The St. Francis Addition is a 72-acre, 80-lot subdivision in the Village of Cross Plains that demonstrates good site planning. Brewery Creek, a cold-water community and a tributary to the cold-water community of Black Earth Creek, runs through the site. During the site planning process, hydric soils to be avoided were mapped, as well as permeable soils where infiltration practices could be located. A soil permeability analysis was prepared for the preliminary plat, identifying the ideal locations for infiltration practices. Following the site analysis, a site plan was prepared which incorporated the following practices:

• naturally vegetated buffer to Brewery Creek
• protected existing wooded areas
• deep tilling to increase infiltration and reduce other effects of soil compaction
• stormwater storage and infiltration swales behind all house sites
• stormwater storage and infiltration swales within cul-de-sacs and boulevards
• narrower street widths (16’ boulevard lanes; 28’ street widths wherever possible)

A comprehensive study was designed to evaluate both the physical and biological in-stream response resulting from this development. The results of this study concluded that the erosion control and stormwater management plans that were designed and implemented by the developer of the subdivision were effective in mitigating the harmful effects of runoff to Brewery Creek. The complete USGS publication can be viewed here.

Many of the practices and techniques discussed above are commonly referred to as “conservation design” or “low-impact design.” Figure 2 compares a conventional subdivision layout with a subdivision incorporating conservation design practices. During plan review, conservation design and other practices will be evaluated to ensure that the plan meets or exceeds ordinance requirements. Less imperviousness results in less runoff to treat or store, resulting in smaller structural practices needed to comply with requirements, resulting in lower development costs (related to structure size). Reducing peak rate and runoff volume also means less land will need to be set aside for peak rate and volume control which may result in less engineering design, and may contribute to lower development costs.

Conventional subdivision layout (left) and conservation subdivision layout (right)

Dane County encourages conservation design by providing incentives in the ordinance for features commonly associated with conservation or low impact practices. When developers incorporate such practices, they reduce the runoff that needs to be treated by other engineered practices. Limiting the disturbed area may reduce construction costs and minimize the need for erosion control practices.

Dane County’s ordinance fee structure also provides an incentive for reducing imperviousness and minimizing land disturbance. Permit fees are calculated based on amount of disturbed area and amount of impervious area.

Dane County’s Erosion Control and Stormwater Management Ordinance does not prohibit regional treatment for stormwater management. A regional stormwater treatment facility can increase efficiency of treatment and ease maintenance. These facilities improve sediment trapping with a larger pool of water in the detention pond, and avoid problems that can arise from many smaller on-site treatment facilities all releasing water at the same time. Regional facilities, however, are not adequate for meeting the soil loss standard during land disturbance and may not be the best strategy for stormwater infiltration.

Cities and villages in Dane County that have adopted regional stormwater plans may want to establish a “fee-in-lieu” program and identify its requirements in their ordinances. Fee-in-lieu programs allow developers to pay a fee rather than install on-site control measures where these may not be desirable. The fee is put into a dedicated fund to recoup the costs for construction, operation, and maintenance of regional or multipurpose detention facilities.

Fee-in-lieu may not be used as an alternative to meeting the county’s minimum standards.

Dane County’s ordinance allows municipalities that establish a fee-in-lieu program to allow owners of sites served by an off-site stormwater management facility to pay a fee-in-lieu of on-site control. However, the ordinance requires that these municipalities only allow this if the regional facility is in place at the time of land disturbance, is designed and adequately sized to provide a level of stormwater control capable of meeting county standards, and has a legally-obligated entity responsible for its long-term operation and maintenance. Regional facilities must be in place at the time of land disturbance to prevent situations where a landowner pays a fee-in-lieu of on-site control, yet the regional facility is never built, or built after a delay of years, resulting in uncontrolled and untreated stormwater runoff.

The Dane County ordinance, while focusing on plans and practices to meet the erosion control and stormwater needs of particular sites, complements watershed-wide planning. Ideally, stormwater management should be conducted as part of a watershed plan.

In watershed-wide planning, communities can work together across municipal boundaries to identify potential locations for regional stormwater treatment facilities, and coordinate on-site basins and outlets to reduce the effect of combined peak discharges after storm events. They can also collectively identify areas where stormwater detention facilities should not be located, e.g. in hydric or alluvial soils, and target areas where they are preferred, e.g. deep sandy soil. Such a collaborative approach may result in significant cost savings from economies of scale and shared responsibility.