Dry Detention Basins 

Description

Dry detention ponds (also called dry ponds, extended detention basins, detention ponds and extended detention ponds) are basins that detain stormwater for some minimum time (e.g., 24 hours) to allow particles and pollutants to settle and reduce peak flow rates. They do not have large permanent pools of water—unlike wet ponds—though they often have small pools at the inlet and outlet of the basin. Although dry detention ponds were once popular for flood control, they are less so now, given their limited ability to provide water quality treatment.

Applicability

Dry detention ponds have traditionally been one of the most widely used stormwater controls. They are appropriate for detaining stormwater from large drainage areas (typically 10 or more acres). They require a large area to construct, so other stormwater controls are more appropriate for smaller sites (see Grassed Swales, Infiltration Basin, Infiltration Trench, Bioretention (Rain Gardens), Permeable Pavements, or Green Roofs). If pollutant removal efficiency is an important consideration, dry detention ponds may not be the most appropriate choice. 

• Regional Applicability
• Stormwater Hot Spots
• Stormwater Retrofit
• Cold Water (Trout) Streams

Siting Considerations

Designers need to ensure that the dry detention pond is feasible at the site in question. This section provides basic guidelines for siting dry detention ponds

• Drainage Area
• Slope
• Soils
• Standing Water

Design Considerations

Specific designs may vary considerably, depending on site constraints, preferences of the designer or community, or local regulations. Common recommended features fall into five basic categories: pretreatment, treatment, conveyance, maintenance reduction and landscaping. For any project, design engineers should follow local requirements.

• Pretreatment
• Treatment
• Conveyance
• Maintenance Reduction
• Landscaping
• Storage Pipes and Tanks
• Arid or Semiarid Climates
• Cold Climates

Maintenance Considerations and Best Management Practices (BMP's)

In addition to incorporating features into the dry detention pond design to minimize maintenance, site operators will need to carry out some regular maintenance and inspection practices. Table 1 outlines some of these practices.

Grassed Swales 

Description

In the context of stormwater controls to improve water quality, a grassed swale is a vegetated, open-channel management practice that treats and reduces stormwater flows for a specified water quality volume. As stormwater flows along these channels, the vegetation slows it down, allowing for sedimentation, soil filtration and/or infiltration into the underlying soils. Variations of the grassed swale include the grassed channel, dry swale, bioswale and wet swale. The specific design features and methods of treatment differ in each of these designs, but all are improvements on the traditional drainage ditch. They incorporate modified geometry and other features for use of the swale as a treatment and conveyance practice. 

Applicability

Planners can use grassed swales in most situations with some restrictions. Swales are linear practices, wellsuited for treating stormwater from highways or residential roads. A swale is also useful as one stormwater control in a series of stormwater controls known as a treatment train: for instance, conveying water to a rain garden and receiving water from filter strips. Furthermore, swales can be integral parts of green infrastructure and better site design approaches1; see the Site Design and Planning Strategies fact sheet for more information. 

• Regional Applicability
• Urban Areas
• Stormwater Hot Spots
• Stormwater Retrofit
• Cold Water (Trout) Streams

Siting Considerations

• Drainage Area
• Slope
• Soils/Topography
• Groundwater

Design Considerations

Designers should consider several features to improve longevity and performance while minimizing maintenance burden. Although there are variations, some considerations are common to all grassed swale designs. An overriding similarity between the types of swales is cross-sectional geometry. Swales often have a trapezoidal or parabolic cross section with relatively flat side slopes (flatter than 3:1), though they can also have rectangular and triangular channels. Flat side slopes increase the wetted perimeter—the length along the edge of the swale cross section where stormwater flowing through the swale contacts the vegetated sides and bottom. This design slows stormwater, and the added contact with vegetation encourages sorption, filtering and infiltration. Flat side slopes also let stormwater entering from the sides of the swale receive some pretreatment along the side slope. In addition to treating stormwater for water quality, it is important that grassed swales convey flows from larger storms safely. Typical designs allow the stormwater from the 2-year storm to flow through the swale without causing erosion. Swales should also have the capacity to convey larger storms (typically a 10-year storm) safely (Philadelphia Water, 2018).  

• Design Variations
  • Grassed Channel
  • Dry Swale
  • Wet Swale
  • Arid Climates

Maintenance Considerations and Best Management Practices (BMP's)

Maintenance of grassed swales mostly involves litter control and maintaining the grass or wetland plant cover. Table 1 lists maintenance activities recommended by typical design documents. Some general recommendations include:

• Not using too much salt or sand around the swale during the winter months.
• Not applying fertilizer (consult a local nursery or botanist if plants aren’t thriving).
• Not piling snow that can crush plants or leach deicing materials into the system.

Rain Gardens 

Description

Bioretention practices, such as rain gardens, are landscaped depressions that treat on-site stormwater discharge from impervious surfaces such as roofs, driveways, sidewalks, parking lots and compacted lawns. They are used to collect stormwater and filter it through a mixture of soil, sand and/or gravel. The designs of bioretention practices mimic volume reduction and pollutant removal mechanisms that work in natural systems. The filtered stormwater soaks into the ground, provides water to plants and can help recharge the local groundwater supply. Through these processes, bioretention practices reduce peak flows within downstream sewer systems and allow pollutant removal through filtration and plant uptake.

Applicability

Bioretention practices are well suited to small sites in urbanized settings and can filter stormwater from small to medium storms. Designers generally bypass stormwater discharges from larger storms past a bioretention practice to a larger stormwater control or the storm drain system.

• Urban Areas
• Stormwater Hot Spots
• Cold Water (Trout) Streams
• Regional Applicability

Siting Considerations

Important site conditions to consider when designing bioretention practices include the size of the drainage area, slopes, soil and subsurface conditions, and the depth of the seasonal high groundwater table. Design engineers can incorporate design features that improve the longevity and performance of the bioretention practice while minimizing maintenance.

• Drainage Area
• Slope/Topography
• Soils
• Groundwater

Design Considerations

Bioretention practice designs can vary considerably, depending on site constraints or preferences of the design engineer or community. Some consistent design features fall into five basic categories described below: pretreatment, treatment, conveyance, maintenance reduction and landscaping.  

• Pretreatment
• Treatment
• Conveyance
• Landscaping
• Design Variations

Limitations

Bioretention practices are not suitable for treating large drainage areas. Surface soil layers can clog over time in areas with excessive sediment loadings. Although bioretention practices typically have small footprints, incorporating them into a parking lot design may reduce the number of parking spaces available if the design did not previously include islands. In addition, bioretention practices should leave space between the system and permanent structures, including buildings (with the exception of the bioretention planter box design variation). Bioretention practices can reduce local flooding but may not provide flood control during extreme storms. They can, however, alleviate the stress on other flood control measures by reducing peak flows and stormwater volumes within their drainage areas. 

Maintenance Considerations and Best Management Practices (BMP's)

Bioretention practices require landscaping maintenance as well as measures to ensure that the practice is functioning properly. Bioretention practices may initially require more labor for maintenance than a traditional landscaped island, but maintenance needs generally decrease over time. If they contain appropriate vegetation, landscaping maintenance may require fewer resources than traditional landscaped islands in parking areas. Table 1 below provides a general overview of the typical maintenance activities, frequency and maintenance notes for bioretention practices. Local stormwater manuals often include specific maintenance considerations.

Stormwater Wetland

Description

Stormwater wetlands (or constructed wetlands) are structural post-construction stormwater controls similar to wet ponds (see the Wet Ponds fact sheet) whose design incorporates shallow zones and vegetation. As stormwater flows through the wetland, it removes pollutants through settling and biological uptake. Wetlands are among the most effective postconstruction stormwater controls in terms of pollutant removal and also offer aesthetic and habitat value. Stormwater wetlands are fundamentally different from natural wetland systems. Engineers design them specifically to treat stormwater, and they typically have less biodiversity than natural wetlands in terms of both plant and animal life. Several variations of stormwater wetlands exist, differing in relative amounts of dry, shallow and deep water zones. Planners should distinguish between using a constructed wetland for stormwater management and diverting stormwater into a natural wetland. They should avoid the latter: altering the hydrology of a natural wetland can in turn alter and, in many cases, degrade the existing system. In most cases, local regulations also prohibit this practice. In all circumstances, communities should protect natural wetlands from the adverse effects of development, including impacts from increased stormwater discharge. This is especially important because natural wetlands provide stormwater and flood control benefits on a regional scale. 

Applicability

Constructed wetlands are widely applicable. They can have limited applicability in highly urbanized settings and arid climates, but they have few other restrictions.  

• Regional Applicability
• Urban Areas
• Stormwater Hot Spots
• Stormwater Retrofit
• Cold Water (Trout) Streams

Siting Considerations

In addition to the broad applicability concerns described above, designers need to consider site-specific conditions such as drainage area, slope, soils/topography and groundwater and incorporate design features that improve the longevity and performance of the practice while minimizing maintenance needs. 

• Drainage Area
• Slope
• Soils
• Groundwater

Design Considerations

Specific designs may vary considerably, depending on site constraints or the preferences of the designer or community. Most constructed wetlands, however, should incorporate certain design features. These fall into five basic categories: pretreatment, treatment, conveyance, maintenance reduction and landscaping. 

• Pretreatment
• Treatment
• Conveyance
• Maintenance
• Landscaping

Design Variations

Wetland designs can vary in terms of volume of the wetland in the deep pool, high marsh and low marsh, and in whether the design allows for detention of small storms above the wetland surface. Other design variations help to make wetland designs practical in cold climates. 

• Shallow Wetland
• Extended Detention Wetland
• Pond/Wetland System
• Pocket Wetland
• Subsurface Flow Wetlands
• Water Reuse Wetland

Regional Variations

• Cold Climates
• Karst Topography

Maintenance Considerations and Best Management Practices (BMP's)

Though design features can minimize their maintenance needs, wetlands still need regular maintenance and inspection. Table 1 outlines these practices.  

Additional Methods of Best Management Practices (BMP's), Infiltration, Filtration and Detention

• Infiltration Basin
• On-Lot Treatment
• Permeable Pavement
• Stormwater Inlet Controls
• Vegetated Filter Strips
• Wet Ponds

Green Parking 

Description

“Green parking” refers to several techniques that, together, reduce stormwater discharge from parking lots. Green parking techniques include setting the maximum number of parking spaces, right-sizing the dimensions of parking spaces, substituting alternative surfaces for asphalt in overflow parking areas, using green infrastructure to treat stormwater, encouraging shared parking and providing economic incentives for structured parking.

Applicability

All green parking practices in the above description are applicable to new developments, and some are applicable to redevelopment projects, depending on site characteristics. In urban areas, practices such as encouraging shared parking and providing economic incentives for structured parking are practical and often necessary. Commercial areas can have excessively high parking ratios (the number of spaces per building area), providing an opportunity to convert impervious surfaces with infrequent use to permeable surfaces.

Implementation

The most straightforward green parking strategy is to right-size the number of parking spaces, ensuring that there are enough spaces for the intended uses. Parking lots typically have far more spaces than necessary. By right-sizing parking, planners, developers and the community can ensure that there is enough parking without creating unnecessary impervious surface. The problem of too much parking results from designing parking ratios around the highest hourly parking needs during peak seasons—a common practice in parking lot design. Designing to average demand, as opposed to the peak demand, results in fewer spaces and less impervious space. For existing developments, design engineers can reduce the number of parking spaces and convert unused spaces to landscaped islands that provide aesthetic benefits and reduce impervious area. Table 1 provides examples of conventional parking requirements and compares them to average parking demand. 

Minimizing the length and width of individual parking spaces is another technique that can minimize impervious area. Planners often cite large sport utility vehicles as barriers to minimization techniques. One option to address this problem is to provide a mix of parking space sizes and designate smaller spaces for compact cars. New technologies also allow for the use of intelligent parking reservation systems that can assign a driver to a parking space according to their vehicle size (Caicedo et al., 2012). Another effective green parking technique is the use of alternative surfaces. Alternative surfaces include permeable surfaces and reduce stormwater discharges by increasing infiltration. They can include gravel, cobbles, wood mulch, brick, grass pavers, turf blocks, natural stone and permeable pavements. Permeable pavements include permeable pavers, pervious concrete and porous asphalt and can be effective substitutes for conventional asphalt and concrete, given their durability. For more information on permeable pavements, refer to the Permeable Pavements fact sheet. Green infrastructure practices such as bioretention practices and grassed swales are other green parking techniques that can effectively treat stormwater before it leaves a parking lot. 

• Bioretention practices are shallow, landscaped areas that temporarily store stormwater. Stored water then filters down through the bed of the system, where it either infiltrates into the subsurface soils or is collected by an underdrain pipe for discharge into a storm sewer system or another stormwater facility. For redevelopment projects, design engineers can convert underutilized parking spaces to bioretention practices.
• Grassed swales are vegetated conveyances that slow stormwater flow, allowing solids to settle. Depending on site conditions and design type, grassed swales can also promote infiltration.

Design engineers can integrate both bioretention and grassed swale stormwater controls into parking lot landscaped areas and maintain them along with other landscaped areas. In mixed-use areas, shared and structured parking can reduce the conversion of land to impervious cover.  

• A shared parking arrangement involves two parties that share one lot. For example, an office that experiences peak demand during weekdays can share its parking lot with an adjacent church that experiences peak demand during weekends and evenings.
• Structured parking, such as above- or below-ground parking garages, can greatly reduce the amount of stormwater-generating area for a given parking demand.

Limitations

Limitations to green parking techniques include applicability, cost and maintenance. For example, shared parking is practical only in mixed-use areas, and the cost of land versus the cost of construction may limit structured parking. The cost of individual green infrastructure practices may also be prohibitive in some cases. Permeable pavements, bioretention practices and grassed swales can be more costly than traditional development— though it is important to take into account the cost savings that can be achieved by reduced stormwater management requirements. The pressure to provide an excessive number of parking spaces can result from the fear of customer complaints about limited parking. These factors can pressure developers into constructing more parking than is necessary. Together, these barriers inhibit the construction of parking lots using the maximum number of green parking techniques.  

Green Roofs 

Description

Green roofs are a green infrastructure alternative to conventional roofs that reduce stormwater discharge and provide a wide range of additional environmental and aesthetic benefits. Through integrative design approaches, they offer opportunities to maximize the beneficial use of spaces traditionally unused for stormwater management. In contrast to traditional asphalt shingles or metal roofing, green roofs absorb, store, infiltrate, and evapotranspire stormwater. They also serve as thermal buffers for the building’s underneath, cooling the buildings during warm weather and insulating them during cold weather. As greenspaces that are often within highly developed landscapes, green roofs can provide habitat for wildlife such as birds and insects and offer aesthetic amenities to building occupants. If communities implement green roofs widely, the localized benefits of green roofs can add up in important and measurable ways. By reducing stormwater discharges, green roofs also reduce impacts to local waterways by reducing stream scouring, lowering water temperatures and improving water quality. Widespread implementation can also reduce combined sewer overflows (CSOs) in areas with combined sewer systems, potentially preventing the discharge of millions of gallons of sewage into local waterways. Through better thermal regulation, green roofs may not only reduce urban heat island effects, they may also increase the energy efficiency of buildings. This reduces heating and cooling energy use, thus helping to reduce greenhouse gas (GHG) emissions.

Applicability

Design engineers can apply green roofs to new construction or retrofit them onto existing residential, commercial and industrial buildings. Many cities, such as Chicago and the District of Columbia, actively encourage green roof construction to reduce stormwater discharges and CSOs. Other municipalities encourage green roof development with tax credits, density credits or grants. In addition, green roofs can often provide several points toward a Leadership in Energy and Environmental Design (LEED) certification.

• Regional Applicability
• Urban Areas
• Stormwater Retrofit

Siting and Design Considerations

• Siting Considerations
• Design Considerations
• Design Variations

Limitations

 In most climates, green roofs should include drought tolerant plant species. In semiarid and arid climates, it can be a challenge to keep plants alive in a green roof shallower than 4 inches (Tolderlund, 2010). Therefore, developers typically prefer semi-intensive or intensive green roof systems in these climates. In arid regions, supplemental irrigation is sometimes a necessity. Structurally, the roof slope and the load-bearing capacity of the building may limit green roof design. Roof slope should not be too steep, as steeper slopes can promote overland flow, uneven drainage or rapid drying of uphill portions. Although sources often cite 30 degrees as a maximum slope, design engineers should exercise caution when considering any slopes that are not flat. In new construction, engineers should design buildings to manage the increased weight associated with a saturated green roof. When designing green roofs for existing structures, engineers should take the load restrictions of the building into account. Green roofs can also entail greater capital costs than conventional alternatives. In recognition of this possible barrier to adoption, a number of large cities have some type of incentive program to reduce upfront costs. 

Maintenance Considerations

Immediately after construction, property owners need to regularly monitor green roofs to ensure that vegetation is healthy. During the first season, owners may need to water green roofs periodically if precipitation is insufficient. After the first season, property owners may only need to inspect and lightly fertilize extensive green roofs approximately once per year. Property owners need to maintain intensive green roofs like any other landscaped area. Maintenance may involve gardening and irrigation in addition to general roof maintenance. Green roofs are less prone to leaking than conventional roofs. In most cases, detecting and fixing a leak under a green roof is no more difficult than doing the same for a conventional roof. Still, a qualified professional should use proper construction techniques and conduct leak testing before planting occurs. Many green roof guidance documents—including this General Services Administration report—provide helpful descriptions of leak detection methods, including flood tests and low voltage leak detection. 

Protection of Natural Resources

Description

Undeveloped sites can have numerous natural features that provide environmental, aesthetic and recreational benefits if preserved and protected from the impacts of construction and development. These features include wetlands, riparian areas, aquifer recharge areas, mature trees, woodlands and other wildlife habitat. Site designs should also protect restricted areas such as floodplains and steep slopes. Natural area protection can also be important on properties undergoing redevelopment. They might have attractive open space, well-drained soils or riparian areas that design engineers should identify and consider for preservation early in the planning process. Design engineers, construction staff, and municipalities can protect natural features and open space—both during development and after a site is in use—through a combination of site planning techniques, construction site stormwater controls and post-construction stormwater controls. 

Implementation

Site Planning Techniques

Developments can incorporate existing environmental features into a site’s design and market them as amenities. This can be accomplished by delineating a “development envelope” for buildings and infrastructure that keeps them from affecting natural features. Current design manuals for cities like Portland, Seattle and Philadelphia (City of Portland, 2016; PWD, 2018; SPU, 2017) build on this concept through incorporation of other smart growth practices, which provide financial and planning incentives to combat urban sprawl, promote compact development and conserve natural lands. The first step in urban conservation is to assemble background information by: 

• Determining the local context (urban, agricultural, forested, etc.)
• Mapping significant features as candidate conservation areas, including floodplains, slopes, soils, wildlife habitats, woodlands, farmland, historical/cultural sites, views, aquifer recharge areas and others.
• Ranking conservation areas based on how special, unique, irreplaceable, environmentally valuable, historic or scenic they are.
• Identifying areas to place buildings and infrastructure so as to minimally impact conservation areas.
• Establishing the layout of buildings and infrastructure, using techniques such as clustering buildings and designing smaller lots, shared driveways and narrower streets.

Site evaluation and design can enable the preservation of significant features while maintaining the desired overall site density (although density in localized parts of the development will be higher when open space is set aside). There can be some negative perceptions associated with high-density residential areas. Developers want to achieve a particular development density when building subdivisions or commercial sites. Also, for residential developments, lot size is an important factor in determining lot prices. Setting aside natural areas can take space away from yards, parking, transportation infrastructure and other built features. Developers can accommodate overall site density using clustering techniques, smaller lots, density increases and more efficient street layouts. To offset lost premiums from smaller individual lots, developers can market a lot’s proximity to natural areas and attractive views as amenities. Also, local zoning codes might restrict the use of clustering, reduced road widths and other techniques for natural area preservation. Developers should work with local regulatory agencies to determine whether they can obtain waivers to protect natural features. 

Stormwater Controls During Construction

Design engineers and construction staff need to take extra care during site preparation to protect environmentally significant areas of the property (see Preservation of Natural or Existing Vegetation). They should indicate a limit of disturbance and the locations of protected areas in design drawings, stormwater pollution prevention plans and on-site maps. They should also post signs with prohibitions and educate workers about the importance of and special considerations for the protected areas. Without training and explicit signage, vehicle traffic, stored waste and materials, and other construction-related activities could damage areas slated for protection. Construction staff should check areas regularly to identify problems and determine if additional controls are needed (e.g., more training, explicit signage, obvious barriers). Operators should also look for signs of unintended consequences of construction activities on the natural areas (e.g., changes in hydrology, flooding, accidental spills) and take appropriate actions to mitigate the damage. Developers and construction site operators can employ the following specific practices to protect each type of resource:

• Mature trees or woodlands
• Steep slopes
• Well-drained soils and aquifer recharge areas
• Wetlands and riparian areas
• Wildlife habitat 
• The presence of threatened or endangered species or habitats critical to their survival on the site might require a consultation with the U.S. Fish and Wildlife Service or the National Oceanic and Atmospheric Administration’s (NOAA’s) National Marine Fisheries Service. Ensure compliance with all regulations and any state or local permit requirements.
• Floodplains

After Site Development

After development, natural areas become amenities for the site’s occupants (e.g., property owners or commercial tenants). These natural areas also become the responsibility of the owner or occupant. Developers should inform the occupant about each natural area or protected feature’s importance and outline activities that the occupant should prohibit to adequately protect the resource. Developers should also provide guidance to occupants on how to maintain these areas. For example, occupants should not mow a preserved prairie or riparian stream buffer or manicure it like turf. Property owners or maintenance crews should employ special procedures to preserve native species, such as integrated pest management practices like hand-weeding and limiting chemical use. They should use the same practices in areas where traditional landscape maintenance activities could threaten water quality, such as in or adjacent to wetlands and riparian areas or where endangered species are present. Property owners can post interpretive signage to educate occupants and visitors about the significance of the features, as well as to describe prohibited activities such as mowing, dumping and vehicle traffic. They can install barriers to protect natural areas from damage without detracting from their aesthetics and function. These barriers can include strategic placement of low fences, walls, bollards or large rocks that unobtrusively limit access to the areas.

Using Conservation Easements

Developers can use conservation easements to maintain open space over the long term. For example, the Minnesota Land Trust implements “subdivision conservation,” protecting thousands of acres and hundreds of shoreline miles along various lakes. This approach involves compacting development areas and preserving part of the development area as natural land. For example, the Fields of St. Croix residential development in Lake Elmo, Minnesota, permanently protected 60 percent of the development’s 226 acres through a conservation easement granted to the Minnesota Land Trust (Anderson, 2014).