Collect and Detain runoff

• Landscape
  • Bioswales
  An open drainage channel that has been explicitly designed to detain or infiltrate the entire runoff volume associated with a water quality storm event. (Metro Seattle, 1992)
  • Checkdams/ small basins
  Checkdams can be used to detain runoff during storm events within a bioswale or other surface detention/infiltration systems (Tom Richman & Associates, 1997. pp. 56).
  • Concave lawn
  According to the San Francisco stormwater document, a square lawn area 50 feet on a side, sloping 2% towards the center will create a low point 6 inches below the outside rim. Surrounding surfaces can sheet flow into this area and water will collect during storm events. In low infiltrating soils this strategy will require an overflow mechanism, but still can provide some infiltration, especially if the soil beneath the lawn is amended with compost, and will contribute to detention of stormwater (Tom Richman & Associates, 1997. pp. 54).
  • Concave planting areas
  The CUWRM document recommends directing stormwater to vegetated areas for detention and infiltration (Konrad, et al. 1995. pp. 16). This strategy is also promoted by the San Francisco stormwater document (Tom Richman & Associates, 1997. pp. 54)
  • Replace lawn & impervious w/ plantings and lawn alternatives (i.e. meadow, forest community, and native associations)
  (See Increase Infiltration/Vegetation entry)
  • Preserve existing native vegetation
  Natural soils and existing vegetation are highly effective sources for stormwater infiltration (Konrad, et al. 1995. pp. 7).
  • Terracing sloped yards
  Sloping yards may be graded, including the use of rockeries, to create flat areas that reduce stormwater runoff. This strategy should not be used where existing native vegetation already exists. It is an approach recommended in the CUWRM document and depending upon the steepness of the slope it may require the services of an engineer (Konrad, et al. 1995. F-4).
  • Detention pond
  Detention ponds can be designed and constructed at a variety of scales. The CUWRM document suggest that they can fit into a residential site in the form of a pond, lined with plastic and soil and planted with native wetland plants (Konrad, et al. 1995. C-2). Guidelines for detention systems can be located in a variety of sources. Schueler’s book, Design of stormwater wetland systems : guidelines for creating diverse and effective stormwater wetlands in the mid-Atlantic Region, although not written for the Pacific Northwest, is a good starting point.
• Roof
  • Cistern from downspout
  An above-ground strategy, a cistern or "rain barrel" can be tight-lined into the roof downspout, providing storage and slow release of roof runoff. The cistern can be fitted with an operable valve, so that water can be saved for later use, such as irrigation. This strategy is proposed in both San Francisco publication and the CUWRM document, where it is noted that they are typically less expensive to install than underground systems and leaks are easier to detect (Tom Richman & Associates, 1997. pp. 52) (Konrad, et al. 1995. C-1). Factors to consider in the design and implementation of above ground tanks are safe overflow mechanisms and structural support of the system. This strategy exhibits high implementability for residential site scale use because of its potentially low cost, the availability of pre-fabricated products, ease of maintenance, and service provided to the homeowner, i.e. summer irrigation savings.
  • Drywell, from downspout
  (See Walks and Driveways entry for drywells and infiltration tanks)
  • Green roofs
  A lightweight roof system utilizing a waterproofing material covered with a thin layer of soil and vegetation. Because of its lightweight nature, the green roof strategy has potential for retrofit projects. According to a draft of the City of Seattle’s Flow Control Manual, a green roof can capture and evapotranspire 10-100% of precipitation, depending on the season.
  • Infiltration Trench
  A deep trench lined with filter fabric and back-filled with washed rock. Downspouts are tight-lined to a perforated pipe that runs through the trench. The trench is then protected by the fabric filter and covered with a layer of soil. A simplified version of this strategy, the dispersion trench, is similar yet it does not utilize a filter fabric and is not covered with a soil layer. Both strategies are recommended in the CUWRM document (Konrad, et al. 1995). Infiltration and dispersion trenches have high potential for implementation at the residential site scale because of their relative simplicity, effectiveness detaining runoff, do it yourself implementability, and low cost.
  • Planting at dripline
  Typically used for buildings with no gutter system, planting at the base of building eaves can provide increased infiltration and protect from erosion. It is important to select plants able to tolerate heavy runoff sheet flows and periodic soil saturation. This strategy is proposed in San Francisco stormwater document (Tom Richman & Associates, 1997. pp. 53).
  • Roof garden
  A heavy weight roof system comprised of an waterproof underlayer with a thick soil/vegetation protective cover. The roof garden can replace a traditional roof, but because of its weight it does not lend itself to retrofit projects. According to the City of Seattle’s draft Flow Control Manual, roof gardens can capture and evapotranspire between 50-100% of precipitation. A roof garden can be combined with paved terrace to allow for human enjoyment of the rooftop.
• Parking Alternatives

• Street
  • Utilize street hierarchy
  In new development settings this strategy can be implemented to reduce impervious cover by sizing streets according to their intended usage. This approach is discussed in the San Francisco stormwater document (Tom Richman & Associates, 1997. pp. 38).
  • Street hybrid w/ gravel for parking edge
  Another strategy for consideration with new development, however potentially applicable to retrofitting, it employs narrow driving lanes with gravel shoulders for parking (Tom Richman & Associates, 1997. pp. 41). The SEA-Street model should be further investigated, as it is a retrofit that reduces lane width, adding angled parking and infiltration planting strips to reduce stormwater, as well as slow traffic and provide aesthetic values to residents.
  • Pervious concrete & porous asphalt
  (See Walks and Driveways entry)
  • Curb/swale system
  A swale inlet replaces the conventional drop inlet, collecting runoff into a surface infiltration system. According to a 1998 study this strategy has shown to provide drainage functions, promote infiltration, trap sediments, and reduce flow velocity along the drainage path (Li, et al. 1998).
  • Cul de sac planting circles
  In new or retrofit projects, the center of a cul de sac can be fit with a central concave landscaped space to reduce impervious surface area and aid in stormwater detention and infiltration. This method is illustrated in the San Franciso stormwater publication (Tom Richman & Associates, 1997. pp. 45). RETURN TO STRATEGIES