4.4.2 Design

Eastern Washington has wide-ranging climate and hydrology. With annual precipitation ranging from 7 inches to more than 75 inches, the character of bioretention facilities will vary considerably throughout the region.

Bioretention systems are placed in a variety of residential and commercial settings and are a visible and accessible component of the site. Design objectives and site context are, therefore, important factors for successful application.

Key design and site suitability principles for eastern Washington bioretention design include:
  • Soils: The bioretention soil media (BSM) and soils underlying and surrounding bioretention facilities are the principal design elements for determining infiltration capacity, sizing, and associated conveyance structures. The BSM placed in the cell or swale is typically composed of a highly permeable sandy mineral aggregate mixed with compost (or other locally available substitutes) and will often have a higher infiltration rate than the surrounding subgrade; however, in some cases (such as outwash soils) the subgrade infiltration rate may be higher. See Section Bioretention components for details.
  • Plantings: In some cases, adapted, drought-tolerant species may be better suited to bioretention facilities than native species. In many areas of eastern Washington, selected plant species will need to be able to tolerate summer drought and low rainfall. Plantings may also need to withstand added stresses associated with snow plowing and snow storage, where applicable. These conditions suggest a minimum plant establishment of two to three years.
  • Site topography: Based on geotechnical concerns, infiltration on slopes greater than 10 percent should only be considered with caution. The site assessment should clearly define any landslide and erosion critical areas and coastal bluffs, and appropriate setbacks required by the local jurisdiction. Thorough geotechnical analysis should be included when considering infiltration within or near slope setbacks. Depending on adjacent infrastructure (e.g., basements and subsurface utilities) and subgrade geology, geotechnical analysis may also be necessary on relatively low gradients. See below for slope setbacks.
  • Depth to hydraulic restriction layer: Separation to a hydraulic restriction layer (rock, compacted soil layer or water table) is an important design consideration for infiltration and flow control performance. Protecting groundwater quality is a critical factor when infiltrating stormwater; however, when determining depth to the water table for bioretention facilities, the primary concern for Ecology is infiltration capacity (as influenced by ground water mounding) and associated flow control performance. When properly designed and constructed, the BSM will provide very good water quality treatment before infiltrated stormwater reaches the subgrade and then groundwater (see Section Bioretention components for recommended BSM depth). The following are recommended minimum separations to groundwater:
    • A minimum separation of 1 foot from the hydraulic restriction layer to the bottom of the bioretention area is recommended where the contributing area has less than 5,000 square feet of pollution-generating impervious surface; and less than 10,000 square feet of impervious surface; and less than ¾-acre of lawn, landscape, and other pervious surface.
    • A minimum separation of 3 feet from the hydraulic restriction layer to the bottom of the bioretention area is recommended where the contributing area is equal to or exceeds any of the following limitations: 5,000 square feet of pollution-generating impervious surface; or 10,000 square feet of impervious surface; or ¾-acre of lawn, landscape, and other pervious surface.
    • Note that recommended separation distances for bioretention areas with small contributing areas are less than the Ecology recommendation of 3-5 feet for conventional infiltration facilities for two reasons: 1) bioretention soil media provides effective pollutant capture; and 2) hydrologic loading and potential for groundwater mounding is reduced when flows are directed to bioretention facilities from smaller contributing areas.
  • Utilities: Consult local jurisdiction requirements for horizontal and vertical separations required for publicly owned utilities, such as water, sewer, and stormwater pipes. Consult the appropriate franchise utility owners for utility separation requirements, which may include communications and/or gas. See Figure 4.4.5 for an example design detail illustrating vertical and horizontal separation requirements for roadway bioretention. Extensive potholing (or excavation to daylight and document utilities) may be needed during project planning and design to develop a complete understanding of the type, location, and construction of all utilities that may be impacted by the project. When applicable separation requirements cannot be met, designs should include appropriate mitigation measures, such as impermeable liners over the utility, sleeving utilities, fixing known leaky joints or cracked conduits, and/or adding an under-drain to the bioretention areas to minimize the amount of infiltrated stormwater that could enter the utility.
  • Setbacks: Consult local jurisdiction guidelines for appropriate bioretention area setbacks from wellheads, on-site sewage systems, basements,foundations, utilities, slopes, contaminated areas, and property lines. General recommendations for setbacks include:
    • Within 50 feet from the top of slopes that are greater than 20 percent.
    • Within 100 feet of an area known to have deep soil contamination.
    • Within 100 feet of a closed or active landfill.
    • Within 100 feet of a drinking water well or aspring used for drinking water supply.
    • Within 10 feet of small on-site sewage disposal drain field (including reserve area) and grey water reuse systems. For setbacks from a “large on-site sewage disposal system,” see Chapter 246-272B WAC.
    • Note: Setback distances are measured from the bottom edge of the bioretention soil mix (e.g., intersection of the bottom and side slope of the bioretention area).
  • Expected pollutant loading and soil and effluent quality: Bioretention can provide very good water quality treatment. For heavy pollutant loads associated with industrial settings, an impermeable liner between the BSM and the subgrade and an under-drain may be required due to soil and groundwater contamination concerns. Areas where infiltration is not recommended, a liner and under-drain should be incorporated due to soil contamination concerns, include:
    • For properties with known soil or groundwater contamination (typically federal Superfund sites or cleanup sites under the state Model Toxics Control Act (MTCA)).
    • Where groundwater modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the groundwater.
    • Wherever surface soils have been found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltration area.
    • Any area where these facilities are prohibited by an approved cleanup plan under MTCA or federal Superfund law, or an environmental covenant under Chapter 64.70 RCW.
  • Phosphorus (P) and Nitrogen (N) considerations: For bioretention systems with direct discharge to fresh water, or located on soils adjacent to fresh water that do not meet the soil suitability criteria in Section 5.4.3 of the 2004 SWMMEW. See Section Bioretention components for recommended designs by pollutant types.
  • Transportation safety: The design configuration and selected plant types should provide adequate sight distances, clear zones, and appropriate setbacks for roadway applications in accordance with the local jurisdiction requirements. Bioretention designs that extend the curb line into the roadway (e.g., chicanes and neck-downs) can provide traffic-calming functions and improve vehicle and pedestrian safety.
  • Ponding depth and surface water draw-down: Plant and soil health, flow control needs, water quality treatment performance, location in the development, and mosquito breeding cycles will determine drawdown timing. For example, front yards and entrances to residential or commercial developments may require more rapid surface dewatering than necessary for plant and soil health due to aesthetic needs. See Section Bioretention components, for details.
  • Infiltration capability: See Sections 5.4.3 and 6.3.2 of the 2004 SWMMEW for recommended minimum infiltration rates for runoff treatment and flow control, respectively.
  • Impacts of surrounding activities: Human activity influences the location of the facility in the development. For example, locate bioretention areas away from traveled areas on individual lots to prevent soil compaction and damage to vegetation, or provide elevated or bermed pathways in areas where foot traffic is inevitable (see Section Bioretention components for details) and provide barriers, such as wheel stops, to restrict vehicle access in parking lot applications.
  • Visual buffering: Bioretention areas can be used to buffer structures from roads, enhance privacy among residences, and for an aesthetic site feature.
  • Site growing characteristics and plant selection: Appropriate plants should be selected for sun exposure, soil moisture, and adjacent plant communities. Native species or hardy cultivars are recommended and can flourish in the properly designed and placed BSM with no nutrient or pesticide inputs and 2 to 3 years irrigation for establishment. Manual invasive species control may be necessary. Pesticides or herbicides should never be applied in bioretention areas.
  • Maintenance: see Section 4.4.6: Maintenance and Appendix E for details.

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Figure 4.4.5
Recommended utility setbacks. Source: AHBL , Inc. and HDR Engineering, courtesy of Low Impact Development Technical Guidance Manual for Puget Sound (2012)