4.4.5 Infeasibility Criteria

The following criteria, adopted from the 2012 Stormwater Management Manual for Western Washington, describe conditions that make bioretention or rain gardens infeasible. If a project triggers any of the below-listed infeasibility criteria, yet the proponent wishes to use a bioretention or rain garden BMP, they may propose a functional design that effectively mitigates the infeasibility issues to the local government.

Note: Criteria with setback distances are as measured from the bottom edge of the bioretention soil mix.

The following infeasibility criteria should be based on an evaluation of site-specific conditions by an appropriate licensed professional (e.g., engineer, geologist, hydrogeologist):
  • Where professional geotechnical evaluation recommends infiltration not be used due to reasonable concerns about erosion, slope failure, or down gradient flooding.
  • Within an area where ground water drains into an erosion hazard, or landslide hazard area.
  • Where the only area available for siting would threaten the safety or reliability of pre-existing underground utilities, pre-existing underground storage tanks, pre-existing structures, or pre-existing road or parking lot surfaces.
  • Where the only area available for siting does not allow for a safe overflow pathway.
  • Where there is a lack of usable space for rain garden/bioretention facilities at re-development sites, or where there is insufficient space within the existing public right-of-way on public road projects.
  • Where infiltrating water would threaten existing below grade basements.
  • Where infiltrating water would threaten shoreline structures.
  • Within setbacks from structures as established by the local government with jurisdiction.
  • Where they are not compatible with surrounding drainage system as determined by the local jurisdiction (e.g., project drains to an existing stormwater collection system where the elevation or location precludes connection to a properly functioning bioretention facility).
  • Where land for bioretention is within area designated as an erosion hazard, or landslide hazard.
  • Where the site cannot be reasonably designed to locate bioretention facilities on slopes less than 8 percent.
  • Within 50 feet from the top of slopes that are greater than 20 percent and over 10 feet of vertical relief.
  • For properties with known soil or ground water contamination (typically federal Superfund sites or state cleanup sites under the Model Toxics Control Act (MTCA):
    • Within 100 feet of an area known to have deep soil contamination.
    • Where ground water modeling indicates infiltration will likely increase or change the direction of the migration of pollutants in the ground water.
    • Wherever surface soils have been found to be contaminated unless those soils are removed within 10 horizontal feet from the infiltrationarea.
    • Any area where these facilities are prohibited by an approved cleanup plan under the state Model Toxics Control Act or federal Superfund Law, or an environmental covenant under Chapter 64.70 RCW.
  • Within 100 feet of a closed or active landfill.
  • Within 100 feet of a drinking water well, or a springused for drinking water supply.
  • Within 10 feet of small on-site sewage disposal drainfield, including reserve areas, and grey water reuse systems. For setbacks from a “large on-site sewage disposal system”, see Chapter 246-272B WAC.
  • Within 10 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is 1,100 gallons or less. (As used in these criteria, an underground storage tank means any tank used to store petroleum products, chemicals, or liquid hazardous wastes of which 10% or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.
  • Within 100 feet of an underground storage tank and connecting underground pipes when the capacity of the tank and pipe system is greater than 1,100 gallons.
  • Where the minimum vertical separation of 1-foot to the seasonal high water table, bedrock, or other impervious layer would not be achieved below bioretention or rain gardens that would serve a drainage area that is: 1) less than 5,000 sq. ft. of pollution-generating impervious surface, and 2) less than 10,000 sq. ft. of impervious surface; and, 3) less than ¾ acres of pervious surface.
  • Where the a minimum vertical separation of 3 feet to the seasonal high water table, bedrock or other impervious layer would not be achieved below bioretention that: 1) would serve a drainage area that meets or exceeds: a) 5,000 square feet of pollution-generating impervious surface, or b) 10,000 square feet of impervious surface, or c) three-quarter (¾) acres of pervious surfaces; and 2) cannot reasonably be broken down into amounts smaller than indicated in (1).
  • Where the field testing indicates potential bioretention/rain garden sites have a measured (e.g., initial) native soil saturated hydraulic conductivity less than 0.30 inches per hour. If the measured native soil infiltration rate is less than 0.30 inches per hour, a bioretention facility with an under-drain may be used to treat pollution-generating surfaces to help meet Core Element #5. If the under-drain is elevated within a base course of gravel, it will also provide some modest flow reduction benefit that will help achieve Core Element #6.


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