4.6.5 Infeasibility Criteria

The following criteria, borrowed from the 2012 SWMMWW, describe conditions that make permeable pavement infeasible. If a project triggers any of the below-listed infeasibility criteria, yet the proponent wishes to use a permeable pavement BMP, they may propose a functional design that effectively mitigates the infeasibility issues to the local government.

These criteria also apply to impervious pavements that would employ stormwater collection from the surface of impervious pavement with redistribution below the pavement.

The following infeasibility criteria should be evaluated based on 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 whose ground water drains into an erosion hazard, or landslide hazard area.
  • Where infiltrating and ponded water below new permeable pavement area would compromise adjacent impervious pavements.
  • Where infiltrating water below a new permeable pavement area would threaten existing below grade basements.
  • Where infiltrating water would threaten shoreline structures such as bulkheads.
  • Down slope of steep, erosion prone areas that are likely to deliver sediment.
  • Where fill soils are used that can become unstable when saturated.
  • Excessively steep slopes where water within the aggregate base layer or at the sub-grade surface cannot be controlled by detention structures and may cause erosion and structural failure, or where surface runoff velocities may preclude adequate infiltration at the pavement surface.
  • Where permeable pavements can not provide sufficient strength to support heavy loads at industrial facilities such as ports.
  • Where installation of permeable pavement would threaten the safety or reliability of pre-existing underground utilities, pre-existing underground storage tanks, or pre-existing road subgrades.
  • Within an area designated as an erosion hazard, or landslide hazard.
  • Within 50 feet from the top of slopes that are greater than 20 percent.
  • 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 infiltration area.
    • 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 spring used for drinking water supply, if the pavement is a pollution-generating surface.
  • Within 10 feet of a 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 any underground storage tank and connecting underground pipes, regardless of tank size. 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 percent or more of the storage volume (including volume in the connecting piping system) is beneath the ground surface.
  • At multi-level parking garages, and over culverts and bridges.
  • Where the site design cannot avoid putting pavement in areas likely to have long-term excessive sediment deposition after construction (e.g., construction and landscaping material yards).
  • Where the site cannot reasonably be designed to have a porous asphalt surface at less than 5 percent slope, or a pervious concrete surface at less than 12 percent slope, or a permeable interlocking concrete pavement surface (where appropriate) at less than 12 percent slope. Grid systems upper slope limit can range from 6 to 12 percent; check with manufacturer and local supplier.
  • Where the native soils below a pollution-generating permeable pavement (e.g., road or parking lot) do not meet the soil suitability criteria (SSC) for providing treatment. See SSC-6 in Section 5.4.3 of the 2004 SWMMEW. Note: In these instances, the local government has the option of requiring a 6-inch layer of media meeting the soil suitability criteria or the sand filter specification as a condition of construction.
  • Where seasonal high ground water or an underlying impermeable/low permeable layer would create saturated conditions within one foot of the bottom of the lowest gravel base course.
  • Where underlying soils are unsuitable for supporting traffic loads when saturated. Soils meeting a California Bearing Ratio of 5 percent are considered suitable for residential access roads.
  • Where appropriate field testing indicates soils have a measured (e.g., initial) native soil saturated hydraulic conductivity less than 0.3 inches per hour (Note: In these instances, unless other infeasibility restrictions apply, roads and parking lots may be built with an underdrain, preferably elevated within the base course, if flow control benefits are desired.)
  • Where the road type is classified as arterial or collector rather than access. See RCW 35.78.010, RCW 36.86.070, and RCW 47.05.021. Note: This infeasibility criterion does not extend to sidewalks and other non-traffic bearing surfaces associated with the collector or arterial.
  • Where replacing existing impervious surfaces unless the existing surface is a non-pollution generating surface over an outwash soil with a saturated hydraulic conductivity of 4-inches per hour or greater.
  • At sites defined as “high use sites” in Section 2.2.5 of the 2004 SWMMEW.
  • In areas with “industrial activity” as identified in 40 CFR 122.26(b)(14).
  • Where the risk of concentrated pollutant spills is more likely such as gas stations, truck stops, and industrial chemical storage sites.
  • Where routine, heavy applications of sand occur in frequent snow zones to maintain traction during weeks of snow and ice accumulation.

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