H.1 Onsite Requirement Infeasibility Criteria Checklists for All Dispersion BMPs

Infeasibility Criteria for All Dispersion BMPs

• A licensed professional recommends dispersion not be used anywhere within project site due to reasonable concerns of erosion, slope failure, or flooding (requires a signed and stamped written determination based on site‑specific conditions from an appropriately licensed professional).
• The dispersion flow path does not provide positive drainage.
• Only available dispersion flow path area is within an erosion hazard or a landslide hazard area (Title 19 KCC).
• Only available dispersion flow path area is in or within 100 feet up‑gradient of a known contaminated site or landfill (active or closed).
• Only available dispersion flow path area is in a critical area (Title 19 KCC), on or above slopes greater than 20% or above erosion hazard areas.
• Only available dispersion flow path area is within the minimum horizontal setback requirements between stormwater control device and onsite sewage system components per Table II-5.2.

Infeasibility Criteria for All Infiltration BMPs

The following criteria each establish that the BMP is infeasible but only if based on an evaluation of site‑specific conditions and documented within a signed and stamped written determination from an appropriately licensed professional (e.g., engineer, geologist, hydrogeologist):

• Infiltration is not recommended due to reasonable concerns about erosion, slope failure, or flooding.
• 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 or subgrades.
• The area available for siting would threaten shoreline structures such as bulkheads.
• Site‑specific infiltration rates are below minimum allowable rates in Table II-5.5.

The area available for siting is within a steep slope area or landslide prone area (or setback) (see Vol II–5.3.2 Determine Infiltration Feasibility)

• The only area available for siting does not allow for a safe overflow pathway.
• Infiltration is restricted due to known contaminated soil or groundwater

The following criteria each establish that the BMP is infeasible, without further justification, though some criteria require professional services:

• The horizontal setback criteria listed Vol II–5.3.2 Determine Infiltration Feasibility cannot be met.

  For most infiltration BMPs, setbacks are measured from the vertical extent of maximum ponding before overflow. For bioretention and rain gardens, setback distances are as measured from the bottom edge of the bioretention or rain garden soil mix (i.e., bioretention cell bottom at the toe of the side slope).

• The following minimum vertical separation to the seasonal high water table or hydraulically‑restrictive layer would not be achieved below the infiltration BMP:

  • 1‑foot separation for a BMP that would serve a drainage area that is:

    1. less than 5,000 square feet of pollution‑generating hard surface (PGHS), and
    2. less than 10,000 square feet of impervious surface; and,
    3. less than three‑quarter (3/4) acres of pervious surface.

    This clearance also applies to permeable pavement facilities regardless of size. Vertical separation requirements are larger if explorations are conducted during the dry season (see Vol II–5.3.2 Determine Infiltration Feasibility).

  • 3‑foot separation for a BMP that would serve a drainage area that meets or exceeds:

    1. 5,000 square feet of PGHS, or
    2. 10,000 square feet of impervious surface, or
    3. three-quarter (3/4) acres of pervious surfaces.

    To use the 3‑foot separation criterion, it must be demonstrated that the drainage areas cannot reasonably be broken down into amounts smaller than the drainage thresholds listed above. Vertical separation requirements are larger if explorations are conducted during the dry season (see Vol II–5.3.2 Determine Infiltration Feasibility).