Onsite Infeasibility Checklists

Portions of the site comprised of till soils with slopes greater than 33% can be considered infeasible for this BMP.

One or more of the infeasibility criteria for Infeasibility Criteria for All Dispersion BMPs apply.
The design criteria for full dispersion (Vol II–5.4.4 Dispersion BMPs) cannot be met.
The dispersion area cannot meet the requirement to have a minimum area 6.5 times the area of the impervious surface draining to it.
Minimum dispersion flow path area and length requirements per Table II-5.1 are unachievable.

One or more of the infeasibility criteria for Infeasibility Criteria for All Dispersion BMPs apply.
The design criteria for splashblock or trench downspout dispersion (Vol II–5.4.4 Dispersion BMPs) cannot be met.
There are no downspouts.
The flow path setbacks to property lines, structures and other flow paths (Vol II–5.4.4 Dispersion BMPs) cannot be achieved.

The vegetated flow path is less than 50 feet.
Greater than 700 square feet of surface area drains to the BMP.
The flow path does not meet the minimum horizontal setback requirements to property lines, structures, and other flow paths (Vol II–5.4.4 Dispersion BMPs).
A 50‑foot minimum flow path for the dispersion area or a maximum of 700 square feet of drainage area to any splashblock is unachievable.

The minimum dispersion trench length of 10 feet for every 700 square foot of drainage area cannot be met.
The vegetated flow path is less than 25 feet.
The flow path is within the setbacks to property lines, structures, and other flow paths (Vol II–5.4.4 Dispersion BMPs).

One or more of the infeasibility criteria for Infeasibility Criteria for All Dispersion BMPs apply.
The design criteria for sheet flow dispersion (Vol II–5.4.4 Dispersion BMPs) cannot be met.
Positive drainage for sheet flow runoff is unachievable.
Area to be dispersed (e.g., driveway, patio) cannot be graded to have less than a 15% slope.
The flow path does not meet the minimum horizontal setbacks to property lines, structures, and other flow paths (see Vol II–5.4.4 Dispersion BMPs).

One or more of the infeasibility criteria for Vol II–Infeasibility Criteria for All Dispersion BMPs apply.
The design criteria for concentrated flow dispersion (Vol II–5.4.4 Dispersion BMPs) cannot be met.
There are no concentrated flows to disperse.
The dispersion device and flow path requirements are unachievable:
- A minimum 10‑foot length of dispersion trench followed by a 25‑foot minimum flow path or a rock pad with a 50‑foot minimum flow path.
- A maximum of 700 square feet of drainage area to any dispersion device.
The flow path does not meet the minimum horizontal setbacks to property lines, structures, and other flow paths (see Vol II–5.4.4 Dispersion BMPs).

The design criteria for bioretention (Vol II–5.4.6 Bioretention Cells, Swales, and Planter Boxes) cannot be met.
Refer to the additional bioretention Infeasibility Criteria in the BMP T7.30 of the Ecology Manual.

The design criteria for rain gardens (Vol II–5.4.5 Rain Gardens) cannot be met.
Refer to the additional rain garden Infeasibility Criteria in the Ecology Manual, Volume V, Chapter 7.

One or more of the infeasibility criteria for Infeasibility Criteria for All Infiltration BMPs apply.
The design criteria for perforated stub-out connections (Vol II–5.4.7 Perforated Stub-Out Connections) cannot be met.
The only location for the perforated pipe portion of the system is under impervious or heavily compacted (e.g., driveways and parking areas) surfaces.
A minimum of 10 feet of perforated pipe per 5,000 square feet of contributing roof area is unachievable.
The seasonal water table is less than 1 foot below the trench bottom.
The site cannot be reasonably designed to locate a catch basin between the perforated stub-out and point of connection to the public system.

The Design Criteria for Permeable Pavement (Vol II–5.4.8 Permeable Pavement) cannot be met.
Refer to additional permeable pavement Infeasibility Criteria in BMP T5.15 of the Ecology Manual.
Note that the infeasibility criteria for Infeasibility Criteria for All Infiltration BMPs are not applicable and the minimum native soil infiltration rate differs, as described in BMP T5.15 of the Ecology Manual).

Space necessary for the mature height, size, and/or rooting depth for tree planting per Title 17 KCC is unachievable.
No existing trees with diameter equal to or greater than 6‑inches diameter at breast height (DBH) on project site. DBH is defined as the outside bark diameter at 4.5 feet above the ground on the uphill side of a tree.
New and/or replaced ground level impervious surface not proposed within 20 feet of existing tree.
For tree(s) with a diameter greater than or equal to 6 inches, significant grading is unavoidable within the dripline.
For tree(s) with a diameter of 4–6 inches, significant grading is unavoidable within 5 feet of tree trunk.
Trees are considered danger trees according to KCC 19.150.230.

H.2 Onsite Infeasibility Checklists