5.3.1 Determine Dispersion Feasibility

Dispersion BMPs include full dispersion, sheet flow dispersion, and concentrated flow dispersion (Vol II–5.4.4 Dispersion BMPs). The following two steps for determining dispersion feasibility are outlined in the subsequent sections:

• Step 1: Evaluate Horizontal Setbacks and Site Constraints

• Step 2: Evaluate Use of Dispersion to Meet Minimum Requirements

Step 1: Evaluate Horizontal Setbacks and Site Constraints

Assess horizontal setbacks, flow path requirements, and site constraints to determine dispersion feasibility for the site, as follows:

Horizontal Setbacks

Horizontal setbacks vary depending on the type of dispersion BMP selected; see Vol II–5.4.4 Dispersion BMPs for horizontal setback requirements for each dispersion BMP type.

Flow Path Requirements

Dispersion BMPs have minimum requirements for a dispersion area and vegetated flow path that can be difficult to achieve on sites with limited space, such as in urban environments. Assess the minimum dispersion flow path requirements per Table II-5.1. This table is intended to help the designer determine whether the minimum flow path requirements can be achieved given site constraints. If dispersion BMPs are feasible based on these minimum requirements, see Vol II–5.4.4 Dispersion BMPs for the comprehensive requirements pertaining to dispersion BMP design.

Table II-5.1. Summary of Minimum Dispersion Flow Path Area and Length Requirements.

Dispersion Type	Flow Path Description	Flow Path Length
BMP T5.30: Full Dispersion – Residential Projects
From Impervious Surfaces	From impervious surface through the area preserved as forest or native vegetation	≥100 feet
	Sheet flow from lawn to landscaping areas associated with the impervious area being mitigated	≥25 feet
From Cleared Areas	Through the cleared area (and leading to the dispersion area)	≤25 feet
	From the cleared area through the dispersion area	For cleared area width ≤25 feet: ≥25 feet For cleared area width between 25 and 250 feet: 25 feet plus additional 1 foot for every 3 feet of width cleared area (beyond the initial 25 feet) up to a maximum width of 250 feet.
BMP T5.30: Full Dispersion – Public Road Projectsa
By Sheet Flow from Uncollected, Unconcentrated Runoff into the Dispersion Area	From impervious area leading to the dispersion area	≤75 feet
	Pervious area leading to the dispersion area	≤150 feet
	Through the dispersion area	Varies by soil type (see Minimum Design Requirements for Public Road Projects in BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual)
Channelized (Collected and Re-dispersed) Stormwater into the Dispersion Area	Shall be re-dispersed to produce longest possible flow path	Varies by soil type (see Minimum Design Requirements for Public Road Projects in BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual)
Engineered Dispersion	–	Varies by soil type (see Minimum Design Requirements for Public Road Projects in BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual)
BMP T5.10B: Downspout Dispersionb
Dispersion Trench	Vegetated flow path	≥25 feet
	Vegetated flow path between outlet and any slope steeper than 15%	≥50 feet
Splashblock	Vegetated flow path	≥50 feet
BMP T5.11: Concentrated Flow Dispersion
All	Vegetated flow path	>25 feet
BMP T5.12: Sheet Flow Dispersion
All	–	≥10 feet
Notes: Review minimum design requirements for Public Road Projects per BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual prior to evaluating these criteria. If vegetated flow path is less than 25 feet, Perforated Stub-out Connections (see Vol II–5.4.7 Perforated Stub-Out Connections) may be used in lieu of downspout dispersion.

Site Constraints

• Landslide-Prone and Erosion Hazard Areas – The dispersion flow path is not typically permitted within landslide hazard areas (Title 19 KCC), on or above slopes greater than 20%, or above erosion hazard areas.

  • Full Dispersion – Dispersion area is not allowed in critical area buffers or on slopes steeper than 20%. Dispersion areas proposed on slopes steeper than 15% and within 50 feet of a geologically hazardous area (RCW 37.07A.030(5)) shall be approved by a geotechnical engineer or engineering geologist.

  • Sheet Flow Dispersion and Concentrated Flow Dispersion – Runoff discharge toward landslide hazard areas shall be evaluated by a geotechnical engineer or qualified geologist. Do not allow sheet flow on or above slopes greater than 20%, or above erosion hazard areas, without evaluation by a geotechnical engineer or qualified geologist and approval by Kitsap County.

  • Downspout Dispersion Systems – Have a geotechnical engineer or a licensed geologist, hydrogeologist, or engineering geologist evaluate runoff discharged towards landslide hazard areas. Do not place the discharge location from splash blocks or dispersion trenches on or above slopes greater than 20% or above erosion hazard areas without evaluation by a licensed engineer in the state of Washington with geotechnical expertise or a licensed geologist, hydrogeologist, or engineering geologist, and approval by Kitsap County.

• Septic Systems and Drain fields – The dispersion flow path is not permitted within 10 feet of a proposed or existing septic system or drain field.

  • Full Dispersion, Downspout dispersion systems – For sites with onsite sewage disposal systems, the discharge of runoff from the dispersion area shall be located downslope of the primary and reserve drain field areas. This requirement may be waived by the permitting jurisdiction if site topography clearly prevents discharged flow from intersecting the drain field.

• Sheet Flow Dispersion, Concentrated Flow Dispersion. The discharge area shall be 10 feet downgradient of the drain field primary and reserve areas (Table II-5.2 and WAC 246‑272A‑0210). Kitsap Public Health District may waive this requirement if site topography clearly prohibits flows from intersecting the drain field.

• Drinking Water Supply Wells or Springs – See Table II-5.3 for infiltration setback requirements to private and public wells. See Kitsap County Board of Health Ordinance 2008A‑01, as now or hereafter amended, for additional setback information.

• Contaminated Sites and Landfills – The dispersion flow path is not permitted within 100 feet of a contaminated site or landfill (active or closed).

Table II-5.2. Minimum Horizontal Setback Requirements between Stormwater BMPs and Onsite Sewage System (OSS) Components.

Stormwater BMPa,b	From Edge of Soil Dispersal Component and Reserve Area (feet)c	From Sewage Tank and Distribution Box (feet)c	From Building Sewer and Nonperforated Distribution Pipe (feet)c
Upgradient
Individual Lot Infiltration System	30	30	N/A
Individual Lot Dispersion System	30	30	N/A
Individual Lot Rain Garden	30	10	N/A
Individual Lot Downspout Splash Blocks	10	10	N/A
Subsurface Stormwater Infiltration or Dispersion Component	30	10	N/A
Down- or Side-Gradient
Individual Lot Infiltration System	10	10	N/A
Individual Lot Dispersion System	30	10	N/A
Individual Lot Rain Garden	10	10	N/A
Individual Lot Downspout Splash Blocks	100	100	N/A
Subsurface Stormwater Infiltration or Dispersion Component	10	10	N/A
Up-, Down-, or Side-Gradient
Regional Infiltration Facility	100	100	N/A
Unlined Detention Ponds/Infiltration Basins	100	50	10
Notes: N/A: Not Applicable. BMP horizontal setback distances shall be measured from closest edge of the BMP to the feature of interest. BMP discharge location(s) and flow path(s) shall be directed away from or around OSS. If the Kitsap County Board of Health setback conflicts with this manual, the Kitsap County Health District setback will take precedence.

Table II-5.3. Minimum Horizontal Setback Requirements Between Stormwater BMPs and Private/Public Wells.

Stormwater BMPa	Private Well (feet)b	Public Well (feet)b
Individual Lot Infiltration System	30	100
Individual Lot Dispersion System	50	100
Individual Lot Rain Garden	50	100
Individual Lot Downspout Splash Blocks	30	50
Unlined Detention Ponds/Infiltration Basins	50	100
Regional Infiltration Facility	100	100
Note: Discharge location(s) and flow path(s) shall be directed away from wells. If the Kitsap County Board of Health setback conflicts with this manual, the Kitsap County Health District setback will take precedence.

Step 2: Evaluate Use of Dispersion to Meet Minimum Requirements

If dispersion is considered feasible for the site, evaluate the feasibility of individual dispersion BMPs (Vol II–5.4.4 Dispersion BMPs) when selecting BMPs for on-site stormwater management (Vol II–5.3.3 Select BMPs for On-Site Stormwater Management), runoff treatment (Vol II–5.3.4 Select BMPs for Runoff Treatment), and flow control (Vol II–5.3.5 Select BMPs for Flow Control).