2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

5.4.4 Dispersion BMPs

5.4.4.1 BMP Description

Dispersion BMPs disperse runoff over vegetated pervious areas to provide flow control. The dispersion BMPs in this section include:

A site (or an area of a site) that applies full dispersion per BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual consists of the following elements:

  • An impervious (or cleared) area. The impervious (or cleared) area is the area that the design is mitigating for by using BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual.

  • A flow spreader. Runoff from the impervious (or cleared) area may need to be routed through a flow spreader (see Volume V, Section 1.4.2 in the Ecology Manual), depending on the site layout and type of impervious surface, as described in this section.

  • A dispersion area. This area defines the limits of the Full Dispersion BMP. The impervious (or cleared) area shall disperse into the preserved dispersion area.

    • The dispersion area shall be forest, native vegetation, or a cleared area depending on the site type. Details are provided for BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual for what amount of vegetation the dispersion area shall contain based on site type.

    • If the dispersion area must be preserved as forest or native vegetation, it may be a previously cleared area that has been replanted in accordance with BMP T5.30 in Volume V, Chapter 3 of the Ecology Manual.

    • The dispersion area shall be situated to minimize the clearing of existing forest cover, to maximize the preservation of wetlands (though the wetland area and any streams and lakes do not count as part of the dispersion area), and to buffer stream corridors.

    • The dispersion area shall be protected through recorded easements for individuals lots or through protective covenants recorded against the property.

    • The dispersion area shall be shown on the site plans and shall be clearly marked during clearing and construction on the site.

    • All trees within the dispersion area at the time of permit application shall be retained, aside from:

      • Dangerous or diseased trees, and

      • Approved timber harvest activities regulated under WAC Title 222. Class IV General Forest Practices that are conversions from timberland to other uses are not acceptable for the preserved area.

    • The dispersion area may be used for passive recreation and related facilities, including pedestrian and bicycle trails, nature viewing areas, fishing and camping areas, and other similar activities that do not require permanent structures. Cleared areas and areas of compacted soil associated with these areas and facilities shall not exceed 8 percent of the dispersion area.

    • The dispersion area may contain utilities and utility easements, but not septic systems. For the purpose of this BMP, utilities are defined as potable and wastewater underground piping, underground wiring, and power and telephone poles.

  • A flow path through the dispersion area. The length of the flow path from the impervious (or cleared) area through the dispersion area varies based on the site layout and type of impervious surface, as described in this section. Regardless of the site layout and type of impervious surface, the flow path shall meet the following criteria:

    • The slope of the flow path shall be no steeper than 15 percent for any 20‑foot reach of the flow path. Slopes up to 20 percent are allowed where flow spreaders are located upstream of the dispersion area and at sites where vegetation can be established.

    • The flow paths from adjacent flow spreaders shall be sufficiently spaced to prevent overlap of flows in the flow path area.

The dispersion of runoff shall not create flooding or erosion impacts.

5.4.4.2 Performance Mechanism

Dispersion of concentrated flows from driveways or other pavement through a vegetated pervious area attenuates peak flows by slowing entry of the runoff into the conveyance system, allowing for some infiltration, and providing some water quality benefits.

5.4.4.3 Applications and Limitations

Full dispersion can be utilized for runoff from impervious surfaces and cleared areas of project sites into areas preserved as forest, native vegetation, or cleared area. The site (or area of the site) that is applying full dispersion per this BMP shall be laid out to allow the runoff from the impervious (or cleared) surface to fully disperse into the preserved dispersion area. (i.e., have full access to and not be intercepted by pipe(s), ditch(es), stream(s), river(s), pond(s), lake(s), or wetland[s]). Projects that successfully apply full dispersion on all or a portion of their site will decrease effective impervious surfaces and may avoid triggering the TDA thresholds in Vol I–4.2.7 Minimum Requirement #7: Flow Control. Downspout dispersion may be used in all subdivision lots where downspout full infiltration, full dispersion, and bioretention/rain gardens are not feasible.

Use sheet flow dispersion for flat or moderately sloping (<15% slope) surfaces such as driveways, sports courts, patios, roofs without gutters, lawns, pastures; or any situation where concentration of flows can be avoided.

Use concentrated flow dispersion in any situation where concentrated flow can be dispersed through vegetation.

Dispersion BMPs can be used to meet on-site stormwater management, basic treatment, and flow control requirements (Table II-5.8). See Volume V, Chapter 3, BMP T5.30 of the Ecology Manual for further discussion on full dispersion applications and limitations.

Table II-5.8. Dispersion Applicability.
BMP

MR #5: On-site Stormwater Management

MR #6: Runoff Treatment

MR #7: Flow Control

List

LID Performance Standard

Basic

Enhanced

Oil Control

Phosphorus

Full

 X

X

      

 

 X

Downspout

X

X

Xa

      

X

Sheet Flow

X

X

Xa

      

X

Concentrated Flow

X

X

Xa

      

X

Notes:

  1. Meets basic runoff treatment requirements when additional requirements for Basic Filter Strip BMPs (Vol II–5.4.21 Vegetated Filter Strips) are met.

5.4.4.4 Site Considerations

The dispersion area shall have a minimum area 6.5 times the area of the impervious surface draining to it. See Volume V, Chapter 3, BMP T5.30 of the Ecology Manual for additional site considerations for residential projects and public road projects.

The following are key considerations in determining the feasibility of dispersion BMPs for a particular site (see Vol II–5.3.1 Determine Dispersion Feasibility for additional discussion of site requirements for dispersion):

  • Dispersion Flow Path – Dispersion BMPs generally require large areas of vegetated ground cover to meet flow path requirements and may be oftentimes infeasible in dense, urban settings.

  • Erosion or Flooding Potential – Dispersion is not allowed in settings where the dispersed flows might cause erosion or flooding problems, either on site or on adjacent properties.

  • Site Topography – Dispersion flow paths are prohibited in and near certain sloped areas.

5.4.4.5 Design Information

See the Minimum Design Requirements for Residential Projects and Minimum Design Requirements for Public Road Projects in Volume V, Chapter 3, BMP T5.30 of the Ecology Manual. See also Volume V, Chapter 4, BMP T5.10B for downspout dispersion, Volume V, Chapter 4, BMPT T5.10C in the Ecology Manual for concentrated flow dispersion, and Volume V, Chapter 3, BMP T5.12 in the Ecology Manual for sheet flow dispersion design requirements.

Flow path design requirements that are common to all dispersion BMPs include the following:

  • The vegetated flow path shall meet the requirements in Vol II–5.3.1 Determine Dispersion Feasibility and shall consist of either undisturbed, well-established native landscape or lawn, or landscape or groundcover over soil that meets the Post Construction Soil Quality and Depth BMP requirements outlined in Vol II–5.4.1 Post Construction Soil Quality and Depth.

  • To ensure that the groundcover is dense to help disperse and infiltrate flows and prevent erosion, the design plans shall specify that vegetation coverage of plants will achieve 90% coverage within 1 year.

  • The flow path topography shall promote shallow sheet flow across a width of no less than 6 feet for dispersion points (i.e., splash blocks or rock pads) or the width of the dispersion device (i.e., trench or sheet flow transition zone).

  • The dispersion flow path is not typically permitted within landslide–hazard areas as defined in Title 19 KCC.

  • The dispersion flow path is not typically permitted within a setback above a steep slope area (Title 19 KCC). The setback is calculated as 10 times the height of the steep slope area (to a 500‑foot maximum setback). Dispersion within this setback may be feasible provided a detailed slope stability analysis is completed by a licensed geotechnical engineer or hydrogeologist. The analysis shall determine the effects that dispersion would have on the steep slope area and adjacent properties.

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

  • For sites with septic systems, the point of discharge to the dispersion device (e.g., splash block, dispersion trench) shall be down-gradient of the drain field primary and reserve areas.

See Table II-5.1 for a summary of flow path requirements and see the Ecology Manual for detailed design guidance for each dispersion BMP. See Figure II-4.1 for dispersion trench design standards.

See BMP T5.11, BMP T5.12, and BMP T5.30 in Volume V, Chapter 5 of the Ecology Manual for guidance on Runoff Model Representation for concentrated flow dispersion, sheet flow dispersion, and full dispersion, respectively.

5.4.4.6 Minimum Construction Requirements

Protect the dispersion flow path from sedimentation and compaction during construction. If the flow path area is disturbed during construction, restore the area to meet the Post Construction Soil Quality and Depth BMP requirements in Vol II–5.4.1 Post Construction Soil Quality and Depth, and establish a dense cover of lawn, landscape or groundcover.

5.4.4.7 Operations and Maintenance Requirements

Key maintenance considerations for dispersion BMPs include the maintenance of splash blocks (where used with downspout dispersion), trenches (where used with downspout dispersion), transition zones (where used with sheet flow dispersion), rock pads at discharge locations (where used with concentrated flow dispersion), and the dispersal area. See Volume V, Appendix V‑A in the Ecology Manual for additional maintenance guidelines pertaining to all dispersion BMPs.