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

4.2.6 Minimum Requirement #6: Runoff Treatment

4.2.6.1 Thresholds

When assessing a project against the following thresholds, only consider those hard and pervious surfaces that are subject to this minimum requirement as determined in Vol I–4.1 Project Applicability of this chapter.

The following require construction of runoff treatment facilities:

  • Projects in which the total of PGHS is 5,000 square feet or more, or;

  • Projects in which the total of PGPS—not including permeable pavements—is 3/4 of an acre or more, and from which there will be a surface discharge in a natural or manmade conveyance system from the site.

The use of Threshold Discharge Areas (TDAs) detailed in the Ecology Manual is not applicable to the implementation of MR #6 in Kitsap County except for rural (outside the UA and UGA) residential project sites that are 5 acres or greater. If a TDA meets either of the following thresholds for a rural residential project site, runoff treatment BMPs are required:

  • TDAs that have a total of 5,000 square feet or more of PGHS, or

  • TDAs that have a total of 3/4 of an acre or more of PGPS – not including permeable pavements, and from which there will be a surface discharge in a natural or manmade conveyance system from the site.

The project proponent must demonstrate that the TDA does not meet either of these thresholds for runoff treatment BMPs to not be required for that TDA. Refer to Volume I, Section I-3.4.6 of the Ecology Manual for details.

4.2.6.2 Runoff Treatment Sizing

Size runoff treatment BMPs for the entire area that drains to them, even if some of those areas are not pollution-generating, or were not included in the project site threshold decisions (see Vol I–4.1 Project Applicability). Runoff treatment BMPs are sized by using either a volume (Water Quality Design Storm Volume) or a flow rate (Water Quality Design Flow Rate), depending on the runoff treatment BMP selected.

4.2.6.3 Water Quality Design Storm Volume

When using an approved continuous runoff model, the water quality design storm volume shall be equal to the simulated daily volume that represents the upper limit of the range of daily volumes that accounts for 91% of the entire runoff volume over a multi-decade period of record.

4.2.6.4 Water Quality Design Flow Rate

The water quality design flow rate is dependent on the location of the runoff treatment BMP relative to the detention BMP(s):

  • Upstream of Detention BMPs or when there are no Detention BMPs: The flow rate at or below which 91% of the runoff volume, as estimated by an approved continuous runoff model, will be treated. Design criteria for treatment facilities are assigned to achieve the applicable performance goal (e.g., 80% TSS removal) at the water quality design flow rate. At a minimum, 91% of the total runoff volume, as estimated by an approved continuous runoff model, must pass through the runoff treatment BMP(s) at or below the approved hydraulic loading rate for the BMP(s).

  • Downstream of Detention BMPs: The water quality design flow rate shall be the full 2‑year release rate from the detention BMP.

4.2.6.5 Runoff Treatment BMP Selection, Design, and Maintenance

Runoff Treatment BMPs shall be:

4.2.6.6 Additional Requirements

The (direct or indirect) discharge of untreated stormwater from pollution-generating hard surfaces to ground water is prohibited. Infiltration or dispersion in accordance with LID BMPs per the List Approach in Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management does not violate this requirement.

Oil control treatment is required for high-use sites, or those sites that typically generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. All stormwater from hard surfaces subject to motor vehicle traffic shall flow through a spill-containment type oil/water separator prior to surface discharge off site.

Kitsap Public Health District conducts regular lake sampling and has determined that Kitsap Lake and Long Lake are phosphorus limited. Therefore, Phosphorus treatment is required for project sites draining into these receiving water bodies.

4.2.6.7 Supplemental Guidelines

See Vol II–5.3.4 Select BMPs for Runoff Treatment for guidance on selection of runoff treatment BMPs. See Volume II, Chapter 5, and Volume V of the Ecology Manual for detailed guidance on design and maintenance of runoff treatment BMPs.

An adopted and implemented basin plan or a TMDL (also known as a Water Clean-up Plan) may be used to develop runoff treatment requirements that are tailored to a specific basin. However, runoff treatment requirements shall meet, at a minimum, the Basic Treatment Performance Goal (see Volume I of the Ecology Manual). Runoff from surfaces that are not pollution-generating do not need to be treated and may bypass the Runoff Treatment BMP(s), if it is not mingled with runoff from pollution-generating surfaces.

Do not mix drainage from areas in native vegetation with untreated runoff from streets and driveways, if possible. It is best to infiltrate or disperse this relatively clean runoff to maximize recharge to shallow ground water, wetlands, and streams.