4.2.7 Minimum Requirement #7: Flow Control

Projects must provide flow control to reduce the impacts of stormwater runoff from hard surfaces and land cover conversions.

If the discharge is to a stream that leads to a wetland, or to a wetland that has an outflow to a stream, both this requirement and Minimum Requirement #8: Wetlands Protection apply.

4.2.7.1 Exemptions

Flow control is not required for projects that discharge directly to or indirectly through an MS4 to a water listed in Appendix I‑A of the Ecology Manual, subject to all of the following restrictions:

• Direct discharge to the exempt receiving water does not result in the diversion of drainage from any perennial stream classified as Types 1, 2, 3, or 4 in the State of Washington Interim Water Typing System (WAC 222‑16‑031), or Types “S,” “F,” or “Np” in the Permanent Water Typing System (WAC 222‑16‑030), or from any Category I, II, or III wetland

• If flow splitters or conveyance elements are applied to route natural runoff volumes from the project site to any downstream Type 5 stream or Category IV wetland, then:

  • Design of the flow splitters or conveyance elements shall be based on approved continuous simulation modeling analysis. The design shall ensure that flows delivered to Type 5 stream reaches will approximate, but in no case exceed, durations ranging from 50% of the 2‑year to the 50‑year peak flow.
  • Flow splitters or conveyance elements that deliver flow to Category IV wetlands shall also be designed using approved continuous simulation modeling to preserve pre-project wetland hydrologic conditions unless specifically waived or exempted by regulatory agencies with permitting jurisdiction.

• The project site must be drained by a conveyance system that is comprised entirely of manmade conveyance elements (e.g., pipes, ditches, outfall protection) and extends to the ordinary high water line of the exempt receiving water.

• The conveyance system between the project site and the exempt receiving water shall have sufficient hydraulic capacity to convey discharges from future build-out conditions (under current zoning) of the site, and the existing condition from non-project areas from which runoff is or will be collected.

• Any erodible elements of the manmade conveyance system must be adequately stabilized to prevent erosion under the conditions noted above.

4.2.7.2 Thresholds

The following projects require achievement of the standard flow control requirement:

• Projects in which the total of effective impervious surfaces is 10,000 square feet or more, or
• Projects that convert 3/4 acre or more of vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture, and from which there is a surface discharge in a natural or manmade conveyance system from the site, or
• Projects that through a combination of effective hard surfaces and converted vegetation areas cause a 0.15 cubic feet per second (cfs) increase in the 100‑year flow frequency as estimated using WWHM or other approved continuous simulation model and 15‑minute time steps.1 For the purpose of applying this threshold, the existing condition is either the pre-project land cover, (for a developed site with an approved stormwater mitigation plan) or the land cover that existed at the site as of a date when the County first adopted flow control requirements into code or rules (September 21, 1987).

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

The use of TDAs detailed in the Ecology Manual is not applicable to the implementation of MR #7 in Kitsap County except for rural (outside the UA and UGA) residential project sites that are 5 acres or greater. If a TDA meets any of the following thresholds for a rural residential project site, flow control BMPs are required:

• TDAs that have a total of 10,000 square feet or more of effective impervious surfaces, or

• TDAs that convert 3/4 acre or more of vegetation to lawn or landscape, or convert 2.5 acres or more of native vegetation to pasture, and from which there is a surface discharge in a natural or manmade conveyance system from the TDA, or

• TDAs that through a combination of effective hard surfaces and converted vegetation areas cause a 0.15 cfs increase in the 100-year flow frequency as estimated using WWHM or other approved continuous simulation model and 15-minute time steps.

Refer to Volume I, Section I-3.4.7 in the Ecology Manual for details.

4.2.7.3 Flow Control Performance Standard

Stormwater discharges shall match developed discharge durations to pre-developed durations for the range of pre-developed discharge rates from 50% of the 2‑year peak flow up to the full 50‑year peak flow. The pre-developed condition to be matched shall be a forested land cover unless reasonable, historical information is provided that indicates the site was prairie prior to settlement (modeled as “pasture” in the approved continuous simulation model).

4.2.7.4 Additional Requirement

Flow Control BMPs shall be selected, designed, and maintained in accordance with Volume II.

4.2.7.5 Supplemental Guidelines

Reduction of flows through infiltration decreases stream channel erosion and helps to maintain base flow throughout the summer months. Infiltration shall follow the guidance in this manual to reduce the chance of threatening ground water quality.

Using LID BMPs reduces the predicted runoff rates and volumes, and thus also reduces the size of required Flow Control BMPs.

Application of certain LID and/or infiltration BMPs can result in reducing the effective impervious area and the converted vegetation areas such that a Flow Control BMP is not required. See the definition of Effective Impervious Surface for details. Application of BMP T5.30: Full Dispersion, also results in eliminating the requirement for a Flow Control BMP for those areas that are “fully dispersed.”