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

5.4 BMP Design

This section presents BMP design information for approved BMPs for meeting Minimum Requirement #5: On-site Stormwater Management, Minimum Requirement #6: Runoff Treatment, and Minimum Requirement #7: Flow Control. On-site Stormwater Management and Flow Control BMPs may also be used to help satisfy Minimum Requirement #8: Wetlands Protection, where applicable.

The remainder of this section details requirements for runoff treatment prior to infiltration BMPs; requirements for BMPs classified as UICs; and BMP-specific design standards, requirements, and guidelines. The BMP-specific design guidance is generally organized into sub-sections for BMP description, discussion of performance mechanisms, applications and limitations (including which minimum requirements can be fully or partially satisfied by the given BMP), site considerations, design information, minimum construction requirements, and operation and maintenance (O&M) requirements. See the following additional references for additional design guidelines and requirements:

In the event of conflicts between the LID Technical Guidance Manual and the Ecology Manual, the Ecology Manual supersedes, and its guidance shall be followed.

Pretreatment

Pre-settling shall be evaluated for most BMPs to protect BMPs from excessive siltation and debris. Pretreatment is required for some Runoff Treatment BMPs, as described in the individual BMP sections (where applicable). Beyond the requirements provided below, pre-settling and pretreatment shall be considered wherever a Basic Treatment BMP or the receiving water may be adversely affected by non-targeted pollutants (e.g., oil), or may be overwhelmed by a heavy load of targeted pollutants (e.g., suspended solids).

Runoff Treatment Prior to Infiltration BMPs

A pretreatment BMP to remove a portion of the influent suspended solids should precede all infiltration BMPs. This is to reduce potential plugging of the soils and prolong the life of the infiltration BMP. Use either a basic treatment BMP, as described in Volume III, Section 1.2 of the Ecology Manual, or a pretreatment BMP as described in Volume V, Chapter 9 of the Ecology Manual. The lower the influent suspended solids loading to the infiltration BMP, the longer the infiltration BMP can infiltrate the desired amount of water, and the longer interval between maintenance activity.

In BMPs such as BMP T7.20: Infiltration Trenches where a reduction in infiltration capability can have significant maintenance or replacement costs, selection of a reliable pretreatment or basic treatment BMP prior to the infiltration BMP with high solids removal capability is preferred. For infiltration BMPs that allow easier access for maintenance and less costly maintenance activity (e.g., BMP T7.10: Infiltration Basins with gentle side slopes), there is a trade-off between using a pretreatment or basic treatment BMP with a higher solids removal capability and a device with a lower capability. Generally, basic treatment BMPs are more capable at solids removal than pretreatment BMPs. Though basic treatment BMPs may be higher in initial cost and space demands, the infiltration BMP should have lower maintenance costs.

BMPs Classified as Underground Injection Controls (UICs)

If the proposed design includes an infiltration BMP that is classified as an UIC, the design shall ensure compliance with the UIC program as well as any other applicable regulatory requirements. UIC Wells include subsurface infiltration systems that are used to discharge fluids from the ground surface into the subsurface (e.g., drywells, infiltration trenches with perforated pipe, storm chamber systems with the intent to infiltrate). (See Volume I, Section 2.14 of the Ecology Manual.)