20.1 BMP F100 - Detention Facilities
Detention facilities include detention ponds (BMP F110), detention tanks (BMP F120), and detention vaults (BMP F130) that are used to capture stormwater, store it, and slowly release the flows to help mimic predeveloped or existing conditions. Detention facilities use flow control structures to allow for slow release of the stormwater - see A500 - Control Structures.
BMP F110 - Detention Ponds, BMP F120 - Detention Tanks, and BMP F130 - Detention Vaults can be used to help ensure compliance with Minimum Requirement 5 - Onsite Stormwater Management for Meeting the LID Performance Standard Only, Minimum Requirement 7 - Flow Control, and Minimum Requirement 8 - Wetlands Protection.
20.1.3 Design Criteria for All Detention Facilities
Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.
The volume and outflow design for detention ponds, detention tanks, and detention vaults must be sized using an Ecology approved continuous simulation model, assuming a 15-minute timestep, using the design criteria in A500 - Control Structures, and using the criteria in Modeling Your Best Management Practices. The design shall be based upon the applicable Minimum Requirement that is required to be met:
Minimum Requirement 5 - Onsite Stormwater Management: See Volume 1, Section 1.4.5.2 - Compliance Methods - Low Impact Development (LID) Performance Standard.
Minimum Requirement 7 - Flow Control: See Volume 1, Section 1.4.7.4 - Flow Control Mitigation.
Minimum Requirement 8 - Wetlands Protection is specific to the individual wetland needs. Mitigation may require detention facilities in order to maintain wetland hydrology.
20.1.3.2 Detention Facility Overflow
In all ponds, tanks, and vaults, a primary overflow (usually a riser pipe within the control structure; see A500 - Control Structures) shall be provided to bypass the developed peak volumetric flowrate calculated using a 10-minute timestep from a Type 1A, 100-year, 24-hour frequency storm using a single event model or the 100-year return period flowrate as estimated by an Ecology approved continuous simulation model, assuming a 15-minute timestep, over or around the restrictor system. The design must provide controlled discharge directly into the downstream conveyance system.