16.1 BMP T610 - Presettling Basin
A presettling basin provides pretreatment of runoff in order to remove suspended solids, which can impact other stormwater treatment BMPs.
Pretreatment basins, when used with other stormwater treatment BMPs, can be used to ensure compliance with Minimum Requirement 6 - Stormwater Treatment when Pretreatment is required.
Pretreatment basins may also be used for ensuring compliance with Minimum Requirement 3 - Source Control for areas where sediment may impact the stormwater system.
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.
Presettling basins shall be installed upstream of other stormwater treatment or flow control BMPs.
The presettling basin shall be designed as a wetpool facility. The wetpool volume shall be at least 30% of the total water quality volume. The water quality volume shall be calculated using an Ecology approved continuous simulation model, assuming a 15-minute timestep.
If the runoff in the presettling basin will be in direct contact with the soil, it must be lined per A100 - Liners.
The presettling basin shall conform to the following:
The length-to-width ratio shall be at least 3:1. Berms or baffles may be used to lengthen the flowpath.
The minimum depth shall be 4 feet; the maximum depth shall be 6 feet.
Inlets and outlets shall be designed to minimize velocity and reduce turbulence. Inlet and outlet structures should be located at extreme ends of the basin in order to maximize particle-settling opportunities.