5.4.14
5.4.14.1 BMP Description
A pre-settling basin (BMP T6.10 in Volume V, Chapter 9 of the Ecology Manual) provides pretreatment of runoff prior to discharge into downstream BMPs, such as bioretention, detention ponds, detention pipes, detention vaults, sand filters, wetponds, and stormwater treatment wetlands. The purpose of the pre-settling basins is to provide pretreatment of runoff in order to remove suspended solids, which can impact the downstream BMP performance, and to help consolidate the removed suspended solids in relatively easy-to-maintain areas.
While this section presents only one BMP for pre-settling basins (BMP T6.10 in Volume V, Chapter 9 of the Ecology Manual), other manufactured devices and BMPs have received a General Use Level Designation for pretreatment through Ecology’s TAPE program.
5.4.14.2 Performance Mechanism
Pre-settling basins slow down the velocity of incoming stormwater, which allows particulates and particulate-bound pollutants to settle.
5.4.14.3 Applications and Limitations
Pre-settling basins on their own do not satisfy any stormwater minimum requirements. They are rather used upstream of other On-site Stormwater Management, Flow Control, and Runoff Treatment BMPs to reduce sedimentation in those BMPs and facilitate drainage system maintenance by focusing sedimentation in preferred locations.
Many BMPs in this chapter, including bioretention, sand filters, and wetponds, have specific pretreatment and/or pre-settling requirements. Use those BMP-specific requirements for pretreatment and pre-settling where provided.
5.4.14.4 Site Considerations
Site constraints are any manmade restrictions such as property lines, easements, structures, etc., that impose constraints on development. Constraints may also be imposed from natural features such as requirements of Title 19 KCC. These should also be reviewed for specific application to the proposed development.
Pre-settling basins shall be a minimum of 20 feet from any structure, property line, and any vegetative buffer required by Title 17 KCC.
Pre-settling basins shall be 100 feet from any septic tank/drain field. See Vol II–5.3.1 Determine Dispersion Feasibility and the Kitsap County Board of Health Ordinance 2008A‑01, as now or hereafter amended, for additional information regarding regulations for septic tank/drain fields.
Pre-settling basins shall be a minimum of 50 feet from any slope greater than 15% and a geotechnical report shall address the potential impact of a wetpond on a steep slope. See reporting requirements in Volume II, Chapter 1.
Embankments that impound water must comply with the Washington State Dam Safety Regulations (Chapter 173‑175 WAC). If the impoundment has a storage capacity (including both water and sediment storage volumes) greater than 10 acre‑feet (435,600 cubic feet or 3.26 million gallons) above natural ground level, or has an embankment height of >6 feet at the downstream toe, then dam safety design and review are required by Ecology. See BMP D.1 in Volume V, Chapter 12 of the Ecology Manual for more detail.
5.4.14.5 Design Information
Refer to BMP T6.10 in Volume V, Chapter 9 of the Ecology Manual for design guidance and criteria.
5.4.14.6 Minimum Construction Requirements
Use a low permeability liner or treatment liner if the basin will intercept the seasonal high ground water table. Provide 1‑foot minimum sediment storage depth.
5.4.14.7 Operations and Maintenance
Maintenance of pre-settling basins shall be similar to that of wetvaults and wetponds. See Volume V, Appendix V‑A of the Ecology Manual.