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

5.4.23 Wetvaults

5.4.23.1 BMP Description

A wetvault (BMP T10.20 in Volume V, Chapter 8 of the Ecology Manual) is an underground structure similar in appearance to a detention vault, except that a wetvault has a permanent pool of water (wetpool) which dissipates energy and improves the settling of particulate pollutants (see the Wetvault figure in Volume V, Chapter 8 of the Ecology Manual). Being underground, the wetvault lacks the biological pollutant removal mechanisms, such as algae uptake, present in BMP T10.10: Wetponds – Basic and Large and BMP T10.30: Stormwater Treatment Wetlands in Volume V, Chapter 8 of the Ecology Manual.

5.4.23.2 Performance Mechanism

Wetvaults are designed to provide runoff treatment by dissipating energy and providing retention time in order to settle out particulate pollutants. Being underground, the wetvault lacks the biological pollutant removal mechanisms, such as algae uptake, present in surface wetponds. Wetvaults are believed to be ineffective in removing dissolved pollutants such as soluble phosphorus or metals, such as copper. Therefore, use of wetvaults shall only be considered when other treatment BMPs are infeasible, and shall be approved by DCD on a case-by-case basis.

5.4.23.3 Application and Limitations

A wetvault may be used for commercial, industrial, or roadway projects if there are space limitations precluding the use of other treatment BMPs. The use of wetvaults for residential development is highly discouraged. Combined detention and wetvaults are allowed; see Vol II–5.4.29 Combined Detention and Wetpool Facilities.

A wetvault is believed to be ineffective in removing dissolved pollutants such as soluble phosphorus or metals such as copper. There is also concern that oxygen levels will decline, especially in warm summer months, because of limited contact with air and wind. However, the extent to which this potential problem occurs has not been documented.

Below-ground structures like wetvaults are relatively difficult and expensive to maintain. The need for maintenance is often not seen and as a result routine maintenance does not occur.

If oil control is required for a project (see Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment), a wetvault may be combined with an American Petroleum Institute (API) oil/water separator; see Vol II–5.4.23 Wetvaults.

Wetvault BMPs can be designed to meet all runoff treatment requirements, including basic, enhanced, oil control, and phosphorus control (see Table II-5.26).

Table II-5.26. Wetvault Applicability.
BMP

MR #5: On-site Stormwater Management

MR #6: Runoff Treatment

MR #7: Flow Control

List

LID Performance Standard

Basic

Enhanced

Oil Control

Phosphorus

Wetvault

  

 

X

Xa

Xb

Xc

 X

Notes:

  1. Can be used to meet enhanced runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Section 1.2 in the Ecology Manual.

  2. Can be used to meet oil control performance goal requirement if combined with API oil/water separator; see Volume V, Chapter 13, BMP T11.10 in the Ecology Manual.

  3. Can be used to meet phosphorus runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Section 1.2 in the Ecology Manual.

5.4.23.4 Site Considerations

The following site considerations can help determine the feasibility of a wetvault for a particular site:

  • Vault location and vault material approval is required and may require geotechnical analysis.

  • Location of the wetvault relative to site constraints (e.g., buildings, property lines) shall be the same as for detention ponds (see Vol II–5.4.26 Detention Ponds).

  • Consider wetvaults where there are space limitations precluding the use of other treatment BMPs.

  • Consider how the wetvault grates and access points fit within a site plan, including restrictions for safety considerations and restriction of pollutants entering through grates. Grates shall not operate as inlets. Generally, the surrounding area shall be sloped away from grates.

  • Consider how access will be provided for Vactor trucks for sediment removal.

5.4.23.5 Design Information

As with wetponds, the primary design factor that determines the removal efficiency of a wetvault is the volume of the wetpool. The larger the volume, the higher the potential for pollutant removal. Performance is also improved by avoiding dead zones (like corners) where little exchange occurs, using large length-to-width ratios, dissipating energy at the inlet, and ensuring that flow rates are uniform to the extent possible and not increased between cells.

Refer to BMP T10.20 in Volume V, Chapter 8 of the Ecology Manual for detailed design guidance and criteria on wetvaults.

5.4.23.6 Minimum Construction Requirements

Sediment that has accumulated in the vault shall be removed after construction in the drainage area is complete. If no more than 12 inches of sediment have accumulated after the infrastructure is built, cleaning may be left until after building construction is complete. In general, sediment accumulation from stabilized drainage areas is not expected to exceed an average of 4 inches per year in the first cell. If sediment accumulation is greater than this amount, it will be assumed to be from construction unless it can be shown otherwise.

5.4.23.7 Operations and Maintenance

Refer to BMP T10.20 in Volume V, Chapter 8 of the Ecology Manual for required maintenance practices for wetvaults.