A wetvault 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 details in Figure 4 - 38: Wetvault). Being underground, the wetvault lacks the biological pollutant removal mechanisms, such as algae uptake, present in surface wetponds.
15.2.2 Applications and Limitations
Wetvaults can be used to help ensure compliance with Minimum Requirement 6 - Stormwater Treatment. 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 wetpools are allowed; see BMP T1040 - Vegetated Filter Strip.
If oil control is required for a project, a wetvault may be combined with an API oil/water separator.
Below-ground structures like wetvaults are more difficult to inspect and maintain.
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.
Wetvaults may be located upstream or downstream of detention facilities.
The wetpool volume shall be equal to or greater than the water quality design storm volume. The wetpool volume shall be calculated using an Ecology approved continuous simulation model, assuming a 15-minute timestep to obtain the water quality design volume.
Typical design details and concepts for the wetvault are shown in Figure 4 - 38: Wetvault.
Same as specified for wetponds (see BMP T1010 - Basic Biofiltration Swale) except for the following two modifications:
The sediment storage in the first cell shall be an average of 1-foot. Because of the v-shaped bottom, the depth of sediment storage needed above the bottom of the side wall is roughly proportional to vault width according to the schedule below:
The second cell shall be a minimum of 3 feet deep since planting cannot be used to prevent re-suspension of sediment in shallow water as it can in open ponds.
Figure 4 - 38: Wetvault
Separate the vault into two cells by a wall or a removable baffle. If a wall is used, provide a 5-foot by 10-foot removable maintenance access for both cells. If a removable baffle is used, the following criteria apply:
The baffle shall extend from a minimum of 1-foot above the WQ design water surface to a minimum of 1-foot below the invert elevation of the inlet pipe.
The lowest point of the baffle shall be a minimum of 2 feet from the bottom of the vault, and greater if feasible.
If the vault is less than 2,000 cubic feet (inside dimensions), or if the length-to-width ratio of the vault pool is 5:1 or greater, the baffle or wall may be omitted and the vault may be one-celled.
Do not divide the two cells of a wetvault into additional subcells by internal walls. If internal structural support is needed, it is preferred that post and pier construction be used to support the vault lid rather than walls. Any walls used within cells must be positioned so as to lengthen, rather than divide, the flowpath.
Slope the bottom of the first cell toward the access opening. Slope shall be between 0.5 percent (minimum) and 2 percent (maximum). The second cell may be level (longitudinally) sloped toward the outlet, with a high point between the first and second cells. Sloping the second cell towards the access opening for the first cell is also acceptable. Alternatively, access openings shall be positioned a maximum of 10 feet from any location within the vault.
Slope the vault bottom laterally a minimum of 5 percent from each side towards the center, forming a broad "v" to facilitate sediment removal.
Note: More than one "v" may be used to minimize vault depth.
The City may allow the vault bottom to be flat if removable panels are provided over the entire vault. Removable panels shall be at grade, have stainless steel lifting eyes, and weigh no more than 5 tons per panel.
The highest point of a vault bottom must be at least 6 inches below the outlet elevation to provide for sediment storage over the entire bottom.
Evaluate buoyancy of the vault and include results in the Stormwater Site Plan.
Wetvaults may be constructed using arch culvert sections provided the top area at the WQ design water surface is, at a minimum, equal to that of a vault with vertical walls designed with an average depth of 6 feet.
Submerge the inlet to the wetvault. The inlet pipe invert shall be a minimum of 3 feet from the vault bottom. Submerge the top of the inlet pipe at least 1-foot, if possible.
Unless designed as an offline facility, the capacity of the outlet pipe and available head above the outlet pipe shall be designed to convey the 100-year design flow for developed site conditions without overtopping the vault. The available head above the outlet pipe must be a minimum of 6 inches.
The outlet pipe shall be back-sloped or have tee section, the lower arm of which shall extend 1 foot below the WQ design water surface to provide for trapping of oils and floatables in the vault.
Center the inlet and outlet pipes over the “V” portion of the vault.
The inlet and outlet should be at opposing corners of the vault.
The number of inlets to the wetvault should be limited, and the flowpath length should be maximized from inlet to outlet for all inlets to the vault.
Minimum 3,000 psi structural reinforced concrete may be used for wetvaults. Provide all construction joints with water stops.
All vaults must meet structural requirements for overburden support and H20 traffic loading. Vaults located under roadways must meet City of Tacoma live load requirements.
Design cast in place wall sections as retaining walls.
Structural design for cast in place vaults must be stamped by a Washington State Licensed Professional Engineer with structural engineering expertise.
Place vaults on stable, well consolidated native material with suitable bedding. Do not place vaults in fill slopes, unless analyzed by a Washington State Licensed Professional Engineer or Washington State Licensed Professional Geologist for stability.
15.2.3.7 Access and Setback Requirements
Provide access opening over the inlet and outlet pipes.
Position access opening a maximum of 50' from any location within the vault. Provide access to each "v" if more than one "v" is provided in the vault floor.
For vaults greater than 1,250 ft2 of floor area, provide a 5' by 10' removable panel over the inlet.
For vaults under roadways, locate the removal panel outside the travel lanes when possible.
All access openings, except those covered by removable panels, may have round, solid locking lids or 3 ft2 locking diamond plate covers.
Vaults with widths 10 feet or less must have removable lids.
The maximum depth from finished grade to the vault invert should be 20 feet.
Provide internal structural walls of large vaults with openings sufficient for maintenance access between cells. Size and situate the openings to allow access to the maintenance "v" in the vault floor.
The minimum internal height should be 7 feet from the highest point of the vault floor (not sump) and the minimum width should be 4 feet. Concrete vaults may be a minimum 3 feet in height and width if used as tanks with access manholes at each end, and if the width is no larger than the height. Also, the minimum internal height requirements may not be needed for any areas covered by removable panels.
Vaults must comply with the OSHA confined space requirements, which includes clearly marking entrances to confined space areas. This may be accomplished by hanging a removable sign in the access riser(s), just under the access lid.
Provide ventilation pipes (minimum 12 inch diameter or equivalent) in all four corners of vaults to allow for artificial ventilation prior to entry of maintenance personnel into the vault. Or, provide removable panels over the entire vault. Vaults providing maintenance hole access at 12 foot spacing need not provide corner ventilation pipes.
Provide a minimum of 50 square feet of grate over the second cell. For vaults in which the surface area of the second cell is greater than 1,250 square feet, grate 4 percent of the top. This requirement may be met by one grate or by many smaller grates distributed over the second cell area.
A grated access door can be used to meet this requirement.
Lockable grates instead of maintenance hole covers are recommended to increase air contact with the wetpool.
Wetvaults shall be a minimum of 20 feet from any structure or property line.
15.2.4 Modifications for Combining with a Baffle Oil/Water Separator
If the project site is a high-use site and a wetvault is proposed, the vault may be combined with a API oil/water separator to meet the stormwater treatment requirements with one facility rather than two.
The sizing procedures for BMP T1200a - API (Baffle Type) Separators shall be run as a check to ensure the vault is large enough. If the oil/water separator sizing procedures result in a larger vault size, increase the wetvault size to match.
An oil retaining baffle shall be provided in the second cell near the vault outlet. The baffle shall not contain a high-flow overflow, or else the retained oil will be washed out of the vault during large storms.
The vault shall have a minimum length-to-width ratio of 5:1.
The vault shall have a design water depth-to-width ratio of between 1:3 to 1:2.
The vault shall be watertight and shall be coated to protect from corrosion.
Separator vaults shall have a shutoff mechanism on the outlet pipe to prevent oil discharges during maintenance and to provide emergency shut-off capability in case of a spill. Also, provide a valve box and riser.
Wetvaults used as oil/water separators must be offline and must bypass flows greater than the offline WQ design flow multiplied by 3.5. This will minimize the entrainment and/or emulsification of previously captured oil during very high flow events.