5.4.28 Detention Vaults

5.4.28.1 BMP Description

Detention vaults (BMP D.3 in the Ecology Manual) are box-shaped underground storage facilities typically constructed with reinforced concrete (see Typical Detention Vault figure in Figure V‑12.16: Typical Detention Vault in Volume V, Chapter 12 of the Ecology Manual). Control structure details are shown in V‑12.2 Control Structure Design in Volume V, Chapter 12 of the Ecology Manual.

5.4.28.2 Performance Mechanism

Detention vaults provide peak flow attenuation and control of erosive flow durations by storing and slowly releasing low flows through an outlet control structure.

5.4.28.3 Application and Limitations

Detention vaults can be applied to partially or fully satisfy flow control requirements (see Table II-5.31).

Table II-5.31. Detention Vault 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
Detention Vaults	X						X

5.4.28.4 Site Considerations

Detention vaults are typically shallower than detention pipes, since they can utilize a greater area. Primary site considerations for a detention vaults include providing sufficient access points for maintenance, incorporating the access requirements into a site, conflicts with existing underground utilities, and site setback requirements.

Detention vaults shall have the following minimum setbacks:

• 5 feet from tract or easement lines.

• 20 feet from any structure, property line, and any vegetative buffer required by conditions of land use approval, Title 19 KCC or other applicable codes.

• 50 feet from the top of any slope greater than 15%. A geotechnical analysis and report shall be prepared addressing the potential impact of the BMP on a steep slope to support setbacks less than 50 feet. See report submittal requirements in Volume II, Chapter 1.

Grading and drainage collection on site are important site considerations that can impact flow control effectiveness. Special care is necessary, particularly with roadway projects, to match BMP sizing to actual runoff collected and conveyed to the BMP.

5.4.28.5 Design Information

See Volume V, Chapter 12, BMP D.3 in the Ecology Manual for detailed design guidance and criteria.

5.4.28.6 Minimum Construction Requirements

Refer to the construction-related issues outlined above as part of the design criteria. Additional construction requirements are as follows:

• Conduct infiltration or exfiltration testing of the detention vault.

• Submit field changes to the flow control device assembly, including elevation changes, to the Engineer of Record for confirmation that the device still meets the design requirements.

5.4.28.7 Operations and Maintenance

See Volume V, Chapter 12, BMP D.3 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for closed detention systems, including detention vaults.