5.4.27
5.4.27.1 BMP Description
Detention tanks are underground detention BMPs typically constructed with large diameter corrugated metal pipe. Standard detention tank details are shown in the Typical Detention Tank and Detention Tank Access Detail figures in Volume V, Chapter 12 in the Ecology Manual. Control structure details are shown in Volume V, Section 12.2 in the Ecology Manual.
5.4.27.2 Performance Mechanism
Detention tanks provide peak flow attenuation and control of erosive flow durations by storing and slowly releasing stored flows through an outlet control structure.
5.4.27.3 Application and Limitations
Detention tanks can be applied to partially or fully satisfy flow control requirements (see Table II-5.30).
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Detention Tank |
|
|
X | ||||
5.4.27.4 Site Considerations
The primary site considerations for detention tanks include conflicts with existing underground utilities and required setbacks, as follows:
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Detention tanks shall be a minimum of 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.
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All detention tanks shall be a minimum of 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.
5.4.27.5 Design Information
See BMP D.2 in Volume V, Chapter 12 of the Ecology Manual for detailed design guidance and criteria. The Engineer shall provide buoyancy calculations to verify that the tank will not move due to floatation forces in the event of high ground water. An appropriate factor of safety shall be used.
5.4.27.6 Minimum Construction Requirements
Construction requirements are as follows:
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Place at least 4 inches of bedding under the tank. The bedding shall fill the trench to a point half–way up the sides of the tank (to the “spring line”).
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Provide at least 2 feet of cover over a detention tank. For single-family and duplex residences, 18 inches of cover is allowable. Prior to permit issuance, a Kitsap County inspector shall approve the installed system, including the detention pipe and the flow control structure, after it is bedded but before it is covered with soil.
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The standard slope for detention tanks is 0.5%. The inlet pipe to the detention tank and the outlet pipe from the flow control structure shall have at least a 2% slope.
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Follow the manufacturer’s recommendations if backfill and bedding requirements are more stringent than the requirements in this section.
Field changes to the flow control device assembly, including elevation changes, require submittal to Kitsap County by the Engineer of Record, prior to installation, for confirmation that the device still meets the design requirements.
5.4.27.7 Operations and Maintenance
See Volume V, Chapter 12, BMP D.2 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for closed detention systems, including detention tanks.