5.4.26
Detention ponds (BMP D.1 in Volume V, Chapter 12 of the Ecology Manual) are basins that provide temporary storage of stormwater runoff resulting from development and are designed to release flows at a controlled rate. Detention ponds can also be combined with runoff treatment BMPs, as described further in Vol II–5.4.29 Combined Detention and Wetpool Facilities.
5.4.26.1 Performance Mechanisms
Detention ponds provide peak flow attenuation and control of erosive flow durations by slowly releasing stored flows through an outlet control structure.
5.4.26.2 Applications and Limitations
Detention ponds can be designed to meet or partially meet the flow control requirement, MR #7 (see Table II-5.29).
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Detention Ponds |
|
|
X | ||||
5.4.26.3 Site Considerations
Detention ponds generally require a large amount of area, and shall also include maintenance access roads to, around, and sometimes into the ponds.
The following setback requirements apply to detention ponds:
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The toe of the exterior slope of a detention pond berm embankment shall comply with the required grading setbacks per Volume II, Chapter 9, and shall be set back a minimum of 5 feet from the tract, easement, property line and any vegetative buffer required by the conditions of the land use approval.
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The tract, easement, or property line on a pond cut slope shall be set back a minimum of 10 feet from the emergency overflow water surface.
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Stormwater facilities shall comply with KPHD regulations for setbacks to onsite sewage systems, wells, and other features regulated by KPHD.
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All detention ponds shall be a minimum of 50 feet away from the top of any slope greater than 15% and a minimum of 200 feet from the top of any slope greater than or equal to 30%. These distances may be reduced based on recommendation and justification by a licensed geotechnical engineer. A geotechnical analysis and report shall be prepared addressing the potential impact of the BMP on slopes 15% or greater or otherwise sensitive slopes. See report submittal requirements in Volume II, Chapter 1.
5.4.26.4 Design Information
See BMP D.1 in Volume V, Chapter 12 of the Ecology Manual for detailed design guidance on detention ponds. Fencing shall be installed according to the Ecology Manual with the following exceptions:
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Bench at Fence Line: A minimum of 5 feet flat bench area with the fencing placed in the middle shall be required for all fencing around ponds where the slope is 3H:1V or greater.
Note that the use of AutoPond in the Western Washington Hydrology Model (WWHM) may not result in a buildable pond that would meet the control structure requirements in Volume V, Section 12.2 of the Ecology Manual. Sizing results from AutoPond shall be compared against the applicable design requirements and pond dimensions shall be modified as needed to meet the requirements.
5.4.26.5 Minimum Construction Requirements
The following construction requirements shall be considered during construction of a detention pond:
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Detention ponds may be used for sediment control during site construction, but sediment shall be removed upon completion.
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Exposed earth on the pond bottom and interior side slopes shall be vegetated or seeded with an appropriate seed mixture.
5.4.26.6 Operations and Maintenance Requirements
Maintenance activities and frequencies for detention ponds are provided in BMP D.1 in Volume V, Chapter 12 of the Ecology Manual and Volume V, Appendix V‑A of the Ecology Manual.
Handle any standing water and sediments removed during the maintenance operation in a manner consistent with Appendix IV‑B of the Ecology Manual.