5.4.22
5.4.22.1 BMP Description
Wetponds (BMP T10.10: Wetponds – Basic and Large in Volume V, Chapter 8 of the Ecology Manual) are constructed stormwater ponds that retain a permanent pool of water (i.e., a wetpool or dead storage) at least during the wet season. The volume of the wetpool is related to the effectiveness of the pond in settling particulate pollutants. As an option, a shallow marsh area can be created within the permanent pool volume to provide additional treatment for nutrient removal. Peak flow control can be provided in the "live storage" area above the permanent pool. Wetpond (Plan View) and Wetpond (Section View) in Volume V, Chapter 8 of the Ecology Manual illustrate a typical wetpond BMP.
As an option, a shallow marsh area can be created within the permanent pool volume to provide additional treatment for nutrient removal. Peak control can be provided in the live storage area above the permanent pool.
5.4.22.2 Performance Mechanism
The volume of the wetpool, which slows down the velocity of influent stormwater, allows particulates and particulate-bound pollutants to settle and is a key factor in determining wetpond effectiveness. Biological uptake also acts as a secondary pollutant removal mechanism.
5.4.22.3 Application and Limitations
A wetpond requires a larger area than a sand filter (see Volume V, Chapter 6 of the Ecology Manual), but it can be integrated to the contours of a site fairly easily. In till soils, the wetpond holds a permanent pool of water that provides an attractive aesthetic feature. In more porous soils, wetponds may still be used, but water seepage from unlined cells could result in a dry pond, particularly in the summer months. Lining the first cell with a low permeability liner (see Volume V, Section 1.3.3 of the Ecology Manual) is one way to deal with this situation. As long as the first cell retains a permanent pool of water, the pond will function as an effective Runoff Treatment BMP.
Wetponds work best when the water already in the pond is moved out en masse by incoming flows, a phenomenon called "plug flow.” Because treatment works on this displacement principle, the wetpool storage of wetponds may be provided below the ground water level without interfering unduly with treatment effectiveness. However, if combined with a detention function, the live storage shall be above the seasonal high ground water level.
Wetponds may be single-purpose BMPs, providing only runoff treatment, or they may be combined with a detention pond to also provide flow control. If combined, the wetpond can often be stacked under the detention pond with little further loss of development area. Refer to Vol II–5.4.29 Combined Detention and Wetpool Facilities for a description of combined detention and wetpool BMPs. See Table II-5.25 for a summary of runoff treatment requirements that wetpond BMPs can be designed to meet.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
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|
Basic Wetpond |
|
X |
Xb |
Xc |
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|
Large Wetponda |
X |
X |
X |
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|
Notes:
|
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5.4.22.4 Site Considerations
Wetponds require a larger area than a bioretention swale or a sand filter but can be integrated into the contours of a site fairly easily and function well for any size project. The following considerations apply to siting of wetponds:
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The location of the wetpond relative to site constraints (e.g., buildings, property lines, etc.) shall be the same as for detention ponds (Vol II–5.4.26 Detention Ponds).
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Access and maintenance roads shall be provided and designed according to the requirements for detention ponds (Vol II–5.4.26 Detention Ponds)
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Access and maintenance roads shall extend to both the wetpond inlet and outlet structures.
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An access ramp (7H minimum:1V) shall be provided to the bottom of the first cell unless all portions of the cell can be reached, and sediment loaded from the top of the pond. See Vol II–5.4.26 Detention Ponds, "Access Requirements" for more information on access alternatives.
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If the dividing berm is also used for access, it shall be built to sustain loads of up to 80,000 pounds.
Refer to BMP T10.10 in Volume V, Chapter 8 of the Ecology Manual for additional wetpond siting requirements.
5.4.22.5 Design Information
Refer to BMP T10.10 in Volume V, Chapter 8 of the Ecology Manual for detailed design criteria. Planting the wetpond is required by Kitsap County. See Appendix D of the Kitsap County Stormwater Pond Retrofit Design Guidance Manual (Herrera 2012) for a recommended plant list for wetponds.
5.4.22.6 Minimum Construction Requirements
Minimum construction requirements include the following:
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Sediment that has accumulated in the pond shall be removed after construction in the drainage area of the pond is complete (unless used for a liner – see below).
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Sediment that has accumulated in the pond at the end of construction may be used in excessively drained soils to meet the liner requirements if the sediment meets the criteria for low permeability or treatment liners in keeping with guidance in Volume V, Section 1.3 of the Ecology Manual.
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Sediment used for a soil liner shall be graded to provide uniform coverage and shall meet the thickness specifications in Volume V, Section 1.3 of the Ecology Manual. The sediment shall not reduce the design volume of the pond. The pond shall be over-excavated initially to provide sufficient room for the sediments to serve as a liner.
5.4.22.7 Operations and Maintenance
Refer to BMP T10.10 in Volume V, Chapter 8 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for wetponds.