Stormwater Management Standards Manual - Toledo Metropolitan ...

5.3.3 Wet Detention Ponds
Wet detention ponds are one of the most widely accepted BMP’s for meeting water quantity
requirements and providing limited stormwater quality treatment. The “wet pond” improves water quality
by detaining the stormwater for an extended period of time in a permanent pool, which allows pollutants
to settle out. Runoff from each rain event is detained and treated in the pond until it is displaced by
runoff from the next storm event. Pollutants removed include suspended solids, organic matter,
dissolved metals, and nutrients.
There are several common modifications that can be made to the ponds to increase their pollutant
removal effectiveness. The first is to increase the settling area for sediments through the addition of a
sediment forebay. Heavier sediments will drop out of suspension as runoff passes through the sediment
forebay, while lighter sediments will settle out as the runoff is retained in the permanent pool. A second
common modification is the construction of shallow ledges along the edge of the permanent pool. The
ledges allow emergent wetland plants to grow, adding some of the biological benefits of a stormwater
wetland (Section 5.4.2).
General Applicability
Land Area
Land constraints, such as small sites or highly developed areas, may preclude the installation of a wet
pond. The wet pond BMP is most applicable for large industrial and commercial facilities and residential
subdivisions. Owners of smaller properties may cooperatively construct one large pond to serve several
sites. Fewer, cooperative ponds are preferred to numerous individual smaller ponds. Wet detention
ponds are effective in meeting both stormwater quantity and quality goals. Additionally, they can provide
an amenity to properties through “water front” designation, fountain pools, wildlife habitat, etc.
Soils
Wet detention ponds must be able to maintain a permanent pool of water. Therefore, similar to
stormwater wetlands, wet ponds are recommended for medium to large drainage areas (greater than 10
acres). Sites with highly permeable soils (e.g. in the "A" or "B" hydrologic groupings) may require clay or
geo-textile liners to help maintain water levels.
Retrofit
Wet ponds provide opportunities for retrofit coverage for existing development. With minor excavation
and/or modification of the outlet, existing dry ponds can be converted to wet ponds. The retrofits can
generate greater water quality benefits for the receiving stream, helping the (community) meet water
quality goals.
Third Edition Stormwater Standards Manual September 2008 Page 94