DEFINITION
Wetponds (a.k.a. wet detention pond) is a type of structural measure used to manage stormwater.
Unlike the typical dry pond, the wetpond contains a permanent pool of water.
It can be designed to control both the quantity and increase the quality of water that is eventually discharged from it.

AKA
retention pond

PERFORMANCE CRITERIA
how is it measured?
The criteria that must be used to design the facility may vary depending on the jurisdiction in which the project is located.
Each jurisdiction can establish their own criteria.
For example, in Florida the design and resulting performance criteria is established in the Florida Administrative Code (F.A.C.) 42.026(4), Specific Design and Performance Criteria) .
Qouting the relevant portion of the F.A.C. :
(4) Wet detention stormwater management systems shall:
(a) Provide a treatment volume of the greater of the following:
1. First one inch of runoff; or
2. 2.5 inches of runoff from the impervious area.
(b) Be designed so that the outfall structures shall bleed down one-half the volume of stormwater specified in paragraph (a), above, within 24 to 30 hours following a storm event, but no more than one-half of this volume will be discharged within the first 24 hours.
(c) Contain a permanent pool of water sized to provide an average residence time of at least 14 days during the wet season (June-October).
(d)1. Provide a littoral zone to be designed as follows:
a. The littoral zone shall be gently sloped (6:1 or flatter). At least 30 percent of the wet detention system surface area shall consist of a littoral zone. The percentage of littoral zone is based on the ratio of vegetated littoral zone to surface area of the pond at the control elevation.
b. The treatment volume should not cause the pond level to rise more than 18 inches above the control elevation unless the applicant affirmatively demonstrates that the littoral zone vegetation can survive at greater depths.
c. Eighty percent coverage of the littoral zone by suitable aquatic plants is required within the first twenty-four months of completion of the system or as specified by permit conditions.
d. To meet the 80% coverage requirement, planting of the littoral zone is recommended. As an alternative, portions of the littoral zone may be established by placement of wetland top soils (at least a four inch depth) containing a seed source of desirable native plants. When utilizing this alternative, the littoral zone must be stabilized by mulching or other means and at least the portion of the littoral zone within 25 feet of the inlet and outlet structures must be planted.
2. In lieu of the requirements of subparagraph 1., above, the applicant may provide either of the following:
a. At least fifty percent additional permanent pool volume over that specified in paragraph (c), above; or
b. Treatment of the stormwater pursuant to subparagraphs 40C-42.024(2)(b)2., 3., 4., or 6., F.A.C., prior to the stormwater entering the wet detention pond.
(e) Be designed so that the mean depth of the permanent pool is between 2 and 8 feet and the maximum depth does not exceed 12 feet below the invert of the bleed down device, unless the applicant affirmatively demonstrates that alternative depths will not inhibit the physical, chemical, and biological treatment processes or cause the resuspension of pollutants into the water column due to anaerobic conditions in the water column.
(f) Be designed so the flow path through the pond has an average length to width ratio of at least 2:1. The alignment and location of inlets and outlets should be designed to maximize flow paths in the pond. If short flow paths are unavoidable, the effective flow path should be increased by adding diversion barriers such as islands, peninsulas, or baffles to the pond. Inlet structures shall be designed to dissipate the energy of water entering the pond.
(g) Be designed so that bleed down devices shall incorporate minimum dimensions no smaller than six square inches of cross section area, two inches wide, and 20 degrees for “v” notches. Bleed down devices incorporating dimensions smaller than six inches minimum width or less than 45 degrees for “v” notches shall include a device to minimize clogging. Examples include baffles, grates, and pipe elbows.
(h) Be designed so that bleed down structure invert elevations are at or above the estimated post-development normal ground water table elevation. If the structure is proposed to be set below this elevation, ground water inflow must be considered in the drawdown calculations, calculation of average residence time, estimated normal water level in the pond, and pollution removal efficiency of the system.
(i) Provide for permanent maintenance easements or other acceptable legal instruments to allow for access to and maintenance of the system, including the pond, littoral zone, inlets, and outlets. The easement or other acceptable instrument must cover the entire littoral zone.
(j) Be designed so that the average pond side slope measured between the control elevation and two feet below the control elevation is no steeper than 3:1 (horizontal:vertical).
(k) Wet detention systems which have direct discharge to Class I, Class II, Outstanding Florida Waters, or Class III waters which are approved, conditionally approved, restricted, or conditionally restricted for shellfish harvesting shall provide either of the following in addition to the requirements in paragraphs (b), (d), and (e)-(j), above:
1. An additional fifty percent of the applicable treatment volume specified in paragraph (a), above, and an additional fifty percent of the applicable permanent pool volumes specified in paragraph (c) or subparagraph (d)2., above; or 2. Treatment pursuant to subsections (1), (2), (3) above, or (5) below, prior to discharging into a wet detention pond designed pursuant to paragraphs (a)-(j), above.


Generic information may be found at http://www.epa.gov/OW-OWM.html/mtb/wetdtnpn.pdf

ILLUSTRATION


CASE STUDIES
Meadowbrook Pond
High Point
Waterworks Gardens

ADDITIONAL RESOURCES