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Jul 02, 2018
2004 Connecticut Stormwater Quality Manual 11-PI-1
DescriptionStormwater ponds are vegetated ponds that retain a permanent pool ofwater and are constructed to provide both treatment and attenuation ofstormwater flows. This section addresses four types of stormwater ponds:
Micropool Extended Detention Pond
Wet Extended Detention Pond
Multiple Pond System
Through careful design, stormwater ponds can be effective at removingurban pollutants. Treatment is primarily achieved by the sedimentationprocess where suspended particles and pollutants settle to the bottom of thepond. Stormwater ponds can also potentially reduce soluble pollutants instormwater discharges by adsorption to sediment, bacterial decomposition,and the biological processes of aquatic and fringe wetland vegetation.
The key to maximizing the pollutant removal effectiveness ofstormwater ponds is maintaining a permanent pool. To achieve this, wetponds typically require a large contributing watershed with either animpermeable liner or an elevated water table without a liner. The pool typ-ically operates on the instantaneously mixed reservoir principle whereincoming water mixes with the existing pool and undergoes treatmentthrough sedimentation and the other processes. When the existing pool isat or near the pond outlet or when the primary flow path through the pondis highly linear, the pond may act as a plug flow system in which incom-ing water displaces the permanent pool, which is then discharged from the pond. The value provided by this process is that a portion of the new, polluted runoff is retained as the old, treated water is dischargedfrom the pond, thereby allowing extended treatment of the water qualityvolume (WQV). For example, when sized to store the WQV, a pond system will retain all of the water from storms that generate runoff less thanor equal to the WQV and result in a significantly increased period of timeavailable for treatment. For storms that generate runoff greater than the WQV, wet ponds still provide a reduced level of treatment through
Treatment Practice Type
Primary Treatment Practice Secondary Treatment Practice
Stormwater ManagementBenefitsPollutant Reduction
Sediment Phosphorus Nitrogen Metals Pathogens Floatables* Oil and Grease* Dissolved Pollutants
Runoff Volume ReductionRunoff Capture Groundwater Recharge
Stream Channel Protection
Peak Flow Control
Key: Significant Benefit Partial Benefit Low or Unknown
*Only if a skimmer is incorporated
Source: Nonpoint Education for Municipal Officials (NEMO).
2004 Connecticut Stormwater Quality Manual11-PI-2
conventional settling and filtration for the additionalrunoff volume that is conveyed through the pond.The pond volume should be greater than or equal tothe WQV to ensure at least one-day retention timewithin the pond.
When properly designed, the permanent poolreduces the velocity of incoming water to preventresuspension of particles and promote settling ofnewly introduced suspended solids. The energy dissi-pating and treatment properties of the permanentpool are enhanced by aquatic vegetation, which is anessential part of the stormwater pond design. In con-trast, dry detention ponds, or dry extended detentionponds that have no permanent pool, are not consid-ered an acceptable option for treating the WQV dueto the potential for resuspension of accumulated sed-iment by incoming storm flows during the earlyportion of a storm event when the pond is empty.
Several design variations of stormwater pondsexist that can fit a wide range of design conditions.Descriptions of these design variations are providedin the following section.
Design VariationsWet Ponds: Wet ponds typically consist of two gen-eral components - a forebay and a permanent wetpool. The forebay provides pretreatment by captur-ing coarse sediment particles in order to minimizethe need to remove the sediments from the primarywet pool. The wet pool serves as the primary treat-ment mechanism and where much of the retentioncapacity exists. Wet ponds can be sized for a widerange of watershed sizes, if adequate space exists.For example, a variation on the conventional wetpond, sometimes referred to as a pocket pond, isintended to serve relatively small drainage areas(between one and five acres). Because of thesesmaller drainage areas and the resulting lowerhydraulic loads of pocket ponds, outlet structurescan be simplified and often do not have safety fea-tures such as emergency spillways and low leveldrains. Figure 11-P1-1 depicts a typical schematicdesign of a conventional wet pond, while Figure11-P1-2 shows a typical schematic design of a mod-ified wet pond or pocket pond.
Several adaptations of this basic design havebeen developed to achieve the specific treatmentgoals of various watershed or site conditions. Thesewet pond design variations are described below.
Micropool Extended Detention Pond: Micropoolextended detention basins are primarily used for peakrunoff control and utilize a smaller permanent poolthan conventional wet ponds. While micropoolextended detention ponds are not as efficient as wetponds for the removal of pollutants, they should be
considered when a large open pool might be unde-sirable or unacceptable. Undesirable conditions couldinclude thermal impacts to receiving streams from alarge open pool, safety concerns in residential areas,or where maintaining a large open pool of waterwould be difficult due to a limited drainage area ordeep groundwater.
Micropool extended detention ponds are alsoefficient as a stormwater retrofit to improve the treat-ment performance of existing detention basins.Figure 11-P1-3 depicts a typical schematic design ofa micropool extended detention pond.
Wet Extended Detention Ponds: These ponds arevery similar to wet ponds with the exception that theirdesign is more focused on attenuating peak runoffflows. As a result, more storage volume is committedto managing peak flows as opposed to maximizingthe wet pool depth. The configuration of the outfallstructure may also differ from typical wet ponddesigns to provide additional storage volume abovethe level of the permanent pool. Figure 11-P1-4depicts a typical schematic design of a wet extendeddetention pond.
Multiple Pond System: Multiple pond systems con-sist of several wet pools that are constructed in aseries following a forebay. The advantage of thesesystems is that they can improve treatment efficiencyby better simulating plug flow conditions as com-pared to a single large wet pool. Also, these systemscan reduce overall maintenance needs since more fre-quent maintenance would be performed within thefirst pool cells as opposed to the large, primary pool.The disadvantage of these systems is that they typi-cally require more land area to treat the same waterquality volume. Figure 11-P1-5 depicts a typicalschematic design of a multiple pond system.
Advantages Can capture/treat both particulate and soluble
pollutants. Stormwater ponds are one of the mosteffective stormwater treatment practices for treat-ing soluble pollutants.
Can provide an aesthetic benefit if open water isdesired as part of an overall landscaping plan.
May provide wildlife habitat with appropriatedesign elements.
Can be adapted to fit a wide range of sites.Design variations allow this control to be uti-lized for both small and large drainage areas.Pollutant removal mechanisms make stormwaterponds efficient in treatment of pollutants-of-concern from a wide range of land uses.
2004 Connecticut Stormwater Quality Manual 11-PI-3
Figure 11-P1-1 Wet Pond
Source: Adapted from NYDEC, 2001.
Pond Buffer(10 to 50 feet)
Native Landscaping Around Pool
Riser in Embankment
Irregular Pool Shape6 to 8 Feet Deep
Anti-Seep Collar or Filter Diaphragm
Extreme Flood Control
Overbank Flood Control
Channel ProtectionSafety Bench
Water QualityWet Pond
2004 Connecticut Stormwater Quality Manual11-PI-4
Limitations Unlined ponds that intercept groundwater
have potential to impact groundwater quality ifdissolved pollutants are present in the runoff.
Lined ponds typically require a minimumdrainage area in order to maintain a perma-nent pool, which may become difficult duringextended dry periods.
Require a relatively large land area that isdirectly proportional to the size of the areadraining to it.
May cause thermal impacts to receiving watersand thereby are not recommended to dischargedirectly to cold water fish habitats.
Require more storage volume (i.e., above perma-nent pool) to attenuate peak flows.
Potential breeding habitat for mosquitoes, partic-ularly for smaller ponds with stagnant water orisolated pockets of standing water (rather thanlarge open water bodies). Circulating water inthe permanent pool may minimize this problem.This may be a more significant problem forlined basins.
Pollutant removal efficiency can be affected incold climates due to ice formation on the perma-nent pool and longer particle settling timesassociated with higher density water during winter months. However, modifications to aponds design can help maintai