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Stormwater Ponds - Connecticut Stormwater Quality Manual 11-PI-1 Stormwater Ponds Description Stormwater ponds are vegetated ponds that retain a permanent pool of water and are constructed

Jul 02, 2018

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  • 2004 Connecticut Stormwater Quality Manual 11-PI-1

    Stormwater Ponds

    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:

    Wet Pond

    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

    Benefit

    *Only if a skimmer is incorporated

    Implementation Requirements

    Cost ........................................ModerateMaintenance.........................Moderate

    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)

    Hardened Pad

    Overflow Spillway

    MaintenanceAccess Road

    Native Landscaping Around Pool

    Aquatic Bench

    Safety Bench

    Riser in Embankment

    Riser/Barrel

    Outfall

    Emergency Spillway

    Irregular Pool Shape6 to 8 Feet Deep

    Forebay

    Plan View

    Section

    Embankment

    Riser

    Emergency Spillway

    StableOutfall

    Anti-Seep Collar or Filter Diaphragm

    BarrelReverse Pipe

    Pond Drain

    Extreme Flood Control

    Overbank Flood Control

    Channel ProtectionSafety Bench

    Water QualityWet Pond

    AquaticBench

    Forebay

    Inflow

    Inflow

    Berm

  • 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

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