v>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
EPA 832-F-99-048
September 1999
Storm Water
Technology Fact Sheet
Wet Detention Ponds
DESCRIPTION
Wet detention ponds are storm water control
structures providing both retention and treatment of
contaminated storm water runoff. A typical wet
detention pond design is shown in Figure 1. The
pond consists of a permanent pool of water into
which storm water runoff is directed. Runoff from
each rain event is detained and treated in the pond
until it is displaced by runoff from the next storm.
By capturing and retaining runoff during storm
events, wet detention ponds control both storm
water quantity and quality. The pond's natural
physical, biological, and chemical processes then
work to remove pollutants. Sedimentation
processes remove particulates, organic matter, and
metals, while dissolved metals and nutrients are
removed through biological uptake. In general, a
higher level of nutrient removal and better storm
water quantity control can be achieved in wet
Principal Release Pipe
Set on Negative Slope
to Prevent Clogging
Riser with Trash Rack
Emergency
Spillway
Deep Water Zone for
Gravity Settling
Riprap
Riprap for Shoreline
Protection
Inlet
Sediment Forebay
Cutoff Trench
Concrete
Base
Low Flow Drain for Pond Maintenance
(should be designed to provide easy access and to
avoid clogging by trapped sediments.)
Source: Maryland Department of the Environment, 1986.
FIGURE 1 TYPICAL LAYOUT OF A WET DETENTION POND
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detention ponds than can be achieved with other
Best Management Practices (BMPs), such as dry
ponds, infiltration trenches, or sand filters.
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, as shown in Figure 1. 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. These
shallow peripheral ledges can be used to establish
aquatic plants that can impede flow and trap
pollutants as they enter the pond. The plants also
increase biological uptake of nutrients. In addition
to their function as aquatic plant habitat, the ledges
also have several other functions, which can include
including acting as a safety precaution to prevent
accidental drowning and providing easy access to
the permanent pool to aid in maintenance. Finally,
perimeter wetland areas can also be created around
the pond to aid in pollutant removal.
APPLICABILITY
Wet detention ponds have been widely used
throughout the U. S. for many years. Many of these
ponds have been monitored to determine their
performance. EPA Region V is currently
performing a study on the effectiveness of 50 to 60
wet detention ponds. Other organizations, such as
the Washington, D.C., Council of Governments
(WMCOG) and the Maryland Department of the
Environment, have also conducted extensive
evaluations of wet detention pond performance.
ADVANTAGES AND DISADVANTAGES
Wet detention ponds provide both storm water
quantity and quality benefits, and provide
significant retrofit coverage for existing
development. Benefits include decreased potential
for downstream flooding and stream bank erosion
and improved water quality due to the removal of
suspended solids, metals, and dissolved nutrients.
While the positive impacts from a wet detention
ponds will generally exceed any negative impacts,
wet detention ponds that are improperly designed,
sited, or maintained, may have potential adverse
affects on water quality, groundwater, cold water
fisheries, or wetlands. Improperly designed or
maintained ponds may result in stratification and
anoxic conditions that can promote the resuspension
of solids and the release of nutrients and metals
from the trapped sediments. In addition,
precautions should be taken to prevent damage to
wetland areas during pond construction. Finally, the
potential for groundwater contamination should be
carefully evaluated. However, studies to date
indicate that wet detention ponds do not
significantly contribute to groundwater
contamination (Schueler, 1992).
The following limitation should also be considered
when determining the feasibility of installing a wet
detention pond:
1. Wet detention ponds must be able to
maintain a permanent pool of water.
Therefore, ponds cannot be constructed in
areas where there is insufficient
precipitation to maintain the pool or in soils
that are highly permeable. In wetter regions,
a small drainage area may be sufficient to
ensure that there is enough water to
maintain a permanent pool; whereas in more
arid regions, a larger drainage area may be
required. In some cases, soils that are highly
permeable may be compacted or overlaid
with clay blankets to make the bottom less
permeable.
2. Land constraints, such as small sites or
highly developed areas, may preclude the
installation of a pond.
3. Discharges from ponds usually consist of
warm water, and thus pond use may be
limited in areas where warm water
discharges from the pond will adversely
impact a cold water fishery.
4. The local climate (i.e., temperature) may
affect the biological uptake in the pond.
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5. Without proper maintenance, the
performance of the pond will drop off
sharply. Regular cleaning of the forebays is
particularly important. Maintaining the
permanent pool is also important in
preventing the resuspension of trapped
sediments. The accumulation of sediments
in the pond will reduce the pond's storage
capacity and cause a decline in its
performance. Therefore, the bottom
sediments in the permanent pool should be
removed about every 2 to 5 years. In most
cases, no specific limitations have been
placed on disposal of sediments removed
from wet detention ponds. Studies to date
indicate that pond sediments are likely to
meet toxicity limits and can be safely
landfilled (NVPDC, 1992). Some states
have allowed sediment disposal on-site, as
long as the sediments are deposited away
from the shoreline to prevent their re-entry
into the pond.
DESIGN CRITERIA
In general, pond designs are unique for each site
and application. Criteria for selecting the site for
installation of the pond should include the site's
ability to support the pond environment, as well as
the cost effectiveness of locating a pond at that
specific site. In addition, the pond should be
located where the topography of the site allows for
maximum storage at minimum construction costs
(NVPDC, 1992). Site-specific constraints for pond
construction may include wetlands impacts,
existing utilities (e.g., electric or gas) that would be
costly to relocate, and underlying bedrock that
would require expensive blasting operations to
excavate.
The site must have adequate base-flow from the
groundwater or from the drainage area to maintain
the permanent pool. Typically, underlying soils
with permeabilities of between 10"5 and 10"6 cm/sec
will be adequate to maintain a permanent pool.
All local, state and federal permit requirements
should be established prior to initiating the pond
design. Depending on the location of the pond,
required permits and certifications may include
wetland permits, water quality certifications, dam
safety permits, sediment and erosion control plans,
waterway permits, local grading permits, land use
approvals, etc.(Schueler, 1992). Since many states
and municipalities are still in the process of
developing or modifying storm water permit
requirements, the applicable requirements should be
confirmed with the appropriate regulatory
authorities.
Wet detention ponds should be designed to meet
both storm water quality and quantity control
requirements. Storm water quantity requirements
are typically met by designing the pond to control
post-development peak discharge rates to
pre-development levels. Usually the pond is
designed to control multiple design storms (e.g. 2-
and/or 10-year storms) and safely pass the 100-year
storm event. However, the design storm may vary
depending on local conditions and requirements.
Storm water quality control is achieved through
pollutant removal in the permanent pool. Removal
efficiency is primarily dependent on the length of
time that runoff remains in the pond, which is
known as the pond's Hydraulic Residence Time
(HRT). As discussed above, wet detention ponds
remove pollutants through both sedimentation and
biological uptake processes, both of which increase
with the length of time runoff remains in the pond.
These processes can be modeled to determine a
design HRT using either the solids settling method
or the eutrophication method, respectively
(Hartigan, 1988).
The calculated HRT will be dependent on the
method selected. HRTs calculated by the
eutrophication method can be up to three times
greater than HRTs calculated by the solids settling
method. The longer HRTs associated with the
eutrophication method appear to be due to the
slower reaction rates associated with the biological
removal of dissolved nutrients (Hartigan, 1988).
Once the design HRT has been determined, the
actual dimensions of the pond must be calculated to
achieve the design HRT. The primary factor
contributing to a pond's HRT is its volume.
Because many wet detention ponds are restricted in
area, pond depth can be an important factor in the
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pond's overall volume. However, the depth of the
pool also affects many of the pond's removal
processes, and so it must be carefully controlled. It
is important to maintain a sufficient permanent pool
depth in order to prevent the resuspension of
trapped sediments (NVPDC, 1992). Conversely,
thermal stratification and anoxic conditions in the
bottom layer might develop if permanent pool
depths are too great. Stratification and anoxic
conditions may decrease biological activity.
Anoxic conditions may also increase the potential
for the release of phosphorus and heavy metals
from the pond sediments (NVPDC, 1992). These
factors dictate that the permanent pool depth should
not exceed 6 meters (20 feet). The optimal depth
ranges between 1 and 3 meters (3 and 9 feet) for
most regions, given a 2 week HRT (Hartigan,
1988).
Other key factors to be considered in the pond
design are the volume and area ratios. The volume
ratio, VB/VR, is the ratio of the permanent pool
storage (VB) to the mean storm runoff (VR).
Larger VBs and smaller VRs provide for increased
retention and treatment between storm events. Low
VB/VR ratios result in poor pollutant removal
efficiencies.
The area ratio, A/As, is the ratio of the contributing
drainage area (A) to the permanent pool surface
area (As). The area ratio is also an indicator of
pollutant removal efficiency. Data from previous
studies indicates that area ratios of less than 100
typically have better pollutant removal efficiencies
(MDDEQ, 1986).
The contours of the pond are also important. The
pond should be constructed with adequate slopes
and lengths. While a length-to-width ratio is
usually not used in the design of wet detention
ponds for storm water quantity management, a 2:1
length-to-width ratio is commonly used when water
quality is of concern. In general, high
length-to-width ratios (greater than 2:1) will
decrease the possibility of short-circuiting and will
enhance sedimentation within the permanent pool.
Baffles or islands can also be added within the
permanent pool to increase the flow path (Hartigan,
1988). Shoreline slopes between 5:1 and 10:1 are
common and allow easy access for maintenance,
such as mowing and sediment removal (Hartigan,
1988). In addition, wetland vegetation is difficultto
establish and maintain on slopes steeper than 10:1.
Ponds should be wedge-shaped so that flow enters
the pond and gradually spreads out. This minimizes
the potential for zones with little or no flow
(Urbonas, 1993).
The design of the wet pond embankment is another
key factor to be considered. Proper design and
construction of the embankments will prolong the
integrity of the pond structure. Subsidence and
settling will likely occur after an embankment is
constructed. Therefore during construction, the
embankment should be overfilled by at least 5
percent (SEWRPC, 1991). Seepage through the
embankment can also affect the stability of the
structure. Seepage can generally be minimized by
adding drains, anti-seepage collars, and core
trenches. The embankment side slopes can be
protected from erosion by using minimum side
slopes of 2:1 and by covering the embankment with
vegetation or rip-rap. The embankment should also
have a minimum top width of 2 meters (6 feet) to
aid in maintenance.
Finally, the internal flow control of the pond must
be considered. Discharge from the pond is
controlled by a riser and an inverted release pipe.
Normal flows will be discharged through the wet
pond outlet, which consists of a concrete or
corrugated metal riser and barrel. The riser is a
vertical pipe or inlet structure that is attached to the
base with a watertight connection. Risers are
typically placed in or adjacent to the embankment
rather than in the middle of the pond. This provides
easy access for maintenance and prevents the use of
the riser as a recreation spot (e.g. diving platform
for kids) (Schueler, 1988). The barrel is a
horizontal pipe attached to the riser that conveys
flow under the embankment.
Typically, flow passes through an inverted pipe
attached to the riser, as shown in Figure 1, while
higher flows will pass through a trash rack installed
on the riser. The inverted pipe should discharge
water from below the pond water surface to prevent
floatables from clogging the pipe and to avoid
discharging the warmer surface water. Clogging of
the pipe could result in overtopping of the
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embankment and damage to the embankment
(NVPDC, 1992). Flow is conveyed through the
near horizontal barrel and is discharged to the
receiving stream. Rip-rap, plunge pools, or other
energy dissipators, should be placed at the outlet to
prevent scouring and to minimize erosion. Rip-rap
also provides a secondary benefit of re-aeration of
the pond discharges.
Planners should consider both the design storm and
potential construction materials when designing and
constructing the riser and barrel. Generally, the
riser and barrel are sized to meet the storm water
management design criteria (e.g. to pass a 2-year or
a 10-year storm event). In many installations, the
riser and barrel are designed to convey multiple
design storms (Urbonas, 1993). To increase the life
of the outlet, the riser and barrel should be
constructed of reinforced concrete rather than
corrugated metal pipe (Schueler, 1992). The riser,
barrel, and base should also provide have sufficient
weight to prevent flotation (NVPDC, 1992).
In most cases, emergency spillways should be
included in the pond design. Emergency spillways
should be sized to safely pass flows that exceed the
design storm flows. The spillway prevents pond
water levels from overtopping the embankment,
which could cause structural damage to the
embankment. The emergency spillway should be
located so that downstream buildings and structures
will not be negatively impacted by spillway
discharges. The pond design should include a low
flow drain, as shown in Figure 1. The drain pipe
should be designed for gravity discharge and should
be equipped with an adjustable gate valve.
PERFORMANCE
The primary pollutant removal mechanism in a wet
detention pond is sedimentation. Significant loads
of suspended pollutants, such as metals, nutrients,
sediments, and organics, can be removed by
sedimentation. Other pollutant removal
mechanisms include algal uptake, wetland plant
uptake, and bacterial decomposition (Schueler,
1992). Dissolved pollutant removal also occurs as
a result of biological and chemical processes
(NVPDC, 1992).
The removal rates of conventional wet detention
ponds (i.e., without the sediment forebay or
peripheral ledges) are well documented and are
shown in Table 1. The wide range in the removal
rates is a result of varying hydraulic residence times
(HRTs), which is further discussed in the Design
Criteria section. Increased pollutant removal by
biological uptake and sedimentation is correlated
with increased HRTs. Proper design and
maintenance also effect pond performance.
Studies have shown that more than 90 percent of the
pollutant removal occurs during the quiescent
period (the period between the rainfall events) (MD
DEQ, 1986). However, some removal occurs
during the dynamic period (when the runoff enters
the pond). Modeling results have indicated that
two-thirds of the sediment, nutrients and trace metal
loads are removed by sedimentation within 24
TABLE 1 REMOVAL EFFICIENCIES
FROM WET DETENTION PONDS
Parameter
Percent Removal
Total
Suspended
Solid
Total
Phosphorus
Soluble
Nutrients
Lead
Zinc
Biochemical
Oxygen
Demand or
Chemical
Oxygen
Demand
Schueler,
1992
50-90
30-90
40-80
70-80
40-50
20-40
Hartigan,
1988
80-90
50-70
1 hydraulic residence time varies
2 hydraulic residence time of 2 weeks
Source: Schueler, 1992 & MD DEQ, 1986.
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hours. These projections are supported by the
results of the EPA's 1993 National Urban Runoff
Program (NURP) studies. However, other studies
indicate that an HRT of two weeks is required to
achieve significant phosphorus removal (MD DEQ,
1986).
The pond's treatment efficiency can be enhanced by
extending the detention time in the permanent pool
to up to 40 hours. This allows for a more gradual
release of collected runoff, resulting in both
increased pollutant removal and control of peak
flows (Hartigan, 1988).
OPERATION AND MAINTENANCE
Wet detention ponds function more effectively
when they are regularly inspected and maintained.
Routine maintenance of the pond includes mowing
of the embankment and buffer areas and inspection
for erosion and nuisance problems (e.g. burrowing
animals, weeds, odors) (SEWRPC, 1991). Trash
and debris should be removed routinely to maintain
an attractive appearance and to prevent the outlet
from becoming clogged. In general, wet detention
ponds should be inspected after every storm event.
The embankment and emergency spillway should
also be routinely inspected for structural integrity,
especially after major storm events. Embankment
failure could result in severe downstream flooding.
When any problems are observed during routine
inspections, necessary repairs should be made
immediately. Failure to correct minor problems
may lead to larger and more expensive repairs or
even to pond failure. Typically, maintenance
includes repairs to the embankment, emergency
spillway, inlet, and outlet; removal of sediment; and
control of algal growth, insects, and odors
(SEWRPC, 1991). Large vegetation or trees that
may weaken the embankment should be removed.
Periodic maintenance may also include the
stabilization of the outfall area (e.g. adding rip-rap)
to prevent erosive damage to the embankment and
the stream bank. In most cases, sediments removed
from wet detention ponds are suitable for landfill
disposal. However, where available, on-site use of
removed sediments for soil amendment will reduce
maintenance costs.
COSTS
Typical costs for wet detention ponds range from
$17.50-$35.00 per cubic meter ($0.50-$1.00 per
cubic foot) of storage area (CWP, 1998). The total
cost for a pond includes permitting, design and
construction, and maintenance costs. Permitting
costs may vary depending on state and local
regulations. Typically, wet detention ponds are less
costly to construct in undeveloped areas than to
retrofit into developed areas. This is due to the cost
of land and the difficulty in finding suitable sites in
developed areas. The cost of relocating pre-existing
utilities or structures is also a major concern in
developed areas. Several studies have shown the
construction cost of retrofitting a wet detention
pond into a developed area may be 5 to 10 times the
cost of constructing the same size pond in an
undeveloped area. Annual maintenance costs can
generally be estimated at 3 to 5 percent of the
construction costs (Schueler, 1992). Maintenance
costs include the costs for regular inspections of the
pond embankments, grass mowing, nuisance
control, debris and liter removal, inlet and outlet
maintenance and inspection, and sediment removal
and disposal. Sediment removal cost can be
decreased by as much as 50 percent if an on-site
disposal areas are available (SEWRPC, 1991).
REFERENCES
1. Center for Watershed Protection, 1998.
Cost and Benefits of Storm Water BMPs.
2. Hartigan, J.P., 1988 "Basis for Design of
Wet Detention Basin BMPs," in Design of
Urban Runoff Quality Control. American
Society of Engineers. 1988.
3. Maryland Department of the Environment,
1986. Feasibility and Design of Wet Ponds
to Achieve Water Quality Control.
Sediment and Storm Water Administration.
4. Northern Virginia Planning District
Commission, Engineers and Surveyors
Institute, 1992. Northern Virginia BMP
Handbook.
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5. Schueler, T.R., 1992. A Current Assessment
of Urban Best Management Practices.
Metropolitan Washington Council of
Governments.
6. Southeastern Wisconsin Regional Planning
Commission, 1991. Costs for Urban
Nonpoint Source Water Pollution Control
Measures. Technical Report No. 31.
7. Urbonas, Ben and Peter Stahre, 1993.
Storm Water Best Management Practices
and Detention for Water Quality, Drainage
and CSOManagement. PTR Prentice Hall,
Englewood Cliffs, New Jersey.
ADDITIONAL INFORMATION
City of Charlotte, North Carolina
Steve Sands
Storm Water Services, Engineering and Property
Management
600 East 4th Street
Charlotte, NC 28202
Illinois EPA
Charles Fellman
Auxiliary Point Source Program, Permit Section,
Division of Water Pollution Control
1021 N. Grand Avenue East, P.O. Box 19276
Springfield, IL 62794
Minnehaha Creek Watershed District
Pete Cangialosi
Gray Freshwater Center, Navarre
2500 Shadywood Road, Suite 37
Excelsior, MN 55331
Southwest Florida Water Management District
Betty Rushton
2379 Broad Street
Brooksville, FL 34609
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for the use by the U.S. Environmental Protection
Agency.
Polk County, Florida
Bob Kollinger
Natural Resources and Drainage Division
4177 Ben Durrance Road
Bartow, FL 33830
City of Reynoldsburg, Ohio
Larry Ward
Storm Water Utility
7806 East Main Street
Reynoldsburg, OH 43068
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
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