Stormwater Management Fact Sheet: Wet Pond
Wet ponds (a.k.a. stormwater ponds, retention ponds, wet extended detention ponds) are constructed basins that have a permanent pool of water throughout the year (or at least throughout the wet season). Ponds treat incoming stormwater runoff by settling and algal uptake. The primary removal mechanism is settling while stormwater runoff resides in the pool. Nutrient uptake also occurs through biological activity in the pond. Wet ponds are among the most cost-effective and widely used stormwater treatment practices. While there are several different versions of the wet pond design, the most common modification is the extended detention wet pond, where storage is provided above the permanent pool in order to detain stormwater runoff in order to provide greater settling.
Wet ponds are a widely applicable stormwater treatment practice. While they may not always be feasible in ultra-urban areas or arid climates, they otherwise have few restrictions on their use.
Wet extended detention ponds can be applied in most regions of the United States, with the exception of arid climates. In arid regions, it is difficult to justify the supplemental water needed to maintain a permanent pool because of the scarcity of water. Even in semi-arid Austin, TX one study found that 2.6 acre-feet per year of supplemental water were needed to maintain a permanent pool of only 0.29 acre-feet (Saunders and Gilroy, 1997). Other modifications and design variations are needed in semi-arid and cold climates, and karst (i.e., limestone) topography (for more information see Stormwater Strategies for Arid and Semiarid Watersheds , Article 66 in the Practice of Watershed Protection and Performance of Stormwater Ponds in Central Texas, Article 74 in the Practice of Watershed Protection).
Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. It is difficult to use wet ponds in ultra urban areas because enough land area may not be available for the pond. Wet ponds can, however, be used in an ultra-urban environment if a relatively large area is available downstream of the site.
Stormwater hotspots are land use or activities that generate highly contaminated runoff that has pollutant concentrations that exceed those typically found in stormwater. A typical example is a gas station or convenience store. Wet ponds can accept runoff from stormwater hotspots, but need significant separation from groundwater if they are used to treat hotspot runoff.
A stormwater retrofit is a stormwater treatment practice (usually structural) put into place after development has occurred, to improve water quality, protect downstream channels, reduce flooding, or meet other watershed restoration objectives. Wet ponds are widely used for stormwater retrofits, and have two primary applications as a retrofit design. In many communities, dry detention ponds have been designed for flood control in the past. It is possible to modify these facilities to develop a permanent wet pool to provide water quality treatment (see "Treatment" under Design Considerations), and modify the outlet structure to provide channel protection. Alternatively, new wet ponds may be installed in streams, or in open areas as a part of a comprehensive watershed retrofit inventory.
Cold Water (Trout) Streams
Wet ponds pose a risk to cold water streams because of their potential to warm streams. When water remains in the permanent pool, it is heated by the sun. A study in Prince Georges County, MD found that wet ponds increased temperatures by about 9 F from the inlet to the outlet (Galli, 1990).
Siting and Design Considerations
Designers need to ensure wet ponds are feasible for the site in question. The following section provides basic guidelines for locating wet ponds.
Wet ponds need sufficient drainage area to maintain a permanent pool. In humid regions, a drainage area of about twenty-five acres is typically needed, but greater drainage areas are needed in arid and semi-arid regions.
Wet ponds can be used on sites with an upstream slope up to about 15%. The local slope within the pond should be relatively shallow, however. While there is no minimum slope requirement, there must be enough elevation drop from the pond inlet to the pond outlet to ensure that water can flow through the system by gravity.
Wet ponds can be used in almost all soils and geology, with minor design adjustments for regions of karst topography (see Design Considerations).
Unless they receive hotspot runoff, ponds can often intersect the groundwater table. However, some research suggests that pollutant removal is moderately reduced when groundwater contributes substantially to the pool volume (Schueler, 1997) (for more information, see Influence of Groundwater on Performance of Stormwater Ponds in Florida, Article 78 in The Practice of Watershed Protection.
There are some design features that should be incorporated into all wet pond designs (see Figure 1). These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information, see the Manual Builder Category).
Pretreatment features are designed to settle out coarse sediment particles before they reach the main pool. By trapping these sediments in the forebay, it is possible to greatly reduce the maintenance burden of the pond. A sediment forebay is a small pool (typically about 10% of the volume of the permanent pool) located near the pond inlet. Coarse sediments are trapped in the forebay, and these sediments are removed from the smaller pool on a five to seven year cycle.
Treatment design features help enhance the ability of a stormwater treatment practice to remove pollutants. Several features can enhance the ability of wet ponds to remove pollutants from stormwater runoff. The purpose of most of these features is to increase the amount of time that stormwater remains in the pond.
One technique to increase pond pollutant removal is to increase the volume of the permanent pool. Typically, ponds are sized to be equal to the water quality volume (i.e., the volume of water treated for pollutant removal). Designers may consider using a larger volume to meet specific watershed objectives, such as phosphorous removal. Regardless of the pool size, designers need to conduct a water balance analysis to ensure that sufficient inflow is available to sustain a permanent pool.
Other design features can increase the amount of time stormwater remains in the pond, and help to eliminate short circuiting. Wet ponds should always be designed with a length to width ratio of at least 1.5:1. In addition, the design should incorporate features to lengthen the flow path through the pond, such as underwater berms designed to create a longer flow path through the pond. Combining these two measures helps ensure that the entire pond volume is used to treat stormwater. Another feature that can improve treatment is to use multiple ponds in series as part of a "treatment train" approach to pollutant removal. This redundant treatment can also help slow the rate of flow through the system.
Stormwater should be conveyed to and from all wet ponds safely and to minimize downstream erosion potential. The outfall of pond systems should always be stabilized to prevent scour. In addition, an emergency spillway should be provided to safely convey large flood events. In order to prevent warming at the outlet channel, designers should provide shade around the channel at the pond outlet.
Several design features can be incorporated to ease the maintenance burden of wet ponds. Maintenance reduction features include techniques to reduce the amount of maintenance needed, as well as techniques to make regular maintenance activities easier.
One maintenance concern in wet ponds is potential clogging of the pond outlet. Ponds should be designed with a non-clogging outlet such as a reverse-slope pipe, or a weir outlet with a trash rack. A reverse slope pipe draws from below the permanent pool extending in a reverse angle up to the riser and establishes the water elevation of the permanent pool. Because these outlets draw water from below the level of the permanent pool, they are less likely to be clogged by floating debris. Another general rule is that no low flow orifice should be less than 3" in diameter (smaller orifices are more susceptible to clogging).
Direct access is needed to allow maintenance of both the forebay and the main pool of ponds. In addition, ponds should generally have a drain to draw down the pond or forebay to enable periodic sediment clean outs.
Landscaping of wet ponds can make them an asset to a community, and can also enhance the pollutant removal. A vegetated buffer should be created around the pond to protect the banks from erosion, and provide some pollutant removal before runoff enters the pond by overland flow. In addition, ponds should incorporate an aquatic bench (a shallow shelf with wetland plants) around the edge of the pond. This feature provides some pollutant uptake, and also helps to stabilize the soil at the edge of the pond and enhance habitat and aesthetic value.
There are several variations of the wet pond design. Some of these design alternatives are intended to make the practice adaptable to various sites and to account for regional constraints and opportunities.
Wet Extended Detention Pond
The Wet Extended Detention Pond combines the treatment concepts of the dry extended detention pond (for more information see Dry Extended Detention Pond Fact Sheet) and the wet pond (see Figure 2). In this design, the water quality volume is split between the permanent pool and detention storage provided above the permanent pool. During storm events, water is detained above the permanent pool and released over 12 to 48 hours. This design has similar pollutant removal to a traditional wet pond, and consumes less space. Wet Extended Detention Ponds should be designed to maintain at least half the treatment volume in the permanent pool. In addition, designers need to carefully select vegetation planted in the extended detention zone to ensure that it can withstand both wet and dry periods.
In this design variation, a pond drains a smaller area than a traditional wet pond, and the permanent pool is maintained by intercepting the groundwater. While this design variation achieves less pollutant removal than a traditional wet pond, it may be an acceptable alternative on sites where space is at a premium, or in a retrofit situation.
Water Reuse Pond
Some designers have used wet ponds to act as a water source, usually for irrigation. In this case, the water balance should account for the water that will be taken from the pond. One study conducted in Florida estimated that a water reuse pond could provide irrigation for a 100-acre golf course at about one seventh the cost of the market rate of the equivalent amount of water ($40,000 versus $300,000).
In arid climates, wet ponds are not a feasible option (see Application), but they may be possible in semi-arid climates if the permanent pool is maintained with a supplemental water source, or if the pool is allowed to vary seasonally. This choice needs to be seriously evaluated, however. Saunders and Gilroy (1997) reported that 2.6 acre-feet per year of supplemental water were needed to maintain a permanent pool of only 0.29 acre-feet in Austin, TX (for more information see Stormwater Strategies for Arid and Semiarid Watersheds, Article 66 in The Practice of Watershed Protection).
Cold climates present many challenges to designers of wet ponds. The spring snowmelt may have a high pollutant load, and large volume to be treated. In addition, cold winters may cause freezing of the permanent pool or freezing at inlets and outlets. Also, high salt concentrations in runoff resulting from road salting may impact pond vegetation, and sediment loads from road sanding may quickly reduce pond capacity.
One means of effectively dealing with spring snowmelt is to use a seasonally operated pond to capture extra snowmelt during the spring, but retain a smaller permanent pool during warmer seasons. In this option, proposed by Oberts (1994), a wet pond has two water quality outlets, both equipped with gate valves. In the summer, the lower outlet is closed. During the fall and throughout the winter, the lower outlet is opened to draw down the permanent pool. As the spring melt begins, the lower outlet is closed to provide detention for the melt event. This method can act as a substitute to using a minimum extended detention storage volume. When wetlands preservation is a downstream objective, seasonal manipulation of pond levels may not be desired (for more information, see Performance of Stormwater Ponds and Wetlands in Winter, Article 71 in The Practice of Watershed Protection). An analysis of the effects on downstream hydrology should be conducted before considering this option. In addition, the manipulation of this system requires some labor and vigilance; a careful maintenance agreement should be confirmed.
Several other modifications help to improve the performance of ponds in cold climates. Designers should consider planting the aquatic buffer with salt-tolerant vegetation if the pond receives road runoff. In order to counteract the effects of freezing on inlet and outlet structures, weirs and larger diameter pipes that are resistant to frost can be used. Designing ponds on-line, which create a continuous flow of water through the pond, also helps prevent freezing of outlet structures. Finally, since freezing of the permanent pool can reduce the effectiveness of pond systems, it may be useful to incorporate extended detention into the design to retain usable treatment area above the permanent pool while it is frozen (for more information, see Performance of Stormwater Ponds and Wetlands in Winter, Article 71 in The Practice of Watershed Protection).
In karst (i.e., limestone) topography, wet ponds should be designed with an impermeable liner to prevent groundwater contamination or sinkhole formation, and to help maintain the permanent pool.
Limitations of wet ponds include:
In addition to incorporating
features into the pond design to minimize maintenance, some regular maintenance
and inspection practices are needed. The table below outlines these practices.
1. Typical Maintenance Activities for Wet Ponds
As Needed Maintenance
5 to 7 year Maintenance
20 to 50 year Maintenance
Stormwater treatment practices can be used to achieve four broad resource protection goals. These include: Flood Control, Channel Protection, Groundwater Recharge, and Pollutant Removal (for more information, see the Manual Builder Category.) Wet ponds can generally provide flood control channel protection, and pollutant removal functions.
One objective of stormwater treatment practices is to reduce the flood hazard associated with large storm events by reducing the peak flow associated with these storms. Wet ponds can easily be designed for flood control, by providing flood storage above the level of the permanent pool.
One result of urbanization is channel erosion caused by increased stormwater runoff. Traditionally wet ponds have been designed to provide control of the two-year storm . It appears that this design storm has not been effective in preventing channel erosion, and recent research suggests that control of a smaller storm may be more appropriate (MacRae, 1996). Choosing a smaller design storm (one-year) and providing longer detention time (12 to 24 hours) is now thought to be the best method to reduce channel erosion.
Wet ponds generally cannot provide groundwater recharge, as infiltration is impeded by the accumulation of organic debris on the bottom of the pond.
Wet ponds are among the most effective stormwater treatment practices at removing stormwater pollutants. A wide range of research is available to estimate the effectiveness of wet ponds. Table 2 provides pollutant removal estimates derived from CWP's National Pollutant Removal Performance Database for Stormwater Treatment Practices:
Table 2. Pollutant Removal Efficiency of Stormwater Wet Ponds (Winer, 2000)
Removal Efficiency (%)
1: ± values represent one standard deviation
There is considerable variability in the effectiveness of wet ponds, and it is believed that properly designing and maintaining ponds may help to improve their performance. The locational and design criteria presented in this sheet reflect the best current information and experience to improve the performance of wet ponds. A recent joint project between the American Society of Civil Engineers (ASCE) and the US EPA Office of Water may help to isolate specific design features that can improve performance. The National Stormwater Best Management Practice (BMP) database is a compilation of stormwater practices which includes both design information and performance data for various practices. As the database expands, inferences about the extent to which specific design criteria influence pollutant removal may be made. For more information on this database, access the ASCE web page at http://www.asce.org.
Wet ponds are relatively inexpensive stormwater practices. The construction costs associated with these facilities range considerably. A recent study (Brown and Schueler, 1997) estimated the cost of a variety of stormwater management practices. The study resulted in the following cost equation, adjusting for inflation:
C = 24.5V0.705
C = Construction, Design
and Permitting Cost
V = Volume in the Pond to Include the 10-Year Storm (cubic feet)
Using this equation, a typical construction costs are:
$ 45,700 for a 1 acre-foot facility
$ 232,000 for a 10 acre-foot facility
$ 1,170,000 for a 100 acre-foot facility
Ponds do not consume a large area (typically 2-3% of the contributing drainage area). Therefore, the land consumed to design the pond will not be very large. It is important to note, however, that these facilities are generally large. Other practices, such as filters or swales, may be "squeezed" into relatively unusable land, but ponds need a relatively large continuous area.
For ponds, the annual cost of routine maintenance is typically estimated at about 3 to 5% of the construction cost. Alternatively, a community can estimate the cost of the maintenance activities outlined in the maintenance section. Ponds are long-lived facilities (typically longer than 20 years). Thus, the initial investment into ponds systems may be spread over a relatively long time period.
In addition to water resource protection benefits of wet ponds, there is some evidence to suggest that they may provide an economic benefit by increasing property values. The results of one study suggest that "pond front" property can increase the selling price of new properties by about 10% (US EPA, 1995). Another study reported that the perceived value (i.e., the value estimated by residents of a community) of homes was increased by about 15 to 25% when located near a wet pond (Emmerling-Dinovo, 1995).
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