Description

Dry extended detention ponds (a.k.a. dry ponds, extended detention basins, detention ponds, extended detention ponds) are basins whose outlets are designed to detain the stormwater runoff from a water quality "storm" for some minimum duration (e.g., 24 hours) which allow sediment particles and associated pollutants to settle out. Unlike wet ponds, dry extended detention ponds do not have a permanent pool. However, dry extended detention ponds are often designed with small pools at the inlet and outlet of the pond, and can also be used to provide flood control by including additional detention storage above the extended detention level.

Applicability

Dry extended detention ponds are among the most widely applicable stormwater treatment practice. While they may not always be applicable in highly urban watersheds, they have few other restrictions.

Regional Applicability
Dry extended detention ponds can be applied in all regions of the United States. Some minor design modifications are needed in cold or arid climates or in regions with karst topography, however.

Ultra Urban Areas
Ultra urban areas are densely developed urban areas in which little pervious surface exists. It is difficult to use dry extended detention ponds in the ultra urban watershed because of the land area each pond consumes. They can, however, be used in these environments if a relatively large area is available downstream of the site.

Stormwater Hotspots
Stormwater hotspots are land use or activities which generate highly contaminated runoff, that have pollutant concentrations far in excess of those typically found in stormwater. Dry extended detention ponds can accept runoff from stormwater hotspots, but need significant separation from groundwater when used for this purpose.

Stormwater Retrofit
A stormwater retrofit is a stormwater management 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. Dry extended detention ponds are very useful stormwater retrofits and have two primary applications as a retrofit design. In many communities, detention basins have been designed for flood control in the past. It is possible to modify these basins to incorporate features that encourage water quality control, and/or channel protection. It is also possible to construct new dry extended detention ponds in open areas of a watershed to capture existing drainage, or create them above a road crossing or culvert.

Cold Water (Trout) Streams
A study in Maryland indicated that dry extended detention ponds can slightly increase stream temperatures (Galli, 1990). Overall, dry extended detention ponds increased temperature by about 5 F in the summer months. In cold water streams, dry extended detention ponds should be designed to detain stormwater for a relatively short time (i.e., less than twelve hours) to minimize the potential amount of stream warming that occurs in the practice.

Siting and Design Considerations

Siting Considerations
While dry extended detention ponds have broad applications, designers need to ensure that they are feasible at an individual site. The following section provides basic guidelines for locating dry extended detention ponds.

Drainage Area

In general, dry extended detention ponds should be used at sites with a minimum drainage area of ten acres. On smaller sites, it may be difficult to provide channel or water quality control because the orifice diameter at the outlet becomes very small, and is thus prone to clogging. In addition, it is generally more cost-effective to control larger drainage areas due to the economies of scale in pond construction (see Cost Considerations).

Slope

Dry extended detention ponds can be used on sites with slopes up to about 15%. The local slope needs to be relatively flat, however, in order to maintain reasonably flat side slopes. While there is no minimum slope requirement, enough elevation drop is needed from the pond inlet to the pond outlet to ensure that flow can move through the system.

Soils /Topography

Extended detention ponds can be used in almost all soils and geology, with minor design adjustments for regions of karst (i.e., limestone) topography or in rapidly percolating soils such as sand. In these areas, extended detention ponds should be designed with an impermeable liner to prevent groundwater contamination or sinkhole formation.

Groundwater

Except for the case of hotspot runoff, the only consideration regarding groundwater is that the base of the extended detention facility should not intersect the groundwater table. A permanently wet bottom may become a mosquito breeding ground. Research in Southwest Florida demonstrated that intermittently flooded systems, such as dry extended detention ponds, produce more mosquitos than other pond systems (Santana et al., 1994), particularly when the facilities remained dry for more than three days following heavy rainfall.

Design Considerations

Specific pond designs may vary considerably, depending on site constraints or preferences of the designer or community. There are some features, however, that should be incorporated into most dry extended detention pond designs. These design features can be divided into five basic categories: pretreatment, treatment, conveyance, maintenance reduction, and landscaping (for more information see Manual Builder Category for more information).

Pretreatment

Pretreatment is intended to capture and remove coarse sediment particles before they enter the practice. Maintenance burden of the pond is reducedwhen these particles are removed from runoff before they reach the pool. In ponds, pretreatment is achieved with a sediment forebay. A sediment forebay is a small pool (typically about 10% of the volume of water to be treated for pollutant removal).

Treatment

Treatment features help enhance the ability of a dry extended detention pond to remove pollutants. Designing dry extended detention ponds with a high length to width ratio (i.e., at least 1.5:1) and incorporating other design features to maximize the flow path effectively increases the detention time in the system by eliminating the potential of flow to short circuit the pond. Designing ponds with relatively flat side slopes can also help to lengthen the effective flow path. Finally, the pond should be sized to detain the volume of runoff to be treated for between 12 and 48 hours.

Conveyance

Conveyance of stormwater runoff into and through a stormwater treatment practice is a critical component of stormwater design. Stormwater should be conveyed to and from dry extended detention ponds safely and to minimize erosion potential. The outfall of dry extended detention ponds should always be stabilized to prevent scour. In order to convey low flows through the dry extended detention ponds, designers should provide a pilot channel. A pilot channel is a surface channel should be used to convey low flows through the pond. In addition, an emergency spillway should be provided to safely convey large flood events. In order to prevent stream warming, designers should provide shade around the pilot channel and the pond outlet.

Maintenance Reduction

In addition to regular maintenance activities needed to maintain the function of dry extended detention ponds. Several design features can ease the maintenance burden associated with a practice. In dry extended detention ponds, a "micropool" at the outlet can prevent resuspension of sediment and outlet clogging. A good design also includes maintenance access to the forebay and micropool.

Another design feature that can reduce maintenance needs is a non-clogging outlet pipe. Typical examples include 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 determines the water elevation of the micropool. Because these outlets draw water from below the level of the permanent pool, they are less likely to be clogged by floating debris.

Landscaping

Designers should maintain a vegetated buffer around the pond, and should select plants within the extended detention zone (i.e., the portion of the pond up to the elevation where stormwater is detained) that can withstand both wet and dry periods. The side slopes of dry ponds should be relatively flat to reduce safety risks.

Design Variations

Dry Detention Ponds

Detention ponds are similar in design to extended detention ponds, except that they do not incorporate features to improve water quality. In particular, these practices do not detain stormwater from small flow events. Therefore, detention ponds provide almost no pollutant removal. However, dry ponds can help to meet flood control, and sometimes channel protection, objectives in a watershed.

Tank Storage

Another variation of the dry detention pond design is the use of tank storage. In these designs, stormwater runoff is conveyed to large storage tanks or vaults underground. This practice is most often used in the ultra urban environment, on small sites where sites since underground storage tends to be expensive. Since drainage areas contributing to tank storage are typically small, the outlet diameter needed to reduce the flow from very small storms tends to be very small. A very small outlet diameter, combined with the tanks being underground, creates the potential for debris being caught in the outlet, and resulting maintenance problems.

Regional Variations

Arid or Semi-Arid Climates

In arid and semi-arid regions, some modifications may be needed to conserve scarce water resources. Any landscaping plans should prescribe drought-tolerant vegetation wherever possible. In addition, the wet forebay may be replaced with an alternative form of pretreatment, such as a dry sediment chamber. In regions which have distinct wet and dry seasons, regional extended detention ponds can be designed to act as a recreation area such as a ball field during the dry season.

Cold Climates

In cold climates, some additional design features are needed to treat the spring snowmelt. One such modification is to increase the volume available for detention to help treat this relatively large runoff event. In some cases, dry ponds may be an option as a snow storage facility to promote some treatment of plowed snow. If a pond is used to treat road runoff, or is used for snow storage, landscaping should incorporate salt tolerant species. Finally, sediment removal from the forebay may need to be completed more frequently than in warmer climates (see Maintenance Consideration for guidelines) to account for sediment deposited as a result of road sanding.

Limitations

While dry extended detention ponds are widely applicable, they have some limitations that may make other stormwater management options preferable. Some limitations include:

• Dry extended detention ponds may become a nuisance due to mosquito breeding
• Habitat destruction may occur during construction, if the practice is designed in-stream or within the stream buffer.
• While wet ponds can increase property values, dry ponds can actually detract from the value of a home (see Cost Considerations).
• Dry extended detention ponds have only moderate pollutant removal when compared to other structural stormwater practices, and are ineffective at removing soluble pollutants (see Effectiveness).

Maintenance Considerations

In addition to incorporating features into the pond design to minimize maintenance, some regular maintenance and inspection practices are needed. The table below outlines some of these practices.

Effectiveness

Structural management 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 Manual Builder Category). Dry extended detention basins can provide flood control, channel protection, and some pollutant removal.

Flood Control
One objective of stormwater treatment practices can be to reduce the flood hazard associated with large storm events by reducing the peak flow associated with these storms. Dry extended detention basins can easily be designed for flood control, and this is actually the primary purpose of most extended detention ponds in the ground today.

Channel Protection
One result of urbanization is channel erosion caused by increased stormwater runoff. Traditionally, dry extended detention basins have been designed to provide control of the two-year storm (i.e., the storm that occurs, on average, once every two years). 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.

Pollutant Removal
Dry extended detention basins provide moderate pollutant removal, provided that the design features described in the Siting and Design section are incorporated. While they can be effective at removing some pollutants through settling, they are less effective at removing soluble pollutants due to the absence of a permanent pool. A few studies are available on the effectiveness of dry extended detention ponds. Typical removal rates, as reported by Winer (2000) are:

There is considerable variability in the effectiveness of ponds, and it is believed that properly designing and maintaining ponds may help to improve their performance. The siting 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.

Cost Considerations

Dry extended detention ponds are the least expensive stormwater treatment practice, on a cost per unit area treated. The construction costs associated with these facilities range considerably. One recent study evaluated the cost of all pond systems (Brown and Schueler, 1997). Adjusting for inflation, the cost of dry extended detention ponds can be estimated with the equation:

C = 12.4V^0.760

Where:

C = Construction, Design and Permitting Cost
V = Volume needed to control the 10-year storm (cubic feet)
Using this equation, a typical construction costs are:
$ 41,600 for a 1 acre-foot pond
$ 239,000 for a 10 acre-foot pond
$ 1,380,000 for a 100 acre-foot pond

Interestingly, these costs are generally slightly higher than the cost of wet ponds on a cost per total volume basis. Dry extended detention ponds are generally less expensive on a given site, however, because they are usually smaller than a wet pond design for the same site.

Ponds do not consume a large area compared to the total area treated (typically 2-3% of the contributing drainage area). It is important to note, however, that each pond is generally large. Other practices, such as filters or swales, may be "squeezed in" in 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. Finally, 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.

Another economic concern associated with dry ponds is that they may slightly detract from the value of adjacent properties. One study found that dry ponds can actually detract from the perceived value of homes adjacent to a dry pond by between three and ten percent (Emmerling-Dinovo, 1995).

References

Brown, W. and T. Schueler. 1997. The Economics of Stormwater BMPs in the Mid-Atlantic Region. Prepared for: Chesapeake Research Consortium. Edgewater, MD. Center for Watershed Protection. Ellicott City, MD.

Center for Watershed Protection (CWP), Environmental Quality Resources and Loiederman Associates. 1997. Maryland Stormwater Design Manual. Draft. Prepared for: Maryland Department of the Environment. Baltimore, MD.

Center for Watershed Protection (CWP). 1997. Stormwater BMP Design Supplement for Cold Climates. Prepared for: US EPA Office of Wetlands, Oceans and Watersheds. Washington, DC.

Denver Urban Drainage and Flood Control District. 1992. Urban Storm Drainage Criteria Manual; Volume 3 - Best Management Practices. Denver, CO.

Emmerling-Dinovo, C. 1995. Stormwater Detention Basins and Residential Locational Decisions. Water Resources Bulletin, 31(3): 515-521

Galli, J. 1990. Thermal Impacts Associated with Urbanization and Stormwater Management Best Management Practices. Metropolitan Washington Council of Governments. Prepared for: Maryland Department of the Environment. Baltimore, MD.

MacRae, C. 1996. Experience from Morphological Research on Canadian Streams: Is Control of the Two-Year Frequency Runoff Event the Best Basis for Stream Channel Protection? IN: Effects of Watershed Development and Management on Aquatic Ecosystems. American Society of Civil Engineers. Edited by L. Roesner. Snowbird, UT. pp. 144-162.

Santana, F., J. Wood, R. Parsons, and S. Chamberlain. 1994. Control of Mosquito Breeding in Permitted Stormwater Systems. For: Southwest Florida Water Management District. Brooksville, FL.

Schueler, T. 2000. Influence of Groundwater on Performance of Stormwater Ponds in Florida, Article 78 in The Practice of Watershed Protection. Center for Watershed Protection. Ellicott City, MD.

US EPA. 1993. Office of Water. Guidance to Specify Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA-840-B-92-002. Washington, DC.

Watershed Management Institute (WMI). 1997. Operation, Maintenance, and Management of Stormwater Management Systems. Prepared for: US EPA Office of Water. Washington, DC.

Winer, R. 2000. National Pollutant Removal Database for Stormwater Treatment Practices: 2^nd Edition. Center for Watershed Protection. Ellicott City, MD.