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Learn about wet ponds


What is a wet pond:

Also called stormwater ponds, retention ponds or wet extended detention ponds, these facilities are basins that have a permanent pool of water throughout the year. The wet pond is constructed to store runoff during and after storms. Wet ponds treat and filter stormwater runoff through settling and through nutrient uptake by plants and other aquatic organisms.

Why consider:
Stormwater management basins are one of the most common methods chosen by engineers and developers to handle stormwater runoff generated from land development activities. Wet ponds also provide water quality benefits dry basins can not offer. Additionally, many existing dry stormwater basins can be converted to wet ponds through some minor adjustments to outlet structures.

Limitations:
Wet ponds are usually inappropriate measures in dense urban areas due to their space requirements. And most experts agree that in order to maintain a permanent water elevation within the pond, they should only be used for sites with drainage areas greater than 5 acres. Wet ponds have regulatory limitations to where they can be placed. For example, they should not be located within wetlands. And while they may not always be feasible in very urban or arid climates, wet ponds otherwise have few restrictions on their use.

There is a potential impact to the stream that receives stormwater from the wet ponds, because the standing water in the pond will be warmed by the sun. However, existing dry detention basins that contain concrete low flow channels drastically heat stormwater runoff, more so than the wet pond. The warming impact of the wet pond can be mitigated by planting taller vegetation around the perimeter and at the discharge point, to shade the water and provide cooling.

Benefits:
Wet ponds offer water quality and flood control benefits not typically associated with dry basins. They are also among the most effective stormwater treatment practices at removing stormwater pollutants. Water contained in the permanent pool mixes with and dilutes the initial polluted runoff from storm events. Additionally, plants, algae, and bacteria in the water consume these pollutants and thereby substantially reduce the total mass of pollutants released downstream.

The ability of a wet pond to store runoff for longer time periods decreases stormwater peak flows. The longer detention times can reduce stream channel erosion, a common result of traditional stormwater practices.

Many wet ponds have been designed as an aesthetic site amenity, to create wildlife habitat or as a development focal point or recreational area. The results of one study suggest that "pond front" property can increase the selling price of a new property by 10% (EPA, 1995). Another study found that the perceived value (value estimated by residents of a community) of homes increased by about 15-25% when located near a wet pond (Emmerling-Dinovo, 1995).

The effectiveness of wet ponds can be further improved by combining them with other innovative stormwater practices such as porous pavement and bioretention areas. Additionally, multiple, smaller wet ponds located in a series create what is called a "treatment train" that can better reduce pollutants and slow the rate of flow through the system.

Cost:
Wet ponds are among the most cost-effective and widely used stormwater treatment practices. The construction costs associated with these facilities range considerably, depending on factors such as size and landscaping regimen. Wet ponds should be designed according to site specific requirements. These are long-lived systems. Thus the initial investment into constructing wet ponds may be spread over a relatively long time.

Maintenance:
All stormwater basins, both wet ponds and dry, require regular maintenance. Some of these maintenance activities include: annual inspections for damage, sediment removal, ensuring the operation of inlet and outlet structures, repairing undercut and eroded areas as needed, and clearing debris. At some point, if the pool volume has become reduced significantly or the pond develops high nutrient levels, wet ponds will need to be drained & dredged. Several design features, such as sediment forebays, trash racks, access easements for maintenance and an outlet drain hole can be incorporated to ease the maintenance burden of wet ponds and increase their longevity.

Landscaping:
The right landscaping plan can transform a wet pond into community asset, while enhancing its pollutant removal ability. Wetland plants should be encouraged, as well as trees, shrubs and native ground covers. Landscaping of this kind will also help discourage resident geese populations.

Safety:
Often, homeowners and municipal officials express safety concerns as the main deterrent to approving wet pond designs. Fencing of ponds is not generally desirable, but may be required by local ordinances. A preferred method is to manage the contours of the pond to incorporate shallow bench areas around the pond and eliminate dropoffs and other safety hazards. Vegetation can also be used to reduce public access to the basin area.