Retention basin

  A retention basin, sometimes called a retention pond, is a type of constructed wetland that is used to contain stormwater or rain runoff. A retention basin provides an area to hold water from a small surrounding drainage area that would otherwise flow into other areas. The water remains in the local area that it was deposited in. This is opposed to a detention basin that holds water for a limited period of time from a larger basin area to prevent flooding and releases all the water contained in a short period of time.

These structures are frequently used to act as a replacement for the natural absorption of a forest or other natural process that was lost when an area is developed. As such, these structures are designed to blend into neighborhoods and viewed as an amenity rather than a basin without value.

Retention basins are more often used in wetter climates so they tend to retain water for extended periods of time and attract and support wildlife such as amphibians and birds. In some parts of the U.S. these structures are required to mitigate the impact of development in previously undeveloped areas.

Retention ponds are most often associated with human development in a given area; as buildings and streets are built and cover up absorbent soil, the ability of the land mass to absorb water from rain or flooding is lessened. Since the water must flow somewhere, a retention pond is built on the site to localize the effects of reduced water absorption. It is common to see cattails and other tall, thick grass that can be used to help purify the water.

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Capacity and size

  These structures are designed to accommodate a specific amount of water. If the flow exceeds this amount, there is some kind of exit provided from the basin for the excess water. However, release of water by these means usually will pose a threat to the areas below the retention basin. Overflow can drain into creeks, yards or neighborhoods depending on where the basin is located.

The size of these structures varies. They are often found in sizes of 1 to 10 acres (4,000 to 40,000 m²) and up to 10 feet (3 m) deep. The designed holding capacity of these structures is determined by the size of the area that drains into the structure and the amount of water that it needs to retain to support the plant and animal populations.

Retention ponds can achieve depths of 15 to 30 feet (5 to 10 meters) when they are also used to control the flow of flood waters.

Design features

The basins function by allowing large flows of water to enter, but limit the outflow by having a small opening at the highest point of the structure. Sometimes these will drain excess flood water at much higher flow rates to a nearby river or stream over an extended period of time until the water reaches normal levels.

Some retention ponds are permanent features of the landscape and have water fountains, parks, and bridges surrounding and crossing over them.

Secondary uses

  Since these structures almost always contain water, they are designed to limit access by people. This enhances their ability to attract and support wildlife. They are usually fenced to prevent entry and minimize risks.

When retention ponds sides use grass that is kept mowed, it can attract nusiance types of wildlife like ducks or Canada Geese by reducing ability of foxes, coyotes and other predators to approach unseen. Such predators tend to hide in the cattails and other tall, thick grass surrounding natural water features.

Specialized types

• Sediment or sedimentation retention basin
• Stormwater retention basin
• Wastewater retention basin

See also

• Detention basin
• River reclamation
• Sustainable urban drainage systems

Other meanings

A retention basin can also be a part of a reactor used to contain a core meltdown.

References

• Virginia retention basin standards
• Detention vs. retention
• Stormwater Ecological Enhancement Project
• The use of retention ponds in residential settings
• Description and Performance of Storm Water Best Management Practices