A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...

s, streams,

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

s or bays to protect against

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

ing and, in some cases, downstream

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...

by storing water for a limited period of time. These basins are also called dry ponds, holding ponds or dry detention basins if no permanent pool of water exists. Detention ponds that are designed to permanently retain some volume of water at all times are called

retention basin A retention basin, sometimes called a wet pond, wet detention basin, or stormwater management pond (SWMP), is an artificial pond with vegetation around the perimeter and a permanent pool of water in its design. It is used to manage stormwater r ...

s. In its basic form, a detention basin is used to manage water quantity while having a limited effectiveness in protecting

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

, unless it includes a permanent pool feature.

Functions and design

Detention basins are storm water best management practices that provide general

flood protection Flood control methods are used to reduce or prevent the detrimental effects of flood waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water levels ...

and can also control extreme floods such as a 1 in 100-year storm event. The basins are typically built during the construction of new land development projects including residential subdivisions or shopping centers. The ponds help manage the excess

urban runoff Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precip ...

generated by newly constructed

impervious surface Impervious surfaces are mainly artificial structures—such as pavements (roads, sidewalks, driveways and parking lots, as well as industrial areas such as airports, ports and logistics and distribution centres, all of which use considerable p ...

s such as

road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...

s, parking lots and

rooftop A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of temper ...

s. A basin functions by allowing large flows of water to enter but limits the outflow by having a small opening at the lowest point of the structure. The size of this opening is determined by the capacity of underground and downstream culverts and washes to handle the release of the contained water. Frequently the inflow area is constructed to protect the structure from some types of damage. Offset

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

blocks in the entrance spillways are used to reduce the speed of entering flood water. These structures may also have

debris Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer to ...

drop vaults to collect large rocks. These vaults are deep holes under the entrance to the structure. The holes are wide enough to allow large rocks and other debris to fall into the holes before they can damage the rest of the structure. These vaults must be emptied after each storm event. Research has shown that detention basins built with real-time control of the outflow from the basin are significantly more effective at retaining

total suspended solids Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus known as sintered glass crucible. TSS is a water qu ...

and associated contaminants, such as heavy metals, when compared to basins without control.

Extended detention basin

A variant basin design called an extended detention dry basin can limit downstream erosion and control of some

pollutants A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...

such as suspended solids. This basin type differs from a

, also known as a "wet pond," which includes a permanent pool of water, and which is typically designed to protect water quality.Water Environment Federation
Alexandria, VA; an
American Society of Civil Engineers
Reston, VA
"Urban Runoff Quality Management."
WEF Manual of Practice No. 23; ASCE Manual and Report on Engineering Practice No. 87. 1998. . Chapter 5.Minnesota Pollution Control Agency
"Stormwater Detention Ponds."
Chapter 5, Minnesota State Permit Guidance Document. While basic detention ponds are often designed to empty within 6 to 12 hours after a storm, extended detention (ED) dry basins improve on the basic detention design by lengthening the storage time, for example, to 24 or 48 hours. Longer storage times tend to result in improved water quality because additional suspended solids are removed.

References

External links

Detention vs. retention
- Project Brays (Harris County, Texas)
Maintaining Your BMPs: A Guidebook for Private Owners & Operators in Northern Virginia
{{Authority control Environmental engineering Hydraulic engineering Hydrology Infrastructure Ponds Water treatment Stormwater management Water supply

Functions and design

Extended detention basin

See also

References

External links