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

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

stream A stream is a continuous body of water, body of surface water Current (stream), flowing within the stream bed, bed and bank (geography), banks of a channel (geography), channel. Depending on its location or certain characteristics, a strea ...

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

s or bays to protect against

flood A flood is an overflow of water (list of non-water floods, 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 of significant con ...

ing and, in some cases, downstream

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

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 retention pond, wet detention basin, or storm water 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 ...

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 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 Precipitati ...

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 ...

s such as

road A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved. Th ...

parking lot A parking lot or car park (British English), also known as a car lot, is a cleared area intended for parking vehicles. The term usually refers to an area dedicated only for parking, with a durable or semi-durable surface. In most jurisdi ...

s and rooftops. 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

culvert A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe (fluid conveyance), pipe, reinforced concrete or other materia ...

s 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 aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

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 waste, 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 ref ...

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 and associated contaminants, such as

heavy metals upright=1.2, Crystals of lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals is a controversial and ambiguous term for metallic elements with relatively h ...

, 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 such as

suspended solids Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to motion of the water. Suspended solids can be removed by sedimentation if their size or density is comparatively large, or by filtration ...

. This basin type differs from a

, also known as a "wet pond," which includes a permanent pool of water.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 typically designed to empty within 6 to 12 hours after a storm, extended detention (ED) dry basins improve the basic detention design by lengthening the storage time, for example, to 24 or 48 hours. Longer detention allows for more settling of suspended solids, resulting in higher-quality water.

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