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A Detention basin is an artificial flow control structure that is used to contain flood water for a limited period of a time, thereby providing protection for areas downstream. This is opposed to a retention basin that holds water for an extended period of time. These basins are generally a part of a larger engineered flood water management system.
In some areas, structures like these are used to assist in separating the storm sewers from the sanitary waste sewers. In older cities, storm and sanitary waste were combined in one pipe. Many cities have sewage treatment facilities that are forced to flush untreated sewage into streams and rivers without treatment during heavy storms since they are not equipped to handle the larger flows. By providing a way to reduce these peak flows, it helps to reduce the frequency of releases of untreated sewage.

Capacity and size
These structures are designed to accommodate a specific amount of water, usually defined in terms of a 50 year flood or a 100 year flood, statistically the 2% and 1% annual chance exceedence flood respectively. If the flow exceeds this amount, there is a spillway which provides an outlet for the excess water. However release of water by this means usually will pose a threat to the areas the detention basin was installed to protect. Overflow can be into washes or streets depending on where the basin is located.
The size of these structures varies. However they are frequently seen in sizes of 10 to 50 acres (4,000 to 20,000 m²) and up to 70 feet (25 m) deep. The designed holding capacity of these structures is determined by the size of the area that drains into the structure and the maximum size of the flood the structure is designed to contain.

Design features
The basins function by allowing large flows of water to enter, but limit the outflow by having a small opening at the lowest point of the structure. The size of this opening is determined by the underground and downstream culverts and washes capacity 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 blocks in the entrance spillways are used to reduce the speed of entering flood water. These structures may also have debris 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 use.

Uses when empty
Since these structures do not see frequent use, especially in desert areas, it is common to have them designed as dual use facilities.
The designed secondary uses include: ball fields, dog parks or other large play areas. Since the fields can be under water, special care needs to be taken when lighting is installed to illuminate the playing fields. These areas also have minimal equipment to limit property damage when in use during floods.

Underground storm water detention
A recent trend in stormwater detention is underground detention. In this method, instead of a pond-like structure, large holding pipes or chambers are buried under parking lots or land where open water detention is not acceptable. As a detention basin, the inlets are larger than the outlets to regulate flow as required. This is often used where real estate prices are at a premium, or there is too little space to otherwise comply with water detention requirements. This is generally more expensive than a standard detention basin. Common uses are urban areas where there isn't space for a standard basin, but it could be under the parking lot; or near airports where open ponds could attract birds dangerous to aircraft operations.