A vegetated depression, normally  dry except after storm events. It is constructed to store water temporarily to attenuate flows.  It may allow infiltration of water to the ground and permits settlement of coarse solids.  Outlets are normally designed to empty over a 24-hour period and the maximum depth of water should not exceed 3m, only occurring for the design storm.

As these features are normally dry, they may be used for other purposes when not required for drainage.  Where further conveyance of flow is required, the base of the detention basin should be designed to drain freely to ensure that standing water is not left on the basin floor.

See also extended detention basin.