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 generated by newly constructed impervious surfaces such  as roads, parking lots 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 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 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 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, when compared to basins without control.