Retention / Detention Systems

Detention systems are designed to release all runoff captured between storm events gradually and completely. The commonly used detention facilities for stormwater management are detention basins (dry ponds) and underground vaults, pipes, and tanks. Most detention basins are designed to empty out the runoff in less than 24 hrs. It serves more as a stormwater quantity control measure for peak discharge than as stormwater quality control to any receiving streams. Suspended solids and related contaminants can be removed via a gravity settling mechanism but the removal efficiency is limited.

Retention systems consist of a permanent pool of water into which stormwater runoff is directed. Runoff from each rain event is detained and treated in the pond until it is displaced by runoff from the next storm. The pollutants in the runoff are removed by the retention system’s natural physical, biological, and chemical processes. Particulates, organic matter, and metals are removed through sedimentation processes. Dissolved metals and nutrients are removed through biological uptake. Therefore, retention systems control both quantity and quality of stormwater runoff.

Maintenance includes repairs to the embankment, emergency spillway, inlet, and outlet; removal of sediment; and control of algal growth, insects, and odors. In most cases, sediments removed from wet detention ponds are suitable for landfill disposal. Well maintained retention basins can also provide esthetic value and habitat for various plants and animals. The commonly seen retention systems are retention basins/ponds and wet ponds.

Copyright @ January,2005 Prepared by Transportation Environmental Resource Center (TERC)