What is the difference between a retention system and a detention system?
A retention system retains all the stormwater it receives and percolates it into the ground. A detention system detains stormwater and slowly releases it into a pond, canal, or other conveyance system; or percolates it into the ground. Retention and detention systems can be either wet or dry. Dry systems dry out within 72 hours after a rainfall event. If they don’t this means they were improperly designed or improperly maintained, or both. A wet detention system is designed to allow material to settle and it slowly releases captured water out of it through an orifice or weir in the outflow structure. Part of a wet detention pond – known as the permanent pool – is always below the level of the drain structure. Sometimes aquatic plants are planted around a pond’s perimeter to help filter sediment in stormwater runoff.
