Detention & Retention
Detention Structure – a permanent stormwater management structure whose primary purpose is to temporarily store stormwateer runoff and release the runoff at controlled rates.
Retention Structure – a permanent struture whose primary purpse is to permanently store a given volume of stormwater runoff. Release of the given volume is by infiltration and/or evaporation.
Detention and Retention Structures (ponds, basins, etc.) are permanent strucutral controls recommended for water quantity measures. These structures are typically implemented when construction sites surpass 5 acres of land disturbing activities in order to meet regulatory quantity dishcarge requirements. The use of a retention structure in lieu of a detention structure is generally dictated by the permeability of the on-site soils. If the soils do not allow for proper infiltration rates, a detention structure would be better suited to control stormwater discharge rates post development.
A single retention or detention strucuture has the ability to be designed to meet both the water quantity and water quality regulatory constraints. This is accomplished by initially constructing a sediment basin to meet quality constraints. Once the site becomes stabilized, the sediment basin may then be converted into the permanent detention or retention structure. This is typically designed when land disturbing activities meet or surpass the 10 acre marker.
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