A stormwater detention vault is an underground structure designed to manage excess stormwater runoff on a developed site, often in an urban setting.  This type of best management practice may be selected when there is insufficient space on the site to infiltrate the runoff or build a surface facility such as a detention basin or retention basin.
Detention vaults manage stormwater quantity flowing to nearby surface waters. They help prevent flooding and can reduce erosion in rivers and streams. They do not provide treatment to improve water quality, though some are attached to a media filter bank to remove pollutants.