Stormwater Detention & Retention
CSI Division 33, Utilities
Underground vaults, surface ponds, or other structures designed to temporarily hold stormwater runoff on site and release it slowly through a controlled outlet, detention, or hold it long-term behind a permanent pool, releasing it gradually through evaporation, infiltration, and a controlled outlet, retention, reducing the peak volume and rate of water leaving a site so it doesn't overwhelm the municipal storm drainage system downstream.
Why it matters
Most municipalities require a site's post-development stormwater runoff rate to not exceed what the same site produced before development, detention and retention systems are what let a project meet that requirement even though paving and roofs replace absorbent natural ground with impervious surface.
On a real project
A shopping center's site design includes an underground detention vault sized to hold the calculated increase in stormwater runoff from the new building and parking lot, releasing it into the municipal storm system at a controlled rate that matches the site's pre-development runoff.
Who this matters most to
A Civil Engineer designs the detention or retention system's sizing based on the site's runoff calculations. A QA/QC Inspector verifies the system is installed to the specified volume and outlet configuration before it's buried and inaccessible.
Where this goes wrong
A contractor installs an underground detention vault but the outlet control structure is built with a larger orifice than the design specifies, letting water drain out faster than the calculated controlled rate. The error isn't caught until the next heavy storm, when downstream flooding complaints prompt a review that traces the problem back to the oversized outlet.