Detention vs Retention Stormwater Facilities
Detention basins release stormwater slowly. Retention facilities hold or infiltrate it. Compare objectives, infiltration, upkeep and local terms.
Civil & Structural · Last reviewed
Short answer: use detention to control the peak flow leaving a site, by storing runoff and releasing it at a limited rate. Use retention where the goal is to keep water on site, by infiltration, evaporation or a permanent pool, or to improve water quality. Local rules use these words differently, so check the definitions that apply.
| Detention | Retention | |
|---|---|---|
| Main purpose | Reduce the peak outflow to the pre-development rate | Hold or reduce the volume of runoff, and treat water quality |
| Normal condition | Usually dry between storms (a dry pond), or draining within days | Holds a permanent pool, or infiltrates, or stores water without surface discharge |
| Discharge | Controlled outlet to a stream or pipe, such as an orifice and a weir | Little or no discharge in smaller storms, with an overflow for large storms |
| Infiltration | Minor, if any | Important in infiltration basins and ponds. Needs suitable soil |
| Site conditions | Needs space and an outlet with enough downstream capacity and fall | Needs soil that drains, a low water table and distance from foundations and wells, or a liner and a permanent pool |
| Water quality | Limited settling during the short detention time | Better, from settling, biological action and infiltration |
| Maintenance | Outlet cleaning, sediment removal, mowing, inspections | Sediment, vegetation, mosquito and algae management, and infiltration performance checks |
| Risks | Outlet clogging, flooding if the volume is too small | Groundwater contamination if poorly sited, standing water issues |
| Terminology | Called a dry pond or extended detention basin | Some places call a wet pond a retention pond, others call it a wet detention pond |
Related tools and labs
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Questions
Can a facility do both?
Yes. Many designs combine a permanent pool for quality with extra storage above it for peak flow control, which is sometimes called wet detention.
Does detention reduce the total runoff?
No. It delays and spreads the same volume. Retention and infiltration reduce the volume that leaves the site.
Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.