Stormwater detention is the temporary storage of rainwater runoff, released slowly to the storm drain system or a stream so that peak flows do not exceed what downstream pipes and channels can handle. Large paved parking areas are among the main sources of runoff that detention is designed to control.
Detention differs from retention. A detention system holds water for hours or days and then empties; a retention system holds water permanently or lets it soak into the ground. Local stormwater rules usually set the required storage volume, release rate and water quality treatment.
Common detention approaches for parking
- Surface basins and swales. Landscaped areas around a lot that fill during storms.
- Underground vaults and chambers. Storage beneath a surface lot, preserving usable land.
- Parking surface storage. Shallow ponding on low areas of a lot, where allowed.
- Rooftop and deck storage. Controlled drains on the top level of a structure. See green roofs.
- Permeable pavement. Storage in the stone base beneath porous paving.
Design considerations
- Coordinate detention with structure and utilities early; underground systems compete with foundations.
- Plan maintenance access to clean sediment and debris.
- Combine detention with treatment for oil and sediment from vehicles.
The Parkonomics view
Every space of pavement creates runoff that someone must manage. Reducing paved area is the cheapest detention there is.
Related terms: Rainwater Harvesting, Green Roofs, Heat Island Effect, Sustainability, Parksmart Certification