Detention ponds are an excellent way to naturalize the stormwater patterns of a larger urban area. Rain gardens, infiltration trenches and bioswales collect stormwater runoff from smaller areas, and are most effective when used to treat the water from one urban land parcel. However, detention ponds are so large, and hold so much water that they are more effective when implemented at the community level.

Detention ponds primarily manage stormwater quantity, but also somewhat contribute to treating the water quality. When it rains over a detention pond’s drainage area, the runoff water either travels to the pond across the land surface or is routed there by gutters or underground pipes. In some cases, the water is passed through a pretreatment wetland or sedimentation tank. These additional treatment cells are included to remove extra sediment and dissolved pollutants from the water. Once the water leaves these pretreatment cells, it then enters into the permanent pond and slightly raises the water level. Because they are a designated area for water storage, detention ponds prevent water from flooding in the community’s developed areas.

The runoff is stored in the pond and slowly released into the stormwater sewer system. The storage-release process reduces the peak runoff volumes and runoff rates. The implications of these reductions in “quantity” are that the stormwater sewer system and treatment facility can be designed to handle smaller volumes. Also, when the water pools in the basin, its velocity is significantly reduced. This allows suspended sediments to sink to the bottom of the pond, and the organic pollutants to be digested by the organisms present in the pond. This improves the water “quality.”