Extended Detention Basin

Extended detention basins are surface depressions that temporarily store stormwater runoff. The slower drain time allows sediment to settle to the basin bottom before water discharges. The amount of water quality treatment is dependent upon the detention time.

Extended detention basin detail.
Image Credit: NJ BMP Manual, 2018

Considerations: Sediment will accumulate at the basin bottom. Designing systems to accommodate more volume than required can accommodate for this volume loss.

Cost: Extended detention basins are generally less expensive per square foot than infiltration basins or bioretention basins due to the lack of planting and soil modifications/amendments. However, they have less benefit per square foot and are likely to require larger footprints to manage water quality and water quantity. Detention basins need to be paired with additional BMPs to achieve groundwater recharge on site and/or water quality rates over 60%.

Maintenance: Regular maintenance activities include inspections for structural damage related to the inlet and outlet structures, ponding within the basin, and the structural integrity of the berm and spillway. Frequent mowing and clearing of debris/sediment is also required.