Performance of Retrofitted Stormwater Extended Detention Wetlands

An existing detention basin was retrofitted to an extended detention basin through the outlet modification. Since bottom of the existing basin was covered extensively by vegetation, it served as the surface wetlands as well. Therefore, the basin after outlet modification became an extended detention basin with dual flood control and pollutant removal functions as well as the surface wetlands with advanced pollutant removal function. It was determined that the retrofitted extended detention-surface wetlands system would be as efficient as other retrofit alternatives in pollutant removal. Sustaining such a system would eliminate the additional retrofit costs, reduce the long-term maintenance costs, and prevent the adverse upstream flooding impacts. Water quality performance of the retrofitted extended detention-surface wetlands system was monitored and evaluated. The retrofitted basin achieved excellent total suspended solids removal and reasonably good nutrient removal.