Post-Construction Monitoring to Confirm Green Infrastructure Performance in Philadelphia

This paper describes how the Philadelphia Water Department analyzes performance of its green stormwater infrastructure systems using post-construction monitoring data. The level of runoff and combined sewer system performance required is determined by an agreement between the Philadelphia Water Department and the Pennsylvania Department of Environmental Protection. Requirements include the average annual combined sewer overflow volume permissible, the minimum degree of capture and treatment of wet weather flows, and pollutant loads. These city-scale requirements have been translated to the site level and incorporated in green infrastructure design criteria. The combined sewer overflow volume reduction is linked to a stormwater storage volume and allowable release rate that allow any given site to meet city-wide performance objectives on a unit-area basis. Removal of pollutants occurs through infiltration, filtration and settling processes. Any flow remaining after infiltration is conveyed to a wastewater treatment facility.