Large-Scale Stormwater Management in an Urban Area Served by Combined Sewers: A Green Approach to Overflow Control

The Philadelphia Water Department's combined sewer overflow control program is part of its “Green Cities – Clean Waters” vision and includes a range of land-based stormwater management techniques. The U.S. EPA's Storm Water Management Model was selected to develop a watershed-scale model of rainfall-runoff processes, land-based stormwater management measures, and combined sewer system hydraulics. The model is calibrated to match existing observed flows, while simulation of stormwater controls is based on a systems approach representing the physical processes involved. Results suggest that significant reductions in stormwater runoff, combined sewer overflow frequency, and combined sewer overflow volume can be achieved. Implementation of stormwater management techniques on a large scale is possible over the long term using a variety of programs targeting public and private land.