Hydrologic and Hydraulic Modeling Methods Developed for Philadelphia’s Long-Term CSO Control Program - Green City, Clean Waters

This paper provides an overview of the elegant and effective hydrologic and hydraulic modeling methods developed for Philadelphia’s Green City, Clean Waters program, the nation’s first CSO Long-Term Control Plan (LTCP) to be adopted based primarily on green stormwater infrastructure and distributed stormwater source control measures. These tools and methods were developed and used for evaluating the performance of green stormwater infrastructure along with traditional infrastructure alternatives as part of Philadelphia’s CSO Long Term Control Plan Update. The evolution of the modeling tools and methods needed to support the program from planning through implementation is presented.Combined sewer overflow (CSO) control standards require effective treatment of combined stormwater and municipal wastewater prior to discharge to receiving waters. Properly managing combined sewer flows in this manner relies on complex large-scale infrastructure projects for the conveyance, storage and treatment of high flows and volumes of stormwater combined with sanitary sewage. The green stormwater infrastructure approach to CSO control manages stormwater at its source before it combines with sanitary sewage and requires increased levels of treatment. This approach relies upon many distributed stormwater management practices to infiltrate, reuse onsite, filter or detain for slow release to existing collection and treatment facilities.The green stormwater infrastructure approach to CSO control is gaining support from regulatory agencies and is becoming adopted more widely in CSO long-term control plans across the nation. This recent trend in using distributed stormwater source controls for CSO management requires new and effective tools to evaluate performance for planning, implementation support, and regulatory reporting. Innovative and elegant system modeling methods, developed by Philadelphia Water, using public domain open source United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) software, can be of benefit to all communities developing and implementing CSO long-term control plans incorporating distributed source control measures.