Evaluating Potential Combined Sewer System Pipe Capacity Issues Due to Climate Change

Abstract Submission: The Philadelphia Water Department has developed a planning-level screening tool to evaluate pipe capacity and level of service of larger pipes (>36”) in its combined sewer service areas. This tool can be used to inform renewal and replacement strategies, detailed future flooding analyses, and climate change adaptation strategies. Given projected future increases in rainfall intensities and volumes, more areas of the city may experience overwhelmed drainage systems and potential flooding during storm events. This tool helps identify areas that are at risk of flooding during rainfall events of varying severity and inform strategies to mitigate these risks. Using planning-level SWMM models of the combined sewer system, a total of 12 design storms were evaluated: 24-hour duration design rainfall with 2-, 5-, 10-, and 25-year average recurrence intervals for existing, mid-century, and 2100 rainfall depths. A capacity fraction was computed for each pipe as the ratio between the 15-minute average maximum observed flow and full flow according to Manning’s equation. The planning level models include 149 miles of sewers. Under existing conditions, 52% of all sewers have a level of service of 25 years while 20% have a level of service of less than 2 years. Projections suggest that by mid-century, only 37% of sewers will have a 25-year level of service of and 57% of sewers will have a level of service of less than 2 years. By 2100, only 32% of sewers will have a 25-year level of service while 64% will have a level of service of less than 2 years.

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