The odd couple: Sewer overflow modeling and the similarities between Pittsburgh, PA, and North Carolina

With the increasing variability in our climate, specifically with respect to rainfall, sewer overflows are becoming a more frequent and often unavoidable occurrence in our society. Notwithstanding the effect of climate change and the increase in population, the capacity and integrity of pipes are also questionable, as some can be dated back to the 1900s.  

Pittsburgh’s combined sewer system collects runoff from catchment basins but has overflow structures that are necessary to prevent flooding, extreme surcharging of hydraulic elements, and backup into service properties. These combine sewer systems operate on the 85% capture rule. North Carolina’s sewer system, which is sanitary only, ideally should not collect water from rainfall or storm events. Permitted overflow structures do not exist, and thus there is no 85% capture regulation; however, infiltration and inflow (I&I) into its sanitary sewers is an issue. These rainfall-derived infiltration and inflow (RDII) flows in extreme weather essentially make these systems into combined sewers. Increasing I&I will eventually cause flows to exceed the capacity of pipes and even treatments plants. RDII into sewers, especially aging sanitary sewer systems, is the concern of any North Carolina municipality.  

We will discuss the modeling of these sewer systems, including the selection of wet weather events, sewer shed areas, loadings, sizing of diversion chambers, weirs orifices, and even flow monitoring systems to meet 85% capture where applicable. Models are only as good as the data provided for their calibration and development. Can all data, especially from flow meters, be trusted? We will discuss how to evaluate data to determine which data should be accepted, and which discarded.  

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