The Pittsburgh region is served by a combined sewer, which collects, conveys, and partially treats both stormwater and wastewater in the same infrastructure system. During periods of heavy rainfall, stormwater can exceed the capacity of combined sewers, which causes overflow into nearby rivers and streams. While these combined sewer overflows (CSOs) mitigate upstream flooding, they release untreated wastewater into receiving water bodies.
Improvements to “gray” infrastructure - pipes, pumps, storage, and treatment facilities - can increase the capacity of the collection system to accommodate more severe wet weather events. Conversely, “green” infrastructure includes features that reduce the stormwater entering the collection system by temporarily retaining or diverting stormwater. Types of green infrastructure vary from completely natural systems, such as converting a parking lot to a park, to single purpose engineered systems, such as pervious paving.
A comprehensive green infrastructure strategy starts with identifying broad areas appropriate for green infrastructure given variation in hydrologic (land use plus topography) and hydraulic (the capacity of pipes, pumps, and treatment) then applying a mix of additional criteria to target specific installations.
Technically feasible installations of green infrastructure include all possible structural installations at their maximum load ratio plus all possible non-structural installations. The technically feasible installations are identified through hydrologic and hydraulic models performed at the sewershed scale.
Cost optimal installations are those technically feasible projects that, taken together, minimize the net cost of our wet weather plans. These projects are identified by applying engineering economic analysis (e.g., cost effectiveness analysis) to scale estimated project-level performance to sewershed or service area level. The cost optimal set of installations include all possible cost effective installations when considering gray plus green strategies in balance.
Not all cost optimal installations are realistic. The realistic potential of green infrastructure considers land use in balance and recognizes any limitations to installations on private property, where property owners significantly influence the feasibility of installation.