Controlling the Source: Identifying Impactful, Cost-Effective Projects using Overflow Reduction Efficiencies and Opportunity Identification Processes

The Allegheny County Sanitary Authority (ALCOSAN) serves an area that spans approximately 310 square miles across 83 municipalities, including the City of Pittsburgh, Pennsylvania. ALCOSAN recently adopted a Clean Water Plan (CWP) to improve and protect the water quality of the region's streams and rivers by controlling combined sewer overflows (CSOs) and eliminating sanitary sewer overflows (SSOs). The CWP is part of a Federal Consent Decree (CD) to comply with the United States Environmental Protection Agency's CSO Control Policy. The CWP is made up of several components including preventing excess water from entering the sewer systems. As part of its green stormwater infrastructure (GSI) and source control (SC) program, ALCOSAN released Controlling the Source that lays out a consistent framework for evaluating GSI/SC in the Pittsburgh region. A key component involves identifying cost€effective and impactful GSI/SC projects capable of reducing sewer overflows to improve water quality. Overflow reduction efficiencies (OREs) were developed throughout the service area using automated modeling techniques to represent different GSI/SC techniques, varying implementation levels, and different system conditions. OREs provide a hydraulically-informed estimate of the relative overflow impacts of projects in different areas, so that effort and attention can be focused in those areas with the greatest potential overflow benefits. To complement the OREs, a GIS-based constraints analysis was performed to provide a geospatially-informed estimate of areas where GSI potential may be limited and/or costlier based on mapped physical and environmental constraints. Individual constraints such as shallow bedrock and slopes were assigned a range of constraint scores based the relative impact to GSI and then overlaid and summed. By targeting areas/sites with high OREs and lower levels of constraints, practitioners have powerful planning-level tools to identify projects most likely to be feasible, cost-effective, and impactful. Using these tools, ALCOSAN identified and evaluated almost 200 potential GSI opportunities. The opportunities were ranked and prioritized based on capture potential and implementation feasibility. Detailed concept plans for 60 of the higher ranked sites were developed and incorporated in discussions with municipalities as a tool to incentivize GSI project implementation in response to public comments received during the development of the CWP. This presentation will describe the development and application of the ORE modeling and the constraint scores as well as the results of the opportunity analysis and will discuss lessons learned for applicability in helping to address wet weather issues in other communities.