A Paper on Balancing CSO Program Investment: The King County Green Infrastructure Scorecard

King County Wastewater Treatment Division (WTD) is in the process of considering Green Infrastructure (GI) projects as part of their Combined Sewer Overflow (CSO) program capital planning, as part of that effort, WTD is working to determine when the investment in GI is commensurate to the cost of gray infrastructure on a unit basis (i.e., cost per gallon of CSO reduction). Additionally, WTD is working to identify what level of additional investment is appropriate for GI as compared to gray infrastructure based on the value of the ancillary benefits of GI, such as reducing stormwater flow to sewer system, providing ecosystem services, urban greening, climate change resiliency, etc.The development of a GI Project Scorecard (Scorecard) intends to provide WTD with the data to identify the types of GI projects, locations, and/or minimum performance necessary to be hydraulically cost-competitive to gray alternatives. This work also intends to provide King County WTD decision-makers with better information on when to pursue a capital GI project, how to select between multiple project concepts, and how to report out the justification for pursing a specific GI project.The Scorecard was developed through the research and review of existing GI cost and performance data, analysis of the co-benefits of GI and related value specific to King County, the development of cost-performance thresholds for GI, and the documentation of the Scorecard’s project evaluation metrics and methods. This presentation will inform conference participants on how those metrics were selected and the methods for quantifying those project benefits that are often considered unquantifiable.