Risk-Based Approach for Controlling CSOs

Seattle Public Utilities (SPU) has developed a method to quantify unknowns and provide a transparent means for senior management to make decisions on facilities to meet CSO control goals. Risk, defined as the product of likelihood and consequence, is quantified to size facilities to meet reduction goals. The likelihood function is created using behavioral parameter sets run through a calibrated hydrologic and hydraulic model. These simulations are evaluated using a goodness-of-fit test. The consequence rating, reflecting the City's values, is determined from a subjective scoring of social, economic, and environmental characteristics of each basin. The likelihood function and consequence rating are used in the Decision Framework to inform a recommended CSO reduction facility sizing. This recommendation provides a numeric goal for reduction and quantifies the confidence that the volume is accurate to meet the City's CSO control goal of one untreated CSO per year per site.