Probabilistic Flood Model to Evaluate Hydrologic Loadings and Evaluate the Operational Conservatisms of the PMP/PMF, An Owner's Perspective

This paper presents the results of a probabilistic flood hazard assessment to support a quantitative risk assessment for Tacoma Power’s Cushman Hydroelectric Project in Washington State. The project comprises two concrete arch dams, Cushman No. 1 (upstream) and Cushman No. 2 (downstream), with a total tributary area of 100 square miles. From the project owner’s perspective, evaluating the conservatism of the Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF) provided valuable insights into the conservatism of current operational strategies. The Cushman No. 1 reservoir is operated with a significantly reduced pool in winter to provide sufficient storage to safely pass the PMF with a 100-year antecedent snowpack. Flood frequency relationships produced by the Stochastic Event Flood Model (SEFM) were used to assess the likelihood of overtopping, the exceedance probability of the reservoir water surface produced by the PMF and improved operational procedures for both dams.