CS28A - Concurrent Session 28A: Probabilistic Flood Model to Evaluate Hydrologic Loadings and Evaluate the Operational Conservatisms of the PMP/PMF, An Owners Perspective

Abstract Description: This paper presents the results of a probabilistic flood hazard assessment to support a quantitative risk assessment for Tacoma Power's Cushman Hydroelectric Project located in the Olympic mountains of 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. Using precipitation-frequency relationships developed for inputs to the stochastic flood model, it was possible to assess the exceedance probability of the PMP. Flood frequency relationships produced by 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.

Key findings:

• Exceedance Probability of the PMP and PMF
A regional precipitation frequency analysis was used to develop watershed precipitation-frequency relationships for input to the SEFM stochastic flood model. Using these relationships, it was found that the 50.1 inch, 72-hour general storm PMP was exceptionally rare, with an annual exceedance probability of 1.4E-09. The exceedance probability of the water surface elevation produced by the PMF was estimated at 9.0 E-09.
 

• Dam Overtopping Probabilities
For Cushman Dam No. 1, reducing the outflow until Lake Cushman (controlled by Dam No. 1) partially fills reduced the likelihood of overtopping of the downstream dam, while maintaining the annual exceedance probability (AEP) of overtopping of Dam No. 1 at an acceptable level, less than 1.0 E-07.
For Cushman Dam No. 2, utilizing the flood storage in Lake Cushman significantly reduced the overtopping probability to below 1.0 E-07.

This analysis underscores the value of probabilistic flood assessments in refining operational protocols and providing flood-frequency information for risk management frameworks. It also offers a measure of the conservatism of the PMF used to develop previous operational strategies for the project, equipping Tacoma Power with critical tools to enhance operational procedures while maintaining acceptable levels of risk.