Probabilistic Hydrologic Hazards vs. PMF for an SQRA: A Case Study of the USACE Greers Ferry Dam

ABSTRACT ONLY - A U.S. Army Corps of Engineers (USACE) Issue Evaluation Study (IES) for the Greers Ferry Dam in Arkansas was completed in support of a Semi-Quantitative Risk Analysis (SQRA). Under this process, both an updated Probable Maximum Flood (PMF) and a Probabilistic Flood Hazards Analysis (PFHA) were completed in parallel, providing a unique opportunity not only to compare outputs and their application for the SQRA, but also the development process for each and required data. As the industry continues to expand the use of PFHA to support risk analysis, some PMF habits are hard to quit.
Traditional PMF studies are central to standards-based compliance for dam safety, and within an SQRA, provide critical inputs for geotechnical and structural stability analyses. PFHA, on the other hand, provides a spectrum of annual exceedance probability hazards curves rather than a single value. For Greers Ferry, a parallel update of both allowed their joint application in the SQRA process. The PMF used a HEC-HMS model, whereas a modified version of the same model was central to the application of the new Risk Management Center (RMC) Rainfall Runoff Frequency Tool (RRFT), RMC-BestFit, and the RMC-RFA applications. Other than the benefit of informing an approximate AEP of the PMF, the PFHA provides return periods of other critical reservoir stages, release rates, and exceedance durations.
The USACE has been a forerunner in the application of Risk Informed Decision Making. In this presentation, we will review the overlaps between the PMF and PFHA analyses with respect to data and tools, and the divergences in how these data and tools are applied in an SQRA framework. We will discuss the latest tools, how both PMF and PFHA analyses are used to inform dam safety decision making at the USACE, and why the PMF endures in the dam safety world.