CS28C - Concurrent Session 28C: Probable Maximums Should Probably Have Rare Probabilities: Implications of Modernizing Probable Maximum Precipitation

Abstract Description: Current estimates of Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF) assume that precipitation and flood magnitudes are bounded by practical or physical limits and cannot be exceeded. These estimates rely on empirical methods using limited data, do not incorporate changing climate conditions or statistical uncertainty, and have been surpassed in certain instances. To address these shortcomings and ensure the safety of critical infrastructure, the National Academies of Sciences, Engineering, and Medicine (NASEM) has proposed a revised definition and methodology for estimating PMP. The proposed revision asserts that PMP estimates should correspond to an extremely low annual exceedance probability (AEP) for a given climate period. This shift from deterministic upper limits to probabilistic estimates will significantly impact dam safety programs and infrastructure decision-making. Furthermore, NASEM’s long-term vision of employing physics-based climate models to estimate PMP for both current and future climates presents technical challenges, including how to reliably extrapolate to rare AEPs and quantify uncertainty across different climate periods. This paper explores the challenges of applying these proposed changes, presents project-specific examples, and outlines preliminary plans from the U.S. Army Corps of Engineers (USACE) for implementing these changes.