Updating PMP to provide better dam and spillway design

Abstract

Probable Maximum Precipitation (PMP) is used as input to derive the Probable Maximum Flood (PMF) for high hazard dams across the United States. The PMF is used to design high hazard dams to ensure they do not overtop and fail during a PMP/PMF event. PMP values were developed for most of the United States by the National Weather Service (NWS) in a series of Hydrometeorological Reports (HMRs) starting in 1940 and continuing through 1999. However, the NWS stopped updating the HMRs and the storms used to derive the PMP values. Since that time the meteorological understanding of extreme rainfall has advanced significantly and numerous large rainfall events have occurred which affect PMP. Because the dam safety community required updated PMP values which incorporated these updated data and meteorological understanding to ensure proper design and rehab of dams, Applied Weather Associates (AWA) has been performing site-specific, statewide, and regional PMP studies to update these data and advance the science regarding PMP. Beginning in the 1990's, AWA has completed more than 100 PMP studies, which have been accepted by state and Federal dam safety regulators. Each of these provide updated PMP values which replace those from the HMRs. In this process, AWA has analyzed hundreds of the most extreme rainfall events, which are required for proper PMP development. This presentation will detail the history of PMP development in the United States, provide an understanding of current PMP development, discuss the storm analysis process, and detail future improvements that are needed for dam safety.