Generalized and Site-Specific Probable Maximum Precipitation (PMP) Studies for Dam safety evaluations

Probable Maximum Precipitation (PMP) is defined as the theoretically greatest depth of precipitation for a given duration that is physically possible over a given storm area at a particular geographic location at a certain time of year. PMP estimates have been used for many years to provide the rainfall values used to compute the Probable Maximum Flood (PMF) for drainage basins where state and/or federal regulations require dams to safely pass the PMF. PMP values for all locations in the United States are provided in Hydrometeorological Reports (HMRs) published by NOAA. These reports provide generalized PMP values for watersheds located within the geographic boundaries defined in the report. Flood specialists use the PMP estimates from these HMRs unless site-specific PMP studies are performed for individual basins. Site-specific PMP studies address local attributes for a particular drainage in more detail than is provided in the generalized HMRs, especially where terrain could significant affect both the magnitude and location of the largest rainfall.

There have been many site-specific PMP studies performed during the past two decades. The results of many of these studies have been used in PMF analyses for both federal and state licensing requirements while others have not been accepted by the regulators for use in determining the PMF. Most site-specific PMP studies follow the basis procedures used in the HMRs but add additional considerations which are either refinements in the HMR procedures or application of new techniques or technologies which provide improvements to the historic procedures.

A discussion of the basic procedures used in the generalized PMP reports along with several refinements and improvements introduced in site-specific PMP studies will be presented. Among these are improvements in moisture analysis, both available storm moisture and maximum moisture climatologies. Additional discussions will present the inclusion of results from paleoflood studies that provide estimates of the maximum floods during many thousands of years. These maximum flood data are used to validate site-specific PMP study results. Examples will be provided from site-specific PMP studies that have been completed in the last several years.