The North Carolina Department of Environmental Quality’s (DEQ) Dam Safety Program, with help from Applied Weather Associates (AWA), is releasing the state’s first Probable Maximum Precipitation (PMP) study and evaluation tool, which is designed to better prepare for future storms and their possible impact on dams in the state.
“We know that storms will keep coming, with increasing frequency and intensity, and this new tool will help us better prepare for storms and their impact on dams,” said DEQ Secretary Reid Wilson. “We will continue to work with federal and local government agencies and other partners to plan proactively since that is our best defense against disasters.”
The PMP study incorporates data from 91 storms in North Carolina and surrounding areas over the past 150 years, tracking the type of storms, the storms’ severity, the size of the area covered and the intensity of rainfall. Recent storms including Hurricane Helene are included in the study. The tool is expected to modernize the state’s emergency response approach to the impacts of severe rainfall events on hydraulic structures such as dams, spillways and flood control structures.
“Before the new PMP tool, the standard for storm preparedness was based on the Hydrometeorological Report 51, a study published in 1978,” said Toby Vinson, Director of the Division of Energy, Mineral and Land Resources, which includes the Dam Safety program. “This new tool is based on the most current scientific methods and incorporates data on a much wider scale. As we continue to see storm systems increase in intensity from year to year, the PMP tool will help us better adapt and prepare for oncoming storm seasons and address dam impoundment storage and spillway design capacities more comprehensively.”
There are three major differences between the PMP Study and the Hydrometeorological Report 51 (HMR51).
The new PMP study evaluates three types of storms:
- Local storms, which include smaller thunderstorm systems with shorter durations of rainfall
- General storms, which are large systems that can cover multiple states and typically have a longer duration of rainfall
- Tropical storms including hurricanes that typically cover large areas and last for longer periods.
Additionally, the PMP takes into account terrain characteristics, a wider range of rainfall durations and smaller watershed areas to include possible impacts on smaller high-hazard dams across the state.
When evaluating storms, the HMR51 only considered one type of storm, and used a minimum rainfall duration of six hours and dams with minimum watershed areas of 10 square miles. The PMP study evaluates storms with durations as short as one hour and includes watershed areas as small as 1 square mile or less, which accounts for most dams in North Carolina.
A local storm might only last an hour or two over a small, localized area, but it can also bring a higher volume of rainfall. For a large dam with a significant watershed area, that storm may be insignificant. But for a small dam with a small watershed area, the volume of rain dropped in those one or two hours could be enough to overtop the dam.
Though the PMP study cannot be used to predict specific impacts of oncoming storms, the new tool can provide more accurate data to inform preparation for oncoming systems, as well as the design of important hydraulic structures such as dams.
The Dam Safety Program has published the PMP study on its website, as well as provided the actual PMP evaluation tool, user manual, tutorials and other usage information.