The North Carolina Department of Environmental Quality (DEQ) has announced the release of a new tool aimed at improving storm preparedness in the state. Developed with Applied Weather Associates (AWA), the state’s first Probable Maximum Precipitation (PMP) study and evaluation tool is designed to better prepare for future storms and their potential impact on dams.
“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. It tracks storm types, severity, area size covered, and rainfall intensity. Recent events such as Hurricane Helene are included in the study. The tool aims to modernize emergency response approaches concerning severe rainfall impacts on hydraulic structures like 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. “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 previous Hydrometeorological Report 51 (HMR51). The new PMP study evaluates three types of storms: local storms with shorter durations; general storms covering multiple states with longer durations; and tropical storms including hurricanes that cover large areas over extended periods.
Additionally, the PMP considers terrain characteristics, a broader range of rainfall durations, and smaller watershed areas to assess impacts on smaller high-hazard dams across North Carolina.
The HMR51 only considered one type of storm with a minimum rainfall duration of six hours for dams with minimum watershed areas of 10 square miles. In contrast, the PMP evaluates storms lasting as short as one hour with watershed areas as small as one square mile or less.
A local storm might last an hour or two over a small area but can bring significant rainfall volumes. For large dams with extensive watershed areas, such a storm may be insignificant. However, for small dams with limited watershed areas, it could be enough to overtop them.
While not predicting specific impacts of upcoming storms, the PMP tool provides more accurate data for preparation efforts regarding incoming systems and designing crucial hydraulic structures like dams.
The Dam Safety Program has made available its findings online along with user manuals, tutorials, and other relevant information about using this new evaluation tool.
