Florence’s impact was record-setting
Study shows deluge set 28 flood marks in the Carolinas
The Carolinas’ impact from Hurricane Florence eclipsed marks set during Matthew two years earlier.
Hurricane Florence, which deluged the Carolinas after it made landfall Sept. 14, set at least 28 flood records, according to a new report by the U.S. Geological Survey.
Florence hit North Carolina as a Category 1 hurricane and then stalled, combining a storm surge and coastal flooding with prolonged, record-breaking rainfall that reached as much as 30 inches in some inland regions. The intense rains caused flash flooding, which was followed — sometimes as much as a week later — by floods along rivers that continued to build as they accumulated runoff.
The report uses preliminary data to show that 18 USGS stream gauges in North Carolina and 10 in South Carolina registered record water levels, or “peaks of record.”
In addition, the “streamflows” — or volume of water passing a fixed point — at 45 stream gauges in North Carolina and 10 in South Carolina were among the top five ever at those sites. USGS scientists working in the field subsequently checked the gauge data by looking for high-water marks.
“The report is basically recapping what we know,” said Laila Johnston, an associate director of the nonprofit American Rivers, which works to preserve the country’s rivers and conserve clean water supplies. Johnston, who is based in South Carolina, said the report shows how “these new rain events are the new normal.”
Many of the new record peaks topped those set just two years earlier during Hurricane Matthew.
It’s rare for two historic events to occur in such quick succession, but it’s not unheard of, according to Toby Feaster, lead author of the study. He pointed to two separate floods a week apart along the Savannah River, where the peak of record on Oct. 2, 1929, came a week after the secondlargest peak of record on Sept 29, 1929. But water years, like financial years, do not match calendar years. The first day of the new water year is Oct. 1, meaning the Savannah River floods are recorded as belonging to two separate years.
The USGS study used data collected from 84 of the 485 USGS stream gauges in North and South Carolina, relying on sites that had records going back at least 10 years, with several going back more than three decades.
Some sites that have data across more than seven decades also broke records, including the Waccamaw River in Freeland, N.C., which set a new peak record five days after Florence’s landfall, at 22.61 feet and with streamflows of 53,600 cubic feet per second. Two days later, the Little Pee Dee River in South Carolina recorded its highest peak in 77 years.
Feaster said the data are used for infrastructure design as well as to look at development close to flood plains.