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Hurricane Florence set at least 28 flood records, according to new USGS report

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Hurricane Florence, the massive storm that deluged the Carolinas after it made landfall on 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 huge 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 rise as they accumulated runoff.

The report, released Tuesday, 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 four 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 lines of debris left behind at the highest points the floodwaters reached.

“The report is basically recapping what we know,” said Laila Johnston, an associate director of the nonprofit organization 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 record peaks topped those set two years earlier during Hurricane Matthew, which caused extensive damage with its torrential rains in 2016.

It’s rare for two historic events to occur in such quick succession, but it’s not unheard of, according to Toby Feaster, a hydrologist with the USGS and lead author of the study. He pointed to two floods just a week apart along the Savannah River, where the peak of record in Oct. 2, 1929, came just a week after the second-largest 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 two Savannah River floods are recorded as belonging to two separate years although they were just days apart.

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 is used for infrastructure design, including building or replacing bridges, and factors into development close to flood plains.

Johnston said the report is important because it informs planning and development, especially as communities put impervious surfaces in place, which can exacerbate runoff and flooding.

“Smart growth is really key,” she said, noting that such water issues are vital as stronger new storms drop record amounts of rainfall. “We blew the old records out of the water.”