Ten flood records were set across South Carolina in the aftermath of Hurricane Florence, according to a new report by the US Geological Survey.
USGS confirmed record flood levels at 10 streamgages in South Carolina’s Pee Dee region. Another four in South Carolina recorded streamflows – the volume of water moving past a fixed point — within the top five measured at those specific sites.
“One thing we discovered while compiling this report was many of the new peaks of record set by Hurricane Florence broke previous records set by Hurricane Matthew in 2016,” lead author Toby Feaster said. “Since several of the streamgage sites we analyzed had more than 30 years of historical data associated with them, it was interesting that a majority of the number one and two records were from back-to-back flooding events.”
While USGS maintains more than 200 gauges in South Carolina, it only examined those with more than 10 years of data. The agency said its Galivants Ferry streamgage along the Little Pee Dee River set a new peak of record September 21, with water levels at 17.21 feet and 66,900 cubic feet per second of water discharge. USGS has more than 77 years of records at the site.
“We made the decision to only include streamgages in this report with at least a decade of historical data, because a new peak of record at a site with only a few years of history doesn’t really provide very useful information,” Feaster said. “We also decided to stick with at least ten years of historical records because that is the minimum required for flood frequency analysis.”
Feaster emphasized that, while two catastrophic flooding incidents so close together from 2016’s Hurricane Matthew and Hurricane Florence this year is unusual, it is not unprecedented. He noted the Congaree River experienced near-record highs near Columbia in both 1928 and 1929, before construction of the Lake Murray dam began regulating the Saluda River upstream.
The report will be used by emergency managers and water resources engineers who often need to know the expected frequency and magnitude of peak streamflows observed during a flood. These flood frequency analyses can provide insight into the likelihood of peak streamflow. Of the 28 sites in both North and South Carolina which set new streamflow records from Florence flooding, nine had a less than 1 in 500 chance of flooding of that magnitude happening in any given year. Three sites had an estimated annual exceedance probability equal to a 1 in 500 chance of flooding. At six other sites, there was between a 1 and 500 chance and a 1 in 100 chance of flooding of that magnitude. The 10 remaining sites had an annual exceedance probabilities of 1 in 67 chance or greater.