USGS: Florence set at least 28 flood records in Carolinas

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The U.S. Geological Survey has confirmed what many residents of the Carolinas already suspected: Hurricane Florence’s rainfalls brought with them record flooding. Preliminary data indicates that 18 USGS streamgages in North Carolina and 10 in South Carolina registered record-setting water levels, called peaks of record.

Another 45 streamgages in North Carolina and four in South Carolina recorded streamflows – the volume of water moving past a fixed point -- within the top five measured at those specific sites.

The information comes from a new report that focused on peak streamflow and water level data measured at 84 U.S. Geological Survey streamgages in the Carolinas. These gauges all had water level records stretching back ten years or more, and they all recorded water levels during Hurricane Florence that were among the top five measured for that site. This report can 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. Decision makers can also use this information for city planning, to update building codes and to help prepare for future storms.

“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,” said Toby Feaster, USGS Hydrologist and lead author of the study. “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.”

There were also some sites with more than 70 years of historical data that set new flood records. The Waccamaw River in Freeland, North Carolina, set a new peak of record September 19, with water levels at 22.61 feet and 53,600 cubic feet per second of water discharge. This was the largest peak of record for this site, which has data going back to 1940. In South Carolina, the Little Pee Dee River in Galivants Ferry 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. This was the largest peak of record in the 77 years the USGS has operated a streamgage at this site.

Since there are more than 485 USGS streamgages in the Carolinas, the authors of the report decided to only focus on the gauges that had a top five peak and longer historical records.

“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.”

These flood frequency analyses can provide insight into the likelihood of peak streamflows of varying magnitudes based on an annual exceedance probability, which is the probability of a peak streamflow at a particular location being achieved or eclipsed during any given year. For example, an annual exceedance probability of 0.01 means there is a 1 percent chance, or a 1 in 100 chance, of that specific streamflow magnitude to be equaled or exceeded in any given year.

Of the 28 streamgage sites in the Carolinas that 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.

The data used in the report came from USGS streamgages affected by Hurricane Florence and from the work of dozens of USGS field crews in the aftermath of Florence. These specialists performed more than 100 streamflow measurements of flooded rivers and streams and collected hundreds of high-water marks across the Carolinas. These high-water marks are thin lines of debris left behind on buildings and structures and are telltale signs scientists can use to determine how high flood waters reached. The high-water marks are also an important way to verify USGS streamgage measurements. ­­­

The full report, “Preliminary peak stage and streamflow data for selected U.S. Geological Survey streamgaging stations in North and South Carolina for flooding following Hurricane Florence, September 2018,” is available here: