Flooding from Hurricane Florence

The U.S. Geological Survey has released their report: "Preliminary Peak Stage and Streamflow Data at Selected U.S. Geological Survey Streamgaging Stations in North and South Carolina for Flooding Following Hurricane Florence, September 2018." The report is by Toby D. Feaster, J. Curtis Weaver, Anthony J. Gotvald, and Katharine R. Kolb. The article below is from that report and the news release.

Area of Study for the report USGS

Early Friday morning on September 14, 2018, Hurricane Florence made landfall as a Category 1 hurricane at Wrightsville Beach, North Carolina (North Carolina Department of Public Safety, 2018). The storm was reported to be nearly 400 miles wide. Once over land, the forward motion of the hurricane slowed to about 2 to 3 miles per hour; and over the next several days, the hurricane drenched the Carolinas,  delivering historic amounts of rainfall across North and South Carolina. In many communities in both states this caused substantial flooding (Feaster and others USGS 2018).

The maximum 4-day rainfall total reached almost 36 inches in some areas of North Carolina and almost 24 inches in some areas of South Carolina, resulting in historic flooding in many communities within both States. In addition to the catastrophic flooding from Hurricane Florence, the coastal and central parts of North and South Carolina experienced previous catastrophic flooding over the years and recently in 2016 and 2015 from two other storm events. In October 2016, Hurricane Matthew brought heavy rainfall to the eastern and central parts of the Carolinas (Weaver and others, 2016). In October 2015 a low-pressure system over the Southeast funneled tropical moisture from Hurricane Joaquin into South Carolina causing historic rainfall amounts (Feaster and others, 2015), which resulted in historic flooding in the central and coastal parts of the State. The scientists at the USGS looked at stream flow data for gauges that had at least 10 years of data to look at both the magnitude of the flooding from Hurricane Florence and the probability of future flooding.

from USGS Feaster and others 2018

“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. “...Several of the 28 streamgage sites we analyzed had more than 30 years of historical data ..., it was interesting that a majority of the number one and two records were from back-to-back flooding events.”

There were some sites with more than 70 years of historical data that set new flood records, but there were others where the peak flood event was more than a hundred years ago. In North Carolina, French Broad River at Asheville, N.C., has one of the longest records with peak streamflows going back to 1896 (fig. 16 id from Feaster and others USGS 2018). The peak of record was recorded on July 16, 1916, at 23.1 ft and a peak streamflow of 110,000 ft3/second, which is about two and one-half times larger than the second largest peak that occurred on September 8, 2004. Historic information indicates the 1916 peak was likely the largest peak since at least 1791(Feaster and others USGS 2018). 

The new report analyzed the estimated annual exceedance probability for the Hurricane Florence peak streamflow to determine how likely a reoccurance at the 28 streamgages studied. They found that 9 of the streamgages was less than 0.2 percent, which in terms of recurrence intervals is greater than a 500-year flood event. At three streamgages, the estimated annual exceedance probability was equal to 0.2 percent, and at six streamgages, it was between 0.2 and 1 percent (between a 500- and 100-year recurrence interval, respectively). For the remaining 10 streamgages, the estimated annual exceedance probability was between 1.5 and 7.1 percent, which in terms of recurrence intervals is approximately a 67- to 14-year event, respectively (Feaster and others USGS 2018). Frequent flooding events is a given for the region and often these events occur with back to back storms. This would explain a real estate requirement that my husband learned from his father- that you should always live on high ground- My father-in-law's people were from North Carolina.

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To read the full report see:

Feaster, T.D., Weaver, J.C., Gotvald, A.J., and Kolb, K.R., 2018, 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: U.S. Geological Survey Open-File Report 2018–1172, 36 p., https://doi.org/10.3133/ofr20181172.