‘Thousand year flood’ leaves widespread damage after Hurricane Helene

Hurricane Helene hit Florida’s western coast as a Category 4 hurricane on September 26 and was accompanied by serious storm surges—but the damage didn’t end there.

Still a Category 2 hurricane when it swept into Georgia, Helene dumped staggering amounts of rain over eastern Tennessee and western North Carolina, far inland and at much higher elevations in the Appalachian Mountains than people often consider to be at risk from hurricanes. All told, Helene is known to have killed more than 250 people, predominantly in North and South Carolina and Georgia—and that count will likely rise. Because the communities most affected are difficult to reach, simply understanding the storm’s total damage is likely to take weeks, says Janey Camp, a civil engineer at the University of Memphis.

AccuWeather's preliminary estimate for total damage and economic loss, meanwhile, was between $95-$110 billion, which would make Helene one of the costliest storms in U.S. history.

In North Carolina, the rainfall totals Friday afternoon were staggering: 29.58 inches for Busick, N.C.; 24.20 for nearby Mount Mitchell State Park; about 13 inches in Boone, some 55 miles away.        

According to the U.S. Geological Survey, the term “1000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1000 chance of occurring in any given year.

1 in 1000 year flood meaning : (A once-in-1,000-year flood is one that has a 0.1 chance of happening in any given year.) The longest period that the National Oceanic and Atmospheric Administration calculates that out to is 60 days, for which a rainfall event in Asheville is considered to be a once-in-1,000-year occurrence if it produces 19.3 inches.

“Probably, above a 1000 year event in Western North Carolina, but even in parts of East Tennessee, right at 1000 year event for rainfall,” said Dr. Joyner. “We’re getting the rain waters from areas that were probably closer to 2000 year events.”

Inland flooding According to the U.S. Geological Survey, the term “1000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1000 chance of occurring in any given year.

Inland flooding from tropical cyclones, even at high altitudes, is a major worry—and one that scientists don’t know enough about.from tropical cyclones, even at high altitudes, is a major worry—and one that scientists don’t know enough about.

Asheville and many surrounding mountain towns were built in valleys, leaving them especially vulnerable to devastating rain and flooding. Plus, the ground already was saturated before Helene arrived, said Christiaan Patterson, a meteorologist with the National Weather Service

“These are historic flooding levels in an area where the terrain is not conducive to being able to withstand those levels of precipitation,” Camp adds. “Unfortunately, it’s a perfect storm for one of the worst-case situations you could have.”

To be clear, Helene would have been devastating no matter where it hit, given that it dropped a truly enormous amount of rain—more than 18 inches across swathes of western North Carolina, with three-day totals that were well above 20 inches at multiple stations. For context, a three-day-long precipitation event in Asheville, N.C., the largest city in the most-affected region, is considered to be a once-in-1,000-year occurrence if it produces 8.4 inches of rain. (A once-in-1,000-year flood is one that has a 0.1 chance of happening in any given year.) The longest period that the National Oceanic and Atmospheric Administration calculates that out to is 60 days, for which a rainfall event in Asheville is considered to be a once-in-1,000-year occurrence if it produces 19.3 inches.

The only place that can endure that sort of rainfall without serious consequences is the ocean, Camp says.

The rain in the days prior to Helene’s arrival also contributed to the extent of flooding. “There was a tremendous amount of rain before the tropical cyclone got very close to North Carolina,” says James Smith, a hydrologist at Princeton University. And when ground is already saturated, any further rainwater will flow off right away.

The most devastated areas are also predominantly rural and lower-income, Camp notes, increasing their vulnerability. “These are not areas that get a lot of attention and investment for resilience and planning and improved infrastructure ,” she says. It’s likely that some local infrastructure wasn’t designed to be resilient even under once-in-100-year or once-in-500-year circumstances, much less the type of flooding Helene produced. Because of the sheer magnitude of the rains, “those design guidelines and standards kind of got thrown out the window; they wouldn’t have really helped,” Camp says. The scale of the events we’re now seeing suggests that even stronger building codes are necessary to protect people and infrastructure.