Why Appalachia was so badly flooded by Helen’s remains

Why Appalachia was so badly flooded by Helen’s remains

By Meghan Bartels

Hurricane Helene hit the west coast of Florida as a Category 4 hurricane on September 26 and was accompanied by severe storm surge, but the damage didn’t stop there.

Still a Category 2 hurricane when it swept through Georgia, Helen dumped staggering amounts of rain on eastern Tennessee and western North Carolina, far inland and at much higher elevations. higher in the Appalachian Mountains, which people often consider threatened by hurricanes. In total, Helene is known to have killed more than 100 people, mostly in North Carolina, South Carolina and Georgia – and that number is likely to rise. Because the hardest-hit communities are hard to reach, it will likely take months to understand the full extent of the storm’s damage, says Janey Camp, a civil engineer at the University of Memphis.

“These are historic flood levels in an area where the terrain is not conducive to withstanding these levels of precipitation,” Camp adds. “Unfortunately, this is a perfect storm for one of the worst situations anyone could encounter.”

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To be clear, Helene would have been devastating no matter where it hit, given that it dropped a truly enormous amount of rain – over 18 inches across swaths of western Carolina of the North, with three-day totals well above 20 inches on several occasions. stations. As a reminder, a three-day precipitation event in Asheville, North Carolina, the largest city in the hardest-hit region, is considered a once-in-1,000-year event if it produces 8.4 inches of rain . (A once-in-1,000-year flood has a 0.1 chance of occurring in a given year.) The longest period calculated by the National Oceanic and Atmospheric Administration is 60 days, for which an event of Rain in Asheville is considered a once-in-1,000-year occurrence if it produces 19.3 inches.

The only place that can handle this type of rain without serious consequences is the ocean, Camp says.

Rain in the days preceding Helen’s arrival also contributed to the extent of the flooding. “There was a lot of rain before the tropical cyclone came very close to North Carolina,” says James Smith, a hydrologist at Princeton University. And when the soil is already saturated, extra rainwater runs off immediately.

The most devastated areas are also predominantly rural and low-income, Camp notes, which increases their vulnerability. “These are not areas that are getting a lot of attention and investment in resilience, planning and improved infrastructure,” she says. It is likely that some local infrastructure was not designed to be resilient, even in circumstances that occur once in 100 years or once in 500 years, much less the type of flooding caused by Helen. “Those design guidelines and standards have kind of been thrown out the window; they wouldn’t have really helped,” Camp says.

Then there is the terrain. In terms of response, mountains mean there are fewer roads leading to a given city, hampering both evacuation and response efforts, Camp says.

Water will always flow downhill no matter what, but the mountainous terrain limits where it can go. This means water cascading down slopes will accumulate more quickly in low-lying areas, worsening the effects – and will speed up as it moves, potentially making flooding even more dangerous.

Although tropical storm systems don’t often reach inland mountains, they can be particularly severe when they do due to these types of factors. “This is a common way to produce catastrophic flooding,” Smith says. “There have been a number of them from the southern Appalachians to New England.” He points in particular to the year 1916, when Asheville itself experienced horrific flooding after back-to-back tropical storms hit in June and July. Helen was able to reach this area and dump so much rain partly because it was so strong upon landing, extremely large, and moving quickly, meaning it retained more of its energy further afield. inland than storms often do.

Despite the known risk of these storms reaching the Appalachians, scientists don’t know much about their behavior once they reach the mountains. For example, high-altitude terrain often causes storm systems to dump more rain on the windward side of mountains, but scientists don’t know if this phenomenon could play a role in cases like Helen’s Flood in the Appalachians. “How tropical cyclones behave over land has received only a fraction of the attention that tropical cyclones over the ocean have received,” says Smith.

And of course, as climate change manifests itself, it could make this type of situation worse – maybe not directly, but certainly in terms of how often the groundwork is laid. Atmospheric and sea surface temperatures are increasing, leading to more extreme precipitation and a higher proportion of more intense tropical storms. “All of this is bad for inland precipitation,” Smith says. “Typically, you don’t want a major hurricane to make landfall and then move inland.”

In Helene’s case, emergency services are still assessing the damage caused, but what we know so far is ominous. The North Carolina Department of Transportation said all roads in the western part of the state were effectively closed, non-emergency travel was prohibited and evacuations from Asheville were occurring via two eastbound highways. About 1.5 million people remain without power in the Carolinas and Georgia. Such a lack of electricity can in turn destroy communication and water infrastructure, among other consequences.

The effects will also be long-lasting, she says. Recovery from such a disaster can be difficult to measure: when does life really return to normal? But given the scale and challenges involved, Camp believes “it could take decades.”