Why was Appalachia so severely flooded by Helen’s debris?

Why was Appalachia so severely flooded by Helen’s debris?

Megan Bartels

Hurricane Helen hit the west coast of Florida as a Category 4 hurricane on September 26, bringing with it severe storm surge, but the damage didn’t end there.

Helen was still a Category 2 hurricane when it hit Georgia, but it affected eastern Tennessee and western North Carolina, far inland, and far beyond what people generally think are at risk from hurricanes. It dumped an incredible amount of rain on the high Appalachian Mountains. All told, Helen is known to have killed more than 100 people, primarily in North Carolina, South Carolina, and Georgia, and that number could be even higher. Janie Camp, a civil engineer at the University of Memphis, said it will likely take months to figure out the total damage caused by the storm because of the difficulty in contacting the hardest-hit areas.

“This is a historic flood level in areas where the terrain cannot withstand that level of precipitation,” Camp added. “Unfortunately, this is a perfect storm for one of the worst possible situations.”

About supporting science journalism

If you enjoyed this article, please consider supporting our award-winning journalism. Currently subscribing. By subscribing, you help ensure future generations of influential stories about the discoveries and ideas that shape the world today.

To be clear, Helen really dumped an enormous amount of rain, considering that all of western North Carolina had over 18 inches of rain, and the three-day total was well over 20 inches in multiple locations. , it would have caused catastrophic damage wherever it fell. station. For context, a three-day precipitation event in Asheville, North Carolina, the largest city in the most affected region, would be considered a once-in-1,000-year event if 8.4 inches of rain fell. Masu. (A 1,000-year flood is one that has a 0.1 chance of occurring in a given year.) The National Oceanic and Atmospheric Administration calculates a maximum duration of 60 days, and a rainfall event in Asheville occurs in If 19.3 inches were produced, it would be considered a once-in-1,000-year occurrence.

The only place that can withstand this much rainfall without serious consequences is the ocean, Camp said.

The rain that fell a few days before Helen’s arrival also had a significant effect on the scale of the flood. “We had a tremendous amount of rain before the tropical storm got very close to North Carolina,” says Princeton University hydrologist James Smith. And if the ground is already saturated, further rainwater will quickly run off.

Camp said the hardest-hit areas are primarily rural and low-income, increasing their vulnerability. “These are not areas that get a lot of attention and investment for resilience, planning and infrastructure improvements,” she says. Perhaps some local infrastructure was not designed to be resilient in a 100-year or 500-year situation, much less the type of flooding that Helen caused. “Those design guidelines and standards were kind of thrown out the window. They really couldn’t have helped,” Camp says.

Then there’s the terrain. In terms of response, Camp said the mountains mean there are fewer roads to get to any town, hampering both evacuation and response efforts.

Water always flows downward no matter what, but mountainous terrain limits where water can flow. This means that water flowing down slopes collects faster in lower elevation areas, exacerbating the effects. Additionally, water can pick up speed as it moves, further increasing the risk of flooding.

Tropical storm systems don’t often reach inland mountains, but when they do, these types of factors can make them particularly vicious. “This is a common way to cause catastrophic flooding,” Smith said. “There’s a lot of damage happening from the southern Appalachians all the way to New England.” In particular, he said, Asheville itself experienced horrific flooding after back-to-back tropical storms in June and July. He points out that in 1916. The reason Helen was able to reach this area and dump so much rain is because it was so powerful, so large, and moved so quickly when it made landfall, which means it could dump more rain than storms typically do. This meant keeping the energy inland.

Despite the known danger of these storms reaching Appalachia, scientists don’t know much about how the storms behave once they reach the mountains. For example, in high-elevation terrain, storm systems often dump more rain on the windward side of mountains, but scientists are unsure whether that phenomenon would have an impact in cases like Helen’s Appalachian Flood. I don’t know. “How tropical cyclones behave on land has received only a fraction of the attention compared to how they behave in the open ocean,” Smith said.

And of course, as climate change advances, this kind of situation is likely to get even worse. Perhaps not directly, but certainly worse in terms of how often the foundations are laid. Atmospheric and sea surface temperatures are rising, leading to more extreme rainfall and an increased incidence of more intense tropical cyclones. “All of these things are bad for inland rainfall,” Smith said. “Generally speaking, you don’t want a major hurricane to make landfall and move inland.”

In Helen’s case, emergency responders are still assessing the damage, but what we know so far bodes ill. The North Carolina Department of Transportation announced that virtually all roads in the western part of the state are closed, non-emergency travel is prohibited, and evacuees from Asheville are concentrated on two eastbound highways. . About 1.5 million people remain without power in the Carolinas and Georgia. Such power shortages can disrupt communications and water infrastructure, among other things.

The effects will be long-lasting, she says. Measuring recovery from such disasters can be difficult. When will life truly return to normal? But given the scale and challenges going on here, “it could be decades,” Camp says.