Why Hurricane Helene was so devastating

The powerful gusts pushed the sea toward the coast, generating unprecedented flooding that inundated coastal communities with meters of seawater. Near Keaton Beach, Florida, the storm surge was estimated to reach at least 15 feet (4.5 meters) high.

And that was just the beginning. After landfall, Helene headed north through Georgia, delivering Atlanta a record 28 centimeters (11 in) of rain in 48 hours, surpassing the previous record of 24 centimeters (9.6 in) set in 1886. When Helene moved to the Appalachian mountains, its rains caused widespread flooding and rapid landslides called debris flows, deadly, unstoppable sludges of water, soil and rock that can surge downhill for miles.

Mountain areas of western North Carolina were especially hard hit, with some places like Jeter Mountain and Busick reporting more than 30 inches (76 centimeters) of rain. Washed out roads and downed power lines caused outages that isolated the city of Asheville, home to nearly 100,000 residents.

As of Oct. 1, the death toll from Hurricane Helene had surpassed 130 people in six states, and that number could rise in the coming days as hundreds of people remain missing. Furthermore, the associated economic damages are estimated at around 150 billion dollars.

To find out how Helene was able to leave such a devastating trail of damage in the mountains, Scientific news spoke with four experts. Charles Konrad is a climatologist at the University of North Carolina at Chapel Hill. Coastal oceanographer Rick Luettich and aquatic ecologist Hans Paerl work at UNC, based in Morehead City. And geologist Brad Johnson of Davidson College in North Carolina studies landslides, erosion and landscape evolution in the southeastern United States. Their responses have been edited for clarity and brevity.

SN: Why was this hurricane’s storm surge so destructive?

Luettich: The thing about Helene is that she was very big, and that means she can push an enormous amount of water with her. (Tropical storm-force winds reached more than 300 miles (480 kilometers) from its center.) Our models predicted that virtually all of the barrier islands, from Estero Island south of Fort Myers to around Tampa Bay, would be underwater. As far as we know, that was pretty accurate. The second thing was that as Helene moved over the Gulf (of Mexico), and particularly as she began to make landfall, she was over very warm waters. That helped him quickly develop a very strong core.

Florida’s western shelf is also quite wide and shallow, making it susceptible to storm surges. Deep water is difficult to capture. And of course, Florida’s Big Bend is shaped like a C, and as you push water into that area, water tends to pool on the hook.

SN: Are there any lingering effects or risks to coastal areas from this storm?

Luettich: Our barrier islands, which are typically made up of sand dunes, are a primary defense against flooding. When a storm like Helene comes through and damages or floods them, a subsequent smaller storm can flood areas that would otherwise be protected.

There is no doubt that Helene has made Florida’s west coast more susceptible to flooding from minor events, should they occur over the next month. There is a storm of some kind brewing in the Gulf right now. We’re not quite sure what it will look like. But something is likely to happen there.

Paerl: All that rain that has fallen turns into runoff and carries all kinds of pollutants. You can imagine a flooded gas station and all the contaminants coming out of it. Or a wastewater treatment plant. There are pesticides, herbicides, PFAS and a whole soup of chemicals in those floods.

And then there are also the nutrients that are removed from fertilizers on farmland. When a storm hits, it can wash these nutrients into our estuaries and coastal areas and cause algae blooms. These blooms can sometimes produce toxins that can be harmful to fish, invertebrates, household pets, and humans, and can last from days to months.

S.N.: Why did Helene hit the Appalachian Mountains so hard?

Konrado: In the mountains, there was what meteorologists call a predecessor event, which occurred just before the hurricane hit. I think the Asheville airport received six or seven inches of rain before the Helene rain hit.

You can consider it as an advantage over rain. Major flooding has already occurred. Soils were saturated and streams were already in mild to moderate flooding.

To make matters worse, the winds were blowing from the southeast and east, and that air had to rise over a large, steep relief in the mountains called the Blue Ridge Escarpment. When air rises to higher altitudes, it encounters lower pressure, which causes it to expand, cool, and release moisture as precipitation. When Helene began pushing air over the escarpment, it caused a massive increase in precipitation in that area.

Johnson: It is not surprising that landslides and debris flows occur in these situations.

The established threshold for mudslides in North Carolina is five inches of rain. If you look at every series of landslides that have occurred, it’s basically always an event where at least that amount of rain falls.