The highly effective gusts pushed the ocean onto the shore, producing record-breaking floods that inundated coastal communities in meters of seawater. Close to Keaton Seashore, Fla., the storm surge was estimated to have reached at the very least 4.5 meters (15 toes) excessive.

Preliminary post-landfall modeling of storm surge from Hurricane #Helene signifies areas throughout the Large Bend area of Florida close to Keaton Seashore, Steinhatchee, and Horseshoe Seashore had water ranges attain greater than 15 ft above floor stage.

— NHC Storm Surge (@NHC_Surge) September 27, 2024

And that was only the start. After making landfall, Helene barreled north by Georgia, delivering to Atlanta a record-breaking 28 centimeters (11 inches) of rain in 48 hours, besting the earlier document of 24 centimeters (9.6 inches) set in 1886. As Helene moved into the Appalachian Mountains, its rainfall triggered widespread flooding and fast-moving landslides known as particles flows, lethal and unstoppable slurries of water, soil and rock that may surge downhill for kilometers.

The mountainous western components of North Carolina had been hit particularly laborious, with some places like Jeter Mountain and Busick reporting greater than 76 centimeters (30 inches) of rainfall. Washed-out roads and downed energy traces brought about outages that remoted town of Asheville, dwelling to just about 100,000 residents.

As of October 1, the demise toll from Hurricane Helene has surpassed 130 individuals throughout six states, and that determine might rise over the approaching days as a whole bunch are nonetheless reported lacking. What’s extra, the related financial damages are estimated to be someplace round $150 billion.

To learn how Helene was in a position to depart behind such a devastating path of harm far into the mountains, Science Information spoke with 4 consultants. Charles Konrad is a climatologist on the College of North Carolina at Chapel Hill. Coastal oceanographer Rick Luettich and aquatic ecologist Hans Paerl are each with UNC, based mostly in Morehead Metropolis. And geologist Brad Johnson of Davidson Faculty in North Carolina research landslides, erosion and the evolution of the Southeastern United States’ panorama. Their solutions have been edited for readability and brevity.

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

Luettich: The factor about Helene was it was actually massive, and meaning it will possibly push an terrible lot of water together with it. [Tropical storm force winds reached over 480 kilometers (300 miles) from its center.] Our fashions predicted that just about the entire barrier islands, from Estero Island south of Fort Myers all the best way up round Tampa Bay, would go underwater. To one of the best of our present understanding, that was pretty correct. The second factor was that as Helene moved over the Gulf [of Mexico], and notably because it began to make landfall, it was over very heat water. That helped it quickly develop a really sturdy core.

The west Florida shelf can be pretty vast and shallow, and that makes it amenable to storm surge. Deep water is difficult to select up. And naturally, the Large Bend of Florida is C-shaped, and as you push water up into that space, water tends to build up within the hook.

SN: Are there any lingering results or dangers in coastal areas from this storm?

Luettich: Our barrier islands, that are usually made from sand dunes, are a major protection towards flooding. When a storm like Helene comes alongside and damages or overwashes them, then a later, lesser storm occasion can flood areas that might in any other case be protected.

There’s no query that Helene has made the west Florida coast extra vulnerable to flooding from lesser occasions, ought to they happen over the following month. There’s a storm of some kind brewing within the Gulf proper now. We’re not very sure of what it’s going to seem like. However one thing is more likely to happen there.

Paerl: All that rain that has fallen, it turns into runoff and it carries all types of contaminants. You’ll be able to simply think about a gasoline station being flooded and all of the contaminants popping out of that. Or a wastewater therapy plant. There are pesticides, herbicides, PFAS, a complete soup of chemical substances in these floodwaters.

After which there’s additionally the vitamins that get washed out of fertilizers on farmland. When a storm comes it will possibly wash these vitamins into our estuarine and coastal areas and might result in algal blooms. These blooms can generally produce toxins that may be dangerous to fish, invertebrates, home pets and people, they usually can final wherever from days to months.

SN: Why did Helene hit the Appalachian Mountains so laborious?

Konrad: Within the mountains, there was what meteorologists name a predecessor occasion, which occurred proper earlier than the hurricane moved in. I believe the Asheville airport acquired six or seven inches of rain earlier than Helene’s rainfall even acquired there.

You’ll be able to consider it as a head begin on the rainfall. There was already vital flooding. Soils had been saturated and streams had been already in minor to reasonable flood stage.

To make issues worse, the winds had been blowing out of the southeast and east, and that air should rise over a big and steep landform within the mountains known as the Blue Ridge escarpment. When air rises into increased elevations it encounters decrease strain, inflicting it to develop, cool and launch moisture as within the type of precipitation. As Helene started pushing air over the escarpment, it brought about large enhancement of rainfall in that space.

Johnson: It’s not stunning to get landslides and particles flows in these conditions.

The established threshold for landslides in North Carolina is 5 inches of rain. If you happen to take a look at each set of landslides which have occurred, it’s mainly all the time in an occasion the place you get at the very least that a lot rain.

When the storm began hitting, each rain gauge I had entry to within the mountains was over eight inches of rain, some had been at 10 inches, and the hurricane was nonetheless 100 miles out within the Gulf. I simply thought, I can’t see a manner out of this that doesn’t have dozens to a whole bunch of landslides.

SN: Are there any lingering hazards within the mountains from this hurricane?

Johnson: The height danger of flooding, landslides and particles flows is throughout the precipitation occasion. In my expertise, as soon as that precipitation occasion has ended, you’re pretty properly within the clear. However there’s different hazards transferring round, with individuals strolling out within the rain with energy traces down, and inevitably there’s flooding within the valley bottoms.

Konrad: Hopefully it’s going to dry out, however the soils are actually moist. I’m positive there’s plenty of locations the place the rainfall has set the stage for landslides and particles flows, in order that it wouldn’t take as a lot rainfall to set off now. Rock slides, too.

Lots of people in these communities aren’t going to have the ability to entry medicines or well being care due to the street harm, and so I believe there’s going to be a variety of what we name oblique deaths. It’s a public well being catastrophe that’s nonetheless unfolding.