Alaskan Coast at Risk of Catastrophic Landslide and Mega-Tsunami
Prince William Sound, a major fishing and recreational area on the south coast of Alaska, is at high risk of experiencing a landslide and tsunami of catastrophic proportions, according to a multi-institute group of Alaskan geoscientists.
Above a fjord on the western side of the sound, a retreating glacier has put a newly exposed mountain slope in danger of complete failure. The resulting landslide would generate a wave that could devastate the hundreds of fishermen and tourists that frequent the area and the town of Whittier further south. The warning was published in an open letter to the Alaska Department of Natural Resources, Division of Geological and Geophysical Survey this month. The department also issued its own statement.
Barry Glacier, as it’s known, has receded nearly two miles in recent years, due to climate change. The rapid retreat has hastened a process known as “de-buttressing,” in which the mountain slopes once supported by the glacier start to relax and expand, like a Bundt cake if the mold was removed too soon. Sometimes portions of the slope fall off. And in the case of Barry Arm, that would involve millions of tons of rock. The researchers estimated that a failure was possible within a year, and likely within 20 years. A number of natural processes could trigger this disaster—an earthquake, or thawing permafrost, or even a heavy rain or snowmelt.
Anna Liljedahl, a hydrologist with Woods Hole Research Center and one of the authors of the letter, was working on a federally funded project mapping permafrost thaw when she and her colleagues found the unstable slope at Barry Arm. The team was working from their homes because of the coronavirus pandemic, and discovered the slope while analyzing satellite imagery.
The slope has actually been inching down the mountainside for years. Sometimes quickly. Sometimes slowly. Sometimes stopping altogether. This has left an enormous scar called a “scarp” across the face of the mountain. “We didn’t believe what we saw, at first, because it was so big,” Liljedahl told GlacierHub.
Over the last century, at least two landslide-generated tsunamis over 500 feet have occurred in Alaska’s Taan Glacier in 2015, and Lituya Bay in 1958, which launched a 1,700-foot wave up the opposite slope of the valley.
The rock masses released in those slides, however, were a tenth of the size of the Barry Arm slide, which is several hundred times the volume of the Hoover Dam. “If the whole thing was to collapse at once into the water it would be so much energy released that it would create a magnitude 7 earthquake,” said Liljedahl.
And none of the previous tsunamis were in regions frequented by people. As many as 500 commercial and recreational boaters can be found in the area at any given time, according to the Alaska Department of Fish and Game. And many more can be further south in the town of Whittier — several thousand if a large cruise ship arrives in port that day. Tsunami models indicate that the town could be swamped by over 30-foot waves in the event of a complete slope failure in Barry Arm.
“It’s really pretty terrifying,” landslide expert Hig Higman told GlacierHub. Higman is a geoscientist and founder of an organization known as Ground Truth Alaska. He was also part of the working group that wrote the open letter.
If the whole slope goes, Higman explained, the immediate result will be violent. The millions of tons of rock and dirt that slam into the fjord will displace enough water that the resulting initial wave will be hundreds of feet high. The area in Barry Arm fjord will be destroyed. Flying rocks and water will sever trees. Anyone in the vicinity will likely die.
“If you picture throwing a cobble into the water and making a splash, that is kind of the picture you should have in your head for how the landslide hits,” Higman said.
Researchers calculate that about 20 minutes after the landslide, 30-foot waves will crash ashore in Whittier. The sheer scale and speed of the event, compounded by confusion among tourists and a railyard hemming in the waterfront, could make it difficult, if not impossible, for people to evacuate in time. “A recurring nightmare of tsunami evacuation is everyone jumps in a car, there’s immediately a traffic jam, and no one can evacuate,” said Higman.
In order to avoid such a calamity, targeted monitoring of Barry Arm is paramount, the researchers say. Higman told GlacierHub he anticipates that such an effort will happen, but the risk is severe and imminent enough that they decided to publish this letter without the extra review and study they would normally like to have. “Releasing this letter was something we decided to do because so many things seemed to point to this being an urgent hazard,” he said.
The emerging threat at Barry Arm is also illuminating. Glaciers are retreating all over the world and are due to continue with climate change. The more the Earth warms, the likelier landslide-generated tsunamis become. “The climate is warming. The glaciers are retreating. They’re exposing this unstable ground, and there you go,” said Liljedahl.
Higman offered a comparison to another geohazard, volcanoes. Humans have learned to identify them as risks. We know what they look like, know they present danger. “Maybe we’re entering a time now where we need to look at glaciated landscapes with the same kind of glasses,” he said.