Josh Roering, a University of Oregon geomorphologist, speaks to the Nolan Center crowd on Jan. 18. Roering is a member of the team of landslide experts who shared some of their findings about the Nov. 20, 2023, landslide.
These landslide sketches by Annette Patton, a landslide expert from University of Oregon, depict how the 11.2-Mile slide capitalized on loose soil deposits on the mountain's bedrock steps to gain both size and speed. During their August site visit, the experts noted seeing surviving blueberry bushes on the edge of one step, proving that the slide cascaded so quickly down the bedrock that the soil quite literally flew over the blueberry bushes, leaving them untouched.
Unique bedrock formations, a thick layer of loose sediment, and a concentrated water drainage system flowing down from the ridgetop were three of the main ingredients that led to the Nov. 20, 2023, landslide that killed six people at 11.2-Mile.
These three causes, all benign on their own, became disastrous when heavy rains down poured on Wrangell Island. Though the airport weather station reported rainfall conducive to a "large but not especially large" storm, members of the public who live close to the landslide location told the team of experts that their personal rainfall data tallied around nine inches in the four days ahead of the slides.
University of Alaska Fairbanks professor of geological engineering Margaret Darrow led the team of experts in their research. Other members of her team, Josh Roering, a University of Oregon geomorphologist, and Annette Patton, a watershed extension specialist from Oregon State University, joined Darrow for the Jan. 18 presentation at the Nolan Center.
The event was well attended, and additional rows of seats were added to accommodate the large crowd eager to hear what the experts had discovered about the slide.
The team of experts' initial takeaway was the sheer size of the landslide.
"This was truly an extreme, anomalous event," Roering said, adding that the 11.2-Mile slide is twice the size of the second-largest recorded landslide on the island.
The main slide ran down the hill more than two-thirds of a mile, then extending an additional 500 feet into Zimovia Strait.
It measured about 150 feet wide at the top, picked up and moved an estimated 63,000 cubic yards of material as it flowed downhill, according to a state report, equal to more than 6,000 standard-size 10-yard dump trucks.
The team wanted to identify what particularly made the slide so large. Though they admitted it is impossible to perfectly weigh the amount of responsibility each different factor held, the experts identified the staircase-like formation of the hillslope as a primary factor.
Wrangell's mountains, especially out the road, are shaped like stairs, alternating between steep vertical pitches and flat plateaus. Because of this, rock, dirt and sentiment often tumble from the steep slopes and rest on the flat part of the step.
So, when the drainage coming down the mountain from the ridgetop seeped into these pockets of loose soil, they became unstable.
"In some ways having channels getting water out is a good thing, but having a lot of channels funnel water to one location is a really bad thing," Roering said.
The team of scientists was able to map the drainage patterns. They shared the map during their presentation, making special note that a lot of the runoff water was draining into what was the initiation zone of the landslide.
Additionally, Patton pointed out that the soil on the slope was not very "sticky," meaning it lacked the clay contents that could potentially fight back against forces that led to the landslide. Once the loose material became heavily saturated, it simply gave way. And since what was below it was more loose material, it just kept on growing.
One potential cause they were able to dismiss as a lead factor in the slide were the trees. Since the slide initiated in the upper elevation old-growth forest, they said that logging at lower elevations likely did not contribute to the 11.2-Mile slide.
They also eliminated a large-scale "blowdown event" as a contributing factor. Using LIDAR (airborne laser images) mapping from before and after the landslide, the team noted that there was no mass treefall caused by the storm on Nov. 20, 2023, thus eliminating this potential cause - though the slide later stripped the hillside of its trees
Though the team of scientists was able to glean loads of information from their site visit last August, they continue to implore the Wrangell community to help them better understand the slides.
"We look at a lot of data, but you folks live here. You see it. You live it," Patton said.
After the presentation, the team reached out and explained two crucial ways that people from Wrangell can better help the scientists. The first thing they suggest is if you are aware of a landslide, no matter how small, you should report it. The best way to do that is to go to the state website: https://dggs.alaska.gov/hazards/landslide-reporter.html.
Additionally, the team is putting a call out for any personal rainfall data from people who live out the road. You can report this by reaching out to the team directly, or by sending your data to the Sentinel, where it will be redirected to the scientists.
Patton also gave some advice for how to best recognize potential landslides. She said people often report seeing brown water coming off mountains in the minutes before a landslide - adding this is something to worry about if the water was running clear and then very quickly turned brown.
"If you see a change to the way the water is moving, that is something to pay attention to. It's not always a definitive yes or no, but it's a definitive warning sign," she said.
Patton also noted that the noise of the slide can be a warning sign, but she admitted that "once you start hearing something, you might only have moments."
The team hopes to release its final academic report about the 11.2-Mile slide later this year. They said a version of the report catered to the public, rather than the scientific academia audience, will be released as well.
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