State geologists were able to more accurately measure the movement and damage from massive landslides that poured across roads in the middle of the island in November because just a few months earlier the state and U.S. Forest Service had collected detailed images and data — literally laser-focused — of the terrain.
The Forest Service and the Alaska Division of Geological and Geophysical Surveys partnered in July to conduct an aerial survey of the entire island, using airborne lasers to map out ground cover, geology and slopes for future reference, said Mort Larsen, who leads the division’s landslide hazards program based in Fairbanks.
That timing was fortuitous as the state flew laser-equipped aircraft over the landslide areas in the days after a deadly flow of mud and trees overran three homes and killed six people near 11.2-Mile Zimovia Highway. A second slide that evening took out the nearby Middle Ridge Road but caused no injuries or private property damage.
The state division issued its preliminary report on the Wrangell slides on Feb. 6. The report does not provide a definitive cause of the slides, but rather explains the size and volume of the slides, the differences between the July images and the post-landslide terrain, and the weather on the day of the slide.
The area was soaked by heavy rains which saturated the thin soils on steep slopes in the days before the slides, with strong winds that evening.
The next step for the division is seeking Federal Emergency Management Agency funding for additional mapping work, Larsen said in an interview on Feb. 9. The goal is to develop detailed maps identifying possible slide areas, based on slopes, terrain and geology, he said.
It’s not possible to predict when a landslide will occur, just as geologists cannot predict earthquakes. But more information could help local officials and residents make their own decisions about where they build and live, such as zoning decisions, Larsen explained.
The July aerial mapping work in Wrangell cost more than $200,000, Larsen said.
“It is a game changer for scientists and geologists,” he said of advances in lidar technology, which is coming with lower costs as its use increases.
Lidar, which stands for light detection and ranging, uses laser pulses, generally sent out by aircraft, to measure distances and characteristics of the earth’s surface as the pulses bounce back to the aircraft. The measurements, “of which there are hundreds of thousands made per second,” the division’s report explains, are collected and assembled to provide information about the ground surface, including trees and structures.
The data can be used to produce two models: A digital surface model represents the tops of trees and structures; and a digital terrain model represents the bare earth, where trees, structures, and other objects have been electronically removed from the image.
The combined use of the two models “reveals changes in vegetation and soils, which provides a complete picture of landslide activity,” the report said.
The borough is working with the state to obtain federal funding for additional mapping, Mason Villarma, interim borough manager, said in a letter to the community on the day the state issued its report.
The borough is applying for a FEMA-Cooperative Technical Partners grant in partnership with the state Division of Geological and Geophysical Surveys, “which will enable us to conduct additional research and analysis to better understand and mitigate landslide risks,” Villarma said in his letter.
The same FEMA program provided funding for mapping work after fatal landslides in Haines (2020) and Sitka (2015).
Alaska Sen. Lisa Murkowski said she is trying to “move more federal money to Alaska for landslide monitoring.” She addressed the issue during video remarks to the Southeast Conference in Juneau, where community, business and government leaders met on Feb. 6.
The state report is available on the borough website, wrangell.com. Just click on the “News” button on the left-hand side of the page.
According to the report, 2.23 inches of rain fell at the Wrangell airport in the 24 hours preceding the Nov. 20 landslides, but it noted that rainfall varies greatly around the island. For example, after the slide, the Alaska Department of Transportation installed a rain gauge in the area of the 11.2-Mile landslide, often measuring up to twice the rainfall as the airport gauge.
And while wind gusts up to 40 mph were reported at sea level at the Wrangell harbor several hours before the landslides, gusts at the 900-foot elevation on Zarembo Island, west of Wrangell, hit 70 mph a couple of hours before the slides, the report said.
The main slide started at almost the 1,500-foot elevation, spilling across approximately 3,750 feet to tidewater, then extending an additional 500 feet into Zimovia Strait.
That slide, which 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 the report. That would equal the loads of more than 6,000 standard-size 10-yard dump trucks.
The Middle Ridge Road slide, which started an hour or two after the main slide, ran about 3,600 feet and more than 100 feet wide, crossing the road in three places and following a stream channel as it moved downhill.
The lidar data indicates the slide moved about 14,000 cubic yards of material.
The state geologists also reported the lidar images showed there were much smaller slides that same day that never reached the highway in the area of 11.5-Mile and 11.8-Mile.
Southeast Alaska, a region packed with steep mountains amid a rainforest, is susceptible to landslides, and scientists say vulnerability has increased as climate change brings more extreme precipitation.
Landslides threaten people and infrastructure elsewhere in the state, too. Among the vulnerable spots are coastal areas of Prince William Sound, where retreating glaciers and the thaw of high-altitude permafrost has destabilized mountain slopes.
Through its landslide hazards program, the Division of Geological and Geophysical Surveys has been working to identify and map high-risk areas.
Yereth Rosen, of the Alaska Beacon, a statewide online news service, contributed reporting for this story.
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