Landslide likely caused by rain, high winds

Heavy rains, high winds and the unique topography of the area likely caused the Nov. 20 landslide at 11-Mile.

The type of landslide that hit Wrangell is called a "debris flow" - a "notably destructive" event that is common in the region, said state geologist Barrett Salisbury at a Nov. 21 press conference. They occur when soil becomes so saturated with water that individual pieces of soil are no longer touching each other, and lack the strength to hold themselves in place.

Debris flows can reach speeds of 35 miles per hour, he said.

By studying the way water moves through the land and identifying the areas where it accumulates, geologists can guess where and how a landslide started.

Data on what triggered the 11-Mile slide is "super preliminary," said Mitch McDonald, a geologist with the state Department of Transportation. But he thinks it's possible that the slide may have started somewhere in the middle of the mountain, not at the top. "There's a feature mid-slope that looks like it could trap water," he said.

The water saturation process is not a single-day event, added McDonald. Instead, areas of saturated soil build up over time, though a single concentrated period of heavy rainfall can cause a slide.

On Nov. 20, more than an inch of rain fell at 11-Mile within a six-hour period - almost twice as much rain as was measured with the airport's rain gauge, according to Salisbury.

A gauge high on Zarembo Island just west of Wrangell also registered a 70 mph gust of wind, "right before we got the first report from the city of Wrangell about this landslide," said Juneau-based meteorologist Andrew Park.

From top to bottom, the slide is about 1,500 feet long. Where it crosses Zimovia Highway, it's 450 feet wide, but is likely more than twice that at its widest point, according to McDonald. He estimates that there are between 3,500 and 4,000 cubic yards of debris on the highway.

Several bedrock ledges cross the slide's path on the way down, and likely "(played) a huge part in how the landslide is shaped top to bottom," said Salisbury.

An old logging road in the slide path directed some of the flow to the north, causing a few offshoots in that direction. Several secondary slides also occurred high in the hills on the same night, including a 3,000-foot one that blocked access to Middle Ridge cabin.

Thanks to the region's rainy climate and steep slopes, debris flows are an ever-present risk in Southeast communities. "Without specialized instruments in place long before an event like this, it's virtually impossible to prevent this type of catastrophe," said Salisbury.

However, some communities do take preventative measures. After experiencing multiple deadly landslides in 2015, Sitka created the Sitka GeoTask Force to map hillslopes and make models that could be used for prediction.

Since the Sitka slides in 2015, the state has been responding to a higher volume of similar disasters. "It's really only since the Kramer landslide (in Sitka) that we seem to be responding to these larger events that impact communities," said McDonald. "Is it growing in intensity? Is it climate change? It's hard to tell. Sometimes, it's just that we had a data gap in the past."

New imaging and reporting technology, along with the prevalence of cell phones and social media, mean that news of any given disaster spreads farther and faster. McDonald doesn't know for sure whether the frequency of landslides in Southeast is actually increasing.

Regardless, the state is expanding its capacity to respond to these events.

"I think what we as agencies are all learning is that this is an interdisciplinary effort," said McDonald. "I think that's really what's coming together in Alaska right now. They're definitely encouraging us to work with other agencies. ... We're making gains in that and getting better."

"This (Wrangell) has been the best-supported response that I've ever been involved with, based on the technology that we have," he continued. He's also responded to landslides in Sitka and Haines.

 

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