A landslide, a tsunami and then a flood: the enormous cascade of dangers that shook the world

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New research reveals the intensity of the cascade of dangers in British Columbia in 2020 as members of the Homalco First Nation continue to pick up the pieces.

By Lauren A. Koenig, Ph.D.science writer (@Lauren_A_Koenig)

At the end of November 2020, a geological mystery appeared on seismographs around the world. A signal comparable to a magnitude 5.0 earthquake emanated from deep within the southern Coast Mountains of British Columbia (BC), Canada.

The cause of this ground-shaking event remained unknown for two weeks, until logging workers crossing the traditional territory of the Homalco First Nation suffered the consequences in the Elliot Creek watershed. The glacier-carved valley, tightly framed by mile-high rock faces, has been decimated by a huge cascade of hazards – a chain reaction of geological events – involving a landslide, tsunami, flood and a plume of sediment. What was once a verdant environment for the area’s famous salmon is now an alley of cinders that unfolds into a sea of ​​debris.

The magnitude of the waterfall can be difficult to understand even when viewing the valley from a helicopter, said Marten Geertsema, a geomorphological researcher with the British Columbia Ministry of Forests and lead author of a new study describing the events.

“It’s amazing when you stay there,” Geertsema said. “It’s kind of hard to understand how powerful it all was.”

The Homalco First Nation and researchers from the Hakai Institute, based in British Columbia, are assessing the long-term ecological impacts on the region, particularly for fisheries. Ongoing unstable conditions in the valley suggest that recovery could take decades. Additionally, Elliot Creek has changed course erratically several times over the past year, which can make restoration plans pointless overnight.

“If we get a massive rain event like last year, the whole river could change again and that’s not money well spent,” said Erik Blaney, an environmental technician for the Tla’amin Nation. who was hired by the Homalco Nation to lead the assessment and remediation efforts. “You are playing with mother nature.

A cascade of unfortunate events

The cascade of dangers began with the fifth-largest landslide on record in British Columbia, involving, according to study co-author Göran Ekström, the equivalent of the combined mass of Canada’s 25 million cars . Ekström is a seismologist at Columbia University. Nearly half of the debris crashed into the toe of the West Grenville Glacier near the base of the valley. The rest made their way up the opposite wall of the valley before gravity pulled them back down. Moving at more than 100 miles per hour (170 kilometers per hour), the landslide plunged into an alpine lake left behind by the glacier as it retreated over the past century.

Like the splash after a jump from a high dive, the impact of the landslide was quick and violent: the falling rocks catapulted enough water out of the lake to reduce its surface area by almost 20%, creating islands in its new shallow depths. In just over a minute, a tsunami wave more than 100 meters high crossed the lake before peaking on the opposite shore, creating what is known as a glacial flood.

The view down the valley showing the eroded creek bed and lack of vegetation. Credit: Briar Stewart/CBC.

The water was then channeled forcefully along the confines of the valley like a marble in a Rube Goldberg machine. Although it usually takes millennia for water to steadily erode deep ravines, the flood carved a 160-foot (50-meter) deep groove in the stream bed within minutes.

As the creek bank gave way and trees were felled, the flood turned into a thick soup of debris that left a huge fan of sand, mud and wood stretching from the mouth of the valley. It contaminated local fresh and marine waterways, creating a plume of sediment – suspended organic matter – that destroyed water quality.

“You need certain elements in place to create these massive domino effects,” Geerstema said. “It shows us the harmful footprint of these events when you have water in the right place.”

Watch with LiDAR

The remote location of the landslide meant that luckily no one was around when the cascade of hazards occurred. To map what happened, Geertsema, who routinely scours satellite imagery for evidence of landslides in high mountain areas, worked with members of Canada’s First Nations, the Hakai Institute and other institutions around the world to simulate the events using numerical modeling and LiDAR. a survey method that emits laser pulses from an aircraft to create 3D representations of the surface.

Geertsema, who compared the post-landslide images with those taken a year earlier, said the team was very lucky to have such detailed images. “We wouldn’t have been able to produce these models without this input data,” he said.

Gray beach surrounded by dark gray mountains with snow.
The view from the lake towards the West Grenville Glacier and the steep vertical face of the slide. Credit: Brian Menounos

Less glaciers, more dangers

British Columbia’s mountainous terrain is no stranger to landslides, floods and tsunamis. Climate change, however, has exacerbated the impacts and frequency of these risks, especially as warming temperatures lead to the melting of permafrost and the glaciers that stabilize the ground.

As glaciers recede, the fragile bedrock loses the support that prevents it from collapsing, said Tom Millard, a geomorphology researcher with the British Columbia Ministry of Forests and co-author of the study. Meltwater lakes left in their wake, such as at Elliot Creek, also tend to enlarge, increasing the risk of a potential tsunami or flood.

Living with the consequences

The chain reaction of geological events has created a cascade of ecological effects that will persist for decades. The flooding destroyed most of the salmon population, as well as the spawning habitat they return to each year. Fish are unable to survive current turbidity levels, which remain more than 25 times higher than normal (especially after a rainstorm), Blaney said.

More than a food, salmon is an important part of the culture and livelihood of the Homalco First Nation. The annual grizzly salmon feast attracts tourism which helps the community thrive. But last year low salmon numbers meant the bears were hungry.

As a coordinator of recovery efforts, Blaney has ideas for sustainable ways to help the ecosystem return to some semblance of normality. One solution is to prune crabapple trees as another food source for bears.

“It’s something our people did before,” Blaney said.

Blaney also plans to install a pad that would provide researchers with a safer way to monitor the salmon population, divert the stream to a more stable area with remnant trees, and plant native vegetation to control erosion. .

However, finding funding for these projects is only one obstacle among others that is part of an even greater challenge: living with the increasingly marked effects of climate change. Severe wildfires in the summer of 2021 scorched across British Columbia, and the Coast Mountains are seeing some of the highest rates of glacier loss in the world, which means hazard waterfalls like Elliot’s Creek could become more frequent.

“I don’t think the average person living in a city can really understand or see the changes we’re seeing and the devastation they’re having on salmon and other important parts of our survival and our culture,” said Blaney. “We’re seeing change, and it’s happening fast and it’s beyond anything we could have imagined.”

Further reading

For the Hakai Institute’s full multimedia functionality – which includes videos, interactive maps, and more. – Click here.

Geertsema, M., Menounos, B., Bullard, G., Carrivick, JL, Clague, JJ, Dai, C., … & Sharp, MA (2022). The November 28, 2020 landslide, tsunami and flood – a cascade of hazards associated with rapid deglaciation in Elliot Creek, British Columbia, Canada. Geophysical Research Letters, 49(6), e2021GL096716.

Menounos, B., Hugonnet, R., Shean, D., Gardner, A., Howat, I., Berthier, E., … & Dehecq, A. (2019). Heterogeneous changes in western North American glaciers related to decadal variability in zonal wind strength. Geophysical Research Letters, 46(1), 200-209.


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