A landslide in a rural part of BC’s central coast two years ago displaced enough water to trigger a tsunami more than 100 meters high, according to a University of Northern BC study
On Nov. 28, 2020, the landslide near West Grenville Glacier in a remote valley resulted in “catastrophic damage” to land and waterways in Homalco First Nation territory, according to a UNBC press release.
New research from UNBC scientists, in collaboration with the Hakai Institute and BC’s Forests Ministry, and published in the journal Geophysical Research Letters, found that a rare cascade of events in the southern Coast Mountains triggered a landslide that triggered the tsunami in a glacial lake and then floods.
According to the report, this has caused extensive damage to the salmon habitat in Elliot Creek and the Southgate River.
Flood water, organic waste and fine sediment entered a fjord where it produced a sediment plume more than 60 kilometers long. That changed water temperature, water chemistry and the amount of suspended solids in the water for weeks, the report said, destroying habitat for forests and salmon.
The scientific team used a variety of observations, including seismic energy, laser mapping, satellite imagery and state-of-the-art computer simulations, according to UNBC.
“Imagine a landslide with a mass equal to all the cars in Canada, hitting a large lake at about 140 kilometers per hour,” said lead author Marten Geertsema, an adjunct professor in UNBC’s ecosystem science program and — management.
Geertsema said the landslide displaced enough water to trigger a tsunami with wave heights of more than 100 meters.
“This drained most of the water from the lake, which then flowed through a (10 km) long channel, causing widespread channel erosion and loss of salmon habitat,” he said. “It took about 30 seconds from when the rock on the slope started to break free for that rock mass to go into the lake and that caused a huge displacement wave. And everything was removed, all the trees and the ground, and so that was pretty spectacular.”
The region was drenched with up to 103 millimeters of rain in the week leading up to the landslide, while snowmelt may also have contributed up to 30 millimeters of water, the report said. Other factors that led to the slide, the researchers say, were fractured bedrock at the base of the slope as a result of the glacier’s retreat.
The report said that this type of danger can occur in high mountains due to rapid deglaciation.
“It’s certainly consistent with what we might expect under a changing climate,” said Brian Menouno, a UNBC geography professor and co-author of the study.
He said these landslides could occur when the glacial ice fills some of these valleys and the supports are removed from these steep, unstable slopes.
“Then those steep unstable slopes are prone to failure. And that’s what we think happened. We’re still not quite sure what the actual trigger was, but we know that removing that ice played an important role.”
Both Menounos and Geertsema said their work could not have been done without the partnership of the Homalco First Nations and the data from the Hakai Institute.
The study notes that while landslides and flooding eruptions are not new to mountainous regions, salmon now have low yields due to a number of factors, including climate change and habitat degradation.
“In a warmer climate, we know that glaciers will continue to retreat. And will continue to do so, largely because of its greenhouse gas emissions. And as they do that, not only are they exposing steep terrain, but they can also start exposing areas of large lakes,” Menounos said.
Landslides have caused tsunamis before. For example, there was one just over five years ago at Mount Colonel Foster on Vancouver Island and it set off a 50-meter wave.