City could face ‘lake tsunami’

Shuswap Lake: Professor suggests major rock slide has the potential to cause wave damage.

Chehalis Lake: A view of a massive landslide at the remote lake

Large destructive waves are a problem that most would think should only concern those who live on the ocean, but an SFU professor says lake communities should be concerned as well, and Salmon Arm could fall victim.

Professor John Clague studies landslide generated tsunamis, which can occur in lakes.

“I got engaged in this when we had a very large event of this sort at a lake called Chehalis Lake,” Clague said.

Two million cubic metres of earth slid down a hillside at Chehalis Lake in 2007 creating a displacement wave that devastated the surrounding shoreline, destroying trees as far as 25 metres from the water’s edge.

“The thing that startled us is the size of the displacement wave,” Clague said.

Chehalis Lake was home to several forestry service campsites, however the campsite was vacant so no one was injured.

“I did mention Shuswap Lake (in the study), not because there’s an obviously threatening slope there, but because there are large rock faces that border the lake. If there ever were to be a landslide there then you’d have a big problem in a place like Salmon Arm,” Clague said.

Calvin van Buskirk, local engineer and geoscientist, agreed there is potential for large landslides in the Shuswap area, but added only a truly massive landslide has the potential to cause a lake tsunami.

“There are lots of large-scale landslide features around Shuswap and Adams Lakes. It’s just a question of whether they fail catastrophically,” he said.

Van Buskirk offered the example of a slide from Bastion Mountain in December 1959 as one that although large, did not cause destructive waves. The British Columbian newspaper reported the 1959 slide was large enough to cover approximately 1,000 feet of what is now Sunnybrae-Canoe Point Road with as much as six feet of rubble, temporarily isolating eight families.

The article in The British Columbian refers to a legend of a large “Tidal Wave” caused by landslides which destroyed a First Nations village on the Shuswap sometime in the 19th century.

Although Clague says there is no indication that Bastion Mountain or any other large rock faces on the Shuswap are unstable, he believes that another tsunami-like event caused by a large rockslide is inevitable in B.C.

“Given the amount of rock faces we have adjacent to populated shorelines it’s only a matter of time before something injurious or damaging happens,” he said.

With the exception of BC Hydro monitoring rock faces near their reservoirs to prevent a potentially catastrophic dam failure, he wasn’t aware of any large scale efforts to prevent large rockslides at lakes, Clague said.

Very slow movements of slopes precede their failure, making damaging landslides detectable and possibly even preventable.

There is a satellite tool for measuring slope movements called Interferometric Synthetic Aperture Radar (INSAR), he said.

INSAR is used extensively in Norway, where over 170 people have died in the last century from tsunamis in fjords, he added.

Van Buskirk isn’t aware of anyone studying or monitoring rock formations near Shuswap Lake

 

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