Harrison Lake: newly discovered, large, ancient landslides in Canada

There is increasing interest in the hazards associated with large landslides into enclosed bodies of water, such as fjords and lakes.  Examples include the famous Lituya Bay landslide and the ongoing concerns about the potential for a major failure at Barry Arm, both in Alaska.  Clearly large landslides are a hazard in themselves, but in general the main worry is the potential for a destructive landslide-induced tsunami that can travel large distances in an enclosed water body.

An interesting open access paper has just been published in the journal Landslides (Hughes et al. 2020), which examines newly discovered deposits of three large landslides in the bed of  Harrison Lake, in southwest British Columbia.  The landslides were discovered during a bathymetric survey of the lake floor, and have then been investigated in more detail through seismic imaging.  This is the location, as shown on Google Earth:-

The location of Harrison Lake, as shown on Google Earth

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The largest failure, termed the Mount Douglas landslide, which was a subaerial rock avalanche, has a volume of 2.4 million cubic metres.  Of the other two, the Mount Breakenridge landslide, which was a rockslide, has a volume of 1.3 million cubic metres and the nicely named Silver Mountain landslide has a volume of 200,000 cubic metres. The landslide deposits are blocky and flow-like, suggesting that they were emplaced rapidly.  Given the volumes, the two largest landslides into Harrison Lake, which both had a runout distance of over a kilometre, are likely to have been tsunamigenic.

As the Google Earth image above shows, the banks of Harrison Lake are populated, meaning that any future event could have serious consequences.  Interestingly, Hughes et al. (2020) found that there is significant bulging at the base of Mount Breakenridge, “indicating that the slope continues to slowly deform and could be the site of a future landslide.”  This clearly needs further investigation.

Understanding past landslides into lakes is a key to understanding future hazard.  This study provides a really useful insight into the nature of previous rockslope failures into Lake Harrison.  It would now be really interesting to see a model of the resultant tsunamis and also to date the landslide events.

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Quickslide 1:The duel hydroelectric landslides in Vietnam

Rescuers have now recovered the 13 bodies of the soldiers killed by the second of the two landslides associated with the Rao Trang 3 hydroelectric scheme in Vietnam.  That these people had been sent to rescue the vicims of the earlier landslide feels particularly poignant.

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Quickslide 2: A nice landslide video from Costa Rica

With thanks to Tom Hodgson, there is a nice video of a large roadside landslide in Costa Rica on the CRHoy website.  The landslide, which was triggered by heavy rainfall, occurred on the South Interamerican highway.

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Reference

Hughes, K.E., Geertsema, M., Kwoll, E. et al. 2020. Previously undiscovered landslide deposits in Harrison Lake, British Columbia, Canada. Landslides . https://doi.org/10.1007/s10346-020-01514-3.