The Old Fort Landslide: 160 m of movement so far

The reactivation of the Old Fort landslide in Canada continues to cause substantial disruption to the population of the Old Fort subdivision.  Reports yesterday indicate that the landslide had moved about 160 metres in the most recent movement event, at rates of about 2 metres per hour.  There is an excellent archive of images in a post on the Alaska Highway News website.

The scale of the movement can be seen in these two Planet Labs images of the site, taken five days apart:-

Satellite images of the Old Fort Landslide, showing the movement over five days. The image on the left was collected on 19 June 2020 whilst the one on the right was collected on 24 June 2020. Copyright Planet Labs, used with permission

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Note in particular the displacement of the road, which can just be made out in the second image.  The toe of the landslide has also displaced, and there are signs of significant amounts of deformation towards the crown of the landslide.

In the gallery of images the most interesting one may be this image of the crown area of the landslide:-

Image of the crown area of the Old Fort landslide. Image from BC Transport and Infrastructure.

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There is much to note in this fascinating image.  The main slide extends into the distance – the now destroyed road can just be seen.  In the foreground the rotational component of the system is clear. On the lower left of the image there is a rear scarp, several metres high.  Below this (in the centre right of the image) there is a large block that has displaced and rotated.  This block is starting to break up – note the large tension crack running across it, and the rent on the right hand side of the block.  Finally, note the very clear lateral shear just to the left of centre of the image, beside the hillock.

Given the weather, I would expect movement of this slide to reduce in the coming days.  However, the system is primed to reactivate in future heavy rainfall events, with similar impacts.  Stabilisation of such a large, complex landslide is unlikely to be economic.

This landslide reminds me of the Tessina landslide in northern Italy.  Back in the mists of time I wrote a paper (Petley et al. 2005) that examined the ways in which the style of movement of these landslides change as the blocks that feed the system transition from rotational movement to flow.

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On reflection 1: The building Asian monsoon

The summer monsoon is rapidly developing across Asia.  In Nepal, warnings have been issued for heavy rainfall today amid reports of ongoing landslide induced disruption.  In China, heavy rainfall warnings have been issued across a wide area.

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On reflection 2: Heavy rainfall causes debris flows in Ukraine

Meanwhile, very heavy rainfall in Ukraine has induced debris flows and flooding, impacting in particular on the ski resort at Bukovel.

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Reference and acknowledgement

Petley, D.N., Mantovani, F., Bulmer, M.H.K.., and Zannoni, F. 2005. The interpretation of landslide monitoring data for movement forecasting.  Geomorphology, 66 (1-4), 133-147.

Planet Team (2020). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/

Thanks to Susan DeSandoli for highlighting fascinating reports on the Old Fort landslide.