17 March 2016

The Val Strem rockslide – a dramatic long run out landslide in Switzerland

Posted by dr-dave

Val Strem rockslide

The Val Strem rockslide occurred on Monday night in Canton Graunbunden in Swtizerland.  This large rockslide appears to have had a dramatically long run out given its volume.  There are a number of good reports about the landslide (in German) on the web, with some very high quality images.  This one, via RTR, provides a nice overview of the landslide:

The Val Strem rockslide

The Val Strem rockslide via RTR

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There are a number of interesting features of this rockslide.  The source of the collapse, which had a volume of about 200,000 cubic metres, was high on the valley wall on the right.  The debris has slid down, super-elevated on the opposite valley wall, turned 90 degrees and then travelled down the main valley.  Note the dust mantling the opposite valley wall.  The landslide appears to have been very mobile (the estimated runout is about 1 km) – this is not really captured in the image above, but this one, also from RTR, shows it much more clearly:

The Val Strem rockslide

The Val Strem rockslide via RTR – detail of the runout

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Reports suggest that the source of the rockslide, the Cuolm is da Vinci, was known to be unstable and had been monitored.  It will be interesting to see what the data shows.  The long runout might be explained by the fall onto a layer of snow, which might have provided a low friction surface.  However, the snow depth does not look large to me, so I have some doubts about this.  It could have been the effects of a frozen ground surface, but I suspect that a more important factor might be the constant valley gradient, which may have been sufficient to keep the debris moving.  Interestingly, the landslide does appear to have been losing mass (depositing material) the whole way along its track.  The image below, again from RTR, shows the upper part of the track in more detail – note the coarse material in the centre of the track, with finer grained material on the margins:

The Val Strem rockslide

The Val Strem rockslide – detail of the track via RTR

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Note the dust cloud from ongoing slides in the background.  The Val Strem rockslide did not cause any loss of life or damage to property, but it has damaged the water supply the local communities and the inlets for a small hydro-electric plant.