Wolhusen rockfall

On Monday a large rockfall occurred on a cliff in the town of Wolhusen in Switzerland.  The rockfall itself was large – estimates suggest around 500 cubic metres, with a reported maximum travel distance of 200 metres.  Significantly though, the rockfall blocked and diverted the Kleine Emme river.  Water flowed through a part of the town, causing extensive damage.

The best set of images that I have found is in this report in the Luzerner Zeitung (the report is in German of course).  This aerial image gives the setting rather nicely, showing both the deposit and the diverted river flowing through a part of Wolhusen:

Drone footage of the Wolhusen rockfall, via Luzerner Zeitung

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Whilst this image gives a good view of the rockfall itself:

Wolhusen rockfall via Luzerner Zeitung

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It does appear that the rockfall itself was unusually energetic.  The Luzerner Zeitung report also includes this image of damage to one of the adjacent building, appearing to show impact marks from debris extending to a high level.  It is fortunate that no-one was in the way of one of these pieces of rock.  This can be explained by the timing of the rockfall, which was at night:

Rockfall impact marks on an adjacent building, via Luzerner Zeitung

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The report suggests that the damage will cost over a million Swiss Francs (which is about US$1 million).  Note from the first image that the landslide occurred on the outside of a gentle curve of the river, which would suggest that the key mechanism here might have been undercutting of the rock face.  In the second image the unweathered surface shows the origin of the landslide – it appears that there is at least some recent fracturing here to allow the final detachment to occur. I am unclear as to whether there was a distinct triggering event.