6 September 2019
Veslemannen in Norway – major movement, and possibly a significant failure, reported
Over the last few days, the Velsemannen rockslide in Norway has undergone another major movement event. This large rock mass has undergone repeated movement events in recent years, responding in particular to phases of wet weather when the ground is not frozen. There has been a substantial impact on the local community – the 11 residents in the valley below the Velsemannen rockslide have been evacuated on 16 separate occasions over the last five years as movement crises have developed.
This time though the events are clearly different. Yesterday (Thursday 5th September) the landslide showed movement rates far higher than had been recorded previously, with the lower part of the rock mass reportedly moving 70 cm. In comparison, the mass moved a total of 58 cm in the whole of 2018. Rockfalls were repeatedly observed on the unstable mass, a sign that it was undergoing internal deformation and fracturing:-
Overnight Thursday and into early Friday a large failure appears to have occurred. News reports suggest that this might be in the order of 100,000 cubic metres, although more analysis is clearly needed to validate these numbers, and that the initial visual inspection indicates that a major part of the unstable mass has now failed and collapsed. Pleasingly, the properties at the foot of the slope do not seem to have been impacted so far.
Forecasting the behaviour of Velsemannen has been extremely challenging as the rockslide has gone through repeated movement crises, without accelerating to final failure. A collapse event was inevitable, such that evacuations were essential, but the behaviour has been extremely unpredictable. The best case scenario appeared to be a collapse event in a series of smaller failures, preventing the long runout that would have destroyed the properties. So far, his appears to have been the outcome.
Work will now be needed to monitor the remaining unstable rock mass, and to determine its susceptibility to a collapse. We have seen elsewhere that an initial event can sometimes trigger a further large failure. But for the people living below Velsemannen, and for the geologists, this must be a major step towards resolving the problems.