31 October 2014

The Mannen rockslide: still standing

Posted by Dave Petley

The Mannen rockslide – an update

The Mannen rockslide in Norway continues to confound predictions of its imminent demise, and as of this morning it remains intact.  A press conference was held yesterday morning in which it was reported that the rate of movement had declined from over 4 cm per day to about 1.5 cm / day.  Some rockfall activity continues to occur, but the major collapse is still some indeterminate time away.  The live web cams continue to broadcast from the Mannen rockslide site, although the public comments express an increasing level of frustration amongst those taking an interest.

.The scientific effort for the Mannen rockslide is being led by Lars Harald Blikra of NGU.  He is vastly experienced in rockslope monitoring – there is no-one better – and his team are using appropriate techniques to monitor the slide.  However, the processes of detachment of large rock blocks are extremely complex and inadequately understood, so forecasting (and predicting) a collapse event is very challenging.   The recent snowfall will not have helped the efforts to monitor the landslide, given that radar is a key technique being deployed.  This graph, from Nyheter, shows the movement of the landslide over a 24 hour period earlier this week:

Mannen Rockslide



I suspect that the emergency management people are now in a very challenging situation.  The continued movement of the slope will be edging it closer to failure, but it is not clear when this might occur.  They will be anxious to reopen the railway line and to lift the evacuation order, but very cautious about the ongoing risks from the landslide.  I am sure that they are hoping that it will just fail.  I suspect that they will come under increasing pressure to try to fail the slope artificially – water bombers are being mentioned – but it will be very difficult to get enough water in the right place for this to be effective.  Explosives could also be used, but emplacing the charges would carry a very high level of risk.  Thus, this may continue to be a waiting game for the time being.  I have no doubt that Lars and his colleagues are under great stress.

The currently active block is part of a much larger landslide at Mannen.  This image from Nyheter shows the multiple fractures at the top of the slope:

Mannen rockslide



This whole slope is moving, albeit at a much lower rate than the active block.  Whilst there is no suggestion that the main slide will fail, the active deformation will have probably created a much more fragmented rock mass below the active slide.  The failure of the active block will almost certainly cause more detachment of material downslope. That the downslope material is likely to be fragmented, and thus more easily entrained, explains the estimates that the final mobile volume may be in the order of 2 million cubic metres.