4 November 2014
The Mount Mannen rockslide
The Mount Mannen rockslide in Norway is continuing to behave in an unpredictable manner, no doubt to the intense frustration of all involved. The tv2 website (in Norwegian, though Google Translate does a good job), which has the live webcam, has produced a series of graphs showing the movement of the landslide. This one, complete up to yesterday, shows the cumulative displacement of the landslide over the last month:
The lines show different parts of the monitored slope of the Mount Mannen rockslide. Points 1 and 2 in the upper part of the active slide, points 3 and 4 are in the lower section of the active block. So, it appears that the upper part of the landslide is still moving quickly (about 15 mm per day) and has now displaced about 24 cm in a month, whilst the lower part has moved about 10 cm. The difference between the two is not unusual or surprising for this type of landslide. The behaviour on about 30th October is interesting as the landslide appears to have slowed dramatically in the upper portion and actually stopped briefly lower down, before starting to move again. We see similar behaviour in lab tests that simulate creep movements, although we cannot fully explain these episodes. Whilst the movement record is quite noisy (which is unsurprising for an area that receives regular snowfall), the medium term trend is still an accelerated creep movement pattern, and my view would be that the slightly longer-term rate of creep is still increasing, with variations caused by changes in the environmental forcing (i.e. temperature and rainfall).
In other words the authorities are correct to maintain the evacuation. I remain skeptical that much can be done to speed up this natural process. Although it is frustrating, nature probably needs to be allowed to take its course.