The Aknes rockslide: Monitoring, threshold values and early-warning

The Aknes/Tafjord project is a large investigation, monitoring and early-warning project related to some large unstable rockslopes in western Norway. The risk is due to the generation of large tsunamis when rockslides plunge into the fjords. The Aknes rockslide is a large rockslide of possible 30-40 million m(3) moving with a velocity of 3-10 cm/year. The monitoring systems is today based on surface extensometers, single lasers, GPS, total station, geophones, climate station and borehole instrumentation (inclinometers and piezometers). The data is implemented in an integrated web-based system. Major challenges are linked to the steep terrain, remote setting and problems with rockfalls and snow avalanches. Major effort has been put on to get reliable operational power and communications systems. The movement data so far demonstrates a continuous movement during the entire year, but with significant seasonal changes. During snow melt in the spring and heavy precipitation events, the movement rate can increase up to 10 times the yearly mean. This means a movement of up to 1 mm/day. Based on the historical data from the Aknes rockslide and information from historical rockslide events elsewhere, preliminary early-warning levels have been proposed (green, blue, yellow, orange and red levels).