Analysis of an artificially triggered avalanche at the nepheline syenite mine on Stjernøya, Alta, Northern Norway

Since 1961, a Nepheline Syenite mine is operated on the island of Stjernøya in the Altafjord, Northern Norway. The facilities are located in Lillebukt, on the southern side of the island. Above the facilities, the Nabbaren mountain is rising to a height of 727 m a.s.l. Rockfall during summer season and snow avalanches during wintertime pose potential hazards from its slopes. Due to this setting, the mining company has long experience with both physical and non-physical hazard mitigation measures. Apart from physical installations against rockfall and snow avalanches, artificial triggering of the Nabbaren avalanche forms part of this mitigation strategy.The winter of 2013/2014 was characterized by an unusual snow scarcity between December 2013 and March 20, 2014. After this date, large amounts of snow fell during a short period. Due to this new snow loading, together with intensive snowdrift, the mining company decided to artificially trigger the Nabbaren avalanche on April 8, 2014. A D4 slab avalanche was released, subsequently evolving into a mixed dry avalanche of impressive scale. In contrast to avalanches triggered in other years, this avalanche overtopped the avalanche deflecting wall at its one end causing slight damages to some of the factory installations. In order to document the avalanche, an on-site study was carried out shortly after the event. In addition, a WorldView-1 panchromatic satellite image was obtained to map the non-accessible parts of the avalanche. Here, we present findings from the field visit, from image analyses and first modellings of the avalanche run-out.