Supporting structures stopped most of an avalanche
Avalanche above the health care centre of a town in north Iceland in 2012
A snow avalanche was released from the Tvísteinahlíð mountain by the health care centre at Engihlíð in Ólafsvík on 19 March 2012. The fracture line of the avalanche was near the top of the slope and extended from the gully to the west of the hillside and into supporting structures that have been installed as protection measures to reduce avalanche danger in the settlement near the mountainside (see fig. 1).
An avalanche slab was released inbetween the rows of supporting structures. The snow was mostly stopped by the structures and piled up against them (see fig. 2) but part of the snow jumped over the rows. A slab was also released below the lowermost row. The avalanche stopped near the foot of the slope to the west of the structures but before it reached the bottom of the slope below the structures (see the map below). The supporting structures clearly prevented the release of a much larger avalanche.
Map of Tvísteinahlíð showing the outline of the avalanche that was released on 19 March 2012 (thick, red curve). The narrow curves show outlines of historical avalances. The health care centre is the large H-shaped building close to the hillside. The rows of supporting structures are shown as black dashed lines.
It may be estimated that 5,000-10,000 m³ of snow were stopped in the rows of supporting structures. This is many times the volume of snow in the avalanche tongue below the structures. The snow cover above the topmost row did not move or break up. It is possible that this is due to the supporting structures but as the fracture line to the west of the structures is approximately at the elevation of the top row it is not clear whether the snow cover above the structure would have been released as an avalanche in the absence of the structures.
In a few locations one could see that the avalanche slab had started to break up but was nevertheless moving as a more or less solid block when it was stopped by the supporting structures. It may be assumed that the avalanche would have been many times larger, and that it might have reached the health care centre, if the supporting structures had not reduced the released snow volume by an order of magnitude.
This is the first example of supporting structures preventing or reducing the size of a snow avalanche in Iceland.
Supporting structures for preventing snow avalanches have been installed at Neskaupstaður and Siglufjörður, in addition to Ólafsvík in recent years. Supporting structures are the most frequently used protection measure against snow avalanches for settlements in the Alps but they have not been much used in Iceland until recently. Further installations of supporting structures are planned in the next 5-10 years, for example at Ísafjörður and Siglufjörður.
The fractured avalanche slab that was released between the rows of supporting structures that stopped and piled up on the structures before it disintegrated fully. Photo: Tómas Jóhannesson.
Additionally, a map on a selected front page shows recent observations of avalanches.