NGU is systematically mapping unstable rock slopes in Troms county. So far, 117 unstable rock slopes have been identified that could develop into rock avalanches in the future.

Picture: The unstable rock slope at Fallsnesfjellet, south of Skibotn (Photo: I. Henderson, NGU).

Troms is a county in northern Norway that is characterized by many fjords and high mountains. The steep mountainsides have resulted in many large rock avalanches since the last deglaciation. The treeline in Troms is generally quite low, resulting in a large proportion of exposed bedrock. In this way, Troms is an ideal natural laboratory for studying unstable rock slopes. With the help of both aerial photography and satellite-based radar interferometry (InSAR), many unstable rock slopes have been identified throughout the county. The highest concentration is in a zone from Balsford in the south to Reisadalen in the north, with Kåfjord standing out with an exceptional number of unstable rock slopes and deposits from prehistoric avalanches.

Many of the rock avalanche deposits in Troms cover large areas, with the deposit mapped in Grovfjorden being the largest. That avalanche is estimated to have involved a volume of about 100 million cubic metres, and has travelled more than 4 km across the valley bottom. A number of the avalanche deposits in Troms have been radiometrically dated, and indicate that the highest number of avalanches occurred shortly after the last deglaciation, between 11 500 and 10 500 year ago. I Troms, there are only a few historic rock avalanches recorded in our database (Skrednett). One example is the rock avalanche from Pollfjellet, in Lyngen in 1810, which created a displacement wave along Storfjord that killed 14 people.

The systematic mapping of unstable rock slopes in Troms began in 2005, mostly by NGU geologists, and is still ongoing. Of the more than 300 localities that have been mapped, 117 unstable rock slopes have been identified that could develop into rock avalanches in the future. This makes Troms the county with highest number of mapped unstable rock slopes in Norway. All unstable rock slopes will eventually be classified using the standardized risk and hazard classification system and entered into our database. For each location, the potential runout area will be calculated in order to determine the consequences. In addition, tsunami wave modelling will be carried out for relevant locations along the fjord. Where necessary, periodic displacement measurements will be made.

Periodic displacement measurements are currently being taken for 40 unstable rock slopes in Troms, mostly in the northern part of the county, using for example differential Global Navigation Satellite Systems (commonly known as GPS), terrestrial laser scanning, satellite-based and ground-based InSAR or tape extensometers. Significant displacements have been measured on 7 unstable slopes (Dorrisdalen in Reisadalen, Fuglen on Reinøya, Gamanjunni 3 in Manndalen, Gavtavarri above Kåfjorden, Hengfjellet and Revdalsfjellet on Nordnes peninsula over Storfjorden, and Oksfjellet in Kåfjorddalen). Displacement rates measured using GPS range from 3 mm to 5 cm per year. In addition, numerous rock slopes have small displacements measured by InSAR. Two unstable rockslopes on Nordnesfjellet (Jettan and Indre Nordnes) are being continually monitored by Nordnorsk Fjellovervåkning (now taken over NVE).

NGU reports:

  • NGU reportt 2013.021
  • NGU report 2011.031
  • NGU report 2010.021
  • NGU report 2009.023
  • NGU report 2009.026
  • NGU report 2008.025
  • NGU report 2007.041
  • NGU report 2006.040
Det ustabile fjellpartiet Gamanjunni 3 i Manndalen (Photo: H.S.S. Bunkholt, NGU). Fjellpartiet har allerede forflyttet seg totalt 150 m og målt hastighet i fast fjell er opp mot 6 cm per år.
The unstable rock slope called Gamanjunni 3, in Manndalen
(Photo: H.S.S. Bunkholt, NGU).
The unstable rock mass has already moved 150 m and
the current displacement rate has been measured to be up to 6 cm per year.