Rock avalanches in Norway

Due to Norway’s special geographic conditions, with deep fjords penetrating more than 100 km into its alpine landscape, rock avalanches are the type of landslide that has most frequently caused disastrous events. Displacement waves, created by the impact of a rock avalanche into a water body, have often led to the loss of numerous lives. Historically, such events have occurred 2-4 times per century.
Figure 1: Unstable slope at Kassen above Bandak lake, Telemark (Photo: R. Hermanns, NGU).
Figure 1: Unstable slope at Kassen above Bandak lake, Telemark (Photo: R. Hermanns, NGU).

Rock avalanches from an unstable rock slope (Figure 1), and any related displacement waves, are events that only last minutes (Figure 2A), and have such high energy that they cannot be mitigated by any physical measure. The events themself leave traces in the landscape that prevail, in the case of displacement waves, for decades (strong disturbance in the vegetation, as shown on Figure 3) and in the case of rock avalanches, leave deposits that are visible in the landscape for thousands of years and longer (Figure 2B). 

	Figure 2: A) The rock avalanche event that takes place within minutes, B) the deposit of a rock avalanche that is a witness of past rock avalanches in the landscape, C) unstable slope that can fail in a rock avalanche in future.
Figure 2: A) The rock avalanche event that takes place within
minutes, B) the deposit of a rock avalanche that is a
witness of past rock avalanches in the landscape,
C) unstable slope that can fail in a rock
avalanche in future.

The goal of geological mapping carried out at NGU is to find unstable slopes (Figure 1 and 2C) that can collapse as rock avalanches in future and to predict the area they can affect. If society has developed in the area of potential impact and risk levels are medium, specialists will have their eyes on the sites. If the risk level is unacceptable, the sites are continuously followed up and early warning practises by NVE guarantee that people will be evacuated from the hazard zone before an event.

Although Norway is the 68th largest country on earth, it has the 7th longest shoreline of all countries due to its fjord coast. This coastline is often characterized by steep rock slopes that are the product of multiple mountain building periods and glaciations cutting into that high relief. Rock slope failures of small sizes happen along the coast frequently. Most of them are not registered as they do not cause any damage. However, in the landslide event database there are 136 events of rock slope failures that impacted into fjords or lakes that caused damage and/or loss of life. Among those are 12 rock avalanches registered that account for 283 fatalities.

Large rock slope failures are not only destructive when they impact a still water body. In narrow valleys rock avalanches can produce landslide dams that flood the valleys upriver and also cause downriver flooding if the dam breaches.

Besides these secondary effects that impact large areas are the devastating direct impacts of rock avalanches on infrastructure or housing. Because it is impossible that society in Norway only develops outside the mountain area, a two-stage approach was selected by society in order to adopt to the rock avalanche risk by 1) mapping and 2) monitoring. The first step is carried out at NGU by mapping mountain areas for those areas that are potentially unstable and might collapse in the future following the approach to map unstable rock slopes. All potentially unstable slopes are classified according to their hazard and risk using a standardized methodology. In the second step, all sites that show movement and where people live within the hazard zone (despite risk level being considered as acceptable) are followed up with periodic displacement measurements. On the other hand, all sites that have unacceptable risk levels, are followed up with continuous monitoring and early-warning practises are put in place. This work is carried out by NVE.

 

Figure 3: Trim line that reaches 60 m above the sea level after the April 21st 2007 displacement wave in Aysén fjord and related rock avalanche scar at other shoreline, Chile (Photo: R. Hermanns, NGU).
Figure 3: Trim line that reaches 60 m above the sea level after the April 21st 2007 displacement wave in Aysén fjord and related rock avalanche scar at other shoreline, Chile (Photo: R. Hermanns, NGU).