Rock avalanches in Norway
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).
NGUs task is to map, hazard and risk assess and classify unstable mountain areas that can develop to rock avalanches. This is done by distant analysis from satellites, planes, helicopters, drones and further through thorough field studies. The unstable mountain areas are classified after a scale of low, middle and high risk. Risk includes both the probability of a rock avalanche to occur and the potential loss of lives.
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.
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.