Towards a national susceptibility map for rock avalanches

Several rock avalanches with significant consequences have taken place in Norway during the last centuries. This has caused a high awareness with respect to this natural hazard. As a result, mapping of unstable slopes was initiated in 2006 and several high-risk unstable rock slopes have been identified and investigated in detail and today are monitored. Furthermore, the mapping program of unstable rock slopes has become systematic. Under this initiative, so far five out of eleven Norwegian counties have been analysed systematically for unstable rock slopes and the mapping has been completed for one of these counties. Registered slopes are mapped and classified based on a systematic hazard and risk classification system, established in 2012. This process is time intensive, and currently attention might not be given to the highest risk objects.

In order to get a rapid, complete national overview of potential large rock slope failures, as well as their total hazard and consequence potential, a national overview mapping project has been started. This will make it possible to better prioritize high risk objects in the systematic mapping program. The project will be divided into several steps: (1) systematic analysis of remote sensing data (e.g. detailed DEM, orthophoto and InSAR data) to locate potential unstable rock slopes; (2) a simplified hazard ranking; (3) semi-automated volume estimation; (4) automated run-out assessment; (5) and empirical displacement wave run-up height assessment.

In order to minimize the area that needs to be analysed in Step 1, presently known unstable rock slopes have been analysed. Results indicate that the study area can be restricted based on available relief, presence of inhabitants and distance to the shorelines (fjords and lakes). This makes it possible to reduce the study area significantly, from the total land area of Norway down to roughly one third of this. Furthermore, for this quick overview assessment we use a simplified hazard ranking that is based on signs of activity, visible grade of development and its volume.

The resulting susceptibility map will serve as a source to prioritize mapping and mitigation efforts, with respect to other natural hazards in Norway as well.