Recommended hazard and risk classification system for large unstable rock slopes in Norway
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Catastrophic failure of large rock slopes in Norway has several times per century led to rock avalanches or large rock falls, which impacted into settlements directly, but also caused either a displacement wave when impacting a water body or damming of narrow valleys with often fatal consequences. Such events will also occur in the future. Therefore, the Geological Survey of Norway (NGU), following its obligation towards society and the Norwegian Water and Energy Directorate (NVE) carries out systematic geologic mapping of potentially unstable rock slopes that might fail catastrophically. Within the last years mapping in only three of the 17 relevant counties of Norway has revealed more than 300 sites of potential future rock slope failures. This number necessitates a systematic mapping approach that focuses on the relevant geological data for assessing the likelihood of failure. Furthermore, it requires prioritization of follow-up activities, such as periodic or permanent monitoring, early-warning systems, and other mitigation measures. As the likelihood of failure cannot be given quantiatively with today's scientific knowledge, the risk analysis is built on a qualitative hazard analysis and a quantitative consequence analysis.