SYSTEMATIC MAPPING AND HAZARD AND RISK CLASSIFICATION OF UNSTABLE ROCK SLOPES WITH A POTENTIAL OF FORMING DISPLACEMENT WAVES IN NORWAY

Large rockslides that impact water bodies, causing several tens of meter high displacement waves, occur repeatedly in Norway. Three such displacement waves resulted in a high number of casualties in the 20th century. Therefore, systematic mapping of unstable rock slopes has been carried out in Norway since 2005. More than 300 unstable or potential unstable rock slopes have been detected so far. This requires a standardized hazard and risk classification system that was established in 2012. Based on that, a new iterative mapping approach was set up, starting with remote assessments to quickly exclude zero risk and low risk sites with low likelihood of failure from expensive geological field investigations. However, the higher the hazard/risk level of a site is, the larger the amount of geological information that is collected.

The hazard analysis is qualitative and includes analyses of the morphological indications of deformation (back scarp, lateral release surface, underlying basal rupture surface), structural analyses and a kinematic feasibility test, as well as analyses of displacement rates, acceleration of displacement rates, rock fall activity, and the analyses of past events. The uncertainty of those observations are expressed with uncertainty margins that result in uncertainty margins of the hazard classification. For each unstable rock slope, a run-out analysis is performed, followed by a displacement wave assessment that shows the area of potential impact of a wave. These analyses are increasing in quality with the risk level.

The risk analysis is focused on loss of life only and is quantitative. We analyze the number of persons that settle in the impact area and those that are transient (e.g. at work, school, in hotels, shops, tourists). As transient persons are also counted, the consequence analysis has uncertainty margins. The final product is a risk classification that is based on a qualitative hazard classes and a quantitative consequence analysis. Relevant data for land-use planning will be published online on http://www.skrednett.no/. Initially, the run-out area and displacement wave run-up area will be published as susceptibility zones. However, a linkage to the quantitative hazard regulation of the Norwegian building code is envisioned.