The hazard and risk classification for large unstable rock slopes in Norway
Abstract
Catastrophic failure of large rock slopes led several times per century in Norway 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 that impacred settlements in distance with often deadly consequences. Such events will also occur in future. In order to make mitigation of the negative consequences of such events possible, 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 systematic mapping in only three of the 17 counties of Norway has revealed more than 300 sites of potential future rock slope failures. This number necessitates prioritization of follow-up activities, such as periodic and permanent monitoring and early-warning. This quantitative hazard and risk classification system was build to compare sites so that political and financial decisions can be taken. The classification system is scenario-based as intensity and rate of displacement, as well as the geological structures activated by the sliding rock mass vary significantly on slopes. In addition, for each scenario the secondary effects, such as generation of displacement waves or landslide damming of valleys with the potential of later outburst floods, have to be evaluated. The hazard classification is based on two sets of criteria: 1) Structural site investigations including analysis of the development of the back-scarp, lateral limits and basal sliding surface. This includes a kinematic analysis that tests if rock sliding or toppling is kinematically feasible with respect to the slope orientation, the persistence of main structures and the morphologic expression of the sliding surface. 2) The analysis of the activity of the slope is primarily based on the slide velocity, but also includes the change of deformation rates (acceleration), observation of rockfall activity and historic or prehistoric events. The classification system is organized in a decision tree were probabilities to each observation can be given. The final output is a cumulative frequency distribution divided into several classes, which are interpreted as susceptibility classes. As the Norwegian building law is formulated using annual probabilities (<1/100yr, <1/1000yr, <1/5000yr) the susceptibility classes are ranked accordingly, hence represent arbitrary hazard classes. This was done as there is not enough knowledge today to predict large rock slope failures for these time periods. In addition to hazard, the risk analysis also deals about consequences, which focuses on the potential fatalities to the rock slide scenarios and its secondary effects only.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMNH13A1573H
- Keywords:
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- 4302 NATURAL HAZARDS / Geological;
- 4315 NATURAL HAZARDS / Monitoring;
- forecasting;
- prediction;
- 4328 NATURAL HAZARDS / Risk;
- 4355 NATURAL HAZARDS / Miscellaneous