Integrated analysis of past, and potential future rock slope failures of various size from Rombakstøtta, Nordland
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Catastrophic failure of large rock slopes has led to fatalities in Norwegian settlements several times per century. The Geological Survey of Norway (NGU) currently carry out systematic geological mapping of potentially unstable rock slopes in Norway, on assignment from the Norwegian Water Resources and Energy Directorate (NVE). In this context, a hazard analysis and preliminary consequence assessment of the unstable rock slope at Rombakstøtta in Narvik kommune, Nordland fylke has been carried out. In addition, an analysis of rock fall run-out lengths and frequencies since deglaciation has been carried out in the modelling software Rockyfor3D. A fragmentation cycle analysis, used to assess the fragmentation during a failure and to separate rock fall deposits from rock avalanche deposits, has been developed and tested. The study area is located in a north facing slope along a fjord ca. seven kilometers east of the city Narvik. Based on delimiting lineaments observed in the field, aerial photos, photo panoramas and digital elevation models, eight failure scenarios (1.A-B, 2.A, 3.A and 4.A-D) are defined at Rombakstøtta. Application of NGU´s hazard analysis resulted in one scenario being assigned to the medium/low hazard class, six to the medium hazard class, and one to the high hazard class. Volume estimation and run-out analysis were carried out for all scenarios as a part of the preliminary consequence assessment. Resulting volumes for the scenarios range from 10 000 m3 to 4 650 000 m3, and four of the scenarios have modelled run-out reaching houses, Ofotbanen, the E6 highway, and the fjord. An additional two scenarios have run-out reaching Ofotbanen. The rock fall analysis show that rock fall blocks larger than ~7.8 m3 have reached coarse rock avalanche deposits with a possible frequency of 5-13 blocks per 1x1 m since deglaciation. This could affect cosmogenic nuclide dating of rock avalanche deposits, carried out within the CryoWALL project to which this thesis has affiliations. The calculation is based on rock fall modelling and an extrapolation of a dataset with ~100 years of rock fall registrations. The fragmentation cycle analysis has been developed and applied inspired by Charrière et al. (2016). Results suggest that rock avalanche deposits at Rombakstøtta underwent 0-3 fragmentation cycles during failure. And that rock fall deposits below Rombakstøtta generally experienced more than 4 fragmentation cycles during failure. Results are discussed and compared to the results of Charrière et al. (2016) from the Frank Slide, Canada.