The Unstable Rock Slopes Træstane and Hest in Sørfjorden - Structural Mapping and Hazard assessment for Different Failure Scenarios
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The geological survey of Norway carries out a systematic mapping of potential unstable rock slopes in Norway. In context of this, a hazard analysis and preliminary consequence assessment has been undertaken at the two unstable rock slopes Træstane and Hest in Ullensvang municipality, Hordaland. The investigated area lies in an east facing slope on the western side of the 38 km long Sørfjorden, mainly above Aga. Seven main failure scenarios are defined in the two instabilities, based on delimiting cracks and obvious geomorphological features. Two of these are located at Hest (2A and B), while five are located at Træstane (1A-E). Four of these lies in the moderate hazard class, while three lies in the low hazard class. For the consequence assessment, volume and run-out analysis has been carried out for the different scenarios. The scenarios at Træstane range from 0.16-2.82 million m3, while the whole instability is estimated to 37.6 million m3. The largest scenario is considered of low probability due to its characteristics and when comparing to historical events in Norway. At Hest, the two scenarios are estimated to 0.53 and 1.65 million m3. The run-out analysis show that the two largest scenarios at Træstane may reach settlements in Aga, and reach Sørfjorden where further empirical displacement wave analysis has been carried out. The run-up heights show that a displacement wave may have consequences for settlements all along Sørfjorden. Two other scenarios from Træstane may reach individual buildings in the uppermost part of Træstane. At Hest, the largest scenario may reach settlements at Syreflot and reach the fjord, but this run-out is considered of lower likelihood.The structural mapping has resulted in three main joint sets (J1, J2 and J3), where J1 (~275/90 (dip/dip direction)) and J2 (~208/89) are the most dominating. The third joint set, J3, vary between the two instabilities (340/85 to 135/72) and is not as prevalent. A fourth joint set, J4 (101/48), interpreted as an exfoliation joint, was recognized at the backscarp of Træstane through further structural analysis based on photogrammetric point cloud models. The undulating foliation is also dominating, which dips on average towards Sørfjorden with the orientation of 120/16. The exception is within the instability of Træstane, where a back-rotation or back-tilt of the investigated blocks show that the foliation dips into the slope. The rotation of the blocks has been investigated, by comparing matching discontinuity sets in the relative stable area and on the blocks within the instability. This analysis show that the blocks have tilted in a mainly W-NW direction towards the backscarp.Additionally, an analysis of a larger rockfall failure which occurred at backscarp of Træstane in the autumn of 2016 has been undertaken. This included comparing two photogrammetric point clouds models from before and after the event, and a simple comparison and back-calculation of the actual run-out with the Rockyfor3D simulation model.