Preprints
https://doi.org/10.5194/nhess-2020-234
https://doi.org/10.5194/nhess-2020-234

  09 Sep 2020

09 Sep 2020

Review status: this preprint is currently under review for the journal NHESS.

GIS-based topographic reconstruction and geomechanical modelling of the Köfels Rock Slide

Christian Zangerl1, Annemarie Schneeberger1,2, Georg Steiner1,3, and Martin Mergili1,4 Christian Zangerl et al.
  • 1Institute of Applied Geology, University of Natural Resources and Life Sciences (BOKU), Vienna, 1190 Austria
  • 2Institute of Geography, University of Innsbruck, Innsbruck, 6020, Austria
  • 3Amt der Kärntner Landesregierung, Klagenfurt, 9021, Austria
  • 4Department of Geography and Regional Science, University of Graz, Graz, 8010, Austria

Abstract. The Köfels Rock Slide in the Ötztal Valley (Tyrol, Austria) represents the largest known extremely rapid landslide in metamorphic rock masses in the Alps. Although many hypotheses for the trigger were discussed in the past, until now no scientifically proven trigger factor has been identified. This study provides new data about the i) pre-failure and failure topography, ii) failure volume and porosity of the sliding mass, and iii) shear strength properties of the gneissic rock mass obtained by back-calculations. Geographic information system methods were used to reconstruct the slope topographies before, during and after the event. Comparing the resulting digital terrain models leads to volume estimates of the failure and deposition masses of 3.1 km3 and 4.0 km3, respectively and a sliding mass porosity of 26 %. For the back-calculations the 2D discrete element method was applied to determine the shear strength properties of the reconstructed basal shear zone. Results indicated that under no groundwater flow conditions, a very low friction angle below 24° is required to promote failure, whilst, with groundwater flow, the critical value increase to 28°. Such a low friction angle is unexpected from a rock mechanical perspective for this strong rock and groundwater flow, even if high water pressures are assumed, may not be able to trigger this rock slide. Additional conditioning and triggering factors should be identified by further studies, for example focussing on the impact of dynamic loading.

Christian Zangerl et al.

 
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Christian Zangerl et al.

Christian Zangerl et al.

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Short summary
The Köfels Rock Slide in the Ötztal Valley (Tyrol, Austria) represents the largest known extremely rapid landslide in metamorphic rock masses in the Alps and was formed in the early Holocene 9527–9498 cal BP. Although many hypotheses for the conditioning and triggering factors were discussed in the past, until now no scientifically accepted explanatory model has been found. This study provides new data to better understand the cause and triggering factors of this gigantic natural event.
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