Influences of snow cover on thermal and mechanical processes in steep permafrost rock walls: internal response
Zusammenfassung der Projektergebnisse
In this project, the influence of snow cover on thermal and mechanical processes in steep permafrost rockwalls was systematically researched. (1) Snow distribution is controlled by micro-topographical factors. Increasing slope angle does not decrease snow depth, however, snow distribution is more heterogeneously in steeper terrain. Furthermore, snow deposition show similar patterns of snow depth and distribution from year to year with maximum depth between 1.5 and 2 m at Gemsstock and up to 3.8 m at Steintaelli. In addition, snow ablation is influenced by local shading effect which can reverse the usual earlier ablation on the south-exposed slopes. (2) The timing, initial thickness and duration of the snow cover strongly influence the thermal regime of rockwalls. At both research areas, a cooling effect by late snow cover onset and long-term insulation and a warming effect by early snow cover onset were observed. The insulation by snow cover is influenced by snow cover properties and started at a threshold of 0.2 m where snow depth becomes the major factor controlling the thermal regime. (3) To address spatial and temporal variation of active-layer, we developed a novel laboratory-calibrated SRT approach. The approach was combined with thermal data from borehole and temperature modelling and demonstrated that the cooling effect of a long lasting snow cover preserves frozen conditions or decreases active layer thaw while areas of the earlier snow disappearance show much deeper active or permafrost absence. (4) The mechanical regime strongly reflects the snow cover and the thermal. During snow-free periods, high frequency thermal expansion and contraction occurred which can be amplified by volumetric expansion when temperature drops below -10°. In contrast, insulating snow prevents thermal expansion or contraction and favours ice segregation. Basal ice layer formation prevents snowmelt infiltration into open joints in spring and, thus, prevents the development of perched groundwater levels. In addition, ice filling in fractures is required to prevent water seepage from fractures and hydrostatic pressure development. Rock fractures show an oscillating behaviour without persistent crack opening is observed, however, high-frequent high-magnitude thermal expansion and contraction and low frequent high-magnitude ice segregation can lead to rock fatigue which reduces long-term rock stability. (5) We united our findings into a rock-ice mechanical model and a conceptual model to explain the influence of snow cover on thermal and mechanical processes in permafrost rockwalls. On seasonal scale, snow cover controls the timing of rock instabilities which can occur during two time windows of instability in summer and autumn. On a long-term scale, rock bridge cracking due to ice segregation and or thermal stress can change system state from rock-mechanical to more sensitive ice-mechanical state. Increased active-layer thaw due to enhanced warming by climate change can further increase sensitivity to failure.
Projektbezogene Publikationen (Auswahl)
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Thermal and mechanical responses resulting from spatial and temporal snow cover variability in permafrost rock slopes, Steintaelli, Swiss Alps. Permafrost and Periglacial Processes
Draebing, D., Haberkorn, A., Krautblatter, M., Kenner, R. & M. Phillips, M.
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(2012): P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model. The Cryosphere, 6:1163‐1174
Draebing, D. & M. Krautblatter
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(2013): Why permafrost rocks become unstable: a rock–ice-mechanical model in time and space. Earth Surface Processes and Landforms, 38(8), 876‐887
Krautblatter, M., Funk, D. & F.K. Günzel
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(2014). A two-phase mechanical model for rock-ice avalanches. Journal of Geophysical Research ‐ Earth Surface, 119 (10): 2272‐2290
Pudasaini, S. & Krautblatter, M.
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(2014): Interaction of thermal and mechanical processes in steep permafrost rock walls: a conceptual approach. Geomorphology, 266:226-235
Draebing, D., Krautblatter, M. & R. Dikau
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(2014): Pseudo 3D-P-wave refraction seismic monitoring of permafrost in steep unstable bedrock. Journal of Geophysical Research ‐ Earth Surface, 119 (2): 287‐299
Krautblatter, M. & D. Draebing
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(2015). Snow as a driving factor of rock surface temperatures in steep rough rock walls. Cold Regions Science and Technology, 118, 64‐75
Haberkorn, A., Hoelzle, M., Phillips, M., Kenner, R.
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(2015): Glacier- and permafrost-related slope instabilities. In: C. Huggel, M. Carey, J.J. Clague & A. Kääb (Eds.), The High-Mountain Cryosphere. Cambridge University Press, Cambridge, 147‐165
Krautblatter, M. & K. Leith
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(2015): Quantifying rock fatigue and decreasing compressive and tensile strength after repeated freeze-thaw cycles. Permafrost and Periglacial Processes, 26(4), 368‐377
Jia, H., W. Xiang & M. Krautblatter
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(2016): Application of Refraction Seismics in Permafrost Studies: A review. Earth Science Reviews
Draebing, D.
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(2016): Seasonally intermittent water flow through fractures in a rock ridge: Gemsstock, central Swiss Alps. Cold Regions Science and Technology
Phillips, M., Haberkorn, A., Draebing, D., Krautblatter, M., Rhyner, H. & R. Kenner
