Abstract. P-wave refraction seismics is a key method in permafrost research but its applicability to low-porosity rocks, which constitute alpine rock walls, has been denied in prior studies. These studies explain p-wave velocity changes in freezing rocks exclusively due to changing velocities of pore infill, i.e. water, air and ice. In existing models, no significant velocity increase is expected for low-porosity bedrock. We postulate, that mixing laws apply for high-porosity rocks, but freezing in confined space in low-porosity bedrock also alters physical rock matrix properties. In the laboratory, we measured p-wave velocities of 22 decimetre-large low-porosity (< 10%) metamorphic, magmatic and sedimentary rock samples from permafrost sites with a natural texture (> 100 micro-fissures) from 25 °C to −15 °C in 0.3 °C increments close to the freezing point. When freezing, p-wave velocity increases by 11–166% perpendicular to cleavage/bedding and equivalent to a matrix velocity increase from 11–200% coincident to an anisotropy decrease in most samples. The expansion of rigid bedrock upon freezing is restricted and ice pressure will increase matrix velocity and decrease anisotropy while changing velocities of the pore infill are insignificant. Here, we present a modified Timur's two-phase-equation implementing changes in matrix velocity dependent on lithology and demonstrate the general applicability of refraction seismics to differentiate frozen and unfrozen low-porosity bedrock.
Abstract. Identifying precursors of gravity-driven slope instabilities in inhomogeneous fractured rock masses is a challenging task. Recent laboratory studies have brought upon an enhanced understanding of rock fatigue and fracturing in cold environments but were not successfully confirmed by field studies. In this study we monitor environmental conditions, rock temperatures and fracture dynamics at 3500 m a.s.l. on the steep, strongly fractured Hörnligrat of Matterhorn (Swiss Alps). Here we analyze seven years of continuous data of the long term evolution of fracture dynamics in permafrost offering unprecedented level of detail and observation duration. The fracture dynamics consists of reversible and irreversible movement components resulting from a combination of temporal varying driving and resisting forces. As irreversible motion is suspected to occur prior to global gravity-driven slope failure, we developed a statistical model, assuming the reversible deformation is caused by thermo-mechanical induced strain, and tested it successfully with field measurements from steep permafrost bedrock. We apply this linear regression model to our data set of fracture dynamics and rock temperature in order to separate the residual irreversible movement component. From this, we produce a new metric that quantifies relative irreversibility of fracture dynamics and enables a better interpretation of the data. This index of irreversibility is based on in situ measurements and enables a local assessment of rock wall stability. Here we show how environmental forcing causes reversible and irreversible rock mass deformations that might be relevant in preconditioning rock slope instability. In general, all locations instrumented show a trend of fracture opening, but at variable rate between locations. At each individual location, the temporal pattern of deformation is very similar every year. All but one sensors show a reversible deformation component caused by thermo-mechanical induced strain. For many sensors, we observe an irreversible enhanced fracture deformation in summer, starting when rock temperatures reach above zero. This likely indicates thawing related process, such as melt water percolation into fractures, as a forcing mechanisms for irreversible deformation. Most likely, such water or thawing leads to a decrease of the cohesion and friction along fracture in the shear zone. For a few fractures instrumented, we find an irreversible deformation with the onset of freezing period, which suggest that cryogenic processes act as a driving factor through increasing ice pressure. It further highlights that irreversible fracture deformation can even at locations in close proximity not be explained by one single process.
<p><span>In the context of climate change, permafrost degradation is a key variable in understanding rock slope failures in high mountain areas. Permafrost degradation imposes a variety of environmental, economic and humanitarian impacts on infrastructure and people in high mountain areas. Therefore, new high-quality monitoring and modelling strategies are needed.</span></p><p><span>Electrical Resistivity Tomography (ERT) is the predominant permafrost monitoring technique in high mountain areas. Its high temperature sensitivity for frozen vs. unfrozen conditions, combined with the resistivity-temperature laboratory calibration on Wettersteinkalk (Zugspitze) (Krautblatter et al. 2010) gives us quantitative information on site-specific rock wall temperatures (Magnin <em>et al.</em> 2015). Long-term ERT-Measurements (2007/2014 &#8211; now) were taken at the Kammstollen along the northern Zugspitze rock face. Two high-resistivity bodies along the investigation area reach resistivity values &#8805;10<sup>4.5</sup></span>&#937;<span>m (</span><span>&#8764;</span><span>&#8722;0.5 </span><span>&#176;</span><span>C), indicating frozen rock, displaying a core section with resistivities &#8805;10<sup>4.7</sup></span>&#937;<span>m (</span><span>&#8764;</span><span>&#8722;2 </span><span>&#176;</span><span>C) (Krautblatter <em>et al.</em>, 2010). We can differentiate seasonal variability, seen by laterally aggrading and degrading marginal sections (Krautblatter <em>et al.</em>, 2010) and singular effects due to environmental factors and extreme weather events.</span></p><p><span>Here, we present a new local high-resolution numerical, process-orientated thermo-geophysical model (TGM) for steep permafrost rock walls. The model links apparent resistivities, the ground thermal regime and meteorological forcings as seasonality and long-term climate change to validate the ERT and project future conditions. The TGM comprises a surface energy balance model, conductive energy transport, turbulent and seasonal heat fluxes (sensible, latent, melt and rain heat fluxes) including phase-change, as well as a multi-phase rock wall composition.</span></p><p><span>Finally, we can reproduce the natural temperature field in the rock wall, assess the spatial-temporal permafrost evolution in alpine rock walls, validate the ERT measurements via the new TGM and the applicability of the laboratory derived resistivity-temperature relationship by Krautblatter et al. (2010) for natural rock-wall conditions.</span></p><p><span>&#160;</span></p><p><span>Krautblatter, M., Verleysdonk, S., Flores-Orozco, A. & Kemna, A. (2010): Temperature- calibrated imaging of seasonal changes in permafrost rock walls by quantitative electrical resistivity </span><span>tomography</span><span> (Zugspitze, German/Austrian Alps). <em>J. Geophys. Res. </em>115: F02003.</span></p><p><span>Magnin, F., Krautblatter, M., Deline, P., Ravanel, L., Malet, E., Bevington, A. (2015): Determination of warm, sensitive permafrost areas in near-vertical rockwalls and evaluation of distributed models by electrical resistivity tomography. <em>J. Geophys. Res. Earth Surf.</em>, 120, 745-762.</span></p>
Oversteepened valley walls in western Norway have high recurrences of Holocene rock-slope failure activity causing significant risk to communities and infrastructure. Deposits from six to nine catastrophic rock-slope failure (CRSF) events are preserved at the base of the Mannen rock-slope instability in the Romsdal Valley, western Norway. The timing of these CRSF events was determined by terrestrial cosmogenic nuclide dating and relative chronology due to mapping Quaternary deposits. The stratigraphical chronology indicates that three of the CRSF events occurred between 12 and 10 ka, during regional deglaciation. Congruent with previous investigations, these events are attributed to the debuttressing effect experienced by steep slopes following deglaciation, during a period of paraglacial relaxation. The remaining three to six CRSF events cluster at 4.9 ± 0.6 ka (based on 10 cosmogenic 10 Be samples from boulders). CRSF events during this later period are ascribed to climatic changes at the end of the Holocene thermal optimum, including increased precipitation rates, high air temperatures and the associated degradation of permafrost in rock-slope faces. Geomorphological mapping and sedimentological analyses further permit the contextualisation of these deposits within the overall sequence of post-glacial fjord-valley infilling. In the light of contemporary climate change, the relationship between CRSF frequency, precipitation, air temperature and permafrost degradation may be of interest to others working or operating in comparable settings.
Abstract Geomorphological evidence of incised bedrock channels is widespread in all mountain landscapes worldwide. However, the processes controlling incision and gorge formation in bedrock have not directly been observed in an actualistic way. Here, we show a LiDAR change detection deciphering the erosive power of a 60,000 m 3 hyperconcentrated flow (transition between flood and debris flow) in a deeply incised rock gorge in June, 2020. The flow laterally eroded up to 1 m of massive limestone and widened a 4 m narrow section of the gorge by up to 15%. Sinuosity, convergence, and gradient of the channel were proven to not influence erosivity indicating the hyperconcentrated nature of erosion. Furthermore, other than in prior studies no abrasion of thin rock veneer dominates erosion but mechanically excited breakout of rock fragments. Magnitude-frequency relations of eroded volumes mimic subaerial rock wall retreat. We show how single hyperconcentrated flows can erode bedrock channels far more efficient than decades of turbulent flows and hypothesise that repeated hyperconcentrated flows in phases of enhanced precipitation or by elevated material supply could control erosion boosts in gorge formation, e.g. in the Lateglacial or during climatic fluctuations.
