Detection and quantification of permafrost change in alpine rock walls and implications for rock instability

The perennial presence of ice in permafrost rock walls alters thermal, hydraulic and mechanic properties of the rock mass. Temperature-related changes in both, rock mechanical properties (compressive and tensile strength of water-saturated rock) and ice mechanical properties (creep, fracture and cohesive properties) account for the internal mechanical destabilisation of permafrost rocks. Two hypothetical ice-/rock mechanical models were developed based on the principle of superposition. Failure along existing sliding planes is explained by the impact of temperature on shear stress uptake by creep deformation of ice, the propensity of failure along rock-ice fractures and reduced total friction along rough rock-rock contacts. This model may account for the rapid response of rockslides to warming (reaction time). In the long term, brittle fracture propagation is initialised. Warming reduces the shear stress uptake by total friction and decreases the critical fracture toughness along rock bridges. The latter model accounts for slow subcritical destabilisation of whole rock slopes over decades to millennia, subsequent to the warming impulse (relaxation time). To gain further understanding of thermal, hydraulic and mechanic properties of permafrost rocks, multidimensional and multi-temporal insights into the system are required. This Ph.D. adopted, modified and calibrated existing ERT (electrical resistivity tomography) techniques for the use in permafrost rocks. Laboratory analysis of electrical properties of eight rock samples from permafrost summits brought upon evidence that the general exponential temperature-resistivity relation, proposed by McGinnis (1973), is not applicable for frozen rocks, due to the effects of freezing in confined space. We found, that separate linear temperature-resistivity (T- ρ) approximation of unfrozen, supercooled and frozen behaviour offers a better explanation of the involved physics. Frozen T-ρ gradients approach 29.8 ±10.6 %/°C while unfrozen gradients were confirmed at 2.9 ±0.3 %/°C. Both increase with porosity. Path-dependent supercooling T-ρ behavior (3.3 ±2.3 %/°C) until the spontaneous freezing temperature -1.2 (±0.2) °C resembles unfrozen behavior. Spontaneous freezing subsequent to supercooling coincides with sudden self-induced temperature increases of 0.8 (±0.1) °C and resistivity increases of 2.9 (±1.4) kΩm. As temperature-resistivity gradients of frozen rocks are steep, temperature-referenced ERT with accuracies in the range of 1 °C is technically feasible in frozen rock. Technical design for field measurements in permafrost-affected bedrock was developed from 2005 to 2008 in consecutive measurements at a rock crest in the Swiss Alps (Steintaelli, 3150 m a.s.l., Matter Valley) and in a gallery along a north face in the German/ Austrian Alps (Zugspitze, 2800 m a.s.l.). 2D measurements in the Steintaelli along S-, NE-, NW- and Wfacing rock walls showed that ERT provides information on temporal and spatial patterns of thawing, refreezing, cleftwater flow and permafrost distribution in a decameter scale. Monthly, annual and multiannual data were compared using a time-lapse inversion technique and showed consistent results. Seasonal thaw at the Zugspitze was observed in February and monthly from May to October 2007 with high-resolution ERT (140 electrodes). An error model based on the measured offset of normal-reciprocal measurements was operated to empirically fit inherent error. A smoothness-constrained, error-controlled inversion routine (CRTomo) was applied to gain quantitatively reliable ERT data. Application of temperature-referenced laboratory data is consistent with temperature data observed in the adjacent borehole and with temperature logger data. Calculated temperature changes are in accordance with slow thermal conduction away from the rock surface and subsequent refreezing from the rock face in September/October. Smoothness-constrained, error-controlled inversion was transferred to pseudo-3D measurements collated from five 2D-transects with an offset of 4 m across a NE-SW facing ridge in the Steintaelli. In spite of the enormous topography, ERT transects were capable of resolving permafrost and thaw dynamics at the NE facing slope and along ice-filled crevices as well as disclosing unfrozen rock on the SW-facing rock slope. Consecutive measurements of 2006, 2007 and 2008 provide coherent results in line with temperature logger data. ERT measurements confirm that aspect is the most important control of permafrost distribution in rock walls, for a given altitude. At 3150 m a.sl., rock permafrost was found in NE-, NW- and E-facing rock walls in the Steintaelli but not in S-facing transects. Multiannual 3D data show that all NE-facing rock slopes still comprise decameter large permafrost bodies, but the 104.5 Ωm (31.6 kΩm) line which represents a definite transition to the –2 °C range is not reached in any of the transects apart from the surrounding of ice-filled clefts or at the surface. Semiconductive effects of centimetre to decimetre wide frozen fractures significantly cool ambient bedrock and have a dominant influence on the spatial distribution of permafrost under the crestline. Multiannual 2D data reveal that cleftwater inundation in two fracture systems can effectively prevent a decametre large rockwall from cooling below –1 °C (20 kΩm) during two years with permafrost aggradation (August 2005 to August 2007) in sheltered positions. An adjacent rockwall with similar surface characteristics but no hydraulic interconnectivity cooled significantly below –3 °C (> 60 kΩm) in the same time. Steep, highly dissected rock masses can create local permafrost occurrences of meter size even on SW-facing rock slopes. Seasonal thaw of rock permafrost occurs much faster than expected. Monthly measurements at the Zugspitze showed that maximum thaw depth in 2007 was already reached in July/August. In May, rapid warming of permafrost rocks with a resistivity increase equivalent to 1.5 °C warming and more was observed along a fracture zone with active cleftwater flows up to 30 m away from the rock face. Eighteen extensometer transects along the 3D-ERT array in the Steintaelli indicate that rock deformation on the permafrost-affected crest line and in the NE-facing slope is 3-4 times higher than in the non-perennially-frozen SW-facing slope. The velocity of rock displacements in late summer is 20 times higher than in all-season measurements. Velocities along a directly ERT-approved permafrost rock slope respond exponentially to mean air temperature during observation period with an R2; of 0.86. These findings support the hypothesised rapid sliding response to temperature change due to enhanced ice-creep and failure of ice in fractures

Quantifying lateral bedrock erosion caused during a hyperconcentrated flow in a narrow alpine limestone gorge

Here, we show results of an unprecedented LiDAR dataset quantitatively determining the lateral bedrock erosion of a narrow limestone gorge during an extreme hyperconcentrated flow. The comparison of two point clouds prior and post to the June 15th hyperconcentrated flow event provide information about the massive breakout of particles and abrasion of the channel walls. With a multiscale model to model cloud comparison analysis, we can show that particles from 0.0001 m3 and 3.5 m3 were eroded from the subvertical limestone gorge walls. A total of 20.9 m3 of massive bedrock was eroded in the observable part of the channel with 90 % of the particles being smaller than 0.15 m3. We delimited two main erosion processes during the hyperconcentrated flow event: shearing of particles that reach into the flow and particles with predefined failure surfaces, and abrasion along the whole channel, detectable by LiDAR if the changes are > 3 cm. This study provides quantitative evidence for massive rock erosion processes in alpine gorges that could also control rock gorge formation and evolution over Holocene/Lateglacial time scales

Time-lapse capacitive resistivity imaging: a new technology concept for the monitoring of permafrost

The British Geological Survey, in partnership with the Universities of Sussex and Bonn, is investigating and seeking to prove a new technology concept for the non-invasive volumetric imaging and routine temporal monitoring of the thermal state of permafrost (Figure 1), a key indicator of global climate change. Capacitive Resistivity Imaging (CRI), a technique based upon a low-frequency, capacitively-coupled measurement approach (Kuras et al., 2006) is applied in order to emulate Electrical Resistivity Tomography (ERT) methodology, but without the need for galvanic contact on frozen soils or rocks. Recent work has shown that temperature-calibrated ERT using galvanic sensors (Figure 2) is capable of imaging recession and re-advance of rock permafrost in response to the ambient temperature regime. However, the use of galvanic sensors can lead to significant practical limitations on field measurements due to high levels of and large variations in contact resistances between sensors and the host material as it freezes and thaws Figure 3). The capacitive technology developed here overcomes this problem and provides a more robust means of making high-quality resistance measurements with permanently installed sensors over time. Reducing the uncertainty associated with uncontrolled noise from galvanic sensors increases the value of time-lapse ERT datasets in the context of monitoring permafrost

Long-term destabilization of retrogressive thaw slumps (Herschel Island, Yukon, Canada)

Retrogressive thaw slumps (RTS) are a common thermokarst landform along Arctic coastlines and provide a large amount of material containing organic carbon to the nearshore zone. The number of RTS has strongly increased since the last century. They are characterized by rapidly changing topographical and internal structures e.g., mud flow deposits, seawater-affected sediments or permafrost bodies and are strongly influenced by gullies. Furthermore, we hypothesize that due to thermal and mechanical disturbance, large RTS preferentially develop a polycyclic behavior. To reveal the inner structures of the RTS several electrical resistivity tomography (ERT) transects were carried out in 2011, 2012, and 2019 on the biggest RTS on Herschel Island (Qikiqtaruk, YT, Canada), a highly active and well-monitored study area. 2D ERT transects were conducted crossing the RTS longitudinal and transversal, always reaching the undisturbed tundra. Parallel to the shoreline, and crossing the main gully draining the slump, we applied 3D ERT which was first measured in 2012 and repeated in 2019. The ERT data was calibrated in the field using frost probing to detect the unfrozen-frozen transition and with bulk sediment resistivity versus temperature curves measured on samples in the laboratory. The strong thermal and topographical disturbances by gullies developing into large erosional features like RTS, lead to long recovery rates for disturbed permafrost, probably taking more than decades. In this study we demonstrate that ERT can be used to determine long-lasting thermal and mechanical disturbances. We show that they are both likely to prime the sensitivity of RTS to a polycyclic reactivation

Impact of an 0.2 km 3 Rock Avalanche on Lake Eibsee (Bavarian Alps, Germany) – Part II: Catchment Response to Consecutive Debris Avalanche and Debris Flow

The ~0.2 km3 Eibsee rock avalanche impacted Paleolake Eibsee and completely displaced its waters. This study anal- yses the lake impact and the consequences, and the catchment response to the landslide. A quasi‐3D seismic reflection survey, four sediment cores from modern Lake Eibsee, reaching far down into the rock avalanche mass, nine radiocarbon ages, and geomorphic analysis allow us to distinguish the main rock avalanche event from a secondary debris avalanche and debris flow. The highly flu- idized debris avalanche formed a megaturbidite and multiple swashes that are recorded in the lake sediments. The new calibrated age for the Eibsee rock avalanche of ~4080–3970 cal yr BP indicates a coincidence with rockslides in the Fernpass cluster and sub- aquatic landslides in Lake Piburg and Lake Plansee, and raises the possibility that a large regional earthquake triggered these events. We document a complex history of erosion and sedimentation in Lake Eibsee, and demonstrate how the catchment response and rebirth of the lake are revealed through the complementary application of geophysics, sedimentology, radiocarbon dating, and geo- morphology

Thermal and mechanical responses resulting from spatial and temporal snow cover variability in permafrost rock slopes, steintaelli, swiss alps

The aim of this study is to investigate the influence of snow on permafrost and rock stability at the Steintaelli (Swiss Alps). Snow depth distribution was observed using terrestrial laser scanning and time-lapse photography. The influence of snow on the rock thermal regime was investigated using near-surface rock temperature measurements, seismic refraction tomography and one-dimensional thermal modelling. Rock kinematics were recorded with crackmeters. The distribution of snow depth was strongly determined by rock slope micro-topography. Snow accumulated to thicknesses of up to 3.8 m on less steep rock slopes (<50°) and ledges, gradually covering steeper (up to 75°) slopes above. A perennial snow cornice at the flat ridge, as well as the long-lasting snow cover in shaded, gently inclined areas, prevented deep active-layer thaw, while patchy snow cover resulted in a deeper active-layer beneath steep rock slopes. The rock mechanical regime was also snow-controlled. During snow-free periods, high-frequency thermal expansion and contraction occurred. Rock temperature locally dropped to -10 °C, resulting in thermal contraction of the rock slopes. Snow cover insulation maintained temperatures in the frost- cracking window and favoured ice segregation. Daily thermal-induced and seasonal ice-induced fracture kinematics were dominant, and their repetitive occurrence destabilises the rock slope and can potentially lead to failure

Seasonally intermittent water flow through deep fractures in an Alpine Rock Ridge: Gemsstock, Central Swiss Alps

Geological investigations and seismic refraction tomography reveal a series of 70° steep, parallel and continuous fractures at 2950 m asl within the Gemsstock rock ridge (Central Swiss Alps), at the lower fringe of alpine permafrost. Temperature measurements in a 40 m horizontal borehole through the base of the ridge show that whilst conductive heat transfer dominates within the rock mass, brief negative and positive temperature anomalies are registered in summer. These have very small amplitudes and coincide with summer rainfall events lasting longer than 12 h. In contrast, a complete lack of anomalous thermal signals during spring snowmelt suggests that runoff does not penetrate the open joints, despite high snow water equivalents of around 400 mm. This is attributed to the development of an approximately 20 cm thick, continuous and impermeable basal ice layer which forms at the interface between the snow cover and the cold rock on the shady North facing rock wall during snowmelt. Spring snowmelt water therefore does not affect rock temperatures in the centre of the rock mass, despite the presence of deep open joints. The mechanical impact of snowmelt infiltration on rock wall stability at depth is thus assumed to be negligible at this site

Monitoring rock freezing and thawing by novel geoelectrical and acoustic techniques

Automated monitoring of freeze-thaw cycles and fracture propagation in mountain rockwalls is 23 needed to provide early warning about rockfall hazards. Conventional geoelectrical methods 24 such as electrical resistivity tomography (ERT) are limited by large and variable ohmic contact 25 resistances, requiring galvanic coupling with metal electrodes inserted into holes drilled into 26 rock, and which can be loosened by rock weathering. We report a novel experimental 27 methodology that combined capacitive resistivity imaging (CRI), ERT and microseismic event 28 recording to monitor freeze-thaw of six blocks of hard and soft limestones under conditions 29 simulating an active layer above permafrost and seasonally frozen rock in a non-permafrost 30 environment. Our results demonstrate that the CRI method is highly sensitive to freeze-thaw 31 processes; it yields property information equivalent to that obtained with conventional ERT and 32 offers a viable route for non-galvanic long-term geoelectrical monitoring, extending the benefits 33 of the methodology to soft/hard rock environments. Contact impedances achieved with CRI are 34 less affected by seasonal temperature changes, the aggregate state of the pore water (liquid or 35 frozen), and the presence of low-porosity rock with high matrix resistivities than those achieved 36 with ERT. Microseismic monitoring has the advantage over acoustic emissions that events were 37 recorded in relevant field distances of meters to decameters from cracking events. For the first 38 time we recorded about 1000 microcracking events and clustered them in four groups according 39 to frequency and waveform. Compared to previous studies, mainly on ice-cracking in glaciers, 40 the groups are attributed to single- or multiple-stage cracking events such as crack coalescence