Search

Mega retrogressive thaw slumps (MRTS, >106 m3) are a major threat to Arctic infrastructure, alter regional biogeochemistry, and impact Arctic carbon budgets. However, processes initiating and reactivating MRTS are insufficiently understood. We hypothesize that MRTS preferentially develop a polycyclic behavior because the material is thermally and mechanically prepared for subsequent generation failure. In contrast to remote sensing, geophysical reconnaissance reveals the inner structure and relative thermal state of MRTS decameters beneath slump surfaces, potentially controlling polycyclicity. Based on their life cycle development, five (M)RTS were studied on Herschel Island, an MRTS hotspot on the Canadian Beaufort coast. We combine >2 km of electrical resistivity tomography (ERT), 500 m of ground‐penetrating radar (GPR) and annual monitoring of headwall retreat from 2004 to 2013 to reveal the thermal state, internal structure, and volume loss of slumps. ERT data were calibrated with unfrozen‐frozen transitions from frost probing of active layer thickness and shallow boreholes. In initial stage MRTS, ERT displays surficial thermal perturbations a few meters deep, coincident with recent mud pool and mud flow development. In early stage polycyclic MRTS, ERT shows decameter deep‐reaching thermal perturbations persisting even 300 years after the last activation. In peak‐stage polycyclic MRTS, 3D‐ERT highlights actively extending deep‐reaching thermal perturbations caused by gully incisions, mud slides and mud flows. GPR and headwall monitoring reveal structural disturbance by historical mud flows, ice‐rich permafrost, and a decadal quantification of headwall retreat and slump floor erosion. We show that geophysical signatures identify long‐lasting thermal and mechanical disturbances in MRTS predefining their susceptibility to polycyclic reactivation. Plain Language Summary: Mega retrogressive thaw slumps, hereafter slumps, are a giant type of erosional landform linked to permafrost thaw. They endanger Arctic infrastructure and are an important source of carbon emission in the Arctic. Processes that initiate and reactivate slumps are poorly understood. To better understand the internal structures and processes, we used geophysical techniques to study five slumps in a hotspot area on Herschel Island, YT, Canada. We compared geophysical measurements with field measurements of slump erosion (2004–2013) to gain information about the internal structures and material volume loss. In the initial slump stage, geophysical methods show shallow thawing underneath recent mud pools and mud flows. Reactivated slumps show deep reaching thermal influences in an early stage of development that can still be detected 300 years after the last activation. Highly active reactivated slumps show deep‐reaching thermal disturbances caused by gully incisions, organic layer removal, mud slides and mud flows. The applied methods can show the influence of former mud flows and ice‐rich permafrost and how they persist over decades and prepare slumps for future reactivation. Here we show that geophysical methods can be used to identify long‐lasting thermal and mechanical disturbances in slumps that predefine their likelihood for reactivation. Key Points: Mega retrogressive thaw slumps develop polycyclic behavior as material is thermally and mechanically prepared for future generation failuresGeophysical images show decameter‐deep thermal perturbations by warm, wet mud deposition, gully incision, and lateral slumping along gulliesWe provide a summary classification of polycyclic reworking stages based on electrical resistivity tomography and ground‐penetrating radar structures and geomorphological features [ABSTRACT FROM AUTHOR]

The anticipation of massive rock slope failures is a key mitigation strategy in a changing climate and environment requiring a precise understanding of pre-failure process dynamics. Here we exploit >4 years of multi-method high-resolution monitoring data from a large rock slope instability close to failure. To quantify and understand the effect of possible drivers (water from rain and snowmelt, internal rock fracturing, and earthquakes), we correlate slope displacements with environmental data, local seismic recordings, and earthquake catalogues. During the snowmelt phase, displacements are controlled by meltwater infiltration with high correlation and a time lag of 4–9 d. During the snow-free summer, rainfall induces accelerations with a time lag of 1–16 h for up to several days without a minimum activation rain sum threshold. Rock fracturing, linked to temperature and freeze–thaw cycles, is predominantly near the surface and unrelated to displacement rates. A classic Newmark analysis of recent and historic earthquakes indicates a low potential for immediate triggering of a major failure at the case site, unless it is already very close to failure. Seismic topographic amplification of the peak ground velocity (PGV) at the summit ranges from a factor of 2–11 and is spatially heterogeneous, indicating a high criticality of the slope. The presented in-depth monitoring data analysis enables a comprehensive rockfall driver evaluation and indicates where future climatic changes, e.g. in precipitation intensity and frequency, may alter the preconditioning of major rock slope failures. [ABSTRACT FROM AUTHOR]

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

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.

Rock slope instabilities and failures from permafrost are among the most significant alpine hazards in a changing climate and represent considerable threats to high-alpine infrastructure. While permafrost degradation is commonly attributed to rising air temperature and slow thermal heat propagation in rocks, the profound impact of water flow in bedrock permafrost on warming processes is increasingly recognized. However, quantifying the role of water flow remains challenging, primarily due to the complexities associated with direct observation and the transient nature of water dynamics in rock slope systems. To overcome the lack of quantitative assessment that inhibits thermal and mechanical modelling, we perform a joint analysis of electrical resistivity measurements and borehole temperature, combining datasets of monthly repeated electrical resistivity tomography acquired in 2013 and 2023, rock temperature measured in two deep boreholes (2016–2023), and site-specific temperature-resistivity relation determined in laboratory with samples from the study area. Field measurements were carried out at the permafrost-affected north flank of the Kitzsteinhorn (Hohe Tauern range, Austria), characterized by significant water outflow from open fractures during the melt season. Borehole temperature data demonstrate a seasonal maximum of the permafrost active layer of 4–5 m. They further show abrupt temperature changes (∼ 0.2–0.7 °C) during periods with enhanced water flow, which cannot be explained by conductive heat transfer. Monthly repeated electrical resistivity measurements reveal a massive decrease in resistivity from June to July and the initiation of a low-resistivity (< 4 kΩm) zone in the lower part of the rock slope in June, gradually expanding to higher rock slope sections until September. We hypothesize that the reduction in electrical resistivity of more than one order of magnitude, which coincides with abrupt changes in borehole temperature, provides certain evidence of pressurised water flow in fractures. This study shows for the first time that, in addition to slow thermal heat conduction, permafrost rocks are subject to sudden push-like warming events, favoring accelerated bottom-up permafrost degradation, and contributing to the build-up of hydrostatic pressures potentially critical for rock slope stability. [ABSTRACT FROM AUTHOR]

Massive sediment pulses in catchments are a key alpine multi-risk component. Substantial sediment redistribution in alpine catchments frequently causes flooding, river erosion, and landsliding and affects infrastructure such as dam reservoirs as well as aquatic ecosystems and water quality. While systematic rock slope failure inventories have been collected in several countries, the subsequent cascading sediment redistribution is virtually unaccessed. For the first time, this contribution reports the massive sediment redistribution triggered by the multi-stage failure of more than 130 000 m3 from the Hochvogel dolomite peak during the summer of 2016. We applied change detection techniques to seven 3D-coregistered high-resolution true orthophotos and digital surface models (DSMs) obtained through digital aerial photogrammetry later optimized for precise volume calculation in steep terrain. The analysis of seismic information from surrounding stations revealed the temporal evolution of the cliff fall. We identified the proportional contribution of > 600 rockfall events (> 1 m3) from four rock slope catchments with different slope aspects and their volume estimates. In a sediment cascade approach, we evaluated erosion, transport, and deposition from the rock face to the upper channelized erosive debris flow channel, then to the widened dispersive debris flow channel, and finally to the outlet into the braided sediment-supercharged Jochbach river. We observe the decadal flux of more than 400 000 m3 of sediment, characterized by massive sediment waves that (i) exhibit reaction times of 0–4 years in response to a cliff fall sediment input and relaxation times beyond 10 years. The sediment waves (ii) manifest with faster response times of 0–2 years in the upper catchment and over 2 years in the lower catchments. The entire catchment (iii) undergoes a rapid shift from sedimentary (10 2 –10 3 mma-1) to massive erosive regimes (10 2 mma-1) within single years, and the massive sediment redistribution (iv) shows limited dependency on rainfall frequency and intensity. This study provides generic information on spatial and temporal patterns of massive sediment pulses in highly sediment-charged alpine catchments. [ABSTRACT FROM AUTHOR]