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Rock slope failures often result from progressive rock mass damage which accumulates over long timescales. In deglaciating environments, rock slopes are affected by stress perturbations driven by mechanical unloading due to ice downwasting and concurrent changes in thermal and hydraulic boundary conditions. Since in‐situ data are rare, the different processes and their relative contribution to slope damage remain poorly understood. Here, we analyze borehole monitoring data from a rock slope adjacent to the retreating Great Aletsch Glacier (Switzerland) and compare it to englacial water levels, climate data, and decreasing ice levels. Rock slope pore pressures show a seasonal signal controlled by infiltration events as well as effects from the connectivity to the englacial hydrological system. We find that reversible and irreversible strains are driven by: (a) hydromechanical effects caused by englacial pressure fluctuations and infiltration events, (b) stress transfer related to changing mechanical glacial loads from short‐term englacial water level fluctuations and longer term ice downwasting, and (c) thermomechanical effects from annual temperature cycles penetrating the shallow subsurface, which primarily result in reversible deformation. We relate most observed irreversible strain (damage) to mechanical unloading from ice downwasting. Damage is strongest directly at the ice margin and moves through the slope at the pace of glacial retreat and advance. Locations with many retreat/advance cycles are very sensitive to landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area. Plain Language Summary: The formation of rock slope instabilities is a long‐term process related to continuous rock mass weakening. For rock slopes in glaciated valleys, increased rates of rock mass damage are assumed to occur during ice retreat due to a reduction of the glacier weight and concurrent changes in rock temperatures and slope groundwater pressures. However, because direct field measurements are missing, governing processes and their relative importance are still poorly understood. Here, we analyze borehole monitoring data and compare it to englacial water levels, climate data, and decreasing ice levels. We find that the glacial hydrology influences the dynamics of the groundwater pressures in the adjacent rock slope at various timescales. We record micrometer‐scale reversible (elastic) deformation and irreversible rock mass damage and identify the involved drivers. Several processes contribute to progressive rock mass weakening at different timescales and magnitudes, but most damage is related to mechanical unloading from ice downwasting and is strongest directly at the ice margin. Locations with many retreat/advance cycles are sensitive to increased damage accumulation and landslide formation. The current climate warming impacts very sensitive valley sectors, which is confirmed by landslide distributions and activity in the study area. Key Points: We monitored subsurface pore pressures and micrometer scale strain in a rock slope during strong glacial retreat caused by climate warmingWe identified thermo‐hydro‐mechanical drivers for reversible deformation and irreversible rock mass damage acting at various timescalesSlope damage intensity can be related to landslide spatial and temporal distribution [ABSTRACT FROM AUTHOR]

In deglaciating environments, rock mass weakening and potential formation of rock slope instabilities is driven by long-term and seasonal changes in thermal- and hydraulic-boundary conditions, combined with unloading due to ice melting. However, in-situ observations are rare. In this study, we present new monitoring data from three highly instrumented boreholes, and numerical simulations to investigate rock slope temperature evolution and micrometer-scale deformation during deglaciation. Our results show that the subsurface temperatures are adjusting to a new, warmer surface temperature following ice retreat. Heat conduction is identified as the dominant heat transfer process at sites with intact rock. Observed non-conductive processes are related to groundwater exchange with cold subglacial water, snowmelt infiltration, or creek water infiltration. Our strain data shows that annual surface temperature cycles cause thermoelastic deformation that dominate the strain signals in the shallow thermally active layer at our stable rock slope locations. At deeper sensors, reversible strain signals correlating with pore pressure fluctuations dominate. Irreversible deformation, which we relate with progressive rock mass damage, occurs as short-term (hours to weeks) strain events and as slower, continuous strain trends. The majority of the short-term irreversible strain events coincides with precipitation events or pore pressure changes. Longer-term trends in the strain time series and a minority of short-term strain events cannot directly be related to any of the investigated drivers. We propose that the observed increased damage accumulation close to the glacier margin can significantly contribute to the long-term formation of paraglacial rock slope instabilities during multiple glacial cycles. [ABSTRACT FROM AUTHOR]

We present a new inventory of large rock slope instabilities and an extensive structural data set for northwestern Bhutan. Our data set is largely based on satellite observations, such as optical images and high‐resolution digital surface models, complemented with field observations. Kinematic analyses over seven different structural domains recognized in the study region were carried out to investigate structural control over the landslide distribution at a regional scale. In order to account for the sampling heterogeneity in the different data sets, a weighting system to the structural data is proposed. The results show that structural control exists in five out of seven domains, with foliation strongly influencing the sliding mechanisms. A lithological control appears also evident, with sedimentary rocks showing the highest landslide densities in the region. This study demonstrates how methodologies built on remote sensing data are suitable to investigate landslide predisposition at regional scales especially in largely inaccessible areas. Key Points: Rock slope instabilities and structural inventories for NW Bhutan based on optical images and high‐resolution DSMWeighting system applied to the data set improves its reliability and the assessment of limitationsLarge‐scale kinematic analyses for rock slope instabilities reveal structural control over a large part of the study region [ABSTRACT FROM AUTHOR]

Subglacial water pressures influence groundwater conditions in proximal alpine valley rock slopes, varying with glacier advance and retreat in parallel with changing ice thickness. Fluctuating groundwater pressures in turn increase or reduce effective joint normal stresses, affecting the yield strength of discontinuities. Here we extend simplified assumptions of glacial debuttressing to investigate how glacier loading cycles together with changing groundwater pressures generate rock slope damage and prepare future slope instabilities. Using hydromechanical coupled numerical models closely based on the Aletsch Glacier valley in Switzerland, we simulate Late Pleistocene and Holocene glacier loading cycles including long‐term and annual groundwater fluctuations. Measurements of transient subglacial water pressures from ice boreholes in the Aletsch Glacier ablation area, as well as continuous monitoring of bedrock deformation from permanent Global Navigation Satellite Systems stations, help verify our model assumptions. While purely mechanical glacier loading cycles create only limited rock slope damage in our models, introducing a fluctuating groundwater table generates substantial new fracturing. Superposed annual groundwater cycles increase predicted damage. The cumulative effects are capable of destabilizing the eastern valley flank of our model in toppling‐mode failure, similar to field observations of active landslide geometry and kinematics. We find that hydromechanical fatigue is most effective acting in combination with long‐term loading and unloading of the slope during glacial cycles. Our results demonstrate that hydromechanical stresses associated with glacial cycles are capable of generating substantial rock slope damage and represent a key preparatory factor for paraglacial slope instabilities. Plain Language Summary: Glacial melt produces large amounts of water keeping water pressures at the base of alpine glaciers high. This meltwater in turn influences the groundwater table in adjacent valley flanks. Water seeps into the subsurface through joints and faults reducing the strength of these discontinuities. In a glaciated alpine valley, the position of the glacier changes over time and the groundwater table changes in accordance. Meanwhile, seasonal infiltration from snowmelt raises and lowers the water table in valley flanks, creating measureable movements. These changes in groundwater can alter the strength of a rock mass and together with the changing ice load generate damage in the floors and walls of alpine valleys. This damage helps prepare future slope instabilities, creating fractures that may ultimately define a landslide failure surface. We present new data and models from the Great Aletsch Glacier in Switzerland exploring our hypothesis that groundwater‐glacier coupled interactions are an important mechanism generating damage in alpine valley rock slopes. Our results show that higher water pressures reduce the strength of rock discontinuities and can result in failure, especially as glaciers retreat. This mechanism is an important part of the strength and stress changes in valley slopes that accompany glacial cycles. Key Points: Subglacial water pressures in ablation areas are high and vary with ice thickness, affecting the groundwater table in adjacent valley flanksIncreasing groundwater pressure during glacier advance reduces effective stresses in joints, partly offsetting the increased ice loadHydromechanical coupled stresses during glacial cycles create and propagate fractures in rock slopes, preparing future instabilities [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of a dramatic rock slope acceleration that occurred in fall 2016 at the Moosfluh Landslide, located at the glacier tongue of the Great Aletsch Glacier (Switzerland). The acceleration that occurred in 2016 was unanticipated and exposed the valley bottom and an adjacent damned lake to high risk. This acceleration occurred in an active deep‐seated gravitational slope deformation (DSGSD) controlled primarily by deep block‐flexural toppling. In 2013, a highly accurate displacement monitoring system was developed and installed in the surroundings of the Great Aletsch Glacier, including a time‐lapse camera, GNSS, and robotic total stations. This monitoring system provided unique data during the 2016 slope acceleration which are used in this study to assess failure mechanisms, landslide volumes, and subsurface displacement geometry. Based on a novel displacement vector analysis, we find that three retrogressive secondary rockslides developed during the first six weeks of the slope acceleration, with rupture surface depths of 30 to 40 m, and estimated volumes between 1 and 4 Mm3. These rockslides display complex deformation features, including head and lateral scarps, which developed during the slope acceleration. The kinematics of these secondary rockslides changed through time, from primarily toppling to combined toppling and sliding. Our results provide a uniquely detailed understanding of the spatial and temporal evolution of deformation features and movement kinematics that occur when several sectors of a slowly moving DSGSD transitions into rapid rockslides. Plain Language Summary: The slow moving (~30 cm per year) deep‐seated (up to 170‐m depth) landslide of Moosfluh experienced a substantial acceleration of slope movement in September 2016. A comprehensive monitoring system recorded that several compartments in the center of the Moosfluh Landslide suddenly started to move rapidly with velocities of up to several meters per week. This caused a strong modification of the slope surface morphology and structure which is documented in detail in this study. Based on monitored changes in the directions of slope movement over time, a new method was developed to interpret subsurface movement types and geometries. With this method we found that on top of the Moosfluh deep‐seated landslide several shallow (30–40 m) rockslides developed during the Moosfluh slope acceleration in 2016. We are able to assign depths and inclinations of the failure surfaces and failure mechanisms. In this case the deep‐seated landslide is formed by rotation (toppling) of steep rock columns, whereas the shallow secondary rockslides within the deep‐seated landslide show sliding movements along planar rupture planes and mixtures of both sliding and toppling. Key Points: The rapid formation of 3 secondary rockslides within a toppling mode DSGSD (Moosfluh, CH) is monitored with DIC from webcam images, aerial photogrammetry and total station monitoringThis transition corresponds to an acceleration of the landslide from a few dm/a to 50 m/a in the lowermost sectors of the instabilityA novel method developed to interpret total station reflector data revealed a change in landslide kinematics (toppling, sliding) through time [ABSTRACT FROM AUTHOR]

Cycles of glaciation alter the temperature field in proximal alpine valley flanks, driving rock slope damage through thermomechanical stresses. Here we extend simplified assumptions of glacial debuttressing to quantitatively examine how paraglacial bedrock temperature changes, acting in concert with changing ice loads during Late Pleistocene and Holocene glacial cycles, create damage in adjacent rock slopes and prepare future slope instabilities. When in contact with temperate glacier ice, valley walls maintain near isothermal ~0 °C surface temperatures and are shielded from daily and annual cycles. With retreat, rock walls are rapidly exposed to strongly varying temperature boundary conditions, a transition we term "paraglacial thermal shock." Using detailed, conceptual numerical models based on the Aletsch Glacier in Switzerland, we show that including thermomechanical stresses during simulated glacial cycles creates significantly more rock slope damage than predicted for purely mechanical ice loading and unloading. Glacier advances are especially effective in generating damage as rapid cooling drives contraction of the rock mass reducing joint normal stresses. First time exposure to annual temperature cycles during deglaciation induces a shallow damage front that follows the retreating ice margin, generating damage in a complementary process at shorter time scales. Acting on a reduced strength rock mass, modeled thermomechanical cycles enhance the development of a slope instability with similar attributes as observed in our study area. Our results demonstrate that thermomechanical stresses acting in conjunction with changing ice loads are capable of generating considerable rock slope damage with spatial and temporal patterns controlled by glacier extents. Key Points: Subsurface temperature changes on glacial time scales can generate rock mass damage through thermomechanical coupled stressesGlacier advances are most effective in producing rock slope damage as cooling drives contraction of subglacial bedrockThermal shock during deglaciation creates a shallow damage front as bedrock is first exposed to annual temperature cycles [ABSTRACT FROM AUTHOR]