Influences of Snow Cover on Thermal and Mechanical Processes in steep Permafrost Rock Walls

Degradation of rock permafrost can cause instability due to influences on rock- and ice-mechanical properties. Permafrost conditions can be altered by thermal processes and, thus, also mechanical properties of rocks. Snow cover controls the seasonal occurrence of thermal processes. A conceptual approach is presented to explain snow cover influences on steep permafrost rock walls. This approach combines snow cover with thermal and mechanical processes. To support the conceptual approach, empirical data is presented to evaluate snow cover, the thermal and the mechanical regime. A combination of temperature data loggers, photos of automatic cameras and avalanche probe measurements allows the reconstruction of the temporal and spatial development of snow cover. Four snow stages can be distinguished and an overall cooling effect derived. In laboratory measurements, p-wave velocities of 22 different alpine rocks are tested and the influence of ice pressure on seismic velocities is evaluated. P-wave velocity increases dependent on lithology due to freezing and increase is dominated by an increase of the velocity of the rock matrix due to ice pressure. These findings are incorporated into a novel time-average equation and provide the basis for the applicability of refraction seismics in permafrost rock walls. The influence of snow cover on the thermal regime was investigated with the use of Seismic Refraction Tomography (SRT), Electrical Resistivity Tomography (ERT) and thermal modelling. Long lasting snow cover in 2013 delayed heat transport processes by insulating the underground and prevented active-layer thaw while snow cover absence resulted in deep thawing in 2012. Thus, snow cover plays a key role of permafrost evolution on slope facet scale. Snow cover is the main controlling factor of discontinuity movement and rock decay. The snow cover controls the occurrence of thermal expansion/contraction and volumetric expansion as it prevents these processes, while favouring ice segregation due to isolation. Volumetric expansion increases short-term cryostatic pressure, whereas ice segregation leads to seasonal cryostatic pressure. Active-layer thaw decreases shear strengths during summer and increases instability seasonally. The conceptual approach explains rock stability on seasonal and system scale. Therefore, this study delivers the basis in the understanding of stability of permafrost rock walls.Einflüsse der Schneedecke auf thermale und mechanische Prozesse in steilen Permafrost-Felswänden Die Degradation von Fels-Permafrost kann durch den Einfluss auf fels- und eismechanische Eigenschaften Felsinstabilität verursachen. Thermale Prozesse können die Permafrost-Bedingungen verändern und dadurch auch die mechanischen Eigenschaften von Felsen. Die Schneedecke kontrolliert das saisonale Auftreten dieser thermalen Prozesse. Ein konzeptioneller Ansatz wird vorgestellt, um den Einfluss der Schneedecke auf steile Permafrost-Felswände zu erklären. Dieser Ansatz kombiniert die Schneedecke mit thermalen sowie mechanischen Prozessen. Um den konzeptionellen Ansatz zu belegen, werden empirische Daten zur Schneebedeckung, zum thermalen und mechanischen Regime ausgewertet. Die Kombination von Temperaturdatenloggern, automatischen Kamerafotos sowie Lawinensonden-Messungen ermöglicht die Rekonstruktion der zeitlichen und räumlichen Schneedeckenentwicklung. Vier Schneephasen können unterschieden werden und ein überwiegend kühlender Effekt abgeleitet werden. In Labormessungen wurden P-Wellengeschwindigkeiten an 22 alpinen Felsproben getestet und der Einfluss des Eisdrucks auf seismische Geschwindigkeiten evaluiert. P-Wellengeschwindigkeiten steigen in Abhängigkeit der Lithologie durch Gefrieren und dieser Anstieg wird dominiert durch einen vom Eisdruck verursachten Anstieg der Felsmatrixgeschwindigkeit. Diee Erkenntnisse sind in eine neue Durchschnittszeit-Gleichung eingeflossen und stellen die Basis für die Anwendung der Refraktionsseismik in Permafrost-Felswänden dar. Der Einfluss der Schneedecke auf das thermale Regime wurde mit Hilfe von Refraktionsseismik-Tomographie (SRT), elektrischer Widerstandstomographie (ERT) und thermaler Modellierung untersucht. Die lang anhaltende Schneebedeckung in 2013 verzögerte Wärmetransportprozesse durch Isolierung des Untergrundes und verhinderte das Auftauen der Auftauschicht während die Abwesenheit der Schneedecke 2012 zu tiefgründigem Auftauen führte. Die Schneedecke spielt folglich eine Schlüsselrolle in der Entwicklung des Permafrostes auf der Hang-Fazies-Skale. Die Schneedecke ist der Hauptkontrollfaktor von Trennflächenbewegungen und Felszersatz und kontrolliert das Auftreten thermaler Expansion/Kontraktion sowie Volumenexpansion. Sie verhindert diese Prozesse, während die Eissegregation durch die Isolierungswirkung des Schnees begünstigt wird. Volumenexpansion führt zum kurzfristigen Anstieg kryostatischen Drucks, wohingegen die Eissegregation zu einem saisonalem kryostatischen Druckanstieg führt. Das Auftauen der Auftauschicht verringert die Scherfestigkeiten im Sommer und verursacht saisonal Instabilität. Der konzeptionelle Ansatz erklärt Felsstabilität auf saisonaler wie auch Systemskale. Diese Studie liefert damit die Grundlage zum Verständnis der Felsstabilität in Permafrost-Felswänden

Topographic and geologic controls on frost cracking in Alpine rockwalls

Frost weathering is a major control on rockwall erosion in Alpine environments. Previous frost cracking model approaches used air temperatures as a proxy for rock temperatures to drive frost weathering simulations on rockwall and on mountain scale. Unfortunately, the thermal rockwall regime differs from air temperature due to topographic effects on insolation and insulation, which affects frost weathering model results and the predicted erosion patterns. To provide a more realistic model of the rockwall regime, we installed six temperature loggers along an altitudinal gradient in the Swiss Alps, including two logger pairs at rockwalls with opposing aspects. We used the recorded rock surface temperatures to model rock temperatures in the upper 10 m of the rockwalls and as input data to run four different frost cracking models. We mapped fracture spacing and rock strength to validate the model results. Our results showed that frost cracking models are sensitive to thermal, hydraulic and mechanical parameters that affect frost cracking magnitude but frost cracking patterns in terms of peak location and affected rock mass remained consistent between varying input parameters. Thermo‐mechanical models incorporate rock strength and hydraulic properties and provided a frost cracking pattern at the rockwall scale that better reflects the measured fracture spacing. At the mountain scale, these models showed a pattern of increasing frost cracking with altitude, which is contrary to purely thermal models but consistent with observations of existing rockfall studies.Plain Language Summary: Frost weathering is an important mechanism in shaping rockwalls in Alpine environments. Previous studies developed either purely thermal or thermo‐mechanical models incorporating mechanical and hydraulic parameters to simulate this process. Both model types provide valuable insights about a process that is hard to measure. Previous model approaches used air temperature as input data. However, rock temperatures differ from air temperatures due to topography that changes the insolated surface of rockwalls and insulating snow cover. We measured rock temperatures directly at six rockwalls with different aspects along a large range of altitude. We used our data to run four existing frost weathering models. Our results show that rock type, the strength of rocks and water availability influence the frost weathering magnitude, but the location of cracking and the rockwall depth affected does not change. The frost cracking pattern should be reflected by the fracture network and the strength of rockwalls. We mapped fractures and measured rock strength and our results correspond better to thermo‐mechanical model results. Thermo‐mechanical model results show an increase in frost weathering with increasing altitude. This pattern is consistent with rockfall observations. In contrast, purely thermal models showed an inverse relationship with higher frost cracking at lower altitudes.Key Points: Temperature loggers provide rock temperature data that incorporates topographic effects on insolation and insulation. Sensitivity tests on frost cracking models showed differences of frost magnitude while frost cracking depth patterns were consistent. Thermo‐mechanical models incorporating rock strength and hydraulic properties produced more realistic altitudinal frost cracking patterns.Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/50110000165

P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model

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

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

Geology and vegetation control landsliding on forest-managed slopes in scarplands

Landslides are important agents of sediment transport, cause hazards and are key agents for the evolution of scarplands. Scarplands are characterized by high-strength layers overlying low-inclined landslide-susceptible layers that precondition and prepare landsliding on geological timescales. These landslides can be reactivated, and their role in past hillslope evolution affected geomorphometry and material properties that set the framework for present-day shallow landslide activity. To manage present-day landslide hazards in scarplands, a combined assessment of deep-seated and shallow landsliding is required to quantify the interaction between geological conditions and vegetation that controls landslide activity. For this purpose, we investigated three hillslopes affected by landsliding in the Franconian scarplands. We used geomorphic mapping to identify landforms indicating landslide activity, electrical resistivity to identify shear plane location and a mechanical stability model to assess the stability of deep-seated landslides. Furthermore, we mapped tree distribution and quantified root area ratio and root tensile strength to assess the influence of vegetation on shallow landsliding. Our results show that deep-seated landslides incorporate rotational and translational movement and suggest that sliding occurs along a geologic boundary between permeable Rhätolias sandstone and impermeable Feuerletten clays. Despite low hillslope angles, landslides could be reactivated when high pore pressures develop along low-permeability layers. In contrast, shallow landsliding is controlled by vegetation. Our results show that rooted area is more important than species-dependent root tensile strength and basal root cohesion is limited to the upper 0.5 m of the surface due to geologically controlled unfavourable soil conditions. Due to low slope inclination, root cohesion can stabilize landslide toes or slopes undercut by forest roads, independent of potential soil cohesion, when tree density is sufficient dense to provide lateral root cohesion. In summary, geology preconditions and prepares deep-seated landslides in scarplands, which sets the framework of vegetation-controlled shallow landslide activity

Alpine rockwall erosion patterns follow elevation-dependent climate trajectories

Mountainous topography reflects an interplay between tectonic uplift, crustal strength, and climate-conditioned erosion cycles. During glaciations, glacial erosion increases bedrock relief, whereas during interglacials relief is lowered by rockwall erosion. Here, we show that paraglacial, frost cracking and permafrost processes jointly drive postglacial rockwall erosion in our research area. Field observations and modelling experiments demonstrate that all three processes are strongly conditioned by elevation. Our findings on catchment scale provide a potential multi-process explanation for the increase of rockwall erosion rates with elevation across the European Alps. As alpine basins warm during deglaciation, changing intensities and elevation-dependent interactions between periglacial and paraglacial processes result in elevational shifts in rockwall erosion patterns. Future climate warming will shift the intensity and elevation distribution of these processes, resulting in overall lower erosion rates across the Alps, but with more intensified erosion at the highest topography most sensitive to climate change