Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T < −1 • C) at high elevations near the surface, but warm (T > −1 • C) at lower elevations or deeper in the snowpack. The intake of warm snow is believed to be of major importance to increase the temperature of the snow composition in the avalanche and eventually cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete when the deposit at runout is characterized only by warm snow or as partial if there is a warm flow regime, but the farthest deposit is characterized by cold snow. We introduce a transition index F t , based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude H s , which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that F t is related to the snow cover properties, i.e. approximately proportional to H s . Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find that, if H s is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great impor-tance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

The warming of rock permafrost affects mechanical stability and hydro-cryostatic pressures in rock walls. The coincident decrease in slope stability frequently affects infrastructure by creep and subsidence and promotes the generation of rockfalls and rockslides. The increasing hazard posed by warming permafrost rock walls and the growing exposure of infrastructure and individuals create a demand for quantitative monitoring methods. Laboratory-calibrated electrical resistivity tomography provides a sensitive record for frozen versus unfrozen bedrock, presumably being the most accurate quantitative monitoring technique in permafrost areas where boreholes are not available. The data presented here are obtained at the permafrost-affected and unstable Steintaelli Ridge at 3100 m a.s.l. and allow the quantification of permafrost changes in the longest electrical resistivity tomography time series in steep bedrock. Five parallel transects across the rock ridge have been measured five times each, between 2006 and 2019, with similar hardware. Field measurements were calibrated using temperature-resistivity laboratory measurements of water-saturated rock samples from the site. A 3D time-lapse inversion scheme is applied in the boundless electrical resistivity tomography (BERT) software for the inversion of the data. To assess the initial data quality, we compare the effect of data filtering and the robustness of final results with three different filters and two time-lapse models. We quantify the volumetric permafrost distribution in the bedrock and its degradation in the last decades. Our data show mean monthly air temperatures to increase from −3.4°C to −2.6°C between 2005-2009 and 2015-2019, respectively, while simultaneously permafrost volume degraded on average from 6790 m 3 (±640 m 3 rock in phase-transition range) in 2006 to 3880 m 3 (±1000 m 3 ) in 2019. For the first time, we provide a quantita-

Abstract. Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T < −1 °C) at high elevations near the surface, but warm (T > −1 °C) at lower elevations or deeper in the snow pack. The intake of thermal energy in the form of warm snow is believed to cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete, when the deposit at runout is characterized only by warm snow, or as partial, if there is a warm flow regime but the furthest deposit is characterized by cold snow. We introduce a transition factor Ft, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude Hs, which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that Ft is related to the snow cover properties, i.e. approximately proportional to Hs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find, that, if Hs is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

&lt;p&gt;One of the most important but still unknown destabilizing factors of rock faces in periglacial environments is the contribution of water in terms of hydrostatic pressure (e.g. Piz Cengalo in 2017). Its presence has often been registered in major rock failures, but it has never been quantified. Perched water table &gt;&gt;20m above virtually impermeable permafrost bedrock can cause excessive hydrostatic stress on affected rockwalls. Climate change related intensification of rainstorms as well as permafrost degradation promote water accumulation. An increase in rockfall activity due to higher water pressure peaks is therefore expected, thus intensifying the risk for humans and infrastructures.&lt;/p&gt;&lt;p&gt;Here we conduct a hydromechanical stability analysis at two study sites in the Northern Calcareous Alps where this effect has been observed. We use the distinct element method developed in the software UDEC (Itasca); the required geometric and mechanical model input parameters were obtained from previous studies with direct investigations and laboratory tests in frozen/unfrozen conditions. Infiltration from rainfall or snow/ice melting is expected to create extreme pressure peaks, especially when permafrost seals fractured rock.&lt;/p&gt;&lt;p&gt;Here we present results from:&lt;/p&gt;&lt;ol&gt;&lt;li&gt;the permafrost affected Zugspitze summit (Wetterstein Range), where sealing permafrost allows the meltwater to accumulate in the active layer. This causes high hydrostatic pressure, evaluated by relative gravimetry methods and with the help of a fracture mapping.&lt;/li&gt; &lt;li&gt;a preparing high-magnitude rock fall at the Hochvogel (Allg&amp;#228;u Alps), where perched water could destabilize up to 260&amp;#8217;000 m&amp;#179;. Displacement measurements on the summit showed acceleration following intense precipitation.&lt;/li&gt; &lt;/ol&gt;&lt;p&gt;Our model proves that a column of water can bring the Zugspitze north face to instable equilibrium. This happens with different intensities according to frozen/unfrozen conditions and various depth of the active layer, if the hydrostatic pressure is adequate (0.2-0.4 MPa = 20-40 m water column).&lt;/p&gt;&lt;p&gt;Water could also increase the destabilization rates of the south-east face of Hochvogel by adding hydrostatic pressure. A Factor of Safety &lt; 1 is reached when other water-related factors are considered, like: (i) reduction of cohesion in saturated joints, (ii) decrease of the interface friction angle in fractures and (iii) accelerates weathering along the shear plane&lt;/p&gt;

&lt;p&gt;The warming of alpine bedrock permafrost in the last three decades and consequent reduction of frozen areas has been well documented. Its consequences like slope stability reduction put humans and infrastructures at high risk. 2020 in particular was the warmest year on record at 3000m a.s.l. embedded in the warmest decade.&lt;/p&gt;&lt;p&gt;Recently, the development of electrical resistivity tomography (ERT) as standard technique for quantitative permafrost investigation allows extended monitoring of this hazard even allowing including quantitative 4D monitoring strategies (Scandroglio et al., in review). Nevertheless thermo-hydro-mechanical dynamics of steep bedrock slopes cannot be totally explained by a single measurement technique and therefore multi-approach setups are necessary in the field to record external forcing and improve the deciphering of internal responses.&lt;/p&gt;&lt;p&gt;The Zugspitze Kammstollen is a 850m long tunnel located between 2660 and 2780m a.s.l., a few decameters under the mountain ridge. First ERT monitoring was conducted in 2007 (Krautblatter et al., 2010)&amp;#160;and has been followed by more than one decade of intensive field work. This has led to the collection of a unique multi-approach data set of still unpublished data. Continuous logging of environmental parameters such as rock/air temperatures and water infiltration through joints as well as a dedicated thermal model (Schr&amp;#246;der and Krautblatter, in review)&amp;#160;provide important additional knowledge on bedrock internal dynamics. Summer ERT and seismic refraction tomography surveys with manual and automated joints&amp;#8217; displacement measurements on the ridge offer information on external controls, complemented by three weather stations and a 44m long borehole within 1km from the tunnel.&lt;/p&gt;&lt;p&gt;Year-round access to the area enables uninterrupted monitoring and maintenance of instruments for reliable data collection. &amp;#8220;Precisely controlled natural conditions&amp;#8221;, restricted access for researchers only and logistical support by Environmental Research Station Schneefernerhaus, make this tunnel particularly attractive for developing benchmark experiments. Some examples are the design of induced polarization monitoring, the analysis of tunnel spring water for isotopes investigation, and the multi-annual mass monitoring by means of relative gravimetry.&lt;/p&gt;&lt;p&gt;Here, we present the recently modernized layout of the outdoor laboratory with the latest monitoring results, opening a discussion on further possible approaches of this extensive multi-approach data set, aiming at understanding not only permafrost thermal evolution but also the connected thermo-hydro-mechanical processes.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Krautblatter, M. et al.&amp;#160;(2010) &amp;#8216;Temperature-calibrated imaging of seasonal changes in permafrost rock walls by quantitative electrical resistivity tomography (Zugspitze, German/Austrian Alps)&amp;#8217;, Journal of Geophysical Research: Earth Surface, 115(2), pp. 1&amp;#8211;15. doi: 10.1029/2008JF001209.&lt;/p&gt;&lt;p&gt;Scandroglio, R. et al.&amp;#160;(in review) &amp;#8216;4D-Quantification of alpine permafrost degradation in steep rock walls using a laboratory-calibrated ERT approach (in review)&amp;#8217;, Near Surface Geophysics.&lt;/p&gt;&lt;p&gt;Schr&amp;#246;der, T. and Krautblatter, M. (in review) &amp;#8216;A high-resolution multi-phase thermo-geophysical model to verify long-term electrical resistivity tomography monitoring in alpine permafrost rock walls (Zugspitze, German/Austrian Alps) (submitted)&amp;#8217;, Earth Surface Processes and Landforms.&lt;/p&gt;