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In June 2005, a series of major rockfall events completely wiped out the Bonatti Pillar located in the legendary Drus west face (Mont Blanc massif, France). Terrestrial lidar scans of the west face were acquired after this event, but no pre-event point cloud is available. Thus, in order to reconstruct the volume and the shape of the collapsed blocks, a 3-D model has been built using photogrammetry (structure-from-motion (SfM) algorithms) based on 30 pictures collected on the Web. All these pictures were taken between September 2003 and May 2005. We then reconstructed the shape and volume of the fallen compartment by comparing the SfM model with terrestrial lidar data acquired in October 2005 and November 2011. The volume is calculated to 292 680 m3 (±5.6 %). This result is close to the value previously assessed by Ravanel and Deline (2008) for this same rock avalanche (265 000 ± 10 000 m3). The difference between these two estimations can be explained by the rounded shape of the volume determined by photogrammetry, which may lead to a volume overestimation. However it is not excluded that the volume calculated by Ravanel and Deline (2008) is slightly underestimated, the thickness of the blocks having been assessed manually from historical photographs.

Rockfalls and rock avalanches are active processes, including hazards for infrastructures and outdoor activities.Present rockfalls are well surveyed and documented in the Mont Blanc massif thanks to a network of observersset up in 2007, and composed of hut keepers, mountain guides, alpinists and infrastructure workers. Frequencyover the past 150 years of massif rockfall, studied by comparison of historical photographs, has strongly increasedduring the last 30 years, especially during very hot summer (2003 and 2015), likely due to permafrost degradationdriven by the climate change.In order to understand at a longer timescale the relationship between rockfall frequency and climate dynamicsin the Mont Blanc massif, we use Terrestrial Cosmogenic Nuclide (TCN) dating to obtain the exposure ages ofLateglacial and Holocene rockfall scars and old rockwall surfaces, and glacial and climate proxies to verify thehypothesis that rockfalls were more frequent in warm periods.55 samples have been collected at 9 sites of the Glacier du Géant basin, at elevation in the range 3300-3800 m a.s.l.These new exposure ages were completed by the 25 others TCN ages sampled during two previous campaigns andrecalculated using the newest input parameters. A total of 72 ages were obtained, between 0.04 ± 0.02 and 100.50± 8.50 ka.We found four age clusters. Two clusters are related to the Holocene Warm Period (∼ 6.1-7.4 ka) and RomanWarm Period (∼ 1.6-2.3 ka); a cluster of LIA-post-LIA ages is mainly composed by smaller rockfalls, consideredas the ‘normal’ erosion. A forth cluster have been recognized at ∼ 4.2-5.0 ka.Data suggest a relationship between the reflectance spectral data of the scar surfaces and exposure ages, mainlyin the E-SE-S aspects. Fresh rock surfaces of recent rockfall scars are light grey, whereas long-time exposedweathered rock surfaces range from light orange to dark red. This confirms the initial hypothesis: the redder a rocksurface, the older its age. Reflectance spectroscopy is used to quantify the granite surface colour and allow theestablishment of a robust relationship between scar exposure ages and colour of the rockwalls in the Mont Blancmassif.

Poster; In response to the overcrowding of some tourist sites, as a result of the worldwide growth of this economicsector, innovative strategies emerged around the world. Geotourism offers new destinations and among themdinosaurian geosites hold an important place.The birth of dinosaurian geotourism, can be dated from 1957, the year of the inauguration of the Carnegie Quarry,Utah, the first site museum devoted to this theme. Since then, dinosaur deposits, "high places" of paleontologicaldiscoveries have evolved into geotourism destinations contributing to local economic development based on theconstruction of specific territorial resources through the patrimonialization of these exceptionnals geosites.Three types of deposits characterize dinosaurian geosites:The cemeteries, composed by numerous fossilized bones, allow the paleontologists’ work to be staged, by theselective removal of the most beautiful elements. For example, the Ischigualasto site in Argentina, inscribed onthe UNESCO World Heritage List in 2000, has just inaugurated the William Sill site museum.Nesting areas, where dinosaur nests and/or eggs can be found. Thus, the Dinosaur Egg Geological Museum,established in 2016 on the Qinglong Mountain National Geological Park site, is one of China’s 10 most beautifulmuseums.Footprints and dinosaur tracks left in suitable environments, numerous all over the world and whose interpretationmakes it possible to describe certain behaviors of these disappeared animals. For example, the Cretacico Park,which highlights the tracks discovered in 2002 in Cal Orcko, Bolivia, has become the most visited tourist site inthe country with 170,000 visitors a year.The geotouristic valorisation of these sites, which is based on strategies enhanced at different territorial scales, hasbeen in full expansion for some twenty years.This presentation analyzes how was build the attractiveness of these new tourist destinations by comparing thegraphs of networks of actors involved. As part of the field of digital humanities, the corpus of data relating tothe networks that actors weave have been collected on the web via the links they create between themselvesand with others. The analysis shows that geotourism clusters have been developed creating business ecosystemswhose spontaneous or constructed origin may explain the greater or lesser success and / or sustainability of thesedestinations.Tourism globalization must meet the preservation of these exceptional geoheritage in order to develop sustainabletourism practices.

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International audience

Rockfalls strongly influence the evolution of steep rocky landscapes and represent a significant hazard in mountainous areas. Defining the most probable future rockfall source areas is of primary importance for both geomorphological investigations and hazard assessment. Thus, a need exists to understand which areas of a steep cliff are more likely to be affected by a rockfall. An important analytical gap exists between regional rockfall susceptibility studies and block-specific geomechanical calculations. Here we present methods for quantifying rockfall susceptibility at the cliff scale, which is suitable for sub-regional hazard assessment (hundreds to thousands of square meters). Our methods use three-dimensional point clouds acquired by terrestrial laser scanning to quantify the fracture patterns and compute failure mechanisms for planar, wedge, and toppling failures on vertical and overhanging rock walls. As a part of this work, we developed a rockfall susceptibility index for each type of failure mechanism according to the interaction between the discontinuities and the local cliff orientation. The susceptibility for slope parallel exfoliation-type failures, which are generally hard to identify, is partly captured by planar and toppling susceptibility indexes. We tested the methods for detecting the most susceptible rockfall source areas on two famously steep landscapes, Yosemite Valley (California, USA) and the Drus in the Mont-Blanc massif (France). Our rockfall susceptibility models show good correspondence with active rockfall sources. The methods offer new tools for investigating rockfall hazard and improving our understanding of rockfall processes.

Rockfalls and rock avalanches are a recurrent process in high mountain areas like the Mont Blanc massif. These processes are surveyed due to the hazard they present for infrastructure and alpinists. While rockfalls and rock avalanches have been documented for the last 150 years, we know very little about their frequency since the Last Glacial Maximum (LGM). In order to improve our understanding, it is imperative to date them on a longer timescale. A pilot campaign using Terrestrial Cosmogenic Nuclide (TCN) dating of five samples was carried out in 2006 at the Aiguille du Midi (3842 m a.s.l.). In 2011, a larger scale study (20 samples) was carried out in five other test sites in the Mont Blanc massif. This paper presents the exposure ages of the 2011 TCN study as well as the updated exposure ages of the 2006 study using newer TCN dating parameters. Most of these exposure ages lie within the Holocene but three ages are Pleistocene (59.87 ± 6.10 ka for the oldest). A comparison of these ages with air temperature and glacier cover proxies explored the possible relationship between the most active rockfall periods and the warmest periods of the Holocene: two clusters of exposure ages have been detected, corresponding to the Middle Holocene (8.2–4.2 ka) and the Roman Warm Period (c. 2 ka) climate periods. Some recent rockfalls have also been dated (

International audience

Sedimentary archives are used to infer past climate and environment thanks to their physical and chemical properties. Sampling methods (millimetric or centimetric) and routine analysis are time and material consuming. The use of some specific spectroscopic methods and data analysis, allow to develop and perform some robust methods capable of (i) fast high resolution (ii) performed at low costs (iii) non-destructive and (iv) monitor concentration variations of major sediment compounds. X-ray fluorescence spectroscopy is one of these techniques, but it is able to detect just mineral geochemistrys. Whereas hyperspectral imaging (VNIR 400-1000 nm, SWIR 1000-2500 nm) allow, behind each voxel (a pixel with several wavelengths), to define spectral fingerprint of organic or mineral chemical compounds. This type of data can be analyzed by statistical techniques. Many (pseudo-) univariate coefficients are available for the quantification of some molecules (RABD845 for BPhe a, RABD660-670 for chlr-a and chlorins). But in this study we choose to applied multivariate methods that take into account all spectra variations. To achieve such study we can use technic that usually applied in classical spectroscopy or for satellite data that can be unsupervised or supervised. For unsupervised techniques, without any prior knowledge of the sample, exploratory algorithms are used to determine groups in the data. Then, these groups can be interpreted with the comparison to other analytical methods. It is possible to find pure signal that corresponds to one or several organic or mineral sedimentary compounds by (i) endmembers techniques, (ii) spectral unmixing, or (iii) clustering. For supervised techniques, we can use the knowledge of sample chemical and physical properties to create prediction models, then it is possible to observe variations of a specific property along the core. To develop qualitative and quantitative model for focused spectral properties we can applied classification and regression techniques. They allow to discriminate spectral domains or some wavelengths for some interest property. In the present study, Lake Le Bourget (Savoie, France) and Black Sea (Northwest margin) sediment cores are used. From this two different environmental systems we could create and test several prediction models. The high-resolution acquisition is done with two hyperspectral cameras: VNIR (400-1000 nm) and SWIR (1000-2500 nm) with spatial resolution of several dozen micrometers. Both sensors are well designed to create predictive models for either physical or chemical properties. In order to improve prediction models and make them more robust, we can pair these two cameras and add XRF core scanner data. For the black sea sediment, we use unsupervised techniques to determine groups and define interesting spatial areas to take samples for analytic analysis. Whereas for the Lake le Bourget sediment, several previous studies allow us to have many available data, thus supervised techniques are used to observe along core variations. For some properties, we could try to use models of the Lake Le Bourget in the Black Sea data, for example if we create a grain-size model, chemical elements ratio or organic compounds.

International audience

Poster

International audience

International audience

Glacial shrinkage and the gravitational processes associated with it, which are characteristic of global warming effects in high mountain environments, are affecting mountaineering routes more and more, including access routes to high altitude refuges. These changes have almost never been studied. Also, this research seeks to identify and explain the evolution of access trails to five refuges, located in the Mer de Glace basin (Mont Blanc massif), over more than a century. This glacier is the largest in France (L = 11.5 km, SA = 30 km²) is a major Alpine tourist attraction since 1741 and is the birthplace of mountaineering. This work is based on a three-step methodology: 1) collection and analysis of maps, climbing guidebooks and photographs, 2) semi-structured interviews and 3) analysis of high-resolution digital terrain models obtained through terrestrial laser scanning.While there was not a significant evolution during most of the 20th century, glacier thickness loss and the associated erosion of lateral moraines have resulted in numerous modifications made to the access trails since the 1990s. Despite these adaptations, the danger they pose continues to rise, and the necessity to equip them is ever more important (633 m. of ladders at present), which raises doubts about the future accessibility of such high mountain refuges., Le retrait glaciaire et les processus gravitaires associés, caractéristiques de l’évolution des milieux de haute montagne induite par le réchauffement climatique, affectent de plus en plus fortement les itinéraires d’alpinisme dont les sentiers d’accès aux refuges de haute altitude. Alors que ces changements n’ont quasiment fait l’objet d’aucune étude, cette recherche tente de caractériser et d’expliquer l’évolution sur un peu plus d’un siècle des accès aux cinq refuges situés autour de la Mer de Glace (massif du Mont Blanc), le plus grand glacier français (L = 11,5 km, S = 30 km2), haut lieu du tourisme alpin depuis 1741 et berceau de l’alpinisme. Ce travail repose sur une méthodologie en 3 étapes : 1) récolte et analyse de cartes, de topo-guides et de photographies, 2) conduite d’entretiens semi-directifs et 3) analyse de modèles numériques de terrain à haute résolution acquis par balayage laser terrestre. Si l’essentiel du xxe siècle ne présente pas d’évolution marquée, la perte d’épaisseur du glacier et la dégradation associée des moraines latérales ont donné lieu à de nombreuses modifications des itinéraires à partir des années 1990. Malgré ces adaptations, leur dangerosité s’accroit et la nécessité de les équiper est sans cesse plus importante (633 m d’échelles actuellement), remettant en question l’accessibilité future des refuges.

Poster; International audience; Snow bridges that form over the crevasses of the Alpine glaciers allow mountaineers and skiers to cross them easily but constitute an important danger in case of rupture. Between 2008 and 2014, 37 injured persons and 13 deaths related to falls into crevasse were recorded (i.e. an average of two deaths per year) on the glaciers of the French side of the Mont Blanc massif - out of the famous Vallée Blanche ski route, which however embodies an important part of the aid related to falls into crevasses. To understand the construction and evolution of these fragile structures, instrumentation was set up on the Glacier du Géant, at 3450 m a.s.l., near the Aiguille du Midi (3842 m a.s.l.), on the French side of the Mont Blanc massif, close to a crevasse whose bridge had recently collapsed over a length of 37 m. The maximum width of the crevasse in this area is 6 m. At the top of a 7-m-high pole - to prevent future snowfalls -, sensors have been installed in September 2016 to measure different snow and weather parameters: air temperature, wind speed and direction, snow height. An automatic camera surveys the crevasse and the snow bridge geometry. Several other sensors monitor the temperature of snow and air in the crevasse. In addition, an extensometer was installed into the crevasse to measure the evolution of its width. The results of the first 6 months of survey are presented, including the formation of the bridge in mid-November, during a period of snowfall associated with a strong wind. Although the instrumentation is well suited to the high mountain conditions, its maintenance is delicate due to the strong instability of the environment (glacier movements and extreme weather conditions, primarily) but the results of this work will bring new glaciological knowledges which should participate in a better safety on glaciers.

Poster; Thermal state of steep permafrost-affected rock faces is crucial to assess the safety and reliability of mountain infrastructure as current permafrost degradation affects the rock slope stability. In the Mont-Blanc massif, 23 infrastructures are built on such a rock face with 13 of them that are characterized by a high risk of destabilization (Duvillard et al., 2015), including the upper station of the Grands Montets cable car (3325 m a.s.l.) as well as the Cosmiques hut (3613 m a.s.l.) on which we will focus. These two buildings have already been affected by different geomorphological processes. A rockfall event (600 m3) occurred for example on the SE face on the Arête inférieure des Cosmiques on the 21st of August 1998 (Ravanel et al., 2013) and the Grands Montets case shows a slow subsidence of the stairway over the last decade. In order to better assess the role of the permafrost in these processes and to gain insight on possible future geomorphic activity, we characterized the current permafrost conditions and simulated its changes up to the end of the 21st century using two complementary approaches: (i) the result of ERT (Electrical Resistivity Tomography) surveys carried out in October 2016 on the northern and southern faces right below the Cosmiques hut (at the level of the foundations) and at the Aiguilles des Grands Montets; (ii) the modeling of mean annual rock surface temperature for 2016 and at the end of the 21st century (Magnin et al., in rev.). Duvillard P.-A., Ravanel L., Deline P. (2015). Risk assessment of infrastructure destabilisation due to global warming in the high French Alps. Journal of Alpine Research, 103 (2). Magnin F., Josnin J.-Y., Ravanel L., Pergaud J., Pohl B., Deline P. (in rev.). Modelling rock wall permafrost degradation in the Mont Blanc massif from the LIA to the end of the 21st century. The Cryosphere Discuss.

National audience; Dynamics and evolution of the major glaciers of the Mont-Blanc massif have been vastly studied since the XXth century. Ice/snow covers on steep rock faces as part of the cryosphere however remain poorly studied with only qualitative descriptions existing. The study of ice/snow covers is primordial to further understand permafrost degradation throughout the Mont-Blanc massif and to improve safety and prevention for mountain sports practitioners. This study focuses on quantifying the evolution of ice/snow covers surface during the past century using a specially developed monoplotting tool using Bayesian statistics and Markov Chain Monte Carlo algorithms. Combining digital elevation models and photographs covering a time-span of 110 years, we calculated the ice/snow cover surface for 3 study sites-North faces of the Tour Ronde (3792 m a.s.l.) and the Grandes Jorasses (4208 m a.s.l.) and Triangle du Tacul (3970 m a.s.l.)-and deduced the evolution of their area throughout the XXth century. First results are showing several increase/decrease periods. The first decrease in ice/snow cover surface occurs between the 1940's and the 1950's. It is followed by an increase up to the 1980's. Since then, ice/snow covers show a general decrease in surface which is faster since the 2010's. Furthermore, the gain/loss during the increase/decrease periods varies with the considered ice/snow cover, making it an interesting cryospheric entity of its own.

International audience; Dynamics and evolution of the major glaciers of the Mont-Blanc massif have been vastly studied since the XXthcentury. Ice/snow covers on steep rock faces as part of the cryosphere however remain poorly studied with onlyqualitative descriptions existing. The study of ice/snow covers is primordial to further understand permafrost degradationthroughout the Mont-Blanc massif and to improve safety and prevention for mountain sports practitioners.This study focuses on quantifying the evolution of ice/snow covers surface during the past century using a speciallydeveloped monoplotting tool using Bayesian statistics and Markov Chain Monte Carlo algorithms. Combining digitalelevation models and photographs covering a time-span of 110 years, we calculated the ice/snow cover surfacefor 3 study sites — North faces of the Tour Ronde (3792 m a.s.l.) and the Grandes Jorasses (4208 m a.s.l.) andTriangle du Tacul (3970 m a.s.l.) — and deduced the evolution of their area throughout the XXth century.First results are showing several increase/decrease periods. The first decrease in ice/snow cover surface occurs betweenthe 1940’s and the 1950’s. It is followed by an increase up to the 1980’s. Since then, ice/snow covers show ageneral decrease in surface which is faster since the 2010’s. Furthermore, the gain/loss during the increase/decreaseperiods varies with the considered ice/snow cover, making it an interesting cryospheric entity of its own.

International audience

Since the early 2000s, a remarkable amount of rockfalls has been observed in permafrost areas of the mid-latitude mountain ranges concurrently to hot summers. This study explores the seasonal thaw (ST) in permafrost rock walls of the Aiguille du Midi site (3842 m a.s.l., Mont Blanc massif). We first analyse six years of temperature records in three 10 m-deep boreholes against air temperature (AT) and a four-year time series of pictures showing the snow conditions on two rock faces. Then, we test the sensitivity of the active layer against eight snow fall scenarios using the 1-D surface energy balance and heat conduction model CryoGrid 3 forced by in-situ measurements from a vertical face. Snow falls occur all the year round at this elevation and play an important role for the active layer thickness (ALT), but the snow cover and its control are highly heterogeneous. A long-lasting of a snow cover during spring/early summer delays the ST and reduces the ALT. The thicker and the more spatially-continuous is the snow cover, the stronger are the delay and ALT reduction. Convective clouds could also reinforce this pattern. The summer AT and heat waves are the dominant controlling factors of the ALT. But summer snow falls can sometimes persist for several days on the rock surface and reduce the effect of the heat waves. Active layer can thicken during the early fall, except if the snow starts to accumulate on the rock surface and favours the refreezing. The timing of the snow fall is the most critical parameter to determine the snow effect on the ALT. This study suggests that the characteristics of the bedrock and snow accumulation (steepness, surface roughness, and sun-exposure) must be taken into account to better understand the formation and changes of the active layer and its possible implications for rockfall triggering.

International audience

International audience; Since the early 2000s, a remarkable amount of rockfalls has been observed in permafrost areas of the mid-latitude mountain ranges concurrently to hot summers. This study explores the seasonal thaw (ST) in permafrost rock walls of the Aiguille du Midi site (3842 m a.s.l., Mont Blanc massif). We first analyse six years of temperature records in three 10 m-deep boreholes against air temperature (AT) and a four-year time series of pictures showing the snow conditions on two rock faces. Then, we test the sensitivity of the active layer against eight snow fall scenarios using the 1-D surface energy balance and heat conduction model CryoGrid 3 forced by in-situ measurements from a vertical face. Snow falls occur all the year round at this elevation and play an important role for the active layer thickness (ALT), but the snow cover and its control are highly heterogeneous. A long-lasting of a snow cover during spring/early summer delays the ST and reduces the ALT. The thicker and the more spatially-continuous is the snow cover, the stronger are the delay and ALT reduction. Convective clouds could also reinforce this pattern. The summer AT and heat waves are the dominant controlling factors of the ALT. But summer snow falls can sometimes persist for several days on the rock surface and reduce the effect of the heat waves. Active layer can thicken during the early fall, except if the snow starts to accumulate on the rock surface and favours the refreezing. The timing of the snow fall is the most critical parameter to determine the snow effect on the ALT. This study suggests that the characteristics of the bedrock and snow accumulation (steepness, surface roughness, and sun-exposure) must be taken into account to better understand the formation and changes of the active layer and its possible implications for rockfall triggering.

International audience; In order to test the geomorphological hypothesis on the link between permafrost degradation and rock wall destabilisation, we survey all the rockfalls that occur in the central part of the Mont-Blanc massif using a network of observers since 2007. 511 rockfalls (100 < V < 45,000 m3) have been documented, year 2015 included. Between 2007 and 2014, the average number of destabilizations was 44 (from 17 in 2014 with a cold summer to 72 in 2009 with a relatively hot summer). In 2015, 160 events were recorded i.e. 4 times more than the annual average of the previous years. That makes the year 2015 similar to 2003 that was characterized by its summer heatwave triggering 152 rockfalls in the area currently covered by the network of observers, as shown by the analysis of a SPOT-5 image. Observations of 2015 are discussed and crossed with a statistical model of the Mean Annual Rock Surface Temperature (MARST) for the 1961-1990 period, implemented on a 4-m-resolution DEM of the Mont Blanc massif, and temperature measurements in three 10-m-deep boreholes at the Aiguille du Midi (3842 m a.s.l.), where the summer 2015 active layers have been the thickest since the start of measurements in 2009 (e.g. 3.6 m in the NE face against 2.9 m in average during the previous years). Before 2015, 90 % of the inventoried rockfalls occurred in areas where MARST is in the range -5 to 1°C, whereas only 50 % of the whole rock wall area above 2000 m a.s.l. covers this temperature range. With an air 0°C isotherm which sometimes exceeded the summit of Mont Blanc (4809 m a.s.l.) during the 2015 Summer, conditions were particularly unfavorable for mountaineering. Numerous rescues were carried out to climbers technically blocked by uncommon conditions or injured by rockfalls. On the normal route to the summit of Mont Blanc, two administrative closures of the Goûter hut (3835 m a.s.l.) were necessary to prevent climbers from the huge risk of rockfalls in the access couloir, known for its rockfall activity since its snow/ice cover thaws earlier and earlier in the hot season. This raises the question of the future of mountaineering in certain high altitude areas in the context of global warming.

International audience; Due to ongoing global warming, high alpine environments are affected by significant changes, such as glacial retreat and permafrost warming, which can trigger mass movements in rock slopes or superficial deposits. These processes generate a risk of direct destabilisation for high mountain infrastructures (huts, cable cars, etc.). To help prevent such risks, an inventory of all the high mountain infrastructures in the French Alps was carried out using a Geographic Information System. This combined several data layers, including the Alpine Permafrost Index Map and glacier inventories since the end of the Little Ice Age. 1,769 infrastructures were identified in areas probably characterised by permafrost and/or possibly affected by glacier shrinkage. An index of destabilisation risk was constructed to identify and rank infrastructures at risk. This theoretical risk index includes a characterisation of hazards and a diagnosis of vulnerability. 10 % of the infrastructures were characterised by a high risk of destabilisation.

Rockfalls and rock avalanches are active processes in the Mont Blanc massif, with infrastructure and alpinists at risk. Thanks to a network of observers (hut keepers, mountain guides, alpinists) set up in 2007 present rockfalls are well surveyed and documented. Rockfall frequency over the past 150 years has been studied by comparison of historical photographs, showing that it strongly increased during the three last decades, especially during hot periods like the summer of 2003 and 2015, due to permafrost degradation driven by the climate change. In order to decipher the possible relationship between rockfall occurrence and the warmest periods of the Lateglacial and the Holocene, we start to study the morphodynamics of some selected high-elevated (>3000 m a.s.l.) rockwalls of the massif on a long timescale. Contrary to low altitude, deglaciated sites where study of large rockfall deposits allows to quantify frequency and magnitude of the process, rockfalls that detached from high-elevated rockwalls are no more noticeable as debris were absorbed and evacuated by the glaciers. Therefore, our study focuses on the rockfall scars. Their 10Be dating gives us the rock surface exposure age from present to far beyond the Last Glacial Maximum, interpreted as the rockfall ages. TCN dating of rockfalls has been carried out at the Aiguille du Midi in 2007 (Boehlert et al., 2008), and three other sites in the Mont Blanc massif in 2011 (Gallach et al., submitted). Here we present a new data set of rockfall dating carried out in 2015 that improves the 2007 and 2011 data. Furthermore, a relationship between the colour of the Mont Blanc granite and its exposure age has been shown: fresh rock surface is light grey (e.g. in recent rockfall scars) whereas weathered rock surface is in the range grey to orange/red: the redder a rock surface, the older its age. Here, reflectance spectroscopy is used to quantify the granite surface colour. Böhlert, R., Gruber, S., Egli, M., Maisch, M., Brandová, D., Haeberli, W., Ivy-Ochs, S., Christl, M., Kubik, P.W., Deline, P. (2008). Comparison of exposure ages and spectral propierties of rock surfaces in steep, high alpine rock walls of Aiguille du Midi, France. Proceedings of the 9th International Conference on Permafrost, 143-148. Gallach, X. et al. (submitted). Timing of rockfalls in the Mont Blanc massif (western Alps). Evidences from surface exposure dating with cosmogenic 10Be. Landslides.

International audience; Permafrost degradation through the thickening of the active layer and the rising temperature at depth is a crucial process of rock wall stability. The ongoing increase in rock falls observed during hot periods in mid-latitude mountain ranges is regarded as a result of permafrost degradation. However, the short-term thermal dynamics of alpine rock walls are misunderstood since they result of complex processes related to the interaction of local climate variables, heterogeneous snow cover and heat transfers. As a consequence steady-state and long-term changes that can be approached with simpler process mainly related to air temperature, solar radiations and heat conduction were the most common dynamics to be studied so far. The effect of snow on the bedrock surface temperature is increasingly investigated and has already been demonstrated to be an essential factor of permafrost distribution. Nevertheless, its effect on the year-to-year changes of the active layer thickness and of the permafrost temperature in steep alpine bedrock has not been investigated yet, partly due to the lack of appropriate data. We explore the role of snow accumulations on the active layer and permafrost thermal regime of steep rock walls of a high-elevated site, the Aiguille du Midi (AdM, 3842 m a.s.l, Mont Blanc massif, Western European Alps) by mean of a multi-methods approach. We first analyse six years of temperature records in three 10-m-deep boreholes. Then we describe the snow accumulation patterns on two rock faces by means of automatically processed camera records. Finally, sensitivity analyses of the active layer thickness and permafrost temperature towards timing and magnitude of snow accumulations are performed using the numerical permafrost model CryoGrid 3. The energy balance module is forced with local meteorological measurements on the AdM S face and validated with surface temperature measurements at the weather station location. The heat conduction scheme is calibrated with the temperature measurements in the S-exposed borehole. Results show that the snow may be responsible for permafrost presence while it is absent in the surrounding snow free bedrock. The long lasting of the snow at high elevation, where it can remain until the mid-summer has a delaying effect on the seasonal thaw, which contributes to the lowering of the active layer thickness.

Poster; On January 9, 2016 at 4:57, inhabitants of the municipality of Entremont-le-Vieux were awakened by the sound ofa large rock collapse detached from the Mont Granier (1933 m, Savoie, France), iconic mountain of the Chartreusemassif located between Chambery and Grenoble. Its north face, a 900-m-high natural geological cross section inUrgonian limestone, Hauterivian marls, Valanginian limestone and Berriasian marls, was affected in 1248 by ahuge collapse (500 million m3) that caused hundreds of fatalities.The SW pillar, shaped in the upper Urgonian limestone, climbed for the first time in 1964 and several times duringthe warm and dry autumn 2015, collapsed throughout its height (180 m) over a width of up to about 85 m witha volume certainly much higher than 100,000 m3. Blocks rolled down the western slope on about 700 m beforestopping in the forest. It is now notched over a hundred meter wide. No infrastructure was affected.The Granier has many predispositions to large instabilities. This is the western remaining part, largely fractured, ofa much eroded perched syncline whose inclination is oriented to the east. Fracturing and orientation of the stratacontributes to pre-cut limestone and marl. The Granier is also a major karstic area: the Granier plateau with itsmultilevel karst network (90 km of galleries explored) is a good model of polyphased karst network. The region isalso frequently affected by small earthquakes but seismicity does not appear to be a triggering factor. Conversely,the collapse has produced an M 2.2 earthquake. Besides rock fatigue related to what has just been mentioned anda vertical or overhanging topography, it is likely that heavy rains of days before the event and after a long periodof drought have unleashed the destabilization.The collapse is not comparable to the historical event of 1248, which had reshaped the entire northern side.However, its dimensions make the January 2016 event one of the major events of recent decades in the limestonewestern Alps. Events of this size are very rare in the currently available rockfall data series.There is now a significant residual risk of falling boulders while a large overhang just formed suggests probablecollapses in the short or medium term, with volumes exceeding 10,000 m3.

International audience; Processes that control climate-dependent rockfall from permafrost-affected rock slopes are still poorly understood.In this study, we present the results of a Wireless Sensor Network, integrated within the Swiss project PermaSenseand developed in 2012, to measure rock temperature and geotechnical parameters in the steep rockwalls of theAiguille du Midi (AdM, 3842 m a.s.l., Mont Blanc massif, France). Accessible year round by cable car, the AdMcomprises two main peaks: (i) the Piton Nord with the cable car arrival station, where 4 crack-meters are placed onfour major fractures, and (ii) the Piton Central with many touristic infrastructure, equipped with three 10-m-deepboreholes with 15 temperatures sensors since 2009, and where 2 crack-meters are installed along a major fracture.Three major kinematic regimes are observed: (i) opening of clefts when the rock temperature becomes positive,followed by closing during the cold period, (ii) summer opening continued by a winter opening, and (iii) closingduring the warm period followed by opening in winter.

National audience; Rockfalls and rock avalanches are active processes in the Mont Blanc massif, with infrastructure and alpinists at risk. Thanks to a network of observers (hut keepers, mountain guides, alpinists) set up in 2007 present rockfalls are well surveyed and documented. Rockfall frequency over the past 150 years has been studied by comparison of historical photographs, showing that it strongly increased during the three last decades, especially during hot periods like the summer of 2003 and 2015, due to permafrost degradation driven by the climate change. In order to decipher the possible relationship between rockfall occurrence and the warmest periods of the Lateglacial and the Holocene, we start to study the morphodynamics of some selected high-elevated (>3000 m a.s.l.) rockwalls of the massif on a long timescale. Contrary to low altitude, deglaciated sites where study of large rockfall deposits allows to quantify frequency and magnitude of the process, rockfalls that detached from high-elevated rockwalls are no more noticeable as debris were absorbed and evacuated by the glaciers. Therefore, our study focuses on the rockfall scars. Their 10Be dating gives us the rock surface exposure age from present to far beyond the Last Glacial Maximum, interpreted as the rockfall ages. TCN dating of rockfalls has been carried out at the Aiguille du Midi in 2007 (Boehlert et al., 2008), and three other sites in the Mont Blanc massif in 2011 (Gallach et al., submitted). Here we present a new data set of rockfall dating carried out in 2015 that improves the 2007 and 2011 data. Furthermore, a relationship between the colour of the Mont Blanc granite and its exposure age has been shown: fresh rock surface is light grey (e.g. in recent rockfall scars) whereas weathered rock surface is in the range grey to orange/red: the redder a rock surface, the older its age. Here, reflectance spectroscopy is used to quantify the granite surface colour. Böhlert, R., Gruber, S., Egli, M., Maisch, M., Brandová, D., Haeberli, W., Ivy-Ochs, S., Christl, M., Kubik, P.W., Deline, P. (2008). Comparison of exposure ages and spectral propierties of rock surfaces in steep, high alpine rock walls of Aiguille du Midi, France. Proceedings of the 9th International Conference on Permafrost, 143-148. Gallach, X. et al. (submitted). Timing of rockfalls in the Mont Blanc massif (western Alps). Evidences from surface exposure dating with cosmogenic 10Be. Landslides.

International audience; Due to ongoing global warming, high alpine environments are affected by significant changes, such as glacial retreat and permafrost warming, which can trigger mass movements in rock slopes or superficial deposits. These processes generate a risk of direct destabilisation for high mountain infrastructures (huts, cable cars, etc.). To help prevent such risks, an inventory of all the high mountain infrastructures in the French Alps was carried out using a Geographic Information System. This combined several data layers, including the Alpine Permafrost Index Map and glacier inventories since the end of the Little Ice Age. 1,769 infrastructures were identified in areas probably characterised by permafrost and/or possibly affected by glacier shrinkage. An index of destabilisation risk was constructed to identify and rank infrastructures at risk. This theoretical risk index includes a characterisation of hazards and a diagnosis of vulnerability. 10 % of the infrastructures were characterised by a high risk of destabilisation.; Dans le contexte du changement climatique actuel, les milieux de haute montagne sont soumis à de profonds bouleversements tels que la fonte des glaciers ou la dégradation du permafrost, à l’origine de mouvements de versant au sein des parois rocheuses comme des formations superficielles. Ces processus impliquent des risques directs de déstabilisation pour les infrastructures (refuges, remontées mécaniques, etc.). Dans un but de prévention de ces risques, un recensement des infrastructures de haute montagne dans les Alpes françaises a été réalisé à l’aide d’un Système d’Information Géographique constitué de plusieurs couches de données dont des cartes de la répartition potentielle du permafrost et de l’extension des glaciers depuis la fin du Petit Âge Glaciaire. 1769 infrastructures ont été identifiées dans des secteurs à permafrost probable et/ou éventuellement affectés par le retrait glaciaire. Un indice de risque a été construit afin d’identifier et classer les infrastructures potentiellement à risque. Il inclut une caractérisation de l’aléa et un diagnostic de la vulnérabilité. 10 % des infrastructures sont caractérisées par un risque fort de déstabilisation.

The objective of this paper is to analyze the actors' interactions around this tourism diversification process based on outdoor sports, and in particular trail running, in the Pays of Allevard. To understand the tourism diversification process, it is necessary to analyze the way the local actors use the sport, take ownership of it, and transform it according to their interests, in particular to make it a tourism resource.

International audience

Poster; In the current context of imbalance of geosystems in connection with the rising air temperature for several decades,high mountain environments are especially affected by the shrinkage of glaciers and the permafrost degradationwhich can trigger slope movements in the rock slopes (rockfall, rock avalanches) or in superficial deposits (slides,rock glacier rupture, thermokarst). These processes generate a risk of direct destabilization for high mountain infrastructure(huts, cable-cars...) in addition to indirect risks for people and infrastructure located on the path ofmoving rock masses. We here focus on the direct risk of infrastructure destabilization due to permafrost degradationand/or glacier shrinkage in the French Alps.To help preventing these risks, an inventory of all the infrastructure was carried out with a GIS using different datalayers among which the Alpine Permafrost Index Map and inventories of the French Alps glaciers in 2006-2009,1967-1971 and at the end of the Little Ice Age. 1769 infrastructures have been identified in areas likely characterizedby permafrost and/or possibly affected by glacier shrinkage.An index of risk of destabilization has been built to identify and to rank infrastructure at risk. This theoretical riskindex includes a characterization of hazards and a diagnosis of the vulnerability. The value of hazard is dependenton passive factors (topography, lithology, geomorphological context...) and on so-considered active factors(thermal state of the permafrost, and changing constraints on slopes related to glacier shrinkage). The diagnosis ofvulnerability has meanwhile been established by combining the level of potential damage to the exposed elementswith their operational and financial values. The combination of hazard and vulnerability determines a degree ofrisk of infrastructure destabilization (from low to very high). Field work and several inventories of infrastructuredamages were used to validate it.The application of this risk index for infrastructure in the French Alps indicates 999 infrastructures potentiallyat risk, among 0.2 % are characterized by a very high risk and 4.4 % by a high risk of destabilization. The riskunequally affects massifs: 55 % of the infrastructure at risk are in the Vanoise massif (Savoie) due to the largenumber of high-altitude ski resorts in this area. The Mont-Blanc massif (Haute-Savoie) includes only 6.5 % of theinfrastructure at risk. Furthermore, 71 % of the exposed infrastructure are ski-lifts.

Poster; The Pilatte hut is located at 2572 m a.s.l. at Saint-Christophe-en-Oisans (south of the Ecrins Massif, France), ata 3-hours-walk from La Bérarde hamlet. Its capacity is 120 beds for hikers and climbers who are engaged in theascent of Les Bans (3669 m a.s.l.).Built on a rocky ledge on the right side of the Pilatte Glacier (A = 2.64 km2; L = 2.6 km), it currently dominatesthe glacier by about 150 m. This relief results from the retreat of the glacier since the end of the Little Ice Age, asthe till around the hut was deposited during this stage. The glacier has lost about 1.8 km in length during the sameperiod.A first wooden hut was built in 1925 and presently serves as a winter refuge. In 1954, the growth of mountainactivities led to the construction of a larger hut made of cemented stones. An extension to the west made ofreinforced concrete was built in 1994. But in the late 1980s, severe damages to the 1954 part of the building werealready recognized: vertical cracks lining the north and south facades, ,subsidence (c. 10 cm downstream) of theground floor, cracked interior walls.

International audience

International audience; Rockfall is one of the main natural hazards in high mountain regions and its frequency is growing, especially since two decades. Collapses at high elevation are with an increasing certainty assumed to be a consequence of the climate change through the warming permafrost. In the Mont Blanc massif, data on present rockfalls (occurrence time when possible, accurate location, topographical and geological settings, volume, weather and snow conditions) were acquired for 2003 and for the period 2007-2014 thanks to a satellite image of the massif and a network of observers in the central part of the massif, respectively. The study of the 533 so-documented rockfalls shows a strong correlation at the year scale between air temperature and rockfall. Along with this data acquisition, a statistical model of the Mean Annual Rock Surface Temperature (MARST) for the 1961-1990 period has been implemented on a 4-m-resolution DEM of the Mont Blanc massif. The model runs with Potential Incoming Solar radiation (PISR) calculated with GIS tools and air temperature parameters computed from Chamonix Météo France records. We cross here the data on rockfalls with the permafrost distribution model to show that: (i) rockfall occurs mainly over modeled negative MARST (context of permafrost); (ii) simulated warm permafrost areas (> -2°C) are the most affected by instabilities; (iii) as the 1961-1990 period is supposed to be representative of the conditions at depth that are not affected by the climate warming during the two last decades, the latest results are mainly valuable for rockfalls related to pluri-decadal signal; and (iv) the higher (close to 0°C) the MARST, the deeper the detachment (possibly related to the deepening of the permafrost active layer). These results and field observations confirm that warming permafrost corresponds to the main required configuration for rockfall triggering at high elevation. In addition, we show that rockfalls for which ice observed in their scar indicates the presence of permafrost can be used to validate the permafrost distribution model.

International audience

International audience

Despite the rising interest in mountain permafrost due to climatic changes and a noticed increase of registered rockfall events in the European Alps and other mountain ranges, little is known about transient thermal conditions in the detachment areas of rockfalls. Temperature conditions prior to the rockfall events of 144 past events in the European Alps were modelled with a physically based ground temperature model. To minimise the impact that uncertainty has on interpretations, only relative values were used, that is, percentiles obtained from cumulative distribution functions of the modelled ground surface temperatures from the beginning of the meteorological measurement series up to the event dates. Our results suggest that small and mid‐sized rockfalls (volumes up to 100 000 m3) from high elevation occurred mainly during short‐term periods of unusually high temperatures. This was neither found to be a result of the seasonal distribution (most analysed events in higher elevations occurred from July to September) nor of the longer‐term temporal distribution (most analysed events occurred after 2000) only. Plausible explanations are either a destabilisation related to advective thaw or failure due to stress redistribution caused by large temperature variations. Large deep‐seated rock slope failures (≥100 000 m3) in high elevation occurred all year round.

International audience; Rock mass movements are dominant in the morphodynamics of high Alpine rock slopes and are at the origin of significant risks for people who attend these areas and for infrastructures that are built on (e.g. huts, cable cars). These risks are increasing because of permafrost degradation and glacier retreat as consequences of the global warming. These two factors may affect slope stability by changing mechanical properties of the interstitial ice and modifying the mechanical constraints in these rock slopes. The monitoring of rock slopes is thus an essential element for risk management. Our study focuses on two particularly active areas of the Western Alps: the lower Arête des Cosmiques (3613 m a.s.l., Mont Blanc massif, France) on which is located the very popular Refuge des Cosmiques, and the Col des Gentianes (2894 m a.s.l., Valais, Switzerland) where is located a cable car station. Discussed on the basis of geophysical and glaciological data, the evolutions monitored by terrestrial laser scanning probably result from the combination between permafrost activity/degradation and glacier shrinkage.

The steep rockwalls of the Mont Blanc massif have been affected by an increase in rockfall activity in the last decades. Permafrost degradation is suggested as the most likely triggering factor. To better understand geomorphic processes we investigate permafrost distribution and address questions on its pattern in steep alpine bedrock. We use GIS-modeling to simulate Mean Annual Rock Surface Temperature (MARST) distribution. Rock temperature measurements including three 10-m-deep borehole monitoring at the Aiguille du Midi (AdM, 3842 m a.s.l) serve to estimate the temperature offset (i.e. temperature difference between rock surface and depth of negligible inter-annual temperature varibility). The estimation of the lower extent of permafrost distribution is derived from a combination of both approaches and hypotheses on permafrost occurrence are evaluated with Electrical Resistivity Tomography (ERT) measurements. The MARST model indicates that the 0 °C isotherm extends down to 2600 m a.s.l in the most shaded faces and rises up to 3800 m in the most sun-exposed areas. According to recent literature and the AdM borehole thermal profiles, we postulate that permafrost could extends down below MARST reaching up to 3°C due to temperature offset processes. ERT measurements performed along 160-m-long profiles at six different sites which the top are located from 3360 m a.s.l to 2760 m a.s.l and the MARST range from 3°C are the first of this kind. Five of sites are located in the granite area making them directly comparable. They all show high resistivity values at depth (>200 k) interpreted as permafrost bodies. Lower resistivity values (< 90 k ) are found either above the high resistivity bodies and interpreted as thawed active layer, or below MARST warmer than 2-3°C and interpreted as non-perenially frozen rock. Two sites were measured in autumn 2012 and autumn 2013 allowing for time-lapse investigation which demonstrates the change in resistivity in repeated measurements. These preliminary results could confirm that steep alpine bedrock permafrost exists below surface tempera- ture reaching up to 3 °C. A temperature-resistivity calibration will be performed in a freezing laboratory at the Technical University of Munich to better assess ERT results and their interpretation in terms of permafrost occurrence and interannual changes.