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In high mountains environments, permafrost degradation causes rockwall instabilities, sometimes leading to rock slope failures. These processes could be a threat for human lives and activities, it is therefore essential to improve our knowledge about their triggering and propagation mechanisms.This study focuses on two rock slope failures which occured in the Vallon d'Étache (Maurienne Valley, France) and the Crête des Grangettes (Écrins massif, France) in 2020. These events have respectively volumes around 270000 m3 and 35000 m3. In both cases, ice in the rockfall scars suggests the presence of permafrost, but its local distribution and its role in the triggering of the events remain to be confirmed. The aims of this study are (i) to assess thermal conditions in which both events were triggered and (ii) to model their propagation. To do so, we first propose a method that combines rockwalls temperature monitoring, statistical modelling of permafrost distribution, geophysical surveys (Electrical Resistivity Tomography) and laboratory measurements (petrophysical model) to explain the thermal conditions under which the rockfalls occurred. Then, 3D models acquired by photogrammetry are used to obtain high-resolution DEMs in order to measure their volumes, and to provide consistent inputs for runout modelling.This multi-method approach allows to understand the context and the processes leading to rockfall triggering of two events in the French Alps. We also propose depth-averaged flow simulations to model the runout characteristics of the Vallon d’Étache rock slope failure., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

It is increasingly recognized that the atmosphere composition of snow covered regions – especially polar regions – is noticeably affected by air-snow interactions. Indeed, the snowpack is a multiphase reactor, but physico-chemical processes which take place inside are still poorly understood. A detailled understanding of snow-atmosphere interactions is essential for understanding and modeling properly the composition and reactivity of the atmosphere above snow covered regions. Reconstructions of past trends in atmospheric composition using ice cores also require to understand snowpack processes that affected the composition of interstitial air and burried snow after its deposition.Nitrate (NO3-) present in the snowpack plays an important role as it photochemically produces nitrogen oxides (NOx=NO+NO2), which affect the oxidative capacity of the atmosphere through ozone production.This thesis thus aimed at studying physico-chemical processes which take place inside the snowpack and modify nitrate concentration.In a first part, a reaction mechanism to reproduce nitrate photochemistry in snow were developed, based on previous studies. The main hypothesis was that chemical reactions take place in a quasi-liquid layer located on the surface of snow cristals. However, the properties of this ice-air interface are poorly known, and it appeared that this approach had too many uncertainties to be continued.Then, a thorough discussion were carried out to assess current attempts in snow chemistry modeling, and to propose another approach which could prevail given current knowledge on this topic.In a second part, physico-chemical exchange processes between air and snow were studied and modeled. This concerned adsorption, solid phase diffusion and co-condensation. Among the results that arise, it appeared that current parameterizations of nitrate surface coverage are unable to reproduce measured concentrations, in the studied case of Dome C surface snow, and further reveal sizeable overestimations. On the contrary, simultaneous modeling of solid phase diffusion and co-condensation allows a qualitatively good reproduction of measurements, which cover more than a year, thus including both austral summer and winter with their specific features.This study reveals the importance of exchange processes for snow chemistry modeling, and give basis for future work on this topic., Il est aujourd'hui avéré que la composition chimique de l'atmosphère des régions enneigées – et notamment des régions polaires – est sensiblement affectée par les échanges d'espèces chimiques réactives entre l'air et la neige. En effet, le manteau neigeux constitue un véritable réacteur photochimique multiphasique, mais les mécanismes physico-chimiques à l'œuvre en son sein sont encore mal connus. Une compréhension détaillée des processus s'y déroulant est indispensable pour modéliser correctement la composition et la réactivité de l'atmosphère au-dessus des régions enneigées. De plus, la reconstitution de l'évolution post-dépôt des composés chimiques stables de la neige est également un préalable indispensable pour permettre l'interprétation paléoclimatique de leurs profils de concentration enregistrés dans les carottes de glace.Le nitrate (NO3-) présent dans la neige joue un rôle fondamental, car sa photolyse induit notamment l'émission d'oxydes d'azote (NOx = NO + NO2) par le manteau neigeux, qui modifient la capacité oxydante de l'atmosphère via la production d'ozone. L'objet de cette thèse a donc été d'étudier par modélisations les processus physico-chimiques intervenants dans l'évolution de la concentration du nitrate dans la neige.Une première approche, prolongeant des études préexistantes, a visé à identifier un mécanisme réactionnel pour la photochimie du nitrate dans la neige, en postulant notamment l'existence d'une couche quasi-liquide à la surface des grains de neige. Néanmoins, les propriétés exactes de l'interface air – glace sont, à l'heure actuelle, encore mal caractérisées, et il est apparu que cette démarche présentait de trop larges incertitudes pour être poursuivie.Une discussion approfondie a alors été menée afin d'évaluer les tentatives actuelles de modélisation de la chimie de la neige, et dans le but de proposer une nouvelle approche plus réaliste au regard du niveau de connaissance actuel.Ainsi, dans une seconde partie, l'ensemble des processus d'échange physico-chimiques du nitrate entre l'air et la neige ont été étudiés puis modélisés : adsorption à la surface, diffusion en phase solide et co-condensation. Parmi les résultats obtenus, il est apparu que les paramétrisations actuelles de la couverture surfacique en nitrate étaient incapables de reproduire les concentrations mesurées, dans le cas de la neige de surface à Dome C, et révèlent d'importantes surestimations. A contratio, la prise en compte conjointe de la diffusion en phase solide ainsi que d'un processus de co-condensation permet de bien reproduire qualitativement les séries temporelles de plus d'un an, couvrant donc à la fois l'été et l'hiver austral, qui présentent chacun des caractéristiques distinctes en terme de concentration mesurées.Cette étude révèle ainsi l'importance de ces processus physico-chimiques d'échange dans la modélisation de la chimie de la neige, et pose les bases des mécanismes à prendre en compte dans le cadre de développements futurs.

International audience; The structure of the snowpack near Barrow was studied in March-April 2009. Vertical profiles of density, specific surface area (SSA) and thermal conductivity were measured on tundra, lakes and landfast ice. The average thickness was 41 cm on tundra and 21 cm on fast ice. Layers observed were diamond dust or recent wind drifts on top, overlaying wind slabs, occasional faceted crystals and melt-freeze crusts, and basal depth hoar layers. The top layer had a SSA between 45 and 224 m2 kg-1. All layers at Barrow had SSAs higher than at many other places because of the geographical and climatic characteristics of Barrow. In particular, a given snow layer was remobilized several times by frequent winds, which resulted in SSA increases each time. The average snow area index (SAI, the dimensionless vertically integrated SSA) on tundra was 3260, higher than in the Canadian High Arctic or in the Alaskan taiga. This high SAI, combined with low snow temperatures, imply that the Barrow snowpack efficiently traps persistent organic pollutants, as illustrated with simple calculations for PCB 28 and PCB 180. The average thermal conductivity was 0.21 Wm-1 K-1, and the average thermal resistance on tundra was 3.25 m2 K W-1. This low value partly explains why the snow-ground interface was cold, around -19°C. The high SAI and low thermal resistance values illustrate the interplay between climate, snow physical properties, and their potential impact on atmospheric chemistry, and the need to describe these relationships in models of polar climate and atmospheric chemistry, especially in a climate change context.