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Bernd Zimanowski and Ralf Büttner shared with the authors the experiment design and development. They are gratefully thanked. The paper greatly benefited from the revision of Gert Lube and Kristen Fauria. Published with permission of the Executive Director of British Geological Survey (UKRI). Data can be found at the Zenodo repository with the following link https://zenodo.org/search?page=1&size=20&q=dellino, with DOI 10.5281/zenodo.3408690., The entrainment rate of pyroclastic density currents is investigated by large‐scale experiments. The ground flows are initiated by the impact on the terrain of a dense gas‐particle fountain issuing from a cylindrical conduit, similarly to natural volcanic events. On impact, the excess density with respect to the surrounding atmosphere was up to 11.6 kg/m3, making the currents non‐Boussinesq. A power law model of the entrainment rate is developed, which is similar to that proposed for snow avalanches by Ancey (2004, https://doi.org/10.1029/2003JF000052) and is verified for the Richardson's number range between 0.25 and 5.95. Rapid changes of the entrainment are caused by (i) strong accelerations at the fountain impact on the ground; (ii) break in slope; and (iii) topographic obstacles. Such changes, together with the sedimentation rate, influence flow mobility. The use of the power law is suggested for modeling the motion of unsteady hazardous geophysical mass flows such as pyroclastic density currents and snow avalanches.

For the 20th century, high mountain areas, such as the Alps, have undergone a significant warming with temperature increase twice as much as the global average. Such warming strongly alters the cryosphere components. It induces, for example, a shift from solid to liquid precipitation, more frequent and more intense snowmelt phases or a strong decrease in the amount and duration of snow cover, especially at the elevation of the snowrain transition. In the future, climate models forecast that snow depth will be significantly reduced and that snow properties such as snow stability will be modified. These changes in snow cover characteristics and amounts are expected to induce significant changes in snow avalanches activity. At the same time, afforestation induced by the abandonment of agriculture and grazing, the democratization of winter recreation activities and the urban sprawl on the slopes have strongly modified the mountain landscapes since the mid-18th century as well as the exposition of individuals to snow avalanches. In this context, a precise documentation of past snow avalanche activity is crucial to decipher and to understand the impacts of the undergoing climate warming on the snow avalanche activity. To date, this documentation usually relies on historical chronicles or systematic observations. However, the firsts are often discontinuous and focused on catastrophic events. The seconds are limited to the second half of the 20th century thus precluding a comparison from climatically distinct period. On forested paths, the dendrogeomorphic approach is theoretically a reliable approach complement to historical archives and series of systematic observation to infer past snow avalanche activity. Yet, so far, the robustness of this approach has been poorly questioned and no regional chronology, crucial to disentangle potential interferences between snow avalanche activity, climate fluctuations and socio-economic changes, has been developed in the Alps. In this PhD thesis proposes new methodological frameworks to (1) detect avalanche events from tree-ring series, (2) remove non-stationarities related to the decreasing number of trees over time in the reconstruction and (3) aggregate locals reconstructions in regional chronologies. Based on these development, homogenized multicentennial regional chronologies developed (4) for 10 paths of the Goms Valley (Valais canton, Swiss Alps, 1880-2014) and (5) 11 paths from the Queyras Massif (French Alps, 1560-2016) are confronted to climatic fluctuations and land use changes. At Goms, the absence of clear climatic signal in the regional chronology evidence the interference with local nonstationarities and question the need for a sampling strategy at the regional scale to create a robust chronology. In the Queyras massif, the strong decrease of avalanche activity observed over the 20th century is attributed to global warming and to the afforestation process., Au 20ème siècle, les massifs montagneux, dont les Alpes, ont connu un réchauffement significatif avec une augmentation des températures deux fois plus importante que la moyenne mondiale. Un tel réchauffement altère les composantes de la cryosphère. Elle induit, par exemple, un passage des précipitations solides aux précipitations liquides, des phases de fonte des neiges plus fréquentes et plus intenses, ainsi qu’une forte diminution de la quantité de neige et une réduction de la durée de la couverture neigeuse. Aux horizons 2050–2100, les modèles climatiques prévoient que l'épaisseur du manteau neigeux sera considérablement réduite et que les propriétés de la neige, et notamment la stabilité du manteau neigeux, seront modifiées. Ces changements devraient entraîner des modifications importantes dans l'activité des avalanches. Parallèlement, l’afforestation induite par la déprise agro-sylvo-pastorale, la démocratisation des sports d'hiver et l'urbanisation des versants ont profondément modifié les paysages de montagne depuis le milieu du 18ème siècle, de même que l'exposition des individus. Dans ce contexte, une documentation précise de l'activité passée des avalanches est cruciale pour mettre en évidence et comprendre les impacts du réchauffement climatique sur l'activité avalancheuse. Jusqu'à présent, cette documentation s'appuyait sur des chroniques historiques ou des observations systématiques. Cependant, les premières sont souvent discontinues et axées sur des événements catastrophiques, tandis que les secondes se limitent à la seconde moitié du 20ème siècle, excluant toute comparaison avec des périodes climatiques distinctes – tel que les phases froides du Petit Âge Glaciaire, par exemple. Sur les versants forestiers, l'approche dendrogéomorphique apparaît être un complément fiable aux archives historiques et aux séries d'observations systématiques, car elle permet de reconstruire l'activité passée des avalanches, en continu, à l‘échelle des plusieurs siècles, avec une résolution annuelle. Pourtant, jusqu'à présent, même si de nombreuses reconstructions locales ont été proposées, la fiabilité de l’approche a été peu souvent analysée et aucune chronologie régionale – cruciale pour distinguer les interférences potentielles entre l'activité des avalanches, les fluctuations climatiques et les changements socio-économiques – n'a été développée dans les Alpes. Dans cette thèse de doctorat, des avancées méthodologiques significatives ont été réalisées afin (1) d’améliorer la détection des avalanches dans les cernes de croissance, (2) d’éliminer les non-stationnarités liées à la diminution du nombre d'arbres au cours du temps dans les reconstructions et (3) d’agréger les reconstructions locales en chronologies régionales. Sur la base de ces développements, des chronologies régionales pluriséculaires homogénéisées ont été développées (4) pour 10 couloirs d’avalanche de la vallée de Goms (Valais, Alpes suisses, 1880-2014) et (5) 11 couloirs du massif du Queyras (Alpes françaises, 1560-2016). Ces dernières ont été confrontées aux fluctuations climatiques et aux changements d'occupation du sol. À Goms, l'absence de signal climatique clair dans la chronologie régionale souligne les interférences induites par les non-stationnarités locales et démontre qu’une stratégie d'échantillonnage à l'échelle régionale devra nécessairement constituer un préalable au développement d’une chronologie robuste. Dans le massif du Queyras, la forte diminution de l'activité avalancheuse observée au cours du 20ème siècle est attribuée au réchauffement climatique et au processus d’afforestation des versants

Snow avalanches can be triggered by strong earthquakes. Most existing models assume that snow slab avalanches happen simultaneously during or immediately after their triggering. Therefore, they cannot explain the plausibility of delayed avalanches that are released minutes to hours after a quake. This paper establishes the basic mechanism of delays in earthquake-induced avalanche release using a novel analytical model that yields dynamics consistent with three documented cases, including two from Western Himalaya and one from central Italy. The mechanism arises from the interplay between creep, strain softening and strain-rate sensitivity of snow, which drive the growth of a basal shear fracture. Our model demonstrates that earthquake-triggered delayed avalanches are rare, yet possible, and could lead to significant damage, especially in long milder slopes. The generality of the model formulation opens a new approach for exploring many other problems related to natural slab avalanche release. ISSN:1364-5021 ISSN:1471-2946 ISSN:0080-4630 ISSN:0950-1207

The natural and anthropic processes that intervene in the landscape dynamics of the Sierra del Aramo are analyzed. Particularly, we focus on the analysis of slope dynamics and climate warming, as well as the depopulation and changes of land uses in mountain environments. The methodology has combined field work, imagery and climate data analyses, as well as demographic and agrarian sources. The depopulation and abandonment of traditional activities explain the decline of pastures. However, the intensity of the current snow avalanches prevents the reforestation in some slopes. In the Sierra del Aramo 3.7 km2 are deforested by the recurrence of such avalanches. The relevance of snow avalanches in the dynamics of the landscape of the middle mountains in temperate latitudes are revealed. Se analizan los procesos naturales y antrópicos que intervienen en la dinámica del paisaje de la Sierra del Aramo, en especial la dinámica de laderas y el calentamiento atmosférico, así como el despoblamiento y los cambios de usos del suelo en los medios de montaña. La metodología ha combinado el trabajo de campo, la fotointerpretación de imágenes aéreas, el análisis de datos climáticos, fuentes demográficas y agrarias. El despoblamiento y el abandono de las actividades tradicionales explican la disminución de los pastizales. No obstante, la intensidad de los aludes de nieve actuales interfiere en buena medida en la recuperación de los bosques. En efecto, 3,7 km2 de la Sierra del Aramo se hallan deforestados por la recurrencia de tales avalanchas. Se muestra, por tanto, la relevancia de los aludes de nieve en la dinámica del paisaje de las montañas medias en las latitudes templadas.

Knowledge about frequency and location of snow avalanche activity is essential for forecasting and mapping of snow avalanche hazard. Traditional field monitoring of avalanche activity has limitations, especially when surveying large and remote areas. In recent years, avalanche detection in Sentinel-1 radar satellite imagery has been developed to improve monitoring. However, the current state-of-the-art detection algorithms, based on radar signal processing techniques, are still much less accurate than human experts. To reduce this gap, we propose a deep learning architecture for detecting avalanches in Sentinel-1 radar images. We trained a neural network on 6345 manually labeled avalanches from 117 Sentinel-1 images, each one consisting of six channels that include backscatter and topographical information. Then, we tested our trained model on a new synthetic aperture radar image. Comparing to the manual labeling (the gold standard), we achieved an F 1 score above 66%, whereas the state-of-the-art detection algorithm sits at an F 1 score of only 38%. A visual inspection of the results generated by our deep learning model shows that only small avalanches are undetected, whereas some avalanches that were originally not labeled by the human expert are discovered.

Mathematical models of different degrees of complexity, describing the motion of a snow avalanche along a path with given center line and spatially varying width, are formulated and compared. The most complete model integrates the balance equations for mass and momentum over the cross-section and achieves closure through an entrainment function based on shock theory and a modified Voellmy bed friction law where the Coulombic contribution to the bed shear stress is limited by the shear strength of the snow cover. A simplified model results from integrating these balance equations over the (time-dependent) length of the flow and postulating weak similarity of the evolving avalanche shape. On path segments of constant inclination, it can be solved for the flow depth and speed of the front in closed form in terms of the imaginary error function. Finally, the very simplest model assumes constant flow height and length. On an inclined plane, the evolution of flow depth and velocity predicted by the simplified model are close to those from the full model without entrainment and with corresponding parameters, but the simplest model with constant flow depth predicts much higher velocity values. If the friction coefficient is varied in the full model with entrainment, there can be non-monotonous behavior due to the non-linear interplay between entrainment and the limitation on the Coulomb friction.

Observations of the deposits, flow marks, and damages of three mixed-snow avalanches of widely different size were analyzed with regard to flow regimes, velocities, pressures, densities, flow depths, erosion modes, and mass balance. Three deposit types of different density and granulometry could be clearly discerned in these avalanches. They are attributed to dense, fluidized, and suspension flow regimes, respectively. Combining observations, we estimated the density in the fluidized layer as 35&ndash, 100 kg m - 3 , in good agreement with inferences from pressure measurements. Upper bounds for the suspension layer density, arising from the run-up height, velocity, and damage pattern, are about 5 kg m - 3 at the valley bottom. An approximate momentum balance of the dense layer suggests that the snow cover was eroded to considerable depth, but only partly entrained into the flow proper. The suspension layer had largely lost its erosive power at the point where it separated from the denser parts of the avalanche. Our estimates shed doubt on collisions between snow particles and aerodynamic forces at the head of the avalanche as sole mechanisms for creating and upholding the fluidized layer. We conjecture that the drag from air escaping from the snow cover as it is being compressed by the overriding avalanche could supply the missing lift force.

Author(s): Weit, A; Roche, O; Dubois, T; Manga, M | Abstract: Dilute gas-particle mixtures in which the particles are carried by the turbulent fluid are found in various geophysical contexts, from cold snow avalanches to hot pyroclastic density currents. Though previous studies suggest that such mixtures have maximum particle concentrations of a few volume percent, the dependence of this maximum concentration on the Reynolds number is unclear. We addressed this issue through laboratory experiments in a vertical pipe, where dilute gas-particle mixtures were created by injecting a turbulent air flow from below. Nearly monodisperse mixtures of glass beads of different grain sizes (77 to 1,550nμm) were used with varying bulk concentrations from 0.025 to 8 vol. %. To create quasi-static mixtures, the mean air velocity matched the terminal settling velocity for the grain sizes investigated. The maximum Reynolds numbers of the mixtures were ~104–106. The air pressure indicated full support of the particle weight at concentrations down to 0.025 vol. %. Above a critical particle concentration, at which all the particles were suspended, subsequent additional particles were not maintained in the mixture and led to the formation of clusters that settled downward in the pipe to form a dense fluidized bed. Maximum mean particle concentrations of the dilute mixtures increased from ~1 to ~2.8 vol. % and reached a plateau at increasing mixture Reynolds number. These results give insights into the maximum particle concentrations of geophysical turbulent gas-particle mixtures and may serve to constrain observations as well as the input and output data of models.

The work contains quantitative analysis of differentiation of altitudinal position and sinuosity of two lines determining the course of forest limit, i.e. timberline and treeline within Babia Gora Mt. (1725 m a.s.l.) homoclinal flysch ridge (the Western Carpathians). The course of the empiric timberline was delimited basing on aerial photographs with details from spatial data of Aerial Laser Scanning conducted in 2012. On the N slope, the course of timberline is exclusively conditioned by natural factors, whereas on the S slope this line was shifted downwards as a result of sheep and cattle grazing (however it has shown progression for the last 80 years). In the course of theoretical treeline conditioned by macrotopography and local climate, the mass-elevation effect is visible, and on the N slope, additionally, a sub-summit downward shift (the Diablak effect) occurs. The sequence of natural factors, according to their positive or negative influence on timberline and treeline courses was determined.

The paper presents examples of the change in snow avalanches and debris flows activity due to the anthropogenic pressure on vegetation and relief. The changes in dynamical characteristics of selected snow avalanches and debris flows due to the anthropogenic activity are quantified. The conclusion is made that the anthropogenic effects on the snow avalanches and debris flows activity are more pronounced than the possible effects of the climate change. The necessity is expressed on the unavoidable changes of the natural environment as the result of a construction and of use of the constructed infrastructure to be account for in corresponding planning of the protection measures.

We quantify the synchronization between snowfall and natural avalanches in relation to terrain properties at the detachment zone. We analyze field statistics of 549 avalanche events in terms of slope, aspect, timing, coordinate, and release area, identified by a georeferencing procedure applied on terrestrial photography. The information from the digital pictures, together with associated meteorological data, provides us with the input needed for model calibration, namely, the magnitude of snowfall, the snow compaction rate, and the timing of precipitation and of avalanche events. Synchronization between snowfall and avalanches is established for different slope categories. We obtain an average probability of release after a snow event of 30% and 16% for the high- and low-slope categories (average slope 44 degrees and 36 degrees, respectively). Using the notion of information entropy, we quantify the uncertainty in predicting avalanche occurrence from a snow event. The steeper slopes correspond to a larger entropy in avalanche prediction. Further, the presented method allows us to establish the return period of avalanches without requiring a long series of data. When considering events regardless of their release depth, the avalanches had a return period of 48 days (higher slopes) and 88 days (lower slopes). Finally, we determine the average daily detachment rate as a function of snow depth and the return period of avalanches as a function of the release depth.

A comparison was done of the timberline course from the mid-20th and beginning of 21st century, in Mengusovská Valley (Slovakia) and Rybi Potok Valley (Poland). These are two valleys in the High Tatra Mts. Aerial photos and satellite images were used to assess the changes of the timberline in the two valleys. The course of the timberline ecotone in both valleys is similar. In both valleys, the stable timberline section is almost half of the total timberline length. In both valleys there has been an increase in the elevation of the timberline (on average by 10 m in the Mengusovská Valley and 15 m in the Rybi Potok Valley), and free spaces have been increasingly closing up. The progressive changes of the timberline are mainly due to the limit placed on human economic activity, and to climate warming. Inactive avalanche paths have led to an enlargement of the forest area in both valleys. The reduction of avalanche activity is the direct result of climate warming in the Tatra Mts. and from the decrease in the amount of snow in winters.

Natural conditions that explain the triggering of snow avalanches are becoming better-known, but our understanding of how socio-environmental changes can influence the occurrence of damaging avalanches is still limited. This study analyses the evolution of snow avalanche damage in the Asturian Massif (NW Spain) between 1800 and 2015, paying special attention to changes in land-use and land-cover patterns. A damage index has been performed using historical sources, photointerpretation and fieldwork-based data, which were introduced in a GIS and processed by means of statistical analysis. Mapping allowed connecting spatiotemporal variations of damage and changes in human-environment interactions. The total number of victims was 342 (192 dead and 150 injured). Results show stability in the number of avalanches during the study period, but a progressive decrease in the damage per avalanche. Changes in land use explain the evolution of damage and its spatial/temporal behaviour. The role played by vegetation cover is at the root of this process: damage was the highest during the late 19th and early 20th centuries, when a massive deforestation process affected the protective forest. This deforestation was the result of demographic growth and intensive grazing, disentailment laws and emerging coal mining. Since the mid-20th century, the transformation of a traditional land-management system based on overexploitation into a system based on land marginalization and reforestation, together with the decline of deforestation due to industrial and legal causes, resulted in the decrease of avalanches that affected settlements (mostly those released below the potential timberline). The decrease of damage has been sharper in the western sector of the Asturian Massif, where oak deforestation was very intense in the past and where lithology allows for a more successful ecological succession at present. Taking into account that reforestation can be observed in mountain environments of developed countries worldwide, and considering present initiatives conducted to counteract its negative cultural effects by means of grazing and clearing operations, planning is imperative, and this research provides useful information for environmental management policies and risk mitigation in avalanche prone areas.

降雨と融雪が重なる現象はrain-on-snow eventと呼ばれ，近年，世界各地で大規模な融雪災害をもたらしている．北海道においても，冬期におけるROS記録は各地に存在し，今後の降雨・積雪特性の変化が融雪災害に与える影響が懸念される．そこで，本研究では厳冬期のROS災害の特性を把握することを目的に室内水路実験を実施した．結果，融雪期の最高気温を想定した場合は，降雨に伴う積雪層のせん断強度低下と重量増加とが重なることで湿性雪崩が生じた．一方，本実験条件下で気温が10℃付近の場合は，降雨の影響は比較的少なく，雨は積雪層をゆっくり鉛直浸透し，流出した．ただし，降雨の影響は積雪層が新しい雪で構成されるほど大きく，北海道のような積雪寒冷地では，今後の気候変動に伴う積雪や降雨特性の変化が冬期の積雪層挙動や河川融雪災害に影響を与える可能性を留意する必要がある．, In recent years, rain-on-snow (ROS) events has caused large-scale flooding disasters in winter all over the world. In Hokkaido located in the northernpart of Japan, ROS events also has been recorded in various places in the past and has been conserned to be increased in future due to the rise of temperatures over snowy-cold region. In this study, authors conduted flume experiments to understand the effects of rainfall on snow melt and snowmelt runoff, which affects ROS disasters during winter. The results showed that in the cases, estimated maximum temperature in winter (snow melting season), snow avalanches occurred due to the rainfall. In contrast, where the temperature is below 10 ℃, the rainfall slowly penetrate and formed the flow path at the bottom of snow layer, leading into an increase of 1.23 times of the runff discharge to the low channel path.

This paper presents a snow avalanche hazard map of the most avalanche-prone mountain range in the Czech Republic, the Krkonose Mountains. The map was prepared using historical records of 1132 avalanches which occurred over the last 54 years and state-of-the-art modelling of avalanche propagation and the spatial distribution of potential avalanche source areas. The map provides not only reliable and easy to understand information for the Mountain Rescue Service of the Czech Republic and mountain tourists, but also for land use managers to identify areas where new avalanche paths may develop under favourable conditions, including the total removal of forest cover.

В первой главе проекта рассмотрены разделы: общие сведения о лавинах, классификация лавин, основные факторы лавинообразования. Во второй главе рассмотрены инженерные противолавинные сооружения. На основании чего делается вывод о необходимости совершенствования ныне существующих методов борьбы со снежными лавинами. В третьей главе на основе расчета описаны преимущества усовершенствованной противолавинной системы., The section such as general information about snow avalanches, the classification of snow avalanches and avalanche main factors were considered in the first chapter of the project. Engineering avalanche structures were discussed in the second chapter. Based on this it was concluded that the need to improve existing methods of struggle against avalanches. The advantages of the improved system was described on the basis of calculation in the third chapter.

Predicting the occurrence and spatial extent of extreme avalanches is a longstanding issue. Using field data pooled from various sites within the same mountain range, authors showed that the avalanche size distribution can be described using either an extreme value distribution or a thick-tailed distribution, which implies that although they are much larger than common avalanches, extreme avalanches belong to the same population of events as "small" avalanches. Yet, when looking at historical records of catastrophic avalanches, archives reveal that a few avalanches had features that made them "extra-ordinary." Applying avalanche-dynamics or statistical models to simulate these past events runs into considerable difficulty since the model parameters or the statical properties are very different from the values usually set to model extreme avalanches. Were these events genuine outliers (also called "dragon-kings")? What were their distinctive features? This paper reviews some of the concepts in use to model extreme events, gives examples of processes that were at play in extreme avalanches, and shows that the concept of dragon-king avalanches is of particular relevance to describing some extreme avalanches.

In recent years, the Government of the Russian Federation considerably increased attention to the exploitation of the Russian Arctic territories. Simultaneously, the evaluation of snow avalanches danger was enhanced with the aim to decrease fatalities and reduce economic losses. However, it turned out that solely reporting the degree of avalanche danger is not sufficient. Instead, quantitative information on probabilistic parameters of natural hazards, the characteristics of their effects on the environment and possibly resulting losses is increasingly needed. Such information allows for the estimation of risk, including risk related to snow avalanches. Here, snow avalanche risk is quantified for the Khibiny Mountains, one of the most industrialized parts of the Russian Arctic: Major parts of the territory have an acceptable degree of individual snow avalanche risk (1×10-4) degree of individual snow avalanche risk (0.5 and 2% of the total area) correspond to the Southeast of the Khibiny Mountains where settlements and mining industries are situated. Moreover, due to an increase in winter tourism, some traffic infrastructure is located in valleys with an admissible or unacceptable degree of individual snow avalanches risk.

Natural Hazards, 59 (3), ISSN:0921-030X, ISSN:1573-0840

The purpose of this paper is to develop a model to describe the snow avalanche dynamics emphasizing the phenomenon of entrainment, the shape variation and the velocity profile thanks to the peculiar features of non-Newtonian fluids, in particular those showing shear thinning and Bingham-like constitutive behaviours. Two different approaches are proposed to simulate the avalanches numerically. The first considers the transformation of the avalanche domain into a simple shape domain that does not change in time. The second is based on the level set method, which is suitable for free boundary problems. Finally, the characteristics of the variation of the interface between avalanche and snowcover under a similarity hypothesis is put forward. The model is validated through the comparison with some experimental data.

_, Geomorphology for Society, Conference proceedings

With the aim of better understanding avalanche risk in the Catalan Pyrenees, the present work focuses on the analysis of major (or destructive) avalanches. For such purpose major avalanche cartography was made by an exhaustive photointerpretation of several flights, winter and summer field surveys and inquiries to local population. Major avalanche events were used to quantify the magnitude of the episodes during which they occurred, and a Major Avalanche Activity Magnitude Index (MAAMI) was developed. This index is based on the number of major avalanches registered and its estimated frequency in a given time period, hence it quantifies the magnitude of a major avalanche episode or winter. Furthermore, it permits a comparison of the magnitude between major avalanche episodes in a given mountain range, or between mountain ranges, and for a long enough period, it should allow analysis of temporal trends. Major episodes from winter 1995/96 to 2013/14 were reconstructed. Their magnitude, frequency and extent were also assessed. During the last 19 winters, the episodes of January 22–23 and February 6–8 in 1996 were those with highest MAAMI values, followed by January 30–31, 2003, January 29, 2006, and January 24–25, 2014. To analyze the whole twentieth century, a simplified MAAMI was defined in order to attain the same purpose with a less complete dataset. With less accuracy, the same parameters were obtained at winter time resolution throughout the twentieth century. Again, 1995/96 winter had the highest MAAMI value followed by 1971/72, 1974/75 and 1937/38 winter seasons. The analysis of the spatial extent of the different episodes allowed refining the demarcation of nivological regions, and improving our knowledge about the atmospheric patterns that cause major episodes and their climatic interpretation. In some cases, the importance of considering a major avalanche episode as the result of a previous preparatory period, followed by a triggering one was revealed.

Modelling of dynamic processes in physical geography is really quickly developing discipline. This trend is noticeable in case of snow avalanche modelling too. Following bachelor thesis is mapping snow avalanche modells and describes theirs basic principes. In the aplication part is thesis focused on use RAMMS numerical avalanche model. Experimentally verifies suitabiliti of friction coeficients for use in condicions prevailing in the Giant Mountains (Krkonoše). Friction coeficients was experimentaly stated in the Swiss Alps and tabooed in user manual. Results of RAMMS avalanche model are based on a combinaton of 36 values of friction coeficients. These results of numerical models are compared with real avalanche from 10. 2. 2015. Between real avalanche and modeled avalanche is compared run- out and area of acumulation. Friction coeficients asumed in Swiss Alps can be used for modelling avalanches in Giant Mountains. For correct dessignation of friction parameters is appropriately use run out. Results of modells strongly overvalue area of acumulation. Abstrakt Modelování fyzickogeografických procesů je dynamicky rozvíjející se obor, tudíž stejný trend sledujeme v případě modelování sněhových lavin. Tato bakalářská práce modely sněhových lavin mapuje a popisuje jejich základní principy. V aplikační...

Road accessibility is highly strategic for the maintenance of economic activities but also for the emergency services. In mountains, snow avalanches are a particularly strong threat because, in addition to the victims and direct damage, they cause a loss of accessibility more or less prolonged when the networks are already strongly altered by seasonal closures. Specifically, risk to traffic roads caused by snow avalanches has been very rarely assessed at a regional scale. To assess the physical, human and functional vulnerabilities of road networks in three Alpine departments (Alpes-de-Haute-Provence, Hautes-Alpes, Alpes-Maritimes), the first step of this research was to geo-locate and harmonize within a GIS all information sources about the consequences of avalanches on roads. This allowed identifying the road impacts of avalanches since 1937, to characterize the intensity and typology of damages and to evaluate the functional vulnerability of networks. The second step was to produce simple risk indexes of dysfunction and isolation at this regional scale. These indicators were modeled using the graph theory in a GIS framework, integrating avalanche activity indicators derived from the past activity with the road network. The obtained output maps should facilitate the decision support for crisis management and a comparative spatial analysis at the regional scale.

Long term avalanche risk quantification for mapping and the design of defense structures is done in mostcountries on the basis of high magnitude events. Such return period/level approaches, purely hazardoriented,do not consider elements at risk (buildings, people inside, etc.) explicitly, and neglect possiblebudgetary constraints. To overcome these limitations, risk based zoning methods and cost-benefit analyseshave emerged recently. They combine the hazard distribution and vulnerability relations for the elementsat risk. Hence, the systematic vulnerability assessment of buildings can lead to better quantify the riskin avalanche paths. However, in practice, available vulnerability relations remain mostly limited to scarceempirical estimates derived from the analysis of a few catastrophic events. Besides, existing risk-basedmethods remain computationally intensive, and based on discussable assumptions regarding hazard modelling(choice of few scenarios, little consideration of extreme values, etc.). In this thesis, we tackle theseproblems by building reliability-based fragility relations to snow avalanches for several building types andpeople inside them, and incorporating these relations in a risk quantification and defense structure optimaldesign framework. So, we enrich the avalanche vulnerability and risk toolboxes with approaches of variouscomplexity, usable in practice in different conditions, depending on the case study and on the time availableto conduct the study. The developments made are detailed in four papers/chapters.In paper one, we derive fragility curves associated to different limit states for various reinforced concrete(RC) buildings loaded by an avalanche-like uniform pressure. Numerical methods to describe the RCbehaviour consist in civil engineering abacus and a yield line theory model, to make the computations asfast as possible. Different uncertainty propagation techniques enable to quantify fragility relations linkingpressure to failure probabilities, study the weight of the different parameters and the different assumptionsregarding the probabilistic modelling of the joint input distribution. In paper two, the approach is extendedto more complex numerical building models, namely a mass-spring and a finite elements one. Hence, muchmore realistic descriptions of RC walls are obtained, which are useful for complex case studies for whichdetailed investigations are required. However, the idea is still to derive fragility curves with the simpler,faster to run, but well validated mass-spring model, in a “physically-based meta-modelling” spirit. Inpaper three, we have various fragility relations for RC buildings at hand, thus we propose new relationsrelating death probability of people inside them to avalanche load. Second, these two sets of fragilitycurves for buildings and human are exploited in a comprehensive risk sensitivity analysis. By this way,we highlight the gap that can exist between return period based zoning methods and acceptable riskthresholds. We also show the higher robustness to vulnerability relations of optimal design approaches ona typical dam design case. In paper four, we propose simplified analytical risk formulas based on extremevalue statistics to quantify risk and perform the optimal design of an avalanche dam in an efficient way. Asensitivity study is conducted to assess the influence of the chosen statistical distributions and flow-obstacleinteraction law, highlighting the need for precise risk evaluations to well characterise the tail behaviour ofextreme runouts and the predominant patterns in avalanche - structure interactions., La quantification du risque avalanche à long terme dans un but de zonage et d'optimisation des moyens de protection est fait dans la plupart des pays sur la base de la connaissance des événements de forte intensité. Ces approches fondées sur les périodes de retours, centrées uniquement sur l'aléa, ne considèrent pas explicitement les éléments à risque étudiés (bâtiments, personnes à l'intérieur, etc.) et négligent les possibles contraintes budgétaires. Afin de palier à ces limitations, les méthodes de zonage basés sur le risque et les analyses coût-bénéfice ont récemment émergées. Elles combinent la distribution de l'aléa avec les relations de vulnérabilité des éléments étudiés. Ainsi, l'évaluation systématisée de la vulnérabilité des bâtiments permet de mieux quantifier le risque dans un couloir d'avalanche donné. Cependant, en pratique, les relations de vulnérabilité disponibles restent principalement limitées à de rares estimations empiriques déduites de l'analyse de quelques catastrophes survenues. De plus, les méthodes existantes basées sur le risque font face à des calculs encore lourds, et les hypothèses sur la modélisation de l'aléa sont discutables (choix de quelques scénarios, faible considération des valeurs extrêmes, etc.). Dans cette thèse, ces problèmes sont abordés en construisant grâce à une approche fiabiliste des relations de fragilité de différents configurations de bâtiments en béton armé (BA) sollicités par des avalanches de neige et également des relations de fragilité pour les personnes potentiellement à l'intérieur de ces bâtiments. Ces relations sont ensuite utilisées dans un cadre de quantification du risque et de recherche de structure de défense optimale. L'apport de cette thèse est donc l'enrichissement de la caractérisation de la vulnérabilité et du risque face aux avalanches par des approches de complexités variables utilisables en fonction de la spécificité du cas et du temps imparti pour conduire l'étude. La thèse est composée de quatre volets. D'abord, les courbes de fragilité associées à différents états limites de murs en BA soumis au chargement uniforme d'une avalanche sont obtenues à partir d'approches classiques de dimensionnement du BA. Ensuite, l'approche est étendue à des modèles numériques de bâtis plus riches (modèle masse-ressort) permettant de décrire en particulier l'évolution temporelle de la réponse du système. A partir de ces relations de fragilité, de nouvelles relations pour les personnes à l'intérieur de ces bâtiments sont proposées. Ces relations pour les bâtiments et les personnes sont utilisées dans une analyse complète de sensibilité du risque. Enfin, une formule analytique du risque basée sur la statistique des valeurs extrêmes est proposée pour efficacement quantifier le risque et obtenir une caractéristique optimale de digue paravalanche.