Les systèmes amorphes "mous", loin de leur transition vitreuse, comprennent les verres colloïdaux de hautefraction volumique, les émulsions concentrées, les mousses, etc...L’échelle de temps pour leur relaxationmicroscopique est divergente, et ils ne se mettent en écoulement que lorsque la contraint appliquée estsuffisamment grande. Cette transition dynamique d’un état apparemment solide à un état apparemmentliquide, suivant la contrainte imposée, est appelé transition d’écoulement. Cette transition est étudiéedans cette thèse par l’intermédiaire d’une modélisation mésoscopique, basé sur un modèle d’élémentsélasto-plastiques en interaction.Après une brève introduction à la transition vitreuse et aux systèmes réels supposés être décrit parle modèle élast-plastique, une formulation du modèle différente de celle qui est habituellement présentéedans la littérature est introduite, pour à la fois incorporer les protocole à taux de cisaillement fixé et leprotocole à contraint fixée. A travers des approximations, un modèle mésoscopique de type champ-moyen(à l’origine décrit par Hébraud et Lequeux) est déduit à partir du modèle elasto-plastique qui contient lesinformations spatiales.En appliquant le protocole à taux de cisaillement fixé, le transition d’écoulement est dans un premiertemps étudiée à travers la dépendance de la statistique des avalanches (chutes de contrainte) en taux decisaillement. Une transition d’un comportement de champ moyen à un comportement corrélé est observéen variant le taux de cisaillement. Les lois d’échelle observées dans la limite des petits taux de cisaillementsupportent l’idée que la transition d’écoulement appartient à une certaine classe d’universalité de transitiondynamique. L’étude de la symétrie de la forme moyenne des chutes de contrainte en fonction de leur durée,de la taille de système et du taux de cisaillement appliqué, conduit à l’interprétation que les chutes decontrainte résultent d’une superposition d’avalanches individuelles possédant une longueur coopérative etun temps coopératif.En étudiant les fluctuations de contrainte macroscopique, la longueur coopérative l_c est identifiéepar un crossover en taille de système en-dessous de laquelle le loi d’échelle avec la taille de système1/L^d impliquée par la théorème de limite centrale, ne fonctionne plus. En complément, une échelle detemps de saturation T_c est trouvé dans le séries temporaire de taux de cisaillement plastique, tempsen-dessous duquel la dynamique de la contrainte peut être décrite par un mouvement Brownien. Le tempsde saturation, pour les systèmes de taille plus petit que l_c obéit à une loi d’échelle avec la taille de systèmeT_c~(l_c)^z. Cette dernière peut être interprétée comme la loi d’échelle entre la longueur coopérative et letemps coopératif des avalanches individuelles.En appliquant le protocole de contrainte imposée, la transition d’écoulement est étudiée en simulantdes expériences de fluage sur les systèmes amorphes. Les modèles mésoscopiques (le modèle elasto-plastiqueet le modèle champ moyen de Hébraud-Lequeux) sont capables de reproduire la réponse du taux decisaillement macroscopique pour une contrainte imposée légèrement au-dessus de la contrainte, et ceciqualitativement en accord avec les expériences. A travers cette étude, il apparaît que le condition initialeinfluence significativement le comportement de fluage des systèmes amorphes. / Amorphous systems deep blow the glass transition, as well as colloidal glasses at high packing fractions,concentrated emulsions, foam systems, etc. exhibit divergent microscopic relaxation time scales and flowonly upon a large enough external loading. This dynamical phase transition of amorphous systems fromthe apparent solid state to the apparent liquid state mediated by the external loading, is called theyielding transition. This transition is studied throughout this thesis by a mesoscopic modeling approach,specifically versions of the so-called elasto-plastic model.After introducing a general background of the glass transition and experimental systems, that are thetarget of the elasto-plastic model description, a formulation of the elasto-plastic model, slightly differentfrom the conventional ones used in the literature, is introduced for incorporating both the shear ratecontrol and the stress control protocols. It is also shown that the mean-field Hebraud-Lequeux model canbe derived from the spatially resolved elasto-plastic model by assuming some approximations.Using the shear rate control protocol, the yielding transition is firstly probed by studying the shearrate dependence of the avalanche statistics close to criticality. A crossover from a non mean-field behaviorto an apparent mean-field behavior with respect to an increasing shear rate is evidenced. Scaling laws in thezero shear rate limit, support the idea that the yielding transition belongs to a non mean-field universalityclass of a dynamical phase transition. The dependence of the symmetry of the average shape of the stressdrops on the stress drop duration, the system size and the shear rate, leads to the interpretation that stressdrops at finite shear rates result from the superposition of individual avalanches possessing a cooperativelength and time scale.By studying the macroscopic stress fluctuation, the cooperative length scale l_c is identified as thecrossover size below which the scaling relation with the system size 1/L^d implied by the central limittheorem breaks down. Further a saturation time scale T_c can be defined in the analysis of the timeseries of macroscopic plastic strain rate. Below this time scale one observes the manifestation of Browniandynamics. The saturation time for systems of sizes smaller than the cooperative length l_c scales withthe system size as a power law T_c~(l_c)^z, which can be interpreted as the scaling relation between thecooperative time and the cooperative length of individual avalanches.Further using the stress controlled protocol, the yielding transition is studied by simulating typical creep experiments of the amorphous systems. The mesoscopic models (the elasto-plastic model aswell as the mean-field Hébraud-Lequeux model) are shown to be capable to reproduce the response ofthe macroscopic shear rate to an imposed stress slightly above the yielding point in qualitatively goodagreement with several experiments. Within the mesoscopic modeling approach, the results reveal thatthe creep behavior depends strongly on the initial condition of the amorphous system submitted to creepexperiments.

L'auteur n'a pas fourni de résumé en français / Experimental study of the influence of protection structures on avalanches and impact pressures Abstract: In the frame of snow avalanche protection, the optimisation of defence structure design depends on the understanding of the flow dynamics and on a exhaustive knowledge of the flow-obstacle interaction. The study presented here utilises a mainly experimental approach. Small-scale laboratory tests were combined with field measurements and observations. Dense snow avalanches are modelled by granular materials. Dry cohesionless and mono-dispersed glass beads are released down an inclined channel. Reference tests (with no obstacles) were carried out in order to characterise the flow dynamic properties, and an obstacle was then mounted and force measurements were taken. The geometry of two obstacles was tested: a large wall spanning the whole width of the flow and a narrower wall allowing lateral flows. Results showed that an influence zone forms uphill from the obstacle and plays an important role in the flow dynamics. An analysis of this zone was carried out, together with precise measurements of the flow depth (laser technique), surface velocity (PIV) and impact forces (force sensors). In relation to density currents, powder snow avalanches are modelled by a dyed salt solution flowing down an inclined channel immersed in a water tank. We investigated the influence of two different catching-dam-type obstacles on the flow behaviour with respect to reference conditions. The maximum flow height and its front and core velocity were measured by means of image processing and ultrasound Doppler velocimetry. Results mainly showed the higher effectiveness of a dam with vertical uphill face rather than inclined and underscored the importance of the velocity norm in the computation of the total incoming flow velocity. In-situ full-scale measurements complement small-scale laboratory tests. A new full-scale experimental site was implemented on the existing avalanche defence system of Taconnaz (Chamonix, France). Three breaking mounds were equipped with velocity and pressure sensors. The reference event, the rough site conditions, the need for data synchronization and remote access defined the design constraints. In December 2010, the first event was recorded, which proved that the conceived system works effectively and also provided the first set of data. Preliminary results showed very high pressure peaks with high impact pressures even for low velocity regimes and thus a drag coefficient which increases when the Froude number decreases. Isolated rocks or ice blocks struck the sensors, contributing significantly to the total energy released by the avalanche. This work provided effective inputs for numerical and analytical models and enhanced the current knowledge of avalanche dynamics in order to optimise the future design guidelines for avalanche protection structures. Keywords: Snow avalanches, impact pressure, laboratory experiments, dense avalanches, granular flows, influence zone, dead zone to granular jump transition, powder avalanches, density currents, ultrasound Doppler velocimetry, full-scale measurements.

Caractériser les propriétés mécaniques de la neige est un défi majeur pour la prévision et la prédétermination du risque d’avalanche. Du fait du grand nombre de types de neige et de la difficulté à effectuer des mesures sur ce matériau très fragile, la compréhension de la relation entre la microstructure de la neige et ses propriétés mécaniques est encore incomplète. Cette thèse aborde ce problème par le biais d’une approche de modélisation mécanique basée sur la microstructure tridimensionnelle de neige obtenue par microtomographie aux rayons X. Tout d’abord, afin d’automatiser et améliorer la segmentation des images microtomographiques, un nouvel algorithme tirant profit de la minimisation de l’énergie de surface de la neige a été développé et évalué. L’image air-glace est ensuite utilisée comme entrée géométrique d’un modèle éléments finis où la glace est supposée élastique fragile. Ce modèle permet de reproduire le comportement fragile en traction et révèle le comportement pseudoplastique apparent causé par l’endommagement microscopique, ainsi que la forte hétérogénéité des contraintes dans la matrice de glace. Pour reproduire les grandes déformations impliquant le ré-arrangement de grains, un modèle par éléments discrets a ensuite été développé. Les grains sont identifiés dans la microstructure en utilisant des critères géométriques dont la pertinence mécanique a été démontrée, et décrits dans le modèle par des blocs rigides de sphères. Le comportement simulé en compression est dominé par le rôle de la densité mais révèle également des différences liées au type de neige. Enfin, pour distinguer le degré de cohésion entre les types de neige, un indicateur microstructurel a été développé et s’est avéré être fortement corrélé aux propriétés mécaniques et physiques du matériau. / Characterizing the complex microstructure of snow and its mechanics is a major challenge for avalanche forecasting and hazard mapping. While the effect of environmental conditions on the snow metamorphism, which leads to numerous snow types, is fairly known, the relation between snow microstructure and mechanical properties is poorly understood because of the very fragile nature of snow. In order to decipher this relation for dry snow, this thesis presents a modeling approach of snow mechanics based on the three-dimensional microstructure of snow captured by X-ray microtomography and the properties of ice. First, in order to automatically process the microtomographic data, we take advantage of the minimization of the snow surface energy through metamorphism to efficiently binary segment grayscale images. Second, assuming an elastic brittle behavior of the ice matrix, the tensile strength of snow is modeled via a finite element approach. The model reveals an apparent pseudo-plastic behavior caused by damage, and the highly heterogenous stress distribution in the ice matrix. Third, we develop a discrete element model, accounting for grain-rearrangements and the creation/failure of inter-granular contacts. The grains, geometric input of the model, are detected in the microstructure with mechanically-relevant criteria and described as rigid clumps of spheres. The model evidences that the compression behavior of snow is mainly controlled by density but that the first stage of deformation is also sensible to the snow type. Last, the inter-granular bonds, recognized to be critical for the mechanical properties, are characterized through a new microstructural indicator, which effectively highly correlates with the simulated mechanical and physical properties.

Over a span of six days from November 2-7, 2006 approximately 43 cm of precipitation fell over the Hood River Basin in Oregon. A lahar was initiated on the Eliot Branch of the Middle Fork Hood River by two or more landslides that occurred on the lateral moraines of the Eliot Glacier on the early part of November 7th, 2006. The Eliot Branch lahar was embedded within the larger regional flood that was occurring in the Hood River Basin and traveled a total of 48 km from the initiation points on the north flank of Mount Hood to the Hood Rivers confluence with the Columbia River. The initiating landslides abruptly transformed into a debris flow upon mixing with flood waters of the Eliot Branch. The debris flow traveled a distance of ~28 km at which point it was transformed first to a hyperconcentrated flow and then to water flow via selective deposition of coarse sediment and progressive dilution by channel flow waters from the East and West Fork Hood Rivers. The transformation from debris flow to hyperconcentrated streamflow was recorded by a thickening wedge of hyperconcentrated streamflow sediments found above and below progressively fining debris flow sediments over a reach of 22 km. Finally, the hyperconcentrated-flow phase of the lahar transformed to water flow and then traveled an additional 20 km to the Hood River delta. Upon reaching the apex of the Hood River delta, depositing sediments led to an expansion of the delta. Debris-flow sediments were predominantly gravel (36.0-69.7% by wt.) with sand (22.1-55.9% by wt.) and fines (4.7-7.8% by wt.). Hyperconcentrated flow deposits contained a larger sand fraction of (66.8-99.2% by wt.) with few gravel clasts (0-26.0% by wt.) and fines (0-8.8% by wt.). Water flow deposits averaged 90.5% (wt.) sand with 6.0% (wt.) gravel and 3.0% (wt.) fines. Sorting was a key factor in flow identification and showed progressive improvement downstream from the initiation point. Sorting values for the flow types are as follows: debris flow deposits ranged from 3.3Φ (very poorly sorted) to 1.8Φ (poorly sorted), hyperconcentrated flow deposits ranged from 2.4Φ (very poorly sorted) to 0.8Φ (moderately sorted), and water flood deposits ranged between 1.4Φ (poorly sorted) to 0.6Φ (moderately sorted).

published_or_final_version / Applied Geosciences / Master / Master of Science

Thesis (Ph.D.)--University of Washington, 1995. / Vita. Includes bibliographical references (leaves [101-116).

Thesis (M.S.)--West Virginia University, 1999. / Title from document title page. Document formatted into pages; contains ix, 61 p. : ill. (some col.), maps (some col.) Includes abstract. Includes bibliographical references (p. 55-59).

Thesis (M.S.)--Brigham Young University. Dept. of Geological Sciences, 2007. / Includes map in back cover pocket. Includes bibliographical references (p. 29-30).

Various dendrochronological methods for avalanche reconstructions was applied on tree-ring records of dominant Norway spruce (Picea abies) in the Giant Mountains National Park. The individual growth chronologies from studied avalanche paths (Pramenný důl, Navorská jáma, Pančavská stěna, Velká Studniční jáma and Úpská jáma) proved that climate is the main driving factor of intra-annual variation of tree growth. Traumatic resin ducts are formed with delay up to 5 years. Combination of two and more tree-ring characteristics is suitable for reconstruction of avalanche events. I achieved to reconstruct undocumented avalanches. I conclude that dendrochronology is suitable tool for avalanche reconstruction; however, high pollutant emission highly reduced the potential of gained results.

No description available.