Shallow water waves generated by subaerial solid landslides

Subaerial landslides are common events, which may generate very large water waves. The numerical modelling and simulation of these events are thus of primary interest for forecasting and mitigation of tsunami disasters. In this paper, we aim at describing these extreme events using a simplified shallow water model to derive relevant scaling laws. To cope with the problem, two different numerical codes are employed: one, SPHysics, is based on a Lagrangian meshless approach to accurately describe the impact stage whereas the other, Gerris, based on a two-phase finite-volume method is used to study the propagation of the wave. To validate Gerris for this very particular problem, two numerical cases of the literature are reproduced: a vertical sinking box and a 2-D wedge sliding down a slope. Then, to get insights into the problem of subaerial landslide-generated tsunamis and to further validate the codes for this case of landslides, a series of experiments is conducted in a water wave tank and successfully compared with the results of both codes. Based on a simplified approach, we derive different scaling laws in excellent agreement with the experiments and numerical simulation

Correction: Experimental and theoretical study of magnetohydrodynamic ship models

[This corrects the article DOI: 10.1371/journal.pone.0178599.]

Les instabilités hydrodynamiques dans les écoulements granulaires géophysiques

De par le vaste champ d’applications qu’elle offre, l’étude des écoulements granulaires a connu une expansion considérable au cours de ces vingt dernières années, autant du point de vue industriel que géophysique. Ces écoulements granulaires ont une grande influence dans le monde qui nous entoure. Or, différentes instabilités hydrodynamiques peuvent naitre en leur sein, entrainant alors des changements importants des propriétés mêmes de l’écoulement. Les instabilités dues à la ségrégation par taille de particules, le développement d’ondes de surface ou encore l’apparition de ressauts dans l’écoulement en sont des exemples marquants

The kinematics of bidisperse granular roll waves

Small perturbations to a steady uniform granular chute flow can grow as the material moves downslope and develop into a series of surface waves that travel faster than the bulk flow. This roll wave instability has important implications for the mitigation of hazards due to geophysical mass flows, such as snow avalanches, debris flows and landslides, because the resulting waves tend to merge and become much deeper and more destructive than the uniform flow from which they form. Natural flows are usually highly polydisperse and their dynamics is significantly complicated by the particle size segregation that occurs within them. This study investigates the kinematics of such flows theoretically and through small-scale experiments that use a mixture of large and small glass spheres. It is shown that large particles, which segregate to the surface of the flow, are always concentrated near the crests of roll waves. There are different mechanisms for this depending on the relative speed of the waves, compared to the speed of particles at the free surface, as well as on the particle concentration. If all particles at the surface travel more slowly than the waves, the large particles become concentrated as the shock-like wavefronts pass them. This is due to a concertina-like effect in the frame of the moving wave, in which large particles move slowly backwards through the crest, but travel quickly in the troughs between the crests. If, instead, some particles on the surface travel more quickly than the wave and some move slower, then, at low concentrations, large particles can move towards the wave crest from both the forward and rearward sides. This results in isolated regions of large particles that are trapped at the crest of each wave, separated by regions where the flow is thinner and free of large particles. There is also a third regime arising when all surface particles travel faster than the waves, which has large particles present everywhere but with a sharp increase in their concentration towards the wave fronts. In all cases, the significantly enhanced large particle concentration at wave crests means that such flows in nature can be especially destructive and thus particularly hazardous

Multiple solutions for granular flow over a smooth two-dimensional bump

Geophysical granular flows, such as avalanches, debris flows, lahars and pyroclastic flows, are always strongly influenced by the basal topography that they flow over. In particular, localised bumps or obstacles can generate rapid changes in the flow thickness and velocity, or shock waves, which dissipate significant amounts of energy. Understanding how a granular material is affected by the underlying topography is therefore crucial for hazard mitigation purposes, for example to improve the design of deflecting or catching dams for snow avalanches. Moreover, the interactions with solid boundaries can also have important applications in industrial processes. In this paper, small-scale experiments are performed to investigate the flow of a granular avalanche over a two-dimensional smooth symmetrical bump. The experiments show that, depending on the initial conditions, two different steady-state regimes can be observed: either the formation of a detached jet downstream of the bump, or a shock upstream of it. The transition between the two cases can be controlled by adding varying amounts of erodible particles in front of the obstacle. A depth-averaged terrain-following avalanche theory that is formulated in curvilinear coordinates is used to model the system. The results show good agreement with the experiments for both regimes. For the case of a shock, time-dependent numerical simulations of the full system show the evolution to the equilibrium state, as well as the deposition of particles upstream of the bump when the inflow ceases. The terrain-following theory is compared to a standard depth-averaged avalanche model in an aligned Cartesian coordinate system. For this very sensitive problem, it is shown that the steady-shock regime is captured significantly better by the terrain-following avalanche model, and that the standard theory is unable to predict the take-off point of the jet. To retain the practical simplicity of using Cartesian coordinates, but have the improved predictive power of the terrain-following model, a coordinate mapping is used to transform the terrain-following equations from curvilinear to Cartesian coordinates. The terrain-following model, in Cartesian coordinates, makes identical predictions to the original curvilinear formulation, but is much simpler to implement

Tsunami generated by a granular collapse down a rough inclined plane

In this Letter, we experimentally investigate the collapse of initially dry granular media into water and the subsequent impulse waves. We systematically characterize the influence of the slope angle and the granular material on the initial amplitude of the generated leading wave and the evolution of its amplitude during the propagation. The experiments show that whereas the evolution of the leading wave during the propagation is well predicted by a solution of the linearized Korteweg-de Vries equation, the generation of the wave is more complicated to describe. Our results suggest that the internal properties of the granular media and the interplay with the surrounding fluid are important parameters for the generation of waves at low velocity impacts. Moreover, the amplitude of the leading wave reaches a maximum value at large slope angle. The runout distance of the collapse is also shown to be smaller in the presence of water than under totally dry conditions. This study provides a first insight into tsunamis generated by subaerial landslides at low Froude number

Granular collapse into water: toward tsunami landslides

International audienc

Oedenburg : une agglomération d'époque romaine sur le Rhin Supérieur : fouilles francaises, allemandes et suisses sur les communes de Biesheim et Kunheim (Haut-Rhin)

L’établissement gallo-romain d’Oedenburg à Biesheim et Kunheim (Haut-Rhin, France) est fouillé depuis 1998 par une équipe internationale (École pratique des hautes études, Paris ; université de Fribourg-en-Brisgau, Allemagne ; université de Bâle, Suisse). Le site, au bord du Rhin actuel, en face de l’oppidum celtique de Breisach, était, dans l’Antiquité, installé dans un paysage sensiblement différent, traversé alors par différents chenaux du fleuve. Dans l’état actuel des connaissances, les lieux semblent occupés, au début du règne de Tibère, par un camp militaire romain dépendant de la zone de commandement de Vindonissa, au nord du territoire rauraque. Un établissement civil se développe dans le même temps, faisant peut-être suite à une occupation indigène sporadique. Dès le milieu du Ier s. apr. J.-C. sont perceptibles des traces de parcellaire dans le plan d’urbanisme, mais l’agglomération (qu’on identifie de manière hypothétique avec Argentovaria) n’est pas organisée comme un chef-lieu de cité ; on n’y rencontre en effet ni centre public ni trame viaire régulière, mais un complexe religieux composé de plusieurs temples de type indigène. Après le départ des militaires, vers 69-70, l’établissement civil se développe de manière autonome. D’importantes études archéozoologiques et paléobotaniques, favorisées par des conditions de conservation exceptionnelles, mettent en évidence les processus de romanisation et l’introduction de pratiques alimentaires nouvelles. On ignore pour l’instant si le site fut remilitarisé dans la seconde moitié du IIIe s., au moment où le Rhin redevint frontière. Les constructions de l’Antiquité tardive se concentrent alors sur les zones qui échappent aux inondations. C’est seulement sous Valentinien qu’on voit apparaître sur la rive gauche un palais fortifié, qui répond sans doute à la forteresse de Breisach. Le site n’est pas abandonné après les invasions du début du Ve s. Plusieurs églises successives et une nécropole s’installent ensuite sur les ruines de l’ancienne fortification