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

Nonconservative higher-order hydrodynamic modulation instability

The modulation instability (MI) is a universal mechanism that is responsible for the disintegration of weakly nonlinear narrow-banded wave fields and the emergence of localized extreme events in dispersive media. The instability dynamics is naturally triggered, when unstable energy side-bands located around the main energy peak are excited and then follow an exponential growth law. As a consequence of four wave mixing effect, these primary side-bands generate an infinite number of additional side-bands, forming a triangular side-band cascade. After saturation, it is expected that the system experiences a return to initial conditions followed by a spectral recurrence dynamics. Much complex nonlinear wave field motion is expected, when the secondary or successive side-band pair that are created are also located in the finite instability gain range around the main carrier frequency peak. This latter process is referred to as higher-order MI. We report a numerical and experimental study that confirm observation of higher-order MI dynamics in water waves. Furthermore, we show that the presence of weak dissipation may counter-intuitively enhance wave focusing in the second recurrent cycle of wave amplification. The interdisciplinary weakly nonlinear approach in addressing the evolution of unstable nonlinear waves dynamics may find significant resonance in other nonlinear dispersive media in physics, such as optics, solids, superfluids and plasma

Run Up of Surface and Internal Waves

The evolution of breaking waves propagating towards the shore and more specifically the run-up phase over the swash-zone for surface as well as for internal waves is considered. The study is based on a) laboratory run up experiments for surface waves ; b) laboratory stratified flow experiments and c) on field data describing the internal wave run up. The presentation is focused on the breaking and energy transfer mechanisms common to surface and internal waves in the swash-zone. The mathematical model taking into account turbulent mixing and dispersion effects is discussed

Space-time measurement of breaking wave kinematics and void fraction in the surf zone

Particle Image Velocimetry (PIV) measurements were performed in a wave tank under regular water waves propagating and breaking on a 1=15 sloping beach. To obtain more accurate measurements, the domain from the initiation of the breaking to the swash zone is splited in fourteen overlapping windows. The full field is reconstructed at each time step by gathering the fourteen synchronized PIV fields. We measured then the complete space-time evolution of velocity field over the whole surf zone. From these data the ensemble-period-average and phase-average components of the flow with their associated fluctuating parts were computed. Moreover using the light intensity of the PIV images and velocity measurements, we estimated the void fraction in each point of the surf zone and at each time step

Propriétés cinématiques des vagues déferlantes sur une plage inclinée / Breaking wave kinematics on sloping beaches

12 p.National audienceDes mesures de champs de vitesses utilisant la technique de vélocimétrie par image de particules ont été réalisées pour des vagues déferlantes sur une plage inclinée. Le problème dans ce type d'écoulement est que pour obtenir une définition suffisante en vitesse, la taille des images de mesure doit rester petite, alors que le phénomène observé prend place sur plusieurs longueurs d'onde. Pour obtenir une information pertinente, sur toute la zone de surf, le champ de mesure a été scindé en 13 parties de taille identique et les images ont été assemblées en synchronisant les données laser et les données de sondes à vagues. A partir de ces champs assemblés, il est possible d'obtenir des informations statistiques telles que les moyennes de phase et les fluctuations associées. Une attention toute particulière est donnée au calcul des gradients permettant d'obtenir les diverses grandeurs physiques liées aux fluctuations

Laboratory PIV Measurements of Wave Breaking on Beach

ISBN 1-880653-62-1Experiment were conducted in the ESIM wave tank in Marseilles in order to study the space/time evolution of the flow field under waves breaking on an 1/15 beach slope. Fluid particles velocities were measured at different depths, before, during and after the breaking event using the Particle Imaging Velocimetry (PIV) technique. The mean and fluctuant components of the flow were calculated

Effondrement granulaire dans l'eau : application à la génération de tsunami

Qu'il s'agisse du remplissage d'un silo à grain ou de la distance parcouru par un glissement de terrain, la dynamique des écoulements granulaires est présente aussi bien dans des domaines industriels que géophysiques. Si les différentes configurations d'écoulements granulaires secs sont assez bien prédites par la rhéologie mu(I), il en est tout autrement lorsqu'on ajoute de l'eau. L'objectif de cette étude est de caractériser le comportement d'un effondrement granulaire initialement sec dans l'eau le long d'une pente d'angle variable et de déterminer son influence sur les vagues générées. Pour cela plusieurs séries d'expériences ont été menées dans un aquarium de 2.20 m de long, 0.40 m de haut et 0.20 m de large. Afin de quantifier l'influence des principaux paramètres de ce type d'écoulement, l'angle de la pente, la hauteur d'eau, la masse initiale de granulaire ainsi que le type de grains utilisés ont été modifiés. Pour assurer une condition de non-glissement, les pentes et fonds ont été réalisés en collant à chaque fois le même type de matériaux que l'effondrement. Une caméra rapide a été utilisée pour enregistrer l'évolution du front granulaire au cours du temps. La propagation des vagues générées est enregistrée à l'aide de quatre sondes réparties le long de l'aquarium. Les expériences montrent que l'amplitude maximale atteint par la première vague dépend directement de la masse de granulaire et de la pente. D'autres résultats moins intuitifs ont également été observés comme le fait que la vitesse et l'épaisseur du glissement granulaire restent constant au cours de l'effondrement. Afin d'interpréter ces résultats expérimentaux, une étude numérique est en cours en modélisant l'écoulement granulaire par un fluide non-Newtonien. Le développement d'une loi d'échelle sur l'amplitude de la vague générée est également menée en parallèle

Evolution of water waves generated by subaerial solid landslide

International audienceWaves generated by aerial and subaerial landslides are studied experimentally, theoretically and numerically. A set of experiments are done in a wave tank of 18 m long, 0.65 m wide and 1.5 m deep. Numerical simulations are in good agreement with the experiments. Basedon numerical and experimental results, we derive different scaling laws which show a good agreement with the experiments and the simulations. These scaling laws allow thus to predict the time evolution of the maximum amplitude wave generated by an aerial solid landslide, which is a relevant quantity for wave forecast

Numerical study of the hydrodynamics of regular waves breaking on a sloping beach

International audienceIn the last three decades, great improvements have been brought to the knowledge of the hydrodynamics and general processes occurring in the surf zone, widely affected by the breaking of the waves. Nevertheless, the turbulent flow structure is still very complicated to investigate. The aim of this work is to present and discuss the results obtained by simulating twodimensional breaking waves by solving the Navier-Stokes equations, in air and water, coupled with a dynamic subgrid scale turbulence model (Large Eddy Simulation, LES). First, the ability of the numerical tool to capture the crucial features of this complicated turbulent two-phase flow is demonstrated. Numerical results are compared with experimental observations provided by Kimmoun and Branger [1]. Spilling/plunging breaking regular waves are considered. Generally, there is good agreement and the model provides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore

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