Experimental evidence of a hydrodynamic soliton gas

We report on an experimental realization of a bi-directional soliton gas in a 34~m-long wave flume in shallow water regime. We take advantage of the fission of a sinusoidal wave to inject continuously solitons that propagate along the tank, back and forth. Despite the unavoidable damping, solitons retain adiabatically their profile, while decaying. The outcome is the formation of a stationary state characterized by a dense soliton gas whose statistical properties are well described by a pure integrable dynamics. The basic ingredient in the gas, i.e. the two-soliton interaction, is studied in details and compared favourably with the analytical solutions of the Kaup-Boussinesq integrable equation. High resolution space-time measurements of the surface elevation in the wave flume provide a unique tool for studying experimentally the whole spectrum of excitations.Comment: accepted for publication in Physical Review Letter

Experimental study of integrable turbulence in shallow water

We analyze a set of bidirectional wave experiments in a linear wave flume of which some are conducive to integrable turbulence. In all experiments the wavemaker forcing is sinusoidal and the wave motion is recorded by seven high-resolution side-looking cameras. The periodic scattering transform is implemented and power spectral densities computed to discriminate linear wave motion states from integrable turbulence and soliton gas. Values of the wavemaker forcing Ursell number and relative amplitude are required to be above some threshold values for the integral turbulence to occur. Despite the unavoidable slow damping, soliton gases achieve stationary states because of the continuous energy input by the wavemaker. The statistical properties are given in terms of probability density distribution, skewness and kurtosis. The route to integrable turbulence, by the disorganization of the wavemaker induced sinusoidal wave motion, depends on the non-linearity of the waves but equally on the amplitude amplification and reduction due to the wavemaker feedback on the wave field

Physical and numerical modelling of sand liquefaction in waves interacting with a vertical wall

International audienceWave induced liquefaction at a coastal structure is studied. Experiments in a glass-wall flume filled with a partially saturated bed of light-weight sediment are presented. Periodic waves and single wave loadings are simulated. For large enough wave conditions an excess pore pressure is recorded within the soil and a liquefaction threshold is reached. Velocity fields obtained from video recordings display large zones of the bed that behave as a fluid. Phases of soil compaction and dilatation are identified. Moreover, a Discrete Element Method - Pore-scale Finite Volume model is used to simulate the wave-sediment interactions. The computation of the coupling between the flow and the motion of the particles enables to reproduce the excess pore pressure that lead to liquefaction and the progressive compaction of the bed

Etude de couches limites oscillantes par vélocimétrie laser Doppler

International audienceLe transport sédimentaire induit par les vagues à l'approche de la côte est piloté par des processus non linéaires et turbulents. Les non-linéarités des vagues se caractérisent par une dissymétrie de vitesse (les crêtes hautes des vagues sont de courte durée et les creux peu profonds de longue durée) et une asymétrie de vitesse (ou dissymétrie d'accélération, caractérisant la raideur des fronts). Des études récentes indiquent que des fronts raides (vagues asymétriques) produisent des vitesses dissymétriques dans la couche limite. Ainsi, pour développer des formules de prédiction de transport des sédiments, la compréhension détaillée de la dynamique de la couche limite de fond et des contraintes de cisaillement sous les ondes de surface apparaît essentielle. Ceci justifie de chercher à réaliser des mesures de vitesse dans les tous premiers millimètres au-dessus du lit.Par ailleurs, la caractérisation de la turbulence sous les vagues déferlantes reste une question ouverte, en particulier pour chercher à évaluer la part provenant de la vague déferlée de celle produite par frottement au fond.Actuellement, l’essentiel de notre connaissance des couches limites oscillantes est issu de mesures réalisées sur des fonds fixes horizontaux. En laboratoire, l’évolution des non-linéarités des vagues, lors de leur propagation et de leur déferlement, a été principalement étudiée pour des plages de pentes relativement fortes (> 1:40). Cependant, des études de terrain récentes sur des plages réelles de pentes moins raides (1:80) ont montré que certains processus non-linéaires sont différents par rapport aux cas des plages de pentes relativement raides (> 1:40). Cette constatation a motivé le lancement d’une série d'expériences de laboratoire dans le cadre du projet européen GLOBEX sur une plage à fond fixe de pente 1:80

Measurements of surf zone sand bed dynamics under irregular waves

International audienceMeasurements of velocity profiles, sediment concentration, pore-pressure and sheet flow layer dynamics are analysed in order to better assess the relative importance of the processes that contribute to destabilise the bed of a sandy beach. These measurements were conducted in the surf zone of irregular waves, in a 30 m x 30 m wave tank. The effects of flow acceleration, excess pore-pressure vertical gradients across the bed and infiltration/exfiltration seepage flow are studied. The appearance of a sheet flow layer is apparently initiated by the strong accelerations in the wave fronts. These results are compared to field measurements at Truc-Vert beach (Atlantic coast of France). Probably due to the partial saturation of the bed made of finer sand in the wave tank experiments, the upwards-directed excess pore-pressure gradient could enhance the bed destabilisation compared to that observed in the field.Des mesures de profil de vitesses, de concentration en sédiment, de pressions intersticielles et de la dynamique de la couche de fond sont analysées afin de mieux juger de l’importance relative des processus susceptibles de déstabiliser le lit d’une plage. Ces mesures ont été réalisées en laboratoire, dans un bassin de 30 m × 30 m, dans la zone de déferlement de vagues irrégulières et sur la plage du Truc Vert (côte Aquitaine, France). Les effets d’accélération, du gradient vertical de pression à travers le lit et de l’écoulement d’infiltration/exfiltration sont étudiés. L’apparition d’une couche mobile et fortement concentrée semble initiée par les fortes accélérations précédant le passage de la crête des vagues. Probablement dû à un lit partiellement saturé et constitué de grains plus fins dans les expériences en bassin, la déstabilisation du lit par un gradient de pression interstitielle orienté vers le haut n’apparaît pas négligeable comme dans les mesures de terrain

Flume experiments on wave non-linear interactions effects on beach morphodynamics

Proceedings of the 11th International Coastal Symposium, Szczecin, PolandInternational audienceWaves in the coastal zone are organized in groups. The breaking of those groups generates long waves. The effects of these long waves on the beach morphology are quite misunderstood. Experiments in similitude with nature were conducted in the LEGI wave flume in order to better understand non-linear interactions structures and to assess their influence on morphodynamics. Bichromatic waves were generated in order to develop different structures of standing waves. All conditions were tested on similar profiles characterized by a surf zone confined on the upper beach. On the lower beach including the shoaling zone, the morphological evolutions are found to be only induced by wave non-linearities and not influenced by breaking related effects such as undertow or turbulence. Morphological evolutions are very different from a wave condition to the others: bar formation, bar washing, bar displacement. Modulations of skewness and asymmetry were observed, related to both recurrence phenomena and long wave modulation. They explain the morphological evolution. Although a role of infragravity waves has been observed, bar formation is not consistently located under nodes or anti-nodes of the standing waves

Effects of horizontal pressure gradients on bed destabilization under waves

International audienceWe report on new experiments designed to investigate bed destabilization processes ina two-dimensional wave flume physical model of a beach. The mobile bed consists ofnon-cohesive granular material of low density. The wave conditions are provided byrepeating a cycle of waves made of two bichromatic groups of different period. Thehorizontal and vertical velocities are acoustically profiled vertically from free-streamelevation down to the still bed level in the surf zone. Additional measurementsof the fluid pressure at positions closely aligned horizontally and vertically in andslightly above the sediment bed are undertaken. Mobile bed interfaces, still bedand top interface, are detected via acoustic and optical methods. Both methods arecross-compared and give similar results. Flow turbulence over the bed is analysed,the Reynolds turbulent shear stress is found negligible compared to the orbital flowinduced momentum diffusion. The shear stress and the horizontal pressure gradientare computed at near-bed elevation and used in the bed incipient plug flow model ofSleath (Cont. Shelf Res., vol. 19 (13), 1999, pp. 1643–1664). Both the model and themeasurements confirm that destabilization occurs when the non-dimensional pressuregradient (or Sleath number) exceeds the threshold value of 0.3 which is simultaneouswith strong flow acceleration. The near-bottom fluid shear stress detected during theseflow accelerations at steep wave fronts is found experimentally to be negative, whichis retrieved with an unsteady plug flow model. This is suggesting that the fluid abovethe bed resists the sediment layer motion at these particular phases

Infragravity Waves in Mobile-Bed Laboratory Experiments Coastal Sediment

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Long waves and beach profile evolutions

International audienceExperiments were carried out in a flume 36 m long and 55 cm wide equipped with a piston wave generator. The sloping bottom consists of a loose material of low density (1190 kg m-3) with a median diameter d 50 =0.6 mm in order that Shields and Rouse numbers are of the same magnitude as those of natural environments. Time and length scales ratios are roughly 1/3 and 1/10. Irregular waves were generated according to a JONSWAP spectrum. The waves were measured along the flume and bottom profiles were recorded in between repeated wave sequences. A wave climate (characterized by its peak frequency and root mean square wave height) was run for several tens of hours, so as to reach bottom equilibrium conditions. H rms and infragravity mode amplitudes along the flume were obtained for transient and equilibrium bottom profiles. The long waves node positions and structure conform to model solutions of the linearized Saint-Venant equations. On the equilibrium bottom profile they are more energetic and the correlation between infragravity waves and the incident short wave envelope clearly indicate that they conform on both breaking point and bound long wave release mechanisms