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International audience; The propagation of horizontal dam-break flows induced by the release of a rectangular column of a liquid phase into a negligible ambient fluid is investigated by combining numerical simulations and laboratory experiments. Varying both the geometry of the initial column and liquid properties, the parameter space covered allows the investigation of the transition from inertial to viscous slumping barely considered so far. In light of the Reynolds number Re based on the initial parameters of the fluid column, we report a newly investigated transient regime following the inertial slumping regime and prior to the viscous-dominated one. The transient intermediate regime is shown (i) to only exist for $Re\gtrsim10$ and (ii) to be characterized by an inertial overshoot followed by a strong deceleration of the front position accompanied by an excess and a deficit of fluid at the front and at the origin, respectively, with respect to the expected theoretical prediction of the viscous-dominated regime. Beyond the intuitive idea of an adaptive transition from the inertial to fully viscous slumping, these results highlight and quantify the influence of initial inertia on the spreading of a liquid, which may be particularly useful for modeling industrial and geophysical applications.

Co-auteur étranger; International audience; Granular flows that propagate down a mountainside, then reach the sea, a lake or a river and finally, travel underwater, is a common event on the Earth’s surface. To help the description of such events, laboratory experiments on gas-fluidized granular flows entering water are performed, analyzed, and compared to those propagating in air. The originality of this study lies in the fluidization process, which improves the laboratory modelling of geophysical flows by taking their high mobility into account. Qualitatively, the presence of the water body promotes the generation of a granular jet over the water surface, a leading and largest wave, and a particle-driven gravity current underwater. Hydrodynamic forces mainly play a dissipative role by slowing and reducing the spreading of the granular mass underwater, but a low amount of grains are still transported by the turbulent fluid as a gravity current far away. The temporal evolution of the granular jet and the particle-driven gravity current are well described by ballistic motion theory and scaling laws of homogeneous gravity currents, respectively. Most currents propagate with a constant flow-front velocity along the horizontal bottom, which is controlled by the flow height depending on the water depth. In constrast, the bulk volume concentration of particles in the current is estimated to be nearly constant, interpreted as a critical concentration above which the excess of particles cannot be maintained by the turbulent fluid. This experimental study highlights the complexity of the dynamics and deposits of granular masses when they encounter a water body.

International audience; Novel laboratory experiments of fluidized granular flows entering water are reported, for the purpose of investigating tsunamis generated by pyroclastic flows. Qualitatively, the impact of a fluidized granular flow into water leads to (i) an initial vertical granular jet over water, (ii) a leading and largest wave, and (iii) a turbulent mixing zone forming a turbidity current. The present study focuses on the leading wave features in the near‐field region, as a function of the mass flux per width qm and the volume per width υ of the flow, the maximum water depth Ho, and the slope angle θ of the inclined plane. The obtained waves are of Stokes and cnoidal types, for which the generation is mostly controlled by qm and υ. By contrast, Ho plays no role on the wave generation that occurs in the shallowest region. Moreover, a comparison between fluidized granular, dry (nonfluidized) granular, and water flows entering water is addressed under similar flow conditions. The dimensionless amplitude scales as A/Ho=f(ζ), where urn:x-wiley:jgrb:media:jgrb54190:jgrb54190-math-0001 is a dimensionless parameter depending on the Froude number Fr, the relative slide thickness S, the relative mass M, and the slope angle θ. Data of fine fluidized granular, fine dry granular, and water flows collapse on a master curve, which implies that the nature of the flowing material is of lesser importance in the current setup. By contrast, coarse granular flows generate lower amplitude waves, which is attributed to the penetration of water into the porous granular medium.

This study presents the results of paleo-tsunami investigation in marshes of the southern and eastern coasts of Martinique Island (Lesser Antilles, France). A sand unit dated 1726–1813 cal. CE at 30 cm depth at Anse Meunier represents the most convincing evidence of high-energy coastal flooding of the last 5000 years on the island. Sedimentological analyses combined with tsunami simulations demonstrate that this sand deposit likely corresponds to the 1755 CE Lisbon transatlantic tsunami. No other tsunami or hurricane event was identified on the 11 marshes investigated. The 1755 Lisbon tsunami is thus the only tsunami recorded in the stratigraphy of coastal marshes of the southern and eastern coasts of Martinique Island during the last 3500 years, even if we cannot exclude the possibility of larger-magnitude but lower-frequency tsunamis that would not be recorded in coastal stratigraphy during this period.

Tsunamis generated by the collapse of a granular mass into water represent a low amount of documented tsunamis in the world. However, they can locally cause severe human and material damages as they occur near coastlines. In order to identify physical mecanisms controlling the features of induced waves, the simple configuration of a granular material entering into water has been widely investigated. This study extends the configuration to the case of a fluidized granular flow, representative of tsunamis generated by pyroclastic flows during volcanic eruptions. We show that, in dam-break configuration, the fluidization process enhances the mobility of the granular flow with larger front velocities and larger propagation distances of the mass. The consequence is a significant increase of the maximum wave amplitude in comparison to an analogous situation without fluidization. The influence of the impact velocity and of the mass on the amplitude of the wave is also supported by varying the initial height of the granular column released. Finally, we show that the level of water does not affect the characteristics of the wave, in the range of parameters considered here., Les tsunamis générés par l'entrée d'une masse granulaire dans l'eau ne représentent qu'une faible proportion des tsunamis répertoriés dans le monde. Toutefois, ils peuvent localement causer d'importants dégâts humains et matériels car ils sont généralement créés proche des côtes. Afin d'identifier les méca-nismes physiques contrôlant les propriétés des vagues induites, la configuration simplifiée d'un matériau granulaire impactant l'eau a été largement étudiée en laboratoire. Cette étude étend la configuration au cas d'un écoulement granulaire fluidisé, représentatif des tsunamis générés par des écoulements py-roclastiques lors d'éruptions volcaniques. Nous montrons qu'en configuration de rupture de barrage (dam-break), le processus de fluidisation améliore la mobilité de l'écoulement granulaire, caractérisée par une augmentation de la vitesse du front et de la distance de propagation de la masse. La consé-quence est une amplitude de vague plus grande par rapport à une situation analogue sans fluidisation. L'influence de la vitesse d'impact et de la masse sur l'amplitude de la vague est également confirmée en variant la hauteur initiale de la colonne granulaire relâchée. Finalement, nous montrons que la hauteur d'eau n'affecte pas les caractéristiques de la vague, dans la gamme des paramètres considérées ici.

International audience; Laboratory experiments on the collapse of a liquid-saturated granular column on a horizontal plane are reported. The trigger and the resulting dynamics of the collapse when occurring, as well as the shape of the final deposit, are characterized and analyzed in light of some dimensionless parameters, namely, the "column" Bond number Bo, the grain diameter to capillary length ratio d/l c , the initial aspect ratio a, the Stokes number St, and the initial volume fraction φ, by varying the properties of the interstitial fluid and of the grains, the geometry, and the compaction of the initial granular column. The main contribution of this study is to: (i) provide a diagram of the different regimes of collapse shown to be mostly controlled by capillary effect, (ii) develop simple criteria that capture the transitions between each regime in terms of critical values of the Bond number and the ratio of the grain diameter to the capillary length, (iii) extend a predictive model of the runout for dry collapses to the more general case of liquid-saturated granular collapses, and (iv) quantify the influence of a, St, and φ on the collapse dynamics and the shape of the final deposit in the capillary-free regime. A perspective description of the role of the interstitial fluid on the spreading of the granular medium, and more particularly of the driving role of the fluid, is discussed and argued on the basis of the present set of experiments.

International audience; Laboratory experiments of the granular collapse of an initially dense-packing column in a fluid are reported. By extracting the temporal evolution of the granular material height profile, both the dynamics of the granular flow and the final deposit are characterized depending on the Stokes number St, based a on dissipative process at the grain scale, the grain-fluid density ratio r, and the aspect ratio a of the initial column. A full description of the granular collapse including the transient dynamics of the flow and the characterization of the final shape deposit is proposed. The main contribution of the present study is to provide (i) the St dependence on the granular collapse beyond the effect of the aspect ratio a already reported in previous studies, (ii) the characterization of granular flow regimes in the ((d/Hi) 1/2 St, (d/Hi) 1/2 r) plane, where d/Hi is the particle diameter to initial column height ratio, and (iii) simple correlations to describe the granular collapse which would be of interest for geophysical purposes.

Experiments on the collapse of non-colloidal and neutrally buoyant particles suspended in a Newtonian fluid column are presented, in which the initial volume fraction of the suspension $\unicode[STIX]{x1D719}$, the viscosity of the interstitial fluid $\unicode[STIX]{x1D707}_{f}$, the diameter of the particles $d$ and the mixing protocol, i.e. the initial preparation of the suspension, are varied. The temporal evolution of the slumping current highlights two main regimes: (i) an inertial-dominated regime followed by (ii) a viscous-dominated regime. The inertial regime is characterized by a constant-speed slumping which is shown to scale as in the case of a classical inertial dam-break. The viscous-dominated regime is observed as a decreasing-speed phase of the front evolution. Lubrication models for Newtonian and power-law fluids describe most of situations encountered in this regime, which strongly depends on the suspension parameters. The temporal evolution of the propagating front is used to extract the rheological parameters of the fluid models. At the early stages of the viscous-dominated regime, a constant effective shear viscosity, referred to as an apparent Newtonian viscous regime, is found to depend only on $\unicode[STIX]{x1D719}$ and $\unicode[STIX]{x1D707}_{f}$ for each mixing protocol. The obtained values are shown to be well fitted by the Krieger–Dougherty model whose parameters involved, say a critical volume fraction $\unicode[STIX]{x1D719}_{m}$ and the exponent of divergence, depend on the mixing protocol, i.e. the microscale interaction between particles. On a longer time scale which depends on $\unicode[STIX]{x1D719}$, the front evolution is shown to slightly deviate from the apparent Newtonian model. In this apparent non-Newtonian viscous regime, the power-law model, indicating both shear-thinning and shear-thickening behaviours, is shown to be more appropriate to describe the front evolution. The present experiments indicate that the mixing protocol plays a crucial role in the selection of a shear-thinning or shear-thickening type of collapse, while the particle diameter $d$ and volume fraction $\unicode[STIX]{x1D719}$ play a significant role in the shear-thickening case. In all cases, the normalized effective consistency of the power-law fluid model is found to be a unique function of $\unicode[STIX]{x1D719}$. Finally, an apparent viscoplastic regime, characterized by a finite length spreading reached at finite time, is observed at high $\unicode[STIX]{x1D719}$. This regime is mostly observed for volume fractions larger than $\unicode[STIX]{x1D719}_{m}$ and up to a volume fraction $\unicode[STIX]{x1D719}_{M}$ close to the random close packing fraction at which the initial column remains undeformed on opening the gate.