Dynamique tourbillonnaire dans un champ de houle

Cette étude présente des résultats préliminaires sur la dynamique de tourbillons générés par une plaque plane horizontale dans un canal à houle. Sous l'effet de l'écoulement orbital et fortement cisaillé de la houle, les tourbillons ondulent et se déstabilisent. Les caractéristiques des tourbillons sont déterminées, dans un premier temps, dans un plan 2D perpendiculaire à l’axe du tourbillon par méthode PIV

Wave overtopping and overflow hazards: application on the Camargue sea-dike

Hydrodynamic

Dynamique tourbillonnaire - Application à l'impact environnemental de structures immergées

The study was performed experimentally. The vortices are generated by interaction between a monochromatic wave and a horizontal thin plate used as vortex generator. Their characterisation has been done in two and three dimensions. The two-dimensional vortex dynamics is characterized using PIV measurements. We observed the formation of downstream and upstream vortex pairs which are mutually advected toward the flume bottom and the free surface respectively. These evolutions are characterised through mean vortex trajectories computed thanks to λ2 criteria. It shows the presence of two stagnation point at the flume bottom due to the vortex pair advection and to the presence of two adjacent recirculation cells under the plate. These points would be a location of scouring phenomenon in the case of a sea bed. The three-dimensional vortex destabilisation has been studied thanks to the stereo-videography. The vortex cores are visualised with hydrogen micro bubbles generated at the edges of the plate by electrolysis. The deformation modes study, using a Fourier and a wavelet analysis, revealed that the deformation modes vary over the wave period. Strong axial velocity had been visualised on the vortex cores from the beginning of their development. These velocities are characteristics of strong destabilisations and vortex bursting.L'étude de la dynamique tourbillonnaire autour des structures immergées dans la mer est entreprise pour comprendre l'impact de ces implantations sur l'environnement. Les tourbillons sont étudiés en considérant l'interaction entre une plaque horizontale et une houle monochromatique. Cette houle est générée dans un canal à houle vitré possédant un batteur et une plage d'amortissement. La Vélocimétrie par Image de Particule (PIV) et la Stéréo-Vidéographie sont utilisées pour suivre les tourbillons dans leur évolution. La PIV, qui est une technique de visualisation 2D, a permis de montrer que les tourbillons jouent un rôle essentiel dans la dynamique de l'écoulement autour de la plaque. En effet, l'écoulement est très différent de celui prédit par les modèles analytiques qui ne prennent pas en compte la dynamique tourbillonnaire. Émis aux bords de plaque, ces tourbillons évoluent par paires et sont advectés dans le sillage de la plaque en amont et en aval. De très fortes vitesses sont induites lors de l'advection des tourbillons et rendent l'écoulement autour de la plaque fortement asymétrique. Cette asymétrie est aussi constatée lorsque la circulation autour de la plaque est calculée. Elle induit des efforts de portance non négligeables sur la plaque. À chaque nouvelle période de houle, les paires de tourbillons alimentent des cellules de recirculation autour de la plaque. Au fond du canal, ces dernières induisent des points de stagnation et conduiraient à de l'affouillement dans le cas d'un lit sédimentaire. La Stéréo-Vidéographie a, quant à elle, permis de représenter l'évolution spatio-temporelle des filaments tourbillonnaires. Ces représentations, ainsi qu'une analyse spectrale, ont permis de montrer qu'il n'existe pas de longueurs d'onde caractéristiques de déformation des filaments tourbillonnaires. Cependant, trois perturbations de grande longueur d'onde ont pu être identifiées. Leur longueur d'onde et leur position varient au cours du temps. Elles pourraient être associées au développement d'ondes, type soliton, observées au cœur du tourbillon. De fortes vitesses axiales ont été mesurées au cœur des tourbillons dans le sillage de ces ondes. Ces vitesses sont caractéristiques de fortes déstabilisations ainsi que d'éclatements tourbillonnaires

Water Depth Inversion From a Single SPOT-5 Dataset

International audienceKnowing bathymetry at intermediate depth, over large areas, and at a reasonable cost is a key issue. Spaceborne remote sensing techniques must play an essential role in retrieving such bathymetry. In this paper, a method is proposed that aims to address this issue without any in situ measurements by exploiting the characteristics of the SPOT-5 satellite dataset. The proposed method is designed to provide bathymetry from two optical SPOT-5 satellite images separated by a time lag DT of 2.04 s. It relies on the estimation of several clouds of wave celerity and wavelength pairs using wavelet and cross-correlation techniques and on the linear wave dispersion relation. This method has been applied to two SPOT-5 images on a test site characterized by complex bathymetry (Saint-Pierre, La Réunion Island). A comparison of the retrieved bathymetry with in situ bathymetric measurements reveals good morphological agreement. The mean relative error is less than 30% in the 3–80-m water depth range. The methodological choices made during method development are discussed based on additional computations, and guidelines for using the proposed method on other images at other sites are provided

InSAR monitoring of ground motions impacts for in-situ sea level measurement: The example of Dakar (Senegal)

International audienceTide gauges are a unique source of information on past sea level changes over the 20th century. However, these instruments can be affected by local ground motions, which are often poorly known. Here, InSAR can provide valuable information to understand the ground deformation context at city scale. In this study, we evaluate ground motion velocities from 13 interferograms of 10 Envisat images. Within the precision limitations of the technique, our results suggest that the city of Dakar displays relatively uniform ground motions, except in the harbor. Further investigation is necessary to examine the particular case of a recent tide gauge, which could be affected by a moderate subsidence with respect to the permanent GPS station of Dakar

Erosion de surface d'un sol : influence des propriétés du sol sur la résistance à l'érosion

International audienceOne type of internal erosion, identified as concentrated leak erosion, consists in loss of particles of soil along a preferential flow paths existing in hydraulic earthen work. To assess the vulnerability of soils to this phenomenon, and more generally to surface erosion, several experimental devices exist, among them the Hole Erosion Test. We investigate here the effect of several parameters depending on the nature of soil considered, on erosion characteristics. A comprehensive campaign was conducted on soil, varying clay content, coarse grain shape and degree of compaction. In agreement with previous results, it is shown that presence of clay allows a better resistance to erosion; in the same way, the denser the sample, the more resistant it is. Concerning the grains shape, the experiments show two results: samples with sub-rounded to rounded particles exhibit a better resistance concerning the critical shear stress, (i.e. the shear stress needed to initiate erosion), but when erosion begins, these samples are eroded faster than those with sub-angular to angular particles.L’érosion de conduit se produit lorsqu’il y a départ de particules de sol le long d'un chemin d'écoulement préférentiel existant dans un ouvrage hydraulique. Pour évaluer la vulnérabilité des sols vis-à-vis de ce phénomène, et plus généralement à l’érosion de surface, plusieurs dispositifs expérimentaux de laboratoire existent, parmi lesquels le Hole Erosion Test. Nous étudions ici l'impact de plusieurs paramètres constitutifs du sol sur sa résistance à l’érosion, à l’aide d’une campagne expérimentale réalisée sur des sols de teneur en argile, forme des grains grossiers et taux de compactage variables. En accord avec des résultats antérieurs, il est montré que la présence d'argile confère une meilleure résistance à l'érosion ; de même, plus l'échantillon est dense, plus il est résistant. Concernant la forme des grains, les essais réalisés montrent deux résultats : d’une part, et contre intuitivement, les échantillons avec des particules sub-arrondies à arrondies présentent une meilleure résistance vis-à-vis de l’initiation de l’érosion, cependant, une fois l'érosion enclenchée, ces échantillons s’érodent plus rapidement que ceux ayant des particules de forme sub-angulaire à angulaire

Vortex kinematics around a submerged plate under water waves. Part II: Numerical computations

International audienceThis paper presents numerical computations of the flow generated by a horizontal plate immersed in a regular wave field, and associated loads acting on the plate. This numerical work is the continuation of the Poupardin et al. (2012) experimental study. This numerical study is original in the way that the vortical aspects of the flow are not neglected. Therefore, a 2D Lagrangian Vortex method is used as a numerical scheme. These methods are particularly well suited for the computation of unsteady and highly rotational flows in an open domain. The velocity field is decomposed into rotational and potential components, using the Helmholtz decomposition. The rotational part of the velocity is calculated by the Biot-Savart equation using vortex particles. The plate is modelled by a distribution of normal dipoles and the wave field is taken into account by means of a Stokes formulation, which completes the potential part of the velocity. Firstly, the numerical code is validated by means of comparison with the experimental results of Poupardin et al. (2012). In particular, the complex vortical activity and the mean flow velocity field are well reproduced and physically analysed. Secondly, forces acting on the plate are analysed on a wide range of parameters by varying the plate immersions and lengths. In the end, a new scaling is found for the lift forces acting on the plate based on the modified Stokes velocity (i.e. the bottom Stokes velocity for a water depth equals to the plate immersion) and the square of the plate length

A combined experimental and numerical strategy to assess the influence of model geometric distortion in laboratory scale modelling of urban flooding

Accurate modelling of urban flood hazard remains hampered by a lack of suitable validation data. In contrast with water marks and inundation extent, discharge partition in-between streets and flow fields are generally unknown, while they have a strong influence on flood risk (human destabilization, scour, contaminant transport). Laboratory experiments may provide a valuable complement to field data to achieve robust validation of flood hazard models. However, urban flooding is a multiscale phenomenon, with horizontal length scales (~ 103 m) are considerably larger than the vertical ones (~ 1 m). Therefore, recent experimental studies of urban flooding used geometrically distorted scale models, with a vertical scale factor smaller than the horizontal one. Though, little is known so far on the bias induced by model geometric distortion in the case of urban flooding. To address this issue, we have combined computational modelling with laboratory experiments. Based on 100+ numerical simulations, the bias induced by model geometric distortion on flow depth and discharge partition was found of the order of 10 %, which is not negligible compared to other uncertainties involved in urban flood hazard modelling. Moreover, when the geometric distortion is varied, the induced bias shows an intriguing non-monotonous evolution, which we could relate to a competition between frictional and secondary head losses. New tailored laboratory experiments are on-going on a “series” of laboratory scale models, i.e. several laboratory models representing the same urban layout at various scales. The outcomes of these experiments will make more robust the conclusions drawn from computational modelling

Influence of model geometric distortion in laboratory scale modelling of urban flooding

peer reviewedUrban flooding at the block- or district-level is characterized by comparatively larger horizontal length scales than vertical ones. Therefore, a number of laboratory experi-ments of urban flooding were based on geometrically distorted scale models. Here, we investigate the influence of model geometric distortion on the flow processes in two simple settings representative of urban flooding: a single prismatic street and a street junction. We specifically compare the upstream flow depths predicted from laboratory observations using different geometric distortions. The predicted upstream flow depths consistently decrease as the model geometric distortion is increased. The bias induced by model geometric distortion, estimated as the relative difference between predictions from distorted and undistorted models is of order of 15% and 7% for the prismatic street and the street junction, respectively. This difference may be attributed to a difference in the relative importance of frictional losses and local head losses in the two settings. The outcomes of the study provide valuable guidance for the design of laboratory models of urban flooding

Deep Submarine Landslide Contribution to the 2010 Haiti Earthquake Tsunami

The devastating Mw 7, 2010, Haiti earthquake was accompanied by local tsunamis that caused fatalities and damage to coastal infrastructure. Some were triggered by slope failures of river deltas in close vicinity of the epicenter, while others, 30 to 50 km to the north across the Bay of Gonâve, are well explained by the reverse component of coseismic ground motion that accompanied this mostly strike-slip event. However, observations of run-up heights up to 2 m along the southern coast of the island at distances up to 100 km from the epicenter, as well as tide gauge and DART buoy records at distances up to 600 km from the epicenter have not yet received an explanation. Here we demonstrate that these observations require a secondary source, most likely a submarine landslide. We identify a landslide scar 30 km from the epicenter off the southern coast of Haiti at a depth of 3500 m, where ground acceleration would have been sufficient to trigger slope failure in soft sediments. This candidate source, 2 km3 in volume, matches observation remarkably well assuming that the sediment collapse obeys a viscous flow with an initial apparent viscosity of 2 × 105 Pa s. Although that particular source cannot be proven to have been activated in 2010, our results add to a line of evidence that earthquake-triggered submarine landslides can cause significant tsunamis in areas of strike-slip tectonic regime