Extreme scenarios for the evolution of a soft bed interacting with a fluid using the Value at Risk of the bed characteristics

International audienceWe show how to introduce the Value at Risk (VaR) concept in the analysis of the adaptation of the shape of a soft bed to a flow knowing the Probability Density Function (PDF) of the responses of the shape to flow perturbations (bed receptivity). Our aim is to quantify our confidence on simulation scenarios by an available morphodynamic model for the shape. The approach permits to perform this task at low complexity as it does not require any sampling of the bed receptivity parameter space. The paper goes beyond stationarity for the variability of the bed receptivity by linking its dynamics to the bottom morphodynamics through an original transport equation for the local bed receptivity standard deviation. The approach has been applied to the analysis of bed morphodynamics based on minimization principles. The results show the importance of including uncertainty information during the coupling and not only eventually through simple margins on the results

Rheology, force transmission, and shear instabilities in frictional granular media from biaxial numerical tests using the contact dynamics method

International audienceBy means of the contact dynamics discrete element method, we investigate the quasi-static behavior of granular media composed of rigid frictional particles. Eluding specific modeling of the contact rheology, this method is suitable for numerical simulation of the plastic deformations of granular materials. We studied the macroscopic stress-strain and volume-change behavior, as well as force transmission and shear instabilities, in a two-dimensional biaxial geometry for dense samples composed of 5000 rigid disks. The peak and residual strengths and shear bands were analyzed by varying the confining pressure and the coefficient of friction between particles. The results are consistent with well-known features of the plasticity of noncohesive granular media. The mechanical behavior is rigid-plastic governed by a Mohr-Coulomb yield criterion and showing strain hardening and softening. Conjugated shear bands characterize plastic failure. The volumetric strain is globally dilatant with considerable expansion observed along shear bands. The macroscopic coefficient of friction, determined from peak and residual strengths, increases nonlinearly and saturates to a constant value as a function of contact friction. The strong force chains are mostly parallel to the major principal stress axis, yet deviations are observed near the shear bands. These chains are often composed of particles that are larger than the average. The deviatoric stress shows small fluctuations often in the form of rapid falls that are correlated with tiny contractional events. This behavior is interpreted in terms of the propagation of dynamic shear instabilities along the shear bands, in close analogy with stick-slip behavior

High Performance Pre-Computing: Prototype Application to a Coastal Flooding Decision Tool

International audienceAfter defining the High Performance Pre- Computing --referred as HPPC-- concept, the aim of the present study is to develop a prototype whether to approve or not the benefits of this concept. Our application case tries to answer the geophysical issue of coastal flooding. This is an example of an alert system based on the HPPC architecture, thus on pre-computed scenarios. The prototype provides the scientists with an ergonomic and on-demand tool allowing the run of scenarios of any implemented numerical models. These runs are available through a web application which submits the corresponding jobs on the remote french public cluster of HPC@LR. In this study we simulate the waves propagation over a Mediterranean grid using the wave model WaveWatch III⃝R . A reference simulation using usual conditions is approximated using the k-NN algorithm over 12, 98 and then 980 pre-computed scenarios. This simple experiment demonstrates how useful the pre-computing of scenarios is for alert systems as far as enough and relevant scenarios are pre-computed. This is the reason why searches continue in each critical points of the HPPC architecture such as the design of experiment, the approximation of the results by meta-models and the research of the closest scenarios in this big data context

Field evidence of swash groundwater circulation in the microtidal rousty beach, France

International audienceThis manuscript reports on a novel field experiment carried out on a microtidal beach in Camargue, France. For the first time in the field, a comprehensive description of the groundwater dynamics under sandy beach swash zone is presented. A cross-shore network of 15 buried pressure sensors is combined with terrestrial LiDAR measurements to study the swash-groundwater dynamics. The presented data focus on the decay of a moderate storm which allows to monitor the evolution of the groundwater pressure field in response to the retreat of the swash zone. Both horizontal and vertical head gradients are measured within the porous sand soil to estimate the groundwater flow field using Darcy’s law. Time-averaged analysis demonstrates the presence of a rather consistent groundwater circulation pattern under the swash zone, shifting offshore with the swash zone. The main tendency is an offshore directed flow, with infiltration/exfiltration in the upper/lower parts of the swash zone. Time-resolved analysis highlights the typical groundwater response to swash events which consists mainly of an overall infiltration flow during the bed inundation by the swash tongue, a seaward flow during the swash retreat and, for long backwash events, a localized exfiltration flow under the next incoming uprush

Interaction houle/sédiment (La rampe carbonatée du bassin du Sud-est au Jurassique terminal)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Two numerical approaches: 2D and 3D SPH model to simulate extreme waves over a barrier island

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Architectures de dépôts et facteurs de contrôle d'un système côtier à faibles apports sédimentaires (le littoral languedocien (Golfe du Lion, Sud de la France))

Actuellement, la prévision de l'évolution d'un système littoral représente un enjeu social important. De nombreuses études sont réalisées afin de mieux connaitre la dynamique côtière et donc de comprendre les facteurs qui influencent son évolution. Cette dynamique est contrôlée par la morphologie, les apports sédimentaires, l'hydrodynamique, l'eustatisme... qui agissent à des échelles de temps différentes et avec de fortes interactions. A l'échelle géologique, l'architecture des dépôts, contrôlée par l'eustatisme et la subsidence, est parfaitement décrite dans les zones à forts apports sédimentaires. Par contre, dans les zones à faibles apports sédimentaires, l'enregistrement des cortèges côtiers est réduit et est difficile à interpréter. L'analyse de 250 kilomètres de profils sismiques THR sur le littoral languedocien (Nord du Golfe du Lion, France), de carottages et de données de terrain a permis de déterminer la géométrie en trois dimensions des cortèges sédimentaires côtiers depuis le Quaternaire jusqu'à l'actuel. Ces observations montrent l'importance de la dynamique longshore sur les littoraux pauvres en apports sédimentaires directs. De plus, l'étude d'un système lagunaire sur ce littoral met en évidence les concepts de lagunes protégées et lagunes isolées ainsi que l'importance des vallées incisées dans la génèse de ces environnements. Enfin, la modélisation en laboratoire des processus d'incision de chenaux sous contrôle eustatique et climatique permet de discuter le rôle des vallées incisées en milieu côtierAt present, the forecasting of littoral evolution presents a considerable social stake. Many studies have been achieved to better understand coastal dynamics. Such dynamics are controlled by morphology, sediment supply, hydrodynamics, eustacy... at different time scales. At geological time scale, deposits geometry, controlled by eustacy and subsidence, is succesfully analysed where sediment supply is high. But the littoral zones present important sediment supply variability. In low sediment supply areas, the record of coastal tracts is smaller and harder to interprete. Analyses of about 250 kilometres of new VHR seismic profiles acquired along the languedocian coast (Northern Gulf of Lion, France), tens of short cores and outcrops data allowed us to determine the three-dimensional high-resolution geometry of the coastal tracts from Quaternary to present-day. This highlighted the role of alongshore dynamics in coast where direct sediment supply is low. The concepts of protected lagoon and isolated lagoon were broughted to the fore and the effect of inherited incised valley morphology was discussed. Last, a complementary study by analogue modeling of the channel incision processes controled by eustatic and climatic changes allowed to discuss the role of incised valleys in coastal areasMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Modelling beaches morphodynamic by Hadamard sensitivity analysis

International audienceThe paper presents a morphodynamic model which can be coupled with any wave model capable of producing time/spectral averaged wave quantities. This model based on a wave energy minimization principle highlights the morphodynamic phenomenology, such as the sandbar creation. Such a model can be used in solving engineering optimization problems. It is also developed to illustrate the idea that beach sand transport can be thought as a non-local phenomenon. We used wave calculations from SWAN and XBeach in our model, and we compared the morphodynamic results to LIP and SANDS hydro-morphodynamic benchmark as well as open-sea simulations. Using supplementary mathematical development, we improved the minimization method using the Hadamard derivative

A Generic Beach Morphodynamic Model by Minimization Principle

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