Michel Benoit - Academia.edu (original) (raw)

Papers by Michel Benoit

Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd... more Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd, 2019

Coastal Engineering Proceedings, 2017

Tsunami generation have commonly been modeled by using the Okada method (see Okada (1992)), i.e. ... more Tsunami generation have commonly been modeled by using the Okada method (see Okada (1992)), i.e. from seismic data, the final ground deformation is calculated and applied to the free surface as an initial deformation. Using this method, lot of aspects of the generation are neglected, including timescales. The aim of this study is to measure the influence of temporal parameters during a simplified kinematic generation. In this purpose, a junction between the work of Hammack (1973) and Todorovska and Trifunac (2001) is done to built a linear semi-analytical solution of the free surface deformation depending simultaneously of the the rise time, tr, and the rupture velocity, vp. They characterize the vertical and horizontal motions respectively. This solution is compared and validated with numerical simulations and the influence of the temporal parameters is measured by varying their values in a large range. A resonance phenomenon appears, as noticed by Todorovska and Trifunac (2001), b...

This report presents the work related to Deliverable 3.3. It is dedicated to the estimation of Ae... more This report presents the work related to Deliverable 3.3. It is dedicated to the estimation of Aero-Servo-Hydro-Elastic (ASHE) model uncertainty for what concerns the hydrodynamic loading (section 4) and the aerodynamic loading (section 5). Indeed, the models which are commonly used to predict the loads on the main components of an Offshore Wind Turbine (OWT), given some conditions of wind and waves, simplify the physics to keep the computation time affordable. Note that the influence of submarine current is not considered in this report for the sake of simplification. These "engineering" models are thus introducing

European Journal of Mechanics - B/Fluids

To simulate the interaction of ocean waves with marine structures, coupling approaches between a ... more To simulate the interaction of ocean waves with marine structures, coupling approaches between a potential flow model and a viscous model are investigated. The first model is a fully nonlinear potential flow (FNPF) model based on the Harmonic Polynomial Cell (HPC) method, which is highly accurate and best suited for representing long distance wave propagation. The second model is a CFD code, solving the Reynolds-Averaged Navier-Stokes (RANS) equations within the OpenFOAM® toolkit, more suited to represent viscous and turbulent effects at local scale in the body vicinity. Two one-way coupling strategies are developed and compared in two dimensions, considering fully submerged and fixed structures. A domain decomposition (DD) strategy is first considered, introducing a refined mesh in the body vicinity on which the RANS equations are solved. Boundary conditions and interpolation operators from the FNPF results are developed in order to enforce values at its outer boundary. The second coupling strategy considers a decomposition of variables (functional decomposition, FD) on the local grid. As the FNPF simulation provides fields of variables satisfying the irrotational Euler equations, complementary velocity and pressure components are introduced as the difference between the total flow variables and the potential ones. Those complementary variables are solutions of modified RANS equations. Extensive comparisons are presented for nonlinear waves interacting with a horizontal cylinder of rectangular cross-section. The loads exerted on the body computed from the four simulation methods (standalone FNPF, standalone CFD, DD and FD coupling schemes) are compared with experimental data. It is shown that both coupling approaches produce an accurate representation of the loads and associated hydrodynamic coefficients (inertia and drag) over a large range of incident wave steepness and Keulegan-Carpenter number, for a small fraction of the computational time needed by the complete CFD simulation.

Journal of Fluid Mechanics, 2016

In this paper, we present a novel extension of impedance (Liebau) wave pumping to a free-surface ... more In this paper, we present a novel extension of impedance (Liebau) wave pumping to a free-surface condition where resonance pumping could be used for hydraulic energy harvesting. Similar pumping behaviours are reported. Surface envelopes of the free surface are shown and outline two different dynamics: U-tube oscillator and wave/resonance pumping. The latter is particularly interesting, since, from an oscillatory motion, a unidirectional flow with small to moderate oscillations is generated. A linear theory is developed to evaluate pseudo-analytically the resonance frequencies of the pump using eigenfunction expansions, and a simplified model is proposed to understand the main pumping mechanism in this type of pump. It is found that the Stokes mass transport is driving the pump. The conversion of energy from paddle oscillation to mean flow is evaluated. Efficiency up to 22 % is reported.

HAL (Le Centre pour la Communication Scientifique Directe), Nov 22, 2022

La détermination des coefficients hydrodynamiques d'un objet soumis à un champ de vagues est un e... more La détermination des coefficients hydrodynamiques d'un objet soumis à un champ de vagues est un enjeu majeur du design des structures offshore. On étudie et valide ici la capacité d'extraction de ces coefficients à partir de simulations CFD (neptune_cfd, EDF R&D et OpenFoam®, IFPEN). Une attention particulière est portée sur les coefficients de traînée, , critiques en termes d'effets physiques, et qui ne peuvent être obtenus par les simulations potentielles linéaires, largement utilisées aujourd'hui dans l'industrie. Une comparaison code à code est effectuée sur plusieurs corps simples soumis à différentes conditions de vague, pour valider non seulement les séries temporelles d'efforts obtenues, mais aussi comparer les coefficients hydrodynamiques extraits avec les données de la littérature. Une étude prospective est ensuite réalisée dans le cadre d'un flotteur complet, dont plusieurs sections caractéristiques sont sélectionnées et analysées, toujours avec l'objectif d'obtenir une représentation en de l'ensemble du flotteur. Summary The determination of the hydrodynamic coefficients of an object subjected to a wave field is a major issue in the design of offshore structures. The ability to extract these coefficients from CFD simulations (neptune_cfd, EDF R&D and OpenFoam®, IFPEN) is studied and validated here. Particular attention is paid to the drag coefficients, , which are critical in terms of physical effects, but which cannot be obtained from linear potential simulations, widely used nowadays in the industry sector. A code-to-code comparison is performed on several simple bodies subjected to a wide range of wave conditions, to validate not only the time series of forces obtained, but also to compare the hydrodynamic coefficients obtained with data from the literature. A prospective study is then carried out on a complete floater, from which several characteristic sections are selected and analyzed, always with the objective of obtaining a representation of the whole floater.

XVIIèmes Journées, Chatou

In the last decade, a period of extreme storm conditions, the 2013-2014 winter, occurred in the n... more In the last decade, a period of extreme storm conditions, the 2013-2014 winter, occurred in the northeastern Atlantic Ocean and impacted significantly the French coasts. Several energetic events were recorded, in deep and intermediate water depths. In the present work, the main purpose is to model those events with the 3 rd generation wave model TOMAWAC (BENOIT et al., 1996), in the framework of the construction and calibration of the version 3 of the ANEMOC sea state database (see RAOULT et al., 2018) for a progress report in the building of this database). The wave numerical database is built using two computational grids: one at a larger scale, covering the whole Atlantic Ocean (up to 0.15° mesh resolution), and a second one, nested into the former, covering the Atlantic, the English Channel and North Sea French coast (up to 0.01° mesh resolution). The wave-tide-current interactions are considered in the later model. The wave generation and propagation, as well the hydrodynamic calculations are modelled through the forcing of CFSR (1979-2010) and CFSv2 (2011-2021) wind fields reanalyses, available with a spatial resolution of 0.3° and 0.2° respectively, and a time step of 1 h. The numerical results are compared with buoy measurements during the energetic period of the winter season 2013-2014 taken offshore and approaching the French coast, with sensitivity analysis regarding different physical processes involved in the wave generation and propagation modelling, models' resolution and interaction of waves with tidal currents. A particular attention is devoted to improving the performance of the model to simulate the extreme significant wave heights recorded at the peaks of the various storms.

XVIIèmes Journées, Chatou

Pour protéger les zones côtières contre l'aléa de submersion par les vagues en conditions de temp... more Pour protéger les zones côtières contre l'aléa de submersion par les vagues en conditions de tempête, des protections côtières sont nécessaires. Celles-ci sont, en grande partie, dimensionnées par rapport à un débit admissible de franchissement par les vagues. Dans la littérature, de nombreuses références, régulièrement mises à jour, proposent des méthodes pour estimer ce débit (cf. par exemple le guide EurOtop 2018). Cependant, l'application de ces méthodes à un état de mer bimodal combinant une houle, provenant du large, et un clapot, formé par un vent local, pose question. Est-ce que les caractéristiques moyennes de cet état de mer complexe (i.e. une hauteur et une période caractéristiques) suffisent pour estimer le débit de franchissement ? Pour répondre à cette problématique, une maquette de digue côtière a été construite dans le canal à vague de l'OSU Institut Pythéas à Marseille. En considérant une digue de pente 3:2 (H:V), deux types de revêtements ont été testés : un revêtement lisse et imperméable et un revêtement avec enrochements naturels. Les essais réalisés ont montré que, pour des choix de digues et pour les états de mer considérés, les formules empiriques existantes tendent à surestimer les débits de franchissements lorsque la proportion de clapot est grande dans l'énergie totale de l'état de mer. En particulier, cette surestimation est plus marquée en présence d'un revêtement rocheux. Il semblerait que les talus en enrochements amortissent plus efficacement les courtes longueurs d'onde, par rapport au cas d'une pente lisse.

HAL (Le Centre pour la Communication Scientifique Directe), Nov 27, 2018

Lorsque des vagues périodiques se propagent dans un milieu de profondeur d'eau uniforme hormis su... more Lorsque des vagues périodiques se propagent dans un milieu de profondeur d'eau uniforme hormis sur une zone présentant une série d'ondulations régulières, de type rides, une réexion importante des vagues incidentes peut se produire dans certains cas. Ce phénomène, appelé résonance de Bragg, se produit d'après la théorie linéaire asymptotique établie par Mei (1985) [6], lorsque la longueur d'onde des vagues est exactement le double de la longueur d'onde des ondulations du fond. Récemment, Couston et al. (2015) [1] ont étendu cette théorie au cas de deux zones de rides séparées par une zone de profondeur uniforme et montré que sous certaines conditions une amplication importante des vagues peut se produire dans la zone entre les rides (résonateur) : ce phénomène correspond à la résonance de Fabry-Pérot initialement étudiée en optique. Dans ce travail, nous étudions par le biais de simulations numériques (réalisées avec la version linéaire du code potentiel de vagues whispers3D), les eets de certaines des hypothèses de la théorie ALAT, notamment pour ce qui est de la formulation de la condition à la limite sur le fond ondulé. Nous montrons en particulier que lorsque l'amplitude relative des rides augmente, la résonance de Fabry-Pérot se produit pour un nombre d'onde des vagues incidentes légèrement plus faible que celui prédit par ALAT ('wave number downshift'). La forme de la courbe de résonance est également étudiée, et il apparaît que la gamme de nombres d'onde susceptibles d'entrer en résonance est réduite, et ce d'autant plus que l'amplitude relative des rides augmente.

This work has been performed by a French national consortium within the framework of the national... more This work has been performed by a French national consortium within the framework of the national project Tandem, with aim to improve knowledge about tsunami risk on the French coasts. Workpackage #1 of this project was the opportunity to build a database of benchmark cases to assess the capabilities of 18 codes, solving various set of equations with different numerical methods. 14 test cases were defined from the existing literature with validation data from reference simulations, theoretical solutions or lab experiments. They cover the main stages of tsunami life: 1) generation, 2) propagation, 3) run-up and submersion, and 4) impact. For each case several of the numerical codes were compared in order to identify the forces and weaknesses of the models, to quantify the errors that these models may induce, to compare the various modelling methods, and to provide users with recommendations for practical studies. In this paper, 3 representative cases are selected and presented with a...

Journal of Ocean Engineering and Marine Energy, 2019

Journal of Renewable and Sustainable Energy, 2015

Wave energy converters (WECs) extract energy from ocean waves and have the potential to produce a... more Wave energy converters (WECs) extract energy from ocean waves and have the potential to produce a significant amount of electricity from a renewable resource. However, large “WEC farms” or “WEC arrays” (composed of a large number of individual WECs) are expected to exhibit “WEC array effects”. These effects represent the impact of the WECs on the wave climate at an installation site, as well as on the overall power absorption of the WEC array. Tests have been performed in the Shallow Water Wave Basin of DHI (Denmark) to study such “WEC array effects”. Large arrays of up to 25 heaving point absorber type WECs have been tested for a range of geometric layout configurations and wave conditions. Each WEC consists of a buoy with a diameter of 0.315 m. Power take-off was modeled by realizing friction based energy dissipation through damping of the WECs' motion. The produced database is presented: WEC response, wave induced forces on the WECs, and wave field modifications have been mea...

XVIIèmes Journées, Chatou

Un modèle mathématique reposant sur un formalisme potentiel est présenté pour simuler la propagat... more Un modèle mathématique reposant sur un formalisme potentiel est présenté pour simuler la propagation des vagues côtières, avec un aperçu des méthodes numériques associées. Ce modèle utilise une approximation spectrale du potentiel sur la verticale, à l'aide d'une base de polynômes de Tchebychev tronquée à un ordre NT fixé par l'utilisateur. Cet ordre NT contrôle la précision du modèle sur les propriétés dispersives et non-linéaires, avec une plage de valeurs recommandées de 7 à 12 pour les applications pratiques, y compris des cas de vagues irrégulières. Cette précision est en particulier vérifiée sur la relation de dispersion du modèle linéarisé, avec une erreur relative sur la célérité de phase qui ne dépasse pas 2,5 % pour des profondeurs relatives atteignant kh = 100 (où k est le nombre d'onde des vagues et h la profondeur d'eau). Parmi les nombreux cas de validation du modèle, deux sont présentés en détail ici, reproduisant des expériences en canal à vagues avec une barre trapézoïdale immergée. Sur le premier cas, en vagues régulières, il est vérifié que le modèle capture bien la combinaison des effets physiques de dispersion et de non-linéarité au passage de la barre, avec des transferts d'énergie vers les harmoniques supérieurs du mode fondamental. Une analyse de la vitesse horizontale sous les vagues est également menée. Ensuite, sur un cas en vagues irrégulières, on montre que le modèle reproduit très fidèlement les évolutions au passage de la barre : (i) du spectre de variance, (ii) des moments statistiques skewness et kurtosis (à la fois pour la surface libre et la vitesse horizontale 0,048 m sous le niveau moyen), et (iii) de la distribution statistique des hauteurs de vagues, y compris les vagues extrêmes, pour certaines dépassant le critère H > 2Hs, habituellement considéré pour identifier les vagues scélérates. Le travail présenté dans cette communication a bénéficié de nombreuses collaborations, dont les plus significatives sont : Marissa Yates (chercheuse), Bruno Simon et Christos Papoutsellis (post-doctorants), Cécile Raoult et Jie Zhang (doctorants).

Climate change and sea level rise ; coastal vulnerability ans societal impacts,

XIVèmes Journées, Toulon, 2016

L'objectif de cette étude est de caractériser et d'optimiser l'efficacité de différentes structur... more L'objectif de cette étude est de caractériser et d'optimiser l'efficacité de différentes structures immergées parallèles à la côte (brise-lame immergé, boudin simulant un géotube, dalle anti-affouillement) pour réduire l'érosion des plages. Des modèles réduits de plages avec des structures immergées (échelle 1/10) et une procédure expérimentale sont proposés afin de comparer l'efficacité des structures. Pour une hauteur de submersion constante et un scénario de tempête donné, les effets de la forme et de la distance au trait de côte des structures sont comparés. La sensibilité des résultats aux scénarii de tempête ou au profil initial de la plage est par ailleurs analysée.

Coastal Engineering 1996, 1997

Coastal Engineering Proceedings, 2014

The knowledge of the statistical distribution of extreme sea levels at the coast is of utmost imp... more The knowledge of the statistical distribution of extreme sea levels at the coast is of utmost importance for the characterization of flood risks in coastal areas. In this study we consider that the sea level results from two components: the (astronomical) tide and the (meteorological) surge, without considering the effects of waves. We focus our attention on the dependence of the surge height on the tidal level. At sites with a strong tidal range, the classical analysis methods rely on working only with high tide data (namely high tidal levels and skew surges). A statistical method of adjustment of extreme values is applied to the surge component, leading to the Revisited Joint Probability Method. In that case, we consider that surge and tide components are independent. However, comparisons with measured data show that in several cases this procedure leads to an overestimation of the water levels for a given return period. We therefore propose here to study the dependence of skew surges on high tidal levels, with two different approaches: one based on a so-called seasonal dependence, and the other one based on the interaction between surge and tide. Three methods are adapted or developed to test the influence of these two forms of dependence. They are applied to a series of 19 French harbours along the Atlantic and English Channel coasts of France for which more than 10 years of data are available. The results show that the seasonal dependence does not affect the result significantly, while the interaction between the skew surge and the high tidal level appear to be significant for over half the harbours studied. A revisited model proposed here, as an extension of the model by Dixon and Twan (1994), seems to be more satisfactory at least for most harbours studied.

The storm surges coming ashore in the northeastern Bay of Biscay provoke extensive flooding of it... more The storm surges coming ashore in the northeastern Bay of Biscay provoke extensive flooding of its low-lying coasts. We present the results of hindcasting of several storm surges observed in this region during the winter 1999-2000. The numerical model employs the TELEMAC and TOMAWAC software to simulate the surge, tide and wave propagation. A high resolution finite element grid is used and the model is nested to a larger scale model of the surge, tide and wave propagation on the European Shelf. It is demonstrated that the wave-wind-surge interactions resulting in the wave-dependent surface and bottom stress turn out to be crucial for the accurate simulating of the surge peaks in the region and, consequently, must be included in the flood forecasting systems. It is also shown how the specific coupling between tidal and wind-induced currents in shallow embayments of the NE Bay of Biscay results in the surge oscillations with the period of several hours but different from that of the tide.

Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd... more Presented at: Coastal Structures Conference 2019, Hannover, Germany, September 30th – October 2nd, 2019

Coastal Engineering Proceedings, 2017

Tsunami generation have commonly been modeled by using the Okada method (see Okada (1992)), i.e. ... more Tsunami generation have commonly been modeled by using the Okada method (see Okada (1992)), i.e. from seismic data, the final ground deformation is calculated and applied to the free surface as an initial deformation. Using this method, lot of aspects of the generation are neglected, including timescales. The aim of this study is to measure the influence of temporal parameters during a simplified kinematic generation. In this purpose, a junction between the work of Hammack (1973) and Todorovska and Trifunac (2001) is done to built a linear semi-analytical solution of the free surface deformation depending simultaneously of the the rise time, tr, and the rupture velocity, vp. They characterize the vertical and horizontal motions respectively. This solution is compared and validated with numerical simulations and the influence of the temporal parameters is measured by varying their values in a large range. A resonance phenomenon appears, as noticed by Todorovska and Trifunac (2001), b...

This report presents the work related to Deliverable 3.3. It is dedicated to the estimation of Ae... more This report presents the work related to Deliverable 3.3. It is dedicated to the estimation of Aero-Servo-Hydro-Elastic (ASHE) model uncertainty for what concerns the hydrodynamic loading (section 4) and the aerodynamic loading (section 5). Indeed, the models which are commonly used to predict the loads on the main components of an Offshore Wind Turbine (OWT), given some conditions of wind and waves, simplify the physics to keep the computation time affordable. Note that the influence of submarine current is not considered in this report for the sake of simplification. These "engineering" models are thus introducing

European Journal of Mechanics - B/Fluids

To simulate the interaction of ocean waves with marine structures, coupling approaches between a ... more To simulate the interaction of ocean waves with marine structures, coupling approaches between a potential flow model and a viscous model are investigated. The first model is a fully nonlinear potential flow (FNPF) model based on the Harmonic Polynomial Cell (HPC) method, which is highly accurate and best suited for representing long distance wave propagation. The second model is a CFD code, solving the Reynolds-Averaged Navier-Stokes (RANS) equations within the OpenFOAM® toolkit, more suited to represent viscous and turbulent effects at local scale in the body vicinity. Two one-way coupling strategies are developed and compared in two dimensions, considering fully submerged and fixed structures. A domain decomposition (DD) strategy is first considered, introducing a refined mesh in the body vicinity on which the RANS equations are solved. Boundary conditions and interpolation operators from the FNPF results are developed in order to enforce values at its outer boundary. The second coupling strategy considers a decomposition of variables (functional decomposition, FD) on the local grid. As the FNPF simulation provides fields of variables satisfying the irrotational Euler equations, complementary velocity and pressure components are introduced as the difference between the total flow variables and the potential ones. Those complementary variables are solutions of modified RANS equations. Extensive comparisons are presented for nonlinear waves interacting with a horizontal cylinder of rectangular cross-section. The loads exerted on the body computed from the four simulation methods (standalone FNPF, standalone CFD, DD and FD coupling schemes) are compared with experimental data. It is shown that both coupling approaches produce an accurate representation of the loads and associated hydrodynamic coefficients (inertia and drag) over a large range of incident wave steepness and Keulegan-Carpenter number, for a small fraction of the computational time needed by the complete CFD simulation.

Journal of Fluid Mechanics, 2016

In this paper, we present a novel extension of impedance (Liebau) wave pumping to a free-surface ... more In this paper, we present a novel extension of impedance (Liebau) wave pumping to a free-surface condition where resonance pumping could be used for hydraulic energy harvesting. Similar pumping behaviours are reported. Surface envelopes of the free surface are shown and outline two different dynamics: U-tube oscillator and wave/resonance pumping. The latter is particularly interesting, since, from an oscillatory motion, a unidirectional flow with small to moderate oscillations is generated. A linear theory is developed to evaluate pseudo-analytically the resonance frequencies of the pump using eigenfunction expansions, and a simplified model is proposed to understand the main pumping mechanism in this type of pump. It is found that the Stokes mass transport is driving the pump. The conversion of energy from paddle oscillation to mean flow is evaluated. Efficiency up to 22 % is reported.

HAL (Le Centre pour la Communication Scientifique Directe), Nov 22, 2022

La détermination des coefficients hydrodynamiques d'un objet soumis à un champ de vagues est un e... more La détermination des coefficients hydrodynamiques d'un objet soumis à un champ de vagues est un enjeu majeur du design des structures offshore. On étudie et valide ici la capacité d'extraction de ces coefficients à partir de simulations CFD (neptune_cfd, EDF R&D et OpenFoam®, IFPEN). Une attention particulière est portée sur les coefficients de traînée, , critiques en termes d'effets physiques, et qui ne peuvent être obtenus par les simulations potentielles linéaires, largement utilisées aujourd'hui dans l'industrie. Une comparaison code à code est effectuée sur plusieurs corps simples soumis à différentes conditions de vague, pour valider non seulement les séries temporelles d'efforts obtenues, mais aussi comparer les coefficients hydrodynamiques extraits avec les données de la littérature. Une étude prospective est ensuite réalisée dans le cadre d'un flotteur complet, dont plusieurs sections caractéristiques sont sélectionnées et analysées, toujours avec l'objectif d'obtenir une représentation en de l'ensemble du flotteur. Summary The determination of the hydrodynamic coefficients of an object subjected to a wave field is a major issue in the design of offshore structures. The ability to extract these coefficients from CFD simulations (neptune_cfd, EDF R&D and OpenFoam®, IFPEN) is studied and validated here. Particular attention is paid to the drag coefficients, , which are critical in terms of physical effects, but which cannot be obtained from linear potential simulations, widely used nowadays in the industry sector. A code-to-code comparison is performed on several simple bodies subjected to a wide range of wave conditions, to validate not only the time series of forces obtained, but also to compare the hydrodynamic coefficients obtained with data from the literature. A prospective study is then carried out on a complete floater, from which several characteristic sections are selected and analyzed, always with the objective of obtaining a representation of the whole floater.

XVIIèmes Journées, Chatou

In the last decade, a period of extreme storm conditions, the 2013-2014 winter, occurred in the n... more In the last decade, a period of extreme storm conditions, the 2013-2014 winter, occurred in the northeastern Atlantic Ocean and impacted significantly the French coasts. Several energetic events were recorded, in deep and intermediate water depths. In the present work, the main purpose is to model those events with the 3 rd generation wave model TOMAWAC (BENOIT et al., 1996), in the framework of the construction and calibration of the version 3 of the ANEMOC sea state database (see RAOULT et al., 2018) for a progress report in the building of this database). The wave numerical database is built using two computational grids: one at a larger scale, covering the whole Atlantic Ocean (up to 0.15° mesh resolution), and a second one, nested into the former, covering the Atlantic, the English Channel and North Sea French coast (up to 0.01° mesh resolution). The wave-tide-current interactions are considered in the later model. The wave generation and propagation, as well the hydrodynamic calculations are modelled through the forcing of CFSR (1979-2010) and CFSv2 (2011-2021) wind fields reanalyses, available with a spatial resolution of 0.3° and 0.2° respectively, and a time step of 1 h. The numerical results are compared with buoy measurements during the energetic period of the winter season 2013-2014 taken offshore and approaching the French coast, with sensitivity analysis regarding different physical processes involved in the wave generation and propagation modelling, models' resolution and interaction of waves with tidal currents. A particular attention is devoted to improving the performance of the model to simulate the extreme significant wave heights recorded at the peaks of the various storms.

XVIIèmes Journées, Chatou

Pour protéger les zones côtières contre l'aléa de submersion par les vagues en conditions de temp... more Pour protéger les zones côtières contre l'aléa de submersion par les vagues en conditions de tempête, des protections côtières sont nécessaires. Celles-ci sont, en grande partie, dimensionnées par rapport à un débit admissible de franchissement par les vagues. Dans la littérature, de nombreuses références, régulièrement mises à jour, proposent des méthodes pour estimer ce débit (cf. par exemple le guide EurOtop 2018). Cependant, l'application de ces méthodes à un état de mer bimodal combinant une houle, provenant du large, et un clapot, formé par un vent local, pose question. Est-ce que les caractéristiques moyennes de cet état de mer complexe (i.e. une hauteur et une période caractéristiques) suffisent pour estimer le débit de franchissement ? Pour répondre à cette problématique, une maquette de digue côtière a été construite dans le canal à vague de l'OSU Institut Pythéas à Marseille. En considérant une digue de pente 3:2 (H:V), deux types de revêtements ont été testés : un revêtement lisse et imperméable et un revêtement avec enrochements naturels. Les essais réalisés ont montré que, pour des choix de digues et pour les états de mer considérés, les formules empiriques existantes tendent à surestimer les débits de franchissements lorsque la proportion de clapot est grande dans l'énergie totale de l'état de mer. En particulier, cette surestimation est plus marquée en présence d'un revêtement rocheux. Il semblerait que les talus en enrochements amortissent plus efficacement les courtes longueurs d'onde, par rapport au cas d'une pente lisse.

HAL (Le Centre pour la Communication Scientifique Directe), Nov 27, 2018

Lorsque des vagues périodiques se propagent dans un milieu de profondeur d'eau uniforme hormis su... more Lorsque des vagues périodiques se propagent dans un milieu de profondeur d'eau uniforme hormis sur une zone présentant une série d'ondulations régulières, de type rides, une réexion importante des vagues incidentes peut se produire dans certains cas. Ce phénomène, appelé résonance de Bragg, se produit d'après la théorie linéaire asymptotique établie par Mei (1985) [6], lorsque la longueur d'onde des vagues est exactement le double de la longueur d'onde des ondulations du fond. Récemment, Couston et al. (2015) [1] ont étendu cette théorie au cas de deux zones de rides séparées par une zone de profondeur uniforme et montré que sous certaines conditions une amplication importante des vagues peut se produire dans la zone entre les rides (résonateur) : ce phénomène correspond à la résonance de Fabry-Pérot initialement étudiée en optique. Dans ce travail, nous étudions par le biais de simulations numériques (réalisées avec la version linéaire du code potentiel de vagues whispers3D), les eets de certaines des hypothèses de la théorie ALAT, notamment pour ce qui est de la formulation de la condition à la limite sur le fond ondulé. Nous montrons en particulier que lorsque l'amplitude relative des rides augmente, la résonance de Fabry-Pérot se produit pour un nombre d'onde des vagues incidentes légèrement plus faible que celui prédit par ALAT ('wave number downshift'). La forme de la courbe de résonance est également étudiée, et il apparaît que la gamme de nombres d'onde susceptibles d'entrer en résonance est réduite, et ce d'autant plus que l'amplitude relative des rides augmente.

This work has been performed by a French national consortium within the framework of the national... more This work has been performed by a French national consortium within the framework of the national project Tandem, with aim to improve knowledge about tsunami risk on the French coasts. Workpackage #1 of this project was the opportunity to build a database of benchmark cases to assess the capabilities of 18 codes, solving various set of equations with different numerical methods. 14 test cases were defined from the existing literature with validation data from reference simulations, theoretical solutions or lab experiments. They cover the main stages of tsunami life: 1) generation, 2) propagation, 3) run-up and submersion, and 4) impact. For each case several of the numerical codes were compared in order to identify the forces and weaknesses of the models, to quantify the errors that these models may induce, to compare the various modelling methods, and to provide users with recommendations for practical studies. In this paper, 3 representative cases are selected and presented with a...

Journal of Ocean Engineering and Marine Energy, 2019

Journal of Renewable and Sustainable Energy, 2015

Wave energy converters (WECs) extract energy from ocean waves and have the potential to produce a... more Wave energy converters (WECs) extract energy from ocean waves and have the potential to produce a significant amount of electricity from a renewable resource. However, large “WEC farms” or “WEC arrays” (composed of a large number of individual WECs) are expected to exhibit “WEC array effects”. These effects represent the impact of the WECs on the wave climate at an installation site, as well as on the overall power absorption of the WEC array. Tests have been performed in the Shallow Water Wave Basin of DHI (Denmark) to study such “WEC array effects”. Large arrays of up to 25 heaving point absorber type WECs have been tested for a range of geometric layout configurations and wave conditions. Each WEC consists of a buoy with a diameter of 0.315 m. Power take-off was modeled by realizing friction based energy dissipation through damping of the WECs' motion. The produced database is presented: WEC response, wave induced forces on the WECs, and wave field modifications have been mea...

XVIIèmes Journées, Chatou

Un modèle mathématique reposant sur un formalisme potentiel est présenté pour simuler la propagat... more Un modèle mathématique reposant sur un formalisme potentiel est présenté pour simuler la propagation des vagues côtières, avec un aperçu des méthodes numériques associées. Ce modèle utilise une approximation spectrale du potentiel sur la verticale, à l'aide d'une base de polynômes de Tchebychev tronquée à un ordre NT fixé par l'utilisateur. Cet ordre NT contrôle la précision du modèle sur les propriétés dispersives et non-linéaires, avec une plage de valeurs recommandées de 7 à 12 pour les applications pratiques, y compris des cas de vagues irrégulières. Cette précision est en particulier vérifiée sur la relation de dispersion du modèle linéarisé, avec une erreur relative sur la célérité de phase qui ne dépasse pas 2,5 % pour des profondeurs relatives atteignant kh = 100 (où k est le nombre d'onde des vagues et h la profondeur d'eau). Parmi les nombreux cas de validation du modèle, deux sont présentés en détail ici, reproduisant des expériences en canal à vagues avec une barre trapézoïdale immergée. Sur le premier cas, en vagues régulières, il est vérifié que le modèle capture bien la combinaison des effets physiques de dispersion et de non-linéarité au passage de la barre, avec des transferts d'énergie vers les harmoniques supérieurs du mode fondamental. Une analyse de la vitesse horizontale sous les vagues est également menée. Ensuite, sur un cas en vagues irrégulières, on montre que le modèle reproduit très fidèlement les évolutions au passage de la barre : (i) du spectre de variance, (ii) des moments statistiques skewness et kurtosis (à la fois pour la surface libre et la vitesse horizontale 0,048 m sous le niveau moyen), et (iii) de la distribution statistique des hauteurs de vagues, y compris les vagues extrêmes, pour certaines dépassant le critère H > 2Hs, habituellement considéré pour identifier les vagues scélérates. Le travail présenté dans cette communication a bénéficié de nombreuses collaborations, dont les plus significatives sont : Marissa Yates (chercheuse), Bruno Simon et Christos Papoutsellis (post-doctorants), Cécile Raoult et Jie Zhang (doctorants).

Climate change and sea level rise ; coastal vulnerability ans societal impacts,

XIVèmes Journées, Toulon, 2016

L'objectif de cette étude est de caractériser et d'optimiser l'efficacité de différentes structur... more L'objectif de cette étude est de caractériser et d'optimiser l'efficacité de différentes structures immergées parallèles à la côte (brise-lame immergé, boudin simulant un géotube, dalle anti-affouillement) pour réduire l'érosion des plages. Des modèles réduits de plages avec des structures immergées (échelle 1/10) et une procédure expérimentale sont proposés afin de comparer l'efficacité des structures. Pour une hauteur de submersion constante et un scénario de tempête donné, les effets de la forme et de la distance au trait de côte des structures sont comparés. La sensibilité des résultats aux scénarii de tempête ou au profil initial de la plage est par ailleurs analysée.

Coastal Engineering 1996, 1997

Coastal Engineering Proceedings, 2014

The knowledge of the statistical distribution of extreme sea levels at the coast is of utmost imp... more The knowledge of the statistical distribution of extreme sea levels at the coast is of utmost importance for the characterization of flood risks in coastal areas. In this study we consider that the sea level results from two components: the (astronomical) tide and the (meteorological) surge, without considering the effects of waves. We focus our attention on the dependence of the surge height on the tidal level. At sites with a strong tidal range, the classical analysis methods rely on working only with high tide data (namely high tidal levels and skew surges). A statistical method of adjustment of extreme values is applied to the surge component, leading to the Revisited Joint Probability Method. In that case, we consider that surge and tide components are independent. However, comparisons with measured data show that in several cases this procedure leads to an overestimation of the water levels for a given return period. We therefore propose here to study the dependence of skew surges on high tidal levels, with two different approaches: one based on a so-called seasonal dependence, and the other one based on the interaction between surge and tide. Three methods are adapted or developed to test the influence of these two forms of dependence. They are applied to a series of 19 French harbours along the Atlantic and English Channel coasts of France for which more than 10 years of data are available. The results show that the seasonal dependence does not affect the result significantly, while the interaction between the skew surge and the high tidal level appear to be significant for over half the harbours studied. A revisited model proposed here, as an extension of the model by Dixon and Twan (1994), seems to be more satisfactory at least for most harbours studied.

The storm surges coming ashore in the northeastern Bay of Biscay provoke extensive flooding of it... more The storm surges coming ashore in the northeastern Bay of Biscay provoke extensive flooding of its low-lying coasts. We present the results of hindcasting of several storm surges observed in this region during the winter 1999-2000. The numerical model employs the TELEMAC and TOMAWAC software to simulate the surge, tide and wave propagation. A high resolution finite element grid is used and the model is nested to a larger scale model of the surge, tide and wave propagation on the European Shelf. It is demonstrated that the wave-wind-surge interactions resulting in the wave-dependent surface and bottom stress turn out to be crucial for the accurate simulating of the surge peaks in the region and, consequently, must be included in the flood forecasting systems. It is also shown how the specific coupling between tidal and wind-induced currents in shallow embayments of the NE Bay of Biscay results in the surge oscillations with the period of several hours but different from that of the tide.