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Wind resource assessment is a crucial step for the development of offshore wind energy. It relies on the installation of measurement devices, whose placement is an open challenge for developers. Indeed, the optimal sensor placement for field reconstruction is an open challenge in the field of sparse sampling. As for the application to offshore wind field reconstruction, no similar study was found, and standard strategies are based on semi-empirical choices. In this paper, a sparse sampling method using a Gaussian mixture model on numerical weather prediction data is developed for offshore wind reconstruction. It is applied to France's main offshore wind energy development areas: Normandy, southern Brittany and the Mediterranean Sea. The study is based on 3 years of Météo-France AROME's data, available through the MeteoNet data set. Using a Gaussian mixture model for data clustering, it leads to optimal sensor locations with regards to wind field reconstruction error. The proposed workflow is described and compared to state-of-the-art methods for sparse sampling. It constitutes a robust yet simple method for the definition of optimal sensor siting for offshore wind field reconstruction. The described method applied to the study area output sensor arrays of respectively seven, four and four sensors for Normandy, southern Brittany and the Mediterranean Sea. Those sensor arrays perform approximately 20 % better than the median Monte Carlo case and more than 30 % better than state-of-the-art methods with regards to wind field reconstruction error.

With the flourishing of offshore wind projects there is a new socio-economic interest to better our knowledge and forecasting ability of winds within the coastal marine atmospheric boundary layer (MABL). Air-sea fluxes of enthalpy and momentum greatly influence the turbulent and mean winds in the MABL. Already at moderate but certainly at high winds, wave breaking is a key driver of air-sea fluxes and the sea spray generated by whitecaps is thought to be a crucial component when modelling air-sea interactions. Most studies so far have focused on the role of sea spray in enhancing tropical cyclone intensity. Here we investigate its impacts on the MABL under strong orographic wind forcing. A coupled model framework was developed within the scope of the CASSIOWPE project aiming at characterizing the physical environment in the Gulf of Lion (NW Mediterranean Sea) in the prospective of future floating wind farms development. It consists of the non-hydrostatic mesoscale atmospheric model of the French research community Meso-NH, the 3rd generation wave model WAVEWATCH III®, and the oceanic model CROCO. Sea-spray physics were incorporated into the Meso-NH’s surface model SURFEX. Added parametrizations will be detailed and a series of test cases will be presented to illustrate how sea spray alters the MABL under Mistral and Tramontane winds. Several sea-state dependent sea spray generation functions (SSGF) are considered in the present study. The variability in simulated fields linked to the choice of wave forcing or coupling will be showcased to evaluate their suitability in varying fetch conditions. Sea spray production remains to be adequately quantified. Existing measurement derived SSGFs span several orders of magnitude resulting in uncertainties in simulated fields which will be discussed.

Some properties of chaotic dynamical systems can be probed through features of recurrences, also called analogs. In practice, analogs are nearest neighbors of the state of a system, taken from a large database called the catalog. Analogs have been used in many atmospheric applications including forecasts, downscaling, predictability estimation, and attribution of extreme events. The distances of the analogs to the target state usually condition the performances of analog applications. These distances can be viewed as random variables, and their probability distributions can be related to the catalog size and properties of the system at stake. A few studies have focused on the first moments of return-time statistics for the closest analog, fixing an objective of maximum distance from this analog to the target state. However, for practical use and to reduce estimation variance, applications usually require not just one but many analogs. In this paper, we evaluate from a theoretical standpoint and with numerical experiments the probability distributions of the K shortest analog-to-target distances. We show that dimensionality plays a role on the size of the catalog needed to find good analogs and also on the relative means and variances of the K closest analogs. Our results are based on recently developed tools from dynamical systems theory. These findings are illustrated with numerical simulations of well-known chaotic dynamical systems and on 10-m wind reanalysis data in northwest France. Practical applications of our derivations are shown for forecasts of an idealized chaotic dynamical system and for objective-based dimension reduction using the 10-m wind reanalysis data.

Analogs are nearest neighbors of the state of a system. By using analogs and their successors in time, one is able to produce empirical forecasts. Several analog forecasting methods have been used in atmospheric applications and tested on well-known dynamical systems. Although efficient in practice, theoretical connections between analog methods and dynamical systems have been overlooked. Analog forecasting can be related to the real dynamical equations of the system of interest. This study investigates the properties of different analog forecasting strategies by taking local approximations of the system's dynamics. We find that analog forecasting performances are highly linked to the local Jacobian matrix of the flow map, and that analog forecasting combined with linear regression allows to capture projections of this Jacobian matrix. The proposed methodology allows to estimate analog forecasting errors, and to compare different analog methods. These results are derived analytically and tested numerically on two simple chaotic dynamical systems., Article submitted to the Journal of Atmospheric Sciences

The deployment of tidal turbines requires a precise hydrodynamic characterisation of the production site. Acoustic Doppler Current Profilers (ADCP), usually employed for measuring the time-mean characteristics of environmental flows, could also be used for assessing the main features of turbulence. ADCP measurements are sensitive to many sources of uncertainties associated mainly with the spreading of the beams or the assumptions made on flow homogeneity. The ability of ADCPs to accurately measure the hydrodynamic parameters of a given flow can be tested on a synthetic dataset. However, it is difficult to generate a dataset representative of a real environmental flow. In this work, large-eddy simulation of a high Reynolds flow over a rough seabed is performed and used to assess the accuracy of two, coupled, 4-beam ADCP systems forming an 8-beam arrangement. The study confirms the relevance and efficiency of the tested 8-beam configuration for the characterisation of turbulence. The results near the seabed are of a lower quality, with up to 50 % error on the Reynolds stresses for elevations under twice the roughness height, which questions the interpretation of ADCP measurements in the lower part of the water column. Also, the spatial averaging over ADCP cells leads to an underestimation of the turbulence intensity of 10 % to 20 %.

This paper presents a novel method for obtaining the probability wave of breaking (Pb) of deep water, dominant wind‐sea waves (that is, waves made of the energy within ±30% of the peak wave frequency) derived from Gaussian wave field theory. For a given input wave spectrum we demonstrate how it is possible to derive a joint probability density function between wave phase speed (c) and horizontal orbital velocity at wave crest (u) from which a model for Pb can be obtained. A non‐linear kinematic wave breaking criterion consistent with the Gaussian framework is further proposed. Our model would allow, therefore, for application of the classical wave breaking criterion (that is, wave breaking occurs if u/c > 1) in spectral wave models which, to the authors’ knowledge, has not been done to date. Our results show that the proposed theoretical model has errors in the same order of magnitude as six other historical models when assessed using three field datasets. With optimization of the proposed model's single free parameter, it can become the best performing model for specific datasets. Although our results are promising, additional, more complete wave breaking datasets collected in the field are needed to comprehensively assess the present model, especially in regards to the dependence on phenomena such as direct wind forcing, long wave modulation and wave directionality. Plain Language Summary Waves will break if the speed of the water particles on the wave crest is greater than the speed of the wave itself, causing the wave crest to overtake the front part of the wave, leading to wave breaking. Precisely simulating real ocean waves requires, therefore, a particle‐by‐particle description of the water motion, which is too expensive for the current computers to handle in real‐world applications. Instead, wave models describe waves by means of their statistical properties, that is, averaged over a large number of waves. In this paper, we present a mathematical formulation that allows to calculate the combined probability between the speed of particles on the wave crest and the wave speed based only on statistical properties. From these combined probabilities, we model the probability of wave breaking. Our results indicate that our model performed relatively well when compared to six other models using three historical datasets. Because of a lack of observed data to assess our model, we recommend that future research should focus on collecting more wave breaking data measured in the field. Future advances on this line of research could lead, for example, to improvements on operational weather forecast models.

This paper presents a novel method for obtaining the probability wave of breaking Pb of deep water, dominant wind-sea waves (that is, waves made of the energy within +-30% of the peak wave frequenc...

Stereo imaging of the sea surface elevation provides unique field data to investigate the geometry and dynamics of oceanic waves. Typically, this technique allows retrieving the 4-D ocean topography (3-D space + time) at high frequency (up to 15–20 Hz) over a sea surface region of area ~104 m2. Stereo data fill the existing wide gap between sea surface elevation time-measurements, like the local observation provided by wave-buoys, and large-scale ocean observations by satellites. The analysis of stereo images provides a direct measurement of the wavefield without the need of any linear-wave theory assumption, so it is particularly interesting to investigate the nonlinearities of the surface, wave-current interaction, rogue waves, wave breaking, air-sea interaction, and potentially other processes not explored yet. In this context, this open dataset aims to provide, for the first time, valuable stereo measurements collected in different seas and wave conditions to invite the ocean-wave scientific community to continue exploring these data and to contribute to a better understanding of the nature of the sea surface dynamics., Measurement(s)stereo image • wave height • peak wave period • wave directionTechnology Type(s)Camera Device • computational modeling techniqueFactor Type(s)geographic locationSample Characteristic - Environmentsea surface layerSample Characteristic - LocationBlack Sea • Adriatic Sea • Yellow Sea • Iroise Sea Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12181158

This works aims to investigate the impact of wind forcing datasets and wave breaking parameterizations on spectral wave model performance under extremely energetic conditions. For this purpose we used the wave model WaveWatch III to simulate the evolution of the highly energetic storms that occurred in winter 2013/2014 in the North-East Atlantic. We forced the wave model with two different wind datasets: one proceeding from the ECMWF ERA5 reanalysis dataset and the other from satellite observations. Moreover, two wave energy dissipation parameterizations were tested: Test471 and Test500. The model accuracy was assessed by comparing the output datasets with buoy data both in deep and coastal water. Moreover, wave height measurements from satellite were used to assess the model accuracy along storm tracks across the ocean. The accuracy of simulated results shows a significant dependence on the wind forcing and wave dissipation parameterization used. Error metrics computed under storm conditions at wave buoys are consistent with those computed along storm tracks. At the wave buoy locations, all datasets tend to underestimate wave parameters at the peaks of the storms.

Wave breaking is an important process for energy dissipation in the open ocean and coastal seas. It drives beach morphodynamics, controls air-sea interactions, determines when ship and offshore structure operations can occur safely, and influences on the retrieval of ocean properties from satellites. Still, wave breaking lacks a proper physical understanding mainly due to scarce observational field data. Consequently, new methods and data are required to improve our current understanding of this process. In this paper we present a novel machine learning method to detect active wave breaking, that is, waves that are actively generating visible bubble entrainment in video imagery data. The present method is based on classical machine learning and deep learning techniques and is made freely available to the community alongside this publication. The results indicate that our best performing model had a balanced classification accuracy score of ~90% when classifying active wave breaking in the test dataset. An example of a direct application of the method includes a statistical description of geometrical and kinematic properties of breaking waves. We expect that the present method and the associated dataset will be crucial for future research related to wave breaking in several areas of research, which include but are not limited to: improving operational forecast models, developing risk assessment and coastal management tools, and refining the retrieval of remotely sensed ocean properties., Comment: Under review in Scientific Reports

A system of two coupled four-beam acoustic Doppler current profilers was used to collect turbulence measurements over a 36-h period at a highly energetic tidal energy site in Alderney Race. This system enables the evaluation of the six components of the Reynolds stress tensor throughout a large proportion of the water column. The present study provides mean vertical profiles of the velocity, the turbulence intensity and the integral lengthscale along the streamwise, spanwise and vertical direction of the tidal current. Based on our results and considering a tidal-stream energy convertor (TEC) aligned with the current main direction, the main elements of turbulence prone to affect the structure (material fatigue) and to alter power generation would likely be: (i) the streamwise turbulence intensity ( I x ), (ii) the shear stress, v ′ w ′ ¯ , (iii) the normal stress, u ′ 2 ¯ and (iv) the vertical integral lengthscale ( L z ). The streamwise turbulence intensity, ( I x ), was found to be higher than that estimated at other tidal energy sites across the world for similar height above bottom. Along the vertical direction, the length ( L z ) of the large-scale turbulence eddies was found to be equivalent to the rotor diameter of the TEC Sabella D10. It is considered that the turbulence metrics presented in this paper will be valuable for TECs designers, helping them optimize their designs as well as improve loading prediction through the lifetime of the machines. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

(IF 2.98; Q1); International audience; Modelling three-dimensional wave-current-turbulence interactions in extreme tidal environments is still challenging and necessary for the development of the tidal industry, particularly for the dimensioning of tidal converters. Following this objective, we focus our study on the Alderney Race (France) where the strongest tidal current in Western Europe and various sea states occur. Due to the strong tidal current at this location, wave-current interactions were poorly studied by the past. Therefore, we propose to assess the impacts of wave-current interactions in Alderney Race by the use of a 3D fully-coupled wave-current numerical model and in-situ measurements. The following wave-current interactions were observed : i) Stokes drift e ects inducing an increase/decrease in the current depending on the angle between waves and current, ii) wave enhancement of the bottom friction reducing the tidal current, iii) refraction of waves by the current, generating changes in waves directions, and iv) wave breaking ascribed to tidal current. The impact of such interactions were maximal near the reverse event ( 1h30) and for a weak/medium tidal range. A non-stationary time delay, varying within a same tidal cycle, was noted at both locations, which is reduced by including the local wind e ects and by adjusting the bottom stress formulation. This study shows that wave-current interactions play a non-negligible role in Alderney Race and that they need to consider by the tidal industry.

Wave group focusing gives rise to the formation of large gravity waves at the surface of the ocean, some of which are called rogue waves and represent a natural hazard for ships and offshore platforms. For safety purposes, it is crucial to predict when and where these large waves will appear and how large they will be. This work focuses on crest velocities, a quantity that is relatively easy to extract from sea surface elevation fields. It is shown that there is a direct link between crest velocity gradient and wave group linear dispersive focusing. Studying analytically the focusing of one-dimensional Gaussian wave packets under linear evolution makes it possible to derive estimates of quantities at focus, based only on crest velocity measurements. In this way, the focusing time, focusing size and focusing amplitude (relative to instantaneous amplitude) of an isolated Gaussian wave packet can be estimated. Our work is also applicable to second-order non-linear waves. Limitations due to higher-order non-linear effects are studied in numerical simulations of the non-linear Schrodinger equation.

Velocity measurements collected by an upward-looking acoustic Doppler current profiler were used to provide the first study of ambient turbulence in Alderney Race. Turbulence metrics were estimated at mid-depth during peak flooding and ebbing tidal conditions. The dissipation rate e and the integral lengthscale (L) were estimated using two independent methods: the spectral method and the structure function method. The spectral method provided e and (L) estimates with standard deviations twice lower than that obtained from the structure function method. Removal of wave and Doppler noise-induced bias when estimating the dissipation rate was shown to be a crucial step in turbulence characterization. It allowed for a significant refining in (L) estimates derived from the spectral and structure function methods of 35 % and 20 % respectively. The integral lengthscale was found to be 2–3 times the local water depth. It is considered that these findings could be valuable for current turbine designers, helping them optimizing their designs as well as improving loading prediction through the lifetime of the machines.

In this study we show that spectral and phase-resolving wave models, used for coastal engineering applications, can provide significant differences in the local wave heights and wave power magnitude in the presence of strong bottom-induced refraction. We present here methodologies to compare, in a consistent manner, model outputs to an alternative source of spatial data derived from Synthetic Aperture Radar (SAR) satellite images. Normalized amplitude-derived data are computed from SAR images, compared to both normalized amplitude-derived map from a phase-resolving model outputs and normalized significant wave height-derived map from a spectral model. Two methodologies of SAR images analysis are developed in this paper. While one appears unsuitable due to stochastic wave group modulations, the second methodology provides encouraging similarities between the SAR data and the phase-resolving model outputs.

International audience

This paper presents results from an experiment designed to improve the understanding of the relationship between extreme breaking waves and their mechanical loading on heritage offshore lighthouses. The experiment, conducted at La Jument, an iconic French offshore lighthouse, featured several records of wave, current and structure accelerations acquired during severe storm conditions, with individual waves as high as 24 m. Data analysis focuses on a storm event marked by a strong peak in the horizontal accelerations measured inside La Jument. Thanks to stereo-video wave measurements synchronized to the acceleration record we were able to identify and describe the breaking wave responsible for this intense loading. Our observations suggest that this giant wave (19 m high) had a crest elevation high enough to directly hit the lighthouse tower, above the substructure. This paper reveals the potential for conducting ambitious field experiments from offshore lighthouses in order to collect valuable storm waves and wave loading observations. This offers a possible second service life for these heritage structures as in situ laboratories dedicated to the study of the coastal hydrodynamics and its interaction with marine structures. This article is part of the theme issue ‘Environmental loading of heritage structures’.

A large body of work has been devoted to the accurate detection and simulation of wave breaking in coastal areas. It is a key process for a wide range of engineering activities and environmental issues. This has motivated the development of a variety of breaking onset criteria, such as kinematic criteria based on a maximum value (usually unity) of the ratio u c / c , of the horizontal particle velocity at the wave crest u c to its phase velocity c, both taken in the direction of wave propagation. Here, we investigate numerically the validity of this criterion in capturing breaking onset for solitary and quasi-regular two-dimensional shallow water waves using the Fully Nonlinear Potential Flow (FNPF) model by Grilli and Subramanya (1996). With this model, the propagation up to overturning of solitary waves over plane slopes, and solitary and quasi-regular waves over a submerged bar, both initially specified as numerically exact FNPF waves, is simulated. In all cases, waves break as spilling or plunging breakers, initiated by the formation of a breaker jet near the wave crest. Results show that the location of the maximum fluid velocity | | u | | m on the free surface closely coincides with the location where the overturning jet is initiated. Based on this, a new breaking onset criterion is proposed as | | u | | m / c ≃ 1 , which is shown to be more universal for accurately detecting wave breaking initiation than existing criteria based on the crest velocity.

Two coupled four-beam acoustic Doppler current profilers were used to provide simultaneous and independent measurements of the turbulent kinetic energy (TKE) dissipation rate ε and the TKE production rate P over a 36 h long period at a highly energetic tidal energy site in the Alderney Race. The eight-beam arrangement enabled the evaluation of the six components of the Reynolds stress tensor which allows for an improved estimation of the TKE production rate. Depth-time series of ε, P and the Reynolds stresses are provided. The comparison between ε and P was performed by calculating individual ratios of ε corresponding to P . The depth-averaged ratio ε / P averaged over whole flood and ebb tide were found to be 2.2 and 2.8 respectively, indicating that TKE dissipation exceeds TKE production. It is shown that the term of diffusive transport of TKE is significant. As a result, non-local transport is important to the TKE budget and the common assumption of a local balance, i.e. a balance between production and dissipation, is not valid at the measurement site. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Wave runup is known to depend on offshore wave conditions and coastal morphology. While most field studies on wave runup have focused on low‐to‐mild‐sloping sandy beaches, runup measurements on steep and irregular rocky cliff profiles are still very scarce. Here we investigate the physical processes controlling wave runup in such environments and the range of applicability of empirical runup formula. This study focuses on the steep rocky cliffs (0.1 < tanβ < 0.4) of Banneg Island, a small island located in the Molène archipelago, Brittany, France, occasionally flooded during extreme water level events. A statistical parameter for extreme runup is derived from the measurements of pressure sensors deployed in the intertidal zone. Deep water wave parameters are used to force a high‐resolution wave model, and nearshore wave parameters and high‐resolution topographic data are analyzed concurrently with runup time series in order to assess the dependence of the runup on hydrodynamic conditions and morphological parameters. The wave runup is shown to be strongly related to the square root of the offshore significant wave height times the offshore wavelength. The measurements also reveal the depth dependence of the runup, which is mainly attributed to the curvature of the foreshore profile. In comparison to empirical relation obtained for a mild‐sloping beach, the present data show a significant reduction in normalized wave runup, that is attributed to enhanced bottom friction over the rocky bottom. Plain Language Summary When waves reach the shores, they travel up and down the beach before being reflected seaward. The maximum vertical excursion of the waterline relative to the still water level, called the wave runup, is a key parameter for the design of coastal structures and the prediction of overtopping volumes during storm events. Most runup studies in natural environments have focused on smooth and mild‐sloping sandy beaches, and empirical formula to predict the wave runup has been derived for these environments. Here we study the process of wave runup over steep rocky cliffs. Using pressure sensors deployed in the intertidal zone of Banneg Island—a small island located west of Brittany, France, exposed to very high waves—we measured runup events. Then we investigated the link between these runup data, offshore wave parameters, and cliff slopes. Our results reveal that the wave runup is linearly dependent to the square root of the offshore wave height times the offshore wavelength. In addition, a significant reduction of the runup is found compared to sandy environments, which is attributed to the frictional effect exerted by the rocky bottom on the flow.

International audience; Tidal turbine will be installed in area with high current and high turbulence level. A characterisation of this last is required. The aim of the project THYMOTE is to characterize and understand the generation of eddies from smaller to several tens of meters. Three technics are used: Numerical modelling, Physical modelling, field measurements. Physical and numerical modelling show clearly the appearance of the eddies close to the bottom in presence of dunes or rocks and their motion towards the free surface.

Hawaii's access to the ocean and remoteness from fuel supplies has sparked an interest in ocean waves as a potential resource to meet the increasing demand for sustainable energy. The wave resources include swells from distant storms and year-round seas generated by trade winds passing through the islands. This study produces 10 years of hindcast data from a system of mesoscale atmospheric and spectral wave models to quantify the wind and wave climate as well as nearshore wave energy resources in Hawaii. A global WAVEWATCH III (WW3) model forced by surface winds from the Final Global Tropospheric Analysis (FNL) reproduces the swell and seas from the far field and a nested Hawaii WW3 model with high-resolution winds from the Weather Research Forecast (WRF) model capture the local wave processes. The Simulating Waves Nearshore (SWAN) model nested inside Hawaii WW3 provides data in coastal waters, where wave energy converters are being considered for deployment. The computed wave heights show good agreement with data from satellites and buoys. Bi-monthly median and percentile plots show persistent trade winds throughout the year with strong seasonal variation of the wave climate. The nearshore data shows modulation of the wave energy along the coastline due to the undulating volcanic island bathymetry and demonstrates its importance in selecting suitable sites for wave energy converters.

International audience; Large clastic cliff-top storm deposits (called CTSDs) are one of the most remarkable signatures that characterizes extreme storm wave events on coastal cliffs. Hence, the study of CTSDs is of key importance for understanding and predicting the impacts of extreme storm wave events on rocky coasts or establishing proxies for storm intensity. The present study uses new data including hydrodynamic measurements in both deep and intertidal waters, and records of CTSDs displacement and deposition across Banneg Island during the stormy winter 2013–2014. Two drone-based surveys were carried out in January 2013 (pre-storms) and in April 2014 (post-storms). In addition, complementary field observations were carried out during the winter, providing a comprehensive and detailed dataset. Concerning the hydrodynamic measurements, nine pressure sensors deployed along four cross-shore profiles on the western coast of the island, recorded wave and water level conditions between December 2013 and April 2014. Aerial orthorectified photographs and digital elevation model of differences provide a detailed spatial description of cliff erosion and the reworking of cliff-top storm deposits. After the storms, 172 fresh scars corresponding to quarrying in the bed rock were localized above high spring tide water level, 507 blocks were transported up to 40 m inland, at altitudes of 8 to 14 m above mean sea level, and the tracks of 170 of them were identified. The water levels and wave parameters estimated from the pressure sensor measurements provided accurate information on the maximum water elevation reached during the largest storms at the sensor locations and were extrapolated to provide run-up elevation along the four survey profiles for the whole winter 2013–2014. The results demonstrate that a large number of overwash episodes occurred, at times of high tides and energetic waves. These episodes generally correspond to morphological changes. This study confirms the major role played by extreme storm wave events on CTSDs quarrying, transport, and deposition on Banneg Island, and clearly identify the events of major evolutions.

The French research EMACOP project aims at characterising wave power nearby onshore structures. This paper presents the application of the non-hydrostatic wave-flow model SWASH to wave propagation and transformation on two hot spots in Brittany. The numerical simulations were performed for dominant wave conditions and three tide levels. The results of wave simulations allow us to characterise wave energy resources and define Wave Energy Converters (WEC)'s promising positions on both sites., Un des objectifs du projet de recherche EMACOP (Energies MArines, COtières et Portuaires) est de caractériser le potentiel houlomoteur près des structures portuaires et côtières, comme les digues et jetées. L’article présente l’application d’un modèle nonhydrostatique de propagation et de transformation des vagues en proche côtier SWASH pour deux sites exposés de Bretagne. Les simulations numériques sont effectuées avec SWASH pour les conditions de houle dominante et trois niveaux de marée. Les résultats présentés ici permettent de caractériser le potentiel houlomoteur et de définir les zones énergétiques propices à l’installation de systèmes de récupération sur les deux sites

This study examines the implementation and validity of phase-averaged spectral models in describing wave transformation over fringing reefs. The wave breaking mechanism over the abrupt reef crest, dissipation due to bottom friction, and nonlinear energy transfer over the reef flat set the problem apart from general applications in non-tropical settings. The Simulating Wave Nearshore (SWAN) model, which is widely used in coastal engineering applications, has embedded parameterizations of these surf-zone processes for examination. In addition, we extend the model to include a recently implemented bottom-friction source term for tropical reefs. Comparisons of model predictions with laboratory and field observations over fringing reefs identify promising wave-breaking and bottom-friction parameterizations for calibration. The calibrated parameterizations enable SWAN to reproduce the observed wave height and setup and extend the applicability of spectral models to fringing reef environments. SWAN, however, lacks the parameterization to describe the nonlinear energy transfer toward the infra-gravity band that was observed in the laboratory and field studies.

New parameterizations for the spectra dissipation of wind-generated waves are proposed. The rates of dissipation have no predetermined spectral shapes and are functions of the wave spectrum and wind speed and direction, in a way consistent with observation of wave breaking and swell dissipation properties. Namely, the swell dissipation is nonlinear and proportional to the swell steepness, and dissipation due to wave breaking is non-zero only when a non-dimensional spectrum exceeds the threshold at which waves are observed to start breaking. An additional source of short wave dissipation due to long wave breaking is introduced to represent the dissipation of short waves due to longer breaking waves. Several degrees of freedom are introduced in the wave breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr. 1991). These parameterizations are combined and calibrated with the Discrete Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the nonlinear interactions. Parameters are adjusted to reproduce observed shapes of directional wave spectra, and the variability of spectral moments with wind speed and wave height. The wave energy balance is verified in a wide range of conditions and scales, from gentle swells to major hurricanes, from the global ocean to coastal settings. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Some systematic defects are still present, but the parameterizations yield the best overall results to date. Perspectives for further improvement are also given., Comment: revised version for Journal of Physical Oceanography

This work presents the analysis of wave and turbulence measurements collected at a tidal energy site. A new method is introduced to produce more consistent and rigorous estimations of the velocity fluctuations power spectral densities. An analytical function is further proposed to fit the observed spectra and could be input to the numerical models predicting power production and structural loading on tidal turbines. Another new approach is developed to correct for the effect of the Doppler noise on the high frequencies power spectral densities. The analysis of velocity time series combining wave and turbulent contributions demonstrates that the turbulent motions are coherent throughout the water column, rendering the wave coherence-based methods not applicable to our dataset. To avoid this problem, an alternative approach relying on the pressure data collected by the ADCP is introduced and shows appreciable improvement in the wave-turbulence separation.

A pair of self-contained acoustic Doppler current profilers (SC-ADCPs) operating with different frequencies were moored on a muddy sea bottom at about 20 m depth in the Bay of Vilaine off the French Atlantic coast. With their acoustic beams oriented upwards, the SC-ADCPs ensonified most of the water column. The results of several months of in situ recorded echo intensity data spanning 2 years (2003 to 2004) from the dual-frequency ADCPs are presented in this paper. The aim was to estimate suspended particle mass concentration and mean size. A concentration index CI is proposed for the estimation of particle concentration. Based on theory the CI—unlike the volume backscatter strength—does not depend on particle size. Compared with in situ optical data, the CI shows reasonable precision but not increased with respect to that of the highest-frequency backscatter strength. Concerning the mean particle size, despite a lack of quantitative validation with optical particle-size measurements, the method yielded a qualitative discrimination of mineral (small) and organic (large) particles. This supports the potential of dual-frequency ADCPs to quantitatively determine particle size. A cross-calibration of the transducers of each ADCP shows that a specific component of the precision of the backscatter strength measured by ADCP depends on the acoustic frequency, the cell thickness and the ensemble integration time. Based on these results, the use of two ADCPs operating with distinctly different frequencies (two octaves apart) or a single dual-frequency ADCP is recommended.

Dans un contexte de developpement durable, la tendance actuelle est de se tourner vers des sources alternatives de production d’energie. Le projet National de Recherche EMACOP (Energies MArines COtieres et Portuaires) a entre autres pour objectif de definir le potentiel houlomoteur aux abords d’ouvrages cotiers. Dans le present travail, une simulation numerique des transformations induites aux vagues a l’approche du site d’Esquibien (Finistere) est realisee. La modelisation numerique des vagues est effectuee avec le code de calcul SWASH (ZIJLEMA et al., 2011). Pour chaque simulation, des resultats d’elevation temporelle, de hauteur significative et de spectre directionnel sont obtenus. Les analyses des simulations portent sur l’etude des conditions hivernales moyennes, mais aussi sur l’influence des directions de provenance des vagues, des hauteurs et periodes des houles et des differents niveaux d’eau pour le site d’Esquibien.

Whitecaps are the main sink of wave energy and their occurrence has been related to the steepness of the waves. Recent parameterizations of the wave dissipation in numerical models are based on this property, but wave models have seldom been verified in terms of whitecap properties. Here we analyze and adjust the breaking statistics used in two recent wave dissipation parameterizations implemented in the spectral wave model WAVEWATCH III® and now used operationaly at NOAA/NCEP. For dominant breaking waves, the reduction of breaking probabilities with wave age is well reproduced. Across the spectrum, the parameterizations produce a reasonable distribution of breaking fronts for wave frequencies up to three times the dominant frequency, but fail to reproduce the observed reduction in breaking front lengths for the shorter waves. Converted to whitecap coverage, the breaking parameterizations agree reasonably well with the classical empirical fits of whitecap coverage against wind speed and the global whitecap coverage estimated from space-borne radiometry.

[1] A new wave-breaking dissipation parameterization designed for phase-averaged spectral wave models is presented. It combines wave breaking basic physical quantities, namely, the breaking probability and the dissipation rate per unit area. The energy lost by waves is first explicitly calculated in physical space before being distributed over the relevant spectral components. The transition from deep to shallow water is made possible by using a dissipation rate per unit area of breaking waves that varies with the wave height, wavelength and water depth. This parameterization is implemented in the WAVEWATCH III modeling framework, which is applied to a wide range of conditions and scales, from the global ocean to the beach scale. Wave height, peak and mean periods, and spectral data are validated using in situ and remote sensing data. Model errors are comparable to those of other specialized deep or shallow water parameterizations. This work shows that it is possible to have a seamless parameterization from the deep ocean to the surf zone.

International audience; no abstract

[1] Breaking probabilities and breaking wave height distributions (BWHDs) in deep, intermediate, and shallow water depth are compared, and a generic parameterization is proposed to represent the observed variability of breaking parameters as a function of the nondimensional water depth. In intermediate and deep water, where waves of different scales may have markedly different breaking probabilities, a BWHD as a function of wave frequency is proposed and validated with intermediate‐depth and deep water observational data. The current study focuses on waves with frequencies between 0.55 and 3.45 times the peak frequency fp. For the dominant frequency, the integration of the frequency‐dependent BWHD provides a breaking probability that reproduces the known threshold‐type behavior of the breaking probability for dominant waves. In shallow water, the present breaking statistics parameterization is consistent with other independent formulations validated by shallow water‐breaking observations.

A new source term intended to spectral models is presented. It has the advantage to be based on physical variables that may be validated by observations, it relies in particular on the breaking probability parameterization per wave scales developped by FILIPOT et al. (in press). The dissipation, first calculated in the physical space is then distributed over the spectrum. Moreover, this source term represents, in the mean time, the wave breaking energy dissipation from deep ocean to shallow water. The ability of this source term has been tested in various situations from global scale to depth-induced breaking and the first results are encouraging.

The dissipation of wave energy is generally parameterized empirically with an arbitrary separation between white-capping and depth-induced breaking. Here we follow recent developments in phase-averaged spectral models and extend the semi-empirical parameterization to include the loss of short wave energy due to the breaking of longer waves. The physical basis of this parameterization is discussed and an extension is proposed, giving a unified parameterization for both white-capping and depth-induced breaking. That physical formulation is the product of two verifiable quantities that are the energy dissipation rate per unit length of breaking wave front and the breaking probability for each wave scale.

International audience; The wave runup is known to depend on the offshore wave conditions and the beach slope. While most field studies on wave runup have focused on low-to-mild sloping sandy beaches, runup measurements on steep and irregular rocky beach profiles are still very sparse. Hence, little is known on the physical processes affecting the wave runup in such environments, and the range of applicability of empirical runup formula requires further investigation. This study focuses on the steep (0.1 < tan β < 0.4) rocky cliffs of Banneg Island, a small island of the Iroise Sea occasionally flooded during extreme water level events. A statistical parameter for extreme runup is derived from the measurements of pressure sensors deployed in the intertidal zone. Deep water wave parameters and highresolution topographic data are analyzed concurrently with runup time-series in order to assess the dependence of the runup on hydrodynamic conditions and foreshore slopes. The wave runup is shown to be strongly related to the surf-similarity parameter times the offshore significant wave height. Given the large topographic variability of the beach profile, the method used to compute the beach slope is shown to strongly affects the results.

International audience; A sample of observations gathered during the PROTEVS-VAGUES 2012 field experiment is presented. A focus is done on the analysis of five instruments deployed over Sein Island south coast. Waves and their effects on the mean water level are examined in the case of a long wave event that occurred on February 11, 2013. The nonlinear wave processes observed in the nearshore are found to be similar to those found over tropical reefs. Preliminary simulations of the Boussinesq type model, BOSZ, show encouraging results. BOSZ captures the main hydrodynamic processes such as refraction, shoaling, and wave breaking, but also accounted for second-order processes such as wave setup and the inherent recirculation in the surf zone.