Extreme set-up and run-up on steep cliffs (Banneg Island, France)

International audienceno abstrac

A data set of sea surface stereo images to resolve space-time wave fields

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

Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation

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 Oceanograph

Ocean Surface Current Airborne Radar (OSCAR): a new instrument to measure ocean surface dynamics at the sub-mesoscale

Oceans form Space V Symposium, 24-28 october 2022, Venice, Italy.-- 2 pages, 3 figuresThe ocean interacts with the atmosphere, land and ice on multiple spatial scales including fine submesoscales that are often observed in high resolution optical images. Little is known about their dynamics however. SeaSTAR is an innovative satellite mission concept that proposes to address this gap by mapping ocean current and wind vectors at 1 km resolution. In this paper, we present the OSCAR instrument - an airborne demonstrator of the SeaSTAR concept - and the first results from a scientific campaign over the Iroise Sea in May 2022. The capabilities of OSCAR are demonstrated against ground truth data with very promising first results. These results open the door to using OSCAR as a scientific tool to provide unique 2D synoptic views of ocean and atmosphere dynamics at km-scalesThis work was supported by ESA/ESTEC Contract Number 4000116410/16/NL/BJ for the OSCAR development and ESA/ESTEC contract number 400017623/22/NL/IA for the campaign over Iroise SeaPeer reviewe

Observation and estimation of Lagrangian, Stokes and Eulerian currents induced by wind and waves at the sea surface

The surface current response to winds is analyzed in a two-year time series of a 12 MHz (HF) Wellen Radar (WERA) off the West coast of France. The measured currents, with tides filtered out, are of the order of 1.0 to 1.8% of the wind speed, in a direction 10 to 40 degrees to the right of the wind. This Lagrangian current can be decomposed as the vector sum of a quasi-Eulerian current U_E, representative of the top 1 m of the water column, and a part of the wave-induced Stokes drift Uss at the sea surface. Here Uss is estimated with an accurate numerical wave model, thanks to a novel parameterization of wave dissipation processes. Using both observed and modelled wave spectra, Uss is found to be very well approximated by a simple function of the wind speed and significant wave height, generally increasing quadratically with the wind speed. Focusing on a site located 100 km from the mainland, the estimated contribution of Uss to the radar measurement has a magnitude of 0.6 to 1.3% of the wind speed, in the wind direction, a fraction that increases with wind speed. The difference U_E of Lagrangian and Stokes contributions is found to be of the order of 0.4 to 0.8% of the wind speed, and 45 to 70 degrees to the right of the wind. This elatively weak quasi-Eulerian current with a large deflection angle is interpreted as evidence of strong near-surface mixing, likely related to breaking waves.Comment: Submitted to J. Phys. Oceanogr. le 16/10/2008. Revised 18/02/2009, Accepted 03/04/201

Breaking waves and their applications to the offshore wind sector: Habilitation to drive research (Habilitation à Diriger des Recherches, HDR).

This document presents research efforts dedicated to improve the characterizationof breaking waves and of their multiple effects on offshore wind turbines.The manuscript focuses first on the definition of the breaking onset criterionthat is important for the design of offshore wind turbines. It further presentsrecent advances in the parameterization of the wave breaking statistics, neededto identify the design sea states, when slamming loads are considered. This isfollowed by a description of efforts to improve the spectral wave models, used toobtain the wave climatology at offshore wind farms location.Observational works to capture area of wave energy focusing (potentially featuringbreaking waves) are then introduced. The document further presents experimentalefforts to observe interactions between large amplitude breaking wavesand offshore structures.Finally, different perspectives targeting the improvement of the above-citedresearch axes are given, and extension to other applications are discussed. Theseapplications cover the effects of breaking waves on the characterization of the windproperties with spaceborne measurements and ocean-wave-atmosphere couplednumerical models

Wave breaking parameterization and spectral wave modelling

Bien que le déferlement est le plus important puits d énergie pour les vagues, il est encore assez mal compris. Le travail présenté dans cette thèse vise à unifier la dissipation d énergie associée au déferlement du large à la côte à travers le développement d un nouveau paramétrage destiné aux modèles spectraux de prévision des états de mer. Le premier résultat de cette thèse est un paramétrage de la probabilité de déferlement applicable du large à la côte et permettant de séparer les différentes échelles pouvant être présentes dans un champ de vagues. Ce paramétrage a servi de base à un nouveau terme source de dissipation d énergie liée au déferlement et destiné aux modèles spectraux de prévision des états de mer. Ce terme repose en outre sur le paramétrage de deux autres grandeurs fondamentales pour le déferlement : le taux de dissipation par unité de longueur de crête de déferlante et la densité de longueur de crête par mètre carré. Les résultats de la validation réalisée à l échelle globale ainsi qu à l échelle d une plage suggèrent qu il est possible de représenter la dissipation par déferlement par un unique paramétrage, du large à la côte. D autre part, deux études portant sur la directionalité du déferlement et sur les modulations du déferlement des vagues courtes par les vagues longues ont été abordées. Ces deux aspects peuvent aussi avoir des conséquences importantes sur la dissipation d énergie par déferlement.Although breaking is the most important energy sink term for wind-generated waves, the underlying physical processes are the least understood. The work presented in this thesis s aimed at unifying the wave breaking induced dissipation from deep to shallow water through the development of a novel parameterization designed for spectral wave models. The first result of this work is a parameterization of the breaking probability enabling to discriminate among the wave scales present within a given wave field and applicable from the deep ocean to the surf zone. This parameterization was used as the basis of a new spectral source term for the wave breaking induced-dissipation. This source function is based on the parameterization of three basics physical quantities: the breaking probability, the dissipation rate per unit crest length and the crest length density per unit area. The results of the validation performed at global scale and at beach scale suggest that it is possible to represent the wave breaking induced dissipation by a single formulation. We shall besides discuss a study of the breaking directionality and the impact of the presence of long waves on the breaking of short waves. Both effects may as well have important on the breaking dissipation parameterization.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocSudocFranceF

A unified deep-to-shallow water wave-breaking probability parameterization

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

Extreme sea states and wave breaking characterization for MRE activities

This paper presents a eld experiment part of the DiMe project and aiming at capturing extreme sea states with a focus on their wave breaking properties. These observations collected from La Jument lighthouse, are intended to help in re ning the design conditions for Marine Renewable Energy (MRE) Converters. Preliminary analyses from the eld observations are presented, which include stereo-video, x-band radar and accelerometers data.Cet article presente une experience du projet DiMe visant a capturer des etats de mer extremes et leur deferlement. Ces observations, collectees depuis le phare de La Jument, sont destinees a ameliorer la connaissance des conditions extremes a considerer pour le dimensionnement des convertisseurs d'energies marines renouvelables. Des analyses prelimaires des observations sont presentees, incluant les donnees de stereo-video, de radar en bande-X et d'accelerometres