Analysis and Interpretation of Frequency-Wavenumber Spectra of Young Wind Waves

The energy level and its directional distribution are key observations for understanding the energy balance in the wind-wave spectrum between wind-wave generation, nonlinear interactions, and dissipation. Here, properties of gravity waves are investigated from a fixed platform in the Black Sea, equipped with a stereo video system that resolves waves with frequency f up to 1.4 Hz and wavelengths from 0.6 to 11 m. One representative record is analyzed, corresponding to young wind waves with a peak frequency f(p) = 0.33 Hz and a wind speed of 13 m s(-1). These measurements allow for a separation of the linear waves from the bound second-order harmonics. These harmonics are negligible for frequencies f up to 3 times f(p) but account for most of the energy at higher frequencies. The full spectrum is well described by a combination of linear components and the second-order spectrum. In the range 2f(p) to 4f(p), the full frequency spectrum decays like f(-5), which means a steeper decay of the linear spectrum. The directional spectrum exhibits a very pronounced bimodal distribution, with two peaks on either side of the wind direction, separated by 150 degrees at 4f(p). This large separation is associated with a significant amount of energy traveling in opposite directions and thus sources of underwater acoustic and seismic noise. The magnitude of these sources can be quantified by the overlap integral I(f), which is found to increase sharply from less than 0.01 at f = 2f(p) to 0.11 at f = 4f(p) and possibly up to 0.2 at f = 5f(p), close to the 0.5 value proposed in previous studies

Infragravity waves: From driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Observation and modelisation of wave breaking

The recent parameterizations used in spectral wave models provide today interesting results in terms of forecast and hindcast of the sea states. Nevertheless, many physical phenomena present in these models are still poorly understood and therefore poorly modeled, in particular the dissipation source term due to breaking. First, the work presented in this thesis is aimed at analyzing and criticizing the existing parameterizations of the dissipation through the explicit modeling of the underlying properties of breaking. The finding of the failure of these parameterizations to reproduce the in situ and satellite observations, a new method for the observation and the analysis of breaking is proposed using stereo video systems . This method allows the observation of breaking waves on the high-resolution stereo-reconstructed sea surfaces. Therefore, a complete method for reconstruction of the sea surfaces in the presence of breaking waves is proposed and validated. The detection of breaking waves on the images and their reprojection on reconstructed surface is also discussed. Although too few acquisitions are available to draw firm results, an overview of the various observable parameters through the use of stereo video is given. This work shows the importance of stereo video systems to a better observation and understanding of the breaking waves, required in order to improve dissipation source term in spectral wave models.Les récentes paramétrisations utilisées dans les modèles spectraux de vagues offrent des résultats intéressants en termes de prévision et rejeux des états de mer. Cependant, de nombreux phénomènes physiques présents dans ces modèles sont encore mal compris et donc mal modélisés, notamment le terme de dissipation lié au déferlement des vagues.Le travail présenté dans cette thèse vise dans un premier temps à analyser et critiquer les paramétrisations existantes de la dissipation, au travers de la modélisation explicite des propriétés du déferlement sous-jacentes. Du constat de l’échec de ces paramétrisations à reproduire les observations in situ et satellite du déferlement, une nouvelle méthode d’observation et d’analyse des déferlements est proposée à l’aide de systèmes de stéréo vidéo. Cette méthode permet l’observation des déferlements sur des surfaces de mer reconstruites à haute résolution par stéréo triangulation. Ainsi, une méthode complète de reconstruction des surfaces de mer en présence de vagues déferlantes est proposée et validée. La détection des vagues déferlantes sur les images et leur reprojection sur les surfaces reconstruites est également discutée. Bien que peu d’acquisitions soient disponibles, les différents paramètres observables grâce à l’utilisation de la stéréo vidéo sont mis en avant. Ce travail montre l’intérêt des systèmes vidéo stéréo pour une meilleure observation et compréhension du déferlement des vagues, pour le développement des paramétrisations de la dissipation dans les modèles spectraux de vague

Observation et modélisation du déferlement des vagues

The recent parameterizations used in spectral wave models provide today interesting results in terms of forecast and hindcast of the sea states. Nevertheless, many physical phenomena present in these models are still poorly understood and therefore poorly modeled, in particular the dissipation source term due to breaking. First, the work presented in this thesis is aimed at analyzing and criticizing the existing parameterizations of the dissipation through the explicit modeling of the underlying properties of breaking. The finding of the failure of these parameterizations to reproduce the in situ and satellite observations, a new method for the observation and the analysis of breaking is proposed using stereo video systems . This method allows the observation of breaking waves on the high-resolution stereo-reconstructed sea surfaces. Therefore, a complete method for reconstruction of the sea surfaces in the presence of breaking waves is proposed and validated.The detection of breaking waves on the images and their reprojection on reconstructed surface is also discussed. Although too few acquisitions are available to draw firm results, an overview of the various observable parameters through the use of stereo video is given.This work shows the importance of stereo video systems to a better observation and understanding of the breaking waves, required in order to improve dissipation source term in spectral wave models.Les récentes paramétrisations utilisées dans les modèles spectraux de vagues offrent des résultats intéressants en termes de prévision et rejeux des états de mer. Cependant, de nombreux phénomènes physiques présents dans ces modèles sont encore mal compris et donc mal modélisés, notamment le terme de dissipation lié au déferlement des vagues.Le travail présenté dans cette thèse vise dans un premier temps à analyser et critiquer les paramétrisations existantes de la dissipation, au travers de la modélisation explicite des propriétés du déferlement sous-jacentes. Du constat de l’échec de ces paramétrisations à reproduire les observations in situ et satellite du déferlement, une nouvelle méthode d’observation et d’analyse des déferlements est proposée à l’aide de systèmes de stéréo vidéo. Cette méthode permet l’observation des déferlements sur des surfaces de mer reconstruites à haute résolution par stéréo triangulation. Ainsi, une méthode complète de reconstruction des surfaces de mer en présence de vagues déferlantes est proposée et validée. La détection des vagues déferlantes sur les images et leur reprojection sur les surfaces reconstruites est également discutée. Bien que peu d’acquisitions soient disponibles, les différents paramètres observables grâce à l’utilisation de la stéréo vidéo sont mis en avant. Ce travail montre l’intérêt des systèmes vidéo stéréo pour une meilleure observation et compréhension du déferlement des vagues, pour le développement des paramétrisations de la dissipation dans les modèles spectraux de vague

Characterization of Seiches in Small-Harbors

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Spectral wave modelling of the extreme 2013/2014 winter storms in the North-East Atlantic

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 ERAS 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

Extreme wave runup over the steep rocky cliffs of Banneg Island, France

International audienceOcean waves are constantly shaping coastal morphology. When storm waves are combined with high water levels, the wave runup can reach unexpected elevations and become a threat for coastal populations, through dike overtopping, dune breaching or accelerated coastline erosion. Therefore, the wave runup has been largely investigated over the last decades for its key importance in coastal engineering and risk management. Although the strongly nonlinear nature of swash flows prevented theoreticians and modellers from accurately predicting its kinematics, numerous laboratory and field studies provided a means for establishing wave runup empirical formula, which are now commonly used in engineering practice. However, most of the field studies only concerned gently sloping sand beaches, and run-up formula have barely been validated against field observations in steep rocky environments, where data are still very sparse. This study presents water elevation data acquired with pressure sensors solidly fixed to the bedrock of Banneg Island, France, during winter 2013/14. Offshore wave parameters and water levels were also measured during this winter and recorded storm events with Hm0 up to 9 m in spring tide conditions, which caused flooding and boulder transports across the island. A methodology to infer R 2% from the local pressure measurements was implemented. The 4-month time-series of run-up measurements at the top and at the bottom of subvertical cliff profiles (with slopes ranging from 20% to 30%) were compared with offshore wave parameters and revealed a strong dependence of R 2% to the Hunt parameter (ξ.Hm0). Several period parameters based on the spectral moments were also tested to compute ξ and the best correlations were obtained with T(m0,-1). Finally, the exceptional run-up values (up to 8 m) measured at the cliff top during the major storms allowed to test the validity of existing run-up formula for a range of conditions that exceeds any other observations, to our knowledge

Dissipation source terms and whitecap statistics

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 (R) 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

A New Probabilistic Wave Breaking Model for Dominant Wind‐sea Waves Based on the Gaussian Field Theory

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

Non-hydrostatic modelling of extreme water levels on Banneg Island, France

International audienceExtreme water level events were observed during recent winters on Banneg Island [Ardhuin et al., 2011; Suanez et al., 2009], a small island off western Brittany well exposed to the large North Atlantic swells and characterized by steep rocky cliffs on its western part. Based on geomorphic evidences (bedrock scars, overturned boulders, debris lines) and hydrodynamic data, Fichaut and Suanez [2011] investigated the quarrying and transport of cliff-top storm deposits induced by giant wave events and partial flooding of the island (Fig.1), while Suanez et al. [2009] provided a retrospective analysis of extreme water levels based on a 30-year wave model hindcast and empirical run-up formula. Later, Sheremet et al. [2014] deployed pressure sensor measurements to this site and applied a 1D nonlinear mild-slope model to reveal that the highest water levels could exceed 6.5 m above the astronomical tide during major storms, mainly induced by large infragravity waves. In order to improve our understanding of the storm-induced hydrodynamics in the Iroise Sea, a wave buoy was deployed to the west of Banneg and additional pressure sensors were installed on the western part of the island (Section 2). In addition, a phase-resolving wave model based on the nonlinear shallow water equations, including non-hydrostatic pressure was applied to the study site and the model results were validated against in-situ observations (Section 3). The hydrodynamic data collected during the major storms of February 2014 were analyzed and the model was forced with the hydrodynamic conditions of the morning high tide of February 5, when some of the highest water levels were observed on the island (Section 4). Based on this combined model-data analysis, the incident and infragravity wave dynamics in the nearshore, and the associated sporadic flooding of the island were investigated in the light of previous studies on Banneg Island (Section 5)