Bathymetric features such as nearshore sandbars can alter local nearshore hydrodynamic processes such as the production of infragravity energy. These bathymetric features may act to reduce or increase the amount of infragravity energy that reaches the shoreline. To determine the influence of the bathymetric features on infragravity energy, the numerical nearshore processes model XBeach was used to simulate infragravity energy at the shoreline. Numerical simulations were completed for three types of bathymetric scenarios: continuous alongshore bar, bar-rip, and no-bar. The presence of the bar reduces the amount of infragravity energy at the shoreline when compared with the no-bar scenario. This reduction was characterized by modifying an empirical parameterization for significant infragravity swash developed by Stockdon et al. (2006) for barred beaches. Results show that the amount of infragravity energy in the form of swash is dependent on the bar height and depth, in addition to the offshore wave height and wavelength. The bar-rip bathymetry produces significant alongshore variation in infragravity energy. The alongshore variations may be due to refracted wave energy or the production of an edge wave by the rip. The magnitude of infragravity energy in the alongshore direction is found to be correlated with the surf zone width. Finally, erosion for the bar-rip scenario is studied qualitatively. The shape of the shoreline is modified during storm events, and is found to take the shape of the alongshore distribution of infragravity energy. Since infragravity swash influences beach erosion, results of this research may be used as part of an erosion vulnerability scale. Such information on erosion vulnerability is important for the design of coastal protection systems and the protection of coastal communities.

Infragravity waves are low frequency (0.005-0.04 Hz) waves that can dominate the spectrum of water motions and sediment transport processes within the inner surf zone. Despite the established importance of infragravity waves in shaping our coasts and numerous studies dating back to the 1950s, several aspects of infragravity wave analysis, generation and dissipation remain poorly understood. As much of the recent infragravity research has focussed on fetch-limited coasts, less is known about the climatology of these waves on energetic coastlines subject to both swell and fetch-limited waves. It has been postulated that bed friction only plays a significant role in the dissipation of infragravity waves where the bed is exceptionally rough, but the precise impact of bed roughness is not fully understood, particularly on extremely rough rock platforms. Finally, although there have been many methodologies proposed for the decomposition of reflective wave fields (an essential tool for studying infragravity wave dynamics), very little attention has been given to evaluating their accuracy, particularly the impact of uncorrelated noise. This study aims, primarily through the collection of an extensive field dataset and the establishment of accurate analysis tools, to provide new insight into the propagation, dissipation and reflection of infragravity waves on energetic coastlines of varied roughness, subject to both swell and fetch-limited waves. To ensure the accurate decomposition of infragravity wave signals into their incident and reflected components, a sensitivity analysis into the effect of uncorrelated noise on an array separation method is performed. Results show that signal noise, often prevalent in field data, introduces a significant bias to estimates of incident and reflected wave spectra, and corresponding reflection coefficients. This bias can exceed 100% for signal-to-noise ratios of < 1. Utilising the systematic change in coherence with noise, a correction function is developed which is effective at reducing bias by up to 90%. When applied to field data, results imply that infragravity reflection coefficients can be overestimated by > 50% if signal noise is unaccounted for. Consequently, noise reduction should form an integral part of future infragravity wave studies. New research from a dissipative, fetch-unlimited sandy beach (Perranporth, Cornwall, UK) and a macrotidal, rocky shore platform (Freshwater West, Pembrokeshire, UK) uniquely demonstrates that the level of infragravity wave energy close to shore is linearly dependent on the offshore short wave energy flux H_o^2 T_p (r^2 = 0.93and 0.79, respectively). Infragravity waves approach the coast as bound waves lagging slightly (~4 s) behind the wave group envelope and are released in the surf zone where their heights can exceed 1 m. Considerable infragravity dissipation is observed in the surf zone and is a function of both frequency and H_o^2 T_p. Complex Empirical Orthogonal Function (EOF) analysis reveals (quasi-)standing waves at low infragravity frequencies < 0.017 Hz. Conversely, at higher frequencies (>0.017 Hz), infragravity waves demonstrated progressively more dissipation (up to 90%) and progressive wave characteristics, with increasing frequency. Much of the observed dissipation occurs very close to shore (h < 0.8 m) and the dependence of the reflection coefficient on a normalised bed slope parameter implies a mild sloping bed regime at these high infragravity frequencies, suggesting that the observed dissipation is dominated by wave breaking processes. This is supported by the results of bispectral analysis which show predominantly infragravity-infragravity interactions in shallow water and the development of infragravity harmonics indicative of steepening and eventual breaking of the infragravity waves. This study presents the first simultaneous field observations of infragravity waves on a macrotidal, rocky shore platform and adjacent sandy beach. Infragravity wave dissipation is observed on both the platform and beach and occurs at statistically similar rates, demonstrating that frictional dissipation due to bed roughness is not the dominant dissipation mechanism, even in this extreme case. Sea-swell waves are also unaffected by the extreme roughness of the platform, with relative wave heights on the beach and platform (γ = 0.38 and 0.43, respectively) scaling well with their respective gradients and are in very close agreement with formulations derived from sandy beaches. Overall, bed roughness is shown to have no significant impact on infragravity or sea-swell wave transformation, with offshore forcing and bed slope being the main controlling factors, particularly under moderate to high energy offshore forcing.

Les vagues de surface générées par le vent, généralement appelées houle ou mer de vent, sont omniprésents à la surface de I'océan. Ils sont et ont des périodes variant entre 2 et 25 secondes et de longueur d'onde variant de quelques mètres à plusieurs centaines de mètres. Il existe aussi des ondes plus longues et, à plus basse fréquence appelés ondes infragravitaires (IG), qui sont associés aux vagues courtes, générées par le vent. Ces ondes ont des périodes dominantes comprise entre 30 secondes et 10 minutes et, quand ils se propagent librement, avec des longueurs d'ondes pouvant atteindre plusieurs dizaines de kilomètres. En dehors de la zone de surf, I'amplitude verticale de ces ondes infragravitaires est de I'ordre de 1 à 10 cm, tandis que I'amplitude des vagues courtes est de I'ordre de 1-10 m.Malgré leurs petites échelles d'amplitude, ces ondes infragravitaires peuvent avoir une importance non-négligeable dans certaines situations. Elles peuvent par exemple exciter des phénomènes de seiches dans les ports et mettre en résonance des structures en mer et des lames de glaces dansI'Arctique ou I'Antarctique. Le champ d'ondes infragravitaires constituera probablement aussi une fraction significative du signal mesuré par la future mission du satellite Surface Water Ocean Topography (SWOT). Ce champ d'onde infragravitaire devra être caractérisé pour atteindre la précision attendue sur les mesures de hauteur de mer dynamiques. Il est probable que la précision visée ci-dessus ne soit pas possible pour les forts états de mer avec de longues houles. L'un des objectifs de cette thèse était de fournir une première quantification de ces incertitudes associées.Au-delà de la mission SWOT, la quantification du champ d'ondes IG est un problème clé pour la compréhension de plusieurs autres phénomènes géophysiques tels que la compréhension des microséismes. / Wind-generated surface gravity waves are ubiquitous at the ocean surface. Their period varies between 2 and 25 seconds, with wavelength varying between a few meters to several hundreds of meters. Longer and, lower frequency surface gravity waves, called infragravity (IG) waves, are associated to these short, high frequency wind-generated waves. These infragravity waves have dominant periods comprised between 30 seconds and 10 minutes, and, when they propagate freely, with horizontal wavelengths of up to tens of kilometres, as given by the linear surface gravity wave dispersion relation. Outside of surf zones, the vertical amplitude of these infragravity waves is of the order of 1-10 cm, while the amplitude of wind-generated waves is of the order of 1-10 m.Given the length scales of the infragravity wavelengths, and despite the fact that the infragravity wave field exhibits much smaller vertical amplitudes than the high frequency wind-driven waves, the infragravity wave field will be a significant fraction the signal measured by the future Surface Water Ocean Topography satellite (SWOT) mission. This infragravity wave field will have to be characterized in order to achieve the expected precision on dynamic height measurements. It appears likely that the above mentioned precision will not be feasible for high sea states and long and steep swells. One of the aims of this thesis was to provide a first quantification of these associated uncertainties. Beyond the SWOT mission, the quantification of the IG wave field is a key problem for the understanding of several geophysical phenomena, such as the understanding of microseisms and ice shelves break up.

Cette thèse présente l’étude de la signature sédimentaire des submersions marines de tempête, dans le domaine rétrolittoral. Cette signature peut être extrêmement variable principalement en fonction des paramètres météo-océaniques menant à la submersion, de la morphologie de l’avant-côte et du domaine rétrolittoral et du disponible sédimentaire. Premièrement, l’étude de dépôts de washover mis en place lors de l’hiver 2013-2014, au sud de l’île d’Oléron, particulièrement énergétique en terme de climat de vagues, via une approche couplant processus hydrodynamiques et sédimentologie, a permis (1) de mettre en évidence l’importance des ondes infragravitaires, sur une côte dissipative à caractère macrotidal, dans le déclenchement de l’overwash de la barrière et la mise en place de washovers ; et de (2) proposer un nouveau modèle de faciès des dépôts de washover mis en place par des ondes infragravitaires combinées à la marée haute. Deuxièmement, la caractérisation de la signature sédimentaire des submersions marines en domaine rétrolittoral plus distal a montré que dans le contexte morphologique des marais de Charente-Maritime, les apports extrêmement faibles de sédiments en provenance du domaine continental, entre deux évènements de submersion marine, empêche la distinction des différents niveaux de submersion. Cependant cette étude a permis de valider des proxies tels que la microfaune ou l’isotopie de la matière organique pour l’identification de niveaux de submersion marine, permettant en partie de pallier aux limites inhérentes à l’anomalie granulométrique généralement utilisée. Cette étude a aussi permis la mise en évidence d’une variabilité dans la signature sédimentaire des submersions marines entre deux marais géomorphologiquement contrastés et plus ou moins exposés aux houles en provenance de l’océan. / This thesis presents the study of the sedimentary signature of marine submersions triggered by storms, in the back barrier area. This signature can be extremely variable mainly according to the meteo-oceanic parameters leading to the submersion of the back barrier area, the morphology of the nearshore, shore and back barrier area, and sediment availability. Firstly, the study of washover deposits emplaced during the winter of 2013-2014 on the southern end of the Oléron Island, characterized by an exceptional wave climate, via a coupled hydrodynamical and sedimentological approach, allowed us to (1) highlight the importance of infragravity waves, on macrotidal and dissipative coasts, in triggering the overwash and emplace washover deposits ; and (2) to suggest a new facies model of washover deposits emplaced by infragravity waves combined to high tides. Secondly, the characterization of the sedimentary signature of marine submersions in the distal part of the back barrier area showed that in the morphological context of the Charente-Maritime coastal marshes, the extremely limited amounts of sediments in provenance from the continent, between two submersion events, prevent the distinction of the different marine submersion sedimentary layers. Nevertheless, this study allowed validating new proxies as microfauna and organic matter isotopy for the identification of marine submersion sedimentary signature, allowing to work independently of the granulometric anomaly proxy and its known limits. This study also allowed evidencing the variability in the sedimentary signature of marine submersions in geomorphologically contrasted marshes and that this variability was linked to the ocean waves exposure.

Les non-linéarités des ondes de surface, qui se caractérisent par leur aspect dissymétrique, sont reconnues comme l'un des principaux moteurs du transport de sédiments en zone littorale. Cependant, l'estimation du transport reste imparfaite, en partie du fait d’une description inexacte des non-linéarités de la vitesse de l'écoulement orbital. Les ondes infra-gravitaires, qui coexistent avec les ondes courtes en zone littorale, sont des ondes de longue période (20-200 s) associés aux groupes d'ondes courtes. Les mécanismes de génération, propagation et dissipation de ces ondes sont maintenant relativement bien compris, mais leur effet sur le transport sédimentaire est encore mal connu. Afin (i) d’améliorer les paramétrisations existantes des non-linéarités de vitesse et (ii) d’étudier le rôle des ondes infra-gravitaires dans le transport sédimentaire, de nouvelles expériences ont été réalisées dans deux canaux à houle, l’un avec un fond fixe et l'autre avec un fond mobile constitué de sédiments de faible densité. Les données obtenues avec ces modèles physiques ont été utilisés conjointement à des données de terrain et des simulations numériques pour étudier les deux sujets.}TEXT{}{Les paramétrisations classiques pour les non-linéarités de vitesse prennent seulement en compte l'influence de paramètres locaux (la hauteur de vague, la longueur d'onde et la profondeur d’eau), ce qui entraîne des erreurs importantes dans les estimations, en particulier les valeurs maximales de non-linéarité. Ce travail montre que les non-linéarités de vitesse ont aussi une dépendance avec des paramètres qui ne sont pas locaux: (i) la cambrure de vague et (ii) la largeur de la bande spectrale au large, ainsi que (iii) la pente de la plage. Une nouvelle paramétrisation est proposée, qui réduit d’environ 50% l'erreur sur les résultats obtenus avec les paramétrisations existantes. Les résultats expérimentaux obtenus dans le canal à houle avec un fond mobile montrent que des conditions de vagues avec la même énergie d'ondes courtes, mais avec une modulation basse fréquence différente, modèlent des profils de plage différents. L'influence des ondes infra-gravitaires sur le transport sédimentaire est mis en évidence par deux mécanismes distincts: (i) l’advection par les ondes infra-gravitaires de sédiment mis en suspension par l'action des ondes courtes, qui dépend de la hauteur des ondes infra-gravitaires et de leur phase par rapport aux groupes d'ondes courtes et (ii) les ondes infra-gravitaires modifient les non-linéarités des ondes courtes, que ce soit directement ou indirectement, par modulation de la hauteur de la colonne d'eau. Les variations morphologiques du haut de plage induites par les ondes infra-gravitaires sont associées à des changements du profil de la plage dans les zones de déferlement et de levée, en mettant en évidence le lien entre les différentes régions du profil transversal de la plage. / Wave nonlinearities have long been recognised as being among the main drivers of sediment transport in the coastal zone. However, there are still significant errors in the prediction of this transport associated, partially due to inaccurate predictions of the velocity nonlinearities. The infragravity waves, which coexist with the short waves in the coastal zone, are long-period waves (with 20-200 s) associated to the short-wave groups. Their generation, propagation and dissipation mechanisms are already reasonably well understood, but their influence on sediment transport is still very poorly characterised. In order to (i) improve current predictions of velocity nonlinearities and (ii) investigate the role of infragravity waves in sediment transport, new experiments were carried out both in a fixed-bed wave flume and in a light-weight-sediment wave flume. The physical-modelling data set is used in combination with field data and numerical simulations for studying both subjects. Existing parameterizations of velocity nonlinearities account only for the influence of local wave parameters (e.g. wave height, wave length and water depth), which leads to considerable estimation errors, especially of the maximal values of nonlinearity. This work shows that the velocity nonlinearities depend also on non-local wave parameters: (i) offshore wave steepness, (ii) offshore spectral bandwidth and (iii) beach slope. A new parameterization is proposed, which reduces by about 50% the root-mean-square error relatively to former parameterizations. The experimental results in the light-weight-sediment wave flume demonstrate that wave conditions with the same short-wave energy, but different low-frequency modulation, shape different equilibrium beach profiles. The influence of the infragravity waves on the sediment transport is confirmed and depends on two different mechanisms: (i) advection of the short-wave suspended sediment by the infragravity-waves, which is dependent on the infragravity-wave height and phasing with the short-wave groups and (ii) modulation of short-wave nonlinearities by infragravity-wave motion, both directly and indirectly, through water-depth modulation. Changes in the beachface morphology induced by infragravity waves are connected to beach-profile changes in the surf and inner-shoaling zones, highlighting the existent link between the different zones of the cross-shore beach profile.

Video monitoring of beaches is becoming the preferred method for observing changes to nearshore morphology. Consequently this work investigates a new technique for predicting the probability of inundation that is based on measuring runup variation using video. Runup is defined as the water-level elevation maxima on the foreshore relative to the still water level and the waterline is defined as the position where the MWL intersects the beach face. Tairua, and Pauanui Beaches, on the north east coast of the North Island of New Zealand, were used as the field site in this study and represent two very different beaches with the same incoming wave and meteorological conditions. Tairua is most frequently in an intermediate beach state, whereas Pauanui is usually flatter in nature. In order to rectify runup observations, an estimate of the runup elevation was needed (Z). This was estimated by measuring the variation of the waterline over a tidal cycle from time-averaged video images during a storm event and provided beach morphology statistics (i.e. beach slope (α) and beach intercept (b)) used in the rectification process where Z=aX+b. The maximum swash excursions were digitized from time-stacks, and rectified to provide run-up timeseries with duration 20 minutes. Field calibrations revealed a videoed waterline that was seaward of the surveyed waterline. Quantification of this error gave a vertical offset of 0.33m at Tairua and 0.25m at Pauanui. At Tairua, incident wave energy was dominant in the swash zone, and the runup distributions followed a Rayleigh distribution. At Pauanui, the flatter beach, the runup distributions were approximately bimodal due to the dominance of infragravity energy in the swash signal. The slope of the beach was a major control on the runup elevation; runup at Pauanui was directly affected by the deepwater wave height and the tide, while at Tairua there was no correlation. Overall, the results of the study indicate realistic runup measurements, over a wide range of time scales and, importantly, during storm events. However, comparisons of videoed runup and empirical runup formulae revealed larger deviations as the beach steepness increased. Furthur tests need to be carried out to see if this is a limitation of this technique, used to measure runup. The runup statistics are consistently higher at Tairua and suggests that swash runs up higher on steeper beaches. However, because of the characteristics of flatter beaches (such as high water tables and low drainage efficiencies) the impact of extreme runup elevations on such beaches are more critical in regards to erosion and/ or inundation. The coastal environment is of great importance to Māori. Damage to the coast and coastal waahi tapu (places of spiritual importance) caused by erosion and inundation, adversely affects the spiritual and cultural well-being of Māori. For this reason, a chapter was dedicated to investigating the practices used by Māori to protect and preserve the coasts in accordance with tikanga Māori (Māori protocols). Mimicking nature was and still is a practice used by Māori to restore the beaches after erosive events, and includes replanting native dune plants and using natural materials on the beaches to stabilize the dunes. Tapu and rahui (the power and influence of the gods) were imposed on communities to prohibit and prevent people from free access to either food resources or to a particular place, in order to protect people and/ or resources. Interpretations of Māori oral histories provide insights into past local hazards and inform about the safety and viability of certain activities within an area. Environmental indicators were used to identify and forecast extreme weather conditions locally. Māori knowledge of past hazards, and the coastal environment as a whole, is a valuable resource and provides a unique source of expertise that can contribute to current coastal hazards management plans in New Zealand and provide insights about the areas that may again be impacted by natural hazards.

Ces travaux de thèse concernent la morpho-dynamique d'une plage de poche sableuse, située en environnement macrotidal. L'étude est menée sur la plage de Porsmilin, une plage de type intermédiaire avec une terrasse de basse mer. Cette étude s’appuie sur des observations in situ acquises lors de campagnes de mesures, organisées au cours de cette thèse. Le principal objectif est de caractériser le rôle de la pente, dans l'équilibre dynamique existant entre les processus hydrodynamiques et la morphologie de la plage. La dynamique des niveaux d’eau, de la propagation des ondes infragravitaires et de la turbulence sont examinés et mis en regard des variations morphologiques de la plage, en particulier de la pente locale de la plage. Les niveaux d’eau sont globalement sous-estimés par les formulations empiriques proposées dans la littérature. Des formules adaptées au site de Porsmilin sont présentées. L’impact de la brisure de pente observée le long du profil de plage sur la dynamique de la plage, est étudié. La variabilité de la position de la brisure de pente est impliquée dans les processus d’accommodation à court terme de la plage. Cela se traduit par un retrait vers le haut de plage de la brisure de pente, permettant une dissipation plus efficace de l’énergie incidente par la terrasse de basse mer. En revanche, ce retrait peut conduire, à une concentration d’énergie importante en haut de plage, lors de conditions particulièrement énergétiques, associées à un marnage important. La position de la brisure de pente pourrait être un bon indicateur de l'état de vulnérabilité de la plage face à des évènements énergétiques, ce qui constitue une information primordiale dans le contexte actuel d’érosion généralisée des littoraux sableux. / This PhD work is focused on the morphodynamic of a pocket beach, located in a macrotidal environment. The beach type is intermediate, with a low tide terrace. The study is based on in situ observations, collected during field campaigns organized during the PhD. The objective is to characterize the beach slope impact on hydrodynamics processes and beach morphology. The water levels dynamic, infragravity waves propagation and turbulence generation in the swash zone, are examined and linked to the beach slope variability.The empirical formulations proposed in the literature show large under-estimations of water levels in the site. Empirical formulations adapted to the beach are proposed.The characteristic break slope presents along the cross-shore profile of low tide terrace beaches is studied. The variability of the break slope location is relevant to explain the short term accommodation of the beach. Under incident wave energy, the location of the beach slope moves shoreward, allowing higher wave dissipation by the low tide terrace. Nevertheless, the reflective part is reduced, which leads to a vulnerable state of the upper beach under strong forcing associated with high tide. The location of the beach slope is found to be a good indicator of the beach vulnerability under a given wave forcing. It provides valuable information in the globalized beach erosion context.