Coastal Change in Tropical Overseas and Temperate Metropolitan France Inferred from a National Monitoring Network: A Summary from the Current Special Issue

International audienceThis paper provides an overview of the papers published in this Special Issue « Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France » of Journal of Coastal Research. This special issue reflects the large diversity of mainland and tropical French coast and highlights the complex, site-and timescale-specific, combination of factors driving coastal evolution. Many contributions reveal a strong climate control on storm wave activity and, in turn, coastal response. In some contributions, the inherited geology and anthropogenic factors clearly appear affect coastal change. This special issue emphasises the need to monitor the coast combining different means to improve our understanding and predicting capacities of the natural variability of coastal response in a changing climate

Modélisation de l'hydrodynamique sédimentaire au-dessus des barres sableuses soumise à l'action de la houle : application à la Côte Aquitaine

Cette thèse présente la première étude complète de la dynamique des vagues, des courants moyens induits et des barres sableuses sur les plages d’Aquitaine. La démarche suivie repose sur la modélisation des processus physiques, en interaction avec l’imagerie satellite et aérienne, les observations sur le terrain, ainsi que le traitement et l’analyse in-situ. Du 12 au 19 Octobre 2001 sur la plage aquitaine du Truc Vert, la campagne de mesures PNEC 2001 a permis de collecter des données hydrodynamiques et sédimentaires pendant des conditions de houle énergétiques. A partir de l'analyse de ces données, le module hydrodynamique du modèle morphodynamique développé au cours de cette thèse a été validé, et la dynamique des courants moyens au-dessus de la plage du Truc Vert a été décrite. A partir de simulations au-dessus des systèmes barre/baïne et de systèmes de barres en croissant de la côte aquitaine, on montre que la modulation tidale des processus est intense. La formation des courants sagittaux est favorisée par les houles longues et frontales tandis qu’un courant de dérive oscillant est induit par les houles d’incidence oblique. Le couplage morphodynamique complet avec prise en compte de la marée montre que le mécanisme d'auto-organisation est à l'origine de la formation des systèmes barre/baïne dans la zone intertidale et des systèmes de barres en croissant dans la zone subtidale. Les caractéristiques morphologiques des systèmes formés sont en accord avec les observations sur le terrain. Le développement de ces barres est également étudié, ainsi que sa sensibilité au forçage au large. L’étude aboutit à un nouveau modèle conceptuel de la morphologie des barres sableuses observées sur la côte aquitaine.This thesis presents the first complete study of wave dynamics, wave-induced currents, and morphodynamics of nearshore sandy bars on the Aquitanian coast beaches. This study is based on a physical modeling approach, associated with satellite imagery as well as treatment and analysis of field data. From the 12th to the 19th of October 2001 at Truc Vert Beach, hydrodynamic and sedimentary data were collected during PNEC 2001 field measurements for energetic swell conditions. From the analysis of these data, the hydrodynamic module of the morphodynamic model developed during this thesis has been validated, and the dynamics of waves and wave-induced currents has been described. Simulations over Aquitanian coast ridge and runnel systems and nearshore crescentic bar systems show an intense tidal modulation of physical processes. Rip currents are induced by shore normal incidence long swells and an oscillating longshore current is induced by oblic incidence swells. The morphodynamic coupling including tidal cycles shows that self-organization mechanisms are responsible for the formation of ridge and runnel systems in the intertidal domain and crescentic bar systems in the nearshore zone. The morphological characteristics of simulated systems are in agreement with observations. The development of these bars is also studied, as well as its sensitivity to wave forcing. The study leads to a new conceptual model of sandy bars morphology on the Aquitanian coast

Assimilation de données et inversion bathymétrique pour la modélisation de l'évolution des plages sableuses

Cette thèse présente une plateforme d'assimilation de données issues de l'imagerie vidéo et intégrée au modèle numérique d'évolution de profil de plage 1DBEACH. Le manque de jeux de données bathymétriques haute-fréquence est un des problèmes récurrents pour la modélisation morphodynamique littorale. Pourtant, des relevés topographiques réguliers sont nécessaires non seulement pour la validation de nos modèles hydro-sédimentaires mais aussi dans une perspective de prévision d'évolution morphologique de nos plages sableuses et d'évolution de la dynamique des courants de baïnes en temps réel. Les récents progrès dans le domaine de l'imagerie vidéo littorale ont permis d'envisager un moyen de suivi morphologique quasi-quotidien et bien moins coûteux que les traditionnelles campagnes de mesure. En effet, les images dérivées de la vidéo de type timex ou timestack rendent possible l'extraction de proxys bathymétriques qui permettent de caractériser et de reconstruire la morphologie de plage sous-jacente. Cependant, ces méthodes d'inversion bathymétrique directes sont limitées au cas linéaire et nécessitent, selon les conditions hydrodynamiques ambiantes, l'acquisition de données vidéo sur plusieurs heures voire plusieurs jours pour caractériser un état de plage. En réponse à ces différents points bloquants, ces travaux de thèse proposaient l'implémentation puis la validation de méthodes d'inversion bathymétrique basées sur l'assimilation dans notre modèle de différentes sources d'observations vidéo disponibles et complémentaires. A partir d'informations hétérogènes et non redondantes, ces méthodes permettent la reconstruction rapide et précise d'une morphologie de plage dans son intégralité pour ainsi bénéficier de relevés bathymétriques haute fréquence réguliers.This thesis presents data-model assimilation techniques using video-derived beach information to improve the modelling of beach profile evolution.The acquisition of accurate and recurrent nearshore bathymetric data is a difficult and challenging task which limits our understanding of nearshore morphological changes. This is particularly true in the surf zone which exhibits the largest degree of morphological variability. In addition, surfzone bathymetric data are crucial from many perspectives such as numerical model validation, operational rip current prediction or real-time nearshore evolution modelling. In parallel, video imagery recently arose as a low-cost alternative to direct measurement in order to daily monitor beach morphological changes. Indeed, bathymetry proxies can be extracted from video-derived images such as timex or timestacks. These data can be then used to estimate underlying beach morphologies. However, simple linear depth inversion techniques still suffer from some restrictions and require up to a 3-day dataset to completely characterize a given beach morphology. As an alternative, this thesis presents and validates data-assimilation methods that combine multiple sources of available video-derived bathymetry proxies to provide a rapid, complete and accurate estimation of the underlying bathymetry and prevent from excessive information.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

A new climate index controlling winter wave activity along the Atlantic coast of Europe: The West Europe Pressure Anomaly

International audienceA pioneering and replicable method based on a 66-year numerical weather and wave hindcast is developed to optimize a climate index based on the sea level pressure (SLP) that best explains winter wave height variability along the coast of western Europe, from Portugal to UK (36–52 ∘ N). The resulting so-called Western Europe Pressure Anomaly (WEPA) is based on the sea level pressure gradient between the stations Valentia (Ireland) and Santa Cruz de Tenerife (Canary Islands). The WEPA positive phase reflects an intensified and southward shifted SLP difference between the Icelandic low and the Azores high, driving severe storms that funnel high-energy waves toward western Europe southward of 52 ∘ N. WEPA outscores by 25–150% the other leading atmospheric modes in explaining winter-averaged significant wave height, and even by a largest amount the winter-averaged extreme wave heights. WEPA is also the only index capturing the 2013/2014 extreme winter that caused widespread coastal erosion and flooding in western Europe

Modélisation du festonnage des barres sableuses d'avant-côte : application à la côte aquitaine, France

Modeling of crescentic pattern development of nearshore bars: Aquitanian Coast, France. Nearshore crescentic bars play key role in nearshore morphodynamics. These bars are observed all along the Aquitanian Coast, with a mean wavelength of about 700 m. A non-linear stability analysis is undertaken to simulate the development of crescentic patterns. Results show that self-organization mechanism can lead alone to the development of these alongshore rhythmic features. Simulated wavelengths are in agreement with observations on the Aquitanian Coast

On the use of linear stability model to characterize the morphological behaviour of a double bar system. Application to Truc Vert Beach (France).

10 pages, 9 figures, 3 tableauxInternational audienceSandy barred beaches are often characterized by the presence of rhythmic patterns such as crescentic bars. In this paper, a linear stability analysis (LSA) model is used to characterize the morphological behaviour of the double bar system of Truc Vert beach. Using a limited number of combination of representative bathymetries, wave classes and water levels, the morphodynamic response of the system is analysed, focussing on the geometrical characteristics of 3D patterns generated with the model. These characteristics are described and then compared with available observations. The shapes and the wavelengths of the instabilities predicted by the model compare well with field observations. Thus, the use of linear stability model, with representative hydrodynamic conditions and bathymetries of the considered site, allows a characterization of the global morphodynamic behaviour of a double-barred system

A Bayesian network approach to modelling rip-current drownings and shore-break wave injuries

A Bayesian network (BN) approach is used to model and predict shore-break-related injuries and rip-current drowning incidents based on detailed environmental conditions (wave, tide, weather, beach morphology) on the high-energy Gironde coast, southwest France. Six years (2011–2017) of boreal summer (15 June–15 September) surf zone injuries (SZIs) were analysed, comprising 442 (fatal and non-fatal) drownings caused by rip currents and 715 injuries caused by shore-break waves. Environmental conditions at the time of the SZIs were used to train two separate Bayesian networks (BNs), one for rip-current drownings and the other one for shore-break wave injuries. Each BN included two so-called “hidden” exposure and hazard variables, which are not observed yet interact with several of the observed (environmental) variables, which in turn limit the number of BN edges. Both BNs were tested for varying complexity using K-fold cross-validation based on multiple performance metrics. Results show a poor to fair predictive ability of the models according to the different metrics. Shore-break-related injuries appear more predictable than rip-current drowning incidents using the selected predictors within a BN, as the shore-break BN systematically performed better than the rip-current BN. Sensitivity and scenario analyses were performed to address the influence of environmental data variables and their interactions on exposure, hazard and resulting life risk. Most of our findings are in line with earlier SZI and physical hazard-based work; that is, more SZIs are observed for warm sunny days with light winds; long-period waves, with specifically more shore-break-related injuries at high tide and for steep beach profiles; and more rip-current drownings near low tide with near-shore-normal wave incidence and strongly alongshore non-uniform surf zone morphology. The BNs also provided fresh insight, showing that rip-current drowning risk is approximately equally distributed between exposure (variance reduction Vr=14.4 %) and hazard (Vr=17.4 %), while exposure of water user to shore-break waves is much more important (Vr=23.5 %) than the hazard (Vr=10.9 %). Large surf is found to decrease beachgoer exposure to shore-break hazard, while this is not observed for rip currents. Rapid change in tide elevation during days with large tidal range was also found to result in more drowning incidents. We advocate that such BNs, providing a better understanding of hazard, exposure and life risk, can be developed to improve public safety awareness campaigns, in parallel with the development of more skilful risk predictors to anticipate high-life-risk days.Marier les objectifs de défense côtière avec ceux de la protection du milieu naturel grâce aux dunes sableuse

MODEX: Laboratory experiment exploring sediment spreading of a mound under waves and currents

The dispersal of sand from submerged mounds in the nearshore is driven by the interplay of processes such as converging and recirculating flows, changing roughness, bed slope effects and wave focusing/refraction. This morphological diffusivity is key to understanding sand bars in shallow seas, tidal inlets, estuaries, and the nearshore response to human interventions such as nourishments and dredging. Most of the work on the evolution of submerged mounds has been based on fluvial studies, focusing on flow without waves. In these cases, circular mounds tend to deform to crescentic (barchan) shapes. In contrast, observations of sandbars and berms in the nearshore subjected to waves show much more complex translation and deformation behavior. This contribution introduces the laboratory MOrphological Diffusivity Experiment (MODEX) aimed at examining morphological diffusivity under different forcing conditions. The experiment particularly addresses the linkages between small scale (local) effects (e.g. bed slope, bedforms) on the adjustment of sandy mounds.Peer ReviewedPostprint (published version

Comment les surfeurs sauvent des vies sur les plages françaises

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