Search

Les embouchures tidales sont des environnements très dynamiques soumises à la fois aux forçages associés aux vagues, aux courants de marée et à l’hydrologie continentale. L’association de ces dynamiques rend la compréhension fine des processus hydro-sédimentaires et l’expression des aléas érosion ou submersion particulièrement complexes. Or, et de surcroît en conditions énergétiques lors de la période hivernale, il existe peu de données hydrodynamiques in situ du fait des complexités logistiques liées à l’étendue, à l’accessibilité, à la rapidité et l’ampleur des évolutions morpho-sédimentaires. Cette contribution détaille les données acquises dans le cadre du projet ARCADE, lors de deux campagnes de mesures long terme réalisées pendant les hivers 2021 et 2022. Les données de vagues, de courants et de marée ont été acquises de manière simultanée depuis le large jusqu’à l’intérieur du Bassin d’Arcachon. Ces jeux de données permettent d’améliorer les connaissances sur la complexité des processus associés à la propagation des vagues et leurs transformations du large vers l’intérieur de la lagune. Un effort particulier est mis ici sur la caractérisation des ondes infragravitaires (ondes longues allant de plusieurs dizaines de secondes à plusieurs minutes) depuis l’embouchure jusqu’au fond du Bassin et du clapot (ondes courtes générées localement dont les longueurs d’ondes sont inférieures à 4 s) dans le bassin lors de différentes conditions de forçage de vent, de marée et de houle au large.

Gabarrou, S.; Le Cozannet, G.; Parteli, E.J.R.; Pedreros, R.; Guerber, E.; Millescamps, B.; Mallet, C., and Oliveros, C., 2018. Modelling the Retreat of a Coastal Dune under Changing Winds. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 166–170. Coconut Creek (Florida), ISSN 0749-0208.Coastal dunes can move in response to winds and cause serious hazard to human assets. Changes in wind patterns, potentially occurring as a consequence of climate change or variability, could affect rates of aeolian transport and migration velocity of coastal dunes. However, most of previous modeling studies were conducted assuming that aeolian bedforms are subject to constant wind velocities. This article presents a modeling of the mobility of the Dune du Pilat in Aquitaine, a coastal transverse dune exposed to winds of varying intensity. It applies well-established models and empirical formula for aeolian dune migration, including a model previously validated against measurements of real profiles of desert and coastal dunes. The average migration velocity of Pilat dune predicted by the models is about 3m/year, which is in good agreement with in-situ measurements. To test the response of transverse dune mobility to changing winds, virtual wind time series are generated using a stochastic model. Modelling experiments suggest that more frequent storms have less impacts than more intense winds. Due to the size of the Pilat Dune (altitude of about 100m), these result in moderate changes in the average dune velocity. This study shows that the approach of combining a stochastic model for winds with a morphodynamic dune model can provide valuable insight into how aeolian bedforms respond to changes in flow conditions potentially caused by climate change.

International audience

With apparent uniformity, the 230 km of the Aquitanian sandy coast presents many differences in geomorphological characteristics and evolutions at the event, seasonal, annual and pluriannual time scales. This contribution highlights work based on extensive airborne LiDAR coverage campaigns realized in 2011, 2014, 2016 and 2017. Geomorphological evolutions are studied at the scale of the sedimentary cells and subcells of the Aquitanian sandy coast, specifically through the automatic detection of dune erosion scarps and incipient foredunes, planimetric indicators analysis, and the sediment budget of the beach-dune interface. Analyses were performed for two periods: 2011–2014 (including the erosional impact of the 2013–2014 winter) and 2014–2017, presenting continuous and extensive beach recovery at the beach-dune interface. Dune erosion scarps and incipient foredune characteristics are analyzed. A classification based on the alongshore length of the erosion scarp shows great diversity of erosional dynamics, which are directly linked to the geomorphologic characteristics of the beach sediment availability and the nearshore bar system. This diversity includes homogeneously large erosional bands above a 1 km alongshore length, periodic Mega cusp embayments from 200 to 800 m, and a local erosion mark less than 200 m long. Analysis for the following period shows a prompt and massive recovery at the upper beach and dune foot proxies on the majority of the coast. However, the recovery process as well as erosion is strongly marked by a North–South gradient. The dune front was massively impacted in Gironde, with a retreat exceeding 25 m that was poorly prograded since 2014, whereas in Landes a progradation of the front dune is observed compared to 2011. During the first period (including the 2013–2014 winter), more than 15 500 × 103 m3 of sediment was eroded from the beach-dune interface, which represents on average a loss of 66 m3 per linear meter. The gains during the following period (2014–2017) are also massive, above 13 500 × 103 m3, with an average gain of 57 m3 per linear meter. Given the budget between the two periods, 86.5% of the sand eroded in 2014 had moved back in 2017 at the beach-dune interface but 90 % of the sand stocks are localised between the upper beach and the dune foot. The progressive recovery tendency at the beach-dune interface is massive but fragile because the sediments are potentially removable by storm marine processes in case of energetic events. At the pluriannaual, decadal and pluridecadal scales, the different sections of the coast follow various trends concerning erosion as well as recovery processes resulting in a contrasted long term coastline evolution from the north to the south of the coast.

International audience; Peu urbanisée et riche d’une grande diversité d‘environnements côtiers comprenant notamment les dunes littorales girondines et landaises, la côte rocheuse et les plages de poche du pays basque, les cordons littoraux et les marais maritimes du Bassin d’Arcachon, la côte aquitaine n’en est pas moins l’une des côtes métropolitaines les plus exposées à l’érosion côtière. En moyenne, la côte sableuse (hors Bassin d’Arcachon) recule de 2,5 m/an en Gironde et 1,7 m/an dans les Landes ; la côte rocheuse basque recule quant à elle de 0,25 m/an en moyenne.Réseau d’experts au service du littoral, l’Observatoire de la Côte Aquitaine (OCA) est chargé du suivi des phénomènes d’érosion et de submersion sur le littoral de l’ex-région Aquitaine depuis 1996. Porté techniquement par le BRGM et l’ONF, et en coordination avec les organismes de recherche et d’expertise néo-aquitains (Universités, CNRS, CEREMA, Centre de la Mer de Biarritz, …), il apporte à l’ensemble des acteurs du littoral, des outils scientifiques et techniques d’aide à la décision, à la gestion et à la prévention des risques côtiers. Les techniques de suivi en constante évolution (DGPS, LiDAR, photographies aériennes, images satellites haute résolution, lasers 3D…) ont pu servir à de multiples réalisations scientifiques analysant les évolutions morphologiques du littoral à l’échelle régionale et aux échelles temporelles évènementielle, annuelle et pluriannuelle. L’objet de cette communication est de mettre en valeur le savoir scientifique accumulé au cours des 20 années consacrées à l’observation du littoral, présenter les produits de diffusion de la connaissance développés par l’OCA, issus d’une démarche intégrée de développement des outils de mesure et du réseau.

International audience

Coastal environment systems are always exposed to natural phenomena such as erosion and submersion, and climate change is likely to increase these phenomena and their related vulnerability. The decision whether or not to protect the coast from an extreme weather event is not only based on technical data, but must also take into account its social acceptability. The involvement of stakeholders thus appears as a risk governance option. By using a scenario (both physical and socio-economic) and workshop meth- odology, we compare the deliberations and recommendations made by stakeholders facing a storm scenario in two different locations on the French Atlantic coast in the future (2030): Truc Vert and La Tresson-Noirmoutier. Group deliberations were content-analyzed in order to reveal the main directions taken by the debate. A conservative ‘wait and see’ option was favored in the less occupied region (Truc Vert), and a more protective option was preferred in the polderised and more intensively inhabited region of La Tresson-Noirmoutier. info:eu-repo/semantics/publishedVersion

International audience; About 10% of the global population is currently livingalong the coasts. In many regions, populations are exposed to a variety of natural hazards (e.g., extreme weather such as damaging cyclones and their associated storm surges), as well as to the effects of global climate change (e.g., sea level rise), and to the impacts of human activities (e.g., urbanization). Today, our knowledge regarding these processes still remains limited by the lack of observations. For example, the proportion of the world's shorelines currently affected by erosion still remains uncertain. This lack of information not only prevents us from addressing important scientific questions, but it has also practical implications for coastal managers in charge of managing coastal risks and adapting to climate change. In this poster, we present the outcome of the International Space Science Institute (ISSI) Forum on " Monitoring the evolution of coastal zones under various forcing factors using space-based observing systems " (http://www.issibern.ch/forum/costzoneevo/) held at ISSI, Bern, Switzerland on 11-12 October 2016. This poster first reviews the scientific questions with high societal significance, where improved remote sensing observations are needed: this includes (1) separating the contributions of climate-induced sea-level changes and vertical ground motions (uplift and subsidence) in relative (coastal) sea-level changes; (2) understanding the roles, for each different coastal geomorphological setting, of human interventions, extreme events, seasonal interannual and multidecadal variability and trends in driving coastal evolution. In a second step, we review theobservations currently available or needed to address these questions. Overall, we show that since the publication of the latest IGOS report on coastal zone observational requirements (2006), the availability of high resolution topographic data, hydrometeorological reanalysis (e.g., wind, waves, pressures) and historical surge databases have greatly improved the ability to understand and model coastal flooding. In addition, there is a continued need for tide gauges collocated with GNSS and other geodetic data. However, research is needed in many other topics such as the retrieval of changing topographic and bathymetric features at the required accuracy and frequency, and in processing radar altimetry measurements in the coastal ocean. Concerning ocean color, global analyses are expected to provide useful information (e.g. on suspended materials). Besides the improvements of the current observing infrastructure, there is a need of strengthening the exchanges between different scientists and stakeholders concerned with coastal risks and climate change. Today, information on the evolution of coastal zones is managed at local to regional scales by coastal observatories. These entitieslinkscienceinformation to operational observations (including space-based)and coastal stakeholders. We argue that establishing links between global providers of Earth Observation data (such as space agencies), and the emerging networks of coastal observatories, can be beneficial to both coastal science and the management of coastal risks.

Idier, D., Castelle, B., Charles, E. and Mallet, C., 2013. Longshore sediment hindcast: spatio-temporal variability along the SW Atlantic coast of France ! The Aquitanian coast (SW France) is exposed to high-energy waves, and subject to an overall shoreline erosion of 1 to 3 m/year. Engineering practices in this area requires the knowledge of longshore sediment fluxes. The reference values of direction and variability have been estimated 33 years ago with data available at that time. More recent estimations have been done on limited area and time span. The present study aims at: (1) updating and discussing the longshore sediment flux on this coast, (2) comparing the longshore drift spatial variability with measured shoreline evolution, (3) analyzing the interannual variability, in comparison with NAO indexes. Using a recent wave hindcast spanning the period 1958–2001, and the Kamphuis (1991) formula, the obtained gradients in longshore drift appear roughly consistent with the shoreline evolution ...

International audience; The succession of severe storms in the North-Eastern Atlantic during the 2013-2014 winter has generated exceptional erosion along the Gironde coast (SW France). Meanwhile, at the Southern extremity of this 110-km long sandy coastal stretch, the Cap Ferret barrier-spit terminus remained relatively stable. The spit terminus is flanked by the Bay of Arcachon tidal inlet which generates strong tidal currents that help local waves to build massive swash bars. Such a bar is seen as the main explanation to the contrasting behaviour observed throughout the winter.

International audience; This paper aims to investigate the seabed morphodynamics of the south Aquitaine inner shelf in the area known as “La Salie” (150 km2, Atlantic ocean, west coast of France), through a descriptive and comparative analysis (time lapse of 29 years) of geophysical and sedimentological datasets.At a water depth of 24–50 m, four orders of sedimentary body types were observed at different scales. The first order are large cross-shore “morphological ridges,” corresponding with the properties of very large sorted bedforms. The second order consisted in patchy sorted bedforms, composed of alternately medium to fine sand patches (0.5–2 m in thickness), cut by smaller, elongated coarse sediment depressions. In particular, the data from the sub-bottom profiler revealed that sand patches predominantly overlayed the coarse-grained blankets on the eastern (shoreward) extremities, while coarse-grained blanket wedges were found in front of the sand patches (southwestward) or locally overlying them on the southwestern extremities. The third order of bedforms involved groups of dune-like features (fine/medium sand), lying in wide areas of coarse-grained sediment. Finally, in the fouth order, the entire inner shelf was covered with wave-generated ripples, oriented N15°, that were larger where sediments were coarse (wavelengths of 2.2 m) than where sediments were fine (wavelengths of 0.3 m). Over the past 29 years, at a large scale of observation, patchy sorted bedforms have remained remarkably persistent, as has their overall appearance. However, at a smaller scale, weak but constant movements were observed. The coarse depressions have become elongated at their extremities (by a maximum of 300 m over 15 years), and certain coarse/fine sediment boundaries have moved toward the northeast and southeast (by a maximum of 75 m over 12 years). The general movement has been shoreward as has the migration of third-order submarine dune-like features. The persistence of sorted bedforms thus appears to be the consequence of sediment sorting feedback and recurrent storm events.

Resume A la suite d'une etude pluridisciplinaire, nous presentons les modalites de l'erosion oceanique sur les formations nord-pyreneennes de la cote basque francaise. Nos travaux sont bases sur l'analyse de documents anciens, ainsi que sur l'etude geomorphologique et geologique de terrain. Au cours de cette etude, une alterite d'extension regionale a ete mise en evidence. Cette observation, que nous tenons pour majeure, est utilisee pour discuter l'evolution recente et actuelle du littoral basque et les modalites de recul de la ligne de cote. La modelisation de ces phenomenes permet de proposer une perspective d'evolution du littoral. Pour citer cet article : A. Genna et al., C. R. Geoscience 337 (2005).

International audience; n this study, the Capbreton canyon head, just off the coast, is investigated using high-resolution multibeam bathymetry datasets, sediment samples and numerical modeling. The HR bathymetty analysis reveals a morphological connection between the longshore trough and the head of the canyon. The analysis of recent sediment samples shows a clear correlation between the sediment of the canyon head and that of the nearshore. Hydrodynamic modeling (a coupled wave-flow model) shows that for high-energy waves, the rotational nature of surfzone circulation reverses and wave-induced currents have the potential to transport large quantities of nearshore sands toward the canyon head. All these arguments support the assumption that the canyon head captures a part of the sand transported by longshore drift. Over the 15 years of observation (1998-2013), time-lapse bathymetry shows that the floor of the canyon head and one lateral gully network experienced significant morphological reworking. In terms of hazards, despite this strong activity, the position of the canyon head and the profile of its longitudinal slope remained stable and appear as a comforting factor. However, the activity of unusual lateral erosions needs to be monitored.

Sediment distribution is well defined in the mouth of the Gironde estuary but the sediment dynamics is still poorly understood. The aim of this work is to analyse residual circulation of non-cohesive sediment due to the action of the main hydrodynamic factor: tidal currents. A depth-averaged numerical model emphasises the residual circulation of the Gironde estuary, which is characterised by residual tidal gyres. The comparison between sediment transport calculated from the numerical model and from a statistical analysis of the sedimentary distribution leads to the definition of a simple scheme of non-cohesive sediment circulation in the Gironde estuary’s mouth.

This study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment. The sandbar is elongated and attached to a headland that is located in the Gironde estuary (southwestern France). Bathymetric surveys, side-scan-sonar sonograms, grain-size trends of surficial sediment, and 2DH numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar. The sandbar is oriented anticlockwise with respect to the main flow. The channel-oriented bar face is ebb-dominated, whereas the opposite face is flood-dominated, resulting in a clockwise residual transport. The line of sediment flux convergence corresponds to the crest of the bar. The residual flow over the bar appears to originate from the regional estuarine flow. The dominant process responsible for the maintenance of the sandbar seems to be the depth-averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed. To generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents (Pingree and Maddock 1979, Zimmerman 1981, Huthnance 1982, and others). The seabed stability approach developed by Huthnance (1982) seems to be the most reliable theory to explain the Saint-Georges Bank dynamics, even though many natural factors are not taken into account.

International audience; The main objective of the VULSACO (VULnerability of SAndy COasts to climate change and anthropic pressure) project was to investigate present day and potential future vulnerability of sandy coasts at the 2030 horizon, i.e. on a time scale related to climate variability. The method, based on a multidisciplinary approach bringing together geologists, geographers, physicists , social psychologists, engineers and stakeholders, was structured around 4 axes: field data analysis; numerical modelling; analysis of governance and stakeholder perceptions; and development of vulnerability indexes. This approach was designed to investigate vulnerability at a local scale and was applied to 4 contrasting beaches located in France: Sète Lido (Mediterranean Sea), Truc Vert and La Tresson beaches (Atlantic Ocean), and Dewulf (English Channel). The results focus on decadal and multi-annual beach trends at the Truc Vert beach site. There is almost no trend in beach volume at Truc Vert beach, although there is a variation in this parameter on a cycle of 2 to 3 yr, with variations related to wave energy and probably to indexes of climate variability. Numerical modelling identified the sensitivity of beach responses to changes in wave height and direction, especially in terms of subtidal morphology and the potential development of shoreline instability. Together with the observed offshore wave angle at the Biscay Buoy, these model results suggest that a potential change in wave angle due to climate variability could significantly modify the bars' morphology. The combination of data analysis and numerical modelling contributed to the development of vulnerability indexes designed for sandy coasts, which take into account climate-dependant variables such as waves. This allowed the differentiation of the sites in terms of vulnerability to erosion: Sète Lido and Truc Vert beach were the most and least vulnerable sites, respectively. These indexes help in identifying the dominant components of beach vulnerability, and provide potential for the study of how anthropogenic factors affect vulnerability. The study of stakeholder perceptions and decision-making with regard to climate-related risk also highlighted potential anthropogenic effects on beach vulnerability, and identified possible site-specific outcomes.

Resume : Parmi ses missions de protection et de surveillance du littoral atlantique, l'Observatoire de la Cote Aquitaine assure le suivi du trait de cote en Aquitaine a l'aide d'observations DGPS (40 transects repartis sur les 240 km de plage sableuse). Afin de completer les observations in situ, et de produire une carte complete des facies sedimentaires a l'echelle de l'ensemble de la cote sableuse aquitaine, le potentiel de l'imagerie satellite haute resolution Formosat-2 a ete teste. Trois jeux successifs de 14 scenes multispectrales ont ete acquis en aout 2007, mai 2008 et mai 2009. Une approche par classification de pixels a ete definie, laquelle appliquee a des mosaiques Formosat permet de restituer des cartes thematiques des facies littoraux. Le trait de cote (defini comme le pied de dune) est deduit de l'analyse des cartes doublee de l'analyse des scenes Formosat filtrees a l'aide de l'algorithme de Sobel. Cette approche a ete validee a l'aide de plus de 1300 mesures DGPS in situ. La precision globale des cartes produites ("overall accuracy") est de 73% pour 5 classes ("Foret", "Dune grise", "Dune blanche", "Haut de plage" et "Plage intertidale") et 2 limites geomorphologiques ("Forest/Dune" et "Dune/Plage"). Les evolutions ont ete analysees a l'echelle du littoral aquitain en se basant sur l’imagerie spatiale Formosat completee de donnees plus anciennes. A l'exception de quelques secteurs montrant des phases d'accretion ou d’erosion prononcees (>45 m d'amplitude), le deplacement du trait de cote reste globalement limite. Le potentiel et les limites de la teledetection pour le suivi du trait de cote a l'echelle de la cote sableuse en Aquitaine sont detailles dans ce manuscrit afin de definir un cadre prospectif precis pour le suivi du littoral sableux par un outil de suivi operationnel. Mots-cles : Teledetection – Cartographie – Trait de cote – Gestion – Prevention de l’alea erosion

Un outil d’aide a la gestion environnementale des parcs ostreicoles du Bassin d’Arcachon est presente ici. Il repose sur une methode de determination semi-automatisee de la nature et de l’etat des concessions, exploitant des images de teledetection tres haute resolution spatiale. La validation de la carte de teledetection montre que l’etat de la surface (entretenu ou en friche) et le mode de culture sont detectes dans 92% et 85% des cas, respectivement. D’autre part, les superficies de friche obtenues par teledetection excedent de 20% celles mesurees sur le terrain, 75% des friches etant retrouvees in situ. Au total, 96% des parcelles sont correctement cartographiees. La teledetection tres haute resolution est donc, au vu de ces resultats chiffres, un outil de gestion du domaine pertinent et efficace, qui, en outre, permet d’enrichir le cadastre d’attributs d’etat et de couches vectorisees representant les friches ostreicoles. Abstract: An environmental management tool used for mapping oyster-farming activities of the Arcachon lagoon is presented here. It is based on a semi-automatized detection method of the nature and status (clean/dirty) of oyster farms. It exploits very high resolution remote sensing imagery. Validation of the remotely sensed map shows that the surface status and the type of practice are detected in 92% and 85% of the cases, respectively. The simulated surface associated to waste lands exceeds by about 20% the observed ground-field one, 75% of them corresponding to real “dirty” farms. Altogether, 96% of the oyster-farming parcels are correctly mapped. Therefore, very high resolution remote sensing is an efficient and valuable management tool that, moreover, allows enhancing the cadaster with attributes relative to the status of oyster farms and underlying waste lands as vector layers. Keywords: Remote sensing; Mapping; Oyster-farming; Domain status; Management.

Les falaises sont des écosystèmes littoraux riches et complexes qui sont en proie à une érosion côtière plus ou moins rapide. La présence d'enjeux anthropiques ou patrimoniaux conduit parfois à mener des actions afin de réduire, voire stopper le recul des falaises pour quelques dizaines d'années (durée de vie des ouvrages). Face à ce recul inéluctable des falaises sur le long terme, les décideurs sont amenés à définir une stratégie : faut-il protéger, laisser faire, reculer ? Quelques suggestions de réponses illustrées par des exemples sont apportées dans cet article.