Search

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; 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.

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.

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

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.