Rapport sur le suivi morphosédimentaire du cordon dunaire de la plage du Vougot pour l'année 2011

Cette étude s'inscrit dans l'un des objectifs Natura 2000 défini par la commune de Guissény : la conservation et l'entretien des milieux dunaires (objectif A3) qui se traduit par une fiche action N°6 " réhabiliter les zones dunaires dégradées ". A ce titre, deux actions ont été décidées : (i) la restauration, la mise en défend et l'entretien des milieux dunaires financés par le biais des Contrats Natura 2000 (financement Etat et Europe) ; (ii) le suivi scientifique de ces opérations (pour évaluer la pertinence des actions) financé par le Conseil Régional de Bretagne entre 2004 et 2008 dans le cadre d'un Contrat Nature. Pour l'année 2009, ce suivi a été financé par la commune de Guissény. À ce titre, un suivi morphosédimentaire du cordon dunaire de la plage du Vougot a été lancé au mois de juillet 2004 ; ce travail s'est de plus accompagné de deux études qui ont été également réalisées par le laboratoire GEOMER - UMR LETG 6554 CNRS (Université de Bretagne Occidentale) ; la première portant sur la cinématique du cordon dunaire de la plage du Vougot depuis 50 ans, et la seconde sur l'évaluation du risque de submersion (Suanez, 2004 ; Suanez et Sparfel, 2005 ; Suanez et al., 2006 ; Cariolet et Suanez, 2007 ; Sparfel et Suanez, 2007 ; Suanez et al., 2007 ; Suanez et Cariolet, 2008 ; Cariolet et Suanez, 2009 ; Suanez et al., 2009 ; Suanez et al., 2010 ; Suanez et Cariolet, 2010 ; Suanez et al., 2012). Le travail mené durant l'année 2011 se situe dans la continuité de ce suivi, il a concerné (i) la poursuite du suivi topo-morphologique du cordon dunaire à partir de mesures de terrain à haute fréquence du système cordon dunaire / plage intertidale ; (ii) la réalisation d'un levé du front de dune afin de poursuivre l'analyse de sa cinématique sur l'année écoulée (2010-2011) ; (III) la quantification du bilan sédimentaire de la plage orientale du Vougot depuis l'année 2009

Monitoring of recent morphological changes of the dune of Vougot beach (Brittany, France) using differential GPS

International audienceThe dune of Vougot beach is a massive drifting sand body approximately 250 to 400 m wide and 2 km long. It is located in the municipality of Guissény, on the north coast of Finistère (Brittany). This dune, with a southwest to northeast position, protects a vast polder area which was disconnected from the sea by a dike construction in 1834. For several decades the eastern part of this dune experienced erosion mainly due to the construction of an artificial jetty in 1974 (Curnic jetty), which entirely modified the hydrodynamics and sedimentation processes. In order to determine the actual trend of evolution, the advance rate, and the resultant sand drift that is occurring, a survey of the dune was achieved between 2004 and 2009. Shoreline changes were monitored on a yearly basis; in addition, two detailed topography surveys (2005 and 2009) were carried out. Topography surveys were made using a Trimble DGPS respectively with 5 cm and 1,5 cm of horizontal and vertical accuracy. ArcView GIS was used to process the data and display the results. Shoreline change rates were determined using Digital Shoreline Analysis (DSA) ArcView tools to measure erosion and accretion transects. 3D surface analysis was based on Digital Elevation Model (DEM) calculations using Surfer software. Foredune changes showed that dune evolution since 2004 is in accordance with the trend observed during the last decades. However, the speed of dune retreat has increased from 0,6 m/yr to 1,5 m/yr. The comparison between DEMs obtained from the 2005 and 2009 surveys confirmed this evolution. A volume of eroded sand from the dune amounting to -10,677 ± 110 m3 with an erosion of the foreshore beach of -10,933 ± 1,396 m3 was determined. These results confirm the fact that the Curnic jetty is constantly interrupting the sand drift inducing an increase in sediment loss from the Vougot beach/dune system

Extreme wave activity during 2013/2014 winter and morphological impacts along the Atlantic coast of Europe

Studies of coastal vulnerability due to climate change tend to focus on the consequences of sea level rise, rather than the complex coastal responses resulting from changes to the extreme wave climate. Here we investigate the 2013/2014 winter wave conditions that severely impacted the Atlantic coast of Europe and demonstrate that this winter was the most energetic along most of the Atlantic coast of Europe since at least 1948. Along exposed open-coast sites, extensive beach and dune erosion occurred due to offshore sediment transport. More sheltered sites experienced less erosion and one of the sites even experienced accretion due to beach rotation induced by alongshore sediment transport. Storm wave conditions such as were encountered during the 2013/2014 winter have the potential to dramatically change the equilibrium state (beach gradient, coastal alignment, and nearshore bar position) of beaches along the Atlantic coast of Europe

Acquisition d'un état topo-morphologique initial sur les sillons et les marais maritimes de la rade de Brest

Dans le cadre de l'étude préalable à la mise en place d'un Contrat Nature portant sur les Sillons de la rade de Brest, les sites du Pédel, de Pont-Callec, du Bourg, du Bindy et de Mengleuz sont apparus comme présentant une valeur patrimoniale forte, mais fortement dégradée par l'érosion côtière des cordons littoraux, les actions anthropiques et l'invasion des schorres par la spartine alterniflore (Stéphan et al., 2012). Sur chacun de ces sites, un ensemble approprié d'actions de réhabilitation a été proposé, notamment la suppression de remblais mis en place aux dépends des marais maritimes et le rechargement sédimentaires de cordons littoraux en cours d'érosion. La présente étude a été réalisée à la demande de Brest- Métropole Océane. Elle a pour objectif l'acquisition d'un état topo-morphologique des Sillons et des marais maritimes attenants, sur les secteurs de Pont-Callec, du Pédel, de Mengleuz, du Bindy et du Bourg pour l'année 2013. Ce travail permet d'obtenir une donnée précise au centimètre près de la topographie de chacun des sites (cordons, zones humides attenantes et secteurs sur lesquels il conviendrait d'intervenir). Cette donnée topographique aide à déterminer très précisément les volumes concernés par les travaux et les cotes afférentes. Ainsi, sur les secteurs où un déblaiement sera prévu, les volumes à extraire ont été calculés et les marges d'erreurs liées à ce calcul ont été estimées. Sur les secteurs où un rechargement sédimentaire est prévu, les volumes nécessaires ainsi que le profil que l'on souhaite donner aux cordons littoraux ont été calculés

A new Holocene relative sea-level curve for western Brittany (France): Insights on isostatic dynamics along the Atlantic coasts of north-western Europe

International audienceThis study presents new Relative Sea Level (RSL) data that were obtained in the Finistère region (Western tip of Brittany, France) and the implications those data have for the understanding of the isostatic dynamics across north-western Europe, and more specifically along the Atlantic and Channel coasts. New stratigraphic sequences were obtained and analyzed to derive 24 new Sea-level Index Points, in which 6 are basal. These new data considerably increase the knowledge we have of the RSL evolution along the coasts of Western Brittany since the last 8 kyr B.P. From this new dataset, RSL was estimated to rise continuously over the last 8 kyr with a major inflection at ca. 6 kyr cal. BP. Our results show large vertical discrepancies between the RSL records of Brittany and South-Western UK, with the latter plotting several meters below the new data. From this comparison we suggest that the two regions underwent a very different pattern and/or amplitude of subsidence during the last 8 kyr which has implications for the spatial and temporal pattern of the peripheral bulge of the European ice sheets. We compared our data against predictions from Glacio-Isostatic Adjustment models (GIA models). There are large misfits between RSL observations and the predictions of the global (ICE-5G (VM2a) – Peltier, 2004, GLAC1-b – Tarasov and Peltier, 2002; Tarasov et al., 2012, Briggs et al., 2014) and regional UK models (“BIIS” – Bradley et al., 2009; Bradley et al., 2011; “Kuchar”- Kuchar et al., 2012), which can't be resolved through significant changes to the deglaciation history and size of the British–Irish Ice sheet. Paleo-tidal modelling corrections indicate regional changes in the tidal ranges played a negligible role in the data-model misfits. Hence, we propose that the misfits are due to some combination of: (i) unaccounted mass-loss of far-field ice-sheets (Antarctic ice-Sheet or Laurentide Ice-Sheet), (ii) unresolved differences in the deglaciation history and size of the Fennoscandian Ice sheet or, more likely, (iii) significant lateral variations in the Earth's structure across the English Channel

40 years of research on boulder dynamics along the Brittany coast

International audienc

Quarrying, transport and deposition of cliff-top storm deposits during extreme events: Banneg Island, Brittany

International audienceOn 10 March 2008, a particularly high energy storm hit the French Atlantic coast and the western part of the English Channel. This storm did not generate exceptionally high waves: significant and maximum wave heights recorded in the Iroise Sea (Brittany) reached 10.85 and 18.17 m respectively, whereas during the 1989-90 winter storms, they had reached 12 and 20 m, respectively. The exceptional character of the March 2008 storm event arises from the fact that it occurred during an exceptional spring tide. From a morphogenetic point of view, the effects of this storm in terms of block quarrying, transport and deposition on Banneg Island (Brittany) were significant. This study shows that the weight of the blocks displaced during the event was between 0.07 and 42.64 tons, with a median value of 0.72 tons. More than 60% of the blocks were quarried from the wave-scoured cliff-top platform, demonstrating that the favoured zone for supplying material was located inland of the cliff. Two dominant modes of transport were involved depending on the relationship between extreme water levels and cliff height. At the centre of the island, the height of the waves breaking over the top of the cliffs on the western coast resulted in a torrential surge that flowed out towards the eastern coast of the island over a landward-sloping platform. On the western coast of the island, blocks weighing between 0.3 and 1.4 tons were displaced between 50 and 90 m from the cliff edge by this flow. At the flow outlet on the eastern coast (90 m from the western cliff edge), a pit 1.6 m deep was excavated at Porz ar Bagou cove and some of the mobilised blocks were deposited in two parallel lobes about 40 m seaward of the eastern shoreline. Elsewhere on the island, block transport occurred by airborne projection although wave heights were lower than the altitude of the cliffs. The pressure exerted by breaking waves on the bedrock was sufficient to quarry and displace blocks. A temporal comparison of the changes recorded in the double and triple boulder ridges showed that the most seaward ridges were practically untouched with the most important changes occurring in the second and third ridges. These observations allow us to propose a spatio-temporal model for the accretion of cliff-top storm deposits (CTSDs), with the various stages of CTSD formation being directly related to the morphological evolution of the cliff

Shoreline management and its implications for coastal processes in the eastern part of the Rhône delta

International audienceThe coastal fringe of the eastern part of the Rhône delta is a large area where activities related to ecological and economical interests have increased since the beginning of the century, including the salt industry of theCompagnie des Salins du Midi et des Salines de l'Est, the industrial complex ofFos-sur-Mer, tourism managed by the local authorities ofPort-Saint-Louis andArles, and dune restoration of the La Gracieuse spit undertaken by thePort Autonome de Marseille. The vulnerability of the area for changes brought about by these activities is linked to the extent of several coastal changes, whereas these changes are related to the physical functioning of the coastal fringe, such as coastal processes, but also to external components such as relative sea-level rise, subsidence, reduction of sediment input, human impact, etc. For many decades, coastline 'protection' in the eastern part of the Rhône delta has been undertaken to withstand erosional processes that offset beaches and retreat of the littoral. The use of different engineering structures illustrates the heterogeneity and differences in age of the various management policies. At the eastern side of the Rhône, thePort Autonome de Marseille, dealing with management, has opted for soft engineering structures using fences for dune restoration on the La Gracieuse spit. At the western side of the mouth, theCompagnie des Salins du Midi et des Salines de l'Est has been using hard engineering structures to combat erosion (rocks, groins, tetrapods, etc.). A brief assessment of the management efficiency is presented, including an analysis of the data collected concerning the survey of the zones where the structures were built. Their varying efficiency shows the important role played by the sediment supply to the littoral (solid river discharge) and the coastal hydro-sedimentary processes. In the eastern part of the Rhône river the success of the dune restoration is mainly due to the important sediment supply transported in this area by eastern drift currents. At the western part of the mouth, the decrease of sediment supply linked to erosional processes in the shallow off-shore beach has made the structures more vulnerable, and therefore, the activities of the salt industry. For this last threatened zone, three management scenarios are discussed by taking into account economical, physical and ecological parameters

Approche méthodologique pour une cartographie du risque de submersion des côtes basses

Article faisant suite au Colloque SHF "Nouvelles approches sur le risques côtiers", Paris 30-31 janvier 2008.International audienceCette étude présente une méthode visant à cartographier le risque de submersion des côtes basses en prenant pour exemple deux plages situées dans le Finistère nord. Cette démarche reprend la méthode élaborée par les PPR-L. Toutefois, l'estimation des niveaux marins extrêmes a été réalisée en considérant les effets du wave setup. De même, les paramètres dynamiques tels que l'érosion du trait de côte et l'élévation du niveau marin pour 2100 ont été pris en compte