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While recent studies highlighted the great mobility of boulder beaches related to the impact of storm waves, numerous researches are still needed to better understand the morphodynamic of coastal boulder accumulations. This paper provides original data about pluri-annual morphological and sedimentological changes, as well as storm-induced geomorphic processes and their impact on the Valahnúkur boulder barrier, in the south-west of Iceland. First described by Etienne & Paris (2010), this massive accumulation of boulders shows numerous evidences of coastal barrier flooding and intense boulder mobility.Between May 2015 and May 2021, a topo-morphological survey was undertaken to analyze and quantify both cross-shore and longshore morphosedimentary processes of the Valahnúkur boulder barrier and surrounding areas. Annual campaigns of UAV (Unmanned Aerial Vehicle) flights were conducted in order to produce a set of Digital Elevation Models (DEMs) and Orthophotographs using Structure-from-Motion photogrammetry. DEMs were compared to deduce the morphological changes. Orthophotographs were analyzed from a deep learning-based method to estimate the changes in the spatial distribution of boulder sizes. Hydrodynamic conditions were reconstructed using wave buoy and tide gauge measurements. Wave runup were calibrated from the analysis of a wave/swash motion data set acquired by video monitoring related to different hydrodynamic conditions. The chronology of extreme water levels has been reconstructed on this high refective beach to estimate the elevation of the wave action during the survey period.From the sediment budget calculation, a significant northward longshore drift of around 1000 m3/a is highlighted on the beachface. The ridge and back-barrier areas were dominated by accretion related to cross-shore boulder transport from the beach. The volume of transported boulders is highly variable according to hydrodynamic conditions. Although the morphogenic impact of winter storms is difficult to assess, the observed morphological changes suggest processes dominated by swash, overtopping and overwash. The boulder size analysis highlights a link between the boulders mass and their movement landward. Video monitoring indicates that the amplitude of the swash is quite similar whether the hydrodynamic conditions are normal or energetic. The high porosity of the barrier appears to be a key factor in the dissipation of wave energy.

International audience; The morphodynamic functioning of the Sillon de Talbert gravel barrier spit is analyzed using a high-frequency survey carried out between September 2012 and December 2019. It is based on beach profile measurements along two transects, modeling offshore wave data (WW3), tide gauge records, and shallow waves and water levels recorded in the intertidal zone. A barrier retreat of –23 to –30 m over the 7-year survey (i.e., –3.3 to –4.3 m.y-1) is measured. This retreat is not related to long-term SLR (macroscale of 102 to 103 yr), but to mesoscale (100 to 102 yr) morphogenic events combining storm wave and high spring tide. Over 87% to 90% of the barrier retreat is due to three significant events (February 1-2, 2014, February 9, 2016, and January 3, 2018). The storm impact scale model of Orford and Carter (1982) is tested. The estimation of the wave runup for the calculation of extreme water levels (i.e., peak overflow elevation (Oe) component), is based on the calibration of an equation performed from in situ measurements of the swash elevation. The flow depth (Od,q) overtopping the crest of the barrier (Bh) is thresholded by taking into account the morphological response of the barrier in order to define regimes corresponding to overtopping, discrete overwash, and sluicing overwash. While the Orford and Carter model is generally successful in reproducing the morphodynamic evolution of the Sillon de Talbert, the wave energy flux (F) must be considered as an additional parameter in order to improve the fit of the model, so far as it contributes in some cases to change the morphodynamic regime. Thus, the wave energy flux constitutes a key component in the quantification of the water flow across the barrier (Od,q) corresponding to the hydrodynamic forcing of the model, which becomes (Od,F).

International audience; Cliff-top boulder deposits (CBDs) are morphological indicators of high-energy conditions. Since 2014, a monitoring of CBDs dynamics has been undertaken on the south-western coast of Iceland (Reykjanes Peninsula) to monitor their long-term activation (quarrying, transport and deposition) as a proxy of the inter-annual winter storminess variations and basaltic cliff erosion processes in a context of rocky coast progradation. Annual topomorphological surveys of four study sites were conducted and Structure-from-Motion photogrammetry was performed to quantify CBDs displacements. Hydrodynamic conditions were analyzed based on offshore waves and water level. Results show that CBDs activation occurs every winter, regardless of the variability of hydrodynamic conditions. Depending on the site and the year, more than 2% and 17% of the CBDs accumulated above 8 m to 10 m asl at the top of the cliffs are regularly mobilized. While inland movements represent the main mode of transport of blocks (between 50% to 60%), seaward and longshore movements are also well represented (10% to 20%). Longshore displacement is favored by the wide tabular morpho-structural setting of the wave-scour cliff-top platforms, which is explained by the structure of pāhoehoe lava flows. The activation of CBDs –measured from the volumes of displaced boulders–, shows a good correspondence with the frequency and duration of storms. However, as was the case during the winter of 2018-2019, it was rather the intensity of two highly morphogenic episodes combining storm waves and especially very high spring tide water levels, that generated the largest boulders displacements. Substantial interannual activation of the CBDs confirms that they constitute an important and still understudied proxy of the morphogenic system of high-energy rocky coasts, whose the dynamic in terms of carrying, transport, and deposition, could significantly increase with rising sea level.

High-energy waves such as tsunamis and storm waves can dislodge and transport enormous clasts. With a growing body of literature that uses subaerial coarse clasts (individual clast sized between tens of centimetres and several metres in the intermediate axis) to interpret paleotsunamis or storms on the one hand, and characterizes coarse-clast deposits formed in recent events on the other, this chapter provides an introduction to wave-deposited coarse-clast research. It (1) addresses how to correctly identify wave-transported coarse-clast deposits; (2) provides an overview of different patterns of coarse-clast deposits, including isolated megaclasts, boulder fields, boulder ridges, coral-rubble ridges, and other gravel-sized deposits; and (3) discusses the potentials for inferring extreme-wave characteristics and deposition mechanisms (tsunami vs. storm) from coarse-clast deposits.

International audience; The Sillon de Talbert is situated on the Northern coast of Brittany; it is a large 3.5 km-long swash-aligned gravel spit barrier comprising a volume of sediment of 1.23x106 m3. Since 2002 a morphodynamic survey was carried. It is based on annual DEMs and monthly beach profile measurements. Waves and water levels are also surveyed using modelling data and field measurements. The 15-years (2002-2017) sediment budget calculation shows that cross-shore transfers are dominant and represent a total volume of 370,000 m3, while the longshore sediment transfer is estimated at about 50,000 m3. However, considering the volume of sediment eroded from the beachface (as much as –411,000 m3), and the cross-shore and longshore sediment accumulation (reaching +420,000 m3), the global sediment budget is balanced for the whole survey period. The landward displacement of the spit due to rollover processes has increased almost doubled during the last 15 years with rates of retreat of 2 m.yr-1 between 2002 and 2017, vs 1.2 m.yr-1 between 1930 and 2010. Storm events control more than 95% of this retreat due to catastrophic overwash/inundation processes. The spit retreat has led to the opening of a breach in the proximal section at the beginning of March 2018, which has experienced a rapid enlargement (35 vs 15 m) and deepening (3.4 vs 1.25 m) over the last seven months. The morphological evolution of the Sillon de Talbert is driven by anthropogenic forcing (i.e., impact of coastal defence structures, cutting off of longshore sediment transport), and natural forcing such as the depletion of the supply of sediment from the platform, and the increase of extreme storm events over the last few years.; Le Sillon de Talbert est situé sur la côte nord de Bretagne ; c’est une flèche littorale de galets faisant face à la houle, qui s’étend sur 3,5 km et dont le volume de sédiment atteint 1,23 106 m3. Depuis 2002 un suivi morphodynamique a été entrepris.Il est basé sur des levés annuels, permettant de calculer des MNT, et des mesures mensuelles de profils de plage. Le calcul du bilan sédimentaire sur les 15 ans (2002-2017) montre que les transits transversaux sont dominants et représentent un volume total de 370 000 m3, alors que le transit longitudinal est estimé à environ 50 000 m3. Toutefois, considérant le volume de plus de –411 000 m3 de sédiment érodé à la plage avant, et le volume transversal et longitudinal accumulé atteignant +420 000 m3, le bilan sédimentaire global sur l’ensemble de la période de suivi est équilibré. Le déplacement vers la terre lié au phénomène de rollover a doublé durant les quinze dernières années avec un taux de recul de 2 m/an entre 2002 et 2017, contre 1,2 m/an entre 1930 et 2010. Les évènements tempétueux contrôlent plus de 95 % de ce recul lors des processus catastrophiques de submersion/inondation. Le recul de la flèche a conduit, au début du mois de mars 2018, à l’ouverture d’une brèche dans la partie proximale qui s’est rapidement élargie et approfondie, passant respectivement de 15 à 35 m, et de 1,25 à 3,4 m, au cours des 7 derniers mois. L’évolution morphologique du sillon de Talbert est commandée par des forçages anthropiques (i.e., l’impact des ouvrages de défense côtière qui bloquent le transit sédimentaire longitudinal), et des forçages naturels, comme la pénurie de sédiments accumulés sur la plateforme pour l’alimentation de la flèche, ou l’augmentation des évènements extrêmes durant les dernières années.

International audience; This paper aims to analyze the shoreline changes of coastal accumulations (sandy and gravel beaches/barriers) of Brittany (Western France). Three long, medium, and short term spatio-temporal scale observations are taken into consideration for the assessment of shoreline dynamics at this regional scale. Firstly, the long-term shoreline position evolution is based on a comparison of two sets of aerial orthophotos (1949-1952 and (2006-2009). A total of 652 beaches were analyzed in order to map and quantify erosion (35% of the total studied coastline), stability (38%), and accretion (27%) over the last 60 years. In detail, these percentages vary significantly according to the beach/barrier morphologies (spits vs pocket beaches), sediment composition (sandy vs gravelly), and hydrodynamic context (exposed vs sheltered). Secondly, a pluri-annual (i.e., medium-term) shoreline change analysis based on five representative beaches was conducted. This analysis was also based on image processing using sets of aerial photos taken every five years over the last 60 years (1948-2013). Results show an alternation of significant erosion- and accretion-dominated periods (respectively EDP and ADP), with six main EDP (i.e., periods 1962-1968, 1977-1978, 1980-1985, 1987-1990, 1993-1997, and 2013-2014) related to an increase in the frequency of extreme water levels associated with storm events. Finally, the short-term change analysis based on high-frequency monitoring of 11 sites was carried out over the period 1998-2017. These surveys, based on field topo-morphological measurements, highlight the impact of five morphogenetic events associated with significant storm events: 1998-2000 (storms Lothar and Martin in December 1999), 2008 (storm Johanna on March 10, 2008), the winter of 2013-2014 (a cluster of storms in January, February, and March 2014), 2016 (storm Ruzica/Imogen on February 8, 2016) and 2018 (storm Eleanor on January 2, 2018). A relevant recovery phase, which took place between 2008 and 2012 due to the calm and cold winters, was also recognized. The identification of parameters involved in shoreline variations at these three timescales is important for future management options of the Brittany coast.

International audience; High–energy supratidal coastal boulder deposit (SCBD) dynamics were investigated on Vierge Island and Pors Carn Point, north and south of western Brittany, France, respectively. Morphological changes induced by boulder transport and quarrying were quantified using high–resolution topographic survey data taken between 2012 and 2017. Additional in–situ wave parameters and water levels were also recorded over this period (2014–2017) in order to compute the maximum water levels and assess the relationship between SCBD morphological changes and specific hydrodynamic conditions. During extreme water levels (for maximum water levels exceeding a one in ten year event), SCBDs were broadly reworked (up to 40% of the total volume). During lower intensity events, for which maximum water levels were still very high, morphological changes represented 1% to 5% of the total volume. These morphological and hydrodynamic observations were then used to calibrate a chronology of SCBD activation events based on 70 years of hindcast winter maximum water levels. These long–term time–series showed great interannual variability in SCBD activation but no significant long–term trend. Winter–frequency SCBD activation was better correlated to the WEPA index (r = 0.46) than the NAO index (r = 0.1). Therefore, the WEPA index can be considered to be a more significant climate proxy for assessing storm–related geomorphic changes in the temperate latitudes of the N–E Atlantic basin (36°–52° N), including the Brittany coast. The potential of SCBDs as a morphological storm proxy for macrotidal high–energy rocky coasts is addressed.

La présente étude s’inscrit dans la continuité du suivi topo‐morphologique du Sillon de Talbert initié en 2003 à la demande de la commune de Pleubian et du Conservatoire du Littoral. L’objectif est d’analyser les modalités d’évolution de la flèche depuis l’enlèvement de l’enrochement en 2004 (Stéphan et al., 2007, 2008, 2009, 2011, 2012, 2015 ; Fichaut et al., 2010, 2013). Dans le cadre de ce rapport, nous présentons les principaux changements morphologiques survenus entre les levés topographiques effectués aux mois d’octobre 2016 et septembre 2017 à l’échelle du Sillon. La méthode que nous avons employée pour reconstituer la topographie de l’estran respecte en partie le protocole de mesure détaillé dans nos précédents rapports. Elle s’appuie sur l’acquisition de mesures topographiques au DGPS (type TopCon HiperV) à partir d’une station fixe installée sur la borne IGN située sur le sémaphore de Créac’h Maout, dont les coordonnées géodésiques sont accessibles sur le site de l’IGN (www.ign.fr/ rubrique Géodésie). Notons qu’en 2016, nous avons réalisé nos mesures à partir d’une méthode impliquant le survol du Sillon au drone et l’acquisition d’images aériennes. Cette technique n’a pas été employée en 2017 pour des raisons que nous expliquerons dans la section 2 de ce rapport. Enfin, un travail important a été réalisé au cours de l’année 2017 à travers la rédaction d’un article scientifique soumis à la revue Ocean & Coastal Management. Dans cette publication, nous avons approfondi l’analyse morphologique du Sillon de Talbert sur la période 2002‐2017. Une synthèse des bilans sédimentaires a été réalisée. Un travail plus fin a également été mené sur le secteur de la taille de guêpe en voie de rupture. Des propositions concrètes en vue d’un éventuel rechargement sédimentaire de ce secteur ont été formulées. Cet article figure en annexe de ce rapport.

Between December 2013 and March 2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency. It was in February that these storm events were the most frequent and particularly virulent. The significant wave heights measured off Finistere reached respectively 12.3 m and 12.4 m during Petra and Ulla storms on February 5 and 14. However, analysis of hydrodynamic conditions shows that only three episodes promoted extreme morphogenetic conditions because they were combined with high spring tide level. The first one occurred on January 1 to 4, it was followed by events from February 1 to 3, and March 2-3. During these three extreme events observed tide levels were above highest astronomical tide level (HAT). Maximum surge level (0.97 m) was reached during Ulla storm of February 14-15. For comparison, we must go back in the winter of 1989-90 to find such extreme storm frequency. High frequency topomorphological measurements were achieved on more than ten coastal zones distributed around Brittany peninsula to assess the effects of these storms on shoreline erosion. They show that during the first phase (December-January), meeting it's climax from 1,1 to 4(th) of January 2014, shoreline erosion has been limited, with the exception of southern Brittany. This is due to the SW orientation of waves. For all monitoring sites, it has averaged -2.7 m, the averaged minimum equal to 0.6 m, and the averaged maximum at -6.20 m. During the second phase from mid-January to mid-February, reaching it's climax on 1-2 of February storm corresponding to the most morphogenetic event of the winter, the average of shoreline retreat reached -4.2 m, the averaged minimum reached approximately -1.5m, the averaged maximum -95 in. It is essentially the Northern and Western coast of Brittany that experienced largest shoreline retreat due to W-NW storm wave orientation. During the third and last phase, running from mid-February to mid-March, and characterized by the March 2-3 extreme morphogenetic event, shoreline retreat was very low. It reaches -1m on average, for an averaged minimum of -0.6 m and an averaged minimum of -1.9m. Considering the whole winter 2013-14 period, shoreline erosion for all monitoring sites reached -6.3 m on average, with a minimum of about -0.2 in and a maximum of -30.1 m. Depending on the type of environment, it appears that the dunes have retreated the most, followed by gravel or sandy barriers; the lowest erosion rates concern beaches backed by low cliffs cut in highly consistent materials such as periglacial deposits (head). Considering the three morphogenous episodes, the morphological response in terms of shoreline retreat of beaches and barriers was different. Storm occurring at the beginning of February induced the largest erosive rates partly explained by the large morphological sensitivity of beaches and barriers which were weakened by the previous storm events in the beginning of January. Conversely, the storm of March induced very few impacts. These elements show that there is no cumulative of storm effect attested. Over a long period marked by a cluster of storms, beyond a certain threshold in the shoreline retreat process, the erosive action of morphogenesis events is no longer significant, regardless of their intensity., Entre les mois de décembre 2013 et mars 2014, une douzaine de tempêtes ont touché la pointe bretonne avec une fréquence exceptionnelle. C’est au mois de février que ces évènements ont été les plus fréquents et particulièrement virulents. Les hauteurs significatives des vagues mesurées au large du Finistère ont atteint respectivement 12,3 m et 12,4 m lors des tempêtes Petra et Ulla du 5 et du 14 février. L’analyse des conditions hydrodynamiques montre toutefois que seuls trois épisodes ont été particulièrement morphogènes car ils ont été combinés à des fortes marées de vive-eau ; il s’agit des évènements couvrant les périodes du 1 au 4 janvier, du 1 au 3 février, et du 2-3 mars 2014 durant lesquels les niveaux de marée observée ont été supérieurs au niveau des plus hautes marées astronomiques (PHMA). Les surcotes maximum (entre 0,80 m et 0,97 m) ont été atteintes lors de la tempête Ulla du 14-15 février. Il faut remonter à l’hiver 1989-1990 pour connaître une situation hivernale similaire, même si les tempêtes du mois de février 1990 avaient été encore plus nombreuses et plus violentes. Des mesures topo-morphologiques à haute fréquence réalisées sur une dizaine de cordons littoraux répartis sur le pourtour de la péninsule bretonne permettent d’évaluer les effets de ces évènements sur le recul du trait de côte. Elles montrent que durant la première phase (décembre-janvier), marquée notamment par l’épisode morphogène du 1 au 4 janvier 2014, l’érosion du rivage a été limitée, à l’exception du sud Bretagne. Cela s’explique par l’orientation plutôt sud-ouest de la houle incidente. De façon générale, le recul a atteint en moyenne -2,7 m (le minimum moyenné sur l’ensemble des sites : -0,6 m ; le maximum moyenné sur l’ensemble des sites : -6,20 m). Durant la seconde phase de mi-janvier à mi-février, caractérisée par la tempête du 1-2 février qui a été la plus morphogène de l’hiver, le recul moyen a atteint -4,2 m (le minimum moyenné sur l’ensemble des sites : -1,5 m ; le maximum moyenné sur l’ensemble des sites : -9,5 m). C’est essentiellement la côte nord et ouest de la Bretagne qui a enregistré les reculs les plus importants car la houle était alors orientée O-NO. Durant la troisième et dernière phase qui s’étend de mi-février à mi-mars, marquée par l’épisode morphogène du 2-3 mars, le recul du trait de côte à été très faible. Il a atteint -1 m en moyenne (le minimum moyenné sur l’ensemble des sites : -0,6 m ; le maximum moyenné sur l’ensemble des sites : -1,9 m). À l’échelle de l’hiver 2013-14, l’érosion du rivage pour l’ensemble des sites observés a atteint -6,3 m en moyenne (minimum : -0,2 m ; maximum : -30,1 m). Lorsque l’on observe plus en détail ces chiffres, il apparaît que ce sont les cordons dunaires qui ont le plus reculé, viennent ensuite les formes fuyantes à pointe libre de type flèches sableuses ou les cordons de galets ; les reculs les plus faibles concernent les plages adossées ou les falaises entaillées dans des matériaux cohérents comme les plages pléistocènes ou les falaises de head. La réponse des plages aux trois épisodes morphogènes a été différente. Celui du début du mois de février reste le plus érosif et s’explique en grande partie par une plus forte sensibilité des cordons préalablement fragilisés par l’épisode du début du mois de janvier. À l’inverse, la tempête du mois de mars n’a pratiquement eu aucun impact. Ces éléments montrent qu’il n’y a aucun effet cumulatif attesté de l’effet des tempêtes. Sur une longue période marquée par une série d’évènements tempétueux, au-delà d’un certain seuil dans le processus de recul du rivage, l’action érosive des épisodes morphogènes n’agit plus de façon significative, quelle que soit leur intensité.

International audience; The Sillon de Talbert is a large swash-aligned gravel barrier spit of 3.5 km length, situated on the Northern coast of Brittany. Over the last decades, the spit experienced landward migration by rollover reaching 1.1 m.yr-1 at least since 1930, with maximum retreat rates of 1.35 m.yr-1 affecting the proximal section. This evolution has led to the construction of coastal defense structures (rip-rap and groin) during the 70’s and 80’s to stabilize the spit. In 2001, when the Sillon de Talbert became the property of the French public office called “Conservatoire du Littoral”, a different management strategy led to the removal of these hard coastal defence structures. At the same time, a topo-morphological survey was undertaken to analyze and quantify both cross-shore and longshore morphosedimentary processes of the spit. This monitoring began in 2002 and is still ongoing. The results show that the spit landward displacement has increased during the last fifteen years with rates of retreat almost twice as great as prior to 2002 when the monitoring began (2 m.yr-1 vs 1.2 m.yr-1). The most efficient migration process occurs when a high tide level coincides with storm waves inducing sluicing overwash and/or inundation regime. In that context, the spit barrier retreat reaches several tens of meters through rollover processes. However, following such episodes of overwashing the crest of the spit may rise rapidly during fair meteorological periods. Longshore sediment transfer through cannibalization processes is also driving the evolution of the Sillon de Talbert. Due to both of these dynamics, the spit is actually threatening to break in its proximal section in a zone called “wasp waist” where the landward retreat has been the most important since the survey began in 2002. Two strategies in terms of coastal erosion management are drawn according to the policy of the “Conservatoire du Littoral”, as owner of the Sillon de Talbert area, and the duty of the municipality of Pleubian to manage the coastal risks on its communal land. The first option is to remove the existing hard coastal defence structures in order to allow the spit to recover its natural morphodynamic. This option would imply the relocation of several buildings to prevent coastal erosion/flooding risk due to the withdrawal of coastal defence structures. The second option consists of sediment replenishment in the threatened zone with pebbles extracted from available sediment sources. The topo-morphological survey provided relevant scientific expertise in terms of volumetric requirements and existing sources to support this option.

International audience

International audience; Large clastic cliff-top storm deposits (called CTSDs) are one of the most remarkable signatures that characterizes extreme storm wave events on coastal cliffs. Hence, the study of CTSDs is of key importance for understanding and predicting the impacts of extreme storm wave events on rocky coasts or establishing proxies for storm intensity. The present study uses new data including hydrodynamic measurements in both deep and intertidal waters, and records of CTSDs displacement and deposition across Banneg Island during the stormy winter 2013–2014. Two drone-based surveys were carried out in January 2013 (pre-storms) and in April 2014 (post-storms). In addition, complementary field observations were carried out during the winter, providing a comprehensive and detailed dataset. Concerning the hydrodynamic measurements, nine pressure sensors deployed along four cross-shore profiles on the western coast of the island, recorded wave and water level conditions between December 2013 and April 2014. Aerial orthorectified photographs and digital elevation model of differences provide a detailed spatial description of cliff erosion and the reworking of cliff-top storm deposits. After the storms, 172 fresh scars corresponding to quarrying in the bed rock were localized above high spring tide water level, 507 blocks were transported up to 40 m inland, at altitudes of 8 to 14 m above mean sea level, and the tracks of 170 of them were identified. The water levels and wave parameters estimated from the pressure sensor measurements provided accurate information on the maximum water elevation reached during the largest storms at the sensor locations and were extrapolated to provide run-up elevation along the four survey profiles for the whole winter 2013–2014. The results demonstrate that a large number of overwash episodes occurred, at times of high tides and energetic waves. These episodes generally correspond to morphological changes. This study confirms the major role played by extreme storm wave events on CTSDs quarrying, transport, and deposition on Banneg Island, and clearly identify the events of major evolutions.

International audience; Large clastic cliff-top storm deposits (called CTSDs) are one of the most remarkable signatures that characterizes extreme storm wave events in the Northeastern Atlantic Basin. Hence, the study of CTSDs is of key importance for understanding and predicting the impacts of extreme storm wave events on rocky coasts. This study is based on a new set of data including hydrodynamic measurements in both deep and intertidal waters, and CTSDs displacement and deposition across Banneg Island during the stormy winter 2013-2014. Two drone-based surveys were carried out in January 2013 (pre-storm) and in April 2014 (post-storm). In addition, complementary field observations were carried out during the winter, providing a large and accurate dataset. Concerning the hydrodynamic measurements, nine pressure sensors were deployed along four cross-shore profiles on the western coast of the island, to record wave and water level conditions between December 2013 and April 2014. Aerial orthorectified photographs and digital elevation model of differences enabled a detailed spatial description of the reworking of cliff-top storm deposits. After the storms, 172 fresh scars corresponding to quarrying in the bed rock were localized above high spring tide water level, 507 boulders were transported up to 40 m inland, at altitudes of 8 to 14 meters above mean sea level, and the tracks of 170 of them were identified. The water levels and wave parameters estimated from the pressure sensor measurements provided accurate information on the maximum water elevation reached during the largest storms. After calibrating the Stockdon’s formula against field measurements, runup elevation were computed along the four survey profiles for the whole winter 2013-2014. Then, the maximum runup excursion were computed based on the observed tide level. The results revealed a large number of overwash episodes, associated with high tidal level, atmospheric surges and large waves, which were partially confirmed by morphological observations. This study confirms the major role played by extreme storm wave events on CTSDs quarrying, transport, and deposition on Banneg Island, as hypothesized in previous studies.

International audience; The present study focuses on the distribution and morphology of cliff-top storm deposits (CTSDs) in the Reykjanes Peninsula in southwest Iceland. The study area stretches over a 35 kilometre coastline and consists mainly of cliffs not exceeding 15 m, cut into post-glacial lava beds aged 14 500 to 800 yr BP. The origin of these deposits cannot be related to tsunamis since none has been recorded in this area within this period. It is due to the action of North Atlantic waves whose significant height, measured between 1994 and 2004, can reach 17.4 m in this area. During a field survey from April to June, 2014, 35 CTSD sites were studied mainly through qualitative observations and topomorphological measurements. The analysis of the data shows that the organisation and distribution of CTSDs in the Reykjanes peninsula do not depend on the cliff height. In contrast, the lithostructural characteristics of the lava beds and local variations in hydrodynamism, both of which determine the morphology of cliffs, appear to be decisive factors in explaining these aspects of CTSDs in this area.; Cette étude porte sur la répartition et la morphologie des dépôts grossiers de tempêtes (25,6 cm ≤ l ≤ 820 cm) qui se trouvent en retrait du sommet des falaises de la péninsule de Reykjanes, au sud-ouest de l’Islande. Les 35 km de linéaire côtier sur lesquels porte ce travail sont constitués de falaises, hautes au maximum de 15 mètres, taillées dans un substrat basaltique postglaciaire d'âge compris entre 14 500 et 800 ans BP. L’origine de ces dépôts est marine et est attribuée à l’action des vagues engendrées par les dépressions atlantiques, vagues dont les hauteurs significatives pour une période de retour de 100 ans atteignent 17,4 mètres dans ce secteur. Entre les mois d’avril et juin 2014, un travail de terrain a permis d’étudier 35 accumulations supratidales de blocs à partir d’observations qualitatives et de relevés topomorphologiques. L’analyse de ces données montre que l’altitude du sommet des falaises n’est pas un facteur limitant à la formation de ces dépôts sur la péninsule de Reykjanes. Au contraire, le contexte lithostructural des coulées de lave et les spécificités hydrodynamiques locales qui influent sur la morphologie des falaises sont prédominantes dans la formation et la disposition des différents types d’accumulations.

International audience; Extreme water level events were observed during recent winters on Banneg Island [Ardhuin et al., 2011; Suanez et al., 2009], a small island off western Brittany well exposed to the large North Atlantic swells and characterized by steep rocky cliffs on its western part. Based on geomorphic evidences (bedrock scars, overturned boulders, debris lines) and hydrodynamic data, Fichaut and Suanez [2011] investigated the quarrying and transport of cliff-top storm deposits induced by giant wave events and partial flooding of the island (Fig.1), while Suanez et al. [2009] provided a retrospective analysis of extreme water levels based on a 30-year wave model hindcast and empirical run-up formula. Later, Sheremet et al. [2014] deployed pressure sensor measurements to this site and applied a 1D nonlinear mild-slope model to reveal that the highest water levels could exceed 6.5 m above the astronomical tide during major storms, mainly induced by large infragravity waves. In order to improve our understanding of the storm-induced hydrodynamics in the Iroise Sea, a wave buoy was deployed to the west of Banneg and additional pressure sensors were installed on the western part of the island (Section 2). In addition, a phase-resolving wave model based on the nonlinear shallow water equations, including non-hydrostatic pressure was applied to the study site and the model results were validated against in-situ observations (Section 3). The hydrodynamic data collected during the major storms of February 2014 were analyzed and the model was forced with the hydrodynamic conditions of the morning high tide of February 5, when some of the highest water levels were observed on the island (Section 4). Based on this combined model-data analysis, the incident and infragravity wave dynamics in the nearshore, and the associated sporadic flooding of the island were investigated in the light of previous studies on Banneg Island (Section 5).

La présente étude s’inscrit dans la continuité du suivi topo‐morphologique du Sillon de Talbert initié en 2003 à la demande de la commune de Pleubian et du Conservatoire du Littoral. L’objectif est d’analyser les modalités d’évolution de la flèche depuis l’enlèvement de l’enrochement en 2004 (Stéphan et al., 2007, 2008, 2009, 2011, 2012, 2015 ; Fichaut et al., 2010, 2013). Dans le cadre de ce rapport, nous présentons les principaux changements morphologiques survenus entre les levés topographiques effectués aux mois de septembre 2014 et septembre 2015 à l’échelledu Sillon. La méthode que nous avons employée pour reconstituer la topographie de l’estran respecte en partie le protocole de mesure détaillé dans nos précédents rapports. Elle s’appuie sur l’acquisition de mesures topographiques au DGPS (type TopCon HiperV) à partir d’une station fixe installée sur la borne IGN située sur le sémaphore de Créac’h Maout, dont les coordonnées géodésiques sont accessibles sur le site de l’IGN (www.ign.fr/ rubrique Géodésie). Toutefois, il faut noter que quelques changements ont été introduit cette année et nécessite de faire un nouveau point méthodologique. Il faut noter que le levé de 2015 fait suite aux grosses tempêtes de l’hiver 2013‐ 2014. Ce dernier a été l’un des plus tempétueux qu’a connu la Bretagne au cours des deux dernières décennies. Entre les mois d’octobre 2013 et de mars 2014, une douzaine de tempêtes majeures se sont succédé sur les côtes françaises de l’Atlantique et de la Manche, associant des vents violents, de fortes pluviométries et de fortes houles océaniques (Blaise et al., 2015). Ce contexte, particulièrement propice au mouvement des sédiments sur le littoral, a entraîné un démaigrissement de nombreuses plages de Bretagne. La forte fréquence de ces événements durant l’hiver a abouti, à plusieurs reprises, à la conjonction d’une mer très agitée et d’une marée de vive‐eau. Durant les pleines mers, l’action des vagues a été portée dans les parties hautes de l’estran, rarement atteintes par la mer. Conjuguée à des coefficients de marée importants et des surcotes significatives, la forte agitation marine a généré des niveaux d’eau instantanés très élevés. Les vagues ont donc franchi de nombreux cordons de galets. Compte tenu de son exposition aux houles, le Sillon de Talbert a été frappé de plein fouet par ces phénomènes. Toutefois, la comparaison des levés réalisés entre mars 2014 et septembre 2014 a montré que le cordon était en cours de régénération post‐tempête, liée aux conditions météo‐océaniques plus calmes qui ont agi durant cette période (Stéphan et al., 2015). Il s'agissait donc de voir si le levé de l'année 2015 confirmait ou pas cette tendance à la régénération du sillon de Talbert.

International audience; While recent studies highlighted the great mobility of boulder beaches related to the impact of storm waves, numerous researches are still needed to better understand the morphodynamic of coastal boulder accumulations. This paper provides original data about storm-induced geomorphic processes and their impact on the Valahnúkur coastal barrier. First described by Etienne & Paris (2010), this massive accumulation of boulders is located between the Valahnúkur hyaloclastite cone and prehistoric lava flows of the Reykjanesta Point, in the south-west of Iceland. Numerous evidences of barrier flooding and intense boulder mobility were observed in the field.Between May 2015 and May 2017, three kite surveys were realized over the Valahnúkur boulder barrier and surrounding areas. The Digital Elevation Models (DEMs) produced using Structure-from-Motion photogrammetry were compared to deduce the morphological changes. Three orthophotographs were analyzed to estimate the changes in the spatial distribution of boulder sizes. Hydrodynamic conditions were reconstructed using wave buoy and tide gauge measurements. Wave runup were calibrated from the analysis of a wave/swash motion data set acquired by video monitoring related to different hydrodynamic conditions. The chronology of extreme water levels has been reconstructed on this high refective beach to estimate the elevation of the wave action during the survey period. Results indicate a high mobility of the Valahnúkur boulder beach. From May 2015 to May 2016, the clast mobility was moderate and was restricted to the sea face of the barrier. These changes were attributed to fair weather conditions during the winter. From May 2016 to May 2017, the results highlighted a strong alongshore sediment transfer, a crest reworking and a landward projection of high number of boulders due to several intense storm events associated with high water levels during the winter.

Investigating waves in the nearshore zone is of key importance for coastal management, marine spatial planning, coastal risk mitigation and also for the understanding of coastal geomorphology. Among the existing systems to measure wave parameters in the nearhsore, the deployment of pressure sensors in the intertidal zone is an easy, low cost and energy-efficient solution. This solution extensively used to investigate the hydrodynamics of sandy beaches, has been less used in the case of steep rocky cliffs where the breaking process can be extremely energetic. Hence, the instruments deployed in such environments need to be able to resist extreme conditions. In this study, we present the pressure-sensor based measurement of wave parameters deployed on Banneg Isalnd, in the Molène archipelago, during four consecutive winters. This system is adequate for extreme wave conditions and can be used for long-term monitoring., L’étude des vagues en zone littorale est primordiale pour l’aménagement du littoral, la planification spatiale marine, la gestion du risque côtier ou encore la compréhension de la géomorphologie du littoral. Parmi les dispositifs existants pour mesurer les paramètres de vagues à la côte, le déploiement de capteurs de pression en zone intertidale représente une solution simple à mettre en oeuvre, peu onéreuse et faiblement consommatrice d’énergie. Ce dispositif, très largement utilisé pour étudier l’hydrodynamique des plages sableuses, reste peu usité dans le cas des falaises rocheuses escarpées où le déferlement des vagues peut s’avérer extrêmement énergique. L’instrumentation de ces environnements doit donc pouvoir résister à ces conditions extrêmes. Dans cette étude nous présentons un dispositif de mesures reposant sur des capteurs de pression, déployé sur l’île de Banneg, dans l’archipel de Molène, durant quatre hivers consécutifs. Ce dispositif adapté à des environnements extrêmes permet une observation des vagues sur le long terme., Dodet Guillaume, Leckler Fabien, Outré Michel, Autret Ronan, Floc'h France, Le dantec Nicolas, Suanez Serge. Mesures de vagues extrêmes le long de falaises rocheuses escarpées, île de Banneg, archipel de Molène, Bretagne. In: Collection EDYTEM. Cahiers de géographie, numéro 19, 2017. Monitoring en milieux naturels. Retours d’expériences en terrains difficiles. pp. 257-262.

International audience; Sur les côtes à falaises rocheuses exposées aux houles de tempêtes, les vagues peuvent générer des dépôts de blocs supra-tidaux communément appelés CTSDs (pour « cliff-top storm deposits » dans la littérature anglo-saxonne). Leur disposition prend en général la forme d’amas ou de remparts le long du sommet des falaises ou plus en arrière sur leurs plateformes sommitales. Cette étude porte sur la distribution et la morphologie des dépôts grossiers de tempêtes (25,6cm ≤ l ≤ 820cm) qui se trouvent sur le sommet des falaises de la péninsule de Reykjanes, au sud-ouest de l’Islande. Les 40 km de linéaire côtier sur lesquels se basent ces travaux sont majoritairement constitués de falaises taillées dans des coulées de lave basaltique dont les âges sont respectivement compris entre 14500 et 800 ans. Les vagues, engendrées par les dépressions atlantiques, atteignent régulièrement des hauteurs voisines de 15 mètres dans ce secteur, tandis que la probabilité d’occurrence passée et actuelle d’un tsunami est négligeable. Au cours des mois d’avril, mai et juin 2014, 35 accumulations de blocs supra-tidales ont fait l’objet d’observations de terrain et de relevés topomorphologiques. Leur analyse montre que l’altitude du sommet des falaises n’est pas un facteur contraignant à la formation dépôts de blocs supra-tidaux sur la péninsule de Reykjanes. En effet, un barrage de blocs est situé en retrait des falaises de roche cohérente les plus hautes du secteur, à la pointe de Skarfasetur. Au contraire, la structure lithologique des coulées de lave a une importance prédominante dans la formation et la disposition de ces accumulations.