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On the embayed beaches of La Magdalena, located in the western mouth area of the estuarine Bay ofSantander, dredging sands were dumped from the 70s to increase the beach width. Until the year 2000, the sands were systematically nourished by suction dredger but they were later replaced by transfers with trucks from the western beach to the eastern ones and the mechanical redistribution to create artificial berms. A recent project aimed to solve the sand losses after each storm by building two breakwaters, the western one to retain the E-W sandy beach drift. The eastern breakwater was built in early summer of 2018, and wave storms in November 2018 and February 2019 have continued to dismantle the reconstructed upper beach areas and eroded with the same magnitude other segments where they previously retreated. Two field campaigns were carried out with laser TLS and GNNS, one after the storms in November 2018 and another in mid-March 2019 to have a database that will be complemented with future records after the summer of 2019, typical of calm periods. The first indicative results of a sandy loss are exposed, establishing a hypothesis of dynamo-sedimentary functioning., En las playas de La Magdalena, situadas en la bocana occidental de la bahía estuarina de Santander, se vertieron arenas de dragado desde los años 70 para incrementar su superficie. Hasta el año 2000, se aportaron sistemáticamente chuponas y, posteriormente, fueron sustituidas por trasvases desde la playa occiedntal a las orientales para su redistribución y la creación de mermas artificiales. Un proyecto actualizado asegura solucionar las pérdidas arenosas después de cada temporal mediante la construcción de dos diques, el occidental para retener el flujo arenoso por deriva de la playa E-O. Finalizado el oriental en el verano de 2018, los temporales de noviembre y febrero de 2019 han desmantelado las partes reconstruidas y erosionado con la misma magnitud los tramos donde anteriormente hubo retrocesos. Se cartografiaron las bermas y talud intermareal, y escarpes erosivos, incluyendo los afloramientos de piedras, y se realizaron dos campañas con equipos láser TLS y GNNS, una después de los temporales de primeros de noviembre de 2018 y otra a mediados marzo de 2019, cuya base de datos que se complementará con registros después del verano de 2019, en períodos de calma. Se exponen los primeros resultados indicativos de una pérdida arenosa, estableciendo una hipótesis de funcionamiento dinamo-sedimentario.

International audience; Shorelines are the locus of significant and diverse economic activity as well as ecosystem services and conservation interest. At a time of global climate change and sea level rise, the type of management being implemented has direct influence on the future of the shoreline and its resources. Several approaches can be taken, but in practical terms, the overwhelming response has been, and continues to be, to armour or nourish the shoreline and prevent it moving. This removes the shoreline's natural resilience, damages the coastal ecosystem and the resources it provides, and has a direct economic cost (in design, construction and maintenance) and indirect costs (through its impact on recreation, scenic quality, tourism potential and several other economic activities). The coast of south Brittany in France is subject to severe erosion. This coast is characterized mainly by seacliffs and headland embayed and pocket beaches with a "low-tide-terrace" active morphology. Wrack deposits (macroalgae: mainly Soleiria Chordalis) also form significant seasonal accumulations on the beaches, thus impacting their morphology and ecosystem. Because of the absence of sediment stocks that could be used for beach nourishment, shoreline management has had recourse, thus far, to "hard" engineering techniques to hold the coastline. The use of "soft" engineering techniques to protect dunes and beaches in south Brittany has been limited to the implementation of sand fences on the dunes to limit human access to these fragile systems. The aeolian sediment transport potential, with a relative abundance of onshore winds on this coast, has unfortunately been under-exploited. In order to gain maximum profit from the aeolian transport potential and from the nutrient release generated by wrack decomposition (which may facilitate plant growth and sediment fixing), we proposed an experimental tool, the "AlgoBox", which consists in sand-trapping fence cases filled by wrack after partial aeolian sand deposition. The fixing of sand in the "AlgoBox" by vegetation colonization is facilitated by the nutrients released by wrack decomposition during spring and summer seasons (Sedrati and Cochet, 2015). This paper presents the design and evaluation of the "AlgoBox" system which has now been successfully experimented at too sites in South Brittany coast (Penvins beach since July 2014 and Kerjouano beach since June 2016) including the regular topographic dune-beach profile surveys, algae wrack decomposition and nutrient generation, vegetation colonization processes and future considerations for the optimal design and improvement of the system as a new tool for ecological engineering of beach-dune systems.

International audience; The shoreface is defined as the transition zone between the beach and the inner shelf. Its morphodynamics are driven by weather-wave climate interaction where, during fair weather, shoaling waves favour onshore sediment transport, whereas high-energy events produce offshore directed currents which transport sediment offshore up to the inner shelf. Significant changes on the shoreface only occur at long temporal time scales and over large spatial domains, making their study challenging. Consequently, the role of the shoreface in beach and shoreline change has often been overlooked. As a result of the shoreface acting as both a sediment source and a sediment sink, as well as facilitating or impeding sediment transport, its contribution to nearshore dynamics is usually complex. When considering the longer term and larger-scale evolution of coastal systems however, the morphodynamics of the shoreface appears to be a determining factor in coastal and shoreline evolution. Along high-energy coastal areas of (west) Ireland and northwestern France, dominated by geomorphologi-cal features such as embayments, pocket beaches, sea cliffs and rock outcrops, the geological background plays a major role in the control of the coastal morphodynamics. This geological control acts to limit accommodation space, sediment availability and longshore sediment transport thus helping constrain the shape of the shoreface and therefore its dynamics and evolution. The study of shoreface morphodynamics requires the combination of multiple datasets at variable temporal and spatial scales. In this study, the west coast of Ireland and France (Brittany), provides spatially extensive sea to shore continuous topo-bathymetry (multi-beam) datasets to allow the study of beach and shoreface morphology as well as examination of why particular coastal dynamics are at play. Furthermore, the existence of wave data from offshore buoys and nearshore wave models, will also allow the analysis of the hydrodynamic behaviour across a range of shoreface configurations. The overall aim of the work is to advance our understanding of the morphodynamics of the shoreface in geologically complex regions like the west of Ireland and north west France and to examine its link to dune-beach system behaviour and shoreline evolution patterns.

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The Agadir coastline represents an interesting zone to understand the influence of natural and anthropogenic processes on sedimentary dynamics. This study presents a methodology based on three complementary approaches: 1) Mineralogical and granulometric characterization of sediments, 2) A long-term analysis of bathymetric data of the seabed of the two periods (1972-1993 et 1993-2012), 3) A numerical simulation of wave and current. The granulometric and mineralogical characterisation of the sandy beach of Agadir was performed by analysing the spatial evolution of the characteristics of the sand along the beach and at every geomorphologic unit. The results relative to granulometric indices have shown that it’s about medium size sand, well sorted and almost symmetrical. However, variability in the spatial distribution of sand highlights the combined effect of forcing agents (wave and current), the nature of the seabed and sediment sources. Also, the bathymetric differencing allowed the calculation of sands volume corresponding to surfaces in erosion or accretion of sediment budget. The results allow distinguishing a northern sector in accretion and a southern sector in erosion. The total sediment budget of the bay is strongly negative; sediment losses amounted to more than 145000 m3/year over a surface of 8.5km2 ranging from a depth of 0 to -12 m/Zh. The simulation of wave and currents allowed understanding the origin of the changes that took place at the shoreface: Accretion in the north and erosion in the south are largely explained by human intervention especially the implementation of the main jetty of the commercial harbour that has blocked the littoral drift and disrupted the energy spectrum of swell, Le littoral d’Agadir est une zone particulièrement intéressante pour comprendre l’influence des processus naturels et anthropiques sur la dynamique sédimentaire. Cette étude présente une méthodologie basée sur trois approches complémentaires; 1) une caractérisation minéralogique et granulométrique des sédiments 2) une analyse à long terme des levées bathymétriques des fonds marins des deux périodes (1972-1993 et 1993-2012) et 3) une simulation numérique de la houle et des courants. La caractérisation granulométrique et minéralogique de la plage sableuse d’Agadir a été effectuée en analysant l’évolution spatiale des caractéristiques du sable le long de la plage et au niveau de ses différentes unités géomorphologiques. Pour cela, une campagne d’échantillonnage a été réalisée en janvier 2014 le long de 19 profils perpendiculaires à la ligne de rivage. Les résultats relatifs aux indices granulométriques et leurs distributions en fonction des unités morphologiques ont montré qu’il s’agit, généralement, de sables de taille moyenne, bien classés et presque symétriques. Cependant une variabilité de la distribution spatiale des grains de sable met en évidence l’effet combiné des agents de forçage (vagues et courants), la nature des fonds de l’avant côte et les sources des sédiments. Par ailleurs, la superposition des levés bathymétriques a permis le calcul des volumes de sables (cubatures) correspondant aux surfaces en érosion ou en accrétion et du bilan sédimentaire. Les résultats permettent de distinguer un secteur Nord en accrétion et un secteur Sud en érosion. Le bilan sédimentaire global de la baie est fortement négatif, les pertes de sédiments se chiffrent à plus de 145,000 m3 /an sur une surface de 8.5km2 en allant de la profondeur 0 à -12 m/Zh. Les simulations de la houle du courant ont permit de comprendre l’origine des changements qui ont eu lieu au niveau du shoreface : L’accrétion au nord et l’aggravation de l’érosion au sud sont en grande partie expliquées par les interventions humaines surtout la mise en place de la jetée principale du port de commerce qui a bloqué la dérive littoral et a perturbé le spectre de la houle.

International audience; Several studies on barred beaches in settings with mesotidal to macrotidal regimes have focused on crossshoreand alongshore bar mobility. However, the role of coastal defense structures (breakwaters, groins, seawalls) onbar-trough systems morphodynamics have received more much less attention. The main aim of this paper is to highlightthe contrasting behavior of a natural and protected barred beach under several hydrodynamics conditions. This paperpresents detailed hydrodynamic and morphological data from a field experiment undertaken in Matalascañas beach, amesotidal protected vs natural barred beach in the Southern Spanish coast. The non-protected beach shows a lesspronounced bar-trough system than the protected zone. Under low energy conditions, onshore bar migration rate in theprotected area was more important than the non-protected area. This migration was associated with an onshoresediment transport, resulting from the erosion of the bar’s seaward slope. In the same moment, a clear longshore barmigration was observed in the non-protected zone. During few energetic tides, the protected and non-protected zonesshow a flattening bars processes. The findings of the present study suggest that cross-shore vs longshore bar mobilitymay even be mitigated by the presence of the groins, which favor onshore than longshore bar migration.Key words: Dissipative beach, bar dynamics, groins, Matalascañas, Huelva.; Numerosos estudios en playas disipativas con barras en entornos de régimen meso y macromareal se hancentrado en la movilidad de las barras tanto lateral como longitudinalmente. Por menos que el impacto de las estructurasde defensa costera (rompeolas, espigones, malecones) en la morfodinámica de estas barras ha recibido mucho menosatención y los estudios experimentales en el campo de estos ambientes son escasos. El objetivo de este trabajo escontrastar bajo varias condiciones hidrodinámicas el comportamiento de una playa natural y otra protegida por peinesde espigones transversales. Este trabajo presenta datos hidrodinámicos y morfológicos de un experimento de campo queabarca 10 días en la playa de Matalascañas, una playa mesomareal con un tramo protegido y otro natural en la costasuroeste de España. Bajo condiciones de baja energía, la migración hacia tierra de la barra en el área protegida fue másimportante que el área no protegida. Al mismo tiempo, una clara migración longitudinal de la barra también fueobservada en la zona no protegida con la ausencia de este proceso en la zona protegida durante algunas mareasenergéticas. Los resultados sugieren que la movilidad de barra longitudinal puede ser mitigada por la presencia deespigones, que favorecen la migración de las barras hacia tierra a la vez que minimizan su movilidad en el sentidolongitudinal.

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National audience; From experimental topographic and hydrodynamic data, this paper highlights the principal morphological and volumetric variations in four semi-sheltered closely spaced pocket beaches along the southern shoreface of Rhuys Peninsula (South Britanny, France), during a period of high energy, culminating in Storm Xynthia. These four beaches are subjected to a mixed hydrodynamic regime, with a mesotidal range. In the western part of Rhuys Peninsula, three beaches adjacent to each other are oriented north-west/south-east while in the eastern part, an embayed beach, larger in size, exhibits a more variable shoreline geometry and is more exposed to an oceanic wave climate. Along this latter beach, during the storm, three currentmeters comprising wave gauges were deployed (at the two ends and in the centre of the bay) in the lower intertidal area. Results show that tidal currents are more forced in direction than in intensity and an important storm surge is measured (0.5-0.63 m). This leads to significant overwash or erosion of the foredune depending on its morphology. Topographic surveys, carried out within 24 hours, highlight a substantial erosion rate exceeding 5.5% of the initial sediment budget. During this period of high energy culminating in the storm, the embayed beach profiles underwent a total loss up to 10% of their sediment budget. On the contrary, the three western pocket beaches accreted to about +2% of their initial sediment budget. It clearly appears that such an energetic event does not lead to a widespread erosion of all the beaches of Rhuys Peninsula. The intertidal morphology and the local subtidal morphobathymetric context govern the sedimentary exchanges, promoting storage or by-pass of sediment between these two sections in topographic incisions. Thus, along this shoreface, each sandy system adopts a single morphodynamic signature in response to an energetic event.; À partir de mesures expérimentales topographiques et hydrodynamiques in situ, cet article présente les principaux résultats des variations morphologiques et volumétriques de quatre plages de poche, espacées de quelques kilomètres le long du littoral méridional semi-abrité de la presqu’île de Rhuys (Bretagne sud, France), au passage d’une période de forte énergie finissant par un évènement tempétueux exceptionnel : la tempête Xynthia. Ces quatre plages sont soumises à un régime hydrodynamique mixte, à marnage mésotidal. À l’ouest de la presqu’île de Rhuys, trois plages adjacentes sont orientées nord-ouest/sud-est alors qu’au sud-ouest, une plage de baie de plus grande dimension présente une orientation plus variable et une exposition plus prononcée. Au passage de la tempête, trois courantomètre-houlographes étaient disposés aux extrémités et au centre de la baie, sur le domaine intertidal inférieur de cette dernière. Le passage de la tempête a contraint davantage les courants en direction qu’en intensité. Une surcote significative est observée (0,5-0,63 m). Cette surcote entraîne une importante submersion marine de la crête du front dunaire, lorsque celui-ci est bas, ou une importante érosion et un recul de celui-ci lorsqu’il est taillé en falaise verticale. Les levés topographiques, espacés de 24 heures, effectués au passage de la tempête mettent en évidence une importante érosion et une perte significative jusqu’à 5,5 % du budget sédimentaire initial. Pendant cette période de forte énergie, qui a fini par la tempête Xynthia, l’ensemble des levés topographiques souligne une perte conséquente de plus de 10 % du volume total de sédiment. Au contraire, les plages de poche situées à l’ouest de la presqu’île de Rhuys enregistrent un gain de volume sédimentaire de l’ordre de 2 % du volume initial. Il apparaît clairement que l’impact d’un tel évènement n’engendre pas systématiquement une érosion sur l’ensemble des systèmes sableux de la presqu’île de Rhuys. La géomorphologie de cette frange littorale ainsi que la nature et la morpho-bathymétrie des domaines intertidaux et subtidaux locaux conditionnent les échanges sédimentaires préférentiels, entre ces deux domaines, en favorisant des zones de by pass et de stockage dans les incisions et les dépressions du substratum. Ainsi, chaque système adopte une signature morphodynamique unique à un évènement énergétique impactant cette portion du littoral sud breton.

National audience; This study presents a classification of morphodynamic beach states monitored along the South Brittany coast, on the western part of France. The investigated area faces the Atlantic Ocean and it is exposed to prevailing wind and waves mainly coming from the NW to SW and experience similar waves and wind climates as most beaches located on the French Atlantic coast. However, morphological settings of Brittany coast display more specific characteristics which contribute to make the morphodynamic beach behaviours more complex. To characterise the morphodynamic beach states, a beach survey programme was carried out from February 2008 to June 2009. Twenty five beaches were surveyed. Beach profiles were levelled from the toe of the dune to the water level line using a TRIMBLE electronic theodolite. Wave data were obtained from two offshore buoys, named l’Ile d’Yeu Nord (8503) and Le plateau du Four (04403), and owned by CETMEF (Centre d'Etudes Techniques Maritimes Et Fluviales). Wave characteristics used include significant wave height (Hs) and its associated period (Ts). Morphodynamic beach states were determined by the surf scaling and the surf similarity parameters using breaker wave’s height values derived from offshore wave height. Temporal changes of morphodynamic beach states were assessed in relation with beaches profiles and hydrodynamic factors variations. Six different morphodynamic beach behaviours were identified, mainly varying between reflective and intermediate states. Multivariate statistical analyses were then carried out to assess the main factors driving morphologic and morphodynamic beach disparities. Obtained results are related to both coastline orientation and nearshore bedrocks distribution, enhancing the role of these specific morphologic constraints in the complex behaviour of South Brittany coast.; Cette étude présente une classification des plages des côtes morbihannaises (Bretagne) basée sur l’identification de leurs comportements morphodynanamiques à court terme. Ces côtes sont exposées à des houles et vents dominants provenant d’un quart NW-SW, caractéristiques de la façade Ouest Atlantique de la France. Toutefois, à une échelle régionale, la complexité morphologique et bathymétrique des littoraux morbihannais confère à la zone d’étude une forte originalité d’un point de vue géomorphologique. Afin de caractériser le comportement morphodynamique de ces plages, plusieurs campagnes de levés topographiques ont été mises en place entre février 2008 et juin 2009. Trente-cinq plages ont été étudiées. Les profils de plage ont été levés lors des basses mers à l’aide d’un théodolite électronique TRIMBLE, à partir du pied de dune et jusqu’à la limite des eaux de mer au moment du levé. Les données de houle proviennent de deux bouées, gérées par le CETMEF (Centre d'Etudes Techniques Maritimes Et Fluviales), situées au large de la zone d’étude. Les caractéristiques des vagues retenues pour l'analyse sont la hauteur significative des vagues (Hs) et sa période associée (Ts). Les états morphodynamiques associés à chaque profil de plage ont été estimés à partir du calcul des paramètres de réplication et d’échelonnement de barres. Afin d’obtenir ces indicateurs, la hauteur des brisants a été estimée à partir de la houle au large à l’aide de la formule de Komar et Gaughan (1972). L’évolution des états morphodynamiques a été mise en relation avec les variations des profils de plage et l’évolution des contraintes hydrodynamiques. Au total, l’analyse a permis d’identifier 6 types de comportements morphodynamiques, évoluant principalement entre des états réfléchissants et intermédiaires. Des analyses multivariées ont été réalisées à partir de l’ensemble de ces données de façon à identifier les principaux facteurs associés à la disparité des comportements morphologiques et morphodynamiques observés. Les principaux résultats mettent en avant le rôle majeur joué par l’orientation du trait de côte et la distribution des zones de roches en bas de plage et dans les petits fonds, soulignant ainsi l’importance de ces contraintes morphologiques dans le fonctionnement des plages morbihannaises.

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