Coastal boulder deposits (CBD) provide relevant information on coast-erosion wave events, -defined here as extreme events combining energetic storm wave and high spring tide level-, and are crucial for understanding giant storm wave impacts on coastal geomorphology. Banneg Island, located off the coast of Brittany (France), faces Atlantic storm waves that are not attenuated in this highly exposed area. This island is characterized by CBD resulting from the erosion of western cliffs, that have accumulated at the top of the island as a result of storm wave/inudation processes. These accumulations have been subjected to topo-morphological survey for the last two decades (using GNSS measurements from 2005 to 2012, and using UAV-based measurements since 2012). Hydrodynamic conditions (wave and water levels) were also analyzed. We used these data to analyse the frequency of extreme events over the last 20 years. The results show a good relationship between boulder transport (measured as volume of boulder displacement) and the frequency of extreme events inundating the island. Over the survey period, we found significant interannual variability in the occurrence of these extreme events, depending on winter weather conditions. This storminess variability is significantly correlated with the WEPA climatic index, calculated on the 3 winter months (December to February). Finally, the results from Banneg Island were compared with long-term monitoring carried out in other sectors along the coast of Brittany, to build a comprehensive picture of storminess and its impact on coastal change in Brittany over the last 20 years.

Le site du Gouffre à Plougrescant est un géotope breton inscrit à l’inventaire des objets géologiques remarquables (OGR) de Bretagne établi en 1994 par la Société Géologique et Minéralogique de Bretagne (SGMB). Ces caractéristiques géologiques et géomorphologiques remarquables ont contribué à sa mise en valeur touristique et culturelle par les collectivités locales à partir des années 1980. Aujourd’hui, le Conservatoire du littoral, en tant que gestionnaire du site, doit gérer les impacts d’une fréquentation qui ne cesse d’augmenter, et composer avec les aspirations/exigences d’une population riveraine installée sur cet espace depuis le XIXe siècle. Dans ce contexte, une étude morphosédimentaire des cordons de galets et des relations qu’ils entretiennent avec le fonctionnement hydrologique des lagunes rétrolittorales a été réalisée à la demande du Conservatoire du littoral. Le but de ce diagnostic est d’apporter des connaissances scientifiques pertinentes afin d’orienter/accompagner la politique de gestion et de renaturation du site. Si l’état morphologique et sédimentaire des cordons de galets ne présente aucune dégradation majeure, le fonctionnement hydrologique des lagunes a quant à lui été mis à mal par un ensemble d’interventions humaines. De toute évidence, il conviendra dans le futur d’intervenir sur ce point afin de redonner au site du Gouffre l’aspect naturel qui était le sien avant l’installation de l’Homme.

While many studies have highlighted the high mobility of gravel/pebble beaches due to storm waves, further research is needed to improve understanding of the morphodynamic of coastal boulder accumulations. This paper provides original data about storm-induced morphogenetic processes for the Valahn & uacute;kam & ouml;l boulder barrier located in the Reykjanes Peninsula (SW Iceland). The study is based on annual topo-morphological surveys conducted from 2015 to 2023 and a shoreline change analysis for the period 1978-2023 using a series of aerial photos. In addition, hydrodynamic conditions (wave and water levels) were analysed using field records and model results. Results show a good correspondence of the morphological changes of the barrier to the frequency/energy of morphogenetic events. Interannual surveys (2015-2023) indicate a high mobility of boulders across the beachface regardless of the variability of hydrodynamic conditions. In contrast, boulders located on the crest and the back-barrier moved more episodically by overtopping and overwashing processes during extreme events combining storm waves and high spring tide levels. A massive landward transfer of boulders resulting in the most significant barrier retreat, -6.1 m, was recorded during the winter 2022. Between 1978 and 2023, the mesoscale shoreline changes of the barrier retreated landward by rollover. The pluri-decadal variability in shoreline retreat rates seemed to be strongly controlled by extreme hydrodynamic conditions. However, this cross-shore dynamic responses to episodic overwash depended on specific geological constraints. This survey provides a first quantitative assessment of the morphological impact of significant morphogenetic events within the highly energetic hydrodynamic context of SW Iceland. These data complement surveys undergone through a wide range of coastal morphologies elsewhere along the Western coasts of the North Atlantic basin. Such assessments may prove useful in anticipating the potential intensification of hydrodynamic conditions at mid-latitudes due to climate change.

Composite beaches, defined by a steep gravel berm fronting a low-angle sandy foreshore, remain understudied despite their increasing relevance as models for nature-based coastal defenses called dynamic cobble berm revetments. To support development of effective designs in diverse coastal settings and improve understanding of coastal change and resilience across beach types, detailed field observations of composite beaches are required. Since September 2020, a regular monitoring program has been conducted on two adjacent high-energy composite beaches — La Palue and Lostmarc’h on the Crozon peninsula (northwest France). Using drone photogrammetry, this program investigates the morphodynamic response of gravel berms to varying wave energy and changes in sand level on the beach face. The results reveal consistent temporal patterns, including a pronounced seasonal signal: The gravel berm tends to widen over winter, when sand levels are lower, whereas gravel exposure decreases during calmer, low-energy periods. Berm and foreshore morphologies also exhibit large interannual variability driven by fluctuations in winter wave energy and storm characteristics. Integration of high-frequency satellite imagery further demonstrates that gravel exposure adjusts rapidly to shifts in wave forcing and sand levels. These findings highlight the dynamic and coupled nature of gravel-sand interactions on composite beaches and underscore the need for comparative investigations across sites with differing hydrodynamic and sedimentological settings.

In intertidal rocky shores, sessile organisms are subject to local variations due to a complex interplay of physical and biological drivers. Here, we examined the patterns and the dynamics of the distribution of a key benthic ecosystem-engineer species, the mussel Mytilus galloprovincialis. We characterized areas of loss, gain, and persistence of mussel space occupancy using drone images with a resolution of 1 cm across two rocky shores spanning 3121 m2 and 3499 m2. Then, we assessed the effect of microhabitat slope, microhabitat orientation, and intertidal height on the characteristics of mussel distribution for the two rocky shores. Notably, substantial losses ranged from 56 % to 80 % depending on the rocky shore. The hierarchical impact of habitat topographic features on mussel occupancy diminishes in the order of rocky shore, intertidal height, and microhabitat features. To unravel the dynamics further, we calculated persistence, resilience, and the time to recover from disturbance using a Markov Chain model. Persistence time decreased at the extremes of the rocky shore -both its top and bottom edges. Contrary to expectations, mussels at these extremities exhibit higher resilience, preventing their disappearance despite significant losses. Our results provide insights into the structural dynamics and emphasize the need to consider long-term responses to environmental changes. This study underscores the potential of integrating models with drone technology for monitoring intertidal populations, offering a powerful tool to comprehend and anticipate the enduring impacts of environmental shifts on spatial distribution.

This paper presents an extended reality (XR) application designed to raise awareness about coastal flooding risks linked to climate change. Built from open-source geospatial data and high-resolution photogrammetry surveyed by the team, the user is able to explore climate scenarios by combining global projections from the IPCC with local sea-level data. The experience offers model-scale interaction and immersive first-person navigation.

The Gouffre site in Plougrescant is a Breton geotope listed in the inventory of remarkable geological objects in Brittany established in 1994 by the "Soci & eacute;t & eacute; G & eacute;ologique et Min & eacute;ralogique de Bretagne" (SGMB). These remarkable geological and geomorphological features have contributed to its tourist and cultural development by local authorities since the 1980s. Today, the Conservatoire du littoral, as the site manager, has to deal with the impact of ever-increasing visitor numbers and the aspirations/demands of a coastal population that has lived in this area since the 19th century. In this context, a morphosedimentary study of the gravel barriers and their relationship to the hydrological functioning of the sites' lagoons was carried out at the request of the Conservatoire du Littoral. The purpose of this assessment is to provide relevant scientific knowledge to guide and support the site's management and ecological restoration strategy. Although the morphological and sedimentary condition of the gravel barriers shows no major deterioration, the hydrological functioning of the lagoons has been compromised by a series of human interventions. This key issue must be addressed in the future to help restore the Gouffre site to the more natural state it had before human modifications.

Coastal barriers are a natural defence against the marine submersion of low-lying back-barrier areas. In March 2018, a breach opened up in the proximal section of the Sillon de Talbert spit barrier (North Brittany, France), causing great concern among local residents and elected representatives. A study to assess the risk of submersion of the low-lying areas of the Laneros Peninsula, located behind the barrier, was carried out in 2020. Hydrodynamic conditions (waves, currents, and water levels) were modeled using Telemac-2D coupled with the phase-averaged wave model TOMAWAC. Simulations were compiled for the storm of February 1, 2014, corresponding to the most morphogenic event to occur in the last two decades. Five topomorphological configurations were considered for the numerical simulations: configurations #1 and #2 correspond to the ante-breach and post-breach morphological settings, while configurations #3, #4, and #5 correspond to hypothetical scenarios, from the enlargement of the breach (from 300 to 500 m width), to complete crest lowering of the barrier after a complete removing of the crest due to catastrophic overwash events. The results show that the breach accelerates the draining of the back-barrier sandflat, thereby reducing the height of extreme water levels at the coastline. This is mainly due to different tidal impacts on either side of the barrier, with a higher water level and the tidal peak occurring 15 min later and the on the back-barrier than on the ocean-side zone. As a result, ebb currents begin to flow before the high tide level is reached, inducing a significant draining with a lowering of the extreme water level at the back-barrier zone. However, these simulations do not take into account the effects of climate change over the coming years, in particular the future rise in sea level and/or the acceleration of erosion processes already underway.

Gravity on Earth is of great interest in geodesy, geophysics, and natural resource exploration. Ship-based gravimeters are a widely used instrument for the collection of surface gravity field data in marine regions. However, due to the considerable distance from the sea surface to the seafloor, the spatial resolution of surface gravity data collected from ships is often insufficient to image the detail of seafloor geological structures and to explore offshore natural minerals. Therefore, the development of a mobile underwater gravimetry system is necessary. The GraviMob gravimeter, developed for a moving underwater platform by Geo-Ocean (UMR 6538 CNRS-Ifremer-UBO-UBS), GeF (UR4630, Cnam) and MAPPEM Geophysics, has been tested over the last few years. In this study, we report on the high-resolution gravity measurements from the GraviMob system mounted on an Autonomous Underwater Vehicle, which can measure at depths of up to several kilometres. The dedicated GraviMob underwater gravity measurements were conducted in the Mediterranean Sea in March 2016, with a total of 26 underwater measurement profiles. All these measurement profiles were processed and validated. In a first step, the GraviMob gravity measurements were corrected for temperature based on a linear relationship between temperature and gravity differences. Through repeated profiles, we acquired GraviMob gravity measurements with an estimated error varying from 0.8 to 2.6 mGal with standard deviation after applying the proposed temperature correction. In a second step, the shipborne gravity data were downward continued to the measurement depth to validate the GraviMob measurements. Comparisons between the corrected GraviMob gravity anomalies and downward continued surface shipborne gravity data revealed a standard deviation varying from 0.8 to 3.2 mGal and a mean bias value varying from −0.6 to 0.6 mGal. These results highlight the great potential of the GraviMob system in measuring underwater gravity.

Les aléas côtiers (érosion, submersion, migration dunaire) et les dynamiques côtières sont souvent méconnus par le grand public. Or, dans un contexte de changement climatique et d’évolution de la législation (loi « Climat et résilience » en 2021) sur la gestion des aléas et des risques côtiers (risque(s) combinant aléa(s) et enjeu(x)), le suivi de l’évolution du littoral au travers d’indicateurs des aléas côtiers (position du trait de côte, niveau de la plage, niveau d’eau sur un repère lors d’une submersion, etc.) devrait être l’affaire de tous. Jusqu’à présent, aucune application citoyenne pour smartphone ou tablette ne permettait de suivre conjointement les indicateurs des aléas côtiers d’érosion et de submersion, alors que ces deux aléas sont souvent liés. Pour y remédier, dans le cadre d’un projet de recherche et de formation (2019-2022) regroupant des étudiants en géographie, en biologie et en informatique, nous avons créé l’application Android CoastAppli. CoastAppli, qui vise un public large (habitants, touristes, scolaires, gestionnaires du littoral, etc.) et a été expérimentée à Guissény (Finistère) entre janvier et octobre 2022 auprès de 86 personnes, permet de réaliser un suivi citoyen des indicateurs d’aléas côtiers. Grâce à l’implication des utilisateurs sur les quatre sites de cette commune, les résultats soulignent la fiabilité des données quantitatives (jusqu’à une erreur globale de ± 4 cm) et/ou qualitatives, ainsi que la haute fréquence d’acquisition (mensuelle). Par ailleurs, cela semble un bon moyen pour comprendre, discuter et réfléchir entre acteurs du territoire aux défis de gestion du littoral d’aujourd’hui et de demain.

Cliff-top boulder deposits (CBDs) are morphological indicators of high-energy conditions. Since 2014, a moni-toring 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 top-omorphological 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 hydro-dynamic conditions. Depending on the site and the year, >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 pa over bar hoehoe 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.

Coastal hazards (erosion, flooding, dune migration) and coastal dynamics are often unknown by the general public. However, with climate change and changes in legislation (<< Climate and resilience >> law in 2021) on the management of coastal hazards and risks (hazard(s) combined with assets), monitoring the coastal evolution through indicators of coastal hazards (position of the coastline, level of the beach, the water level on a benchmark during a flooding event, etc.) should be everyone's concern. Up to now, no smartphone or tablet application allowed the general public to participate in the joint monitoring of indicators of coastal erosion and flooding hazards whereas these hazards are often linked. To address this, as part of a research and training project (2019-2022) involving geography, biology and computer science students, we have created the Android app CoastAppli. CoastAppli, which targets a wide audience (inhabitants, tourists, pupils, coastal managers, etc.) and was tested in Guisseny (Finistere) from January to October 2022 with 86 users, allows to realise a citizen monitoring of indicators of coastal hazards. flanks to the involvement of the users on the four sites in this commune, the results underline the reliability of the quantitative (up to +/- 4 cm of root-mean-square error) and/or qualitative data, as well as the high frequency of acquisition (monthly). It also seems to be a relevant way to understand, discuss and think with actors of the territory about the current and future challenges of coastal management.

The purpose of this paper is to present the design, development and testing of an innovative instrument called GraviMob, which allows performing dynamic measurements of underwater gravity anomalies. After recalling the interest in underwater gravimetry, we describe the system, the core of which consists of triads of accelerometers rigidly attached to an Autonomous Underwater Vehicle (AUV). The article also presents the mathematical methods for estimating the east, north and vertical components of the local gravity vector. An unscented Kalman filter, integrating AUV position and orientation data, performs estimation of gravity in a frame adapted to its interpretation. To assess its performance, GraviMob was tested in the Mediterranean Sea during the year 2016. A comparison of the surface gravimetric signal previously acquired by the French Navy indicates that the maximum discrepancy between the vertical gravity component and its reference is below 4 mGal. Components of the vertical deflection calculated from GraviMob’s measurements were compared with those calculated from recent gravity field models. While a remarkable agreement was found on the north component, there remains a discrepancy (7 arcsec) on the east component which can be largely reduced by refining the estimation of the orientation of GraviMob’s sensitive axes in the AUV.

Advances in image-based remote sensing using unmanned aerial vehicles (UAV) and structure-from-motion (SfM) photogrammetry continue to improve our ability to monitor complex landforms over representative spatial and temporal scales. As with other water-worked environments, coastal sediments respond to shaping processes through the formation of multi-scale topographic roughness. Although this topographic complexity can be an important marker of hydrodynamic forces and sediment transport, it is seldom characterized in typical beach surveys due to environmental and technical constraints. In this study, we explore the feasibility of using SfM photogrammetry augmented with an RTK quadcopter for monitoring the coastal topographic complexity at the beach-scale in a macrotidal environment. The method had to respond to resolution and time constraints for a realistic representation of the topo-morphological features from submeter dimensions and survey completion in two hours around low tide to fully cover the intertidal zone. Different tests were performed at two coastal field sites with varied dimensions and morphologies to assess the photogrammetric performance and eventual means for optimization. Our results show that, with precise image positioning, the addition of a single ground control point (GCP) enabled a global precision (RMSE) equivalent to that of traditional GCP-based photogrammetry using numerous and well-distributed GCPs. The optimal model quality that minimized vertical bias and random errors was achieved from 5 GCPs, with a two-fold reduction in RMSE. The image resolution for tie point detection was found to be an important control on the measurement quality, with the best results obtained using images at their original scale. Using these findings enabled designing an efficient and effective workflow for monitoring coastal topographic complexity at a large scale.