Video-Based Nearshore Bathymetric Inversion on a Geologically Constrained Mesotidal Beach during Storm Events

Although geologically constrained sandy beaches are ubiquitous along wave-exposed coasts, there is still a limited understanding of their morphological response, particularly under storm conditions, which is mainly due to a critical lack of nearshore bathymetry observations. This paper examines the potential to derive bathymetries from video imagery under challenging wave conditions in order to investigate headland control on morphological beach response. For this purpose, a video-based linear depth inversion algorithm is applied to three consecutive weeks of frames collected during daylight hours from a single fixed camera located at La Petite Chambre d’Amour beach (Anglet, SW France). Video-derived bathymetries are compared against in situ topo-bathymetric surveys carried out at the beginning and end of the field experiment in order to assess the performance of the bathymetric estimates. The results show that the rates of accretion/erosion within the surf zone are strongly influenced by the headland, whereas the beach morphological response can be classified into three main regimes depending on the angle of wave incidence θp: (1) under deflection configuration (θp>0°), the alongshore sediment transport was trapped at the updrift side of the headland, promoting sand accretion. (2) Under shadowed configuration (θp<0°), the interruption of the longshore current drove a deficit of sand supply at the downdrift side of the headland, leading to an overall erosion in the surf zone. (3) Under shore-normal configuration (θp=0°), rip channels developed, and up-state beach transition was observed. A comparison between video-derived bathymetries and surveys shows an overall root mean square error (RMSE) around 0.49 to 0.57 m with a bias ranging between −0.36 and −0.29 m. The results show that video-derived bathymetries can provide new insight into the morphological change driven by storm events. The combination of such inferred bathymetry with video-derived surface current data is discussed, showing great potential to address the coupled morphodynamics system under time-varying wave conditions.Comprendre et prévoir l'évolution contemporaine du traite de côte dans un contexte de changement climatique par assimilation de données satellite dans les modèles hybride

Observation and Modeling of the Equilibrium Slope Response of a High-Energy Meso-Macrotidal Sandy Beach

Beach slope is a critical parameter to, e.g., beach safety, wave reflection at the coast and longshore transport rate. However, it is usually considered as a time-invariant and profile-average parameter. Here, we apply a state-of-the-art equilibrium model to hindcast beach slope variability from the time scales of days to years at the high-energy meso-macrotidal sandy beach of Truc Vert, southwest France. We use 9 years of bimonthly beach surveys to compute beach slope time series at different elevations. Results show that beach slope exhibits an equilibrium response with contrasting behaviors along two distinct areas of the beach profile. From 0 to 2 m above mean sea level, which is located under the berm crest, a slope response predominantly at the storm time scale is observed. The beach slope steepens under low energy waves, with the equilibrium model explaining up to 40% of the observed beach slope variability. In contrast, from 2.5 to 4 m above mean sea level, which is above the berm crest, the beach slope steepens under high-energy waves. Within this region of the beach profile, the response time scale increases upwards from seasonal (~2.5 m) to seasonal (~4 m), with the model explaining up to 65% of the observed beach slope variability. Such behaviors are found to be enforced by the berm dynamics developing from the end of the winter to early autumn, providing new perspectives to model and predict beach slope on sandy beaches

Swash Bar Effects On The Response Of A Large Barrier-Spit Terminus To Extreme Wave Climate: The Cap Ferret Example

International audienceThe succession of severe storms in the North-Eastern Atlantic during the 2013-2014 winter has generated exceptional erosion along the Gironde coast (SW France). Meanwhile, at the Southern extremity of this 110-km long sandy coastal stretch, the Cap Ferret barrier-spit terminus remained relatively stable. The spit terminus is flanked by the Bay of Arcachon tidal inlet which generates strong tidal currents that help local waves to build massive swash bars. Such a bar is seen as the main explanation to the contrasting behaviour observed throughout the winter

Coastal dune response to extreme storms and multi-annual recovery along the atlantic coast of Europe

International audienceCoastal dunes are natural barriers buffering storm waves, protecting coastal communities from flooding and rising sea level, and providing a valuable source of biodiversity for the surrounding environment (Grootjans et al., 2008). Although coastal dunes have received quite a lot of attention over the last decades (Hesp and Walker, 2013), knowledge gaps remain, and our understanding of long-term (years to decades) coastal dune evolution remains limited. The large diversity of coastal dunes along the Atlantic coast of Europe and the sequence of extreme storms observed during the 2013/14 winter, considered as the most energetic storms since at least 1948 (Masselink, 2016), represent a unique opportunity to study the spectrum of coastal dune response and recovery from an extreme winter

Alongshore-Variable Beach and Dune Changes on the Timescales from Days (Storms) to Decades Along the Rip-dominated Beaches of the Gironde Coast, SW France

International audienceThe high-energy meso-macrotidal 110-km long Gironde coast, SW France, is primary composed of quasi-straight sandy beaches bordered by high and wide coastal dunes. Beaches are intermediate double-barred and are essentially morphologically variable alongshore with ubiquitous rip channels incising both bars. These rip channels enforce a strong alongshore variability in the morphology of the dry beach and/or of the dune, morphological patterns referred to as megacusp embayments. In this study, we use 70-year diachronic shoreline data, 3.5-year semi-annual in situ shoreline surveys since 2014, combined with 12.5-year monthly to semi-monthly topographic surveys collected since 2005 at Truc Vert beach. Results show that 2 types of megacusp can be identified: (1) accretive megacusps on the upper beach, forming through a sequence of accretionary beach states following a storm event, are enforced by inner-bar rip channels with a spacing of O(100 m) and a typical lifetime of a few months and (2) erosive megacusps cutting the dune, forming during severe-storm driven erosive events, which are primarily enforced by the outer-bar morphology with a spacing of O(1000 m). These erosive megacusps do not migrate alongshore and can persist for years to decades. The outstanding winter of 2013/2014 drove the formation of erosive megacusps all along the coast, dramatically altered the coastal landscape and also impacted the behaviour and mean spacing of the accretive megacusps during the subsequent years. Overall, the study demonstrates the complex interplay between the nearshore morphology and the alongshore-variable changes of the foreshore/backshore from the timescales of days to decades, with accessional outstanding winters having the potential to deeply affect beach morphology and rhythmicity on the time scale of a few years, at least

LX-Shore : Un nouveau modèle d'évolution du trait de côte pour les littoraux sableux dominés par l'action des vagues.

International audienceComprendre et prévoir les évolutions du trait de côte est crucial pour informer et guider les gestionnaires du littoral. Jusqu'à très récemment la plupart des modèles de trait de côte ne permettaient pas de répondre à ces objectifs le long des littoraux sableux dominés par l'action des vagues du fait de limitations numériques et/ou physiques. Cette étude présente un nouveau modèle numérique à complexité réduite, nommé LX-Shore, permettant de simuler les évolutions du trait de côte le long de ces littoraux, sur des échelles de temps allant de l'heure à quelques décennies, et avec des temps de calcul très réduits. Ce modèle est applicable à la plupart des géométries de côtes sableuses (ex. : flèches sableuses, îles) et prend en compte l'existence de zones non-érodables (ex. : ouvrages de défense, caps rocheux). LX-Shore s'appuie sur une approche one-line originale, où les évolutions du trait de côte résultent non seulement des gradients de transport sédimentaire longshore, mais aussi du transport sédimentaire cross-shore dû à la variabilité temporelle de l'énergie des vagues incidentes. LX-Shore est couplé avec le modèle spectral de vagues SWAN pour simuler la transformation des vagues au-dessus de bathymétries complexes et assurer une rétroaction de l'évolution du trait de côte sur la propagation des vagues. Les cas d'application présentés montrent par exemple que LX-Shore est capable de simuler la formation d'instabilités du trait de côte (ex. : flèches sableuses) ou résultant de l'implémentation de structures en dur (ex. : épis et caps rocheux) sur une large gamme d'échelles temporelles. Cet outil ouvre la voie vers : (1) une meilleure évaluation des processus physiques contrôlant les évolutions du trait de côte et de leurs contributions respectives, (2) et la réalisation de projections

Observation and numerical modeling of tidal dune dynamics

International audienceTidal sand dune dynamics is observed for two tidal cycles in the Arcachon tidal inlet, southwest France. An array of instruments is deployed to measure bathymetric and current variations along dune profiles. Based on the measurements, dune crest horizontal and vertical displacements are quantified and show important dynamics in phase with tidal currents. We observed superimposed ripples on the dune stoss side and front, migrating and changing polarity as tidal currents reverse. A 2D RANS numerical model is used to simulate the morphodynamic evolution of a flat non-cohesive sand bed submitted to a tidal current. The model reproduces the bed evolution until a field of sand bedforms is obtained that are comparable with observed superimposed ripples in terms of geometrical dimensions and dynamics. The model is then applied to simulate the dynamics of a field of large sand dunes of similar size as the dunes observed in situ. In both cases, simulation results compare well with measurements qualitatively and quantitatively. This research allows for a better understanding of tidal sand dune and superimposed ripple morphodynamics and opens new perspectives for the use of numerical models to predict their evolution

16 years of topographic surveys of rip-channelled high-energy meso-macrotidal sandy beach

International audienceSandy beaches are highly dynamic environments buffering shores from storm waves and providing outstanding recreational services. Long-term beach monitoring programs are critical to test and improve shoreline, beach morphodynamics and storm impact models. However, these programs are relatively rare and mostly restricted to microtidal alongshore-uniform beaches. The present 16year dataset contains 326 digital elevation models and their over 1.635 × 10 6 individual sand level measurements at the high-energy meso-macrotidal rip-channelled Truc Vert beach, southwest France. Monthly to bimonthly topographic surveys, which coverage progressively extended from 300 m to over 2000 m to describe the alongshore-variable changes, are completed by daily topographic surveys acquired during a 5-week field campaign. The dataset captures daily beach response at the scale of a storm to three large cycles of interannual variability, through the impact of the most energetic winter since at least 75 years and prominent seasonal erosion/recovery cycles. The data set is supplemented with high-frequency time series of offshore wave and astronomical tide data to facilitate its future use in beach research

Surf zone retention in a laboratory rip current

Proceedings of the 11th International Coastal Symposium, Szczecin, PolandInternational audienceField and numerical studies recently challenged the traditional paradigm of rip currents systems that states that rip currents produce a continuous interchange of waters between the surf zone and shelf. Instead it is suggested that rip current flow fields consist of semi-enclosed, large-scale vortices that retain floating material (e.g. drifters) at a rate of about 80-90%. In this paper is presented a laboratory rip current experiment over eight contrasting nature-like beach morphologies involving deployment of a large number of drifters. When the rip current was symmetric over a typical bar and rip morphology (4 out of the 8 cases), only about 10% of the drifters entering the rip exited the surf zone, whereas when the mean rip current was asymmetric, more drifters (~30-45%) entering in the rip exited the surf zone compartment. Drifters exiting the surf zone compartment were not systematically caught by a pulsating jet. More frequently, these drifters were likely caught in a vortex being shed offshore, as they often looped track in the vicinity of the rip head before exiting the surfzone compartment. This confirms new thoughts on rip currents that are very important from the perspective of both mixing in the nearshore and beach safety: rip currents systems only sporadically produce intense interchange between the waters of the surf zone and the shelf. Results additionally suggest that asymmetric rip current retain less floating material than symmetric rip currents