Search

Coastal dunes have global importance as ecological habitats, recreational areas, and vital natural coastal protection. Dunes evolve due to variations in the supply and removal of sediment via both wind and waves, and on stabilization through vegetation colonization and growth. One aspect of dune evolution that is poorly understood is the longshore variation in dune response to morphodynamic forcing, which can occur over small spatial scales. In this paper, a fixed wing unmanned aerial vehicle (UAV), is used to measure the longshore variation in evolution of a dune system in a megatidal environment. Dune sections to the east and west of the study site are prograding whereas the central portion is static or eroding. The measured variation in dune response is compared to mesoscale intertidal bar migration and short-term measurements of longshore variation in wave characteristics during two storms. Intertidal sand bar migration is measured using satellite imagery: crescentic intertidal bars are present in front of the accreting portion of the beach to the west and migrate onshore at a rate of 0.1–0.2 m/day; episodically the eastern end of the bar detaches from the main bar and migrates eastward to attach near the eastern end of the study area; bypassing the central eroding section. Statistically significant longshore variation in intertidal wave heights were measured using beachface mounted pressure transducers: the largest significant wave heights are found in front of the dune section suffering erosion. Spectral differences were noted with more narrow-banded spectra in this area but differences are not statistically significant. These observations demonstrate the importance of three-dimensionality in intertidal beach morphology on longshore variation in dune evolution; both through longshore variation in onshore sediment supply and through causing longshore variation in near-dune significant wave heights.

International audience; A consequence of Global Change might be an increase of coastal risks. Indeed, sea-level rise conjugated with spring tides, can lead to hazardous sea-levels, in worldwide coastal areas subject to unprecedented demographic densification(exposure),thusrisks.Coastalecogeosystemshavethepotentialtoalleviatehazardsbyreducingwave energyduetotheirtopographiccomplexity(dragcoefficient).Thisattenuationserviceisnonlinear,itevolveswith time and depends on meteorological and environmental factors. This study focusses on the variability of the wave attenuation service (WAS) provided by saltmarshes and shelly cheniers, both major ecogeomorphic features of the Mont-Saint-Michel Bay (France). The protocol consists in WAS comparison of six geo-ecosystems configurations: (1) lower marsh, (2) middle marsh, (3) upper marsh, (4) single shelly chenier, (5) multiple shelly cheniers, and (6) levelled shelly cheniers. All field sites are within a 4 km distance. Wave measurements, using pressure sensors, were carried out for three tide cycles at high water levels (> 12m above the national tidal datum epoch) in order to maximize the likelihood of extreme sea-level conditions. Wave height data are recorded, simultaneously, at a 2 Hz frequency for 13 stations along cross-shore transects. For each tide cycle, approximatively 15 million data allow to compute the wave attenuation along saltmarshes and cheniers. This comparative study has enabled each system configuration to be scored according to its mean wave attenuation over three tides, but also in respect to its time variability in attenuation. Such added values about natural coastal barriers hold great promise to be integrated into expected spatially-explicitly modelling of the wave attenuation service by ecogeosystems.

International audience; The morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in Normandy, France, was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts. Although there have been numerous studies on ebb-tidal bar morphology and on the processes affecting such bars, these concern only microtidal or mesotidal settings, and are mainly based on observations. No detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features, especially along macro and megatidal coasts. In this study, the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process (swash, surf, shoaling waves) over a representative bar cross-shore profile, and sediment transport patterns deduced from fluorescent tracer. The results show that the bar migrated exclusively shoreward during moderate to storm conditions (Hs > 0.7 m at the bottom of the bar seaward slope). During low energy wave conditions, no bar movement was observed. Swash action was not the dominant process, mainly due to its duration, which did not exceed 8 min in the course of a semi-diurnal tidal cycle. The movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents. The increase in the significant wave height disturbed the general mean current behaviour, which was parallel to the bar crest during low energy conditions. Under surf conditions, mean flows are directed onshore, with an absence of bed return flow; this allows sediment to be transported towards the bar crest causing a landward bar migration. An offshore-directed cross-shore current (not favourable for bar migration) is recorded with larger water depths and no breaking waves (shoaling conditions). This study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting, which induce a rapid shift in processes (such as bed shear and sediment re-working) across the shore-normal profile. These conditions explain the relative low rate of migration of the bar over the delta platform (about 33 m/year). This rate is much less than those reported from micro-mesotidal environments. Calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement, notwithstanding the megatidal character of the field site.

The Bay of Mt-St-Michel is a megatidal environment, the Holocene sedimentary infill of which began some 8000 years ago. The present-day Bay consists in various sub-environments: an embayment in the West, an estuary in the East, and a sandy barrier subject to wave action in the NE. Thanks to VHR seismic data and cores, different architectures are highlighted: progradational to aggradational in the embayment, aggradational to chanelized in the estuary and along the NE shoreline. The infill model established with all the data is characterised as follows: a transgressive systems tract of low volume; a well-developed highstand systems tract; a tidal ravinement surface of large extent, reaching locally the substrate; a wave ravinement surface locally present along the estuary margins; an accommodation for sequence preservation of tidal origin in the estuary, and of eustatic origin along the margins. At the basin scale, discrepancies in time-and-space of the preservation potential of sedimentary sequences observed into the infill are very important. As soon as the transgression rate slow down from 6500 yB.P., the key-control of local factors (hydrodynamics, sediment supply, substrate morphology) is evidenced. Sequences preserved along the estuary margins record environmental changes interpreted as the result of climate crisis of 1500-year period. Although the origin of these climate cycles is uncertain (Bond cycles, long term tidal cycles, combination of both?), their control on coastal system evolution should be considered as major., La Baie du Mt-St-Michel est un milieu mégatidal dont le remplissage holocène a débuté il y a 8000 ans BP. La Baie présente aujourd'hui des environnements contrastés : une baie sensus stricto à l'Ouest, un système estuarien à l'Est, et un littoral sableux exposé aux houles au NE. A partir de données de sismique THR et de carottage, des architectures de remplissage différentes sont mises en évidence : progradante puis aggradante dans la baie, aggradante puis chenalisante dans le domaine estuarien et le long du littoral NE. L'ensemble des résultats permet d'établir un modèle de remplissage caractérisé par : un cortège transgressif de faible volume ; un cortège de haut niveau marin très développé ; une surface de ravinement tidale de grande extension, pouvant localement atteindre le substratum ; une surface de ravinement par la houle présente en marge de l'axe estuarien ; une préservation de séquences sédimentaires liée à une accommodation d'origine tidale dans l'axe estuarien, et d'origine eustatique sur les marges. A l'échelle du bassin, les disparités spatio-temporelles du potentiel de préservation des séquences sont très importantes. Dès que la vitesse de la transgression holocène diminue à partir de 6500 B.P., l'importance des facteurs locaux (hydrodynamisme, apports, morphologie du substratum) est mise en évidence. Les séquences préservées en marge de l'estuaire enregistrent des changements environnementaux interprétés comme le résultat de crises climatiques d'une périodicité de 1500 ans. Si l'origine de ces crises est incertaine (cycles de Bond, cycles tidaux long terme, combinaison des deux), leur rôle dans l'évolution des littoraux doit être considéré comme majeur.