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Phosphorites composed by linguliform brachiopod shells represent a poorly known phosphorus source. As mineral precursors for P-fertilizers, they are an increasingly important resource for agricultural activities. inguliform brachiopod shells have a stratiform organo-phosphatic microstructure, which produces bioclasts characterized by their low density and laminar shape; as of yet, no other studies on their hydrodynamic behaviour have been carried out. The objective of this work is to shed some light on the concentration mechanisms acting on organo-phosphatic particles and finally leading to phosphorite formation; and also, to betterunderstand general processes of segregation in mixed bioclastic-siliciclastic sediments. In this work we present preliminary results of experiments made on fragments of Lingula anatina shells, a modern linguliform brachiopod species. We performed settling velocity and entrainment velocity measurements, as well as current-flow experiments on a mixed-sediment bed to investigate particle segregation. Lingula anatina shells show a variable degree of mineralization, and thus density, throughout the shell: measured densities ranged from 1010 – 1330 kg/m3 in marginal areas to 1370 – 1730 kg/m3 in the central part of the shell. Their fragments have platy shapes, with shape factors between 0.02 – 0.39, a range similar to the one reported for platy carbonate bioclasts (Li et al., 2020). Settling velocity values were about 50% of similarly shaped and sized carbonate bioclasts, and 20% of similarly sized quartz sand. Settling velocities are controlled jointly by particle diameter, shape and density, which are strongly variable in Lingula anatina bioclasts; when plotted against each other, these parameters show high dispersion and complex interrelationships. By contrast, the ratio between the particle mass and its maximum projected area (M:A) shows a strong correlation with settling velocity. Drag coefficients were calculated by nonlinear regression. The obtained values are comparable with those calculated from the data of Li et al. (2020), suggesting that the lower settling velocities in organo-phosphatic bioclasts are explained mostly by their lower density and not by differences in their falling behaviour. Threshold near-bed velocities (u*c) ranged between 1.0 – 4.3 cm/s for organo-phosphatic bioclasts ranging between 0.7 – 5 mm (sieve-diameter). The obtained values are comparable with those of carbonate bioclasts and quartz-sand with similar diameters ( Weill et al. 2013, Rieux et al. 2018). Dual-behaviour has been noted in carbonate bioclasts, where low settling velocities and significant resistance to erosion contributes to their segregation from siliciclastic sediments ( Weill et al. 2013, Li et al. 2020); this type of behaviour should be more pronounced in organo-phosphatic bioclasts given their smaller settling velocities. Entrainment and segregation experiments were conducted on a mixed-sediment bed composed of fine siliciclastic sand and organo-phosphatic particles (3 mm-sieve diameter). For the bed set-up, a concentrated slurry of siliciclastic and organo-phosphatic particles was set on the empty flume, which was later filled with water. At sub-threshold velocities, little sand transport took place with no bioclast motion; this was followed by incipient migration of small sandy ripples, occasional bioclast motion and imbrication. At this stage, armoured bioclast-rich patches developed through sand winnowing. Above threshold velocities the sandy ripples grew taller; bioclasts were not easily eroded, excepted in high-shear and turbulence areas such as stoss faces and downstream of lee faces of ripples. Once incorporated into the flow they were rapidly transported downstream and redeposited in bioclast-rich areas (with higher bed rugosity) or out of the test section. By contrast, sand seemed to be transported with a lower velocity corresponding to that of the migrating ripples. The final rippled bed was noticeably sandier than the original mixed-sediment massif; by contrast, a bioclast-rich deposit was formed downstream of the test section. Contrasting hydrodynamic behaviour in terms of settling and entrainment velocities favoured the segregation between both sediment fractions; however, sorting mechanisms are also strongly dependent on less predictable sedimentary processes such as bed armouring, and increased form-related shear stress and turbulence due to ripple migration. Extensive experimental studies should be carried out to further investigate segregation in mixed bioclasticsiliciclastic sediments.

The southern coast of Argentina is composed of many coarse-grained coastal barriers, which are mostly concentrated at the mouth of a few large rivers that drains the Andean mountains. As the Patagonian estuaries are characterized by tidal ranges that are among the largest in the world, coastal barriers are exposed to high amplitude tide-induced sea-level changes as well as strong tidal currents. In order to better understand the influence of tides in wave-built sedimentary bodies, an in-depth analysis of the architecture of barrier systems has been realized at the mouth of the Santa Cruz estuary (50°S). Maximum tidal range is 12 m in this estuary. A great variety of morphologies compose both sides of the estuary inlet, from simple spits and barrier spits to beach ridges. Barrier spit and beach ridges characterize the southern side, whereas elongated simple spits and spit barriers isolating large tidal flats are more developed on the northern side. As a consequence of this configuration the northern system is much wider than the southern one (6 km vs 1 km). The site was investigated using ground-penetrating radar (400 MHz GSSI antenna), along with the analysis of digital elevation models and some sedimentological observations. Cross-shore and long-shore profiles, with a penetration depth up to 4 meters, show a large range of radar facies attributed to erosional surfaces, beach face progradation, along-drift elongation, hooks and spit terminus development, and washover deposits. The analysis of the radar architecture allows reconstructing the development of individual morphology, and helps to differentiate the influence of wave- and tide-related processes. The most open-sea barrier spit systems show a classic development controlled by the wave-related littoral drift entering into the estuary (onshore directed), and enhanced due to shore parallel flood currents into the inlet. In contrast, in the innermost part of the northern side, spits and barrier spits feature contrasting morphologies, structures and directions of elongation, either landward or seaward, reflecting a complex combination between wave-, tide-and fluvial dynamics on their construction. One of the spits imaged with the GPR shows an intense reflection that has been associated with the top of the tidal flat on which the spit migrates (Fig. 1). The GPR lines highlight the control of underlying substrate on spit morphologies A same morphology is thus characterized by different architectures; in Figure 1B the prograding sequence follows a transgressive sequence, while in Figure 1C the transgressive sequence cannot be observed in the architecture due to the highest elevation of the tidal flat. In the southern side of the estuary, the barriers show extensive progradational sequences with different slopes that seem to depend on the spit morphology, and therefore on the exposure to ocean swells.

International audience; La compréhension de la morphodynamique des barrières littorales, et de son archivage dans l’architecture des corps sédimentaires recèle encore de nombreuses incertitudes. Les flèches sédimentaires se prêtent particulièrement bien à cette analyse en raison d’un fort potentiel de préservation lié à leur élongation rapide sous l’action de la dérive littorale. Si de nombreux travaux se sont penchés sur la question des flèches sableuses, il n’en est pas de même pour leurs équivalents graveleux. Cette étude se focalise sur la caractérisation de l’architecture sédimentaire d’une flèche littorale mixte sablo-graveleuse, en relation avec son évolution morphologique pluri-décennale. Pour se faire, une prospection au géoradar (GPR GSSI 400 MHZ) a été menée sur la flèche de La Mollière, qui s’intègre dans la grande barrière de la baie de Somme. L’histoire de ce corps sédimentaire unique en France métropolitaine débute dans les années 1940. La flèche s’étend sur environ 5 km et est constituée d’une multitude de cordons et de crochons. Les résultats permettent de proposer un modèle synthétique de dépôt et d’architecture stratigraphique spécifique aux flèches de galets avec trois unités "morpho-sédimentaires" : L'Unité 1, située à la racine de la flèche. Elle se caractérise par des structures principalement progradantes : L'Unité 2, dans la partie intermédiaire, constituée de cordons bien distincts comprenant un noyau au sommet duquel se surimpose une structure aggradante associée à une dynamique dominante cross-shore : L'Unité 3, à l’extrémité de la flèche, où l’élongation des cordons est fortement impactée par la dynamique des bancs sableux et le développement de la plateforme sableuse. Une prospection géoradar plus étendue permettrait de mieux comprendre comment ces flèches littorales à développement rapide (quelques décennies à siècles) s’intègrent dans la dynamique de construction de la grande barrière de la baie de Somme à une échelle plurimillénaire.

International audience; Barrier spits and beach ridges are different types of coastal barriers controlled predominantly by wave dynamics. They represent the two end members of a continuum of wave-built features reflecting varying orientations of net sediment transport (longshore vs. cross-shore). In hypertidal environments, high amplitude tide-induced sea-level changes as well as strong tidal currents necessarily combine with wave action in the construction of spits and beach ridges. In this study, we image the internal architecture of two coarse-grained coastal barriers, both located in hypertidal environments: the mixed sand-and-gravel spit at the mouth of the hypertidal Somme estuary in the English Channel (Northern France), and the sand-and-gravel barrier systems at the inlet of the hypertidal Santa Cruz estuary in Southern Patagonia (Southern Argentina). Maximum tidal ranges are 9 and 11 m in the French and Argentinian sites respectively. A great variety of morphologies compose the two systems, from simple spits and barrier spits to beach ridges. Both sites were investigated using ground-penetrating radar (400 MHz GSSI antenna), along with the analysis of aerial photographs and digital elevation models. Cross-shore and long-shore profiles, with a penetration depth between 2 and 4 meters, show a large range of radar facies attributed to erosional surfaces, beachface progradation, along-drift elongation, hooks and spit terminus development, and washover deposits. The in-depth analysis of radar architecture allows reconstructing the development of individual morphology in the two systems, and helps to differentiate the influence of wave- and tide-related processes. The most open-sea barrier-spit systems (Somme estuary and Santa Cruz external mouth) show a classic development controlled by the wave-related littoral drift, probably enhanced due to shore parallel flood currents. In contrast, the most internal systems, located along the northern edge of the Santa Cruz inner inlet, feature contrasting morphologies, structures and directions of elongation, either landward or seaward, reflecting a complex balance between wave- and tide-dominated dynamics on their construction.

International audience; Understanding the hydrodynamic behaviour of bioclastic particles is necessary to refine our interpretation of depositional environments in the fossil record and to improve predictive numerical models for coastal zone management. Many coastal sediments are partly composed of biogenic particles, which shapes and densities differ strongly from classic rounded quartz grains. This results in particular hydrodynamic behaviours. Characteristics of biogenic particles can also vary significantly between species. If numerous studies have investigated the hydrodynamic behaviour of bioclastic sediments derived from reef-dwelling organisms, there is a paucity of research focusing on “cool-water carbonate” bioclastic particles (i.e. mollusc shell debris, calcareous algae,...). The aim of the present research is to characterize the influence of faunal composition on the settling velocities and the entrainment threshold of mollusc shell debris from temperate regions. Shells have been sampled in the southern coast of Mont-Saint-Michel bay (Brittany, France) which is bordered by a coarse, shelly coastal barrier, before being ground and separated into individual sieve fractions. Eight species representative of the faunal composition in the area have been studied: four wild species (Cerastoderma edule, Scrobicularia plana, Anomia ephippium, Ostrea edulis) three reared species (Crassostrea gigas, Mutilussp., Ruditapes philippinarum) and one introduced (Crepidula fornicata). Settling velocities of the eight species have been measured in a settling tube for several debris sizes. A set of experiments have been performed in a small recirculating flume. Threshold of motion of the species under unidirectional current for different grain sizes have been characterized using an Acoustic Doppler Velocimeter Profiler. Critical bed shear stress values (tau_cr) were derived from velocity profiles in the boundary layer, by a logarithmic regression of the “law of the wall”.Results show consistent interspecific differences in settling velocity and critical bed shear stress, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Mollusc shells, according to their composition (calcite and/or aragonite), have either compact, foliated or a more complex structure. Debris derived from foliated shells (Anomia for example) moves earlier and settles slower than debris derived from compact shells (Scrobicularia and Crepidula for example). Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. Debris from species with dense compact shells behave like siliciclastic particles while particles from foliated (or chalky) shells show lower critical shear stress values. The results of the Movability number suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris.

International audience; Coastal areas of the north-eastAtlantic region are hit by storms mostly during the wintertime. Storms have an impact on coastal communities and economic activities. In western Cotentin, in Normandy, France, settlements are at threat due to heavy dune erosion; however, the history of coastal storm is quite well documented but due to coastal retreat and a prompt construction period for the last 5 to 7 decades along the coastline, sedimentary archives of these events have not been preserved. Trees located along the coast, either isolated or as stands, do represent a potential archive. The present study aims at testing the tree-ring reliability potential to document storminess in areas that are lacking any document, over short and long time periods, as tree-ring patterns are deeply affectedby the mechanical impact of low-frequency-high-intensity winds.

International audience; Wave-dominated coastal barriers are dynamic systems responding to sea-level fluctuations at different time scales induced by storm surges, tides (from diurnal to pluri-annual cycles), and climate (century to millenia). They are composed of siliciclastic sand, bioclastic debris or a mixture of both. Sediment-starved epicontinental platforms of temperate areas are frequently the location of intense biologically-controlled carbonate production. Consequently, coastal sediments of these systems are mixed, composed of siliciclastic sand (mostly quartz grains) and shell debris, essentially derived from mollusc shells. In some environments of very high biological productivity, such as the English Channel and more specifically the Normandy-Brittany Gulf, shell debris can largely dominate the overall composition of coastal sediments. With time, due to natural factors (climate changes, sea-level fluctuations) and human activities (shellfish farming, fishing, species introduction, marine aggregates extraction) the proportion between siliciclastic and bioclastic grains can vary. It is critical to understand how these modifications in sediment composition can affect sedimentary processes involved in the construction of coastal barriers and potentially their stability. This is the general objective of the present study mainly based on physical modelling.After two experimental studies performed in a settling tube and in a flume under unidirectional current, which show the complex hydrodynamic behaviour of different mollusc shell species (Rieux et al., 2019), a set of three wave-tank experiments were performed under varying mean water level and regular wave forcing, in order to reproduce barrier morphologies using bioclastic (B) / siliciclastic (S) sediment mixture in different proportions. Bioclastic debris were sampled in the Mont-Saint-Michel bay (NW France) whereas siliciclastic particles consist in a mix of different marine sands. Three sediment compositions have been created: 90% B – 10% S; 50% B – 50% S; 25% B –75% S. Barrier morphology was surveyed using a laser telemeter and Structure-from-Motion photogrammetry. The internal architecture was studied using side-photographs and lacquer-peels. The results show a great variability of migration rate, morphology and internal structure of barriers depending on the sediment composition. These variations can be explained by the modification of the permeability and by sorting processes occurring in the breaker and swash zones where the settling velocity of sediment particles plays a leading role. Barriers rich in bioclastic debris absorb the energy of the waves, which involve an aggradation of bioclastic particles on the upper beach, a steeper beach face, infrequent ridge overwash and subsequently a shorter internal face. On the contrary, a sediment composition dominated by siliciclastic grains, involve a less permeable sediment, a less steep beach face, many ridge owerwash and a longer internal face.

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International audience; Tidal bores provide a significant influence on sediment erosion and deposition in estuarine systems. Evolution of suspended sediment concentration close to the channel bed and in the water column during the passage of several tidal bores is proposed thanks to extensive field data acquisitions and laboratory calibrations. An Acoustic Doppler Velocimeter (ADV) was used to measure longitudinal, vertical flow velocities and turbulence features. Suspended Sediment Concentration (SSC) evolution was monitored using an Argus Surface Meter (144 OBS sensors) and direct water sampling at different elevations above the channel bed. The ASM has been calibrated in laboratory using a recirculating water column with natural sediment. ASM did provide at least accurate informations of relative evolution of the SSC in the water column, with a temporal and spatial resolution which is difficult to achieve with usual systems. A water sampling system was deployed in association with the ASM, and provided absolute references of the SSC to the relative measurements of the optical system. The sampling tubes were positioned at 0, 20, 40 and 60 cm above the channel bed. Water was sampled simultaneously at the four elevations every 2 seconds. The sampling started a few seconds before the passage of the tidal bore, and lasted for at least 40 minutes. Finally 800 samples per tidal bore were collected and processed at the laboratory. To characterize the influence of various parameters on the ADV measurements, a laboratory protocol was also defined. We mainly considered suspended sediment concentration and turbulence intensities. The tests were conducted in a tank of 500 l. The turbulence was created by a rotating steel grid driven by a linear motor with a dimmer in order to adjust the turbulence rate. Ten levels of turbulence and 8 levels of sediment concentration were tested. In the field, the highest sediment concentration was recorded at the channel bed a few seconds just after the passage of the bore front. This important sediment resuspension is due to a highly sheared flow during the bore passage, resulting in a 10 cm thick fluid mud layer with concentration values up to 50 g/L, travelling on the channel bed as the bore is propagating. Few minutes after the bore passage, an upward advection of suspended sediment was observed, that resulted in the homogenization of the water column concentration up to 15g/L. The development of a tidal bore results in very high sediment fluxes initiated by the bore front and impact on the bottom, of the order of 10 to 40 times higher than sediment fluxes without tidal bore.

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International audience; Understanding barrier spit formation and evolution is increasingly important in order to assess how these soft-sediment coastal systems will respond and adapt to expected future environmental changes. Barrier spit development is the result of complex multiscale interactions between wave and tide dynamics, fluctuations in sea level, storm impact and sediment supply along with geological and morphological inheritance. The stratigraphy and internal architecture of barrier systems preserve information about the dominating processes and controls that governed past coastal evolution. In coastal areas with relative high levels of tidal energy and low levels of wave energy, tide-dominated deposits preferentially make up the sedimentary record and wave-dominated deposits are rarely preserved. Hence, these systems are often considered to hold little or no information of past changes in wind and storm climate. In this study we investigate a Holocene recurved barrier spit situated along the west coast of the Cotentin Peninsula in the English Channel, France. This coast is subject to a tide-dominated energy regime with a tidal range exceeding 14 m and a mean significant wave height of less than 0.5 m. Integrated analysis of core and ground-penetrating radar data suggest that the barrier spit has a complex sedimentary architecture consisting of tide and wave dominated sedimentary bodies. We present a new sedimentological model for recurved barrier spit development under hypertidal conditions. The model describes how the spit extents downdrift by clinoformal progradation resulting from littoral drift caused by longshore sediment transport and due to swash bar welding along the distal end of the barrier. Lateral and vertical growth of the spit terminus results from sediment convergence due to landward migration of swash bars and seaward migration of tidal dunes. Radiocarbon and optically stimulated luminescence datings reveal that spit evolution and progradation probably occurred in two stages from about 900 to 1200 years ago and from 300 to 400 years ago. We suggest that increased windy conditions during the Little Ice age and resulting increased littoral drift may be responsible for the latter period of spit progradation.

International audience; Coastal barriers represent around 15% of the world’s oceanic shorelines, occurring in energetic, wave-dominated environments, and play an important role as early warning indicators of environmental changes. They are typically composed of coarse-grained siliciclastic particles, but in many cases biogenic carbonates also represent a significant proportion of the bulk sediment. Shelly chenier ridges are lag-concentrations of mollusc skeletal remains separated by prograding intertidal mudflats. They have been identified along several non-tropical coasts, such as U.K., or western Korea, among others. This study focuses on shelly cheniers of the hypertidal Mont Saint Michel bay, which is located within the cool temperate shallow carbonate Normandy Brittany Gulf (western English Channel). Previous works have focused on the hydrodynamic behaviour of biogenic particles with flume tank experiments, and on the internal architecture of cheniers imaged by sub-surface radar. The aim of this study is to focus on 1) meso-scale spatial variability in the distribution of mollusc skeletal remains across the 5-km wide intertidal mudflat, and 2) identify the processes involved in the spatial distribution of cheniers along the coast. A bathymetric survey of the intertidal flat was carried out in 1957 with a mono-beam echo sounder. More than 4500 soundings have been digitised and interpolated to reconstruct a spatially-explicit bathymetry of the study area at that time, which was compared with a LiDAR survey carried out in 2002. The 1957-2002 sediment budget is weak but globally positive (+13 cm). Difference bathymetry map shows a cross-shore pattern of alternating ca. 1 km wide corridors of erosion (up to50 cm) and accretion (up to +50 cm). Erosion corridors coincide with areas of chenier development on the coast, as evidenced by sets of aerial photographs taken between 1947 and 2014. Molluscs have been sampled in the first 5 cm of surface sediment at 25 stations scattered along 5 cross-shore transects corresponding to contrasting values of sediment budget. Live and dead individuals (> 1mm) have been sorted, identified, counted and weighed separately. Bivalve molluscs account for more than 95% of the biogenic carbonate particles found on the intertidal mudflat. Short-lived intertidal species clearly dominate both the live and death assemblage, including Cerastoderma edule, Macoma balthica and Scrobicularia plana. Diversity of the live assemblage is very low (6 species), resulting in a poor live-dead agreement related to incorporation in the death assemblage of subtidal species transported out of habitat (Abra alba), and to time-averaged accumulation of rare taxa, in accordance with previous works. High amounts of fragmented shell hash in surface sediments (1 to 2 kg.m-2) were found in erosion corridors. These results suggest that wave and current propagation across the intertidal flat is focused along corridors, favouring the fragmentation of shells and their coastward transportation, and leading to preferential areas of chenier accumulation. High-resolution numerical hydrodynamic modelling is now being tested to identify the factors controlling corridor pattern of propagation.

International audience; Coastal barriers have been intensively studied since they constitute natural protections against wave attack and marine submersion. In a context of climate change and sea-level rise acceleration, understanding their future development is critically. Here we present the main results of a research project that aimed in reconstructing and comparing the evolution of two sandy barriers (sandspits). The originality of the study was to examine sandspits located along macrotidal coasts since most barriers are considered in their context of wave-built sedimentary bodies. In order to examine when and how these sandspits developed, a stratigraphic reconstruction based on ground penetrating radar (GPR) investigations were performed, completed with vibrocore acquisitions for sediment facies analyses, GPR ground-truthing, and chronology. Historical land and sea charts were used to refine the time frame of evolution. To be able to infer the role of tides vs. wave climate in sandspit evolution, two sandspits subject to significant difference in tidal ranges were studied. The Saint-Germain spit (NW France, English Channel coast, tidal range up to 14 m); The Arcay spit (SW France, Atlantic Ocean coast, tidal range up to 7 m). Both sites are influenced by relatively similar low to moderate wave dynamics (Hs~0.5 m). The comparison of historical charts demonstrates that the formation of the two spits occurred between 1650 and 1700 AD, within the period of the Little Ice Age (LIA). Our study suggests that LIA climate conditions (enhanced storm and wind activities) were favourable for spit formation. Despite the convergence in time of formation, the depositional history of the two spits differs significantly. It appears that the Arcay spit experienced a regular elongation that is still ongoing today. On the contrary, the evolution of Saint-Germain spit was periodic and very fast. Already about 1750 AD it reached its current extension and morphology, suggesting that in half a century or maybe less, the entire spit formed. The GPR and core data reflect these contrasted evolutions of the spits. Data from Arçay demonstrate that the spit complex is mainly made of three wave-dominated units (spit platform, beach, washover) and evidence a dominant shore-parallel mode of construction. 14C ages for Arcay spit are consistent with an onset of construction during the 1600 s. Data from the Saint-Germain spit display two main units that are respectively tide-dominated (estuary, embayment) and mixed tide, wave and wind-influenced (sand spit body). 14C and OSL ages constrain the onset of sandspit formation to around 1000 years ago, and the most recent sand construction, around 400 years ago. The Saint-Germain spit complex results both from a vertical and progradational / longshore accretion. Differences in sediment supply are assumed to explain these two contrasted construction modes (quick progradational vs progressive longshore mode). This difference can be partly related to tidal ranges. Sand fluxes, both wave and wind-induced, are much higher on hypertidal sites, with large intertidal foreshore/flats, than those on macrotidal beaches. This difference was probably enhanced during the severe wind conditions of the LIA.

International audience; Understanding bottom sediment erodibility is necessary to refine our interpretation of depositional environments in the fossil record and to improve predictive numerical models for coastal zone management. Many coastal sediments are partly composed of biogenic particles, which shapes and densities differ strongly from classic rounded quartz grains. This results in particular hydrodynamic behaviours. Characteristics of biogenic particles can also vary significantly between species. If numerous studies have investigated the hydrodynamic behaviour of bioclastic sediments derived from reef-dwelling organisms, there is a paucity of research focusing on “cool-water carbonate” bioclastic particles (i.e. mollusc shell debris, calcareous algae,...). The aim of the present research is to characterize the influence of faunal composition on the entrainment threshold of mollusc shell debris from temperate regions. Shells have been sampled in the southern coast of Mont-Saint-Michel bay (Brittany, France) which is bordered by a coarse, shelly coastal barrier, before being ground and separated into individual sieve fractions. Eight species representative of the faunal composition in the area have been studied: four wild species (Cerastoderma edule, Scrobicularia plana, Anomia ephippium, Ostrea edulis) three reared species (Crassostrea gigas, Mutilus sp., Ruditapes philippinarum) and one introduced (Crepidula fornicata). A set of experiments have been performed in a small recirculating flume. Threshold of motion of the eight species under unidirectional current for several debris sizes have been characterized using an Acoustic Doppler Velocimeter Profiler. Critical bed shear stress values (tau_cr) were derived from velocity profiles in the boundary layer, by a logarithmic regression of the “ law of the wall”. Depending on the species, the evolution of taucr with increasing grain diameters follows either an asymptotic or a more linear trend. Significant disparity of entrainment threshold is observed between species, which increases with particle sizes. Three groups can be discerned: Anomia ephippium, Crassostrea gigas and Ostrea edulis show the smallest tau_cr, Crepidula fornicata, Scrobicularia plana, Mutilus sp. for an intermediate tau_cr and Cerastoderma edule; Ruditapes philippinarum for the highest taucr. For example, for the sieve fraction 2-3.15 mm, Anomia ephippium is the first to move with a tau_cr of 0.38 N.m-2, and Cerastoderma edule shows the highest critical shear stress (tau_cr = 2.13 N.m-2). Oyster shells (Ostrea and Crassostrea sp.) are mainly composed of foliated sheets of calcite, sometimes interstratified with soft, porous and chalky material. These two structures present very different thresholds of motion (e.g. for 2-3.15 mm sieve fraction: 0.54 N.m-2 for the chalky material, and 1.08 N.m-2 for the foliated calcite sheets). Globally, the interspecies variations of hydrodynamic behaviour can be attributed to differences in shell density, shell structure, grain shape, or to a combination of these three parameters.

International audience; Tidal deposits have been a specific research topic for about 40 years, and whilst this has resulted in a proliferation of papers in scientific journals, there have only been a few book-length syntheses. Over the years, tidal sedimentology has been reinforced by fluid mechanics and numerical modelling but has remained rooted in facies and stratigraphic studies. Recent developments in tidal sedimentology lean toward a more quantitative assessment of the imprint of tides in the facies record of intertidal and shallow subtidal areas. They highlight the increasing relevance of tidal deposits studies, from high resolution subsurface reservoir geology to climate change and sea-level rise. This volume gathers 17 contributions to the Tidalites 2012 congress held in Caen, France. It reflects current advances in the sedimentology and stratigraphy of tidal deposits, in both ancient and modern environments. It shows the current diversity of this field of research, through a wide spectrum of methods including remote sensing, in-situ hydrodynamical measurements, and ichnology, in addition to classic field studies and petrography

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(IF 0.61; Q2); International audience; A precise chronology of the sea level rise occurring since the Last Glacial Maximum is difficult to establish. Geological evidences are generally located on the coastal fringe, a privileged interface for ocean/continent interactions. Two complementary approaches, the archaeological one and the sedimentological one, have been used in order to describe the different steps of sea level rise in the Bay of Quiberon (south of Brittany, France). Numerous megalithic sites have been discovered on the eastern coast of Bay of Quiberon and in the Morbihan Gulf. These man-made rows of standing stones, erected 6,500 years ago, extend below the present sea level. In order to detect the extension of these subtidal megaliths, lateral and interferometric sonar and seismic surveys have been conducted and provided a high resolution bathymetry together with sonar imageries of this area. Thus, Neolithic constructions such as megaliths were identified below the current sea level. Their extension limit corresponds to the Neolithic palaeo-shoreline situated around -5.5 m below the NGF zero. The sedimentological approach, combining seismic profiles with a long core, gives details on the frequency of high energetic events like storms. The end of the Carnacean phenomenon may be related to the time interval characterised by stormy climatic events combined to sea level rise increase, around 4,300 yr BC.

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During the last few years, many studies dealing with the Holocene evolution of tide-dominated estuaries and embayments, pointed out the major role of rapid climate changes on the morphodynamics behaviour of such coastal systems (Chaumillon et al., 2010). Examples include the Holocene sedimentary infill of incised valleys along the Atlantic and English Channel coasts of France (Billeaud et al., 2009; Sorrel et al., 2009, Sorrel et al., 2010; Tessier et al., 2011).

Forcing parameters of chenier ridges formation and internal structure are investigated using field data and wave flume experiments. This work focuses on modern, coarse bioclastic beach ridges such as those located on the uppermost part of the tidal flat in Mt. St. Michel Bay (NW France), in the context of a prograding megatidal chenier plain. These ridges migrate landward over the upper tidal flat and salt marshes by washover processes during coincidence of high spring tide and enhanced wave activity, until they are stabilized and integrated in the chenier plain. The internal architecture of these ridges has been investigated on the field using high-frequency ground-penetrating radar (GPR). Three types of ridges were identified, that represent a continuum of evolution between active transgressive, mature transgressive (Fig. 1), and mature progradational ridges (Fig. 2). Each type reflects major differences in external morphology and internal structure. The altitude of the banks regarding to the level of tidal flooding, as well as local sediment supply, are assumed to be important forcing parameters in chenier development and evolution.

The Mont Saint Michel estuary is a megatidal setting (tidal range up to 14 m). It is characterized by a strong tidal asymmetry during spring tides, with the flood stage much shorter and quicker than the ebb, reaching commonly a velocity of 2m/s into the estuarine channels. In estuaries with tidal ranges greater than 6 m, the difference of elevation between the rising tide and the river creates a discontinuity of velocity and pressure, called tidal bore (or "mascaret" in French). Visually, a tidal bore can be described as a wave or series of waves propagating upstream. This study takes place into a national project "ANR Mascaret". Part of the field work we performed recently on the tidal bores that propagate into the Mt St Michel estuary, aims in studying the impact of the bore fluid dynamics on sediment transport. This is an important issue for a better understanding of the complex fluid-sediment interactions and for the operation of restoration of the Mont-Saint-Michel's maritime character

Date du colloque: 07/2012; International audience

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