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L'intérêt porté aux barrières littorales est étroitement lié à la forte pression anthropique exercée sur les milieux côtiers. Néanmoins, la compréhension de la morphodynamique de ces objets, 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. Pourtant, ces barrières littorales grossières sont souvent le résultat d'un équilibre fragile, en raison d'une source de sédiment moins abondante. Cette étude se focalise sur la caractérisation de l'architecture sédimentaire d'une flèche littorale de galets, en relation avec son évolution morphologique pluri-décennale. Pour se faire, une prospection géophysique au géoradar (GPR GSSI à 400 MHz) a été menée sur la flèche de La Mollière, qui s'intègre dans la grande barrière de la rive sud de la Baie de Somme. Ce corps sédimentaire unique en France métropolitaine, et dont l'histoire débute dans les années 1950, s'est montré propice à une approche morpho-stratigraphique. La flèche s'étend sur environ 5 km et est constituée d'une multitude de cordons et de crochons. Elle est alimentée par une dérive littorale sud-nord permettant le transport des galets de silex sur une distance de presque 16 km depuis les falaises de craie crétacée. Les résultats permettent de proposer une ébauche de modèle synthétique de dépôt et d'architecture stratigraphique spécifique aux flèches de galets. L'interprétation des données radar rend possible la distinction entre des phases de construction cross-shore (géométries progradantes) et longshore (géométries aggradantes). La combinaison entre ces données et la morphologie générale de la flèche, permet d'identifier 3 unités "morpho-sédimentaires" d'amont en aval. L'Unité 1, située à la racine de la flèche, se caractérise par des structures principalement progradantes, assimilables à celles d'un beach ridge. L'Unité 2, dans la partie intermédiaire, est marquée principalement par le développement long-shore des cordons, au sommet desquels se surimpose une dynamique cross-shore. L'Unité 3, à l'extrémité de la flèche, est dominée par des structures aggradantes avec des faciès particulièrement sableux. L'analyse en 3 dimensions d'un point d'inflexion des crochons permet de proposer un premier schéma montrant l'architecture résultant de l'abandon d'un crochon au profit d'une nouvelle phase d'élongation. Une prospection géoradar plus étendue et dans des parties plus anciennes du littoral 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 flèche de la Baie de Somme à une échelle plurimillénaire.

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In risk analyses, two components are taken into account: (1) hazard analysis, including susceptibility and temporal occurrence, and (2) consequence analysis, including characterization of elements at risk (EaRs) and their vulnerability. This study focused on characterization of EaRs, in which items are spatially displayed and impacted by natural events. Several methods can assess these EaRs through expert or engineering approaches. Among the expert approaches, the multicriteria method is more flexible and allows integration of a wide range of information in order to characterize and discretize different EaRs. Traditionally, the mapping and criteria accuracy of EaRs is the same at all spatial analysis scales, while the hazard accuracy changes according to the spatial scales. Therefore, we propose an approach based on the selection of different information/criteria among several private or open access multiple geographical databases to adapt the mapping and criteria accuracy of each EaR according to the hazard analysis spatial scale. After harmonizing the different databases and merging them under GIS, a single database per work scale is created through a specific procedure with interoperability of results between scales. Thus, the number of criteria used to describe these EaRs will depend on the scale of work and the spatial scale of the analysis. To develop and test the transposability of this method, three experimental coastal study sites subject to several hazards (multirisk) have been selected in Normandy (France) with error estimations ranging between 10% and 20%. Subsequently, these data can be integrated into risk and multirisk analyses.

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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.

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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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Physics-based modeling of karst systems remains almost impossible without enough accurate information about the inner physical characteristics. Usually, the only available hydrodynamic information is the spring discharge at the karst outlet. Numerous works in the past decades have used and proven the usefulness of time-series analysis applied to spring discharge, precipitations or even physico-chemical parameters, for interpreting karst hydrological functioning. The main objective of this work is to provide additional insights of to what extent the informative content of the hydrodynamic signal at karst springs is sensitive to karst aquifers internal physical properties. To address this issue, we undertake an empirical approach based on the use of both distributed and physics-based models and on synthetic systems responses. The link between karst hydraulic and physical properties was studied. For this purpose, forward modeling of flow through several simple, constrained and synthetic cases in response to precipitations is undertaken. It allows us to quantify how the statistical characteristics of flow at the outlet are sensitive to changes (1) one hydraulic parameter of the model and (2) in conduit network geometry. The matrix/conduit exchange parameter appeared clearly as a determinant model parameter in the spring discharge simulation. The auto-and cross-correlation functions seem to be of particular interest for the understanding of the karst inner physics. Indeed, these functions are always different, despite not so pronounced configuration differences. This would highlight that there is an informative content within the spring discharge time series and the usefulness of such analysis methods.

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Since a few years, land use management aims to reduce and control water erosion processes in watersheds but there is a lack of quantitative information on the contribution of the sources of transported sediment. This is most important in agricultural areaswhere soils are sensitive to erosion. The geology of these areas is often characterized by large expanses of relatively homogeneous quaternary silts. The possibility of distinguishing the sources of erosion according to their geological substratum is thus very delicate. This information is important because its lack can lead to the mis-implementation of erosion control measures. To address this request, a confluence-based sediment fingerprinting approach was developed on the Canche river watershed (1274 km2; northern France), located in the European loess belt, an area that is affected by diffuse and concentrate erosion processes. Suspended particulate matter was collected during five seasonal sampling campaigns using sediment traps at the outlet of each tributary and confluencewith the main stream of the Canche river. The final composite fingerprintwas defined using physico-chemical and statistical analyses. The best tracer parameters for each tributary were selected using stepwise discriminant function analyses. These parameters were introduced into a mass balance mixing model incorporating Monte-Carlo simulations to represent the uncertainty. Estimates of the overall mean contributions fromeach tributary were quantified at different temporal scales. The annual sediment flux tributaries contributions range from 3 to 22% at the outlet of the Canche river, and annual sediment flux range from 0.87 to 40.7 kt yr−1. The Planquette and the Créquoise tributaries appear to be those producing the largest sediment flux. In contrast, tributarieswith the highest number of erosion control on their area exhibit the lowest values of sediment flux. Our results indicate a positive impact of recent land management policies in the Canche river watershed.

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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.

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