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This study describes the building of a common erosion law for fine sand and mud, mixed or not, in the case of a typical continental shelf environment, the Bay of Biscay shelf, characterized by slightly energetic conditions and a seabed mainly composed of fine sand and muddy sediments. A 3D realistic hydro-sedimentary model was used to assess the influence of the erosion law setting on sediment dynamics (turbidity, seabed evolution). A pure sand erosion law was applied when the mud fraction in the surficial sediment was lower than a first critical value, and a pure mud erosion law above a second critical value. Both sand and mud erosion laws are formulated similarly, with different parameters (erodibility parameter, critical shear stress and power of the excess shear stress). Several transition trends (linear or exponential) describing variations in these erosion-related parameters between the two critical mud fractions were tested. Suspended sediment concentrations obtained from simulations were compared to measurements taken on the Bay of Biscay shelf with an acoustic profiler over the entire water column. On the one hand, results show that defining an abrupt exponential transition improves model results regarding measurements. On the other hand, they underline the need to define a first critical mud fraction of 10 to 20%, corresponding to a critical clay content of 3–6%, below which pure sand erosion should be prescribed. Both conclusions agree with results of experimental studies reported in the literature mentioning a drastic change in erosion mode above a critical clay content of 2–10% in the mixture. Results also provide evidence for the importance of considering advection in this kind of validation with in situ observations, which is likely to considerably influence both water column and seabed sediment dynamics.

The rising interest in environmental and ecosystem dynamics have lead coastal oceanographers to not only investigate the “traditional” physical parameters describing the ocean state and its dynamics (e.g. temperature, salinity, currents, water levels in coastal areas), but to also account for the dynamics of parameters describing its biogeochemical components. To that end, MARS3D regional and coastal modelling system has been coupled to ecosystem modules (ECO-MARS3D, ECO3M) as well as sediment dynamics modules (MARS3D-SEDIM): sediment, nutrient and primary production contents can be considered as the lower level environment and ecosystem descriptors of the “biogeochemical” ocean. Early investments into physical and biological analysis at the regional scale have led to the development of several operational configurations within PREVIMER since 2006 for physical and biological parameters, providing 3 to 5-day forecasts as well as hindcasts. The more recent introduction of sediment-related parameters into the operational chain required validating computed sediment transport at the regional scale. Such validation is mostly accessible through indirect measurements – namely turbidity measurements in the water column or derived from satellite data. This paper describes the main features of MARS3D sediment module, the sensitivity analyses and the validation procedures based on dedicated data acquisition, as well as the assessment of the operational configuration focused on the Bay of Biscay continental shelf. Comparison between in situ measurements and satellite data shows a fairly systematic overestimation of the satellite-derived SPM in Southern Brittany; this result stresses the need for further investigation regarding the correct quantitative satellite SPM determination at all times and all places. On the other hand, numerical results highlight the difficulty to simultaneously predict the correct magnitude of bottom and surface concentrations.

This paper investigates the main characteristics of the fine sediments dynamics in a very complex and extensive inner shelf area located in South America, the Río de la Plata. The results derive from a 5 yr international research program that allowed intensive data acquisition throughout the area, as well as the development of the first numerical model investigating sediment dynamics at a regional scale in the area. The model is based on the MARS3D suite: it computes the regional tide- and wind-induced circulation, as well as sediment mixtures dynamics (erosion, advection and deposition of several sedimentary variables). Erosion due to wave action is taken into account thanks to the use of SWAN. The hydrodynamic and wave models were calibrated against several measurements in different zones of the Río de la Plata. Turbidity data (time series and vertical profiles) were then used in order to calibrate the sediment dynamics model. The model is shown to accurately reproduce the sediment concentration variability during both fair and stormy weather conditions in the intermediate and in the outer Río de la Plata. The paper also shows that the suspended sediment dynamics in the Río de la Plata is mainly driven by water/bottom exchanges.

Les fortes vitesses rencontrées à l’entrée de la Baie du Mont Saint-Michel font de cet environnement macro-tidal un endroit privilégié, en termes de ressources énergétiques, pour l’implantation d’une ferme d’hydroliennes. Afin d’établir plus précisément son potentiel énergétique et d’évaluer l’impact de l’implantation de telles machines sur son hydrodynamisme, une modélisation hydrodynamique de la Baie du Mont Saint-Michel est mise en oeuvre à l’aide du modèle de circulation régionale MARS3D (LAZURE & DUMAS, 2008) dans sa version tridimensionnelle. La validation hydrodynamique de cette configuration est réalisée à partir de comparaisons à des mesures courantologiques récentes (KERVELLA, 2010). A partir de cette modélisation, une cartographie du potentiel énergétique de la Baie est établie et les endroits propices à l’installation de structures récupératrices de l’énergie des courants sont identifiés. L’impact de l’implantation d’une ou plusieurs structures à l’un de ces endroits sur l’hydrodynamique de la Baie est évalué : l’effet de l’extraction d’énergie des courants est modélisé par l’implémentation d’un nouveau terme puits, correspondant à une force de retard colinéaire à l’écoulement (DEFNE et al., 2011), dans les équations du mouvement de MARS3D. Les résultats présentés pour 2 valeurs d’extraction de l’énergie disponible (25% et 55%) montrent que l’impact d’un parc de petite superficie sur les courants reste très local.

Mixed sediments are constituted of cohesive and non-cohesive materials with distinct behaviours that numerical models traditionally manage separately. This paper first introduces a rapid state of the Art in sediment transport modelling in order to point out the specific requirements for process-based models applied to mixed sediments. Based on a preliminary study by Waeles et al. (2007), which showed the validity of the advection approach to compute fine sand transport, a complete modelling strategy is described: it is applied to the suspended transport of sand and mud mixtures, and accounts for consolidation of mixed sediments. Special care is paid to the realistic representation of the structure and density of sand and mud mixtures, and to the segregation in consolidating sediment layers. The model state variables are the different classes of particles, generally classified according to their size, and grouped into categories that are either transported as bedload or in suspension. The bed is described as thin layers characterised by a distribution of these classes. The erosion law for fine sands and for sand and mud mixtures is a function of the excess shear stress calibrated against measurements in a small flume. The transition between cohesive and non-cohesive behaviours is parameterised through a critical mud fraction that depends on the sand grain size: the coarser the sand, the higher the mud content before the sediment becomes cohesive. The consolidation module is based on Gibson equation formulated for each class, and modified to account for segregation. Constitutive relationships are calibrated by means of laboratory settling tests. In the deposition module, new deposits may be managed in different ways (creation of a new layer or integration into the existing surficial layer) depending on the mud fraction and its relative concentration. When deposited material is mixed with the surficial sediment, pores between coarser particles are first filled up with

Sur de nombreux sites ostréicoles, d’importants mouvements sédimentaires modifient les conditions d’exploitation. Les mouvements naturels sont modifiés par la présence des tables à huîtres, les phénomènes en jeu étant de deux ordres : la perturbation des courants de marée et la modification des conditions de houle. Nous présentons dans ce document la méthodologie mise en place pour améliorer la compréhension des perturbations physiques du milieu engendrées par l’implantation de tables ostréicoles et caractériser les modifications de l’écoulement. Les résultats obtenus, tant expérimentaux que numériques, sont présentés et les développements pour une extension du modèle à l’échelle d’un parc ostréicole sont discutés. Des essais expérimentaux réalisés à l’échelle 1/2 ont notamment permis de caractériser l’impact en champ proche sur l’écoulement. Nous avons mis en évidence des zones préférentielles de dépôts de sédiments dues à l’interaction entre la couche limite se créant sur le dessous de la table et la couche limite de fond. Les contraintes de cisaillement mesurées près du fond ne sont quant à elles que très peu modifiées par la présence de la structure. Seule la contrainte de cisaillement sur l’ensemble de la colonne d’eau est amplifiée au passage des tables. Avec l’objectif de généraliser ces travaux, un modèle numérique a été développé à partir du code de CFD Fluent, les tables étant modélisées à l’aide de milieux poreux. La comparaison des résultats numériques et expérimentaux montre une bonne concordance : de 3 à 10% d’erreur moyenne sur l’ensemble du domaine d’étude suivant la configuration étudiée. Les choix effectués pour représenter la structure permettent d’envisager l’extension du modèle à des échelles plus importantes (plusieurs centaines de mètres) pour une structure seule ou en interaction avec d’autres placées en espace proche.

Oyster tables are artificial obstacles which are laid over muddy or sandy-muddy floors in intertidal areas. This study aims to show the current modifications related to the presence of the oyster table thanks to LDV measurement in a flume tank. Boundary layer development in different geometric configuration is underlined and suggests some important areas of velocity decrease and sediment transport modification. Shear stresses are also calculated near the bottom and around the table. The total shear stress in the water column exhibits an important increase when the flow passes through the oyster table, hence a flow energy decrease through dissipation. This study helps to understand the reasons for the increased sedimentation observed in some natural sites.

In order to define and quantify the rhythm and amplitude of the processes of erosion and sedimentation on broad tidal flats, in situ measurements of sediment elevation have been performed in the western part of the Mont-Saint-Michel bay. The amplitude of reworking is generally low (< ± 5 cm), and variable from west to east. The wind influence on tidal currents and surficial reworking emphasizes erosion phases rapidly followed by accretion of similar amplitude on the whole tidal flat. A numerical model for hydrodynamics and sediment transport is applied to Mont Saint Michel Bay, where ecological concerns are related to sediment transfers., Les rythmes et l'ampleur des processus « érosion-sédimentation » ont été suivis, à trois échelles de temps, sur le vaste estran sablo-vaseux de la partie occidentale de la baie du Mont-Saint-Michel. Les mesures in situ du niveau de l'interface eau-sédiment montrent que l'amplitude générale des remaniements est généralement inférieure à 5 cm et variable d'ouest en est. L'influence du vent sur les remaniements superficiels souligne des phases d'érosion rapidement suivies par des phases de sédimentation de même amplitude sur l'ensemble de l'estran. Un modèle numérique hydro-sédimentaire, validé par des mesures sur l'estran, permet de simuler les courants et les transferts de sédiments en zone intertidale.

A numerical model for hydrodynamics and sediment transport is applied to Mont Saint Michel Bay, where ecological concerns are related to sediment transfers. Recent current datasets recorded on the tidal flat show the wind influence on the currents during neap tide. They are compared to model outputs. The simulation of sediment transfers driven by tides and waves accurately mimics the sediment repartition in the Bay., Un modèle numérique hydro-sédimentaire de la Baie du Mont Saint-Michel a été développé pour l'étude des transferts de sédiments en relation avec les préoccupations écologiques du site. De récentes mesures de courant sur l'estran montrent l'influence du vent sur les courants de morte eau, et sont comparées aux résultats hydrodynamiques. La modélisation du transport des sédiments fins sous l'action de la marée et des vagues reproduit correctement leur répartition dans la Baie.

This article presents a methodology developed to evaluate the instability of submarine slopes that extend over a large area. Special attention was paid to (1) the complex geometry (bathymetry) and the expanse of the slope, (2) the heterogeneity of the sediment, and (3) the distribution of the pore pressure. The safety factor was considered as a spatially varying quantity. The General Formulation (GLE, Fredlund and Krahn 1977), which fully satisfies equilibrium conditions, was used for evaluating the stability of the marine slope. The submarine slope failure, which occurred on 16 October 1979 during the construction of the new Nice airport, was studied in order to test the developed model. Geotechnical parameters were taken from experimental tests carried out by IFREMER on 19 cores extracted at different depths (from 27 m to 1300 m) (Cochonat, Bourillet and Savoye, 1993; Mulder et al., 1994). Many scenarios were proposed in order to explain the cause of the Nice slope failure (Habib, 1994). In this article, two of those scenarios were tested. Simulation results are presented and discussed.