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The Bay of Biscay (Bob) is a hot spot for the generation of internal tides and nonlinear internal waves (NLIW). However, no studies have focused on internal waves on the continental shelf of the Bob. Here, we present 22 days of collocated temperature, velocity and backscatter profiles within a water depth H of 65 m. The background stratification evolved from two pycnoclines, with the strongest one near the sea bed, to a continuous profile due to wind-driven upwelling.Under the double pycnocline situation, we observed trains of elevation emerging from each internal tidal front with amplitude reaching up to H/4 and propagating at speeds between 0.1 and 0.35 m/s. Sporadically depression waves were measured within the train and can propagate substantially faster (between 0.36 and 0.54 m/s). With the continuous stratification, the trains of NLIWs of elevation and containing opposite polarities were no longer observed.These observations suggest that depression waves can cross the train of elevation waves. Resulting interactions could have significant impacts on sediment dynamics over the shelf. The double pycnocline regime and the impact of the stratification modification due to wind will be investigated numerically in future work.

A new special collection invites studies on a new era of models and knowledge that provide predictions or insights into predictability in coastal geomorphology.

The Bay of Biscay (Bob) is a hotspot of internal tide and non-linear internal wave generation. Yet, the internal waveson the inner shelf have never been observed nor described. An oceanographic campaign has been conducted overthe Landes shelf (Southern BoB) during the summer of 2017 to better characterize internal wave dynamics. Usingvelocity current profiles, thermistor chains and MVP (Moving Vessel Profiler) measurements in 65m water depth, wehere describe a part of the first field observations of non-linear internal waves (NLIWs) on the southern shelf of theBay of Biscay. Frequent NLIWs, characterized by isotherm excursions up to 20m and vertical currents of 0.2m/s, wereobserved. NLIWs of depression propagate close to the surface and polarity reversal was evidence for some of thoseNLIWs. No polarity reversals were observed when background stratification was maximum close to the seafloor.Trains of NLIWs of elevation, associated with a maximum density gradient close to the bottom, were observed at thebeginning of the campaign. The latter observation seems to be linked to a deep cold water advection. Velocity measurements show two flow regions within the wave frame of reference. The backscatter signal is used to proxysuspended materials and showed different responses depending on the flow region. In conclusion, we observechanges in the NLIWs response with the background stratification. Clear elevation and depression NLIWs exist whenthere was strong near-bed stratification. Twenty days later, maximum stratification was in the middle of the watercolumn and packets of mixed polarity waves were observed. Increased backscatter signal within the core of someNLIWs suggests that they could potentially impact the dynamics of near-bed material. However, the processesinducing increased backscatter have not been identified, and dedicated measurements will be conducted in the futureto better understand the underlying mechanisms.

This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.

Sediment fluxes at the estuary-sea interface strongly impact particle matter exchanges between marine and continental sources along the land-sea continuum. However, human activities drive pressures on estuary physical functioning, hence threatening estuarine habitats and their ecosystem services. There is an increasing societal need to better predict the potential trajectories of estuarine sediment fluxes resulting from natural and anthropogenic pressures, but the concomitance of human-induced and meteorological-induced changes makes the responses ambiguous. Therefore, this study explores a 22-year numerical hindcast, experiencing contrasted meteorological conditions and human-induced morphological changes (i.e., estuary deepening and narrowing), in order to disentangle the relative contributions of meteorological and anthropogenic changes on net sediment fluxes between a macrotidal estuary and its adjacent coastal sea. Our results highlight that intense wave events induce fine sediment (≤100 µm) export to the sea but coarser sediment (≥210 µm) import within the estuary. Remarkably, moderate to large river flows support mud import within the estuary. Over 25 years, the reduction of intense wave and river flow events reduces fine sediment export to the sea. In addition, the estuary morphological changes due to human activities increase fine sediment import within the estuary, shifting the estuary from an exporting to importing system. We propose a conceptualization of mud flux response to river flow and wave forcing, as well as anthropogenic pressures. It provides valuable insights into particle transfers along the land-sea continuum, contributing to a better understanding of estuarine ecosystem trajectories under global changes.

Estuaries are subject to extensive morphological changes through human activities, such as deepening and narrowing via dredging and channelization. The estuary sediment load, characterised by the estuarine turbidity maximum (ETM), can severely increase in response to channel deepening, shifting the estuary from a natural to hyperturbid state. The main processes driving the estuarine circulation, hydrology and sediment dynamics are relatively well known. However, their relative influence on suspended sediment concentration (SSC), as well as their role in the transition toward hyperturbid estuaries, is still a subject of debate. Therefore, this study aims at investigating the relative contribution of key estuarine drivers (gravitational circulation and tidal pumping mechanisms) on the ETM dynamics under significant estuary morphological changes. In a 3D numerical model of the Seine Estuary (France), three bathymetries from the last 50 years (1960, 1975 and 2010) were implemented, characterising the gradual estuary deepening and narrowing. The morphological changes induce an expected tidal amplification in the main channel, associated with a decrease of the tidal duration asymmetry. The salinity front slightly migrates up-estuary and the stratification strengthens, resulting in a higher and upper-estuary SSC distribution. Starting from a dominantly natural system driven by the tidal pumping mechanism in 1960, the contribution of the gravitational circulation strongly increased in the present-day anthropogenically-controlled system. Contrastingly, the maximum ETM mass barely changes with the morphological changes and dredging activities have hardly any influence. Hence, the maximum ETM mass in the Seine Estuary would principally result from the tidal forcing rather than the morphology.

Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants transported by sediments from continental areas to the sea is crucial for marine resources protection. However, the complexity of estuarine dynamics makes it difficult to quantify sediment fluxes from field measurements alone and requires numerical modeling. Thus, using a realistic 3D hydrodynamic and sediment transport model, this study aims at evaluating the influence of model empirical parameters on sediment fluxes and estimating uncertainties on mud and sand transfers at a macrotidal estuary mouth. A sensitivity analysis, considering changes in sediment transport parameters, revealed that the system is not only sensitive to settling and erosion parameterizations, but also to the spin‐up period and the sediment sliding process. Both estuarine circulation and tidal pumping induce a residual up‐estuary transport, which is balanced by seaward export during spring tides. Although more fine sediments are exported within the surface turbid plume during high river discharge, the net mud transport is directed up‐estuary due to increased baroclinic circulation. Besides, model results highlighted a strong seasonal variability in sediment fluxes with a short and high import during high river flow and a long and weak export during low river flow. Uncertainties associated with the simulated fluxes were about 93% for mud and 51% for non‐cohesive classes, based on the best performing parameter sets for surface suspended sediment concentrations. These results can be reliably extrapolated to similar macrotidal estuarine systems. Plain language summary Estuaries are transitional zones between terrestrial and marine environments (freshwater vs saltwater). Because of their potential to transport nutrients and contaminants, quantifying the amount of sediment particles (mud and sand) exchanged at this interfacial area is essential for marine resources protection. Here, we use a realistic numerical model of sediment transport applied to an estuary and its adjacent continental shelf. Some parameters in the model are not well‐known and require calibration. This study aims at evaluating the impact of various model calibrations on simulated sediment exchanges at the estuary mouth. We found that the simulated sediment behavior is very sensitive to the selected model parameters. The quantity and direction of simulated exchanges are influenced by the parameterization of sediment erosion and settling (i.e. the rate at which particles are suspended and settle out). The dominant physical processes driving these exchanges are strongly influenced by river flow and tide amplitude. Sediment transfers are very intense and directed upstream during a short period in winter and compensated by weak export seaward during a long period in summer. Besides, uncertainties associated with simulated sediment exchanges are about 93% for mud and 51% for sands, which can reliably be applied to similar estuarine systems.

In the Seine estuary, maintenance dredging is operated continuously and yearly dredged volumes have the same order of magnitude as natural sediment fluxes at the mouth of the estuary. Prior to 2017, the main dumping site was in the vicinity of the estuarine turbidity maximum (ETM), therefore impacting sediment dynamics in the area. In this context, the aim of our research is to understand and model the contributions of maintenance dredging to the ETM characteristics and to sediment fluxes, and to compare natural fluxes to the anthropogenic ones related to these activities. Our approach relies on a 3D multi-class hydro-sedimentary model validated by (Grasso et al. in J Geophys Res: Oceans 12(3):558–577, 2018), in which dredging and dumping were implemented as processes following rules defined by port authorities of Rouen and Le Havre. Model results are first evaluated regarding the temporal and spatial variability of dredging in the Seine Estuary, and secondly regarding the stability of dumping sites. In order to follow the fate of dredged materials, the latter are numerically marked by assigning them to specific state variables of the model that respect the particle characteristics (gravel, sands or mud). The corresponding concentrations of these new variables can be computed either in the water column (suspensions) or in the layered sediment. Their contributions to the total suspended sediment mass of the estuary and to the siltation rate are evaluated. Results show that most of the dredged material remains in the proximity of the dumping site but the rest contributes to the turbidity and to the siltation pattern in the estuary. The similitude between the global siltation pattern and the distribution of re-deposited dredged material reveals a strong mixing of the latter within the whole estuary mouth. After 3 years of marking, the contribution of dredged sediment to the ETM can be estimated around 1%.

Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants brought by sediment particles from the continental regions to the sea is of major interest for marine resource protection. Estuaries form a transition zone between river and maritime environments and therefore represent dynamic areas in terms of hydrodynamics, hydrology and sediment transport. The hydrodynamics is strongly dominated by asymmetrical tidal currents, modulated by neap and spring phasing. Moreover, the interaction between fresh and saltwater induces salinity gradients, and therefore density gradients, influencing the sediment dynamics. The complexity and the spatial variability of this interface area make it difficult to quantify the actual sediment transfers between the estuary and the ocean, either by in situ measurements, remote sensing or numerical simulation. This study is based on a 3D realistic hydrodynamic model of the Gironde estuary (South–West France) and the continental shelf seaward coupled with a multi-layer process-based sediment model accounting for sand and mud mixtures. In order to better understand the sediment dynamics between the estuary and the continental shelf, the objective is to provide a quantification of suspended sediment mass uncertainties associated with sediment transport parameters. The sensitivity analysis is conducted considering changes in the key sediment transport parameters, such as the settling velocity of the mud class and the sediment initial distribution. A better understanding of the uncertainties associated with simulated sediment transport will enhance the degree of confidence that can be placed in the quantification of the estuarine sediment exchanges with the sea. Such a quantification provides knowledge about the intra-estuarine exchanges with lateral intertidal mudflats, as well as sediment export to the sea, possible trapping in the subtidal muddy areas seaward of the mouth (e.g. the West Gironde Mud Patch) and dispersion along the adjacent coasts.

Understanding the sediment dynamics in an estuary is important for its morphodynamic and ecological assessment as well as, in case of an anthropogenically controlled system, for its maintenance. However, the quantification of sediment fluxes and budgets is extremely difficult from in-situ data and requires thoroughly validated numerical models. In the study presented here, sediment fluxes and budgets in the lower Seine Estuary were quantified and investigated from seasonal to annual time scales with respect to realistic hydro- and meteorological conditions. A realistic three-dimensional process-based hydro- and sediment-dynamic model was used to quantify mud and sand fluxes through characteristic estuarine cross-sections. In addition to a reference experiment with typical forcing, three experiments were carried out and analysed, each differing from the reference experiment in either river discharge or wind and waves so that the effects of these forcings could be separated. Hydro- and meteorological conditions affect the sediment fluxes and budgets in different ways and at different locations. Single storm events induce strong erosion in the lower estuary and can have a significant effect on the sediment fluxes offshore of the Seine Estuary mouth, with the flux direction depending on the wind direction. Spring tides cause significant up-estuary fluxes at the mouth. A high river discharge drives barotropic down-estuary fluxes at the upper cross-sections, but baroclinic up-estuary fluxes at the mouth and offshore so that the lower estuary gains sediment during wet years. This behaviour is likely to be observed worldwide in estuaries affected by density gradients and turbidity maximum dynamics.

Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants brought by sediments from continental areas to the sea is of major interest for marine resource protection. The complexity of the intra-estuarine dynamics, associated with the strong variability of meteorological forcing makes it difficult to quantify the residence time of particles within the estuary and the accumulation and dispersion areas offshore the mouth. Moreover, the dynamics of fine sediment trapping areas in the adjacent continental shelf (e.g. temporary or permanent storage such as the “West-Gironde Mud Patch” (WGMP) ) remains extremely challenging to address. Based on a realistic process-based numerical model, the aim of this work is to investigate the dynamics of the sediment accumulation and dispersion areas in order: (i) to describe the role played by the intertidal mudflats and the subtidal mud patches on sediment trapping and further resuspension, and (ii) to investigate the influence of hydro-meteorological events (e.g. high/low river discharges, storms) on sediment fluxes.

An equilibrium dune-erosion model is used every six years to assess the capability of the most seaward dune row on the Dutch Wadden islands to withstand a storm with a 1 in 10,000 probability for a given year. The present-day model is the culmination of numerous laboratory experiments with an initial cross-shore profile based on the central Netherlands coast. Large parts of the dune coast of the Wadden islands have substantially different dune and cross-shore profile characteristics than found along this central coast, related to the presence of tidal channels, ebb-tidal deltas, beach-plains and strong coastal curvature. This complicated coastal setting implies that the predictions of the dune-erosion model are sometimes doubtful; accordingly, a shift towards a process-based dune-erosion model has been proposed. A number of research findings based on recent laboratory and field studies highlight only few of the many challenges that need to be faced in order to develop and test such a model. Observations of turbulence beneath breaking waves indicate the need to include breaking-wave effects in sand transport equations, while current knowledge of infragravity waves, one of the main sand transporting mechanisms during severe storm conditions, is strongly challenged by laboratory and field observations on gently sloping beaches that are so typical of the Wadden islands. We argue that in-situ and remote-sensing field observations, laboratory experiments and numerical models need to be the pillars of Earth Scientific research in the Wadden Sea area to construct a meaningful process-based dune-erosion tool.

International audience; Intertidal mudflats play a critical role in estuarine exchange, connecting marine and continental supplies of nutrients and sediments. However, their complex morphodynamics, associated with a wide range of physical and biological processes, are still poorly understood and require further field investigation. In addition, mudflats are challenging areas for Structure-from-Motion (SfM) photogrammetric surveys. Indeed, the mudflats generally hold back residual tidal water, which can make stereo restitution particularly difficult because of poor correlations or sun-glint effects. This study aims to show the potential of light UAVs (Unmanned Aerial Vehicles) for monitoring sedimentary hydrodynamics at different spatial scales in a silty estuary. For each UAV mission an orthophotograph and a Digital Elevation Model (DEM) are computed. From repeated surveys the diachronic evolution of the area can be observed via DEM differencing. Considering the ground texture in such a context, the stereo restitution process is made possible because of the high spatial resolution of the UAV photographs. Providing a synoptic view as well as high spatial resolution (less than 4 cm), the UAV dataset enables multi-scale approaches from the study of large areas to the morphodynamics of smaller-scale sedimentary structures and the morphodynamics impact of plant ground cover.

On steep beaches, the cross-shore movement of sand in response to ‘erosive’ storm waves and ‘accretive’ swell waves can lead to temporal changes between a barred winter profile and a non-barred summer profile with a pronounced berm in the upper swash zone. Despite recent improvements in predicting berm formation and evolution within process-based morphodynamic models, substantial demand for improvement in understanding swash processes and associated surf–swash sand exchange remains. Here, we analyze bed level data collected on a near-prototype, 4.5-m high and 75-m wide sandy beach (median grain diameter D50 = 430 μm) with a lagoon situated at its landward side. In particular, we distinguish between surf–swash sand exchange (time scale of tens of minutes to hours), the net effect of single and multiple swash events on the entire beach face (time scale of a few seconds to hours), and instantaneous bed variability at 3 cross-shore locations within individual swashes. During ‘erosive’ waves (Hs = 0.8 m, Tp = 8 s) sand on the initially 1:15 planar profile was predominantly eroded from the inner surf zone to be deposited in the outer surf zone as a sandbar, indicating minimal surf–swash sand exchange. Subsequent ‘accretive’ waves (Hs = 0.6 m, Tp = 12 s) caused substantially larger surf–swash sand exchange: the pre-existing sandbar migrated onshore and decayed, with the sand ending up on the beach face in a prominent (up to 0.7 m high), steep (1:6) berm. We found the dynamics of the berm to be governed primarily by wave conditions and the antecedent morphology, with ground water gradients of additional importance when morphodynamic feedback between swash flow and the berm is small. The observed bed level change within a swash and averaged over a swash event could be substantial (several centimeters) during all wave conditions, but the net (i.e., averaged over multiple swash events) bed level change was strongly suppressed because erosive and accretionary swashes nearly balanced. In addition, the local beach face slope could be instantaneously ≈ 25 % steeper or shallower than the median slope, or the initial or final slope. We anticipate that our data will stimulate new model development, as to increase the range of conditions and settings in which morphodynamic models can be applied realistically and reliably.

Wave breaking is the primary driver of beach erosion, injecting breaking-induced turbulence at the sea surface and diffusing bed boundary layer turbulence at the sea bed. The limited understanding of the vertical turbulence structure under natural breaking waves, and hence sand entrainment, is one of the reasons that coastal-evolution models produce inadequate estimates of storm response. Here we use a recently collected field dataset to analyze turbulence dissipation under breaking waves and bores on the intertidal beach at Truc Vert, France. The vertical structure of the turbulent dissipation rate indicates that wave breaking is the dominant source of turbulence dissipation. The current-induced turbulence represents no more than 50% of the turbulent dissipation rate close to the bed (at 10% of the water column), even when alongshore currents reach 1 m/s. The data further illustrate that the turbulent dissipation rate is almost depth-uniform under breaking waves, whereas it decreases profoundly toward the bed under bores. Moreover, we found that the fraction of wave energy flux decay dissipated below wave-trough level is about 1% under breaking waves and about 10% under bores. These results imply that the turbulent dissipation rate in the surf zone is severely underestimated by coastal-evolution models that do not consider breaking-induced turbulence as a surface boundary condition. Consequently, they will underestimate sand stirring and transport by mean currents during severe storms.

Ce papier presente le developpement d'un modele simple d'evolution de profil de plage sableuse base sur une approche a phase moyennee et sur un couplage vagues/transport sedimentaire/evolution bathymetrique. Une nouvelle parametrisation du transport sedimentaire tenant compte de l'asymetrie des vagues et de l'acceleration pres du fond est proposee et etudiee ici. Le modele reproduit la migration des barres vers la plage pendant les episodes de temps calme et vers le large pour des conditions energetiques. La prise en compte de l'asymetrie d'acceleration ameliore sensiblement les performances du modele. La comparaison du modele avec les experiences realisees en canal et avec l'evolution d'une plage australienne sur une periode de 2 mois montre des resultats encourageants.

Sediment transport modelling in estuarine environments, characterised by cohesive and non-cohesive sediment mixtures, has to consider a time variation of erodibility due to consolidation. Generally, validated by settling column experiments, mud consolidation is now fairly well simulated; however, numerical models still have difficulty to simulate accurately the sedimentation and consolidation of mixed sediments for a wide range of initial conditions. This is partly due to the difficulty to formulate the contribution of sand in the hindered settling regime when segregation does not clearly occur. Based on extensive settling experiments with mud-sand mixtures, the objective of this study was to improve the numerical modelling of mixed-sediment consolidation by focusing on segregation processes. We used constitutive relationships following the fractal theory associated with a new segregation formulation based on the relative mud concentration. Using specific sets of parameters calibrated for each test—with different initial sediment concentration and sand content—the model achieved excellent prediction skills for simulating sediment height evolutions and concentration vertical profiles. It highlighted the model capacity to simulate properly the segregation occurrence for mud-sand mixtures characterised by a wide range of initial conditions. Nevertheless, calibration parameters varied significantly, as the fractal number ranged from 2.64 to 2.77. This study investigated the relevance of using a common set of parameters, which is generally required for 3D sediment transport modelling. Simulations were less accurate but remained satisfactory in an operational approach. Finally, a specific formulation for natural estuarine environments was proposed, simulating correctly the sedimentation-consolidation processes of mud-sand mixtures through 3D sediment transport modelling.

Estuarine sediment transport models require that a time variation of erodibility due to consolidation be taken into account. If numerical modelling of mud sedimentation and consolidation is currently satisfactory, simulation of mud-sand mixture consolidation is more problematical. This is partly due to mixed-sediment processes, as hindered settling and segregation, not well understood yet. Hence, based on extensive settling columns experiments, the aim of this study is to improve our understanding of mud-sand mixture consolidation for varying concentrations (54 to 600 kg/m 3 ) and large sand contents (15 to 80%). Firstly, we observed that for a given initial mass concentration and sand content the final consolidation appeared independent from the initial sediment sample height. For a given initial sand content, the settling phase started earlier for lowconcentrated mixtures and the final consolidation rates ranged between 40 and 90% for high- and low-concentrated mixtures, respectively. Intriguingly, consolidation tests with the lowest initial mass concentrations led to the largest final mass concentrations. A vertical analysis revealed that for high-concentrated sediments the pore water pressure did not fully dissipate, limiting the entire sediment consolidation. In addition, we observed larger final consolidation rates for larger initial sand contents, leading to larger final mass concentrations. Sand segregation was not always observed in our experiments. Interestingly, the initial relative mud concentration appeared as a potential segregation indicator and of interest to estimate the initial gelling concentration. Finally, these experiments represent a rich dataset to validate numerical modelling of mixedsediment settling and consolidation.

In 2012, large-scale laboratory experiments were carried out in the Deltagoot in the framework of the Hydralab IV-funded BARDEXII project. The overall project aims were to examine the effect of swash/groundwater interactions to sand transport and morphological development in the swash zone and, additionally, to investigate the sediment exchange between the swash and surf zone. Work package 5 in the BARDEXII project focused on this second aim and, therefore, paid particular attention to the effect of surface-generated turbulence on sand suspension and on the direction and magnitude of the subsequent sand transport. We know from earlier, predominantly small-scale laboratory experiments that breaking-induced surface-generated eddies can penetrate through the water column to hit the sea bed and suspend sand. The inherently intermittent nature of this turbulence and hence suspension is strikingly at odd with sand transport equations that assume sand transport to be related to the orbital flow to some power. Although attempts to include breaking-induced turbulence into sand-transport equations have been performed to improve the modeling of swash-surf sand exchange, our quantitative understanding of the effects of breaking-induced turbulence on sand suspension is limited because of a lack of simultaneous and accurate measurements of turbulence and sand suspension under breaking waves. This paper will describe the experimental design for this work package and present preliminary results on (1) the bathymetric evolution during the series of tests and (2) the vertical turbulence structure beneath shoaling and breaking waves.

[1] Waves breaking in the shallow surf zone near the shoreline inject turbulence into the water column that may reach the bed to suspend sediment. Breaking-wave turbulence in the surf zone is, however, poorly understood, which is one of the reasons why many process-based coastal-evolution models predict coastal change during severe storms inadequately. Here, we use data collected in two natural surf zones to derive a new parameterization for the stability functionCμ that determines the magnitude of the eddy viscosity νtin two-equation turbulent-viscosity models,νt = Cμk2/e, where k is turbulent kinetic energy and e is the turbulence dissipation rate. In both data sets, the ratio of turbulence production to dissipation is small (≈0.15), while vertical turbulence diffusion is significant. This differs from assumptions underlying existing Cμ parameterizations, which we show to severely overpredict observed Cμ for most conditions. Additionally, we rewrite our new Cμparameterization into a formulation that accurately reproduces our Reynolds-stress based estimates of turbulence production. This formulation is linear with strain, consistent with earlier theoritical work for large strain rates. Also, it does not depend oneand can, therefore, also be applied in one-equation turbulent-viscosity models. We anticipate our work to improve turbulence modeling in natural surf zones and to eventually lead to more reliable predictions of coastal evolution in response to severe storms.

[1] The waveshape effects on sediment transport are investigated for cross-shore beach profile changes. This study is based on experiments performed in the Laboratoire des Ecoulements Geophysiques et Industriels wave flume for irregular waves. The interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load, ripple, and sheet flow regimes. Net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry, undertow, and ripple occurrence. Onshore bar migration is mainly associated with onshore-directed sediment transport, whereas terrace profile and offshore bar formation correspond to offshore sediment transport. As for natural beaches, energetic (moderate) wave climates mostly induce offshore (onshore) sediment fluxes. For a given significant wave height, an increase (decrease) in the wave climate peak period is associated with an increase (decrease) in wave skewness and leads mostly to offshore (onshore) sediment fluxes. The experiments are fully characterized by unsteady behavior. Consequently, several conditions exhibit phase-lag effects where the sediment is mobilized by the wave crest and transported by the following trough, which produces a net offshore transport even for a weak undertow. The presence of ripples clearly contributes to enhance this behavior. An original concept, due to its application to skewed asymmetric irregular waves, presents the important interaction between wave nonlinearities driving the sediment fluxes. The net sediment transport rate under strongly skewed waves is either offshore directed due to phase-lag effects or onshore directed when the wave asymmetry is large enough. Both these mechanisms probably largely contribute to bar formation and migration.

International audience; Laboratory experiments are conducted in a wave facility with a loose sediment beach in order to estimate the wave characteristics needed for sediment transport modeling. A roller model and a Boussinesqtype model are used for comparing and discussing the experimental results. It is shown that the velocity skewness and acceleration skewness evolutions along the beach profile can be estimated from measurements of surface elevations. The roller model correctly reproduces the set-up and undertow measurements as long as the cross-shore variations of the second-order moment of the free surface displacement are taken into account. The Boussinesq model correctly reproduces the wave non-linearities with some discrepancies in the breaking region over the bar topography.

This study focuses on barred beach shoreface nourishments physically simulated in a wave flume. The attack of a schematic storm on three different nourishments is analysed. The apex and waning storm phases lead respectively to offshore and onshore sediment transports. Nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore. The bar acts as a wave filter and reduces shore erosion (lee effect). In contrast, nourishment on the beach face leads mostly to shore feeding and reconstruction (feeder effect). With successive nourishments, the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates. The surface grain size analysis reveals marked variations. Coarser sediments are sorted on the bar and the upper beach face. These locations correspond to large wave dissipation zones during the storm apex.

This study is based on experimental simulations carried out in the LEGI flume for irregular waves. Sediment transport associated to cross-shore beach profile changes has been analysed using wave height, velocity and acceleration skewness, undertow current, and Shields number. For energetic waves conditions and a wave climate with a Dean number =2.5, the mean sediment transport rate is mainly offshore directed. In contrast, moderate wave conditions and a wave climate with =3.6 are associated to an onshore sediment transport. The wave asymmetries are shown to play a determinant role on the sediment transport direction and intensity.

Laboratory experiments on cross-shore beach morphodynamics are presented. A lightweight sediment (density rho(s) = 1.19 g cm(-3)) model is used in order to fulfill a Shields number and Rouse number scaling. This choice aims at correctly reproducing bed load transport as well as suspension dynamics. Terraces and barred beach profiles obtained in the experiments also present close similarities with profiles observed in the field. In order to question the concept of equilibrium beach profile, wave forcings conforming to a JONSWAP spectrum were imposed over long periods (up to more than a hundred hours). An average bottom evolution velocity is defined and used to determine when the profile reaches equilibrium. Usually, beach profiles are characterized according to the Wright and Short (1984) classification based on the Dean number W. This well-known classification is investigated and refined in the intermediate range, that is, for 1

Des techniques d'optimisation de formes sont utilisées dans des problèmes du Génie Côtier pour la protection des plages. En collaboration avec le bureau d'étude Bas-Rhône Languedoc ingénierie, un projet d'aménagement concret de la plage du Lido de Sète, utilisant des ouvrages immergés en géotextile, a été étudié. Des solutions optimales, visant à réduire la mise en suspension des sédiments, pour les structures de défenses ont été obtenues à partir d'un modèle hydrodynamique de propagation de houle. Une étude complémentaire à partir d'un modèle de circulation et des travaux expérimentaux en canal à houle permettent ensuite de valider les résultats optimaux numériques obtenus. Shape optimization techniques, focusing on coastal structures design, are used to solve coastal engineering problem. In collaboration with the Bas-Rhône Languedoc engineering Company, a actual development plan concerning the beach of the Lido of Sète has been studied. Optimal solutions, aimed at reducing sediments suspended, for defense structures have been obtained with a hydrodynamic water wave model. An additional study using a circulation model and experimental works in flume tend to validate optimal numerical results.

This paper presents the development of a simple coupled, wave-averaged, crossshore waves-currents-bathymetry evolution model. A new parameterization involving wave skewness and acceleration is used to compute sediment transport. The model reproduces onshore bar migration during low wave energy condition and offshore migration for higher energy waves. Comparison with simulations undertaken in a wave flume shows encouraging results. Mots-cles : Barres sableuses – migration - modele a phase moyennee – asymetrie des vagues – transport sedimentaire

The main objectives of this study are on one end to test with a physical model the concept of equilibrium beach profile for a wave climate. Our experiments were conducted in the sedimentary LEGI wave-flume. We were able to identify eight wave climates leading to an equilibrium. Equilibrium profiles have been sorted according to their Dean number. On the other end, the evolution of profiles correctly reproduces the different scenarios observed in nature, such as erosion and sediment accretion at the shore and bar migrations.

One way to recover beaches consists in beach nourishment. Sometimes the grain size of the borrow sand is however too thin to be in equilibrium on the beach. A solution can be to make shoreface nourishment, where sand is generally thinner.

Colloque avec actes et comité de lecture. Internationale.; International audience; Un modèle de type Boussinesq incluant une paramétrisation du déferlement est utilisée pour reproduire les propriétés non-linéaires de la houle en zone de surf ainsi que les ondes basses fréquences associées. Les performances du modèle sont comparées à des experiences en canal de laboratoire. Le modèle est appliqué à la propagation d'ondes irrégulières sur une bathymétrie qui correspond à un fond sédimentaire en équilibre. La génération des ondes infragravitaires est analysée.

La dynamique sedimentaire a l’embouchure de l’estuaire de Seine est principalement controlee par les forcages hydrometeorologiques (debit, vent, vagues, maree) et les fortes pressions anthropiques (endiguement, dragage, clapage). La comprehension et la quantification de la dynamique des flux sedimentaires en suspension (sable/vase) est un enjeu majeur, autant d’un point vu ecologique (ex. turbidite, envasement) que financier (ex. gestion des chenaux de navigation). La mise en place de reseaux de mesure in situ permet une meilleure comprehension de la dynamique sedimentaire de l’estuaire, mais leur caractere local rend difficile toute estimation des flux et bilans sedimentaires. Pour repondre a ces questions, cette etude s’appuie sur la modelisation numerique, validee par de nombreuses series de mesures in situ, afin de quantifier les flux et bilans sedimentaires a differentes echelles caracteristiques de temps et d'espace. Les concentrations simulees dans la zone d'extension du bouchon vaseux sont du meme ordre que les observations et leur dynamique associee aux differentes phases et cycles de la maree, ainsi qu'au cycle hydrologique, est satisfaisante. L’analyse des flux residuels et bilans sedimentaires aux echelles annuelle et trimestrielle a permis d’identifier un schema de circulation a l’embouchure de l’estuaire et de quantifier l’importance du forcage hydrologique (crue/etiage) sur les echanges sedimentaires. Enfin, ce travail se poursuivra par l’etude de ces flux et bilans pour differents scenarios hydrometeorologiques contrastes.

Dans le cadre du programme de recherche Seine-Aval 5, différentes stratégies de modélisation numérique sont mises en place pour aider à répondre aux questionnements scientifiques et sociétaux associés au fonctionnement physique, biologique, chimique et écologique de l'estuaire de la Seine (ex. projets RESET et HYMOSED). Les forçages météo-marins (vent, vague, marée) et hydrologique (débit) contrôlent le fonctionnement hydro-morpho-sédimentaire et biogéochimique de l'estuaire, modifiant ainsi les paramètres environnementaux de l’estuaire amont jusqu’à la baie de Seine (ex. nature des fonds, courant, turbidité, salinité, température, oxygène dissous, chlorophylle-a). La modélisation de ces paramètres, ainsi que leur évolution face à différents scénarios climatiques et/ou anthropiques, représentent une source de données essentielle à l’étude écosystémique de l’estuaire et de ses trajectoires futures. Au sein du programme Seine-Aval 5, l’outil de modélisation nourrit des projets portant sur le devenir des nutriments conditionnés par différentes stratégies de régulation hydro-agricole et leur impact sur la qualité de l’eau en estuaire, sur l’aide à la décision pour des projets de restauration écologique, ou encore sur l’étude des interactions physique/biologie à l’interface eau-sédiment. Enfin, les résultats de ces travaux de modélisation ont également pour objectif de participer au développement du SIG « Habitats Fonctionnels » porté par le GIPSA.

En se basant sur les travaux des programmes de recherche Seine-Aval 4 & 5, et plus particulièrement le modèle numérique hydro-sédimentaire MARS3D « CurviSeine » (HYMOSED-SA5), le projet ARES (« Analyse de Rejeux hydro-sédimentaires en Estuaire de Seine ») du programme de recherche Seine-Aval 6 avait pour objectifs de produire un référentiel de données validées de la dynamique hydro-sédimentaire de l’estuaire pour deux périodes de 11 ans (1990-2000 et 2005-2015). Le présent document décrit : (i) le modèle ainsi que les paramètres de modélisation (forçages, grille bathymétrique, etc…) utilisés ; (ii) la comparaison des sorties hydrologiques (température, salinité, sédiments en suspension) du modèle avec les données du réseau PHRESQUES (bouée SCENES et stations aval SYNAPSES) ; (iii) une étude de sensibilité du modèle à la grille bathymétrique pour l’année 2014, simulée avec la grille bathymétrique de 2010 et celle de 2016, en période d’évolution morphologique douce de l’estuaire de Seine ; et (iv) des résultats préliminaires concernant l’historique des conditions hydrologiques de l’estuaire de Seine, aux échelles décennales et annuelles. L'ensemble des rejeux hydro-sédimentaires sont disponible sur le ftp : ftp://ftp.ifremer.fr//ifremer/dataref/ocean-coastal-model/SEINE/curviseine_ares_hindcast/