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GNSS/A is a proven method to localize seafloor geodetic stations and, thus, to monitor seafloor tectonic phenomena. This approach combines GNSS positioning of a surface platform with acoustic ranging between this platform and acoustic beacons on the sea bottom, with the objective to localize the beacons in a global frame within a centimeter or so. As part of the FOCUS project, a network of seafloor stations was deployed offshore Sicily in Italy, at about 1900 meters depth. It was surveyed from a small autonomous surface vehicle (ASV) equipped with two GNSS receivers. Here we present the post-processing applied to the GNSS surface navigation data in order to reduce the uncertainty budget of the GNSS/A algorithm. We found significant discrepancies between GNSS receivers in terms of signal degrading effects (multipaths, cycle slips, signal interruptions). We also found that GNSS antennas are less subject to these effects when mounted on an ASV rather than on the main vessel mast. The correlation found between solution accuracy and signal degrading effects potency turned out to be inadequate to predict the former knowing the latter. Then, we compared the accuracy of kinematic solutions from three GNSS PPP-AR post-processing packages to a few/several centimeters level. From our analysis, we find that the best accuracy was reached using the CSRS-PPP service.

Low-lying coasts and small islands, such as in the Lesser Antilles, are particularly vulnerable to hurricane-induced marine floods. In September 2017, Category 5 Hurricane Irma, with winds up to 360 km/h, hit the northern Caribbean islands and caused the destruction of 95 % of the structures on Barbuda Island. We investigated the geomorphological impacts and the sedimentological record of this hurricane in Barbuda’s Codrington Lagoon. Following Hurricane Irma, two wide inlets developed across the Codrington sandy barrier. One of these inlets enlarged and was still opened four years later. From available data, it seems that this barrier remained continuous for the last 250 years before Hurricane Irma. At a longer time scale, very high-resolution seismic exploration combined with sediment cores sampled in Codrington Lagoon were used to investigate Irma deposit and environmental changes for the last 3700 years. The evolution from a low-energy small and shallow lagoon to the modern wide and high-energy lagoon recorded by the lagoon sediment-fill was related to both long-term sea-level rise and subsidence. The top of the lagoon fill consists of a thick and extensive sand sheet recording an abrupt increase in energy. Given its location at the top of the cores, its very recent age supported by short-lived radionuclides data, together with large inlets opening and barrier erosion after Irma implying a large sand supply to the lagoon, this sand sheet was attributed to Hurricane Irma. From our cores, it appears that this deposit is unique over more than 3700 years. Both the opening of a new inlet and the thick upper sand sheet support the exceptional character of Irma at the scale of centuries to millennia. Our study reinforces the idea that Hurricane Irma was exceptional in terms of intensity and may be associated with global warming.

Siltation is a major concern in dynamic and complex ecosystems, such as ports. The mud must be regularly dredged to avoid disturbing the navigation channels. Sediments are carried by the waters entering the port and are partially trapped by harbour structures. Numerous studies have been conducted on the physical factors influencing siltation in port areas, whereas, few have focused on the role of biotic factors in mud formation. However, research in other contexts has shown that organisms that are abundant in pontoons, such as bivalves and tunicates, play an important role in this siltation process. All of these organisms belong to the filter feeder group. The sediments sucked in by the filter feeders are excreted in the form of faeces or mucus-bound pseudo-faeces. These waste materials, called bioproducts, settle efficiently and are involved in the composition of the mud. This study aimed to highlight the role of filter feeders in the siltation process in port areas and to determine the factors that influence the production of bioproducts by filter feeders. To investigate the role of filter feeders in the siltation processes, an experimental analysis was conducted in the largest marina in Europe (La Rochelle, France). It is divided into four basins with distinct filter feeder communities and environmental conditions, allowing for a detailed study of the environmental factors that influence the production of bioproducts. This analysis consisted of recovering and studying the bioproducts generated by the filter feeders using sediment traps fixed under pontoons. To explore the evolution of this biological production, 16 campaigns were conducted from January to March 2020 and May to July 2020. The total amount of dry matter produced was constant between seasons at approximately 130 g/m

In the framework of the European Research Council (ERC) funded project – FOCUS, testing laser reflectometry in a fiber optic cable to detect movement across an active submarine fault in real time, an array of eight acoustic beacons has been set up for monitoring motions across the fault and calibrating the observation from the fiber optic cable (see Gutscher et al. abstract in session SM2.1). The two experiments jointly started in October 2020. The selected North-Alfeo Fault is located at the foot of Mount Etna and shows evidence of right-lateral strike-slip motion.The geodetic array forms a triangular web of 28 baselines, 16 of which cross the fault and 4 of which are parallel to the sections of fiber-optic cable cutting the fault. Beacon depths range from 1910 to 1806m. Each baseline, 400 to 1800 m long, is measured 4 times a day in both directions. Additional sensors simultaneously monitor the temperature (at ±0.001˚C) and pressure (at ±0.01dbar), so that sound-speed can be derived, and acoustic ranging (at ±1 microsecond) converted into distances. Inclinometers monitor the stability of the beacons (at ±0.05˚), mounted on 3m-high tripods, which, so far, have remained stable on the seabed.The collected data (Jan. 2022) show transient and inhomogeneous environmental changes, due to cold bottom-water flows or mixing that last from days to weeks, and hence causing transient changes in the sound-speed and measured acoustic flight-times between beacons. Sound-speed (SSP) varies up to 0.1 m/s, inducing changes up to 25 microsecondes in one-way flight-times (equivalent to a 4-cm displacement at a constant SSP). Unfortunately, such an episode occurred when the optic fiber detected a significant elongation (20 - 40 microstrain) at two fault crossings, between 19 and 21 November 2020. Further processing is underway to extract possible actual displacements from the first 14 months of continuous acoustic ranging.

International audience; Precise underwater geodetic positioning remains a challenge. Measurements combining surface positioning (GNSS) with underwater acoustic positioning are generally performed from research vessels. Here we tested an alternative approach using a small Unmanned Surface Vehicle (USV) with a compact GNSS/Acoustic experimental set-up, easier to deploy, and more cost-effective. The positioning system included a GNSS receiver directly mounted above an Ultra Short Baseline (USBL) module integrated with an inertial system (INS) to correct for the USV motions. Different acquisition protocols, including box-in circles around transponders and two static positions of the USV, were tested. The experiment conducted in the shallow waters (40 m) of the Bay of Brest, France, provided a data set to derive the coordinates of individual transponders from two-way-travel times, and direction of arrival (DOA) of acoustic rays from the transponders to the USV. Using a least-squares inversion, we show that DOAs improve single transponder positioning both in box-in and static acquisitions. From a series of short positioning sessions (20 min) over 2 days, we achieved a repeatability of ~5 cm in the locations of the transponders. Post-processing of the GNSS data also significantly improved the two-way-travel times residuals compared to the real-time solution.

La Rochelle marina, in the south-west of France, is one of the largest recreational port in Europe. As many ports around the world, La Rochelle marina is not spared by siltation and has to face serious dredging operations over a period of 8 months per year. Consequently, there is a big interest in determining sediment fluxes in the area and implementing measures to minimise siltation and/or to optimise dredging operations. In the present study, a three-dimensional hydro-sedimentary model (TELEMAC-3D) was calibrated with field measurements from 2014 to 2019. Analysis of the in situ measurements (turbidity, bathymetric surveys, seabed elevation) showed that the macro-tidal influence and local wind-wave regime significantly contributed to the siltation in the marina. In order to minimise the cost and time associated with maintenance dredging, multiple scenarios have been tested numerically and subsequently analysed. Numerous sediment trap configurations were defined to focus the deposition of sediment in dredged traps and constructional measures were also assessed in terms of siltation reduction (e.g. entrance closure, narrowing entrances, Current-Deflecting-Wall). Numerical simulations showed that the best solution is to create a sediment trap at the downstream corner of the marina, along the main navigation channel, to intercept sediment particles arriving from the ocean. The constructional measures as well as the use of sediment traps inside the marina are not efficient in reducing siltation. These results highlight the importance of using operational numerical model to define a more sustainable strategy to fight against siltation in a more efficient and less costly way.

International audience; This study presents unpublished field observations of infragravity waves, collected at the dissipative beach of Saint‐Trojan (Oléron Island, France) during the storm Kurt (3 February 2017), characterized by incident short waves of significant heights reaching 9.5 m and peak periods reaching 22 s. Data analysis reveals the development of exceptionally large infragravity waves, with significant heights reaching 1.85 m close to shore. Field observations are complemented by numerical modeling with XBeach, which well reproduces the development of such infragravity waves. Model results reveal that infragravity waves were generated mainly through the bound wave mechanism, enhanced by the development of a phase lag with the shortwave energy envelope. Spectral analysis of the free surface elevation shows the generation of superharmonic and subharmonic infragravity waves, the latter dominating the free surface elevation variance close to shore. Modeling results suggest that subharmonic infragravity waves result, at least partly, from infragravity‐wave merging, promoted by the combination of free and bound infragravity waves propagating across a several kilometer‐wide surf zone. Due to the steeper slope of the upper part of the beach profile, observed and modeled reflection coefficients under moderate‐energy show a strong tidal modulation, with a weak reflection at low tide (R2

International audience; Harbor siltation is a problem that will exist as long as harbors exist and it is intrinsically linked to their primary function: providing 6 shelter for anchorage and operative conditions for loading and unloading ships. In these semienclosed basins, flow characteristics are one of 7 the main factors influencing siltation and water quality. One of the largest recreational ports of Europe, La Rochelle Marina (southwestern 8 France), is not spared by siltation, which requires serious dredging operations during a major part of the year. In this context, a three-9 dimensional model (5 TELEMAC-3D) has been used to investigate its hydrodynamics. Using a simplified approach, floating structures were 10 implemented in the model. Comparison with observations has demonstrated the need to consider these structures in our study. They sig-11 nificantly reduce velocity in the inner parts of the marina and concentrate current on access channels. Numerical results also highlight the joint 12 role of the macrotidal regime and wind stress in the movement of water masses and their residual circulation.

International audience; In this paper, we investigate the water renewal of a highly populated marina, located in the southwest of France, and subjected to a macro-tidal regime. With the use of a 3D-numerical model (TELEMAC-3D), three water transport timescales were studied and compared to provide a fully detailed description of the physical processes occurring in the marina. Integrated Flushing times (IFT) were computed through a Eulerian way while a Lagrangian method allowed to estimate Residence Times (RT) and Exposure Times (ET). From these timescales, the return-flow (the fraction of water that re-enters the marina at flood after leaving the domain at ebb) was quantified via the Return-flow Factor (RFF) and the Return Coefficient (RC) parameters. The intrinsic information contained in these parameters is thoroughly analysed, and their relevance is discussed. A wide range of weather-marine conditions was tested to provide the most exhaustive information about the processes occurring in the marina. The results highlight the significant influence of the tide and the wind as well as the smaller influence of the Floating Structures (FS) on the renewal. Besides, this study provides the first investigation of the water exchange processes of La Rochelle marina. It offers some content that interest researchers and environmental managers in the monitoring of pollutants as well as biological/ecological applications.

International audience; The Lesser Antilles are a densely populated region where local populations and industrial facilities are concentrated at the coastlines, and are therefore exposed to many rapid-onset hazards such as hurricanes and tsunamis. However, the historical catalog of these events is too short to allow risk assessment and return period estimations, and it needs to be completed with long-term records of washover deposits in coastal sedimentary environments such as lagoons. In this study, two sediment cores were taken in March 2018 in a small coastal lagoon on Scrub Island (northeastern Caribbean). Sedimentological, geochemical, microfaunal and chronological analyses enabled us to identify 25 sandy layers resulting from high-energy-marine floods. Two of these layers were interpreted as tsunami deposits based on sedimentological (rip-up clast of the underlying cohesive substrate and internal mud laminae), and geochemical evidence. The most recent deposit is associated with the transatlantic tsunami triggered by the 1755 CE Lisbon earthquake. The older one is the thickest sandy layer recorded in the lagoon, with an age range of 1364-1469 cal. CE, as determined using 14 C dating. This event was recorded in sedimentary archives of both the northern and the southern part of the Caribbean, with its large spatial extent, supporting a distant tsunamigenic origin. The 23 remaining sandy layers were interpreted as storm deposits, based on sedimentological and chronological data, with the three most recent layers being correlated with historical hurricanes. This new 1600 year-long record displays similarities with that of the Bahamas, with the periods of intense hurricane activity being in antiphase with those of the northeastern US coast. This regional comparison may provide evidence for a latitudinal forcing of hurricane tracks through time in relation to climate fluctuations.

International audience

The processes associated with infragravity waves (IGW) and their role on the hydro-sedimentary dynamics of a wave-dominated inlet are investigated using field observations collected at the Albufeira Lagoon (Portugal) during storm Leslie in October 2018. During the storm, significant IGW heights reached up to 1.0 m in the surf zone and 0.4 m inside the lagoon. IGW frequencies were blocked by ebb currents at the flood-delta and this effect was stronger for higher IGW frequencies. Therefore, low-frequency IGW were able to propagate further into the inner lagoon. The application of a 1D energy balance equation suggests that depth-induced breaking and bottom friction contributed equally to dissipate IGW energy at the flood-delta and that this dissipation was stronger during the flood than during the ebb. Large horizontal velocities under IGW crests increased the instantaneous suspended sediment transport rate by up to two orders of magnitude during flood at the flood-delta. The net suspended sediment transport rate and its variability, associated with the wave motion, was positive (landward) during flood and negligible during ebb. These positive values were associated with large wave-induced orbital velocities, suggesting that waves were more effective than tidal currents in inducing suspended sediment transport. The oscillatory suspended sediment transport rate was directed towards the lagoon and dominated by IGW frequencies. The observed sediment accretion of the northern channel occurred during the most energetic IGW. The measurements presented in this study and their analyses demonstrate how IGW play a major role in the sedimentary dynamics of wave-dominated inlets during storm conditions.

International audience

Wave setup corresponds to the increase in mean water level along the coast associated with the breaking of short-waves and is of key importance for coastal dynamics, as it contributes to storm surges and the generation of undertows. Although overall well explained by the divergence of the momentum flux associated with short waves in the surf zone, several studies reported substantial underestimations along the coastline. This paper investigates the impacts of the wave-induced circulation that takes place in the surf zone on wave setup, based on the analysis of 3D modelling results. A 3D phase-averaged modelling system using a vortex force formalism is applied to hindcast an unpublished field experiment, carried out at a dissipative beach under moderate to very energetic wave conditions ( H m 0 = 6 m at breaking and T p = 22 s ). When using an adaptive wave breaking parameterisation based on the beach slope, model predictions for water levels, short waves and undertows improved by about 30%, with errors reducing to 0.10 m, 0.10 m and 0.09 m/s, respectively. The analysis of model results suggests a very limited impact of the vertical circulation on wave setup at this dissipative beach. When extending this analysis to idealized simulations for different beach slopes ranging from 0.01 to 0.05, it shows that the contribution of the vertical circulation (horizontal and vertical advection and vertical viscosity terms) becomes more and more relevant as the beach slope increases. In contrast, for a given beach slope, the wave height at the breaking point has a limited impact on the relative contribution of the vertical circulation on the wave setup. For a slope of 0.05, the contribution of the terms associated with the vertical circulation accounts for up to 17% (i.e. a 20% increase) of the total setup at the shoreline, which provides a new explanation for the underestimations reported in previously published studies.

Tidal asymmetry is a phenomenon that characterises estuarine hydrodynamics and has a strong impact on sediment dynamics. Extensive research has been dedicated to studying tidal dynamics in semidiurnal macrotidal estuaries, highlighting several general principles. The ratio of flood to ebb peak velocities and differences in ebb and flood durations are often used to characterise the asymmetry encountered in estuaries. In the Charente estuary (French Atlantic coast), water surface elevation data obtained using an ADCP (Acoustic Doppler Current Profiler) and a tide gauge show that the duration asymmetry undergoes inversions during the spring-neap tidal cycle. A two-dimensional hydrodynamics model is used to investigate the connection between spring-neap inversions of the tidal asymmetry and the harmonic composition of the tide. Different constituents (M2, S2, M4 and MS4) are considered at the open boundary. The results show that M4 and MS4 play a key role in the occurrence of these inversions. The influence of the morphology is also discussed and modifications of the bathymetry are performed to evaluate its impact. In the Charente estuary, the existence of both externally and internally generated overtides thus results in a spatially and fortnightly variable tidal asymmetry. The modelled barotropic tidal currents are used to estimate the possible impact on sediment dynamics. The results suggest that asymmetry inversions tend to create sediment accumulation in an intermediate zone between the river mouth and Rochefort, located approximately 20 km upstream.

International audience; Understanding estuarine sediment dynamics, and particularly turbidity maximum dynamics, is crucial for the management of these coastal systems. Various processes impact the formation, movement and structure of the turbidity maximum. Several studies have shown that tidal asymmetry and density gradients are responsible for the presence of this suspended sedimentary mass. The Charente estuary is a highly turbid system (with suspended sediment concentrations mostly in excess of 5 g/L) that remains poorly understood, despite its strong impact on local activities. In this study, a three-dimensional hydrosedimentary model is developed to represent the sediment dynamics of this estuary. Model validation demonstrates good accuracy, especially on reproducing semi-diurnal and spring-neap variability. Several simulations are performed to evaluate the influence of tides and river discharge on the turbidity maximum. Mean and maximum suspended sediment concentrations (S S C) and sediment stratification, are calculated. S S C transects are also used to visualise the suspended sediment distribution along the estuary. The turbidity maximum generally oscillates between the river mouth and the Rochefort area (20-30 km upstream). The model shows strong variations at different time scales, and demonstrates that S S C is mainly driven by deposi-tion/resuspension processes. Spring-neap comparisons show that the turbidity maximum is not well-defined during neap tides, for low and mean runoff conditions. Simulations of spring tides and/or high runoff conditions all result in a compact suspended sedimentary mass. Performing simulations without taking density gradients into account demonstrates that tidal asymmetry is the main mechanism leading to the formation of the turbidity maximum. However, density gradients contribute to maintaining the stability of the turbidity maximum. Vertical stratification traps sediments at the bottom. Longitudinal stratification ensures a sharper edge at the downstream limit of the suspended sedimentary mass, preventing a massive export of sediments.

Salt marshes in the St. Lawrence Estuary are subjected to strong seasonal variations with sub-arctic winter conditions. The present paper explores sedimentary processes in the Pointe-aux-Epinettes marsh during the ice-free period. Currents, waves, suspended sediment, and sedimentation rates have been measured in June, August and October 2009. Vegetation growth was monitored during one year on a monthly basis to understand the seasonal impact on the marsh. Vegetation attenuates currents and waves, but this attenuation changes over the year as vegetation disappears along winter. Results show that suspended sediment transport and deposition are controlled by vegetation, wave height, currents, distance from the marsh edge and distance from sediment sources. Suspended sediment concentrations and sedimentation rates were significantly correlated to wave height, highlighting the importance of waves for sediment resuspension transport. Transport was lowest in August when vegetation was high and wave occurrence low. However, vegetation growth didn't change fundamentally sediments dynamic of the marsh. Within one tide, an important part of sediment transport is only local within the marsh, as shown by the maximum sedimentation rates occurring near unvegetated areas of the marsh. Data provide a spatial understanding of summer sedimentology on cold climate marshes.

In macrotidal estuaries, tides and the river flow are in constant interaction. The consequent sedimentary structures are varying following different temporal scales. This study focuses on the sediment dynamics of the Charente estuary, located on the French Atlantic coast. The turbidity maximum is reproduced using 3D hydrosedimentary modelling. Several tidal and fluvial forcings are considered. Results show that the tide remains the dominant process at the river mouth. However, the river discharge has a strong influence on the turbidity maximum characteristics (concentration, extension). Cumulated sedimentary fluxes suggest that, in the case of an average river discharge, the estuary tends to import sediments.