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Abstract. Decadal time series of chlorophyll-a concentrations sampled at high and low frequencies are explored to study climate-induced changes on the processes inducing interannual variations in the Initiation of the Phytoplankton Growing Period (IPGP) in early spring. In this study, we specifically detail the IPGP in two contrasting French coastal ecosystems: the Bay of Brest and the Bay of Vilaine. A large interannual variability in the IPGP is observed in both ecosystems in connection with variations of environmental drivers (solar radiation, sea temperature, wind direction and intensity, precipitation, river flow, sea level, currents and turbidity). We show that the IPGP is delayed by around 30 days in 2019 in comparison with 2010. The use of a one-dimensional vertical model coupling hydrodynamics, biogeochemistry and sediment dynamics shows that the IPGP is generally dependent on the interaction between several drivers. Interannual changes are therefore not associated with a unique driver (such as increasing sea surface temperature). Extreme events also impact the IPGP. In both bays, IPGP is sensitive to cold spells and flood events. The interannual variability of the IPGP is significant and strongly conditioned, at the local scale, by a combination of several environmental parameters, with a larger sensitivity to sea temperature and light conditions, linked to the turbidity of the system. While both bays are hydrodynamically contrasted, the processes that modulate IPGP are similar. Keywords Phytoplankton biomass, Long-term in situ observations, Coastal ecosystems, Extreme events, Climate change.

20 pages, 15 figures, 3 tables.-- Data availability: Data will be made available on request, A simulation based on a hydro-sedimentary model was conducted for the period between summer 2010 and spring 2012 in the Gulf of Lion (northwestern Mediterranean Sea) to understand the spatial and temporal variability of sediment transport, erosion and deposition on the continental shelf and slope. Datasets of both simulated and observed current, temperature and suspended matter from the shelf and the Cap de Creus Canyon which is the main export route towards the continental slope, were first compared to assess the reliability of the simulation. The simulation shows the massive sediment accumulation near the Rhone River mouth (∼56 % of the inputs), the accretion along the mid-shelf mud belt, and the impact of dense shelf water cascading on sediment resuspension and erosion inside the Cap de Creus Canyon. The two studied autumn–winter periods were strongly contrasted in terms of meteorological conditions and subsequent impacts on the sediment dynamics. During the first period (2010–2011) dominated by marine storms, the shelf sediment underwent strong changes, the Rhone River sediment load accumulated in a relatively small area, stock of littoral sands moved to the inner shelf (20–40 m) while inner shelf fine particles fed the mid-shelf mud belt and the upper Cap de Creus Canyon. During the second period (2011–2012) with very little marine wind and a particularly cold winter, sediment on the shelf underwent little change except for a southwestward growth of the Rhone River prodeltaic deposit. Sediment from the southwestern end of the shelf as well as from the upper Cap de Creus Canyon was flushed toward deeper reaches by dense shelf water cascading. Cascading also had a more moderate impact in the various canyons incising the continental shelf. Our work supports the view of an unbalanced sedimentary system, with a deficit mainly over the inner shelf, whose main driver is probably the strong and fast reduction of particulate matter inputs from the Rhone River (by a factor of 4 in less than one century), This study (including the PhD and Postdoc of G. Mikolajczak) was mainly funded by the AMORAD-project (grant ANR-11-RSNR-0002 of the French program Investissement d’Avenir run by the Agence Nationale de la Recherche). We also benefited from the support of the CHIFRE and DeltaRhone projects of the national EC2CO program. The simulations were performed using HPC resources from CALMIP (Grant P09115) and CINES OCCIGEN (Grant A0040110088). This research was also supported by grant PID2020-114322RB-I00 (FAR-DWO) funded by MCIN/AEI/ 10.13039/501100011033, With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)

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.

Coastal environments are highly impacted by the combined influence of anthropogenic climate change and the occurrence of persistent episodes of extreme meteorological events: warming of sea waters and changes in nutrient inputs due to the modification of precipitation patterns and consequently on river flows. Here, we study the impact of climate driven changes on phytoplankton biomass dynamics by analyzing high and low frequency of phytoplankton fluorescence and chlorophyll measurements in two French eutrophic ecosystems (Bay of Brest 2000-2019 and Bay of Vilaine 2011-2019). While the frequency and intensity of blooms in the Bay of Vilaine is driven by the supply of nutrients from rivers, no clear relationship is detected in the Bay of Brest where the residence time of water masses is short and the nutrient limitations infrequent. Long-term changes in phytoplankton biomass in these two contrasted ecosystems revealed a strong interannual variability in the timing, intensity and magnitude of blooms that occurred during the growing period (mid-March to mid-September). We observed that the first spring bloom is initiated in 2019 about 30 days earlier than 20 years ago in the Bay of Brest while it is delayed by 20 days in a decade in the Bay of Vilaine. These modifications in the timing of the phytoplankton bloom are related to a “shift” in sea surface temperature and changes in solar irradiance, as originating from the influence of large-scale hydro- climatological processes.

Long term and high-frequency monitoring of water quality, particularly the suspended particulate matter (SPM) concentration are crucial to decipher the health and sustainable development of marine ecosystems. However, in-situ measurements based on indirect optical or acoustic techniques are often associated with large uncertainties due to the dynamics of natural SPM, especially throughout the land-sea continuum. Therefore, this study aims to improve the accuracy of long term in-situ measurements by quantitatively elucidating the physical mechanisms by which sand and fine sediment respond to multi-wavelength optical and multi-frequency acoustic signals. We hypothesize that whilst fine sediment is very sensitive to optical signals, the coarser particles are more sensitive to acoustic signals, and vice versa. We further hypothesize that the SPM compositions and variability can be differentiated and derived based on such sensitivities and differences in behaviors of sand and fine sediment under different types of signals, i.e., optical and acoustic. Before testing the hypotheses, a novel laboratory device that is capable of 1) generating homogeneous suspended concentration and 2) providing sufficient space for multiple sensors to operate simultaneously must be developed. The new device, DEXMES (dispositive experimental de quantification des matières en suspension), primarily consists of two main components. The upper part is a cylindrical tank with an inner diameter of 0.96 m and 1.4 m high. To break up the large vortexes and mitigate the vortex-induced bubbles (e.g., generated by the impeller), four baffles with dimensions of 0.09 x 1.31 m are evenly attached to the inner side of the tank. The bottom part of the DEXMES device is a convex, elliptical Plexiglas bed. Turbulent flow is generated by an impeller with a diameter of 0.36 m placed approximately 1 m below the water surface. The speed of the impeller, ranging from 0 to 235 rpm, is regulated by a controller box.To test the hypotheses, 30 experiments, consisting of 6 concentrations and 5 mixture ratios (by mass) of Bentonite and fine sand (d50 = 100 µm), i.e., 100/0, 75/25, 50/50, 25/75, and 0/100, were thoroughly investigated using three acoustic sensors (ADV, AQUAscat, LISST-ABS) and three optical sensors (Wetlabs, HydroScat, LISST-100X). On average, each data point is the averaged value of 10 min of recording at 1 or 32 Hz. First, results show logarithmic/linear relationships between concentration and acoustic/optical signals respectively for a given bentonite/sand. Second, the slope of this relation is a function of the Bentonite/sand ratio. Third, the results confirm the hypotheses that coarser particles are more sensitive to acoustic signals and fine sediment is more sensitive to optical signals. Simple regression models were developed for different pairs of acoustic and optical sensors based on their relative sensitivity to SPM characteristics. The correlation coefficient, bias, and RMSE between observed and predicted concentrations then were examined. The results also show that it is possible to use a combination of one acoustic and one optical sensor to infer the concentration and the ratio of fine/coarse sediment in suspension with minimum use of water samples calibration.

Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to mud particles ( < 63μm) and flocs, whereas acoustic backscatter systems are more responsive to larger sand grains ( > 63μm). It is thus challenging to estimate the relative proportion of sand or mud in environments where both types of sediment are present. The suspended sediment concentration measured by these devices depends on the composition of that sediment, thus it is also difficult to confidently measure concentration with a single instrument when the composition varies and extensive calibration is not possible. The objective of this paper is to develop a methodology for characterizing the relative proportions of sand and mud in mixed sediment suspensions by comparing the response of simultaneous optical and acoustic measurements. We derive a sediment composition index (SCI) that is used to directly predict the relative fraction of sand in suspension. Here we verify the theoretical response of these optical and acoustic instruments in laboratory experiments, and successfully apply this approach to field measurements from Ameland ebb-tidal delta (the Netherlands). Increasing sand content decreases SCI, which was verified in laboratory experiments. A reduction in SCI appears during more energetic conditions when sand resuspension is expected. Conversely, the SCI increases in calmer conditions when sand settles out, leaving behind mud. This approach provides crucial knowledge of suspended sediment composition in mixed sediment environments. Plain Language Summary Sand and mud particles are the building blocks of our coastlines. Counting and describing sand and mud particles floating through the water is essential to managing coasts. We commonly do this with devices that send out a sound (acoustic) or light (optical) signal into the water. The sensors measure the strength of the signal reflecting back off of any sand and mud particles passing by. Optical instruments are better at “seeing” mud than sand, and acoustic instruments are better at “hearing” sand than mud. If both sand and mud are present, a single instrument will not accurately estimate the total amount of sediment because of these different sensitivities. Instead, we can use both types of instrument together and compare what we “see” with what we “hear”. This comparison allows us to estimate whether there are more sand or mud particles floating through the water. The relationship between “seeing” and “hearing” can be described in a single number, the sediment composition index (SCI). We successfully tested this approach in laboratory experiments and then applied it to a site on the coast of the Netherlands. This approach gives us a new way to understand environments that are both sandy and muddy.

La station COAST-HF MESURHO permet d’observer la variabilité hydrologique et hydro-sédimentaire à proximité de l’embouchure du Rhône, en étant positionnée au sud-est du prodelta, à approximativement 17m de profondeur, s’appuyant sur une BFI des Phares et Balises. La station est équipée de sondes multiparamètres en sub-surface (-2m) et à 2m au-dessus du fond, mesurant notamment la température, la salinité et la turbidité. Elle bénéficie également du déploiement d’un ADCP600kHz Sentinel RDI, positionné à proximité de la BFI, et mesurant les paramètres de vagues et le profil vertical de courant et d’intensité acoustique rétrodiffusée sur l’ensemble de la colonne d’eau. Dans le cadre de l’OSR5, les mesures acoustiques de l’ADCP ont été utilisées pour estimer la concentration en matières en suspension à MesuRho. Pour les calibrer, ces mesures ont été couplées à des mesures optiques de turbidité et des mesures satellitales. La dynamique des concentrations en MES à l’embouchure du Rhône est contrôlée par les apports en provenance du bassin versant, avec des concentrations qui atteignent leur maximum en période de crue, dépassant 10mg/l pour des débits supérieurs à 2000m3/s, et pouvant atteindre 50mg/l. Le rôle des crues et des tempêtes a pu être étudié. Enfin, entre 2010 et 2019, une accrétion de plus de 2m est observée à la station MESURHO.

Residual suspended sediment flux in estuaries is dependent on water level, velocity, and suspended sediment concentration (SSC), but complex interactions between these variables and other forcing mechanisms can lead to drastic differences in the magnitude and direction of sediment flux. The goal of this study was to quantify residual suspended sediment flux in a shallow, macrotidal estuary, and to determine its most important forcing mechanisms, using the Dyer flux decomposition equation and a simplified analytical model. Water level, velocity, and acoustic backscatter were measured in the Aulne River estuary in Brittany, France, and acoustic backscatter converted to SSC. The vertical tide was slightly flood dominant near the mouth, but strongly flood dominant upstream. Velocity was ebb dominant throughout the estuary. The magnitude and direction of total residual suspended sediment flux changed with position in the estuary and seasonally. The Eulerian flux was dominant at the mouth, but the tidal pumping and Stokes drift components increased in importance landward. Residual suspended sediment flux in the Aulne is dependent on several processes in addition to those included in the simplified model. The strong spring-neap control and tidal resuspension of sediments in the Aulne and the presence of higher-order tidal velocity terms contribute in a non-negligible way to residual suspended sediment flux. Finally, all of the first five components of the Dyer flux decomposition equation are needed to accurately represent residual suspended sediment flux in the Aulne.

Context To protect the marine environment more effectively, the European Union adopted the Marine Strategy Framework Directive (MSFD) in 2008, aiming to achieve the Good Environmental Status (GES) of the EU's marine waters by 2020 and to protect the resource base upon which marine-related economic and social activities depend. A major challenge in the implementation of the MSFD is to achieve the necessary scientific knowledge on the marine environment, its processes and the methods to monitor them. The focus of INDI67 is on the evaluation and development of indicators to monitor GES of descriptor 6 and 7. Seafloor integrity (descriptor 6) refers to the structure and functions of the benthic ecosystems. It relates to the comprises physical, chemical and biological properties as well as to spatial and temporal connectedness, avoiding artificial fragmentation of habitats or temporal sealing due to ephemeral sediment deposits or armouring. Hydrographic conditions (descriptor 7) imply that the nature and scale of any long-term changes to the prevailing hydrographical conditions resulting from anthropogenic activities (individual and cumulative), do not lead to significant negative impacts on the benthic and pelagic habitats, functioning or on hydro-geomorphological impacts on the seabed. Objectives The overall objective of INDI67 is to develop and evaluate tools and methods to support the monitoring of MSFD descriptors 6 and 7. The subject is the monitoring of seafloor integrity and hydrography using both modelling and measurements of hydro- and sediment dynamic processes and seabed characteristics. Three parameters have been selected as key indicator, i.e. turbidity, bottom shear stress and seabed/habitat type. These parameters are all related to sea floor dynamics and are strongly linked as changes in seafloor integrity and turbidity occur as a result of the combined force that waves and currents exert on the sea floor. Furthermore, they are witnesses of changes induced by human activities (dredging/disposal, aggregate extraction, constructions, fishery). Turbidity (used both in terms of suspended particulate matter(SPM) concentration and light availability) and bottom shear stress are currently measured and modelled. Bottom shear stress and seabed/habitat type are included in the Belgian MSFD monitoring programme, while turbidity is not yet included as GES indicator. Conclusions The major conclusion with respect to the measuring and modelling of the key indicators turbidity/SPMC, bottom shear stress and seabed/habitat type are: 1) Despite calibration to a reference solution and the use of ISO-normed optical turbidity sensors, model calibration may vary considerably in recorded turbidity for a same SPMC solution across different instruments resulting in instrument-specific turbidity-SPMC relation. Turbidity (or dB for acoustic sensors) should therefore not be used as it is not standardized and will diminish the comparability of the data. Instead, the optical and acoustic sensor output should be transformed into a mass concentration, a unit that is comparable in time and between regions. Monitoring in situ SPMC should follow common guidance and protocols to restrict their measurement uncertainties. The main challenge is now to evaluate model results uncertainty and improve the formulation of natural processes, such as flocculation and bottom shear stress, together with the effects of pressures in the models.

Suspended particulate matters (SPM) in coastal waters were investigated with an approach combining suspended particulate matter concentrations (SPMCs) measured by an optical backscatter sensor (OBS), particle size distributions measured by a laser in situ scattering transmissometer (LISST), and the fractal theory. The aim was to investigate whether changes in the fractal dimension indicate variations in the SPM composition. The method relies on the fractal theory that relates the floc excess density ∆ρ to the ratio between floc size Df and primary particle size Dp as follows: Δρ = (ρp – ρw)[Df/Dp]nf − 3. The best-fit fractal dimension nf was determined by matching the OBS-derived SPMC and the SPMC calculated from the LISST assuming fixed primary particle size and density. This method was applied to measurements from four tidal cycles characterized by different organic matter (OM) contents. Spurious data caused by schlieren were identified and discarded. The fractal dimension was relatively homogeneous over each tidal cycle except one where a strong apparent decrease in the fractal dimension was related to an important episode of sediment resuspension. This apparent decrease could result from the limit of the LISST to measure valid data when SPMC is high enough to induce multiple scattering and/or a change in the SPM population in suspension. Results showed also strong differences between the tidal cycles such as an increase of the fractal dimension with increasing OM content. Sensitivity analyses of fractal dimension and settling velocity were performed on major parameters: primary particle size and density, slope of the OBS calibration relationship, and optical model of inversion of the LISST. Results showed that the assumed primary particle size and the OBS calibration relationship significantly affect fractal dimension and settling velocity calculations.

International audience; The present paper reports on a series of field experiments aiming to characterise the functioning of a man-engineered strongly forced salt-wedge estuary : the lower estuary of the Adour river, France. Bottom-moored velocity measurements and surface boat surveys have been performed under low river discharge conditions, for both neap and spring tides, in order to provide a well-documented reference framework to understand the dynamics of water masses, turbulence and suspended particulate matter (SPM) transport in the lower estuary. An additional campaign has been carried out in high river discharge conditions. This first documented in-situ study of the Adour lower estuary demonstrates its variability in terms of hydrological regimes, from salt-wedge to partially mixed regimes depending on tidal and discharge conditions. Turbulent properties showed a significant response to the variations of salinity structure, with higher values when stratification is minimal. At spring tide, a tidal variation between mixing conditions on the ebb and the flood is revealed by ADCP measurements, with higher values extended up to the surface during the ebb. The link between turbulent mixing and suspended sediment concentration is straightforward during the ebb. During the flood, the suspended sediment concentration (SSC) seems related to the salt-wedge entrance re-suspension and stratification-induced turbulence damping. No stable Estuarine Turbidity Maximum (ETM) has been observed during the field experiment in the lower Adour estuary.

(IF 3.25 [2018]; Q1); International audience; Measurement of suspended particulate matter concentration (SPMC) spanning large time and geographical scales have become a matter of growing importance in recent decades. At many places worldwide, complex observation platforms have been installed to capture temporal and spatial variability over scales ranging from cm (turbulent regimes) to whole basins. Long-term in situ measurements of SPMC involve one or more optical and acoustical sensors and, as the ground truth reference, gravimetric measurements of filtered water samples. The estimation of SPMC from optical and acoustical proxies generally results from the combination of a number of independent calibration measurements, as well as regression or inverse models. Direct or indirect measurements of SPMC are inherently associated with a number of uncertainties along the whole operation chain, the autonomous field deployment, to the analyses necessary for converting the observed proxy values of optical and acoustical signals to SPMC. Controlling uncertainties will become an important issue when the observational input comprises systems of sensors spanning large spatial and temporal scales. This will be especially relevant for detecting trends in the data with unambiguous statistical significance, separating anthropogenic impact from natural variations, or evaluating numerical models over a broad ensemble of different conditions using validated field data.The aim of the study is to present and discuss the benefits and limitations of using optical and acoustical backscatter sensors to acquire long-term observations of SPMC. Additionally, this study will formulate recommendations on how to best acquire quality-assured SPMC data sets, based on the challenges and uncertainties associated with those long-term observations. The main sources of error as well as the means to quantify and reduce the uncertainties associated with SPMC measurements are also illustrated.

(IF 3.35; Q1); International audience; The aim of this study is to investigate the complex response of optical turbidity sensors (side- and back-scattering sensors) to Suspended Particle Matter (SPM) characteristics and the consequences when investigating SPM dynamics from long-term high frequency monitoring networks. Our investigation is based on the analysis of a unique dataset of monthly 12 h cycle measurements of SPM characteristics such as turbidity, concentration, floc size distribution, floc density and organic matter content in the macrotidal Seine Estuary (France) between February 2015 and June 2016. Results reveal that despite calibration to a Formazin standard, turbidity sensor response to SPM concentrations (in the range of 7–7000 mg L−1) are strongly variable, from the tidal scale to the annual scale and in different compartments of the Seine Estuary. The variability in the calibration relationships is related to changes in the sensor sensitivity according to (i) the sensor intern technology (mainly due to optical geometry) and (ii) the variability in inherent optical properties (IOP) of SPM.Side-scattering optical instruments (measuring scattering at 90°) provide at the annual scale a more stable optical response than backscattering sensor (measuring scattering at angles larger than 100°) for a wide variety of floc size and density in the estuarine environment, while at the tidal scale the backscatter sensors are the most accurate. Sensor sensitivity is strongly affected by floc characteristics, i.e. their median size D50, dry density ρ and the scattering efficiency Qb. Results highlight that the median particle diameter contribute to modify the scattering efficiency Qb as well as the dry density: Qb increases with increasing floc size, and for a given floc size, Qb increases with floc density.In this study, the results are next applied to turbidity data from the long-term automated monitoring network in order to estimate SPM concentrations and estimate the related uncertainties.

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.

Since the eighties, several French organizations (Ifremer, CNRS, and marines universities) have joined their forces to operate the first high frequency automated network around France. Now days, 14 Instrumented platforms are in operation along the North Sea, the Atlantic and Mediterranean French coasts. Furthermore, this organization is now largely contributing to the French national Research Infrastructure named "I.R. ILICO".

Regions Of Freshwater Influence (ROFI) are of particular interest in a source-to-sink approach in terms of sediment advection, settling, and deposition in the coastal zone. An experiment was carried out in the ROFI of the Rhone River in February 2016 to describe the properties of suspended particulate matter (SPM) during a flood event. A digital holographic camera (LISST-HOLO, 20–2000 μm) was used to estimate the variability of fine sediment floc properties (size, nature and shape) formed in the Rhone mouth. An automatic image toolbox was developed to classify the different constituents of the SPM (as diatoms, bubbles and flocs) and to describe the diversity of floc shapes existing in the material in suspension. We estimated the fractal dimension (DF3D), the aspect ratio (AR) and the settling velocity of flocs (Ws). The estimated DF3D ranged between 2.0 and 2.5 highlights the complexity of floc shape, which was used as a proxy of the flocculation mechanism functioning in the Rhone mouth. Additionally, we performed a sensitivity analysis on the estimate of Ws using different shape-related coefficients (α/β) and primary particle size (dP). The results highlighted the impact of the flocculation of fine sediments on the increase of Ws from 0.01 to 3 mm s−1 when floc sizes increase from 30 to 500 μm. Ws showed a decrease of 41% considering the sphericity of flocs that emphasized the need to consider the floc shape to properly estimate their settling velocity. We showed that an increase of dP from 1 to 12 μm induces a fivefold increase of Ws that showed the need for an adequate system to properly estimate the size of primary particles. These results emphasized the need to take into account such variability in future model of floc dynamics in ROFI to properly estimate plume sinking rate and SPM dynamics.

Moskalski, S.; Floc'h, F.; Verney, R.; Fromant, G.; Le Dantec, N., and Deschamps, A., 2018. Sedimentary dynamics and decadal-scale changes in the macrotidal Aulne River estuary, Brittany, France. Studies of macrotidal estuaries have been dominated by large, high-turbidity systems like the Gironde and Severn, but some macrotidal estuaries are different from these systems. The goal of this study was to document the state of hydrosedimentary processes in the macrotidal, low-turbidity Aulne River estuary and, where possible, compare it to a previous study. Study sites from a 1977 study were revisited in 2013. Observations of velocity, salinity, and suspended sediment concentration were analyzed and compared to the 1977 study, with particular attention given to hourly data. The results illustrate the impact of shallowing on hydrosedimentary processes. The estuary is strongly dominated by spring–neap variability and shallowing, with increasing tidal distortion in the landward direction and a turbidity ...

An innovative experiment was carried out in the vicinity of the Rhône River mouth in February 2014. An instrumental package, composed of a CTD, a LISST-100 type B (1.25–250 μm) and a LISST-HOLO (20–2000 μm), was used to characterize the hydrological parameters and suspended particles properties (concentration, size, composition, shape and effective density) in the region of freshwater influence (ROFI) of the Rhône River. Besides, a coastal SLOCUM glider, equipped with a CTD and optical backscattering sensors at several wavelengths, was deployed to detail the spatial description of the hydrological parameters and some particle properties. Large river discharge (annual flood ~ 5000 m3 s− 1) and strong wind conditions favored the dispersal of the river plume on the shelf. Surface suspended particulate matter concentrations decreased rapidly seaward from 20 mg L− 1 next to the river mouth to 1.5 mg L− 1 at the shelf break. A persistent bottom nepheloid layer was observed across the shelf with concentrations decreasing from 8 mg L− 1 at the coast to 1 mg L− 1 at the shelf break. Observations showed that most of suspended particles were mainly flocculated in micro and macro-flocs (30–400 μm) in inner shelf waters. The particle assemblage in the Rhône River plume and in the bottom nepheloid layer became progressively finer seaward and the associated effective density increased from 370 to 1600 kg m− 3. Outside the plume, planktonic organisms increasingly contributed to the total volume concentration. Finally, we demonstrated the ability of gliders, equipped with optical backscattering sensors at several wavelengths, to describe the fine scale distributions of suspended particles, and provide an index of their size distribution.

The coastal city of Toulon, located in southern France, is the homeport of the French navy's nuclear-powered vessels, and is the largest nuclearized harbour in Europe. Therefore, an accidental release of radioactive material would have major consequences for the marine ecosystem and the economy of the region. A hydro-sedimentary model is presented to better understand the dispersion of a hypothetical release of radionuclides and to gain insights into the potential effects on the surrounding ecosystem, especially on sea sediments. The aim of the model is to provide the foundation of a management tool for experts and stakeholders in decision making in case of an accidental release. We simulate a numerical radionuclide discharge in the military harbour under several climatic conditions and we compute 137Cs, which might have substantial impacts on the marine ecosystem, and 239Pu for its high affinity for solid phase. Results show that wind conditions and solid discharges from the Las River are the primary influences on sediment-bed contamination, and the hydro-sedimentary dynamics following the first days of release have an important influence on the distribution of radionuclides.

We describe and demonstrate algorithms for treating cohesive and mixed sediment that have been added to the Regional Ocean Modeling System (ROMS version 3.6), as implemented in the Coupled Ocean Atmosphere Wave Sediment-Transport Modeling System (COAWST Subversion repository revision 1179). These include: floc dynamics (aggregation and disaggregation in the water column); changes in floc characteristics in the seabed; erosion and deposition of cohesive and mixed (combination of cohesive and non-cohesive) sediment; and biodiffusive mixing of bed sediment. These routines supplement existing non-cohesive sediment modules, thereby increasing our ability to model fine-grained and mixed-sediment environments. Additionally, we describe changes to the sediment bed-layering scheme that improve the fidelity of the modeled stratigraphic record. Finally, we provide examples of these modules implemented in idealized test cases and a realistic application.

International audience; The export of riverine suspended particulate matter (SPM) in the coastal ocean has major implications for the biogeochemical cycles. In the Mediterranean Sea (France), the Rhone River inputs of SPM into the Gulf of Lion (GoL) are highly variable in time, which severely impedes the assessment of SPM fluxes. The objectives of this study are (i) to investigate the prediction of the land-to-ocean flux of SPM using the complementarity (i.e., synergy) between a hydrodynamic sediment transport model and satellite observations, and (ii) to analyze the spatial distribution of the SPM export. An original approach that combines the MARS-3D model with satellite ocean color data is proposed. Satellite-derived SPM and light penetration depth are used to initialize MARS-3D and to validate its predictions. A sensitivity analysis is performed to quantify the impact of riverine SPM size composition and settling rate on the horizontal export of SPM. The best agreement between the model and the satellite in terms of SPM spatial distribution and export is obtained for two conditions: (i) when the relative proportion of ''heavy and fast'' settling particles significantly increases relative to the ''light and slow'' ones, and (ii) when the settling rate of heavy and light SPM increases by fivefold. The synergy between MARS-3D and the satellite data improved the SPM flux predictions by 48% near the Rhone River mouth. Our results corroborate the importance of implementing satellite observations within initialization procedures of ocean models since data assimilation techniques may fail for river floods showing strong seasonal variability.

International audience; In coastal environments, river plumes are major transport mechanisms for particulate matter, nutriments and pollutants. Ocean color satellite imagery is a valuable tool to explore river turbid plume characteristics, providing observations at high temporal and spatial resolutions of suspended particulate matter (SPM) concentration over a long time period, covering a wide range of hydro-meteorological conditions. We propose here to use the MERIS-FR (300m) Ocean Color archive (2002-2012) in order to investigate Rhone River turbid plume patterns generated by the two main forcings acting on the north-eastern part of the Gulf of Lions (France): wind and river freshwater discharge. Results are exposed considering plume metrics (area of extension, south-east-westernmost points, shape, centroid, SPM concentrations) extracted from satellite data using an automated image-processing tool. Rhone River turbid plume SPM concentrations and area of extension are shown to be mainly driven by the river outflow while wind direction acts on its shape and orientation. This paper also presents the region of influence of the Rh delta ne River turbid plume over monthly and annual periods, and highlights its interannual variability.

A three-dimensional model has been modified to describe the complex interactions between hydrodynamics, sediment dynamics and biological parameters in the presence of Zostera noltei. The model treats seagrass leafs as flexible blades that bend under hydrodynamic forcing and alter the local momentum and turbulence fluxes and, therefore, the benthic shear conditions; these changes cause related changes to the mass balance at the boundary of the bed, in turn affecting the suspended matter in the column and ultimately primary productivity and the growth of the dwarf-grass. Modelling parameters related to the impact of Z. noltei to the local flow and to erosion and deposition rates were calibrated using flume experimental measurements; results from the calibration of the model are presented and discussed. The coupled model is applied in the Arcachon Bay, an area with high environmental significance and large abundance of dwarf-grass meadows. In the present paper, results from preliminary applications of the model are presented and discussed; the effectiveness of the coupled model is assessed comparing modelling results with available field measurements of suspended sediment concentrations and seagrass growth parameters. The model generally reproduces sediment dynamics and dwarf-grass seasonal growth in the domain efficiently. Investigations regarding the effects of the vegetation to the near-bed hydrodynamics and to the sediment suspension in the domain show that dwarf-grass meadows play an important part to velocity attenuation and to sediment stabilisation, with flow and suspended sediment concentrations damping, compared to an unvegetated state, to reach 35–50 and 65 %, respectively, at peak seagrass growth.

The Arcachon lagoon (Atlantic coast, SW France) is a mesotidal embayment where seagrass beds colonize the majority of intertidal areas. In recent years, the surface area of Zostera noltii meadows has considerably decreased, with possible consequences for the sediment balance of the lagoon. However, such interactions are poorly understood, and knowledge of the relationship between hydro-sedimentary processes and small intertidal meadows, such as Z. noltii , is limited. An intertidal mudflat, with variable meadow coverage, was studied during an annual survey. The study consisted in continuous high-frequency monitoring of bed altimetry, tidal elevation and waves. Sediment parameters and meadow characteristics were analyzed using samples collected monthly. Acoustic altimetry was validated as an efficient method to measure bed elevation in a vegetated environment, despite the presence of leaves under the transducer. The acoustic altimeter was also shown to have the potential to accurately estimate canopy height in a submerged environment. Our survey data showed centimetric bed accretion at all vegetated stations. Accretion was positively correlated with seasonal growth of the meadows. During seasonal degeneration, the meadow prevented erosion of the sea bed. These results highlight the important role of seagrasses as ecosystem engineers.

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.

This study proposes a method to simulate the images of the future European geostationary sensor dedicated to ocean color sensor: the geostationary ocean color advanced permanent imager (GeoOCAPI), and it demonstrates the sensor capabilities to monitor the water composition throughout the day. The temporal variation of the coastal seascape is obtained from biogeochemical and hydrosedimentary models, the ocean-atmosphere radiance is obtained from the water and atmosphere radiative transfer model. The GeoOCAPI images are simulated with 400-m resolution, 18 spectral bands with associated signal-to-noise ratio (SNR) and with 1-h acquisition frequency, on the Gulf of Lion (Marseille, France) during a pollution event caused by the urban outfall of Cortiou in Marseille. These images describe the water color dynamic in the Gulf of Lion due to the river transport and the urban outfall. The validation with real medium resolution imaging spectrometer (MERIS) images showed that the image simulator was reliable with an average relative error (RE) at 4.76% for visible bands and at 16.51% for near infrared bands. The quasi-analytical algorithm (QAA) inversion method was tested. The suspended particulate matter ( SPM ) and the colored dissolved organic matter ( CDOM ) can be retrieved with good accuracy; the error is, respectively, 7.69% and 12.21%. The chlorophyll concentration ( chl) is misestimated (58.10%) due to the low concentration in this area ( ${ ) compared to SPM ( ${ > } 1 {\text g}\cdot {\text m}^{-3}$ ). The study showed that the future geostationary sensor GeoOCAPI will be able to monitor the water composition in coastal areas through the day and detect and monitor an urban outfall discharge.

Ocean color satellite sensors are powerful tools to study and monitor the dynamics of suspended particulate matter (SPM) discharged by rivers in coastal waters. In this study, we test the capabilities of Landsat-8/Operational Land Imager (OLI), AQUA&TERRA/Moderate Resolution Imaging Spectroradiometer (MODIS) and MSG-3/Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensors in terms of spectral, spatial and temporal resolutions to (i) estimate the seawater reflectance signal and then SPM concentrations and (ii) monitor the dynamics of SPM in the Rhône River plume characterized by moderately turbid surface waters in a micro-tidal sea. Consistent remote-sensing reflectance (Rrs) values are retrieved in the red spectral bands of these four satellite sensors (median relative difference less than ~16% in turbid waters). By applying a regional algorithm developed from in situ data, these Rrs are used to estimate SPM concentrations in the Rhône river plume. The spatial resolution of OLI provides a detailed mapping of the SPM concentration from the downstream part of the river itself to the plume offshore limits with well defined small-scale turbidity features. Despite the low temporal resolution of OLI, this should allow to better understand the transport of terrestrial particles from rivers to the coastal ocean. These details are partly lost using MODIS coarser resolutions data but SPM concentration estimations are consistent, with an accuracy of about 1 to 3 g·m-3 in the river mouth and plume for spatial resolutions from 250 m to 1 km. The MODIS temporal resolution (2 images per day) allows to capture the daily to monthly dynamics of the river plume. However, despite its micro-tidal environment, the Rhône River plume shows significant short-term (hourly) variations, mainly controlled by wind and regional circulation, that MODIS temporal resolution failed to capture. On the contrary, the high temporal resolution of SEVIRI makes it a powerful tool to study this hourly river plume dynamics. However, its coarse resolution prevents the monitoring of SPM concentration variations in the river mouth where SPM concentration variability can reach 20 g·m-3 inside the SEVIRI pixel. Its spatial resolution is nevertheless sufficient to reproduce the plume shape and retrieve SPM concentrations in a valid range, taking into account an underestimation of about 15%–20% based on comparisons with other sensors and in situ data. Finally, the capabilities, advantages and limits of these satellite sensors are discussed in the light of the spatial and temporal resolution improvements provided by the new and future generation of ocean color sensors onboard the Sentinel-2, Sentinel-3 and Meteosat Third Generation (MTG) satellite platforms.

L’objectif de ce travail est mettre en place un modele hydro-sedimentaire de dispersion des radionucleides, comme outil d’aide a la decision suite a une hypothetique contamination marine accidentelle. Applique a la Rade de Toulon, il utilise le modele MARS-3D pour l’etude hydrodynamique et le module MIXSED pour la dynamique des differentes classes de sediments. Les cas d’application presentes concernent le cesium et le plutonium, qui ont des affinites tres differentes avec les sediments fins. Les simulations de rejet, effectuees en fond de baie, montrent une variabilite saisonniere marquee, due aux conditions meteorologiques typiques, mais aussi aux conditions de debit et de charge sedimentaire associee du fleuve Las, qui se jette dans la Rade. Ainsi, les simulations par forts vents, qui favorisent la circulation et les echanges de masses d’eau, presentent une diminution rapide des activites dissoutes. Par ailleurs, le Vent d’Est hivernal, qui engendre d’importantes crues du Las et un apport de sediments a la Rade, favorise le piegeage des radionucleides dans les sediments de fond, ou les deuxtiers de la contamination initiale sont pieges apres une simulation de deux mois.

A high resolution 3D coastal ocean model has been applied to the forecast of the oceanic circulation off Marseille (from the Rhone River to the Cap Sicie). The realistic numerical simulations were forced by the Rhone River, meteorological fields and by the surrounding general circulation. A lot of observations providing the stratification and currents in the area were acquired at moorings and from vessel cruises during the last decade. In order to validate the nested forecasting system, quantitative comparisons between observations and model results were performed for the years 2007 and 2008. Statistics over long time series of temperature at moorings were computed to show the model skills in capturing the monthly to seasonal variability of the thermal structure. The comparison also demonstrated the model's ability to reproduce the observed features over the shelf, like the warming or cooling of the sea due to upwelling and downwelling events. The model also reproduced an intrusion of the Rhone River plume observed into the southern bay of Marseille during a flood event, inducing a salinity decrease at the Frioul station under westerlies and south-easterlies. Hence, although the wind constrains the coastal circulation, the general circulation is also of great influence, with intrusions of the Northern Current from the south of the domain observed and reproduced by the model in winter, due to the penetration of meanders over the narrow eastern part of the shelf. This process was better reproduced by the nested high resolution model than by the North Western Mediterranean Sea model. The nested model was also found to be more suitable to reproduce the plumes of diluted pollutants emanating from coastal discharges.

The present contribution aims to provide an insight into the sediment dynamics of the Bay of Marseille (BoM), France, an area characterised by a mostly rocky, steep-sloped, and protected shoreline. In terms of sediment composition, the northern part of the study area is dominated by fines, whereas sands of mean grain size 0.2 < d(50) < 1 mm occur at most other locations. Posidonia oceanica meadows occupy many nearshore areas. Critical bed-shear stress was determined through tests in a unidirectional flume using 15 natural cores. Measured values ranged from 0.04 N/m(2) to 0.46 N/m2 and were expressed as a function of the logarithm of d50. Mud content had no observable effect on sediment cohesion, whereas a weak positive trend between critical shear stress and clay content was discerned. Autonomous benthic stations (Acoustic Doppler Current Profiler/Conductivity, Temperature, Depth/Turbidity/ALTUS Data Collection System) were deployed for periods of 2-3 months, and several suspension and erosion events were identified for significant wave heights exceeding 1.5 m. Grain-size trends showed that the protected BoM appeared to favour accumulation of sediments advected from the surrounding areas. The southern sector of the BoM was found to be the area with the highest sediment agitation and erosion rates but with weak transport trends. The northern sector, exposed to dominant SW waves, was shown to be the second most active area in terms of sediment dynamics, whereas the several Posidonia oceanica meadows and sheltered locations along the study area appeared to be possible sediment sinks. The latter included locations close to port installations and the western part. of the Cortiou area, known to have water quality issues, which, according to the present findings, are linked to low sediment mobility and/or dispersion of pollutants.

Ability of an acoustic echosounder (Altus) intended to quantify bed level variations in coastal areas to correlation studies with wave features and surficial sediment deposits Quantitative evaluation of erosion/deposition events of surficial sediments is crucial to understand the processes and establish sedimentary budgets in coastal areas. The ALTUS system was designed for more than a decade, to meet this objective. The possibilities offered by this altimeter (short-term or several months measurements) are particularly fitted for validation of sedimentary transport numerical models. The recent integration of a large capacity memory card has allowed to optimize the system. Indeed, it is well established that wave activity is one of the key parameters for the erosion and hence govern sediment transport in intertidal/shallow water areas. The ALTUS system, in addition to bottom level variation recordings, allows access to wave characteristics (wave height, significant period, bottom shear stress…) Correlations can be established between a recorded erosion and dynamics (waves, human impact…) In this paper it is particularly reported laboratory tests carried out in order to assess the ability of ALTUS to detect relatively "rough" changes of the physical characteristics of surficial sediment deposits., L'évaluation quantitative fine des séquences d'érosion/dépôt des sédiments superficiels est primordiale pour comprendre les processus et établir des bilans sédimentaires en domaine littoral. Le système ALTUS a été conçu, depuis plus d'une décennie, pour répondre à cet objectif. Les possibilités de mesures offertes par cet altimètre (mesures à court terme ou sur plusieurs mois) sont particulièrement appropriées pour permettre une validation des modèles numériques hydro-sédimentaires. L'intégration récente d'une carte mémoire de grande capacité a permis d'optimiser ce système. En effet, il est bien établi que la houle est un paramètre clé pour la reprise des sédiments en zones intertidales, gouvernant ainsi le transport sédimentaire. Le système ALTUS, outre l'enregistrement du suivi altimétrique, permet d'accéder aux caractéristiques de vagues (hauteur, période significative, frottement sur le fond…) Des corrélations peuvent ainsi être établies entre une érosion constatée et la dynamique du milieu (vagues, impacts anthropiques…) Dans ce papier il est également fait état de tests en laboratoire réalisés dans le but d'évaluer la capacité d'ALTUS à détecter une modification relativement "brutale" des caractéristiques physiques du sédiment des dépôts superficiels., Bassoullet Philippe, Verney Romaric, Kervella Youen, Kervella Stéphane, Jestin Hervé, Voineson Guillaume. Utilisation d’un altimètre (ALTUS) destiné à la quantification des dépôt/érosion en domaine littoral pour l'étude des corrélations avec les caractéristiques de vagues et les interfaces de dépôt.. In: 31èmes Journées de l’Hydraulique de la Société Hydrotechnique de France. Morphodynamique et gestion des sédiments dans les estuaires, les baies et les deltas. 22 et 23 SEPTEMBRE 2009. 2009.

The objective of this study is to propose an original approach to the analysis of the formation of intertidal rhythmites, their preservation, and the evaluation of sedimentation rates on estuarine mudflats. Three mudflats, one from each of three estuaries, were analysed using a combination of long-term (>a year), high-frequency (1 burst/10 or 20 min), and high-resolution (0.2 cm) altimeter datasets and X-ray images of sediment cores collected during topographic surveys. The results highlight the roles played by sediment supply, hydrodynamics, and morphology of the lower estuaries on the sedimentation rhythms. While the sediment-starved Medway estuary (Kent, UK) remains stable at different time-scales, the annual sedimentation rates on the sheltered Authie mudflat (Pas-de-Calais, France) and the open Seine mudflat (Normandy, France) are relatively high at 18 and 15 cm y(-1), rrespectively. On the Authie mudflat, sedimentation rhythms correspond to the semi-lunar cycle, with a good correlation between tidal range and deposit thickness. Sedimentation occurs at the beginning of the recovery (mean value of 0.25 cm per semi-diurnal cycle), and is not disturbed by wind-induced waves. In the lower Seine estuary, semi-diurnal mechanisms of deposition occur mainly when the mudflat is covered by a minimal water height (tidal range threshold value=7.1 m). Sedimentation rhythms are discontinuous and deposition occurs only during the highest spring tides. Mean deposit thickness is 0.6 cm per semi-diurnal cycle, controlled by the turbidity maximum and the long high tide slack (2-3 h). The fluid mud is sensitive to wind-waves in this open funnel-sbaped estuary, which undergoes wind-induced erosion (0.2 to 2 cm) about 10 times per year. Comparison of altimeter datasets and lithology of the sediments cored at the same points provided improved understanding of the sediment record rhythms and the sedimentation events. On the sediment-starved Medway mudflat, the result of sediment processes is a single superficial lamina. The elementary deposit in the Authie bay corresponds to a semi-lunar-linked layer, typical of sheltered environments. On the lower Seine mudflat, sedimentation rhythms are linked to the highest spring tides (i.e., the lunar cycle), resulting from increased sediment availability related to the high suspended matter concentration in the turbidity maximum. These results underline the complex response of intertidal mudflats to hydrodynamics and sediment supply conditions, from the semi-diurnal to the annual scales. (C) 2007 Elsevier B.V. All rights reserved.

Semi-diurnal and fortnightly surveys were carried out to quantify the effects of wind- and navigation-induced high-energy events on bed sediments above intertidal mudflats. The mudflats are located in the upper fluvial part (Oissel mudflat) and at the mouth (Vasiere Nord mudflat) of the macrotidal Seine estuary. Instantaneous flow velocities and mudflat bed elevation were measured at a high frequency and high resolution with an acoustic doppler velocimeter (ADV) and an ALTUS altimeter, respectively. Suspended particulate matter concentrations were estimated by calibrating the ADV acoustic backscattered intensity with bed sediments collected at the study sites. Turbulent bed shear stress values were estimated by the turbulent kinetic energy method, using velocity variances filtered from the wave contribution. Wave shear stress and maximum wave-current shear stress values were calculated with the wave-current interaction (WCI) model, which is based on the bed roughness length, wave orbital velocities and the wave period (T-S). In the fluvial part of the estuary, boat passages occurred unevenly during the surveys and were characterized by long waves (T-S > 50 s) induced by the drawdown effect and by short boat-waves (T-S < 10 s). Boat waves generated large bottom shear stress values of 0.5 Nm(-2) for 2-5 min periods and, in burst of several seconds, larger bottom shear stress values up to 1 N m(-2). At the mouth of the estuary, west south-west wind events generated short waves (T-S < 10s) of H-S values ranging from 0.1 to 0.3 m. In shallow-water environment (water depth < 1.5 m), these waves produced bottom shear stress values between 1 and 2 N m(-2). Wave-current shear stress values are one order of magnitude larger than the current-induced shear stress and indicate that navigation and wind are the dominant hydrodynamic forcing parameters above the two mudflats. Bed elevation and SPM concentration time series showed that these high energy events induced erosion processes of up to several centimetres. Critical erosion shear stress (tau(ce)) values were determined from the SPM concentration and bed elevation measurements. Rough tau(ce) values were found above 0.2 N m(-2) for the Oissel mudflat and about 1 N m(-2) for the Vasiere Nord mudflat. These results demonstrate the advantages of combining the measurement of instantaneous velocity and bed elevation to determine in situ the erosion and deposition processes as a function of bottom shear stress variations. (c) 2006 Elsevier Ltd. All rights reserved.

International audience; Innovative flume experiments were conducted in a recirculating straight flume. Zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth. The aims of this study were: (i) to quantify the combined effects of leaf flexibility and development characteristics of Zostera noltei canopies on their interaction with hydrodynamics; and (ii) to quantify the role of Zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses. Velocity within the canopy was significantly damped. The attenuation in velocity ranged from 34 to 87% compared with bare sediments and was associated with a density threshold resulting from the flow-induced canopy reconfiguration. The reduction in flow was higher in dense canopies at higher velocities than in less dense canopies, in which the reduction in flow was greater at low velocities. These contrasted results can be explained by competition between a rough-wall boundary layer caused by the bed and a shear layer caused by the canopy. The velocity attenuation was associated with a two to three-fold increase in bottom shear stress compared with unvegetated sediment. Despite the increase in near-bed turbulence, protection of the sediment against erosion increased under a fully developed meadow, while sediment properties were found to be the main factor controlling erosion in a less developed meadow. Deposition fluxes were higher on the vegetated bed than on bare sediments, and these fluxes increased with leaf density. Fewer freshly deposited sediments were resuspended in vegetated beds, resulting in an increase in net sediment deposition with meadow growth. However, in the case of a very high leaf area index, sediment was mostly deposited on leaves, which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow.

The aim of this study is to analyse and to quantify the fine particles transfer (i.e. 7.1 m). Then, deposition of SPM from the turbidity maximum becomes important, allowing fine particles to settle on the mudflat, where the current velocities are low (< 0.4 m s− 1). On tides of lower range (< 7.1 m) bed elevation levels suggest that erosion of the mudflat surface dominates, however dewatering processes also occur in the soft mud deposits resulting in compaction. During periods of wave activity and periods of increasing river discharge, the mudflat surface experiences rapid erosion. These periods of erosion (sudden or progressive) imply a transfer of fine-grained material from the mudflat surface back into the TM. The long term (∼ 22 months), high frequency (1 measurement every 10 min) and high resolution (0.6 mm) dataset obtained from the altimeter allows quantification of the volume of material exchanged between the intertidal area and the estuarine standing stock of SPM. The maximum of deposition on the studied mudflat during the study was of 27 cm, with highest deposition occurring after the periods of highest river flow and implying that between 25 and 40% of the maximum estimated mass of SPM held within the estuarine TM (300,000–500,000 T) had been temporarily deposited on the mudflat. Likewise following periods of resuspension by wave activity, bed elevations reduced by an order of ∼ 1 cm, suggesting a release of between 1–1.5% of the TM load from the mudflat back into suspension. The study shows that lateral exchanges of sediment between the intertidal reaches of the estuary and the water column are an important part of the sediment budget, and that the supply of sediment available for deposition (i.e. the TM) has a causal relationship with sedimentation rates experienced by intertidal mudflats.

Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (

In order to understand the hydrodynamic parameters that control the fluvial sediment dynamics on an intertidal mudflat located in a sheltered zone in the upper part (fluvial part) of the macrotidal Seine estuary (France), a two-year field study of high-frequency field measurements was carried out. The bed-level evolution of the mudflat surface was measured from the semi-diurnal period to annual time scales using a high-resolution altimeter. The data showed that the sedimentary patterns on the mudflat were mainly controlled by river flows and tides. During high river flows in winter, sedimentation dominated; suspended particulate matter concentrations were higher, submersion was constant and at semi-diurnal scale, sedimentation duration was more important than erosion due to an asymmetrical tide. By contrast during low river flows in summer, erosion dominated mainly as a result of immersion/emersion of tidal flats during semi-diurnal cycle. From this annual sedimentation–erosion cycle we identify a temporary storage of 10–30% of the fine-grained ( River-related sediment fluxes were estimated from the measurement of fine-grained sedimentation zones in the fluvial part of the estuary. The erosion/sedimentation processes were perennial, and the amounts of contributing sediments were directly related to the solid river load. Our results indicate that mudflats in the fluvial part of the Seine estuary play an important role in the downstream transfer of fine-grained suspended particulate matter (SPM) towards the turbidity maximum and the Rouen docks particularly during low river flows, when roughly 30–50% of the SPM originates from the eroded intertidal flats.