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A three-dimensional coupled hydrodynamic and morphodynamic numerical model was developed to analyze sediment dynamics in the Rance estuary, in response to the tidal power station (TPS) built near the mouth in the 1960s. The Rance estuary is a relatively small low-discharge steep-sided ria , located along the Brittany coast in northern France, with a maximum spring tidal range of 13.5m. Taking advantage of this significant tidal regime, the first and currently the second largest operational tidal power station in the world was built at the estuary's mouth, with peak output capacity of 240MW. After calibration and validation of the model for present-day conditions, suspended sediment concentration (SSC) and bed level evolution were evaluated at tidal and fortnightly scales for different scenarios, with and without TPS. Peak SSC are reached during spring tides and specifically during the estuary's infilling (flood) stage where both turbines and sluice gates are open. Unbalanced with sediment transport during ebb, sediments are accumulated in the main channel of the upper estuary. Under natural tidal forcing (i.e., without TPS), simulations show that an estuarine turbidity maximum forms in the upper estuary, and sediment deposition is significant. However, sedimentation rates are two times lower than those observed in the presence of the TPS. A possible alternative for reducing sediment accumulation in the upper estuary would be the opening of sluice gates simultaneously with the turbines during falling tide, to enhance ebb currents that would allow particles transport towards the estuary's downstream. [ABSTRACT FROM AUTHOR]

Dune dynamics and associated sedimentary fluxes have been estimated over a period of a week in the river Seine characterised by limited sediment supply. Distributed into two corridors, typical medium and small dunes associated with fluvial environments have been characterised from multibeam data acquired on four dates, 23-29 March 2021, after a limited peak flood. Dune morphometric parameters have been quantified on cross-dune 1 m spacing of bed elevation profiles and dune migration rates have been calculated by analysing successive multibeam acquisitions. Evidence of limited sediment supply conditions have been recognised from areas uncovered by mobile sediment in the troughs between medium dunes. Under such conditions, bedload transport associated with dune migration has been estimated using three methods: differential bathymetry, dune tracking and the Van Rijn empirical formula using a correction factor accounting for limited sediment supply conditions. The three methods provide similar bedload fluxes (less than an order of magnitude difference), and the small differences are related to input data uncertainties and the capability of these methods to capture sediment transport processes, such as sediment bypassing dunes. Based on field observations, this paper brings new constrains to the relationships between dune morphological parameters, migration rates and sedimentary fluxes and environmental parameters (sediment grain size, flow velocity and water depth) under limited supply conditions in a real river. • Dune morphologies and shapes are constrained by sediment availability • Limited supply of sediment restricts bedload transport associated with dune migration • Differential bathymetry captures a portion of suspended bedload bypassing dunes [ABSTRACT FROM AUTHOR]

Over the last two centuries, rivers worldwide have been affected by human interventions, including dams, river training, and gravel mining. Such actions have often involved river fragmentation and sediment starvation. In this regard, gravel augmentation is an increasingly common restoration practice for mitigating sediment starvation in gravel-bed rivers. However, uncertainties remain on how to better implement and design such operations. This is the case for the Rhône River at Péage-de-Roussillon, France, where 6885 m3 of gravels were augmented in 2017. This work raised some concerns from river managers about the potential threat posed to a downstream reservoir from the arrival of the sediment, and explains why they were interested in the timeframe of sediment propagation, and travel distances and velocities of the augmented sediment. To answer these questions, we propose a modelling framework for simulating the long-term downstream propagation of a pulse of augmented sediment, with this framework being based on a combination of particle tracking data collected in the field and bedload transport capacity estimates. This workflow allowed us to successfully model propagation of gravels along the study reach, and provided useful information on how the sediment wave would behave over the long term. We believe that the methodology proposed in this study has much potential for exploring and investigating the kinematics of sediment wave propagation in gravel-bed rivers. [ABSTRACT FROM AUTHOR]

Abstract: Soil erosion is one of the major drivers of landscape evolution in Western Europe. However, depending on the land use characteristics and on the geological and topographical settings, miscellaneous forms of erosion may lead to a very diverse morphological evolution. A key element is the connection between the hillslope and the river. In this context, the objective of this study is to investigate to what extent eroded soil particles contribute to river sediment exports for the major French basins. The hillslope erosion rates are deduced from a distributed pan-European assessment of hillslope, rill, and interrill erosion by water, which was quantified by interpolation of erosion plot data as a function of topographical, land use, and pedological parameters. Sediment fluxes were mainly assessed from the French river quality database using an improved rating curve approach from the daily discharge data (IRCA), which allows estimation of the mean annual suspended sediment load. A similar approach based on the same datasets was used to calculate the mean annual dissolved load over the same period. The resulting mean annual suspended sediment loads show that French rivers export approximately 50 Mt of sediments per year to the seas, including ca. 17 Mt as particulate matter, corresponding to a specific sediment yield of ca. 0.4tha−1 yr−1. No relationship was found between the mass of the exported sediment and the size of the drainage basins, but large differences were observed between the different basins. For the Seine river basin and the Rhône river basin, the estimated average gross erosion rates are comparable, with ca. 1.14tha−1 yr−1 for the Rhone and ca. 1.80tha−1 yr−1 for the Seine. In contrast, the Rhône exports ca. 10 times more than the Seine River. The ratio of gross erosion to sediment exports calculated in this work ranges from 6.8% for the Seine to more than 100% for the Rhône. To explain these variations, we propose the use of indices that represent the landscape patchiness and connectivity. [Copyright &y& Elsevier]

Abstract: Various transformation mechanisms can generate turbidity currents from subaqueous debris flows. Different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits, but the two approaches have not generally been linked. This has made the genetic interpretation and comparison of deposits difficult. In this paper a generic classification scheme of debrite–turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments. Five different flow types (called A–E herein) and their resulting deposits are detailed, but they are all part of a continuous spectrum, and a mixture of types is likely to be found in the field. Type A flows are strong, dense debris flows that undergo little transformation. Their deposit will be a debrite overlain by a thin turbidite, which is separated from it by a clear grain size break. Type B flows are weaker and can develop waves at the debris flow-turbidity current interface. The deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type A flows. For type C flows, the interfacial waves will grow so much that the debris flow disintegrates into separate parts. The deposit will consist of debrite lenses encased in a turbidite. Type D flows will undergo even more mixing than type C flows so that the debrite parts will be mixed. Their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed. Type E flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration. The deposit will be largely turbiditic. The flow types and deposits will be illustrated with some examples from two field areas: the Polish Carpathians and the French Maritime Alps. [Copyright &y& Elsevier]

Temporal and spatial variability of the Abra alba–Pectinaria koreni and Macoma balthica communities was examined in the northern part of the Seine estuary (North Channel) over different space and time scales in order to assess the role that the hydrologic regime and/or anthropogenic influences play in defining benthic communities over time. Sediment in the North Channel displayed strong spatial and temporal variability, sustained by intense sediment transport episodes. Total macrobenthic abundances ranged widely on the course of the year and there was no evidence of a seasonal signal for the density fluctuations, whatever the spatial scale considered. The bio-sedimentary dynamics can be divided into two periods: the first corresponds to the high flow rate period (January–May) during which fauna is influenced by fine silt/clay deposition, and the second to the low flow rate period (June–December) during which sandy deposits prevail. Despite the absence of significant correlations between sediment composition and abundance, episodes of sediment transport seem to be an important structuring mechanism in the Seine estuary. As a consequence, the faunal composition varied throughout the year. The winter and spring fauna, characterised by species living on muddy fine-sands or muds, were enriched during the summer and autumn by species living in clean fine sand, such as Donax vittatus, Nephtys cirrosa or Spio decoratus, mainly represented by adult individuals. Secondary settlement of drifters may explain the rapid structuration of assemblages a few days after the sandy deposits. Our results suggest the importance of the bentho-pelagic coupling, primarily induced by the sedimentary instability, on the macrobenthic fauna dynamics. The intra-annual variability of assemblages at the mouth of the Seine river and the silted situation of the North Channel might simply be the result of the silting up and alteration of the inner estuary, generated by several decades of man-made modifications and natural processes. [ABSTRACT FROM AUTHOR]

Forty-nine sediments were sampled along the coast of the Seine bay: 19 from the Port-en-Bessin harbor and 30 from the Bay des Veys. Metallic and polychlorinated biphenyl contaminations were estimated in harbor sediments by their comparison to the OSPAR Convention guidelines. Sediments in Bay des Veys were considered as nonpolluted materials and metal concentrations were chosen as regional reference data to calculate anthropogenic factor and gave human contributions of each metal in harbor sediments. The source of the principal pollutants was principally associated with the careening activities and the use of antifouling paints. A statistical study of data showed strong relationships between metals, organic matter, and fine-grained fraction (<50 μm) of sediments, assuming mobility of metals to water column in case of removal. [ABSTRACT FROM AUTHOR]

The greatest change in particle roundness, sphericity, and size in most fluvial systems generally occurs in the initial first few kilometers of sediment transport. Observations in natural settings and laboratory experiments have shown that changes, such as increased grain rounding and decreased size due to particle breakage, wear, and attrition, tend to be progressive along the dispersal path. In marked contrast, this study records highly variable and poorly developed particle-texture trends downslope along short (<5 km) channels that extend directly from a high-relief carbonate source area down to the shore of Lake Annecy in the western Alps, SE France. The observed inconsistent patterns are measured in both a region-wide analysis using all collected samples and also in a more specific survey of samples in six different channels incised on the steep, topographically irregular mountain flanks that border the lake. The major factor responsible for the irregular downslope patterns of roundness, sphericity, and size in the Annecy study area is the lateral introduction of sediment into the fluvial channels along dispersal paths. Episodic failure of mechanically unstable sediments and their transport down steep valley walls bring less-abraded sediment of variable shape and size along the dispersal paths. The valley wall sediment influx factor warrants further examination to better interpret sedimentation processes in dissected, high-gradient terrains that border coastal areas. [ABSTRACT FROM AUTHOR]

For three years (2001-2003), sediment samples were extracted from about 100 silt traps (STs) spread out all over the combined sewer network of Paris. These STs, whose volume varied from 21 to 325m³, were cleaned out as soon as their filling capacities were reached. All these sediment samples were analysed for physicochemical parameters (pH, organic matter (OM) content, grain size distribution), total hydrocarbons (THs), 16 polycyclic aromatic hydrocarbons (PAHs) selected from the priority list of the USEPA, and heavy metals (Al, Ag, As, Cd, Cu, Cr, Sn, Fe, Mn, Hg, Ni, Pb, Zn). For each silt trap, six sediment samples were extracted before the clean out procedure: three samples were extracted from the sediment surface (5-10 cm depth) and three other samples were extracted from a deeper sediment layer (approximately at 1m depth). The location of these sampling points allowed us to take into account the possible spatial fluctuation of pollutant loads in each ST. The first results showed that there were some important inter-site variations of pollutant contents. These variations have to be taken into account by the sewer manager for the fate of the ST sediments. Therefore, we decided to assess the grain size distribution of some pollutants. OM, heavy metals and PAHs have been investigated on the five grain size fractions (>20 mm, 8-20 mm, 0.5-8 mm, 50-500 µm, <50 µm) for 9 STs, which have been selected on their heavy metal content basis. This work aims at understanding the distribution of the pollutant contents and at improving the knowledge of the ST sediment pollution. [ABSTRACT FROM AUTHOR]

• Low and high bar modes originate from water discharge variations. • In turn, this induces complex compound bar formation. • Sediment sorting patterns differ between unit and compound bar patterns. • Sediment sorting does not modify bar morphodynamics if sediment mobility is high. • Floods decrease the degree of sediment sorting. Bars are large sediment deposits alternating with deeper areas that arise from alluvial river bed instability and forcing. The present study aimed at investigating the combined influence of flow and longitudinal width variations on the co-evolution between bar pattern and sediment sorting in a sandy-gravel river reach. To this goal, a fully non-linear 2D numerical model was developed to reproduce the morphodynamic behavior of bars in a reach of the Loire River consisting in a typical channel expansion/contraction. Numerical results showed that varying water discharge promoted a competition between low and high bar modes: i.e., from alternate to multiple bar patterns. Low bar modes were associated with coarse sediment over bar tops and fine sediment in pools, and this sorting pattern was inverted for higher bar modes. Surface sediment was coarser and the degree of sediment sorting was greater after periods of low than high flow. Due to high sediment mobility, sediment sorting did not significantly modify bar morphodynamics. [ABSTRACT FROM AUTHOR]

The shoreface is an important transitional zone between the beach and the inner shelf. Significant change on the shoreface occurs at long temporal time scales and over large spatial domains, making its study challenging. The shoreface's contribution to nearshore dynamics is also usually complex, acting as both a sediment source and a sediment sink, as well as facilitating or impeding sediment transport. To date, no universal agreement exists for the definition of the shoreface nor its exact spatial extent.When considering the longer term and larger-scale evolution of coastal systems, the morphodynamics of the shoreface appears to be a determining factor in coastal and shoreline evolution. Along high-energy coastal areas, for instance western Ireland and North-western France, dominated by geomorphological features such as embayments, pocket beaches, sea cliffs and rock outcrops, the geological background plays a major role in the control of the coastal morphodynamics. This geological control acts to limit accommodation space, sediment availability and longshore sediment transport thus helping constrain the shape of the shoreface and therefore its dynamics and evolution.The study of shoreface morphodynamics requires the combination of multiple datasets at variable temporal and spatial scales. In this study, the west coast of Ireland provides spatially extensive bathymetry (multi-beam) datasets to allow the study of shoreface morphology and extent. Furthermore, the existence of wave data from offshore buoys and reanalysis wave models, also allows the analysis of hydrodynamic behaviour (using SWAN modelling) and the identification of the seaward shoreface limit. The overall aim of the work is to advance our understanding of the morphodynamics of the shoreface in complex regions. [ABSTRACT FROM AUTHOR]

In the last two centuries, many areas have undergone a rather fast demographic, industrial andurban development. This intense land occupancy affected the quality of surface waters, whichbecome the receptacle of anthropogenic effluents from various origins. In this context, severalrivers in North-Eastern France were strongly modified (rectification of river bed, dambuilding) and received high amounts of industrial and domestic effluents due to paststeel-making activities installed nearby water resources. As a consequence of these pasteffluent inputs in the river, and although part of the settled material was dredged andremoved from the riverbeds, these often remain contaminated. During flood events, theremobilization of these riverbed sediments can strongly impact water and evensoil quality. In this context, there is consequently a clear need for predicting thepotential resuspension and transport of sediment in these heavily polluted riversystems. Hydromorphodynamic models are powerful tools for predicting the potential mobilizationand transport of sediment in river ecosystems. Recent studies even showed that they are ableto predict suspended sediment matter concentration in small river systems satisfyingly.However, modelling exercises often neglect suspended sediment properties (e.g.particle site distribution and density), even though such properties are known todirectly control the sediment particle dynamics in the water column during floodevents. This study has two objectives. On the one hand, it aims to further develop an existinghydromorphodynamic model based on the dynamic coupling of TELEMAC-3D (v7p1) andSISYPHE (v7p1) in order to enable an enhanced parameterization of the sediment grain-sizeand density distributions. On the other hand, it evaluates and discusses the added-value of thenew development for improving sediment transport and riverbed evolution predictions. Tothis end, we evaluate the sensitivity of the model to the parameterization of sedimentgrain-size distribution, sediment density and suspended sediment concentration at theupstream boundary. As a test case, the model is used to simulate a flood event in asmall-scale river, the Orne River in north-eastern France. The results show substantialdiscrepancies in bathymetry evolution depending on the model setup. Moreover,the sediment model based on an enhanced sediment grain-size distribution (10classes) and with distributed sediment density outperforms the model with only twosediment grain-size classes in terms of simulated suspended sediment concentration. [ABSTRACT FROM AUTHOR]