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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]

Submarine channels are the main conduits and intermediate stores for sediment transport into the deep sea, including organics, pollutants, and microplastics. Key drivers of morphological change in channels are upstream-migrating knickpoints whose initiation has typically been linked to episodic processes such as avulsion, bend cutoff, and tectonics. The initiation of knickpoints in submarine channels has never been described, and questions remain about their evolution. Sedimentary and flow processes enabling the maintenance of such features in non-lithified substrates are also poorly documented. Repeated high-resolution multibeam bathymetry between 2012 and 2018 in the Capbreton submarine canyon (southeastern Bay of Biscay, offshore France) demonstrates that knickpoints can initiate autogenically at meander bends over annual to multi-annual time scales. Partial channel clogging at tight bends is shown to predate the development of new knickpoints. We describe this initiation process and show a detailed morphological evolution of knickpoints over time. The gradients of knickpoint headwalls are sustained and can grow over time as they migrate through headward erosion. This morphology, associated plunge pools, and/or development of enhanced downstream erosion are linked herein to the formation and maintenance of hydraulic jumps. These insights of autogenically driven, temporally high-frequency knickpoints reveal that cut-and-fill cycles with depths of multiple meters can be the norm in submarine systems. [ABSTRACT FROM AUTHOR]

Carboniferous-Permian strata in basins within the Central Pangean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (Pangea assembly, ice-age collapse, megamonsoon inception). The voluminous (~1.5 km) succession of exclusively finegrained red beds that comprises the Permian Salagou Formation (Lodève Basin, France) has long been interpreted to record either lacustrine or fluvial deposition, primarily based on a local emphasis of subaqueous features in the upper ~25% of the section. In contrast, data presented here indicate that the lowermiddle Salagou Formation is dominated by up to 15-m-thick beds of internally massive red mudstone with abundant pedogenic features (microscale) and no evidence of channeling. Up-section, limited occurrences of ripple and hummocky cross-stratification, and mudcracks record the intermittent influence of shallow water, but with no channeling nor units with grain sizes exceeding coarse silt. These data suggest that the most parsimonious interpretation for the Salagou Formation involves eolian transport of the sediment and ultimate deposition as loess in shallow, ephemeral lacustrine environments. Provenance analyses of the Salagou Formation indicate coarse-grained protoliths and, together with geochemical proxies (chemical index of alteration [CIA] and tNa) that correspond respectively to a low degree of chemical weathering and a mean annual temperature of ~4 °C, suggest that silt generation in this case is most consistent with cold-weathering (glacial and associated periglacial) processes in the Variscan highlands. Together with previous studies that detailed voluminous Permian loess in western equatorial Pangea, this work shows a globally unique distribution of dust at low latitudes that can be linked either directly to glaciated alpine terranes or to reworked and deflated deposits of other types (e.g., fluvial outwash) where fine-grained material was originally generated from glacial grinding in alpine systems. These results further support a revised model for early Permian climate, in which extratropical ice sheets coexisted with a semiarid tropics that may have hosted significant ice at moderate elevation. [ABSTRACT FROM AUTHOR]

Abstract: The Fleuve Manche (English Channel) palaeoriver represents the largest river system that drained the European continent during Pleistocene lowstands, particularly during the Elsterian (Marine Isotope Stage 12) and Saalian Drenthe (MIS 6) glaciations when extensive ice advances into the North Sea area forced the central European rivers to flow southwards to the Bay of Biscay (NE Atlantic). Numerous long piston cores and high-resolution acoustic data from the northern Bay of Biscay have been correlated using a well-constrained chronostratigraphical framework, and for the first time allow the estimation of the discharge of the Fleuve Manche palaeoriver at the end of the last glacial period (i.e. Weichselian). Our results, discussed in the light of relationships between river loads and drainage area characteristics, strongly suggest that the Bay of Biscay collected sediment-laden meltwater from the Northern European lowlands and the southern part of the Fennoscandian ice sheet at the end of the last glacial period and particularly during the paroxysmal phase of the European deglaciation ca 20–18ka ago. This demonstrates that the Fleuve Manche drainage area at that time was equivalent to those dating from the major Elsterian and Saalian Drenthe glaciations. Based on our data, we propose a revision of the palaeogeographical and palaeoglaciological evolution of the North Sea area while showing that the British and Fennoscandian ice sheets entered the North Sea basin until ca 18ka initiating isostatic adjustment of the basin. This allowed the North Sea fluvial system to flow southwards through the Dover Strait. Finally, we discuss the Fleuve Manche freshwater discharge during the maximum European ice sheet retreat ca 18ka. The estimated discharge at this time appears important and sufficiently sustained to possibly contribute to the profound destabilisation of the thermohaline circulation recognised in the North Atlantic from ca 18 to 16ka. [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]

Abstract: As a part of the ANR-Forclim experiment, particle mass fluxes and sedimentation processes were investigated on the slope of Aquitanian margin of the Bay of Biscay, between the canyons of Cap-Breton and Cap-Ferret. Interface sediments were collected along a depth transect from 145 to 2000m; simultaneously a mooring line was deployed at the deepest station (WH, 2000m) with two traps (800 and 1700m) for a 16-month period (June 2006–November 2007). 210Pb activities of settling particles and of interface sediments were determined to study transport processes of particles. Sediment and mass accumulation rates, calculated from excess 210Pb profiles in the sediment column, show the expected decreasing trend with depth, as usually observed on margins. Mean particulate mass fluxes at 800 and 1700-m depth at site WH are, respectively, 27 and 70gm−2 a−1. The 210Pb budget points out events of temporary high lateral input of particles. The comparison of mass and 210Pb fluxes between the water column and the seabed indicates that lateral transport plays an important role in particle accumulation on the Aquitanian margin. Regarding the objectives of the ANR-Forclim program, which aims to improve significantly the interpretation of fossil foraminifera signals, as a proxy for hydrological changes in the North Atlantic ocean, these results highlight advection processes must be considered when interpreting fluxes of foraminifers on the Aquitanian margin. [Copyright &y& Elsevier]

Abstract: Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5mg/l. In Cap de Creus canyon head (located ∼14km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists. [Copyright &y& Elsevier]

Abstract: In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a hydrodynamic–sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1Mt during the study period. [Copyright &y& Elsevier]

The article describes a study that compares the concentration of suspended sediment in two small streams in France to discharges. Both suspended sediment and discharges were measured at the outlets on a yearly and monthly basis, with flood and non-flood periods taken into account. Data revealed that the relationship between suspended sediment concentration and discharge varied greatly, leading to the conclusion that cattle trampling and hydraulic erosion of stream banks are the two chief causes of sediment accumulating in stream catchments.

This paper presents continuous field experiments carried out during 4 years in a man-entry egg-shaped combined sewer in Lyon, France in order to contribute to the knowledge and the modelling of sediment accumulation and sediment removal by means of a Hydrass flushing gate. The 250 μm sediments are mainly mineral, and their physical and chemical characteristics appear as rather stable in time and space. Long-term sediment monitoring reveals: i) a regular asymptotic increase of both the sediment mass and the slope of its longitudinal profile, ii) a clear correlation between local sediment profile irregularities and sewer ancillaries but without significant influence on the global and long term accumulation. Simple sediment accumulation modelling shows: i) a good suitability of a three parameters conceptual model to reproduce asymptotic sediment volume accumulation, and ii) a good suitability of the Velikanov model to reproduce sediment profiles. Both models reproduce observations with an acceptable margin of uncertainty for operational management purposes but are very sensitive to input data and parameter values. The Hydrass flushing gate is efficient and it appeared that the mass of sediments moves downstream linearly with the number of flushes. [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]

The formation and long-term evolution of an estuarine bank of the Gironde mouth, SW France (the Saint-Georges Bank) is studied through the comparison of nine historical bathymetric maps (1677, 1723, 1825, 1874, 1892, 1920, 1.940, 1960, and 1992) and information obtained during a recent bathymetric survey (! 999). The data show that the initial formation of Saint-Georges Bank occurred at the end of the 17th or the beginning of the 18th century and it could be related to the other morphological changes which took place at the estuarine inlet and particularly to the gradual destruction of the downstream estuarine shoal of Grande Barre, which (a) changed the inshore hydrodynamic regime, (b) allowed large quantities of marine sediments to move further upstream along the coast and (c) made large quantities of sand-sized sediments available. The Saint Georges Bank growth during the past centuries can be differentiated into 3 stages: Stage 1 (1723–1892), which has been characterized by a vertical and lateral (down-stream) bank .growth (with an average rate of accretion of about 0.3 ⊗ 106 m³ yr-1); Stage II (1892–1960), which is characterized by a relatively slow development (with an average growth rate of about 0.15 ⊗ 106 m³ yr-1); and Stage III (1960–1999), which is characterized by a rapid bank growth (with an accretion rate of about 0.7 ⊗ 106 m³ yr-1) and a anticlockwise rotation of the crest. The morphological and sedimentological data (side-scan sonar and sub-bottom profiling data and surficial sediment samples) which were collected during the present investigation suggest that Saint Georges Bank is likely to be maintained by a complex hydrodynamic/sediment dynamic regime consisting of a clockwise residual tidal circulation, headland associated secondary flows and trapping of sediments in a zone contained between mutually evasive sediment transport paths. [ABSTRACT FROM AUTHOR]