Search

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]

A regional 2DH hydrodynamic model is used to estimate the tidal stream resource of a site located in a macrotidal environment with extreme tidal velocities. The study site is the Alderney Race (Raz Blanchard in French) which is a straight located in the English Channel between the Alderney Island and La Hague cape (France). The estimation of the resource is used to build two realistic tidal energy extraction scenarios consisting in placing a 290 MW tidal turbine array in two different areas. Then, we analyze the impact of turbines on the hydrodynamics and the sediment transport. The hydrodynamic perturbation is restricted to the vicinity of the array where the mean current velocity reduction reaches 0.3 m/s locally (corresponding to 15% of the baseline velocity). Focusing on the variable driving the bedload (the critical erosion threshold exceedance), we confirm that tidal energy extraction tends to reduce the bedload rate and to deflect the sediment fluxes. Our simulations with a simple model for the suspended sediment transport suggest that tidal energy extraction has a significant effect on the area of deposition of the particles transiting through the tidal farm. For the baseline, the sediment particles transiting through the tidal farm deposit mostly in the eastern part of the English Channel. Depending on the location of the tidal farm, the sediment mass balance between the eastern and the western part of the English Channel changes drastically. [ABSTRACT FROM AUTHOR]

The vertical distribution of longshore sediment transport was measured on sandy macrotidal barred beaches of Northern France under low to moderate wave energy conditions, using streamer traps collecting sand at different elevations above the bed. Longshore sediment flux measurements and wave and current data were collected at several locations along cross-shore transects in order to analyse the relation between the vertical patterns of suspended sand transport and the forcing hydrodynamics across the bar-trough topography. Suspended sediment fluxes generally showed a typical upward decrease in longshore transport, revealing that near-bed transport represents the major transport mode on these macrotidal beaches, notably over the bars where the highest rates of sediment transport were measured. Conversely to previous studies in which the shape of the vertical distribution was either independent of the location in the surf zone or which showed an increase in suspended transport over intertidal bars, our results revealed a strong upward decrease in longshore sediment transport over bars, especially under higher wave energy conditions, whereas more uniform vertical distributions were observed in troughs. These differences in the vertical distribution of longshore sediment flux can be explained by spatially variable wave breaking and surf zone processes over the bar-trough topography and by the distribution of small-scale bedforms across the intertidal zone, which influence sand resuspension processes and control the shape of the vertical profiles of sediment transport. Larger longshore sediment fluxes over bars can be explained by the action of breaking waves that are responsible for increased sediment remobilization, but the dominance of spilling breakers on the studied beaches likely results in significant turbulence dissipation, leading to limited sand resuspension in the water column. Comparatively, landward propagating surf bores over bars favour an increase in sediment concentration at higher elevations above the bottom in the adjacent trough. [ABSTRACT FROM AUTHOR]

Wind waves breaking at an angle with the shoreline force the drifting of littoral sediments, which is known for contributing to the formation and growth of barrier spits. Intriguingly, increased rates of longshore wave power have also been associated with the erosion of some barrier spits on the updrift margin of tidal inlets. Therefore, a numerical experiment was designed and is presented here, which investigates the possible links between the longshore wave power and the shortening of these elongated coastal barriers. Based on a process-based model, the experiment provides new insights into the forces at play in the redistribution of sediments between a sandspit and its adjacent inlet, respectively the Cap Ferret and the Bay of Arcachon's tidal inlet, in SW France. More particularly, model scenarios were defined that show how combined waves and tide create gradients of residual sediment transport responsible for a sediment deficit at the spit – inlet boundary. The deficit was also found to deepen with increasing longshore wave energy, as if the transfer of sediment from the spit to inlet shoals was accelerated. This physically explains the previously observed retreat of the spit's distal end during periods dominated by the positive phase of North Atlantic Oscillation (NAO) in winter. Indeed, according to model results, higher and/or more oblique waves associated with the positive phase of the NAO are expected to increase the transfer and storage of the drifting sediments to and by the inlet shoals, and this at the expense of the spit. While these conclusions remain valid, we noticed that the sensitivity of model results to the bottom friction enhanced the importance of accurately representing the spatio-temporal distribution of bed roughness when investigating the morphodynamic interactions between real-world tidal inlets and their margins. • Residual sediment transport is modelled at a mixed energy tidal inlet. • Waves with tides create transport gradients near the inlet's updrift spit extremity. • Larger or more oblique waves deepen this gradient and reinforce the sand deficit. • Physically sounded explanation to enhanced spit erosion during NAO + winters. • Bed roughness models need to be improved for 2DH inlet-spit morphodynamic modelling. [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]