Orateur
Description
Bedload, the coarser material transported near the bed by turbulent flow in stream channels, has major consequences for public safety, and environmental sustainability. Size segregation is responsible for our limited ability to predict sediment flux and river morphology, hence to mitigate risks, especially in mountains where steep slopes drive an intense transport of a wide range of grain sizes.
Specific experiments [1] were carried out in a 10% steep, narrow channel, with a constant turbulent flow in two stages. In the first stage, coarse spherical beads of diameter 5 mm were introduced at a constant feed rate to obtain a sediment bed in one-size-equilibrium (constant flow rate and slope; sediment outflux equal to influx). In the second stage, finer beads of different diameters (from 0.7 mm to 4mm) were introduced at different feed rates, the coarse feed rate remaining constant. The objective was to study the influence of (1) the grain size ratio (coarse to fine) and (2) the percentage of the fine feed rate in the total feed. Depending on these parameters, the slope of the bed evolved eventually reaching a new two-size equilibrium value either larger (aggradation) or smaller (degradation) than the one-size slope [1].
Each experiment was recorded at high temporal image frequency allowing further insight at the grain scale. Particle tracking algorithms based on a continuous minimisation energy method [2] were developed to detect the coarse beads, and to track their trajectories. We will analyse herein the depth profiles of particle streamwise velocity, concentration and sediment transport rate of the coarse beads once the two-size equilibrium is reached. Depending on the grain size ratio and the percentage of the finer feed rate in the total feed, a variable thickness layer of fine grains on top of which coarse grains moved was observed. For the same constant coarse sediment rate, coarse grains moved either in concentrated low velocity clusters or individually at higher velocity.
Better understanding of bedload size segregation at the grain scale should permit upscaling to river morphology modelling.
- Dudill A, Lafaye de Micheaux H, Frey P, Church M. 2018. Introducing finer grains into bedload: The transition to a new equilibrium. Journal of Geophysical Research: Earth Surface 123(10): 2602-2619. https://doi.org/10.1029/2018JF004847
- Frey, P., Ducottet, C., 2025. Particle Tracking with Continuous Energy Minimization for the Study of Segregation in Bedload Transport. Experiments in Fluids 66, 150. https://doi.org/10.1007/s00348-025-04072-3