Torrents Avalanches Grenoble Bedload sediment transport Discrete element

Torrents, Avalanches Grenoble Bedload sediment transport Discrete element modelling P. Frey, G. Marsan Séminaire Gestrans, Drôme, 5 November 2010

Bedload • Over a century of research (Du Boys 1879, Gilbert 1914) • Mostly from a fluid perspective – What is the solid flux given the fluid flux • What about the granular perspective ?

Bedload at the particle scale Slope : 10 %, D=6 mm, 20 beads/s

Microstructural study in a 2 D flume ! Turbulent and supercritical water flow 2 D live-bed Image analysis to access each bead The idea: simplest representation of bed load on steep slopes to gain access to variables of each bead Böhm

Flume schematic First, equilibrium is established

Velocity and number-density profiles Red : moving , yellow: non-moving S 10%, transport concentration 3%, / c=1. 7

Velocity and concentration profiles dry granular flow Data courtesy, P. Richard, Univ. of Rennes

Principle of DEM Calculation cycle in PFC 3 D

Contact model Normal direction Shear direction

Hydrodynamic forces • Buoyancy • Drag • Lift (necessary)

« All or nothing » Velocity profile y u(m/s) Too simple !

Velocity profile

Bedline 0

Numerical results

Conclusion • • Promising method Needs extensive sensitivity analysis Well adapted for rolling Fluid/coupling needs refining for saltation

Perspectives • Segregation S=10%, 1 small bead every two second, about 1% of the input sediment rate t=0 in (Calantoni and Thaxton JGR-Oceans 2008) at t=36 s, after 6 wave periods

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