Title: Armoring and morphology of a granular bed below the critical Shield's number in shear flow

Author (Talk): Benjamin Allen, Clark University


We investigate transient movement of a granular bed under shear fluid flow leading to armoring and bed morphology before the onset of steady state erosion. Our experiment is designed to directly visualize the particle movement inside the granular bed using index matching techniques and also allows the stress to be calculated from measurements of the torque exerted on the bed by the fluid in cone and plate geometry. Visual imaging allows us to examine the bead positions as we change the shear rate across the bed from a stationary state, no particles moving, to above the onset of steady-state erosion, a constant flux of particles. Starting with a freshly sedimented bed we find that below a small Shield's number $\tau*_t\approx .02$ there is no movement of the particles over the time scale of at least a week. Above $\tau*_t$ particles in the bed shift and there is a transitional period as the particles shift and resettle with an over-all lowering of the bed height by $8\%$ over the timescale of an hour with the majority of particle movement in the first 10 minutes. At low shear rates during this compaction transition there is no net transport of particles. As the shear rate rises particles on the top surface start to be transported from pocket to pocket in the bed as they search for a stable packing, and the time scale of the settling and packing extends longer than an hour although as before most of the compaction happens in the first 10 minutes. When the Shield's stress reaches $\tau*_c=0.19$ the particles continue to move across the top of the bed and creep inside as we reach steady state erosion.

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