Fig. 2

Experimental particle and segregation dynamics. a Vertical velocity profile for small and large grains for the interval Δt = [0:20] min at the beginning of shearing at Shields number \(\tau _s^* = 4.1\tau _{cs}^*\). The elevations of the bed surface (z _s ) and transition to creep (z _c ) are indicated. Inset shows the horizontal streamwise velocity u _x profile for all grains and vertical velocity for large grains u _z,l in logarithmic scale for the bedload zone. The streamwise particle velocity measurements have error-bar (one standard deviation/mean) value ~0.3%, whereas vertical particle velocity measurements have error-bar (one standard deviation/mean) value ~1%. b Vertical mean-square displacement (MSD) for large grains as a function of time Δt. MSD shown for various depths of the granular bed, defined with a colorbar. The top and bottom red-dashed lines indicate the limiting behaviors of advection and diffusion, respectively. Boundary between bed load and creep is indicated. Note that near-surface grains are advective at short times; creeping grains show no change in MSD at short times indicating caged dynamics, but transition to diffusive behavior at longer times