  We performed molecular dynamics simulations of granular beds driven by a
model hydrodynamic shear flow to elucidate general grain-scale mechanisms that
determine the onset and cessation of sediment transport. By varying the Shields
number (the nondimensional shear stress at the top of the bed) and particle
Reynolds number (the ratio of particle inertia to viscous damping), we explore
how variations of the fluid flow rate, particle inertia, and fluid viscosity
affect the onset and cessation of bed motion. For low to moderate particle
Reynolds numbers, a critical boundary separates mobile and static states.
Transition times between these states diverge as this boundary is approached
both from above and below. At high particle Reynolds number, inertial effects
become dominant, and particle motion can be sustained well below flow rates at
which mobilization of a static bed occurs. We also find that the onset of bed
motion (for both low and high particle Reynolds numbers) is described by
Weibullian weakest-link statistics, and thus is crucially dependent on the
packing structure of the granular bed, even deep beneath the surface.
