Entrainment of particles from a granular bed in a turbulent shear flow: experiments and stochastic modeling

We study experimentally and theoretically the incipient motion of individual grains from a granular bed sheared by a weak turbulent fluid flow. This problem underlies numerous applications in Earth sciences since sediment transport in flowing water is a major source of landscape instability. Our experiments indicate that individual particles remain stationary on the bed over timescales characterized by statistical distributions. We model these "resting time" distributions by considering particle entrainment into motion as a first passage time problem, where the fluctuating fluid forces act to tip particles beyond a point where gravity can restore their stability on the granular bed. The solution of this first passage time problem is obtained by numerical and approximate analytical methods, and the modeled distributions show close correspondence to the experimental data.