Fluctuations and time scales for bed-load sediment motion over a smooth bed

Results are presented for experiments of bed-load sediment transport over a plane, smooth bed. The smooth-bed configuration, though not adequate for mimicking natural streams, enables the effects of bed roughness to be filtered out, thus, highlighting the role of flow turbulence for particle dynamics. Sediments were individually tracked along their paths, measuring position and velocity of the individual grains. A number of analyses were then applied to the data: probability density function, auto-correlation, and spectra of the grain velocity. Several Lagrangian time scales of particle motion were obtained and compared to available data for the turbulent flow field to determine a phenomenological interpretation of the process.