Flow of sand along the bed of rivers: laboratory and numerical 
experiments of turbulence and grain motion


It has been generally accepted that the flux of sand at the bed of 
rivers is primarily by the process of saltation, in which particles 
move in a series of ballistic trajectories between collisions with the 
bed. Tracking of particles using high-speed video reveals that 
saltation models do a poor job of predicting the full distribution of 
grain motions; many grains move more slowly and for shorter distances 
than by saltation. The video particle tracking results also show that 
the temporal variability of grain motion due to turbulence is large at 
all mean transport rates. Numerical results of bedload transport using 
large eddy simulation (LES) of turbulence coupled to a distinct 
element model (DEM) of grains reproduce the exponential-like 
distribution of sediment velocity. These numerical simulations suggest 
that penetration of turbulence structures into the bed plays a key 
role in grain entrainment and may be important in the growth and 
stability of ripples and dunes.