Influence of sand supply and grain size on upper regime bedforms

Notwithstanding the large number of studies on bedforms such as dunes and
antidunes, performing quantitative predictions of bedform type and
geometry remains an open problem. Here we present the results of
laboratory experiments specifically designed to study how sediment supply
and caliber may impact equilibrium bedform type and geometry in the upper
regime. Experiments were performed in a sediment feed flume with flow
rates varying between 5 l/s and 30 l/s, sand supply rates varying between
0.6 kg/min and 20 kg/min, and uniform and non-uniform sediment grain sizes
with geometric mean diameter varying between 0.22 mm and 0.87 mm. The
analysis of the experimental data and the comparison with datasets
available in the literature revealed that the ratio of the volume
transport of sediment to the volume transport of water Qs/Qw plays a prime
control on the equilibrium bed configuration. The equilibrium bed
configuration transitions from washed out dunes, to downstream migrating
antidunes for Qs/Qw between 0.0003 and 0.0007. For values of Qs/Qw greater
than those typical of the downstream migrating antidunes, the bedform
wavelength increases. Equilibrium bed configuration for fine sands is
characterized by upstream migrating antidunes or cyclic steps, and
significant suspended sand. At these high values of Qs/Qw the equilibrium
bed for coarse sands is plane with bedload transport in sheet flow mode.
Standing waves form at the transition between downstream migrating
antidunes and bed configurations with upstream migrating bedforms or
bedload transport in sheet flow mode.