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.