The variability of antidune morphodynamics on steep slopes

Steep streams on rough beds are generally characterised by supercritical flow conditions under which antidunes can develop and migrate over time. In this paper, we present flume experiments that we conducted to investigate the variability of antidune geometry and migration celerity, a variability observed even under steady-state conditions. Quantifying this variability is important for river morphodynamics, hydraulics and paleohydraulics. We imposed moderate to intense bedload transport rates at the flume inlet to assess their effects on antidune morphodynamics for near-constant values of the mean bed slope. The bed elevation profile was monitored for each experiment with high spatial and temporal resolution. Upstream migrating antidunes were observed along most of the flume length. Considering single values for wavelength and celerity was not sufficient to describe the antidune behaviour in these experiments. By using spectral analysis, we identified the variability ranges of bedform shape and celerity. Interestingly, migration celerity increased with increasing antidune wavelength; the opposite trend was reported for dunes in other studies. Antidunes were more uniform and migrated faster for higher sediment feeding rates. Scaling the spectra made it possible to find a general dimensionless relationship between antidune wavelength and celerity. This framework provides a novel method for estimating the mean bedload transport rate in the presence of upstream migrating antidunes.