The variability of antidune morphodynamics on steep slopes

Abstract

Steep streams on rough beds are generally characterised by supercritical flow condi-tions under which antidunes can develop and migrate over time. In this paper, wepresent flume experiments that we conducted to investigate the variability ofantidune 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 transportrates 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 foreach experiment with high spatial and temporal resolution. Upstream migrating anti-dunes were observed along most of the flume length. Considering single values forwavelength and celerity was not sufficient to describe the antidune behaviour inthese experiments. By using spectral analysis, we identified the variability ranges ofbedform shape and celerity. Interestingly, migration celerity increased with increasingantidune 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 relationshipbetween antidune wavelength and celerity. This framework provides a novel methodfor estimating the mean bedload transport rate in the presence of upstream migratingantidune.