Supercritical flows: the sedimentology of the illustrious upper flow-regime

Bedforms are the morphological patterns on the sediment bed originating from coherent structures in fluid flows. The classical grouping of bedforms into the lower and upper regimes follows the transition from subcritical to supercritical flow as the Froude number passes a critical value. The revival of academic interest in supercritical flows and their products over the past two decades is attributed to the most recent addition to the supercritical palette: cyclic steps. This alternating pattern of subcritical and supercritical flow results from the flow overstepping the boundary between stable and unstable behaviour as predicted by the Vedernikov number. Waves at the upper flow boundary are key to understanding the transitions between the stable subcritical (ripples and dunes), stable supercritical (antidunes) and unstable supercritical (cyclic steps) regimes. In this talk we review our knowledge on the sedimentological aspects of supercritical flows and explore which questions remain to be answered.