River bedforms influence fluvial hydraulics by altering bed roughness.
With increasing flow velocity, subaqueous bedforms transition from flat
beds to ripples, dunes, and an Upper Stage Plane Bed. Although prior
research notes increased bedform height variation with flow strength and
rapid shifts between bed configurations, the latter remains
understudied. This study reanalyzes data from earlier experiments, and
reveals a bimodal distribution of dune heights emerges beyond a
transport stage of 18. Dune heights flicker between a low and high
alternative state, indicating critical transitions. Potentially
triggered by local sediment outbursts, these shifts lead to dune
formation before returning to an Upper Stage Plane Bed. This flickering
behavior challenges the adequacy of a single snapshot to capture the
system's state, impacting field measurements and experimental designs,
and questions the classical equilibrium equations. This study calls for
further research to understand and quantify flickering behavior in
sediment beds at high transport stages.