Bimodality in subaqueous dune height suggests critical transition at high flow

Abstract River bedforms influence fluvial hydraulics by altering bed roughness. With increasing flow velocity, the riverbed transitions from a flat bed 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, revealing a bimodal distribution of dune heights 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 a washed-out state. 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 in geomorphology. This study calls for further research to understand and quantify flickering behavior in sediment beds at high transport stages.