Bimodality in subaqueous dune height suggests flickering behavior at high flow.

Abstract

River bedforms influence fluvial hydraulics by altering bed roughness. With increasing flow velocity, the sand-bedded river 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. Here, we reveal flickering between low and high dune heights for transport stages exceeding 18, based on data from earlier experiments and a complementary experiment. Above this transport stage, the second mode in the dune height distributions becomes increasingly distinctive, suggesting a critical transition. The emergence of the second mode is potentially triggered by temporal changes in suspended sediment concentration impacting turbulence, or might result from dune kinematics enabling larger dunes to grow and persist longer. This flickering behavior challenges the adequacy of a single snapshot to capture the system's bed geometry, impacting field measurements and experimental designs, and questions a classical equilibrium equation in geomorphology. Our study calls for further research to understand and quantify flickering behavior in sediment beds at high transport stages.