Predicting the generation of ripples downstream of bluff obstacles, such as sediment mounds, is critical to estimating current speed from bedforms and predicting interfacial exchanges of solutes and fine particles. Herein, observations from previous studies of sediment transport downstream of bed roughness on flat sand beds are condensed into a single threshold for the generation of ripples. This threshold is based on two dimensionless numbers: one describing the impact of the roughness on near-bed turbulence and another comparing the bed shear stress to the threshold for sediment motion. Results from additional flume experiments confirmed this threshold for sediment mounds of both fine and coarse sands. Measurements of the velocity field near sediment mounds revealed changes in the vortices shed from bed roughness depending on the flow and roughness scales. When vortices begin to coalesce, the magnitude of velocity fluctuations downstream of the mound increases. Even though mean conditions are below Shields' threshold, sediment is transported when the local bed shear stress due to vortex shedding exceeds the critical shear stress for initiation of sediment motion. Including the critical shear stress in the threshold for ripple generation makes it applicable to more complex boundaries, such as biologically bound and heterogeneous sediments. The threshold for ripple generation, and the mechanisms behind it, describe a potentially important sediment transport process.

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