Transportational cyclic steps created by flow over an erodible bed. Part 1. Experiments

Froude-supercritical flow over a flat, erodible bed with copious transport of sediment in suspension appears to be fundamentally unstable. Under a broad range of conditions the flow and bed co-evolve into a series of spatially periodic, or cyclic steps. Each step has a gently sloping Froude-subcritical upstream portion followed by a steeply sloping Froude-supercritical downstream portion, ending in a hydraulic jump which reduces bed shear stress to the threshold value for bed erosion. The steps migrate upstream while preserving form, with net erosion over most of the supercritical region and net deposition over most of the subcritical region. Erosion and deposition balance over one wavelength, so that no average bed erosion results. The successive passage of steps reworks the bed, however, to a depth corresponding to the height of the steps. In the case of silt, the morphology has a strongly step-like character. With fine sand, however, the steps can devolve into the limiting case of chute-and-pool morphology, for which the region of supercritical flow is not so steep.