Abstract

Previous hypotheses for the absence of current ripples in coarse sands and for the stability of lower stage plane beds are critically reviewed and refuted. It is proposed that the lower stage plane bed remains stable and does not ripple because flow separation over bed defects is inhibited by the enhanced vertical mixing of boundary layer fluid over transitional to rough boundaries. This vertical mixing neutralises the increased static pressure over bed defects and prevents separation and re-attachment so that the defects are unable to amplify and propagate downstream. Eventually, as flow strength is increased, larger defects advance as bar bedforms which are regarded as kinematic waves. Further increases of flow strength give rise to dune bedforms related to effective flow re-attachment from pre-existing bar crests and whose downstream repeat distances are controlled by the magnitude of burst events occupying the whole turbulent boundary layer.

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