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

Kazuo Taki, Gary Parker

Research output: Contribution to journalArticlepeer-review


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 position 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.

Original languageEnglish (US)
Pages (from-to)488-501
Number of pages14
JournalJournal of Hydraulic Research
Issue number5
StatePublished - 2005
Externally publishedYes


  • Bedforms
  • Cyclic steps
  • Erodible bed
  • Sediment
  • Supercritical flow
  • Suspension

ASJC Scopus subject areas

  • Civil and Structural Engineering
  • Water Science and Technology


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