Abstract
Assemblages of transverse dune-like bed forms that are polymodal in wavelength and height typify sand-bed rivers of large discharge ratio and/or rate of change of discharge. A numerical model of dune time-lag in unsteady flows operated with hydraulic variables representative of such rivers also affords bed-form populations that are polymodal in wavelength and height. The polymodality arises because the experimental systems are self-regulating in such a way that dune creation—destruction is concentrated into certain time-intervals (equivalent to ranges of stage), repeated identically from one flow cycle to the next. The results suggest that it may be erroneous to conclude that natural polymodal assemblages of transverse dune-like bed forms comprise individuals of more than one bedform class, in the sense of a hydrodynamically distinct category.
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