Pulsations in Bedload Transport Rates Induced by a Longitudinal Sediment Sorting: A Flume Study Using Sand and Gravel Mixtures

Laboratory experiments, in which two different sized materials were continuously fed into a flume, revealed that a longitudinal sediment sorting produced a rhytmic fluctuation in the bedload transport rates. Bedload pulsation was immediately noticeable for conditions in which the finer size fraction was poor in the mixture. Three types of bed state—congested, transitional and smooth—repeated along the channel in an orderly manner; this is the longitudinal sediment sorting.Abundant sand particles thoroughly filled up the interstices of a gravel framework in the smooth state, which in turn invited mixing effects and gave a greater mobility. Sands and gravels were transported rapidly from reaches having the smooth state. An excess of sediment supply, however, was stored within the antecedent congested state, which created a bed surface closely packed by gravel particles. Gravels were intermittently in motion there under these conditions. The longitudinal sediment sorting was also accompanied by aggrading—degrading sequences and/or steep slope—gentle slope events. A descriptive model applicable to some problems in gravel-bed rivers was obtained.