Fujiko Iseya

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

ABSTRACTLaboratory 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 aggra...

Response of a Channel with alternate bars to a decrease in supply of mixed‐size bed load: A Flume Experiment

The response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand-gravel mixture. After a series of quasi-stationary alternate bars were formed under equilibrium sediment transport, feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant. The primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased. After the first feed rate reduction, the channel incised by twice the mean water depth, on average, and caused distal bar surfaces to emerge as terracelike features. Bar roughness decreased, and mean boundary shear stress exerted on bed surface particles increased. Little incision occurred after the second feed rate reduction.

Evolution of a sediment wave in an experimental channel

The routing of bed material through channels is poorly understood. We approach the problem by observing and modeling the fate of a low-amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave. The wave essentially dispersed upstream and downstream without translation, although there was inconclusive evidence of translation late in the experiment when the wave was only 10–20 grain diameters high. Alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume, celerity, or transport rate. Sediment that overpassed migrating bars was apparently responsible for dispersion of the wave. The evolution of the wave was well predicted by a one-dimensional model that contains no adjusted empirical constants. Numerical experiments demonstrate, however, that the theory does not predict sediment waves that migrate long distances downstream. Such waves can only be explained by the following processes not represented by the theory: selective bed load transport, spatial variations in bar and other form roughness, the mechanics of mobile armor, and perhaps other mechanisms.