Experimental Investigation on Temporal Lag Effect of Graded Sediment Transport in Unsteady Flows

Unsteady flood hydrograph flows and graded bed sediments are two distinct characteristics of many natural alluvial systems. At present, however, there is a lack of understanding of the nature of interactions between these physical properties and resulting temporal variability in graded sediment transport rates. A series of parametric experiments were undertaken using two bed sediment mixtures (unimodal and bimodal particle size distributions) with the same overall mean grain size and standard deviation. These graded sediments were subjected to different hydrograph flow conditions of constant overall water volume, characterized by variable rising and falling limb durations and unsteadiness parameters. Temporal lag effects between the flow hydrograph and measured bedload transport rates were found to be significant, but varied between finer, medium and coarser sediment fractions due to the unsteady flow conditions and inter-granular interactions at the bed surface. Specifically, coarse gravel fractions were found to respond more actively during the accelerating rising limb of the flow hydrographs, attaining their peak bedload transport rate prior to the peak flow rate. Conversely, finer grade particles tended to become more gradually exposed at the bed surface, with peak transport rates measured in the decelerating falling hydrograph limbs, subsequent to the peak flow rate. Furthermore, results for similar hydrograph conditions indicated that lower total and fractional bedload yields were generated for the unimodal sediment mixture than for the bimodal sediment mixture, suggesting more pronounced hiding effects for fine size fractions and greater overall bed stability are achieved with the unimodal mixture.