Experimental investigations of graded sediment transport under unsteady flow hydrographs

Abstract

Natural fluvial channels can experience significant variations in sediment transport rates under unsteady flow conditions, especially during flood hydrograph events. At present, however, there is a distinct lack of understanding of the interaction between unsteady hydrograph flow properties and temporal variability in graded sediment transport rates. In the current study, a series of parametric experiments were conducted to investigate the response of two-graded sediment beds to a range of different unsteady hydrograph flow conditions. Investigations of the total and fractional bed-load sediment transport rates revealed strong temporal variations in transport over the hydrographs, with size-dependent temporal lag effects observed between peak flow conditions and peak bed-load transport rates. Specifically, coarse gravels had increased mobility during the rising limb of the hydrographs, attaining their peak bed-load transport rate either prior to, or near, peak flow conditions. By contrast, the finer grades tended to have enhanced mobility during the receding limb of the hydrographs, with peak transport rates measured after peak flow conditions had passed. Grain size distributions measured from the collected bed-load samples also indicated material coarsening over the rising limb and fining during the receding limb, while corresponding image analysis measurements of bed surface composition showed only marginal variation over the hydrographs. Computation of total and fractional sediment yields revealed that the bimodal sediment mixture tested was transported at significantly higher rates than the uni-modal mixture over all hydrograph conditions tested. This finding indicated that the uni-modal sediment bed was inherently more stable than the bimodal bed due to the increased abundance of medium-sized gravels present in the uni-modal sediment grade. The parametric dependences established in the study have clear implications for improved understanding of fractional inter-granular effects at the bed surface and their influence on graded sediment transport processes within natural fluvial channels under flood flow hydrographs.