The response of bed-load sediment transport and bed evolution under unsteady hydrograph flows

Fluvial channels can experience significant variations to both sediment transport rates and deformations in channel morphology under unsteady flow conditions, especially during flood hydrograph events. Within the current study, a series of parametric experiments was conducted to investigate the response of both quasi-uniform (sand) and graded (sand-gravel) sediment beds to a range of different unsteady hydrograph flow conditions, defined by three non-dimensional variables: (i) an unsteadiness parameter γ_HG representing hydrograph flashiness; (ii) an asymmetry parameter η defining the ratio of rising and falling limb durations; and (iii) a total flow work parameter W _k. Investigations of bed-load sediment transport rates for quasi-uniform sediment indicated different temporal lag effects between the peak flow and peak bed-load transport rates for different hydrograph conditions. Fractional transport rates for the graded sediment runs also revealed differing lag effects for different sediment fractions, with coarser fractions reaching their peak transport rate prior to peak flow (i.e. during the rising limb), while finer fractions typically reached their peak transport rate following peak flow (i.e. during the falling limb). This behavior was attributed, at least in part, to the role of sub-threshold, antecedent base flow conditions in restructuring the bed surface grading through (i) stabilization of finer sediment fractions in bed surface interstices (i.e. hiding effects) and (ii) increased exposure of the coarser grains at the bed surface. Measurements of total and fractional transport rates during the rising and falling limbs revealed that an increase in hydrograph asymmetry η resulted in a larger proportion of transport occurring during the rising limb, compared to the falling limb, while the overall influence of W_k and γ_HG was less clear. Finally, bed elevation measurements for the quasi-uniform sediment runs revealed the formation of bed ripples, with mild net upstream entrainment and mid-stream deposition patterns observed when W _k and γ_HG values were low. By contrast, strong upstream entrainment coupled with mid- to downstream formation of alternate bars was found to occur in quasi-uniform beds under higher W_k and γ_HG conditions. Erosion and deposition patterns for the graded sediment beds were patchier with no indication of ripples or alternate bars.