Experimental Investigation of Bedload Sediment Transport and Bed Evolution in Double-Peaked Hydrograph

In natural rivers, significant bedload transport and variability in sediment bed geomorphology is generally observed to occur under unsteady flows, especially during flood events. In this study, a double-peaked hydrograph event, consisting of two identical and consecutive flood waves (i.e. sub-hydrographs), was performed in a large titling flume to investigate the behavior of bedload transport and response of sediment bed change within both uniform and fine-graded sediment beds, respectively. The results indicate that the total bedload transport rates for both uniform and fine-grained sediment beds tend to be reduced during the second sub-hydrograph, in comparison to the first sub-hydrograph, with more significant reductions observed to occur during the rising limb compared to the falling limb. In terms of fractional transport rates for the fine-graded sediment, the results also show that the bedload transport rates for the coarsest fraction are most significantly reduced in the second sub-hydrograph, compared to the first, during the rising limb. A similar overall trend is also shown for the corresponding fine and medium sediment fractions. In terms of the bed evolution over the duration of the double-peaked hydrograph, detailed measurement of bed surface elevation within both uniform and graded sediment beds runs reveal that the well-defined alternate bars are generated in uniform sediment bed, while only slight variations in elevation are found to occur in the fine-graded sediment bed with no indication of the generation of bed forms.