Bend sorting of bed load in a rotating annular flume
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River bends are characterised by a transversely sloped bed profile, which is steepest in the outer bend (thalweg) and shallowest in the inner bend (point bar). Sediment, consisting of a range of grain sizes, is sorted laterally by bend flow and the transverse bed slope effect, and vertically by bed forms. It is unclear however how the degree of bend (=lateral) sorting is related quantitatively to these processes; field data and experimental research on this matter are limited, and the effects on and of sorting are not well captured in current fluvial numerical models. Therefore, it is the objective of this study to examine the effect of transverse bed slope and sediment mobility on the degree of bend sorting in a meander. To this end, a near-unimodal sediment mixture (d50=0.75mm) was used in a rotating annular flume. A rotating lid steers the flow and counter-rotation of the flume itself generates a centrifugal force on the flow low in the water column, weakening the secondary flow intensity. By varying the rotation speeds of the lid and flume, the effect of the transverse bed slope and sediment mobility were isolated. Two sets of experiments were conducted: sorting experiments and morphology experiments. The former were experiments of which the bed resembled natural river morphology best, namely aimed at transverse slopes between 0 and 0.25. Digital elevation models were generated for both sets, and only the sorting experiments were analysed in further detail, involving taking photographs and sediment samples. Bend sorting becomes more distinct with steeper transverse slope. Especially slopes over 0.15 show well developed bend sorting. Sediment mobility has an insignificant effect on the sorting process as long as all sediment is transported as bed load. Only at or just below the threshold of motion is sediment mobility hypothesised to influence lateral sorting; hiding and exposure effects become prominent and a lag layer in the outer bend may form. Longitudinal variation in bend sorting is observed on the bed form length scale. This is caused by dunes acting as obstacles of flow, leading to high secondary flow intensity, and therefore steep slopes, in the dune troughs. Also, dunes sort sediment vertically that is made available by lateral sorting, in a fining upward trend. On the natural river scale, most pronounced bend sorting is expected in sand bed rivers with high sinuosity (predominantly meandering and sinuous rivers). Least pronounced sorting is expected in mixed sand-gravel bed and gravel bed rivers, especially in case of threshold channels. Hiding and exposure effects were mitigated in this study, but are argued to influence the transverse slope development and bend sorting. Elaboration on the annular flume experiments with wide sediment mixtures and field campaigns in mixed sand-gravel bed rivers are proposed.