Flume study of mechanisms responsible for particle sorting in gravel-bed meandering channels
Date
2019
Authors
White, Daniel, author
Nelson, Peter, advisor
Morrison, Ryan, committee member
Wohl, Ellen, committee member
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Abstract
Meandering gravel-bed rivers tend to exhibit bed surface sorting patterns with coarse particles located in pools and fine particles on bar tops. The mechanism by which these patterns emerge has been explored in sand-bed reaches; however, for gravel-bed meandering channels it remains poorly understood. Here we present results from a flume experiment in which bed morphology, velocity, sediment sorting patterns, and bed load transport were intensively documented in a single-bend meandering channel. The experimental channel is 1.35 m wide, 15.2 m long, and its centerline follows a sine-generated curve with a crossing angle of 20 degrees. Water and sediment input were held constant throughout the experiment at 104.8 L/s and 230 kg/h, respectively, and measurements were collected under quasi-equilibrium conditions once the sediment input and output were approximately equal and the bed was essentially unchanging. Measurements of the three-dimensional velocity field indicate the development of a helical flow where near-bed velocity is directed toward the inner bank and flow at the surface is directed toward the outer bend. Calculated cross-stream bed load transport rates show that the trajectories of fine and coarse particles cross downstream of the bend apex, with fine sediment directed inward toward the point bar and coarse sediment directed toward the outer pool. Boundary shear stress, calculated from near-bed velocity measurements, indicates that in a channel with mild sinuosity, deposition of fine particles on bars is a result of divergent shear stress at the inside bend of the channel just downstream of the apex. The strong inward secondary currents that developed near the outside bend of the channel have little impact on the fine sediment deposition occurring on the bar under the conditions of this study. Boundary shear stress at equilibrium in the upstream half of the pool was below the critical value for coarse particles (>8 mm), which were only found in the pool. Selective transport toward the sloped region connecting the pool and bar top was responsible for winnowing of fine particles in the pool. Similarly, boundary shear stress near the bar front at equilibrium was below the critical value for particles near the Dā
ā of the bulk sediment feed (ā¤4 mm). Here, only fine particles were mobilized and transported downstream to the bar top. Fine and coarse sediment followed essentially identical trajectories through the meander bend, which contrasts earlier studies of sand-bedded meanders where fine and coarse particles cross paths. This suggests a different sorting mechanism for gravel bends. This experiment shows that a complex interaction of quasi-equilibrium bed topography, selective sediment transport, and currents that develop as a result of curved channel geometry are responsible for the sorting patterns seen in gravel bed, meandering channels.
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Subject
sediment sorting
shear stress
flume
structure-from-motion
sediment transport