Effect of grain sorting on gravel bed river evolution subject to cycled hydrographs: Bed load sheets and breakdown of the hydrograph boundary layer: Grain Sorting and Breakdown of the HBL

An, Chenge  (ORCID:0000000312625326); Fu, Xudong  (ORCID:0000000307440546); Wang, Guangqian; Parker, Gary  (ORCID:0000000159735296)

doi:10.1002/2016JF003994

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. 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°. Water and sediment input were held constant throughout the experiment and measurements were collected under quasi‐equilibrium conditions. 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, downstream of the apex. Boundary shear stress in the upstream half of the pool was below critical for coarse particles (>8 mm), leading to an armored pool. Inward directed selective transport was responsible for winnowing of fine particles in the pool. Fine and coarse sediment followed similar trajectories through the meander bend, which contrasts earlier studies of sand‐bedded meanders where the loci of 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 secondary currents are responsible for the sorting patterns seen in gravel‐bed, meandering channels.

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