Flume investigation of bed morphology, flow field, and bed load
transport as mechanisms responsible for particle sorting in gravel-bed
meandering channels
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 meters wide, 15.2 meters 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
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.