Finer grained sediment pulses decrease variability in entrainment thresholds of bed material

Pulses of sediment finer than existing channel bed material can modify sediment transport dynamics, but they can also pass without a lasting effect. How a channel adjusts to a sudden influx of finer grained sediment is even more uncertain when sediment pulses are punctuated by extended periods of low sediment supply conditions. Such oscillations in sediment supply can be a byproduct of flood-control dams that only temporarily maintain reservoirs, sending pulses of finer grained sediment each time the reservoir is drained. Using a seasonally operated flood-control dam as a natural experiment, we quantified the entrainment thresholds of cobble-sized sediment through seasonal cycles in sediment supply conditions. The channel bed ranged from being without finer grains when dam gates were closed to being completely buried by them after dam gates opened. We paired tracer clasts embedded with accelerometers with a hydraulic flow model to determine the stresses acting on coarse grains at the time of motion across bed conditions. The influence of the sediment pulses was most evident in reduced variability in Shields numbers at the onset of motion relative to the expectations for a sand-free cobble bed. The observed variance in Shields numbers at the onset of motion best matched the expectation for cobbles moving over a bed of sand, likely reflecting the finer grains from the sediment pulses limiting hiding and protrusion effects on the mobility of coarser grains. While cobbles downstream of the flood-control dam were periodically disturbed, the pulses of finer grains were not able to disrupt the established armour layer.