How does flow history affect sediment mobility?
Critical Shields stress divides the hydrograph into two portions, an above-threshold region associated with significant bedload transport, and that below-threshold, where little bedload transport is observed. Most studies concentrate on the portion of the hydrograph above this threshold, leaving the subcritical region and its subsequent impact on bedload transport rates largely unexplored. The duration & magnitude of these below-threshold conditions is highly variable and a number of observations suggest that critical Shields stress is history dependent, exhibiting memory of previous conditions. We explore how variable duration & magnitude of low flow fluid forcing impacts grain-scale bed topography, bedload transport, and the threshold for motion. We find that small-scale changes in bed topography not captured by reach averaged quantities result in large changes in observed bedload transport rates. In a natural channel, we find that prior flow magnitude is significantly correlated to observed changes in the threshold for motion. We identify a range of flows over which the threshold for motion is a history-dependent quantity, challenging the assumptions of widely used bedload transport models.
tl;dr Yes. Prior flow history affects the threshold for motion! Subtle changes in bed topography (even w/o measurable transport) lead to a history-dependent threshold for motion.
Masteller, C.C., Finnegan, N.J., (2017), Interplay of grain protrusion and bedload transport rates in an experimental flume: Journal of Geophysical Research – Earth Surface. [link]
in prep:
Masteller, C.C., Turowski, J.M., Finnegan, N.J., Rickenmann, D., History-dependent variation to the onset of motion revealed by continuous bedload transport measurements in a steep mountain stream
Undergraduate research assistant, Sergio Martinez, preparing the bed for a flume experiment, Richmond Field Station, UC Berkeley
Fellow graduate student, Alex Nereson, and high school student, Tiffany, preparing the flume for Structure from Motion Photogrammetry, Richmond Field Station, UC Berkeley
