Smartrock transport in a mountain stream: bedload hysteresis and
changing thresholds of motion
Abstract
Bedload movement is fundamentally probabilistic. Our quantitative
understanding of gravel transport is particularly limited when flow
conditions just exceed thresholds of motion, in part because of
difficulties in measuring transport statistics during floods. We used
accelerometer-embedded tracer clasts to precisely measure the timing of
grain motions and rests during snowmelt floods in Halfmoon Creek, a
gravel-bed mountain stream in Colorado, USA. These new data let us
explore how probabilities of tracer movement vary with snowmelt
discharge. Bedload hysteresis occurred over both daily and seasonal
timescales, and included clockwise, counter-clockwise, and figure-eight
patterns. We quantitatively explain these observations in terms of how
thresholds of motion progressively evolved over 22 days during a
seasonal snowmelt flood. Our results suggest that thresholds of motion
are functions of both (a) cumulative shear stress and (b) temporal
changes in shear stress during floods.