Why is plane-bed topography unstable under certain flow conditions? We
investigate the grain-scale mechanisms responsible for topographic
instability at the onset of bedform development. Measurements of
fluorescent tracer particle motion were used to estimate the ensemble
mean particle activity, entrainment rate, hop distance, travel time, and
particle velocity characteristic of flow conditions straddling the
threshold stress for bedform development. Based on these data, we
propose two hypotheses to explain the destabilization of planar
topography with rising transport conditions. Hypothesis 1: plane-bed
topography is unstable above a theory-predicted entrainment rate
threshold that varies primarily as a function of particle diameter.
Hypothesis 2: plane-bed topography is unstable above a threshold
particle collision frequency that is proportional to bedload flux.The
threshold particle collision frequency is predicted analogously to the
propensity for congestion shockwaves in vehicular traffic flow theory.