  We revisit the longstanding question of the onset of sediment transport
driven by a turbulent fluid flow via laboratory measurements. We use particle
tracking velocimetry to quantify the fluid flow as well as the motion of
individual grains. As we increase the flow speed above the transition to
sediment transport, we observe that an increasing fraction of grains are
transported downstream, although the average downstream velocity of the
transported grains remains roughly constant. However, we find that the fraction
of mobilized grains does not vanish sharply at a critical flow rate.
Additionally, the distribution of the fluctuating velocities of non-transported
grains becomes broader with heavier tails, meaning that unambiguously
separating mobile and static grains is not possible. As an alternative
approach, we quantify the statistics of grain velocities by using a mixture
model consisting of two forms for the grain velocities: a decaying-exponential
tail, which represents grains transported downstream, and a peaked distribution
centered at zero velocity, which represents grains that fluctuate due to the
turbulent flow but remain in place. Our results suggest that more sophisticated
statistical measures may be required to quantify grain motion near the onset of
sediment transport, particularly in the presence of turbulence.
