Temporal Resolution Controls on Ensemble Statistics of Bedload Transport

The movement of bedload sediment results from episodic grain motion involving hops and rests
controlled by hydrodynamic forces and collisions with the bed surface. Despite extensive research
studying the velocity distributions, there is no firm consensus regarding particle-scale dynamics,
particularly whether velocity distributions exhibit exponential-like or gamma-like characteristics.
In this work using high-resolution LES–DEM simulations, we investigate how observational
thresholds and flow conditions shape grain motion statistics. Our results demonstrate that velocity
threshold choices critically affect the observed distributions as including low-velocity events
produces exponential-like streamwise (v_x) and Laplace-like cross-stream (v_y) velocity distributions,
while filtering these events yields gamma and Gaussian forms. The streamwise velocity
distribution maintains its exponential character across flow intensities, but cross-stream
distributions evolve from Gaussian to Laplace-like as turbulent forcing strengthens. Fine-scale
analysis of velocity–acceleration phase space exposes highly asymmetric acceleration signatures
controlled by impact-driven collisions, whereas coarser temporal averaging generates symmetric
patterns. Hop distances exhibit Weibull-type distributions with stable scale factors, while the
relationship between hop length and duration transitions from quadratic to linear dependence
across all flow regimes, revealing inherent scale-dependent transport mechanisms. Throughout the
investigated conditions, bedload transport rates increase predominantly via nonlinear growth in the
number of mobile particles (particle activity), with mean particle velocities showing minimal
variation. These findings reconcile contradictory literature on bedload kinematics, emphasizing the
dominant role of particle mobilization dynamics, and reveal how measurement protocols introduce
systematic biases in grain-scale statistical characterization.