A Field-Based Estimation of the Variability of Particle Entrainment in Coarse-Bed Rivers

The determination of critical shear stresses is fundamental to bedload sediment transport prediction in gravel-bed rivers. Due to the heterogeneous shape and arrangement of the individual clasts in a riverbed, critical shear stresses typically show a large spatial variability, which is not adequately captured by the reach-averaged description followed in common studies. In this regard, there is a general paucity of field data on this spatial variability of the critical shear stress, largely due to the lack of a standardized measurement method. In an attempt to fill this gap, we propose a field-based workflow to estimate the frequency distribution of dimensionless critical shear stress (also named critical Shields number), which is based on the measurement of a series of variables related to the position, orientation and resistance to motion of individual clasts in a gravel-bed river, combined with a probabilistic approximation to drag and lift coefficients. Following this workflow, the patch-scale variability of particle incipient-motion conditions was determined in a gravel bar of the Upper Cinca River, Spain. The results are consistent with what is known about sediment entrainment in gravel-bed rivers. We consider this method to have great potential to advance our understanding of particle initiation of motion in gravel-bed rivers as it provides valuable systematic field information.