Field Measurement of the Probability of Coarse-Grained Sediment Entrainment in Natural Rivers

Abstract

This study takes advantage of recent advances in field-based sediment motion monitoring to report direct field measurements of the bedload pick-up, or entrainment, function in natural gravel-bed channels. Bedload sediment tracers with radio frequency identification (RFID) tags were placed upstream of five run-of-river dams that locally change the energy grade slope of the river, allowing for the initiation of tracer motion to be investigated across a range of slopes, flow depths, and surrounding bed grain sizes. Results based on transect-averaged shear stresses differ from those based on location-dependent shear stresses determined using a two-dimensional (2D) flow model. Using 2D shear stresses, the results show that as the tracer size decreases below the median size of the surrounding particles on the bed, both the mean and standard deviation of the Shields value for mobilization increase, reflecting the impact of both hiding and protrusion effects. The increase in the average Shields value for mobilization with decreasing relative grain size is consistent with the predictions of proposed hiding functions. As the relative tracer size increases, the entrainment of the tracers approaches that predicted by the model of Einstein (1950), which ignores intergranular bed geometry effects.