Statistical Description on the Role of Turbulence and Grain Interference on Particle Entrainment from Gravel Beds

A complete understanding of the role of grain-scale particle-flow interaction in sediment entrainment and transport has still not been achieved in spite of recent technological advancement in measurement capabilities. In this study, the initial motion of natural sediment particles in a gravel deposit was detected and combined with simultaneous local measurements of the velocities on a horizontal plane located above the bed surface using a three-component stereoscopic PIV. A series of experimental tests with increasing low values of boundary shear stress were conducted. The acquisition system allowed coupling between streamwise and vertical near-bed velocity and the entrainment of more than 900 individual grains. Initial analysis agreed with previous observations on the predominance of sweeps (Quadrant IV), and to a lesser extent, of outward interactions (Quadrant I) in entraining gravel particles. However, the latter were found to move sediments just as efficiently as sweeps impacting on particles that had long periods of rest and so were exhibiting higher levels of stability. This behavior suggests that sweep-induced lift based on Bernoulli’s principle does not entirely explain the generation of vertical forces on highly stable bed particles. Closer inspection of the data revealed that many entrainments were correlated to occasions when stable bed grains interacted with grains travelling in their close vicinity. Approximately 30% of the entrained population was observed to initiate motion in this type of situation. For this subsample of entrainment events, outward interactions were found to be comparatively more effective than for the noninterference case, whereas the relative contribution of sweeps exhibited an opposite trend.