Thresholds of particle entrainment in a poorly sorted sandy gravel-bed river

A series of bedload samples and hydraulic measurements were made in the poorly sorted sandy-gravel bed of the Arbúcies River to determine the threshold conditions for initiation of motion. Entrainment has been analysed using two different initial-motion methods: the largest-grain method and the reference or fractional transport rate method. Both methods yield different results in terms of the equal mobility hypothesis. The deviation between scaled and non-scaled initial motion relations are clear: (a) the largest-grain critical shear stress varies to the power −0.68 of Di/D50, indicating a tendency towards size-selective entrainment; but (b) computation of the critical shear stress based on the reference transport method indicates only a very weak dependence on grain size (power −0.91). The question is whether such divergent results are caused by real differences or methodological problems. On one hand, estimations of τc from bedload maximum particle size are methodologically problematic. The systematic error associated with determining Di makes it an unreliable estimator of dimensionless critical shear stress. Much of the observed variation between Di and τ may be more shear-stress estimations or bedload sampling effects than a consequence of selective entrainment. Any conclusion following this method cannot be substantiated, at least until more hydraulic and bedload data is available. On the other hand, scaling problems are less important in the reference transport method, because fractional rates incorporate scaling considerations. Results indicate a strong tendency towards equal mobility, a relatively common situation described in many other transport data, although the coefficient still suggests weak particle dependence. Bedload does not become coarser as shear stress increases, but its grain-size distribution remains constant, as expected under prevailing equal mobility conditions.