Rubey (1933) noted that the velocity of a current required to entrain and transport a sediment grain is similar in magnitude to the grain's settling velocity in still water. This observation contains implications as to the nature of the flow drag exerted on the grain at the time of threshold, and a pivoting analysis of grain entrainment can yield a relationship between the threshold shear velocity u (sub * t ) and settling velocity w m , rather than being directly dependent upon the grain's diameter and density. Data sets for the threshold of natural sands have been reanalyzed to establish the relationship between u (sub * t ) and w m , a correlation which does differ for each grain density but to a much smaller degree than for u (sub * t ) versus the grain diameter D. The ratio u (sub * t ) /w m versus the Reynolds numbers u (sub * t ) D/v or w m D/v do form "universal" threshold curves, similar to the standard Shields threshold curve. Comparisons between the threshold measurements and the grain-pivoting model yield reasonable agreement, although many aspects of this analysis remain uncertain and require further testing.

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