Hydraulic Interpretation of Sand Size Distributions

Grain size distributions commonly show discontinuities and straight line segments when plotted (using the phi transformation) on probability paper. The different components making up the distribution have commonly been interpreted as subpopulations of grains moved by different transport mechanisms. The paper considers this hypothesis for the restricted case of the two coarsest components observed in many samples of river bed materials. By comparing analyses of bed materials and suspended load, it is shown that the components may be interpreted as (C) traction, and (A) intermittent suspension subpopulations, moved by the dominant discharge of the river. From an analysis of theory, experiments and field data on turbulence in open channels, it is shown that a satisfactory though approximate hydraulic criterion for separating the two mechanisms of sediment transport is as follows: grains move by traction if their settling velocity is larger than the shear velocity of the stream, and by intermittent suspension if their settling velocity is smaller than the shear velocity. This criterion is confirmed by comparison with hydraulic and sediment data for the Middle Loup, Niobrara, Elkhorn, Missouri, Mississippi and Rio Grande Rivers. Consequently, it should be possible to determine the dominant shear velocity of ancient fluvial sands by determining the size "break" between their two coarsest components.