Entrainment threshold of natural grains in liquids determined empirically from dimensionless settling velocities and other measures of grain size

Abstract

Grain size plays a crucial role in sediment transport, but different methods of size analysis do not yield comparable results. Particle diameters obtained by settling tube, for example, cannot be employed in equations based on sieve sizes. Although the majority of experiments on the initiation of sediment motion has been based on sieve data, settling tube analysis is probably superior as both the settling rate and the entrainment threshold of grains are determined by the same factors of size, shape and density. In this paper, empirical equations are provided which allow the determination of a non-dimensional critical shear stress directly from the equivalent sedimentation diameter. The equations apply to the entrainment of well sorted grains with a high sphericity on flat sediment beds, and are valid for particles and liquids of different densities.