Abstract

A comparison is presented of three methods currently used to measure particle settling velocity beyond Stokes' range. Contrasting natural sediments are analyzed in a settling tube and in a modified bottom withdrawal tube, and velocities which are estimated to represent single-particle behavior are calculated empirically from separate measures of particle size, and density and shape. Velocities calculated empirically for aggregated sediments are lower than measured by either sedimentation method, due primarily to aggregate breakdown that occurs during measurement of size by a wet-sieve method. For all sediment types, multi-particle velocities measured in a settling tube are very close to calculated single-particle velocities, but multi-particle velocities measured in a modified bottom withdrawal tube are higher. The increased velocities measured during this "progressive" or "batch" settling are attributed primarily to "hydrodynamic wake capture" that occurs at Reynolds' numbers > 0.1 when particles of different sizes and velocities settle together in a polydisperse suspension. For particles that settle in a polydisperse suspension, temperature correction based on single-particle behavior appears to be inadequate.

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