Measurement of soil aggregate settling velocities. 1. A modified bottom withdrawal tube method

Abstract

Sediment eroded by water consists largely of soil aggregates. The settling velocity of such aggregates and primary soil particles is of fundamental importance to the processes of sediment transport and deposition in water. A modified bottom withdrawal tube method is presented for the direct measurement of the settling velocity distribution of soil aggregates or particles of different sizes that settle together in a polydisperse suspension. The modified method overcomes experimental error and analytical deficiencies present in the initial method and is suitable for measuring settling velocities up to 0.5 m s-1. The method employs the bottom withdrawal principle; new exact theory is presented for interpretation of sedimentation data thus measured. By complementing the pipette or hydrometer methods which measure settling velocities less than 10-3 ms-1, this modified method allows analysis of the entire range of settling velocities encountered in natural sediment. Settling velocity distributions measured for a variety of sediment types are illustrated.