Sand Motion Initiation by Waves: Two Asymptotes

The new calculation procedure for sand motion initiation on a level bed in oscillatory flow simplifies the Shields criterion adopted from steady flow, and agrees better with available measurements. For fine sand and high flow frequency, published laboratory measurements agree with a model of motion initiation by threshold drag force on a grain in a laminarly shearing boundary layer. For coarse sand and low flow frequency, measurements agree with another model: a threshold flow energy above a thoroughly mixed boundary layer. The unified criterion presented predicts critical peak near-bed velocities between 10 cm/s and 50 cm/s (4 in./sec and 20 in./sec) in —10% agreement with a majority of adequately documented results for quartz sand in water. Critical velocity is determined by sand diameter, ratio of sand to fluid density, oscillation frequency and to a lesser extent fluid vescosity. The effect of oscillation frequency vanishes for usual ocean situations, but not for wave-tank conditions. Needs for additional test data are pointed out.