Effects of acceleration skewness on oscillatory boundary layers and sheet flow sand transport

Surf zone waves are characterized by their forward leaning ‘sawtooth’ shape, leading to stronger accelerations in the onshore direction compared to the offshore direction. Previous research indicates that this so-called ‘acceleration
skewness’ induces a net onshore-directed sand transport, despite onshore- and offshore-directed velocities being equal in magnitude. Large-scale laboratory experiments are conducted in which sawtooth-type flows, with varying degree of
asymmetry, are generated over (i) fixed rough beds and (ii) mobile sand beds. The fixed bed experiments involved detailed (intra-wave) measurements of near-bed velocities and turbulence using PIV and LDA. The mobile bed experiments included measurements of net sand transport rates and detailed concentration and velocity measurements in the sheet-flow layer. The fixed bed results show that bed shear stress is higher under the highly accelerating positive flow compared to the more gradual accelerating negative flow. The net transport experiments confirm previous research that net transport is non-zero, is always in the direction of the largest acceleration and increases with increasing
acceleration skewness. Particularly interesting are the large positive transport rates for fine sands, considering the large negative transport rates previously measured in velocity-skewed flows. These are due in some part to the relatively
short time between maximum negative velocity and the negative-to-positive flow reversal during which sand has limited time to settle. The significant effect of acceleration skewness is seen in all of the results. Data and new insights
from these experiments are used to develop a new semi-unsteady practical sand transport model.