Surf zone cross-shore boundary layer velocity asymmetry and skewness: An experimental study on a mobile bed

An experimental study on a physical model of a beach in a two‐dimensional wave flume was designed to investigate velocity nonlinearities in the wave boundary layer. The cross‐shore velocity was measured in the surf zone along a vertical profile every 3mm from free‐stream elevation down to the still bed level. The skewness and the asymmetry of the phase averaged velocity were computed at each elevation. Observations indicate that the free‐stream asymmetry transforms into bottom velocity skewness. A linear experimental relation between free‐stream asymmetry to skewness ratio and bottom skewness to free‐stream skewness ratio is established. A theoretical linear relationship is discussed, which predicts the phase lead of the bottom velocity. This phase lead is also determined by Fourier analyzing the velocity time series. The first two Fourier components yield the same phase lead at the bed that is found to be about 30° and nearly constant over all the experiments made. Key PointsSmall‐scale laboratory waves breaking over a bed of lightweight sedimentVelocity profiles measurements within the wave bottom boundary layerAn increase in free‐stream asymmetry induces an increase in bottom skewness