Wave boundary layer dynamics on a low sloping laboratory beach

Abstract : Wave-induced cross-shore sediment transport is driven by non-linear processes such as boundary layer streaming, skewness and asymmetry, and the generation of infragravity waves. Most of our understanding of wave nonlinearity and associated wave boundary layer dynamics originates from studies involving relatively steep beach slopes (>1:40). Non-linearity on lower sloping beaches is expected to be of different character, which has prompted a series of laboratory experiments as part of the GLOBEX project on a 1:80 beach slope involving random, bichromatic and regular wave conditions. In this paper preliminary results are presented of high resolution wave boundary layer measurements in the surf zone obtained with a 2-component LDA system. The focus is on the time-averaged velocity near the bed, and the development of skewness and asymmetry within the boundary layer. Results show that for all conditions there is a strong decrease in velocity asymmetry within the boundary layer which coincides with an increase in velocity skewness, consistent with previous findings. When the vertical coordinate is appropriately scaled, a linear relationship appears between the velocity skewness in the boundary layer and the velocity asymmetry and skewness in the free-stream. Parameterizing this relationship for rough turbulent flow conditions could improve predictive capability of cross-shore sediment transport formulae.