Wave orbital velocity skewness and linear transition ripple migration: Comparison with weakly nonlinear theory

Abstract

Field observations of linear transition ripples during an autumn storm exhibit high correlation between cross-shore ripple migration rate and the skewness of the near-bed wave orbital velocity. For a bimodal spectrum of mixed sea and swell, negatively skewed near-bed orbital velocities were accompanied by offshore ripple migration, while for unimodal swell, ripples migrated onshore under positively skewed velocities. Bispectral analysis and weakly nonlinear wave theory are used to show that the positive and negative velocity skewness arose from higher-order interactions between frequency components of the incident wave spectrum. There is good quantitative agreement between predicted and observed velocity skewness, indicating a connection between ripple migration and wave-wave interactions.