Linear transition ripple migration and wave orbital velocity skewness: Observations

Abstract

Field observations were made in 3–4 m water depth of linear transition ripple geometry and migration using a high‐resolution laser‐video bed profiling system and acoustic scanning sensors during both the growth and decay phases of an autumn storm event. Linear transition ripples are long‐crested, low‐steepness bedforms of the anorbital ripple type and were observed to occur here at relatively high wave energies just below the flatbed threshold, with wavelengths of 8.5±0.5 cm and heights of 0.3±0.1 cm. The maximum observed migration rate was 0.7 cm/min. Migration was offshore during storm growth and onshore during storm decay. The observed ripple migration velocities were highly correlated (r2 > 0.7) with nearbed wave orbital velocity skewness in both cross‐shore directions. During storm growth the incident wave spectrum was bimodal and the orbital velocity skewness was negative. During storm decay the wave spectrum was unimodal and the velocity skewness was positive. Bispectral analysis shows that the main contribution to negative velocity skewness during storm growth was due to a difference interaction between the two principal (sea and swell) components of the bimodal velocity spectrum. Positive skewness during storm decay was due to self‐self interaction of the narrowband residual swell. The negative velocity skewness observed during storm growth is consistent with prediction by a two‐frequency second‐order wave theory.