Linear transition ripple migration and wave orbital velocity skewness: Observations

Field observations mere made in 3-4 m mater depth of linear transition ripp le geometry and miff ration using a high-resolution laser-video bed profili ng system and acoustic scanning sensors during both the growth and decay ph ases of an autumn storm event. Linear transition ripples are long-crested, low-steepness bedforms of the anorbital ripple type and were observed to oc cur 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 maxi mum observed migration rate was 0.7 cm/min. Migration was offshore during s torm growth and onshore during storm decay, The observed ripple migration v elocities were highly correlated (r(2) > 0.7) with nearbed wave orbital vel ocity skewness in both cross-shore directions. During storm growth the inci dent wave spectrum was bimodal and the orbital velocity skewness was negati ve. During storm decay the wave spectrum mas unimodal and the velocity skew ness was positive. Bispectral analysis shows that the main contribution to negative velocity skewness during storm growth was due to a. difference int eraction between the two principal (sea and smell) components of the bimoda l velocity spectrum. Positive skewness during storm decal was due to self-s elf interaction of the narrowband residual smell. The negative velocity ske wness observed during storm growth is consistent with prediction by a two-f requency second-order wave theory.