Transient ripples in oscillatory flows

Laboratory measurements are reported of the change in ripple profile with time in response to a step change in flow conditions. The ripples were formed in beds of sand in an oscillating tray rig. Spectra of the bed profiles were obtained with a miniature echo sounder traversed over the bed. Two different growth mechanisms for the new bed profile were observed. In one case the new profile grows from an existing small perturbation of the bed at the same wavenumber as that of the final bed. In the second mechanism the ripple wave length gradually changes with time to fit the new conditions. Rates of growth of the new bed and decay of the old bed are analysed and empirical formulas are derived. These formulas are used to estimate transition times for rippled beds responding to changed flow conditions. Comparison with measured transition times shows significant scatter.