Laboratory experiments on the role of hysteresis, defect dynamics and initial perturbation on wave-generated ripple development

We performed a series of laboratory experiments to study the growth and development of wave-generated ripples under constant and varying conditions. There are two aspects of our study. First, under time varying wave conditions, we measure the rate of ripple adjustment and the equilibrium mean ripple wavelength before and after the change in forcing. Our experiments with increasing wave forcing (i.e., increasing wave amplitude or wave period) lead to faster ripple growth and larger final wavelengths. Decreasing wave forcing leads to shorter ripple wavelengths and hysteresis. Existing predictors provide a reasonable description of both the final equilibrium spacing and of the spacing evolution after wave conditions are changed. The presence of defects (crestline irregularities) impacts the final equilibrium ripple wavelength. Second, we investigate the role of initial perturbation on ripple growth and geometry. Larger initial perturbations in the bed lead to faster growth of ripples and larger initial ripple wavelength. Our results suggest that irregularities at the pattern scale, such as defects and initial bed perturbations, are likely to impact ripple evolution and can be a control factor on hysteresis.