Ripple migration and its implication in sediment transport: two series of wave-flume experiments

Two series of wave-flume experiments were performed with a focus on the effects of flow unsteadiness of oscillatory flow on ripple morphodynamics. One series of experimentation looked at ripple migration under oscillatory flow with velocity ‘hiatus’, during which surface waves are too small to affect the bed sediment. In this experimental series, the direction of ripple migration corresponded well with that of the flow immediately before onset of the velocity hiatuses. The velocity of ripple migration was highly correlated with acceleration skewness of near-bed flow. The other series of wave-flume experimentation investigated the effect of ‘phase lag’ on ripple morphodynamics. The test results shed light on the gap that lies between the physics of sediment transport associated with ripple migration and the performance of a standard bedload transport formula in terms of bed shear concept. The extent of deviations of the Ribberink's bedload transport formula from the measured performance becomes systematically apparent under conditions of increasing settling time factor Ωs (= η/(w0T); η is the ripple height, w0 the settling velocity and T the wave period). In a summary, the present discussion emphasizes the importance of considering the flow unsteadiness, which is inherently included in natural wave-induced flow to a varying degree, for a better understanding of ripple morphodynamics.