Flume experiments on the migration rate of wave-formed ripple marks

The migration rate of wave-formed ripple marks was investigated using a two-dimensional wave flume. Well sorted fine-grained sand (0.2mm in diameter) was placed on a 1/20-fixed slope and an adjacent part of the horizontal flume bottom. The onshore migration rates of ripples and near-bottom orbital velocities were measured at three sites along the slope with different water depths and at one site on the horizontal bed. Ripples which developed fully and reached the steady state migrated at constant rates. The ripple migration speed generally increases as the maximum bottom velocity becomes higher. Ripples in the lowest part of the slope migrated more slowly than those on the horizontal bed although the maximum velocities were nearly equal. A detailed comparison of velocity data between these areas indicated that the time-variation pattern of the bottom velocities slightly differed from each other. Considering sediment transport on ripples, such a slight difference in the velocity patterns was found to give an influence on the ripple migration rate. A comparison using mobility number between the results of this study and the previous data suggested that ripple migration is much more complex in the field where irregular waves are dominant.