We studied sand particle movement on migrating combined-flow ripples in an experimental flume. The water-surface waves propagated against the flow in the flume. The resultant combined-flow ripples have characteristic rounded profiles. Motion pictures reveal that vortices, generated only close to the lee sides of ripples, are responsible for the rounded profile. These vortices excavate rounded troughs and lift sand particles. The lee sides are nourished by the sand, contributing a convex-upward profile. The flow that separates at the crests is dragged into the vortices at the troughs, so that there is no clear reattachment point with focused erosion, as is the case for current ripples. Sand that fulls to the bed from a floating cloud over the stoss side nourishes the convex upward rounded stoss side. The relative strength of the oscillatory flows controls the positions at which the vortices lift sand particles to deposit, and determine whether the ripple profiles are rounded or sharp. A lower relative oscillatory flow velocity yields smaller vortices, which cause rounded ripple profiles. Conversely, a higher relative oscillatory flow velocity results in larger vortices and sharper crests. The movement of sand particles is unlike that on ripples beneath either purely oscillatory flows or unidirectional flows.

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