SAND PARTICLE MOVEMENT ON MIGRATING COMBINED-FLOW RIPPLES

We studied sand particle movement on migrating combined-flow ripples i n an experimental flume, The water-surface waves propagated against th e flow in the flume. The resultant combined-flow ripples have characte ristic rounded profiles. Motion pictures reveal that vortices, generat ed only close to the lee sides of ripples, are responsible for the rou nded profile, These vortices excavate rounded troughs and lift sand pa rticles, The lee sides are nourished by the sand, contributing a conve x-upward profile, The flow that separates at the crests is dragged int o the vortices at the troughs, so that there is no clear reattachment point with focused erosion, as is the case for current ripples, Sand t hat falls to the bed from a floating cloud over the stoss side nourish es the convex upward rounded stoss side. The relative strength of the oscillatory flows controls the positions at which the vortices lift sa nd particles to deposit, and determine whether the ripple profiles are rounded or sharp, A lower relative oscillatory flow velocity yields s maller vortices, which cause rounded ripple profiles, Conversely, a hi gher relative oscillatory flow velocity results in larger vortices and sharper crests. The movement of sand particles is unlike that on ripp les beneath either purely oscillatory flows or unidirectional flows.