Sand Suspension by Vortex Pairing

In studies of oscillatory flow above rippled beds, appreciable quantities of suspended sediment are reported up to 10 ripple heights,(eta R), above the bed. The well-known convective and diffusive processes alone cannot account for this phenomenon. Evidence from a flow visualisation study suggests that vortex pairs generated during flow separation over ripple crests can generate jet-like flows capable of ejecting entrained sediments far from the bed. The present study examines this hitherto unrecognised flow phenomenon and uses a simple model to show that particles can attain velocities greater than the propagation speed of a vortex pair and flows between vortex pairs have a potential to eject particles more than 10(eta R), from the bed. Once free, sediment velocity is quickly reduced by the forces of drag and gravity and particles settle back to the bed.