Abstract: Vortex ripple is widely formed in the coastal region,and the dynamics of vortex is responsible for the sediment transport.The evolution of the initially flat sandy bed is studied in a laboratory wave tank under cnoidal wave conditions.The Acoustic Doppler Velocimeter(ADV)was used to acquire detailed velocity measurements at different positions.The nonlinear wave flow induces asymmetric rippled bed.CCD images were analyzed and the data on topography changes were obtained.The flow structure in the asymmetric rippled bed under the action of the cnoidal wave,and the complete process of the vortex formation,evolvement and disappearance were observed.The formation and the evolution of ripple were analyzed.A separation of the near-bed velocity field of the cnoidal waves over the asymmetric ripples indicates that the near-bed flow field is dominated by the turbulent motion,and the level of turbulence decreases with the increase of the distance away from the bed.The turbulence is anisotropic.The experiment result shows that the mechanism by which sand ripples maintain their shape is in the form of the dynamic equilibrium.Sediment may be lifted from the ripple face,picked up in suspension by the local velocity,and deposited over the crest and in the lee of the ripples.