The significance of turbulent coherent structures on the initiation and evolution of sub-orbital ripples

Abstract

Abstract Motivated by the observation of vortices generated by ripples, numerous studies have focused on how vortex dynamics drive sediment transport and shape ripple geometry. However, during ripple initiation from a flat bed and for low-steepness ripples, such vortices are absent. Turbulent coherent structures (TCS), in contrast, exist within the boundary layer regardless of ripple steepness. This study investigates the role of TCS in ripple initiation and evolution under oscillatory flows using fully coupled three-dimensional Eulerian two-phase large-eddy simulations performed with SedFoam . The results show that TCS initiate ripple marks through localized pockets of turbulent sweep events, supporting the hypothesis that TCS drive ripple initiation from a flat bed. Quadrant analysis further reveals a strong preferential coupling between ejection (sweep) events and particle upward (downward) motion, demonstrating how TCS organize local sediment mobilization and redistribution during ripple initiation. The resulting ripple marks initially exhibit a cross-hatched pattern and subsequently evolve into quasi-two-dimensional rolling-grain ripples. Quantitative analyses further support the hypothesis that sub-orbital ripple evolution involves a transition from TCS-dominated initiation to increasingly wave-orbital-controlled growth and equilibrium development as ripple dimensions exceed the turbulence integral length scale. These findings provide new insights into the mechanisms governing sub-orbital ripple evolution and may help improve the prediction of low-steepness ripple geometry in coastal environments.