Geometric characteristics of coarse-sand ripples generated by oscillatory flows: A full-scale experimental study

Abstract

The shape of vortex ripples generated by wave boundary layers determines the local hydrodynamics and sediment motions, and therefore is critical for understanding coastal sediment transport in the rippled-bed regime. In this study, coarse-sand vortex ripples were generated by periodic oscillatory flows mimicking full-scale field conditions in an oscillatory water tunnel. A laser-based bottom profiler was applied for measuring the movable bed's profile. The observed ripple development from a flat bed is in agreement with many previous studies. Small ripple marks appeared within a couple of flow periods, and gradually became 3-dimensional (3-D) transient ripples, which eventually evolved to equilibrium 2-dimensional (2-D) ripples after O(100–1000) flow periods. The shape of equilibrium 2-D ripple was studied based on a representative ripple profile obtained by ensemble averaging a periodic ripple train. The shape of ripples formed under sinusoidal flows become increasingly sinusoidal as the flow becomes stronger (larger Shields parameter) or wave period becomes longer, which is possibly because the influence of coherent vortex becomes more significant. Wave nonlinearities have different effects on the ripple shape. Generally speaking, wave skewness makes ripples onshore-leaning while wave asymmetry makes ripples' crests sharper and troughs flatter. Our measurements also revealed some detailed geometric characteristics, i.e., the roundness of ripple crests and the maximum local slope along ripple flanks.