In the present study, a three-dimensional numerical model has been developed, in order to study the sediment dynamics that occur in oscillatory flow over ripples. A morphology module has also been developed in order to perform two-dimensional simulations of the bed form evolution under hydrodynamic forcing. Results are presented for oscillatory flow over fixed ripples, at Reynolds number 23,163. The suspended sediment rise is highly correlated to the elevation of the flow vortices after their generation due to flow separation at the lee side of the ripples. The influence of the ripple height on the behaviour of suspended sediment is also significant. Concerning the evolution of the bed form, results of ripple creation, growth, merging, and annihilation are presented.
Surface waves in the coastal zone induce oscillatory flow motions in the vicinity of the seabed. These wave-induced coastal flows interact with the sandy seabed and modify the bed shape by generating coherent small-scale bed structures, which are generally known as ripples. The presence of ripples in oscillatory flows is important due to the impact they have on the seabed roughness. The bed roughness directly affects the near-bed boundary layer hydrodynamics, which in turn controls sediment transport in coastal areas. Consequently, accurate prediction of sediment transport rates is an important element in morphological studies in coastal marine environments.
In oscillatory flow over ripples, the behavior of suspended sediment is highly correlated to the development of the coherent vortices. During each half-cycle, vortices are generated at the lee side of the ripple. Sediment is first hurled over the lee vortex, and at flow reversal, carried with that vortex as it is ejected into the outer flow. According to this vortex formation - ejection mechanism, Bagnold (1946) characterized these bed forms as "vortex ripples". Since then, vortex ripples have been observed both in experimental (Sleath, 1982; Sato et al., 1984; Osborne and Vincent, 1996; Fredsoe et al., 1999) and in numerical simulations (Sato et al., 1984; Blondeaux and Vittori, 1991; Hansen et al., 1994; Fredsoe et al., 1999; Zedler and Street, 2006).
