The present work is focused on the large-eddy simulation (LES) of oscillatory flow and sediment transport dynamics over a rippled bed. A morphological module was also developed and used to perform simulations of the bed form evolution under hydrodynamic/sediment forcing. Results are presented for oscillatory flow over fixed ripples of different steepness values. The suspended sediment rise is highly correlated to the level reached by the vortices of the flow. Suspended sediment concentration increases with increasing mobility parameter values. As long as the evolution of the bed form is concerned, results of ripples adapting to flow conditions are presented.
Surface waves in the coastal zone induce oscillatory flow motions in the vicinity of the seabed which interact with the sandy bottom and modify the bed shape by generating coherent small-scale bed structures generally known as ripples. The geometry of such structures strongly affects the wave-induced bottom boundary layer processes, which in turn control 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 coherent vortices, generated at the lee side of the ripple during each half-cycle. Sediment is first hurled over the lee vortex, and at flow reversal is 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". Clifton and Dingler (1984) separated vortex ripples in three classes. Orbital ripples with wavelengths linearly dependent on the wave orbital amplitude, ao, suborbital ripples with wavelengths dependent both on the wave orbital amplitude and on the grain diameter, Dg, and anorbital ripples with wavelengths depending only on the grain diameter.
Studies of ripples under oscillatory flow date back to the end of the 19th century, but it was not until the late 1950s that large-scale experimental studies took place. The size of the facilities in more recent laboratory experiments has increased significantly compared to earlier works, as well as the quality and quantity of the data obtained. Since 1990, a series of new laboratory datasets with measured sediment transport rates in full-scale, wave-induced, oscillatory flows became available, as a result of a continuing experimental research program in oscillatory water tunnels, such as the Large Oscillating Water Tunnel (LOWT) at WL|Delft Hydraulics (Al-Salem, 1993; Ribberink and Al-Salem, 1994; van der Werf et al., 2007), the Aberdeen Oscillatory Flow Tunnel (AOFT) at the University of Aberdeen (O'Donoghue and Clubb, 2001; O'Donoghue et al., 2006) and the Large Oscillatory Water-Sediment Tunnel (LOWST) at the VenTe Chow Hydrosystems Laboratory at the University of Illinois (Pedocchi and Garcia, 2009).
