Wave generated flow over sand ripples and the resulting entrainment and suspension of sediment
A theoretical simulation of the wave generated flow and associated suspension of sediment over steep, round crested, two dimensional ripples has been developed. The two main aims of this simulation are (i) to increase understanding of the vortex shedding regime over ripples, and (ii) to provide details of sediment concentrations over rippled beds. For these purposes, a time-stepping 'discrete vortex' hydrodynamical model has been developed to recreate the flow over ripples. In turn, this hydrodynamical model drives a separate, but co-existant boundary layer model. Sediment is entrained at the crest at a rate dictated by the boundary layer model. Once in suspension, it is moved in a purely convective, Lagrangian fashion, with a fall component. Results from the hydrodynamical model provide details of the motion of vortices for various values of the ratio of orbital excursion to ripple wavelength (d/A). Results from the suspended sediment simulation include both instantaneous and time-averaged concentration profiles, as well as snap-shot plots of sediment motion. The first aim of the simulation was successful, with the hydrodynamical model providing much useful infonnation on the flow structure over ripples. The second aim has met with mixed success. A wealth of data comparisons suggest that the present simulation performs well in replicating key features of the suspended sediment regime over ripples. However, it tends to underestimate sediment concentrations. Two other allied studies were undertaken. The first concerned the force acting on the bed 'per ripple', illustrated by the behaviour of the friction factor f. and the energy dissipation factor fe• The second concerned the usage of the simulation to predict ripple stability, which involved the calculation of sediment transport rates over the entire ripple profile.