A literature review on sand transport under oscillatory flow conditions in the rippled-bed regime

Abstract

Experimental results are very important to understand the complex nature of sand transport. They give insight in the relevant processes and can be used to validate model concepts. Most of the laboratory experiments on wave-related transport processes are carried out in oscillating water tunnels and in wave flumes. There are large-scale and small-scale oscillating water tunnels and wave flumes. In large-scale tunnels and flumes, the velocities close to the bed and the wave periods can be comparable to the velocities occurring in nature. Therefore, it is possible to perform full-scale experiments. The simulated flow feld in oscillating water tunnels differs from the flowfield in nature (and from the flowfield in wave flumes). In contrast with the orbital motion under real propagating waves, the same phase occurs at every location along oscillating water tunnels. Furthermore, vertical orbital motions are not simulated. A review of these laboratory data sets on wave-related transport mechanisms in the ripple regime shows the following. 1. During one wave cycle two concentration peaks occur above the ripple crest and ripple trough (lower peaks, larger time lags): one just after °ow reversal probably generated by lee-side vortices and one around maximum °ow probably generated by stoss-side vortices. 2. The phase of eddy shedding and suspended cloud ejection is possibly linked to the orbital diameter normalized with the ripple length. 3. The velocity and concentration fields above the ripple structure are so complex that it appears to be impossible to relate the local instantaneous sediment concentration to a local instantaneous fluid velocity. 4. There is empirical evidence that the vertical distribution of the time-averaged concentration (for symmetric, asymmetric, regular, and irregular waves) can be described with an analytical solution of the advection-diffusion equation with a constant decay length. However, proper expressions for this decay length and the reference concentration do not exist. The decay length is possibly linked to the ripple height. 5. Despite the larger onshore orbital velocities, the net wave-related transport over ripples is in most cases offshore-directed due to phase lags between velocities and concentrations caused by vortices on the lee-side of ripples. 6. The number of data sets on wave-related transport processes in the ripple regime with relatively large mobility numbers (psi > 100) and wave periods (T > 5 s) is limited. This especially accounts for time-dependent concentration and net transport measurements.