Grain sorting and decay of sand ripples under oscillatory flow and turbulence

Abstract

The results of laboratory experiments on formation and dynamics of sand ripples under oscillatory flow as well as ripples decay under weak oscillatory flow and turbulence are reported. Two series of experiments were conducted in a water channel with an oscillatory tray. Firstly, the ripple formation and dynamics under variable flow intensity was studied in homogeneous and bimodal sediments. The results show that a bimodal mixture is more stable than homogeneous sediment of equivalent size, and it modifies ripple characteristics. To explain the results of observations, an effective grain size for mixture was proposed, and our previous models for homogeneous sediments (Testik et al., Phys. Fluids, 17, 2005) were modified for the case of bimodal mixtures. It was also demonstrated that selective grain transport leads to a characteristic sediment-sorting pattern with coarse grain accumulating along the ripple crests and fine grain along the ripple troughs. Secondly, the ripple decay in homogenous sediment under oscillatory flow and turbulence was studied, where established ripples were subjected either to weak oscillatory flow (below the threshold for ripple formation) or turbulence generated by an oscillating grid/plate or a combination thereof. In all cases, decrease in ripple height was documented and explained using a simple ripple diffusion model. The values of effective ripple diffusivity were estimated in experiments and compared with available field data. The results obtained will be useful in improving acoustic sensing of seafloor and modelling of wave attenuation by bottom roughness produced by ripples.

Keywords:

• vortex ripples
• ripple decay
• sediment sorting
• turbulence
• ripple diffusivity

Acknowledgment

This research was supported by the Office of Naval Research (ONR) Grant N00014-04-1-0626. The authors thank the two referees for their useful comments, which helped to improve the manuscript substantially.