Self-organising behaviour of sand ripples under waves

Abstract

The mutual interplay between sand ripples and oscillatory flow forms a complex system affecting the hydrodynamic roughness of seabeds and impacting on energy dissipation and sediment mobilisation on the coast. Discrete self-organising computer simulations, in which local rules lead to the emergence of global patterns, currently exist to model the formation of sand ripples under various conditions. However, limited laboratory validation or extension of these models to oscillatory flow has been conducted. In the current study, work was undertaken to demonstrate that ripple formation under waves can be considered a self-organising process, and modelled as such. A laboratory study enabled visual validation of self-organising behaviour, as well as quantification of the rate of ripple growth and re-growth when condition change. A numerical model, based on earlier work in wind-driven ripples, developed ripples of a plausible shape and wavelength and also simulated many of the self-organising characteristics observed in the laboratory. The results of both the laboratory study and model indicate that a self-organising process provides a probable behavioural scheme for ripple development under hydraulic oscillatory flow.