Computer simulations of self-organized megaripples in the nearshore

Megaripples are bed forms with heights of 20-50 cm and lengths of 1-10 m that are common in the surf zone of natural beaches. They affect sediment transport, flow energy dissipation, and larger-scale hydro- and morphodynamics. They are thought to be dynamically similar to bed forms in deserts, rivers, and deeper marine environments. Here a self-organization model (similar to models for subaerial bed forms) is used to simulate the formation and development of megaripples in the surf zone. Sediment flux is determined from combined wave and current flows using stream power and bed shear stress formulations as well as a third formulation for transport based on simple rules, which represent sheet flow. Random bed irregularities, either imposed or resulting from small variations in transport representing turbulence, are necessary seeds for bed form development. Feedback between the bed and the flow, in the form of a shadow zone downstream of a bed form and increasing flow acceleration with elevation over the crests of bed forms, alter the transport such that organized bed forms emerge. Modeled bed form morphology (including cross-sectional shape and plan view) and dynamics (including growth and migration) are similar to natural megaripples. The model can be used to extend the field observations of Clarke and Werner (2004), which suggest that, if conditions remain the same, megaripples will continue to grow. Contrary to many bed form models, this model supports the idea that bed form spacing grows continually.