Computer simulations of self-organized megaripples in the nearshore

Abstract

[1] 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.

Observations of megaripples in the surf zone

“…The megaripple domain was characterised by higher levels of turbulence than the wave ripple domain (Fig. 13) consistent with their presence under breaking waves and this supports the hypothesis proposed by Gallagher (2011), according to which megaripples appear in the surf zone following a bed perturbation, for example in the form of scour pits generated by breaking waves. Scour pits were identified on the seabed as the breaker zone passed the instruments on the ebbing tide on Day 1 and preceding the (re)appearance of megaripples.…”

“…Under shoaling waves and lower bed shear stresses, wave ripples (re-)appeared. These observations support the model proposed by Gallagher (2011) according to which megaripples appear spontaneously when conditions allow, and are possibly preceded by bed perturbations in the form of scour holes formed by breaking waves. In the first few hours of their existence (i.e.…”

“…According to Gallagher (2011) formation from a flat bed requires an initial bed perturbation and Clarke and Werner (2004) observed that megaripple formation occurred within a time span of 1.7-3.1 h after surf zone conditions had been established; bedform growth was linear over the first 12 h and subsequently logarithmic with time.…”

Observations of bedforms on a dissipative macrotidal beach

“…Self-organization theory suggests that nearshore bedforms including cusps, ripples and megaripples either grow or remain stable in size, depending on the forcing conditions (Clarke and Werner 2004;Gallagher 2011). Where hydrodynamic conditions do result in bedform modification, the timeframe for change may be long (Soulsby 1997), leading to the potential for the hydrodynamic conditions to be out of equilibrium with the seabed.…”

Megaripple dynamics on a dissipative sandy beach

“…Self-organization based models have provided a mechanism for wavelength growth, and are able to predict both stable wavelengths and replicate growth characteristics (Coco and Murray, 2007;Gallagher, 2011). Gallagher (2011) modelled selforganized megaripples in the nearshore and found that the selforganization model of bedform spacing leads to the prediction that the megaripples will grow continually.…”