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Abstract. A recent numerical investigation of "self-organization" [Werner and Fink, 1993] suggests that the feedback process between currents and sediment response can result in "self-organized" patterns and can be used to predict beach cusp formation and spacing. A similar model based on self-organization is tested here in order to understand the processes occurring during beach cusp formation and development, to evaluate the sensitivity toward the parameters used, and to examine how the model might relate to field observations. Results obtained confirm the validity of the self-organization approach and its capacity to predict beach cusp spacing, with values in fair agreement with the available field measurements, with most of the input parameters primarily affecting the rate of the process rather than the final spacing. However, changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long-term variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over developed topography. Simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed. Results indicate that preexisting large-amplitude cusps are destroyed if their spacing is substantially different from that expected under self-organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach. These findings support the hypothesis that self-organization is a robust mechanism for beach cusp formation.

[1] Extensive measurements of ripple characteristics and dynamics along with associated suspended sediment fluxes and hydrodynamic conditions were made in the shoaling and surf zones of a macrotidal coarse grained beach at Sennen Cove, Cornwall, England (median grain diameter 0.69 mm). Suborbital vortex ripples were observed during the majority of the study period with height $5 cm and length $35 cm. The scale and shape of the ripples did not vary significantly as the bed shear stress increased during wave shoaling and breaking. However, ripple migration rates (onshore directed) were strongly dependent on their location relative to the breakpoint, increasing from $0.1 cm min À1 under shoaling waves to 2 cm min À1 in the outer surf zone during low-energy conditions. Farther inside the surf zone, ripples persisted but migration rates slowed, probably owing to the presence of the offshore-directed mean flow which impedes landward migration of the ripples. Under low-wave conditions (during which measured sediment fluxes peaked around the outer surf zone and decreased through the saturated surf zone), bed form transport rates under shoaling waves were of the same magnitude as net suspended sediment fluxes but at least an order of magnitude smaller in the outer surf zone. Under high-energy conditions (during which suspended sediment fluxes in the surf zone were offshore directed owing to the presence of the seaward directed mean flow), bed form transport rates were several orders of magnitude smaller than suspended fluxes.Citation: Masselink, G., M. J. Austin, T. J. O'Hare, and P. E. Russell (2007), Geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach,

[1] Cross-shore sediment transport processes are investigated with measurements of horizontal velocity and sediment suspension obtained with electromagnetic current meters and optical backscatter sensors on five different beaches across Europe. Data were gathered under a wide variety of hydrodynamic and morphological conditions spanning the swash, surf, and shoaling zones. Results show that the near bed velocity moments, normalized by the local energy level (hu t 2 i n ), have consistent shapes (shape functions) when plotted against normalized cross-shore depth (h/h b ). According to the energetics approach, near bed velocity moments are good descriptors of sediment transport processes, consequently, the shape functions describe and quantify the cross-shore distribution of the most relevant cross-shore sediment transport processes. The pattern consists of net onshore transport in the swash zone, offshore transport in the surf zone, and onshore transport in the shoaling zone thereby producing divergence of sediment in the inner surf/swash zone and convergence in the breaker zone. In consequence, the shape function supports the breakpoint hypothesis for sandbar generation. This behavior is a product of the balance between multiple opposing mechanisms including undertow, coupling between mean flows and short (long) wave stirring, short (long) wave skewness, and long wave coupling with short wave variance. The cross-shore structure of the measured and normalized cross-shore sediment fluxes is consistent with the velocity moment shape functions, showing the robustness of the approach. Apart from the shortcomings bound to the energetics approach, the present parameterization is also limited due to the difficulty of defining with confidence a breaker depth (h b ) in bar-trough systems.