Investigation of a self-organization model for beach cusp formation and development

A recent numerical investigation of "self-organization'' [Werner and Fink, 1993] suggests that the feedback process between currents and sediment resp onse can result in "self-organized" patterns and can be used to predict bea ch cusp formation and spacing. A similar model based on self-organization i s tested here in order to understand the processes occurring during beach c usp formation and development, to evaluate the sensitivity toward the param eters used, and to examine how the model might relate to field observations . Results obtained confirm the validity of the self-organization approach a nd its capacity to predict beach cusp spacing, with values in fair agreemen t 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 cyc les reveal a dynamical system with significant unpredictable behavior. Cusp spacing tends to change with time, and cusp regularity shows large long-te rm variations. Cusps are found to be accretionary in the swash zone, and in agreement with most observations, mean flows are horn divergent over devel oped topography. Simulations over nonplanar slopes characterized by the pre sence of preexisting nonrhythmic or cuspate features have been performed. R esults indicate that preexisting large-amplitude cusps are destroyed if the ir spacing is substantially different from that expected under self-organiz ation and that the final spacing is consistent with that predicted by the m odel for an equivalent plane beach. These findings support the hypothesis t hat self-organization is a robust mechanism for beach cusp formation.