Regularity and randomness in the formation of beach cusps

Controversy still exists on whether rhythmic features such as beach cusps a re the result of the presence of standing edge wave motions in the hydrodyn amics or if they are the result of self-organising processes. The compatibi lity between the two mechanisms is here investigated through the use of a n umerical model simulating the formation and development of beach cusps. Sim ulations characterised by different forcing conditions have been performed. A series of 'random' simulations, each individual run simply differing fro m the others in the seed used in the random number generator, showed the ty pical self-organisation behaviour with the features appearing at different locations and even with slightly different spacing. 'Regular' series of sim ulations have been run by changing the wavelength of the template superimpo sed by the hydrodynamic forcing and the number of cycles when the template was present. Results surprisingly indicate that even a minimum number of cy cles with a regular forcing can deterministically induce the final shorelin e configuration. Furthermore, if the forcing template has the same waveleng th as the one resulting from purely random simulations, growth rates are mu ch faster than those obtained with random conditions. Implications for the kind of field measurements necessary to discern which of the two mechanisms is responsible for beach cusp formation have also been considered. (C) 200 1 Elsevier Science B.V. All rights reserved.