Beach cusps: A comparison of data and theories for their formation

In recent years, there has been considerable debate about whether beach cus p formation is associated with the presence of standing edge waves, results from self-organising feedback between changing topography and swash motion or is attributable to a number of other less popular mechanisms. In this paper we utilise a large amount of data from laboratory experiments and field studies (from lakes and sea coasts, and from calm to storm condi tions) published over the last 50 years to test the predictions of the two main cusp forming hypotheses. After a review of the relevant theories, comp arison is made between measured cusp spacing and edge wave wavelength in or der to test the edge wave theory. The self-organisation theory is examined by considering the variation of cusp spacing with some metric describing a swash length. These analyses, using more data than previous attempts, confi rm that there is a possible link between cusp development and both edge wav es and swash-sediment feedback, and that it is not possible to produce conc lusive support for one theory above the other with the simple measurements that have been made previously. Furthermore, we report evidence for a specific breaker type (plunging) bein g associated with cusp presence and suggest that if sub-harmonic standing e dge waves are present on a beach (which does not seem to be likely for a la rge number of the measurements considered, and lead to cusp development) th ere will necessarily be a link between their spacing and swash length in th e form predicted by Werner and Fink (1993).