FIELD OBSERVATIONS OF BEACH CUSPS AND SWASH MOTIONS

We tested the hypothesis of cusp formation by the longshore structure of synchronous or subharmonic, mode-zero edge waves using detailed fie ld measurements of foreshore topography, incident waves and swash moti ons. Two distinct intervals of cusp formation were observed. Alongshor e separated swash measurements were analyzed to detect structures at w avelengths and frequencies consistent with the measured cusp spacings. For both events, no statistical support for cusp generation by the tr aditional subharmonic or synchronous edge wave mechanisms was found. H owever, once cusps became well developed, there was evidence of intera ction between fluids and topography at a wavelength equal to the cusp spacing, but at the period of the incident swash. We propose that this interaction could have been responsible for the generation of the sec ond set of cusps with a spacing of half the pre-existing cusp waveleng th.