Test of edge wave forcing during formation of rhythmic beach morphology

Abstract

This paper reports on concurrent measurements of evolving rhythmic beach morphology and the associated water motion on a macrotidal, coarse sandy beach. Beach cusps with a spacing of 19 m developed over a 3‐day period of low‐energy swell after which they were destroyed by energetic wind waves. The beach cusps were clearly accretionary features and developed mainly during the rising tide. Detailed hydrodynamic measurements were conducted at three locations in the swash and inner surf zone in the vicinity of a developing cusp horn and revealed that energetic standing, mode‐zero, subharmonic edge waves were not present during the initiation of the beach cusps. Specifically, analysis of the hydrodynamic data indicated that: (1) the amount of subharmonic energy was limited (<10% of the total energy); (2) none of the computed energy spectra exhibited a significant peak at the subharmonic frequency; (3) the cross‐shore velocity amplitude at the subharmonic frequency was 2–3 times larger than the longshore velocity amplitude; (4) the longshore current motion at the subharmonic frequency was barely coherent between adjacent locations in the surf zone; and (5) the time series of pressure and cross‐shore current were in phase at either side of the developing cusp horn. It is concluded that, at least during this event of beach cusp formation, energetic standing edge waves are not a prerequisite to initiate the development of rhythmic beach morphology.