Longshore structure of infragravity wave motions

Abstract

Synchronous runup records were collected from 14 locations spaced irregularly over a 7 km stretch of a low‐slope beach. The significant runup height, herein defined as the significant vertical excursion of water level at the shoreline, was typically 2 m, 60% of the incident significant wave height at the breakpoint. The runup spectra were dominated by the energy at low frequencies, with 99.9% of the variance in motions with periods longer than 20 s and 83% for periods longer than 50 s. A peak between 0.005 and 0.010 Hz was present in all spectra. Analysis of this band showed the motion to be a standing edge wave with wavelength between 5 and 10 km and significant edge wave height at the antinode of 1.3 m, accounting for half of the local significant runup height. The edge wave period was approximately 140 s and the mode number in the range 3–7. An apparently linear sand bar was present 175 m offshore, roughly the same location as the first node of cross‐shore velocity for the edge wave. While the edge wave could not have been the cause of the bar (a standing edge wave does not create a linear bar), the opposite may be true, with the bar potentially providing a topographic resonance. As the local head‐lands extend only a small fraction of the offshore distance of the edge wave, they are unlikely to be significant reflectors. The single rip current observed occurred at the edge wave node.