Longshore structure of infragravity wave motions

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