Observations of infragravity wave frequency selection

Field measurements of sea-surface elevation, cross-shore and longshore velocities were obtained during a storm event on the north coast of Zealand, Denmark from a cross-shore array of co-located pressure sensors and current meters. During the major part of this storm, statistically significant spectral peaks at a frequency of f≈0.025 Hz were identified at several cross-shore locations. Based on examinations of cross-shore coherence and phase relationships, it appeared that these motions were due to cross-shore standing wave structures rather than caused by shear wave activity. Comparisons with synthetic edge wave spectra, which were computed assuming a white shoreline surface elevation spectrum, suggested that the identified spectral peaks were not artifacts of a cross-shore standing wave nodal structure but an indication that those frequencies where the peaks occurred were preferentially forced. Analyses of local and spatial phase relationships of velocity and sea-surface elevation, as well as comparisons with numerically calculated theoretical cross-shore wave structures suggested that these waves were standing edge waves with mode number (n)⩾3. The selected frequency of f≈0.025 Hz corresponds to one of the cut-off frequencies that can theoretically occur on this bathymetric geometry which asymptotes to a constant depth offshore.