Infragravity waves over a natural barred profile

Abstract

Measurements of cross‐shore flow were made across the surf zone during a storm as a nearshore bar became better developed and migrated offshore. Measured infragravity band spectra were compared to synthetic spectra calculated numerically over the natural barred profile assuming a white run‐up spectrum of leaky mode or high‐mode edge waves. As in earlier studies, the spectra compared closely; however, for some frequencies the energy of the measured spectrum exceeded the energy of the synthetic spectrum, suggesting that the run‐up spectrum was not white but had dominating frequencies. Utilizing cross‐shore flow data and synthetic spectra from a number of cross‐shore locations, an equivalent run‐up spectrum was calculated for each day. On the first day of the storm, the equivalent run‐up spectrum indicated a dominant wave that had a node in velocity reasonably close to the bar crest. Later during the storm, when the bar had migrated farther offshore, there was no evidence for a dominant motion having a velocity node at the bar crest. The structure of the equivalent run‐up spectrum compared well with spectra of direct measurements of run‐up obtained several hundred meters away. We have no clear evidence in support of the theory that infragravity waves might form or force the offshore migration of a bar. To confirm this finding, longer records obtained synoptically over a developing bar are required. The dominant wave observed early in the storm was consistent with Symond and Bowen's (1984) theoretical prediction of resonant amplification of discrete frequencies over a barred profile.