Assessing the Relative Energetics of "Infragravity" Motions in Lakes and Bays

Abstract

Discriminating between specific wave modes at low frequencies is difficult, especially for complex wave fields during storms. Often, the relative importance of long-period motions is assessed on the basis of total spectral variance integrated across frequency space less than some arbitrary frequency cutoff. Through popular usage, frequency cutoffs appropriate to marine systems are becoming convention. In small-scale systems, such as lakes or bays, this convention is not realistic because wave motions generally associated with the term "infragravity" often occur at frequency ranges beyond those characteristic of marine systems. Analysis procedures are suggested that allow for a more realistic interpretation of surf-zone motions in both marine and lacustrine systems. Spectra are divided into three frequency bands (low, mid, and high) rather than the usual two (low and high) according to proposed guidelines. The mid-frequency band includes wave motions that are of a mixed variety, and they should not be integrated directly with wave motions in the high (incident) or low (infragravity) frequency bands until extensive mode discrimination procedures reveal their mode or origin. Normalization of the band-integrated variances in the low-, mid-, and high-frequency bands by total variance in the spectrum removes trends in storm energy levels and facilitates the examination of the frequency distribution of energy in the wave field as the storm evolves. Calculation of white noise energy levels for each of the frequency bands provides a useful baseline for comparative purposes.