Long-period waves on a natural beach

Abstract

A field experiment is described in which three two‐component electromagnetic flowmeters were used to measure simultaneously the longshore (υ) and onshore‐offshore (u) velocity components along a line normal to the shoreline and up to 100 m offshore. Spectral analysis of the data from this experiment reveals the presence of a set of discrete spectral peaks of low frequency (0.014–0.05 Hz) which dominate over the wind wave peak close to the shoreline and which decay in amplitude with distance from the shore. The amplitudes of the velocity components for each of the four lowest‐frequency (and clearest) peaks have been plotted against distance from the shoreline and are found to compare satisfactorily with the calculated edge wave amplitude variation for the beach. Phases between u, υ at one flowmeter and between u, u at different distances offshore have also been found from cross spectra and confirm that the peaks are due to edge waves. It is suggested that each of these low‐frequency peaks corresponds to a progressive edge wave mode at the cutoff frequency for the beach. Ball (1967) and Guza and Inman (1975) calculate that the cutoff frequency υ for an edge wave of mode n should be proportional to [n(n + 1 )]1/2, where the constant of proportionality is dependent on the subaqueous beach profile. The frequencies of the four lowest‐frequency peaks agree well with the predicted frequencies for the lowest modes of cutoff edge waves (n = 1, · · ·, 4), and the observed offshore decay of amplitude at each frequency indicates that assigning the peaks to these modes is correct. Energy exchange between these cutoff modes, through nonlinear interaction, is also suggested by the results. The significance of observing progressive edge waves on this beach is briefly discussed.

Spectral signatures for swash on reflective, intermediate and dissipative beaches

“…Short-wave swash derives from locally-generated wind waves and swell (e.g., Waddell, 1976;Hughes, 1992;Holland and Puleo, 2001), whereas long-wave swash (or infragravity swash) includes leaky-mode standing waves and edge waves (e.g., Huntley, 1976;Aagaard, 1991;Holland et al, 1995;Holland and Holman, 1999). A fundamental difference between incident short-waves and surf zone long-waves is that the former usually break before reaching the beach face whereas the latter typically do not.…”

Modelling infragravity motions on a rip-channel beach

“…T m01,lo is affected by the frequency distribution of the infragravity energy density, and is therefore sensitive to the nodal structure of the standing infragravity waves (e.g. Suhayda, 1974;Huntley, 1976;Holman, 1981;Guza and Thornton, 1985, and others). As such, it is expected to vary in the cross-shore.…”

“…infragravity waves that radiate away from the surfzone, and trapped long waves (edge waves) that cannot escape from the shoreline due to strong refraction. Previous measurements (Suhayda, 1974;Huntley, 1976;Holman, 1981;Wright et al, 1982;Guza and Thornton, 1985, among others) have shown the increased contribution of infragravity motions to the total gravity wave spectrum with decreasing water depth. This effect is associated with the wave-breaking induced saturation of the incident waves, whereas the infragravity waves continue to shoal without breaking, consequently their relative contribution increases rapidly as the shoreline is approached and can reach energy levels significantly higher than the incident wind waves (Wright et al, 1982;Guza and Thornton, 1985).…”

“…However, measurements of infragravity motions often give little idea about the alongshore variation of infragravity conditions due to the fact that the measurements are isolated (Elgar et al, 1992;Okihiro et al, 1992), or restricted to the cross-shore (Huntley, 1976;Guza and Thornton, 1985;Ruessink, 1998a), or performed at what would be considered an alongshore uniform beach (Huntley et al, 1981;Oltman-Shay and Guza, 1987;Herbers et al, 1995). Most notably, measurements at a number of beaches in SouthEastern Australia suggested the expected coupling between (complex) nearshore bathymetry and the infragravity (edge-)wave field (Wright et al, 1979), due to the presence of preferentially forced infragravity frequencies in the measured surface elevation and velocity spectra.…”

Modification of a linear bar-trough system by a standing edge wave

“…Holman and Bowen (1982) proposed a model that incorporated low frequency, edge wave motions, and demonstrated theoretically that the bottom drift patterns associated with complex wave fields looked similar to many commonly observed nearshore topographic features. There can be little doubt as to the existence of edge waves (e.g., Huntley, 1976;Huntley et al, 1981;Katoh, 1981;Oltman-Shay and Guza, 1987), and Bowen and Huntley (1984) point out that there is ample evidence to suggest that "low-frequency motions are dominant over a significant region in the surf zone 0025-3227/90/$03.50 8.0. BAUER AND B. GREENWOOD and that this dominance increases in very severe conditions, precisely the conditions in which the most active sediment transport is expected to occur.…”