Bar‐trapped edge waves and longshore currents

Abstract

Abstract. Edge waves react to the presence of a mean longshore current as though the current was a change in bottom topography. In this way, the cross-shore shape of the edge wave can be amplified over the current, relative to the shoreline, in the same way as the edge wave shape can be amplified over bars. On a beach with both longshore currents and bars, the degree of edge wave amplification (or trapping) depends on the strength and location of the longshore current shear relative to the steepness of the bottom slope and also on the direction of edge wave propagation relative to the longshore current. Weak longshore current shear over well-developed bars does not significantly alter edge wave trapping over bars simply enhancing or reducing the existing bar trapping effect. Conversely, strong current shears over gentle topography can govern the existence and location of edge wave trapping, the extreme example being edge wave trapping on plane beaches where the amplification is entirely due to the existence of mean longshore currents. In reality the possibilities are restricted, since the shape of the longshore current profile is primarily determined by the pattern of incident wave breaking, which itself depends on the beach profile. The importance of realistic current shear relative to realistic bottom slope in determining edge wave shape is demonstrated using frequency-wavenumber spectra taken during the Duck Experiment on Low-Frequency and Incident-band Longshore and Across-shore Hydrodynamics (DELILAH) at Duck, North Carolina. These frequency-wavenumber spectra show a general dominance of edge waves traveling with the longshore current. In cases where the current shear is strong relative to the bottom slope, the location of edge wave trapping changes as the longshore current profile changes with the tide. When the bar was well defined, the longshore current shear was rarely strong enough to influence the existence or location of edge wave trapping, but could have a enhancing effect. While past work [i.e. Sch6nfeldt, 1995] has suggested that edge waves, trapped and amplified on longshore currents, may cause bars to form on plane beaches and bars to move on barred beaches, these observations suggest that under most conditions, edge wave trapping on longshore currents is a subtle effect, modulated by sea level changes, and is unlikely to produce significant morphological change unless the current is very strong.

Field observations of swash zone infragravity motions and beach cusp evolution

“…In addition, only one profile extending across the surf zone was available during the experiment, possibly resulting in errors in predictions of high-mode edge waves, which are sensitive to the location of a surf zone sandbar. Previous surf zone studies [Huntley et al, 1981;Guza, 1987, 1993;Bryan and Bowen, 1998] have shown favorable agreement of current meter data with numerical predictions of edge waves. Recent numerical modeling [Reniers et al, 2004] suggests that the interaction between rhythmic surf zone morphology and incident waves forces the development of edge waves.…”

“…Differences between the energy-weighted, zero-forced, regression lines in the wave number -frequency domain and numerical predictions using a model that accounts for the effects of cross-shore variations to alongshore currents and bathyme- try [Falqués and Iranzo, 1992;Howd et al, 1992] can be well above 50% (Figure 4). Such differences could not be related to the height or slope of the cusps, nor to tidal levels, but potentially could originate from approximations in the edge wave model, which is linear, does not account for swash zone morphology and circulation patterns, uses the average alongshore beach profile, does not consider alongshore current nonuniformity, and is sensitive to changes in the beach profile [Bryan and Bowen, 1998]. In addition, only one profile extending across the surf zone was available during the experiment, possibly resulting in errors in predictions of high-mode edge waves, which are sensitive to the location of a surf zone sandbar.…”

“…Within the region where edge waves have been observed in the past [Huntley et al, 1981;Oltman-Shay and Guza, 1987;Howd et al, 1992;Bryan and Bowen, 1998], lines of energy were observed in some of the spectra. A least squares technique (described in Appendix A) was used to determine the dimensionless slope of and relative amount of energy (g) along these lines.…”

“…[3] Edge waves are ubiquitous in the surf zone [Huntley et al, 1981;Oltman-Shay and Guza, 1987;Howd et al, 1992;Bryan and Bowen, 1998;Ö zkan-Haller et al, 2001]. In contrast, field observations of infragravity motions in the swash zone [Guza and Thornton, 1982;Holman and Sallenger, 1985;Holland et al, 1995;Raubenheimer and Guza, 1996;Ruessink et al, 1998;Holland and Holman, 1999;Raubenheimer, 2002] have not been used to show how the edge wave dispersion relationship might be modified by the presence of swash morphology.…”

Infragravity wave contribution to surf zone sediment transport — The role of advection

“…For the ideal case of rhythmic crescentic bar development, the model requires systematically alongshore-varying magnitudes of sediment advection, for example caused by narrow-banded edge waves (Aagaard and Bryan, 2003) or, in the more common case of broad-banded edge waves, by the trapping and (local) amplification of cross-shore edge wave length scales fitting the underlying linear bar structure (e.g. Symonds and Bowen, 1984;Bryan and Bowen, 1998) leading to dominant alongshore length scales over the bar crest.…”