Bar‐trapped edge waves and longshore currents

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. Schönfeldt, 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.