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Effects of wave-current interaction on shear instabilities of longshore currents

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  • We examine the effects of wave-current interaction on the dynamics of instabilities of the surf zone longshore current. We utilize coupled models for the simulation of the incident waves and the wave-induced nearshore circulation. The coupling between the models occurs through radiation stress gradient terms (accounting for the generation of nearshore circulation) and through wave-current interaction terms (leading to the modification of the wave field by the generated circulation field). Simulations are carried out with a realistic barred beach configuration and obliquely incident waves for two frictional regimes. The results show that the shear instabilities of the longshore current have a significantly altered finite amplitude behavior when wave-current interaction effects are included for beaches with relatively high frictional damping. The primary effects are a reduction of the offshore extent of the motions and a delay of the onset of instabilities. In addition, the energy content of the motions within two surf zone widths is reduced, the propagation speed increases, and tendency to form offshore directed jets is reduced. The horizontal mixing induced by the instabilities is also reduced when wave-current interaction is considered, leading to a larger peak mean longshore current and a larger offshore current shear. These effects appear to be primarily linked to a feedback mechanism, whereby the incident wave field gains energy at locations of offshore directed currents. For more energetic shear instability fields that occur when frictional damping is small, this feedback affects the propagation speed and energy content of the instabilities near and onshore of the current peak only minimally. However, the offshore extent of the motions and the tendency to shed vortices offshore are still reduced. A reduction in the mixing due to the instabilities is evident offshore of the current peak, hence the mean longshore current profile is only affected offshore of the current peak. The inclusion of wave-current interaction significantly affects the shear instability signature observed in the shoreline runup for either frictional regime. These results indicate that the energy content and frequency extent of the shoreline response is increased markedly due to the wave-current interaction process. This effect appears to be related to variations in the forcing of the circulation that arise due to the refraction of the incident waves around offshore directed features of the circulation.
  • Keywords: longshore currents, hydrodynamic instability, wave-current interactions, surf zone circulation
  • Keywords: longshore currents, hydrodynamic instability, wave-current interactions, surf zone circulation
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  • Özkan-Haller, H. T., and Y. Li, Effects of wave-current interaction on shear instabilities of longshore currents, J. Geophys. Res., 108(C5), 3139, 2003.
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  • 108
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