BOTTOM STRESS MODIFICATION BY BREAKING WAVES WITHIN A LONGSHORE CURRENT MODEL

J.C. Church, E.B. Thornton

Abstract


Early radiation stress models of longshore current generation (Bowen 1969, Longuet-Higgins 1970a, 1970b, Thornton 1970) employing monochromatic wave models produced reasonable cross-shore current distributions over planar beaches, but relied heavily on horizontal mixing for smoothing of the velocity profile. Such mixing is required because the radiation stress associated with the alongshore component of the wave-induced momentum flux, Syx, is, in theory, conserved outside the surf zone, but at the singular location of breaking predicted for monochromatic waves experiences instantaneous decay, and so an infinite gradient in radiation stress. Waves observed in nature are seldom monochromatic and so more recent models of wave height transformation employ random wave height descriptions. This randomness is normally invoked through use of a representative statistic, such as H^, via either a probabilistic (eg. Thornton and Guza (1983)) or a deterministic/Monte Carlo (eg. Dally et al. (1985)) approach. Thornton and Guza (1986) found that for the near-planar beach at Santa Barbara, the distribution of breaker locations produced through such randomness, and the resulting smoothing of the rms-wave height decay, yielded a satisfactory velocity profile without the inclusion of a horizontal mixing term. The random wave height model is not, however, able to explain longshore currents on barred beaches. The same radiation stress approach which performs well on a planar beach now predicts two maxima in forcing (over the bar and at the shore) and, if mixing is omitted, two maxima in longshore current velocity. This is in direct conflict with observations from the DELILAH experiment, (an acronym for Duck Experiment on Low-frequency and Incident-band Longshore and Across-shore Hydrodynamics), which generally show a single maximum in longshore current over the trough, where the radiation stress gradient is hear zero.

Keywords


bottom stress; breaking waves; longshore current; current model

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