Study On Effects Of Wave Breaking On Evolution Of Low-frequency Wave Energy Flux Over Near-shore Shoals

Low-frequency waves contribute considerably to nearshore hydrodynamics and engineering problems,such as harbor resonance,sediment transport and dune and barrier breaching.Studies have shown that the shoal has amplifying effect on the lowfrequency waves under the condition of unbroken short waves,yet the effects of breaking waves on evolution of low-frequency waves over near-shore shoals still needs to be further uncovered.The effects of wave breaking on evolution of low-frequency wave energy flux over near-shore shoals were studied by means of numerical simulations with the nonhydrostatic wave model SWASH for waves advancing over nearshore shoals.First,it was verified that SWASH can accurately simulate the evolution of low-frequency waves on the near-shore shoals under the condition of wave breaking.Then,according to the results of SWASH simulation,the evolution of low-frequency wave energy flux in the front slope,the top platform and the back slope of the shoals and the final magnifications were closely related to wave breaking degree.When waves broke slightly on the front slope and wave breaking was confined within the outer surf zone,the low-frequency wave energy flux kept increasing not only over the front slope,but also throughout the rear slope of the shoal,and its amplitude was significantly magnified after passing over the entire shoal.When wave breaking was violently on the front slope and extends to the inner surf zone,the low-frequency wave energy flux turned to attenuate in the inner surf zone rather than being magnified although was magnified in the outer breaking zone,and the final magnifications of the low-frequency wave energy flux after passing over the shoal became much smaller than those induced by non-breakers.The evolution of low-frequency wave energy flux exhibited a trend of being magnified as the slope becomes milder and the topography scale extends horizontally for a consequence.Under the condition of a fixed slope,length of the surf zone extended as shallowing of the water depth on the top of the platform.Then,the final magnifications of the low-frequency wave energy flux increased first,and then decreased compared with those of non-breakers.Therefore,there must be a water depth of the top platform that makes the final magnification of the low-frequency wave energy flux reach a peak value.It was also observed that such a value was close to the value corresponding to the water depth that maximum low-frequency wave energy flux gradient achieves.Also,a shallower water depth of the top platform made the final magnification of the low-frequency wave energy flux for breakers equal to that of nonbreakers.The smaller the peak period was,the larger the amplification of low-frequency wave energy flow behind the shoal was;the smaller the spectral type cusp factor was,the larger the amplification of low-frequency wave energy flow behind the shoal was.