浅滩地形上波浪破碎对低频波能放大的影响

Effects of wave breaking on the evolution of low-frequency wave energy over shoal topography

低频长波对港湾共振、泥沙输运、波浪爬高与越浪等过程有重要影响。采用非静压模型SWASH模拟了不规则波在浅滩上的传播及破碎过程,重点探讨了滩顶短波破碎程度对低频波能演变的影响。模拟结果显示,当滩顶短波处于临界破碎状态时,低频波能流沿浅滩持续增长,浅滩对低频长波的放大效应(滩后与滩前低频波能流之比)显著;当滩顶短波轻微破碎、破波仅发生在外破波区时,低频波能流的沿滩增长率进一步变大;当滩顶短波剧烈破碎、破波延续到内破波区时,低频波能流沿滩先增长而后转为衰减,滩后放大率较临界破碎时明显减小。研究结果表明,浅滩顶部水深改变了短波破碎程度,进而影响低频长波的演化过程,浅滩上长波总体增长率随滩顶水深的减小呈现先增大后减小的规律,在短波轻微破碎时最大。

Low-frequency waves contribute considerably to nearshore hydrodynamics and engineering problems,such as the issues of harbor resonance,sediment transport,and wave runup and overtopping.This study simulated the transformation of irregular waves over near-shore shoals using the non-hydrostatic wave model SWASH,with emphasis on the effects of short⁃wave breaking on the evolution of low-frequency wave energy flux.It was found that,when short waves over the top of the shoal were in a critical breaking state,low⁃frequency wave energy flux kept increasing over the entire shoal,and the amplification effect of the shoal on low⁃frequency waves(quantified by the ratio of low-frequency wave energy flux behind the shoal to that in front of the shoal)was significant;When short waves broke slightly and wave breaking was confined within the outer surf zone,the increase of low⁃frequency wave energy flux along the shoal was further more significant;When wave breaking was violent and the breaking process lasted into the inner surf zone,the low-frequency wave energy flux turned to attenuate in the inner surf zone rather than being magnified.The results of this study indicate that the water depth on top of the shoal influences the evolution of low⁃frequency waves by altering the intensity of short-wave breaking;Amplification of low-frequency waves by the shoal increases first and then decreases with decrease of water depth on top of the shoal and achieves a maximum when short waves break slightly over the shoal.