Edge waves and beach cusps
Edge waves and beach cusps
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DOI：
10.1029/jc080i021p02997
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发表时间：
1975-07
影响因子：
--
通讯作者：
R. Guza;D. Inman
中科院分区：
文献类型：
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作者：
R. Guza;D. Inman
从成因上讲，海滩尖点至少有两种类型：与涌动的、主要被反射的入射海浪相联系的海滩尖点(反射系统)，以及在波浪破碎和近岸环流很重要的海滩上产生的海滩尖点(耗散系统)。在实验室和某些选定的自然情况下，在反射波条件下形成的一些尖点的间距与假设的模型是一致的：(1)零模数的亚谐边缘波(周期是入射波的两倍)或(2)同步(等于入射波的周期)的低模边缘波。实验表明，在实验室不可擦除的平面海滩上，只有当入射波在海滩上强烈反射时，才会产生可见的亚谐边缘波，并且这种观测是量化的。边波共振理论和实验表明，同步势边波的产生也可以发生在反射海滩上，是一种比亚谐振型更高阶、更弱的共振。在耗散系统中，沿岸周期性运动的模式，而不是潜在的边缘波，在控制环流单元和海滩地貌的沿岸尺度方面可能是重要的。在反射平面的实验室海滩上，最初大的次谐边缘波将沙子示踪剂反射成类似于天然海滩尖端的形状，但边缘波的幅度随着尖端的增长而减小。因此，尖端的生长受到从尖端到边缘波激发过程的负反馈的限制。较小的边缘波可以通过在适当地位于激流带的护堤上提供不稳定的扰动来形成沿岸周期性地貌。在这种情况下，由边缘波引起的后退入射波浪在护堤中被引导到护堤中，并且从地形到岸边周期性扰动有初始的正反馈。
Genetically, beach cusps are of at least two types: those linked with incident waves which are surging and mostly reflected (reflective systems) and those generated on beaches where wave breaking and nearshore circulation cells are important (dissipative systems). The spacings of some cusps formed under reflective wave conditions both in the laboratory and in certain selected natural situations are shown to be consistent with models hypothesizing formation by either (1) subharmonic edge waves (period twice that of the incident waves) of zero mode number or (2) synchronous (period equal to that of incident waves) edge waves of low mode. Experiments show that visible subharmonic edge wave generation occurs on nonerodable plane laboratory beaches only when the incident waves are strongly reflected at the beach, and this observation is quantified. Edge wave resonance theory and experiments suggest that synchronous potential edge wave generation can also occur on reflective beaches and is a higher-order, weaker resonance than the subharmonic type. In dissipative systems, modes of longshore periodic motion other than potential edge waves may be important in controlling the longshore scale of circulation cells and beach morphologies. On reflective plane laboratory beaches, initially large subharmonic edge waves rear-rage sand tracers into shapes which resemble natural beach cusps, but the edge wave amplitudes decrease as the cusps grow. Cusp growth is thus limited by negative feedback from the cusps to the edge wave excitation process. Small edge waves can form longshore periodic morphologies by providing destabilizing perturbations on a berm properly located in the swash zone. In this case the retreating incident wave surge is channelized into breeches in the berm caused by the edge waves, and there is an initially positive feedback from the topography to longshore periodic perturbations.