Flash rips and surf eddies are transient horizontal structures of the order of 10 to 100 m, which can be generated in the surfzone in the absence of bathymetric irregularities. They are traditionally evaluated in a depth averaged setting which involves intrinsic horizontal shear instabilities and the direct generation of vorticity by short-crested waves. In this article, we revisit the processes of surf eddy generation with a new three-dimensional wave resolution model (CROCO) and provide a plausible demonstration of new 3D non-hydrostatic instability and turbulent cascade. We first present a quick overview of a compressible free surface approach suitable for nearshore dynamics. Its ability to simulate the propagation of surface gravity waves and nearshore wave-driven circulation is validated by two laboratory experiments. Next, we present a real world application from Grand Popo Beach, Benin, forced by waves with frequency and directional spreading. The generation of surf eddies by the 3D model differs from depth-averaged models, due to the vertical shear associated with shallow breaking waves. In this case, the generation of eddies from both horizontal shear instability and the breaking of short-crested waves is hampered, the former by stretching the alongshore current and the latter by inhibiting the inverse energy cascade. Instead, the vertical shear flow is subjected to forced wave group variability and Kelvin–Helmholtz type instability at an inflection point. Primary and secondary instabilities generate spanwise and streamwise vorticity connecting small-scale eddies to larger horizontal surfzone structures. Streamwise filaments, appearing as 5 m wide ribs or mini-rips, can extend beyond the surfzone but with moderate energy. These results appear consistent with the velocity spectra and observed patterns of tracers and suspended sediments at Grand Popo Beach. The timescale associated with the mean shear-induced turbulence is several times the wave period and suggests an intermediate range between breaker-induced turbulence and large-scale surf eddies.

闪光裂隙和涡流是大约10至100 m的瞬态水平结构，可以在没有测深不规则性的情况下在冲浪带中生成。传统上，它们是在深度平均设置中进行评估的，该设置涉及固有的水平剪切不稳定性和短波引起的涡旋直接产生。在本文中，我们使用新的三维波浪分辨率模型（CROCO）回顾了涡流生成的过程，并为新的3D非静水不稳定性和湍流级联提供了合理的证明。我们首先简要介绍适用于近海动力学的可压缩自由表面方法。通过两个实验室实验验证了其模拟表面重力波传播和近海波驱动的环流的能力。下一个，我们展示了来自贝宁大波波海滩的现实世界中的应用，它受到了具有频率和方向性扩展的波浪的强迫。3D模型产生的涡流与深度平均模型不同，这归因于与浅破碎波相关的垂直剪切力。在这种情况下，水平剪切不稳定性和短波的破裂都会阻碍涡流的产生，前者通过拉伸近海流，后者则通过抑制逆能量级联而受到阻碍。取而代之的是，垂直切变流在拐点处受到强迫波群的变化和Kelvin-Helmholtz型不稳定性的影响。初级和次级不稳定性会产生沿水平和沿流的涡旋，将小涡旋连接到较大的水平海浪带结构。细流丝 出现为5 m宽的肋骨或小裂痕，可以延伸到海浪区之外，但能量适中。这些结果与大波波海滩的示踪剂和悬浮沉积物的速度谱和观察到的模式一致。与平均切变引起的湍流相关的时间尺度是波浪周期的几倍，并且表明在破碎机引起的湍流和大规模涡旋之间处于中间范围。