The height of ripples, representing roughness in the shallow-water area, is an important factor in the prediction of coastal processes. However, the characterization of ripples under violent flow conditions has proven challenging. For instance, in shallow water in the swash zone, significant water fluctuation occurs owing to interactions between the up-rush and down-rush of waves. Because such phenomena shows strong nonlinearity, sufficient knowledge of swash-ripple formation remains lacking. In this study, experiments generating ripples in an oscillating water tank are conducted. Furthermore, numerical simulation using the three-dimensional DEM-MPS coupled method is performed. By comparing numerical and experimental results, validation of the present numerical simulation is demonstrated, followed by an investigation of the mechanics of ripple formation. Numerical results show that not only the drag force but pressure gradient force resulting from the water surface gradient considerably affect sand-particle motion, revealing the need for accurate evaluation of the free surface flow. Moreover, pressure gradient force plays an important role in the ripple formation process, so it should be reflected in the numerical model for ripple simulations.

代表浅水区粗糙度的波纹高度是预测沿海过程的重要因素。但是，事实证明，在剧烈流动条件下表征波纹非常具有挑战性。例如，在斜流带的浅水中，由于波浪的上冲和下冲之间的相互作用，发生了明显的水波动。由于这种现象显示出很强的非线性，因此尚缺乏充分的关于斜波波纹形成的知识。在这项研究中，进行了在振荡水箱中产生波纹的实验。此外，使用三维DEM-MPS耦合方法进行了数值模拟。通过比较数值和实验结果，证明了本数值模拟的有效性，然后研究波纹形成的机理。数值结果表明，不仅阻力，而且由水表面梯度产生的压力梯度力都会极大地影响砂粒运动，这表明需要对自由表面流进行准确评估。此外，压力梯度力在波纹形成过程中起着重要作用，因此应将其反映在波纹仿真的数值模型中。