Understanding the boundary layer flow structure at the tip of a swash flow is important for improving predictions of wave run-up, coastal flooding and sediment transport processes on beaches, but this is difficult to investigate experimentally. Recently, there has been debate regarding the mechanics of boundary layer growth during the uprush flow. Here, an extensively validated RANS model is used to investigate the swash tip dynamics during a dam-break driven swash event. The model enables the analysis of the spatial and temporal variation of important parameters, in particular the water surface gradients, the bed shear stress, and the non-uniformity of the velocity profile. The rate of flow convergence of surface particles toward the tip, and the ratio of the depth-averaged flow velocity and celerity of the swash tip are also obtained. The flow depth varies with distance behind the wave tip approximately as a power law, with a power of 1/2–3/4. The surface elevation dips offshore at distances greater than 0.2 m behind the front. The shear stress decreases quite slowly in the spatial region immediately behind the wave tip, and is in very good agreement with the flat-plate boundary layer model of Barnes and Baldock (2010). The model results indicate that the boundary layer structure is well-developed or depth-limited at the wave tip, and fits a power-law velocity profile with an exponent of order 1/3, consistent with a rough bed. The normalised vertical variation in the velocity profile is very uniform in the region 1 m behind the wave tip, and matches that for a 1/3 power law. Alternate logarithmic fits to the velocity profile yield consistent values of bed roughness at different locations, but corresponding to a grain size that is about twice that used in the experiment. The rate of flow convergence of surface particles toward the wave tip is also consistent with a fully developed boundary layer and a 1/3 power law.

中文翻译：

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在粗糙床上传播的斜盘尖端流动结构的数值研究

了解斜流尖端的边界层流结构对于改进对波浪爬高、沿海洪水和海滩沉积物输送过程的预测很重要，但这很难通过实验进行研究。最近，关于上升流期间边界层生长的机制一直存在争论。在这里，一个经过广泛验证的 RANS 模型用于研究溃坝驱动的斜盘事件期间的斜盘尖端动力学。该模型能够分析重要参数的空间和时间变化，特别是水面梯度、床层剪切应力和速度剖面的不均匀性。还获得了表面粒子流向尖端的收敛速率，以及斜盘尖端的深度平均流速和速度之比。流动深度随波尖后面的距离而变化，近似为幂律，幂为 1/2-3/4。地表高程在锋面后方大于 0.2 m 的距离处向近海倾斜。在紧接波尖后面的空间区域中，剪切应力下降得非常缓慢，并且与 Barnes 和 Baldock (2010) 的平板边界层模型非常吻合。模型结果表明，波尖边界层结构发育良好或深度受限，符合指数为1/3的幂律速度剖面，与粗糙层一致。在波尖后面 1 m 的区域内，速度剖面的归一化垂直变化非常均匀，与 1/3 幂律相匹配。速度剖面的交替对数拟合在不同位置产生一致的床粗糙度值，但对应的晶粒尺寸约为实验中使用的两倍。表面粒子流向波尖的收敛速率也符合完全发展的边界层和 1/3 幂律。