Vegetation is an essential component of natural rivers and has significant effects on flow and morphodynamic processes. Although progress has been made in characterizing flow resistance in vegetated flows, the impact of vegetation on bed shear stress, a key driver of sediment transport, still needs better characterization and understanding. This research, explores bed shear stress and near-bed flow characteristics in sparse arrays of rigid emergent cylinders mimicking vegetation over a rough bed. For this purpose, a novel adaptation of a shear plate was used to measure bed shear stress at the canopy scale. These measurements were analyzed in relation to spatially averaged near-bed flow quantities for different array densities. The results show that, for a constant water depth, the investigated cylinder canopy enhances the ratio between bed shear stress and bulk flow velocity (i.e., Darcy-Weisbach bed friction factor) compared to unobstructed open-channel flows, and that this ratio increases with array density. Moreover, higher near-bed velocities were observed for higher array densities. On the other hand, no influence of the cylinder array on near-bed values of turbulent kinetic energy and turbulent stresses was observed. Finally, it is shown that the thickness of the near-bed layer is a suitable parameter to scale the ratio between bed shear stress and bulk flow velocity in vegetated rough bed flows.

中文翻译：

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刚性挺水植被稀疏阵列的床切应力和近床流

植被是天然河流的重要组成部分，对水流和形态动力过程具有重要影响。尽管在表征植被流的流动阻力方面已经取得了进展，但植被对河床剪应力（沉积物输送的关键驱动因素）的影响仍然需要更好的表征和理解。这项研究探讨了模拟粗糙床上植被的稀疏刚性涌现圆柱体阵列中的床剪应力和近床流动特性。为此，使用了一种新颖的剪切板改造来测量冠层规模的床剪切应力。根据不同阵列密度的空间平均近床流量对这些测量结果进行了分析。结果表明，对于恒定的水深，与无阻碍的明渠水流相比，所研究的圆柱冠层提高了床层剪应力和总体流速之间的比率（即达西-魏斯巴赫床层摩擦系数），并且该比率随着阵列密度。此外，对于较高的阵列密度，观察到较高的近床速度。另一方面，没有观察到圆柱阵列对近床湍流动能和湍流应力值的影响。最后，结果表明，近床层的厚度是衡量植被粗糙床流中床剪应力与总体流速之比的合适参数。