Widely distributed in natural rivers and coasts, vegetation interacts with fluid flows and sediments in a variable and complicated manner. Such interactions make it difficult to predict associated drag forces during sediment transport. This paper investigates the drag coefficient for an emergent vegetated patch area under nonuniform flow and mobile bed conditions, based on an analytical model solving the momentum equation following our previous work (Zhang et al., 2020, https://doi.org/10.1029/2020WR027613). Emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different vegetation coverage ∅. Laboratory flume tests were conducted to measure variations in both the water and bed surfaces along a vegetated patch on a sand bed. Based on the experimental and theoretical analyses, a dimensionless drag model integrating both terms of flow properties and bed effects is proposed to predict the drag coefficient C_d over a mobile bed. The calculated values of C_d exhibit two different trends, that is, nonmonotonically or monotonically increasing along the streamwise direction, due to the combined effect of water surface gradient and bed slope. The morphodynamic response of the mobile bed to nonuniform flow manifests as an evolution in the bed slope within the vegetated patch. Ongoing scouring directs the flow's energy toward overcoming the rising C_d and bed slope, leading to a relatively stable stage with a low sediment transport rate. This study advances the existing understanding of the drag coefficient's role over a mobile bed within nonuniform flows. It also enhances the applicability of vegetation drag models in riverine restoration.

中文翻译：

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移动沙床上浅层非均匀流中挺水植被的阻力系数

植被广泛分布在天然河流和海岸中，与流体和沉积物以变化而复杂的方式相互作用。这种相互作用使得预测沉积物运输过程中相关的阻力变得困难。本文基于我们之前的工作求解动量方程的分析模型，研究了非均匀流动和移动床条件下新兴植被斑块区域的阻力系数（Zhang et al., 2020, https://doi.org/10.1029 /2020WR027613）。挺水植被采用不同植被覆盖度∅交错排列的刚性圆柱体进行建模。进行实验室水槽测试是为了测量沙床上植被斑块上水和床表面的变化。基于实验和理论分析，提出了一种结合流动特性和床效应的无量纲阻力模型来预测移动床的阻力系数C _d 。由于水面梯度和河床坡度的共同作用， C _d的计算值沿流向呈现非单调增加和单调增加两种不同的趋势。移动床对不均匀流动的形态动力学响应表现为植被斑块内床坡度的演变。持续的冲刷引导水流的能量克服不断上升的Cd和河床坡度，从而形成一个相对稳定的阶段_，泥沙输送率较低。这项研究增进了对非均匀流中移动床阻力系数作用的现有理解。它还增强了植被阻力模型在河流恢复中的适用性。