Abstract Vegetation is a key source of flow resistance in natural channels and floodplains. It is therefore important to accurately model the flow resistance to inform decision makers and managers. However, it is challenging to predict the resistance of real vegetation, because vegetation models are based on relatively small-scale lab experiments with mostly artificial vegetation. Experimental tests of real vegetation under field conditions are scarce. The purpose of this study is to measure the flow resistance of a submerged willow patch, where small herbaceous vegetation was allowed to grow in between the willow stems to simulate field conditions. Detailed flow velocity measurements were performed during an full scale experiment of flow around a submerged patch of willows. The parameter values of the willow vegetation model, as well as the friction coefficients of the vegetated banks and unvegetated channel bed, were computed simultaneously using Bayesian inference using a 2D hydrodynamic model. Results show that the presence of understory growth greatly affects flow patterns and the value of the effective vegetation density parameter. Measured flow velocities in the patch with understory growth were very low, and the patch has relatively high deflection. After removal of this undergrowth, flow velocities in the patch increased and deflection of the vegetation canopy decreased. We show that estimating vegetation density using an often-used rigid cylinder estimator based on vegetation sampling, underestimated the effective value by more than an order of magnitude. We argue that proposed extensions to existing vegetation models, which can take into account understory growth and reconfiguration, could be tested under field conditions using the approach followed in this paper.

中文翻译：

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溪流尺度流量实验揭示林下生长对植被粗糙度的巨大影响

摘要 植被是天然河道和漫滩中流动阻力的主要来源。因此，准确地模拟流动阻力以告知决策者和管理者是很重要的。然而，预测真实植被的抵抗力是具有挑战性的，因为植被模型是基于相对小规模的实验室实验，主要是人工植被。在野外条件下对真实植被的实验测试很少。本研究的目的是测量沉水柳树斑块的流动阻力，其中允许在柳树茎之间生长小草本植物以模拟田间条件。详细的流速测量是在水下柳树周围的全尺寸流动实验中进行的。柳树植被模型的参数值，以及植被堤岸和无植被河床的摩擦系数，使用二维流体动力学模型使用贝叶斯推理同时计算。结果表明，林下生长的存在极大地影响了流动模式和有效植被密度参数的值。在有林下生长的斑块中测得的流速非常低，斑块的偏转相对较高。去除这种灌木后，斑块中的流速增加，植被冠层的偏转减少。我们表明，使用基于植被采样的常用刚性圆柱体估计器来估计植被密度，低估了有效值一个数量级以上。我们认为，对现有植被模型的拟议扩展，