River bars often form at river confluences due to variation in flow discharges or in sediment transport capacity; once these bars are grown, they constitute favourable habitats for vegetation development. In this work, we analysed the effect of vegetation above confluence bars on the hydrodynamics of a river reach. The case study considered is the Arno River reach at the Greve junction, where confluence bars expanded towards the opposite bank. A two‐dimensional hydraulic model was implemented through BASEMENT software varying the flow discharge. Three‐dimensional topographic data were used to generate the calculation mesh. Dendrometric surveys were carried out to describe the current state of the vegetation. Seven different vegetative scenarios were considered for numerical simulations, depending on different vegetation management conditions. Such scenarios are characterised by various plant densities, diameters, heights and species (herbaceous, shrub and arboreal), and, accordingly, different formulations were used to estimate the Manning coefficients. The outcomes of this study illustrate that the denser vegetation settled on confluence bars, the more the backwater effect is emphasised and the more the flow is shifted toward the opposite banks of the confluence thus inducing possible erosion phenomena at the toe of the outer bank. Moreover, the observed large bar extensions are surprising, when considering the mild confluence angle and the very low discharge ratio. We showed that the vegetation may be the key driver of the junction hydrodynamics and that vegetated confluences might not follow the classic hydraulics‐related models which may describe only a formative configuration.

中文翻译：

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汇合处植被对河流水动力的影响：以格雷夫交界处的阿尔诺河为例

由于汇流量或泥沙输送能力的变化，河道经常形成河道。这些条一旦长大，便构成了有利于植被发展的栖息地。在这项工作中，我们分析了汇流条上方的植被对河段水动力的影响。考虑的案例研究是在格里夫交界处的阿尔诺河河段，汇合条向对岸扩展。通过BASEMENT软件实现了二维水力模型，以改变流量。使用三维地形数据生成计算网格。进行了树木密度调查，以描述植被的当前状态。根据不同的植被管理条件，为数值模拟考虑了七个不同的植物生长场景。这种情景的特征是各种植物的密度，直径，高度和种类（草皮，灌木和乔木），因此，使用了不同的公式来估计曼宁系数。这项研究的结果表明，密度较高的植被落在汇合条上，越强调回水作用，水流越多地流向汇合的相反两岸，从而在外岸脚趾处诱发可能的侵蚀现象。此外，当考虑到适度的汇合角和非常低的排放比时，观察到的大条形延伸非常令人惊讶。我们表明，植被可能是交汇处水动力的主要驱动力，而植被汇合处可能不遵循经典的与水力学有关的模型，后者只能描述形成性构造。