Abstract Headwater sediment dynamics is very important for fluvial processes in downstream reaches, but have been poorly studied until now, and existing sediment processes are not well understood. In fact, quantifying bedload transport and assessing river changes is challenging, in mountain environments, and traditional bedload formulas tend to overestimate sediment fluxes. This work aims to analyze the coarse sediment transfer processes acting in the high-altitude headwater reach of Rio Cordon (eastern Italian Alps). Pebbles provided by Passive Integrated Transponders (PITs) were deployed in the study site and their position was periodically monitored over 20 months, in order to quantify sediment mobility (i.e., travel distances and virtual velocities) and, the bedload volumes. The geomorphic changes due to flood events were analyzed through high resolution DEMs (0.05 m cell), derived with Structure from Motion (SfM) in three different periods (October 2017, September 2018 and June 2019), allowing to compute the DEMs of Difference (DoDs). The study period was characterized by two distinct flood events, a short and intense summer rainstorm in August 2018 (recurrence interval RI > 2 years) and a severe cyclonic rainfall event (i.e. Vaia storm, RI > 50 years) in October 2018. The combination of tracer and DoDs analysis shows that ordinary flood maintained the reach in an apparent equilibrium state, with balanced volumes of erosion and deposition. Differently, the high-magnitude flood was characterized by bedload fluxes an order of magnitude higher than ordinary event and an evident net erosion. However, in both events the volumes mobilized in the headwater did not contribute significantly to the sediment fluxes delivered at the Rio Cordon basin outlet. This highlighted the reduced efficiency in sediment propagation mainly due to a marked dis-connectivity associated with the presence of a hanging valley in the middle part of the basin. This study discusses the complex dynamism of headwater streams, stressing the importance of understanding sediment fluxes and connectivity conditions that can strongly affect human activities and infrastructures even in lowland reaches.

中文翻译：

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高山源水流中的粗大沉积物转移和地貌变化

摘要 上游泥沙动力学对于下游河段的河流过程非常重要，但迄今为止研究甚少，对现有泥沙过程的了解也不是很清楚。事实上，在山区环境中，量化底泥输送和评估河流变化具有挑战性，而传统的底泥公式往往会高估泥沙通量。这项工作旨在分析作用于 Rio Cordon（意大利东部阿尔卑斯山）高海拔源头河段的粗沉积物转移过程。由被动集成转发器 (PIT) 提供的鹅卵石被部署在研究地点，并在 20 个月内定期监测它们的位置，以量化沉积物的流动性（即移动距离和虚拟速度）和床负载量。洪水事件引起的地貌变化通过高分辨率 DEM（0.05 m 单元）分析，从三个不同时期（2017 年 10 月、2018 年 9 月和 2019 年 6 月）的运动结构（SfM）得出，允许计算差异的 DEM（国防部）。研究期间的特点是两次不同的洪水事件，2018 年 8 月的短暂而强烈的夏季暴雨（重复间隔 RI > 2 年）和 2018 年 10 月的严重气旋降雨事件（即 Vaia 风暴，RI > 50 年）。示踪剂和 DoD 分析表明，普通洪水将河段保持在明显的平衡状态，侵蚀和沉积量平衡。不同的是，高强度洪水的特点是床载通量比普通事件高一个数量级，并且有明显的净侵蚀。然而，在这两个事件中，源头中流动的体积对 Rio Cordon 盆地出口处的沉积物通量没有显着贡献。这突出了沉积物传播效率的降低，这主要是由于与盆地中部存在悬谷相关的显着断开连接。本研究讨论了源头流的复杂动态，强调了解沉积物通量和连通性条件的重要性，这些条件即使在低地河段也能强烈影响人类活动和基础设施。这突出了沉积物传播效率的降低，这主要是由于与盆地中部存在悬谷相关的显着断开连接。本研究讨论了源头流的复杂动态，强调了解沉积物通量和连通性条件的重要性，这些条件即使在低地河段也能强烈影响人类活动和基础设施。这突出了沉积物传播效率的降低，这主要是由于与盆地中部存在悬谷相关的显着断开连接。本研究讨论了源头流的复杂动态，强调了解沉积物通量和连通性条件的重要性，这些条件即使在低地河段也能强烈影响人类活动和基础设施。