Abstract The transport of coarse material strongly controls the stability and evolution of mountain fluvial systems but, despite this, bedload dynamics are not yet fully understood especially in mountain streams. In this sense, particular attention was paid on the bedload magnitude (volume) expressed at event-scale and on the long-term, while few studies were focused on when and how the transport of the coarse material occurs. The aim of this work was to investigate the bedload mobility in the Rio Cordon, a mountain stream characterized by cascade and step-pool morphologies with a rough streambed. Here, the critical conditions for initiation of motion, transport distance and virtual velocity expressed by the coarse streambed material were assessed and their relationships with hydraulic forcing conditions and grain size were analyzed. To this end, a monitoring program based on bedload tracing was maintained over 7 years, allowing to analyze the bedload mobility during persistent high frequency/low magnitude flow conditions. To investigate the bedload mobility, 250 tracers were released between 2011 and 2012 and their propagation along study site was monitored until 2018. Overall, 14 tracer inventories were realized, determining 1697 tracer localizations. During the study period, the bedload dispersion resulted well described by the peak discharge magnitude (Qp, qp and ωp − ωc), while no significant relationships were observed with duration of competent flow (tover) and effective runoff volume (ER). Transport distance and tracer grain size were negatively correlated, whereas virtual velocity increased with increasing particle size. In this sense, the propagation velocity seems to be affected by the high frequency flows that, on the one hand triggered mobilization of the coarser tracers only through limited and impulsive events and, on the other hand favored a slowdown on the finer particles due to bedforms disturbance. Compared to other study sites, the Rio Cordon exhibited accentuated threshold conditions with lower transport distance and virtual velocity, confirming that steep mountain streams are generally influenced by a reduced transport efficiency due to protruding bedforms and macro-roughness that cause a pronounced energy dissipation. Interestingly, such condition seems to have progressively reduced from under- to near-bankfull flows. The results were compared to the bedload mobility observed in the Rio Cordon during 1993–1998, enabling to quantitatively assess how the bedload dispersion varied between a setting of stable armouring layer with protruding bedforms (2012–2018) and a partial alteration of these (1993–1998). An evident difference was observed in terms of transport distance, while critical conditions did not significantly change. Lastly, the long-term bedload tracing investigation highlighted that a certain legacy on the transport efficiency was produced by the persistent high frequency flow conditions. Over the study period, a general decrease of transport distance was observed that could be explained by a progressive stabilization of streambed material. Previous studies suggested that such condition can be interrupted by high magnitude/low frequency floods.

中文翻译：

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陡峭山涧中低强度水流条件对底物流动性的影响

摘要 粗物质的运输强烈控制着山地河流系统的稳定性和演化，但尽管如此，床载动力学尚未完全了解，尤其是在山间溪流中。从这个意义上说，特别关注以事件规模和长期表示的床负荷量级（体积），而很少有研究关注粗物质运输的时间和方式。这项工作的目的是调查 Rio Cordon 的河床流动性，这是一条山间小溪，其特征是具有粗糙河床的梯级和阶梯池形态。在这里，评估了粗流床材料表达的运动开始的临界条件、运输距离和虚拟速度，并分析了它们与水力条件和粒度的关系。为此，一个基于床负荷追踪的监测计划维持了 7 年之久，允许在持续的高频/低幅度流量条件下分析床负荷流动性。为了调查床负载流动性，2011 年至 2012 年间发布了 250 种示踪剂，并在 2018 年之前监测了它们沿研究地点的传播。总体而言，实现了 14 种示踪剂库存，确定了 1697 种示踪剂定位。在研究期间，最大流量大小（Qp、qp 和 ωp - ωc）很好地描述了床负荷分散，而与有效流量（tover）和有效径流体积（ER）的持续时间没有观察到显着关系。传输距离和示踪剂粒度呈负相关，而虚拟速度随着粒度的增加而增加。在这个意义上，传播速度似乎受到高频流动的影响，高频流动一方面仅通过有限的脉冲事件触发较粗示踪剂的动员，另一方面有利于由于床型扰动而导致较细颗粒的减速。与其他研究地点相比，Rio Cordon 表现出更高的阈值条件，具有较低的传输距离和虚拟速度，证实陡峭的山间溪流通常受到由于突出的床型和宏观粗糙度导致明显的能量耗散而导致的传输效率降低的影响。有趣的是，这种情况似乎已经从低到接近满仓的流量逐渐减少。将结果与 1993-1998 年间在 Rio Cordon 观察到的床负载流动性进行了比较，能够定量评估在具有突出床型的稳定装甲层设置（2012-2018）和这些床型的部分改变（1993-1998）之间的床载分散如何变化。在运输距离方面观察到明显差异，而临界条件没有显着变化。最后，长期的床负荷追踪调查强调，持续的高频流动条件对运输效率产生了一定的影响。在研究期间，观察到运输距离普遍减少，这可以通过河床材料的逐渐稳定来解释。以前的研究表明，这种情况可能会被高强度/低频率的洪水中断。