The processes underlying sediment transport mechanics are still debated. Here, we present novel ensemble particle bedload measurements, obtained with high‐frequency imaging of particle velocities and activity over various bedform migration stages. We show that the bedload particle flux quickly responds to the unsteady non‐uniform flow over the bed. Mean particle velocity linearly increases over the dune slopes towards the crest. In contrast, particle activity exhibits a power law increase over the dune slopes, with the top layer almost saturating in the saltation regime. Furthermore, the spatial and temporal variability of particle motion over dune slopes is high. For lower flow, spatial and temporal variability from small‐scale topography, such as superposed bedforms, spurs, and irregular crestlines, are shown to be important for quantifying sediment dynamics. With increasing flow, however, the cross‐stream particle velocities increase significantly, influenced by the large‐scale dune topography (transverse slopes). Overall, variability in the particle flux increases with both flow strength and frequency of turbulent events. For the mean particle flux over the dune slopes, however, differences between neighboring dunes are large, which are attributed to dune topography variations and flow acceleration. The presented insights are essential to validate numerical simulations of sediment dynamics in three‐dimensional flow conditions over dune beds.

中文翻译：

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沙丘迁移过程中的沉积物颗粒速度和活动

沉积物运移力学的基本过程仍在争论中。在这里，我们介绍了新颖的整体颗粒床量测量结果，它是通过对各种床形迁移阶段的颗粒速度和活性进行高频成像而获得的。我们表明，床载颗粒通量快速响应了床上的不稳定非均匀流动。在沙丘坡度上，平均粒子速度沿波峰线性增加。相反，颗粒活性表现出在沙丘坡度上的幂律增加，在盐分作用下顶层几乎饱和。此外，沙丘斜坡上的粒子运动的时空变化很大。对于小规模地形（例如叠加的床形，马刺和不规则的c线）的较低流量，时空变化，被证明对于量化沉积物动力学很重要。但是，随着流量的增加，受大型沙丘地形（横向坡度）的影响，横流颗粒速度会显着增加。总体而言，粒子通量的可变性随流动强度和湍流事件频率的增加而增加。但是，对于沙丘坡度上的平均粒子通量，相邻沙丘之间的差异很大，这归因于沙丘地形变化和流动加速度。提出的见解对于验证沙丘床上三维流动条件下的沉积物动力学数值模拟至关重要。粒子通量的可变性随流动强度和湍流频率的增加而增加。但是，对于沙丘坡度上的平均粒子通量，相邻沙丘之间的差异很大，这归因于沙丘地形变化和流动加速度。提出的见解对于验证沙丘床上三维流动条件下的沉积物动力学数值模拟至关重要。粒子通量的可变性随流动强度和湍流频率的增加而增加。但是，对于沙丘坡度上的平均粒子通量，相邻沙丘之间的差异很大，这归因于沙丘地形变化和流动加速度。提出的见解对于验证沙丘床上三维流动条件下的沉积物动力学数值模拟至关重要。