The morphodynamics of alluvial rivers is controlled by the mobilization of bed material. However, the details of mobilization of mixed-texture bed materials at low flows, increasingly common due to climate change, are still unclear. The 161-km-long Hungarian alluvial reach of the Drava River, downstream of sections where flow characteristics have been heavily modified by human interference, was investigated in 2019. A monitoring campaign at cross-sections, on average 5.55 km apart, was launched to study channel morphology, bedload entrainment dynamics with regard to texture. For the survey, a sonar, an ADCP and a Helley–Smith bedload sampler mounted on a double-hull vessel was used. Our research pointed out an abrupt fining between river kms (hereafter: rkm) 175 and 170 (distance from the mouth), probably due to reduced armouring. The d₆₀ fraction was found to be finer than in 2003 and 2012 for the upstream stations of Botovo and Bélavár, and showed a good correspondence with the records of the Barcs and Drávaszabolcs stations. Temporal fining and higher entrainment rate are due to (a) changing climate of the catchment, that is, diminishing flow between the monitoring dates (2003, 2012 and 2019); (b) reduced armouring, (c) variability of cross-sectional position of sampling points and (d) the different mesh size of the bedload samplers employed. Calculations of shear velocity, Reynolds and Shields numbers indicate more dynamic sediment motion than observed by previous studies. Our reach-scale results may be relevant for the alluvial sections of other alpine and subalpine, partially channelized rivers of similar size, flow dynamics and mixed bedload.

中文翻译：

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部分渠道化河流中的河床夹带动力学：以匈牙利德拉瓦河为例

冲积河流的形态动力学受河床物质的流动控制。然而，由于气候变化而日益普遍的低流量混合质地床材料的动员细节仍不清楚。2019 年对 161 公里长的匈牙利冲积河段进行了调查，该河段下游的水流特征因人为干扰而严重改变。在平均相距 5.55 公里的横截面开展了监测活动，以确保研究通道形态，与质地有关的床负载夹带动力学。在调查中，使用了安装在双壳船上的声纳、ADCP 和 Helley-Smith 床载采样器。我们的研究指出河流 kms（以下简称：rkm）在 175 和 170（与河口的距离）之间突然罚款，可能是由于装甲减少。这d ₆₀发现 Botovo 和 Bélavár 的上游台站的分数比 2003 年和 2012 年更精细，并且与 Barcs 和 Drávaszabolcs 台站的记录有很好的对应关系。临时罚款和较高的夹带率是由于 (a) 流域气候的变化，即监测日期（2003、2012 和 2019）之间的流量减少；(b) 减少装甲，(c) 采样点横截面位置的可变性和 (d) 所用床载采样器的不同网格尺寸。剪切速度、雷诺数和盾牌数的计算表明比以前的研究观察到的更多的动态沉积物运动。我们的河段尺度结果可能与其他高山和亚高山、部分渠道化河流的冲积断面相关，这些河流具有相似的大小、流动动态和混合床量。