Abstract The surface of the streambed in gravel-bed rivers is commonly coarser than the underlying bed material. This surface coarsening, or ‘armouring’, is usually described by means of the ratio between surface and subsurface grain-size metrics (the ‘armour ratio’). Such surface coarsening is typical of river reaches that are degrading due to a deficit in sediment supply (e.g. gravel-bed reaches below dams or lakes), but non-degrading gravel-bed streams may also exhibit some degree of armouring in relation to specific hydrological patterns. For instance, selective transport during the recession limbs of long lasting floods may coarsen the bed more significantly than flash floods. Consequently, regional differences in bed coarsening should exist, reflecting in turn the variability in sediment and water regimes. In this paper, we explore the trends linking armour ratios to sediment supply, taking into account the differences in hydrological context. We based our analysis on a large data set of bedload and grain size measurements from 49 natural gravel-bed streams and four flume experiments compiled from the scientific literature. Our main outcome documents how the balances between sediment yields and transport capacities have a quantifiable reflection on the armour ratios measured in the field: we report statistically significant correlations between bedload fluxes and surface grain-size, and an asymptotic rise in armour ratios with the decline of sediment supply. Hydrological controls are also observed. Additionally, the trends observed in the field data are comparable to those previously documented in flume experiments with varying sediment feed. In this regard, different kinds of bedforms and particle arrangements have been commonly described with progressive reductions in sediment inputs and the subsequent coarsening of the streambed. Hence, armour ratios serve as a proxy for the general organization of the streambed of gravel-bed streams, and our results quantify this streambed adjustment to the dominant sediment regime.

中文翻译：

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砾石床河流中沉积物供应的质地特征：重新审视装甲比

摘要 砾石床河流的河床表面通常比下伏河床材料更粗糙。这种表面粗化或“铠装”通常通过表面和次表面晶粒尺寸度量之间的比率（“铠装比”）来描述。这种表面粗化是由于沉积物供应不足而正在退化的河流河段的典型特征（例如，大坝或湖泊下方的砾石床河段），但未退化的砾石床溪流也可能表现出与特定水文相关的某种程度的装甲模式。例如，在长期洪水的衰退分支期间，选择性输送可能比山洪暴发更显着地使河床变粗。因此，床粗化的区域差异应该存在，反过来又反映了沉积物和水情的变化。在本文中，考虑到水文环境的差异，我们探索了将装甲比率与沉积物供应联系起来的趋势。我们的分析基于来自 49 条天然砾石床溪流的大量床荷载和粒度测量数据以及从科学文献中汇编的四项水槽实验。我们的主要成果记录了沉积物产量和运输能力之间的平衡如何对现场测量的装甲比进行可量化的反映：我们报告了床载通量与表面粒度之间的统计学显着相关性，以及装甲比随着下降而逐渐上升沉积物供应。还观察到水文控制。此外，在现场数据中观察到的趋势与先前在不同沉积物进料的水槽实验中记录的趋势相当。在这方面，不同类型的河床和颗粒排列通常被描述为沉积物输入的逐渐减少和随后的河床粗化。因此，装甲比作为砾石床流河床一般组织的代表，我们的结果量化了这种河床对主要沉积物制度的调整。