The critical shear stress (τc) for grain entrainment is a poorly constrained control on bedload transport rates in rivers. Direct calculations of τc have been hindered by the inability to measure the geometry of in situ grains; i.e., the shape and location of each grain relative to surrounding grains and the bed surface. We present the first complete suite of three-dimensional (3-D) grain geometry parameters for 1055 water-worked grains, and use these to parameterize a new 3-D grain entrainment model and hence estimate τc. The 3-D data were collected using X-ray computed tomography scanning of sediment samples extracted from a prototype scale flume experiment. We find that (1) parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size; (2) τc is primarily controlled by grain protrusion, not pivot angle; and (3) larger grains experience larger forces as a result of projecting higher into the flow profile, producing equal mobility. We suggest that grain protrusion is a suitable proxy for assessing gravel-bed stability.

中文翻译：

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X 射线计算机断层扫描显示颗粒突起控制夹带河流砾石的临界剪切应力

谷物夹带的临界剪切应力 (τc) 是对河流中床载传输速率的一种很好的约束控制。由于无法测量原位晶粒的几何形状，因此无法直接计算 τc；即，每个颗粒相对于周围颗粒和床面的形状和位置。我们为 1055 个水加工谷物提供了第一套完整的三维 (3-D) 谷物几何参数，并使用这些参数化新的 3-D 谷物夹带模型，从而估计 τc。3-D 数据是使用 X 射线计算机断层扫描扫描从原型水槽实验中提取的沉积物样品收集的。我们发现（1）包括枢轴角和比例晶粒暴露在内的参数不随相对晶粒尺寸而系统地变化；(2) τc 主要受晶粒突起控制，不是枢转角度；(3) 较大的颗粒由于向流动剖面中投射得更高而承受更大的力，从而产生相等的流动性。我们建议颗粒突起是评估砾石床稳定性的合适代表。