X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels
X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels
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DOI：
10.1130/g46883.1
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发表时间：
2019-11
期刊：
影响因子：
5.8
通讯作者：
R. Hodge;H. Voepel;J. Leyland;D. Sear;Sharif Ahmed
中科院分区：
文献类型：
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作者：
R. Hodge;H. Voepel;J. Leyland;D. Sear;Sharif Ahmed
泥沙夹带的临界剪应力（τc）对河床输运率的控制约束较差。由于无法测量原位颗粒的几何形状，τc的直接计算受到了阻碍；即，每个颗粒相对于周围颗粒和床面的形状和位置。我们提出了第一个完整的1055个水化颗粒的三维（3-D）晶粒几何参数，并使用这些参数化了一个新的3-D晶粒携带模型，从而估计τc。三维数据是通过x射线计算机断层扫描从原型尺度水槽实验中提取的沉积物样本收集的。研究发现：(1)轴角和比例晶粒暴露等参数不随相对晶粒尺寸的变化而系统变化；(2) τc主要受颗粒突出度的控制，而不是枢轴角的控制；(3)较大的颗粒由于在流动剖面上的突出而受到较大的力，从而产生相同的流动性。我们认为，颗粒突出度是评价砾石床稳定性的合适指标。
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.