Flow competence: A criticism of a classic concept
Flow competence: A criticism of a classic concept
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心流能力：对经典概念的批评
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发表时间：
1992
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通讯作者：
P. Wilcock
中科院分区：
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作者：
P. Wilcock
在输送的泥沙样品中发现的最大颗粒通常被用来估计水流能力。利用一定范围的水流样品，可以推导出水流与最大流动颗粒之间的关系，并用来估计不同粒径颗粒在底泥中初始运动的临界剪应力，或者相反地，估计从运输样品中发现的最大颗粒产生的水流的大小。然而，由于这些估计是基于输运颗粒尺寸分布的极值，它们容易产生很大的误差，并且对样本大小的影响很敏感，而样本大小往往在来自自然水流的泥沙输送样本中变化很大。此外，基于最大采样移动颗粒的临界剪切应力的估计不能以允许在分数之间进行合理比较的方式进行缩放。样本大小和比例问题使分数临界剪应力的最大颗粒估计偏离真实关系的程度无法准确预测，尽管这种偏离的方向是可以证明的。巨大的误差和未知的偏差表明，最大的采样流动颗粒并不是临界剪应力或流动大小的可靠预测因子。根据输送颗粒大小分布的中心值，可以定义整个混合物的单一流动能力。这样的测量相对稳定，不需要分数间的比例，并且似乎得到了观察的很好支持。
The largest grains found in samples of transported sediment are commonly used to estimate flow competence. With samples from a range of flows, a relationship between the flow and the largest mobile grain can be derived and used to estimate the critical shear stress for incipient motion of the different grain sizes in the bed sediment or, inversely, to estimate the magnitude of the flow from the largest grain found in a transport sample. Because these estimates are based on an extreme value of the transport grain-size distribution, however, they are subject to large errors and are sensitive to the effect of sample size, which tends to vary widely in sediment transport samples from natural flows. Furthermore, estimates of the critical shear stress based on the largest sampled moving grain cannot be scaled in a manner that permits reasonable comparison between fractions. The degree to which sample size and scaling problems make largest-grain estimates of fractional critical shear stress deviate from a true relationship cannot be predicted exactly, although the direction of such a deviation can be demonstrated. The large errors and unknown bias suggest that the largest sampled mobile grain is not a reliable predictor of either critical shear stress or flow magnitude. It is possible to define a single flow competence for the entire mixture, based on a central value of the transport grain-size distribution. Such a measure is relatively stable, does not require between-fraction scaling, and appears to be well supported by observation.