Grain shape effects in bed load sediment transport

     Bed load sediment transport, in which wind or water flowing over a bed of sediment causes grains to roll or hop along the bed, is a critically important mechanism in contexts ranging from river restoration to planetary exploration. Despite its widespread occurrence, predictions of bed load sediment flux are notoriously imprecise. Many studies have focused on grain size variability as a source of uncertainty, but few have investigated the role of grain shape, even though shape has long been suspected to influence transport rates. Here we show that grain shape can modify bed load transport rates by an amount comparable to the scatter in many sediment transport datasets. We develop a theory that accounts for grain shape effects on fluid drag and granular friction and predicts that the onset and efficiency of transport depend on the coefficients of drag and bulk friction of the transported grains. Laboratory experiments confirm these predictions and reveal that the effect of grain shape on sediment transport can be difficult to intuit from the appearance of grains. We propose a shape-corrected sediment transport law that collapses our experimental measurements. Our results enable greater accuracy in predictions of sediment transport and help reconcile theories developed for spherical particles with the behaviour of natural sediment grains.

  【译文】从修复一条河流乃至探索一颗行星，推移质运动，即在风力或水力的作用下河床上的沉积物颗粒翻滚升降的机制都是研究重要的一环。尽管这种运动现象广泛存在，但对推移质输沙量的预测始终难以精确。许多研究都将预测的不确定性归因于颗粒粒度的变异性，却很少有研究涉及颗粒形状的影响，尽管学界一直推测颗粒形状对输沙率会产生一定作用。本研究发现，颗粒形状对推移质输沙率有着重要影响，且影响程度与许多泥沙输移数据的分散程度相当。据此提出了一个理论，可解释颗粒形状对流体阻力和颗粒摩擦的作用，并预测输移运动的发生和输移效率取决于被输移颗粒的阻力系数和体摩擦（bulk friction）系数。实验证实了这些预测，也表明实际上很难从颗粒外观直观判断颗粒形状对输沙率的影响。研究人员提出了形状校正的输沙规律，但超出了实验测量能力。研究结果能够提高输沙预测的准确度，并有助于球形粒子相关理论与自然沉积物颗粒行为研究的相互促进。