Resistance Is Not Futile: Grain Resistance Controls on Observed Critical Shields Stress Variations
Resistance Is Not Futile: Grain Resistance Controls on Observed Critical Shields Stress Variations
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DOI：
10.1029/2018jf004817
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发表时间：
2018-12
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影响因子：
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通讯作者：
E. Yager;M. Schmeeckle;A. Badoux
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作者：
E. Yager;M. Schmeeckle;A. Badoux
对泥沙运动开始的估计对于防洪和河流管理是不可或缺的，但往往非常不准确。颗粒夹带的临界剪应力(τ*c)通常被认为是恒定的，但不同河流之间的测量值可能会有几乎一个数量级的变化。这种变化通常是由测量方法、粒度分布或流动水力学的不同来解释的，而颗粒的运动阻力在很大程度上被认为是恒定的。我们证明了颗粒阻力随床层结构的不同而变化很大，床层结构由周围沉积物(突出物，p)和颗粒间摩擦(ϕf)以上的颗粒高度所包裹。我们将这些参数结合到一种新的理论中，该理论正确地预测了实验室、现场和数值模型中估计的阻力。我们的理论挑战了现有的模型，这些模型严重高估了床位移动性。在我们的理论中，p和ϕf的微小变化可以引起τ*c的大变化，而不需要引起测量方法或颗粒尺寸的变化。数据汇编还表明，τ*c经验值的分散可以部分解释为河流之间p的差异。因此，床层结构的时空变化可以部分解释τ*c偏离假设常量的原因。鉴于已知河床结构随所施加的剪切力和上游泥沙供应而变化，我们得出结论，恒定的τ*c是不可能的。τ*c的值不能在流之间互换，甚至不能在给定流中随时间互换，因为它们是用频道历史编码的。
Estimates of the onset of sediment motion are integral for flood protection and river management but are often highly inaccurate. The critical shear stress (τ*c) for grain entrainment is often assumed constant, but measured values can vary by almost an order of magnitude between rivers. Such variations are typically explained by differences in measurement methodology, grain size distributions, or flow hydraulics, whereas grain resistance to motion is largely assumed to be constant. We demonstrate that grain resistance varies strongly with the bed structure, which is encapsulated by the particle height above surrounding sediment (protrusion, p) and intergranular friction (ϕf). We incorporate these parameters into a novel theory that correctly predicts resisting forces estimated in the laboratory, field, and a numerical model. Our theory challenges existing models, which significantly overestimate bed mobility. In our theory, small changes in p and ϕf can induce large changes in τ*c without needing to invoke variations in measurement methods or grain size. A data compilation also reveals that scatter in empirical values of τ*c can be partly explained by differences in p between rivers. Therefore, spatial and temporal variations in bed structure can partly explain the deviation of τ*c from an assumed constant value. Given that bed structure is known to vary with applied shear stresses and upstream sediment supply, we conclude that a constant τ*c is unlikely. Values of τ*c are not interchangeable between streams, or even through time in a given stream, because they are encoded with the channel history.