In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near-bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect.

中文翻译：

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几何形状和粘度在细沉积物移动粗沉积物中的作用


在河流中，已知在较粗的河床上添加较细的沉积物可以增加较粗部分的流动性。为此提出了两种机制：几何机制，较小尺寸使床层平滑，增加近床速度，从而增加较大尺寸的流动性；粘性机制，形成过渡平滑的湍流边界层，使较粗的颗粒更具流动性。在这里，我们报告了使用两种沉积物混合物的实验，以更好地理解这些提出的机制。在混合物 1 中，我们使用 0.5 和 5 毫米颗粒，在混合物 2 中，我们使用 2 和 20 毫米颗粒。如果较细沉积物夹带粗砾石是纯粹的几何效应，则添加较细材料应该对两种混合物的较粗材料的流动性产生相同的影响，因为它们具有相同的尺寸比。我们表明，添加更细的材料对两种混合物有不同的影响。我们观察到两种混合物的粗粒部分的流动性均有所增加，但混合物 1 的粗粒部分流动性的增加几乎是混合物 2 的两倍。我们的实验表明，除了几何效应外，粗砾石运输的增强还包括：更细的沉积物也是由粘性效应驱动的。