Hassan Marwan A, Parker Gary, Hassan Yarra, An Chenge, Fu Xudong, Venditti Jeremy G
Department of Geography, The University of British Columbia, Vancouver, BC V6T 1Z2, Canada.
Department of Earth Science and Environmental Change, University of Illinois Urbana-Champaign, Urbana, IL 61801.
Proc Natl Acad Sci U S A. 2024 Sep 17;121(38):e2409436121. doi: 10.1073/pnas.2409436121. Epub 2024 Sep 12.
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.
在河流中，向较粗的河床添加较细的沉积物会增加较粗部分的流动性。对此提出了两种机制：一种是几何机制，即较小颗粒使河床变得平滑，增加近床速度，从而提高较大颗粒的流动性；另一种是粘性机制，即形成过渡性平滑的湍流边界层，使较粗颗粒更易移动。在此，我们报告使用两种沉积物混合物进行的实验，以更好地理解这些提出的机制。在混合物1中，我们使用了0.5毫米和5毫米的颗粒，在混合物2中，我们使用了2毫米和20毫米的颗粒。如果较细沉积物对粗砾石的夹带是一种纯粹的几何效应，那么添加较细物质对两种混合物中较粗物质的流动性应产生相同的影响，因为它们具有相同的粒径比。我们表明，添加较细物质对两种混合物有不同的影响。我们观察到两种混合物中粗颗粒部分的流动性均有所增加，但混合物1中粗颗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，较细沉积物对粗砾石输运的增强还受到粘性效应的驱动。
