泥沙负荷决定河流的形状
Sediment load determines the shape of rivers
摘要：
了解河流如何适应它们所携带的沉积物负荷对于预测景观的演变至关重要。然而，目前，没有一个基于物理的模型可靠地捕捉基本的河流属性，如它的形状或坡度，对泥沙排放量的依赖，即使在简单的情况下，实验室河流。在这里，我们展示了如何流体应力和重力之间的平衡作用于沉积物颗粒，随着横流扩散的沉积物，确定的形状和沉积物通量剖面的层流实验室河流携带泥沙作为推移质。使用这个模型，它可靠地再现了实验，没有任何调整，我们证实了假设，最初提出的帕克（1978），河流被限制存在接近泥沙运动的阈值（约20%）。这一界限是由水沙相互作用决定的，与河流的水沙负荷无关。因此，随着总输沙量的增加，河流的输沙量强度（单位宽度的输沙量）达到饱和，河流只能通过加宽来输送更多的泥沙。在这种大流量状态下，水流中动量的横流扩散允许泥沙输运。相反，在弱输运状态下，输沙集中在河流中心附近，而不会显著改变河流形状。如果这一理论适用于自然河流，那么河流的长宽比就可以成为泥沙排放量的替代指标--这一数量在实地测量中是出了名的困难。
英文摘要：
Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers which carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker (1978), that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid-sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can only transport more sediment by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge - a quantity notoriously difficult to measure in the field.
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