Quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution
Quantifying the significance of abrasion and selective transport for downstream fluvial grain size evolution
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量化磨损和选择性输送对下游河流粒度演变的重要性
DOI：
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发表时间：
2014
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通讯作者：
G. Domokos
中科院分区：
文献类型：
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作者：
K. Miller;Tímea Szabó;D. Jerolmack;G. Domokos
众所周知，在河流下游，卵石直径有系统地减小。磨损的贡献是不确定的，部分原因是：(1)直径不足以表征由于磨损造成的卵石质量损失；(2)实验室实验中测量的磨损率不能轻易地外推到现场。最近的一项几何理论将磨损描述为一个曲率相关的过程，该过程会产生两个阶段的演变：在第一阶段，最初块状的鹅卵石变为光滑的凸形，轴线尺寸几乎没有减少；然后，在第二阶段，光滑的、凸的鹅卵石慢慢地减小它们的轴尺寸。在这里，我们提供了强有力的证据，证明两相磨损发生在自然环境中，通过研究热带山地溪流中超过10公里的数千块鹅卵石的形状和大小的下游演变。几何理论在该河流系统中得到验证，使用各种人工和基于图像的形状参数，为量化磨损的重要性提供了一种通用的方法。第一阶段发生在约1公里的上游基岩段，主要是磨损，沉积物储存有限。在下游冲积河段，即第二阶段发生的地方，我们观察到预期的卵石直径指数下降。通过对沉积的离散磨损模型（所谓的“盒子方程”），我们推断，磨损会使鹅卵石的质量减少三分之一以上，但粒径选择性分选在鹅卵石直径的下游变化中起主导作用。总体而言，磨蚀是研究河流中下游卵石质量（而非直径）减少的主要过程，对卵石的迁移和细粒沉积物的产生具有重要意义。
It is well known that pebble diameter systematically decreases downstream in rivers. The contribution of abrasion is uncertain, in part because (1) diameter is insufficient to characterize pebble mass loss due to abrasion and (2) abrasion rates measured in laboratory experiments cannot be easily extrapolated to the field. A recent geometric theory describes abrasion as a curvature‐dependent process that produces a two‐phase evolution: in Phase I, initially blocky pebbles round to smooth, convex shapes with little reduction in axis dimensions; then, in Phase II, smooth, convex pebbles slowly reduce their axis dimensions. Here we provide strong evidence that two‐phase abrasion occurs in a natural setting, by examining downstream evolution of shape and size of thousands of pebbles over ~10 km in a tropical montane stream. The geometric theory is verified in this river system using a variety of manual and image‐based shape parameters, providing a generalizable method for quantifying the significance of abrasion. Phase I occurs over ~1 km, in upstream bedrock reaches where abrasion is dominant and sediment storage is limited. In downstream alluvial reaches, where Phase II occurs, we observe the expected exponential decline in pebble diameter. Using a discretized abrasion model (the so‐called “box equations”) with deposition, we deduce that abrasion removes more than one third of the mass of a pebble but that size‐selective sorting dominates downstream changes in pebble diameter. Overall, abrasion is the dominant process in the downstream diminution of pebble mass (but not diameter) in the studied river, with important implications for pebble mobility and the production of fine sediments.