The gravel‐sand transition: Sediment dynamics in a diffuse extension
The gravel‐sand transition: Sediment dynamics in a diffuse extension
复制标题
砾石-砂转变：扩散延伸中的沉积物动力学
DOI：
--
复制
发表时间：
2015
期刊：
影响因子：
--
通讯作者：
C. Rennie
中科院分区：
文献类型：
--
作者：
J. Venditti;N. Domarad;M. Church;C. Rennie
由于砾石层河流在下游方向很细，它们通常表现出从砾石层到砂层条件的突然过渡。为什么突然出现砾石-砂过渡的流行理论是基于双峰沉积物的推移质分选。这种不稳定性被认为是砾石-砂混合物一旦达到床面上的临界覆盖层，砂就会压倒砾石-砂混合物的结果。悬浮液在砾石-砂过渡发展中的作用尚未得到充分认识。位于不列颠哥伦比亚省的弗雷泽河是一个典型的突然砾石-砂过渡带，其“扩散延伸”由砂床和一些砾石组成。我们研究流动，剪切应力和悬浮泥沙通量的扩散延伸，以更好地了解沉积动力学的砂床出现。泥沙悬浮在主要的砾石-泥沙过渡带上游，但在扩散延伸带中，泥沙以推移质和悬浮质的形式移动。我们没有观察到下游梯度的剪切应力或悬浮沙通量通过扩散延伸，这将表明一个逐渐的“雨出”的沙子向下游移动，这就提出了一个问题，是如何形成的砂床？沉积物平流长度尺度表明，除了非常细的沙子，移动作为洗负荷的扩散延伸，馏分粗于中值砂尺寸不能进行悬浮超过一个通道宽度。这表明砂在扩散延伸开始时沉积，在低洪水流量时形成沉积物段塞，在高洪水流量时向下游涂抹，形成砂河段。
As gravel‐bedded rivers fine in the downstream direction, they characteristically exhibit an abrupt transition from gravel‐ to sand‐bedded conditions. The prevailing theory for why abrupt gravel‐sand transitions emerge is based on bed load sorting of a bimodal sediment. The abruptness is thought to be a consequence of sand overwhelming the gravel‐sand mixture once it reaches a critical coverage on the bed. The role suspension plays in the development of gravel‐sand transitions has not been fully appreciated. The Fraser River, British Columbia, is an archetypical abrupt gravel‐sand transition with a “diffuse extension” composed of a sand bed with some patches of gravel. We examine flow, shear stress, and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges. Sand is carried in suspension upstream of the primary abrupt gravel‐sand transition, but in the diffuse extension, sand is moved as both bed load and suspended load. We do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual “rain out” of sand moving downstream, which raises the question, how is the sand bed formed? Sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension, fractions coarser than the median sand size cannot be carried in suspension for more than one channel width. This suggests that sand is deposited en masse at the beginning of the diffuse extension, forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach.