Sediment transport and bedform development in the lee of bars: evidence from fixed- and partially-fixed bed experiments
Sediment transport and bedform development in the lee of bars: evidence from fixed- and partially-fixed bed experiments
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DOI：
10.1201/b17133-158
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发表时间：
2014-09
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影响因子：
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通讯作者：
A. Reesink;D. Parsons;Robert E. Thomas
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文献类型：
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作者：
A. Reesink;D. Parsons;Robert E. Thomas
天然河道中沙洲、沙丘和波纹的共存、相互作用和重复（重新）建立是许多重要的流态流动力学的原因。小床型不断地形成，并叠加在大床型上，特别是在受大床型产生的大规模次级环流模式影响的地区。这些叠加的床型迁移到下游的大尺度斜坡上，在那里它们i)产生额外的形状粗糙度，ii)改变沉积物的输运动力学，iii)控制床型的分裂和合并，iv)改变主背风斜坡的几何形状，v)改变最终的沉积结构。我们对叠合床型发育的理解来自于对均匀流动中平坦床型发育的研究，并没有充分描述明显不均匀流动和大规模二次循环地区的床型发育。为了扩大我们对床型起始的理解，本文提出了固定床和部分固定床实验，研究了宿主床型的分离流动对槽内较小的次级床型发育的影响。结果表明：1)随着坝顶流速的增大，坝槽冲刷深度增大，下游冲刷长度减小；2)坡面起爆点向下游移动，初始波纹幅值随流速的增大而减小；(3)坝槽波纹背风处分离流的波峰-波谷速度梯度和相干性取决于它们相对于大尺度宿主床型分离流的位置，并有向下游增大的趋势。这些观察结果表明，次级床型的发育受到主床型分离流动的阻碍，并且还取决于下游应力坡的长度。床型放大的减小归因于次级床型背风处分离流的强度和相干性的降低，这是主床型分离流增强的结果。因此，这项研究为更全面地理解复杂地形和流场局部变化地区的河床形成和发展迈出了一步
The co-existence, interaction, and repeated (re-)establishment of bars, dunes and ripples in natural channels is responsible for many important flow-form-flow dynamics. Small bedforms are constantly generated, superimposed on larger ones, particularly in zones affected by large-scale secondary circulation patterns produced by the larger bedforms. These superimposed bedforms migrate onto the downstream stoss slope of larger-scale forms where they i) generate additional form-roughness, ii) change sediment transport dynamics, iii) control bedform splitting and merging, iv) alter the geometry of the host lee slope, and v) change the resultant sedimentary structures. Our understanding of superimposed bedform development is de-rived from investigations of bedform development on flat beds in uniform flow and does not adequately de-scribe bedform development in distinctly non-uniform flows and areas with large-scale secondary circulations. In order to expand our understanding of bedform initiation, this paper presents fixed-bed and partially-fixed-bed experiments that investigate the effect of a host-bedform’s separated flow on the development of smaller, secondary bedforms in its trough. The results show that: 1) scour in the trough of bars increases in depth and decreases in downstream length with increasing flow velocity over the crest; 2) the point of bedform initiation moves downstream and the amplitude of the incipient ripples decreases with increasing flow velocity; 3) crest-trough velocity gradients and coherence of the separated flows in the lee of ripples in bar troughs depend on their position relative to the separated flow of the larger-scale host bedform and tend to increase down-stream. These observations indicate that the development of secondary bedforms is hindered by the host bedform’s separated flow and is also dependent on the length of the downstream stoss slope. The reduction of bedform amplification is attributed to the reduced strength and coherence of the separated flows in the lee of the secondary bedforms as a result of the stronger separated flow of the host bedform. Thus, this study presents a step towards a fuller understanding of bedform initiation and development in areas with complex topography and local variability in the flow field