The adaptation of dunes to changes in river flow
The adaptation of dunes to changes in river flow
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DOI：
10.1016/j.earscirev.2018.09.002
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发表时间：
2018-10
影响因子：
12.1
通讯作者：
A. Reesink;D. Parsons;P. Ashworth;J. Best;R. Hardy;B. Murphy;S. McLelland;C. Unsworth
中科院分区：
文献类型：
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作者：
A. Reesink;D. Parsons;P. Ashworth;J. Best;R. Hardy;B. Murphy;S. McLelland;C. Unsworth
覆盖大多数冲积河道河床的沙丘的大小和形状随时间和空间的变化而变化，这反过来又影响河流的水流和沉积物输运动态。然而，这种适应沙丘的精确机制，以及控制这些过程的水力变量，仍然没有得到充分的理解。本文概述了沙丘的维持和适应过程，为沙丘动态分析提供了新的工具，并将其应用于一系列定制的实验中。沙丘生长竞争空间，沙丘腐烂需要脱落多余的沉积物。因此，沙丘适应必然涉及沉积物在沙丘上和沙丘之间的重新分配。沉积物再分布的细节不能用沙丘高度和波长等平均几何参数来描述。因此，新的沙丘运动学，床高分布，沙丘变形的分析，在这里提出的援助识别和分析沙丘dynamic.Dune适应通常被描述为一个形态响应水深变化的速度，取决于沉积物的流动性，这本身是一个产品的流动深度和速度。然而，在洪水波的通道，深度和速度是异相的，它们在空间上变化横跨河流从深泓线酒吧顶部，下游沿着河流剖面。为了加深对沙丘形态和运动学的水力学控制的理解，进行了一系列实验，以研究沙丘在完全流动的沙地中的响应（D_（50）= 240 μm），水深和流速对沙丘适应性的影响不同，沙丘的波峰和波谷不会同时响应水流的变化。槽冲刷增加流速，但超高的沙丘波峰似乎只表现出一个弱的关系与流深。沙丘顶部的变平与深度的减小和流速的增加有关。床形叠加是床形运动学的一个关键特征，与水流深度的增加有关，但也与上游沙丘消散后峰间距离的局部增加系统地有关。因此，当地的流动形式的相互作用有一个显着的影响，其中沉积物重新分配的方式和沙丘之间。沙丘的分裂减少下游方向沿着水槽的长度，说明沙丘继续相互作用，即使沙丘高度已经稳定。其他过程，如差异迁移和沙丘合并，在所有流动条件下普遍存在。这些不同的反应支持的概念，沙丘适应过程随时间和空间而变化，通过检查连续沙丘剖面之间的互相关性的残差沙丘变形分析表明，本地的沉积物源和汇存在于移动的沙丘领域。这些研究结果强调，沙丘适应流量的变化是一个动态的反应，涉及多个相互连接的沙丘。沙丘改变形状和适应新的条件所需的沉积物重新分布预计是沉积物迁移变化的一个重要原因。这些详细的分析和研究结果为进一步研究地球以及其他行星上不同环境下的沙丘动力学提供了基础。
The dunes that cover the beds of most alluvial channels change in size and shape over time and in space, which in turn affects the flow and sediment-transport dynamics of the river. However, both the precise mechanisms of such adaptation of dunes, and the hydraulic variables that control these processes, remain inadequately understood. This paper provides an overview of the processes involved in the maintenance and adaptation of dunes, provides new tools for the analysis of dune dynamics, and applies these to a series of bespoke experiments.Dunes that grow compete for space, and dunes that decay need to shed excess sediment. Therefore, dune adaptation necessarily involves the redistribution of sediment over and among dunes. The details of sediment redistribution are not captured by mean geometric parameters such as dune height and wavelength. Therefore, new analyses of dune kinematics, bed-elevation distributions, and dune deformation are presented herein that aid the identification and analysis of dune dynamics.Dune adaptation is often described as a morphological response to changes in water depth at a rate that depends on sediment mobility, which itself is a product of flow depth and velocity. However, depth and velocity are out-of-phase during the passage of flood waves, and they vary spatially across rivers from the thalweg to bar tops, and downstream along the river profile. In order to improve our understanding of the hydraulic controls on dune morphology and kinematics, a series of experiments was performed to investigate the response of dunes in fully-mobile sand (D50= 240 μm) to changes in flow depth and velocity.The experimental results illustrate that water depth and flow velocity have separate effects on the processes that control dune adaptation, and that the crests and troughs of dunes do not respond simultaneously to changes in flow. Trough scour increases with flow velocity, but superelevation of the dune crests appear to show only a weak relation with flow depth. Flattening-out of dune crests is related to decreasing depth and increasing flow velocity. Bedform superimposition, a key feature of bedform kinematics, was associated with increased flow depth, but was also systematically associated with local increases in the crest-to-crest distance following the dissipation of an upstream dune. Thus, local flow-form interactions have a significant effect on the manner in which sediment is redistributed over and among dunes. The splitting of dunes decreased in the downstream direction along the length of the flume, illustrating that the dunes continue to interact even after dune height has stabilised. Other processes, such as differential migration and dune merging, are ubiquitous during all flow conditions. These varied responses support the notion that the processes of dune adaptation vary over time and in space.Analysis of dune deformation through examination of the residuals of cross-correlations between successive dune profiles illustrates that local sources and sinks of sediment exist within mobile dune fields. These findings highlight that dune adaptation to changes in flow is a dynamic response involving multiple interconnected dunes. The redistribution of sediment that is required for dunes to change shape and adapt to new conditions is expected to be an important cause of variability in sediment transport. These detailed analyses and findings provide a foundation for further study of dune dynamics in different environments on Earth as well as other planetary bodies.