The vertical turbulence structure of experimental turbidity currents encountering basal obstructions: implications for vertical suspended sediment distribution in non-equilibrium currents
The vertical turbulence structure of experimental turbidity currents encountering basal obstructions: implications for vertical suspended sediment distribution in non-equilibrium currents
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DOI：
10.1111/j.1365-3091.2011.01297.x
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发表时间：
2012-04-01
期刊：
影响因子：
3.5
通讯作者：
McCaffrey, William D.
中科院分区：
文献类型：
--
作者：
Eggenhuisen, Joris T.;McCaffrey, William D.
在现代海底和露头的浊积砂岩层下，经常观察到由海底侵蚀引起的大的粗糙度特征。本文旨在研究这些粗糙度元素对湍流速度场的影响及其对水流携沙能力的影响。实验浊度电流通过一个矩形通道，在一些运行中，一个单一的粗糙度元件固定在底部。利用超声多普勒速度剖面技术测量基底阻塞下游区域垂直速度分量的垂直剖面，确定粗糙度元素对湍流速度场的影响。这些实验是为了回答两个研究问题而设置的。(i)单个粗糙度元素如何改变垂直湍流强度的分布？（ii）改变后的剖面在下游方向如何演变？在平面基板上运行的结果与之前提供的数据相似，并且显示在通道底部附近有较低的湍流最大值，与速度最大值相关的湍流最小值，以及与上部流动界面相关的湍流最大值。在受单一粗糙度因素扰动的运行中，低湍流最大值的强度增加了41%至81%。当它进一步向下游移动时，这种多余的湍流在流动中向上消散，但在最远的测量位置（距离大约是元件下游粗糙度高度的39倍）仍然可以观察到。所有的结果都指向了之前作者提出的浊度流的近床湍流结构与自由表面剪切流之间的相似性，并且基于这种相似性的剪切速度估计方法的明显成功进一步支持了这一命题。运用悬沙的湍流扩散理论，讨论了观测到的单个大粗糙度元的湍流效应如何影响现实浊流中悬沙的分布。由此得出结论，这种影响可能包括悬浮沉积物的非平衡净向上输送，抵消了密度分层。因此，由天然浑浊水流前缘形成的侵蚀性底物地形可能会在随后的水流阶段将上部区域的沉积物浓度超提升到平衡值以上，从而延迟水流通过沉积而枯竭并增加其流出距离。
Large roughness features, caused by erosion of the sea floor, are commonly observed on the modern sea floor and beneath turbidite sandstone beds in outcrop. This paper aims to investigate the effect of such roughness elements on the turbulent velocity field and its consequences for the sediment carrying capacity of the flows. Experimental turbidity currents were run through a rectangular channel, with a single roughness element fixed to the bottom in some runs. The effect of this roughness element on the turbulent velocity field was determined by measuring vertical profiles of the vertical velocity component in the region downstream of the basal obstruction with the Ultrasonic Doppler Velocity Profiling technique. The experiments were set up to answer two research questions. (i) How does a single roughness element alter the distribution of vertical turbulence intensity? (ii) How does the altered profile evolve in the downstream direction? The results for runs over a plane substrate are similar to data presented previously and show a lower turbulence maximum near the channel floor, a turbulence minimum associated with the velocity maximum, and a turbulence maximum associated with the upper flow interface. In the runs in which the flows were perturbed by the single roughness element, the intensity of the lower turbulence maximum was increased between 41% to 81%. This excess turbulence dissipated upwards in the flow while it travelled further downstream, but was still observable at the most distal measurement location (at a distance ca 39 times the roughness height downstream of the element). All results point towards a similarity between the near bed turbulence structure of turbidity currents and free surface shear flows that has been proposed by previous authors, and this proposition is supported further by the apparent success of a shear velocity estimation method that is based on this similarity. Theory of turbulent dispersal of suspended sediment is used to discuss how the observed turbulent effects of a single large roughness element may impact on the suspended sediment distribution in real world turbidity currents. It is concluded that this impact may consist of a non-equilibrium net-upwards transport of suspended sediment, counteracting density stratification. Thus, erosive substrate topography created by frontal parts of natural turbidity flows may super-elevate sediment concentrations in upper regions above equilibrium values in following flow stages, delay depletion of the flow via sedimentation and increase their run-out distance.