Relating the Flow Processes and Bedforms of Steady-State and Waning Density Currents
Relating the Flow Processes and Bedforms of Steady-State and Waning Density Currents
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DOI：
10.3389/feart.2020.535743
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发表时间：
2020-10-22
影响因子：
2.9
通讯作者：
McCaffrey, William D.
中科院分区：
文献类型：
--
作者：
de Cala, Isabel;Ohata, Koji;McCaffrey, William D.
浊流和移动基质之间的相互作用可以导致不同类型的床形的发展。尽管对明渠水流下的河床发育进行了大量研究，但对重力流减弱下的河床发育的研究相对较少。因此，与密度流相关的床形发展的分析依赖于明渠流动相图。我们报告了一项实验研究，旨在评估稳定和减弱的盐水密度流下床形的发展。实验密度流形成了阶梯式密度分布，其中较高密度的基底区域被中等密度的区域与环境流体分开；任何形成的河床形态都包含在水流的底层内。在不同的条件下，观察到波纹、沙丘、下游迁移的反沙丘和长波长反沙丘的形成，并可以根据它们与水流下部较密层的上表面的相互作用和相位关系来区分。由于上层水流和下层水流之间的混合有限以及水流动量的维持，以缓慢的排放流量设置的水流在水流的下层中保持稳定的流速。结果，在该下层内实现了持续的床形形成条件，而减弱的水流条件影响了其余的流动。在洋流减弱的情况下，人们发现先前存在的河床状态如何决定河床形态的后续演变。这说明了现有相图的局限性，因为它们没有考虑流经不同床型相空间的轨迹或通过速率。为了在流态和床形发育类型之间建立可靠的定量关联，采用合适的分层流弗劳德数计算方法至关重要。更新的密度电流相图表明，由于重力减小的影响，超临界流可以在比明渠流更低的流速下实现。在露头和现代海底发现的床状沉积结构提供的数据有助于解释形成它们的水流的水动力和沉积学特征。因此，了解密度流下床形发育所涉及的过程将能够更准确地估计流动的特性。
The interaction between turbidity currents and mobile substrates can lead to the development of different types of bedforms. Although much research has been conducted on bedform development beneath open channel flows, research into bedform development beneath waning gravity currents is relatively rare. Analysis of density current-related bedform development has therefore relied upon open channel flow phase diagrams. We report on an experimental study designed to assess the development of bedforms under steady and waning saline density currents. The experimental density currents developed stepped density profiles in which a higher-density basal zone was separated from the ambient fluid by a zone of intermediate density; any bedforms that developed were contained within the bottom layer of the current. Under different conditions ripples, dunes, downstream migrating antidunes and long wavelength antidunes were observed to form and could be distinguished based on their interactions and phase relationships with the upper surface of the lower denser layer of the current. Due to limited mixing between the upper and lower layer of the current and maintenance of current momentum, currents set with slowing discharge flow rates maintained a steady flow velocity in the lower layer of the flow. As a result, sustained bedform formative conditions were achieved within this lower layer, while waning current conditions effected the rest of the flow. Under waning currents, it was seen how pre-existing bed states can determine the subsequent evolution of bedforms. This illustrates the limitations of existing phase diagrams as they do not account for trajectory or rate of passage of flows through different bedform phase spaces. In order to establish a reliable quantitative association between the flow regime and the type of bedform development, it is critical to adopt an appropriate Froude number calculation method for stratified flow. The updated density current phase diagram indicates supercritical flow can be achieved at lower flow velocities than for open channel flows due to the effects of reduced gravity. Bedform depositional structures found in outcrop and on the modern sea floor provide data that helps to interpret the hydrodynamic and sedimentological character of the current that formed them. Therefore, understanding the processes involved in bedform development beneath density currents will enable more accurate estimation of the properties of flows.