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Flow, sediment transport and bedforms under combined flows.

机译：联合流动下的水流，泥沙输送和床形。

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摘要

Combined flow, a combination of unidirectional and wave-induced oscillatory flows, is omnipresent in natural environments and generates a range of bedforms on sandy beds. Combined-flow bedforms are abundant in coastal and lacustrine environments, as well as in the rock record. Few experimental studies have focused on the relationship between the morphology and formative hydraulic conditions of combined-flow bedforms. There is a wide range of unexplored stability conditions for such bedforms, with intermediate oscillation periods. The aim of this research is to undertake new experimental work in the Large Oscillatory Water-Sediment Tunnel (LOWST) in order to address this gap in knowledge. The LOWST has a test section 12.5 m long, 0.8 m wide and 1.2 m high, with the oscillatory motion generated by three pistons. Unidirectional currents were superimposed on these water oscillations using two centrifugal pumps. Fifty-five experiments were conducted in the LOWST, both with and without an initially attened bed. Bedform development in a 250 mum diameter sand bed was studied under pure oscillatory and combined flow conditions with oscillation periods of 4, 5 and 6 s. The maximum orbital velocity (Uo) was varied from 0.10 to 0.70 ms-1 while the unidirectional component (Uu) was varied from 0 to 0.50 ms-1.;This thesis presents data on bedform initiation and development under unidirectional, oscillatory and, under combined flows. This study was able to populate zones of the Terra incognita region previously defined in the literature (Southard, 1991). In this thesis, the stable bedform configurations under a diverse range of flow conditions were studied and divided into no motion (NM), 2D symmetric ripples (SR), 3D symmetric ripples (SR), 3D symmetric dunes (SR), 3D asymmetric ripples (AR), 3D quasi-asymmetric ripples (QAR), 3D asymmetric dunes (AD), 3D current ripples (CR), 3D current dunes (CD) and upper-stage plane bed (USPB). Each of these bedform stages was described, characterized and reproduced in dimensional and dimensionless phase diagrams. A complete re-evaluation of the nomenclature for combined ow bedforms is proposed, which includes their planform and cross-sectional geometries in order to better represent the bed morphologies. This new nomenclature was carefully selected in order to integrate the bedform studies both in the unidirectional and oscillatory flow literature. One of the main changes that allows the integration with the nomenclature used in unidirectional ows is the reclassification of large ripples as dunes. Additionally, the introduction of the planform and cross-sectional geometries as properties to classify bedforms leads to the definition of a stable phase space for two-dimensional symmetrical ripples and three-dimensional quasi-asymmetrical ripples. Furthermore, the experimental data collected under unidirectional flows larger than 0.30 ms-1 allows expansion of the current understanding of the bed configurations within the Terra Incognita zone (Southard, 1991), where the phase boundary between combined ow bedforms and current ripples was uncertain. Based on dimensionless analysis, the oscillatory and unidirectional mobility numbers were used to represent the dimensionless phase diagram under combined flows. A quantitative analysis of the bedform cross-sectional geometries has allowed development of new bedform shape predictors based on the formative flow and sediment transport conditions. Moreover, based on the bedform initiation and development experiments, it was concluded that the genesis and growth processes are unique for all types of flows. This result was reflected in the same geometric pattern and development-path of bedforms regardless of the flow conditions. Furthermore, the development-path or bedform growth exhibit the same general trend for different bedform size (e.g., ripples vs dunes), bedform shape (e.g., symmetric or rounded), bedform planform geometry (e.g., 2D vs 3D) and sediment grain sizes. The development of bed defects during the genesis of bedforms shows a strong relationship with the direction and magnitude of the bed shear stress throughout the oscillation. In conditions with a symmetric bed shear stress, the defects grew and propagated symmetrically, whereas when the bed shear stress was asymmetric, the defects grew and propagated with a predominant downstream direction. Furthermore, for the case of current-dominated combined flow, the maximum upstream bed shear stress was not large enough to entrain sediment in the upstream direction, resulting in solely downstream migration transport. Bedform development was divided and characterized into four main stages: (1) incipient bedforms, (2) growing bedforms, (3) stabilizing bedforms, and (4) fully-developed bedforms, consistent with the scheme proposed by Baas (1994, 1999) for pure unidirectional ows. Finally, a probabilistic model based on the cross-sectional bedform geometries is proposed in order to differentiate between unidirectional, oscillatory and combined flows from their preserved strata.
机译：组合流是单向流和波浪引起的振荡流的组合，在自然环境中无处不在，并在沙质床层上产生一系列地层。在沿海和湖泊环境以及岩石记录中，混合流床形丰富。很少有实验研究集中于联合流床形的形态与形成水力条件之间的关系。对于这种床型，在中间振荡周期内，存在大量未开发的稳定性条件。这项研究的目的是在大型振荡水沙隧道（LOWST）中开展新的实验工作，以解决这一知识差距。 LOWST的测试段长12.5 m，宽0.8 m，高1.2 m，由三个活塞产生振荡运动。使用两个离心泵将单向电流叠加在这些水振荡上。在LOWST上进行了55次实验，无论有没有初始护理床。研究了在纯振荡和混合流动条件下，振荡周期为4、5和6 s的250微米直径砂床上的床形发育。最大轨道速度（Uo）从0.10变化到0.70 ms-1，而单向分量（Uu）从0变化到0.50 ms-1。合并流。这项研究能够填充先前在文献中定义的陆龟地带区域（Southard，1991）。本文研究了在各种流动条件下的稳定床形结构，并将其分为无运动（NM），2D对称波纹（SR），3D对称波纹（SR），3D对称沙丘（SR），3D非对称波纹（AR），3D准非对称波纹（QAR），3D非对称沙丘（AD），3D电流波纹（CR），3D电流沙丘（CD）和上层平面床（USPB）。这些床形阶段中的每个阶段均已在尺寸和无量纲相图中进行了描述，表征和复制。提议对组合流床形的命名进行完整的重新评估，包括其平面形和横截面几何形状，以便更好地表示床的形态。仔细选择了这个新的术语，以便将单向和振荡流动文献中的床形研究整合在一起。允许与单向流中使用的术语集成的主要更改之一是将大波纹重新分类为沙丘。另外，引入平面形状和横截面几何形状作为对床形体进行分类的属性导致了为二维对称波纹和三维拟非对称波纹定义稳定的相空间。此外，在大于0.30 ms-1的单向流下收集的实验数据可以扩大对Terra Incognita区域（Southard，1991年）中床构造的当前了解，那里组合流床形和电流脉动之间的相位边界是不确定的。基于无量纲分析，使用振荡和单向迁移率数表示组合流下的无量纲相图。对床架横截面几何形状的定量分析已允许根据形成的水流和泥沙输送条件开发新的床架形状预测器。此外，基于床形的起始和发育实验，得出的结论是，对于所有类型的流动，其发生和生长过程都是独特的。无论流动条件如何，该结果都反映在床形的相同几何图案和发展路径中。此外，对于不同的床形尺寸（例如，波纹与沙丘），床形（例如，对称或圆形），床形平面形状（例如，2D与3D）和沉积物粒度，发展路径或床形的增长呈现出相同的总体趋势。 。在床形发生期间床缺陷的发展与整个振荡过程中床剪切应力的方向和大小有很强的关系。在具有对称床剪切应力的条件下，缺陷以对称方式生长和扩展，而当床剪切应力不对称时，缺陷以主要下游方向生长和扩展。此外，对于以电流为主的联合流，最大上游河床剪切应力不足以在上游方向夹带沉积物，仅导致下游迁移运移。床形的发展分为四个主要阶段：（1）初始床形；（2）生长的床形；（3）稳定的床形；（4）充分发展的床形，与Baas（1994，1999）提出的方案一致用于纯单向流。最后提出了一种基于横断面床形几何形状的概率模型，以便从保留的地层中区分出单向流，振荡流和组合流。

著录项

  • 作者

    Perillo, Mauricio M.;

  • 作者单位

    University of Illinois at Urbana-Champaign.;

  • 授予单位 University of Illinois at Urbana-Champaign.;
  • 学科 Sedimentary geology.;Civil engineering.;Marine geology.
  • 学位 Ph.D.
  • 年度 2013
  • 页码 314 p.
  • 总页数 314
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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