Processes of Channel Bifurcation in Fluvial Systems

河流系统形成和演化的最基本过程之一是河道分叉过程，其中单个河道分裂成两个(或更多)较小的河道。传统上，根据分叉是出现在水道内(封闭的)还是出现在静止的水体(无封闭的)内，分叉被分为两种类型。前者包括辫状扇分叉和冲积扇分叉，后者包括建造在海湾和洪水冲积平原中的三角洲和决口展布。关于这两种分叉的起源仍然存在许多问题：1)分叉河道的水力几何和流量的统计数据是什么？2)分叉河道的流量通常是相等的，还是它们显示了上述辫状分叉的稳定性理论预测的比率？3)不同地貌环境下的分岔角的统计数据是什么，它们与流量和泥沙类型等基本参数是否显示出统计上的显著相关性？4)在无侧限情况下，哪些因素决定了分叉角度？5)哪些因素决定了无侧限分叉的纵向间距？6)什么物理过程决定了无侧限分叉的纵向间距？形成新出现的分叉河道外壁的水下堤坝的生长？7)扩张射流内的二次环流是中心沙洲生长的必要条件吗？8)在悬移质系统中，什么条件使分叉的两个臂保持开放？为了回答这些问题，将对分叉过程进行基于物理和模型的研究，目的是了解三维水流速度场所起的作用，二次环流和泥沙输运在造成无限制航道分叉中起作用。虽然重点是无约束类型，但这两种类型的最终流体动力原因可能是相似的。为了回答上述问题，课题组将：1)在具有输沙能力的流域中进行物理试验，以研究泥沙-水流的三角洲淤积；2)从文献、地图和照片中收集关于两种分流河道的几何和水力学数据(同时强调无侧限流)；3)基于下文概述的概念模型，建立湍流以及床沙和悬移质输沙的三维大涡模拟(LES)模型；以及4)使用该模型进行数值试验。大涡模拟的结果将有助于理解引起水槽和现场观测的流体动力学。LES结果与观测结果的比较将告诉我们，我们的无约束分叉成因概念模型是否能够预测无约束分叉的基本几何和行为。这项工作的智力价值：对渠道分叉的更好理解和预测能力将改善滩地和渠道结构的规划和发展，渠道设计，以及河流恢复工作的成功。这项研究的贡献在于加深了我们对河流稳定性和河道形态的理解。本研究产生了广泛的影响：这项工作构成了论文的主题，并为一名已入驻的博士研究生提供了资金支持。两名本科生将接受有关沉积体系地貌动力学建模科学方法的培训。该项目将构成本科生毕业论文的核心。所有的学生都将受益于一个需要综合地貌学、泥沙输运和水动力学的问题。

ABSTRACTOne of the most fundamental processes in the creation and evolution of fluvial systems is the process of channel bifurcation wherein a single channel splits into two (or more) smaller channels. Bifurcations traditionally have been grouped into two types based on whether they arise within a channel (confined), or within a standing body of water (unconfined). The former case includes braid and alluvial fan bifurcations; the latter includes deltas and crevasse splays building into bays and flooded alluvial plains. Numerous questions remain concerning the origin of both types of bifurcations: 1) What are the statistics of hydraulic geometries and water discharges for bifurcating channels? 2) Are the bifurcate channels usually equal in discharge or do they show the ratios predicted by stability theory for braid bifurcates noted above? 3) What are the statistics of angles of bifurcation in various geomorphologic settings and do they show statistically significant correlations with such basic parameters as water discharge and sediment type? 4) What factors set the angle of bifurcation in the unconfined case? 5) What factors set the longitudinal spacing of unconfined bifurcations? 6) What physical processes are responsible for the growth of the sub-aqueous levees that form the outer walls of the emerging bifurcate channels? 7) Is secondary circulation within the expanding jet a necessary condition for central bar growth as it is in braided streams? 8) What conditions are necessary to keep both arms of a bifurcation open in suspended load systems?To answer these questions a physical- and model-based study of the bifurcation process will be conducted with the purpose of understanding the roles that three-dimensional flow velocity fields, secondary circulation, and sediment transport play in causing unconfined channel bifurcations. Although the emphasis is on the unconfined type, the ultimate fluid dynamical causes in both types may be similar. To answer the above questions the research group will: 1) conduct physical experiments in a basin with sediment feed capability suitable for studying delta deposition from sediment-water flows; 2) collect data from the literature and maps and photos on the geometries and hydraulics of both types of bifurcating channels (while emphasizing the unconfined type); 3) construct a three-dimensional, large eddy simulation (LES) model of turbulent flow and bed- and suspended-load sediment transport based on the conceptual model outlined below, and 4) use the model to conduct numerical experiments. LES results will help understand the hydrodynamics that are giving rise to the flume and field observations. Comparisons of LES results with observations will tell us whether our conceptual model for unconfined bifurcation genesis is capable of predicting the basic geometries and behaviors of unconfined channel bifurcations.The Intellectual Merit of this Work: A better understanding and predictive capability for channel bifurcations would improve the planning and development of floodplain and channel structures, channel designs, and the success of stream restoration efforts. The contributions of this study include advancing our understanding of stream stability and channel morphology.Broader Impacts Resulting from This Study: This work constitutes the dissertation topic and financial support for one Ph. D. candidate (already in residence). Two undergraduates will be trained in the scientific methods of morphodynamic modeling of sedimentary systems. This project will form the core of senior theses for the undergraduates. All students will benefit from exposure to a problem requiring the integration of geomorphology, sediment transport, and hydrodynamics.