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，即一条河道分叉成两条（或多条）较小的河道。分叉传统上被分为两种类型的基础上，他们是否出现在一个通道（封闭），或在一个常设的水体（开放）。前一种情况包括辫状河和冲积扇分叉;后者包括三角洲和决口扇，形成海湾和洪泛冲积平原。关于这两种类型的分叉的起源仍然存在许多问题：1）分叉通道的水力几何形状和水流量的统计数据是什么？2)分叉通道的流量是否通常相等，或者它们是否显示出上述辫状分叉稳定性理论所预测的比例？3)在不同的地貌背景下，分叉角的统计数据是什么？它们与流量和泥沙类型等基本参数之间是否存在统计上的显著相关性？4)在无侧限情况下，什么因素决定了分叉角？5)什么因素决定了无侧限分叉的纵向间距？6)形成分叉河道外壁的水下堤坝的生长是由什么物理过程造成的？7)扩张射流中的二次环流是中心沙生长的必要条件吗？8)在悬挂荷载系统中，保持分叉的两臂张开需要什么条件？为了回答这些问题的物理和模型为基础的研究的分叉过程中，将进行与理解的作用，三维流速场，二次环流，泥沙输运在造成无约束通道分叉的目的。虽然重点是无侧限型，但两种类型的最终流体动力学原因可能相似。为了回答上述问题，研究小组将：1）在具有沉积物供给能力的盆地中进行物理实验，以研究沉积物-水流的三角洲沉积; 2）从文献、地图和照片中收集关于两种类型分叉通道的几何形状和水力学的数据（同时强调无限制类型）; 3）在以下概念模型的基础上，建立了三维大涡模拟（LES）湍流、推移质和悬移质泥沙输移模型，（4）利用该模型进行数值试验。LES结果将有助于理解引起水槽和现场观测的流体动力学。LES的观测结果的比较将告诉我们，我们的概念模型无约束分叉成因是能够预测的基本几何形状和行为的无约束通道bifurcations.The智力的优点这项工作：更好地了解和预测能力的通道分叉将提高规划和发展的洪泛区和渠道结构，渠道设计，并成功的流恢复工作。这项研究的贡献包括推进我们对河流稳定性和河道形态的理解。更广泛的影响，从这项研究所产生的：这项工作构成了博士学位论文的主题和财政支持。候选人（已经在居住）。两名本科生将接受沉积系统形态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。所有的学生将受益于接触到一个问题，需要整合的地貌，沉积物输送和流体动力学。

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.