Processes of Channel Bifurcation in Fluvial Systems

摘要河流系统形成和演化的最基本过程之一是河道分岔过程，即单个河道分裂成两个（或多个）较小的河道。传统上，分岔被分为两种类型，基于它们是出现在河道（受限）内，还是出现在固定水体（非受限）内。前者包括辫状分支和冲积扇分支；后者包括三角洲和裂缝洼地，形成海湾和被淹没的冲积平原。关于这两种分岔的起源仍然存在许多问题：1)分岔河道的水力几何和水量的统计数据是什么？2)分叉河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预测的比率？3)在不同的地貌环境中，分岔角的统计情况如何？它们与流量、泥沙类型等基本参数是否具有统计学上的显著相关性？4)在无约束情况下，哪些因素决定了分岔角度？5)什么因素决定了无侧限分岔的纵向间距？6)形成新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)扩张射流内的二次循环是否像在辫状流中那样是中央坝生长的必要条件？8)在悬挂负载系统中，什么条件能使分岔的两个分支保持张开状态？为了回答这些问题，将对分岔过程进行基于物理和模型的研究，目的是了解三维流速场、二次循环和泥沙输运在造成无限河道分岔中所起的作用。虽然重点是无约束型，但两种类型的最终流体动力学原因可能是相似的。为了回答上述问题，课题组将：1)在一个具有泥沙供沙能力的盆地进行物理实验，该盆地适合研究泥沙-水流的三角洲沉积；2)从文献和地图、照片中收集两类分岔河道的几何和水力学数据（同时强调无限型）；3)基于本文提出的概念模型，构建了湍流与河床和悬沙输沙的三维大涡模拟（LES）模型，并利用该模型进行了数值实验。LES的结果将有助于理解引起水槽和实地观测的水动力学。LES结果与观测结果的比较将告诉我们，我们的无约束分岔成因概念模型是否能够预测无约束河道分岔的基本几何形状和行为。这项工作的智力价值：更好地理解和预测河道分岔的能力将改善洪泛区和河道结构的规划和发展，河道设计，以及溪流恢复工作的成功。这项研究的贡献包括促进我们对河流稳定性和河道形态的理解。本研究产生的更广泛影响：本工作构成了一名博士候选人（已在校）的论文主题和资金支持。两名本科生将接受沉积体系形态动力学建模的科学方法培训。本课题将构成本科生毕业论文的核心。所有学生都将受益于接触到一个需要整合地貌学、泥沙运输和水动力学的问题。

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.