Collaborative Research: A Field and Numerical Study of the Morphology, Flow, Sedimentary Processes, and Stability of Sand-Bed Fluvial Bifurcations

河道分叉，其中一个河道分裂成两个，是编织、吻合和分流河道网络的组成部分。最近的研究表明，在分叉处最常见的流动分配是不对称的，而这种不对称似乎促进了稳定性。为什么会这样，在什么样的水文和沉积条件下会这样，仍然是一个谜。在此，我们建议对高剪切应力、自然河流分岔的形态、流动和沉积过程进行综合的实地和理论研究，目的是理解和预测其动力学行为。实地研究将包括两个具有不同沉积特征的分支系统：i)萨斯喀彻温河苔藓三角洲的分流河道分支，我们在那里进行了关于分流形态动力学的初步研究；ii)路易斯安那州蜡湖三角洲系统的分支，那里的活跃沉积导致了分流河道的快速增长。在每个位点的几个分支将被选择跨越一系列的形态类型。河床形态、水面地形、流速和泥沙输运率将在几个流动阶段的紧密间隔网格上的每个分岔处进行测量。现场数据将为数值实验提供边界和初始条件，旨在定义导致固有不稳定分岔的过程和形态条件，并详细说明控制稳定性的边界条件。使用Delft3D-FLOW将改进早期的数值模型，Delft3D-FLOW是一个形态动力学模型，它考虑了：(1)三维湍流非定常非均匀流动，(2)河床地形、水流、河床和悬浮荷载之间的相互作用，以及(3)可侵蚀河岸和河床之间的形态动力学相互作用。数值实验将使我们能够确定促进稳定分岔的反馈过程，并预测在面对扰动时哪种通道配置是稳定的。我们的目标是生成一个渠道分岔的稳定性图，并评估形式动力学和下游渠道变化对分岔稳定性的影响。更好地理解和预测河道分叉行为的能力将改善洪水预报、洪泛区和河道结构的规划和发展、河道设计以及河道修复工作的成功。这项研究的结果也将提高我们对河流冲刷发生的时间和地点的认识。就其变革性而言，它将解决相互竞争的稳定性理论之间的差异，并产生一个模型，该模型受最佳可用现场数据的约束，可用于预测这些无处不在的水文和地貌节点的稳定性和行为。本研究成果：本研究为一名博士研究生和一名博士后提供论文选题和资金支持。来自萨斯喀彻温省坎伯兰House Cree Nation的两名美国原住民高中生将接受科学方法的培训。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、水动力学和形态动力学建模的问题。

Channel bifurcations, wherein one channel splits into two, are the building blocks of braided, anastomosed, and distributary channel networks. Recent work has shown that the most common flow partitioning at bifurcations is asymmetric, and that this asymmetry seems to promote stability. Why and under what hydrographic and sedimentologic conditions this should be so, remain a puzzle. Here we propose an integrated field and theoretical investigation of the morphology, flow, and sedimentary processes of higher shear-stress, natural river bifurcations, with the objective of understanding and predicting their dynamical behavior. The field studies will be conducted to encompass two bifurcating systems with different sedimentation characteristics: i) distributary channel bifurcations of the Mossy Delta of the Saskatchewan River, where we have conducted pilot studies concerning bifurcation morphodynamics, and ii) bifurcations within the Wax Lake delta system, Louisiana, where active sedimentation is leading to rapid distributary channel growth. Several bifurcations at each site will be chosen to span a range of morphologic types. Bed morphology, water surface topography, flow velocity, and sediment transport rate will be measured at each bifurcation on a closely spaced grid at several flow stages. The field data will provide boundary and initial conditions for numerical experiments designed to define the processes and morphological conditions that lead to inherently unstable bifurcations, and detail the controlling boundary conditions for stability. Earlier numerical models will be improved by using Delft3D-FLOW, a morphodynamic model that accounts for: (1) three-dimensional turbulent unsteady, nonuniform flow, (2) interaction between bed topography, flow, and both bedload and suspended load, and (3) morphodynamically interacting erodible banks and bed. Numerical experiments will allow us to determine the feedback processes that promote stable bifurcations and predict which channel configurations are stable in the face of perturbations. We aim to yield a stability diagram for channel bifurcations and assess the influence of barform dynamics and downstream channel change in influencing bifurcation stability. A better understanding and predictive capability of channel bifurcation behavior would improve flood forecasting, planning and development of floodplain and channel structures, channel designs, and the success of stream restoration efforts. The results of this study also will advance our knowledge of when and where river avulsions will take. Insofar as it is transformative, it will resolve the differences among competing stability theories, and produce a model, constrained by the best-available field data, which can be used to predict the stability and behavior of these ubiquitous hydrologic and geomorphic nodes. Resulting from This Study: This work constitutes the dissertation topic and financial support for one Ph. D. candidate and will support one post-doctoral researcher. Two Native American high school students from the Cumberland House Cree Nation, Saskatchewan will be trained in its scientific methods. All students will benefit from exposure to a problem requiring the integration of geomorphology, sediment transport, hydrodynamics, and morphodynamic modeling.