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

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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.