CSDMS 2016 annual meeting poster JohnShaw
A mass-conservation approach to predicting the distance to river mouth channel bifurcations
Brandon McElroy, University of Wyoming Laramie Wyoming, United States. bmcelroy@uwyo.edu
Kim Miller, University of Wyoming Laramie Wyoming, United States. litwinmiller@gmail.com
Abstract:
Channel bifurcation is an important process in fluvio-deltaic morphodynamics and resulting stratigraphic architecture of prograding river deltas. We develop and test a new theory for the formation of channel bifurcations based on fluid mass conservation and system-averaged transport conditions rather than local hydrodynamics. 29 experimental deltas were built under a variety of boundary conditions to examine the inception and growth of bars and channel bifurcations. From the initial condition of water and sediment entering a still basin of uniform depth as a wall-bounded turbulent jet, delta growth begins with the formation of a lunate bar as predicted by the hydrodynamics of jet spreading. However, the lunate bar diverts water and sediment laterally causing the bar to widen into a radially symmetric flow expansion extending from the channel axis to the flume walls. This feature is stable to perturbations, and its distal limit progrades basinward while maintaining a roughly constant flow depth of ~10 times the median grain diameter (H=2-3 mm). Bar formation and channel bifurcation occur on top of the apron at the distance where shear stress applied by radially-averaged flow velocity falls below the threshold of sediment motion. Our model predicts that the distance to the first channel bifurcation should scale with water discharge, scale inversely with flow depth over the apron, and scale with median grain diameter to the negative one half.
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