Modeling long-term delta dynamics reveals persistent geometric river avulsion locations

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2021-04-01

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Abstract

River deltas grow through repeated stacking of sedimentary lobes, the location and size of which are determined by channel avulsions (relatively sudden changes in river course). We use a model coupling fluvial and coastal processes to explore avulsion dynamics under a range of wave energies and sea-level-rise rates and find that the primary control on avulsion location and delta lobe size in our model is the critical superelevation ratio (SER), the amount of channel aggradation relative to the surrounding floodplain that is required to trigger an avulsion. The preferred avulsion location arises because of geometric constraints – a preferential avulsion node occurs at the break in floodplain slope that develops as the river progrades and/or sea level rises. This concavity develops in our model because the river profile aggrades and erodes via linear diffusion, whereas the diffusion of the floodplain topography is limited to episodic crevasse splays. These results are in contrast to recent modeling work, which was motivated by laboratory experiments and assumes a union between river channel and floodplain aggradation rates, and where avulsion nodes are driven by backwater hydrodynamics. The preferred avulsion length in our model scales well with laboratory, field, and model results without including hydrodynamic backwater effects. This work suggests an alternative mechanism to explain avulsion locations on deltas where floodplain topography aggrades and/or diffuses more slowly than the river channel profile, and it points to the need to elucidate river channel and floodplain connectivity over large space and time scales, and how the connectivity varies from one type of delta to another.

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10.1016/j.epsl.2021.116786

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Ratliff, KM, EWH Hutton and AB Murray (2021). Modeling long-term delta dynamics reveals persistent geometric river avulsion locations. Earth and Planetary Science Letters, 559. pp. 116786–116786. 10.1016/j.epsl.2021.116786 Retrieved from https://hdl.handle.net/10161/25677.

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Scholars@Duke

Murray

A. Brad Murray

Professor of Geomorphology and Coastal Processes

Murray, a geomorphologist, studies how Earth-surface environments are shaped, and how they change over time, especially in response to changing forcing. He has addressed phenomena in desert, artic, alpine, and riverine environments, although most of his recent research focuses on coastal environments. Much of his research addresses couplings between physical and ecological processes, and couplings between natural and human dynamics. Murray approaches natural systems, and human/natural coupled systems, with the perspective and techniques developed in the study of nonlinear dynamics and complex systems, looking for possibly simple, emergent interactions that could explain apparently complicated behaviors. He develops and uses relatively simple, numerical models to test such hypotheses, and uses observations in developing hypotheses and testing models (using strategies and types of model predictions most effective for testing the usefulness of the type of model in question, in specific scientific contexts). Murray’s most recent research falls under three umbrellas, investigating: 1) how changes in the size and shape of river deltas can be driven by couplings between river processes, coastal processes, and sea-level rise, and by couplings between physical and ecological processes; 2) how coastlines (sandy and rocky) are shaped and reshaped over time, including the effects of changing storm climates; 3) how coastal barriers and back-barrier marshes and bays respond to changing rates of sea-level rise and storm impacts. Some of the research under each of these umbrellas addresses couplings between human actions and landscape/ecosystem evolution.


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