Unsteady and heterogeneous boundary conditions and their impact on sediment transport and landscape morphology




Daniller-Varghese, Max Sasha

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While sediment transport and landscape morphology are well-characterized under the simplifying conditions of steady and uniform inputs, the effects of heterogeneity and unsteadiness are less well-constrained. This dissertation investigates the effects of heterogeneous and fluctuating inputs on upland sediment transport, coastal margin morphology and sediment routing, and turbidity current dynamics and deposition. In my first project, I characterize how cycles of flooding and normal flow influence delta island shape and evolution. Using the Surface Transport and Earth-surface Processes (STEP) Basin, located at the Morphodynamics Lab at the J. J. Pickle Research Center, I determine the morphological effects of mixed transport type, where sediment is alternately suspended and delivered as bedload in floods and normal flow. The cumulative effect of these types of transport forms a flow bifurcation around a delta island. I find that delta islands are a consequence of mixed transports, rather than an invariant consequence of turbulent jet expansion into a still basin. Second, I investigate the effects that surface waves have in affecting turbidity current sediment transport and mixing. I conducted experiments in the Long Flume at the Experimental Sedimentology Lab, running simulated turbidity currents through a wave field. After applying a mixing model, I determine that a wave field fundamentally changes the mixing and turbulent behavior of a turbidity current, leading to an increase in downslope sediment transport. Lastly, I investigate dense tracer particle transport in an experimental fluvial bedform, also in the Long Flume. I find that dense tracer minerals' transport is decoupled from bulk quartz sediment bedload transport. Collectively, these investigations represent a synoptic view of sediment transport and the effect that unsteadiness and heterogeneity have on dynamics of sediment transport and consequent morphology of sedimentary landscapes.


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