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Holistic Evaluation of Basal Stress Evolution in Sinuous Submarine Channel Levee Systems: Towards Process-Based Forward Stratigraphic Modelling

Abstract

Submarine channel systems are the main conduit through which turbidity currents transport sediment to the deep ocean and comprise a key reservoir class. Their evolution, driven by the current dynamics, is controlled via two primary basal processes: erosion and deposition. Because deep ocean locations make the acquisition of field data challenging at best, conceptual process models, based upon seismic data, physical modelling and outcrop studies, provide us with the best overview of how channel systems evolve. However, these approaches are limited and the underpinning dynamics are still not well understood. Here, a combination of laboratory and numerical simulation data is presented which aims to close the gap between channel evolution process models and the controlling flow dynamics. An unsteady Reynolds-averaged Navier-Stokes (URANS) model, combined with with a shear stress transport (SST) turbulence model, is able to accurately replicate the flows produced in the laboratory and is subsequently used to study flows propagating with a range of flow magnitudes within a sinuous channel. Maps of basal stress (an indicator of erosive power) and deposition rate, produced from basal velocity and density gradients, show several features which highlight the complex relationship between current and channel. Foremost of these is the lack of direct correlation between stress rate and current magnitude. Currents can develop over the course of a series of bends via overbank loss and entrainment of ambient fluid. The simulations show that currents with a smaller initial magnitude can evolve via these mechanisms to have higher rates of stress and deposition than relatively larger initial currents. Stress and deposition maps can be combined to create net aggradation maps, which show explicitly how a channel may be affected by an individual current. By analysing these maps for a range of input conditions, correlations can be drawn between flow properties and the resulting grain size, morphology and patterns of the respective deposits. Additional flow phenomena are identified within these datasets, including the rotation of overbank flow towards the direction of intrachannel flow. This rotation, observed in experimental, numerical and field data, forms the basis of a model capable of estimating real-world turbidity current durations.