Seafloor gravity currents: flow dynamics in overspilling and sinuous channels

Turbidity currents are the largest agent of global sediment transport and their deposits, submarine fans, are the largest sedimentary structures on Earth. Submarine fans consist of networks of seafloor channels, which are vital pathways for sediment and nutrient transport to the deep ocean. This work focusses on flow dynamics within these channels, with the aim of understanding the role of the channel form on flow development and identifying implications for the development of channels and, ultimately, for submarine fans. Laboratory experiments have been conducted of continuous saline gravity currents traversing fixed-form channel models with a range of planform geometries. Both velocity and density data were gathered to investigate the effect of a channel on the flow field. Numerical simulations have also been conducted, using a Reynolds-averaged Navier Stokes model and a shear stress transport turbulence closure. These allow an extension of the laboratory analysis, both in terms of physical domain size, data resolution and measured variables. Velocity data reveal how partial confinement exerts a first order control on the vertical variation in flow structure. The channel half-depth acts to limit the height of the velocity maximum, resulting in the development of a confined, high-velocity flow core. The channel form also constrains the lateral and three-dimensional flow structure. Secondary flow rotation, characterised by a local reversal in the radial pressure gradient, is shown here to be inhibited by low channel sinuosity and large levels of overbank fluid losses. A change in cross-sectional channel profile is capable of switching the dominant cross stream basal flow direction of these structures. Furthermore, channels are shown to cause flow tuning, whereby flows of differing magnitudes entering a channel reach are rapidly modified to show a much restricted magnitude range, that remains quasi-stable thereafter. For the cases studied, this quasi-equilibrium state is characterised by a symmetrical cross channel basal stress profile. The existence of such a state could explain how seafloor channels can achieve a degree of planform stability