Physical Modelling of Turbidity Current Flow Equilibration to a Fixed Multibend Sinuous Channel Form

Hunter, Katrina M.; McCaffrey, William D.; Keevil, Gareth M.; Kane, Ian A.

Submarine channels are important morphological features, observable on the present sea floor and preserved in the rock record, that transport sediment from the continental shelf into deep marine basins. Individual channel-levees appear to evolve towards a form of equilibrium slope and channel profile through processes of erosion and deposition as sequences of flows undergo equilibration to the channel form. Due to their nature and scale, modern submarine channel systems are largely inaccessible for direct study and therefore experimental modelling provides key insight into the mechanisms and feedbacks which govern their development.

The feedback mechanisms that govern the rate of growth of confining levees and the degree of confinement of overbanking flows are very important for channel development, but remain poorly understood. Unfortunately such feedbacks are challenging to model directly and so indirect insight is sought through modelling flows through fixed channel forms. Here, a series of nominally identical silica flour turbidity currents were released into a long sinuous channel model consisting of 15 bends. Characterisation of flow and overspill was achieved by measuring individual flows at specific locations, with data being combined to build a picture of flow evolution along the length of the channel. In-channel velocities were recorded using Ultrasonic Velocity Doppler Profiling (UVDP). The 3-D overbank velocity was measured using profiling Acoustic Doppler Velocimetry (ADV), and the concentration and grainsize fractionation was measured via analysis of siphon samples.

Flow morphology evolves and adjusts to the channel form along the whole channel, largely through the process of flow stripping. Although flow stripping occurs at each bend, the volume and velocity of the overspill is greatest in the proximal bends and decreases down channel. Three distinct flow regimes can be identified within the channel form, here associated with differing styles and degrees of equilibration. In the initial stage, flows over-fit to the channel equilibrate to it over c.5 bends via rapid mass loss. The final stage, where flows are under-fit to the channel over the last c.5 bends, may be representative of a retrogressional system. In the intervening stage (bends 6-10), flows may approximate to some form of equilibrium. The associated overspill flux rate may therefore indirectly characterise the longitudinal form of a system that is close to equilibrium everywhere.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013