Spatio-Temporal Flow Evolution of Turbidity Current Flow Structure, Linked to Deposit Development

Simon P. Barker¹, William D. McCaffrey², Jaco H. Baas², Peter D. W. Haughton³, and Caroline Choux^2
1 BG Group, Reading, United Kingdom
² University of Leeds, Leeds, United Kingdom
³ University College Dublin, Dublin, Ireland

A programme of laboratory experiments was used to characterise the instantaneous longitudinal flow structure and sediment distribution in polydisperse turbulent gravity currents. Integration of data from repeat runs captured hydraulic fractionation of different grain sizes spatially, rather than as a conventional time series. This highlighted the ability of the flow to self-organise after initial input as a jet, with the progressive development of flow stratification, partitioning of the flow above and below the level of Vmax, the onset of coarse sediment transport lag, and the efficient fractionation of fines to the upper and rear of the flow. Deposit sampling allowed temporal coupling between the deposit and the flow structure to be established, showing how fluctuations in near-bed velocity, turbulence and sediment distribution are associated with vertical textural variation in the deposit. In addition a comparison can be made between the grain-size of the sediment carried in suspension at the base of the flow and the aggrading sediment bed beneath. The deposit is systematically coarser than the suspension directly above it suggesting a competence control on sedimentation. This, combined with the flow self-organisation, suggests that these analogue experiments allow insights into the development of natural-scale turbidity currents on longer spatial and temporal scales, with conclusions regarding the effectiveness of high-concentration flows as a long-range transport mechanism, interpretation of vertical fabric evolution, and linked debrite formation.