Sustained turbidity currents
University of Hull, Geography Department
Director :
Professor L Frostick
l.e.frostick@hull.ac.uk
01482 466069
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Investigators: Dr. Jan Alexander1 and Prof. Mike Leeder1 and Dr. Stuart McLelland2
1 School of Environmental Sciences, University of East Anglia, UK.2 Department of Geography, University of Hull, UK.
Introduction
Long-duration, sustained turbidity currents are important transporters of sediment in the natural environment but less well understood than surge-type turbidity currents, partly because of the difficulty inherent in their study. The TES makes study of sustained turbidity currents more practicable and this study used it to investigate two aspects of flow behaviour of particular significance to submarine fan architecture: break of slope and the removal of lateral constraint which both occur at the mouth of submarine channels or canyons.Methodology
The objective of this study was to run fully turbulent sustained currents down a sloping channel onto a flat or nearly flat ~50 m2 surface where they were free to spread laterally. Initial sediment concentration, discharge and channel slope were varied independently and the velocity and sediment transport characteristics of the flows were monitored. Sediment and water were pumped, through a dispersive input box into the submerged, 0.3 m wide, 0.35 m deep channel (the flows were fully contained over the full channel length). Seven acoustic Doppler velocimeters (ADVs) were used to measure turbulent velocities within the channel and where the current spread beyond the slope break.A multi-frequency acoustic backscatter system (ABS) was used to measure sediment concentration and grain size profiles within the body of the flows, and to derive current velocity profiles from cross correlation analysis to provide a direct link between sediment transport and turbidity current flow. Six of the ADVs and the ABS were deployed from a movable gantry and this was moved once during each run giving suites of data in proximal and more distal locations, allowing the spatial velocity pattern to be established for the steady phase of the flows. Three digital video cameras and a digital still camera recorded visible flow features and sediment dynamics. The suspension within the currents was sampled using a siphoning system to recover sets of 5 samples at varying heights above the bed. Samples of the deposits and siphon samples from within the flows were analysed for grain size using a laser particle sizer. These methods together produced a prodigious amount of data that are now being analysed.
Results
The flow development can be divided into three phases: an initial channelised phase, a head-expansion phase, and a steady wall-jet phase. During the head-expansion phase (that lasts for a relatively short period from when the head exits the channel) radial spreading of the head ring-vortex is rapid and head height declines. Up-flow margins of the ring-vortex have vertical axis separation with some deposition. After a few seconds the forward momentum of the main following flow creates a wall jet that pushes through the initial vortex ring. From this point the head begins to thicken with distance. The traces of the lateral most ring vortex expansion are preserved as a thin apron of deposits, but the main bulk of lobe deposition is account for by the steady wall jet phase. The speeding behaviour has a greater influence on deposit distribution than proximal slope angle. This flow development behaviour has significant implications for the understanding of ancient submarine fans as it controls the sediment deposition.