Links between Dynamic Pressure Variations and Internal Velocity Strucure in Experimental Turbidity Currents: Implications for Flow-Substrate Interactions

William D. McCaffrey
University of Leeds, Leeds, United Kingdom

Experiments are described in which a high frequency time series of the normal pressure exerted on the bed during the passage of experimental turbidity currents was measured from an array of differential pressure transducers, and linked to simultaneous measurements of the internal velocity structure of the flows. Both dynamic and hydrostatic components of pressure can be recognized. Normal pressure begins to increase in advance of the arrival of the current, reflecting the passage of a pressure wave in front of the head; it continues to rise after arrival of the head, reflecting the hydrostatic effect of the suspended sediment. A second-order, dynamic variation in pressure can also be recognized in the time interval corresponding to the passage of the head. This correlates well with variations in the internal velocity structure of the flow, with pressure increases positively correlated with episodes of downward fluid movement. These velocity variations are thought to be linked to the passage of eddies within the flow. Within these short-lived, waning, experimental currents only a few eddies impact the bed. Within longer-lived flows, however, it seems likely that trains of eddies may cyclically load the substrate. The implications of pressure effects on the substrate at the natural scale are considered, with respect to dewatering, slope destabilisation, and pressure variations associated with inflexions in lateral confining slopes.