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Surge Propagation Within Turbidity Currents

Abstract

It is commonly inferred that turbidity currents have a relatively straightforward longitudinal velocity structure, such that velocity maxima occur in or near the head of the flow, with progressively reducing velocities towards the flow tail. Yet many turbidity currents are generated in such a way that they will initiate with more complex longitudinal velocity patterns. For example, a retrogressive slumping flow generation mechanism in which successive failures increase in magnitude may yield a flow in which successive pulses of flow are of higher velocity. Similarly, the magnitude and hence velocity of turbidity currents generated by hyperpycnal fluvial outflow may mimic the hydrograph of the source river. Finally, the patterns of ground movement responsible for seismically-generated turbidity currents are thought to be reflected in the longitudinal structure of the flows. Here we report on the results of simple physical models of surge propagation from a small lock-release flume. The flume was modified with eight potential lock gate increments and up to four lock gates. The multiple locks allow the instigation of multiple pulses within the dense fluid. This configuration allowed investigation of a range of conditions, where both the volume and concentration of the dense fluid in up to four locks was studied. The release of the lock gate(s) was controlled by a mechanised system that allowed for a high degree of repeatability and precise timing of the release of the gate(s). An array of digital cameras where used to record high resolution imagery of the current propagation. Ultrasound velocity profiling (UVP) was used to record time-series of the current velocity profiles. Analytical models of flow propagation are developed using depth-averaged shallow water theory and verified using the experimental data. The research goal is to assess the degree to which the source signal can be modulated during flow runout, as faster pulses advance forward through the flows. The results have implications for interpretation of flow generation mechanisms and assessment of proximity using flow deposits. A further phase of the research will extend these models, adding in sediment transport dynamics and resultant bed stratigraphy, under the assumption that flow buoyancy is continuous.