Direct Monitoring of Turbidity Currents: New Insights, Challenging Preconceptions and Future Directions

Abstract:

Turbidity currents are, volumetrically, the most important process for the transportation of sediment on the face of our planet. The combination of large volume and fast speeds can damage globally important seafloor cables and offshore structures and may transport sediment over hundreds of kilometres. Despite their significance for sediment flux and as geohazards, very few examples of direct monitoring of real-world turbidity currents exist. Until recently, there has been a reliance on depositional records, scaled-down experiments and numerical models to understand the nature of turbidity currents. The results of direct monitoring obtained over the past few years now provide us with ground-breaking insights into the real-world behaviour of full-scale turbidity currents. We present results of recent flow monitoring acquired using an array of acoustic and geophysical tools, from multiple sites worldwide, including the deep-sea Congo Canyon, Canadian fjords, and a dredging experiment offshore Holland.

This advent in turbidity current monitoring, largely driven by step-changes in technology, has reinforced some existing interpretations, but also challenges some preconceptions. Our results are based on monitoring using multibeam sonars, sub-bottom and acoustic Doppler current profilers. First, we provide insights into the triggering of flows that include landslides, tidal and wave effects, and other more cryptic events with no clear initiation point. Second, the influence of dense layers at the base of flows is shown to be important for sediment transport and bedform migration; however, most acoustic techniques struggle to penetrate. Initial results from a novel Chirp profiler provide imaging of the lowermost part of the flow. Third, the morphology of the flow and its development through time are shown to deviate from that observed in classical flume tank experiments. Finally, we summarise some future directions for flow monitoring to push forward our understanding of these dynamic flows.