First high resolution direct measurements for powerful turbidity currents that reach the deep ocean

Submarine turbidity currents are arguably the volumetrically most important process for moving sediment across our planet. They form the largest sediment accumulations (submarine fans) on earth, and single flows can transport ten times the annual flux from all of the world's rivers. However, the most remarkable feature of turbidity currents is how few direct measurements are available from these flows, as they are notoriously difficult to monitor in action. This is a stark contrast to other major sediment transport processes, such as rivers for which we have many thousands of direct measurements.

Powerful long run-out turbidity currents are especially difficult to monitor, yet it is these flows that build submarine fans. Such flows are important because they break sea-floor cables that carry > 95% of global data traffic, including internet and financial markets that underpin daily lives. The velocity of turbidity currents that reach beyond the continental slope had previously been measured in just five locations, primarily from cable breaks that only record averaged front velocities. Their sediment concentration had never been measured directly. This globally important sediment transport process is therefore poorly understood, and laboratory or numerical models for such flows are poorly validated.

This PhD student will analyse a remarkable dataset comprising the first synchronous velocity and concentration profiles for turbidity currents beyond the continental slope, collected at a cost of > $1M by CASE partner Chevron and co-workers in the Congo Canyon (from 2009-2013). This is the first time that high temporal resolution (>1/min) synchronous profiles of both velocity and concentration have been measured for turbidity currents beyond the continental slope. They are also the fastest (2.5 m/s) turbidity currents yet measured by instruments. The data were collected for a major oil and gas pipeline that will need to cross the Congo Canyon. This is a challenging project as previous cable breaks show the canyon is regularly swept by powerful flows. The data comes from moorings with downward pointing Acoustic Doppler Velocity Profilers (ADCPs) that measure velocity and acoustic backscatter. Backscatter is partly dependent on grain size, but also records changes in sediment concentration.

Initial results were surprising for two reasons. First, flows had surprising durations of several days, with average speeds of ~1 m/s. Interestingly, it was observed that the larger flows always had a similar duration of ~6 days. This seems to indicate the establishment of an equilibrium flow configuration over the first 150 km of the canyon. Several hypothesis have been put forward to explain this behaviour, however, none of them have yet been validated. Second, the measured turbulence intensity decreased as flow speeds increased, this counter-intuitive relation suggests damping of turbulence by elevated suspended sediment concentrations. Although it has been speculated previously that turbulence damping by sediment may be of fundamental importance, such damping has never previously been documented in direct observations from full scale flows in the field. Thus, the relation between turbulence damping and sediment concentration remains to be validated in real submarine flows.

A numerical model is needed to test hypotheses that equilibrium flow configurations produce multi-day flows, and to explore how turbulence dampening may affect submarine flows. Such numerical model will benefit from the well-mapped bathymetry of the Congo canyon (from Chevron and past publications by IFREMER). In this proposal we will use a state-ofthe-art fully three-dimensional numerical model that has been developed over the last eight years by project partner Complex Flow Design AS (CFD), Norway. This model is unique due to its 3D approach and its capability to introduce
sediment concentration effects into its turbulence model.