Seafloor avalanches travel fast and far

Transoceanic pipelines and cables can be damaged by underwater avalanches, or turbidity currents, that transport sediment from the continents to the deep ocean. The longest and most powerful turbidity currents measured in action propagate along the Congo Canyon, which extends 800 km from western Africa and reaches a depth of 5000 m. However, without measurements of sediment concentrations in the flows, it’s difficult to predict when a current might occur and how damaging it might be. Research by Steve Simmons at Hull University, UK, as part of an initiative led by Peter Talling of Durham University, has now made those measurements. The findings provide a fundamental understanding of flow structure that could help geophysicists model the origin and outcome of turbidity currents around the world.

To measure the concentrations of active flows, Simmons and his team developed a method based on acoustic Doppler current profilers, devices commonly used to track flow velocities in rivers and oceans. The profilers transmit acoustic pulses and then detect sound that is scattered from material suspended in the water column. By measuring backscatter and using an established inversion method, the researchers could determine where in a flow the sediment was concentrated. Analysis of 10 separate events revealed that a turbidity current comprises a dense, fast-moving flow front followed by a dilute, slower-moving body and tail.

The analysis indicates that sediment concentrations are very dilute in most of the flow, which means that widely used models are likely to overestimate the force of friction between flow and seafloor. As a result, those models may underestimate how fast and far the flows go and the extent of the damage they can inflict. (S. M. Simmons et al., J. Geophys. Res. Oceans, 2020, doi:10.1029/2019JC015904 ; P. Talling et al., EGU General Assembly 2020, EGU2020-2407 .)