Florian Pohl, Joris T. Eggenhuisen, Mike Tilston, Matthieu Cartigny

In the ocean, particle-laden gravity flows, turbidity currents, flow in river-like channels across the ocean floor. These submarine channels funnel sediment, nutrients, pollutants and organic carbon into the ocean basins and can extend over 1,000’s of kilometers. At the end of these channels, turbidity currents lose their confinement, decelerate and deposit their sediment load. This is what we read in textbooks. However, sea-floor observations have shown exactly the opposite: turbidity currents are prone to eroding the seafloor upon losing confinement. Such erosion features are commonly linked to a rapid flow transition associated with a hydraulic jump. This hypothesis has not been validated due to a lack of field measurements and scaling problems that prevented erosional turbidity currents to form in physical experiments. Here we use a state-of-the-art scaling method to produce the first experimental turbidity currents that erode upon leaving a channel. The experiments reveal a novel flow mechanism, here called ‘flow relaxation’ that explains the erosion. Flow relaxation is the rapid, internal flow deformation resulting from the loss of confinement, which enhances basal shearing of the turbidity current, thus promoting local scouring. This flow mechanism provides a new explanation of scour formation at the end of channels and its role in the propagation of submarine channel systems.