New flow relaxation mechanism explains scour fields at the end of submarine channels

Particle-laden gravity flows, called turbidity currents, flow through river-like channels across
the ocean floor. These submarine channels funnel sediment, nutrients, pollutants and organic
carbon into ocean basins and can extend for over 1000’s of kilometers. Upon reaching the
end of these channels, flows lose their confinement, decelerate, and deposit their sediment
load; this is what we read in textbooks. However, sea floor observations have shown the
opposite: turbidity currents tend to erode the seafloor upon losing confinement. 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 this erosion. Flow relaxation is rapid flow deformation resulting
from the loss of confinement, which enhances basal shearing of the turbidity current and
leads to scouring. This flow mechanism plays a key role in the propagation of submarine
channel systems.