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

Abstract

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.

The influence of basin setting and turbidity current properties on the dimensions of submarine lobe elements

“…, 2015, Pohl et al. , 2019) is a transient feature and therefore rarely observed in ancient outcrops (cf. Brooks et al.…”

“…, 2016; Pohl et al. , 2019) was applied to create turbidity currents that allow for investigation of depositional processes.…”

“…The entire set-up is covered by unconsolidated substrate with a similar composition to that used to generate the turbidity currents (sand/silt mixture with d 50 of 133 lm) allowing for erosion by the incoming turbidity currents. Shields scaling (de Leeuw et al, 2016;Pohl et al, 2019) was applied to create turbidity currents that allow for investigation of depositional processes.…”

“…This suggests that lobe deposits formed during low versus high sediment supply may have distinctive morphologies as well as relations with the channelized slope, raising the question of whether the erosive channel-lobe transition zone (Palanques et al, 1995;Wynn et al, 2002;Hofstra et al, 2015, Pohl et al, 2019) is a transient feature and therefore rarely observed in ancient outcrops (cf. Brooks et al, 2018).…”

Entangled external and internal controls on submarine fan evolution: an experimental perspective

“…To realistically represent a natural system that can erode and transport sediment in suspension downslope, the experimental turbidity currents of this study utilised Shields scaling (Shields, 1936). This approach follows de Leeuw et al 2016and Pohl et al (2019), using the Shields parameter( * ), which is the ratio between bed shear stress and gravitational forces acting on the sediment, and the particle Reynolds number (Re p ), which controls the hydrodynamic condition at the base of the flow ( Figure S9). A Shields parameter comparable to natural systems has been achieved in these experiments by using a high sediment concentration (17% of total volume) and a steep (11°) slope ( Figure S9; Table S1; Xu et al, 2014;Azpiroz-Zabala, et al, 2017).…”