Are Trains of Knickpoints in Submarine Channels Shaped by a Cyclic Step Instability in Turbidity Currents?

Submarine channels enable the transport of globally significant volumes of sediment, nutrients and organic carbon into the deep sea. Notwithstanding their importance, little in known on the morphodynamics of these channels, as the turbidity currents that form them are unpredictable, difficult to access and destructive in nature. The lack of field measurements leave many questions on the working of these channels unanswered. This study focuses on the puzzling observations that trains of knickpoints are common in sandy submarine channels, while such trains of knickpoints are known to be rarely sustainable in sandy river channels. One explanation for this contrast if that turbidity current in submarine channel are often Froude supercritical, while this is rare in rivers in fluvial channels. Here, we present a combination of field measurements in Bute Inlet (Canada) and numerical modelling results to explore the validity of the Froude supercritical hypothesis. Direct observations show that the migration of these knickpoints is directly related to powerful turbidity currents. The observations of these flows are consistent with a Froude supercritical flow state and a cyclic step instability driving the maintenance of the knickpoint. However, it is not possible to proof the hypothesis based on the field observations. Numerical modelling further support the cyclic step hypothesis and highlight other important processes that are likely to play a role in the formation and maintenance of cyclic steps.