A conceptual Investigation of Turbidity Current Trigger from Alongshelf
Current-supported Turbidity Currents
Abstract
Wave- and current-supported turbidity currents (WCSTCs) are one of
the sediment delivery mechanisms from the inner shelf to the shelf
break. Therefore, they play a significant role in the global cycles of
geo-chemically important particulate matter. Recent observations suggest
that WCSTCs can transform into self-driven turbidity currents close to
the continental margin. However, little is known regarding the critical
conditions that grow self-driven turbidity currents on WCSTCs. This is
in part due to the knowledge gaps in the dynamics of WCSTCs regarding
the role of density stratification. Especially the effect of sediment
entrainment, and the parameters thereof, on density stratification and
the amount of sediment suspension, has been overlooked. To this end,
this study revisits the existing theoretical framework for a simplified
WCSTC, in which waves are absent, i.e., alongshelf current-supported
turbidity current (ACSTC). A depth-integrated advection model is
developed for suspended sediment concentration. The analyses of the
model, which are verified by turbulence-resolving simulations, indicate
that the amount of suspended sediment load is regulated by the
equilibrium among density stratification, positive feedback between
entrainment and cross-shelf gravity force, and settling flux dissociated
with density stratification. It is also found that critical density
stratification is not a necessary condition for equilibrium. A
quantitative relation is developed for the critical conditions for
self-driven turbidity currents, which is a function of bed shear stress,
entrainment parameters, bed slope, and sediment settling velocity. In
addition, the suspended sediment load is analytically estimated from the
model developed.
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