MEAM Seminar: “Exploring the Structure of Sediment-Laden Turbidity Currents”

Turbidity currents are sediment-laden turbulent shear flows that run over a sloping bed, submerged beneath a deep layer of quiescent ambient fluid, driven by the excess hydrostatic pressure. As the current travels downslope, the flow interacts with the ambient fluid layer above through entrainment at the interface. In this process, the ambient fluid is continuously incorporated into the current and the thickness of the current increases. Simultaneously, the current also interacts with the bottom bed both depositing and resuspending sediment. As a result, turbidity currents are responsible for massive emplacement of sediment as turbidites, which with their large amounts of organic-matter deposits now form the richest oil and gas reserves.

Due to extreme paucity of direct field observations and measurements, the structure and dynamics of real turbidity currents remain poorly understood. There are many elementary questions pertaining to the global behavior of the currents that are of substantial consequence to erosion and deposition of sediment. Some important aspects of turbidity currents have puzzled scientists: (i) It has been observed that turbidity currents travel hundreds of kilometers confined within submarine canyons. How is this possible, if the turbidity current is turbulent and mixes with the surrounding? (ii) After traveling for hundreds of kilometers, they suddenly drop a significant portion of the suspended sediment to form massive deposits. What triggers such catastrophic events? (iii) What is the effect of sediment size on ambient fluid entrainment and basal drag? (iv) What generates large-scale bedforms like cyclic steps?

We try to answer these questions with high-fidelity direct and large eddy multiphase flow simulations of turbidity currents. The delicate interplay between wall-turbulence, turbulence in the shear-layer, sediment transport and the back effect of sediment on turbulence through stratification presents a fascinating contrast between supercritical and subcritical currents. First, we find that long running currents in the subcritical regime present a near-wall region that behaves like a turbulent channel flow with a lutocline acting as a lid. Furthermore, we observe supercritical currents to form three families of interacting coherent hairpin vortex structures that control transport of turbulence. We elucidate on the effect of sediment size on ambient fluid entrainment and basal drag. Finally, we explore a transcritical current and its possible connection to cyclic steps and sediment waves. While the current slowly evolves in the subcritical and supercritical regimes in a near self-similar manner, the transcritical current with its unique cyclical evolution exhibits a limit-cycle like behavior.