Transcript Document

The Environmental Fluid Dynamics Lecture Series
Presents a Seminar
Professor Gary Parker
W.H. Johnson Professor of Engineering and also Geology
University of Illinois
Champaign Urbana
Tuesday, January 21
Rm. 117 Hayes-Healy
11am-12noon
Emplacement of Massive Turbidities Linked to Extinction
of Turbulence in Turbidity Currents
(by Mariano Cantero, Tzu-hao Yeh and Gary Parker)
ABSTRACT
Turbulent flows that carry suspended sediment are naturally self-stratifying. Such sediment
settles toward the bed with a flux that is linearly dependent upon fall velocity. Maintenance
of this sediment in suspension requires a balancing upward Reynolds flux, further dictating
that mean suspended sediment concentration should decrease upward. Since higher
concentration implies larger density, the implication is that sediment suspension generates
its own stable stratification. In the case of rivers, this self-stratification can act to damp
turbulence, so decreasing the ability of the flow to maintain the suspension. The flow itself,
however, is not extinguished, because the driving force of the flow results from the
downstream component of gravity acting on the water, rather than the sediment phase.
Turbidity currents, i.e. dense bottom currents driven by suspended sediment, have many
similarities to river flows. They differ, however, in that the driving force of the flow acts
directly on the sediment. In this case, sufficient self-stratification can kill the entire flow,
causing the emplacement of a massive deposit of sediment. Conditions for this extinguishing
of the flow can be reached as, for example, a turbidity current traverses ever-lower slopes.
These results are demonstrated using direct numerical simulations of channel flows and
turbidity currents.