Experiments from Substrate Erosion Via Subcritical Turbidity Currents

Brandon McElroy, David Mohrig, and Jim Buttles
Dept. of Geological Sciences, University of Texas, Austin, TX

Existing models of turbidity currents parameterize vertical mixing and suspended sediment transport as a function of turbulence produced solely through interaction with the underlying bed. Implicit to this type of closure is that mixing at the upper fluid-fluid interface can be neglected because it contributes insignificantly to turbulence in the basal flow where sediment transport is focused. Here we present a laboratory scale demonstration that turbulence produced at the current-ambient fluid interface can augment vertical mixing throughout the entire current affecting local sediment transport and patterns of bed erosion.

Initial conditions in our 1-D channel consisted of a 3m flat upper bed segment, an abrupt roll-over to a 3m segment with a 10 degree bed slope, and a transition back to a horizontal run-out section. Salt-laden currents traversed the upper flat portion with peak velocities of 15cm/s and thicknesses near 10cm. With bulk excess densities in the range 1.4% to 2.7%, stratification of the underflow was sufficient to inhibit any significant mixing at the interface between currents and overlying ambient clear water. Currents accelerated over the slope break and thinned commensurate with the spatial acceleration. Acceleration was sufficient to initiate interface mixing via Kelvin-Helmholtz instabilities. The instabilities grew to a diameter of half the flow thickness before breaking. ADV measurements indicate that at the wave-breaking position near-bed turbulence intensities were the largest in the system. Additionally, the local mean Bulk and Gradient Richardson numbers were self-sustained throughout the experiment at or just below critical. Erosion of the sediment bed was also focused directly beneath the zone of breaking waves, highlighting the importance of turbulence produced at a current's upper interface.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas