Non-Cohesive,
Mixed and Cohesive High-Density Turbidity Currents: Internal Flow Properties
Inferred from Laboratory Experiments
Manica, Rafael1,
Jaco H. Baas2, Ana Luiza de Oliveira Borges1, Rogério
Dornelles Maestri1, Jeff Peakall2, William. D. McCaffrey2
(1) Universidade Federal do
Non-cohesive, mixed and cohesive
high-density turbidity currents were generated in the laboratory to investigate
their flow-dynamic (velocity and concentration) and depositional properties.
Each of the three types of turbidity current was studied at five different
densities between 2.5% and 30% by volume. Head, body and tail velocities (at 10
different heights) and concentrations (at 4 different heights) were measured
simultaneously, using Ultrasonic Doppler Velocimetry Profiling probes and
Ultrasonic High Concentration Meters, respectively. The velocity and
concentration profiles disclosed predictable changes in flow character with
progressive changes in suspended sediment concentration and cohesive character.
These changes were expressed best in flow geometry, height of maximum velocity,
near-bed suspended distribution and settling character. Moreover,
Kelvin-Helmholtz instabilities were observed to modify the internal properties
of the flows well below their upper boundary. The results indicate that in
non-cohesive and mixed flows the concentration and velocity data collapse well
whereas at concentrations above 10% in cohesive currents the presence of kaolin
modifies drastically the hydrodynamic behaviour (and possibly the rheology) of
the flow. Three depositional models are distinguished. In the decelerating
non-cohesive flows, abrupt deposition of sediment prevails, whilst in the mixed
flows deposition is more gradual. In the cohesive flows, a high-concentrating
basal layer forms just after the passage of the head, which
seems to behave as a slowly moving fluid mud. Above the fluid mud, the
flow is more dilute, faster moving and more turbulent. The implications for the
spatial evolution of natural high-density turbidity currents and their deposits
are discussed.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California