Abstract
Large roughness features, caused by erosion of the sea floor, are commonly
observed on the modern sea floor and beneath turbidite sandstone beds in
outcrop. This paper aims to investigate the effect of such roughness elements
on the turbulent velocity field and its consequences for the sediment carrying
capacity of the flows. Experimental turbidity
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currents were run through a
rectangular channel, with a single roughness element fixed to the bottom in
some runs. The effect of this roughness element on the turbulent velocity field
was determined by measuring vertical profiles of the vertical velocity
component in the region downstream of the basal obstruction with the
Ultrasonic Doppler Velocity Profiling technique. The experiments were set up
to answer two research questions. (i) How does a single roughness element
alter the distribution of vertical turbulence intensity? (ii) How does the altered
profile evolve in the downstream direction? The results for runs over a plane
substrate are similar to data presented previously and show a lower turbulence
maximum near the channel floor, a turbulence minimum associated with the
velocity maximum, and a turbulence maximum associated with the upper flow
interface. In the runs in which the flows were perturbed by the single
roughness element, the intensity of the lower turbulence maximum was
increased between 41% to 81%. This excess turbulence dissipated upwards in
the flow while it travelled further downstream, but was still observable at the
most distal measurement location (at a distance ca 39 times the roughness
height downstream of the element). All results point towards a similarity
between the near bed turbulence structure of turbidity currents and free
surface shear flows that has been proposed by previous authors, and this
proposition is supported further by the apparent success of a shear velocity
estimation method that is based on this similarity. Theory of turbulent
dispersal of suspended sediment is used to discuss how the observed turbulent
effects of a single large roughness element may impact on the suspended
sediment distribution in real world turbidity currents. It is concluded that this
impact may consist of a non-equilibrium net-upwards transport of suspended
sediment, counteracting density stratification. Thus, erosive substrate
topography created by frontal parts of natural turbidity flows may superelevate
sediment concentrations in upper regions above equilibrium values in following flow stages, delay depletion of the flow via sedimentation and
increase their run-out distance.
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