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| Titel |
Turbulent behaviour of non-cohesive sediment gravity flows at unexpectedly
high flow density |
| VerfasserIn |
Megan Baker, Jaco H. Baas, Jonathan Malarkey, Ian Kane |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250123135
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| Publikation (Nr.) |
 EGU/EGU2016-2335.pdf |
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| Zusammenfassung |
| Experimental lock exchange-type turbidity currents laden with non-cohesive silica-flour were
found to be highly dynamic at remarkably high suspended sediment concentrations.
These experiments were conducted to produce sediment gravity flows of volumetric
concentrations ranging from 1% to 52%, to study how changes in suspended sediment
concentration affects the head velocities and run-out distances of these flows, in natural
seawater.
Increasing the volumetric concentration of suspended silica-flour, C, up to C =
46%, within the flows led to a progressive increase in the maximum head velocity.
This relationship suggests that suspended sediment concentration intensifies the
density difference between the turbulent suspension and the ambient water, which
drives the flow, even if almost half of the available space is occupied by sediment
particles. However, from C = 46% to C = 52% a rapid reduction in the maximum head
velocity was measured. It is inferred that at C = 46%, friction from grain-to-grain
interactions begins to attenuate turbulence within the flows. At C > 46%, the frictional
stresses become progressively more dominant over the turbulent forces and excess
density, thus producing lower maximum head velocities. This grain interaction
process started to rapidly reduce the run-out distance of the silica-flour flows at
equally high concentrations of C ≥ 47%. All flows with C < 47% reflected off
the end of the 5-m long tank, but the head velocities gradually reduced along the
tank.
Bagnold (1954, 1963) estimated that, for sand flows, grain-to-grain interactions start to
become important in modulating turbulence at C > 9%. Yet, the critical flow concentration at
which turbulence modulation commenced for these silica-flour laden flows appeared to be
much higher. We suggest that Bagnold’s 9% criterion cannot be applied to flows that carry
fine-grained sediment, because turbulent forces are more important than dispersive forces,
and frictional forces start to affect the flows only at concentrations just below the cubic
packing density of spheres of C = 52%. These experimental results also imply that natural
flows may be able to transport vast volumes of non-cohesive sediment with relative
ease, especially considering that the experimental flows moved on a horizontal
slope.
References
Bagnold, R. A. (1954). Experiments on a Gravity-Free Dispersion of Large Solid Spheres
in Newtonian Fluid under Shear. Proceedings of the Royal Society series A: Mathematical,
Physical and Engineering Sciences, 225(1160), 49–63.
Bagnold, R. A. (1963). Beach and nearshore processes: Part 1. Mechanics of marine
sedimentation. In: Hill, M. N. (Ed.) The Earth Beneath the Sea, vol. 3. Wiley-Interscience,
London, 507–533. |
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