Environmental Engineering Reference
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border was rich in coarse-grained materials (Coulomb frictional behavior), whereas
the core was fine-grained (viscoplastic behavior). This self-organization has a great
influence on the flow behavior; notably the run-out distance can be significantly
enhanced as a result of levee formation limiting lateral spreading.
Parsons
et al
. [PAR 01] ran a series of experiments to investigate the transition
between viscoplasticity- and friction-dominated regimes. They used a semi-circular
inclined flume and measured the velocity profile at the free surface; in addition, they
estimated the bulk viscosity and yield stress using independent tests. Different slurries
were prepared by altering the sand, clay and silt fractions. They obtained muddy
slurries, when the matrix was rich in silt and clay, and poorly ordered mixtures, when
the silt and clay contents were reduced. Surprisingly enough, the change in fine-
particle content did not significantly modify the appearance of the body, whereas it
markedly altered the composition of the front and its behavior. In all the experiments,
they found that the Herschel-Bulkley performed well since the velocity profile and
the plug position were properly estimated. Reducing the fine fraction in the slurries
induced a radical change of behavior in the front (see Figure 1.7):
- For muddy slurries, the front takes the form of a blunt nose. Lack of slip along
the flume bottom caused a conveyer-belt-like flow at the front.
- For coarse-grained slurries, the front takes the form of a dry granular locked nose
slipping along the bed as a result of the driving force exerted by the fluid accumulating
behind the snout. Additional material was gradually incorporated into the snout, which
grew in size until it was able to slow down the body.
Interestingly enough, the changes in the rheological properties mainly affected the
structure of the flow, especially within the tip region.
(a)
(b)
Figure 1.7.
Schematic of the behavior contrast between fine- and coarse-grained flows.
(a) Conveyer-belt-like flow at the front. (b) Formation of a frictional front.
After Parsons et al. [PAR 01]
