Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 138

observations: kinetic filtering is the chief mechanism for
sorting and grain migration in multi sized granular flows,
the commonest situation in Nature.
A further intriguing complication is demonstrated by a
vibrated granular mass in a container of equal-sized grains
containing one larger grain. The vibrations induce inter-
granular collisions and a pattern of advection within the
container, with the smaller grains continuously migrating
down the walls of the container, while the larger grain, and

adjacent smaller grains move up the center. Patterns also
arise at the free surface of vibrating grain aggregates, the
newly discovered oscillons creating much interest among
physicists in the mid-1990s.
The wider environment of Earth's surface provides
many examples of the flow of particles: witness the peri-
odic downslope movement of dune sand, screen deposits,
or the spectacular sudden triggering of powder snow or
rock avalanches (Figs 4.62 and 4.63).

4.12 Turbidity flows

As we saw in Section 3.6, buoyancy flows in general owe
their motion to forces arising from density contrasts
between local and surrounding fluid. Density contrasts
due to temperature and salinity gradients are common-
place in the atmosphere (Section 6.1) and ocean (Section
6.4) for a variety of reasons. In turbidity flows it is sus-
pended particles that cause flow density to be greater than
that of the ambient fluid. In this chapter we consider sub-
aqueous turbidity flows; we consider the equivalent class of
volcanic density currents in the atmosphere in Section 5.1.
The fluid dynamics of turbulent suspensions is a highly
complicated field because the suspended particles (1) have
a natural tendency to settle during flow, (2) affect the tur-
bulent characteristics of the flow. The trick in understand-
ing the dynamics of such flows therefore involves
understanding the means by which sediment suspension is
reached and then maintained during downslope flow and
deposition. It is probable that natural turbidity flows span
the whole spectrum of sediment concentration, but it
seems that many are dominated by suspended mud- and
silt-grade particles.

without massive entrainment of ambient seawater, and this
is not possible across the irrotational flow front of a debris
flow. Instead, debris flows must transform along their
upper edges by turbulent separation (Fig. 4.64).
Turbidity currents also form from direct underflow of
suspension-charged river water in so-called hyperpycnal
plumes , also better termed as turbidity wall jets . These have
been recorded during snowmelt floods in steep-sided basins
like fjords and glaciated lakes, in front of deltas, and in river
tributaries whose feeder channels have extremely high loads
of suspended sediments. As noted below, these freshwater
underflows may undergo spectacular behavior during the
dying stages of their evolution. Underflows are expected to
give rise to predominantly silty or muddy turbidites.
Finally, collection of sediment by longshore drift in the
nearshore heads of submarine canyons may also lead to
downslope turbidity flow. The process is most efficient
during and following storms and tends to lead to the trans-
port and deposition of sandy sediment.

4.12.2 Experimental analogs for turbidity currents

4.12.1 Origins of turbidity currents

Turbidity currents are difficult to observe in nature and to
maintain in correctly-scaled laboratory experiments. We may
best illustrate their general appearance by studying saline
and scaled particle currents (Fig. 4.65) using lock-gate tanks
or continuous underflows. In the former, as the lock-gate is
removed, a surge of dense fluid moves along the horizontal
floor of the tank as a density current with well-developed
head and tail regions. Under these zero-slope conditions
the head is usually 1.5-2 times thicker than the tail, with the
ratio approaching unity as the depth of the ambient fluid
approaches the depth of the density flow. Close examination
of the head region shows it to be divided into an array of
bulbous lobes and trumpet-shaped clefts. Ambient fluid
must clearly pass into the body of the flow under the over-
hanging lobes and through the clefts. A greater mixing of

The majority of turbidity currents probably originate by
the flow transformation of sediment slides and slumps
caused by scarp or slope collapse along continental mar-
gins (Fig. 4.64). These are often, but not invariably,
caused by earthquake shocks and are undoubtedly facili-
tated at sea level lowstands when high deposition rates
from deltas, grounding ice masses, or iceberg “graveyards”
provide ample conditions for slope collapse. A role for
methane gas hydrates in providing regional mass failure
planes in buried sediment is suspected in some cases. Slides
are thought to transform to liquefied and fluidized slumps
and then to disaggregate into visco-plastic debris flows.
These cannot transform further into turbulent suspensions