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(a)
20
19
20
Head
Nose
overhang
132
127
120
Fine sediment
Mixing
Body
(b)
45
44
44
56
103
147
98
106
Mixing
Body
Fig. 4.65 Experimental turbidity flows (photos are c .0.5 m vertical extent) traveling down progressively steeper ramps (columns of slopes
0
) onto flat horizontal surfaces. (a) Turbidity flow heads (first row) and bodies (second row) on increasing ramp slopes. (b) Turbidity flow
heads (first row) and bodies (second row) on flat horizontal floors leading from upstream ramp slopes as indicated in (a).
-9
turbidity flows generated from finite-volume sediment
slumps or debris flows on small slopes (
or basins. Such flows are wall jets , that is, aperture flows
released onto the floor of large volume reservoirs (volcanic
equivalents are discussed in Section 5.1). An interesting
transformation takes place for the case of dissipating turbid
freshwater underflows . These were particularly important in
oceanic sedimentation during the melting phases after
glaciations when vast quantities of sediment-laden freshwa-
ter were released to the oceans. In such systems, deposition
progressively reduces the buoyancy force, the bulk flow
density eventually decreases below that of sea water. The
flow comes to a complete halt, with the now positively
buoyant fluid rising upward to spread out within density
interfaces in the ocean or at the surface as a plume. The
process has been termed lofting and leads to widespread
suspension and eventual deposition of suspended muds.
) must deceler-
ate because the supply of denser fluid from behind the
head is finite and the buoyancy force driving the flow is
insufficient to overcome frictional energy losses. The head
thus shrinks until it is completely dissipated.
On slopes from at least 0.5
1
, head velocity of con-
tinuous underflows is independent of slope and varies
according to density contrast. Head velocity is approxi-
mately 60 percent of the tail velocity in the slope range 5
to 5
to 50
, leading to the head increasing in size as it travels
downslope. Entrainment of ambient fluid also causes head
growth, increasingly so at higher slopes, and the momen-
tum transferred from the current to this new fluid acts as a
retarding force to counteract the buoyancy force due to
any increased slope. This “steady velocity/growing head”
behavior is also a characteristic of starting thermal plumes
but has not been investigated from the point of view of
turbidity current deposition and erosion.
Rapid dissipation of channelized turbidity flows with
consequent deposition occurs as they undergo vertical
expansion and lateral spreading on entering wide reservoirs
4.12.3 Velocity and turbulence characteristics of
experimental turbidity flows
How quickly can dense underflows travel? We might guess
that a solution would be to treat the surge as a moving
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