Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 141

2.5

2.0

Saline
current

1.0

Turbidity
current

0.5

u max

0.0

0 0.5 1.0 -4 -20246 0 1 2 3 4 0.5 11.52

u / u max w / u max

u 2 max

ke / u 2 max

t xx / T

Fig. 4.66 Mean velocities ( u , w ), turbulent stress ( xx ), and turbulent kinetic energy ( T ke ) contrasts between experimental saline and turbidity
flows of wall jet type. Dimensionless height is with reference to z 0.5 , the height at which u reaches value 0.5 u max .

bore or shallow-water wave. If so, the conversion of potential
energy (due to mean height above the tank floor) to the
kinetic energy of motion gives the velocity of motion, u ,
and proportional to the square root of water depth, h . We
might also guess that u should depend directly upon the
density difference,

u

d 1

d 0

Type C, strong wave

d 1 / d 0 >3

, between the current and the ambi-
ent fluid, or more precisely the action of reduced gravity, g .

Internal velocity profiles taken through experimental
density and turbidity currents reveal a positive velocity gra-
dient in the lower part of the flows; this follows the normal
turbulent law-of-the-wall. Above a velocity maximum,
there is a negative gradient up to the top of the flow
(Fig. 4.66). The latter is due to frictional interactions
and overall retardation of the flow by the ambient fluid
in the form of production of large-scale eddies of
Kelvin-Helmholtz type. Turbulent stresses are much
increased for particulate turbidity flows over saline analogs
(Fig. 4.64).

u

Residual forward flow

Type B, intermediate wave

2< d 1 / d 0 <3

Ramp

u

Bulge of fluid travels
back as internal wave

Type A, weak wave

1< d 1 / d 0 <2

Fig. 4.67 Forward turbidity flow meets opposing topographic ramp
slope and reflects back under residual forward flow as an internal
solitary wave.

4.12.4 Reflected density and turbidity flows

evident that the process can cause upslope deposition on
submarine highs. Reflection may be accompanied by the
transformation of the turbidity flow into a series of trans-
lating symmetrical waves, which have the properties of
solitary waves or bores (Section 4.9). They travel back in
the up-source direction undercutting the slowly-moving
nether regions of the still-moving forward current, trans-
porting fluid and sediment mass as they do so (Fig. 4.67).
Such internal bores have little vorticity, as witnessed by
their smooth forms.

Because turbidity flows derive their motive force from the
action of gravity, they are easily influenced by submarine
slope changes. Flows may partially run up, completely run
up and overshoot, or be partially or wholly blocked,
diverted, or reflected from topographic obstacles. The
process of run-up and full or partial reflection (“sloshing”)
is particularly interesting and the effects may be seen by
inserting ramps into the kind of lock exchange tanks
described previously (Fig. 4.67). Run-up elevations are
approximately 1.5 times flow thickness and in nature it is