CEE 598, GEOL 593
TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS
LECTURE 1
WHAT IS A TURBIDITY CURRENT?
Turbidity current driven by crushed coal moving
down bottom of flume containing fresh water:
1
Garcia Tank, SAFL, University of Minnesota
A TURBIDITY CURRENT IN ACTION
Turbidity current driven by plastic particles:
Experiment of O. Sequeiros and H. Naruse
2
conn13.avi
AN ANALOG OF A TURBIDITY CURRENT:
POWDER SNOW AVALANCHE
3
Video clip courtesy P. Gauer
AvalancheFin01GauerP.avi
AN STARTING POINT: THE BOSPHORUS
Black Sea
Istanbul
Bosphorus
TURKEY
Sea of Marmara
4
THE SETTING:
THE MEDITERRANEAN AND BLACK SEAS
Istanbul
Black Sea
Sea of
Marmara
Mediterranean
Sea
5
DIFFERENCE BETWEEN THE MEDITERRANEAN
SEA AND BLACK SEA
Mediterranean
Sea
Bosphorus
Black Sea
Sea of
Marmara
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THE MEDITERRANEAN SEA AND THE BLACK SEA
• The Mediterranean Sea receives little
freshwater inflow, and has a high
evaporation rate.
Black Sea:
less salty
• Mediterranean water is thus rather salty.
• The Black Sea receives a substantial
flow of fresh water.
flow
• Black Sea water is thus less salty.
• The saltier the water, the higher is the
density.
• The net flow through the Bosphorus is
from the Black Sea to the Mediterranean
Sea (through the Sea of Marmara).
Sea of Marmara:
more salty
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FLOW THROUGH THE BOSPHORUS
The flow through the Bosphorus is so strong that in ancient times,
ships could neither row nor sail into the Black Sea.
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SO HOW DID SHIPS GET UP THE BOSPHORUS TO THE
BLACK SEA IN ANCIENT TIMES?
Black Sea
Byzantium/
Nea Roma/
Constantinople/
Istanbul
Sea of Marmara
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CURRENT AND COUNTERCURRENT!
light fresh water
heavy salty water
Sea of Marmara
Black Sea
The strong surface flow of less salty water from the Black Sea to
the Sea of Marmara is accompanied by a less strong (but still very
strong) flow of more salty water from the Sea of Marmara
(ultimately Mediterranean Sea) to the Black Sea:
Dense bottom flow.
10
THE ANCIENT SOLUTION:
THE WATER SAIL!
light fresh water
heavy salty water
Sea of Marmara
Black Sea
WATER SAIL!
11
TURBIDITY CURRENTS OBTAIN THEIR DRIVING FORCE
FROM THE EXTRA WEIGHT OF SEDIMENT IN SUSPENSION
sediment-free water
water with high concentration of
suspended sediment
gravity pulls the
water-sediment
mixture downslope
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ARCHIMEDES’ PRINCIPLE
body
=
amb
V
g
=
=
=
density of material in a “body” (control volume? sediment
grain?)
density of the ambient fluid in which it is immersed
volume of the “body”
gravitational acceleration
Fbuoy
The weight of the body W is given as
W  body gV
The buoyant force Fbuoy acting on the body is given as
Fbuoy  amb gV
W
The effective immersed weight of the body Wimm is then given as
Wimm  W  Fbuoy  body  amb gV
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FRESH AND SEA WATER DENSITY
Fresh water density depends on
Temperature (C)
Sea water density depends on:
Salinity (mg/l of salt ~ ppm of salt)
Temperature (C)
“Standard” density of fresh water:
1.00 ton/m3 = 1000 kg/m3
“Typical” density of salt water:
1.027 tons/m3 = 1027 kg/m3
(but can vary considerably)
Dead Sea
http://www.reliefmart.com/deadsea/dead_sea_sunset.jpg
Dead Sea saltwater density:
~ 1.17 tons/m3
Water density calculator:
http://www.csgnetwork.com/h2odenscalc.html
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WHAT CAUSES FLOW DOWN A SLOPE?
CASE: WATER UNDER AIR ~ RIVER
a = density of air ( ~ 1.2 kg/m3: use gas law)
w = density of water ( ~ 1000 kg/m3 for fresh water)
 = bed slope angle, so that slope S = tan
The control volume is full of water under air. It has length L and crosssectional area A.
The immersed weight of the control
volume is
A
Wimm  (w  a )gLA
L
Fgd
The downslope component of this
immersed weight Fgd drives the flow
downslope:

Wimm
Fgd  ( w  a )gLA sin 
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BUT FOR WATER UNDER AIR,
a = density of air ( ~ 1.2 kg/m3: use gas law)
w = density of water ( ~ 1000 kg/m3 for fresh water)
So
( w   a )
1
w
A
The immersed weight of the control
volume is
Wimm  w gLA
L
Fgd
The downslope component of this
immersed weight Fgd drives the flow
downslope:

Wimm
Fgd   w gLA sin 
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WHAT CAUSES FLOW DOWN A SLOPE?
CASE: WATER UNDER THE SAME WATER
w = density of water ( ~ 1000 kg/m3 for fresh water)
 = bed slope angle, so that slope S = tan
The control volume is full of water under the same water. It has length L and
cross-sectional area A.
The immersed weight of the control
volume is
Wimm  (w  w )gLA  0
A
L
The downslope component of this
immersed weight Fgd drives the flow
downslope:
Fgd

Wimm
Fgd  ( w   w )gLA sin   0
No flow!
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WHAT CAUSES FLOW DOWN A SLOPE?
CASE: SALINE WATER UNDER FRESH WATER
f = density of fresh water ( ~ 1000 kg/m3)
sal = density of saline water ( ~ 1027 kg/m3 for sea water)
 = bed slope angle, so that slope S = tan
The control volume is full of saline water immersed in fresh water. It has
length L and cross-sectional area A.
The immersed weight of the control
volume is
A
Wimm  (sal  f )gLA
L
Fgd

Wimm
The downslope component of this
immersed weight Fgd drives the flow
downslope:
Fgd  (sal   f )gLA sin 
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LET’S COMPARE
Fresh water flowing under air = open channel flow =
RIVER
Saline water flowing under less saline (e.g. fresh) ambient water =
SALINE BOTTOM UNDERFLOW
Let’s compare the downslope driving force Fgd,saline for standard seawater
under fresh water versus fresh water under air Fgd,river
Fgd,saline
Fgd,riv er
(sal   f )gLA sin  (sal   f )


( f  a )gLA sin 
( f   a )
Using f = 1000 kg/m3, sal = 1027 kg/m3 and a = 1.22 kg/m3,
Fgd,saline
Fgd,riv er
~ 0.027
The saline underflow has only 2.7% of the driving force of a
corresponding river!
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WHAT CAUSES FLOW DOWN A SLOPE?
CASE: SEDIMENT-LADEN WATER UNDER SEDIMENT-FREE
WATER: TURBIDITY CURRENT
w = density of water ( ~ 1000 kg/m3)
t = density of underflowing water + sediment = density of turbidity current
> w
How do we compute t?
sed = density of sediment (quartz ~ 2650 kg/m3)
c = volume concentration of sediment in suspension
c = (volume sediment)/[volume sediment + volume water]
Density f of sediment-water mixture is given as
 t   w (1  c )  sedc
or
 t   w 1  Rc  , R 
sed
 1  1.65
w
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WHAT CAUSES FLOW DOWN A SLOPE?
CASE: TURBIDITY CURRENT
w = density of ambient water ( could be fresh or saline: ~ 1000 kg/m3)
t = density of turbid water in flow
 = bed slope angle, so that slope S = tan
The control volume is full of turbid water. It has length L and cross-sectional
area A.
The immersed weight of the control
volume is
A
L
Wimm  (t  w )gLA
The downslope component of this
immersed weight Fgd drives the flow
downslope:

Fgd  ( t   w )gLA sin 
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CASE: TURBIDITY CURRENT
Driving force of a turbidity current
Fgd,turb  (t  w )gLA sin 
t  w  wRc
Thus
Fgd,turb   wRcgLA sin 
A
L
Compare the driving forces of a
river and a turbidity current:
Fgd,turb

Fgd,riv er
 Rc
So how large can c be?
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HOW LARGE CAN THE SEDIMENT CONCENTRATION IN A
TURBIDITY CURRENT BE?
The sediment in a turbidity current that derives it must be in suspension,
i.e. dispersed in the water column away from the bed.
Both rivers and turbidity currents carry suspended sediment
To qualify as a river suspension or turbidity current, the concentration of
suspended sediment must be dilute, so that
c  1
Thus since R ~ 1.65,
Fgd,turb
Fgd,riv er
 Rc  1
A turbidity current thus has much less driving force than a river carrying the
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same concentration of suspended sediment!
WHAT HAPPENS IF THE CONCENTRATION IS NOT DILUTE?
• Turbidity currents and submarine (subaqueous)
debris flows are members of the class of dense bottom
flows that includes thermohaline bottom flows (e.g.
Straits of Gibraltar or Bosporus).
• The presence of a dilute suspension of
sediment in the water of a turbidity current
renders it slightly heavier than the ambient water.
• A submarine (subaqueous) debris flows consists of a dense sediment-water
slurry that is much heavier than the ambient water, so creating its own sedimentwater rheology.
• In both cases gravity pulls the sediment downslope, and sediment pulls the
water downslope.
• Turbidity currents and submarine (subaqueous) debris flows differ from a
thermohaline underflows in that it is free to exchange sediment with the bed.
• A turbidity current is the subaqueous analog of a river. A submarine
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(subaqueous) debris flow is the subaqueous analog of subaerial debris flow.
AN EXAMPLE OF A SUBAERIAL DEBRIS FLOW
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rte-bookjapandebflow.mpg
AN EXAMPLE OF A SUBAQUEOUS DEBRIS FLOW
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IlstMain_cam3.avi
THE FUNDAMENTAL DIFFERENCE BETWEEN A RIVER AND
A TURBIDITY CURRENT
A river flows downslope under the influence of gravity acting on the water.
The water then drags the sediment with it.
The suspended sediment it carries adds only slightly to the driving force as
long as c << 1).
Fgd,riv er   w (1  Rc )gLA sin 
A turbidity current flows downslope under the influence of gravity acting on
the sediment.
The sediment then drags the water with it.
Fgd,turb   wRcgLA sin 
Note that turbidity currents must die, but river flows do not die, as c  0.
Turbidity currents must find a way to keep their sediment in suspension if
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they are to sustain themselves!
VELOCITY AND SUSPENDED SEDIMENT CONCENTRATION
PROFILES IN RIVERS AND TURBIDITY CURRENTS
A river is sediment-laden water flowing under air.
A turbidity current is sediment-laden water flowing under sediment-free
water.
In the image below u = streamwise flow velocity, c = volume suspended
sediment concentration
Rivers (usually) form sharp
interfaces with the air
above, and turbidity
currents (usually) form
more diffuse interfaces with
the water above.
air
river
clear
water
u
c
u
c
turbidity
current
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Source Material (for Parker only)
TurbCurrAAPGApril06.ppt
ExxonMobilShortCourse06.ppt
TurbidityCurrentMinutes.ppt
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