Transcript Slide 1

CEE 598, GEOL 593
TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS
LECTURE 7
TRIGGERING MECHANISMS OF TURBIDITY CURRENTS
What causes turbidity currents?
Scripps
and
La Jolla
Submarine Canyons, San Diego,
California
http://clasticdetritus.com/category/
marine-science/
1
A PARTIAL LIST OF TRIGGERING MECHANISMS
1. Hyperpycnal flows
2. Surface waves
3. Breaching
4. Double-diffusive phenomenon
5. Transformation from slope failures and submarine debris
flows
6. Internal waves
7. Wave-supported sheet turbidity currents
2
DENSITY OF FRESH WATER
The density of (sediment-free) water varies with temperature (e.g.  C),
salinity (e.g. grams/liter) and pressure (~ depth according to the hydrostatic
law). Here we consider the effect of temperature and salinity.
Fresh water has a maximum density of 1 ton/m3 at 4 C.
1.00000
0.99500
Density tons/m3
0.99000
0.98500
0.98000
0.97500
0.97000
0.96500
0.96000
0.95500
0
20
40
60
Temperature deg C
80
100
Double-click to activate
Excel spreadsheet.
Density
Temperature
Deg C
tons/m3
0 0.999868
2 0.999968
4
1
10
0.9997
20
0.9982
30
0.9957
40
0.9922
50
0.9881
60
0.9832
70
0.9778
80
0.9718
90
0.9653
3
100
0.9584
DENSITY OF SEA WATER
The “standard” density of sea water is ~ 1.026 tons/m3. This corresponds to
a salinity of 35000 mg/liters (35 grams/liter) and a temperature of 14 C.
Increasing temperature (usually) makes water lighter, and increasing salinity
makes it heavier.
You can find a calculator of seawater density at
http://www.csgnetwork.com/h2odenscalc.html
The density of seawater can, however, vary considerably. In brackish
coastal waters near river mouths, the density can approach that of
freshwater, i.e.
1.000 tons/m3.
In nearly-enclosed seas with high evaporation rates, such as the Red Sea,
the salinity can be as high as 40000 mg/liters. Assuming, for example, a
water temperature of 20 C, the density is about
1.029 tons/m3
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1. PLUNGING AND HYPERPYCNAL FLOWS
Reuss River plunging into Lake Lucerne,
Switzerland: flood of summer, 2005
5
WHAT IS PLUNGING?
WHAT IS A HYPERPYCNAL FLOW?
Plunging is a phenomenon when sediment-laden river flow is heavier than
the body of water it flows into. The sediment water immediately sinks,
forming a continuous turbidity current.
Plunging is usually associated with muddy flows, whereas (most of the)
sand (most of the time) tends to deposit in a delta upstream.
Plunging is very common in lakes and reservoirs. The resulting turbidity
currents can emplace sediment over long distances.
Lake Mead
Delta of
Colorado River
6
plunge line
PLUNGING IN LAKE MEAD
Logjam near plunge point
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Image from USBR
PLUNGING CREATES A HYPERPYCNAL FLOW
Hyper  excess, pycnal  density. So a hyperpycnal flow is a continuous
bottom turbidity created by the excess density of the river flow relative to the
ambient standing water due to the presence of sediment.
Coarser
sediment
deposits in
delta (topset
and foreset).
A (usually muddy) turbidity
current is created by
hyperpycnal conditions.
Finer sediment (mud)
deposits in bottomset.
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HYPOPYCNAL FLOWS
A hypopycnal flow is a sediment-laden surface plume created when the
sediment-laden river water is less dense than the ambient water into which it
flows. Sediment gradually rains out from the surface plume (hemipelagic
sedimentation) to emplace the bottomset.
Surface plume of muddy water
Coarser
sediment
deposits in
delta (topset
and foreset).
Sediment gradually rains out to
emplace the bottomset.
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HYPOPYCNAL SURFACE PLUMES ALONG THE
ADRIATIC SHORELINE OF ITALY
Hypopycnal plumes along the Adriatic
margin of Italy: image from J. Syvitski.
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TURBIDITES IN LAKE MEAD EMPLACED BY
PLUNGING TURBIDITY CURRENTS
Twichell et al. (2006)
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TURBIDITES IN LAKE MEAD EMPLACED BY
PLUNGING TURBIDITY CURRENTS
Seismic image of deposits in the west end of the Virgin Basin.
Twichell et al. (2006)
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RELATION BETWEEN SEDIMENT CONCENTRATION
IN MG/L AND WATER DENSITY
Let X denote the concentration in mg/l. Where f equals the density of the
sediment-laden flow, rw is the density of the river water without sediment
and s denotes the density of sediment. The volume concentration C is
given as
s
1x106 X
C
, R
 1  1.65
f R  1
f
The density of the sediment-laden flow is thus
f  rw (1 RC)
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HOW EASY IS IT TO CREATE PLUNGING IN
FRESHWATER?
The minimum condition for plunging is:
f  rw (1 RC )  l

1  l
C  
 1
R  rw


R  1  l

X  1x10  f
 1
R  rw

6
Critical condition for
hyperpycnal flow
3
rw
0.9982 t/m
3
l
0.9997 t/m
C
0.0009
X
2413.4 mg/liter
Suppose the lake has a temperature of 10 C (l = 0.9997 t/m3). We
consider two cases: the river water has a temperature of 10 C (l = 0.9997
t/m3) and 20 C (rw = 0.9982 t/m3). In the latter case there is a temperature
barrier to plunging). The minimum concentration is for plunging can be
calculated with the spreadsheet. (Double-click to activate.) The minimum
concentrations are 0 mg/l and 2413 mg/l.
As shown on the next slide, even a concentration of 2413 mg/l during floods
14
is not uncommon in mountain streams.
SUSPENDED SEDIMENT CONCENTRATION IN RIVERS
Sediment concentrations are based on mean flows rather than flood
flows. From USGS website.
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SUSPENDED SEDIMENT CONCENTRATION IN THE
MINNESOTA RIVER
Suspended Sediment Concentration Minnesota River
Mankato
10000
X mg/liter
1000
100
10
1
1
10
100
Q (m3/s)
1000
10000
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CAN PLUNGING AND HYPERPYCNAL FLOWS OCCUR
IN THE OCEAN?
Let f equals the density of the sediment-laden flow, rw is the density of the
river water without sediment and sea denotes the density of sea water. The
minimum concentration for plunging into seawater is:
f  rw (1 RC )  sea

1  seal
C  
 1
R  rw


R  1  sea

X  1x10  f
 1
R  rw

6
Critical condition for
hyperpycnal flow
3
t/m
rw
1
3
t/m
sea
1.026
C
0.0158
X
41758 mg/liter
If rw = 1 ton/m3 and sea = 1.026 t/m3, C must be at least 0.0158, and X
must be at least ~ 40,000 mg/l in order to get plunging.
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THIS CONDITION IS RARELY MET IN RIVERS
Sediment concentrations are based on mean flows rather than flood
flows. From USGS website.
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AND IT IS ESPECIALLY RARELY MET IN LARGE,
LOWLAND RIVERS FLOWING INTO PASSIVE MARGINS
Mississippi River Tarbert Landing
Suspended Sediment
Concentration mg/liter
3000
2500
2000
1500
1000
500
0
01-Oct- 27-Jun- 24-Mar- 18-Dec- 13-Sep- 10-Jun- 06-Mar- 30-Nov- 27-Aug- 23-May1949
1952
1955
1957
1960
1963
1966
1968
1971
1974
Day
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AND SO HYPERPYCNAL FLOWS ARE NOT LIKELY TO
BE RESPONSIBLE FOR THE EMPLACEMENT OF MOST
LARGE SUBMARINE FANS ON PASSIVE MARGINS.
Mississippi Submarine Fan
meandering channel
on fan
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AND YET SOME RIVERS SOMETIMES DO FORM
HYPERPYCNAL FLOWS WHEN THEY REACH THE SEA
And when they do they can move enormous amounts of sediment into the
sea.
From International Journal
of Sediment Research
Plunging of the Yellow River into the Bohai Sea, China
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NOW WHERE MIGHT
RIVERS FLOWING INTO
THE SEA HAVE SUCH
HIGH
CONCENTRATIONS OF
SUSPENDED
SEDIMENT?
Active margins undergoing
rapid uplift!
16 of the rivers listed by Mulder
and Syvitski (1995) that go
hyperpycnal at least once per 100
years are in Taiwan
Erosion rates in mm/year for
Taiwan.
Image courtesy C. Stark
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A DOCUMENTED CASE OF HYPERPYCNAL FLOW TO
THE OCEAN
The Eel River in Northern California carries a very high sediment load. It is
estimated to become hyperpycnal once every 10 years.
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HYPERPYCNAL EVENT ON THE EEL RIVER
A hyperpycnal event was recorded and documented in the flood of 1995.
From Imran and Syvitski (2000)
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2. TURBIDITY CURRENTS IN CANYONS GENERATED
BY COASTAL SURFACE WAVE ACTION
One of the first field measurements of turbidity currents in the ocean was
performed in Scripps Submarine Canyon off San Diego, California (Inman et
al., 1976).
The turbidity currents
were generated by
wave action, which
was in turn driven by
a winter storm.
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LITTORAL DRIFT
The canyon is not located near a river mouth. The sediment (fine sand) is
delivered from river mouths to the head of the canyon by littoral drift, mostly
during the summer. The sediment piles up at the head of the canyon.
shore
Littoral drift is an alongcoast flow of sediment
Alongshore
driven by incident waves
sediment
that are not parallel to the
transport
shore.
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Incident waves
Reflected waves
STORMS, INCIDENT WAVES, AND EDGE WAVES
Incident waves
edge waves
Incoming waves from storms oscillate in the cross-shore direction. These
can generate trapped edge waves, which are standing nearshore waves that
oscillate in the along-shore direction.
shore
The antinodes of these
edge waves locate
themselves at
depressions, e.g.
canyon heads.
A’
A
A
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A’
THESE EDGE WAVES STIR UP SEDIMENT AT THE
CANYON HEAD
The sedimentladen
seawater so
created is
heavier than
the ambient
sediment-free
seawater.
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AND THE RESULT IS A DOWN-CANYON TURBIDITY
CURRENT
The current so created is sustained as long as the edge waves are present
and there is sediment available in the canyon head (hours, or even days). A
storm immediately subsequent to one that generated a turbidity current often
creates no turbidity current, because there is no longer any sediment
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available.
THESE TURBIDITY CURRENTS CAN BE QUITE STRONG!
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4. TURBIDITY CURRENTS CREATED BY BREACHING
Breaching is the sustained, slow failure of a near-vertical subaqueous
face of slightly overdensified clean, fine sand by spalling of sediment
from the face.
See van den Berg et al. (2002); Mastbergen
and van den Berg (2003).
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BREACHING HAS BEEN USED BY THE DREDGING INDUSTY
IN THE NETHERLANDS TO INSTIGATE SELF-SUSTAINED
REMOVAL BY MEANS OF SPALLING TO A TURBIDITY
CURRENT
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A SLOW BREACH FAILURE LAUNCHES A BOAT
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BREACHING IN THE LABORATORY (courtesy T. Muto)
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WHY THE NEAR-VERTICAL FACE?
As a particle starts to fail off a modestly overdensified face of clean, fine
sand, the negative pore pressure created by the gap prevents it from failing
catastrophically, and instead leads to slow grain-by-grain spalling.
negative pore
pressure pulls particle
back as it tries to fail
K
p
cb
Eb
= hydraulic conductivity of sand [L/T]
= porosity of sand [1]
= retreat speed of breach [L/T]
= volume erosion rate/surface area of sediment [L/T]
cb  25k , Eb  (1 p )cb  25(1 p )k
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GENERATION OF THE TURBIDITY CURRENT
vertical breach face slowly retreats
as sediment spalls off grain-by-grain
breaching generates a quasicontinuous turbidity current
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TURBIDITY CURRENT GENERATED BY BREACHING IN
THE LABORATORY
37
SOME TURBIDITY CURRENTS IN THE MONTEREY
SUBMARINE CANYON APPEAR TO BE GENERATED BY
BREACHING
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Xu, J. P., M. A. Noble, and L. K. Rosenfeld (2004)
A DOCUMENTED TURBIDITY CURRENT IN THE
MONTEREY SUBMARINE CANYON
• Sustained event: lasted
5 - 8 hours
• Max. velocity ~ 1.9 m/s
• Thicker downcanyon?
• Not caused by storm or
hyperpycnal flow (failure
of dredge spoil by
breaching?)
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4. TURBIDITY CURRENTS GENERATED BY DOUBLE
DIFFUSION
Double-diffusion phenomena were discovered in the context of the ocean.
As noted in a previous slide, (sediment-free) oceanic water varies in both
salinity and temperature. A higher salinity makes water heavier. A higher
temperature makes water lighter. In the ocean, the density of water is
controlled by both factors.
In oceanic waters, heat and salt can be fluxed by both convection and
molecular diffusion.
Here we are interested in flux by molecular diffusion. Diffusion fluxes a
quantity from a zone of high concentration to low concentration. Consider
any quantity per unit volume b (e.g. heat in joules per unit volume or salt in
grams per unit volume. We further assume that this quantity decreases in
the x direction.
Diffusive flux transports a quantity from high concentration to low
concentration, or in this case in the positive x direction.
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WHAT IS DOUBLE DIFFUSION? contd.
Let Fbd,x denote the diffusive flux of quantity b (quantity crossing
face/time/face area) in the x direction. If b decreases in x, then Fbd > 0
Fbd,x
b
 0  Fbd,x  0
x
b
x
Thus any quantity diffuses down its spatial gradient.
Now let  denote temperature and s denote salinity.
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WHAT IS DOUBLE DIFFUSION? contd.
We denote the temperature as  and the salinity as s. The diffusive flux of
heat and salt in the x-direction are denoted as Fhd, x and Fsd,x. These terms
are given as
Fhd,x

  w c pDh
x
, Fsd,x
s
 Ds
x
where w is the density of the water, cp is the specific heat of water at
constant pressure (e.g. no. of joules required to raise 1 kg by 1 C) and Dh
and Ds denotes the kinematic diffusivity of heat and salt (e.g. m2/s).
Typical values of cp, Dh and Ds are 4.18 x 103,1.45 x 10-7 m2/s and
1.35 x 10-9 m2/s
The point to note here is:
Ds / Dh  1
This means that in a relative sense, salt diffuses much less rapidly than
heat.
42
THE FIRST CASE OF DOUBLE DIFFUSION: HENRY
STOMMELS PERPETUAL SALT FOUNTAIN
Consider an aluminum tube that extends vertically from
the deep ocean to the surface. We assume that warm,
saline water at the surface overlies cold, less saline
water at depth. The water is stably stratified, i.e. the
deep water is denser than the surface water.
But suppose we start a vertical flow in the pipe. The
flow will sustain itself creating a “perpetual” salt
fountain!
Warm,
less salty
Warm,
more salty
Heat
diffuses
in, but not
salt
Why? As the flow in the pipe rises, heat diffuses
across the pipe walls, warming the pipe water to the
temperature of the seawater surrounding it.
But salt cannot diffuse in due to the walls. So the
water from depth arrives at the surface with the same
temperature as the surface water, but a lower salinity.
Being lighter than seawater, it fountains upward!
cold,
less
salty
43
DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY
CURRENTS
Consider hypopycnal river freshwater entering the sea. For simplicity, we
assume that both are at the same temperature. The coarse sediment
deposits to form a delta, and the fine sediment, i.e. mud, forms a surface
plume.
fresh, sediment-laden
water
surface plume
delta
saline water
44
DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY
CURRENTS contd.
The kinematic diffusivity of mud, which is governed by Brownian
motion, is far less that the kinematic diffusivity of salt.
Consider a blob of mud-laden, fresh water at the interface.
fresh, sediment-laden
water
surface plume
delta
saline water
45
DOUBLE DIFFUSIVE MECHANISM FOR TURBIDITY
CURRENTS contd.
Parsons and Garcia (2000)
Salt can diffuse into the
blob much faster than
sediment diffuses out. As
a result, the blob can get
heavier than the
surrounding saltwater and
sink.
46
THE BLOBS CAN JOIN TOGETHER TO CREATE A
CONTINUOUS BOTTOM TURBIDITY CURRENT
fresh, sediment-laden
water
surface plume
delta
saline water
Note: this mechanism is as yet unverified in the field.
47
5. TRANSFORMATION FROM SLOPE FAILURES AND
SUBMARINE DEBRIS FLOWS
Submarine landslides and debris flows can generate turbidity currents
as sediment is entrained into suspension from their heads (and also
their bodies). The landslide/debris flow can come to rest, but the
turbidity current can run out much father distances.
antecedent seafloor profile
turbidity current
(can go much farther before coming to rest)
slide scar
landslide/debris flow
(comes to rest)
48
THE GRAND BANKS SUBMARINE LANDSLIDE/
TURBIDITY CURRENT
http://earthnet-geonet.ca/communities/earthquake_e.php
An earthquake in 1929 generated the Grand Banks failure, which
produced a huge submarine landslide that devolved into a debris flow,
and then into a turbidity current that ran long distances. The layer of
sand deposited by the turbidity current covered a surface the size of
Quebec. Maximum turbidity current velocities may have been as high
as 18 m/s, as evidenced from the timing of transatlantic submarine
cable breaks. This event served to catalyze interest in turbidity currents
as a submarine process for the redistribution of sediment (e.g. Heezen
49
and Ewing, 1952; Kuenen, 1952).
EXPERIMENTAL EVIDENCE FOR TURBIDITY CURRENTS
GENERATED BY SUBMARINE DEBRIS FLOWS
50
6. TRIGGERING BY INTERNAL WAVES ON THE
CONTINENTAL SHELF
Surface gravity waves travel on
at the ocean-air interface, and
break at the shoreline. The
ocean-air interface is a zone of
sharp density contrast.
Another, more diffuse interface
with a much smaller density
difference is the oceanic
thermocline. It is a zone of
relatively rapid density increase
with depth, from well-mixed,
warmer surface water to lessmixed, colder water at depth.
http://upload.wikimedia.org/wikipedia/en/8/82/Thermocline.jpg
51
INTERNAL WAVES BREAKING ON THE CONTINENTAL
SLOPE
Internal waves can form and propagate along the thermocline. It has
been hypothesized that internal waves breaking on the continental slope
could be responsible for the generation of turbidity currents (D.
Cacchione).
shelf
slope

turbidity
current
rise
http://pof.aip.org/pof/gallery/2006-Koseff.jsp
52
A VIEW OF BREAKING INTERNAL WAVES IN THE
LABORATORY
53
http://physoce.mlml.calstate.edu/PDFs/McPhee-Shaw_oceanmixingconf.pdf
7. WAVE-SUPPORTED SHEET TURBIDITY CURRENTS THAT
DIE ON (AND CONSTRUCT) THE CONTINENTAL SLOPE
54
WAVE-SUPPORTED SHEET TURBIDITY CURRENTS
wave-current boundary
layer on shelf edge
net-depositional turbidity
current on slope
turbidity current becomes
extremely thick and dilute as it
decelerates
55
WAVE-SUPPORTED SHEET TURBIDITY CURRENTS
CAN BE GENERATED BY WAVE-CURRENT BOUNDARY
LAYERS ON THE CONTINENTAL SHELF
waves
wave orbital amplitude
sediment supply
above
wave base
below
wave base
continental
slope
suspended sediment
concentration profile
Wave-supported
turbidity current
sea
continental
shelf
land
56
SUCH CURRENTS BUILD OUT A CLINOFORM
wave-current boundary
layer on shelf edge
net-depositional turbidity
current on slope
turbidity current becomes
extremely thick and dilute as it
decelerates
Migrating clinoform
57
THE CLINOFORM THUS BUILDS
OUTWARD, EXTENDING THE SHELF
Wave resuspension
Diluted muddy
turbidity currents
Late Holocene Adriatic clinoform
Your first seismic image!
10 ms
Slide courtesy F. Trincardi
1 km
58
GANGES-BRAHMAPUTRA DELTA
59
GANGES-BRAHMAPUTRA CLINOFORM: KUEHL ET AL.
(1997)
Topset deposit = deposit on
shelf
Foreset deposit = deposit
on slope
Bottomset deposit =
deposit on slope
base/rise
60
References to collect
Mulder and Syvitski (1995)
Parsons and Garcia (2000)
Imran and Syvitski (2000)
Van den Berg et al. (2002)
Mastbergen et al. (2003)
Trincardi
Kuehl
Xu, J. P., M. A. Noble, and L. K. Rosenfeld (2004), In-situ measurements of velocity
structure within turbidity currents, Geophys. Res. Lett., 31, L09311,
doi:10.1029/2004GL019718.
Kuenen, P. H. (1952) Estimated size of the Grand Banks [Newfoundland] turbidity current,
American Journal of Science, 250, 874-884.
Heezen, B.C., and Ewing, M. (1952) Turbidity currents and submarine slumps and the 1929
Grand Banks earthquake, American Journal of Science, 250, 849-873.
Mohrig, D. and Marr, J. G. (2003) Constraining the efficiency of turbidity current generation
from submarine debris flows and slides using laboratory experiments, Marine and
Petroleum Geology, 20(6-8), 883-899.
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