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1
LECTURE 5CONCEPTS FROM RIVERS THAT CAN BE APPLIED
TO TURBIDITY CURRENTS
CEE 598, GEOL 593TURBIDITY CURRENTS: MORPHODYNAMICS AND DEPOSITS
Image courtesy M. Jaeggi
Reuss River plunging into Lake Lucerne, Switzerland: flood of summer, 2005
2
GRAIN SIZE CLASSIFICATION
Type D (mm) Notes
Clay < 0.002 < -9 > 9 Usually cohesive
Silt 0.002 ~ 0.0625 -9 ~ -4 4 ~ 9 Cohesive ~ non-cohesive
Sand 0.0625 ~ 2 -4 ~ 1 -1 ~ 4 Non-cohesive
Gravel 2 ~ 64 1 ~ 6 -6 ~ -1 “
Cobbles 64 ~ 256 6 ~ 8 -8 ~ -6 “
Boulders > 256 > 8 < -8 “
Mud = clay + silt
22D)2(n
)D(n)D(og2
3
SEDIMENT FALL VELOCITY IN STILL WATER
wheregD
vsf
RR)(f pf ReR and
andvs = fall velocityD = grain sizeR = (sed - w)/w = submerged specific gravity of sediment =
1.65 for quartz (sed = sediment density, w = water density
g = gravitational acceleration = 9.81 m/s2
= kinematic viscosity of water ~ 1x10-6 m2/s
DRgDpRe
Relation of Dietrich (1982):
})](n[b)](n[b
)](n[b)(nbb{exp4
p53
p4
2p3p21f
ReRe
ReReR
b1 2.891394
b2 0.95296
b3 0.056835
b4 0.002892
b5 0.000245
The original relation also includes a correction for shape.
4
USE OF THE WORKBOOK RTe-bookFallVel.xls
A view of the interface in RTe-bookFallVel.xls is given below. It can be downloaded from: http://cee.uiuc.edu/people/parkerg/morphodynamics_e-book.htm
5
SOME SAMPLE CALCULATIONS OF SEDIMENT FALL VELOCITY
(Dietrich Relation)
g = 9.81 m s-2
R = 1.65 (quartz) = 1.00x10-6 m2 s-1 (water at 20 deg Celsius) = 1000 kg m-3 (water)
D, mm vs, cm/s
0.0625 0.330
0.125 1.08
0.25 3.04
0.5 7.40
1 15.5
2 28.3
The calculations to the left were performed with RTe-bookFallVel.xls.
6
MODES OF SEDIMENT TRANSPORT
Bed material load is that part of the sediment load that exchanges with the bed (and thus contributes to morphodynamics of the river bed).
Wash load is transported through without exchange with the bed.In rivers, material finer than 0.0625 mm (silt and clay) is often approximated as wash load. Washload does exchange with the floodplain. Washload moves in suspension.
Bed material load is further subdivided into bedload and suspended load.
Bedload:sliding, rolling or saltating in ballistictrajectory just above bed.role of turbulence is indirect. Suspended load:feels direct dispersive effect of eddies.may be wafted high into the water column.
7
VIDEO CLIP ILLUSTRATING BEDLOAD IN A MODEL RIVER IN THE LABORATORY
Wong et al. (2007)
8
VIDEO CLIP ILLUSTRATING BEDLOAD AND SUSPENDED LOAD CARRIED NEAR THE BED OF
THE TRINITY RIVER, CALIFORNIA
Clip courtesy A. Krause
9
VIDEO CLIP ILLUSTRATING BEDLOAD AND SUSPENDED LOAD CARRIED BY A TURBIDITY
CURRENT
Cantelli et al. (2008)
10
APPLICATION TO TURBIDITY CURRENTS
RIVER: The downslope component of gravitational force Fgd acting on the control volume to drive the flow is
RcgLASF wgd
a
LA
gLAS)Rc1(F wgd
TURBIDITY CURRENT: The downslope component of gravitational force Fgd acting on the control volume to drive the flow is
where c is the volume concentration of suspended sediment
)tan(S a
11
CRITICAL ROLE OF SUSPENDED SEDIMENT TO DRIVE TURBIDITY CURRENTS
RIVER: Suspended sediment is NOT NECESSARY to drive the flow.
RcgLASF wgd
a
LA
gLAS)Rc1(F wgd
TURBIDITY CURRENT: Suspended sediment is NECESSARY to drive the flow!
The suspended sediment in turbidity currents is composed of mud and/or sand.
)tan(S a
12
BEDLOAD TRANSPORT BY TURBIDITY CURRENTSThe same size of sand can participate in both transport mechanisms, whereas gravel is usually moved only as bedload.
Gravel/sand deposit in the River Wharfe, U.K.
Image courtesy D. Powell
Turbidity currents can transport sand, and sometimes gravel as bedload.
Gravel/sand deposit (likely) emplaced by a turbidity current, Cerro
Gordo formation, Patagonia, Chile.
13
TURBIDITY CURRENTS CAN MOVE BEDLOAD, BUT BEDLOAD DOES NOT DRIVE TURBIDITY
CURRENTS
Suspended mud and sanddrove the turbidity currents that emplaced these deposits.
Some of the currents also moved and emplaced sand and gravel moving as bedload.
(Gravel/sand deposits can also be emplaced by submarine debris flows.)
Gravel/sand deposit emplaced by a turbidity
current, Cerro Gordo formation, Patagonia,
Chile.
Mud/gravel/sand deposits emplaced by a turbidity current, Cerro Gordo formation,
Patagonia, Chile.
14
THE REASON WHY BEDLOAD CANNOT DRIVE TURBIDITY CURRENTS
Bedload: moves by sliding, rolling or saltating in ballistic trajectories just above bed. Bedload particles are dragged by the flow. Suspended particles drag the flow with them.
15
BEDLOAD AND SUSPENDED LOAD IN AN EXPERIMENTAL DELTA WITH A PLUNGING
TURBIDITY CURRENT
Kostic and Parker (2003)
16
1R
X10x1c 6
SUSPENDED SEDIMENT CONCENTRATION
Suspended sediment concentration is often expressed in units of mg/liter, i.e. the weight of sediment in milligrams per liter of sediment-water mixture, here denoted as X.
The corresponding volume concentration c i.e. the volume of pure sediment per unit volume of sediment-water mixture, is related to X as
Conversion from X to c
R 1.65X 2000 mg/literc 0.000755
Double-click to open the spreadsheet.
17
A GARDEN-VARIETY SAND-BED RIVER: THE MINNESOTA RIVER NEAR MANKATO
Image courtesy P. Belmont
18
Suspended Sediment Concentration Minnesota River Mankato
1
10
100
1000
10000
1 10 100 1000 10000
Q (m3/s)
X m
g/li
ter
SUSPENDED SEDIMENT CONCENTRATION IN A GARDEN-VARIETY RIVER
Q = flow discharge
Note: X is never higher than ~ 3000
mg/l
19
SUSPENDED SEDIMENT CONCENTRATION IN A GARDEN-VARIETY RIVER contd.
Suspended Sediment Concentration Minnesota River Mankato
c = 1E-05(Q)0.388
0.000001
0.00001
0.0001
0.001
0.01
0.1
1
1 10 100 1000 10000
Q (m3/s)
c
Note: c is never higher than ~ 0.001:
highly dilute suspension
20
BED GRAIN SIZE DISTRIBUTION IN A GARDEN-VARIETY RIVER
Bed Grain Size Distributions, Minnesota River at Mankato
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100
D (mm)
Pe
rce
nt
Fin
er
GSD1
GSD2
GSD3
GSD4
GSD5
GSD6
GSD7
GSD8
GSD9
GSD10
GSD11
GSD12
GSD13
Average
Where’s the mud?
21
FRACTION OF SUSPENDED LOAD THAT IS MUD IN A GARDEN-VARIETY RIVER
Percent of Suspended Load Finer than 0.062 mm:Minnesota River at Mankato
Fload<62 = [-0.0245(Q) + 87.828]/100
0
10
20
30
40
50
60
70
80
90
100
0 200 400 600 800 1000 1200
Q (m3/s)
Per
cen
t m
ud
The suspended load is mostly mud!
22
IMPLICATIONS FOR TURBIDITY CURRENTS (??)
Turbidity currents are also driven by dilute (c << 1) suspensions of sand and mud.
Mud has a smaller fall velocity than sand, and is thus easier to keep in suspension.
Mud is a good driver to carry both sand (in suspension and as bedload) and gravel into deep water.
23
THE CASCADIA AND ASTORIA SUBMARINE
CHANNELS OFF THE PACIFIC
COAST OF THE USA
Nelson et al., 2000
24
CORES SHOW THAT THE CHANNELS MOVE MUD, SAND AND GRAVEL TO DEEP WATER
Nelson et al., 2000
25
RIVERS AND FLOODPLAINS
Strickland River, New Guinea
Image courtesy J. W. Lauer
Mostly mud-free channel, Mud-rich floodplain (but with sand also)
26
RIVERS AND FLOODPLAINS
Minnesota River, Minnesota
Image courtesy J. W. Lauer
Sand load moves as bedload and suspended load. Exchanges mostly with bed, but with floodplain as well.
Mud moves as suspended wash load.Exchanges with the floodplain.
27
SAND AND MUD
Paraná River, Argentina
Sand rich Mud rich
28
APPLICATION TO LEVEED CHANNELS CREATED BY TURBIDITY CURRENTS
Bengal Fan: Schwenk, Spiess,Hubscher, Breitzke (2003)
Crati Fan off Italy, Ricci Lucchi et al. (1984); Morris and Normark
(2000)
Floodplain levee
Channel: predominantly sandy (some mud)
Levees: predominantly muddy (some sand)
29
SCALE FOR GRAVITATIONAL FORCE:RIVERS AND TURBIDITY CURRENTS
flow = denote the density of the flowing amb = density of the ambient fluidU = flow velocityC = volume concentration of suspended sedimentR = (sed - f)/f = submerged specific gravity of sedimentH = depth (layer thickness) and width of control volumeWimm = immersed weight in control volume
H
H 3
ambflowimm gHW
Flowing fluid
ambient fluid
30
SCALE FOR GRAVITATIONAL FORCE:RIVERS AND TURBIDITY CURRENTS
CASE OF A RIVER:flow = w(1+RC) (fresh
water with sediment)
amb = air (air)R = (sed - w)/w 1.65
H
H
CASE OF A TURBIDITY CURRENT:flow = w(1+RC) (fresh or
sea water with sediment)
amb = w (fresh or sea water)R = (sed - w)/w 1.65
3airwimm gH)RC1(W 3
wwimm gH)RC1(W
Flowing fluid
ambient fluid
H
31
x Ut
A
U
The tube shown below has rectangular cross-section with area A. The fluid velocity is U and the fluid density is flow
At time t = 0 we mark a parcel of fluid, the downstream end of which is bounded by an orange face.
In time t the leading edge of the marked parcel moves downstream a distance Ut, so that volume UtA and mass flowUtA has crossed the face in time t.
UtA
VOLUME, MASS AND MOMENTUM DISCHARGE
32
x Ut
A
U
The discharge of any quantity is the rate at which it crosses a section per unit time
The volume that crosses the section in time t is AUtThe mass that crosses is flowAUtThe momentum that crosses is UflowAUt
The volume discharge Q = UAThe mass discharge Qmass = flowUAThe momentum discharge Qmom = flowU2AU
UtA
VOLUME, MASS AND MOMENTUM DISCHARGE contd.
33
MOMENTUM DISCHARGE AND INERTIAL FORCE
Aim a jet of water at a plate perpendicular to the jet.The jet flows into the control volume in the x direction.The jet flows out of the control volume perpendicular to the x direction.
What is the (inertial) force Finert that the plate must exert on the jet in order to deflect it without moving? (Jet has cross-sectional area A.)
Finert
Control volume
x
Force balance:
/t(x-momentum in c.v.) =
Inflow rate – outflow rate – Finert
Steady flow: no outflow of x-momentum:
inert2
flow F0AU0
AUF 2flowinert
34
THE DENSIMETRIC FROUDE NUMBER:A SCALE OF THE RATIO OF INERTIAL TO GRAVITATIONAL
FORCES
H
H
Flowing fluid
ambient fluid
22flowinert HUF 3
ambflowimm gHW
H
Densimetric Froude number Frd:
gH)(
U
gH)(
HU
W
F
ambflow
2flow
3ambflow
22flow
imm
inert2d
Fr
35
THE DENSIMETRIC FROUDE NUMBER:RIVER AND TURBIDITY CURRENT
RIVER: airambwflow ,)RC1(
gH
U
gH))RC1(
U)RC1( 2
airw
2w2
d
Fr
Now for R ~ 1.65, C << 1 and air/w << 1,
TURBIDITY CURRENT:
wambwflow ,)RC1(
Now for R ~ 1.65 and C << 1,
RCgH
U
gH))RC1(
U)RC1( 2
ww
2w2
d
Fr
36
THE FROUDE NUMBERS:
gH
Ud FrFr
RCgH
Ud Fr
RIVER:
TURBIDITY CURRENT:
Most of the concepts based on Froude number for open channel (river) flow generalize to turbidity currents!
Frd < 1: subcritical (tranquil) flowFrd = 1: critical flowFrd > 1: supercritical (shooting) flow
37
EXAMPLE: ENTRAINMENT OF AMBIENT FLUID
In rivers, supercritical flow favors entrainment of ambient fluid (air) into the flow, making a diffuse interface, and subcritical flow favors the absence of entrainment, with a sharp interface.
Sangamon River, Illinois; Fr << 1
River in Maine; Fr > 1
38
EXAMPLE: ENTRAINMENT OF AMBIENT FLUID
In turbidity currents as well, supercritical flow favors entrainment of ambient fluid (sediment-free water) into the flow, making a diffuse interface, and subcritical flow favors the absence of entrainment, with a sharp interface.
Mixing with ambient fluid is easier in the case of a turbidity current, because water and air are immiscible, whereas dirty water and clear water are miscible.
Subcritical: Frd < 1 Supercritical: Frd > 1
Water surface
internal hydraulic jump
Image courtesy N. Strong
39
IN THE CASE OF A HIGHLY SUBCRITICAL TURBIDITY CURRENT, THE INTERFACE CAN BE VERY SHARP INDEED
Water surface
Turbidity current interface
Toniolo et al. (2006)
40
BED SHEAR STRESS AND FLOW VELOCITY
For simplicity, approximate a river as having a wide, rectangular cross-section, so that B/H >> 1, whereB = width [L]H = depth [L]
Now denoteQw = flow discharge [L3/T]U = cross-sectionally averaged flow velocity [L/T] = Qw/BH = water density [M/L3]b = bed shear stress (force per unit bed area) [ML-1T-2]
Then bed shear stress is related to flow velocity using a dimensionless friction (resistance) coefficient Cf, so that
2b
f UC
41
SHEAR VELOCITY AND DIMENSIONLESS CHEZY RESISTANCE COEFFICIENT
bu
The dimensionless Chezy resistance coefficient Cz is defined as
The shear velocity u [L/T] is defined as
u
UCz
42
NORMAL OPEN-CHANNEL FLOW IN A WIDE CHANNEL
Normal flow is an equilibrium state defined by a perfect balance between the downstream gravitational impelling force and resistive bed force. The resulting flow is constant in time and in the downstream, or x direction.
Parameters:
x = downstream coordinate [L]H = flow depth [L]U = flow velocity [L/T]qw = water discharge per unit width [L2T-1]B = width [L]Qw = qwB = water discharge [L3/T]g = acceleration of gravity [L/T2] = bed angle [1]b = bed boundary shear stress [M/L/T2]S = tan = streamwise bed slope [1]
(cos 1; sin tan S) = water density [M/L3]
The bed slope angle of the great majority of alluvial rivers is sufficiently small to allow the approximations
1cos,Stansin
xB
x
gHxBS
bBx
H
43
THE DEPTH-SLOPE RELATION FOR NORMAL OPEN-CHANNEL FLOW
UHBBqQUHq www
Conservation of downstream momentum:Impelling force (downstream component of weight of water) = resistive force
xBxSgHBsinxgHB b
gHSb
Reduce to obtain depth-slope product rule for normal flow:
Conservation of water mass (= conservation of water volume as water can be treated as incompressible):
xB
x
gHxBS
bBx
H
gHSu
44
FLOW REYNOLDS NUMBER, SHIELDS NUMBERAND DIMENSIONLESS CHEZY NUMBER
45
CRITERIA FOR THE ONSET OF MOTION AND SIGNIFICANT SUSPENSION
46
THE SHIELDS DIAGRAM
47
THE DEPTH-SLOPE RELATIONSHIP FOR SHEAR STRESS IN RIVERS
48
THE CONCEPT OF BANKFULL DISCHARGE IN RIVERS
QQbf
Let denote river stage (water surface elevation) [L] and Q denote volume water discharge [L3/T]. In the case of rivers with floodplains, tends to increase rapidly with increasing Q when all the flow is confined to the channel, but much less rapidly when the flow spills significantly onto the floodplain. The rollover in the curve defines bankfull discharge Qbf.
Minnesota River and floodplain, USA, during the
record flood of 1965
49
PARAMETERS USED TO CHARACTERIZE BANKFULL CHANNEL GEOMETRY
In addition to a bankfull discharge, a reach of an alluvial river with a floodplain also has a characteristic average bankfull channel width and average bankfull channel depth. The following parameters are used to characterize this geometry.
Definitions:
Qbf = bankfull discharge [L3/T]Bbf = bankfull width [L]Hbf = bankfull depth [L]S = bed slope [1]Ds50 = median surface grain size [L]= kinematic viscosity of water [L2/T]R = (s/ – 1) = sediment submerged specific gravity (~ 1.65 for natural
sediment) [1]g = gravitational acceleration [L/T2]
50
FROUDE NUMBER AT BANKFULL FLOW
51
DIMENSIONLESS CHEZY RESISTANCE COEFFICIENT AT BANKFULL FLOW
52
BANKFULL FLOW AND THE SHIELDS DIAGRAM
53
VELOCITY AND SUSPENDED SEDIMENT PROFILES IN A RIVER
54
COMPARISON BETWEEN RIVERS AND TURBIDITY CURRENTS
55
REFERENCES
Under construction
Dietrich, W. E., 1982, Settling velocity of natural particles, Water Resources Research, 18 (6), 1626-1982.
Morris, W. R. and Normark, W. R., 2000, Sedimentologic and geometric criteria for comparing modern and ancient turbidite elements. Proceedings, GCSSEPM Foundation Annual
20th Research Conference, Deep-water Reservoirs of the World, Dec. 3 – 6, 606-623. Nelson, H., Goldfinger. C, Johnson, J. E. and Dunhill, G., 2000, Variation of modern turbidite
systems along the subduction zone margin of the Cascadia Basin and implications for turbidite reservoir beds. Proceedings, GCSSEPM Foundation Annual 20th Research Conference, Deep-water Reservoirs of the World, Dec. 3 – 6, 714-738.
Toniolo et al. (2006)
Wong et al. (2007)Cantelli et al. (2008)Schenk et al. (2003)Ricci Lucchi et al. (1984)Kostic and Parker (2003)Nelson?????Lamb?????