1 High mobility of subaqueous debris flows and the lubricating layer model Anders Elverhøi Fabio De Blasio Trygve Ilstad Dieter Issler Carl B. Harbitz International Centre for Geohazards Norwegian Geotechnical Institute, Norway Dep. of Geosciences, University of Oslo, Norway. .
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1 High mobility of subaqueous debris flows and the lubricating layer model Anders Elverhøi Fabio De Blasio Trygve Ilstad Dieter Issler Carl B. Harbitz.
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High mobility of subaqueous debris flows and
the lubricating layer modelAnders ElverhøiFabio De Blasio
Trygve IlstadDieter Issler
Carl B. Harbitz
International Centre for GeohazardsNorwegian Geotechnical Institute, Norway
Dep. of Geosciences, University of Oslo, Norway..
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Debris flow
How can we explain that 10 - 1000 km3 of sediments can
• move100 - > 200 km• on < 1 degree slopes• at high velocities ( -20 - > 60 km/h)
Basic problem!
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Experimental settingsSt. Anthony Falls Laboratory
10 m
turbidity current
debris flow
6° slope
Experimental Flume: “Fish Tank”
Video (regular and high speed) and
pore- and total pressure measurements
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Flow behavior Clay rich debris flows
Hydroplaning front “Auto-acephalation” 32.5 wt% clay cited from G. Parker
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Pressure measurements at the base of a clay rich debris flow as pressure develops during the flow
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Flow behavior:Debris flow at high mass fraction of clay
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Material from the base of the debris flow is eroded and incorporated into the lubricating layer.
L1
L2
Ls
H1
H2Hs
Downslope gravitational forces
Bottom shear stresses
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Grossly simplified detachment/stretching dynamics
• Tensile force in the neck• Viscoplastic stretching of the neck is volume-
conserving– The growth rate of the length is the product of the
stretching rate with the neck length
• Solution of the simplified stretching equations:– The neck stretches and thins at a rate that increases
with time, until the height becomes zero after a finite time
• detachment occurs
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Neglected physics:• Changing tension due to slope and velocity
changes• Friction, drag and inertial forces on neck• Changes in material parameters of neck due to
– shear thinning, accumulated strain and wetting, crack formation
More sophisticated treatment is possible Coupled nonlinear equations, use a numerical modelMain difficulty is quantitative treatment of crack
formation and wetting effects
Detachment/stretching dynamics
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Tension in the “neck”:
Viscoplastic stretching of the neck is volume-conserving:
Solution of the simplified stretching equations:
Grossly simplified detachment dynamics:
ssss
s
ytyt
LdtdLHdtdH
aH
b
/,/
''
)(sin')(sin' 22222
11111 U
H
Lg
H
HLU
H
Lg
H
HL
sssst
)(
)0()0()(,1)0()(
'/
tH
HLtLeHtH
s
sss
t
y
t
y
tss
y
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Simulation of the giant Storegga slide400-500 km runout
• Clay-rich sediments– Visco-plastic
materials:
• Model approach:– “Classical” BING– BING: Remolding of
the sediment during flow
– H-BING: Hydroplaning
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Velocity profile of debris flows Bingham fluid
shear stress
yield strength
dynamic viscosity
shear rate
y
uy
Plug layer
Shear layer
Yield strength: constant during flow
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Water film shear stress reduction in a Bingham fluid
Water, w, w, uw
Mudm, m, um
Lid(Debris flow)
=1=1-
u=1
Shear layer
Plug layer
1+
R(1+)/
1
1+
1
1
1-
u(R-)/
1
1u
1
1-
Velocity Shear stress
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Simulation: final deposit of the large-scale Storegga
Initial deposits
Present deposits
= 10 kPa
= 10 kPa with remoulding to 0,5 kPa
= 10 kPa with remoulding to 0,1 kPa
= 5 kPa with hydroplaning
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Conclusions• Experiments
– water enhances the mobility of debris flows via the formation of a lubricating layer/stretching
• The giant Storegga slide – BING
• reproduced with extremely low yield stresses, 200-300 Pa
– R-BING• starting from yield stresses between 6 and 10kPa, residual
stress of 200 Pa
– Hydroplaning • extreme runout distances, even with stiff sediments
independence of sediment rheology
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Future directions (II)
• Modification of the existing models– Incorporation of water in the slurry – Detachment mechanism of a hydroplaning
head
• Parameterizations of the rheological properties as a function of water content
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Subaqueous conditions -increased mobility
Basic concept – based on experimental studies:– Hydroplaning– Lubricating– Stretching (not yet implemented)
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Comparison between Storegga slides and selected cases
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Future direction (I)
• Modification of the existing models– Incorporation of water in the slurry – Detachment mechanism of a hydroplaning head
• Parameterizations of the rheological properties as a function of water content (and stretching?)
• Important question: How is the basal “water” layer distributed?
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2W ten WD
1 1 1 1
FdU 1gsin C U
dt D L WD 2 L
yten
2 2 2
FdVgsin
dt L WD D
D1
D2
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Velocity profile of debris flows Bingham fluid – with remolding
Plug layer
Shear layer
Yield strength at start: high; 10–20 kPa
Yield strength at stop: low; < 1 kPa
The yield stress is allowed to vary according to:
eyyy )y,0,(,(
initial yield stress
residual yield stress
total shear deformation
dimensionless coefficient quantifying the remolding efficiency