1 Flow, disintegration and lubrication of clay – sandy debris flows: from the laboratory to the...

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Flow, disintegration and lubrication of clay – sandy debris flows: from

the laboratory to the fieldAnders ElverhøiFabio De Blasio

Dieter IsslerJohann Petter Nystuen

Peter GauerCarl B. Harbitz

Jeff Marr

International Centre for GeohazardsNorwegian Geotechnical Institute, Norway

Dep. of Geosciences, University of Oslo, Norway.St. Anthony Falls Laboratory, University of Minnesota

.

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SHELF EDGE (STAGING AREA)• Storage Capacity• Volume of Failure• Canyon Positioning relative to delta (sequence stratigraphy)

DEPOSITIONAL CONTROLS-Basin Relief-Local Gradient-Evolving Seabed Topography

SHELF

• WIDTH

• GRADIENT

• ACCOMODATION

• SEDIMENTS

TRAPS

Controls on submarine fan facies distribution

Adopted from Statoil, Frode Hadler-Jacobsen

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What is the origin of deep-water sand bodies?

• Turbidity currents? (low density, dominant turbulence)

• Debris flows? (high density, laminar, 1) cohesive? 2) non-cohesive?)

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Experimental settingsSt. Anthony Falls Laboratory

Experimental Flume: “Fish Tank”

Video (regular and high speed) and

pore- and total pressure measurements

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Debris flows- high clay content

A: 32.5 wt% clay, hydroplaning front Dilute turbidity current

B: 25 wt% clay hydroplaning frontD: Behind the head, increasing concentration in overlaying turbidity current

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Debris flows- low clay content (5%)

Turbulente front Deposition of sand

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Anatomy of a sand-rich experimental debris flow

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Low clay content – video record

Turbidity current

Sand waves

Dense flow

Deposition of sand.

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Close up pictures/ sand waves

High speed video showing:

1) The turbidity current (TC)

2) Debris flow/fluidized layer (DF)

3) Settling layer

New layer of sand, stacked sequences?

Sand

wave

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High clay content-- Plug flow- “Bingham”

High sand content-Macro-viscous flow?-Divergent flow in the shear layer

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Some examples of velocity field – one image

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Velocity for the whole flow (proportional to

colour)

Deposited materials

”Debris flow”

Turbidity current

Convection cells ?

Hei

ght

[pix

el]

1m =

640

0 pi

xel

Time [frame] 1 frame = 0.04s

Velocity m/s

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Thickness of sandy deposits – versus clay content

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Flow behavior:Mass flow at high sand fraction

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Numerical experiment 1

• Transport of a single sand particle through a free surface laminar flow

• Lift and Drag force• Results: a few km

runout at most: not an efficient process

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Fluidized layer: what is its potential for sand transport?

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Numerical experiment 2: A simple model for fluidized sand

• Calculation of settling velocity and runout of an inclined granular bed.

• Settling is hindered by water expelled from beneath (Richardson-Zaki).

• The material not yet settled is modeled as a laminar free-surface flow

• What are the boundary conditions at the top?

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1

ut y

Numerical experiment 2Basic equations

• Richardson-Zaki

• Viscosity

• Conservation of particle number

• Laminar velocity field

max

max2

0

1 if

1 if

v u

y D

y y

ydydygyu

0 ' )''(

)''(1'''sin)(

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Example I:Viscosity=0.02 Pa s ( 10 x water)Thickness=0.5 mSlope=0.1 degreesLimit velocity=1 cm/s

Result: Runout between 400 m and 2 km

Results from the calculations

Example IIViscosity=0.6 Pa sThickness=2 mSlope=0.8 degreesLimit velocity=0.2 cm/s

Result: Runout between 23 km and 100 km

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Processes that could potentially improve sand transport in the ocean

• Turbulence (generally accepted view).

• Fluidization.

• Importance of clay, even little clay increases viscosity/improves cohesion lubricated (hydroplaning) flow

• Sand waves.

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Conclusions and future plans

• CONCLUSIONS• Water increases mobility • Potential for sand

transport in the dense phase 1) sand packed in the clay matrix?) 2) Fluidization?

3) Sand waves?

• Scaling problems

• FUTURE PLANS• Experiments with

different volumes and different grain sizes

• Direct numerical simulation of high-density fluidization

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1

ut y

Model equations

• Conservation of particle number

• Laminar velocity field

• Viscosity

• Richardson-Zaki

20 max

max

( 1 )

( 1 )

if

if

v u

y D

0 y'

1 (y '')u(y) gsin dy ' dy '' dy ''

2 (y '')

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What model for clay-poor debris flows?

• Some existing models: Savage-Hutter (SH), Norem Irgens Schielthorp (NIS), Iverson Denlinger (ID)

• Granular: YES Turbulence: NO• Macroviscous regime not addressed only in NIS

• Direct Models ?

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Prompt disitengration of the debris flow

• 1) disintegration of the mass: the yield stress drops dramatically

• 2) settling and stratification

y k exp C

solid fraction in the slurry

dependent on the clay content

Reference solid fraction

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Existing models: e.g.: NIS model

• Mud with plug and shear layers– plasticity, viscosity, and visco-elasticity

• dry friction (no cohesion in code)• dynamic shear (thinning)• dispersive pressure

r

xexy

r

xuey

r

xuex

dy

ydvmpc

dy

ydvpp

dy

ydvpp

)(tan

)(

)()(

2

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I-D model

• Depth integrated, three-dimensional model • Accounts for the exchange of fluid between

different parts of the slurry due to diffusion and advection.

• Limitations for our purpose: water content of the slurry must not change, no cohesion, no turbulence

2

2

p ' p ' p ' p 'u v

t x z y