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Experiments on subaqueous mass transport with variable sand-clay ratio

Fabio De Blasio Trygve Ilstad

Anders ElverhøiDieter Issler

Carl B. Harbitz

International Centre for GeohazardsNorwegian Geotechnical Institute, Norway

Dep. of Geosciences, University of Oslo, Norway..

In cooperation with the SAFL group, University of Minnesota

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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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Inferring the dynamics of subaqueous debris flow

• Field observations (long runout, outrunner blocks, geometry of sandy bodies, velocity...)

• Experiments:– (Experiments +Numerical modeling) × Extrapolation Field

– composition change

• Physical understanding and numerical simulation

• Important application:– Emplacement of massive sand in deep water– Offshore geohazards

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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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Runout distance in laboratorySame: GSD, % Water, Discharge, Volume

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8

Horizontal Distance (m)

Ele

va

tio

n (

m)

Subaerial

Subaqueous

0.6

0.8

How to explain the various styles of run out!

Subaerial Short and thick

Subaqueous Thin and long

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High clay content – video record

Turbidity current

Hydroplaning debris flow

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High speed video record (250 frames/sec)

Flow behavior - High clay content ( 30 % kaolinite)

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Low clay content – video record

Turbidity current

Dense flow

Deposition of sand

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Debris flows- low clay content (5%)

Turbulent front Deposition of sand

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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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Thickness of sandy deposits – versus clay content

0 3 6 9

0

1

2

3

10 wt% clay

5 wt% clay

15 wt% clay

Dep

ositi

on h

eigh

t (cm

)

Time (s)

Deposition Dense flow Turbidity current

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Pressure interpretation

Flow

Flow

Pressure

Time

Pressure

Time

Pore pressure

Total pressure

Grains in constant contact with bed

Rigid block over a fluid layer

Total pressure

Flow

Pore pressurePressure

Time

Fluidized flow

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Pressure measurements at the base of a debris flow as pressure develops during the flow

Low clay content High clay content

Total pressure

Hydrostatic pressure

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High clay content viscoplastic/hydroplaning/lubrication

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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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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 and lubricating effects

Detachment/stretching dynamics

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Clay rich sediments

• Visco-plastic materials

• Model approach:– ”Classical Bingham fluid” (“BING”)

– R-BING: Remolding of the sediment during the flow

– H-BING: Hydroplaning/Lubricating

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Velocity profile of debris flows Bingham fluid

Plug layer

Shear layer

•Classical Bingham fluid:•Yield strength: constant during flow

•Bingham fluid – with remolding (R-BING):

•The yield strength is allowed to vary during flow

Plug layer

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Water film/lubricating layer 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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What happens during flow at low clay content?

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

• 2) settling and sand stratification within few seconds

y k exp C

solid fraction in the slurry

dependent on the clay content

Reference solid fraction

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Low clay content Turbulence, disintegration, layering

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Existing models adapted to low clay debris flows: 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

21

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Iverson- Dellinger 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

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In short: high clay debris flows

• Viscoplastic behaviour • Vertically quasi-homogeneous• Hydroplaning/lubrication• Dynamical forces important• The material remains compact• Front detachment/outrunner block• Modeling: rheological flow,

– Modified “BING” • THEY ARE VERY MOBILE BECAUSE OF

LUBRICATION

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In short: low clay debris flows

• Granular + turbulent behaviour • Settling and vertical layering (“Brazil Nut Effect” )• Lubrication only at the very beginning • The material breaks up catastrophically• Blocks do not form• Modeling: Fluid dynamics + granular • THEY ARE VERY MOBILE BECAUSE OF

DRAMATIC DROP IN YIELD STRESS AND FLUIDISATION IN THE SAND LAYER

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Conclusions

• Slurries with a high clay content:– transported over long distances preserving the initial

composition

• Slurries with low clay content:– sandy materials may drop out during flow, alternatively

being transformed into turbidity currents

• Flow behavior:– Strongly influenced by the amount of clay versus sand

in the initial slurry

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Iverson-Dellinger model

• the Coulomb frictional force (diminished of the water pressure at the base of the debris flow),

• the fluid viscous shear stress, • the earth-pressure force (namely, the lateral forces

generated in the debris flow due to differences in the lateral pressure),

• the earth-pressure contribution of the bed pressure, • a diffusive term of water escaping from the bottom,• an earth-pressure term along the lateral (z) direction,• the diffusive term of water along the lateral direction, and

finally • the pressure at the base of the debris flow.

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Conclusions

• At high clay content:– a thin water layer intrudes underneath the front part =

lubrication!– progressive detachment of the head – the thin water underneath the head is a supply for water at the

base of the flow– a shear wetted basal layer with decreased yield strength is

formed

• At low clay content: – water entrainment at the head of the mass flow – low slurry yield stress = particles settlement and continuous

deposition – a wedge thickening depositional layer is developed some

distance behind the head – viscous effects in the diluted flow, Coulomb frictional behavior

within the dense flow. High pore pressures → near liquefaction.

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Dispersive pressure

• When solid particles are present

• Particles forced apart

• Ability to move large particles

– proportional to square of the particle size for given

shear rate (Bagnold, 1954)

– larger particles forced towards area of least shear

(up and front)

• Further research required

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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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Debris flows- high clay content

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

B: 25 wt% clay hydroplaning front D: Behind the head, increasing concentration in overlying turbidity current

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Debris flows- low clay content (5%)

Turbulent front Deposition of sand