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Experiments on subaqueous mass transport with variable sand-clay rati oPowerPoint Presentation

Experiments on subaqueous mass transport with variable sand-clay rati o

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Experiments on subaqueous mass transport with variable sand-clay ratio

Fabio De Blasio

Trygve Ilstad

Anders Elverhøi

Dieter Issler

Carl B. Harbitz

International Centre for Geohazards

Norwegian Geotechnical Institute, Norway

Dep. of Geosciences, University of Oslo, Norway.

.

In cooperation withthe SAFL group, University of Minnesota

Basic problem! sand-clay rati

- 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)

- Debris
- flow

Inferring the dynamics of sand-clay ratisubaqueous 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

Experimental settings sand-clay ratiSt. Anthony Falls Laboratory

Experimental Flume: “Fish Tank”

turbidity current

debris flow

6° slope

10 m

Video (regular and high speed) and

pore- and total pressure measurements

How to explain the various styles of run out! sand-clay rati

Subaerial

Short and thick

Subaqueous

Thin and long

High speed video record sand-clay rati(250 frames/sec)

Flow behavior - High clay content( 30 % kaolinite)

Debris flows- low clay content (5%) sand-clay rati

Turbulent front Deposition of sand

- High clay content- sand-clay rati
- - Plug flow- “Bingham”
- High sand content
- Macro-viscous flow?
- Divergent flow in the
- shear layer

Thickness of sandy deposits – versus clay content sand-clay rati

Pore pressure sand-clay rati

Total pressure

Flow

Pore pressure

Flow

Pressure

Flow

Time

Pressure interpretationGrains in constant contact with bed

Total pressure

Pressure

Fluidized flow

Time

Pressure

Rigid block over a fluid layer

Time

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

High clay content pressure develops during the flowviscoplastic/hydroplaning/lubrication

Material from the base of the debris flow is eroded and incorporated into the lubricating layer.

L2

Ls

L1

H2

Hs

H1

Downslope gravitational forces

Bottom shear stresses

Detachment/stretching dynamics incorporated into the lubricating layer.

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 model
- Main difficulty is quantitative treatment of crack formation and wetting and lubricating effects

Clay rich sediments incorporated into the lubricating layer.

- Visco-plastic materials
- Model approach:
- ”Classical Bingham fluid” (“BING”)
- R-BING: Remolding of the sediment during the flow
- H-BING: Hydroplaning/Lubricating

Shear layer incorporated into the lubricating layer.

Velocity profile of debris flows Bingham fluid- 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

Pluglayer

Water film/lubricating layer incorporated into the lubricating layer. shear stress reduction in a Bingham fluid

u=1

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, m, um

1+

1

u

1

1

1-

1-

(R-)/

1

u

Plug layer

Shear layer

Velocity

Shear stress

1+

R(1+)/

Simulation: final deposit of the large-scale Storegga incorporated into the lubricating layer.

What happens during flow at low clay content? incorporated into the lubricating layer.

- 1) disintegration of the mass: the yield stress drops dramatically
- 2) settling and sand stratification within few seconds

dependent on the clay content

Reference solid fraction

solid fraction in the slurry

Low clay content incorporated into the lubricating layer.Turbulence, disintegration, layering

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

- plasticity, viscosity, and visco-elasticity

Iverson- Dellinger model 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

In short: high clay debris flows model

- 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

Granular + turbulent behaviour model

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

In short: low clay debris flowsConclusions model

- 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

Iverson-Dellinger model 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.

Conclusions model

- 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.

Dispersive pressure model

- 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

shear stress model

dynamic viscosity

yield strength

shear rate

Velocity profile of debris flows Bingham fluidPlug layer

Shear layer

Yield strength: constant during flow

u=1 model

Lid(Debris flow)

1

1

=1

Water, w, w, uw

=1-

Mudm, m, um

1+

1

u

1

1

1-

1-

(R-)/

1

u

Water film shear stress reduction in a Bingham fluid

Plug layer

Shear layer

Velocity

Shear stress

1+

R(1+)/

Debris flows- high clay content model

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

Debris flows- low clay content (5%) model

Turbulent front Deposition of sand

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