Overpressure and slope stability in prograding clinoforms implications for marine morphodynamics
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Overpressure and Slope Stability in Prograding Clinoforms : Implications for Marine Morphodynamics. Matthew A. Wolinsky and Lincoln F. Pratson. Presentation by Kevyn Bollinger OCE 582 Seabed Geotechnics 11/13/2008. What are a prograding clinoforms?. Each layer is a clinoform .

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Overpressure and slope stability in prograding clinoforms implications for marine morphodynamics

Overpressure and Slope Stability in ProgradingClinoforms:Implications for Marine Morphodynamics

Matthew A. Wolinsky

and Lincoln F. Pratson

Presentation by Kevyn Bollinger

OCE 582 Seabed Geotechnics

11/13/2008


What are a prograding clinoforms

What are a prograding clinoforms?

  • Each layer is a clinoform.

  • Progradingclinoforms means clinoforms stacking up on top of each other in a prograding sequence.

Depositional Pattern

Spatial Distribution


Clinoform kinematics

Clinoform Kinematics

q[x,t] sediment flux

r[t]=ro+Vtclinoform rollover point

h[x,t] sediment surface- evolves through time

SSlope

VVelocity of progradation


Scale

Scale


Scale1

Scale


Clinoform kinematics1

Clinoform Kinematics

From:

We get

For the basal boundary conditions


Groundwater mechanics

Groundwater Mechanics

Assume:

  • Impermeable basal surface

  • Saturated deposit

  • Small surface slopes (S<<1)

  • Strains uni-axial and infinitesimal

  • Solids and liquids (grains and pores) incompressible

  • Homogeneous

  • Hydraulic conductivity aligned with depositional layers

Overpressure evolution

Gibson (1958, Bedehoeft and Hanshaw 1968

k= hydraulic diffusivity

h[x,z,t]=excess pressure head,

Sediment submerged specific gravity


Non dimensionalizing

Non-dimensionalizing

Three time scales

Non-dimensional Overpressure

x*=x/L z*=z/H h*=h/H h*=h/coRH

Overpressure generation expressed in terms of two dimensionless parameters:

Gibson Number (loading intensity)Effective anisotropy (horizontal flow potential)

Gb<<1vertical diffusion slow compared to loading -> overpressure buildup

Gb>>1vertical diffusion fast compared to loading -> overpressure dissipation

e<<1vertical diffusion dominates

e>>1horizontal diffusion dominates


Overpressure prediction

Overpressure Prediction

Shaded Areas: Time averaged Loading, Gb White Areas: Instantaneous Loading, Gb

A: Convex (“Gibson delta”) – depositional rate decreases with time

B: Linear (“Gilbert delta”) – depositional rate constant with time

C: Oblique (concave) – depositional rate increases with time

D: Sigmoidal (convexo-concave) – depositional rate cyclic


Overpressure prediction1

Overpressure Prediction

Examples: A-Yellow River, B-Gravel delta front Peyto Lake in Banff NP, C-Colorado river delta at lake Meade, D- Gargano subaqueous delta


Slope stability and liquefaction potential

Slope Stability and Liquefaction Potential

Shear failure occurs when:

t= shear stress,

tc=shear strength,

m=internal friction coefficient,

C=cohesion

Assume: Slope small and Curvature small

Liquefaction potential:

Failure:


Failure modes

Failure Modes

  • Surface Liquefaction

    • Liquefaction potential greatest at surface

    • Threshold for liquefaction greater than Gb=~10

  • Basal Slumping

    • Requires exceedance of a critical slope

      Normalized Failure Slope


Surface liquefaction

Surface Liquefaction

Liquefaction at:


Basal slumping

Basal Slumping

Liquefaction at:


Applications

Applications

Test Cases

  • 14 clinoforms

  • Historic maps and surveys

  • Seismic Profiles

  • 210Pb


Results and implications

Results and Implications

All cases have evidence of slumping/liquefaction.

  • Jersey

    • Well below threshold levels

  • Amazon

    • Fluid Muds

    • Permeably sand

      Predicted positive relationship between sediment supply and slope evident?


Limitations of simplified model

Limitations of Simplified Model

  • Compaction

    • Method ignores effects of compations

  • Slope Failure

    • slope failure inherently uncertain due to effects of transient events

  • Heterogeneity and Anisotropy

    • Assumed kz>>kx

  • Boundary Conditions

    • Drained/ Undrained


Conclusions

Conclusions

  • Deposition highly localized in space and time.

  • Model developed predicts overpressure and slope stability as a function of sediment supply

  • Slope is inversely proportional to supply

  • Overpressure is of first order significance to marine morphodynamics


Reference

Reference

Role of Turbidity Currents in Setting the Foreset Slope of ClinoformsPrograding into Standing Fresh Water

Svetlana Kostic, Gary Parker and Jeffrey G. Marr

  • Abstract: Clinoforms produced where sand-bed rivers flow into lakes andreservoirs often do not form Gilbert deltas prograding at ornear the angle of repose. The maximum slope of the sandy foresetin Lake Mead, for example, is slightly below 1°. Most sand-bedrivers also carry copious amounts of mud as wash load. The muddywater often plunges over the sandy foreset and then overridesit as a muddy turbidity current. It is hypothesized here thata muddy turbidity current overriding a sandy foreset can substantiallyreduce the foreset angle. An experiment reveals a reductionof foreset angle of 20 percent due to an overriding turbiditycurrent. Scale-up to field dimensions using densimetric Froudesimilarity indicates that the angle can be reduced to as lowas 1° by this mechanism. The process of angle reductionis self-limiting in that a successively lower foreset anglepushes the plunge point successively farther out, so mitigatingfurther reduction in foreset angle.

  • Highly relevant to paper due to discussion of previous research on sandy delta foreset angle


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