Overpressure and Slope Stability in Prograding Clinoforms:

Implications for Marine Morphodynamics

Matthew A. Wolinskyand Lincoln F. Pratson

Presentation by Kevyn BollingerOCE 582 Seabed Geotechnics

11/13/2008

What are a prograding clinoforms?

• Each layer is a clinoform.• Prograding clinoforms means clinoforms stacking up

on top of each other in a prograding sequence.

Depositional Pattern

Spatial Distribution

q[x,t] sediment fluxr[t]=ro+Vt clinoform rollover pointx,t] sediment surface- evolves through timeS SlopeV Velocity of progradation

Clinoform Kinematics

Scale

Scale

Clinoform Kinematics

For the basal boundary conditions

From:

We get

Groundwater MechanicsAssume:1. Impermeable basal surface2. Saturated deposit3. Small surface slopes (S<<1)4. Strains uni-axial and infinitesimal5. Solids and liquids (grains and pores) incompressible6. Homogeneous7. Hydraulic conductivity aligned with depositional layers

k= hydraulic diffusivityh[x,z,t]=excess pressure head,

Sediment submerged specific gravity

Gibson (1958, Bedehoeft and Hanshaw 1968

Overpressure evolution

Non-dimensionalizingThree time scales

Non-dimensional Overpressurex*=x/L z*=z/H *=/H h*=h/coRH

Overpressure generation expressed in terms of two dimensionless parameters:Gibson Number (loading intensity) Effective anisotropy (horizontal flow potential)

Gb<<1 vertical diffusion slow compared to loading -> overpressure buildupGb>>1 vertical diffusion fast compared to loading -> overpressure dissipation1 vertical diffusion dominates1 horizontal diffusion dominates

Overpressure Prediction

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

A: Convex (“Gibson delta”) – depositional rate decreases with timeB: Linear (“Gilbert delta”) – depositional rate constant with timeC: Oblique (concave) – depositional rate increases with timeD: Sigmoidal (convexo-concave) – depositional rate cyclic

Overpressure PredictionExamples: 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

Shear failure occurs when: = shear stress,c=shear strength, =internal friction coefficient,C=cohesion

Liquefaction potential:

Failure:

Assume: Slope small and Curvature small

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

Liquefaction at:

Basal Slumping

Liquefaction at:

ApplicationsTest Cases• 14 clinoforms• Historic maps and surveys• Seismic Profiles• 210Pb

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

• 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

• 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

ReferenceRole of Turbidity Currents in Setting the Foreset Slope of

Clinoforms Prograding into Standing Fresh Water Svetlana Kostic, Gary Parker and Jeffrey G. Marr

• Abstract: Clinoforms produced where sand-bed rivers flow into lakes and

reservoirs often do not form Gilbert deltas prograding at or near the angle of repose. The maximum slope of the sandy foreset in Lake Mead, for example, is slightly below 1°. Most sand-bed rivers also carry copious amounts of mud as wash load. The muddy water often plunges over the sandy foreset and then overrides it as a muddy turbidity current. It is hypothesized here that a muddy turbidity current overriding a sandy foreset can substantially reduce the foreset angle. An experiment reveals a reduction of foreset angle of 20 percent due to an overriding turbidity current. Scale-up to field dimensions using densimetric Froude similarity indicates that the angle can be reduced to as low as 1° by this mechanism. The process of angle reduction is self-limiting in that a successively lower foreset angle pushes the plunge point successively farther out, so mitigating further reduction in foreset angle.

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