1. Matthew A. Wolinsky and Lincoln F. Pratson
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

2. What are a prograding clinoforms?
Depositional Pattern
Spatial Distribution
Each layer is a clinoform.
Prograding clinoforms means clinoforms stacking up on top of each other in a prograding sequence.

3. Clinoform Kinematics q[x,t] sediment flux
r[t]=ro+Vt clinoform rollover point
h[x,t] sediment surface- evolves through time
S Slope
V Velocity of progradation

4. Scale

5. Scale

6. Clinoform Kinematics
From:
We get
For the basal boundary conditions

7. 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

8. 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<<1 vertical diffusion slow compared to loading -> overpressure buildup
Gb>>1 vertical diffusion fast compared to loading -> overpressure dissipation
e<<1 vertical diffusion dominates
e>>1 horizontal diffusion dominates

9. 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

10. 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

11. 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:

12. Failure Modes Surface Liquefaction Basal Slumping
Liquefaction potential greatest at surface
Threshold for liquefaction greater than Gb=~10
Basal Slumping
Requires exceedance of a critical slope
Normalized Failure Slope

13. Surface Liquefaction
Liquefaction at:

14. Basal Slumping
Liquefaction at:

15. Applications Test Cases 14 clinoforms Historic maps and surveys
Seismic Profiles
210Pb

17. 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?

18. 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

19. 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

20. Reference
Role 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