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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

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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

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q[x,t] sediment flux r[t]=r o +Vt clinoform rollover point x,t] sediment surface- evolves through time SSlope VVelocity of progradation Clinoform Kinematics

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Scale

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Clinoform Kinematics For the basal boundary conditions From: We get

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Groundwater Mechanics Assume: 1.Impermeable basal surface 2.Saturated deposit 3.Small surface slopes (S<<1) 4.Strains uni-axial and infinitesimal 5. Solids and liquids (grains and pores) incompressible 6.Homogeneous 7.Hydraulic conductivity aligned with depositional layers k= hydraulic diffusivity h[x,z,t]=excess pressure head, Sediment submerged specific gravity Gibson (1958, Bedehoeft and Hanshaw 1968 Overpressure evolution

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Non-dimensionalizing Three time scales Non-dimensional Overpressure x*=x/L z*=z/H *= /H h*=h/c o RH Overpressure generation expressed in terms of two dimensionless parameters: Gibson Number (loading intensity)Effective anisotropy (horizontal flow potential) Gb overpressure buildup Gb>>1vertical diffusion fast compared to loading -> overpressure dissipation 1vertical diffusion dominates 1horizontal diffusion dominates

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Overpressure Prediction Shaded Areas: Time averaged Loading, G b White Areas: Instantaneous Loading, G b 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

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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

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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

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

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Surface Liquefaction Liquefaction at:

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Basal Slumping Liquefaction at:

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

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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?

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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 k z >>k x Boundary Conditions – Drained/ Undrained

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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

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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

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