1 / 20

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. What are a prograding clinoforms?. Each layer is a clinoform .

shyla
Download Presentation

Matthew A. Wolinsky and Lincoln F. Pratson

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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

  3. Clinoform Kinematics q[x,t] sediment flux r[t]=ro+Vtclinoform 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 • 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

  16. 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?

  17. 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

  18. 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

  19. 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

More Related