Presentation on theme: "Matthew A. Wolinsky and Lincoln F. Pratson"— Presentation transcript:
1 Matthew A. Wolinsky and Lincoln F. Pratson Overpressure and Slope Stability in Prograding Clinoforms: Implications for Marine MorphodynamicsMatthew A. Wolinskyand Lincoln F. PratsonPresentation by Kevyn BollingerOCE 582 Seabed Geotechnics11/13/2008
2 What are a prograding clinoforms? Depositional PatternSpatial DistributionEach 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 pointh[x,t] sediment surface- evolves through timeS SlopeV Velocity of progradation
6 Clinoform KinematicsFrom:We getFor the basal boundary conditions
7 Groundwater Mechanics Assume:Impermeable basal surfaceSaturated depositSmall surface slopes (S<<1)Strains uni-axial and infinitesimalSolids and liquids (grains and pores) incompressibleHomogeneousHydraulic conductivity aligned with depositional layersOverpressure evolutionGibson (1958, Bedehoeft and Hanshaw 1968k= hydraulic diffusivityh[x,z,t]=excess pressure head,Sediment submerged specific gravity
8 Non-dimensionalizing Three time scalesNon-dimensional Overpressurex*=x/L z*=z/H h*=h/H h*=h/coRHOverpressure 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 dissipatione<<1 vertical diffusion dominatese>>1 horizontal diffusion dominates
9 Overpressure Prediction Shaded Areas: Time averaged Loading, Gb White Areas: Instantaneous Loading, GbA: 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
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=cohesionAssume: Slope small and Curvature smallLiquefaction potential:Failure:
12 Failure Modes Surface Liquefaction Basal Slumping Liquefaction potential greatest at surfaceThreshold for liquefaction greater than Gb=~10Basal SlumpingRequires exceedance of a critical slopeNormalized Failure Slope
17 Results and Implications All cases have evidence of slumping/liquefaction.JerseyWell below threshold levelsAmazonFluid MudsPermeably sandPredicted positive relationship between sediment supply and slope evident?
18 Limitations of Simplified Model CompactionMethod ignores effects of compationsSlope Failureslope failure inherently uncertain due to effects of transient eventsHeterogeneity and AnisotropyAssumed kz>>kxBoundary ConditionsDrained/ Undrained
19 Conclusions Deposition highly localized in space and time. Model developed predicts overpressure and slope stability as a function of sediment supplySlope is inversely proportional to supplyOverpressure is of first order significance to marine morphodynamics
20 ReferenceRole of Turbidity Currents in Setting the Foreset Slope of Clinoforms Prograding into Standing Fresh WaterSvetlana Kostic, Gary Parker and Jeffrey G. MarrAbstract: 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