Grain collapse complete

1
The deposition of cohesive sedimentsLecture 16
Height in flow (z)
Newtonian fluid
?b 1030 kg/m3
?b 1100 kg/m3
Mobile fluid mud (flow)
?b 1200 kg/m3
Stationary fluid mud (bed)

• Grain collapse complete
• pore pressures unaffected by consolidation
  (secondary)
• geostatic load increases linearly with depth
• region of normal geotechnical studies
• NOTE THE COMPLEX SEDIMENT/WATER INTERFACE

The collapse zone
Soil profile non-deforming cohesive bed
Friction angle (f)
Depth in sediment (z)
2

• Stages of bed development
• Newtonian turbid fluid
• non-Newtonian fluid mud (mobile)
• non-Newtonian fluid mud (stationary)
• deforming cohesive bed (surface)
• non-deforming cohesive bed
• general bed structure
• surface biofilm
• pelletized layer
• collapse zone
• primary consolidating layer
• secondary consolidating layer

Diatom layer
Pellet layer
Collapse zone
Laminated sediments
Bed shear strength or critical shear stress
depth
3
Turbid Newtonian fluids

• Newtonian fluids stress strain rate at all
  levels of stress (viscosity 1.0 by definition)
• the sediment has no significant effect on fluid
  viscosity, though density is affected
• it is the region where Stokes Law and floccule
  settling are applicable
• Stokes Law for C lt 300 mg/L) - most estuaries and
  coastal regions
• floccule settling for 300 lt C lt 2000 mg/L - inner
  Bristol Channel
• the bed shear stress may be calculated normally
  (Law of the Wall)
• the boundary layer is assumed to be unaffected by
  turbidity
• the turbulent structure is assumed to be
  unaffected by turbidity
• (a heavily disputed subject !!!!)
• (any level of turbidity makes the fluid
  non-Newtonian)

4
non-Newtonian fluids - mobile
Inertial force

• Turbidity affects the fluid in two major ways
• increases in the Richardsons number (the density
  gradient dominates the velocity gradient (the
  Monin-Obukhov length Ri 1.0)
• increases in fluid viscosity (?) to result in
  either (1) shear thinning (thixotropic) or (2)
  shear thickening (dilatant) behaviour
• the fluid possesses NO yield resistance to shear
  stress and hence is MOBILE.
• The fluid flows down all gradients of the seabed
  infilling depressions and channels resulting in
  fluid mud pools (Bristol Channel on neap tides)
• Rig gradient Richardson number
• Rif flux Richardson number
• Fd flux of sediment (-kdC/dz)
• Where k eddy diffusivity of flow (0.0025Uh)
• Fd gt 0.15 (supercritical flow collapse)
• (Winterwerp et al., 1999)

Buoyant force
5
Concentration profiles

• The concentration (C) profile in a water column
  is governed by
• Fluid density (?)
• Shear velocity (U)
• Particle settling rate (Ws)
• Fup upward flux of sediment
• Fdown downward flux of sediment
• Cm mass concentration at height z
• Cb near bed concentration
• m coefficient for hindered settling (1)
• h total water depth
• B Mass settlingmixing term
• U peak tidal velocity

6
Concentration profiles

• The (linear) concentration (C, kg/m3) profile in
  a water column may be predicted from
• Rc ratio of near-bed concentration to that
  near water surface
• If no data then Rc 3
• and Csurf Cbed/3

7
non-Newtonian fluids - stationary

• C in excess of 20 g/L
• the fluid builds yield resistance to shear
  stresses (fluid or static)
• shear stress stored elastically (elasticplastic
  ratio increasing)
• fluid density lt 1200 kg/m3 (by definition)
• pore pressures typically in excess of the
  geostatic load (?gz)
• sediment particles buoyant, not in contact
  (fluid-supported)
• often found in navigation channels of estuaries
  as organic-rich gels
• turbidity maxima of mid-latitude estuaries

8
deforming cohesive beds

• Defined by bulk densities in excess of 1200 kg/m3
• they may grade from a fluid to a solid or show an
  abrupt lutocline
• sediment particles are in contact the bed weight
  is solidly-transmitted
• open floccules are collapsed and compressed into
  a sediment microfabric normal to the geostatic
  load (gravity)
• a region of complex biological effects
  (bioturbation, biostabilization, adhesion)
• a region of strong biochemical fluxes (oxygen,
  nutrients, metals)
• the sediment is elasto-plastic (plastic
  dominated)
• elastic part is the yield resistance
• plastic part is strain during erosion

9
The rheology of muds how do they flow ?
Bingham plastic
Newtonian fluid

• Rheogram (behaviour under flow)
• shear stress (Pa)
• strain rate (s-1)
• yield strength cohesion erosion threshold
• Newtonian fluid
• Bingham plastic (ideal)
• pseudo-plastics
• shear thinning (thixotropic)
• shear thickening (dilatant)

Shear thickening
Shear stress (Pa)
Shear thinning
Pseudo-plastics
Shear strain rate (1/s)