Fluid muds, grain flows and turbidity currents Lecture 1718

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Fluid muds, grain flows and turbidity
currentsLecture 17/18

• Sediment mass movement - definitions
• slides,
• slumps,
• debris flows,
• grains flows,
• turbidity currents (torrent)
• support mechanisms
• turbulence, upward intergranular flow, grain
  contact, matrix
• the deposits
• turbidites, conglomerates, fluxoturbidites,
  mudstones

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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Slide
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Creep
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, creek, slump, flow or
  fall)

Slump
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Topple
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Rock fall
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Flow
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• Mass movement governed by
• Rate of movement
• Type of material
• Nature of movement (slide, slump, flow or fall)

Torrent
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Sediment mass movement

• There is a continuum of scales from single grain
  transport to major landslides and slumps
• mass movement defined as
• slumps and slides (elasto-plastic)
• high-concentration turbidity currents (viscous)
• low-concentration turbidity currents (inertial)
• grain flows (dispersive pressure)
• debris flows (fluid pore pressure)
• traction
• deposit

Middleton and Hampton, 1976
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Sediment mass movement

• Mass movements take place ANYWHERE where gravity
  acts on a sloping surface i.e. EVERYWHERE
• Submarine debris flows (water)
• Sub-aerial debris flows (air)
• Snow avalanches (ice)
• Volcanic debris flows (gas, ash)
• Submarine landslides (rock)
• Sub-aerial landslides (rock)

Takahashi, 2001 Particulate Gravity Currents
11-43.
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Sediment mass movementvolcanic debris flow in air
Acceleration/remoulding
Deceleration/deposition
Trigger mechanism
Martin and White, 2001 Particulate Gravity
Currents e.g. Vesuvius and Pompei
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Mass failure

• Kitimat delta, Howe Sound
• arcuate head scarps
• debris flow lobes
• longitudinal shears
• pressure ridges
• slides/slumps/turbidity flows
• slide toes

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Classification of mass movementsand their
deposits
Gani, M.R., 2004. The Sedimentary Record 2(3)4-9
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The types of deposits from mass movementmarine
examples
Gani, R.M. 2004. From Turbid to lucid. In. The
Sedimentary Record , 2(3) 4-8.
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The rheology of mass movements
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A rheogram of mass movements
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Concentration, grain size and Re effects
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The cycle of fine-grained sedimentsmarine
examples

• Turbidity currents
• 1929 Grand Banks event
• depositing flows
• non-depositing flows
• turbidites (deposits)
• Fluidized sediment flows
• Grain flows
• river of sand, California
• Debris flows
• Gulf of Corinth, Greece
• scouring flows
• non-scouring flows

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The internal structure of mass movements

• Slump blocks and olistoliths
• northern Cyprus
• Sliding
• movement along shear planes
• no significant deformation
• Slumping
• folded shear planes
• deformation
• Mass flows
• reduction in internal shear
• liquefaction increasing
• remoulding increasing
• Suspensions
• fluid turbulence dominant

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Balance of forces, no sediment transport - flat
bed
height

• Consider first a flat bed with no bedload
• W 0 ? 0
• then the fluid-transmitted stress is balanced by
  the bed shear stress

Boundary layer
Viscous sub-layer
Shear stress
No sediment motion
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The balance of forces approach - flat bed

• The moving load will continue at constant speed
  if the total driving forces balances the
  stabilizing forces
• the driving forces (Fd) are
• fluid shear stress.area (?f .A)
• gravitational force (W) downslope
• buoyancy (W2)
• the stabilizing forces (Fs) are
• immersed weight (W1)
• bed shear stress.area (?o.A)
• solid-transmitted stress.area (Ttot.A)
• at threshold these forces balance

Fs
Pivotal angle - ? angle of repose angle of
grain contact (from vertical)
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Sediment mass movement

• The collision between material in motion
  maintains
• a RADIATION stress (R) which is directed in all
  directions within the flow, and
• A SOLID-TRANSMITTED bed shear stress (T) which is
  directed backwards
• Buoyancy of the moving material due to density
  effect of finer material
• Kinetic sieving which brings coarser material to
  the top

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Balance of forces, sediment transport - flat bed
Free stream region

• For active transport as bedload
• T increases towards the bed in the
• saltation layer
• T decreases towards the bed in the
• mobile layer
• within the bedload layer
• the shear is solid-transmitted
• the fluid shear stress ? zero

height
?f gt 0 Ttot 0
Boundary layer
?f gt 0 Ttot gt 0
Saltation layer
Mobile layer depth
active bedload
Bagnold, R.A. 1954
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Sediment mass movementsimplest mass balance
diagram

• Down-slope weight of mass produces a downslope
  directed flow (Txz) proportional to the bed slope
  (ß)
• Note that this defines perpetual motion (what
  stops to material from accelerating ?)

Straub,S. 2001. IAS 3191-109
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The balance of forces on a sloping bed (1)
Moving mass constant velocity
?f
?o - bed shear stress (at base of motion) ?f -
applied fluid shear stress opposite direction
if still water ? - internal friction angle ? -
bed gradient (slope) mg - sediment immersed
weight Tt - solid-transmitted stress W -
gravitational component Tr - radiation
(dispersive) stress
?o
P
?
?
P
W
Tt
mg
Straub,S. 2001. IAS 3191-109
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Balance of forces on a sloping bed (2)
Moving mass constant velocity
?f

• When ? gt ? slope is unstable
• failure will occur
• when ? ? slope is in balance
• angle of repose
• when ? lt ? slope tends to stability
• At equilibrium

?o
P
?
?
P
W
Tt
mg
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Balance of forces on sloping bed (3)

• For bed slope (?) we must consider the additional
  driving force due to gravity i.e. the downslope
  component of weight
• the stabilizing force is reduced to
• when slope is in direction of drag then Fd W
  Fs P
• when slope is normal to drag then ?(Fd2 Ft2)
  Fs

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Autosuspension - turbidity currents

• Suspension (once initiated) is maintained by the
  fluid flow
• excess density and gravity ? turbulence
• for sediment to be mobile, W and T must balance
• the above formulation specifies perpetual motion
  !!!
• T acts to erode the bed and decelerate the flow

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The balance of forces on sloping bed (4)

• for a transverse slope, the threshold condition
  is
• for a longitudinal slope the threshold condition
  is

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Balance of forces on sloping bed (5)

• Bagnold (1962) the total kinetic energy to hold
  material in suspension
• where C is the dimensionless mass concentration
• power expended per unit volume (area) per unit
  time
• where Ws is the settling rate power supplied by
  gravity

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Balance of forces - flat bed

Re-arranging terms we arrive at note
the left term is the Shields parameter (?)