CLASTIC TRANSPORT AND FLUID FLOW

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CLASTIC TRANSPORT AND FLUID FLOW

• CHAPTER 3

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Chapter-3 Clastic transport and fluid flow

• Weathered rock and minerals fragments are
  transported from source areas to depositional
  sites (where they are subject to additional
  transport and redeposition) by three kinds of
  processes
• 1- dry (non-fluid assisted), gravity-driven mass
  wasting processes such as rock fall and rock
  slides

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• 2- wet (fluid assisted), gravity-driven mass
  wasting processes (sediment gravity flows) such
  as grain flows, mudflows, debris flows, and some
  slumps and
• 3- processes that involve direct fluid flows of
  air, water, and ice.

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• Mass wasting
• Fluid flow
• Reynolds Number
• Froud Number
• Entrainment, transport and deposition of clasts
• Transport

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Mass Wasting

• Mass wasting processes are important mechanisms
  of sediemnt transport.
• Although they move the soil and rock debris only
  short distances downslope , these processes play
  a crucial role in sediment transport by getting
  the products of weathering into the
  longer-distance sediment transport system.

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Mass Wasting

• In dry mass-wasting processes, fluid plays either
  a minor role or no role at all.
• In rock or talus falls, clasts of any size simply
  fall freely.
• Downslope movement of bodies of rocks or sediment
  in slumps or slides glide downslope en masse
  without significant internal folding or faulting.
  Fluids near the base provides lubrication and
  promotes failure along slippage surface.

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Reynolds Number

• Re 2rVp/?
• Sir Osborne Reynolds addressed the problem of how
  laminar flow changes to turbulent flow.
• The transition from laminar to turbulent flow
  occurs as velocity increases, viscosity decrease,
  the roughness of the flow boundary increases,
  and/or the flow becomes less narrowly confined.

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Froud Number

• The Froud Number is the ratio between fluid
  inertial forces and fluid gravitational forces.
• It compares the tendency of a moving fluid (and a
  particle borne by that fluid) to continue moving
  with the gravitational forces that act to stop
  that motion.
• The force of inertia express the distance
  traveled by a discrete portion of the fluid
  before it comes to rest.
• Like reynolds Numbers, Froud numbers are
  dimensionless.

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Froud Number

• Fr fluid inertial forces .
• gravitational forces in flow

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Deposition What forces control the settling of
particles?

• As soon as a particle is lifted above the surface
  of a bed, it begins to sink back again.
• The distance that it travels depend on the drag
  force of the current, and the settling velocity
  of the Particle.
• The velocity at which a clast settles througha
  fluid is calculated using STOKES LAW of settling

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Stokes Law of settling

• The gravitational force pulling the particle down
  versus the drag force of the fluid resisting this
  sinking.
• The particle will be initially accelerate due to
  gravity, but soon the gravitational and drag
  forces reach equilibrium, resulting in a constant
  Terminal Fall Velocity.

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• The drag force exerted by a fluid on a falling
  grain is proportional to the fluid density (?F),
  the diameter (d) of the grain (in centimeters),
  the drag coefficient (CD) and the fall velocity
  (V).
• Drag force CD p (d2/4) (?F V2/2)

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Drag force

• Upward force due to buoyancy of the fluid is
  Fupward 4/3 p (d/2)3 ?Fg
• Downward forces due to gravity
• Fg 4/3 p (d/2)3 ? sg, where ps is the density
  of the particle.
• Drag force Fg - Fupward

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Drag force Fg - Fupward

• CD p (d2/4) (pF V2/2) 4/3 p (d/2)3 ?sg - 4/3 p
  (d/2)3 ?Fg
• V2 4gd (?s- ?F)
• 3 CD ?F
• For low laminar flow at low concentrations of
  particle and low Reynolds numbers, CD is equal to
  24/Re.
• V 1/18 (? s- ? F g d2 /u) - Stokes Law of
  settling
• V-velocity, g-gravity, u-viscocity, even the
  differnce in density (?s- ? F) is constant for a
  given situation. It can be substitute for C
• C (? s- ? F)g
• 18u
• Stokes law reduces to VCD2

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Stokes law reduces to VCD2

• When density and viscosity are constant, settling
  velocity increases with the diameter of the
  particle.
• Larger grains fall faster.
• Settling velocity decreases with higher
  viscosities and increases with denser particles.
  C (?s- ?F)g
• 18u

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Implications of the Stokes Law

• High density minerals settle more rapidly than
  low density minerals.
• Slow-moving, highly viscous fluids such as
  mudflows and density currents can transport
  coarser-grained materials than less viscuos
  fluids such as rivers and the wind, despite the
  normally higher velocity of these less viscous
  fluids.

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• Lower temperatures will increas viscosity
  decreasing the fall velocity.
• Because of turbulence, coarser particles fall
  more slowly than predicted.
• Non-spherical flakes such as mica will settle
  more slowly than spheres with the same density.
• Angular grains will generate small turbulent
  eddies that retard settling velocity.

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Hydraulic equivalency

• The term refers to clasts that settle at
  identical velocities despite substantial
  differences in size, shape, angularity, and
  density.
• ie. Sediment mixes of fine grained, silt-size
  magnetite, fine sand-size biotite flakes, and
  medium sand-size quartz.