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									SEDIMENT EROSION,TRANSPORT,
DEPOSITION, AND SEDIMENTARY STRUCTURES
An Introduction To
Physical Processes of
Sedimentation
PREFACE
    UNESCO’s International Hydrological Programme (IHP)
    launched the International Sediment Initiative (ISI) in
    2002, taking into consideration that sediment production
    and transport processes are not sufficiently understood for
    practical uses in sediment management. Since information
    on ongoing research is an important support to sediment
    management, and bearing in mind the unequal level of
    scientific knowledge about various aspects of erosion and
    sediment phenomena at the global scale, a major mission of
    the ISI is to review erosion and sedimentation-related
    research. The two papers below were prepared in
    conformity with this important task of the ISI, following
    the decision of the ISI Steering Committee at its session in
    March 2004.
SEDIMENT DYNAMICS
SEDIMENT TRANSPORT
   Fluid Dynamics

   COMPLICATED
       Focus on basics
         Foundation
         NOT comprehensive
SEDIMENTARY CYCLE
 Weathering
     Make particle
 Erosion
     Put particle in motion
 Transport
     Move particle
 Deposition
     Stop particle motion
         Not necessarily continuous (rest stops)
DEFINITIONS
 Fluid       flow (Hydraulics)
     Fluid
       Substance that changes shape easily and
        continuously
       Negligible resistance to shear

       Deforms readily by flow
             Apply minimal stress
     Moves particles
     Agents
       Water
       Water containing various amounts of sediment

       Air

       Volcanic gasses/ particles
    DEFINITIONS
   Fundamental Properties
      Density (Rho (r))
        Mass/unit volume
               Water ~ 700x air
        r  = 0.998 g/ml @ 20°C
          Density decreases with increased temperature

       Impact on fluid dynamics
          Ability of force to impact particle within fluid and on bed

          Rate of settling of particles

          Rate of occurrence of gravity -driven down slope movement of
           particles
               rH20 > r air
DEFINITIONS

    Fundamental Properties
       Viscosity
         Mu (m)
               Water ~ 50 x air
           m = measure of ability of fluids to flow (resistance of
            substance to change shape)
             High viscosity = sluggish (molasses, ice)
             Low viscosity = flows readily (air, water)

           Changes with temperature (Viscosity decreases with
            temperature)
               Sediment load and viscosity co-vary
           Not always uniform throughout body
               Changes with depth
TYPES OF FLUIDS:
STRAIN (DEFORMATIONAL) RESPONSE TO
STRESS (EXTERNAL FORCES)
 Newtonian            fluids
    normal fluids; no yield
     stress
        strain (deformation);
         proportional to stress, (water)
 Non-Newtonian
    no yield stress;
        variable strain response to
         stress (high stress generally
         induces greater strain rates
         {flow})
            examples: mayonnaise, water
             saturated mud
WHY DO PARTICLES MOVE?
 Entrainment
 Transport/ Flow
ENTRAINMENT
   Basic forces acting on particle
       Gravity, drag force, lift force
         Gravity:
         Drag force: measure of friction between water and bottom of

          water (channel)/ particles
         Lift force: caused by Bernouli effect
BERNOULI FORCE
  (rgh)   + (1/2 rm2)+P+Eloss = constant
   Static P + dynamic P
  Potential energy= rgh
  Kinetic energy= 1/2 rm2
  Pressure energy= P
  Thus pressure on grain decreases, creates lift
   force

 Faster current increases likelihood that gravity, lift
  and drag will be positive, and grain will be picked
  up, ready to be carried away
 Why it’s not so simple: grain size, friction, sorting,
  bed roughness, electrostatic attraction/ cohesion
FLOW
 Types       of flow
     Laminar
         Orderly, ~ parallel flow lines
     Turbulent
         Particles everywhere! Flow lines change constantly
           Eddies
           Swirls

     Why are they different?
       Flow velocity
       Bed roughness

       Type of fluid
GEOLOGICALLY SIGNIFICANT
FLUID FLOW TYPES (PROCESSES)
 Laminar     Flows:
    straight or boundary parallel flow lines
 Turbulent     flows:
    constantly changing flow lines. Net mass transport in the
     flow direction
FLOW: FIGHT BETWEEN INERTIAL AND
VISCOUS FORCES

 Inertial    F
     Object in motion tends to remain in motion
       Slight perturbations in path can have huge effect
       Perfectly straight flow lines are rare

 Viscous     F
     Object flows in a laminar fashion
     Viscosity: resistance to flow (high = molasses)
       High viscosity fluid: uses so much energy to move it’s
        more efficient to resist, so flow is generally straight
       Low viscosity (air): very easy to flow, harder to resist,
        so flow is turbulent
 Reynolds        # (ratio inertial to viscous forces)
REYNOLD’S #
            Re = Vl/(r/m)
            dimensionless #
              V= current velocity
              l= depth of flow-diameter of pipe

              r= density

              m= viscosity

       u=(r/m)- kinematic viscosity
  Fluids with low u (air) are turbulent
    Change to turbulent determined experimentally
         Low Re = laminar <500 (glaciers; some mud flows)
         High Re = turbulent > 2000 (nearly all flow)
GEOLOGICALLY SIGNIFICANT
FLUID FLOW TYPES (PROCESSES)
 Laminar     Flows:
    straight or boundary parallel flow lines
 Turbulent     flows:
    constantly changing flow lines. Net mass transport in the
     flow direction
 GEOLOGICALLY SIGNIFICANT FLUIDS
 AND FLOW PROCESSES
                                           Debris flow (laminated flow)
     These distinct flow mechanisms
      generate sedimentary deposits
      with distinct textures and
      structures
     The textures and structures can be
      interpreted in terms of
      hydrodynamic conditions during
      deposition
     Most Geologically significant flow
      processes are Turbulent


Traction deposits
(turbulent flow)
WHAT ELSE IMPACTS FLUID FLOW?
 Channels

 Water depth
 Smoothness of Channel Surfaces

 Viscous Sub-layer
1. CHANNEL

  Greater slope = greater velocity
  Higher velocity = greater lift force
        More erosive
    Higher velocity = greater inertial forces
      Higher numerator = higher Re
      More turbulent
2. WATER DEPTH
   Water flowing over the bottom creates shear
    stress (retards flow; exerted parallel to surface)

       Shear stress: highest AT surface, decreases up
       Velocity: lowest AT surface, increases up

       Boundary Layer: depth over which friction
        creates a velocity gradient
         Shallow water: Entire flow can fall within this
          interval
         Deep water: Only flow within boundary layer is
          retarded
       Consider velocity in broad shallow stream vs
        deep river
2. WATER DEPTH
   Boundary Shear stress (o)-stress that opposes the
    motion of a fluid at the bed surface
    (o) = gRhS
         g= density of fluid (specific gravity)
         Rh = hydraulic radius
               (X-sectional area divided by wetted perimeter)
           S = slope (gradient)

     the resistance to fluid flow across bed (ability of fluid
      to erode/ transport sediment)
     Boundary shear stress increases directly with
      increase in specific gravity of fluid, increasing
      diameter and depth of channel and slope of bed (e.g.
      greater ability to erode & transport in larger
      channels)
2. WATER DEPTH
   Turbulence
       Moves higher velocity particles closer to stream bed/
        channel sides
           Increases drag and list, thus erosion


       Flow applies to stream channel walls (not just bed)
3. SMOOTHNESS
   Add obstructions
     decrease velocity around object (friction)
     increase turbulence
         May focus higher velocity flow on channel sides or bottom
         May get increased local erosion, with decreased overall
          velocity
FLOW/GRAIN INTERACTION:
PARTICLE ENTRAINMENT AND TRANSPORT
   Forces acting on particles during fluid flow
                                     Inertial forces, FI, inducing
                                      grain immobility
                               FI = gravity + friction + electrostatics

                                     Forces, Fm, inducing grain
                                      mobility
                               Fm= fluid drag force + Bernoulli force
                                        + buoyancy
DEPOSITION
 Occurs when system can no longer support grain
 Particle Settling
   Particles settle due to interaction of upwardly
    directed forces (buoyancy of fluid and drag)
    and downwardly directed forces (gravity).
 Generally,      coarsest grains settle out first
       Stokes Law quantifies settling velocity
       Turbulence plays a large role in keeping grains
        aloft
GRAINS IN MOTION (TRANSPORT)
    Once the object is set in motion, it will stay in motion
    Transport paths
      Traction (grains rolling or sliding across bottom)
      Saltation (grains hop/ bounce along bottom)
      Bedload (combined traction and saltation)
      Suspended load (grains carried without settling)
         upward forces > downward, particles uplifted stay aloft
          through turbulent eddies
         Clays and silts usually; can be larger, e.g., sands in floods

               Washload: fine grains (clays) in continuous suspension derived
                from river bank or upstream
  Grains    can shift pathway depending on
     conditions
    TRANSPORT MODES AND PARTICLE ENTRAINMENT
   With a grain at rest, as flow velocity increases
                Fm > Fi ; initiates particle motion
   Grain Suspension (for small particle sizes, fine silt; <0.01mm)
        When Fm > Fi
            U (flow velocity) >>> VS (settling velocity)
      Constant grain Suspension at relatively low U (flow velocity)
      Wash load Transport Mode
    TRANSPORT MODES AND PARTICLE ENTRAINMENT
   With a grain at rest, as flow velocity increases
                        Fm     >    Fi ; initiates particle motion
   Grain Saltation : for larger grains (sand size and larger)
        When Fm > Fi
            U > VS but through time/space U < VS
      Intermittent Suspension
      Bedload Transport Mode
THEORETICAL BASIS FOR HYDRODYNAMIC
INTERPRETATION OF SEDIMENTARY FACIES
   Beds defined by
       Surfaces (scour, non-deposition) and/or
       Variation in Texture, Grain Size, and/or Composition

For example:
 Vertical accretion bedding (suspension settling)
       Occurs where long lived quiet water exists
   Internal bedding structures (cross bedding)
       defined by alternating erosion and deposition due to
        spatial/temporal variation in flow conditions
   Graded bedding
       in which gradual decrease in fluid flow velocity results in
        sequential accumulation of finer-grained sedimentary
        particles through time
FLOW REGIME AND
SEDIMENTARY STRUCTURES




   An Introduction To
   Physical Processes of Sedimentation
SEDIMENTARY STRUCTURES
 Sedimentary  structures occur at very
 different scales, from less than a mm (thin
 section) to 100s–1000s of meters (large
 outcrops); most attention is traditionally
 focused on the bedform-scale
  •   Microforms (e.g., ripples)
  •   Mesoforms (e.g., dunes)
  •   Macroforms (e.g., bars)
SEDIMENTARY STRUCTURES
 Laminae      and beds are the basic
  sedimentary units that produce
  stratification; the transition between the two
  is arbitrarily set at 10 mm
 Normal grading is an upward decreasing
  grain size within a single lamina or bed
  (associated with a decrease in flow
  velocity), as opposed to reverse grading
 Fining-upward successions and
  coarsening-upward successions are the
  products of vertically stacked individual beds
BED RESPONSE TO WATER (FLUID) FLOW
   Common bed forms (shape of the unconsolidated bed) due to fluid
    flow in
       Unidirectional (one direction) flow
            Flow transverse, asymmetric bed forms
                 2D&3D ripples and dunes
       Bi-directional (oscillatory)
            Straight crested symmetric ripples
       Combined Flow
            Hummocks and swales
SEDIMENTARY STRUCTURES
Cross stratification

 The angle of climb of cross-stratified deposits
  increases with deposition rate, resulting in
  ‘climbing ripple cross lamination’
 Antidunes form cross strata that dip upstream, but
  these are not commonly preserved

   A single unit of cross-stratified material is known as
    a set; a succession of sets forms a co-set
BED RESPONSE TO STEADY-STATE,
UNIDIRECTIONAL, WATER FLOW
   Upper Flow Regime
       Flat Beds: particles move continuously with no relief on the bed
        surface
       Antidunes: low relief bed forms with constant grain motion; bed
        form moves up- or down-current (laminations dip upstream)
QUESTION?
TEST
 In which year UNESCO launched International Sediment
  Initiative?
 Write the Sedimentary Cycle.

 Write the Bernouli’s Force equation.

 What is Laminar & Turbulent flow?

 Write the equation of Renold’s Equation.

								
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