Sediment Erosion,Transport, Deposition, and Sedimentary Structures

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Sediment Erosion,Transport, Deposition, and
Sedimentary Structures

• An Introduction To
• Physical Processes of Sedimentation

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PREFACE

• UNESCOs 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.

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Sediment Dynamics
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Sediment transport

• Fluid Dynamics
• COMPLICATED
• Focus on basics
• Foundation
• NOT comprehensive

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Sedimentary Cycle

• Weathering
• Make particle
• Erosion
• Put particle in motion
• Transport
• Move particle
• Deposition
• Stop particle motion
• Not necessarily continuous (rest stops)

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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

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Definitions

• Fundamental Properties
• Density (Rho (r))
• Mass/unit volume
• Water 700x air
• 0.998 g/ml _at_ 20C
• 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
• ?H20 gt ? air

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Definitions

• Fundamental Properties
• Viscosity
• Mu (m)
• Water 50 x air
• ? 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

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Types of FluidsStrain (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

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Why do particles move?

• Entrainment
• Transport/ Flow

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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

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Bernouli Force

• (rgh) (1/2 rm2)PEloss 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 its not so simple grain size, friction,
  sorting, bed roughness, electrostatic attraction/
  cohesion

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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

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Geologically SignificantFluid Flow Types
(Processes)

• Laminar Flows
• straight or boundary parallel flow lines
• Turbulent flows
• constantly changing flow lines. Net mass
  transport in the flow direction

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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
  its 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)

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Reynolds

• 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 lt500 (glaciers some mud flows)
• High Re turbulent gt 2000 (nearly all flow)

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Geologically SignificantFluid Flow Types
(Processes)

• Laminar Flows
• straight or boundary parallel flow lines
• Turbulent flows
• constantly changing flow lines. Net mass
  transport in the flow direction

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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)
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What else impacts Fluid Flow?

• Channels
• Water depth
• Smoothness of Channel Surfaces
• Viscous Sub-layer

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1. Channel

• Greater slope greater velocity
• Higher velocity greater lift force
• More erosive
• Higher velocity greater inertial forces
• Higher numerator higher Re
• More turbulent

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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

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2. Water Depth

• Boundary Shear stress (?o)-stress that opposes
  the motion of a fluid at the bed surface
• (?o) gRhS
• ? 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)

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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)

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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

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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

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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

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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 gt 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

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Transport Modes and Particle Entrainment

• With a grain at rest, as flow velocity increases
• Fm     gt    Fi initiates particle motion
• Grain Suspension (for small particle sizes, fine
  silt lt0.01mm)
• When Fm  gt  Fi
• U (flow velocity) gtgtgt VS (settling velocity)
• Constant grain Suspension at relatively low U
  (flow velocity)
• Wash load Transport Mode

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Transport Modes and Particle Entrainment

• With a grain at rest, as flow velocity increases
• Fm     gt    Fi initiates particle motion
• Grain Saltation for larger grains (sand size
  and larger)
• When Fm  gt  Fi
•  U   gt VS  but through time/space U lt VS
• Intermittent Suspension
• Bedload Transport Mode

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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

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Flow Regime and Sedimentary Structures

• An Introduction To
• Physical Processes of Sedimentation

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Sedimentary structures

• Sedimentary structures occur at very different
  scales, from less than a mm (thin section) to
  100s1000s 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)

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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

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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
• 2D3D ripples and dunes
• Bi-directional (oscillatory)
• Straight crested symmetric ripples
• Combined Flow
• Hummocks and swales

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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

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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)

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Question?
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Test

• In which year UNESCO launched International
  Sediment Initiative?
• Write the Sedimentary Cycle.
• Write the Bernoulis Force equation.
• What is Laminar Turbulent flow?
• Write the equation of Renolds Equation.

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