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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
WeatheringMake particle
ErosionPut particle in motion
TransportMove particle
DepositionStop 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 stressMoves particlesAgents
Water Water containing various amounts of sediment Air Volcanic gasses/ particles
DEFINITIONS
Fundamental Properties Density (Rho ())
Mass/unit volume Water ~ 700x air
= 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 H20 > air
DEFINITIONS
Fundamental Properties Viscosity
Mu () Water ~ 50 x air
= measure of ability of fluids to flowresistance 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 fluidsnormal fluids; no yield
stress strain (deformation);
proportional to stress, (water)
Non-Newtonianno 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
gh) + (1/2 2)+P+Eloss = constantStatic P + dynamic P
Potential energy= gh Kinetic energy= 1/2 2
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 flowLaminar
Orderly, ~ parallel flow linesTurbulent
Particles everywhere! Flow lines change constantly Eddies Swirls
Why are they different? Flow velocity Bed roughness Type of fluid
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
FLOW: FIGHT BETWEEN INERTIAL AND VISCOUS FORCES
Inertial FObject in motion tends to remain in motion
Slight perturbations in path can have huge effect Perfectly straight flow lines are rare
Viscous FObject flows in a laminar fashionViscosity: 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/(/dimensionless #
V= current velocityl= depth of flow-diameter of pipe = density = viscosity/kinematic viscosity
Fluids with low (air) are turbulent Change to turbulent determined
experimentally Low Re = laminar <500 (glaciers; some mud flows) High Re = turbulent > 2000 (nearly all flow)
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
GEOLOGICALLY SIGNIFICANT FLUIDS AND FLOW PROCESSES 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
Debris flow (laminated flow)
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
retardedConsider 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) = RhS
= 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 firstStokes Law quantifies settling velocityTurbulence 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 STRUCTURESLaminae 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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