s ediment e rosion,t ransport, d eposition, and s edimentary s tructures an introduction to physical...

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.