2 - Sediment Transport

7/28/2019 2 - Sediment Transport

1/38

Fluid Dynamics & Physicalproperties of sediments

Chapters 2

Contents

Sediment Erosion, Transport &Deposition

Unidirectional currents

Waves

Bedforms and Primary SedimentaryStructures

Density Currents

Entrainment

Ability of a fluid (e.g., air, water, ice) toerode a particle depends on a variety offactors, including the fluid density, fluidviscosity, depth of flow, fluid velocity

Characteristics of the sediment (size,shape, binding by micro-organisms,etc.) also important


2/38

Entrainment

Density: mass per unit volume

Affects the magnitude of forces that act

within a fluid and on the bed, and the rateat which particles will settle

Viscosity: ability of fluids to flow

Low fluid flows readily (e.g., air)

High fluid resists flowage (e.g., ice)

Entrainment

Fluid motions may be either laminaror turbulent depending on the flowvelocity, fluid viscosity and bedroughness

Laminar flow

Streamlines follow parallel paths, velocity

constant along streamlines

Very low fluid velocities & smooth beds

Assumed for most subsurface flows

Entrainment

Turbulent flow Flow moves as a series of constantly

changing and deforming masses (eddies),includes movement perpendicular to themean flow direction

Instantaneous velocity varies around time-average value

Most flow of water/air is turbulent undernatural conditions


3/38

Boggs 2001

Entrainment

Reynolds Number (dimensionless)

Re= UL/

U mean flow velocity

L characteristic length (e.g., flow depth)

kinematic velocity

Ratio of inertial to viscous forces Low Re laminar flow, high Re turbulent

flow

Entrainment

A fluid moving above a bed exerts ashear stress at the bed surface

Boundary shear stress

Force per unit area parallel to the bed

Proportional to flow velocity, fluid density,scale/depth of flow, slope of stream bed


4/38

Entrainment

Several forces affect a grain ofsediment on a bed and below a flowing

fluid

Forces resisting motion

Gravity

Inter-particle friction

Cohesion/electrochemical bonds (clays)

Entrainment

Fluid forces

Drag component a function of boundaryshear stress

Lift component Bernoulli effect

Boggs 2001


5/38

Entrainment

In reality, the critical thresholdfor grainmovement also depends on:

Particle shape, size, sorting

Bed roughness

Cohesion

As such, thresholds have beendetermined experimentally, rather thanpredicted from calculations

Hjulstrom DiagramExperimentally derived

Applies to flow depth of 1 m

Boggs 2001

Sediment Transport

Once a particle has been set in motion,its transport path is a function ofparticle settling velocity, current velocityand turbulence

More energy needed to put a particle intomotion than to keep the particle in motion


6/38

Stokes Law

Defines particle settlingvelocity

Stokes Law only worksfor particles smaller than.1 (fine sand) in water.

Velocities for largerparticles overestimatedbecause of viscousturbulent drag in thewake of the settling grain

Leeder 1999

Sediment Transport

3 modes of particle movement:

Bedload continuous/regular contact withthe bed

Traction (rolling, sliding)

Saltation

Common for sand, gravel and coarser sediment

Sediment Transport


Suspended Load Particles held incontinuous suspension by fluid turbulence

Upward component of fluid flow overcomesgravitational forces

Finer grain sizes (very fine sand to clay)

Intermittent suspension possible


7/38

Sediment Transport


Washload clay-size particles derived from

an up-current source, rather than erodedfrom bed

Large washloads possible even for very lowvelocities

Grain Transport

Leeder 1999

Sediment Deposition

Sediment transported by fluid flow willbe entrained as long as the flow iscompetentenough I.e., velocity is high enough (air and water)

As a flow decelerates, it losescompetence and sediment is deposited Decrease in slope (e.g., rivers)

Spreading of flow (river deltas)

Etc.


8/38

Sediment Deposition

Sediment deposition may be temporaryor permanent

E.g., seasonal variations in discharge cancause deposition/erosion in rivers

Sedimentologists study sedimentarystructures (see later), sedimenttextures, etc. to infer transportmechanisms, depositional environment

Bedforms/Sed Structures

Primary sedimentary structures

develop during the processes of

transport, deposition and by a variety

of other means shortly thereafter. We

need to understand how these

features form in order to be able to

use them to deduce the depositionalenvironment of a sedimentary

deposit.

Boggs 2001


9/38


Contacts between adjacent beds

indicates change in environmental

conditions

1 bed does not necessarily imply 1depositional event


Currents flowing over a mobile bed ofsilt or coarser grain sizes will producebedforms

Type of bedform is a function of thenature of the current(unidirectional/oscillatory, velocity),sediment grain size and otherparameters (e.g., water depth)


Some overlap

Hysteresis effects

Modification of

bedforms during

changing flow

conditions


10/38

Boggs 2001

CurrentRipples

http://www.geo.uu.nl/Research/Sedimentology/Staff/mvhattum/flume.html


11/38

Current RipplesCurrent Ripples

Boggs 2001

http://www.geo.uu.nl/Research/Sedimentology/Staff/mvhattum/flume.html


12/38


13/38

Dunes: 3-D (Sinuous crested)

Scour pit

Trough cross-bedding Festooncross-bedding

Rib and furrow

cross-bedding

Dunes: 2-D (Sand waves)Superimposed

ripples

Planar tabular cross-bedding


14/38


Straight-crested (2-D) dunes -> planartabular cross-bedding

Sinuous-crested (3-D) dunes -> troughcross-bedding

Shape also depends on orientation ofoutcrop with respect to cross-bedorientation


15/38


Upper Plane Bed


16/38

Modern beach

Cretaceous beach

Herringbone cross-bedding/lamination

Opposing orientations (true)

Tidal indictor reversing currents



17/38


Ripples

Wavelength: 5 cm -> 20 cm

Height: 0.5 -> 3 cm

Planform: Straight -> sinuous

Bedforms/Sed StructuresDunes

Larger scale: Heights 10s of cm ->

several m (more for subaerial dunes),

wavelengths 10s of cm -> 10s of m

Fine sand or coarser

Planform: straight (sand waves) to

sinuous

Bedforms/Sed StructuresCross-beds: Sets & Cosets

Coset of tabular sets

Coset of trough

sets

Form sets


18/38

Like unidirectional currents, wave-

induced near-bottom currents willgenerate bedforms in mobile

sediments

Includes purely oscillatory motions,

asymmetrical motions, combined

flows


Waves

Previous discussion on sedimenttransport relates to unidirectionalcurrents

Surface gravity waves can also putsediment into suspension and causesediment transport

Waves, wave-induced currents

Waves

Formed by wind blowing over sea surface

Parameters: height, wavelength, period

Controlled by:

Durationover which wind blows (time)

Fetch distance over which wind blows

Velocity of wind

Water depth


19/38

Wave Transport

Deep Waves

Shallow Waves

Leeder 1999

Waves, wave-induced currents

Waves start interacting with the bottom whenthey move into shallow water (1/2 )

Circular particle motions in deep water replaced byback-and-forth motions (acceleration/deceleration)at seafloor

Acceleration induces near-bottom shear stressesthat can put sediment into suspension

Sediments become available for transport by othercurrents -> combined flows

Very important in shallow-marine, lacustrineenvironments

Classic straight crestlines, symmetrical

in profile, peaked or rounded crests

Crestlines normal to propagating waves

Interference patterns possible

Small features (few cm wavelength, < 1cm

height) in silt to large features (several m

wavelength, few m height) in gravels

Wave ripples



20/38

Wave ripples


Wave ripples

Asymmetric (shoaling) waves produce

asymmetric wave ripples

Similar to ripples produced by

unidirectional currents

Use morphology to distinguish

wave/current ripples



21/38

Wave ripples


Wave ripples

RI = L/H

4 15

RSI = L /LS L

2.5 3

L LL LS

H

Wave WaveCurrent Current


Shoaling wave ripples


22/38

Hummocky cross-stratification

Coarse silt to fine-grained sandstone

Generally sandstones interbedded with

shales

Wave-formed structures, but no modern

analogues?






23/38

Graded Bedding

Beds have coarser-grained sediments near

their bases, become finer-grained towards

their tops (normal grading)

Waning energy conditions

Turbidity currents, overbank deposits, etc.

Reverse grading also possible

Can be a way up indicator


Graded Bedding

More or less regular folds in sedimentation

units (beds, sets, etc.)

Often undeformed above and below

Syndepositional or post-depositional

Convolute bedding and lamination



24/38

Convolute bedding West Texas

Bedding planes (upper and lower) can

reveal much about depositional

environments



25/38

Erosional MarkingsDeposition of some beds is preceded by a

period of erosion (increasing thendecreasing velocity)

Sole marks on bases of beds often

generated during period of erosion

Bases of sandstones interbedded with

shales, positive relief structures

Episodic sedimentation


Flute marks (erosional)

Scour into cohesive bed by turbulent

eddies

Tightly curved, deep nose, open outward

and shallow in down-current direction

Various morphologies and sizes but

generally same morphology on any givebedding plane

Excellent paleocurrent & way-up indicators


Flute marks


26/38

Tool marks

Load Casts (Deformation)

Gravity acting upon density imbalances

between unconsolidated sediments

Generally sands deposited quickly upon soft

mud

Sand sinks into the underlying soft

sedimentRounded, irregular lobes of variable size

(mm -> m)


Load Casts

Load CastsPseudonodules

(Ball and Pillow)



27/38

Miscellaneous Structures

Raindrop imprintsDessication cracks (mud cracks)

Syneresis cracks - subaqueous shrinkage

of muds

Rill marks


Mudcracks

Mudcracks


28/38

Trace FossilsSedimentary structures formed by the

burrowing, boring, feeding, locomotionactivities of organisms (not body parts)

Ichnofossils

Good environmental significance:

organism behaviour a function of salinity,

energy, sedimentation rate, substrate,

water depth, etc.


Laminated-to-burrowed sandstone

Different types of organisms have similar

types of behaviour (crawling, burrowing, etc.)

Similar traces produced by different types of

organisms.

May not always be possible to determine

which organism responsible for a given

structure

Worms, crustaceans, bivalves, insects,

gastropods, trilobites, fish, dinosaurs, etc.



29/38

Trace Fossils

Configuration spatial relationships (orientation, etc.)

Spreite U-in-U, or spiral structures

Miscellaneous

Bioturbate texture extensive bioturbation, fewrecognizable

Bioturbation

Boring firm substrate

Burrow- loose, unconsolidated sediments

Burrow/boring system interconnected

Shaft vertical burrow/boring

Tunnel/Gallery horizontal burrow/boring

Burrow lining thickened burrow wall

Burrow cast

Burrows and Borings

Track single foot

Trackway many tracks

Trail continuous, surface or subsurface

Tracks and Trails

Descriptive/generic classification

Trackway

Trackway


30/38

Trackway

Teichichnus

Paleophycus

Chondrites

Arenicolites


31/38

Increasing Energy Level

Trace Fossils

Trace FossilsIchnofacies

Sediment Gravity Flows

Gravity can cause sediment transportMass-transport processes

Operative in subaerial and subaqueoussettings

Fluids may help suspend sediment, reduceinternal friction, but gravity key driver

Sediment gravity flows four types, eachwith a different grain suspensionmechanism


32/38

Turbid ity Liquefied Grain Debris

Current Flow Flow Flow

Turbulenc e Intergranular Grain Matrix

Flow Interaction Strength

Processes

Processes

Turbidity Currents Grains held in suspension by fluid

turbulence

Generated by submarine failures, riversentering lakes, etc.

Can transport sediment long distances(100s of km)

Slow/stop through mixing with ambientwater or change of slope

Turbidity Currents


33/38

E Pelagic

D Planar laminated

C Ripple cross-

laminated

B Planar laminated

A - Massive

E Pelagic

D Planar laminated

C Ripple cross-

laminated

B Planar laminated

A - Massive

An Ideal Turbidite Bouma Sequence

Processes

Turbidites

Sedimentary structures record waning currents

Commonly normally graded

Thickness variable cm to 10s of cm; tabularbeds

Complete Bouma Sequences not alwaysdeveloped

Use shorthand notation

E.g., ABC, BC, A, ACE


34/38


35/38

Graded carbonate turbidites West Texas

Processes

Fluidized/liquefied flows

Grains held in suspension by intergranular flow(loss of grain contacts)

Pore fluids escaping upward

Loosely packed sands subjected to a shock

Flow freezes from bottom up as it slows and

sediment is redeposited

Dish structures


36/38

Processes

Grain Flows

Grains held in suspension by grain-to-grain

collisions (dispersive pressure)

Relatively steep slopes

Flow freezes from bottom up as it slowsand sediment is redeposited

Processes

Debris Flows

Grains held in suspension by matrixstrength (suspended fines)

Traditionally thought that clays wereneeded, now known not to be true

Wide range of grain sizes transported if

available


37/38

Processes

Four end-member types gradation from onetype to another

Hybrid flows Changes with time, location

Other classification schemes possible

Most experimental work done in 60s and 70s new work changing some ideas

Relatively low submarine slopes needed, & triggermechanism

Cohesive mass movements slides, slumps, etc.

Summary

Competence of a fluid to entrainsediment is a function of severalvariables

Properties of fluid e.g., velocity, depth,density

Properties of particle size, shape,relationship to other grains, etc.


38/38

Summary

Once a particle is set in motion, it maybe transported in a variety of ways Bedload, suspended load, washload

Particles are deposited when flow losescompetence E.g. though deceleration

Deposition can be temporary orpermanent

Summary

As sediment is transported, it may bemolded into a variety of bedforms

Type of structure that forms is afunction of fluid velocity and grain size Phase diagrams used to predict conditions

under which a particular bedform will form

Bedform migration generatescharacteristic sedimentary structures

Summary

Sediment may be transported/depositedby unidirectional flows, waves andwave-induced flows and gravity flows

Characteristics of sediment deposit used toinfer depositional processes, and thereforeenvironment

Documents

2 - Sediment Transport