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S
EDIMENT

E
ROSION
,T
RANSPORT
,
D
EPOSITION
,
AND

S
EDIMENTARY

S
TRUCTURES


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.

S
EDIMENT

D
YNAMICS

S
EDIMENT

TRANSPORT


Fluid Dynamics



COMPLICATED


Focus on basics


Foundation


NOT comprehensive

S
EDIMENTARY

C
YCLE


Weathering


Make particle


Erosion


Put particle in motion


Transport


Move particle


Deposition


Stop particle motion


Not necessarily continuous (rest stops)

D
EFINITIONS


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

D
EFINITIONS


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


r
H
2
0

>
r

air



D
EFINITIONS


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


T
YPES

OF

F
LUIDS
:

S
TRAIN

(
DEFORMATIONAL
) R
ESPONSE

TO

S
TRESS

(
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

W
HY

DO

PARTICLES

MOVE
?


Entrainment


Transport/ Flow

E
NTRAINMENT



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


B
ERNOULI

F
ORCE


(r
gh) + (1/2
rm
2
)+P+E
loss

= constant


Static P + dynamic P


Potential energy=
r
gh


Kinetic energy=
1/2
rm
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


F
LOW


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

G
EOLOGICALLY

S
IGNIFICANT

F
LUID

F
LOW

T
YPES

(P
ROCESSES
)


Laminar Flows:



straight or boundary parallel flow lines


Turbulent flows:



constantly changing flow lines. Net mass transport in the
flow direction

F
LOW
:
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)


R
EYNOLD

S

#



R
e

= 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)


G
EOLOGICALLY

S
IGNIFICANT

F
LUID

F
LOW

T
YPES

(P
ROCESSES
)


Laminar Flows:



straight or boundary parallel flow lines


Turbulent flows:



constantly changing flow lines. Net mass transport in the
flow direction

G
EOLOGICALLY

S
IGNIFICANT

F
LUIDS

AND

F
LOW

P
ROCESSES


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)

W
HAT

ELSE

IMPACTS

F
LUID

F
LOW
?


Channels


Water depth


Smoothness of Channel Surfaces


Viscous Sub
-
layer

1. C
HANNEL



Greater slope = greater velocity


Higher velocity = greater lift force


More erosive


Higher velocity = greater inertial forces


Higher numerator = higher R
e


More turbulent

2. W
ATER

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. W
ATER

D
EPTH


Boundary Shear stress
(

o
)
-
stress that opposes the
motion of a fluid at the bed surface

(

o
) =
g
R
h
S



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. W
ATER

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. S
MOOTHNESS


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

F
LOW
/G
RAIN

I
NTERACTION
:

P
ARTICLE

E
NTRAINMENT

AND

T
RANSPORT


Forces acting on particles during fluid flow


Inertial forces,
F
I
, inducing
grain immobility

F
I

= gravity + friction + electrostatics



Forces,
F
m
, inducing grain
mobility

F
m
=
fluid

drag force + Bernoulli force

+ buoyancy


D
EPOSITION



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

G
RAINS

IN

M
OTION

(T
RANSPORT
)


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


T
RANSPORT

M
ODES

AND

P
ARTICLE

E
NTRAINMENT


With a grain at rest, as flow velocity increases

F
m


>

F
i
; initiates particle motion


Grain Suspension

(for small particle sizes, fine silt; <0.01mm)


When
F
m

>

F
i


U (flow velocity)

>>> V
S
(settling velocity)


Constant grain
Suspension

at relatively low U
(
flow velocity)


Wash load

Transport Mode



T
RANSPORT

M
ODES

AND

P
ARTICLE

E
NTRAINMENT


With a grain at rest, as flow velocity increases

F
m


>

F
i
; initiates particle motion


Grain Saltation

: for larger grains (sand size and larger)


When
F
m

>

F
i



U


>
V
S


but through time/space
U

<

V
S


Intermittent Suspension


Bedload Transport Mode



T
HEORETICAL

B
ASIS

FOR

H
YDRODYNAMIC

I
NTERPRETATION

OF

S
EDIMENTARY

F
ACIES


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

F
LOW

R
EGIME

AND


S
EDIMENTARY

S
TRUCTURES


An Introduction To

Physical Processes of Sedimentation

S
EDIMENTARY

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)


S
EDIMENTARY

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


B
ED

R
ESPONSE

TO

W
ATER

(
FLUID
) F
LOW



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

S
EDIMENTARY

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


B
ED

R
ESPONSE

TO

S
TEADY
-
STATE
,
U
NIDIRECTIONAL
, W
ATER

F
LOW


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)

Q
UESTION
?

T
EST



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.