132 FLUVIAL PROCESSES AND LANDFORMS_Handout

2011/09/15
Fluid agents
EWS132
FLUVIAL PROCESSES AND
LANDFORMS
Some basic concepts
• Running water is one of the four fluid
agents that carry out denudation
• The others are glacial ice, waves and
currents, and wind
• Running water (fluvial action) is by far the
dominant agent shaping the land surface
Surface water flow
• Two types of surface flow
• Before we go on to fluvial processes and
landforms, lets look at some basic
concepts!
– Overland flow occurs when rain falls too
rapidly to infiltrate and runoff occurs on a
smooth surface as a continuous thin film
called sheet flow
– Overland flow eventually contributes to
stream flow which is a much deeper, more
concentrated form of runoff
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Streams and Channels
• Streams can be defined as long narrow
bodies of flowing water occupying
trenchlike depressions or channels and
moving to lower levels under the influence
of gravity
• The channel of a stream is a narrow
trough, shaped by the forces of flowing
water to its most effective form for moving
water and sediment
Turbulent flow
• Moving water is usually affected by
turbulence
• In turbulent flow, water molecules follow
irregular paths and mixing between
adjacent layers involves transfer of
momentum by large scale eddies
• A water molecule will therefore travel a
highly irregular corkscrew path moving in a
upward, downward and sideway direction
Flow velocity
• Friction near the channel
bed and sides slow
down the movement
• In straight and
symmetrical channels
the highest velocity is
located in midstream
• In a curved stream
maximum velocity shifts
toward the bank on the
outside of the curve
Stream discharge
• Stream flow at a given
point is measured by its
discharge
• Discharge is defined as
the volume of water per
unit time passing through
a cross section of the
stream at that location
and is measured as cubic
meters per second (m³/s)
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Effect of gradient
• Cross-sectional area
and average velocity
of a stream can
change over a short
distance even though
the stream discharge
does not change
• These changes occur
because of changes
in the gradient
Landforms
Q = AV = Constant
Denudation
• Denudation is the total action of all
processes by which rock surfaces are
worn down and the resulting sediments
transported to the sea, i.e. an overall
lowering of the land surface
• We refer to landforms shaped by running
water as fluvial landforms
• Erosional landforms are
forms resulting from the
progressive removal of
bedrock mass
• Depositional landforms
are forms resulting from
transported material
deposited elsewhere
away from their original
position
Soil erosion
• Fluvial action starts with soil erosion from
overland flow
• Soil erosion is a natural process that always
occurs where flowing water is active and where
precipitation occurs
• Under stable conditions, soil erosion is slow and
in balance with the soil forming processes,
allowing plant communities to maintain
themselves
• Soil scientists refer to this state as the geologic
norm
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Erosion (Cont.)
Erosion (Cont.)
• Accelerated erosion is when more soil is
removed than is formed, usually due to
human activities that bare the ground from
a protective cover
• Splash erosion is the mobilization of soil
particles through the direct force of falling
raindrops
• Up to 225metric tons of soil per hectare
can be disturbed by a single rainstorm
• Splash erosion can seal the natural soil
openings and so reduce infiltration
• Reduced infiltration permits a greater
depth of overland flow which in turn
enhances the rate of soil erosion
• The term sediment yield (metric tonnes
per hectare) is used for the amount of
sediment removed by overland flow from a
unit area in a given unit of time
Erosion (Cont.)
Erosion (Cont.)
• Sheet erosion is the
removal of soil in thin
uniform layers by
overland flow
• Rill erosion is erosion
in which many closely
spaced channels are
scored into the soil on
steeper slopes
• If rills are not destroyed
by soil tillage they can
soon join together into
gullies which are steepwalled canyonlike
trenches
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Erosion (Cont.)
• In arid environments
erosion rates are
normally high and
where clay formations
dominate, badlands
may develop
Badlands, Bryce Canyon,Utah
Negev, Israel
Wyoming
The geologic work of streams
Stream Erosion
• Occur through four processes
• Erosion
• Transportation
• Deposition
– Hydraulic action
– Abrasion
– Bank caving
– corrosion
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Hydraulic action
Forces involved in hydraulic action
• Hydraulic action is the force of flowing
water that sets up a dragging action on the
bed and bank material
• Two forces play a role
– Lift force
– Surface drag force
Abrasion
• Lift force is operating
at right angles to the
streambed and is
related to the Bernoulli
effect
• Surface drag force is
related to the
difference in hydraulic
pressure on opposite
sides of the particle
Pothole, Ontario, Canada
• Abrasion is the
grinding action of
particles against each
other and the wearing
away of material.
Potholes of Bourke’s
Luck in Mpumalanga
are good examples of
abrasion
Hawaii
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Bank Caving
Corrosion
• Bank caving is the
process of
undermining of river
banks that causes
banks to slump into
the river
Cole River, UK
• Corrosion is the
chemical processes
of rock weathering
that chemically
remove material from
the stream channel
Goodwin Creek, Massachusetts, USA
Cave Entrance, Belize, Mexico
Camooweal caves, Queensland
Montenegro, Yugoslavia
Copyright © Jelena Calic-Ljubojevic 2002
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Stream Transportation
Example of a bed load channel
• Stream load can be carried in three ways
– As dissolved matter and transported invisibly
as chemical ions
– As bed load close to the bed of the channel
by rolling or sliding
– As suspension load when particles float in the
water often giving the water a yellow muddy
colour
Example of a bed load channel
Bed load channel, Costa Rico
Chamonix Valley, France
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Suspended load in ephemeral river,
Namibia
Test site, Negev Dessert, Israel
Suspension load 80000 ppm
Yellow River, China
Confluence of Colorado and Green
River, Utah
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Stream capacity
• Stream capacity is the maximum load a stream
can transport at a particular discharge
• A stream’s capacity to carry suspended material
or to move bed load increases according to the
third or fourth power of its velocity
• This means that when the stream velocity
doubles, its capacity to transport bed load is
increased from eight to sixteen times
• When water flow increases during the flood
stage, rivers will widen and deepen their
channels and deposit material again when the
water slackens
Graded streams (cont.)
Graded streams
• Graded streams are
streams in equilibrium,
meaning they can
carry the sediment
load supplied to them
by their drainage basin
• Graded streams have
smooth longitudinal
profiles that will flatten
with time
Young river without a floodplain
• Geomorphologically young streams have steep
gradients and show deep gorges, waterfalls and
rapids and are still in the process of reaching an
equilibrium or a graded profile
• The first indication that streams are becoming
graded, is when gradients become flat enough
for floodplains to develop i.e. when lateral
erosion starts
• When this happens rivers will start to meander
and over time they will form meanders, cutoffs
and ox-bow lakes
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Relatively narrow floodplain
Example of well-developed
floodplain
Floodplain
Relatively wide floodplain with
meandering
Well-developed meandering,
cutoffs and oxbow lakes
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Deposition
• Graded streams are very sensitive to
upstream changes such as vegetation or
climatic change
• When bed load increases, additional
sediments will be deposited in the channel
• This will raise the elevation of the channel
bed, a process called aggradation
Braided channel
Braided channels
• Aggradation disrupts the
channel by depositing
material in the channel to
form braided channels
• Braided channels are
generally shallow and
characterized by a
network of interconnected
converging and diverging
channels
Braided ephemeral channel
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Anastomozing channels
• Anastomozing stream
patterns are patterns
where channels
bifurcate, branch and
rejoin irregularly to
create a net-like
formation
Degradation
Levees
• Deposition during overbank flooding can
result in building up of natural levees
alongside the river i.e. sand and silt
deposited on both sides of the channel in
the form of ridges
• Yazoo streams are streams that are
prevented from joining the main stream
after a flood has passed because natural
levees prevent them from doing so
Degradation on alluvial fan
• The opposite of
aggradation is
degradation, a
process whereby the
channel is deepened,
very often resulting
from a decrease in
bedload
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Alluvial fan as depositional form
Terraces
• Lateral cutting into an
alluvial deposit will
lead to formation of
terraces
Alluvial terraces
Entrenched meanders
• Tectonic uplift can
cause a meandering
river to incise its
channel to form
entrenched meanders
San Juan River, Utah
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Entrenched meander, Fish River,
Namibia
Stream rejuvenation
• Stream rejuvenation is the renewed incision of a
stream
• Three types of rejuvenation
– Dynamic rejuvenation resulting from epeirogenic uplift
– Eustatic rejuvenation resulting from worldwide sealevel changes. Two types:
• Diastrophic eustatism
• Glacial eustatism
– Static rejuvenation resulting from
• Decreasing bedload
• Increase in discharge due to climatic change
• Increase in discharge due to stream piracy
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