3717 - river processes presentation
TRANSCRIPT
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Paul Warburton. Head of Geography. Manchester High School for Girls.
This presentation should help teachers and students involved with AS
and A2 Level Geography.
Important terms are highlighted, particularly the first time they are
used. Students will need to be able to define these terms and usethem in examination answers.
Use the left and right arrow keys unless otherwise instructed.
Also includes variations along a long profile and
Hjulstrm Curves
ZigZag Education 2010
Photocopiable/digital resources may only be copied by the purchasing
institution on a single site and for their own use
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This is the wearing down of the landscape by a number offluvial processes. These can lead to the development of
erosion landforms. Click on each box below to find out
more about each process.
ABRASION
HYDRAULIC ACTION
ATTRITION
CORROSION
When you are ready to move on use the right arrow or click here.1
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This takes place in a number of different ways click on the bars below
to find out more about each process.
SALTATION
SUSPENSION
BED or TRACTION LOAD
When you are ready to move on use the right arrow or click here.
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There are two important concepts that relate to
transportation in rivers two terms which you willneed to learn.
COMPETENCE This refers to the largest particle
that can be carried in a river at a given velocity.
CAPACITY This refers to the total amount of load
that can be carried in a river at a given velocity.
When will capacity and competence be at their
maximum?
We will finally look at Deposition next slide
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Deposition is relatively straightforward. A river deposits material as the velocity or
discharge slows. Material becomes too heavy to be carried and is dropped.
On this meander the water is flowing slower on the
inside and deposits have accumulated.
In this estuary water is slowing as it meets the sea
and there is plenty of visible deposition. An estuary
is tidal so at low tide the banks of silt and sand are
exposed.
Here braiding has occurred. This is common
towards the end of a river or where it flows across
flatter land and slows. Deposition creates small
islands called eyots. The course is split into many
separate channels.4 Source: US Geological Survey
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Hopefully you will have realised by
now that rivers carry out much oftheir work during bankfull
conditions or when a river floods.
Then discharge is high, the river is
carrying a greater load and
erosion can be carried out more
effectively.
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The rate of formation of erosional landforms increases under
these conditions. Some depositional landforms like levees are
formed when rivers flood. Some of the characteristics of aflood plain are again associated with flooding.
Use your AS textbook to understand this more fully.
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The Hjulstrm curves are based
on laboratory experiments and
they show the relationship betweenvelocity and particle size and how
they interact to affect erosion,
transportation and deposition in a
river. A number of basic ideas are
easy to grasp.
As the river slows particles aremore likely to be deposited.
As velocities increase particles
start to be transported.
At higher velocities particles are
carried at a velocity that enable
them to erode.
Very fine particles of clay and silt need higher velocities than might be expected to disturb
these sizes as they are more cohesive (stick together). They are also so small that they need
higher velocities to be effective in erosion.
The next slide will ask you some questions to see how well you have understood the graph6
Cmsper
second
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QUESTIONS
Think about each question then press the right arrow
key for the answer and again for the next question.
1. At what velocity would a 0.1mm sand particle start
to carry out erosion?
About 25cm/s
2. At what velocity would a 0.01mm silt particle start to
be transported?
About 0.1cm/s
3. Why do gravel particles need a higher velocity to
carry out erosion than sand particles?
Gravel particles are heavier than sand, more energy
(velocity) is needed to lift and carry these particles
so that they can carry out erosion.
4. Does the graph reflect competence or capacity?
Competence because this is about the size of particle that can be carried at a given velocity, i.e.
the relationship between the two. Capacity is the amount that can be carried at a given velocity
this is not in the graph there is no measure here of quantity or volume of load.
We will finally see how these processes vary along the course of a river.7
Cmsper
second
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THE LONG PROFILE OF A RIVER (Cross profiles follow)
The long profile of a river is the course from the source to the mouth. In
reality it is clearly not as smooth as shown here, but all rivers are wearing
down the landscape and moving towards this theoretically smooth orgraded profile. Each stage is marked by dominant processes and typical
landforms these are outlined in a summary on the next slide. Of course
the processes and even landforms can be found in different stages but are
either at a different scale or are less common.
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UPPER MIDDLE LOWER
PROCESSES PROCESSES PROCESSES
Dominated by vertical
erosion
Lateral erosion becomes
more significant
Deposition on inside of
bends
Deposition becomes
dominant as river
slows particularly
near the mouth
LANDFORMS LANDFORMS LANDFORMS Irregular course small
waterfalls and rapids
V shaped valleys as
river erodes vertically
Interlocking spurs
Valley sides more
gentle
Rivers develop more
pronounced meanders
Flood plain develops
Pronounced
meanders
Levees
Braiding may occur
where river slows and
is carrying a high load
Estuary at mouth
Deltas
The next slide takes you to the end
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CHANNEL CROSS PROFILES
This refers to the cross sectional shape of a rivers
channel. The cross sectional area is obtained by
multiplying channel width by the average depth.
UPPER STAGE
In the upper stage, downstream from the source of a
river, it tends to be relatively shallow and irregular. As
this photo shows, the channel contains many
boulders and pebbles it is a rough channel.
The channel roughness leads to friction and lowervelocities. The discharge is low except in times of
flood.
Erosion is slow and is largely in a vertical direction.
A V-shaped valley is slowly developing.
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Click in this box for details of Mannings Roughness Coefficient
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Width/depth ratio and hydraulic radius
One way of describing channel cross section is to consider the width/depth ratio.
This involves measuring the width of the channel and dividing this by the
average depth. Some simple calculations will reveal that the ratio decreases as
the channel becomes more efficient in its shape.
A further way of measuring channel efficiency
is to calculate the hydraulic radius. This
involves dividing a channels cross sectional
area (in m2) by the wetted perimeter (in m).
The wetted perimeter (red line) is the length of
the bed and banks in contact with water. Thegreater the hydraulic radius, the greater the
efficiency of the channel and the less likely
the river is to flood. The highest values occur
when channels are deep, narrow, and semi-
circular in shape.
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MIDDLE STAGE
With distance downstream the
channel becomes deeper and the
depth increases relative to the
width. This is a more efficient
channel as a smaller proportion of
the water is near the bed and
banks. So even though the gradient
may be less, discharge and velocity
increase as there is less friction to
slow the water.
You may also be able to see thatthe channel is less rough this also
reduces friction and encourages a
higher velocity and discharge.
It is useful at this point to introduce two measures of channel efficiency.
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Erosion is now greater in a lateral direction. A
river will have a more meandering course and a
flood plain will develop.
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LOWER STAGE
Towards the end of a river the
channel is smooth and often deep
relative to the width and continues
to be efficient.
Towards the sea though (or a lake)this can change. As a river slows
and loses its energy, deposition
becomes a more dominant
process. The channel can become
shallower and islands of deposits
develop. If a river flows into thesea the mouth will be tidal and a
wide estuary may develop. At low
tide extensive mud and sand flats
will be seen.This is the mouth of the river Mawddach near
Barmouth in North Wales.
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Suggest reasons why the discharge of a
river would be higher in the lower stage.
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UPPER MIDDLE LOWER
Irregular channel, rough
many boulders,
pebbles
Shallow relative to width
Inefficient channel
High width/depth ratio
Low hydraulic radius
Smoother channel
Deeper relative to its
width
More efficient
Lower width/depth ratio
Higher hydraulic radius
Smooth channel
Deep relative to width
Efficient channelalthough reduces towards mouth
Low width/depth ratio
Higher hydraulic radiusalthough lower towards mouth
A SUMMARY OF CHANNEL PROPERTIES AT STAGES
IN THE LONG PROFILE OF A RIVER
14 The next slide takes you to the end of this presentation.
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End the presentation
(move the cursor and click on the square symbol in
the bottom left of the screen then End Show)
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Corrosion is most effective on rocks that contain carbonates such as
limestone. Minerals in the rock are chemically broken down in water
that is slightly acidic and carried away in solution.
The formula is CaCO3 + H2CO3 Ca(HCO3)2Limestone Acidic water Calcium hydrogen carbonate (soluble)
Gordale
gorge in
the
Yorkshire
Dales
Where the watertable reaches
the surface
streams can
flow in
limestone areas even though
the rock is
permeable.
CORROSION
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ATTRITION
This occurs when particles rub and grind against each other in moving water.
Particles gradually become smoother, smaller and more rounded.
What would most particles be like in the upper stage of a river?
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HYDRAULIC ACTION
Turbulent water has a hammering and disturbing effect on the bed and banks. Air
can be trapped, compressed and suddenly released this also helps to break up
loosely consolidated material.
NB the combination of hydraulic action and abrasion below a waterfall is called
CAVITATION this leads to the development of a plunge pool.
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ABRASION
The river cliff here has been created by a combination of hydraulic action and
abrasion. Abrasion is the sandpaper/abrasive effect of material carried in the water rubbing against and impacting on the banks.
Under what circumstances do you think this process would be most effective
/ at its maximum?
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SALTATION
Saltation occurs where particles are too heavy to be carried in suspension
(entrained) but are light enough to be lifted by the flowing water. Particles
therefore bounce along the channel. Such particles disturb others when
they fall to the bed and set off a general movement of material downstream.
This process largely occurs with smaller particles.
What would enable larger particles to be moved in this way?
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SUSPENSION
Suspension occurs when particles are small enough to be lifted andentrained (held in) the moving water. Suspended material often gives a river
a colouration usually brown but other colours depending on the colour of
the sediment. The Yellow River in China (above) is stained here with loess
(wind blown soil) carried in suspension.
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BED or TRACTION LOAD
This is material often rocks
and boulders that is too large
to be lifted from the bed.
Instead it is rolled and dragged
along by friction with, and by
the force of, the moving water.
When or under what conditionswould these larger boulders be
moved?
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MANNINGS ROUGHNESS COEFFICIENT
Manning was an engineer who developed a formula for calculating channel
roughness.
Where Q = discharge, A = cross sectional area, R = hydraulic radius, and S =
slope.
The formula gives a useful approximate value, the higher it is the rougher thebed and banks of the channel.
A(R x S)Q =
n
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