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River Processes, Landforms & Measurements in the river
channel
River Processes & Landforms• Energy is needed in any system – for transfers to take place
• 95 % of a rivers energy is needed to overcome friction
• Most friction occurs on the wetted perimeter (see slides) –where the water comes into contact with the banks and the river bed
• If a river or stream has many rocks in it (like in the upper course), this creates a lot of friction, and the water flows less quickly
• After heavy rain, or a confluence – river volume is increased, less water is in contact with the wetted perimeter, friction is reduced and the river increases its velocity
• The river can then pick up and transport more material, this material is called its load
Upper course
Middle course
Lower course
THE RIVER PROFILE
http://www.youtube.com/watch?v=ptVasZziom8 http://www.slideshare.net/TonyCassidy/pop-river-basin
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Where do you thinkthese landforms will be found,the upper, middle, lower course of the river?
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Processes of transportation in the course of a river
• Rivers transport loads through four processes-
• Traction and saltation along its bed, suspension & solution, within the river itself
movie showing the process of saltation.
http://library.thinkquest.org/28022/transport/traction.html
Activity: Complete the Processes of Transportation tableLook at the diagram on P.282 to help you
est/
microscopic
er
How is material transported downstream?
How is material transported downstream?
Mini presentations: using the slides present on the following
• Transportation processes• Erosion processes• Deposition• V shaped valleys• Waterfalls and gorges
•You will come up to the front – without notes and present using the slides, make sure everyone contributes, use pages 282-283 in the textbook for additionalinformation (participation grade)•Complete the packet as we go through the presentations
Processes of erosion
• Activity: Show the four processes of erosion in the boxes.
• Try to use graphical representation as it will help you remember.
• Brief descriptions are also required.
Attrition
Hydraulic action
Corrasion
Corrosion
Activity: Use P.282 to complete this – then in pairs explain the processesto each other
Attrition• When Boulders and other
material, which are being transported along the bed of the river, collide and break up into smaller pieces.
• This is more likely to occur when rivers are still flowing in highland areas
Hydraulic Action
• When the sheer force of the river dislodges particles from the river’s banks and bed.
Corrasion/ Abrasion• When smaller material,
carried in suspension, rubs against the banks and bed of the river.
• This process is more likely in lowland areas by which time the material will have been broken up small enough to be carried by suspension.
• River banks are warn away by a sand papering action which is also called abrasion
Corrosion
• Acids in the river also dissolve rocks, such as limestone, which form the banks and bed. This can occur at any point in the rivers course
What are the categories of load?
And again!
‘To erode, transport or deposit’: That is the question!
The Hjulstrom Curve
Do you know the difference between erosion and transport?
The River Conwy has deposited material in this section of its course. Suggest reasons why this has happened.
Deposition
Erosion Clips:• http://www.youtube.com/watch?v=YlDSU1aUpt8
&feature=related (Abrasion)
• http://www.youtube.com/watch?v=c4T5p8Sdkg4&NR=1 (corrosion)
• http://www.youtube.com/watch?v=1cW1n_S5W6Y&feature=related (Attrition)
• http://www.youtube.com/watch?v=cUsefpOWIgc&NR=1 (Hydraulic action)
Deposition
• Deposition occurs when a river lacks enough energy to carry its load. Deposition, beginning with the heaviestmaterial first, can occur following a dry spell when the discharge and velocity of the river drop.
• It also occurs where the current slowsdown the inside of a meander bend or where the river enters the sea .
Deposition
Disposition • Activity: Look at P. 283
What are the main processes that operate in the middle and lower course of a river?What landforms are found in the middle and lower course of a river?How are these landforms created?
Lear
ning
obj
ectiv
es
What landforms are found in the middle and lower course?
1) Meanders
2) Oxbow Lakes
3) Flood plains and Leveés
4) Deltas
DEPOSITION created these landforms
DEPOSITION and EROSION create these landforms
Deposition is the geological process by which material
is added to a landform or land mass
Quick Quiz1. What must all systems have and why is it needed? 2. What is most of the energy in a river needed for? 3. What is the name of material transported by a river?4. Explain the process of transportation of traction5. Describe the erosion process of hydraulic action6. Describe the erosion process of corrasion7. Describe the erosion process of corrosion 8. Are the size of the particles transported by saltation
large or small?9. 8) Explain the process of transportation of
suspension10.9) Deposition is when the river deposits its
• A meander is a large bend in river, as the river approaches its mouth it develops increasingly large bends known as meanders
• They constantly change shape and position• When a river reaches a meander most of the
water is directed to the outside of the bend –this erodes that outside bank by corrasion (the sand papering action – particles rubbing on the bank)
• The outside bank will then be undercut
River LandformsMeanders & Ox-Bow Lakes
Real Meanders 1
Meanders
River Meanders 4
Meanders
http://www.bbc.co.uk/northernireland/landscapes/lev2_rivers_meanders_script.shtml
http://www.slideshare.net/juicygeography/oxbow-lakes
Explain the following hypotheses:
The fastest current is always on the outer bend.
All meanders have river cliffs and slip-off slopes.
Meanders
A cross section through a meander
A cross section through a meander
Meander bend on the River Conwy
A B
Explain why there is more deposition at ‘A’ rather than at ‘B’.
A cross section through a meander
Ox bow lakes 1List all the landforms You can see in this image
Ox bow lakes 2
CLIPS
• http://geovideos.fliggo.com/video/HNY7QOzW
• http://geovideos.fliggo.com/video/v64HN28M
• http://www.youtube.com/watch?v=THTasJHgGAE
Meander = a bend in a river
A Meander
flow
Activity: As we go through, label your own
Meander neck
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21606How an oxbow lake forms
Slip-off slope
River cliff
Fastest current
Slowest current
Deepest water
Shallower water
Outside bank
Inside bank
Most erosion
Deposition
Oxbow lakes
Meander neck becomes smaller
new course of the river
oxbow lake
When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.
Explain the formation of an oxbow lake
• Explain how the river in the photograph might form an oxbow lake. Add arrows to the diagram to improve your answer.
• Include the following terms in your answer:-• Fastest flow – on the outside of the bend • Erosion – leading to undercutting of the outside of the bend
(corrasion, hydraulic action)• Meander neck – eventually cut through in a flood to shorten the
course of the river• Deposition – on the inside of the bend – blocks off the previous
course
Ox Bow Lakes
• Explain how the river in the photograph might form an oxbow lake. Add arrows to the diagram to improve your answer.
• Include the following terms in your answer:-• Fastest flow • Erosion • Meander neck • Deposition
Ox Bow Lakes
Meander Review
Activity: Label these in two groups
Slip-off slope
River cliff
Fastest current
Slowest current
Deepest water
Shallower water
Outside bank
Inside bank
Most erosion
Deposition
Shallower water
Slip-off slope
Most erosion
Fastest current
Deposition
Outside bank
River cliff
Inside bank
Slowest current
Deepest water
flow flow
Upper course
Middle course
Lower course
THE RIVER PROFILE
http://www.youtube.com/watch?v=ptVasZziom8 http://www.slideshare.net/TonyCassidy/pop-river-basin
The difference in wetted perimeter between the upper and lower course of
the river
Wetted perimeter: the part of the bed and banks which is in contact with the water in the channel – how do you think hydrologists measure this?
What is the Wetted Perimeter?
The yellow line shows the wetted perimeter
Wetted perimeter: the part of the bed and banks which is in contact with the water in the channel – how do you think hydrologists measure this?
This river has a higher wetted perimeterin comparison to its volume,which increases friction and reducesVelocity (upper course)
This river has a smaller wetted perimeter in comparisonwith its volume, because it hassmooth banks, friction is reducedand this allows velocity to increase(middle/lower course)
Rocks and boulders
5 meters
50 meters
The difference in the wetted perimeter of a river between the upper and Lower course of a river
http://www.youtube.com/watch?v=mgBqfcMK4jI
Calculating the wetted perimeter• This can be done using a weighted tape
measure• If the stream has a rectangular shape, with no
rocks and boulders in the channel like the one below
• The wetted perimeter can be calculated by multiplying the depth by 2, and adding the width
Av. Depth10 meters
Width20 meters
Wetted perimeter=
10 x 2 = 2020 + 20 = 40 meters
Wetted perimeter: the part of the bed and banks which is in contact with the water in the channel – how do you think hydrologists measure this?
Hydraulic radius (Rh): the ratio between the area of the cross-section of a stream and the length of its wetted perimeter – it measures channel efficiency
• A square-shaped stream cross-section has a larger hydraulic radius than a shallow, rectangular-shaped stream cross section
• The higher the hydraulic radius, the faster the river flow (Velocity) and the more efficient the river is, because there is less contact between the water in the channel and the bed and banks, so lower friction.
(cross sectional)
Velocity & Discharge
• Discharge is the amount of water that passes a given point on the stream every second- measured in cubic meters a second (cumecs)
• Velocity is the speed of the water moving, measured as the distance traveled every second.
Measuring discharge in Cumecs (m³/s )• In order to calculate the discharge of a river in
cumecs (cubic meters per second), you need to calculate the cross sectional area (m) (width x depth) and times this by the velocity (speed in meters a second)
Depth = 1.5 meters
Width = 10 meters Speed of flow (velocity) = 10 meters a second
Discharge in cumecs=15 x 10 = 150 cumecs
10 x 1.5 = 15 (cross sectional area)
Av.Depth = 3 meters
Width = 12 meters
Av.Depth = 4.3 meters
Width = 32 meters
Av.Depth = 9.6 meters
Width = 56 meters
Av.Depth = 0.8 meters
Width = 2 meters
Calculate the:1) Cross sectional Area2) Discharge in cumecs 3) The wetted perimeter4) Hydraulic radiusof these hypothetical rivers (show your working out)
1)
2)
3)
4)
Av.Depth = 1.7 meters
Width = 23 meters
5)
Speed of flow (velocity) = 15.6 meters a second
Speed of flow (velocity) = 11.3 meters a second
Speed of flow (velocity) = 34 meters a second
Speed of flow (velocity) = 8 meters a second
Speed of flow (velocity) = 16 meters a second
Av.Depth = 7 meters
Width = 43.7 meters
Av.Depth = 5.8 meters
Width = 32.1 meters
Av.Depth = 11.3 meters
Width = 68 meters
Av.Depth = 15.9 meters
Width = 2.8 meters
Calculate the:1) Cross sectional Area2) Discharge in cumecs 3) The wetted perimeter4) Hydraulic radiusof these hypothetical rivers (show your working out)
6)
7)
8)
9)
Av.Depth = 2.7 meters
Width = 23.7 meters
10)
Speed of flow (velocity) = 0.2 meters a second
Speed of flow (velocity) = 2 meters a second
Speed of flow (velocity) = 2 meters a second
Speed of flow (velocity) = 3.0 meters a second
Speed of flow (velocity) = 2.8 meters a second
Data Analysis1. Which is the most efficient stream?2. Which stream has the narrowest channel?3. Which stream has the largest wetted perimeter?4. Which stream has the highest discharge in cumecs?5. Which stream has the lowest discharge in cumecs?6. Which stream has the deepest channel?7. Which stream has the widest channel?8. Which stream has the fastest velocity?Bonus: Which stream would be causing the most erosion
and transporting the most load? Why?
Write a sentence describing how you calculate each of these:
• 1) Cross sectional Area• 2) Discharge in cumecs • 3) The wetted perimeter• 4) Hydraulic radius
Quick Quiz – answer in pairs1) What must all systems have and why is it needed? 2) What changes can increase the volume of a river?3) What does increased velocity allow the river to now do?4) What is the name of material transported by a river?5) What is the wetted perimeter?6) How is the wetted perimeter different in the upper and lower
course of the river?7) How does the wetted perimeter influence channel flow?14) How might hydrologists measure the wetted perimeter in
fieldwork?15) How does the transportation process of traction work?16) How does the transportation process of saltation work?17) How does the transportation process of suspension work?18) How does the transportation process of solution work?
