1. Channel Processes and Hjulstrom’s Curve
GEOG 1: types of erosion, transportation and deposition, types of load; the Hjulstrom curve.
GEOG 2: Graphical = Logarithmic line graph
Channel Processes and Hjulstrom’s Curve
Key Questions:
How does a river erode and transport its load?
What factors affect these processes?
Why and where does a river deposit its load?
What does Hjulstrom’s curve show us about channel processes? 1

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How much do you already know about erosion, transportation and deposition? 5 2

3. Channel Processes - Velocity - Discharge
Task:
Write a definition of erosion, transportation and deposition from your textbook.
At any one time the dominant process acting in the river is dependent on energy
The amount of energy available is determined by… 10
- Velocity
- Discharge 3

4. Fluvial Erosion
Erosion is dependant on energy, the following variables determine energy in a river:
_____________________________
The four erosional processes are: 5 4

5. Fluvial Transport
Which processes are happening here? 5

6. Fluvial Deposition
Use textbook and notes to copy and complete the following information:
A river deposits it’s load when it’s energy _________, i.e. it’s no longer ______________ to transport it.
Several factors may cause deposition such as
A ______________ in gradient, e.g. when…
A ______________ in velocity, e.g. when…
A ______________ in discharge, e.g. when…
When there is shallow/deep water, e.g. ….
The ____________ of the load, e.g….
The particles are deposited in order of _________ (the largest/smallest first, because…)
Falls or drops, competent, reduction, decrease x 3, size or calibre, 6

7. Types of Load
Bed load
Suspended load
Dissolved load

8. Hjulstrom’s Curve – a logarithmic graph
Hjulström’s curve shows the relationship between the velocity of the river and the size of particles it can carry…
For a particle to be picked up (sediment entrainment) the critical erosion velocity must be reached.
The deposition velocity is the velocity at which particles can no longer be carried
Hjulstrom’s curve shows the relationship between velocity of the river and the size of the particles it can carry
As discharge rises, velocity increases and the river has more energy to pick up particles. As velocity reduces it deposits particles in order of size…
What is the relationship between velocity and particle size for each curve? What do you notice? 8

9. 1) Sand particles are more easily eroded than smaller silts.
Hjulstrom’s Curve
Hjulström’s curve shows 3 interesting relationships…
2) Once entrained particles can be carried at lower velocities than those required to pick them up
1) Sand particles are more easily eroded than smaller silts.
3) The smallest particles may never be deposited on the river bed and only put down in estuaries where flocculation occurs
Hjulstrom’s curve shows the relationship between velocity of the river and the size of the particles it can carry
As discharge rises, velocity increases and the river has more energy to pick up particles. As velocity reduces it deposits particles in order of size… 9

10. Hjulstrom’s Curve
Less energy is needed here to erode a particle as less energy is needed to erode sands than clays.
Very fine particles need higher velocity to erode them than larger particles as materials like clay and sand are cohesive.
Some of the smallest particles can stay in suspension when the water is still
When the particles are boulders, even the smallest drop in velocity can mean they are deposited.

11. Hjulstrom’s Curve TASK
Particles are eroded from the river bed
Particles are deposited on the river bed
Particles are transported within the river flow
Fine silt would still be in suspension at this velocity.
Critical Erosion Curve – the approximate velocity needed to pick up sediment
Sand in suspension will probably fall out here.
It would take this velocity to move a boulder.
Cobbles only can be picked up at this velocity.
Critical settling velocity curve – the velocities at which particles of a given size become too heavy to transport
Draw your own Hjulstrom curve using the textbook.
Add the yellow labels to your curve and explain why you have placed them where you did, using velocity and particle size information to support your answer.
Why might high velocities be required to both erode the finest and coarsest materials?
Hjulstrom’s curve shows the relationship between velocity of the river and the size of the particles it can carry
As discharge rises, velocity increases and the river has more energy to pick up particles. As velocity reduces it deposits particles in order of size… 11

12. SINK OR SWIM ?

13. Silt at 5 cm/sec

14. Clay at 5 cm/sec

15. Sand at 0.5 cm/sec

16. Pebble at 55 cm/sec

17. Gravel at 500 cm/sec

18. Boulder at 100 cm/sec

19. Sand at 0.1 cm/sec

20. Questions
Identify the relationship between river energy and particle size.
What speed must the river be traveling at to erode a particle of size 10mm?
Identify the river process at for a particle size of 1mm with velocity 10cm/sec. Explain your answer.
Explain why it is more difficult to erode clay and silt.
Name the type of sediment that requires the lowest velocity to be eroded.
Name the type of sediment that is likely to be transported at all velocities.
Describe and explain the relationship between water velocity and the erosion of clay and sand particles.
Explain the variation in water velocity that is required to transport and to deposit sediments of different particle diameter.

21. Problems with Hjulström:
Velocity WHERE? Bed? Banks? Mean? (varies enormously within channel, so hard to apply the graph to real river)
Is SIZE (calibre) of the load the important factor? What about different densities?
SHAPE of load is important too (why?)
SHEAR STRESS is key, not VELOCITY (a function of water depth and gradient)

22. Extra Questions
A stream has a baseflow of 10 cm per second. What is the largest size of particle it can transport?
During a storm the velocity increases to 100 cm/s. What is the largest particle it can now erode?
This is followed by a period of drought and the river’s velocity falls to 1cm/s. Describe and explain what happens to the load during this phase
Answers:
1mm
10mm (or just under)
it can’t erode, can only transport up to 0.1mm so only silt and clay, anything over that will be deposited. 22

23. What have we learnt today?
How does a river erode and transport its load?
What factors affect these processes?
Why and where does a river deposit its load?
What does Hjulstrom’s curve show us about channel processes? 23

24. Answers
Identify the relationship between river energy and particle size.
As the particle size increases, a river will require more energy in order to erode or to transport its load.
However, the above rule does not apply to clay and silt particles which requires high river velocity to erode, as it is extremely cohesive.
What speed must the river be traveling at to erode a particle of size 10mm?
100cm/sec

25. Answers
Identify the river process at for a particle size of 1mm with velocity 10cm/sec. Explain your answer.
Traction. At velocity 10cm/sec, gravel is on the verge of being deposited or transported. Hence the mode of transportation is traction as the river drags the particle from its stationary mode.
Explain why it is more difficult to erode clay and silt.
This is because clay and silt sticky in texture and tend to coagulate (amass and stick to one another).
These particles also tend to stick to the riverbed.