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The long profile – changing processes: types of erosion, transportation and deposition, types of load; the Hjulstrom curve.

 

The main river process are summarised in the table below. Generally, erosion mainly occurs in the upper and middle courses and is either vertical or lateral, and deposition mainly occurs in the middle and lower reaches of the river.

Find out more
Get the plan here

1. Try this exercise to show how SHEAR STRESS is a better explanation than velocity for why sediment moves.

2. Look at the bottom of the fluvioglacial page to do a mix and match exercise

3. Using the graph below complete the exercise at the base of the page.

Erosion Processes Transportation Deposition

Hydraulic Action - where the sheer force of the water erodes the stones, bed and banks of the river

Corrasion - where stone sin transport are thrown into the bed and the banks eroding them

Corrosion - where weak acids within the water react with the rocks and bed and bank of the river

Attrition - where stones in transport are thrown into one another

Solution - minerals are dissolved in water and carried along in solution

Suspension -fine light material is carried along in the water

Saltation - small pebbles and stones are bounced along the river bed

 

Traction - large boulders and rocks are rolled along the river bed

When a river loses energy, it will drop or deposit some of the material it is carrying.

Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example after a flood or during times of drought. It can also occur where wetted perimeter increases relative to the cross sectional area.

Flocculation can also occur - where clay particles bond together and hence gain enough mass to sink to the river or sea bed.
The Hjulström curve

The relationship between these variables is best explained by the Hjulström curve.  This is a graph that shows the relationship between the size of sediment and the velocity required to erode (lift it), transport it and deposit it.  Competence is the maximum size of load that a river can carry, and this is largely determined by velocity.  The capacity is slightly different in that this is the total amount of load that is carried. The critical erosion curve shows the MINIMUM velocity required to lift a particle of a certain size.  The critical deposition curve shows the MAXIMUM velocity at which a river can be flowing before a particle of a certain size is deposited.  The zone in-between is the zone of transport, note the velocities for transport are lower than that for erosion, because it takes much more energy to lift sediment than to maintain it in transport (think about carrying your school bag full of A - level geography geek books, it takes more energy to initially lift it than to carry it).  The other strange pattern is that it takes more energy to erode some of the smallest particles.  This is because they are clay particles which are clagged or bonded together, there fore require a lot of energy to be eroded.


Generally;

Larger particles require more velocity to be lifted off the bed

Larger particles will be deposited at higher velocities where smaller particles will remain in transport.

There are different types of load, including bedload, solute or dissolved load and suspended load which held in the water. During low flow periods rivers will tend to carry only dissolved and suspended load, and when velocities pick up they will carry bedload as well.  Finally, the capacity of a river tends to increase with distance downstream as volumes and velocities increase.

 
 


 

Hjulstrom curve

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
Use the following data to help you complete the gaps below;
eroded     0.1     silt     120     transported     150     cobbles     200

At 10 cm per second
is eroded whilst would be deposited. The smallest clay particles require velocities of cm per second to be eroded. For the smallest silts it is approximately cm per second. Cobbles are upwards of 170cm per second.
Deposition starts to occur at
cm per second for particles of approximately 0.01mm in size. Boulders require the smallest velocity for deposition, at only cm per second for the smallest boulders 250mm in size.
Course sand of 0.5 mm in size is
between 3cm per second and 16 cm per second.