978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

Deposition
If the river no longer has energy to transport material it will be deposited.
As the competence (maximum particle size) and capacity (maximum load)
to carry material falls the largest boulders will be deposited first followed by
progressively smaller material. The amount of energy that a river has and the
likelihood it will deposit material is closely linked to flow conditions. Deposition
is more likely to occur:
• following low periods of precipitation where river levels drop
• where the river flow meets the sea
• in areas of slow flow within a channel, such as on meander bends
• when the load suddenly increases above the capacity, for example following
a landslide
• when the water has carried the material outside of the channel, such as in
times of flood.
With the exception of material in solution, which will never be deposited,
river deposits tend to become smaller and more round closer to the sea.
However it must be noted that larger stones may be present along the entire
course of the river as the bed and banks are constantly being acted on by other
processes such as weathering and erosion.
Hjulstrom’s Curve
The relationship between particle size and velocity can be seen using
Hjulstrom’s Curve (Figure 1.17). The mean or critical erosion velocity curve
shows the approximate velocity needed to pick up and transport (in suspension)
particles of various sizes. The capacity of the river is responsible for most of the
subsequent erosion. The mean fall or settling velocity curve shows the velocities
at which particles of a given size become too heavy to be transported and so will
fall out of suspension and be deposited. There are three important features of
Hjulstrom’s curves:
• The smallest and largest particles require high velocities to lift them. For
example, particles between 0.1 and 1 mm require velocities of around 100
mm/sec to be entrained, compared with values of over 500 mm/sec to lift
clay and gravel. Clay resists entrainment due to cohesion, gravel due to
weight.
• Higher velocities are required for entrainment than for transport.
• When velocity falls below a certain level those particles are deposited.
1000
River velocity (cm/sec)
500
100
50
10
5
1
0.5
0.1
0.001
2
particles
transported
clay
4
0.01
silt
figure 1.17 Hjulstrom’s Curve
particles
eroded
1
mean or critical erosion velocity curve
3
particles
deposited
mean fall or settling velocity curve
0.1 1.0 10.0 100.0 1000.0
sand gravel pebbles
cobbles boulders
Particle diameter (mm)
5
1 – particles of sand picked up
2 – clay needs a greater velocity
as particles stick together
3 – gravel also needs higher
velocities due to size and
weight
4 – small particles in transport
require very little velocity
5 – for larger material only a
small drop in velocity may
lead to sedimentation
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Hydrology and fluvial geomorphology