Explaination on hjulstrom curves

1. Sizes of particles:
boulders & cobbles (more than 15 mm), gravel 2 - 15 mm, sand 0.06 - 2 mm, silt
0.004 - 0.06 mm & clay < 0.004 mm
Hjulstrom curve:
Terms:
1. Capacity: largest amount (weight) of debris that the river can carry.
2. Competence: diameter of the largest particles (size) that can be carried.
3. Critical erosion velocity: lowest velocity at which grain of a given size can be
moved.
4. Settling/fall velocity: velocity falls below certain level particle (fall velocity) will be
deposited.
The Hjulstrøm curve is a graph used by hydrologists to determine whether a river
will erode, transport, or deposit sediment. The graph takes sediment size and
channel velocity into account.
Interpretation of the Hjulstrom curve/graph.
 The x-axis shows the size of the particles in mm.
 The y-axis shows the velocity of the river in cm/s.
 Hjulstrom graph shows relationship between stream velocity and ability to
transport materials of varying sizes. (i.e. finest particles; clay & silt to largest
boulders)
 There are two curves lines drawn in a Hjulstrom curve; critical erosion velocity
and settling or fall velocity.
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2. The Hjulstrøm curve shows that particles of a size around 1mm require the least
energy to erode, as they are sands that do not coagulate.
Sand (between 0.06mm and 2 mm) is relatively easily eroded and moved. (at
velocity 20cm/secs, sand will be eroded)
Particles smaller than these fine sands are often clays (smaller than 0.06mm) which
require a higher velocity to produce the energy required to split the small clay
particles which have coagulated. (stick together cohesively)
Larger particles such as pebbles are eroded at higher velocities and very large
objects such as boulders require the highest velocities to erode. (300cm/secs, most
eroded)
When the velocity drops below this velocity called the line of critical velocity, particles
will be deposited or transported, instead of being eroded, depending on the river's
energy.
As the velocity of the river decreases again, particles are not deposited but are
transported by the water.
Boulders are deposited quickly as the velocity decreases, and after that as the size
of particle decreases, the longer it is transported.
For example, particles of 1mm are deposited when the velocity is at 10cm/sec,
however smaller particles of 0.1mm are deposited when the velocity is at 1cm/sec.
Clay particles stay afloat in water even when there is no velocity at all as they are
very small.
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3. Erosion of clay particles needs a velocity similar
A river with high to that for pebbles because of their cohesive
discharge and properties.
velocity
increases its
capacity &
Pebbles, cobblers,
competence
boulders have
allowed
narrow gap between
increase
amount of energy
erosion of river
needed for erosion
channel.
and deposition, i.e.
only small drop in
velocity can caused
Velocity
deposition of larger
Needed to keep
particles.
particles in
suspension
Is less than to
erode them
Velocity needed to
transport clay particles is
almost zero
(clay particles stay afloat
in water i.e. in suspension,
even when there is no
velocity at all as they are
very small)
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