Bellwork
Complete your mini quizzes
(to test your OWN knowledge, not the knowledge
of the person sitting next to you)
Answers!
1. What is evapotranspiration? Evaporation +
Transpiration
2. What is the equation for working out
precipitation? (Clue- it includes the following:
S=Storage, E=Evapotranspiration, O=Total
Runoff (Streamflow))
P= O+E+/-S
3. Is groundwater storage found above or under
the ground? Under the ground
4. What is river discharge measured in? cumecs
5. What is lag time? The amount of time in
between peak rainfall and peak discharge
River Processes
Learning Objectives:
To understand river
processes and the Hjulström
Curve
Learning outcomes
• Name the 4 erosional
processes
• Name the 4 transportational
process
• Interpret the Hjulström
curve
River
Processes
Erosion
Transportation
Deposition
Hydraulic Action
Corrasion/Abrasion
Attrition
Solution/Corrosion
Traction
Saltation
Suspension
Solution
Low river energy
RIVER PROCESSES:
Three river processes:
1. Transportation
2. Deposition
3. Erosion
Information Hunt
• Walk around the room to find the
information about the 3 main river
processes in order to complete your
sheet
• *Extension Task- Draw a diagram to go
with each of the definitions*
River Competence & River Capacity
• River Competence is the maximum size of load a
river is capable of transporting.
• River Capacity refers to the total volume of sediment
a river can transport.
• It is important to note that the velocity has an
influence:
– At low velocity only fine partials may be transported (Clays,
silts, and sands).
– At a higher velocity larger material can be moved.
• Maximum particle mass that can be moved
increases with the power of velocity, so when
discharge levels are high (during a flood for
example), much larger boulders can be moved.
Hjulstrom Curve
River Processes
Hjulstrom curve
•
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.
•
The curve shows several
key ideas about the
relationships between
erosion, transportation
and deposition.
Hjulstrom curve
•
The Hjulstrøm Curve
shows that particles of a
size around 0.5 mm
require the least energy to
erode, as they are sands
that do not coagulate
(such as clay).
•
Particles smaller than
these fine sands are often
clays which require a
higher velocity to produce
the energy required to
split the small clay
particles which have
coagulated.
Hjulstrom curve
•
Larger particles such as
pebbles are eroded at
higher velocities and
very large objects such
as boulders require the
highest velocities to
erode.
•
When the velocity drops
below this velocity,
particles will be
deposited or
transported, instead of
being eroded,
depending on the river's
energy.
THE HJULSTRÖM CURVE
• The hjulström curve illustrates the relationship between velocity
and competence. It shows the velocities at which sediment will
normally be eroded, transported or deposited.
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
Less energy is needed
here to erode a
particle as less energy
is needed to erode
sands than clays.
When the particles
are boulders, even
the smallest drop in
velocity can mean
they are deposited.
Drawing the Hjulström Curve
• In order to aid your understanding of what
the Hjulström curve is showing, copy out
your own version for your notes, noting
carefully the logarithmic scale on the x
axis
Words Fill!
eroded
0.1
silt 120 transported
cobbles 200
150
At 10 cm per second silt has eroded whilst cobbles would
be deposited. The smallest clay particles require
velocities of 200 cm per second to be eroded. For the
smallest silts it is approximately 120cm per second.
Cobbles are eroded upwards of 170cm per second.
Deposition starts to occur at
cm per second
0.1
for particles of approximately 0.01mm in size. Boulders
require the smallest velocity for deposition, at
only 150
cm per second for the smallest boulders
250mm in size.
Course sand of 0.5 mm in size is transported between 3cm
per second and 16 cm per second.
Plenary Critique
• How could we critique the Hjulstrom
curve?