Transcript LO: To understand the river processes and the Hjulstrom Curve

LO: To understand the river processes and
the Hjulstrom Curve
What are the processes of a river?
River Processes
Erosion
Transportation
Hydraulic Action
Corrasion/Abrasion
Attrition
Solution/Corrosion
Traction
Saltation
Suspension
Solution
Deposition
Low river energy
RIVER PROCESSES:
Three river processes:
1. Transportation
2. Deposition
3. Erosion
RIVER TRANSPORTATION
The load is transported by 4 ways:
(i)
Saltation: when pebbles, sand and gravel (bedload) are lifted up
by current and bounced along the bed in a hopping motion.
(ii)
Traction: when largest boulders and cobbles (bedload) roll or slide
along the bed.
(iii)
Suspension: very fine particles such as clay and silt (suspended
load) are dislodged and carried by turbulence in a fast flowing
river.
(iv)
Solution: water flowing within a river channel contains acids (e.g.
carbonic acid from precipitation) dissolve the load such as
limestone in running water and removed in solution.
Types of Load
Bed load
Suspended load
Dissolved load
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.
River erosion.
Erosion: wearing away of river bed and bank.
There are four main process of erosion:
(i)
Corrasion (Abrasion): The rubbing or scouring of the bed and
banks by sediment carried along by the river. This can vary from
fine particles kept in suspension by turbulent flow to heavier
boulders rolled along at times of bank-full flow. Major method by
which river erodes both vertically and horizontally.
Landforms: potholes
(turbulent eddies in the
current can swirl pebbles
around to form potholes
that are hollows in river
bed and pebbles are
likely to become trapped)
potholes
River erosion (continued)
(ii)
(iii)
Attrition: Refers to reduction in the size of the sediment particles as they
collide with each other, the bed and banks. Pieces of sediment become
smaller and more rounded as they move downstream, so it is more
common to find rounder, smaller fragments of rock downstream and
coarser, more angular fragments upstream
Hydraulic action: The sheer force of water eroding the bank and bed, in
a number of ways:
–
The turbulent water current hits river banks and pushes water into
cracks. The air in cracks compressed, pressure increased and in
time bank will collapse.
–
Cavitation- form of hydraulic action caused by bubbles of air
collapsing.
–
The water simply picking up loose sediment by frictional drag of
moving water.
River erosion (continued)
(iv)
Solution/corrosion: This process in independent of river discharge
and velocity. Occurs when rocks dissolve in the water and are
carried away. This is most common when the rocks in the
channel are carbonate (such as limestone and chalk).
Rivers may erode
horizontally and vertically,
and often it happens at
PROCESSES
OF EROSION
the same time.
Vertical Erosion
This is a characteristic of faster flowing rivers where there is enough
sediment to down-cut, also as there is larger and more angular boulders
being moved by the high velocity and there is a more angular bed-load, it
creates a relatively quick lowering of the channel floor, generating steepsided valleys.
Horizontal Erosion (Lateral Erosion)
When a river has a sizeable flood-plain it may meander across the valley,
meaning lateral erosion will dominate. This happens particularly when the
flood-plain is composed of alluvial (clay, silt or gravel) sediments, as
hydraulic action can attack the outside of the meander bend, leading to
undercutting and eventual collapse of river banks.
DEPOSITION
•
•
Deposition happens when a river is no longer competent or has lost the
capacity to carry all of it’s load.
Any reduction in river velocity will mean in material being deposited,
starting with the coarsest sediment as this requires a lot of energy to
remain in suspension.
When does it occur?
1.
2.
3.
4.
5.
6.
There is a sudden reduction in gradient (e.g. foot of a mountain range)
The river enters a lake or the sea
Discharge has been reduced following a period of little rainfall
Where there is shallower water (e.g. The inside of a meander)
There is a sudden increase in the volume of sediment available, such as at
a confluence / where a landslide has occurred
River overflows its bank so velocity outside channel is reduced. (resulting
in floodplain)
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 illustrates the relationship between velocity and
THE HJULSTRÖM CURVE
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.
1. Consider the statements below and place them on your Hjulstrom curve
where you think they should go. There are more than 1 possible location for
each statement.
1.
2.
3.
4.
5.
6.
7.
Cobbles can only be picked up at this velocity
The water still may not be fast enough to lift clays from the bank
Sand in suspension will probably fall out here
It would take this kind of velocity to lift a boulder
As soon as the water slowed, the boulders would fall out here
Fine silt might still be in suspension at this low velocity
Sand particles would start to be lifted around this velocity
2. For each statement, explain why you located it where you did on the graph,
using velocity and particle size information to support your answers.
3. Why might velocities be required to erode both the finest and the coarsest
calibre of materials?
Hjulstrom Curve Questions
a)
b)
c)
d)
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.
Hjustrom curve Qs
1.
Name the type of sediment that requires the lowest velocity to be
eroded. [1]
2.
Name the type of sediment that is likely to be transported at all
velocities. [1]
3.
Describe and explain the relationship between water velocity and
the erosion of clay and sand particles. [4]
4.
Explain the variation in water velocity that is required to transport
and to deposit sediments of different particle diameter. [4]
Answers
a)
Identify the relationship between river energy and particle size.
–
–
b)
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
Hjulstrom Curve
c)
Identify the river process at for a particle size of 1mm with velocity
10cm/sec. Explain your answer.
–
c)
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.
• SINK OR SWIM exercise.
Provide a particle size and a velocity, and
the answer they need to shout out within
10 seconds is "SINK!" or "SWIM!"
Problems with
Hjulström:
1. Velocity WHERE? Bed? Banks?
Mean?
(varies enormously within
channel, so hard to apply the
graph to real river)
2. Is SIZE (calibre) of the load the
important factor?
What about different densities?
Actually a wide
band
150-400 cm/s
3. SHAPE of load is important too
(why?)
4. SHEAR STRESS is key, not
VELOCITY
(a function of water depth and