LO: To understand the river processes and the Hjulstrom Curve What are the processes of a river?

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LO To understand the river processes and the
Hjulstrom CurveWhat are the processes
of a river?
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(No Transcript)
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RIVER PROCESSES

• Three river processes
• Transportation
• Deposition
• Erosion

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RIVER TRANSPORTATION

• The load is transported by 4 ways
• 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.

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• Suspension very fine particles such as clay and
  silt (suspended load) are dislodged and carried
  by turbulence in a fast flowing river.
• 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.

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Types of Load
Bed load
Suspended load
Dissolved load
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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.

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River erosion.

• Erosion wearing away of river bed and bank.
• There are four main process of erosion
• 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
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River erosion (continued)

• 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.

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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).

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PROCESSES OF EROSION
Rivers may erode horizontally and vertically, and
often it happens at 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 steep-sided 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.
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DEPOSITION

• Deposition happens when a river is no longer
  competent or has lost the capacity to carry all
  of its 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?
• 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)

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Hjulstrom Curve
River Processes
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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.

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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.

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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.

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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.

Less energy is needed here to erode a particle as
less energy is needed to erode sands than clays.
Very fine particles need higher velocity to erode
them than larger particles as materials like clay
and sand are cohesive.
When the particles are boulders, even the
smallest drop in velocity can mean they are
deposited.
Some of the smallest particles can stay in
suspension when the water is still
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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.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?

1. Cobbles can only be picked up at this velocity
2. The water still may not be fast enough to lift
   clays from the bank
3. Sand in suspension will probably fall out here
4. It would take this kind of velocity to lift a
   boulder
5. As soon as the water slowed, the boulders would
   fall out here
6. Fine silt might still be in suspension at this
   low velocity
7. Sand particles would start to be lifted around
   this velocity

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Hjulstrom Curve Questions

• 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.

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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

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Answers

• Identify the relationship between river energy
  and particle size.
  
• 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

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Hjulstrom Curve

• Identify the river process at for a particle size
  of 1mm with velocity 10cm/sec. Explain your
  answer.
• 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.

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• 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!"

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• Problems with Hjulström
• Velocity WHERE? Bed? Banks? Mean?(varies
  enormously within channel, so hard to apply the
  graph to real river)
• Is SIZE (calibre) of the load the important
  factor?What about different densities?
• SHAPE of load is important too (why?)
• SHEAR STRESS is key, not VELOCITY(a function of
  water depth and gradient)
• TURBULENT FLOW

Actually a wide band 150-400 cm/s