Subchapter 3 in the first chapter of Hydrology and Fluvial Geomorphology, suitable for AS students, consisting in the following: river processes, velocity, flows and Hjulstrom Curve.

1. River Channel Processes and Landforms
AS GEO 1.3
HYDROLOGY AND FLUVIAL
GEOMORPHOLOGY

2. River Processes
Velocity
Flow
Hjulstrom Curve

3. RIVER
Depends on:
Discharge - amount of water
Velocity - rate of water
movement

4. RIVER
PROCESSES
River carries three types of
work
1. Erosion
2. Transportation
3. Deposition

5. Occurs when
rivers erode
or wear
away the
land surface
over which
they are
flowing
The rock
particles
which are
worn away
from the land
surface are
called
sediments

6. TRANSPORTATION
Is a process where the
sediment produced by
erosion is carried away
downstream by the
river

7. DEPOSITION
Happens when the sediment
may settle either on the river
bed.
Where the water flows more
slowly as in the flood plain, or
eventually on the sea bed.

8. HYDRAULIC ACTION
A process by which the force of the
flowing water may remove particles
from the banks or bed

9. CAVITATION
A form of hydraulic action caused
by bubbles of air collapsing.
The resultant shock waves hit and
slowly weaken the banks.
This is the slowest and least
effective erosion in process.

15. ATTRITION
A process by which as rocks are carried along by a
river, they knock against each other so pieces break
off and the rock fragments are reduced in size and
become rounded.

16. CORRASION
A process by which the rock particles which are then
carried by the river may be used as tools to help
break more rock fragments from the river bed and
banks.
Example: Circular holes called pot holes may be cut in
a rocky river bed.

17. CORRASION

19. A process by which rivers can dissolve some
rocks such as limestone.
Example
At Mulu in Sarawak, rivers have dissolved the
limestone and created huge caverns (large
caves) through which they flow underground.

20. Rivers flow in channels and the sides of the channel are called
banks, with the floor of the channel known as the river bed.
Rivers can erode river channels in four main ways such as:
HYDRAULIC ACTION
A process by which the force
of the flowing water may
remove particles from the
banks or bed
CORRASION
A process by which the rock
particles which are then
carried by the river may be
used as tools to help break
more rock fragments from
the river bed and banksATTRITION
A process by which as rocks
are carried along by a river,
they knock against each
other so pieces break off
and the rock fragments are
reduced in size and become
rounded
SOLUTION
A process by which rivers can
dissolve some rocks such as
limestone
C
A
S
H

21. TRANSPORTATION
Is a process where the sediment
produced by erosion is carried
away downstream by the river
• 3 main processes:
• Bedload
• Suspended Load
• Dissolved/Solution Load

22. BEDLOAD
Larger particles which cannot be
picked up by current may be
moved along the bed of the river
in two ways:
- Traction
- Saltation

23. TRACTIO
N
When the large particles roll or slide
along the river bed. Large rocks are
only moved after heavy rain when the
river has a large volume of water and
is fast flowing.

24. SALTATION
When particles are temporarily lifted
up by the current and bounced along
the bed in a hopping motion.

25. SUSPENDED
LOAD0
Then small particles such as sand and
clay are carried along without touching
the river bed. These small particles are
just floating, and lightest particles are
near to the surface of the water.

26. SOLUTION
When rainwater can slowly dissolve
limestone rock. They cannot be
seen by the naked eye.

27. RIVER TRANSPORT
SUSPENSION
when small particles such as
sand and clay are carried along
without touching the river bed,
small particles are just floating,
and lightest particles are near
to the surface of the water
SOLUTION
when rainwater can slowly
dissolve limestone rock.
SALTATION
when particles are lifted
up by the current and
bounced along the bed
in a hopping motion
TRACTION
when the largest particles
roll or slide along the river
bed, moved after heavy rain
when the river has a large
volume of water and is fast
flowing

28. HOW DOES RIVER TRANSPORT HAPPEN?

29. RIVER DEPOSITION
The speed of flow of a river is reduced the river may no
longer have enough energy to transport its load of
sediment.
The larger particles will sink and settle first while the
finer particles will be carried further before settling, or
they may be carried all the way to the sea. This sinking
and settling of the river’s sediment is called river
deposition.
Deposition may occur on the river bed, or on the inside
curve of a river bend, or on the river banks
The sediment which is deposited in the sea at the river
mouth may build up new land known as delta.

30. RIVER
DEPOSITION
Decrease in velocity  less energy and no longer had
competence and capacity to carry all its load. Therefore,
largest/heaviest particles, materials begins to be
deposited.
Occurs when:
Low discharge following a period of low precipitation
Less velocity when river enter sea or lake
Shallower water occurs on inside of a meander
The load suddenly increase (debris from landslide)
River overflow its bank so velocity outside channel is
reduced (resulting in floodplain)

31. VELOCITY

32. VELOCITY
Velocity is the speed of a river (m/s). Can influence
the turbulence:
High Velocity:
The amount of energy still available after friction
will be greater and so turbulence increases.
The faster the flow of river the larger the quantity
and size of particles (load) which can be
transported.
Low Velocity:
Less energy to overcome the friction.
Turbulence decreases and may not be visible to
human eye.
Sediment will remains undisturbed.
Reduction in turbulence may lead to deposition of
sediment.

35. Velocity of a river is influenced by
three factors:
(1) Channel shape in cross-section.
(2) Roughness of the channel’s bed
and banks.
(3) Channel slope.

36. 1. CHANNEL SHAPE IN CROSS
SECTION
Simply describe by the term ‘Hydraulic
Radius’ (cross section area/wetted
perimeter).
Wetted perimeter - shape of the channel
or its cross section affects the extent to
which water is in contact with its channel.
The greater the wetted perimeter, the
greater the friction between the water
and the banks and the bed of the channel,
and the slower the flow of river.

38. River volume: 6 sq m (2m x 3m)
Wetted perimeter: 7 metres (2m + 3m + 2m).
The 7 metres will be represent the friction
slowing the river down.

39. Volume: 24 sq metres
Wetted perimeter: 14
metres.
Shape of the river 
a major influence.
A river with the same
volume of water as Ex 2 but
with a different shape will
have a different friction
Volume: 24 sq metres
Wetted perimeter is 26
metres almost double that
of Example 2 which means
that the river will be slower
as a larger part of the river
energy is used to overcome
friction. The gradient of the
river channel is only one
factor to influence the
speed of the river.

40. Example
Stream A: larger hydraulic radius
Small amount of water in contact with the wetted
perimeter.
Creates less friction  reduce energy loss
 allows greater velocity
Stream B: smaller hydraulic radius
Large amount of water in contact with the wetted
perimeter.
Creates greater friction  more energy loss
 reduce velocity

41. 2. ROUGHNESS OF THE CHANNEL’S BED AND
BANKS
Material such as rocks
in the channel can
influence the speed.
Whether rocks on the
river bed are smooth
or rough or uneven.
Rocks that protrude
out from the bank can
slow the pace of the
water as friction slows
it down as it passes
the obstacles.

42. In figure A, the channel
is smooth while that in
figure B is rough or
uneven with boulders on
the river bed as well as
rocks that protrude out
from the bank.
A river that flows
through such a river has
to overcome such
obstacles and therefore
there will be more
friction and the velocity
of the river is reduced.
Figure A
Figure B

43. Velocity of a mountain stream is less than that
of a lowland. Mountain stream is likely to pick up
loose material and carry it downstream
Example:
Mountainous / Upper course of a river
Despite high velocity in waterfalls, the large
number of angular rocks, coarse-grained
banks and protrusions increase frictions and
reduce overall velocity
Lower course of a river:
As there is little resistance from the smooth
bed and banks, there is little friction and river
flows faster

44. 3. CHANNEL SLOPE
A river flowing down a steep slope or gradient
has higher velocity than one which flows
down a gentler gradient.
For example, the speed of flow in a river that
plunges down a steep slope in the form of a
waterfall is much higher than the speed of
flow in a river that winds down a gentler slope.

45. CHANNEL SLOPE
Changes in gradient are related to
changes in discharge.
Discharge is higher in the lower
course.
Since gradient decreases as discharge
increases, river can transport the same
quantity and size of sediment load in
the gentler lower course as it can in
the steeper upper course.

46. FLOW

47. PATTERNS OF FLOW
River water  has a certain amount of available energy.
Greatest when there is a large amt of water and when
there is steep gradient.
Most of the river’s energy used up in overcoming friction
with the bed and banks.
Friction  high in the upper reaches of a river where
large boulders may protrude into large river’s flow.
There are three patterns
of flow:
1. Laminar flow
2. Turbulent flow
3. Helicoidal flow

48. LAMINAR FLOW
Horizontal movement of water. Travel
over the sediment in the river bed
without disturbing it. Rare in reality but
common in the lower reaches.
Condition:
Smooth
Straight channel
Shallow water
Non-uniform velocity

49. TURBULENT FLOW
Series of erratic (inconsistent) eddies. Both
vertical & horizontal in downstream
direction. Depends on the amount of
energy available after friction has been
overcome.
Conditions:
Complex channel shape eg. Winding
channels, riffles and pools
Cavitation as eddies trap air in pores,
cracks crevices which is then release

51. T U R B U L E N T F L O W

52. HELICOIDAL FLOW
Usually occur in meanders.
A corkscrew movement in a meander.
It is responsible for moving material
from the outside of one meander bend
and depositing on the inside of the next
bend.

53. HELICOIDAL FLOW

55. HJULSTROM
CURVE

56. HJULSTROM CURVE
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.

57. HJULSTROM
CURVE
Shows that particles of a size
around 1mm require the least
energy to erode, as they are
sands that do not coagulate.
Particles smaller than these fine
sands are often clays  require
a higher velocity to produce the
energy required to split the
small clay particles which have
coagulated.

58. Larger particles
 pebbles are eroded at
higher velocities
Very large objects
 boulders require the
highest velocities to
erode.
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.

61. THE LANGUAGE OF HJULSTROM
CURVE
Critical erosion velocity: The lowest velocity at which
grains of a certain size can be moved.
Critical deposition velocity: The velocity at which
particles of particular sizes are laid down
Entrainment: Materials being picked up by river
Flocculate: Materials stick together in the river
Clay particles: Tiny particles between 0.001 and
0.01mm in size
Sand particles: Sediments between 0.1 and 2mm in
size
Cobbles: Sediments between 20 and 300mm in size

62. HJULSTROM CURVE
Key:
Silt/sand are picked up (entrained) at the
lowest velocities.
Clays are difficult to pick up as pebbles –
although they are small particles, they are
very cohesive and the claybed is very
smooth.
Large boulders are dropped easily.
Clay particles can be transported in
suspension at very low velocities.

63. VOCABULARY CHECK
Hydraulic action
Cavitation
Attrition
Corassion
Solution
Bedload
Suspended load
Solution
Traction
Saltation
Hydraulic radius
Wetted perimeter
Laminar
Turbulent
Helicoidal
Hjulstrom Curve
Critical erosion curve
Critical deposition curve
Entrainment
Flocculate
Clay particles
Sand particles
Cobbles

64. HJULSTROM CURVE
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]

65. ANSWER
1. Sand
2. Clay
3. Clay - requires higher energy to be eroded
- tend to stick together
- are difficult to pick up as pebbles
- although they are small particles, they are very
cohesive
Sand - requires lower energy
- sand particles are unconsolidated (loose)
4. Boulders - require large velocities to be transported
Small particles - Clay & silt – can be held in suspension
area at low velocity.
Energy velocity to transport is always lower than
energy