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Fluvial Processes
At any time and at any point on a river’s journey things are happening – these
processes can be erosion, transportation and deposition. Very often more
than one of these processes is taking place at the same time. Along the river’s
long profile the relative importance of each process varies considerably.
The factor determining which process (s) taking place at any one time is the
amount of energy available in the river.
High energy conditions in a channel occur with sudden or sustained inputs of
water, such as those following rainfall events and snow or ice melt.
Processes of Erosion
A river can erode the soil and rocks which form its channel by the processes
of corrosion, abrasion, attrition and hydraulic action. This eroded material is
carried by the river as sediment load, and adds to the sediment load derived
from weathering and mass movement processes on the valley sides.
Corrosion
This is the chemical weathering of minerals in rocks in contact with the river
water. The minerals in rocks are slowly dissolved by the river water and this
eventually leads the rock particles to break apart. This process will be most
effective where there is a fast flowing river which is not already saturated with
minerals. Rock type is also important. Limestones in particular are also
susceptible to this kind of erosion.
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Abrasion (corrasion)
This is where rocks in the sediment load which is being carried along by the
river hit the bed and banks of the river – wearing them away. This is most
effective if the river is flowing at high velocities and the particles being carried
are made of hard rocks. Thus, abrasion is most effective in times of high flow
and flood conditions. At these times, larger sediment particles, including
boulders are moved. Abrasion is the main process which causes vertical
erosion. Rocks and boulders swirling on the river bed can produce near
circular holes called potholes by this process of abrasion.
Attrition
Attrition is a process whereby the rocks in the sediment load erode by
colliding with each other as they are carried along the river. The result is that
the sediment load becomes more rounded and smaller in size. Upstream
sediments tend to be larger and more angular than sediments in the lower
reaches since attrition hasn’t been acting on them for very long. As a general
rule, sediments become smaller and rounder downstream. This applies to a
sediment particle on its journey from upstream to the lower reaches.
Remember sediment can also be added to the stream all along its course – if
the river flows across a hard rock outcrop in the lower course, large angular
sediments will be added.
Hydraulic Action
This is the force of moving water. This can be powerful and is the reason why
you should never wade into a stream or river above the level of your knees.
Loose sediment found in the middle and lower course is most susceptible to
erosion by hydraulic action. The force applied by the water is able to dislodge
and entrain the fragments. Another related process is called cavitation. This
operates at high velocities. As water speeds up , there is a drop in pressure
which can cause air bubbles to form. As these implode, very tiny jets of water
shoot off at high speeds and hammer the bed and banks. Its effects have
been seen on dam spillways and other structures.
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Processes of Transportation
A river transports its sediment load in a variety of different ways. The
methods of transport are also used to describe the various loads of the river,
i.e. the bed or traction load, suspension load, dissolved load and wash load
for very fine particles held in suspension all the time.
The sediment load varies from river to river, along the course of one river or in
the same place at different times.
These pictures show the same river, the Piscataquis at low and high flow.
This variation is because the velocity of the water is crucial in determining the
way that sediment is transported. The relationship between erosion,
transportation and deposition is complex and is shown on the Hjulstrom curve.
Entrainment is the process of starting sediment moving – the opposite of
settling.
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Critical erosion
velocity line
High velocities result in sediments being transported in the river flow,
while low velocities result in sediment being deposited.
Medium sand (0.25 -0.5mm in diameter) is moved at the lowest
velocities.
Larger, heavier sediments need higher velocities to start movement.
Silt and clay need higher velocities than their size would suggest
because they are cohesive (they stick together) and so in fact, are
bigger than they should be.
Once set in motion, fine particles, can be transported even if the
velocity falls.
Larger course particles are deposited rapidly as velocity falls. In
channels with mainly boulders and gravel transportation only occurs at
high flows.
In natural channels the situation will be more complex. For example,
small particles may be sheltered by larger particles and therefore they
are not moved.
Velocity of flow is variable across and vertically in a natural channel,
and this will affect the processes.
Sediment transported under lower –flow velocities, as bed load, may
become suspended load under high velocities.
Methods of Transportation
Having overcome friction, energy is then used to transport sediment. The
ability of a river to transport sediment is referred to as its competence.
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Capacity represents the maximum load of sediment of all types irrespective
of particle size that the current can transport. It increases with discharge and
/or gradient.
Load is carried in four ways:
1. in Solution – dissolved minerals such as salt and limestone
2. in Suspension – particles are carried along in the body of a stream and
are supported by the water itself
3. by Traction- material is dragged or rolled along the river (sometimes
over other material) bed
4. by Saltation – the jumping of relatively small particles along the bed,
caused largely by turbulence
Most rivers have ¾ of their load made up by suspended sediments, but the
exact proportions of each type of load carried by a particular river will depend
on the rock type, climate and velocity; the proportions may well vary
seasonally.
Processes of Deposition
The velocity at which a sediment particle drops to the channel bed is called
the settling velocity. This depends upon the size and shape and density of
the sediment particle. Deposition may be temporary on the channel bed and
the sediment may be moved again at a time of higher flow. In other situations
there is a net deposition of sediment, and a deposition landform results, e.g.
floodplains and point bars on the inside of meander bends.
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Deposition occurs when:
discharge is reduced, such as after a dry spell
velocity is reduced by the river entering a lake or sea
there is shallow water, such as on the inside bend of a meander
the load is suddenly increased, perhaps due to a landslide into a river
the river floods and overflows its banks resulting in lower velocities
outside the main channel
The sediment load of a river
The sediment load of a river comes from two sources:
Erosion of the channel bed and banks
Material which has been produced and moved downslope by
weathering and mass movement processes, and accumulates at the
slope base. This material is added to the channel as the banks are
laterally and vertically eroded, or in times of higher discharge when the
river is high enough to flow over the sediment and pick it up.
The sediment load is very important because it effects the energy levels of the
river. Energy is used up in transporting sediment as well as in water flow. If
the sediment load increases too much, then the river does not have enough
energy to carry it and deposition results and the river reaches equilibrium
again (the balance of inputs and outputs in a system) If the sediment levels
are reduced, the river will have more energy, which it uses to erode. The
amount and nature of the sediment load depends on several factors:
The sediment load is produced by weathering of rocks on the hillslope.
Therefore, the type of rock in the drainage basin is important, as is the
climate. The sediment from the hillslopes has to reach the channel. This
depends on the vegetation cover, the rainfall intensity, the gradient and the
land uses.
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E.g. A hot wet climate may produce a high amount of sediment due to fast
rates of weathering, but this climate will also result in dense forest cover
which would protect the sediment from rainfall. Sediment is also stored in the
drainage basin for varying lengths of time, with only a small proportion
reaching the river.
Human activity is also an important influence on the sediment load of rivers.
E.g dams reduce sediment loads downstream as sediment is stored in the
reservoir. Sediment yields are increased by cultivation, removal of natural
vegetation cover, mining activity and urban development.
Give an example of a situation where a very high sediment load would result
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Give an example of where a very low sediment yield would result
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The long profile of the river
A river uses energy to carry out its ‘work’ of erosion, transport and deposition
of sediment and to move the water in its channel. This energy is produced
when water flows down a slope, so the height of a river above sea level
determines this. The lowest level of a river is called base level and for most
rivers it is sea level. The long profile of a river is a graph drawn along the
course of the river from its source to its mouth. The axes may be calibrated in
conventional units of height and length or alternatively, as percentages of the
river’s total drop (relief) and length. The study of river long profiles shows
that they have a concave shape
with a steeper upper reach and a gentler lower reach. River processes are
related to long profile because every river is trying to achieve a smooth
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concave long profile. The ideal long profile is called a graded long profile
and in this ideal situation the available energy and the river processes will be
balanced. Where this is not the case the river will be working to smooth out
its long profile by erosion transportation or deposition. Rivers are trying to
erode to base level (usually sea level). If base level falls, as during tectonic
uplift it sets in train rejuvenation. Adjustment begins at the mouth of the river
and works its way upstream, the point of change being the knickpoint. If
base level rises e.g. during an interglacial period, adjustment is by deposition
rather than erosion.
Channel Cross Section
As a river travels downstream the cross section of the river channel also
changes dramatically as a result of river processes.
The most obvious change is that the cross –sectional area changes
downstream – that is the river channel becomes bigger. It also becomes
more efficient. This can be calculated by using the wetted perimeter (the
part of the channel that is in contact with the water at any particular time).
The fuller the channel, the greater the WP and therefore more water exposed
to the frictional drag of the channel sides. The hydraulic radius of a stream
channel is calculated by dividing its cross sectional area by the length of the
WP. It is a measure therefore of how much water there is in relation to the
amount of the channel currently occupied by the stream. The higher the value
of the hydraulic radius the greater the efficiency of the stream or river.
High values are typically associated with streams that have a large discharge
and those that have a cross sectional area that is approximately semi –
circular. Just as streams and rivers seek to achieve the most efficient graded
profile, so in their channels they try to achieve the most efficient cross channel
shape. In contrast, streams with low values are typically wide and shallow.
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