Background information
The main session of the lesson is dealt with in four sections.
1. The processes involved in meander formation.
2. The characteristics of a river meander.
3. The processes involved in floodplain formation.
4. The way material is deposited on the floodplain (creating levees).
Meander formation
Animation is used to ensure an understanding of the processes involved in
meander formation. The teacher may introduce the concept of how centrifugal
force pulls the water flow outwards causing the river bank to erode (depending
on the ability and age of the students). Also explain that most meanders are
associated with deep pools and shallow riffles and the fact that the presence of
these pools and riffles contribute to the formation of the meanders.
Characteristics of meanders
This section shows photographs of a river meander on the River Tywi in
Carmarthenshire. The sketch shows the characteristics. There is scope for
drawing sketch maps from the photographs here.
Notes to accompany photograph 1
As a river flows, the line of fastest flow adopts a sinuous pattern. The force of the
fast flowing water is enough to start to erode the river bank. This causes the river
to start to meander. The water on the outside of the bend has to flow faster than
that on the inside. As the water flows faster it has more power to erode the
outside bank. The outside bend of the meander becomes deeper, reducing
friction and allowing for even faster flow of water, which in turn means more
erosion. On the inside bend of the meander the river is flowing much slower. The
river does not have enough energy to carry its load and therefore deposition
occurs. This flow and related erosion on the outside bank and deposition on the
inside causes the meander to develop even further. In time the river will erode a
river cliff on the outside bend of the meander and deposit a river beach on the
inside bend.
Notes to accompany photograph 2
As it develops, the meander erodes sideways into the bank. This is known as
lateral erosion. Also, the meander tends to migrate down stream. In time this
erosion flattens and widens the bottom of the valley to create a floodplain. The
edge of the floodplain is shown by bluff lines where the meander has eroded into
the side. As it erodes the outside bend of the meander, the river deposits
material on the inside. In time the meanders develop fully and become stable and
their location will remain constant.
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The floodplain is made up of this deposited material called alluvium. Also, the
floodplain will receive material from the river when it floods. As its name
suggests, the floodplain is prone to flooding. When the river overflows its bank,
its velocity will decrease tremendously due to the friction with the floodplain. This
causes the river to deposit its material. This is usually in the form of fine sands,
silt and clay. Successive flooding will increase the height of the floodplain. This is
a very slow process (usually a few centimetres in 100 years).
The heaviest, coarsest material will be deposited first. In time the continued
flooding of the floodplain will cause a build-up of this material on the edge of the
river bank. This natural embankment is called a levee.
Floodplain formation
Animation is used to ensure an understanding of floodplain formation.
Deposition on the floodplain
Animation is used to show how material is deposited on the floodplain.
Classroom discussion could involve how material is graded on the floodplain, and
why coarser material is deposited nearer the river channel.
The plenary session involves photographs and maps. The OS map can be used
to identify the characteristics of the floodplain. Students can be encouraged to
recognise and label the characteristics of meanders and a floodplain on the
whiteboard.
The photograph and sketch should be used for classroom discussion about the
characteristics of the middle course of a river valley.
Further development could involve the influence of man on meanders and
floodplains or vice versa, eg extended work – lateral erosion by the meanders
can cause difficulties to people such as farmers, transport managers and
planners. Explain how this is so and suggest methods of managing this hazard.
©learnthings Ltd 2003