1. No category

Floodplains
BY
Rebecca Hinks &
Charlotte Bootherstone
What are Floodplains?
• A floodplain, or alluvial plain, is a piece of flat land on either side
of a stream or river, that will experience occasional flooding. It
holds the excess water, allowing it to be slowly released into the
river system and/or to infiltrate.
•Flooding is when river discharge exceeds the capacity of the
channel, water rises over the channel banks and floods the
surrounding low-lying lands.
• Floodplains also give time for sediment to settle out of
floodwaters.
•Floodplains often support important wildlife habitats and are
frequently used by humans as recreation areas.
What are Floodplains? 2
•Floodplains generally contain sediments, often accumulations
of sand, gravel, loam, silt, and/or clay. Sections of the Missouri
River floodplain show layers or areas of varying material.
Sometimes the deposits are of coarse gravel, fine sand or silt.
•During its formation a floodplain can be noted by: meanders,
ox-bow lakes, levees and can periodically be completely
covered with water.
•When arresting factors affect the drainage system the floodplain may
become an area of considerable fertility. Floodplains can support rich
ecosystems (with 100 or even 1000 times more species than a river). When
floodplain soil is wet it releases a surge of nutrients: Microscopic organisms
thrive and larger species enter a rapid breeding cycle. This makes
floodplains particularly valuable for agriculture.
Floodplain Formation
Floodplains are formed in two ways: by erosion and by aggradation.
•An erosional floodplain is created as
a stream cuts vertically and laterally
into its channel and banks. A stream
with a steep gradient will tend to
down cut faster than it causes lateral
erosion, resulting in a deep, narrow
channel with little or no floodplain at
all. Such as the Colorado River,
Arizona. As the stream approaches
base level, lateral erosion increases,
creating a broader floodplain.
Floodplain Formation
Formation 2
Floodplain
•An aggradational floodplain is created when a river deposits thick
layers of sediment. This happens when the river's velocity decreases,
forcing it to drop sediment. Consequently the lower course of the
river valley becomes layered with alluvium.
•In times of flood, the larger
discharge of water in the
upper courses erodes away
and carried a larger quantity
of sediment resulting in the
creation of a flat terrain
(plantation) as well as
aggradation. Such as one
stream in la Plato county,
Colorado.
•Aggradational floodplains
are the most common, and
are most usually found near
the estuary’s of large rivers,
such as the Rhine, the Nile,
the Ganges and the
Mississippi, where there are
occasional floods and the
river carries a large load.
Floodplain uses
•The main use of floodplains is for agriculture, however the use of the land
for settlement has become more popular.
•Settlement on floodplains, despite occasional flooding, is attractive because:
- Water is most available.
- The land is more fertile.
- Rivers correspond to cheap modes of transport.
- Flat or gently sloping land is more suitable for development.
Here is an
example of
how
floodplains
have been
used for
settlement
and how the
river flooding
has had to be
controlled.
Human influences on
floodplains
•In areas where floodplains have been used for settlement,
flooding can be a hazard, and therefore it has had to be
controlled.
•Levees are built or strengthened to prevent flooding, which
would help to benefit and protect the urbanised area surrounding
the floodplain.
•Here is an
example of a
levee in an
urban area.
The End