Floodplains

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Floodplains
BY Rebecca Hinks Charlotte Bootherstone
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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.

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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.

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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.

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Floodplain Formation
Floodplain Formation 2

• 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.

• Aggradational floodplains are the most common,
  and are most usually found near the estuarys 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.

• 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.

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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.
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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.

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The End