A floodplain or flood plain is an area of land adjacent to a stream or river
that stretches from the banks of its channel to the base of the enclosing
valley walls and experiences flooding during periods of high discharge. It
includes the floodway, which consists of the stream channel and adjacent areas
that actively carry flood flows downstream, and the flood fringe, which
are areas inundated by the flood, but which do not experience a strong
current. In other words, a floodplain is an area near a river or a stream which
floods when the water level reaches flood stage.
Formation Flood plains are made by a meander
eroding sideways as it travels downstream. When a river breaks its
banks and floods, it leaves behind layers of alluvium. These gradually
build up to create the floor of the flood plain. Floodplains generally
contain unconsolidated sediments, often extending below the bed of the stream.
These are accumulations of sand, gravel, loam, silt, and/or clay, and are often
important aquifers, the water drawn from them being pre-filtered compared to the
water in the river. Geologically ancient floodplains are
often represented in the landscape by fluvial terraces. These are old
floodplains that remain relatively high above the present floodplain and
indicate former courses of a stream. Sections of the Missouri River
floodplain taken by the United States Geological Survey show a great variety
of material of varying coarseness, the stream bed having been scoured at one
place and filled at another by currents and floods of varying swiftness, so that
sometimes the deposits are of coarse gravel, sometimes of fine sand or of
fine silt. It is probable that any section of such an alluvial plain would
show deposits of a similar character. The floodplain during its formation is
marked by meandering or anastomotic streams, oxbow lakes and bayous, marshes
or stagnant pools, and is occasionally completely covered with water. When the
drainage system has ceased to act or is entirely diverted for any reason, the
floodplain may become a level area of great fertility, similar in appearance
to the floor of an old lake. The floodplain differs, however, because it
is not altogether flat. It has a gentle slope downstream, and often, for a
distance, from the side towards the center.
The floodplain is the natural place for a river to dissipate its energy.
Meanders form over the floodplain to slow down the flow of water and when the
channel is at capacity the water spills over the floodplain where it is
temporarily stored. In terms of flood management the upper part of the
floodplain is crucial as this is where the flood water control starts.
Artificial canalisation of the river here will have a major impact on wider
flooding. This is the basis of sustainable flood management.
Ecology Floodplains can support particularly
rich ecosystems, both in quantity and diversity. Tugay forests form an
ecosystem associated with floodplains, especially in Central Asia. They are a
category of riparian zones or systems. A floodplain can contain 100 or even 1,000
times as many species as a river. Wetting of the floodplain soil releases
an immediate surge of nutrients: those left over from the last flood, and those
that result from the rapid decomposition of organic matter that has accumulated
since then. Microscopic organisms thrive and larger species enter a rapid
breeding cycle. Opportunistic feeders move in to take advantage. The
production of nutrients peaks and falls away quickly; however the surge of new
growth endures for some time. This makes floodplains particularly valuable for
agriculture. Interaction with society
Historically, many towns have been built on floodplains, where they are highly
susceptible to flooding, for a number of reasons:
access to fresh water; the fertility of floodplain land for
farming; cheap transportation, via rivers and
railroads, which often followed rivers; ease of development of flat land
Excluding famines and epidemics, some of the worst natural disasters in history
have been river floods, particularly in the Yellow River in China – see list of
deadliest floods. The worst of these, and the worst natural disaster were the
1931 China floods, estimated to have killed millions. This had been preceded
by the 1887 Yellow River flood, which killed around one million people, and is
the second-worst natural disaster in history.
The extent of floodplain inundation depends in part on the flood magnitude,
defined by the return period. In the United States the Federal
Emergency Management Agency manages the National Flood Insurance Program. The
NFIP offers insurance to properties located within a flood prone area, as
defined by the Flood Insurance Rate Map, which depicts various flood risks for a
community. The FIRM typically focuses on delineation of the 100-year flood
inundation area, also known within the NFIP as the Special Flood Hazard Area.
Where a detailed study of a waterway has been done, the 100-year floodplain will
also include the floodway, the critical portion of the floodplain which includes
the stream channel and any adjacent areas that must be kept free of
encroachments that might block flood flows or restrict storage of flood
waters. Another commonly encountered term is the Special Flood Hazard Area,
which is any area subject to inundation by the 100-year flood. A problem is that
any alteration of the watershed upstream of the point in question can potentially
affect the ability of the watershed to handle water, and thus potentially
affects the levels of the periodic floods. A large shopping center and
parking lot, for example, may raise the levels of the 5-year, 100-year, and
other floods, but the maps are rarely adjusted, and are frequently rendered
obsolete by subsequent development. In order for flood-prone property to
qualify for government-subsidized insurance, a local community must adopt
an ordinance that protects the floodway and requires that new residential
structures built in Special Flood Hazard Areas be elevated to at least the level
of the 100-year flood. Commercial structures can be elevated or flood
proofed to or above this level. In some areas without detailed study
information, structures may be required to be elevated to at least two feet
above the surrounding grade. Many State and local governments have, in addition,
adopted floodplain construction regulations which are more restrictive
than those mandated by the NFIP. The U.S. government also sponsors flood
hazard mitigation efforts to reduce flood impacts. The Hazard Mitigation
Program is one funding source for mitigation projects. A number of whole
towns such as English, Indiana, have been completely relocated to remove them
from the floodplain. Other smaller-scale mitigation efforts include acquiring and
demolishing flood-prone buildings or flood-proofing them.
In some tropical floodplain areas such as the Inner Niger Delta of Mali, annual
flooding events are a natural part of the local ecology and rural economy,
allowing for the raising of crops through recessional agriculture. But in
Bangladesh, which occupies the Ganges Delta, the advantages provided by the
richness of the alluvial soil of floodplains are severely offset by
frequent floods brought on by cyclones and annual monsoon rains, which cause
severe economic disruption and loss of human life in this densely populated
region. See also
Flood-meadow, area of grassland or pasture beside a river, subject to
seasonal flooding. Water-meadow, area of grassland or
pasture beside a river, subject to controlled seasonal flooding.
Crevasse splay, sedimentary deposit formed when an overloaded stream breaks
its levee. List of alluvial sites in Switzerland
Red River Floodway as a good example of a floodway.
Floodplain restoration Flood opening, a technique for
mitigating the effects of flooding on structures, mandated in some regions.
References = Notes=
= Bibliography= Powell, W. Gabe. 2009. Identifying Land
Use/Land Cover Using National Agriculture Imagery Program Data as a
Hydrologic Model Input for Local Flood Plain Management. Applied Research
Project, Texas State University. http:ecommons.txstate.edu296/
This article incorporates text from a publication now in the public domain:
Chisholm, Hugh, ed.. "Flood Plain". Encyclopædia Britannica. Cambridge
University Press. External links
Media related to Floodplain at Wikimedia Commons