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A floodplain or flood plain or bottomlands is an area of land adjacent to a
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of wate ...

river
which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences
flood A flood is an overflow of water that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the . Floods are an area of study of the discipline and are of significant concern in , a ...

flood
ing during periods of high discharge.Goudie, A. S., 2004, ''Encyclopedia of Geomorphology'', vol. 1. Routledge, New York. The soils usually consist of clays, silts, sands, and gravels deposited during floods.


Formation

Most floodplains are formed by deposition on the inside of river
meanders Meanders of the '' Rio Cauto'' at Guamo Embarcadero, Cuba A meander is one of a series of regular sinuous curves, bends, loops, turns, or windings in the channel of a river, stream, or other watercourse. It is produced by a stream or river swing ...

meanders
and by overbank flow. Wherever the river meanders, the flowing water erodes the river bank on the outside of the meander, while sediments are simultaneously deposited in a
point bar A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on t ...

point bar
on the inside of the meander. This is described as ''lateral accretion'', since the deposition builds the point bar laterally into the river channel. Erosion on the outside of the meander usually closely balances deposition on the inside of the meander, so that the channel shifts in the direction of the meander without changing significantly in width. The point bar is built up to a level very close to that of the river banks. Significant net erosion of sediments occurs only when the meander cuts into higher ground. The overall effect is that, as the river meanders, it creates a level flood plain composed mostly of point bar deposits. The rate at which the channel shifts varies greatly, with reported rates ranging from too slow to measure to as much as per year for the
Kosi River The Kosi or Koshi ( hi, कोसी, , ne, कोशी, ) is a trans-boundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the ...
of India. Overbank flow takes place when the river is flooded with more water than can be accommodated by the river channel. Flow over the banks of the river deposits a thin veneer of sediments on the floodplain that is coarsest and thickest close to the channel. This is described as ''vertical accretion'', since the deposits build the floodplain upwards. In undisturbed river systems, overbank flow is a frequent occurrence, typically occurring every one to two years regardless of climate or topography. Sedimentation rates for a three-day flood of the
Meuse The Meuse ( , , , ; wa, Moûze ) or Maas ( , ; li, Maos or ) is a major European river, rising in France and flowing through Belgium and the Netherlands before draining into the North Sea The North Sea is a sea of the Atlantic Ocean bet ...

Meuse
and
Rhine River The Rhine (german: Rhein , french: Rhin, nl, Rijn, : ''Rén'', and : ''Rein'', and : ''Ragn'', , , and : ''Rain'', it, Reno, gsw, Rhi(n) including /, , : ''Rhing'', la, Rhenus , : ''Rajna'') is one of the major . The river begins in the Swis ...
s in 1993 found average sedimentation rates in the floodplain of between 0.57 and 1.0 kg/m2. Higher rates were found on the levees (4 kg/m2 or more) and on low-lying areas (1.6 kg/m2). Sedimentation from overbank flow is concentrated on natural levees, crevasse splays, and in wetlands and shallow lakes of flood basins. Natural levees are ridges along river banks that form from rapid deposition from overbank flow. Most of the suspended sand is deposited on the levees, leaving the silt and clay sediments to be deposited as floodplain muds further from the river. Levees are typically build up enough to be relatively well-drained compared with nearby wetlands, and levees in non-arid climates are often heavily vegetated. Crevasses are formed by breakout events from the main river channel. The river bank fails and floodwaters scour a channel. Sediments from the crevasse spread out as
delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, a landform at the mouth of a river * D (NATO phonetic alphabet: "Delta"), the fourth letter of the modern English alphabet * Delta Air Lines, an Ame ...

delta
-shaped deposits with numerous distributary channels. Crevasse formation is most common in sections of rivers where the river bed is accumulating sediments (
aggrading
aggrading
). Repeated flooding eventually builds up an alluvial ridge, whose natural levees and abandoned
meander loops
meander loops
may stand well above most of the floodplain. The alluvial ridge is topped by a channel belt, formed by successive generations of channel migration and meander cutoff. At much longer intervals, the river may completely abandon the channel belt and begin building a new channel belt at another position on the floodplain. This process is called avulsion, and takes place at intervals of 10–1000 years. Historical avulsions leading to catastrophic flooding include the 1855 Yellow River flood and the 2008 Kosi River flood. Floodplains can form around rivers of any kind or size. Even relatively straight stretches of river are found to be capable of producing floodplains. Mid-channel bars in braided rivers migrate downstream through processes resembling those in point bars of meandering rivers and can build up a floodplain. The quantity of sediments in a floodplain greatly exceed the river load of sediments. Thus, floodplains are an important storage site for sediments during their transport from where they are generated to their ultimate depositional environment. When the rate at which the river is cutting downwards becomes great enough that overbank flows become infrequent, the river is said to have abandoned its floodplain, and portions of the abandoned floodplain may be preserved as
fluvial terrace Fluvial terraces are elongated terraces that flank the sides of floodplain File:Flood plain 7991.JPG, Riparian vegetation on the floodplain of the Lynches River, close to Johnsonville, South Carolina. These tupelo and Taxodium, cypress tr ...
s.


Ecology

Floodplains support diverse and productive
ecosystems An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
. They are characterized by considerable variability in space and time, which in turn produces some of the most species-rich of ecosystems. From the ecological perspective, the most distinctive aspect of floodplains is the ''flood pulse'' associated with annual floods, and so the floodplain ecosystem is defined as the part of the river valley that is regularly flooded and dried. Floods bring in detrital material rich in nutrients, and release nutrients from dry soil as it is flooded. The decomposition of terrestrial plants submerged by the floodwaters adds to the nutrient supply. The flooded
littoral zone The littoral zone or nearshore is the part of a sea, lake A lake is an area filled with water, localized in a basin, surrounded by land, apart from any river A river is a natural flowing watercourse, usually freshwater, flowing ...
of the river (the zone closest to the river bank) provides an ideal environment for many aquatic species, so the spawning season for fish often coincides with the onset of flooding. Fish must grow quickly during the flood to survive the subsequent drop in water level. As the floodwaters recede, the littoral experiences blooms of microorganisms, while the banks of the river dry out and terrestrial plants germinate to stabilize the bank. The biota of floodplains have high annual growth and mortality rates, which is advantageous for the rapid colonization of large areas of the floodplain. This allows them to take advantage of shifting floodplain geometry. For example, floodplain trees are fast-growing and tolerant of root disturbance. Opportunists (such as birds) are attracted to the rich food supply provided by the flood pulse. Floodplain ecosystems have distinct biozones. In Europe, as one moves away from the river, the successive plant communities are bank vegetation (usually annuals); sedge and reeds; willow shrubs; willow-poplar forest; oak-ash forest; and broadleaf forest. Human disturbance creates
wet meadow A wet meadow is a type of wetland A wetland is a distinct ecosystem An ecosystem is a community (ecology), community of living organisms in conjunction with the nonliving components of their environment, interacting as a system. These Biot ...
s that replace much of the original ecosystem. The biozones reflect a soil moisture and oxygen gradient that in turn corresponds to a flooding frequency gradient. The primeval floodplain forests of Europe were dominated by oak (60%) elm (20%) and hornbeam (13%), but human disturbance has shifted the makeup towards ash (49%) with maple increasing to 14% and oak decreasing to 25%. Semiarid floodplains have a much lower diversity of species, which are adapted to alternating drought and flood. Extreme drying can destroy the ability of the floodplain ecosystem to shift to a healthy wet phase when flooded. Floodplain forests constituted 1% of the landscape of Europe in the 1800s. Much of this has been cleared by human activity, though floodplain forests have been impacted less than other kinds of forests. This makes them important
refugia Refugium, plural refugia, the Latin for "refuge" or "hideaway", may refer to: * Refugium (fishkeeping), an appendage to a marine, brackish, or freshwater fish tank that shares the same water supply * Refugium (population biology), a location of an i ...
for biodiversity. Human destruction of floodplain ecosystems is largely a result of flood control, hydroelectric development (such as reservoirs), and conversion of floodplains to agriculture use. Transportation and waste disposal also have detrimental effects. The result is the fragmentation of these ecosystems, resulting in loss of populations and diversity and endangering the remaining fragments of the ecosystem. Flood control creates a sharper boundary between water and land than in undisturbed floodplains, reducing physical diversity. Floodplain forests protect waterways from erosion and pollution and reduce the impact of floodwaters. The disturbance by humans of temperate floodplain ecosystems frustrates attempts to understand their natural behavior. Tropical rivers are less impacted by humans and provide models for temperate floodplain ecosystems, which are thought to share many of their ecological attributes.


Flood control

Excluding
famines A famine is a widespread scarcity of food Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual con ...
and
epidemic An epidemic (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approx ...
s, some of the worst natural disasters in history (measured by fatalities) have been river floods, particularly in the
Yellow River The Yellow River (Chinese: , Jin: uə xɔ Mandarin Mandarin may refer to: * Mandarin (bureaucrat), a bureaucrat of Imperial China (the original meaning of the word) ** by extension, any senior government bureaucrat A bureaucrat is ...
in China – see
list of deadliest floods This is a list consisting of the deadliest floods worldwide with a minimum of 60 deaths. List Floods by year Only floods having caused 10 fatalities or more in 21st-century are listed. 2021 2020 2019 2018 2017 2016 2015 201 ...
. The worst of these, and the worst natural disaster (excluding famine and epidemics) were the
1931 China floods The 1931 China floods, or the 1931 Yangtze–Huai River floods, were a series of floods that occurred from June to August 1931 in the Republic of China (1912–1949), Republic of China, hitting major cities such as Hankow, Wuhan, Nanjing and beyo ...
, 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 periodA return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface o ...
. In the United States the manages the
National Flood Insurance Program The National Flood Insurance Program (NFIP) is a program created by the Congress of the United States The United States Congress or U.S. Congress is the bicameral legislature of the federal government of the United States The fed ...
(NFIP). The NFIP offers insurance to properties located within a flood prone area, as defined by the
Flood Insurance Rate MapA flood insurance rate map (FIRM) is an official map of a community within the United States that displays the floodplains, more explicitly special hazard areas and risk premium zones, as delineated by the Federal Emergency Management Agency The ...
(FIRM), 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 in Albania links Lake Butrint with the Straits of Corfu Corfu (, ) or Kerkyra ( el, Κέρκυρα, Kérkyra, ), ; ; la, Corcyra. is a Greek islands, Greek island located in the Ionian Sea, of the Ionian Islands, and, including its small satel ...
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 US government also sponsors flood hazard mitigation efforts to reduce flood impacts.
California California is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper i ...

California
's Hazard Mitigation Program is one funding source for mitigation projects. A number of whole towns such as
English, Indiana English is a town in Crawford County, Indiana, United States. It has served as the county seat of Crawford County since 28 December 1893. The population was 645 at the 2010 United States Census, 2010 census, making it one of Indiana's smallest count ...
, 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 floodplains, such as the
Inner Niger Delta The Inner Niger Delta, also known as the Macina or Masina, is the inland delta of the Niger River. It is an area of fluvial wetlands, lakes and floodplains in the semi-arid Sahel area of central Mali, just south of the Sahara desert. Location a ...
of
Mali Mali (; ), officially the Republic of Mali (french: République du Mali; bm, ߡߊߟߌ ߞߊ ߝߊߛߏߖߊߡߊߣߊ, Mali ka Fasojamana, ff, 𞤈𞤫𞤲𞥆𞤣𞤢𞥄𞤲𞤣𞤭 𞤃𞤢𞥄𞤤𞤭, Renndaandi Maali, ar, جمهورية م ...

Mali
, annual flooding events are a natural part of the local ecology and rural economy, allowing for the raising of crops through recessional agriculture. However, in
Bangladesh Bangladesh (, bn, বাংলাদেশ, ), officially the People's Republic of Bangladesh, is a country in South Asia South Asia is the southern region of Asia, which is defined in both geography, geographical and culture, ethno-c ...

Bangladesh
, which occupies the
Ganges Delta The Ganges Delta (also known as the Sundarbans Delta or the Bengal Delta) is a river delta A river delta is a landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body. Landforms toget ...
, the advantages provided by the richness of the alluvial soil of the floodplain are severely offset by frequent floods brought on by
cyclone In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mo ...

cyclone
s and annual
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

monsoon
rains. These extreme weather events cause severe economic disruption and loss of human life in the densely-populated region.


See also

*, area of grassland or pasture beside a river, subject to seasonal flooding. *, area of grassland or pasture beside a river, subject to controlled seasonal flooding. * as a good example of a floodway. * *, a technique for mitigating the effects of flooding on structures, mandated in some regions.


References


Sources

* Powell, W. Gabe. 2009. Identifying Land Use/Land Cover (LULC) Using
National Agriculture Imagery ProgramThe National Agriculture Imagery Program (NAIP) acquires aerial imagery during the agricultural growing seasons in the continental United States. It is administered by the USDA's Farm Service Agency (FSA) through the Aerial Photography Field Office ( ...
(NAIP) Data as a Hydrologic Model Input for Local Flood Plain Management. Applied Research Project, Texas State University. http://ecommons.txstate.edu/arp/296/ *


External links

* {{Authority control Flood control Fluvial landforms Hydrology Wetlands