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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 ...
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 (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study o ...
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. Because the regular flooding of floodplains can deposit nutrients and water, floodplains frequently have high
soil fertility Soil fertility refers to the ability of soil to sustain agricultural plant growth, i.e. to provide plant habitat and result in sustained and consistent Crop yield, yields of high quality.
; some important agricultural regions, such as the Mississippi river basin and the
Nile The Nile, , Bohairic , lg, Kiira , Nobiin language, Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered ...
, rely heavily on the flood plains. Agricultural regions as well as urban areas have developed near or on floodplains to take advantage of the rich soil and fresh water. However, the risk of flooding has led to increasing efforts to control flooding.


Formation

Most floodplains are formed by deposition on the inside of river 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 ...
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 ( ne, कोशी, , hi, कोसी, ) is a transboundary river which flows through China, Nepal and India. It drains the northern slopes of the Himalayas in Tibet and the southern slopes in Nepal. From a major confluence o ...
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 from the Rhine–Meuse–Scheldt delta. It has a to ...
and
Rhine River The Rhine ; french: Rhin ; nl, Rijn ; wa, Rén ; li, Rien; rm, label=Sursilvan, Rein, rm, label=Sutsilvan and Surmiran, Ragn, rm, label=Rumantsch Grischun, Vallader and Puter, Rain; it, Reno ; gsw, Rhi(n), including in Alsatian dialect, Al ...
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-shaped deposits with numerous distributary channels. Crevasse formation is most common in sections of rivers where the river bed is accumulating sediments ( aggrading). Repeated flooding eventually builds up an alluvial ridge, whose natural levees and abandoned 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 Terrace (geology), terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a "tread", separated from either an adjacent floodplain ...
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, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal areas ...
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 meadows 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 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, caused by several factors including war, natural disasters, crop failure, Demographic trap, population imbalance, widespread poverty, an Financial crisis, economic catastrophe or government policies. Th ...
and
epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics ...
s, some of the worst natural disasters in history (measured by fatalities) have been river floods, particularly in the
Yellow River The Yellow River or Huang He (Chinese: , Standard Beijing Mandarin, Mandarin: ''Huáng hé'' ) is the second-longest river in China, after the Yangtze River, and the List of rivers by length, sixth-longest river system in the world at th ...
in China – see list of deadliest floods. The worst of these, and the worst natural disaster (excluding famine and epidemics) 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 (FEMA) manages the
National Flood Insurance Program The National Flood Insurance Program (NFIP) is a program created by the Congress of the United States in 1968 through the National Flood Insurance Act of 1968 (P.L. 90-448). The NFIP has two purposes: to share the risk of flood losses through floo ...
(NFIP). The NFIP offers insurance to properties located within a flood prone area, as defined by the Flood Insurance Rate Map (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 physical geography Physical geography (also known as physiography) is one of the three main branches of geography Geography (from Ancient Greek, Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘G ...
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 U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
's 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 floodplains, such as the Inner Niger Delta of
Mali Mali (; ), officially the Republic of Mali,, , ff, 𞤈𞤫𞤲𞥆𞤣𞤢𞥄𞤲𞤣𞤭 𞤃𞤢𞥄𞤤𞤭, Renndaandi Maali, italics=no, ar, جمهورية مالي, Jumhūriyyāt Mālī is a landlocked country in West Africa. 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 (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world, with a population exceeding 165 million pe ...
, which occupies the Ganges Delta, 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, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an antic ...
s and annual
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...
rains. These
extreme weather Extreme weather or extreme climate events includes unexpected, unusual, severe weather, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are b ...
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 Program (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