Floodplain
Encyclopedia
A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream
or river
that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flood
ing during periods of high discharge. It includes the floodway, which consists of the stream channel
and adjacent areas that carry flood flows, and the flood fringe, which are areas covered 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 easily.
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 rock and mud. 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 stream.
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, ox-bow lakes and bayou
s, marsh
es 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 down-stream, and often, for a distance, from the side towards the center.
.
The extent of floodplain inundation depends in part on the flood magnitude, defined by the return period
.
In the United States the National Flood Insurance Program
regulates development in mapped floodplains based on the 100-year flood
(1% annual chance of a flood of this magnitude). The Flood Insurance Rate Maps, typically depict both the 100-year floodplain and the 500-year floodplains. 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. When a floodway is shown on the Flood Insurance Rate Maps, the portion of the 100-year floodplain outside of the floodway is known as the flood fringe. 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 floodproofed 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 cyclone
s and annual monsoon
rains, which cause severe economic disruption and loss of human life in this densely-populated region.
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
or river
River
A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...
that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flood
Flood
A flood is an overflow of an expanse of water that submerges land. The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water...
ing during periods of high discharge. It includes the floodway, which consists of the stream channel
Channel (geography)
In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...
and adjacent areas that carry flood flows, and the flood fringe, which are areas covered by the flood, but which do not experience a strong current
Current (stream)
A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometrics...
.
In other words, a floodplain is an area near a river or a stream which floods easily.
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 rock and mud. 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
Bank filtration
Bank filtration is a type of filtration that works by passing water to be purified for use as drinking water through the banks of a river or lake. It is then drawn off by extraction wells some distance away from the water body...
compared to the water in the stream.
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
Missouri River
The Missouri River flows through the central United States, and is a tributary of the Mississippi River. It is the longest river in North America and drains the third largest area, though only the thirteenth largest by discharge. The Missouri's watershed encompasses most of the American Great...
floodplain taken by the United States Geological Survey
United States Geological Survey
The United States Geological Survey is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The organization has four major science disciplines, concerning biology,...
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
Alluvial plain
An alluvial plain is a relatively flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms...
would show deposits of a similar character.
The floodplain during its formation is marked by meandering or anastomotic
Braided river
A braided river is one of a number of channel types and has a channel that consists of a network of small channels separated by small and often temporary islands called braid bars or, in British usage, aits or eyots. Braided streams occur in rivers with high slope and/or large sediment load...
streams, ox-bow lakes and bayou
Bayou
A bayou is an American term for a body of water typically found in flat, low-lying areas, and can refer either to an extremely slow-moving stream or river , or to a marshy lake or wetland. The name "bayou" can also refer to creeks that see level changes due to tides and hold brackish water which...
s, marsh
Marsh
In geography, a marsh, or morass, is a type of wetland that is subject to frequent or continuous flood. Typically the water is shallow and features grasses, rushes, reeds, typhas, sedges, other herbaceous plants, and moss....
es 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 down-stream, and often, for a distance, from the side towards the center.
Ecology
Floodplains can support particularly rich ecosystems, both in quantity and diversity. 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 (particularly birds) 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 agricultureAgriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
.
Interaction with society
Historically, many towns have been built on floodplain, 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
The extent of floodplain inundation depends in part on the flood magnitude, defined by the return period
Return period
A return period also known as a recurrence interval is an estimate of the interval of time between events like an earthquake, flood or river discharge flow of a certain intensity or size. It is a statistical measurement denoting the average recurrence interval over an extended period of time, and...
.
In the United States the National Flood Insurance Program
National Flood Insurance Program
The National Flood Insurance Program is a program created by the Congress of the United States in 1968 through the National Flood Insurance Act of 1968 . The program enables property owners in participating communities to purchase insurance protection from the government against losses from flooding...
regulates development in mapped floodplains based on the 100-year flood
100-year flood
A one-hundred-year flood is calculated to be the level of flood water expected to be equaled or exceeded every 100 years on average. The 100-year flood is more accurately referred to as the 1% annual exceedance probability flood, since it is a flood that has a 1% chance of being equaled or exceeded...
(1% annual chance of a flood of this magnitude). The Flood Insurance Rate Maps, typically depict both the 100-year floodplain and the 500-year floodplains. 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
Stream channel
Stream channel is the physical confine of a stream consisting of a bed and stream banks.Stream channels exist in a variety of geometries. The stream channel development is controlled by both water and sediment movement. There is a difference between low gradient streams and high gradient streams...
and any adjacent areas that must be kept free of encroachments that might block flood flows or restrict storage of flood waters. When a floodway is shown on the Flood Insurance Rate Maps, the portion of the 100-year floodplain outside of the floodway is known as the flood fringe. 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 floodproofed 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
English, Indiana
English is a town in and the county seat of Sterling Township, Crawford County, Indiana, United States. The population was 645 at the 2010 census, making it one of Indiana's smallest county seats.-Geography:English is located at ....
, 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
Mali
Mali , officially the Republic of Mali , is a landlocked country in Western Africa. Mali borders Algeria on the north, Niger on the east, Burkina Faso and the Côte d'Ivoire on the south, Guinea on the south-west, and Senegal and Mauritania on the west. Its size is just over 1,240,000 km² with...
, annual flooding events are a natural part of the local ecology and rural economy, allowing for the raising of crops through recessional agriculture
Recessional agriculture
Recessional agriculture is a form of agricultural cultivation that takes place on a floodplain. Farmers practice recessional agriculture by successively planting in the flooded areas after the waters recede. Thus recessional agriculture serves as a rudimentary form of irrigation. Soil type is an...
. But in Bangladesh
Bangladesh
Bangladesh , officially the People's Republic of Bangladesh is a sovereign state located in South Asia. It is bordered by India on all sides except for a small border with Burma to the far southeast and by the Bay of Bengal to the south...
, which occupies the Ganges Delta
Ganges Delta
The Ganges Delta is a river delta in the South Asia region of Bengal, consisting of Bangladesh and the state of West Bengal, India. It is the world's largest delta, and empties into the Bay of Bengal...
, the advantages provided by the richness of the alluvial soil of floodplains are severely offset by frequent floods brought on by cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...
s and annual monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
rains, which cause severe economic disruption and loss of human life in this densely-populated region.
See also
- Flood-meadowFlood-meadowA flood-meadow is an area of grassland or pasture beside a river, subject to seasonal flooding. Flood-meadows are distinct from water-meadows in that the latter are artificially created and maintained, with flooding controlled on a seasonal and even daily basis.- Examples :* Angel & Greyhound...
, area of grassland or pasture beside a river, subject to seasonal flooding. - Water-meadowWater-meadowA water-meadow is an area of grassland subject to controlled irrigation to increase agricultural productivity. Water-meadows were mainly used in Europe from the 16th to the early 20th centuries...
, area of grassland or pasture beside a river, subject to controlled seasonal flooding. - Crevasse splayCrevasse splayA crevasse splay is a sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain. This breach can cause large deposits that spread in a pattern similar to that of an alluvial fan deposit. Once the levee has been breached the...
, sedimentary deposit formed when an overloaded stream breaks its levee. - List of alluvial sites in Switzerland
- Red River FloodwayRed River FloodwayThe Red River Floodway is an artificial flood control waterway in Western Canada, first used in 1969. It is a long channel which, during flood periods, takes part of the Red River's flow around the city of Winnipeg, Manitoba to the east and discharges it back into the Red River below the dam at...
as a good example of a floodway. - Floodplain restorationFloodplain restorationFloodplain restoration is the process of fully or partially restoring a river's floodplain to its original conditions before having been affected by the construction of levees and the draining of wetlands and marshes....
- Flood openingFlood openingA flood opening or flood vent is an orifice in an enclosed structure intended to allow the free passage of water between the interior and exterior.-Flood Openings in the United States:...
, a technique for mitigating the effects of flooding on structures, mandated in some regions.