Floodplain

This picture shows the flood plain following a 1 in 10 year flood on the Isle of Wight.

Gravel floodplain of a glacial river near the Snow Mountains in Alaska, 1902.

Entrenched river: The Virgin River at the upper end of Zion Canyon, Zion National Park, Utah, has almost no floodplain at all.

Erosional floodplain with indistinct boundary: The Little Laramie River in Albany County, Wyoming, 1905.

Aggradation and planation: The Laramie River meanders across its floodplain in Albany County, Wyoming, 1949.

Aggradational floodplain: This floodplain of a small meandering stream in La Plata County, Colorado, is underlain by silt deposited above a dam formed by a terminal moraine left by the Wisconsin Glacier.

Oxbow lakes on the floodplain of the White River near Des Arc, Arkansas, 1949.

Riparian vegetation on the floodplain of the Lynches River near Johnsonville, South Carolina. These tupelo and cypress trees show the high water mark of flooding.

A floodplain, or flood plain, is flat or nearly flat land adjacent to a stream or river that experiences occasional or periodic flooding. 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.

Formation

Floodplains are formed in two ways: by erosion; and by aggradation. An erosional floodplain is created as a stream cuts deeper into its channel and laterally into its banks. A stream with a steep gradient will tend to downcut faster than it causes lateral erosion, resulting in a deep, narrow channel with little or no floodplain at all. This is the case of entrenched rivers such as the Virgin River in Zion National Park in the U.S. state of Utah and the Colorado River in the Grand Canyon in the U.S. state of Arizona. As the stream approaches its base level, lateral erosion increases, creating an extremely broad floodplain, as in the case of the Platte River flowing across the Great Plains of the United States. There, the boundary between river and floodplain is not clear. In unmodified drainage systems where the terrain is fairly flat and rainfall intermittent, a floodplain may take the place of a river entirely. Instead of a defined streambed, there is simply a broad flat area where water flows from time to time.

An aggradational floodplain is created when a stream lays down thick layers of sediment. This happens when the stream's gradient becomes very slight and its velocity decreases, forcing it to drop sediment brought from higher regions nearer its source. Consequently the lower portion of the river valley becomes filled with alluvium. In times of flood, the rush of water in the high regions tears off and carries down a greater quantity of sediment resulting in planation (creation of a flat terrain) as well as aggradation. Thus, a stream such as the Laramie River in the U.S. state of Wyoming, widens its valley by working in meanders from side to side and covers the widened valley with sediment. Glacial drainage may also form an aggradational floodplain simply by filling up its valley with alluvium.

Aggradational floodplains are more common than erosional ones. Any obstruction across a river's course, such as a band of hard rock, may form a floodplain behind it. Indeed, anything that checks a river's course and causes it to drop its load will tend to form a floodplain. Aggradational floodplains are most commonly found near the mouths of large rivers, such as the Rhine, the Nile, the Ganges and the Mississippi, where there are occasional floods and the river usually carries a large amount of sediment. Natural levees form inside which the river usually flows, gradually raising its bed above the surrounding plain. Occasional breaches during floods cause the overloaded stream to spread in a great lake over the surrounding country, where the silt covers the ground in consequence.

Physical geography

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 being drawn from them being pre-filtered compared to the water in the stream.

Geologically ancient floodplains are often represented in the landscape by stream 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 being 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, and 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 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 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 1000 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 agriculture.

Markedly different species grow in floodplains than grow outside of floodplains. For instance, riparian trees (that grow in floodplains) tend to be very tolerant of root disturbance and tend to be very quick-growing, compared to non-riparian trees.

Interaction with society

Historically, many towns, homes and other buildings have been built on floodplains where they are highly susceptible to flooding, for several reasons:

• This is where water is most available;
• Floodplain land is usually the most fertile for farming;
• Rivers represent cheap sources of transportation, and are often where railroads are located; and
• Flat land is easier to develop than hilly land

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's 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.

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 Niger Inland Delta of Mali, annual flooding events are a natural part of the local ecology and rural economy. 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

References

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. {{cite encyclopedia}}: Missing or empty |title= (help)