floodplain

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floodplain,

level land along the course of a river formed by the deposition of sediment during periodic floods. Floodplains contain such features as levees, backswamps, deltadelta
[from triangular shape of the Nile delta, like the Greek letter delta], a deposit of clay, silt, and sand formed at the mouth of a river where the stream loses velocity and drops part of its sediment load.
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 plains, and oxbow lakes. Floodplains may be extensive, such as below the conflux of the Ohio and the Mississippi, where they have a width up to 80 mi (130 km). Rivers with extensive floodplains are the Nile, Ganges, Danube, and Po. Floodplains are generally very fertile, thus making them rich agricultural lands. The disadvantage of farming on a floodplain is the natural hazard of floodsflood,
inundation of land by the rise and overflow of a body of water. Floods occur most commonly when water from heavy rainfall, from melting ice and snow, or from a combination of these exceeds the carrying capacity of the river system, lake, or the like into which it runs.
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. In the United States there has been extensive house construction on floodplains in recent years, necessitating the construction of new dams to control small, annual floods.

Floodplain

 

the part of a valley floor that is flooded at high water but is above the low-water stage. A floodplain consists of two parts: a base of channel aluvium underlying a flood alluvial layer formed by the annual or less frequent deposition of silt by floodwater. Occasionally the foundation, composed of bedrock or more ancient alluvium, is exposed.

The most intensive accumulation of coarse-grained alluvium, forming crests and ridges, occurs in the parts of the floodplain closest to the channel. Farther into the floodplain fine-grained alluvium settles. Only silt particles are carried close to the high bank, where the floodplain is lower and swampy (terrace flood-plain).

As sediment accumulates on the surface of the floodplain, the banks of some sections are continuously undercut by the river flow, and bars are built up in other sections. As a result, the contours of floodplain change constantly. On the surface of the floodplain there are numerous hollows, which are traces of meanders (oxbows) and branches of the river that formed cutoffs and alternate with levees. The hollows attest to the channel’s meandering. In the parts of the valley where the alluvium accumulation primarily occurs, that is, in the lower course of the river, a solid channel levee often forms along the channel. The surface of the floodplain is frequently divided by a low scarp into a high floodplain and a lower one.

The floodplain is formed when the valley broadens as the result of lateral shifts in the river channel. Vast floodplains, reaching 20–40 km in width, are characteristic of large, level rivers with uneven flow. If the river flows in a tectonic trough, the width of the floodplain is determined by the dimensions of the trough.

The vegetation plays an important role in determining the relief of a floodplain. It helps prevent erosion and promotes the accumulation of alluvium. The center of the floodplain and the terrace floodplain are usually covered by trees and shrubs. Some floodplains are developed for agricultural use. In such cases, a large part of the land is used for meadows, which are among the best feed lands. The floodplain soils, regularly replenished with organic silts, are very fertile. During the flooded period, the floodplain is a spawning ground for fish.

Placer deposits of such useful minerals as gold, platinum, and cassiterite are found in the alluvial deposits making up the flood-plains of small mountain rivers. The sand and gravel are used as building materials.

REFERENCE

Makkaveev, N. I. Ruslo reki i eroziia v ee basseine. Moscow, 1955.

floodplain

[′fləd‚plān]
(geology)
The relatively smooth valley floors adjacent to and formed by alluviating rivers which are subject to overflow.
References in periodicals archive ?
There is simply no way economically or politically that this could be accomplished by turning large areas of flood-plains into parks," he said.
Control infrastructure prevents high flows from entering flood-plains, thus diminishing both natural flood storage capacity and the processes that sustain healthy riverside forests and wetlands," they observed.
Why now, when it could take a year or longer before the city has new flood-plain maps?
For another, even if the site is found to be in the flood plain, the city will enforce flood-plain development conditions.
The development would lead to a net loss of flood storage capacity, which is contrary to our longstanding policy on flood-plain management,'' he said.
Roland Honeybunn, head of leisure services at Rugby Borough Council, which had inaugurated the scheme, said: ``A formal objection has been received from the Environment Agency, which is taking a much firmer stance on any development within flood-plains since the Easter 1998 floods.
Some residents living near the PeaceHealth properties would rather the city resolved the flood-plain questions before moving ahead with a project that could diminish the area's ability to absorb a flood, said Bonnie Ullmann, treasurer for the neighborhood association.
But Philip Farrington, PeaceHealth director of land use planning and development, said his organization will address flooding concerns and respect the city's flood-plain development requirements on both parcels.
Further, he added, the city is only considering tinkering with a policy that generally allows flood-plain development.
The council's job would be far tougher if staff members were pushing a 180-degree turn to disallow all flood-plain development, Kelly said.
Almost 30 people took up the city's request to speak on the flood-plain code, which has confounded the City Council recently amid growing local and federal pressures.
In Springfield, there is confusion surrounding the accuracy of the city's flood-plain maps at a time when big projects are lining up near the rivers - PeaceHealth's new hospital in Gateway, for one.