Floodplains

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.

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.

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.

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 very fertile for farming;
River transportation was a key economic factor in the founding of many communities;
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. 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 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.