A flood plain, in the strict sense of the phrase, is the lowest section of a river’s valley. As a river proceeds from its source, its upper valley will tend to be a deep V shape with a narrow valley floor. Further downstream the valley will widen and become less deep, reflecting the river’s decreased ability to erode vertically into the landscape.
The final section of a valley, the flood plain, begins after the river has left the hills of its source behind, and begins to meander across an ever widening, almost level area, on its approach to the sea. The word plain defines itself as a flat area of land. Plains can be relatively narrow, but often can be extremely wide.
In some countries the term flood plain is more generally used to describe any extensive, flat area of land which is liable to flood. The liability is normally defined as during a certain period of time. This description includes coastal plains which can flood due to unusually high tides, strong onshore winds, tidal waves or a combination of two or more of those.
In north west France a coastal area was so devastated by marine flooding a few years ago that the settlement there has been abandoned totally. The cost of the flooding in terms of insurance and personal expense was clearly enormous, and has been costed in terms of future risk. As global sea levels are tending to rise, the risk is considered too great and potentially costly in the long term, hence the abandonment.
Flood plains by their usual definition lie to each side of a river, often a large river. Heavy rain, anywhere upstream from a flood plain, can cause the volume of water in a river to increase to the extent that it overflows it banks. This will be the main cause of the flood on the plain.
The problem is that flat land can appear to provide the ideal site for urban development, and it seems that in many cases houses and other buildings have been and are being constructed despite the future risk of flooding. Most countries have an Environment Agency of some sort which will have produced maps showing areas at risk of flooding. Anyone considering the purchase of a house on low lying land should consult these maps, which can often be found online.
If the advice of the Environment Agency is disregarded, and a house is purchased or built, the risks can be considerable. An overflowing river will clearly submerge all adjacent land. As it lifts the local water table, it may render the sewage disposal system inoperable. So the combined risk will be of damage to property, furniture and fittings; a risk to human and animal life; telephone and electrical services, particularly if they are underground; and shortage of clean water with the inherent disease risk from polluted water not running off effectively.
The cost of all this to insurance companies may be such that future premiums will be increased. It may, in fact, be difficult to obtain low cost house insurance for a property situated in an area of high flood risk.
In order to reduce the risk of flooding from a river on a plain, embankments or levées are often constructed along the river banks. These are designed to retain an increased volume of water and they are common in Holland and along all large rivers in developed areas. However they can increase the risk if water volume in the river rises to such an extent that it overflows the levées. As the tops of levées are considerably higher than the flood plain they are protecting, a flood over a levée will be flowing downhill and will therefore flood with a considerable current. The increased risk of this circumstance cannot be overestimated.
Along the Mississippi river, particularly in its lower reaches with an enormously wide flood plain to protect, there are levées. But there have been times when the river has flooded over the levées, and the amount of flood damage caused has been considerable. New Orleans comes to mind.
A flood plain then is normally considered to be the flat land on either side of the lower reaches of a river, although in some countries the term is widened to include lake shore areas and coastal regions. The dangers of living there can be considerable and include loss of life and damage to property and mains services.