Natural Hazards: Causes and Effects - Disaster Management Center ...

Chapter 6
Floods
Introduction
People have long been attracted to floodplains. Here rivers deposit the topsoil picked up
elsewhere, so the land is fertile. Floodplains are both flat and near water, so irrigation,
ploughing and transport (usually aided by the river) are all made easier. The heavy settlement
along the lower reaches of Egypt’s Nile, India’s Ganges, Bangladesh’s Brahmaputra-Padma,
the Yellow River and Yangtze of China, and the Tigris and Euphrates of Iraq are all examples of
floodplain civilizations.
Floodplains are desirable places to live, not only in agricultural societies, but also in industrial
countries where the floodplains often host large capitals that use the river water for industry and
its mouth as a harbor for shipping.
The floodplain of a river is a clearly definable physical feature of its valley. It is the almost flat
area that borders the river. A floodplain is built up of layers of sediment deposited by the river
when it periodically overflows its normal banks. Steep narrow valleys in mountain regions have
no floodplains at all, but a large complex system of converging rivers in a lowland region may
have a floodplain over a hundred kilometers wide. There is a natural tendency for a river to
deposit sediment in its channel during times of low flow, so that an equilibrium is arrived at
where the river comfortably fills its main channel under normal conditions. Therefore the river
will spread out automatically onto its floodplain during periods of high flow—after all, floodplains
are for floods.
But expansion of towns soon forces them to spread out—all too commonly onto the floodplains,
where they were immediately in danger. In the United States alone there are currently an
estimated 10 million people living in areas subject to flooding.
A flood is too much water in the wrong place, whether it be an inundated city or a single street
or a field flooded due to a blocked drain. Among the trigger mechanisms are dam or levee
failures; more rain than the landscape can dispose of; the torrential rains of hurricanes;
tsunamis; ocean storm surges; rapid snow melts; ice floes blocking a river; and burst water
mains.
Flooding is generally defined as any abnormally high streamflow that overtops the natural or
artificial banks of a stream. Flooding is a natural characteristic of rivers. The floodplains are
normally dryland areas. They are an integral part of a river system that acts as a natural
reservoir and temporary channel for flood waters. If more runoff is generated than the banks of
a stream channel can accommodate, the water will overtop the stream banks and spread over
the floodplain. The ultimate factor of damage, however, is not the quantity of water being
discharged but how high the water goes above normal restraints or embankments.
Furthermore, floods can form where there is no stream, as for example when abnormally heavy
precipitation falls on flat terrain at such a rate that the soil cannot absorb the water or the water
cannot run off as fast as it falls.
Of all the disasters except droughts, flood disasters affect the most people. But there are many
more flood disasters than droughts, and the number affected by floods is increasing much more
rapidly than those suffering droughts. In fact, flooding is one natural hazard that is becoming a
greater threat rather than a constant or declining one. Floods are caused not only by rain but