Floods occur either because heavy rainfall causes rivers to overflow their banks into flood plains (this may be augmented by melting snow) or because high tides, possibly combined with large freshwater flows, cause coastal flooding. The flood plains affected may be very narrow (as in mountainous regions) or many miles wide. Floods may have fatal consequences for individuals in the affected area; in any case considerable damage is caused to property, particularly where industrial and community development has encroached on the natural channels of larger rivers. Loss of crops and livestock can have severe economic consequences. Much damage may be caused to dwellings, creating many homeless people, particularly in developing countries. Flooding in a given area may only recur infrequently which makes it difficult to justify the cost of extensive preventive measures, especially in the case of freak floods.
Flood waves of high pressure and velocity may disrupt telecommunication lines and traffic, carrying large quantities of mud, rubble and trees, and eroding watercourses and bridge structures. Their effect is short but devastating. The lack of warning makes them particularly dangerous.
Over the period 1980-85, worldwide floods accounted for 33,542 deaths and some US$ 22,170 million in property loss. Few regions are immune from flooding.
The Southeast Asian region is particularly vulnerable, and half the population of India is considered to be continually menaced by flooding. Flooding in the period 1996-1998 has been compounded by the impact of El Nino - the worst in history - with its full devastating force falling most heavily on the poor. In Bangladesh, in 1998, floods that kept two-thirds of the nation under water for more than two months, set back many of the recent social and economic gains. In China there was flooding of the Yangtze River region causing an estimated 3,500 deaths, 5 million homes destroyed, and 200 million lives dislocated.
In the USA it is estimated that flood damage is approximately $300 million annually. In 1993, after only a month of the flooding that devastated the American Midwest, damages were estimated up to $8 billion. In December 1993, Germany, France, Belgium and the Netherlands experienced the worst floods in 60 years during which power supplies were cut, roadways and rail lines were swamped and thousands were forced to leave their homes.
Extensive areas of Europe have experienced extreme floods and droughts in the 1990s, and changes in climatic conditions and land use have increased the potential for further flooding.
Flooding and landslides in Mexico in 1999 caused by continuous rains have seriously affected the States of Veracruz, Tabasco, Puebla, Hidalgo, Oaxaca, Chiapas and Jalisco. The floods, the worst to have occurred in Mexico for 100 years, have caused more than 400 deaths and left 300,000 people homeless. They have also aggravated recent earthquake damage in some of the same States.
Flooding of the Yangtze river in China between late June and mid-August 1996 affected 20 million people and caused economic losses of more than US$20 000 million. Flooding in Central Europe in 1997 caused economic damage estimated at US$2 900 million in Poland and US$1 800 million in the Czech Republic – in EisenhÃ¼ttenstadt the previous flood record of 1854 was exceeded by 62 cm. Severe floods were also recorded in 1997 in Kenya, Myanmar, Somalia, the United States and along the Pacific coast of Latin America.
The main methods of controlling floods are building embankments to contain flood water within rivers and constructing reservoirs to impound flood waters before being released at a slow rate. For over 3000 years both methods have consistently failed. Embankments do not decrease the volume of water they dramatically increase the rate of flow. They simply transfer the threat of floods further down stream. Silt, normally deposited on the flood plains deposited in the river bed and as the bed rises embankments must rise until the river is above the surrounding country side. China's Yellow River bed is now five to ten meters above ground level as it crosses the Yellow Plains. This, in turn, increases the magnitude of the floods when they come. Dams frequently are not solutions to flood control. Very few dams are built solely for the purpose of flood control; out of 1,554 dams listed in "The International Registry of Large Dams", only 17 had been built only for flood control. The water level in flood control dams should be kept as low as possible while the water level in dams built for irrigation or hydro-electrical purposes should be kept as high as possible. This conflict in requirements for multipurpose dams usually means irrigation and electricity generation have priority and flood can result. These structural solutions, coupled with massive deforestation and increased development of flood plains result in disastrous floods with increasing frequency, cost and lose of life. It is not flooding that causes these disasters but man's responses operating in the illusion that floods can be eliminated. Indeed, floods have been used as a part of a sustainable agriculture for irrigating and fertilizing fields.