DISASTER REVIEWS: FLOOD

Flood is a body of water that covers normally dry land. Most floods are harmful. They destroy homes and other property and even carry off the topsoil, leaving the land barren. When people are not prepared, sudden and violent floods may bring huge losses. Rivers, lakes or sea may flood the land. River floods are more common, through lake and seacoast floods can be more serious. But sometimes floods may be helpful. For example, the yearly floods of the Nile River built up the plains of Egypt and made the Nile Valley one of the most fertile regions in the world. These floods brought fertile soil from lands far to the south and deposited the soil on the Egyptian plains.

River floods. Most rivers overflow their normal channels about once every two years. When a river overflows land where people live, it cause a flood. When river overflows land where people do not live, it is said to be in flood.

Common causes of river floods include too much rain at one time and the sudden melting of snow and ice. Under such conditions, rivers may receive more than 10 times as much water as their beds can hold. Heavy rains produce flash floods if the rains cause small rivers or streams to raise suddenly and overflow. For example, in 1952, 230 milimeters of rain fell in 24 hours on the upper valleys of the East and West Lynn rivers in Devron, England. The West Lyn river burst its banks and a torrent swept through the town of Lynmouth, killing 23 people and making about 1,000 people homeless. Most flash floods occur in mountainous areas. They also occur in deserts, when a rare but violent thunderstorm turns wadis (normally dry watercourse) into ranging torrents.

Many major rivers are famous for great floods. The Chinese call the Huang He (Yellow River) “China’s sorrow” because floods have caused so much destruction. The worst floods occurred in 1887, when nearly a million people died. Disastrous floods have also occurred along the Mississippi River and its tributaries in the United States. For example, in 1937, the Ohio and Mississippi valleys experienced floods that killed more than 135 people and left about a million people homeless. In 1993, heavy rains in the American Midwest for about two months resulted in flooding along the upper Mississippi and Missouri river systems. The floods damaged over 10 billion U.S. dollars worth of property and forced about 74,000 people from their homes. In 1972, heavy rains in the states of New York and Pennsylvania caused rivers to overflow. The resulting floods caused about 3 billion U.S dollars worth of damage and left more than 15,000 people without home.

One of the worst floods in Indian history occurred in 1840 when an earthquake occurred in the upper Indus valley. A landslide created a natural rock dam in the valley. A lake formed behind this dam, reaching about 60 kilimetres in length and more than 300 metres in depth. When the da m broke, a torrent surged down the Indus Valley causing disastrous floods and great loss of life.

Australia’s worst flood based on loss of life occurred in June 1852, when 89 people out of a population of 250 were drowned in Gundagai, New South Wales. In December 1916, a fast-rising flood swept through the lowlying areas of Clermont, Queensland, drowning 61 people. In autumn 1975, storms and prolonged rainfall caused flooding in New South Wales and Victoria, Hundreds of homes were damaged or demolished.

Seacoast floods. Most floods from the sea result from hurricanes or other powerful storms that drive water against harbours and push waves far inland. In 1970, a cyclone and a tidal wave in the Bay or Bengal, a part of the Indian Ocean, caused the greatest sea flood disaster in history. Huge waves struck the coast of Bangladesh and killed about 266,000 people. Te flood also destroyed the cattle, crops and homes of millions of other victims.

Depressions (regions of low air pressure) cause coastal flooding in Western Europe. Sometimes, when depressions are over the North Sea, a combination of strong winds and high tides whip up the water into “hump” causing high surge tides along the coast. Such a storm in 1953 caused waves that breached the dykes (sea walls) of the Netherlands, flooding more than four per cent of the country. More flood damage occurred on the east coast of England and in the Thames estuary.

The coasts of southeastern England, including London on the Thames estuary, are especially vulnerable to floods, because geologists estimate that this region is sinking at a rate of 30 centimetres every 100 years. This factor, combined with a possible rise in the sea level caused by global warming, due to the “greenhouse effect,” makes London especially vulnerable to surge tides.

Earthquake and volcanoes also produce high waves that cause coastal floods. A sea wave caused by an earthquake or a volcanic eruption is called a tsunami. For example, in 1883, the volcano Krakatoa in Sunda Strait, west of Java, erupted and caused tsunami up to 35 metres high. About 30,000 people were killed and one ship was carried 2.5 kilometres inland. Seiche.

Other floods. Storms and high winds also cause floods along lakeshores. Some lakeshore floods occur when water moves suddenly from side to side in a seiche. the failure or artificial structures, such as dam, has caused a number of floods. In 1963, the Vaiont Dam in Italy collapsed and the resulting flood killed about 1,800 people.

Flood control. Flooding has been made worse in many areas by human activity, especially deforestation and overintensive farming. On exposed land, rain, instead of being absorbed by the soil and plants, tends to run across the surface directly into rivers, rapidly increasingly their volume and sometimes causing flash floods. Over a few years, eroded soil that is washed into rivers piles up on river beds, raising the level of the water. Flood control in such regions involves building dams to store water and planting trees on eroded slopes. It also involves dredging river beds and strengthening the levees (raised banks along rivers).

Along coastal, engineers build dykes, flood walls and hurricane barriers to keep seawater off the land. For example, in the Netherlands, where two-fifths of the land is below sea level, the Dutch have built a huge system of dykes to hold back the sea.

Most coastal towns have strong stone sea walls to prevent flooding at high tide and many have wooden barriers called groynes to halt the erosion of beaches. In places, dunes (hillocks of sand) are planted with grass and trees. The plants anchor the sand and to help to check tidal flooding.

Some estuaries that experience surge tides are protected by movable barriers. The Thames flood barrier at Woolwich, London, consists of a series of movable steel gates built to form a continuous wall across the Thames. The gates pivot (turn) between concrete piers. When not in use, they lie conceal in the riverbed so that they do not hold up shipping. If the surge tide alarm is given, the gates can be turned upright to keep the water from surging upriver and flooding London. The barrier was completed in 1982.

Reducing flood cost. Engineers work not only to control floods, but also to reduce flood losses. A programme to decrease such losses includes regulations to control permanent construction on the flood plain (land that gets flooded) and to make buildings waterproof. Other programmes aim to help the victims of a flood by improving methods to warn and evacuate people from flood plains and to provide better insurance and relief aid.

Scientists can identify areas that are liable to be flooded. In some countries, such as the United States, the government has required that small strips of land along waterways be left vacant. Such land is called is called a floodway. Many communities establish wider areas along floodways for use as farmland or parkland.

Permanent buildings can be located on a flood plain and withstand flood damage. The technique of keeping water out of buildings is called flood-proofing. It involves raising buildings of the ground or using water-proof construction materials.

Weather forecasts also pay an important part in reducing flood losses by issuing storm and flood warnings. In this way, they hope to greatly reduce loss of life and to lower property damage. However, major disasters still occur and international appeals are sometimes necessary to help flood victims.

Floodwaters can cause great damage. They have often destroyed entire communities. Floods usually occur in the spring, when melting snow and heavy rains combine to raise the level of rivers above their banks.

A flood occurs when a river rises above its normal level and overflows its banks. People have built levees along some rivers to hold back the high water, but a river may overflow even such barriers. Floodwaters generally cover only a river’s flood plain, the nearby low-lying land. But sometimes extremely high waters flood a much larger area.

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