DROUGHT

Drought is a condition that results when the average rainfall for an area drops far below the normal amount for a long period of time. This condition is also called drouth. Streams, ponds, and wells often dry up during a drought. Water supplies for agricultural, industrial, and personal uses are greatly reduced. Often the dry and crumbled topsoil is blown away by hot, dry winds (see Dust storm). In areas that are not irrigated the lack of rain causes farm crops to wither. Livestock may die. Extreme drought can lead to many human deaths. The deaths are rarely from starvation. They are usually from drought-related conditions. In time of drought, people become more vulnerable to disease, and less resistant to illness such as diarrhoea. In some parts of Africa as many as 25 per cent of children under the age of five died during droughts in the 1980's. Throughout the world, drought affects more people than does any other single type of disaster. And drought causes about 20 per cent of all disaster-related deaths.

Countries affected by drought

The continent currently most seriously affected by drought is Africa. The Sahel, a region south of the Sa­hara, and Ethiopia both suffered the worst effects of the catastrophic drought in the 1980's. Drought conditions persisted in this region through the early 1990's when Sudan, in particular, experienced severe famine. Food aid helped to relieve the famine. Nevertheless, many thousands lost their lives. In sub-Saharan Africa, where about 75 out of every 100 people live by agriculture, many farmers were forced to abandon their villages and look for work to support their drought-hit families.

Many people ended up in refugee camps, totally de­pendent on food aid. Others gave up scratching a living from the land and went to live in Africa's towns and cit-

Droughts occur in many developing countries of the world, but they need not necessarily result in severe food shortages, or famines. In India, many regions expe­rienced severe drought from 1971-1973. Maharashtra, one of the worst affected states, organized massive food-for-work schemes. At its peak, about 5 million peo­ple were employed on public works such as dam and road building. Not a single death was attributed to star­vation during this period. Other countries, such as Brazil and Kenya, have also adopted measures to try to reduce the damage done by droughts in those countries. Such measures have met with limited success.

Drought is not confined to countries in tropical and sub tropical regions. From December 1975 to August 1976 a so called "strong drought" covered northwestern Europe. Losses, especially in agriculture, were esti­mated in billions of U.S. dollars.

Areas of the United States have suffered three bad droughts in the 1900's. From 1931 to 1938 the Great Plains region experienced one of the worst droughts in its history, and the effect was felt throughout the coun­try. From 1950 to 1954 the Southwest and the southern Great Plains suffered a severe drought. The worst drought since the 1930s struck the Midwest, the north­ern Great Plains, and part of the Southeast in 1988. It caused serious damage to grain crops.

Major droughts have occurred in Australia 11 times since the mid 1800s, eight of them since 1900. A major cause of drought in Australia is the El Nino-southern os­cillation phenomenon, a disturbance of the air and sea currents of the Indian and Pacific oceans. Drought can occur anywhere in Australia, but more droughts occur in the drier interior than in coastal areas because the rainfall is less dependable further inland.

Drought control. 

The economic damage caused by drought can to some extent be contained by planning. In Australia, for example, when severe droughts cause great economic loss, the federal government usually of­fers financial assistance to farmers. Improved transport routes to enable farmers to move their stock from drought areas, or bring in relief fodder are among the new measures for controlling the effects of drought. The farmers themselves try to improve their properties as a defence against the climate. They extend irrigation schemes or drill for water. They put up more fencing to keep their pastures in better condition. They store fod­der in good years so that they will have food reserves in drought years. In some more remote areas they also plant an edible tree, called top feed.

Long-term measures to beat drought, such as the cre­ation of artificial lakes, have been suggested. Water evaporating from the lakes might set up a rainfall cycle. But climatologists doubt whether such an idea would have a long-term benefit on the climate.

Another method that has been tried for changing the climate in drought-affected areas is cloud seeding. To produce rain, iodine crystals are scattered among clouds from aircraft. The water-vapour molecules in the clouds cluster around the crystals and make them heavy. Eventually the overloaded crystals fall to earth as rain. Unfortunately, this method works well only in areas where rainfall readily occurs naturally.

EARTHQUAKE

Earthquake is a shaking of the ground caused by the sudden breaking and shifting of large sections of the earth’s rocky shell. Earthquakes are among the most powerful events on earth and their results can be terrifying. A severe earthquake may release energy 10,000 times as great as that of the atomic bomb. Rock movements during an earthquake can make rivers change their course. Earthquake can trigger landslides that cause great damage and loss of life. Large earthquakes beneath the ocean can CREATE A SERIES OF HUGE, DESTRUCTIVE WAVES CALLED TSUNAMIS THAT FLOOD COASTS.

Earthquakes almost never kill people directly, instead, many deaths and injuries in earthquakes result from falling objects and the collapse of buildings, bridges and other structures. Fire resulting from broken gas or power lines is another major danger during a quake. Spills of hazardous chemicals are also a concern during an earthquake. In most earthquake zones, land-use planners and engineers design now housing and other building projects, such as bridges and dams, to reduce property damage, injuries and loss of life during quakes.

The force of an earthquake depends on HOW MUCH ROCK BREAKS AND HOW FAR IT SHIFTS. Powerful earthquakes can shake firm ground violently for great distances. During minor earthquakes, the vibration may be no greater than the vibration caused by a passing truck.

On average, a powerful earthquake occurs less than once every two years. At least 40 moderate earthquakes cause damage somewhere in the world each year.  About 40,000 to 50,000 small earthquakes – large enough to be felt but not damaging – occur annually.

How earthquakes occur along a fault – a fracture in the earth’s rocky outer shell where sections of rock repeatedly slide past each other. Faults occur in weak areas of the earth’s rock. Most faults lie beneath the surface of the earth, but some, like the San Andreas Fault in California, U.S.A. are visible on the surface. Stresses in the earth cause large blocks of rock along a fault to strain, or bend. When the bending becomes too much, the rock breaks and snaps a new position, causing the shaking of an earthquake.

Earthquakes usually begin deep in the ground. The point in the earth where the rocks first break is called the focus, also known as the hypocenter of the quake. The focus of most earthquakes lies than 70 kilometres beneath the surface, through the deepest known focuses have been nearly 700 kilometres below the surface. The point on the surface of the earth directly above the focus is known as the epicenter of the quake. The strongest shaking is usually felt near the epicenter.

From the focus, the break travels like a spreading crack along the fault. The speed at which the fracture spreads depends on the type of rock. It may average  about 3 kilometres per second in granite or other strong rock. At that rate, a fracture may spread more than 560 kilometres in one direction in less than three minutes. As the fracture extends along the fault, blocks of the rock on one side of the fault may drop down below the rock on the other side, move up and over the other side or slide forward past the other.

When an earthquake occurs, the violent breaking of the rock releases energy that travels through the earth in the form of vibration called seismic waves. Seismic waves move out from the focus of an earthquakes in all directions. As the waves travel away from the focus, they grow gradually weaker. For this reason, the ground generally shakes less further away from the focus.

There are two chief kinds of seismic waves: (1) body waves and (2) surface waves. Body waves, the fastest seismic waves, move through the earth. Slower surface waves travel along the surface of the earth.

Body waves tend to cause the most earthquake damage. There are two kinds of body waves: (1) compressional waves and (2) shear wave. AS the waves pass through the earth, they cause particles of rock to move in different ways. Compressional waves push and pull the rock. They cause buildings and other structures to contact and expand. Shear waves make rocks bend or slide from side to side, and buildings shake. Compressional waves can travel through solids, liquids, or gases but shear waves can pass only through solids.

Compressional waves are the fastest seismic waves and they arrive first at a distant point. For this reason, compressioonal waves are also called primary (P) waves. Shear waves, which travel slower and arrive later, are called secondary (S) waves.

Body  waves travel faster deep within the earth than near the surface. For example, at depths of less than 25 kilometres, compressional waves travel at about 6.8 kilometres per second. At a depth of 1,000 kilometres, the waves travel more than 1 ½ times that speed.

Surface waves are long, slow waves. They produce what people feel as slow rocking sensations and cause little or no damage to buildings.

There are two kinds of surface waves: (1) Love waves and (2) Rayleigh waves. Love waves travel through the earth’s surface horizontally and move the ground from side to side. Rayleigh waves make the surface of the earth roll like waves on the ocean. Typical Love waves travel at about 4.4 kilometres per second and Rayleigh waves, the slowest of the seismic waves, move at about 3.7 kilometres per second. The two of waves were named after two British physicists, Augustus E.H. Love and Lord Rayleigh, who mathematically predicted the existence of the waves in 1911 and 1885, respectively.

Damage by earthquakes

How earthquakes cause damage. Earthquakes can damage buildings, bridges, dams, and other structures, as well as many natural features. Near a fault, both the shifting and the shaking of the ground due to seismic waves cause destruction. Away from the fault, shaking produces most of the damage. Undersea earthquakes may cause huge tsunamis that swamp coastal areas. Other hazards during earthquakes include rockfalls, ground settling and falling trees or tree branches.

Fault slippage. The rock on either side of a fault may shift only slightly during an earthquake or may move several metres. In some cases, only the rock deep in the ground shift and no movement occurs at the earth’s surface. In an extremely large earthquake, the ground may suddenly heave six metres or more. Any structure that spans a fault may be wrenched apart. The shifting blocks of earth may also loosen the soil and rocks along a slope and trigger a landslide. In addition, fault slippage may break down the banks of rivers, lakes, and other bodies of waste, causing flooding.

Ground shaking causes structures to sway from side to side, bounce up and down and move in other violent ways. Buildings may slide off their foundations, collapse, or be shaken apart.

In areas with soft, wet soils, a process called liquefaction may intensify earthquake damage. Liquefaction occurs when strong ground shaking causes wet soils to behave temporarily like liquids rather than solids. Anything on top of liquefied soil may sink into the soft ground. The liquefied soil may also flow toward lower ground, burying anything in its path.

Tsunami. An earthquake on the ocean floor can give a tremendous push to surrounding seawater and create one or more large, destructive waves called tsunamis, also known as seismic sea waves. Some people call  tsunamis tidal waves, but scientists think the term is misleading because the waves are not caused by the tide. Tsunamis may build to heights of more than 30 metres  when they reach shallow water near shore. In the open ocean, tsunamis typically move at speeds of 800 to 970 kilometres per hour. They can travel great distances while diminishing little in size and can flood coastal areas thousands of kilometres from their source.

Structural hazards. Structures collapse during a quake when they are too weak or rigid to resist strong, rocking forces. In addition, tall building may vibrate wildly during an earthquake and knock into each other.

A major cause of death and property damage in earthquakes is fire. Fires may start if a quake ruptures gas or power lines. The 1906 San Francisco earthquake ranks as one of the worst disasters in United States history because of a fire that raged for three days after the quake. See the history of San Francisco.

Other hazards during an earthquake include spills of toxic chemicals and falling objects, such as tree limbs, bricks and glass. Sewage lines may break and sewage may seep into water supplies. Drinking of such impure water may cause cholera, typhoid, dysentery and other serious diseases.

Loss of power, communication and transportation after an earthquake may hamper rescue teams and ambulances, increasing deaths and injuries. In addition, businesses and government offices may lose records and supplies, slowing recovery from the disaster.

Reducing earthquake damage. In areas where earthquakes are likely, knowing where to build and how to build can help reduce injury, loss of life, and property damage during a quake. Knowing what to do when a quake strikes can also help prevent injuries and deaths.

 Where to build. Earth scientists try to identify areas that would likely suffer great damage during an earthquake. They develop maps that show fault zones, flood plains (areas that get flooded), areas subject to landslides or to soil liquefaction and the sites of past earthquakes. From these maps, land-use planners develop zoning restrictions that can help prevent construction of unsafe structures in earthquake-prone areas.

How to build. Engineers have developed a number of ways to build earthquake-resistant structures. Their techniques range from extremely simple to fairly complex. For small to medium-sized buildings, the simpler reinforcement techniques include bolting buildings to their foundations and providing support walls called shear walls. Shear walls, made of reinforced concrete (concrete with steel rods or bars embedded in it), help strengthen the structure and help resist rocking forces. Shear walls in the centre of a building, often around a lift shaft or stairwell, form what is called a shear core. Walls may also be reinforced with diagonal steel beams in a technique called cross-bracing.

Builders also protect medium-sized buildings with devices that act like shock adsorbers between the building and its foundation. These devices, called base isolators, are usually bearings made of alternate layers of steel and an elastic material, such as synthetic rubber. Base isolators absorb some of the sideways motion that would otherwise damage a building.

Skyscrapers  need special construction to make them earthquake-resistant. They must be anchored deeply and securely into the ground. They need a reinforced framework with stronger joints than an ordinary skyscraper has. Such a framework makes the skyscraper strong enough and yet flexible enough to withstand an earthquake.

Earthquake-resistant homes, schools and workplaces have heavy appliances, furniture and other structures fastened down to prevent them from toppling when the building shakes. Gas and water lines must be specially reinforced with flexible joints to prevent breaking.

Safety precaution s are vital during an earthquake. People can protect themselves by standing under a doorframe or crouching under a table or chair until the shaking has stopped completely. Even then, people should use extreme caution. A large earthquake may be followed by many smaller quakes, called aftershocks . People should stay clear of walls, windows and damaged structures which could crash in an aftershocks.

People who are outdoors when an earthquake hits should quickly move away from tall trees, steep slopes, buildings and power lines. If they are near a large body of water, they should move to higher ground.

Where and why earthquake occur?

Scientists have developed a theory, called plate tectonics, that explains why most earthquakes occur. According to this theory, the earth’s outer shell consists of about 10 large, rigid plates and about 20 smaller ones. Each plate consists of a section of the earth’s crush and a portion of the mantle, the thick layer of hot rock below the crust. Scientists call this layer of crush and upper mantle the lithosphere. The plates move slowly and continuously on the asthenosphere, a layer of hot, soft rock in the mantle.. as the plates move, they collide, move apart or slide past one another.

The movement of the plates strains the rock at and near plate boundaries and produces zones of faults around, the rock becomes locked in place and cannot slide as the plates move. Stress builds up in the rock on both sides of the fault and causes the rock to break and shift in an earthquake.

 There are three types of faults: (1) normal faults, (2) reverse faults, and (3) strike-slip faults. In normal and reverse faults, the fracture in the rock slopes downward, and the rock moves up down along the fracture. In a normal fault, the block of rock on the upper side of the sloping fracture slides down. In a reverse fault, the rock on both sides of the fault is greatly compressed. The compression forces the upper block to slide upward and the lower block to thrust downward. In a strike-slip fault, the fracture extends straight down into the rock and the blocks of rock along the fault slide past each other horizontally.

Most earthquake occur in the fault zones at plate boundaries. Such earthquakes occur in the fault zones at plate boundaries. Such earthquakes are known as interpolate earthquakes. Some earthquakes take place within the interior of a plate and are called intraplate earthquake.

Interpolate earthquakes occur along the three types of plate boundaries: (1) ocean spreading ridges, (2) subduction zones, and (3) transform faults.

Ocean spreading ridges are places in the deep ocean basins where the plates move apart. As the plates separate, hot lava from the earth’s mantle rises between them. The lava gradually cools, contracts and cracks, creating faults. Most of these faults are normal faults. Along the faults, blocks of rock break and slide down away from the ridge, producing earthquakes.

Near the spreading ridges, the plates are thin and weak. The rock has not cooled completely, so it is still somewhat flexible. For these reasons, large strains cannot build and most earthquakes near spreading ridges are shallow and mild or moderate in severity.

Subduction zones are places where two plates collide and the edge of one plate pushes beneath the edge of the other in a process called subduction. Because of the compression in these zones, many of the faults there are reverse fault. About 80 percent of major earthquakes occur in subduction zones encircling the Pacific Ocean. In these areas, the plates under the Pacific Ocean are plunging  beneath  the plates carrying the continents.

The grinding of the colder, brittle ocean plates beneath the continental plates creates huge strains that are released in the world’s largest earthquakes.

The world’s deepest earthquakes occur in subduction zones down to a depth of about 700 kilometres. Below zones depth, the rock is too warm and soft to break suddenly and cause earthquakes.

Transform faults are places where plates slide past each other horizontally. Strike-slip faults occur there. Earthquakes along transform faults may be large, but not large to deep as those in subduction zones.

One of the most famous transform faults is the San Andreas Fault. The slippage there is caused by the Pacific Plate moving past the North American Plate. The San Andreas Fault and its associated faults account for most of California’s earthquakes. See San Andreas Fault.

Interplate earthquakes are not as frequent or as large as those along plate boundaries. The largest intraplate earthquakes are about 100 times smaller than the largest interpolate earthquakes.

Intraplate earthquakes tend to occur in soft, weak areas of plate interiors. Scientists believe intraplate quakes may be caused by strains put on plate interiors by changes of temperature or pressure in the rock. Or the source of the strain may be a long distance away, at a plate boundary. These strains may produce quakes along normal, reverse or strike-slip  faults.

Recording, measuring and locating earthquakes. To determine the strength and location of earthquakes, scientists use a recording instrument known as a seismograph. A seismograph is equipped with sensors called seismometers that can detect ground motions caused by seismic waves from both near and distant earthquakes. Some seismometers are capable of detecting ground motion as small as 1 hundred-millionth of a centimeter. See seismograph.

Scientists called seismologists measure seismic ground movements in three directions: (1) up-down, (2) north-south, and (3) east-west. The scientists use a separate sensor to record each direction of movement.

A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it. The record of the wave, called a seismogram, is imprinted on paper, film or recording tape or is stored and displayed by computers.

Probably the best-known gauge of earthquake intensity is the local Richer magnitude scale, developed in 1935 by United States seismologist Charles R. Richter. This scala, commonly known as the Richter scale, measures the ground motion caused byan earthquakes. Every increase of one number in magnitude means the energy release  of the quake is 32 times greater. For example, an earthquake of magnitude 7.0 releases 32 times as much energy as an earthquake measuring 6.0. An earthquake with a magnitude of less than 2.0 is so slight that usually only a seismometer can detect it. A quake greater than 7.0 may destroy many buildings. There are about 10 times as many earthquakes with magnitude 6.0 as there are with magnitude 7.0. See Richter magnitude.

Although large earthquakes are customarily reported on the Richter scale, scientists prefer to describe earthquakes greater than 7.0 on the moment magnitude scale. The moment magnitude scale measures the total energy released in an earthquake, and it describes  large earthquakes more accurately than does the Richter scale.

The largest earthquake ever recorded on the moment magnitude scale measured 9.5. It was an interpolate earthquake that occurred along the Pacific coast of Chile in South America in 1960. The largest intraplate earthquakes known struck in central Asia and in the Indian Ocean in 1905, 1920 and 1957. These earthquakes had moment magnitudes between about 8.0 and 8.3.

Scientists locate earthquakes by measuring the time it takes body waves to arrives at seismographs in a minimum of three locations. From these wave arrival times, seismologists can calculate the distance of an earthquake from each seismograph. Once they know an earthquake’s distance from three locations, they can find the quake’s focus at the centre of those three locations.

Predicting earthquakes. Scientists can make fairly accurate long-term predictions of where earthquakes will occur. They know, for example, that  about 80 percent of the world’s major earthquakes happen along a belt encircling the Pacific Ocean. This belt is sometimes called the Ring of Fire because it has many volcanoes, earthquakes, and other geologic activity. Scientists are working to make accurate forecasts on when earthquakes will strike.

Related articles: Continental drift, Earth, Japan (Land), Mediterranean Sea, Plate   tectonics, Richter magnitude, San Andreas Fault, Seismograph, Seismology and Tidal wave.

Outline:
How an earthquake begins. 
How an earthquake spreads: Body waves, and Surface waves.
Damage by earthquakes: How earthquakes cause damage, and Reducing earthquakes damage.
Where and why earthquakes: Interplate earthquake, and Intraplate earthquake.
Studying earthquakes: Recording, measuring and locating earthquakes, and Predicting earthquakes.

Questions
Why do buildings collapse during an earthquake?
Where do the world’s largest and deepest earthquakes occurs?
What is a seismograph?
What should people do to be safe during an earthquake?
What type of seismic waves tend to cause the most damage?
Most earthquakes occur near and along the boundaries of the rocky plates that cover the earth’s surface. 

How  an earthquake happen

First of all, what is an earthquake and how does it occur?

An earthquake is a sudden, rapid shaking of the Earth caused by the breaking and shifting of rocks beneath the Earth’s surface. For hundreds of millions of years, the forces of plate tectonics  have  shaped the Earth. The huge plates that form the Earth’s surface move slowly over, under and past each other. Sometimes the movement is gradual.  At other times, the plates are locked together, and are  unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free causing the ground to shake. Most earthquakes occur at the boundaries where the plates meet.

Ground shaking from earthquakes can cause buildings and bridges to collapse; disrupt gas. electricity and phone services; and sometimes trigger landslides, avalanches, flash floods,  fires or huge, destructive ocean waves (tsunamis). Buildings with foundations resting on unconsolidared landfill and other unstable soil, and trailers and homes not tied to their foundations area at risk because they can be shaken off their mountings during an earthquake. When an earthquake occurs in a populated area, it may cause deaths, injuries and extensive property damage.

Earthquakes can strike suddenly, without warning. They can occur at any time of the year and at any time of the day or night. On a yearly basis, 70 to 75 damaging earthquakes occur throughout the world. Estimates of losses from a future earthquake in the United States approach $200 billion.

During an earthquake, the greatest danger exists directly outside buildings, at exits, and alongside   exterior walls. Many of the 120 fatalities from the 1933 Long Beach earthquake occurred when people  ran outside of buildings only to be killed by falling debris from collapsing walls.

During an earthquake always remember to drop, cover and hold on! Move only a few steps  to nearby safe place. It is very dangerous to try to leave a building during an earthquake because obi eats can fall on you and injure or even kill you.

If you are in bed, hold on and stay there, protecting your head with a pillow. You are less likely to be injured staying where you are. Broken glass on the floor has caused injury to those who have rolled to the floor or tried to get to doorways. If you are outdoors, find a clear spot away from buildings trees, streetlights and power lines. Drop to the ground and stay there until the shaking stops. Injuries can occur from falling trees, streetlights, power lines or building debris.

If you are in a vehicle, pull over to a clear location, stop and stay there with your seatbelt faster ea until the shaking has stopped. Trees, power lines, poles, street signs and other overhead items mav  fall during earthquakes. Stopping will help reduce your risk, and a hard-topped vehicle will help protect you from flying or falling objects. Once the shaking has stopped, proceed with caution.

In conclusion, ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related injuries result from collapsing walls, flying glass and falling objects Thus, much of the damage in earthquakes is predictable and preventable. These few simple tips mentioned can help save your life during an earthquake.

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.

Learn more:

What Causes Flooding? 

(Dams, heavy rainfall, hurricanes and tropical storms, levees, flash floods, and others).

Defining Flood Risks

The Costs of Flooding

DISASTER REVIEW1

  

A disaster is a natural or man-made (or technological) hazard resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment. A disaster can be ostensively defined as any tragic event stemming from events such as earthquakes, floods, catastrophic accidents, fires, or explosions. It is a phenomenon that can cause damage to life and property and destroy the economic, social and cultural life of people.

According to ifrc.org, disaster is a sudden, calamitous event that seriously disrupts the functioning of a community or society and causes human, material, and economic or environmental losses that exceed the community’s or society’s ability to cope using its own resources. Though often caused by nature, disasters can have human origins

In contemporary academia, disasters are seen as the consequence of inappropriately managed risk. These risks are the product of a combination of both hazard/s and vulnerability. Hazards that strike in areas with low vulnerability will never become disasters, as is the case in uninhabited regions. Developing countries suffer the greatest costs when a disaster hits – more than 95 percent of all deaths caused by disasters occur in developing countries, and losses due to natural disasters are 20 times greater (as a percentage of GDP) in developing countries than in industrialized countries.

Etymology and Classification

Etymology

The word disaster is derived from Middle French désastre and that from Old Italian disastro, which in turn comes from the Greek pejorative prefix δυσ-, (dus-) "bad" + ἀστήρ(aster), "star". The root of the word disaster ("bad star" in Greek) comes from an astrological theme in which the ancients used to refer to the destruction or deconstruction of a star as a disaster.

Classification

Researchers have been studying disasters for more than a century, and for more than forty years disaster research. The studies reflect a common opinion when they argue that all disasters can be seen as being human-made, their reasoning being that human actions before the strike of the hazard can prevent it developing into a disaster. All disasters are hence the result of human failure to introduce appropriate disaster management measures. Hazards are routinely divided into natural or human-made, although complex disasters, where there is no single root cause, are more common in developing countries. A specific disaster may spawn a secondary disaster that increases the impact. A classic example is an earthquake that causes a tsunami, resulting in coastal flooding.

Natural Disaster.  A natural disaster is a consequence when a natural hazard affects humans and/or the built environment. Human vulnerability, and lack of appropriate emergency management, leads to financial, environmental, or human impact. The resulting loss depends on the capacity of the population to support or resist the disaster: their resilience. This understanding is concentrated in the formulation: "disasters occur when hazards meet vulnerability". A natural hazard will hence never result in a natural disaster in areas without vulnerability.

Various phenomena like earthquakes, landslides, volcanic eruptions, floods and cyclones are all natural hazards that kill thousands of people and destroy billions of dollars of habitat and property each year. However, natural hazards can strike in unpopulated areas and never develop into disasters. However, the rapid growth of the world's population and its increased concentration often in hazardous environments has escalated both the frequency and severity of natural disasters. With the tropical climate and unstable land forms, coupled with deforestation, unplanned growth proliferation, non-engineered constructions which make the disaster-prone areas more vulnerable, tardy communication, poor or no budgetary allocation for disaster prevention, developing countries suffer more or less chronically by natural disasters. Asia tops the list of casualties due to natural disasters.

Man-Made Disaster.  Man-made disasters are the consequence of technological or human hazards. Examples include stampedes, fires, transport accidents, industrial accidents, oil spills and nuclear explosions/radiation. War and deliberate attacks may also be put in this category. As with natural hazards, man-made hazards are events that have not happened, for instance terrorism. Man-made disasters are examples of specific cases where man-made hazards have become reality in an event.

DISASTER REVIEW

A disaster is a natural or man-made (or technological) hazard resulting in an event of substantial extent causing significant physical damage or destruction, loss of life, or drastic change to the environment. A disaster can be ostensively defined as any tragic event stemming from events such as earthquakes, floods, catastrophic accidents, fires, or explosions. It is a phenomenon that can cause damage to life and property and destroy the economic, social and cultural life of people. According to ifrc.org, disaster is a sudden, calamitous event that seriously disrupts the functioning of a community or society and causes human, material, and economic or environmental losses that exceed the community’s or society’s ability to cope using its own resources. Though often caused by nature, disasters can have human origins. In contemporary academia, disasters are seen as the consequence of inappropriately managed risk. These risks are the product of a combination of both hazard/s and vulnerability. Hazards that strike in areas with low vulnerability will never become disasters, as is the case in uninhabited regions. Developing countries suffer the greatest costs when a disaster hits – more than 95 percent of all deaths caused by disasters occur in developing countries, and losses due to natural disasters are 20 times greater (as a percentage of GDP) in developing countries than in industrialized countries.

Disaster is also defined as a sudden event that has very unfortunate consequences for those affected by it. Disasters involve large-scale loss of life and property. The worst disasters in history have killed thousands of people at a time. Some have destroyed whole towns or cities. A volcanic explosion on the Island of Thera (now Santorini) about 1500 B.C. wiped out the entire Minoan civilization on the island of Crete, 120 kilometers away.

Disasters are of two types: (1) natural disasters, usually caused by geological or meterological events, and (2) major accidents involving human actions or technology. Famine, drought  and war are regarded as disasters but their onset is usually rather gradual. This article deals with sudden events.

Natural disasters have usually been far more destructive of life and property than have accidents resulting from human actions. This is because most natural events, such as volcanic eruptions, cyclones, hurricanes and floods, unleash massive forces that humans are rarely able to match.

Historically, earthquakes have been the most damaging natural disasters. They have brought great loss of life and made millions of people homeless. They usually strike without warning and effects to predict them continue to prove fruitless. Falling objects and collapsing buildings cause most deaths and injuries during an earthquake. Perhaps the worst quake ever recorded was that of 1201, centred on northern Egypt or Syria . It killed more than 1 million people.

Earthquakes that take place in or near the sea are usually associated with a destructive tidal wave, or tsunami. In 1992, an earthquake near Maumere on the Island of Flores, Indonesia, triggered tsunamis that devastated the coast of Flores and killed about 2,500 people.

Erupting volcanoes, floods, cyclones and hurricanes cause fewer deaths today than they once did because modern technology allows experts to warn people beforehand. Towns and villages in the path of a lava flow or a violent storm can usually be evacuated quickly and successfully. These occurrences do  however, cause massive damage and destruction to buildings and agricultural land, especially in developing countries.

The most dramatic volcanic eruptions, such as the eruption of Mount Vesuvius in A.D. 79, have been totally unexpected events. Before 79, Vesusvius had not erupted for centuries. The worst volcanic  eruption in modern history was that of Mount Tambora on the island of Sumbawa, Indonesia., in 1815. In a catastrophic explosion, the 4,000-metre Mount Tambora blew off about one-third of its height. The blast killed more than 90,000 people and made thousands more homeless. The eruption hurled debris into the atmosphere and darkened the skies around the world for months. The year 1816 became known as “the year without a summer”.

Floods can cause destruction over a wide area. In 1887, during what was probably the worst flood in recorded history, the Huang He River in eastern China overflowed an area of 130,000 square kilometers and killed about 900,000 people. The worst cyclone ever recorded struck East Pakistan (now Bangladesh) in 1970. Cyclones are common in this region, but the 1970 one was exceptionally destructive. It killed more than a quarter of a million people, left millions homeless and swamped precious agricultural land.

Accidents involving human actions or technology include air crashes, ship collision and sinkings, train crashes and fires. They rarely cause as many deaths as natural disasters but they can be just as destructive of property, The great fire of London in 1666 razed most of the old city of London to the ground but caused the deaths of only six people. In 1989, the oil spill caused by the oil tanker Exxon Valdez in Alaska, U.S.A., fouled 1,600 kilometres  of coastline, including beaches in wildlife reserves . The accident caused no human casualties but killed thousands of birds and other animals.

These are many accidents in history that have cost large numbers of lives. The release in 1984 of poisoned gas from a factory in Bhopal, India, killed 3,500 people. The sinking of the Titanic in the North Atlantic in 1912 drowned about 1,500 people. The worst accident at sea in modern times was a collision in 1987 between a passenger ferry and an oil tanker in the Mindoro Strait in the Philippines. About 1,840 people died in the tragedy. About 800 people died in the worst train crash in history which occurred in 1981 in Bihar, India.

The worst air crash occurred in Japan in 1985 when a Boeing 747 jumbo jet airliner plunged into Mount Okura killing 520 passengers and crew. The worst collision between occured on the ground in 1977 between two 747’s taxiing along intersecting runways at Tenerife airport in the Canary Islands. The accident  killed 583 people aboard the two aircraft. Space exploration has included a few disasters in both the U.S. and former Soviet space programmes. In one of the worst, seven U.S. astronauts died in 1986 when the Space Shuttle Challenger blew up just after liftoff.

Related articles: Earthquake, Cyclone, Flood, Tidal wave, Tornado and Volcano.