OF ALL THE NATURAL hazards in the world, floods are the most widespread. Historically, floods have killed more people and caused more damage than any other type of natural disaster. A flood is defined as an overflowing of water on an area that is normally dry. There are several types of flooding, including river flooding, coastal flooding, urban flooding, inland flooding from hurricanes, and flash flooding. Each has its own causes and characteristics, but all can be deadly and floods can happen anywhere.
Throughout history, people have always built their towns and cities near water, both along the seacoast and along rivers further inland. The water serves many purposes. It is used for drinking, cooking, and bathing. Water in many places has been used as a source of power, to turn wheels that grind grain. Running water is similarly used to create hydroelectric power. Another reason for settling close to water is that it has always been an easy way to travel and to transport goods. The disadvantage of living near water is the possibility of flooding.
The amount of water on Earth is fairly constant all the time. Each day, some of the water on the Earth is lost into the atmosphere through the process of evaporation. Water in rivers, lakes, and oceans is always evaporating at a slow rate. Most water on the Earth is in the form of liquid, but at the poles we find water in its solid or ice form. Water in glaciers is also in the form of ice.
At some places, water is in the form of water vapor in the air. This is the water that has evaporated from the surface of a body of water. A water molecule remains in the atmosphere for approximately 9 or 10 days before it falls as rain or snow. This happens when the amount of water in a cloud becomes heavy enough to form drops of water, which then fall to Earth. Rain that goes into the bodies of water begins the cycle again. Some rain will soak into the ground, where it will spend some time as groundwater.
River flooding is one of the most common types of flooding. It occurs when rainfall throughout the year is unevenly distributed. Too much rain goes into the river at one time and the water exceeds the capacity of the channel. When that happens, the stream will overflow its banks. Another cause of river flooding is melting snow in the spring. In areas with hills or mountains, the snow in the higher elevations melts and the water runs down into the streams. If the snowfall was heavier than usual, floods may occur. Also, spring rains often occur at the same time as the snow is melting, contributing more water to the streams and rivers. The torrential rains that accompany a hurricane or other tropical storm can also cause flooding in rivers.
When heavy rains fall on bare soil, it is churned into mud. This keeps the water from soaking into the ground, and most of it will flow across the surface of the ground, creating gullies leading to rivers and streams.
Coastal flooding occurs when ocean water is driven inland by winds from tropical storms or hurricanes. Sometimes escape routes are blocked by this high water and people cannot escape. Coastal flooding can also be caused by a tsunami, which is a sea wave sometimes referred to as a tidal wave. These huge waves are caused by volcanic activity or an earthquake.
When urban flooding occurs, the streets in a town become swift rivers and basements fill with water. This is partly due to the fact that land that used to be fields or woodlands has been converted to roads and parking lots. The concrete or asphalt cannot absorb any rainfall and it runs off the surfaces of the parking lots and roads.
Flash flooding is flooding that was not expected. Flash floods are the most dangerous of all floods because of the element of surprise. Intense rainfall over a significant period of time can cause flash flooding. The speed with which a flash flood can occur makes it even more dangerous. Flash floods often occur in dry areas where the ground is baked hard. When torrential rains do come, water immediately fills arroyos, or dry stream beds, turning them into raging rivers. The torrent of water rushes through with enough force to wash away sections of concrete road.
Flash flood waters move very quickly and can move boulders, tear out trees, and destroy bridges and buildings. Sometimes a wall of water 10 to 20 ft (3 to 6 m) high carries a huge amount of debris. The worst flash floods are created when a dam or levee breaks and a huge amount of water suddenly pours downstream, destroying everything in its path.
Inland flooding has been the number-one cause of deaths from hurricanes in the past 30 years. In 1999, Hurricane Floyd caused 56 deaths. Fifty of them were people who drowned due to inland flooding. Inland flooding occurs when a slow-moving storm is accompanied by heavy rains. Inland flooding is a threat to communities hundreds of miles from the coast. Over a third of the people who drown during an inland flood are in their cars. Some are trying to escape the flooding, while others are attempting to abandon the cars. Even a foot of water can cause a car to float, and if that water is rushing down a street, the car can be swept away.
All kinds of floods cause problems for people. The worst consequence of floods is loss of lives. Property damage is a major problem with flooding. Houses can be completely demolished or even carried away down a turbulent river. Even houses left standing after a flood usually have substantial water damage and some have structural damage as well. Also, when the water subsides, it leaves behind a thick, sticky coat of mud filled with debris. So even if there is no structural damage, cleaning up after a flood is a big job.
Floods can also spread disease. Water flowing through an area can pick up chemicals and waste products and move them to another place. Most diseases are more easily spread through water than through air. That is one reason it is so important for those in a flooded area to drink only bottled or boiled water.
Besides damaging homes, floods can cause other problems to agricultural areas. Soil can be eroded and crops already in the fields can be ruined. Floods also damage reservoirs and rivers where they deposit the load of soil the water picked up from the fields.
In some areas, flooding is an annual occurrence and is expected. Seasonal flooding like this took place every summer for thousands of years along the NILE RIVER in EGYPT. The monsoons caused hard rains at the source of the river which filled the river channel to overflowing. This made the land along the banks of the river very fertile in an area of desert. This is one of the few positive results of flooding. In most areas, though, people want to prevent floods from happening. Many solutions have been attempted. In LOS ANGELES and other big cities, concrete channels have been constructed. When there is a lot of rain, the water goes into the channels and is carried out of the city. Here the land can absorb the water that could not be absorbed by the concrete and asphalt ground coverings in the city.
Levees, or large walls, have been built along the edges of rivers to keep them from overflowing. They are high enough so that much more water can flow through the channel. They are fairly effective in the areas in which they are built, but they result in much more water being carried downstream, and this causes problems for areas with no levees. All the water that would have been spread out upstream now floods a larger area than it would have. Also, the sediment that used to be deposited over the whole floodplain builds up in the river bottom, causing the riverbed to rise. Then it can contain less water. Levees can break, too, and when that happens, a flash flood occurs, endangering all who live near the river.
DAMS AS FLOOD CONTROL
Dams are sometimes used for flood control as well. When they are built near the headwaters of the stream, they can be made to retain heavy runoff, and the water can then be released gradually during dry periods. Floodways are another way of controlling floodwater. This principle diverts floodwaters into a controlled area so that other areas are not flooded. Reforestation can help in areas where flooding occurs because the forests have been cut and the soil eroded.
Preventative measures have also been taken to prevent coastal flooding. These measures have not been as successful as the dams and levees on the rivers have been. Fences and walls are sometimes built where the water meets the land, in an attempt to lessen the power of the waves and keep them from wearing down the beach. However, such structures also interfere with beach formation. Walls and fences often do not work because of the overpowering force of the ocean. In an attempt to prevent flooding on beaches and the resulting erosion and loss of sand, some groups have tried pumping sand from deep waters onto the beaches. This works for a while, but is expensive. Breakwaters are also built offshore in order to intercept the waves before they hit the beach, but they are an eyesore.
Probably the worst flood in the United States was the Johnstown flood in 1889. Johnstown, PENNSYLVANIA, was a steel town, built on the floodplain at the fork of the Little Conemaugh and Stony Creek rivers. To prevent flooding, a dam had been built 14 mi (22.5 km) up the Little Conemaugh. The South Fork Dam held back a three-mile-long lake on the side of the mountain, 450 ft (137 m) higher than Johnstown. The dam had been poorly maintained. On the afternoon of May 31, after a night of torrential rains, inhabitants heard a “roar like thunder.” Some immediately realized that the dam had broken. Twenty million tons of water rushed down the narrow valley to Johnstown, carrying with it tons of debris from the dam. The wall of water was at times up to 60 ft (18 m) high and leveled everything in its way.
Thousands of people tried to get out of the way of the water, but many were swept downstream with the debris. Some of these people made it downstream to the old Stone Bridge, where the rivers met, by clinging to debris that served as rafts. However, much of the debris caught fire when it hit the bridge. Eighty people who had managed to cling to something as they were swept downstream then died in the fire. Many of the missing were never found, and many bodies could not be identified. The cleanup took years, and bodies were found months after the flood.
Floods on the HUANG (Yellow) River, in CHINA have killed more people than the flooding of any other river in the world. Almost 2 million died in the flood of 1887, and almost twice that many were killed by the flood in 1931. In 1938, there were almost a million casualties. The channel of the river is choked by millions of tons of yellowish mud, which causes the river to overflow often and to change course. In some areas, the river bed is actually higher than the surrounding countryside. Dikes have been built, some as high as 30 ft (9 m) but they don't always hold. For centuries the Chinese have tried to control the river by building levees and dams and digging channels. A huge new dam called the Xiaolangdi Multipurpose Dam was being built in the 2000s. With 10 intake towers, nine flood tunnels, six power tunnels, and an underground powerhouse, the Chinese hope this will finally alleviate their problems with flooding.
As noted earlier, the Nile River floods like clockwork every summer. For many years, the Egyptians took advantage of the flooding. Recently, though, they have been forced to grow more crops to feed their growing population. In 1970, Egypt built the Aswan High Dam across the Nile 600 mi (965 km) south of Cairo. The dam has stopped the flooding by trapping the water in a reservoir. It is then released during the dry season. This has increased the length of the growing season, as farmers no longer have to wait for the annual floods to recede before planting their crops. However, the land is not as fertile now that the fields aren't flooded annually, so farmers must use large amounts of fertilizer.
The MISSISSIPPI RIVER has regularly flooded its banks for thousands of years. A disastrous flood in 1927 killed 246 people in seven states. It flooded 16,570,627 acres (6,705,894.8 hectares) and caused over $400 million in damage. This prompted the government to try to tame the river. The U.S. Army Corps of Engineers went to work building the longest system of levees in the world. Today, the Mississippi has 29 locks and dams, and miles of levees, as well as hundreds of runoff channels. Most of the time the system works very well, but in 1993, 80 percent of the private earthen levees on the river failed. Most of the federal levees held, but torrents of water still poured down on fields and towns.
Flooding is more disastrous in less developed countries for several reasons. There is a lack of flood control in most of these countries, and the people don't have an emergency response system in place. Many don't have the technology to put out or receive early warnings. Flooding in the UNITED STATES kills far fewer people today than it did a hundred years ago. Early warning systems are in effect, and meteorologists are much better able to predict floods. Almost everyone has a television set and can learn of an imminent flood in time to take safety measures. Most cities have disaster plans, complete with evacuation routes to be used in case of flooding. Also preventative measures such as levees, dams, runoff channels and breakwaters work reasonably well and make it unlikely that a flood of the magnitude of the Johnstown flood or the 1927 flood on the Mississippi will occur.