Flood

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Flood

Coastal floods
River floods and flash floods
Historically significant floods
Recent devastating floods
Flooding and climate change
For More Information

A flood is the overflow of water onto normally dry land. Flooding results when the water level rises and overflows the banks of a river or some other low-lying channel, or when high ocean waters wash over the coast. Floods occur frequently in some parts of the world. While for some areas flooding spells disaster, for other areas yearly flooding is necessary to sustain crops.

Floods kill more people than any other weather phenomenon. Flash floods, which are sudden, intense, localized floods, are especially deadly. Worldwide, 40 percent of all deaths from natural disasters are due to floods.

There are two main types of floods: coastal floods and river floods.

Coastal floods

Coastal floods are floods that occur along the coasts of lakes and oceans. This type of flooding is of great concern in many countries because of the high population density along their coastlines. In the United States, for example, almost two-thirds of the U.S. population lives in states along the three major coasts—38 percent along the Atlantic Ocean, 16 percent along the Pacific Ocean coast, and 12 percent along the Gulf of Mexico. According to the National Oceanic and Atmospheric Administration, 53 percent of the U.S. population (153 million people) live in one of the 673 counties bordering the Great Lakes, the Atlantic, the Pacific, or the Gulf of Mexico.

There are two main causes of coastal flooding: high waters and the subsidence, or lowering, of coastal lands.

Hurricanes and tsunamis

Most coastal flooding is produced by high waters associated with hurricanes. As a hurricane crosses over land, it produces a storm surge. A storm surge is a several-foot-high wall of water that results in the flooding of 40 to 100 miles (65 to 160 kilometers) of coastline.

Large waves can also cause flooding. The most common type of waves, those driven by the wind, are called wind waves. The largest wind waves are generated by large, stationary storm systems. Wind waves tend to be largest in the open ocean and diminish in height as they approach land. Large wind waves have the greatest potential for flooding when accompanied by high tide.

WORDS TO KNOW

coastal flood:: an overflow of water onto a coastal area caused by a storm surge, strong winds, or tsunami.
flash flood:: a sudden rush of water in a low-lying area brought on by heavy rain or a dam break; flash floods can occur when the ground is so saturated with water that no more can soak in, or in desert regions where hard rocky soil is unable to absorb or hold water.
flood:: an overflow of water on land that is normally dry.
precipitation:: water in any form, such as rain, snow, ice pellets, or hail, that falls to Earth's surface.
river flood:: a flood caused when a river spills over its banks.
sinkhole:: a natural, steep depression in a land surface caused by collapse of a cavern roof.
stationary front:: a boundary between two air masses at different temperatures which are not moving or are moving slowly.
storm surge:: an abnormal rise of the sea over and above normal tides and due to strong winds and low pressure accompanying a storm or hurricane.
subsidence:: a gradual sinking of the land surface relative to its previous level.
thunderstorm:: a storm resulting from strong rising air currents; characterized by heavy rain or hail along with thunder and lightning.
tornado:: a violent windstorm characterized by a twisting, funnel-shaped cloud.
tsunami:: a huge ocean wave that can travel at speeds up to 600 mph (965 kph) for hundreds of miles over open ocean before it hits land; caused by an earthquake, underwater volcanic eruption, or underwater landslide.
wind wave:: a wave caused by the action of wind on the water surface.

The largest waves, however, are generated not by the wind but by submarine earthquakes, landslides, and volcanic eruptions. Waves produced by these forces are called tsunamis (tsoo-NAH-meez). Tsunamis start out small and grow larger as they near land. They travel at speeds of up to 600 mph (965 kph). It is typical for a tsunami to measure 60 to 100 feet (18 to 30 meters) in height by the time it reaches the shore.

Tsunamis occur most often in the Pacific Ocean. Several tsunamis have affected Alaska and Hawaii. In 1958, a tsunami over 200 feet (60 meters) in height, generated by a minor earthquake and resultant rock fall into the sea, crashed into Lituya Bay, Alaska. It destroyed great tracts of forest land as far as 1,700 feet (520 meters) above sea level. In December 2004, the deadliest tsunami in recorded history occured not in the Pacific, but in the Indian Ocean. A powerful earthquake triggered a series of tsunamis in Indonesia, Malaysia, and Thailand, killing nearly a quarter million people. Because tsunamis are not common in the Indian Ocean, there was little advance warning for the people of southeast Asia.

On the shelves: The Epic of Gilgamesh

The Epic of Gilgamesh is a four-thousand-year-old story about a king from the region that is now Iraq. The story includes a controversial episode concerning a great flood that some historians and theologians say compares to the worldwide flood mentioned in the Bible.

Subsidence

Subsidence is the lowering of land in coastal areas. Subsidence may be caused by the gradual settling of subterranean rocks or sediments or by the removal of ground water by excessive pumping. Subsidence may also be rapid. For example, if the roof of a cave collapses, the result is a large depression called a sinkhole. Whatever the cause, subsidence in coastal areas makes them more susceptible to flooding.

River floods and flash floods

The banks of rivers and streams overflow due to many causes, including excessive rain, the springtime melting of snow, and blockage of water flow by ice. The failure of a dam or aqueduct is another source of flooding. The primary cause of flooding in large rivers is prolonged heavy precipitation over a large area. Large rivers such as the Mississippi, the Ohio, and the Missouri sometimes flood due to precipitation covering hundreds of square miles. In some areas, flooding occurs nearly every spring when the winter snow melts.

The most dangerous form of river flood is a flash flood. Flash floods are the number one weather-related killer in the United States. Throughout the 1980s, each year flash floods killed an average of 110 people and were responsible for an average of $3 billion in property damage. In the first half of the 1990s, the number of deaths due to flash floods rose to an annual average of 140.

A flash flood can be caused by persistent, torrential rain that falls from a slow-moving or stationary severe thunderstorm, which is itself a result of strong, rising air currents and characterized by thunder and lightning. A flash flood may also be caused by the failure of a dam or levee. A dam is a structure that controls the rate of water flow while a levee is a structure built to prevent water from overflowing the banks of a river.

Flash floods rise and recede much more quickly than other types of river floods. They are the most dangerous because they come on so fast that people are often unable to reach higher ground in advance of the floodwaters. A series of flash floods in the streams that feed into a large river may result in the flooding of the large river.

Watch this: River Voices: A Portrait of an American River Community

In this 2002 documentary, written and directed by Nathan Lorentz, survivors recall both the devastation and the outpouring of community strength caused by the great 1937 Ohio River flood in Portsmouth, Ohio.

Flash floods can be caused by thunderstorms that are either slow-moving or stationary. The reason for their lack of movement is that the high-altitude winds that drive weather systems winds are nearly calm. These storms unleash large quantities of rain over a single area. Thunderstorms that move more quickly spread their rain across larger areas and typically don't produce flash floods.

Weather report: Flash flood alerts and safety procedures

A flash flood watch means that heavy rains may cause flash flooding within the designated area. This is the time to prepare to evacuate to higher ground.

A flash flood warning means that a flash flood has been reported and is imminent for the designated area. In some areas, a warning signal sounds when waters reach critical levels. When a warning is issued, it is imperative to move to safe ground immediately. You may only have seconds or minutes before waters become dangerously high.

If you live in an area that's prone to flash flooding, it's wise to take the following precautions:

Learn the evacuation routes to higher ground.
Keep your car's gas tank filled.
Put together a supply kit for your home containing first aid materials, a battery-powered flashlight, a battery-powered radio, extra batteries, rubber boots, rubber gloves, nonperishable food, a nonelectric can opener, and bottled water.
Install check valves in your home's sewer traps to prevent flood water from backing up and coming in through your drains.
Once a flash flood watch has been issued, fill your bathtub and large containers with water for drinking and cooking in case your local water source becomes contaminated.

If you encounter a flash flood, the most important thing to remember is to avoid the water. If you are driving, turn around and go the other way. If you are on foot, climb to higher ground. Do not try to walk, swim, or drive through flood water. The water is moving rapidly and carries dangerous debris. You can be knocked over by as little as 6 inches of rushing water. Deeper water can have an undertow that drags you beneath the surface.

If you are in a car during a storm, watch for flooding where the road dips down, under bridges, and in low areas. These are the places that fill with water first. If you see a flooded area, turn the vehicle around and go the other way. Be especially careful at night, when the beginnings of flash floods are harder to recognize.

If your car stalls or becomes blocked, abandon the car and seek higher ground. A car will float away in just 2 feet of water. Then it is at the mercy of the current, which may carry it into deeper water or overturn it in a ditch. Sixty percent of people who die in flash floods are either in a car or are attempting to leave a car that has been stranded in high water.

A flash flood happens when the amount of rainfall exceeds the capacity of the ground to absorb it. In that case, the rainwater runs over the surface, rather than sinking into the ground. The water flows to the lowest point, which is generally a river, stream, or storm sewer. If the quantity of water is greater than the capacity of the drainage channel, the channel overflows within a few minutes and a flash flood occurs.

As the banks overflow, water rushes forward at speeds of up to 20 mph (35 kph). It takes the form of a sediment-laden wall that can surge as high as 30 feet (10 meters). The floodwaters are capable of dislodging large objects weighing several tons, such as boulders, cars, and even locomotives. These objects may be carried several miles downstream. Flash floods are also known to tear out bridges in their path.

Flash floods are more common in mountainous areas. Rain runs down mountainsides and becomes concentrated in canyons and valleys. Arid and urban regions are also prone to flash floods, since very little rain can soak into hard desert surfaces or concrete. Most cities have storm sewer systems that direct water underground to nearby rivers. If the storm sewers become clogged or overwhelmed by the volume of water, the streets quickly become flooded.

Flash flooding may also result from rains associated with a hurricane, a break in a dam or levee, or the springtime melting of large quantities of snow and ice. Flooding may even be caused by a small amount of rain that falls on a blanket of snow. If the ground beneath the snow is frozen, the melting snow and rain will not soak into the ground, but will run off and drain into a river. The likelihood of flooding is increased if ice blocks water from flowing in the river.

Historically significant floods

In 1889, a dam break in Johnstown, Pennsylvania, resulted in the worst flash flood in the history of the United States. A wall of water 36 to 40 feet (11 to 12 meters) tall overwhelmed the town, killing over 2,200 people. A second flash flood hit Johnstown in 1977, in which seventy-seven people lost their lives.

The Great Mississippi River Flood of 1927 caused widespread damage over a large area, extending from southern Illinois to the Gulf of Mexico. Flooding began after a year of above-normal precipitation. Water levels along the Mississippi, and rivers flowing into the Mississippi, were already high when 9 inches (22 centimeters) of rain fell on southern Missouri and most of Arkansas in April.

Did you know? Floods and the spread of disease

Cholera, dysentery, and typhoid are diseases that typically spread during floods, when sewage treatment systems become overloaded and drinking water supplies become contaminated. An abundance of standing water also promotes the breeding of mosquitoes, which may carry malaria, dengue fever, yellow fever, and encephalitis (pronounced en-cef-a-LIE-tis).

In 1982, flooding in Ecuador, Bolivia, Colombia, Peru, India, and Sri Lanka resulted in epidemics of malaria. In early 1998, a malaria outbreak hit Peru because of flooding. In the Piura region of northwest Peru, home to 1.5 million people, there were some thirty thousand cases of the disease.

Beginning in April, the river flooded over hundreds of square miles. Levees failed in more than 120 places on the Mississippi and its tributaries. Over twenty-six thousand square miles (seven thousand square kilometers) were flooded

across seven states. Six hundred thousand people were forced from their homes and, despite advance warnings, 246 people were killed.

In the summer of 1993, excessive rains caused what is called "The Great Flood of 1993," one of the largest floods in the history of the United States. Water overflowed the banks of the Mississippi and Missouri Rivers, inundating 16,000 square miles (41,000 square kilometers) of land in Iowa, Illinois, Minnesota, Missouri, Wisconsin, South Dakota, Nebraska, and Kansas. The Mississippi River swelled to over 7 miles (11 kilometers) wide in some places. Fifty-six small towns along the river were completely submerged in water.

The flooding was preceded by heavy snowfalls throughout the Midwest in December 1992. The snow melted in March, and then 16 inches (41 centimeters) of rain fell in the upper Mississippi Valley in April. The rains continued through July. In mid-June, a stationary front, which is a slow-moving boundary between two air masses at different temperatures, formed across the upper Midwest and began producing daily thunderstorms.

The water reached record heights at many points along the Mississippi River and more than 60 percent of levees on the river were destroyed. Forty-eight people died. There was $6.5 billion in crop damage, out of a total property damage of $15-20 billion. Eighty-five thousand people were evacuated from their homes. Forty-five thousand homes were damaged or destroyed and 404 counties were declared disaster areas. Counties that are designated "federal disaster areas" are eligible to receive federal disaster aid including low-interest loans.

Extreme weather: Killer flash floods

One of the most destructive flash floods in recent history occurred in Colorado's Big Thompson Canyon, in the mountains west of Denver, in July 1976. Ten to twelve inches (25 to 30 centimeters) of rain, over half the yearly average rainfall for that location, caused the overflow of the river running through the narrow canyon floor. A wall of water 20 feet (6 meters) high rushed forward, killing at least 139 people. More than 400 houses were destroyed, many roads were washed away, and 197 vehicles were lost in the flood. Approximately one thousand people were rescued by helicopter.

In June 1990, a flash flood occurred in Shady-side, Ohio, when 4 inches (10 centimeters) of rain fell in less than two hours. The resultant 30-foot (9-meter) wall of water killed twenty-six people and caused around $7 million in damages.

Cheyenne, Wyoming, was beset by a flash flood in August 1985, after 6 inches (15 centimeters) of rain fell in three hours. The streets were filled with more than 6 feet (about 2 meters) of water. The flood caused the deaths of twelve people and more than $65 million in damage.

Another of the worst floods in U.S. history ravaged parts of the Midwest and Southeast in March 1997. The disaster was touched off when a series of severe thunderstorms and tornadoes, violent funnel-shaped windstorms, swept through Arkansas, Tennessee, and Mississippi and killed at least twenty-five people. Flash floods and gusting winds killed thirty-six more people in Kentucky, Ohio, West Virginia, and Tennessee, bringing the death toll to sixty-one.

Due to the tornadoes, eleven counties in Arkansas were declared federal disaster areas. Due to floods, sixteen counties in Ohio and sixty-three counties in Kentucky were also declared federal disaster areas.

During the floods, occasional heavy rains continued to fall throughout the region, accompanied by melting snow. On a single day, March 1, nearly a foot of rain fell on north-central Kentucky, southern Ohio, and West Virginia. New rainfall records were set in Louisville, Kentucky, on that day. Two days later, up to 9 more inches (23 centimeters) of rain fell on the same area.

Watch this: The Johnstown Flood

Using archival film and photographs, this documentary recreates the story of the Johnstown flood of 1889. The South Fork Dam burst due to very heavy rains and poor maintenance, sending a wall of water and debris downstream to flood the industrial city of Johnstown, Pennsylvania. The film, written and directed by Charles Guggenheim and released in 1989, won an Academy Award for best documentary, short subject.

As a result, flooding occurred along the Ohio, Missouri, and Mississippi Rivers, as well as on numerous tributaries. Rivers throughout Kentucky rose to record levels. Many river towns in Ohio, Kentucky, Tennessee, and West Virginia were swamped. Thousands of people from

flooded river towns were left homeless. The estimated flood damage in Kentucky was $232 million and in Ohio, $40 million.

China's Joy; China's Sorrow

Somewhere between nine hundred thousand and two million five hundred thousand people living in the crowded river valleys perished during the 1887–88 flooding of China's Huàng Hè (Yellow River). This flood was, by far, the most devastating in recorded history.

The Yellow River, nicknamed "China's Sorrow," is one of the world's most flood-prone rivers. Records show that there have been around 1,500 floods on the Huàng Hè over the last 3,500 years, many of which killed large numbers of people.

According to reports by CNN news on March 7, 1997, Falmouth was one of the hardest-hit areas. The town was built on the banks of the Licking River, a tributary of the Ohio River. When Falmouth residents were allowed to return and survey the damage, they saw extensive damage to their homes and businesses. Houses were moved from their foundations, their walls warped and roofs collapsed. The National Guard had plowed the streets, but a thick layer of brown, gooey mud coated everything else. The Cincinnati Enquirer reported on March 2, 1997, on a helicopter flight taken by Kentucky State Police Trooper Jan Wuchner over Falmouth. Wuchner was stunned. His report was short and to the point: "There is no Falmouth."

Recent devastating floods

In July 2006, news media reported that floods in North Korea caused at least hundreds and maybe even thousands of deaths and left many more people homeless. Since North Korea is a closed country that tightly controls information, exact figures on losses of life and property are hard to determine. But the United Nations Food Agency reported sixty thousand homeless, seven thousand homes destroyed or damaged, and almost forty thousand acres (sixteen thousand hectares) of farmland washed away. One indication of the size of the disaster is that the government of North Korea cancelled the Arirang Festival of dance and mass gymnastics, which is a major tourist attraction and a key source of income for the nation. North Korea also closed its border to tourists from China, another major source of income.

Did you know? Dams for flood control

The construction of river dams is a common method of storing water and lessening the impact of drought. Dams are also used in flood control. A dam is a barrier that blocks a river and controls the flow of water. When a river is dammed, the water backs up in the area behind the dam, creating an artificial lake or reservoir (pronounced REH-zer-vwar). Pipes carry the water from the reservoir to factories, homes, and farms. This ready supply of water is especially useful during times of drought. By the year 2000, there were more than sixty thousand dams in use worldwide.

Large dams, defined as those that are more than 492 feet (150 meters) high, or holding back more than 19.6 million cubic yards (15 million cubic meters) of water, have a significant impact on the surrounding community and environment. When a dam is constructed in a valley, it floods the area upstream, putting homes, farmland, and even whole villages underwater. In the process, it displaces the people who inhabit the area. Dams also disrupt a river's natural cycle of flooding. When a river floods a valley, it deposits a layer of silt that enriches the soil. After the river is dammed, the fertile soil is lost. (The soil upstream is buried beneath the reservoir, and the soil downstream remains exposed but becomes less fertile over time.) Dams also disrupt the balance of the river ecosystem, destroying the habitat of birds, fish, and other animals, as well as many species of plants.

China also suffered disastrous floods from at least eight typhoons during the storm season of 2006. On August 10, 2006, Typhoon Saomai struck China's southeast coast with winds of 170 mph (270 kph). It was the most powerful typhoon to hit China in fifty years. The storm sank over one thousand ships and destroyed or damaged at least fifty thousand houses. More than 1.6 million people fled their homes to avoid the storm and at least 399,999 acres (122,000 hectares) were flooded. Saomai was the second major typhoon to strike China that same month. The government reported that prior to Saomai, economic losses in China already exceeded $9.4 billion.

India has also suffered severe flooding. In 2005, the Indian state of Maharashtra, which includes Mumbai (formerly Bombay), experienced floods that caused the deaths of at least one thousand people. The flooding was caused by the eighth heaviest rainfall ever recorded within a twenty-four-hour period. Then, during July 25 and 26, 2006, 37.2 inches (944 mm) of rain fell. Heavy rains continued over the next few days and forced the closing of schools in Mumbai and surrounding areas.

Later that summer, widespread flooding due to the summer monsoon (seasonal rains caused by shifts in prevailing winds) displaced at least 4.5 million people in the states of Orissa and Gujarat. Rajasthan also suffered unusually heavy flooding due to monsoon rains, with over eight hundred thousand people affected and tens of thousands displaced.

Flooding and climate change

In any given week, a river flood or other flood is certain to be happening somewhere in the world. However, in recent years floods seem to be getting more severe. According to the World Meteorological Organization, floods affected 1.5 billion people in the last decade of the twentieth century alone.

Floods in popular music

There have been many songs written about great floods. Some of the songs have become classics of popular music. The song by Randy Newman, "Louisiana 1927," is a heavily sarcastic and fictional account of the government's response to the Great Mississippi River Flood of 1927. It has been recorded many times by different performers. "Requiem" is a song by Eliza Gilkyson that she composed in response to the floods from the Indian Ocean tsunami of 2004. Both of these songs were played often on many radio stations after the floods in New Orleans and other parts of the Gulf Coast caused by Hurricane Katrina.

After the flood waters that devastated the Gulf Coast in 2005 receded, many other performers and songwriters performed or composed songs in response to the tragedy. An unprecedented number of New Orleans tribute records and benefit concerts appeared, with pop stars such as Norah Jones, Prince, and the Dixie Chicks, among others, trying to help. Many of New Orleans's musicians and songwriters have also composed responses to the storm. Allen Toussaint teamed with Elvis Costello to create The River In Reverse. New Orleans native Dr. John's jazz offering was Sippiana Hericane. It included the heart-wrenching song and plea "Clean Water."

While climate researchers and other scientists have not come to universal agreement on whether there is a direct link between short-term extreme weather and long-term climate change, many of the weather patterns observed are similar to those predicted by models of climate change. For example, the landmark Second Assessment Report (AR2) published in 1995 by the United Nations Intergovernmental Panel on Climate Change (IPCC), predicted that global warming would leave southern China drenched with more rains, while the north and west of the country would suffer worsening droughts. This is very similar to the observed pattern, with droughts in the northern provinces and floods in the south.

Extreme weather: The Grand Forks flood of 1997

In April 1997 entire towns were submerged in floodwaters when the Red River, which runs between North Dakota and Minnesota, overflowed its banks. The floods took seven lives in North Dakota and four in Minnesota. In North Dakota about ninety thousand head of livestock died. Property damages throughout the Red River Valley (the areas of North Dakota and Minnesota along the banks of the Red River) totaled $3 billion. Most of the destruction occurred in Grand Forks, North Dakota, 90 percent of which was flooded.

Floods for the region had been predicted as early as February because of the several heavy snowstorms that took place that winter. In North Dakota, houses were buried up to their roofs in snow. In March the snow began to melt and, as predicted, the rivers overflowed.

In any case, the main cause of loss of life and damage to property due to flooding is the tendency of humans to build homes and other structures near water. For thousands of years, humans have depended on rivers for food, for transportation, for reliable sources of drinking water, and for water used in agriculture. Similarly, the oceans have long been an important food source. While many people do need to live and work near the water, the continued building of homes and recreational housing in flood-prone areas will only make the problems of damage and loss of life more difficult to control.