Floods

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Floods

Introduction

A flood is any unusual, temporary submergence of some area of land. Flooding can be caused by intense rainfalls or snowmelts that swell rivers and streams, the breaching of dams or dikes, temporary rises in sea level during hurricanes, or any combination of such factors. For example, a fast spring thaw combined with a heavy rainstorm can send amounts of water into a river that are greater than the capacity of the watercourse. The excess water has to go somewhere, and the result can be flooding of adjacent land.

Floods can destroy property and endanger lives. Over 520 million people are affected by floods every year, with about 25,000 killed. In 2004, the United Nations University forecast that the number of people vulnerable to catastrophic flooding was likely to increase from about 1 billion in the early 2000s to over two billion by 2050 due to the effects of global climate change and other human activities, such as deforestation and increasing settlement of flood-prone areas. Even attempts to control flooding, such as the extensive reengineering of the Mississippi River in the United States, can increase the chances of catastrophic floods.

However, flooding is also a natural and beneficial process in many ecosystems. For example, the fertile soils in many floodplains (low-lying, flood-prone areas of land adjacent to a river) are created by periodic flooding, which delivers soil particles from upstream and spreads them over the land. The Nile River valley is a prime example of fertility created by seasonal flooding—flooding prevented, since 1970, by the Aswan High Dam.

Historical Background and Scientific Foundations

For thousands of years, people have settled in low-lying land next to oceans, rivers, lakes, or other watercourses. Although the fertile soil in these areas is ideal for growing crops, such land is vulnerable to flooding. Flooding remains the most damaging of all natural disasters. About a third of all the natural disasters occurring around the world each year are floods, and more than 50% of disaster-related deaths are due to floods.

In the wake of Hurricane Katrina in August 2005, for example, nearly every levee (raised-earth water barrier) in the downtown region of New Orleans, Louisiana, was breached, and the resulting flooding turned 80% of the city into a lake. The hurricane- and flood-related damage in New Orleans topped $80 billion and more than 1,000 people died. Over the entire affected Gulf Coast region, the death toll exceeded 2,200 and damage approached $130 billion. As of 2008, the region had yet to fully recover.

The devastation of Hurricane Katrina is an example of flooding caused by a storm surge, where strong winds push water onto shore. In the case of Hurricane Katrina, the storm surge was huge; in one documented example, waves that struck a Gulfport, Mississippi, hotel were approximately 10 ft (3 m) high, even though the hotel was 250 yd (230 m) from the shoreline and 19 ft (6 m) above sea level. Calculations revealed that the surging waves were 28 ft (8.5 m) high when they came ashore.

Much smaller flood surges are still capable of causing a great deal of damage. Anyone who has tried to wade into the ocean surf and who has been thrown off balance by waves less than a one ft (0.3 m) tall can attest to the power packed by a wave. In a flood, waves that are several feet tall can wash away an average-sized car. Powerful floods that last hours or days can shove houses off their foundations.

Other floods are caused by heavy rainfall or a sudden snowmelt, which inundates a watercourse with a greater volume of water than the watercourse can handle. Also, the soil that borders the river, stream, or lake becomes saturated with water. This is especially true for clay-rich soil or soil that has been compacted. When this happens, any additional water cannot be absorbed by the soil, and it flows into the watercourse.

A watercourse with a shoreline that is rocky or close to a parking lot (where the water-absorbing soil has been replaced by asphalt) is more prone to flooding than, for example, one with a forested shoreline capable of absorbing water.

Floods can occur in a fairly predictable pattern. A historic example is the Nile River, which, before the construction of dams to control water flow, tended to flood with the increase of rain in the spring. This type of flooding tends to be less destructive because those living near the river anticipate it. In addition, the nutrients that are washed onto the land by the flood are left when the flood waters recede, making the land very fertile for growing crops.

Floods that occur with no warning are known as flash floods. These floods typically occur when a large amount of rain falls in a short time, such as in a monsoon. The sudden appearance of the excessive amount of water in a small watercourse can create a wall of water that rushes downstream. Just as quickly, the water level can return to normal. In July 2007, for example, a series of flash floods caused by heavy seasonal rains ravaged eight states in the African country of Sudan, killing more than 100 people

Undersea earthquakes release a great deal of energy, which pushes the surrounding ocean water outward from the origin of the earthquake. The result is a ripple effect, similar to the ripples created when a rock is dropped into a pond. The series of waves is called a tsunami. Moving at hundreds of miles per hour, the waves can devastate coastal regions. On December 26, 2004, a magnitude 9.0 earthquake off the coast of Sumatra spawned a series of waves that obliterated coastal villages in Sri Lanka, Thailand, Myanmar, India, and elsewhere. More than 155,000 people were killed inSri Lanka alone.

Impacts and Issues

The flash floods experienced in Sudan in the summer of 2007 affected 21 other African nations including Uganda, Ethiopia, Democratic Republic of the Congo, and Ghana, displacing more than 700,000 people and killing at least 300 people. At the same time, regions of the United Kingdom were affected by severe floods triggered by June rains of 5.5 in (14 cm), more than double the historic average for the month. Some regions of Britain experienced the most monthly rainfall ever recorded.

Hurricane Katrina was one of the 28 tropical and subtropical storms, and one of the 15 hurricanes, that

WORDS TO KNOW

CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

MONSOON: An annual shift in the direction of the prevailing wind that brings on a rainy season and affects large parts of Asia and Africa.

STORM SURGE: Rise of the sea at a coastline due to the effect of storm winds.

TSUNAMI: A series of ocean waves that result because of an undersea disturbance such as an earthquake.

occurred during the 2005 hurricane season. As of late 2007, the 2005 season stood as the most active Atlantic hurricane season ever recorded.

Extreme weather events such as these, with attendant flooding, are likely to be made more common by global climate change. Global climate change is the result of several centuries of human burning of fossil fuels and destruction of forests. These activities have added carbon dioxide (CO2) and other gases to Earth’s atmosphere in large quantities, causing it to retain more heat and warming the average global climate. This warming has had many effects on rainfall, snowfall, sea level, and other aspects of the environment. More drought and more flooding are both predicted results of global climate change, as rainfall in many areas becomes, on average, less frequent but more intense.

A pair of 2002 studies published in Nature highlighted the influence of climate on the occurrence of floods. The studies assessed data from all over the globe on the occurrence of 100-year-floods—those that are so severe that they are predicted to occur very rarely—and on 19 climate models. They revealed an increased frequency of major floods since the mid-twentieth century, and related the increase to global warming.

The World Water Council, a think-tank of water users and suppliers concerned with water-related social and economic issues, has predicted that by 2025 about half of Earth’s population will live in areas that will be at increased risk of severe storms and other extreme weather. One consequence could be increased frequency of floods.

Computer models have been used for decades to help understand weather patterns and predict the weather. Daily weather forecasts rely on computer projections along with the monitoring of data. Similarly, flood-modeling software exists. Information on the size, shape, and topography of the area that drains into the

particular watercourse; the types of soil present in the area; typical weather patterns; and land use are used to better understand how changing parameters affect the potential for flooding. Such models help scientists better understand how climate changes in the real world affect flooding.

BIBLIOGRAPHY

Books

DiMento, Joseph, and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Hulme, Mike. Nature’s Revenge?: Hurricanes, Floods and Climate Change. New York: Hodder, 2003.

Periodicals

Milly, P.C.D., R.T. Wetherald, K.A. Dunne, and T.L. Delworth. “Increasing Risk of Great Floods in a Changing Climate.” Nature 15 (2002): 514–517.

Palmer, T.N., and J. Räisanen. “Quantifying the Risk of Extreme Seasonal Precipitation Events in a Changing Climate.” Nature 415 (2002): 512-514.

Web Sites

Canadian Broadcasting Corporation. “In Depth: Forces of Nature—Flooding.” June 20, 2005. http://www.cbc.ca/news/background/forcesofnature/flooding.html (accessed March 24, 2008).

Environment Canada. “Causes of Flooding.” 2004. http://www.ec.gc.ca/water/en/manage/floodgen/e_cause.htm (accessed March 24, 2008).

United Nations University. “Two Billion People Vulnerable to Floods by 2050.” June 13, 2004. http://www.unu.edu/news/ehs/floods.doc (accessed March 24, 2008).

Brian D. Hoyle

Environmental Science: In Context