Floods

Introduction

Climate is an important influence of floods. A winter thaw or rainy spring can send amounts of water into a river that are greater than the capacity of the water-course. The excess water has to go somewhere, and the result can be flooding of adjacent land. The vast majority of scientists have come to accept the reality of global warming—the warming of the atmosphere due to human activities. One consequence of the world's changing climate may be increased rainfall and flooding.

Historical Background and Scientific Foundations

For thousands of years, people have settled in low-lying land next to rivers, lakes, or other watercourses. Although the fertile soil is ideal for growing crops, the land is vulnerable to flooding. In 2007, flooding remained the most damaging of all natural disasters. One-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 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; 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 early 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 1 ft (.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 throw 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 in Sri Lanka alone.

Impacts and Issues

The flash floods experienced in Sudan in the summer of 2007 have 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 as of October 2007. 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 occurred during the 2005 hurricane season. As of late 2007, the 2005 season stood as the most active Atlantic hurricane season ever recorded.

These examples may have their origin in the changing global climate, which the majority of scientists agree is influenced, if not determined, by the warming of the atmosphere. This warming trend, which has been evident since the middle of the nineteenth century, and which coincides with the increased industrialization in developed countries, is popularly known as global warming. Altered air currents in the changing atmosphere can affect weather patterns, with some regions having more rainfall and other regions experiencing drought.

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.

WORDS TO KNOW

CLIMATE MODEL: A quantitative way of representing 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: Local, temporary rise in sea level (above what would be expected due to tidal variation alone) as the result of winds and low pressures associated with a large storm system. Storm surges can cause coastal flooding, if severe.

TSUNAMI: Ocean wave caused by a large displacement of mass under the surface of the water, such as an earthquake or volcanic eruption.

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.

See Also Erosion; Forests and Deforestation; Hurricanes; Rainfall.

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 & Stoughton, 2003.

Morgan, William W. The Biblical Flood: Global Warming & Bush's Harvest. Charleston: Book Surge Publishing, 2007.

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 415 (2002): 514–517.

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

Web Sites

“Causes of Flooding.” Environment Canada. < http://www.ec.gc.ca/water/en/manage/floodgen/e_cause.htm> (accessed October 31, 2007).

“In Depth: Forces of Nature—Flooding.” Canadian Broad casting Corporation (CBC), June 20, 2005. < http://www.cbc.ca/news/background/forcesofnature/flooding.html> (accessed October 31, 2007).