Flood

The River Berounka, Czech Republic, burst its banks in the 2002 European floods and houses in the village of Hlásná Třebaň, Beroun District, were inundated

For other uses, see Flood (disambiguation).

A flood is an overflow of an expanse of water that submerges land, a deluge. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows, with the result that some of the water escapes its normal boundaries. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endangers land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when the strength of the river is so high it flows out of the river channel, particularly at bends or meanders and cause damage to homes and businesses along such rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, man has lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is only evidence that the value of being near the water far exceeds the costs of repeated periodic flooding.

The word comes from the Old English flod, a word common to Teutonic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float).

The term "The Flood," capitalized, usually refers to the great Universal School described in the Bible, in Genesis, and is treated at Deluge.

Principal types of flood

Riverine floods

Flooding of a creek due to heavy monsoonal rain and high tide in Darwin, Northern Territory, Australia

• Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack.
• Fast kinds: flash flood as a result of e.g. an intense thunderstorm.

Estuarine floods

• Commonly caused by a combination of sea tidal surges caused by storm-force winds. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Coastal floods

Flooding near Key West, Florida, United States from Hurricane Wilma's storm surge in October 2005

• Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Catastrophic floods

• Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption).

Muddy floods

• A muddy flood is generated by runoff on cropland.

Other

Flash flooding caused by a severe thunderstorm.

• Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low evaporation).
• A series of storms moving over the same area.
• Dam-building beavers can flood low-lying urban and rural areas, often causing significant damage.

Typical effects

Primary effects

• Physical damage - Can range anywhere from bridges,cars, buildings, sewer systems, roadways, canals and any other type of structure.
• Casualties - People and livestock die due to drowning. It can also lead to epidemics and diseases.

Secondary effects

• Water supplies - Contamination of water. Clean drinking water becomes scarce.
• Diseases - Unhygienic conditions. Spread of water-borne diseases
• Crops and food supplies - Shortage of food crops can be caused due to loss of entire harvest. However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil.
• Trees - Non-tolerant species can die from suffocation.

Tertiary/long-term effects

• Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortage leading to price increase etc.

Flood defences, planning, and management

Autumn Mediterranean flooding in Alicante (Spain), 1997.

Main article: Flood control

In many countries across the world, rivers prone to floods are often carefully managed. Defences such as levees, bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks. Coastal flooding has been addressed in Europe and the Americas with coastal defences, such as sea walls, beach nourishment, and barrier islands.

Europe

London is protected from flooding by a huge mechanical barrier across the River Thames, which is raised when the water level reaches a certain point (see Thames Barrier).

Venice has a similar arrangement, although it is already unable to cope with very high tides. The defenses of both London and Venice will be rendered inadequate if sea levels were to rise.

Flood blocking the road in Jerusalem

The largest and most elaborate flood defenses can be found in the Netherlands, where they are referred to as Delta Works with the Oosterschelde dam as its crowning achievement. These works were built in response to the North Sea flood of 1953 of the southwestern part of the Netherlands. The Dutch had already built one of the world's largest dams in the north of the country: the Afsluitdijk (closing occurred in 1932).

Currently the Saint Petersburg Flood Prevention Facility Complex is to be finished by 2008, in Russia, to protect Saint Petersburg from storm surges. It also has a main traffic function, as it completes a ring road around Saint Petersburg. Eleven dams extend for 25.4 kilometres and stand eight metres above water level.

In Austria, flooding for over 150 years, has been controlled by various actions of the Vienna Danube regulation, with dredging of the main Danube during 1870-75, and creation of the New Danube from 1972-1988.

Americas

flooding near Snoqualmie, Washington, 2009.

Another elaborate system of floodway defenses can be found in the Canadian province of Manitoba. The Red River flows northward from the United States, passing through the city of Winnipeg (where it meets the Assiniboine River) and into Lake Winnipeg. As is the case with all north-flowing rivers in the temperate zone of the Northern Hemisphere, snowmelt in southern sections may cause river levels to rise before northern sections have had a chance to completely thaw. This can lead to devastating flooding, as occurred in Winnipeg during the spring of 1950. To protect the city from future floods, the Manitoba government undertook the construction of a massive system of diversions, dikes, and floodways (including the Red River Floodway and the Portage Diversion). The system kept Winnipeg safe during the 1997 flood which devastated many communities upriver from Winnipeg, including Grand Forks, North Dakota and Ste. Agathe, Manitoba.

In the U.S., the New Orleans Metropolitan Area, 35% of which sits below sea level, is protected by hundreds of miles of levees and flood gates. This system failed catastrophically, in numerous sections, during Hurricane Katrina, in the city proper and in eastern sections of the Metro Area, resulting in the inundation of approximately 50% of the metropolitan area, ranging from a few inches to Template:Htbot in coastal communities. In an act of successful flood prevention, the Federal Government of the United States offered to buy out flood-prone properties in the United States in order to prevent repeated disasters after the 1993 flood across the Midwest. Several communities accepted and the government, in partnership with the state, bought 25,000 properties which they converted into wetlands. These wetlands act as a sponge in storms and in 1995, when the floods returned, the government did not have to expend resources in those areas.

Asia

In China, flood diversion areas are rural areas that are deliberately flooded in emergencies in order to protect cities [1].

Many have proposed that loss of vegetation (deforestation) will lead to a risk increase. With natural forest cover the flood duration should decrease. Reducing the rate of deforestation should improve the incidents and severity of floods.

Africa

In Egypt, both the Aswan Dam (1902) and the Aswan High Dam (1976) have controlled various amounts of flooding along the Nile river.

Flood clean-up safety

Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers include electrical hazards, carbon monoxide exposure, musculoskeletal hazards, heat or cold stress, motor vehicle-related dangers, fire, drowning, and exposure to hazardous materials. Because flooded disaster sites are unstable, clean-up workers might encounter sharp jagged debris, biological hazards in the flood water, exposed electrical lines, blood or other body fluids, and animal and human remains. In planning for and reacting to flood disasters, managers provide workers with hard hats, goggles, heavy work gloves, life jackets, and watertight boots with steel toes and insoles.

Benefits of flooding

There are many disruptive effects of flooding on human settlements and economic activities. However, flooding can bring benefits, such as making soil more fertile and providing nutrients in which it is deficient. Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River, among others. The viability for hydrological based renewable sources of energy is higher in flood prone regions.

Flood modelling

While flood modelling is a fairly recent practice, attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia. The recent development in computational flood modelling has enabled engineers to step away from the tried and tested "hold or break" approach and its tendency to promote overly engineered structures. Various computational flood models have been developed in recent years either 1D models (flood levels measured in the channel) and 2D models (flood depth measured for the extent of the floodplain). HEC-RAS, the Hydraulic Engineering Centre model, is currently among the most popular if only because it is available for free. Other models such as TUFLOW combine 1D and 2D components to derive flood depth in the floodplain. So far the focus has been on mapping tidal and fluvial flood events but the 2007 flood events in the UK have shifted the emphasis onto the impact of surface water flooding.

Deadliest floods

Main article: List of deadliest floods

Below is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.

Death Toll	Event	Location	Date
2,500,000–3,700,000	1931 China floods	China	1931
900,000–2,000,000	1887 Yellow River (Huang He) flood	China	1887
500,000–700,000	1938 Yellow River (Huang He) flood	China	1938
231,000	Banqiao Dam failure, result of Typhoon Nina. Approximately 86,000 people died from flooding and another 145,000 died during subsequent disease.	China	1975
145,000	1935 Yangtze river flood	China	1935
more than 100,000	St. Felix's Flood, storm surge	Netherlands	1530
100,000	Hanoi and Red River Delta flood	North Vietnam	1971
100,000	1911 Yangtze river flood	China	1911

See also

• Flood Risk Assessment
• Flood control in the Netherlands

Dozens of villages were inundated when rain pushed the rivers of northwestern Bangladesh over their banks in early October 2005. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured the top image of the flooded Ghaghat and Atrai Rivers on October 12, 2005. The deep blue of the rivers is spread across the countryside in the flood image.

External links

• American Water Resources Association
• Associated Programme on Flood Management from World Meteorological Organization
• Dartmouth Flood Observatory
• Decision tree to choose an uncertainty method for hydrological and hydraulic modelling, Choosing an uncertainty analysis for flood modeling.
• DeltaWorks.Org Flood protecting dams and barriers project in the Netherlands
• Europe floods 2006
• Flood Risk Management Research Consortium
• International Flood Initiative
• International teaching module "Integrated Flood Risk Management of Extreme Events" (Floodmaster)
• Predictions Off for Global Warming Flood Risk - Study.
• Protecting against the Next Katrina - Scientific American Magazine (October 2005)
• Related articles at Appropedia, a wiki for non-Wikipedia (projects & practical "how to") content.
• Safecoast Knowledge exchange on coastal flooding and climate change in the North Sea region
• Social & Economic Benefits/Costs of Heavy Rain & Flooding NOAA Economics

Further reading

• O'Connor, Jim E. and John E. Costa. (2004). The World's Largest Floods, Past and Present: Their Causes and Magnitudes [Circular 1254]. Washington, D.C.: U.S. Department of the Interior, U.S. Geological Survey.
• Thompson, M.T. (1964). Historical Floods in New England [Geological Survey Water-Supply Paper 1779-M]. Washington, D.C.: United States Government Printing Office.