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

It is usually due to the volume of water within a body of water, such as a river or lake, exceeding the total capacity of the body, and as a result some of the water flows or sits outside of the normal perimeter of the body. It can also occur in rivers, when the strength of the river is so high it flows right out of the river channel , usually at corners or meanders. These of course, are not applicable in such instances as sea 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" usually refers to the great Universal Deluge described 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 snowmelt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foen winds and warm rain affecting snowpack.
• 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.

Coastal floods

• Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane).

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).

For example: Tropical Storm Alberto, the famous 1994 storm, produced heavy flooding across Georgia, Alabama and northwest Florida and created between 400-600 million damage in the Southeastern US in 1994 United States Dollars

Other

• Flooding 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.^[2]
• Trees - Non-tolerant species can die from suffocation^[3]

Tertiary/long-term effects

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

Flood defences, planning, and management

Autumn Mediterranean flooding in Alicante (Spain), 1997.

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 continue to rise.

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).

Flood blocking the road in Jerusalem

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 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 twenty feet 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 didn't have to expend resources in those areas.^[4]

In China, flood diversion areas are rural areas that are deliberately flooded in emergencies in order to protect cities http://www.chinadaily.com.cn/en/doc/2003-07/07/content_243670.htm.

(See Crossing the Lines)

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.^[5] 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^[6], the Hydraulic Engineering Centre model, is currently among the most popular if only because it is available for free. Other models such as TUFLOW^[7] and Flowroute^[8], 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.^[9]

See also

• Flood Risk Assessment

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.

References

External links

• Integrated Flood Management
• American Water Resources Association
• Catastrophic flood database
• 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 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 from NOAA Social & Economics Benefits website initiative

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.

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