Flood.html

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

For other uses, see Flood (disambiguation).

A flood is an overflow of an expanse of water that submerges land, a deluge.¹ 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, exceeding the total capacity of its bounds, with the result that 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 out of the river channel, particularly at bends or meanders.

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 Deluge described in the Bible, in Genesis, and is treated at Deluge.

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1 Principal types of flood
2 Typical effects
3 Flood defences, planning, and management
- 3.1 Flood clean-up safety
4 Benefits of flooding
5 Flood modelling
6 See also
7 References
8 External links
9 Further reading

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.

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 dollars worth of damage in the Southeastern US in 1994 United States Dollars

Other

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.²
Trees - Non-tolerant species can die from suffocation³

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.

In western countries, 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 with coastal defences, such as sea walls and beach nourishment.

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

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.

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 [1].

(See Crossing the Lines)

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.⁵ 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.⁶

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.⁷ 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-RAS⁸, the Hydraulic Engineering Centre model, is currently among the most popular if only because it is available for free. Other models such as TUFLOW⁹ and Flowroute¹⁰, 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.¹¹