A flood is an overflow of water that submerges land which is
usually dry. The European Union (EU) Floods Directive defines a flood as a
covering by water of land not normally covered by water.In the sense of
"flowing water", the word may also be applied to the inflow of the
tide. Flooding may occur as an overflow of water from water bodies, such as a
river or lake, in which the water overtops or breaks levees, resulting in some
of that water escaping its usual boundaries, or it may occur due to an
accumulation of rainwater on saturated ground in an areal flood. While the size
of a lake or other body of water will vary with seasonal changes in
precipitation and snow melt, these changes in size are unlikely to be
considered significant unless they flood property or drown domestic animals.
Floods can also occur in rivers when the flow rate exceeds
the capacity of the river channel, particularly at bends or meanders in the
waterway. Floods often cause damage to homes and businesses if they are in the
natural flood plains of rivers. While riverine flood damage can be eliminated
by moving away from rivers and other bodies of water, people have traditionally
lived and worked by rivers because the land is usually flat and fertile and
because rivers provide easy travel and access to commerce and industry.
Some floods develop slowly, while others such as flash
floods, can develop in just a few minutes and without visible signs of rain.
Additionally, floods can be local, impacting a neighbourhood or community, or
very large, affecting entire river basins.
Etymology
The word "flood" comes from the Old English flod,
a word common to Germanic languages (compare German Flut, Dutch vloed from the
same root as is seen in flow, float; also compare with Latin fluctus, flumen).
Deluge myths are mythical stories of a great flood sent by a deity or deities
to destroy civilization as an act of divine retribution, and they are featured
in the mythology of many cultures.
Principal types and causes
Areal (rainfall related)
Floods can happen on flat or low-lying areas when the ground
is saturated and water either cannot run off or cannot run off quickly enough
to stop accumulating. This may be followed by a river flood as water moves away
from the floodplain into local rivers and streams.
Floods can also occur if water falls on an impermeable
surface, such as concrete, paving or frozen ground, and cannot rapidly
dissipate into the ground.
Localised heavy rain from a series of storms moving over the
same area can cause areal flash flooding when the rate of rainfall exceeds the
drainage capacity of the area. When this occurs on tilled fields, it can result
in a muddy flood where sediments are picked up by run off and carried as
suspended matter or bed load.
Riverine
River flows may rise to floods levels at different rates,
from a few minutes to several weeks, depending on the type of river and the
source of the increased flow.
Slow rising floods most commonly occur in large rivers with
large catchment areas. The increase in flow may be the result of sustained
rainfall, rapid snow melt, monsoons, or tropical cyclones. Localised flooding
may be caused or exacerbated by drainage obstructions such as landslides, ice,
or debris.
Rapid flooding events, including flash floods, more often
occur on smaller rivers, rivers with steep valleys or rivers that flow for much
of their length over impermeable terrain. The cause may be localised convective
precipitation (intense thunderstorms) or sudden release from an upstream
impoundment created behind a dam, landslide, or glacier.
Dam-building beavers can flood low-lying urban and rural
areas, occasionally causing some damage.
Estuarine and coastal
Flooding in estuaries is commonly caused by a combination of
sea tidal surges caused by winds and low barometric pressure, and they may be
exacerbated by high upstream river flow.
Coastal areas may be flooded by storm events at sea,
resulting in waves over-topping defences or in severe cases by tsunami or
tropical cyclones. A storm surge, from either a tropical cyclone or an
extratropical cyclone, falls within this category.
Urban flooding
Urban flooding is the inundation of land or property in a
built environment, particularly in more densely populated areas, caused by
rainfall overwhelming the capacity of drainage systems, such as storm sewers.
Although sometimes triggered by events such as flash flooding or snowmelt,
urban flooding is a condition, characterized by its repetitive and systemic
impacts on communities, that can happen regardless of whether or not affected
communities are located within formally designated floodplains or near any body
of water. There are several ways in which stormwater enters properties: backup
through sewer pipes, toilets and sinks into buildings; seepage through building
walls and floors; the accumulation of water on property and in public
rights-of-way; and the overflow from water bodies such as rivers and lakes.
Catastrophic
Catastrophic flooding is usually associated with major
infrastructure failures such as the collapse of a dam, but they may also be
caused by damage sustained in an earthquake or volcanic eruption. See outburst
flood.
Effects
Primary effects
The primary effects of flooding include loss of life, damage
to buildings and other structures, including bridges, sewerage systems,
roadways, and canals.
Floods also frequently damage power transmission and
sometimes power generation, which then has knock-on effects caused by the loss
of power. This includes loss of drinking water treatment and water supply,
which may result in loss of drinking water or severe water contamination. It
may also cause the loss of sewage disposal facilities. Lack of clean water
combined with human sewage in the flood waters raises the risk of waterborne
diseases, which can include typhoid, giardia, cryptosporidium, cholera and many
other diseases depending upon the location of the flood.
Damage to roads and transport infrastructure may make it
difficult to mobilise aid to those affected or to provide emergency health treatment.
Flood waters typically inundate farm land, making the land
unworkable and preventing crops from being planted or harvested, which can lead
to shortages of food both for humans and farm animals. Entire harvests for a
country can be lost in extreme flood circumstances. Some tree species may not
survive prolonged flooding of their root systems
Secondary and long-term effects
Economic hardship due to a temporary decline in tourism,
rebuilding costs, or food shortages leading to price increases is a common
after-effect of severe flooding. The impact on those affected may cause
psychological damage to those affected, in particular where deaths, serious
injuries and loss of property occur.
Urban flooding can lead to chronically wet houses, which are
linked to an increase in respiratory problems and other illnesses. Urban
flooding also has significant economic implications for affected neighborhoods.
In the United States, industry experts estimate that wet basements can lower
property values by 10-25 percent and are cited among the top reasons for not
purchasing a home. According to the U.S. Federal Emergency Management Agency
(FEMA), almost 40 percent of small businesses never reopen their doors following
a flooding disaster.
Flood forecasting
Anticipating floods before they occur allows for precautions
to be taken and people to be warned so
that they can be prepared in advance for flooding conditions. For example,
farmers can remove animals from low-lying areas and utility services can put in
place emergency provisions to re-route services if needed. Emergency services
can also make provisions to have enough resources available ahead of time to
respond to emergencies as they occur.
In order to make the most accurate flood forecasts for
waterways, it is best to have a long time-series of historical data that
relates stream flows to measured past rainfall events. Coupling this historical
information with real-time knowledge about volumetric capacity in catchment
areas, such as spare capacity in reservoirs, ground-water levels, and the
degree of saturation of area aquifers is also needed in order to make the most
accurate flood forecasts.
Radar estimates of rainfall and general weather forecasting
techniques are also important components of good flood forecasting. In areas
where good quality data is available, the intensity and height of a flood can
be predicted with fairly good accuracy and plenty of lead time. The output of a
flood forecast is typically a maximum expected water level and the likely time
of its arrival at key locations along a waterway, and it also may allow for the
computation of the likely statistical return period of a flood. In many
developed countries, urban areas at risk of flooding are protected against a
100-year flood - that is a flood that has a probability of around 63% of
occurring in any 100 year period of time.
According to the U.S. National Weather Service (NWS)
Northeast River Forecast Center (RFC) in Taunton, Massachusetts, a general
rule-of-thumb for flood forecasting in urban areas is that it takes at least 1
inch (25 mm) of rainfall in around an hour's time in order to start significant
ponding of water on impermeable surfaces. Many NWS RFCs routinely issue Flash
Flood Guidance and Headwater Guidance, which indicate the general amount of
rainfall that would need to fall in a short period of time in order to cause
flash flooding or flooding on larger water basins.
Control
In many countries around the world, waterways prone to
floods are often carefully managed. Defenses such as detention basins, levees,
bunds, reservoirs, and weirs are used to prevent waterways from overflowing
their banks. When these defences fail, emergency measures such as sandbags or
portable inflatable tubes are often used to try and stem flooding. Coastal
flooding has been addressed in portions of Europe and the Americas with coastal
defences, such as sea walls, beach nourishment, and barrier islands.
In the riparian zone near rivers and streams, erosion
control measures can be taken to try and slow down or reverse the natural
forces that cause many waterways to meander over long periods of time. Flood
controls, such as dams, can be built and maintained over time to try and reduce
the occurrence and severity of floods as well. In the USA, the U.S. Army Corps
of Engineers maintains a network of such flood control dams.
In areas prone to urban flooding, one solution is the repair
and expansion of man-made sewer systems and stormwater infrastructure. Another
strategy is to reduce impervious surfaces in streets, parking lots and
buildings through natural drainage channels, porous paving, and wetlands
(collectively known as green infrastructure or sustainable urban drainage
systems [SUDS]). Areas identified as flood-prone can be converted into parks
and playgrounds that can tolerate occasional flooding. Ordinances can be
adopted to require developers to retain stormwater on site and require
buildings to be elevated, protected by floodwalls and levees, or designed to
withstand temporary inundation. Property owners can also invest in solutions
themselves, such as re-landscaping their property to take the flow of water
away from their building and installing rain barrels, sump pumps, and check
valves.
Benefits
Floods (in particular more frequent or smaller floods) can
also bring many benefits, such as recharging ground water, making soil more
fertile and increasing nutrients in some soils. Flood waters provide much
needed water resources in arid and semi-arid regions where precipitation can be
very unevenly distributed throughout the year. Freshwater floods particularly
play an important role in maintaining ecosystems in river corridors and are a
key factor in maintaining floodplain biodiversity. Flooding can spread
nutrients to lakes and rivers, which can lead to increased biomass and improved
fisheries for a few years.
For some fish species, an inundated floodplain may form a
highly suitable location for spawning with few predators and enhanced levels of
nutrients or food. Fish, such as the weather fish, make use of floods in order
to reach new habitats. Bird populations may also profit from the boost in food
production caused by flooding.
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 of hydropower, a
renewable source of energy, is also higher in flood prone regions.
Computer modelling
While flood computer 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. Recent developments in computational
flood modelling have 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) or 2D
models (variable flood depths measured across the extent of a floodplain).
HEC-RAS,the Hydraulic Engineering Centre model, is currently among the most
popular computer models, if only because it is available free of charge. Other
models such as TUFLOW combine 1D and 2D
components to derive flood depths across both river channels and the entire
floodplain. To date, the focus of computer modelling has primarily been on
mapping tidal and fluvial flood events, but the 2007 flood events in the UK
have shifted the emphasis there onto the impact of surface water flooding.
In the United States, an integrated approach to real-time
hydrologic computer modelling utilizes observed data from the U.S. Geological
Survey (USGS), various cooperative observing networks, various automated
weather sensors, the NOAA National Operational Hydrologic Remote Sensing Center
(NOHRSC), various hydroelectric companies, etc. combined with quantitative
precipitation forecasts (QPF) of expected rainfall and/or snow melt to generate
daily or as-needed hydrologic forecasts. The NWS also cooperates with
Environment Canada on hydrologic forecasts that affect both the USA and Canada,
like in the area of the Saint Lawrence Seaway.
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[25] 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
230,000 Indian
Ocean tsunami Indonesia 2004
145,000 1935
Yangtze river flood China 1935
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
In myth and religion
Flood myths (great, civilization-destroying floods as divine
retribution) are widespread in many cultures and religions. As a prime example,
the Genesis flood narrative plays a prominent role in Judaism and Christianity.
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