A flood is an overflow of water (
or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the
tide. Floods are an area of study of the discipline
hydrology and are of significant concern in
agriculture
Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...
,
civil engineering and
public health.
Human changes to the environment often increase the intensity and frequency of flooding, for example land use changes such as
deforestation and
removal of wetlands, changes in waterway course or
flood controls such as with
levees, and larger environmental issues such as
climate change and
sea level rise. In particular climate change's
increased rainfall and
extreme weather events
Extreme weather or extreme climate events includes unexpected, unusual, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are based on a locat ...
increases the severity of other causes for flooding, resulting in more intense floods and increased flood risk.
Flooding may occur as an overflow of water from water bodies, such as a
river,
lake, or ocean, 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
Drowning is a type of suffocation induced by the submersion of the mouth and nose in a liquid. Most instances of fatal drowning occur alone or in situations where others present are either unaware of the victim's situation or unable to offer as ...
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. Flooding can lead to secondary consequences in addition to damage to property, such as long-term displacement of residents and creating increased spread of
waterborne diseases and
vector-bourne disesases transmitted by mosquitos.
Types
Areal
Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either
infiltrate or
run off. The excess accumulates in place, sometimes to hazardous depths. Surface
soil can become saturated, which effectively stops infiltration, where the
water table is shallow, such as a
floodplain, or from intense rain from one or a
series of storms. Infiltration also is slow to negligible through frozen ground, rock,
concrete, paving, or roofs. Areal flooding begins in flat areas like floodplains and in local depressions not connected to a stream channel, because the velocity of
overland flow
Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when th ...
depends on the surface slope.
Endorheic basins may experience areal flooding during periods when precipitation exceeds evaporation.
Riverine (Channel)
Floods occur in all types of
river and
stream channels, from the smallest
ephemeral streams in humid zones to
normally-dry channels in arid climates to the
world's largest rivers. When overland flow occurs on tilled fields, it can result in a
muddy flood
A muddy flood is produced by an accumulation of run-off over agricultural land. Sediments are picked up by the run-off and carried as suspended matter or bed-load. Muddy floods are typically a hill-slope process, and should not be confused with mu ...
where
sediments are
picked up by run off and carried as suspended matter or
bed load
The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load.
Bed load moves by rolling, sliding, and/or saltating (hop ...
. Localized flooding may be caused or exacerbated by drainage obstructions such as
landslides,
ice
Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
,
debris
Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer to ...
, or
beaver
Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are ...
dams.
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
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depend ...
. However, large rivers may have rapid flooding events in areas with dry climate, since they may have large basins but small river channels and rainfall can be very intense in smaller areas of those basins.
Rapid flooding events, including
flash floods
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flow ...
, more often occur on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, or normally-dry channels. The cause may be localized
convective precipitation
In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is g ...
(intense
thunderstorms) or sudden release from an upstream impoundment created behind a
dam
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, a ...
, landslide, or
glacier. In one instance, a flash flood killed eight people enjoying the water on a Sunday afternoon at a popular waterfall in a narrow canyon. Without any observed rainfall, the flow rate increased from about in just one minute.
Two larger floods occurred at the same site within a week, but no one was at the waterfall on those days. The deadly flood resulted from a thunderstorm over part of the drainage basin, where steep, bare rock slopes are common and the thin soil was already saturated.
Flash floods are the most common flood type in normally-dry channels in arid zones, known as
arroyos in the southwest United States and many other names elsewhere. In that setting, the first flood water to arrive is depleted as it wets the sandy stream bed. The leading edge of the flood thus advances more slowly than later and higher flows. As a result, the rising limb of the
hydrograph
A hydrograph is a graph showing the rate of flow ( discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
It can al ...
becomes ever quicker as the flood moves downstream, until the flow rate is so great that the depletion by wetting soil becomes insignificant.
Estuarine and coastal
Flooding in
estuaries
An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...
is commonly caused by a combination of storm surges caused by
winds and low
barometric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
and large waves meeting high upstream river flows.
Coastal areas may be flooded by storm surges combining with high tides and large wave events at sea, resulting in waves over-topping flood defenses 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. Research from the NHC (National Hurricane Center) explains: "Storm surge is an additional rise of water generated by a storm, over and above the predicted astronomical tides.
Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with spring tide, resulting in storm tides reaching up to 20 feet or more in some cases."
Urban flooding
Catastrophic
Catastrophic riverine flooding is usually associated with major infrastructure failures such as the collapse of a dam, but they may also be caused by drainage channel modification from a
landslide,
earthquake or
volcanic eruption. Examples include
outburst flood
In geomorphology, an outburst flood—a type of megaflood—is a high-magnitude, low-frequency catastrophic flood involving the sudden release of a large quantity of water. During the last deglaciation, numerous glacial lake outburst floods were ca ...
s and
lahars.
Tsunamis can cause catastrophic
coastal flooding
Coastal flooding normally occurs when dry and low-lying land is submerged by seawater. The range of a coastal flooding is a result of the elevation of floodwater that penetrates the inland which is controlled by the topography of the coastal lan ...
, most commonly resulting from undersea earthquakes.
Causes
Upslope factors
The amount, location, and timing of water reaching a drainage channel from natural precipitation and controlled or uncontrolled reservoir releases determines the flow at downstream locations. Some precipitation evaporates, some slowly percolates through soil, some may be temporarily sequestered as snow or ice, and some may produce rapid runoff from surfaces including rock, pavement, roofs, and saturated or frozen ground. The fraction of incident precipitation promptly reaching a drainage channel has been observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain on accumulated snow.
Most precipitation records are based on a measured depth of water received within a fixed time interval. ''Frequency'' of a precipitation threshold of interest may be determined from the number of measurements exceeding that threshold value within the total time period for which observations are available. Individual data points are converted to ''intensity'' by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if the ''duration'' of the rainfall event was less than the fixed time interval for which measurements are reported. Convective precipitation events (thunderstorms) tend to produce shorter duration storm events than orographic precipitation. Duration, intensity, and frequency of rainfall events are important to flood prediction. Short duration precipitation is more significant to flooding within small drainage basins.
The most important upslope factor in determining flood magnitude is the land area of the watershed upstream of the area of interest. Rainfall intensity is the second most important factor for watersheds of less than approximately . The main channel slope is the second most important factor for larger watersheds. Channel slope and rainfall intensity become the third most important factors for small and large watersheds, respectively.
[Simon, Andrew L., ''Practical Hydraulics'', John Wiley & Sons, 1981, ]
Time of Concentration is the time required for runoff from the most distant point of the upstream drainage area to reach the point of the drainage channel controlling flooding of the area of interest. The time of concentration defines the critical duration of peak rainfall for the area of interest.
[Urquhart, Leonard Church, ''Civil Engineering Handbook'', McGraw-Hill Book Company, 1959] The critical duration of intense rainfall might be only a few minutes for roof and parking lot drainage structures, while cumulative rainfall over several days would be critical for river basins.
Downslope factors
Water flowing downhill ultimately encounters downstream conditions slowing movement. The final limitation in coastal flooding lands is often the
ocean or some coastal flooding bars which form natural
lakes. In flooding low lands, elevation changes such as tidal fluctuations are significant determinants of coastal and estuarine flooding. Less predictable events like tsunamis and storm surges may also cause elevation changes in large bodies of water. Elevation of flowing water is controlled by the geometry of the flow channel and, especially, by depth of channel, speed of flow and amount of sediments in it
Flow channel restrictions like bridges and canyons tend to control water elevation above the restriction. The actual control point for any given reach of the drainage may change with changing water elevation, so a closer point may control for lower water levels until a more distant point controls at higher water levels.
Effective flood channel geometry may be changed by growth of vegetation, accumulation of ice or debris, or construction of bridges, buildings, or levees within the flood channel.
Coincidence
Extreme flood events often result from coincidence such as unusually intense, warm rainfall melting heavy snow pack, producing channel obstructions from floating ice, and releasing small impoundments like
beaver
Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are ...
dams.
[Abbett, Robert W., ''American Civil Engineering Practice'', John Wiley & Sons, 1956] Coincident events may cause extensive flooding to be more frequent than anticipated from
simplistic statistical prediction models considering only precipitation runoff flowing within unobstructed drainage channels.
[ United States Department of the Interior, Bureau of Reclamation, ''Design of Small Dams'', United States Government Printing Office, 1973] Debris modification of channel geometry is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats and
railway equipment. Recent field measurements during the
2010–11 Queensland floods
1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length&nb ...
showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by velocity and water depth fluctuations.
These considerations ignore further the risks associated with large debris entrained by the flow motion.
Some researchers have mentioned the storage effect in urban areas with transportation corridors created by
cut and fill
In earthmoving, cut and fill is the process of constructing a railway, road or canal whereby the amount of material from cuts roughly matches the amount of fill needed to make nearby embankments to minimize the amount of construction labor.
...
. Culverted fills may be converted to impoundments if the
culvert
A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. In the United Kingdom, ...
s become blocked by debris, and flow may be diverted along streets. Several studies have looked into the flow patterns and redistribution in streets during storm events and the implication on flood modelling.
Climate change
Intentional flooding
The intentional flooding of land that would otherwise remain dry may take place for military, agricultural, or river-management purposes. This is a form of
hydraulic engineering
Hydraulic engineering as a sub-discipline of civil engineering is concerned with the flow and conveyance of fluids, principally water and sewage. One feature of these systems is the extensive use of gravity as the motive force to cause the mov ...
.
Agricultural flooding may occur in preparing
paddy fields
A paddy field is a flooded field of arable land used for growing semiaquatic crops, most notably rice and taro. It originates from the Neolithic rice-farming cultures of the Yangtze River basin in southern China, associated with pr ...
for the growing of semi-aquatic rice in many countries.
Flooding for river management may occur in the form of diverting flood waters in a river at flood stage upstream from areas that are considered more valuable than the areas that are sacrificed in this way. This may be done ''ad hoc'', as in the 2011 intentional breach of
levees by the
United States Army Corps of Engineers in Missouri, or permanently, as in the so-called ''overlaten'' (literally "let-overs"), an intentionally lowered segment in Dutch riparian levees, like the ''Beerse Overlaat'' in the left levee of the
Meuse between the villages of
Gassel and
Linden, North Brabant
Linden is a village in the Dutch province of North Brabant. It is located in the former municipality of Cuijk, about 10 km south of Nijmegen.
History
The village was first mentioned in 1308 as "Kerklienden en Linen". It used to be called ...
.
Military inundation creates an obstacle in the field that is intended to impede the movement of the enemy. This may be done both for
offensive
Offensive may refer to:
* Offensive, the former name of the Dutch political party Socialist Alternative
* Offensive (military), an attack
* Offensive language
** Fighting words or insulting language, words that by their very utterance inflict inj ...
and
defensive
Defense or defence may refer to:
Tactical, martial, and political acts or groups
* Defense (military), forces primarily intended for warfare
* Civil defense, the organizing of civilians to deal with emergencies or enemy attacks
* Defense industr ...
purposes. Furthermore, in so far as the methods used are a form of hydraulic engineering, it may be useful to differentiate between ''controlled'' inundations (as in most historic inundations in the Netherlands under the
Dutch Republic and its
successor states
Succession of states is a concept in international relations regarding a successor state that has become a sovereign state over a territory (and populace) that was previously under the sovereignty of another state. The theory has its roots in 19th- ...
in that area and exemplified in the two
Hollandic Water Lines, the
Stelling van Amsterdam, the
Frisian Water Line, the
IJssel Line
The IJssel (; nds-nl, Iessel(t) ) is a Dutch distributary of the river Rhine that flows northward and ultimately discharges into the IJsselmeer (before the 1932 completion of the Afsluitdijk known as the Zuiderzee), a North Sea natural harbour. ...
, the
Peel-Raam Line
230px, Peel-Raam Line
The Peel-Raam Line (Dutch: Peel-Raamstelling) was a Dutch defence line built in 1939 and attacked and conquered on 10 May 1940 by the German forces.
The defence line was behind the Maas Line (about 9 km to 21 km ...
, and the
Grebbe line in that country) and ''uncontrolled'' ones (as in the
second Siege of Leiden during the first part of the
Eighty Years' War
The Eighty Years' War or Dutch Revolt ( nl, Nederlandse Opstand) ( c.1566/1568–1648) was an armed conflict in the Habsburg Netherlands between disparate groups of rebels and the Spanish government. The causes of the war included the Refo ...
, and the
Inundation of Walcheren, and the
Inundation of the Wieringermeer
On 17 April 1945, the retreating German occupying forces inundated the polder of Wieringermeer, the Netherlands.
Preparations
In 1945 German forces occupying the Netherlands planned to destroy the Zuiderzee Works to cover their retreat. Milit ...
during the
Second World War). To count as ''controlled'', a military inundation has to take the interests of the civilian population into account, by allowing them a timely
evacuation, by making the inundation ''reversible'', and by making an attempt to minimize the adverse
ecological impact of the inundation. That impact may also be adverse in a
hydrogeological
Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aqu ...
sense if the inundation lasts a long time.
Effects
Floods can also be a huge destructive power. When water flows, it has the ability to demolish all kinds of buildings and objects, such as bridges, structures, houses, trees, cars... For example, in Bangladesh in 2007, a flood was responsible for the destruction of more than one million houses. And yearly in the United States, floods cause over $7 billion in damage.
Primary effects
The primary effects of flooding include
loss of life and damage to buildings and other structures, including bridges,
sewerage
Sewerage (or sewage system) is the infrastructure that conveys sewage or surface runoff (stormwater, meltwater, rainwater) using sewers. It encompasses components such as receiving drains, manholes, pumping stations, storm overflows, and scr ...
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
''Cryptosporidium'', sometimes informally called crypto, is a genus of apicomplexan parasitic alveolates that can cause a respiratory and gastrointestinal illness (cryptosporidiosis) that primarily involves watery diarrhea (intestinal cryptos ...
,
cholera
Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
and many other diseases depending upon the location of the flood.
Damage to roads and transport infrastructure may make it difficult to mobilize 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.
Health effects
Fatalities connected directly to floods are usually caused by
drowning; the waters in a flood are very deep and have strong
currents
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (str ...
.
Deaths do not just occur from drowning, deaths are connected with
dehydration,
heat stroke
Heat stroke or heatstroke, also known as sun stroke, is a severe heat illness that results in a body temperature greater than , along with red skin, headache, dizziness, and confusion. Sweating is generally present in exertional heatstroke, b ...
,
heart attack and any other
illness that needs
medical supplies
A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
that cannot be delivered.
Injuries can lead to an excessive amount of morbidity when a flood occurs. Injuries are not isolated to just those who were directly in the flood,
rescue teams and even people delivering supplies can sustain an injury. Injuries can occur anytime during the flood process; before, during and after.
During floods accidents occur with falling
debris
Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer to ...
or any of the many fast moving objects in the water. After the flood rescue attempts are where large numbers injuries can occur.
Communicable diseases
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
are increased due to many
pathogens
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...
and
bacteria that are being transported by the
water.There are many
waterborne diseases such as
cholera
Cholera is an infection of the small intestine by some strains of the bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and ...
,
hepatitis A,
hepatitis E
Hepatitis E is inflammation of the liver caused by infection with the hepatitis E virus (HEV); it is a type of viral hepatitis. Hepatitis E has mainly a fecal-oral transmission route that is similar to hepatitis A, although the viruses are un ...
and
diarrheal diseases, to mention a few.
Gastrointestinal disease
Gastrointestinal diseases (abbrev. GI diseases or GI illnesses) refer to diseases involving the gastrointestinal tract, namely the oesophagus, stomach, small intestine, large intestine and rectum, and the accessory organs of digestion, the liver, ...
and diarrheal diseases are very common due to a lack of clean water during a flood. Most of clean water supplies are contaminated when flooding occurs. Hepatitis A and E are common because of the lack of
sanitation in the water and in living quarters depending on where the flood is and how prepared the
community is for a flood.
When floods hit, people lose nearly their crops, livestock, and food reserves and face starvation.
Urban flooding can cause chronically wet houses, leading to the growth of
indoor mold
Mold (American English) or mould (British English), also sometimes referred to as mildew, is a fungal growth that develops on wet materials. Mold is a natural part of the environment and plays an important part in nature by breaking down dead or ...
and resulting in adverse health effects, particularly respiratory symptoms.
Respiratory diseases
Respiratory diseases, or lung diseases, are pathological conditions affecting the organs and tissues that make gas exchange difficult in air-breathing animals. They include conditions of the respiratory tract including the trachea, bronchi, bron ...
are a common after the disaster has occurred. This depends on the amount of
water damage
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
and
mold that grows after an incident. Research suggests that there will be an increase of 30-50% in adverse respiratory health outcomes caused by dampness and mold exposure for those living in coastal and wetland areas. Fungal contamination in homes is associated with increased allergic rhinitis and asthma.
Vector borne diseases increase as well due to the increase in still water after the floods have settled. The diseases that are vector borne are
malaria,
dengue
Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. Symptoms typically begin three to fourteen days after infection. These may include a high fever, headache, vomiting, muscle and joint pains, and a characteristic ...
,
West Nile, and
yellow fever.
Floods have a huge impact on victims' psychosocial
integrity
Integrity is the practice of being honest and showing a consistent and uncompromising adherence to strong moral and ethical principles and values.
In ethics, integrity is regarded as the honesty and truthfulness or accuracy of one's actions. Inte ...
. People suffer from a wide variety of losses and
stress. One of the most treated illness in long-term health problems are
depression caused by the flood and all the
tragedy that flows with one.
Loss of life
Below is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.
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 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. In the United States,
insurance is available against flood damage to both homes and businesses.
Benefits
Floods (in particular more frequent or smaller floods) can also bring many benefits, such as recharging
ground water
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
, making soil more
fertile and increasing
nutrients in some soils. Flood waters provide much needed water resources in
arid
A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most a ...
and
semi-arid
A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-ar ...
regions where precipitation can be very unevenly distributed throughout the year and kills pests in the farming land. Freshwater floods particularly play an important role in maintaining
ecosystems in river corridors and are a key factor in maintaining floodplain
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') le ...
. Flooding can spread nutrients to lakes and rivers, which can lead to increased
biomass and improved
fisheries
Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, bot ...
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 Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest rive ...
, the
Indus River
The Indus ( ) is a transboundary river of Asia and a trans-Himalayan river of South and Central Asia. The river rises in mountain springs northeast of Mount Kailash in Western Tibet, flows northwest through the disputed region of Kashmir ...
, the
Ganges and the
Yellow River among others. The viability of
hydropower, a renewable source of energy, is also higher in flood prone regions.
Flood safety planning
In the United States, the
National Weather Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
gives out the advice "Turn Around, Don't Drown" for floods; that is, it recommends that people get out of the area of a flood, rather than trying to cross it. At the most basic level, the best defense against floods is to seek higher ground for high-value uses while balancing the foreseeable risks with the benefits of occupying flood hazard zones.
[Eychaner, J.H. (2015) ''Lessons from a 500-year record of flood elevations'']
Association of State Floodplain Managers, Technical Report 7
, Accessed 2015-06-27 Critical community-safety facilities, such as hospitals, emergency-operations centers, and police, fire, and
rescue services, should be built in areas least at risk of flooding. Structures, such as bridges, that must unavoidably be in flood hazard areas should be designed to withstand flooding. Areas most at risk for flooding could be put to valuable uses that could be abandoned temporarily as people retreat to safer areas when a flood is imminent.
Planning for flood safety involves many aspects of analysis and engineering, including:
* observation of previous and present flood heights and inundated areas,
* statistical,
hydrologic, and hydraulic model analyses,
* mapping inundated areas and flood heights for future flood scenarios,
* long-term
land use planning and regulation,
*
engineering design and construction of structures to control or withstand flooding,
* intermediate-term monitoring,
forecasting, and emergency-response planning, and
* short-term monitoring,
warning, and response operations.
Each topic presents distinct yet related questions with varying scope and scale in time, space, and the people involved. Attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia.
In the United States, the Association of State Floodplain Managers works to promote education, policies, and activities that mitigate current and future losses, costs, and human suffering caused by flooding and to protect the natural and beneficial functions of floodplains – all without causing adverse impacts. A portfolio of
best practice
A best practice is a method or technique that has been generally accepted as superior to other known alternatives because it often produces results that are superior to those achieved by other means or because it has become a standard way of doing ...
examples for
disaster mitigation in the United States is available from the Federal Emergency Management Agency.
Control
In many countries around the world, waterways prone to floods are often carefully managed. Defenses such as
detention basin
A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of time. ...
s,
levees,
bunds,
reservoirs, and
weirs are used to prevent waterways from overflowing their banks. When these defenses fail, emergency measures such as
sandbag
A sandbag or dirtbag is a bag or sack made of hessian (burlap), polypropylene or other sturdy materials that is filled with sand or soil and used for such purposes as flood control, military fortification in trenches and bunkers, shielding g ...
s or portable inflatable tubes are often used to try to stem flooding. Coastal flooding has been addressed in portions of Europe and the Americas with
coastal defenses, such as
sea wall
A seawall (or sea wall) is a form of coastal defense constructed where the sea, and associated coastal processes, impact directly upon the landforms of the coast. The purpose of a seawall is to protect areas of human habitation, conservation ...
s,
beach nourishment
Beach nourishment (also referred to as beach renourishment, beach replenishment, or sand replenishment) describes a process by which sediment, usually sand, lost through longshore drift or erosion is replaced from other sources. A wider beach can ...
, and
barrier island
Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a ...
s.
In the
riparian zone near rivers and streams,
erosion control measures can be taken to try to 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 to reduce the occurrence and severity of floods as well. In the United States, the
U.S. Army Corps of Engineers
, colors =
, anniversaries = 16 June (Organization Day)
, battles =
, battles_label = Wars
, website =
, commander1 = ...
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
Permeable paving surfaces are made of either a porous material that enables stormwater to flow through it or nonporous blocks spaced so that water can flow between the gaps. Permeable paving can also include a variety of surfacing techniques ...
, and
wetlands
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free ( anoxic) processes prevailing, especially in the soils. The ...
(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 valve
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have t ...
s.
In some areas, the presence of certain species (such as
beavers
Beavers are large, semiaquatic rodents in the genus ''Castor'' native to the temperate Northern Hemisphere. There are two extant species: the North American beaver (''Castor canadensis'') and the Eurasian beaver (''C. fiber''). Beavers are ...
) can be beneficial for flood control reasons. Beavers build and maintain
beaver dam
A beaver dam or beaver impoundment is a dam built by beavers to create a pond which protects against predators such as coyotes, wolves and bears, and holds their food during winter. These structures modify the natural environment in such a way th ...
s which will reduce the height of flood waves moving down the river (during periods of heavy rains), and will reduce or eliminate damage to human structures,
[Beavers cut flooding and pollution and boost wildlife populations](_blank)
/ref> at the cost of minor flooding near the dams (often on farmland). Besides this, they also boost wildlife populations and filter pollutants (manure, fertilisers, slurry). UK environment minister Rebecca Pow stated that in the future the beavers could be considered a "public good" and landowners would be paid to have them on their land.
Flood risk management
Analysis of flood information
A series of annual maximum flow rates in a stream reach can be analyzed statistically to estimate the 100-year flood
A 100-year flood is a flood event that has a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year.
The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%.Holmes, R.R., Jr. ...
and floods of other recurrence intervals there. Similar estimates from many sites in a hydrologically similar region can be related to measurable characteristics of each drainage basin to allow indirect estimation of flood recurrence intervals for stream reaches without sufficient data for direct analysis.
Physical process models of channel reaches are generally well understood and will calculate the depth and area of inundation for given channel conditions and a specified flow rate, such as for use in floodplain mapping and flood insurance
Flood insurance is the specific insurance coverage issued against property loss from flooding. To determine risk factors for specific properties, insurers will often refer to topographical maps that denote lowlands, floodplains and other areas tha ...
. Conversely, given the observed inundation area of a recent flood and the channel conditions, a model can calculate the flow rate. Applied to various potential channel configurations and flow rates, a reach model can contribute to selecting an optimum design for a modified channel. Various reach models are available as of 2015, either 1D models (flood levels measured in the channel) or 2D models (variable flood depths measured across the extent of a floodplain). HEC-RAS
HEC-RAS is a computer program that models the hydraulics of water flow through natural rivers and other channels. Prior to the 2016 update to Version 5.0, the program was one-dimensional, meaning that there is no direct modeling of the hydraulic ...
, the Hydraulic Engineering Center model, is among the most popular software, 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.
Physical process models of complete drainage basins are even more complex. Although many processes are well understood at a point or for a small area, others are poorly understood at all scales, and process interactions under normal or extreme climatic conditions may be unknown. Basin models typically combine land-surface process components (to estimate how much rainfall or snowmelt reaches a channel) with a series of reach models. For example, a basin model can calculate the runoff hydrograph
A hydrograph is a graph showing the rate of flow ( discharge) versus time past a specific point in a river, channel, or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
It can al ...
that might result from a 100-year storm, although the recurrence interval of a storm is rarely equal to that of the associated flood. Basin models are commonly used in flood forecasting and warning, as well as in analysis of the effects of land use change and climate change.
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. People can evacuate areas to be flooded.
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 aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...
s 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
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
(NWS) Northeast River Forecast Center (RFC) in Taunton, Massachusetts, a rule of thumb for flood forecasting in urban areas is that it takes at least of rainfall in around an hour's time in order to start significant ponding Ponding is the (typically) unwanted pooling of water, typically on a flat roof or roadway. Ponding water accelerates the deterioration of many materials, including seam adhesives in single-ply roof systems, steel equipment supports, and particularly ...
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.
In the United States, an integrated approach to real-time hydrologic computer modelling uses observed data from the U.S. Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...
(USGS), various cooperative observing networks, various automated weather sensors, the NOAA
The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
National Operational Hydrologic Remote Sensing Center (NOHRSC), various hydroelectric companies, etc. combined with quantitative precipitation forecast
The quantitative precipitation forecast (abbreviated QPF) is the expected amount of melted precipitation accumulated over a specified time period over a specified area. A QPF will be created when precipitation amounts reaching a minimum threshold ...
s (QPF) of expected rainfall and/or snow melt to generate daily or as-needed hydrologic forecasts. The NWS also cooperates with Environment Canada
Environment and Climate Change Canada (ECCC; french: Environnement et Changement climatique Canada),Environment and Climate Change Canada is the applied title under the Federal Identity Program; the legal title is Department of the Environment ( ...
on hydrologic forecasts that affect both the US and Canada, like in the area of the Saint Lawrence Seaway
The St. Lawrence Seaway (french: la Voie Maritime du Saint-Laurent) is a system of locks, canals, and channels in Canada and the United States that permits oceangoing vessels to travel from the Atlantic Ocean to the Great Lakes of North Ameri ...
.
The Global Flood Monitoring System, "GFMS", a computer tool which maps flood conditions worldwide, is availabl
online
Users anywhere in the world can use GFMS to determine when floods may occur in their area. GFMS uses precipitation data from NASA's Earth observing satellites and the Global Precipitation Measurement satellite, "GPM". Rainfall data from GPM is combined with a land surface model that incorporates vegetation cover, soil type, and terrain to determine how much water is soaking into the ground, and how much water is flowing into streamflow
Streamflow, or channel runoff, is the flow of water in streams and other channels, and is a major element of the water cycle. It is one component of the movement of water from the land to waterbodies, the other component being surface runoff. Wat ...
.
Users can view statistics for rainfall, streamflow, water depth, and flooding every 3 hours, at each 12-kilometer gridpoint on a global map. Forecasts for these parameters are 5 days into the future. Users can zoom in to see inundation maps (areas estimated to be covered with water) in 1-kilometer resolution.
Society and culture
Myths and religion
Legends of great, civilization-destroying floods are widespread in many cultures. Flood events in the form of divine retribution
Divine retribution is supernatural punishment of a person, a group of people, or everyone by a deity in response to some action. Many cultures have a story about how a deity exacted punishment upon previous inhabitants of their land, causing th ...
have also been described in religious texts. The Genesis flood narrative plays a prominent role in Judaism, Christianity and Islam.
Etymology
The word "flood" comes from the Old English ''flōd'', a word common to Germanic languages (compare German
German(s) may refer to:
* Germany (of or related to)
**Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
''Flut'', Dutch ''vloed'' from the same root as is seen in ''flow, float''; also compare with Latin ''fluctus'', ''flumen''), meaning "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave,".[Harper Douglas, “Etymology of flood,” Online Etymology Dictionary, accessed February 21, 2022, https://www.etymonline.com/word/flood.] The Old English word ''flōd'' comes from the Proto-Germanic ''floduz'' ( Old Frisian ''flod'', Old Norse ''floð'', Middle Dutch
Middle Dutch is a collective name for a number of closely related West Germanic dialects whose ancestor was Old Dutch. It was spoken and written between 1150 and 1500. Until the advent of Modern Dutch after 1500 or c. 1550, there was no overarch ...
''vloet'', Dutch ''vloed'', German ''Flut'', and Gothic
Gothic or Gothics may refer to:
People and languages
*Goths or Gothic people, the ethnonym of a group of East Germanic tribes
**Gothic language, an extinct East Germanic language spoken by the Goths
**Crimean Gothic, the Gothic language spoken b ...
''flodus'' derives from ''floduz).''
See also
* Cold drop
A cold drop (from the Spanish "gota fría") is a term used in Spain that has commonly come to refer to any high impact rainfall events occurring in the autumn along the Spanish Mediterranean coast. In Europe, cold drops belong to the characteris ...
* Cloudburst
A cloudburst is an extreme amount of precipitation in a short period of time, sometimes accompanied by hail and thunder, which is capable of creating flood conditions. Cloudbursts can quickly dump large amounts of water, e.g. 25 mm of prec ...
* Diversion dam
* Emergency management: Disaster preparedness and disaster response.
* Flash flood guidance system
* Flood alert
Flood alerts are issued by weather agencies to alert residents that flood conditions are a possibility.
Types of flood alerts in the United States
In the United States, a flash flood watch is issued by the National Weather Service (NWS) when w ...
* Flood pulse concept
* Flood risk assessment
A flood risk assessment (FRA) is an assessment of the risk of flooding from all flooding mechanisms, the identification of flood mitigation measures and should provide advice on actions to be taken before and during a flood.
The sources of water ...
(FRA)
* Flood stage Flood stage is the water level or stage at which the surface of a body of water has risen to a sufficient level to cause sufficient inundation of areas that are not normally covered by water, causing an inconvenience or a threat to life and property ...
* International Rescue Corps
The International Rescue Corps (IRC) is a volunteer organisation involved in disaster rescue, based in Grangemouth, Scotland.
Description
International Rescue Corps is an independent (i.e. non-governmental funded) United Nations registered d ...
* Inundation
A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrolog ...
* Lifesaving
Lifesaving is the act involving rescue, resuscitation and first aid. It often refers to water safety and aquatic rescue; however, it could include ice rescue, flood and river rescue, swimming pool rescue and other emergency medical services. ...
* List of floods
* Mudflow
** Swift water rescue
Swift water rescue (also called "white water rescue") is a subset of technical rescue dealing in white water river conditions. Due to the added pressure of moving water, swift water rescue involves the use of specially trained personnel, ropes an ...
* Search and rescue
* SMS (hydrology software)
* Storm drain
A storm drain, storm sewer (United Kingdom, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain ( Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surfac ...
* Washout
* Floods by land:
** Chicago flood, man-made flood under downtown Chicago
''Downtown'' is a term primarily used in North America by English speakers to refer to a city's sometimes commercial, cultural and often the historical, political and geographic heart. It is often synonymous with its central business district ...
** Floods in Australia
This is a list of notable recorded floods that have occurred in the country of Australia.
References
Further reading
* Devin, L.B. and D.L. Purcell (1983) ''Flooding in Australia'' Canberra : Australian Government Publishing Service : (Wat ...
** Floods in the Netherlands
This is a chronological list of sea-floods that have occurred in the Netherlands. In addition to these there have been hundreds of river floods during the centuries.
See also
*Flood control in the Netherlands
References
{{reflist
Extern ...
** Flood control in the Netherlands
Flood control is an important issue for the Netherlands, as due to its low elevation, approximately two thirds of its area is vulnerable to flooding, while the country is densely populated. Natural sand dunes and constructed dikes, dams, and f ...
** Lists of floods in the United States
Lists of floods in the United States provide overviews of major floods in the United States. They are organized by time period: before 1901, from 1901 to 2000, and from 2001 to the present.
Lists
* Floods in the United States before 1901
Floo ...
** Storm tides of the North Sea
References
Sources
*
*
External links
Associated Programme on Flood Management
from the World Meteorological Organization
The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics.
The WMO originated from the Internat ...
Flood and natural hazard research
from Bushfire and Natural Hazards CRC
The Bushfire and Natural Hazards Cooperative Research Centre, commonly abbreviated to Bushfire and Natural Hazards CRC, from 2013 to 2021 drew together all of Australia and New Zealand's fire and emergency service authorities, land management a ...
International Flood Initiative
from UNESCO
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