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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 ...
, civil engineering and
public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
. Human changes to the environment often increase the intensity and frequency of flooding, for example land use changes such as
deforestation Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated ...
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 Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cr ...
. 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 A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
, 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 In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
and snow melt, these changes in size are unlikely to be considered significant unless they flood
property Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to consume, alter, share, r ...
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 A waterway is any navigable body of water. Broad distinctions are useful to avoid ambiguity, and disambiguation will be of varying importance depending on the nuance of the equivalent word in other languages. A first distinction is necessary ...
. 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 The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...
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 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 A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
and
stream A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long large streams ...
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 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, debris, or beaver 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. 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, landslide, or
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
. 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 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 environme ...
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 floods and lahars. Tsunamis can cause catastrophic coastal flooding, 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 The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
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 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 Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a p ...
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. Culverted fills may be converted to impoundments if the culverts 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. 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 , colors = , anniversaries = 16 June (Organization Day) , battles = , battles_label = Wars , website = , commander1 = ...
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 Gassel is a village in the Dutch province of North Brabant. It is located in the former municipality of Grave. Since 2022 it has been part of the new municipality of Land van Cuijk. History Gassel was mentioned first in 1485 as Gassel, and mean ...
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 and defensive 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 The United Provinces of the Netherlands, also known as the (Seven) United Provinces, officially as the Republic of the Seven United Netherlands ( Dutch: ''Republiek der Zeven Verenigde Nederlanden''), and commonly referred to in historiograph ...
and its successor states 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, 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 World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
). 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 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 systems, roadways, and canals. Floods also frequently damage power transmission and sometimes power generation, which then has
knock-on effect In the social sciences, unintended consequences (sometimes unanticipated consequences or unforeseen consequences) are outcomes of a purposeful action that are not intended or foreseen. The term was popularised in the twentieth century by Ameri ...
s 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 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. Deaths do not just occur from drowning, deaths are connected with dehydration, heat stroke, 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 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 and bacteria that are being transported by the
water 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 ...
.There are many waterborne diseases such as cholera,
hepatitis A Hepatitis A is an infectious disease of the liver caused by ''Hepatovirus A'' (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop the ...
, hepatitis E and diarrheal diseases, to mention a few. Gastrointestinal disease 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 Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation syste ...
in the water and in living quarters depending on where the flood is and how prepared the
community A community is a social unit (a group of living things) with commonality such as place, norms, religion, values, customs, or identity. Communities may share a sense of place situated in a given geographical area (e.g. a country, village, t ...
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 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 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, West Nile, and yellow fever. Floods have a huge impact on victims' psychosocial integrity. 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 Tragedy (from the grc-gre, τραγῳδία, ''tragōidia'', ''tragōidia'') is a genre of drama based on human suffering and, mainly, the terrible or sorrowful events that befall a main character. Traditionally, the intention of 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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
, 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 Insurance is a means of protection from financial loss in which, in exchange for a fee, a party agrees to compensate another party in the event of a certain loss, damage, or injury. It is a form of risk management, primarily used to hedge ...
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, 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 and kills pests in the farming land. Freshwater floods particularly play an important role in maintaining
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s 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 The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...
and the Yellow River among others. The viability of
hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the gravitational potential or kinetic energy of ...
, 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 Emergency management or disaster management is the managerial function charged with creating the framework within which communities reduce vulnerability to hazards and cope with disasters. Emergency management, despite its name, does not actuall ...
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 basins, levees, bunds, reservoirs, and weirs are used to prevent waterways from overflowing their banks. When these defenses fail, emergency measures such as sandbags 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, and barrier islands. 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, 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
floodwall A flood wall (or floodwall) is a primarily vertical artificial barrier designed to temporarily contain the waters of a river or other waterway which may rise to unusual levels during seasonal or extreme weather events. Flood walls are mainly ...
s 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 A rainwater tank (sometimes called a rain barrel in North America in reference to smaller tanks, or a water butt in the UK) is a water tank used to collect and store rain water runoff, typically from rooftops via pipes. Rainwater tanks are device ...
, sump pumps, and check valves. In some areas, the presence of certain species (such as beavers) can be beneficial for flood control reasons. Beavers build and maintain beaver dams 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
/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. 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, the Hydraulic Engineering Center model, is among the most popular
software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consist ...
, 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 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
waterway A waterway is any navigable body of water. Broad distinctions are useful to avoid ambiguity, and disambiguation will be of varying importance depending on the nuance of the equivalent word in other languages. A first distinction is necessary ...
s, it is best to have a long time-series of historical data that relates
stream flow 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. Wa ...
s 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 characte ...
s is also needed in order to make the most accurate flood forecasts.
Radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, Marine radar, ships, spacecraft, guided missiles, motor v ...
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 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 basin A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
s. In the United States, an integrated approach to real-time hydrologic computer modelling uses 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 US and Canada, like in the area of the Saint Lawrence Seaway. 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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
'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. 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 have also been described in religious texts. The Genesis flood narrative plays a prominent role in
Judaism Judaism ( he, ''Yahăḏūṯ'') is an Abrahamic, monotheistic, and ethnic religion comprising the collective religious, cultural, and legal tradition and civilization of the Jewish people. It has its roots as an organized religion in th ...
,
Christianity Christianity is an Abrahamic monotheistic religion based on the life and teachings of Jesus of Nazareth. It is the world's largest and most widespread religion with roughly 2.38 billion followers representing one-third of the global popula ...
and
Islam Islam (; ar, ۘالِإسلَام, , ) is an Abrahamic monotheistic religion centred primarily around the Quran, a religious text considered by Muslims to be the direct word of God (or '' Allah'') as it was revealed to Muhammad, the ...
.


Etymology

The word "flood" comes from the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlers in the mid-5th ...
''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 Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''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 Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic br ...
''floduz'' (
Old Frisian Old Frisian was a West Germanic language spoken between the 8th and 16th centuries along the North Sea coast, roughly between the mouths of the Rhine and Weser rivers. The Frisian settlers on the coast of South Jutland (today's Northern Fries ...
''flod'',
Old Norse Old Norse, Old Nordic, or Old Scandinavian, is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and their overseas settlement ...
''floð'', Middle Dutch ''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 *
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 The flash flood guidance system (FFGS) was designed and developed by the Hydrologic Research Center, a non-profit public-benefit corporation located in San Diego, CA, US, for use by meteorological and hydrologic forecasters throughout the world. T ...
*
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 (FRA) * Flood stage *
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 * Lifesaving *
List of floods The following is a list of major floods. List of notable floods 20th century BC * Jishi Gorge outburst flood about 1920 BC 14th century * Saint Marcellus's flood a storm tide is also called the "Second St. Marcellus flood". * St. Mary ...
* Mudflow ** Swift water rescue * Search and rescue * SMS (hydrology software) * Storm drain * 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 ** Floods in the Netherlands **
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
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 The United Nations Educational, Scientific and Cultural Organization is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) aimed at promoting world peace and security through international coope ...
{{Authority control Meteorological phenomena Bodies of water Hydrology Weather hazards Natural disasters