Natural types of floods include river flooding, groundwater floodingcoastal flooding andurban flooding sometimes known as flash flooding. Tidal flooding may include elements of both river and coastal flooding processes in estuary areas. There is also the intentional flooding of land that would otherwise remain dry. This may take place for agricultural, military, orriver-management purposes. For example, agricultural flooding may occur in preparingpaddy fields for the growing of semi-aquatic rice in many countries.
Flooding may occur as an overflow of water from water bodies, such as ariver,lake, sea or ocean. In these cases, the water overtops or breakslevees, resulting in some of that water escaping its usual boundaries.[6] Flooding may also occur due to an accumulation of rainwater on saturated ground. This is called anareal flood. The size of a lake or other body of water naturally varies with seasonal changes inprecipitation and snow melt. Those changes in size are however not considered a flood unless they floodproperty ordrowndomestic animals.
Floods can also occur in rivers when the flow rate exceeds the capacity of theriver channel, particularly at bends ormeanders in thewaterway. Floods often cause damage to homes and businesses if these buildings are in the natural flood plains of rivers. People could avoid riverine flood damage by moving away from rivers. However, people in many countries have traditionally lived and worked by rivers because the land is usually flat andfertile. Also, the rivers provide easy travel and access to commerce and industry.
Flooding can damage property and also lead to secondary impacts. These include in the short term an increased spread ofwaterborne diseases andvector-bourne diseases, for example those diseases transmitted by mosquitos. Flooding can also lead to long-term displacement of residents.[7] Floods are an area of study ofhydrology andhydraulic engineering.
A large amount of the world's population lives in close proximity to majorcoastlines,[8] while many major cities and agricultural areas are located nearfloodplains.[9] There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions.[10]
Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can eitherinfiltrate orrun off. The excess accumulates in place, sometimes to hazardous depths. Surfacesoil can become saturated, which effectively stops infiltration, where thewater table is shallow, such as afloodplain, or from intense rain from one or aseries 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 ofoverland flow depends on the surface slope.Endorheic basins may experience areal flooding during periods when precipitation exceeds evaporation.[11]
Slow-rising floods most commonly occur in large rivers with largecatchment areas. The increase in flow may be the result of sustained rainfall, rapid snow melt,monsoons, ortropical cyclones. However, large rivers may have rapid flooding events in areas with dry climates, since they may have large basins but small river channels, and rainfall can be very intense in smaller areas of those basins.
In extremely flat areas, such as theRed River Valley of the North inMinnesota,North Dakota, andManitoba, a type of hybrid river/areal flooding can occur, known locally as "overland flooding". This is different from "overland flow" defined as "surface runoff". The Red River Valley is a former glacial lakebed, created byLake Agassiz, and over a length of 550 mi (890 km), the river course drops only 236 ft (72 m), for an average slope of about 5 inches per mile (or 8.2 cm per kilometer).[12] In this very large area, spring snowmelt happens at different rates in different places, and if winter snowfall was heavy, a fast snowmelt can push water out of the banks of a tributary river so that it moves overland, to a point further downstream in the river or completely to another streambed. Overland flooding can be devastating because it is unpredictable, it can occur very suddenly with surprising speed, and in such flat land it can run for miles. It is these qualities that set it apart from simple "overland flow".
Rapid flooding events, includingflash floods, 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 localizedconvective precipitation (intensethunderstorms) or sudden release from an upstream impoundment created behind adam, landslide, orglacier. In one instance, a flash flood killed eight people enjoying the water on a Sunday afternoon at a popular waterfall in a narrow canyon.[citation needed] Without any observed rainfall, the flow rate increased from about 50 to 1,500 cubic feet per second (1.4 to 42 m3/s) in just one minute.[13] 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 asarroyos 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 thehydrograph becomes ever quicker as the flood moves downstream, until the flow rate is so great that the depletion by wetting soil becomes insignificant.
Coastal areas may be flooded bystorm surges combining with high tides and large wave events at sea, resulting in waves over-topping flood defenses or in severe cases bytsunami or tropical cyclones. Astorm surge, from either atropical cyclone or anextratropical cyclone, falls within this category. A storm surge is "an additional rise of water generated by a storm, over and above the predicted astronomical tides".[14] Due to theeffects of climate change (e.g.sea level rise and an increase inextreme weather events) and an increase in the population living in coastal areas, the damage caused by coastal flood events has intensified and more people are being affected.[15]
Flooding inestuaries is commonly caused by a combination of storm surges caused bywinds and lowbarometric pressure and large waves meeting high upstream river flows.
Flooding inPorto Alegre of theLagoa dos Patos in Brazil during May 2024Urban flooding is the inundation of land or property in cities or otherbuilt environment, caused by rainfall or coastal storm surges overwhelming the capacity of drainage systems, such asstorm sewers. Urban flooding can occur regardless of whether or not affected communities are located within designated floodplains or near any body of water.[16] It is triggered for example by an overflow of rivers and lakes,flash flooding orsnowmelt. During the flood,stormwater or water released from damagedwater mains may accumulate on property and in public rights-of-way. It can seep through building walls and floors, or backup into buildings through sewer pipes, cellars, toilets and sinks.There are several types of urban flooding, each with a different cause. City planners distinguishpluvial flooding (flooding caused by heavy rain),fluvial flooding (caused by a nearby river overflowing its banks), orcoastal flooding (often caused bystorm surges). Urban flooding is ahazard to both the population and infrastructure. Some well knowndisaster events include the inundations ofNîmes (France) in 1998 andVaison-la-Romaine (France) in 1992, theflooding of New Orleans (United States) in 2005, and the flooding inRockhampton,Bundaberg,Brisbane during the2010–2011 Queensland floods in Australia, the2022 eastern Australia floods, and more recently the2024 Rio Grande do Sul floods in Brazil.
Intentional floods
The intentional flooding of land that would otherwise remain dry may take place for agricultural, military or river-management purposes. This is a form ofhydraulic engineering. Agricultural flooding may occur in preparingpaddy fields 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 donead hoc,[17] or permanently, as in the so-calledoverlaten (literally "let-overs"), an intentionally lowered segment in Dutch riparian levees, like theBeerse Overlaat in the left levee of theMeuse between the villages ofGassel andLinden, North Brabant.
Military inundation creates an obstacle in the field that is intended to impede the movement of the enemy.[18] This may be done both foroffensive anddefensive purposes. Furthermore, in so far as the methods used are a form of hydraulic engineering, it may be useful to differentiate between controlled inundations and uncontrolled ones. Examples for controlled inundations include those in the Netherlands under theDutch Republic and itssuccessor states in that area[19][20] and exemplified in the twoHollandic Water Lines, theStelling van Amsterdam, theFrisian Water Line, theIJssel Line, thePeel-Raam Line, and theGrebbe line in that country.
To count ascontrolled, a military inundation has to take the interests of the civilian population into account, by allowing them a timelyevacuation, by making the inundationreversible, and by making an attempt to minimize the adverseecological impact of the inundation. That impact may also be adverse in ahydrogeological sense if the inundation lasts a long time.[21]
Floods are caused by many factors or a combination of any of these generally prolonged heavy rainfall (locally concentrated or throughout a catchment area), highly acceleratedsnowmelt, severe winds over water, unusual high tides,tsunamis, or failure of dams,levees,retention ponds, or other structures that retained the water. Flooding can be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce the supply of vegetation that can absorb rainfall.
During times of rain, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, and the rest travels over the land assurface runoff. Floods occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water.
This has been exacerbated by human activities such as draining wetlands that naturally store large amounts of water and building paved surfaces that do not absorb any water.[24] Water then runs off the land in quantities that cannot be carried withinstream channels or retained in natural ponds, lakes, and human-madereservoirs. About 30 percent of all precipitation becomes runoff[25] and that amount might be increased by water from melting snow.
Upslope factors
Flash flood in Ein Avdat, Negev, Israel
River flooding is often caused by heavy rain, sometimes increased by melting snow. A flood that rises rapidly, with little or no warning, is called aflash flood. Flash floods usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation.
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.[26]
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 tointensity by dividing each measured depth by the period of time between observations. This intensity will be less than the actual peak intensity if theduration 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.[27]
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 30 square miles or 80 square kilometres. 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.[28]
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.[29] 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 theocean or some coastal flooding bars which form naturallakes. In flooding low lands, elevation changes such as tidal fluctuations are significant determinants of coastal and estuarine flooding. Less predictable events like tsunamis andstorm 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[28] 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.
Periodic floods occur on many rivers, forming a surrounding region known as theflood plain. Even when rainfall is relatively light, theshorelines of lakes and bays can be flooded by severe winds—such as duringhurricanes—that blow water into the shore areas.
Climate change
High tide flooding is increasing due to sea level rise, land subsidence, and the loss of natural barriers.[30]
Long-term sea level rise occurs in addition to intermittent tidal flooding.NOAA predicts different levels of sea level rise for coastlines within a single country.[31]
Due to an increase in heavy rainfall events,floods are likely to become more severe when they do occur.[32]: 1155 The interactions between rainfall and flooding are complex. There are some regions in which flooding is expected to become rarer. This depends on several factors. These include changes in rain and snowmelt, but alsosoil moisture.[32]: 1156 Climate change leaves soils drier in some areas, so they may absorb rainfall more quickly. This leads to less flooding. Dry soils can also become harder. In this case heavy rainfall runs off into rivers and lakes. This increases risks of flooding.[32]: 1155
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 likebeaver dams.[33] Coincident events may cause extensive flooding to be more frequent than anticipated fromsimplistic statistical prediction models considering only precipitation runoff flowing within unobstructed drainage channels.[34] Debris modification of channel geometry is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats andrailway equipment. Recent field measurements during the2010–11 Queensland floods 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.[35] These considerations ignore further the risks associated with large debris entrained by the flow motion.[36]
Negative impacts
Flooded walnut orchards inButte County after several atmospheric rivers hit California in early 2023
Floods can 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, and cars. Economical, social and natural environmental damages are common factors that are impacted by flooding events and the impacts that flooding has on these areas can be catastrophic.[37]
Impacts on infrastructure and societies
There have been numerous flood incidents around the world which have caused devastating damage to infrastructure, the natural environment and human life.[37]
Floods can have devastating impacts to human societies. Flooding events worldwide are increasing in frequency and severity, leading to increasing costs to societies.[37]
The primary effects of flooding includeloss of life and damage to buildings and other structures, including bridges,sewerage systems, roadways, and canals. The economic impacts caused by flooding can be severe.[9]
Every year flooding causes countries billions of dollars worth of damage that threatens the livelihood of individuals.[38] As a result, there is also significant socio-economic threats to vulnerable populations around the world from flooding.[38] 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.[39]
Mud was deposited in this house by flooding in the2018 Kerala floods in India. Flooding not only creates water damage, but can also deposit large amounts of sediment.
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.[40]
Flooding in areas where people live also has significant economic implications for affected neighborhoods. In theUnited 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.[41] According to the U.S.Federal Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster.[42] In the United States,insurance is available against flood damage to both homes and businesses.[43]
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.
Injuries can lead to an excessive amount of morbidity when a flood occurs. Injuries do not just affect those who were directly in the flood:rescue teams and even people delivering supplies can sustain an injury. Injuries can occur before, during and after the flood.[44] During floods accidents occur with fallingdebris or any of the many fast moving objects in the water. After the flood rescue attempts are when large numbers injuries can occur.[44]
When floods hit, people can lose nearly all their crops, livestock, and food reserves and face starvation.[45]
Floods also frequently damagepower transmission and sometimespower generation, which then hasknock-on effects caused by the loss of power. This includes loss of drinkingwater 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 withhuman sewage in the flood waters raises the risk of waterborne diseases, which can includetyphoid,giardia,cryptosporidium,cholera and many other diseases depending upon the location of the flood.[Merge this list with previous list]
Damage to roads and transport infrastructure may make it difficult to mobilize aid to those affected or to provide emergency health treatment.
Flooding can cause chronically wet houses, leading to the growth ofindoor mold and resulting in adverse health effects, particularly respiratory symptoms.[46]Respiratory diseases are common after the disaster has occurred. This depends on the amount ofwater damage andmold 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.[47]Vector-borne diseases also increase, due to the residual still water after the floods have settled. The diseases that are vector borne aremalaria,dengue,West Nile, andyellow fever.[44] Floods may have a huge impact on victims' psychosocialintegrity. People suffer from a wide variety of losses andstress. One of the most treated illnesses in long-term health problems aredepression caused by the flood and all thetragedy that flows with one.[44]
Floods (in particular more frequent or smaller floods) can also bring many benefits, such as rechargingground water, making soil morefertile and increasingnutrients in some soils. For this reason, periodic flooding was essential to the well-being of ancient peoples along theTigris-Euphrates Rivers, theNile River, theIndus River, theGanges and theYellow River among others.
Flood waters provide much needed water resources inarid andsemi-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 maintainingecosystems in river corridors and are a key factor in maintaining floodplainbiodiversity.[49] Flooding can spread nutrients to lakes and rivers, which can lead to increasedbiomass and improvedfisheries for a few years.
For some fish species, an inundated floodplain may form a highly suitable location forspawning, with few predators and enhanced levels of nutrients or food.[50] Fish such as theweather fish use floods to reach new habitats. Bird populations may also profit from the boost in food production caused by flooding.[51]
The viability ofhydropower, a renewable source of energy, is also higher in flood-prone regions.
Flood management describes methods used to reduce or prevent the detrimental effects of flood waters. Flooding can be caused by a mix of both natural processes, such asextreme weather upstream, and human changes to waterbodies and runoff. Flood management methods can be either of thestructural type (i.e. flood control) and of thenon-structural type. Structural methods hold back floodwaters physically, while non-structural methods do not. Buildinghard infrastructure to prevent flooding, such asflood walls, is effective at managing flooding. However, it is best practice withinlandscape engineering to rely more onsoft infrastructure andnatural systems, such asmarshes andflood plains, for handling the increase in water.
Flood management can includeflood risk management, which focuses on measures to reduce risk, vulnerability and exposure to flood disasters and providing risk analysis through, for example,flood risk assessment.[52]Flood mitigation is a related but separate concept describing a broader set of strategies taken to reduce flood risk and potential impact while improving resilience against flood events.
Asclimate change has led to increased flood risk an intensity, flood management is an important part ofclimate change adaptation andclimate resilience.[53][54] For example, to prevent or managecoastal flooding,coastal management practices have to handle natural processes liketides but alsosea level rise due to climate change. The prevention and mitigation of flooding can be studied on three levels: on individual properties, small communities, and whole towns or cities.
In theriparian 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, theU.S. Army Corps of Engineers maintains a network of such flood control dams.
In areas prone to urban flooding, one solution is the repair and expansion of human-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, andwetlands (collectively known asgreen infrastructure orsustainable 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 byfloodwalls andlevees, 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 installingrain barrels,sump pumps, andcheck valves.
Flood safety planning
Aftermath of flooding in Colorado, 2013Flood rescue inNangarhar, Afghanistan in 2010Flash flooding caused by heavy rain falling in a short amount of time
In the United States, theNational Weather Service 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.[56]: 22–23 Critical community-safety facilities, such as hospitals, emergency-operations centers, and police, fire, andrescue 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,
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.[57][page needed]
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.[58] A portfolio ofbest practice examples fordisaster mitigation in the United States is available from the Federal Emergency Management Agency.[59]
Flood clean-up safety
Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers includeelectrical hazards,carbon monoxide exposure,musculoskeletal hazards,heat orcold stress,motor vehicle-related dangers,fire,drowning, and exposure tohazardous materials. Because flooded disaster sites are unstable, clean-up workers might encounter sharp jagged debris, biological hazards in the flood water, exposed electrical lines, blood or other body fluids, and animal and human remains. In planning for and reacting to flood disasters, managers provide workers withhard hats,goggles, heavy work gloves,life jackets, and watertight boots with steel toes and insoles.[60]
A series of annual maximum flow rates in a stream reach can be analyzedstatistically to estimate the100-year flood and floods of otherrecurrence intervals there. Similar estimates from many sites in a hydrologically similar region can be related to measurable characteristics of each drainage basin to allowindirect 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 andflood 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, either1D models (flood levels measured in thechannel) or2D models (variable flood depths measured across the extent of a floodplain).HEC-RAS,[61] the Hydraulic Engineering Center model, is among the most popularsoftware, if only because it is available free of charge. Other models such as TUFLOW[62] 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 runoffhydrograph 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 andclimate change.
The Global Flood Monitoring System, "GFMS", a computer tool which maps flood conditions worldwide, is available online.[67] Users anywhere in the world can use GFMS to determine when floods may occur in their area. GFMS uses precipitation data fromNASA's Earth observing satellites and theGlobal 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 intostreamflow.
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.[68]
Anticipating floods before they occur allows for precautions to be taken and people to be warned[69] 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 forwaterways, it is best to have a long time-series of historical data that relatesstream flows to measured past rainfall events.[70] Coupling this historical information withreal-time knowledge about volumetric capacity in catchment areas, such as spare capacity in reservoirs, ground-water levels, and the degree ofsaturation of areaaquifers is also needed in order to make the most accurate flood forecasts.
Radar estimates of rainfall and generalweather 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,[66] 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% (i.e. 1 − 0.99100, or roughly 1 − 1/e) of occurring in any 100-year period of time.
According to the U.S.National Weather Service (NWS) Northeast River Forecast Center (RFC) inTaunton, Massachusetts, a rule of thumb for flood forecasting in urban areas is that it takes at least 1 inch (25 mm) of rainfall in around an hour's time in order to start significantponding of water onimpermeable 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 largerwater basins.[71]
Flood risks can be defined as the risk that floods pose to individuals, property and the natural landscape based on specific hazards and vulnerability. The extent of flood risks can impact the types of mitigation strategies required and implemented.[72]
A large amount of the world's population lives in close proximity to majorcoastlines,[8] while many major cities and agricultural areas are located nearfloodplains.[9] There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions.[10]
Aflood myth or a deluge myth is amyth in which a great flood, usually sent by adeity or deities, destroyscivilization, often in an act ofdivine retribution. Parallels are often drawn between the flood waters of thesemyths and the primevalwaters which appear in certaincreation myths, as the flood waters are described as a measure for thecleansing of humanity, in preparation forrebirth. Most flood myths also contain aculture hero, who "represents the human craving for life".[73]
The word "flood" comes from theOld Englishflōd, a word common toGermanic languages (compareGermanFlut,Dutchvloed from the same root as is seen inflow, float; also compare withLatinfluctus,flumen), meaning "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave".[74][75] The Old English wordflōd comes from theProto-Germanicfloduz (Old Frisianflod,Old Norsefloð,Middle Dutchvloet, Dutchvloed, GermanFlut, andGothicflodus derives fromfloduz).[74]
^Babbitt, Harold E. & Doland, James J.,Water Supply Engineering, McGraw-Hill Book Company, 1949
^Simon, Andrew L.,Basic Hydraulics, John Wiley & Sons, 1981,ISBN0-471-07965-0
^abSimon, Andrew L.,Practical Hydraulics, John Wiley & Sons, 1981,ISBN0-471-05381-3
^Urquhart, Leonard Church,Civil Engineering Handbook, McGraw-Hill Book Company, 1959
^"What is high tide flooding?".National Oceanic and Atmospheric Administration's National Ocean Service. US Department of Commerce.Archived from the original on 16 October 2020. Retrieved12 October 2020.
.jpg)
.jpg)
