View source for Flood

{{Short description|Water overflow submerging usually-dry land}}
{{Other uses}}
{{redirect|Inundation}}
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[[File:Urban flood cropped.jpg|thumb|[[Urban flooding]] in a street in [[Morpeth, Northumberland|Morpeth]], England  ]]
A '''flood''' is an overflow of water ([[list of non-water floods|or rarely other fluids]]) that submerges land that is usually dry.<ref>MSN Encarta Dictionary, [http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861612277 Flood] {{Webarchive|url=https://web.archive.org/web/20110204203836/http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?refid=1861612277 |date=2011-02-04 }}, Retrieved on 2006-12-28, on 2009-10-31</ref> In the sense of "flowing water", the word may also be applied to the inflow of the [[tide]]. Floods are of significant concern in [[agriculture]], [[civil engineering]] and [[public health]]. [[Environmental issues|Human changes to the environment]] often increase the intensity and frequency of flooding. Examples for human changes are [[land use change]]s such as [[deforestation]] and [[Wetland conservation|removal of wetlands]], changes in waterway course or [[flood control]]s such as with [[levee]]s. Global environmental issues also influence causes of floods, namely [[climate change]] which causes an [[Effects of climate change on the water cycle|intensification of the water cycle]] and [[sea level rise]].<ref>Seneviratne, S.I., X. Zhang, M. Adnan, W. Badi, C. Dereczynski, A. Di Luca, S. Ghosh, I. Iskandar, J. Kossin, S. Lewis, F.  Otto, I.  Pinto, M. Satoh, S.M. Vicente-Serrano, M. Wehner, and B. Zhou, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter11.pdf Chapter 11: Weather and Climate Extreme Events in a Changing Climate]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I  to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R.  Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1513–1766, doi:10.1017/9781009157896.013.</ref>{{rp|1517}} For example, climate change makes [[Extreme weather|extreme weather events]] more frequent and stronger.<ref name=":1">{{cite report |url=https://www.nap.edu/read/21852/chapter/7 |title=Attribution of Extreme Weather Events in the Context of Climate Change |publisher=The National Academies Press |doi=10.17226/21852 |pages=127–136 |isbn=978-0-309-38094-2 |access-date=2020-02-22 |archive-url=https://web.archive.org/web/20220215232008/https://www.nap.edu/read/21852/chapter/7 |archive-date=2022-02-15 |url-status=live |year=2016 |place=Washington, DC}}</ref> This leads to more intense floods and increased flood risk.<ref>{{Cite journal|last1=Hirabayashi|first1=Yukiko|last2=Mahendran|first2=Roobavannan|last3=Koirala|first3=Sujan|last4=Konoshima|first4=Lisako|last5=Yamazaki|first5=Dai|last6=Watanabe|first6=Satoshi|last7=Kim|first7=Hyungjun|last8=Kanae|first8=Shinjiro|date=September 2013|title=Global flood risk under climate change|url=https://www.nature.com/articles/nclimate1911/|journal=Nature Climate Change|language=en|volume=3|issue=9|pages=816–821|doi=10.1038/nclimate1911|bibcode=2013NatCC...3..816H|issn=1758-6798}}</ref><ref>{{Cite web|title=How climate change is making record-breaking floods the new normal|url=https://www.unep.org/news-and-stories/story/how-climate-change-making-record-breaking-floods-new-normal|website=United Nations Environmental Program|date=3 March 2020}}</ref>
Natural types of floods include river flooding, groundwater flooding [[coastal flooding]] and [[urban 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, or [[River management|river-management]] purposes. For example, agricultural flooding may occur in preparing [[paddy field]]s for the growing of semi-aquatic rice in many countries.
Flooding may occur as an overflow of water from water bodies, such as a [[river]], [[lake]], sea or ocean. In these cases, the water overtops or breaks [[levee]]s, resulting in some of that water escaping its usual boundaries.<ref>Glossary of Meteorology (June 2000) [http://amsglossary.allenpress.com/glossary/search?id=flood1 Flood] {{webarchive|url=https://web.archive.org/web/20070824054504/http://amsglossary.allenpress.com/glossary/search?id=flood1 |date=2007-08-24 }}, Retrieved on 2009-01-09</ref> Flooding may also occur due to an accumulation of rainwater on saturated ground. This is called an ''areal flood''. The size of a lake or other body of water naturally varies with seasonal changes in [[precipitation]] and snow melt. Those changes in size are however not considered a flood unless they flood [[property]] or [[drowning|drown]] [[domestication of animals|domestic animals]].
Floods can also occur in rivers when the flow rate exceeds the capacity of the [[Channel (geography)|river channel]], particularly at bends or [[meander]]s in the [[waterway]]. 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 and [[Fertility (soil)|fertile]]. 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 of [[waterborne diseases]] and [[Vector (epidemiology)|vector-bourne disesases]], for example those diseases transmitted by mosquitos. Flooding can also lead to long-term displacement of residents.<ref>{{Cite web|title=WHO {{!}} Flooding and communicable diseases fact sheet|url=https://www.who.int/hac/techguidance/ems/flood_cds/en/|archive-url=https://web.archive.org/web/20041231191549/http://www.who.int/hac/techguidance/ems/flood_cds/en/|archive-date=December 31, 2004|access-date=2021-03-28|website=WHO}}</ref> Floods are an area of study of [[hydrology]] and [[hydraulic engineering]].
A large amount of the world's population lives in close proximity to major [[Coastal flooding|coastlines]],<ref name=":2">{{Cite journal |last1=Neumann |first1=Barbara |last2=Vafeidis |first2=Athanasios T. |last3=Zimmermann |first3=Juliane |last4=Nicholls |first4=Robert J. |date=2015-03-11 |title=Future Coastal Population Growth and Exposure to Sea-Level Rise and Coastal Flooding - A Global Assessment |journal=PLOS ONE |language=en |volume=10 |issue=3 |pages=e0118571 |bibcode=2015PLoSO..1018571N |doi=10.1371/journal.pone.0118571 |issn=1932-6203 |pmc=4367969 |pmid=25760037 |doi-access=free}}</ref> while many major cities and agricultural areas are located near [[floodplain]]s.<ref name=":12">{{Cite web |date=2020-11-24 |title=Flood Risk Management in Canada {{!}} Research report |url=https://www.genevaassociation.org/research-topics/climate-change-and-emerging-environmental-topics/flood-risk-management-canada |access-date=2021-10-29 |website=Geneva Association |language=en}}</ref> There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions.<ref name=":42">{{Cite journal |last1=Dieperink |first1=C. |last2=Hegger |first2=D. L. T. |last3=Bakker |first3=M. H. N. |last4=Kundzewicz |first4=Z. W. |last5=Green |first5=C. |last6=Driessen |first6=P. P. J. |date=2016-10-01 |title=Recurrent Governance Challenges in the Implementation and Alignment of Flood Risk Management Strategies: a Review |journal=Water Resources Management |language=en |volume=30 |issue=13 |pages=4467–4481 |bibcode=2016WatRM..30.4467D |doi=10.1007/s11269-016-1491-7 |issn=1573-1650 |s2cid=54676896 |doi-access=free}}</ref>
{{TOC level|3}}
==Types==
[[File:Katrina-new-orleans-flooding3-2005.jpg|thumb|View of flooded [[New Orleans]] in the aftermath of [[Hurricane Katrina]] ]]
[[File:Rapid Creek flooding 1.jpg|thumb|Flooding of a creek due to heavy [[monsoon]]al rain and high tide in [[Darwin, Northern Territory|Darwin]], [[Northern Territory]], [[Australia]]]]
[[File:Jeddah Flood - King Abdullah Street.jpg|thumb|Flood in [[Jeddah]], covering the King Abdullah Street in [[Saudi Arabia]]]]
[[File:Overland flooding near Georgetown, Minnesota, in the Red River Valley of the North.jpg|thumb|Overland flooding near Georgetown, Minnesota, in the Red River Valley of the North]]
=== Areal flooding ===
[[File:House surrounded by flood Ilmajoki Finland.jpg|thumb|In spring time, the floods are quite typical in [[Ostrobothnia (historical province)|Ostrobothnia]], a flat-lying area in [[Finland]]. A flood-surrounded house in [[Ilmajoki]], [[South Ostrobothnia]].]]
Floods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either [[Infiltration (hydrology)|infiltrate]] or [[Surface runoff|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 [[storm train|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 [[Surface runoff|overland flow]] depends on the surface slope. [[Endorheic basin]]s may experience areal flooding during periods when precipitation exceeds evaporation.<ref>{{cite book|last1=Jones|first1=Myrtle|title=Fact Sheet|chapter=Ground-water flooding in glacial terrain of southern Puget Sound, Washington|chapter-url=https://pubs.er.usgs.gov/publication/fs11100 |date=2000|doi=10.3133/fs11100|access-date=2015-07-23|ref=MJones}}</ref>
===River flooding===
Floods occur in all types of [[river]] and [[stream]] channels, from the smallest [[Stream#Intermittent and ephemeral streams|ephemeral streams]] in humid zones to [[Arroyo (watercourse)|normally-dry channels]] in arid climates to the [[Amazon River|world's largest]] rivers. When overland flow occurs on tilled fields, it can result in a [[muddy flood]] where [[sediment]]s are [[Erosion|picked up by run off]] and carried as suspended matter or [[bed load]]. Localized flooding may be caused or exacerbated by drainage obstructions such as [[landslide]]s, [[Ice jam|ice]], [[debris]], or [[beaver]] dams.
Slow-rising floods most commonly occur in large rivers with large [[Drainage basin|catchment areas]]. The increase in flow may be the result of sustained rainfall, rapid snow melt, [[monsoon]]s, or [[tropical 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 the [[Red River of the North|Red River Valley of the North]] in [[Minnesota]], [[North Dakota]], and [[Manitoba]], 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 by [[Lake Agassiz]], and over a length of {{Convert|550|mi|abbr=on}}, the river course drops only {{Convert|236|ft|abbr=on}}, for an average slope of about 5 inches per mile (or 8.2 cm per kilometer).<ref>[https://www.dnr.state.mn.us/watertrails/redriver/index.html "Red River of the North State Water Trail"]. Minnesota DNR. Accessed 8 September 2024</ref> 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, including [[flash 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 localized [[convective precipitation]] (intense [[thunderstorm]]s) or sudden release from an upstream impoundment created behind a [[dam]], 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.{{Citation needed|date=April 2024}} Without any observed rainfall, the flow rate increased from about {{convert|50|to|1500|ft3/s|sigfig=2}} in just one minute.<ref name="Hjalmarson">{{cite journal|last1=Hjalmarson|first1=Hjalmar W.|date=December 1984|title=Flash Flood in Tanque Verde Creek, Tucson, Arizona|journal=Journal of Hydraulic Engineering|volume=110|issue=12|pages=1841–1852|doi=10.1061/(ASCE)0733-9429(1984)110:12(1841)}}<!--|access-date=2015-07-24--></ref> 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 [[Arroyo (watercourse)|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.
=== Coastal flooding ===
{{Main|Coastal flooding}}
[[Coastal flooding|Coastal areas may be flooded]] by [[storm surge]]s 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. A storm surge is "an additional rise of water generated by a storm, over and above the predicted astronomical tides".<ref>{{cite web|url=http://www.nhc.noaa.gov/surge/ |title=Storm Surge Overview|website=noaa.gov|access-date=3 December 2015}}</ref> Due to the [[effects of climate change]] (e.g. [[sea level rise]] and an increase in [[extreme 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.<ref>{{Cite web |title=Report: Flooded Future: Global vulnerability to sea level rise worse than previously understood |url=https://www.climatecentral.org/news/report-flooded-future-global-vulnerability-to-sea-level-rise-worse-than-previously-understood |access-date=2020-11-09 |website=www.climatecentral.org |language=en |archive-date=2020-03-30 |archive-url=https://web.archive.org/web/20200330202024/https://www.climatecentral.org/news/report-flooded-future-global-vulnerability-to-sea-level-rise-worse-than-previously-understood }}</ref>
Flooding in [[estuary|estuaries]] is commonly caused by a combination of storm surges caused by [[wind]]s and low [[barometric pressure]] and large waves meeting high upstream river flows.
=== Urban flooding ===
{{Excerpt|Urban flooding|paragraphs=1-2|file=no}}
=== 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 of [[hydraulic engineering]]. Agricultural flooding may occur in preparing [[paddy field]]s for the growing of semi-aquatic rice in many countries.[[File:1938 June Yellow River.gif|thumb|Chinese Kuomintang soldiers during the [[1938 Yellow River flood]]]]
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'',<ref>{{cite web |title=Army Corps starts to blow up levee, flood 130,000 acres in Missouri |url=http://www.cnn.com/2011/US/05/02/missouri.levee.breach/index.html |access-date=May 23, 2018 |website=CNN}}</ref> 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]].
Military inundation creates an obstacle in the field that is intended to impede the movement of the enemy.<ref>{{cite web |date=1878 |title=Text Book of Fortification and Military Engineering: For Use at the Royal Military Academy, Woolwich, Volume 1 |url=https://books.google.com/books?id=Yvs_AAAAYAAJ |access-date=May 23, 2018 |website=Google Books |publisher=H.M. Stationery Office |page=50}}</ref> This may be done both for [[Offensive (military)|offensive]] and [[Defense (military)|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 and uncontrolled ones. Examples for controlled inundations include those in the Netherlands under the [[Dutch Republic]] and its [[Succession of states|successor states]] in that area<ref>{{cite web |author=Oosthoek, K.J. |title=The watery ally: military inundations in Dutch history (podcast) |url=https://www.eh-resources.org/podcast-77/ |access-date=May 23, 2018 |website=Environmental History Resources}}</ref><ref>{{cite web |author=Tiegs, R. |title=Military Inundations during the Dutch Wars of Independence |url=http://roberttiegs.com/digital-history/brief-history/ |url-status=dead |archive-url=https://web.archive.org/web/20180524151156/http://roberttiegs.com/digital-history/brief-history/ |archive-date=May 24, 2018 |access-date=May 23, 2018 |website=Brief History of Military Inundations during the Dutch Wars of Independence}}</ref> and exemplified in the two [[Hollandic Water Line]]s, the [[Stelling van Amsterdam]], the [[Frisian Water Line]], the [[IJssel Line]], the [[Peel-Raam Line]], and the [[Grebbe line]] in that country.
To count as ''controlled'', a military inundation has to take the interests of the civilian population into account, by allowing them a timely [[Emergency evacuation|evacuation]], by making the inundation ''reversible'', and by making an attempt to minimize the adverse [[Ecology|ecological]] impact of the inundation. That impact may also be adverse in a [[Hydrogeology|hydrogeological]] sense if the inundation lasts a long time.<ref>{{cite journal |last=Vandenbohed |first=Alexander |date=2016 |title=The hydrogeology of the military inundation at the 1914–1918 Yser front (Belgium) |journal=Hydrogeology Journal |volume=24 |issue=2 |pages=521–534 |bibcode=2016HydJ...24..521V |doi=10.1007/s10040-015-1344-0 |s2cid=131534974}}</ref>
Examples for uncontrolled inundations are the [[Siege of Leiden#Second siege, and relief, of Leiden|second Siege of Leiden]]<ref>{{cite web |author=Tiegs, R. |date=13 April 2017 |title=The Past Comes Flooding Back: The War That Almost Sank Holland |url=http://niche-canada.org/2017/04/13/the-past-comes-flooding-back-the-war-that-almost-sank-holland/ |access-date=May 23, 2018 |website=NICHE. Network in Canadian History & Environment. Nouvelle initiative Canadienne en histoire de l'environnement}}</ref> during the first part of the [[Eighty Years' War (1566–1609)|Eighty Years' War]], the [[Battle of the Yser#Battle|flooding of the Yser plain]] during the [[First World War]],<ref>{{cite news |title=Battle of the Yser |url=https://www.britannica.com/topic/Battle-of-the-Yser |work=Encyclopædia Britannica}}</ref> and the [[Inundation of Walcheren]], and the [[Inundation of the Wieringermeer]] during the [[Second World War]]).
==Causes==
[[File:Cyclone Hudhud destruction in Visakhapatnam 2.jpg|thumb|Flood due to [[Cyclone Hudhud]] in [[Visakhapatnam]], India]]
{{See also|Coastal flooding#Causes}}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 accelerated [[snowmelt]], severe winds over water, unusual high tides, [[tsunami]]s, or failure of dams, [[levees]], [[retention pond]]s, 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 as [[surface 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.<ref>{{Cite web |author=Basic Biology |date=2016 |title=Wetlands |url=https://basicbiology.net/environment/land/wetlands}}</ref> Water then runs off the land in quantities that cannot be carried within [[stream channel]]s or retained in natural ponds, lakes, and human-made [[reservoir]]s. About 30 percent of all precipitation becomes runoff<ref name="M">"Flood Control", [[MSN Encarta]], 2008 (see below: [[Flood control#Further reading|Further reading]]).</ref> and that amount might be increased by water from melting snow.
===Upslope factors===
[[File:Ein Avdat Flood 1.JPG|thumb|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 a [[flash 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.<ref>Babbitt, Harold E. & Doland, James J., ''Water Supply Engineering'', McGraw-Hill Book Company, 1949</ref>
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.<ref>Simon, Andrew L., ''Basic Hydraulics'', John Wiley & Sons, 1981, {{ISBN|0-471-07965-0}}</ref>
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 {{convert|30|sqmi|sigfig=1|disp=or}}. 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.<ref name="Simon">Simon, Andrew L., ''Practical Hydraulics'', John Wiley & Sons, 1981, {{ISBN|0-471-05381-3}}</ref>
[[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.<ref name="Urquhart">Urquhart, Leonard Church, ''Civil Engineering Handbook'', McGraw-Hill Book Company, 1959</ref> 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 [[lake]]s. 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 surge]]s 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<ref name="Simon" /> 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 the [[flood plain]]. Even when rainfall is relatively light, the [[shoreline]]s of lakes and bays can be flooded by severe winds—such as during [[Tropical cyclone|hurricanes]]—that blow water into the shore areas.
===Climate change===
{{multiple image
| total_width       = 500
| image1            = 4207Typhoons Krosa Lekima & monsoon tidal flooding in Calumpit, Bulacan 19.jpg
| caption1          = [[Tidal flooding|High tide flooding]] is increasing due to sea level rise, land subsidence, and the loss of natural barriers.<ref>{{Cite web |publisher=US Department of Commerce |website=National Oceanic and Atmospheric Administration's National Ocean Service |title=What is high tide flooding? |url=https://oceanservice.noaa.gov/facts/nuisance-flooding.html |url-status=live |archive-url=https://web.archive.org/web/20201016163020/https://oceanservice.noaa.gov/facts/nuisance-flooding.html |archive-date=16 October 2020 |access-date=12 October 2020 }}</ref>
| alt               = <nowiki>A large flat sheet of water reflects a grey sky with green tropical vegetation in the background]]</nowiki>
| image3            = 2050 Projected sea level rise - United States coasts - NOAA.svg
| caption3          = 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.<ref name=NOAA_SeaLevelRiseTechReport_20220200>{{cite web |title=2022 Sea Level Rise Technical Report |url=https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html |publisher=National Ocean Service, National Oceanic and Atmospheric Administration (NOAA) |archive-url=https://web.archive.org/web/20221129070303/https://oceanservice.noaa.gov/hazards/sealevelrise/sealevelrise-tech-report.html |archive-date=November 29, 2022 |date=February 2022 |url-status=live }}</ref>
}}
{{excerpt|Effects of climate change#Floods|paragraphs=1-2}}
===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.<ref name="Abbett">Abbett, Robert W., ''American Civil Engineering Practice'', John Wiley & Sons, 1956</ref> Coincident events may cause extensive flooding to be more frequent than anticipated from [[100-year flood|simplistic statistical prediction models]] considering only precipitation runoff flowing within unobstructed drainage channels.<ref name="BR">[[United States Department of the Interior]], Bureau of Reclamation, ''Design of Small Dams'', United States Government Printing Office, 1973</ref> 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]] 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.<ref name="Chanson_2011c">{{Cite book |last1=Brown |first1=Richard |url=http://espace.library.uq.edu.au/view/UQ:243550 |title=Turbulent Velocity and Suspended Sediment Concentration Measurements in an Urban Environment of the Brisbane River Flood Plain at Gardens Point on 12–13 January 2011 |last2=Chanson |first2=Hubert |author-link2=Hubert Chanson |last3=McIntosh |first3=Dave |last4=Madhani |first4=Jay |year=2011 |isbn=978-1-74272-027-2 |series=Hydraulic Model Report No. CH83/11 |page=120 |issue=CH83/11}}</ref> These considerations ignore further the risks associated with large debris entrained by the flow motion.<ref name="Chanson_2014">{{cite book |author=[[Hubert Chanson|Chanson, H.]] |url=https://eprints.qut.edu.au/88693/1/88693.pdf |title=Hydraulic structures and society - Engineering challenges and extremes |author2=Brown, R. |author3=McIntosh, D. |date=26 June 2014 |publisher=Proceedings of the 5th IAHR International Symposium on Hydraulic Structures (ISHS2014) |isbn=978-1-74272-115-6 |editor=L. Toombes |location=Brisbane, Australia |pages=1–9 |chapter=Human body stability in floodwaters: The 2011 flood in Brisbane CBD |doi=10.14264/uql.2014.48}}</ref>
==Negative impacts==
[[File:Flooded walnut orchard in Butte County, California-L1001234.jpg|thumb|Flooded walnut orchards in [[Butte County, California|Butte 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.<ref name=":03" />
=== 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.<ref name=":03">{{Cite book |last=SCHANZE |first=JOCHEN |title=Flood Risk Management: Hazards, Vulnerability and Mitigation Measures |chapter=Flood Risk Management – A Basic Framework |date=2006 |editor-last=Schanze |editor-first=Jochen |editor2-last=Zeman |editor2-first=Evzen |editor3-last=Marsalek |editor3-first=Jiri |chapter-url=https://link.springer.com/chapter/10.1007/978-1-4020-4598-1_1 |series=NATO Science Series |language=en |location=Dordrecht |publisher=Springer Netherlands |volume=67 |pages=1–20 |doi=10.1007/978-1-4020-4598-1_1 |isbn=978-1-4020-4598-1}}</ref>
Floods can have devastating impacts to human societies. Flooding events worldwide are increasing in frequency and severity, leading to increasing costs to societies.<ref name=":03" />
Catastrophic riverine flooding can result from major [[infrastructure]] failures, often the [[dam failure|collapse of a dam]]. It can also be caused by drainage channel modification from a [[landslide]], [[earthquake]] or [[volcanic eruption]]. Examples include [[outburst flood]]s and [[lahar]]s. [[Tsunami]]s can cause catastrophic [[coastal flooding]], most commonly resulting from undersea earthquakes.
=== Economic impacts ===
The primary effects of flooding include [[death|loss 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.<ref name=":12" />
Every year flooding causes countries billions of dollars worth of damage that threatens the livelihood of individuals.<ref name=":7">{{Cite journal |last1=Morrison |first1=A. |last2=Westbrook |first2=C.J. |last3=Noble |first3=B.F. |year=2018 |title=A review of the flood risk management governance and resilience literature |journal=Journal of Flood Risk Management |volume=11 |issue=3 |pages=291–304 |doi=10.1111/jfr3.12315 |bibcode=2018JFRM...11..291M |s2cid=134055424|doi-access=free }}</ref> As a result, there is also significant socio-economic threats to vulnerable populations around the world from flooding.<ref name=":7" /> 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.<ref>{{Cite web |last=Society |first=National Geographic |date=2011-11-07 |title=flood |url=http://www.nationalgeographic.org/encyclopedia/flood/ |access-date=2022-03-10 |website=National Geographic Society |language=en }}{{Dead link|date=November 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
[[File:2foot mud.jpg|thumb|Mud was deposited in this house by flooding in the [[2018 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.<ref>Stephen Bratkovich, Lisa Burban, et al., [http://www.na.fs.fed.us/spfo/pubs/n_resource/flood/cover.htm "Flooding and its Effects on Trees"] {{Webarchive|url=https://web.archive.org/web/20160614132703/http://www.na.fs.fed.us/spfo/pubs/n_resource/flood/cover.htm |date=2016-06-14 }}, [[USDA Forest Service]], Northeastern Area State and Private Forestry, St. Paul, MN, September 1993</ref>
Flooding in areas where people live 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.<ref>Center for Neighborhood Technology, Chicago IL [http://www.cnt.org/resources/the-prevalence-and-cost-of-urban-flooding/ "The Prevalence and Cost of Urban Flooding"], May 2013</ref> According to the U.S. [[Federal Emergency Management Agency]] (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster.<ref>[https://archive.today/20130917193030/http://www.fema.gov/protecting-yourbusinesses "Protecting Your Businesses"], last updated March 2013</ref> In the United States, [[National Flood Insurance Program|insurance]] is available against flood damage to both homes and businesses.<ref name="NFIP">{{cite web |title=National Flood Insurance Program |url=https://www.floodsmart.gov/floodsmart/ |archive-url=https://web.archive.org/web/20161222100205/https://www.floodsmart.gov/floodsmart/ |archive-date=2016-12-22 |access-date=2015-07-06 |website=FloodSmart.gov}}</ref>
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.
=== Health impacts ===
[[File:FEMA_-_37590_-_Community_Flooding_in_Florida.jpg|alt=|thumb|[[Coastal flooding]] in a community in Florida, United States]]
[[File:Flooding after 1991 cyclone.jpg|thumb|Flooding after [[1991 Bangladesh cyclone]], which killed around 140,000&nbsp;people]]
Fatalities connected directly to floods are usually caused by [[drowning]]; the waters in a flood are very deep and have strong [[Current (fluid)|currents]].<ref name="alderman">{{cite journal |last1=Alderman |first1=Katarzyna |last2=Turner |first2=Lyle R. |last3=Tong |first3=Shilu |date=June 2012 |title=Floods and human health: A systematic review |url=https://espace.library.uq.edu.au/view/UQ:346602/UQ346602_OA.pdf |journal=Environment International |volume=47 |pages=37–47 |doi=10.1016/j.envint.2012.06.003 |pmid=22750033|bibcode=2012EnInt..47...37A }}</ref> [[Deaths]] do not just occur from drowning, deaths are connected with [[dehydration]], [[heat stroke]], [[heart attack]] and any other [[illness]] that needs [[medical supplies]] that cannot be delivered.<ref name="alderman" />
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.<ref name="alderman" /> 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.<ref name="alderman" />
[[Communicable diseases]] are increased due to many [[pathogens]] and [[bacteria]] that are being transported by the [[water]].There are many [[waterborne diseases]] such as [[cholera]], [[hepatitis A]], [[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]] in the water and in living quarters depending on where the flood is and how prepared the [[community]] is for a flood.<ref name="alderman" />
When floods hit, people lose nearly all their crops, livestock, and food reserves and face starvation.<ref>{{Cite journal |last1=Ahadzie |first1=Divine Kwaku |last2=Mensah |first2=Henry |last3=Simpeh |first3=Eric |date=2022 |title=Impact of floods, recovery, and repairs of residential structures in Ghana: insights from homeowners |url=https://link.springer.com/10.1007/s10708-021-10425-2 |journal=GeoJournal |language=en |volume=87 |issue=4 |pages=3133–3148 |doi=10.1007/s10708-021-10425-2 |bibcode=2022GeoJo..87.3133A |s2cid=234825689 |issn=0343-2521}}</ref>
Floods also frequently damage [[power transmission]] and sometimes [[power generation]], which then has [[knock-on effect]]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.
Flooding can cause chronically wet houses, leading to the growth of [[indoor mold]] and resulting in adverse health effects, particularly respiratory symptoms.<ref>Indoor Air Quality (IAQ) Scientific Findings Resource Bank (IAQ-SFRB), [http://www.iaqscience.lbl.gov/dampness-risks-house.html "Health Risks or Dampness or Mold in Houses"] {{webarchive|url=https://web.archive.org/web/20131004213929/http://www.iaqscience.lbl.gov/dampness-risks-house.html|date=2013-10-04}}</ref> [[Respiratory diseases]] are a common after the disaster has occurred. This depends on the amount of [[water damage]] and [[Mold (fungus)|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.<ref name=":02">{{Cite journal |last=Demain |first=Jeffrey G. |date=24 March 2018 |title=Climate Change and the Impact on Respiratory and Allergic Disease: 2018 |journal=Current Allergy and Asthma Reports |language=en |volume=18 |issue=4 |page=22 |doi=10.1007/s11882-018-0777-7 |issn=1534-6315 |pmid=29574605 |s2cid=4440737}}</ref> [[Vector (epidemiology)|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 virus|West Nile]], and [[yellow fever]].<ref name="alderman" /> Floods have a huge impact on victims' psychosocial [[integrity]]. People suffer from a wide variety of losses and [[Stress (psychological)|stress]]. One of the most treated illness in long-term health problems are [[Depression (mood)|depression]] caused by the flood and all the [[tragedy]] that flows with one.<ref name="alderman" />
==== Loss of life ====
{{Main|List of deadliest floods}}
Below is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.
{| class="sortable wikitable" style="width:95%;"
|- style="background:#ccc;"
! style="width:20%;" |Death toll !! style="width:40%;" |Event !! style="width:20%;" |Location !! style="width:20%;" |Year
|-
||2,500,000–3,700,000<ref>[http://www.nbc10.com/news/4030540/detail.html Worst Natural Disasters In History] {{webarchive|url=https://web.archive.org/web/20080421043514/http://www.nbc10.com/news/4030540/detail.html|date=2008-04-21}} (2012-06-07), Retrieved on 2012-06-12</ref> || [[1931 China floods]] || [[China]] || 1931
|-
||900,000–2,000,000 || [[1887 Yellow River flood]] || [[China]] || 1887
|-
||500,000–700,000 || [[1938 Yellow River flood]] || China || 1938
|-
||231,000 || [[Banqiao Dam]] failure, result of [[Typhoon Nina (1975)|Typhoon Nina]]. Approximately 86,000 people died from flooding and another 145,000 died during subsequent disease. || China || 1975
|-
||230,000 || [[2004 Indian Ocean earthquake|2004 Indian Ocean tsunami]] || [[Indonesia]] || 2004
|-
||145,000 || 1935 [[Yangtze]] river flood || China || 1935
|-
||100,000+ || [[St. Felix's flood]], [[storm surge]]|| [[Netherlands]] || 1530
|-
||100,000 || [[Hanoi]] and [[Red River Delta]] flood || [[North Vietnam]] || 1971
|-
||100,000 || 1911 [[Yangtze]] river flood || China || 1911
|}
== Positive impacts (benefits) ==
Floods (in particular more frequent or smaller floods) can also bring many benefits, such as recharging [[ground water]], making soil more [[Fertility (soil)|fertile]] and increasing [[nutrient]]s 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]]s in river corridors and are a key factor in maintaining floodplain [[biodiversity]].<ref>WMO/GWP Associated Programme on Flood Management, [http://www.apfm.info/publications/policy/ifm_env_aspects/Environmental_Aspects_of_IFM_En.pdf "Environmental Aspects of Integrated Flood Management"] {{Webarchive|url=https://web.archive.org/web/20150713081046/http://www.apfm.info/publications/policy/ifm_env_aspects/Environmental_Aspects_of_IFM_En.pdf |date=2015-07-13 }}, 2007</ref> 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.<ref>[http://kops.ub.uni-konstanz.de/volltexte/2009/7515/pdf/2008_Wantzen_etal_An_extension_of_the_floodpulse.pdf Extension of the Flood Pulse Concept]{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}, Retrieved on 2012-06-12</ref> 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.<ref>[http://africa.ipsterraviva.net/2010/10/15/birdlife-soars-above-botswanas-floodplains Birdlife soars above Botswana's floodplains] {{webarchive|url=https://web.archive.org/web/20110209093150/http://africa.ipsterraviva.net/2010/10/15/birdlife-soars-above-botswanas-floodplains/ |date=2011-02-09 }} (2010-10-15), Retrieved on 2012-06-12</ref>
Flooding can bring benefits, such as making the soil more fertile and providing it with more nutrients. For this reason, periodic flooding was essential to the well-being of ancient communities along the [[Tigris-Euphrates]] Rivers, the [[Nile River]], the [[Indus River]], the [[Ganges]] and the [[Yellow River]] among others.
The viability of [[hydropower]], a renewable source of energy, is also higher in flood prone regions.
== Protections against floods and associated hazards ==
=== Flood management ===
[[File:Humber Weir.JPG|right|thumb|A [[weir]] was built on the [[Humber River (Ontario)]] to prevent a recurrence of a catastrophic flood.]]
{{Excerpt|Flood risk management|file=no}}
==== Flood management examples ====
In many countries around the world, waterways prone to floods are often carefully managed. Defenses such as [[detention basin]]s, [[levee]]s,<ref>{{Cite book |author=Henry Petroski |title=Levees and Other Raised Ground |publisher=American Scientist |year=2006 |volume=94 |pages=7–11 |issue=1}}</ref> [[bunding|bunds]], [[reservoir (water)|reservoirs]], and [[weir]]s are used to prevent waterways from overflowing their banks. When these defenses fail, emergency measures such as [[sandbag]]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 management|coastal defenses]], such as [[sea wall]]s, [[beach nourishment]], and [[barrier island]]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]] 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]], 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]]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]], [[sump pump]]s, and [[check valve]]s.
=== Flood safety planning ===
[[File:Greeley, CO, September 19, 2013 (12441526333).jpg|thumb|Aftermath of flooding in Colorado, 2013]]
[[File:Afghan Air Force - Jalalabad Flood Rescue (4842784567).jpg|thumb|Flood rescue in [[Nangarhar Province|Nangarhar]], Afghanistan in 2010]]
[[File:Trapped woman on a car roof during flash flooding in Toowoomba 2.jpg|thumb|[[Flash flood]]ing caused by heavy rain falling in a short amount of time]]In the United States, the [[National 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.<ref name="Eychaner">Eychaner, J.H. (2015) ''Lessons from a 500-year record of flood elevations'', [http://www.floods.org/ace-files/documentlibrary/publications/asfpmpubs-techrep7_2015.pdf Association of State Floodplain Managers, Technical Report 7] {{Webarchive|url=https://web.archive.org/web/20150627192514/http://www.floods.org/ace-files/documentlibrary/Publications/asfpmpubs-techrep7_2015.pdf|date=2015-06-27}}, Accessed 2015-06-27</ref>{{rp|22–23}} 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, [[Flood forecasting|forecasting]], and emergency-response planning, and
* short-term monitoring, [[Flood warning|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.<ref>Dyhouse, G., "Flood modelling Using HEC-RAS (First Edition)", Haestad Press, Waterbury (USA) 2003-26-41</ref>{{page needed|date=July 2015}}
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.<ref>{{cite web |title=Association of State Floodplain Managers |url=http://floods.org/ |access-date=2015-07-13}}</ref> A portfolio of [[best practice]] examples for [[disaster mitigation]] in the United States is available from the Federal Emergency Management Agency.<ref>{{cite web |title=Best Practices Portfolio |url=http://www.fema.gov/mitigation-best-practices-portfolio |access-date=2015-07-06 |publisher=Federal Emergency Management Agency}}</ref>
=== Flood clean-up safety ===
Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers include [[Electric shock|electrical hazards]], [[carbon monoxide]] exposure, [[musculoskeletal]] hazards, [[Hyperthermia|heat]] or [[Hypothermia|cold stress]], [[motor vehicle]]-related dangers, [[fire]], [[drowning]], and exposure to [[hazardous 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 with [[hard hats]], [[goggles]], heavy work gloves, [[life jackets]], and watertight boots with steel toes and insoles.<ref>{{cite web |publisher=National Institute for Occupational Safety and Health |url=https://www.cdc.gov/niosh/topics/emres/flood.html |title=Storm/Flood and Hurricane/Typhoon Response |work=Emergency Response Resources |url-status=dead |archive-url= https://web.archive.org/web/20240229092656/https://www.cdc.gov/niosh/topics/emres/flood.html|archive-date= Feb 29, 2024 }}</ref>
== Flood predictions ==
=== Mathematical models and computer tools ===
{{See also|Urban flooding#Modeling}}
[[File:Flood102405.JPG|thumb|Flooding near [[Key West]], [[Florida]], United States from [[Hurricane Wilma]]'s [[storm surge]] in October 2005]]A series of annual maximum flow rates in a stream reach can be analyzed [[extreme value analysis|statistically]] to estimate the [[100-year flood]] and floods of other [[return period|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 [[regression analysis|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 [[One-dimensional space|1D]] models (flood levels measured in the [[stream channel|channel]]) or [[Plane (mathematics)|2D]] models (variable flood depths measured across the extent of a floodplain). [[HEC-RAS]],<ref>{{cite web |publisher=United States Army Corps of Engineers|location=Davis, CA |url=http://www.hec.usace.army.mil/ |archive-url=https://web.archive.org/web/20130308150357/http://www.hec.usace.army.mil/ |archive-date=2013-03-08 |title=Hydrologic Engineering Center Home Page |website=hec.usace.army.mil }}</ref> 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<ref>{{cite web |url-status=dead |url=http://www.tuflow.com/ |archive-url=https://web.archive.org/web/20080627014611/http://www.tuflow.com/ |archive-date=2008-06-27 |title=TUFLOW {{!}} Flood, Urban Stormwater, Coastal and Water Quailty computer modelling software |website=tuflow.com }}</ref> combine 1D and 2D components to derive flood depths across both river channels and the entire floodplain.
[[computer simulation|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]].
In the United States, an integrated approach to real-time hydrologic computer modelling uses observed data from the [[U.S. Geological Survey]] (USGS),<ref name="USGS WaterWatch">{{cite web |website=USGS |title=WaterWatch |url=http://waterwatch.usgs.gov/index.php?id=ww_current |access-date=4 February 2013 |archive-date=10 April 2021 |archive-url=https://web.archive.org/web/20210410010251/https://waterwatch.usgs.gov/index.php?id=ww_current |url-status=dead }}</ref> various [[Weather spotting|cooperative observing networks]],<ref name="Community Collaborative Rain, Hail and Snow Network">{{cite web |title=Community Collaborative Rain, Hail and Snow Network |url=http://www.cocorahs.org |access-date=4 February 2013}}</ref> various [[Automated airport weather station|automated weather sensors]], the [[NOAA]] National Operational Hydrologic Remote Sensing Center (NOHRSC),<ref name="National Operational Hydrologic Remote Sensing Center">{{cite web |date=2 May 2012 |title=NOHRSC |url=http://www.nohrsc.noaa.gov |archive-url=https://web.archive.org/web/20210419115014/https://www.nohrsc.noaa.gov/ |archive-date=19 April 2021 |access-date=4 February 2013}}</ref> various [[hydroelectric]] companies, etc. combined with [[quantitative precipitation forecast]]s (QPF) of expected rainfall and/or snow melt to generate daily or as-needed hydrologic forecasts.<ref name="Advanced Hydrologic Prediction System" /> 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 available online.<ref>{{Cite web |title=Global Flood Monitoring |url=http://flood.umd.edu/ |access-date=2024-01-15 |website=flood.umd.edu}}</ref> 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]].
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.<ref>{{Cite web |title=Predicting Floods |url=https://science.nasa.gov/science-news/science-at-nasa/2015/22jul_floods/ |access-date=2015-07-22 |website=science.nasa.gov}}</ref>
=== Flood forecasts and warnings ===
{{Main|Flood forecasting|flood warning|}}
[[File:Natal Brazil Flood.jpeg|thumb|Flooding in a street of [[Natal, Rio Grande do Norte]], [[Brazil]] in April 2013]]Anticipating floods before they occur allows for precautions to be taken and people to be warned<ref>{{cite web|url=http://www.environment-agency.gov.uk/homeandleisure/floods/58417.aspx |title=Flood Warnings|publisher=Environment Agency|date=2013-04-30|access-date=2013-06-17}}</ref> 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]]s, it is best to have a long time-series of historical data that relates [[stream flow]]s to measured past rainfall events.<ref>{{cite web|url=http://www.bom.gov.au/australia/flood |title=Australia rainfall and river conditions|publisher=Bom.gov.au|access-date=2013-06-17}}</ref> Coupling this historical information with [[Real-time data|real-time knowledge]] about volumetric capacity in catchment areas, such as spare capacity in reservoirs, ground-water levels, and the degree of [[Phreatic zone|saturation]] of area [[aquifer]]s is also needed in order to make the most accurate flood forecasts.
[[Weather radar|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,<ref name="Advanced Hydrologic Prediction System">{{Cite journal|url=http://water.weather.gov/ahps |title=Advanced Hydrologic Prediction System|journal=Journal of Geophysical Research|volume=104|issue=D16|pages=19, 655|access-date=4 February 2013|bibcode=1999JGR...10419655C|last1=Connelly|first1=Brian A|last2=Braatz|first2=Dean T|last3=Halquist|first3=John B|last4=Deweese|first4=Michael M|last5=Larson|first5=Lee|last6=Ingram|first6=John J|year=1999|doi=10.1029/1999JD900051|doi-access=free}}</ref> and it also may allow for the computation of the likely statistical return period of a flood. In many developed countries, urban areas at risk of flooding are protected against a 100-year flood – that is a flood that has a probability of around 63% of occurring in any 100-year period of time.
According to the U.S. [[National Weather Service]] (NWS) Northeast River Forecast Center (RFC) in [[Taunton, Massachusetts]], a rule of thumb for flood forecasting in urban areas is that it takes at least {{convert|1|in|mm}} of rainfall in around an hour's time in order to start significant [[ponding]] of water on [[Impervious surface|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]]s.<ref name="Flash Flood Guidance">{{cite web|url=http://www.srh.noaa.gov/rfcshare/ffg.php|title=FFG|access-date=29 January 2013|archive-date=4 March 2013|archive-url=https://web.archive.org/web/20130304094148/http://www.srh.noaa.gov/rfcshare/ffg.php}}</ref>
=== Flood risk assessment ===
{{Main|Flood risk assessment}}
[[Flood risk assessment|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.<ref>{{Cite book |url=https://link.springer.com/book/10.1007/978-90-481-9917-4 |title=Flood Risk Assessment and Management |date=2011 |isbn=978-90-481-9916-7 |editor-last=Schumann |editor-first=Andreas H. |language=en-gb |doi=10.1007/978-90-481-9917-4}}</ref>
A large amount of the world's population lives in close proximity to major [[Coastal flooding|coastlines]],<ref name=":2" /> while many major cities and agricultural areas are located near [[floodplain]]s.<ref name=":12" /> There is significant risk for increased coastal and fluvial flooding due to changing climatic conditions.<ref name=":42" />
== Examples by country or region ==
* Worldwide: [[List of floods]]
* Africa: [[List of floods#Africa]]
* Asia: [[List of floods#Asia]]
* Europe: [[List of floods in Europe]]
** North Sea: [[Storm tides of the North Sea]]
** The Netherlands: [[Floods in the Netherlands]], [[Flood control in the Netherlands]]
* Oceania: [[List of floods#Oceania]]
** Australia: [[Floods in Australia]]
* United States: [[Lists of floods in the United States]]
== Society and culture ==
[[File:A Flood on Java 1865-1876 Raden Saleh.jpg|thumb|People seeking refuge from flood in [[Java]], {{Circa|1865}}–1876]]
=== Myths and religion ===
[[File:Gustave Doré - The Holy Bible - Plate I, The Deluge.jpg|thumb|"The Deluge", frontispiece to [[Gustave Doré]]'s illustrated edition of the [[Bible]]]]
{{excerpt|Flood myth|file=no}}
== Etymology ==
The word "flood" comes from the [[Old English language|Old English]] {{lang|ang|flōd}}, a word common to [[Germanic language]]s (compare [[German language|German]] {{lang|de|Flut}}, [[Dutch language|Dutch]] {{lang|nl|vloed}} from the same root as is seen in ''flow, float''; also compare with [[Latin]] {{lang|la|fluctus}}, {{lang|la|flumen}}), meaning "a flowing of water, tide, an overflowing of land by water, a deluge, Noah's Flood; mass of water, river, sea, wave".<ref name=":0">{{cite encyclopedia |publisher=Harper Douglas |title=Etymology of flood |dictionary=Online Etymology Dictionary |access-date=February 21, 2022 |url=https://www.etymonline.com/word/flood}}</ref><ref>{{Cite web|title=FLOOD English Definition and Meaning|url=https://www.lexico.com/en/definition/flood|archive-url=https://web.archive.org/web/20200702054412/https://www.lexico.com/en/definition/flood|archive-date=July 2, 2020|access-date=2022-02-21|website=Lexico Dictionaries {{!}} English}}</ref> The Old English word {{lang|ang|flōd}} comes from the [[Proto-Germanic language|Proto-Germanic]] ''floduz'' ([[Old Frisian]] {{lang|ofs|flod}}, [[Old Norse]] {{lang|non|floð}}, [[Middle Dutch]] {{lang|dum|vloet}}, Dutch {{lang|nl|vloed}}, German {{lang|de|Flut}}, and [[Gothic language|Gothic]] {{lang|got|flodus}} derives from ''floduz'').<ref name=":0" />
== See also ==
{{portal|Environment|Ecology}}
*{{annotated link|Disaster response}}
*{{annotated link|Diversion dam}}
* {{annotated link|Emergency management}}
* {{annotated link|Flood alert}}
* {{annotated link|Mudflow}}
==References==
{{reflist}}
{{Sister project links|flood}}
{{Natural disasters}}
{{Rivers, streams and springs}}
{{Natural resources}}
{{Authority control}}
[[Category:Flood| ]]
[[Category:Water]]
[[Category:Bodies of water]]
[[Category:Hydrology]]
[[Category:Meteorological phenomena]]
[[Category:Weather hazards]]
[[Category:Natural disasters]]