Editing Flood (section)

==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)|bibcode=1984JHydE.110.1841H }}<!--|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]]).