Editing Flood (section)

==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>