Editing Flood (section)

== Flood predictions ==
{{Main|Flood prediction}}
{{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=https://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 |url-status=live|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|date=20 July 2015 }}</ref>

[[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% (i.e. 1 − 0.99<sup>100</sup>, or roughly 1 − 1/[[e (mathematical constant)|''e'']]) 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" />