FloodFrom Wikipedia, the free encyclopediaFor other uses, seeFlood (disambiguation).

Dozens of villages were inundated when rain pushed the rivers of northwesternBangladeshover their banks in early October 2005. TheModerate Resolution Imaging Spectroradiometer(MODIS) onNASAsTerrasatellite captured the top image of the flooded Ghaghat andAtrai Riverson October 12, 2005. The deep blue of the rivers is spread across the countryside in the flood image.Afloodis an overflow of water that submerges land which is usually dry.[1]TheEuropean Union(EU)Floods Directivedefines a flood as a covering bywaterof land not normally covered by water.[2]In the sense of "flowing water", the word may also be applied to the inflow of thetide. Flooding may occur as an overflow of water from water bodies, such as ariverorlake, in which the water overtops or breakslevees, resulting in some of that water escaping its usual boundaries,[3]or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes inprecipitationand snow melt, these changes in size are unlikely to be considered significant unless they floodpropertyor drowndomestic animals.Floods can also occur in rivers when the flow rate exceeds the capacity of theriver channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat andfertileand because rivers provide easy travel and access tocommerceand industry.Some floods develop slowly, while others such asflash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighbourhood or community, or very large, affecting entire river basins.

EtymologyThe word "flood" comes from theOld Englishflod, a word common toGermanic languages(compareGermanFlut,Dutchvloedfrom the same root as is seen inflow, float; also compare withLatinfluctus,flumen).Deluge mythsaremythicalstories of a great flood sent by adeityor deities to destroycivilizationas an act ofdivine retribution, and they are featured in the mythology of manycultures.Principal types and causesAreal (rainfall related)Floods can happen on flat or low-lying areas when the ground is saturated and water either cannotrun offor cannot run off quickly enough to stop accumulating. This may be followed by a river flood as water moves away from thefloodplaininto local rivers and streams.Floods can also occur if water falls on an impermeable surface, such asconcrete, paving or frozen ground, and cannot rapidly dissipate into the ground.Localised heavy rain from aseries of stormsmoving over the same area can cause arealflash floodingwhen the rate of rainfall exceeds the drainage capacity of the area. When this occurs on tilled fields, it can result in amuddy floodwheresedimentsare picked up by run off and carried as suspended matter orbed load.RiverineRiver flows may rise to floods levels at different rates, from a few minutes to several weeks, depending on the type of river and the source of the increased flow.Slow rising floods most commonly occur in large rivers with largecatchment areas. The increase in flow may be the result of sustained rainfall, rapid snow melt,monsoons, ortropical cyclones. Localised flooding may be caused or exacerbated by drainage obstructions such aslandslides,ice, ordebris.Rapid flooding events, includingflash floods, more often occur on smaller rivers, rivers with steep valleys or rivers that flow for much of their length over impermeable terrain. The cause may be localisedconvective precipitation(intensethunderstorms) or sudden release from an upstream impoundment created behind adam,landslide, orglacier.Dam-buildingbeaverscan flood low-lyingurbanandruralareas, occasionally causing some damage.Estuarine and coastalFlooding inestuariesis commonly caused by a combination of sea tidal surges caused bywindsand lowbarometric pressure, and they may be exacerbated by high upstream river flow.Coastal areas may be flooded by storm events at sea, resulting in waves over-topping defences or in severe cases bytsunamior tropical cyclones. Astorm surge, from either atropical cycloneor anextratropical cyclone, falls within this category.Urban floodingUrban flooding is the inundation of land or property in abuilt environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such asstorm sewers. Although sometimes triggered by events such asflash floodingorsnowmelt, urban flooding is a condition, characterized by its repetitive and systemic impacts on communities, that can happen regardless of whether or not affected communities are located within formally designated floodplains or near anybody of water.[4]There are several ways in whichstormwaterenters properties: backup through sewer pipes, toilets and sinks into buildings; seepage through building walls and floors; the accumulation of water on property and in public rights-of-way; and the overflow from water bodies such as rivers and lakes.CatastrophicCatastrophic flooding is usually associated with major infrastructure failures such as the collapse of adam, but they may also be caused by damage sustained in anearthquakeorvolcanic eruption. Seeoutburst flood.EffectsPrimary effectsThe primary effects of flooding includeloss of life, damage to buildings and other structures, including bridges,seweragesystems,roadways, andcanals.Floods also frequently damagepower transmissionand sometimespower generation, which then hasknock-on effectscaused by the loss of power. This includes loss of drinkingwater treatmentand water supply, which may result in loss of drinking water or severe water contamination. It may also cause the loss of sewage disposal facilities. Lack of clean water combined withhuman sewagein the flood waters raises the risk ofwaterborne diseases, which can includetyphoid,giardia,cryptosporidium,choleraand many other diseases depending upon the location of the flood.Damage to roads and transport infrastructure may make it difficult to mobilise aid to those affected or to provide emergency health treatment.Flood waters typically inundate farm land, making the land unworkable and preventingcropsfrom 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[5]Secondary and long-term effectsEconomic 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.Urban flooding can lead to chronically wet houses, which are linked to an increase inrespiratoryproblems and other illnesses.[6]Urban flooding also has significant economic implications for affected neighborhoods. In theUnited States, industry experts estimate that wet basements can lower property values by 10-25 percent and are cited among the top reasons for not purchasing a home.[7]According to theU.S. Federal Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen their doors following a flooding disaster.[8]Flood forecastingAnticipating floods before they occur allows for precautions to be taken and people to bewarned[9]so that they can be prepared in advance for flooding conditions. For example,farmerscan 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.In order to make the most accurateflood forecastsforwaterways, it is best to have a long time-series of historical data that relatesstream flowsto measured past rainfall events.[10]Coupling this historical information withreal-time knowledgeabout volumetric capacity in catchment areas, such as spare capacity inreservoirs, ground-water levels, and the degree ofsaturationof areaaquifersis also needed in order to make the most accurate flood forecasts.Radarestimates of rainfall and generalweather forecastingtechniques 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 oflead 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,[11]and it also may allow for the computation of the likely statisticalreturn periodof a flood. In many developed countries,urban areasat risk of flooding are protected against a100-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) inTaunton, Massachusetts, a generalrule-of-thumbfor flood forecasting in urban areas is that it takes at least 1 inch (25mm) of rainfall in around an hour's time in order to start significantpondingof water onimpermeable surfaces. Many NWS RFCs routinely issue Flash Flood Guidance and Headwater Guidance, which indicate the general amount of rainfall that would need to fall in a short period of time in order to causeflash floodingor flooding on largerwater basins.[12]ControlMain article:Flood controlIn many countries around the world, waterways prone to floods are often carefully managed. Defenses such asdetention basins,levees,[13]bunds,reservoirs, andweirsare used to prevent waterways from overflowing their banks. When these defences fail, emergency measures such assandbagsor portable inflatable tubes are often used to try and stem flooding. Coastal flooding has been addressed in portions ofEuropeand the Americas withcoastal defences, such assea walls,beach nourishment, andbarrier islands.In theriparian zonenear rivers and streams,erosion controlmeasures can be taken to try and slow down or reverse the natural forces that cause many waterways tomeanderover long periods of time. Flood controls, such asdams, can be built and maintained over time to try and reduce the occurrence and severity of floods as well. In theUSA, theU.S. Army Corps of Engineersmaintains a network of such flood control dams.In areas prone to urban flooding, one solution is the repair and expansion of man-made sewer systems and stormwater infrastructure. Another strategy is to reduce impervious surfaces in streets, parking lots and buildings through natural drainage channels,porous paving, andwetlands(collectively known asgreen infrastructureorsustainable 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 byfloodwallsandlevees, or designed to withstand temporary inundation. Property owners can also invest in solutions themselves, such as re-landscaping their property to take the flow of water away from their building and installingrain barrels,sump pumps, andcheck valves.BenefitsFloods (in particular more frequent or smaller floods) can also bring many benefits, such as rechargingground water, making soil morefertileand increasingnutrientsin some soils. Flood waters provide much needed water resources inaridandsemi-aridregions where precipitation can be very unevenly distributed throughout the year. Freshwater floods particularly play an important role in maintainingecosystemsin river corridors and are a key factor in maintainingfloodplainbiodiversity.[14]Flooding can spread nutrients to lakes and rivers, which can lead to increasedbiomassand improvedfisheriesfor a few years.For some fish species, an inundated floodplain may form a highly suitable location forspawningwith fewpredatorsand enhanced levels of nutrients or food.[15]Fish, such as theweather 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.[16]Periodic flooding was essential to the well-being of ancient communities along theTigris-EuphratesRivers, theNileRiver, theIndus River, theGangesand theYellow Riveramong others. The viability ofhydropower, a renewable source of energy, is also higher in flood prone regions.Deadliest floodsMain article:List of deadliest floodsBelow is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.Death tollEventLocationDate

2,500,0003,700,000[25]1931 China floodsChina1931

900,0002,000,0001887 Yellow River (Huang He) floodChina1887

500,000700,0001938 Yellow River (Huang He) floodChina1938

231,000Banqiao Damfailure, result ofTyphoon Nina. Approximately 86,000 people died from flooding and another 145,000 died during subsequent disease.China1975

230,000Indian Ocean tsunamiIndonesia2004

145,0001935 Yangtze river floodChina1935

100,000+St. Felix's Flood, storm surgeNetherlands1530

100,000HanoiandRed River DeltafloodNorth Vietnam1971

100,0001911 Yangtze river floodChina1911

In myth and religion

India Flood Prone Areas

The states falling within the periphery of "India Flood Prone Areas" areWest Bengal,Orissa,Andhra Pradesh, Kerala,Assam, Bihar, Gujrat,Uttar Pradesh,Haryanaand Punjab. The intense monsoon rains from southwest causes rivers like Brahmaputra, Ganga, Yamuna etc. to swell their banks, which in turn floods the adjacent areas.

Over the past few decades, central India has become familiar with precipitation events like torrential rains and flash floods. The major flood prone areas in India are the river banks and deltas of Ravi, Yamuna-Sahibi, Gandak, Sutlej, Ganga, Ghaggar, Kosi, Teesta, Brahmaputra, Mahanadi, Mahananda, Damodar, Godavari, Mayurakshi, Sabarmati and their tributaries.An over-view about state-wise flood prone areas can be gained by checking the following table

state-wise Flood Prone Areas

StateArea liable to Floods (million Ha.)

Uttar Pradesh7.336

Bihar4.26

Punjab3.7

Rajasthan3.26

Assam3.15

West Bengal2.65

Haryana2.35

Orissa1.4

Andhra Pradesh1.39

Gujarat1.39

Kerala0.87

Tamil Nadu0.45

Tripura0.33

Madhya Pradesh0.26

Himachal Pradesh0.23

Maharashtra0.23

Jammu & Kashmir0.08

Manipur0.08

Delhi0.05

Karnataka0.02

Meghalaya0.02

Pondichery0.01

Total33.516

Highest flood prone areas in India

Though the north-Indian plains prone to flood more, the "India flood prone areas" can be broadly categorized in three divisions: Ganga Basin: The Ganga Basin gets flooded mostly in the northern part by its northern tributaries. The badly affected states of the Ganga basin are West Bengal, Bihar and Uttar Pradesh.Besides the Ganga, rivers like Sarada, Rapti, Gandak and Ghagra causes flood in eastern part of Uttar Pradesh. The Yamuna is famous for flooding Haryana and Delhi. Bihar experiences massive dangerous flood every year. River Burhi, Bagmati, Gandak, Kamla along with many small rivers contribute to that. In West Bengal, rivers like Mahananda, Bhagirathi, Damodar, Ajay etc. causes floods because of tidal effects and insufficient river channels. Brahmaputra and Barak Basins: The river banks of Brahmaputra and Barak gets flooded due to the Surplus water found in the Brahmaputra basin and the Barak basin. These rivers along with their tributaries flood the northeastern states like West Bengal, Assam andSikkim. Jaldakha, Teesta and Torsa in northern West Bengal and rivers in Manipur often overflow their banks. Central India and Deccan Rivers Basin: In Orissa, spilling over of river banks by Mahanadi, Baitarni and Brahmani causes havoc. The deltaic area formed by these three rivers is thickly populated. Even some small rivers of Kerala and mud stream from the nearby hills add on to the destruction. Southern and central India observes floods caused by Narmada, Godavari, Tapi, Krishna and Mahanadi due to heavy rainfall. Cyclonic storms in the deltaic regions of Godavari, Mahanadi and Krishna even floods the coastal regions of Andhra Pradesh, Orissa and Tamil Nadu occasionally.Refer to the table given below to get an idea about the damage caused in the India flood prone areas:Average Annual Flood Damage (1953 - 1999)

StateArea liable to Floods(million Ha.)

Total DamageRs.13,400 million

Area Affected8.11 million hectare

Crop Area Affected3.57 million hectare

Human Lives Lost1579 Nos.

Cattle Lost95,000 Nos.

Flood Map for Planning (from Rivers and the Sea)Information to help you understand the flood map for planningKey to understanding the Flood Map for Planning (rivers and the sea)The map and associated information is intended for guidance, and cannot provide details for individual properties.1. FloodplainA floodplain is the area that would naturally be affected by flooding if a river rises above its banks, or high tides and stormy seas cause flooding in coastal areas.There are two different kinds of area shown on the Flood Map for Planning (rivers and the sea). They can be described as follows: Dark blueshows the area that could be affected by flooding, either from rivers or the sea, if there were no flood defences. This area could be flooded: from the sea by a flood that has a 0.5 per cent (1 in 200) or greater chance of happening each year; or from a river by a flood that has a 1 per cent (1 in 100) or greater chance of happening each year.(For planning and development purposes, this is the same as Flood Zone 3, in England only.) Light blueshows the additional extent of an extreme flood from rivers or the sea. These outlying areas are likely to be affected by a major flood, with up to a 0.1 per cent(1 in 1000) chance of occurring each year.(For planning and development purposes, this is the same as Flood Zone 2, in England only.)These two colours show the extent of the natural floodplain if there were no flood defences or certain other manmade structures and channel improvements. Where there is no blue shading, this shows the area where flooding from rivers and the sea is very unlikely.There is less than a 0.1 per cent (1 in 1000) chance of flooding occurring each year. The majority of England and Wales falls within this area. (For planning and development purposes, this is the same as Flood Zone 1, in England only.)2. Flood DefencesThepurple lineshows all flood defences built in the last five years to protect against river floods with a 1per cent(1 in 100) chance of happening each year, or floods from the sea with a 0.5 per cent(1 in 200) chance of happening each year, together with some, but not all, older defences and defences which protect against smaller floods. Flood defences that are not yet shown, and the areas that benefit from them, will be gradually added.Hatched areasbenefit from the flood defences shown, in the event of a river flood with a 1 per cent(1 in 100) chance of happening each year, or a flood from the sea with a 0.5 per cent(1 in 200) chance of happening each year. If the defences were not there, these areas would be flooded.Flood defences do not completely remove the chance of flooding, however, and can be overtopped or fail in extreme weather conditions.For information on flood defences which are not yet shown on the map, contact your local Environment Agency Office.3. Main RiversThe blue lineshows the main rivers, these are usually larger streams and rivers.Our powers to carry out flood defence work apply to main rivers only. In England, DEFRA decides which are main rivers. The Welsh Government does this in Wales.Risk of Flooding from Rivers and the Sea mapThis map shows our assessment of the likelihood of flooding from rivers and the sea at any location is based on the presence and effect of all flood defences, predicted flood levels, and ground levels.We describe the chance of flooding in one of four categories: High - greater than to equal to 1 in 30 (3.3%) chance in any given year Medium - less than 1 in 30 (3.3%) but greater than or equal to 1 in 100 (1%) chance in any given year Low - less than 1 in 100 (1%) but greater than or equal to 1 in 1,000 (0.1%) chance in any given year Very Low - less than 1 in 1,000 (0.1%) chance in any