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FLOODSFLOODS

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What is Flood ?What is Flood ?

Flood is a state of high water level along a river

channel or on the coast that leads to inundation

of land, which is not usually submerged.

Floods can form where there is no stream, as

for example when abnormally heavy

precipitation falls on flat terrain at such a ratethat the soil cannot absorb the water or the

water cannot run off as fast as it falls.

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Can floods be manmade !Can floods be manmade !

Floods are caused not only by rain but also byhuman changes to the surface of the earth.Farming, deforestation, and urbani"ation increase

the runoff from rains# thus storms that previouslywould have caused no flooding today inundatevast areas.

$ot only do we contribute to the causes of floods,

but reckless building in vulnerable areas, poorwatershed management, and failure to control theflooding also help create the disaster condition.

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Distributional Pattern of floods in India

Floods occur in almost all the river basins of

the country.

The &ulnerability 'tlas of (ndia showspictorially the areas liable to floods.

'round )2 per cent *%+ million hectare of

land in (ndia is prone to floods.

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Distributional Pattern of floods in India

ost of the flood affected areas lie in the

Ganga basin

Brahmaputra basin *comprising of /arak, Tista, Torsa,

0ubansiri, 0ankosh, 1ihang and Luhit

northwestern river basin *comprising helum, Chenab,3avi, 0utle4, /eas and the 5hagra

peninsular river basin *Tapti, $armada, ahanadi,

/aitarani, 5odavari, krishna, 6ennar and the 7averi

coastal regions of 'ndhra 6radesh, Tamilnadu, orissa and

7erela.

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Distributional Pattern of floods in

India

'ssam, 9ttar 6radesh, /ihar and :rissa are some

of the states who have been severely prone to

floods.

:ur country receives an annual rainfall of )2++mm, ;-< of which is concentrated in =% months

i.e une to 0eptember.

1ue to the intense and periodic rain, most of therivers of the country are fed with huge >uantity of

water, much beyond their carrying capacity.

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Flood a!ard "ap

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#auses of Floods#auses of Floods

0ome of the ma4or causes are@♦ Aeavy rainfall

♦ Aeavy siltation of the river bed reduces the water

carrying capacity of the riversBstream.

♦ /lockage in the drains lead to flooding of the area.

♦ Landslides blocking the flow of the stream.

♦ Construction of dams and reservoirs

♦ (n areas prone to cyclone, strong winds accompanied by

heavy down pour along with storm surge leads to

flooding

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Impa$t on Built and %atural

&nvironmentsFlooding can have any of the following effects on housing or other small buildings.

ouses washed awa' due to the impact of the water under high streamvelocity. The houses are commonly destroyed or dislocated so severly that theirreconstruction is not feasible.

Flotation of houses caused by rising waters. This occurs when light=weight,typically wood houses are not securely anchored. They can be removed too farfrom their foundations for relocation and repair.

Damage $aused b' inundation of house. The house may remain intact andon its foundation, but the water damage to materials may be severe. 3epair isoften feasible but may re>uire special procedures to dry out properly.

(nder$utting of house. The velocity of the water may scour and erode thehouseDs foundation or the earth under the foundation. This may result in thecollapse of the house or re>uire substantial repair.

Damage $aused b' debris. assive floating ob4ects such as trees and other

houses may impact on standing houses and cause significant damage.

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Impa$t on Built and %atural

&nvironments

Health-Related Effects

(n floods, deaths usually exceed in4uries.

0urgical needs are low and are generally

only during the first ?2 hours. Floods maycreate conditions that promote secondary

threats of waterborne and vectorborne

diseases. ' slight increase in deaths fromvenomous snake bites has been reported

but not fully substantiated.

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Impa$t on )gri$ulture (n most flood prone countries where economies are based on agriculture, the

largest economic flood=related losses are in the agricultural sector. Even in theindustriali"ed 9nited 0tates %; percent of flood losses in )?- were in agriculture.

:bviously most losses to agriculture result from the drowning of crops.0usceptibility to drowning depends on the type of crop and duration of flooding.0ome are >uickly killed by a relatively small amount of superfluous water. :therscan resist as much as a few days of submersion. Even crops that thrive on largeamounts of standing water will be killed if the water stagnates as in the /angladeshexample. :ther agricultural losses occur in the submersion of crop storage

facilities. 5rains and other crops will >uickly spoil if saturated with water, even for ashort time.

'n additional negative impact on the agricultural sector is the erosion of topsoil bythe floods. Aere the impact is indeed long term, resulting in the reducedproductivity of the land and possibly eventual abandonment.

Flooding, however, is not all bad. For some agricultural areas flooding is a positiveand necessary event. These lands depend on the periodic silt deposits for addednutrients to the soil. Flooding also serves other advantages including the filtering ordilution of pollutants that enter the waterways, flushing of nutrients in river systems,preserving of wetlands, recharging of groundwater, and maintaining of riverecosystems by providing breeding, nesting, feeding and nursery areas for fish,shell fish, migrating waterfowl, and others.

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Impa$t on Development

idespread floods can have a significant effect on the long=termeconomic growth of the affected region. (ndirect and secondary effectson the local and national economy may include reduction in familyincome, decline in the production of business and industrial enterprises,inflation, unemployment, increase in income disparities, and decline innational income.

(n addition, relief and reconstruction efforts often compete withdevelopment programs for available funds. (n countries where floodingoccurs fre>uently, floods can create an enormous financial burden.

The loss of crops and the need to find alternate sources of income haveoften caused smallscale migrations of farmers and skilled workers fromrural areas to cities. :nce established in a city, few return to their

homes or farms. 0mall marginal farms usually cannot survive economically following a

ma4or flood. Farmers are often forced to sell their land because theycannot afford to rehabilitate it. This may result in a substantial increasein the number of people migrating to urban areas, and thus a relatedhousing shortage.

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Typical adverse effectsTypical adverse effects

The most important conse>uence of floods is the loss of life and property.0tructures like houses, bridges# roads etc. get damaged by the gushingwater. /oats and fishing nets also get damaged. There is huge loss to lifeand livestock caused by drowning.

Lack of proper drinking water facilities, contamination of water *well,ground water, piped water supply leads to outbreak of epidemics,diarrhoea, viral infection, malaria and many other infectious diseases.

Flooding also leads to a large area of agricultural land getting inundatedas a 3esult there is a huge crop loss. This results in shortage of food,

and animal fodder.

Floods may also affect the soil characteristics. The land may be renderedinfertile due to erosion of top layer or may turn saline if sea water floodsthe area.

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*'pes of Floods*'pes of Floods

Flash FloodsFlash Floods+iverine floods

Storm SurgeStorm Surge

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Flash floods Flash floods are local floods of great volume and short

duration. ' flash flood generally results from a torrential rain or

GcloudburstH on relatively small and widely=dispersed streams. 3unoff from the intense rainfall results in high flood waves.

1ischarges, >uickly reach a maximum and diminish almost as

rapidly. Flood flows fre>uently contain large concentrations of

sediment and debris. Flash floods also result from the failure of a dam. Flash floods are particularly common in mountainous areas

and desert regions but are a potential threat in any area wherethe terrain is steep, surface runoff rates are high, streams flowin narrow canyons, and severe thunderstorms prevail.

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3iverine floods

3iverine floods are caused by precipitation over large areas orby melting of the winterDs accumulation of snow, or by both.

These floods differ from flash floods in their extent andduration. hereas flash floods are of short duration in small

streams, riverine floods take place in river systems whosetributaries may drain large geographic areas and encompassmany independent river basins

Floods on large river systems may continue for periodsranging from a few hours to many days. Flood flows in largeriver systems are the distribution of precipitation.

The condition of the ground *amount of soil moisture,seasonal variations in vegetation, depth of snow cover,imperviousness due to urbani"ation, etc. directly affectsrunoff.

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0torm surge0torm surge

Storm surgeStorm surge oror tidal surgetidal surge is an offshore rise of water associated withis an offshore rise of water associated witha low pressure weather system, typically a tropical cyclone.a low pressure weather system, typically a tropical cyclone. 0torm surge is caused primarily by high winds pushing on the oceanIs0torm surge is caused primarily by high winds pushing on the oceanIs

surface.surface. The wind causes the water to pile up higher than the ordinary seaThe wind causes the water to pile up higher than the ordinary sea

level. Low pressure at the center of a weather system also has a smalllevel. Low pressure at the center of a weather system also has a small

secondary effect, as can the bathymetry of the body of water.secondary effect, as can the bathymetry of the body of water. (t is this combined effect of low pressure and persistent wind over a(t is this combined effect of low pressure and persistent wind over ashallow water body which is the most common cause of storm surgeshallow water body which is the most common cause of storm surgeflooding problems.flooding problems.

The term Jstorm surgeJ in casual *non=scientific use isThe term Jstorm surgeJ in casual *non=scientific use is storm tidestorm tide##that is, it refers to the rise of water associated with the storm, plus tide,that is, it refers to the rise of water associated with the storm, plus tide,wave run=up, and freshwater flooding.wave run=up, and freshwater flooding.

' bathymetric map or chart usually shows floor relief or terrain as contour lines *called ' bathymetric map or chart usually shows floor relief or terrain as contour lines *called depth $ontoursdepth $ontours oror isobathsisobaths,,

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Flood #hara$teristi$s

1epth of water

1uration

&elocity

3ate of rise

Fre>uency of occurrence

0easonality

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Fore$asting

The ability to forecast flooding is limited to the time duringwhich changes in the hydrological conditions necessary forflooding to occur have begun to develop.

The formulation of a forecast for flood conditions re>uiresinformation on current hydrological conditions such as

precipitation, river stage, water e>uivalent of snowpack,temperature, soil conditions over the entire drainage basin,as well as weather reports and forecasts.

The relatively rapid rate of rise and fall makes the period oftime above flood stage relatively short. (n lower reaches oflarge river systems where rates of rise and fall are slower,it is important to forecast the time when various criticalstages of flow will be reached over the rise and fall.3eliability of forecasts for large downstream river systemsis generally higher than for headwater systems.

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Warning

arning time for peak or over bank conditions can range from a fewminutes in cloudburst conditions to a few hours in small headwaterdrainages to several days in the lower reaches of large river systems.

's with forecasting, the time and reliability of the warning increase withdistance downstream where ade>uate knowledge of upstream

conditions exists. Clearly the amount of information re>uired, the datacollection network necessary for collecting the information, thetechnical expertise re>uired for interpretation, and the communicationsystem needed to present timely information to potential victims areservices that many poor and developing nations find difficult to provide.

The orld eteorological :rgani"ation of the 9nited $ations, through

its orld eather atch and 5lobal 1ata 6rocessing 0ystem, hopesto coordinate efforts to improve forecasting. This is especially important*and difficult when conditions creating floods lie outside of the nationalboundaries of the downstream region.

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1etection and warning1etection and warning

Flood detection systems can range from inexpensive

networks of volunteer rainfall and stream stage observersand simple rule curves to sophisticated networks oftelemetered gauges and computer models. 'n example oflow technology flood detection is an informal system ofobservers who use the existing infrastructure of telephones

to send progressive flood information downstream.

ethods for warning the public should be well thought out,documented, and practiced on an annual basis. ays todisseminate warnings include radio, television, warning

sirens and public address systems. 9sers of detection andwarning systems should be aware that all members of thepublic will not respond in the desired manner to warnings.

'n understanding of how and why people respond towarnings is an essential ingredient in any warning system.

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Possible +is, +edu$tion "easures-

K "apping of the flood prone areasK Land use $ontrolK #onstru$tion of engineered

stru$turesK Flood #ontrol

Detention

Flood proof

$hannelisationK Flood "anagement

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"apping of the flood prone areas

Aistorical records give the indication of the floodinundation areas and the period of occurrence andthe extent of the coverage.

arning can be issued looking into the earliermarked heights of the water levels in case of

potential threat. (n the coastal areas the tide levels and the land

characteristics will determine the submergenceareas.

Flood ha"ard mapping will give the properindication of water flow during floods.

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Land use $ontrol

The number of casualties is related to thepopulation in the area at risk.

(n areas where people already have built their

settlements, measures should be taken to relocateto better sites so as to reduce vulnerability. $o ma4or development should be permitted in the

areas which are sub4ected to high flooding.(mportant facilities like hospitals, schools should

be built in safe areas. (n urban areas, water holding areas can be

created like ponds, lakes or low=lying areas.

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Flood #ontrol

Flood #ontrol aims to reduce flooddamage. This can be done by decreasing

the amount of runoff with the help of

reforestation, protection of vegetation,clearing of debris from streams and other

water holding areas, conservation of ponds

and lakes etc.

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1etention facilties1etention facilties

1etention facilities, such as dams, store flood waters andrelease them at lower rates, thus reducing or eliminatingthe need for ma4or downstream flood control facilities, theconstruction of which would disrupt the developed areas.

6erhaps the greatest disadvantage of detention facilities,assuming a structurally sound facility, is the false sense ofsecurity that such structures create among the generalpublic. These facilities are almost never designed tocontain the probable maximum flood. Thus they re>uire

a spillway to pass discharges in excess of the design flood.

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Floodproofing 3etrofit floodproofing of existing buildings is sometimes a viable solution,

depending upon the location of the structure within the floodplain and its

structural integrity.

There are two basic types of floodproofing@ active and passive. )$tive floodproofing is temporary, re>uiring some positive action on the

part of building owners andBor occupants immediately before a floodevent. (t will not be effective if personnel are not available to perform the

necessary functions when the flood occurs. 'ctive floodproofing re>uiressome type of flood detection and warning system to give time for thepersonnel to install the floodproofing devices. (n flash flood situations thepersonnel may not be available to respond in time.

Passive floodproofing is permanent and does not re>uire any action atthe time of the flood. 'ctive floodproofing is most effective in areas withlong warning lead times# it should not be relied upon, if possible, in flashflood areas.

Flood 6roofing reduces the risk of damage. easures include use ofsand bags to keep flood water away, blocking or sealing of doors andwindows of houses etc. Aouses may be elevated by building on raisedland.

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Channeli"ation

The construction of open channels is a commonly

used method of reducing the si"e of a

floodplain or floodway. To prevent erosion,

channels can be lined with grass, wire=enclosed rock, concrete, riprap or cobblestones placed a

few layers deep. :pen channels allow water to

enter them at almost any point, thus compensatingfor inade>uate tributary collection systems.

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Flood "anagement

(n (ndia, systematic planning for flood management commenced withthe Five ear 6lans, particularly with the launching of $ational6rogramme of Flood anagement in )-%.

1uring the last %; years, different methods of flood protection structuralas well as nonstructural have been adopted in different states

depending upon the nature of the problem and local conditions.

0tructural measures include storage reservoirs, flood embankments,drainage channels, anti erosion works, channel improvement works,detention basins etc. and non=structural measures include floodforecasting, flood plain "oning, flood proofing, disaster preparedness

etc.

The flood management measures undertaken so far have providedreasonable degree of protection to an area of )-.;) million hectaresthrough out the country.

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