DM

1. FLOODS

2. 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 rate that the soil
cannot absorb the water or the water cannot run
off as fast as it falls.
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3. Can floods be manmade ?
 Floods are caused not only by rain but also by
human changes to the surface of the earth.
Farming, deforestation, and urbanization increase
the runoff from rains; thus storms that previously
would have caused no flooding today inundate vast
areas.
 Not only do we contribute to the causes of floods,
but reckless building in vulnerable areas, poor
watershed management, and failure to control the
flooding also create the disaster condition
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4. Distributional Pattern of floods in India
 Floods occur in almost all the river basins of the country.
 Around 12 per cent (40 million hectare) of land in India is
prone to floods.
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5. Distributional Pattern of floods in India
Most of the flood affected areas lie in the
Ganga basin (Ganga and Yamuna)
Brahmaputra basin (comprising of Barak, Tista, Torsa, Subansiri,
Sankosh, Dihang and Luhit)
Northwestern river basin (comprising Jhelum, Chenab, Ravi, Sutlej,
Beas and the Ghagra)
Peninsular river basin (Tapti, Narmada, Mahanadi, Baitarani,
Godavari, Krishna, Pennar and the Kaveri) coastal regions of
Andhra Pradesh, Tamilnadu, orissa and Kerela.
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7. Distributional Pattern of floods in India
 Assam, Uttar Pradesh, Bihar and Orissa are some of
the states who have been severely prone to floods.
 Our country receives an annual rainfall of 1200 mm,
85% of which is concentrated in 3-4 months i.e June
to September.
 Due to the intense and periodic rain, most of the
rivers of the country are fed with huge quantity of
water, much beyond their carrying capacity.
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9. Types of Floods
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1. Flash Floods1. Flash Floods
2. Riverine floods
3. Storm Surge3. Storm Surge

10. Flash floods
 Flash floods are local floods of great volume and short
duration.
 A flash flood generally results from a torrential rain or
“cloudburst” on relatively small and widely-dispersed
streams.
 Runoff from the intense rainfall results in high flood waves
quickly reach the maximum and diminish almost as rapidly.
 Flood flows frequently 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
where the terrain is steep, surface runoff rates are high,
streams flow in narrow canyons, and severe thunderstorms
prevail.
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11. Riverine floods
 Riverine floods are caused by precipitation over large
areas or by melting of the winter’s accumulation of
snow, or by both.
 These floods differ from flash floods in their extent
and duration. Flash floods are of short duration in
small streams whereas riverine floods take place in
river systems whose tributaries may drain large
geographic areas and encompass many independent
river basins
 Floods on large river systems may continue for periods
ranging from a few hours to many days.
 The condition of the ground (amount of soil moisture,
seasonal variations in vegetation, depth of snow cover,
imperviousness due to urbanization, etc.) directly affects
runoff. 11

12. Storm surge
 Storm surge or tidal surge is an offshore rise of water
associated with a low pressure weather system, typically a
tropical cyclone.
 Storm surge is caused primarily by high winds pushing on
the ocean's surface.
 The wind causes the water to pile up higher than the
ordinary sea level.
 It is this combined effect of low pressure and persistent
wind over a shallow water body which is the most common
cause of storm surge flooding problems.
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13. Causes of Floods
Some of the major causes are:
♦ Heavy rainfall
♦ Heavy siltation of the river bed reduces the water
carrying capacity of the rivers/stream.
♦ Blockage in the drains lead to flooding of the area.
♦ Landslides blocking the flow of the stream.
♦ Construction of dams and reservoirs
♦ In areas prone to cyclone, strong winds accompanied by
heavy down pour along with storm surge leads to
flooding
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15. Impact on Built and Natural Environments
Flooding can have any of the following effects on housing or other
small buildings.
 Houses washed away due to the impact of the water under high
stream velocity. The houses are commonly destroyed or dislocated
so severly that their reconstruction 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 far from their foundations for relocation
and repair.
 Damage caused by inundation of house. The house may remain
intact and on its foundation, but the water damage to materials may
be severe. Repair is often feasible but may require special
procedures to dry out properly.
 Undercutting of house. The velocity of the water may scour and
erode the house’s foundation or the earth under the foundation. This
may result in the collapse of the house or require substantial repair.
 Damage caused by debris. Massive floating objects such as trees
and other houses may impact on standing houses and cause
significant damage.
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16. Impact on Built and Natural Environments
Health-Related Effects
 Floods may create conditions that promote secondary threats of waterborne
and vectorborne diseases.
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17. Impact on Agriculture
 In most flood prone countries where economies are based on
agriculture, the largest economic flood-related losses are in the
agricultural sector. Even in the industrialized United States 48 percent
of flood losses in 1975 were in agriculture.
 Obviously most losses to agriculture result from the drowning of
crops. Susceptibility to drowning depends on the type of crop and
duration of flooding. Some are quickly killed by a relatively small
amount of superfluous water.
 Others can resist as much as a few days of submersion. Even crops
that thrive on large amounts of standing water will be killed if the
water stagnates.
 Other agricultural losses occur in the submersion of crop storage
facilities. Grains and other crops will quickly spoil if saturated with
water, even for a short time.
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18. Impact on Development
 An additional negative impact on the agricultural sector is the erosion of topsoil
by the floods. Here the impact is indeed long term, resulting in the reduced
productivity of the land and possibly eventual abandonment.
 Flooding, however, is not all bad. For some agricultural areas flooding is a
positive and necessary event. These lands depend on the periodic silt deposits
for added nutrients to the soil. Flooding also serves other advantages including
the filtering or dilution of pollutants that enter the waterways, flushing of
nutrients in river systems, preserving of wetlands, recharging of groundwater,
and maintaining of river ecosystems by providing breeding, nesting, feeding and
nursery areas for fish, shell fish, migrating waterfowl, and others.
 Widespread floods can have a significant effect on the long-term economic
growth of the affected region. Indirect and secondary effects on the local and
national economy may include reduction in family income, decline in the
production of business and industrial enterprises, inflation, unemployment,
increase in income disparities, and decline in national income.
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19.  In addition, relief and reconstruction efforts often compete
with development programs for available funds. In
countries where flooding occurs frequently, floods can
create an enormous financial burden.
 The loss of crops and the need to find alternate sources of
income have often caused small-scale migrations of
farmers and skilled workers from rural areas to cities. Once
established in a city, few return to their homes or farms.
 Small marginal farms usually cannot survive economically
following a major flood. Farmers are often forced to sell
their land because they cannot afford to rehabilitate it. This
may result in a substantial increase in the number of people
migrating to urban areas, and thus a related housing
shortage.
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20. Typical adverse effects
 The most important consequence of floods is the loss of life and
property. Structures like houses, bridges; roads etc. get damaged by
the gushing water. Boats and fishing nets also get damaged. There is
huge loss to life and livestock caused by drowning.
 Lack of proper drinking water facilities, contamination of water (well,
ground water, piped water supply) leads to outbreak of epidemics,
diarrhea, viral infection, malaria and many other infectious diseases.
 Flooding also leads to a large area of agricultural land getting
inundated as a Result 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
rendered infertile due to erosion of top layer or may turn saline if sea
water floods the area.
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21. ‘Komen’ Flood (2015) Effect in W.Bengal, Bangladesh and NE States of India