Floods: Causes, Impacts and Types

FLOODS
FLOODS
Presented by
Presented by
CT.Lakshmanan B.Arch., M.C.P.
CT.Lakshmanan B.Arch., M.C.P.
SRM School of Architecture
SRM School of Architecture
SRM University
SRM University

prepared by CT.Lakshmanan
prepared by CT.Lakshmanan 2
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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 rate
that 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 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 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 Vulnerability Atlas of India shows
pictorially the areas liable to floods.
 Around 12 per cent (40 million hectare) of
land in India is prone to floods.

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Distributional Pattern of floods in India
Most of the flood affected areas lie in the
Ganga basin
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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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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Flood Hazard Map

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Causes of Floods
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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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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Impact on Built and Natural
Environments
Health-Related Effects
 In floods, deaths usually exceed injuries.
Surgical needs are low and are generally
only during the first 72 hours. Floods may
create conditions that promote secondary
threats of waterborne and vectorborne
diseases. A slight increase in deaths from
venomous snake bites has been reported
but not fully substantiated.

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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 as in the Bangladesh
example. 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.
 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.

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Impact on Development
 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.
 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 smallscale 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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Typical adverse effects
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,
diarrhoea, 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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Types of Floods
Types of Floods
Flash Floods
Flash Floods
Riverine floods
Storm Surge
Storm Surge

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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.
Discharges, quickly reach a 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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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. Whereas flash floods are of short duration in
small streams, 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. Flood flows in
large river 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 urbanization, etc.) directly affects
runoff.

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Storm surge
Storm surge
 Storm surge
Storm surge or
or tidal surge
tidal surge is an offshore rise of water associated
is an offshore rise of water associated
with a
with a low pressure
low pressure weather system, typically a
weather system, typically a tropical cyclone
tropical cyclone.
.
 Storm surge is caused primarily by high
Storm surge is caused primarily by high winds
winds pushing on the
pushing on the ocean
ocean's
's
surface.
surface.
 The wind causes the water to pile up higher than the ordinary
The wind causes the water to pile up higher than the ordinary sea level
sea level
. Low pressure at the center of a weather system also has a small
. Low pressure at the center of a weather system also has a small
secondary effect, as can the
secondary effect, as can the bathymetry
bathymetry of the body of water.
of the body of water.
 It is this combined effect of low pressure and persistent wind over a
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
shallow water body which is the most common cause of storm surge
flooding problems.
flooding problems.
 The term "storm surge" in casual (non-scientific) use is
The term "storm surge" in casual (non-scientific) use is storm tide
storm 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.
A bathymetric map or chart usually shows floor relief or
A bathymetric map or chart usually shows floor relief or terrain
terrain as
as contour lines
contour lines (called
(called depth contours
depth contours or
or isobaths
isobaths),
),

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Flood Characteristics
 Depth of water
 Duration
 Velocity
 Rate of rise
 Frequency of occurrence
 Seasonality

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Forecasting
 The ability to forecast flooding is limited to the time during
which changes in the hydrological conditions necessary for
flooding to occur have begun to develop.
 The formulation of a forecast for flood conditions requires
information on current hydrological conditions such as
precipitation, river stage, water equivalent 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 of
time above flood stage relatively short. In lower reaches of
large river systems where rates of rise and fall are slower,
it is important to forecast the time when various critical
stages of flow will be reached over the rise and fall.
Reliability of forecasts for large downstream river systems
is generally higher than for headwater systems.

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Warning
 Warning time for peak or over bank conditions can range from a few
minutes in cloudburst conditions to a few hours in small headwater
drainages to several days in the lower reaches of large river systems.
 As with forecasting, the time and reliability of the warning increase with
distance downstream where adequate knowledge of upstream
conditions exists. Clearly the amount of information required, the data
collection network necessary for collecting the information, the
technical expertise required for interpretation, and the communication
system needed to present timely information to potential victims are
services that many poor and developing nations find difficult to provide.
 The World Meteorological Organization of the United Nations, through
its World Weather Watch and Global Data Processing System, hopes
to coordinate efforts to improve forecasting. This is especially
important (and difficult) when conditions creating floods lie outside of
the national boundaries of the downstream region.

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Detection and warning
Detection and warning
 Flood detection systems can range from inexpensive
networks of volunteer rainfall and stream stage observers
and simple rule curves to sophisticated networks of
telemetered gauges and computer models. An example of
low technology flood detection is an informal system of
observers who use the existing infrastructure of telephones
to send progressive flood information downstream.
 Methods for warning the public should be well thought out,
documented, and practiced on an annual basis. Ways to
disseminate warnings include radio, television, warning
sirens and public address systems. Users of detection and
warning systems should be aware that all members of the
public will not respond in the desired manner to warnings.
An understanding of how and why people respond to
warnings is an essential ingredient in any warning system.

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Disaster Mitigation Strategies
 The majority of the deaths and much of the destruction created by
floods are largely preventable. A great deal can be done to lessen the
impact of a disaster. First, though, the general public as well as
engineers, planners, politicians and others need to understand the
 nature of the hazard. Based on that understanding, a decision and a
commitment needs to be made to provide mitigation measures to
reduce flood damage.
Reducing the harmful effects of a flood requires actions on
three fronts:
 reducing the vulnerability of the physical settlements and
structures in which people live;
 reducing the vulnerability of the economy; and
 strengthening the social structure of a community so that
community coping mechanisms can help absorb the impact
of a disaster and promote rapid recovery.

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Possible Risk Reduction Measures:
• Mapping of the flood prone areas
• Land use control
• Construction of engineered
structures
• Flood Control
Detention
Flood proof
channelisation
• Flood Management

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Mapping of the flood prone areas
 Historical records give the indication of the flood
inundation areas and the period of occurrence and
the extent of the coverage.
 Warning can be issued looking into the earlier
marked heights of the water levels in case of
potential threat.
 In the coastal areas the tide levels and the land
characteristics will determine the submergence
areas.
 Flood hazard mapping will give the proper
indication of water flow during floods.

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Land use control
 The number of casualties is related to the
population in the area at risk.
 In areas where people already have built their
settlements, measures should be taken to relocate
to better sites so as to reduce vulnerability.
 No major development should be permitted in the
areas which are subjected to high flooding.
Important facilities like hospitals, schools should
be built in safe areas.
 In urban areas, water holding areas can be
created like ponds, lakes or low-lying areas.

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Construction of engineered
structures
Construction of engineered structures in the
flood plains and strengthening of structures to
withstand flood forces and seepage.
The buildings should be constructed on an
elevated area. If necessary build on stilts or
platform.

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Flood Control
 Flood Control aims to reduce flood
damage. 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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Detention facilties
Detention facilties
 Detention facilities, such as dams, store flood waters and
release them at lower rates, thus reducing or eliminating
the need for major downstream flood control facilities, the
construction of which would disrupt the developed areas.
 Perhaps the greatest disadvantage of detention facilities,
assuming a structurally sound facility, is the false sense of
security that such structures create among the general
public. These facilities are almost never designed to
contain the probable maximum flood. Thus they require
a spillway to pass discharges in excess of the design flood.

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Floodproofing
 Retrofit 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.
 Active floodproofing is temporary, requiring some positive action on
the part of building owners and/or occupants immediately before a
flood event. It will not be effective if personnel are not available to
perform the necessary functions when the flood occurs. Active
floodproofing requires some type of flood detection and warning
system to give time for the personnel to install the floodproofing
devices. In flash flood situations the personnel may not be available to
respond in time.
 Passive floodproofing is permanent and does not require any action
at the time of the flood. Active floodproofing is most effective in areas
with long warning lead times; it should not be relied upon, if possible, in
flash flood areas.
 Flood Proofing reduces the risk of damage. Measures include use of
sand bags to keep flood water away, blocking or sealing of doors and
windows of houses etc. Houses may be elevated by building on raised
land.

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Channelization
 The construction of open channels is a commonly
used method of reducing the size 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. Open channels allow water to
 enter them at almost any point, thus compensating
for inadequate tributary collection systems.

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Flood Management
 In India, systematic planning for flood management commenced with
the Five Year Plans, particularly with the launching of National
Programme of Flood Management in 1954.
 During the last 48 years, different methods of flood protection structural
as well as nonstructural have been adopted in different states
depending upon the nature of the problem and local conditions.
 Structural measures include storage reservoirs, flood embankments,
drainage channels, anti erosion works, channel improvement works,
detention basins etc. and non-structural measures include flood
forecasting, flood plain zoning, flood proofing, disaster preparedness
etc.
 The flood management measures undertaken so far have provided
reasonable degree of protection to an area of 15.81 million hectares
through out the country.

Permanent Flood wall
Temporary Flood wall
Embankment

Flood gate
Bypass drain
Storage basin

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