Floods have the greatest damage potential when compared to the other natural disasters, over the environment. Floods are also considered to be both social and economic disasters. This module highlights the details of floods as natural hazards.

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FLOODS AS NATURAL HAZARDS
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth
Science
University of Mysore
India

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Introduction:
Floods have the greatest damage potential
when compared to the other natural
disasters, over the environment.
Floods are also considered to be both social
and economic disasters.

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In this episode, the following aspects of
floods are discussed:
1. Floods as Natural Disasters
2. Causes of Floods
3. Analysis and Types of Floods
4. Environmental Impacts of Floods and the
5. Methods of controlling Floods.

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1. Floods as Natural Disasters
A natural disaster is a major physical event
occurring on the environment causing a
severe impact over the biotic and abiotic
factors.
Some of these events may happen suddenly
or over a period of time.

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The notable natural disasters of the Earth are
Earthquakes,
Tsunamis,
Floods,
Cyclones,
Tornadoes,
Hurricanes,
Storms and
Volcanic eruptions.

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In addition to these,
Landslides,
Forest Fires,
droughts and
desertification
are also causing more devastating effects
over the environment.
All of these are global phenomena and no
part of the world is free from any of these
natural hazards.

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The intensity and impact of a hazard may
vary considerably from place to place.
The deaths, material loss and environmental
damage will severely affect the economy of
a country whether it is developed or
developing.

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For a developing or an under developed
nation, it will have a long-term impact over
its growth.
Every ecosystem will undergo an
unexpected modification with reference to
its biotic and abiotic conditions, after a
disaster including floods.
Earth quakes, Tsunamis and volcanoes are
lithosphere induced hazards.

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Floods, cyclones, Hurricanes and storms
are induced mainly by the meteorological
factors of the atmosphere.
Whether they are induced by one or more of
the earth’s environmental segments, their
effects will be in almost all the spheres of
the globe.

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A flood is a body of water overflowing the
river channels.
Floods are associated with some extreme
natural events that happen over a drainage
basin.
The drainage basin may comprise rural or
urban populations. Flooding in rural sector
is mainly due to rivers.
Floods in urban lands may be due to storm-
water drainage. Runoff is that portion of
precipitation which reaches the streams.

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When runoff joins a stream, it becomes a
steam flow. It is the total outflow of a
catchment or a watershed.
Floods are defined as relatively high flows
which overtakes the natural channels
provided for the runoff. The level at which
water flows along streams is called as the
river stage.

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During flood, the stage of a river is high.
The river water during a flood usually
overflows its banks.
Floodway is the channel of a water course
and those portions of adjoining floodplain
which are reasonably required to carry
and discharge the floodwaters.
A river is a mechanical, ecological and
geotechnical system.

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It offers a good habitat to many living
organisms.
The interaction between the life and the
fluvial corridor determines the strength of
these ecosystems.
As a river water is a carrier of nutrients,
every life prefers staying nearer to it.

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A river is characterized by :
a) a catchment area called as a basin
b) a network of streams and drainages
c) its channel morphology- which is
constituted by its length, depth and width
d) extend of banks and flood plains
e) Discharge of water represented by
volume, duration and velocity of flow
f) Movement of suspended and bed-load
sediments and pollutants.

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The rain water which is collected by the
Catchment zone of a river basin is drained
through its network of streams.
Cumulative flow of water derived from all
tributaries of a river, beyond its channel
capacity, will cause the situation of flooding.
Most of the time, river stage is below the
channel banks. At the time of higher
discharge, water overflowing the banks
cause the flood.

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At bankfull stage, water fills the channel
upto the top of the banks and that is the
channel capacity beyond which the river
starts flooding.
Wider river channels are formed when there
is enormous outflow of water over a flat
region.

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Over a period of time, the running water, as
a geological agent, develops canyons,
gorges, water falls, meanders, flood plains
and deltas.
The successive river terraces of the Grand
canyon of Colarado is a typical example of a
widened river course of the olden days.
A flood plain of a river is a clearly definable
physical feature of its valley. It is the flat
area bordering a river.

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Since it contains mainly alluvium, it is also
called as alluvial plain.
A Flood plain is built up of layers of
sediments deposited by the river when it
periodically overflows its normal banks.
Since they are flat, near-water and desirable
places to live, people have long been
attracted by the floodplains for settlement.

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As they are very fertile lands, farmers love
to cultivate crops along floodplains.
The world's largest alluvial plain is the
region of the valleys of the Brahmaputra,
Ganges (Ganga), and Indus rivers in India
and Bangladesh.
Large and thickly populated cities, industrial
establishments and capitals are located
along river courses.

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When there is severe flood, all these human
activities along it path are destroyed.
A flood is characterized by its volume,
duration and velocity of water flowing
along these courses.
Several things happen to a river when it
undergoes flooding.

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Most of the work performed by streams
takes place during floods.
As the flood subsides, the river drops its
sediment load and the channel aggrades
back to its previous level.
The obstructions to flow in a stream channel
cause local pockets of turbulence leading to
rapid downward Scour.

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Such obstructions include boulders and
bridge tiers, which can be completely
undermined by the scour during a flood.
2. Causes of Floods
Flooding of rivers come due to both
meteorological and physiographic factors.

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The major climatic factors are:
a) Form, intensity and duration of
Precipitation
b) Presence of a large catchment zone
c) Direction of a storm movement
d) Antecedent Precipitation Index
e) High Humidity and low temperature
f) Variation of atmospheric pressure

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The notable physiographic factors are:
a) Type of soil
b) Landuse and land cover
c) Area and Shape of the basin
d) Slope of the catchment
e) Natural and artificial drainage
f) Storage facilities of surface water
Flood is mainly caused by the high intensity
short duration rainfall.

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Even low intensity rainfall for a longer
period also may induce extended flows and
flooding.
If the catchment zone is large, definitely the
outflow will be more causing floods.
A very unique factor causing floods is the
direction of movement of the storm. If the
storm moves in the downstream direction,
then the magnitude of river discharge is
aggravated by the storm.

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Antecedent Precipitation Index (API) is a
measure of the soil moisture condition
existing before the occurrence of the storm.
The runoff will be more, if the API is high
and the soil is quite wet.
Any variation in the atmospheric pressure is
responsible for the movement of storms.
High humidity and low temperature reduces
the rate of evapotranspiration of the
environment.

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When the rate of rainfall is more than the
rate of infiltration of the soil, overland flow
and runoff will be more.
Infiltration is controlled by the texture and
type of soil. Infiltration rates of sandy and
alluvial soils will be more than silt and
clayey soils.
An area covered by fine grained and
compacted soils like the clayey loam will
have more surface runoff than infiltration.

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Coarse grained and loose soils promote
more infiltration than runoff.
The landuse and Landcover of an area also
control the runoff. Built environment and
barren land always encourage overland
flow. Human changes of land by farming,
deforestation and urbanization all try to
increase the runoff from rains.

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Improper landuse and channelisation of
natural water ways and blockage of drains
by structures are some of the contributing
factors.
If the shape of the basin is oval and large,
more water will be collected by the
catchment and will lead to floods in rivers.
Lengthy and elongated basins may show
flooding at the downstream ends.

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If the basin has adequate network of natural
or artificial drainage systems, runoff will be
highly dissipated to the points of confluence.
A steep slope always encourages more
overland flow.
Natural depressions, trenches, pools, lakes
and ponds can hold much amount of water
and control floods.
Open ditches, tile drains, storm sewers and
paved surfaces can encourage runoff.

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When all other factors remain static in a big
basin, floods are mainly caused by heavy
rainfall.
The maximum flood discharge usually
occurs over a relatively short period when
the intensity of rainfall is high and the
meteorological and other conditions are
favourable for producing maximum
discharge.

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3. Analysis and Types of Floods
Floods often cause the channel a stream to
straighten and change its position within the
flood plain.
Over a long period of time, changes in base
level, annual discharge, or bed-load can
cause a stream to erode rapidly downward to
a new level that put the old flood plain out
of reach of flooding.

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A new floodplain will form leaving the old
floodplain standing above as a terrace. It
must be emphasized that flooding is a
normal part of the long range dynamics of a
river and that the floodplain is a feature that
is constructed and maintained by flooding.
Flooding may be an annual event to some
streams. The recurrence of floods is directly
related to the occurrence of heavy rainfall.

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A Flood risk is the chance of experiencing
flood, expressed in terms of the return
period.
Flood discharge is usually expressed in
cu.m/s.
Floods differ in their types based on their
occurrence.

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The major types of floods are:
a) Flash Floods
b) Single event floods
c) Multiple event floods
d) Seasonal floods
e) Coastal floods
f) Estuarine floods
g) Floods caused by Dam failures
h) Floods due to sudden melting of snow and
glacier.

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Flash floods are frequently associated with
violent, convectional storms of short
duration. They are floods of great volume
and short duration. These results from
cloudburst on relatively small and widely-
dispersed streams.
Heavy rainfall or slow moving
thunderstorms may create flash floods.

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Single event flood is the most common type
of flooding. It is caused due to widespread
heavy rains of longer duration of 2 to 3 days.
These are associated with cyclonic
disturbances like storms and depressions.
Multiple event floods are caused due to
successive weather disturbances. Floods in
Indo-Gangetic plains are of this type.

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Seasonal floods are floods occurring during
different seasons. Floods in some parts come
during summer monsoons and in some other
parts during winter monsoon.
The southern half of the Indian Peninsula
experiences floods mostly during winter
seasons.

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Coastal floods come due to the action of
high tides and waves. Storm surges can
create extensive floods along a coast.
Hurricanes and tropical storms can produce
heavy rains or derive ocean water onto the
land. Beaches and coastal houses can be
swept away by the water.
Coastal flooding can also be produced by
killer sea waves called tsunamis.

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Failure of dams and check dams,
embankments and earthen bunds in village
tanks will also create a flood.
Sudden melting of snow and glaciers will
induce severe flooding along the river
courses of a mountainous catchment.
Floods are also classified as Urban floods
and Rural floods. Flood is relatively a
random phenomena.

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It is therefore difficult to predict the exact
maximum flood which can occur in future.
The estimation of flood is also required for
its relief works. It can be estimated only
with a specified probability.
The following are the criteria of recognizing
a flood and its characteristics:
a) Depth of water
b) Duration of rain
c) Velocity of flow
d) Rate of rise in river level and discharge

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e) Frequency of occurrence
f) Seasonality.
The estimation of maximum flood is another
aspect of its analysis. It involves
a) analysis of past flood records correlating
with the basin yield
b) Empirical methods using appropriate
equations
c) Preparation of unit hydrographs
d) Flood frequency charts

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4. Environmental Impacts of Floods
Floods are natural behavior of streams. Most
floods are harmful. They may destroy
homes and other property and even carry off
the topsoil, leaving the land barren.
When people are not prepared, sudden and
violent floods may bring huge losses.
Rivers, lakes, or seas may flood the land.

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River floods are more common, though lake
and seacoast floods can be more serious.
Large boulders in river beds are moved
during floods, and human structures such as
bridge pilings and concrete barriers can be
swept away by the rushing floodwaters.
The water of flash floods move very fast.
They have the power to move boulders, tear
out trees, destroy buildings and obliterate
bridges. Huge amounts of debris are carried
away by these waters.

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The environmental impacts of flood are of
many kinds.
The effects of floods may be
geomorphological, sedimentological,
demographic, social and economical.
A flood may change the configuration of a
normal and natural stream. It can create new
creeks and rills.

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The erosive power of the floods may change
the slope and remove the top soil which is
loose. It can also roll away the boulders
from one place to the other. Sediment
transport is severe during floods.
Floods destroy settlements and kill
thousands of people depending upon their
location.

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Loss of human life and housing, Destroyal
of commercial and industrial properties,
inflow of storm water into sanitary sewers,
combined sewer overflows and promoting
disease vectors, spilling of sewers over the
streetsand delays in public transportation are
the major effects.

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In addition to these there will be adverse
effects on aesthetics, disturbance of wildlife
habitat, economic losses due to washing
away of crop lands, pollution on water ways
and receiving water bodies.
Floods can devastate the land by removing
human population and their establishments.

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Damage to civil engineering structures,
broken mains, stoppage of transport
services and wired communication systems
are the impacts on urban lands.
In many places, impacts of floods are seen
over human health.
Breaking epidemics and spreading of water-
borne diseases are the severe environmental
impacts to life.

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Flood waters are the carriers of disease
vectors. The impact will be from upstream
end to the downstream end. The diseases
that are to be feared of are
various diarrhoea, acute respiratory
infections, measles, tuberculosis and
malnutrition.
Sudden emergence of communicable
diseases are also warranted.

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During a major flood many deaths, caused
by drowning, may occur. It happens to
mainly the weaker sections of the
population, such as infants, elderly and the
sick.
Among the survivors, the most common
medical problem is hypothermia, meaning
loss of body temperature due to long
immersions in water and exposure to wind.

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Floods can cause severe food and food grain
shortage.
Water is a major solvent and carrier of
organic, inorganic and toxic pollutants.
Flood waters can increase the mobility of
these elements over land and water
resources by reducing their residence time in
a matrix.

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The sources of storm water pollution in
urban environments are diffuse and highly
variable.
Pollutants come from roads, parking slots,
residential, commercial and industrial
activities within municipal boundaries.
Fertility of soil will be severely affected by
floods.

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Manures, fertilizers and soil humus
will be leached away from their localities.
Soil erosion will lead to huge siltation in
tanks, lakes and ponds.
Flood is one of the major causes of reservoir
sedimentation. It controls and limits the
space available for holding water and
increase outflow and induce flooding.

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Floods can destroy the cattle, crops, and
homes of millions of items.
Flooding has been recognised as one of the
most recurring, widespread and disastrous
natural hazards in the densely populated
regions of Asia.
In many parts of the Indian Sub-continent,
flooding reaches catastrophic proportions
during monsoons.

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A historically notable example was the
Harappan (Indus) Civilization which was
eroded due to a series of large floods in
Indus river.
Catastropes of outstanding proportions have
occurred in the past and there is no end to
the hazards created by floods.

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It is a continuous and destructive
phenomena in the Ganga and Brahmaputra
River basins in India.
Several lakh hectares of land was affected
by the ravages of floods causing a sever loss
of economy.
In India, the average annual damage is of the
Order of 7500 millions of Indian rupees and
the annual human loss is accounted to be
approximately 1400.

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Floods occur during southwest monsoon. It
is observed from the records that during the
period from 1953 to 1987, about 50,000
people died in India due to flood hazards.
Global statistics show that over 5 million
people were affected so by internationally
recognised floods every year. The rate also
increases year after year.
The expenditure on flood relief also gets
increased proportionately.

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5. Methods of controlling Floods.
The study of causes and effects of floods,
management and protection of flood are
carried out by experts of meteorology,
hydrology,
geomorphology and
engineering.

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Several methods are adopted to control the
floods and minimize the impacts of
flooding.
They are mainly attempted
a. To Modify the flood by construction of
reservoirs, river channels, natural detention
basins, inter-basin water transfer,
construction of levees and flood walls,
diversion canals, bank stabilization, best
agronomic practices, afforestation and
catchment zone management

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b. Flood control measures are done to
modify the susceptibility of flood damage
through flood plain management, structural
changes, flood proofing, disaster
preparedness, flood forecasting and flood
warning
c. The methods are also adopted to reduce
the loss though emergency evacuation,
flood fighting, public health, disaster relief,
flood insurance and tax remission.

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It is possible to lessen the impact of a
disaster. It is necessary to understand the
nature of hazard in order to reduce the
harmful effects.
In flood controls, the main aspect lies in the
identification of risk areas or flood-prone
areas.

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Mapping of flood-prone zones of a river
basin can be done through an integrated
hydrological and meteorological analysis.
Flood Management is to be attempted with
specific goals, objectives and
methodologies, depending upon the
location, area and drainage systems of
basins and the frequency of floods.
Assessment of past floods is of primary
importance in controlling future floods.

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The first step in assessing flood hazard is
estimating the probable future runoff rates
and associated characteristics( depths of
water and velocities) on the basis of
rainfall patterns,
topography,
hydrology of soils,
landuse pattern and
the morphology of the natural and artificial
water courses.

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This will result in flood extend maps
showing the boundaries of flood damage.
These hazard zonation maps will certainly
help the people to plan for controlling and
managing the flood events.
It helps in redistributing waters, providing
gravel beds, growing shelter belts,
flood proofing, land treatment, tree
management, fencing and construction of
flood walls.

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Flood controls can never eliminate floods.
But it can minimize the loss and save all life.
It needs political and scientific attention in
order to mitigate the effects of natural
hazards world-wide.
International bodies like the UNESCO and
the WMO emphasized the needs of
computing flood intensities right from 1967.
The period of 1990s was declared as the
International Decade of Natural Disaster
Reduction.

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In India about 40 Mha of land is flood-
prone, which is about 12% of the total
geographical area.
The National Flood commission was setup
in the year 1976. For flood controls a large
number of measures such as construction of
storage reservoirs, levees and flood walls are
adopted.

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Estimation of river discharge is done in
almost all major rivers having potential
threats.
Creating public awareness about the floods
and their impacts and methods of
minimizing their impacts are very much
needed in flood disaster management.
Computation of the recurring interval is
also an attempt to arrange for flood
preparedness.

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Flood warning systems are established to
inform the people of flood plains well in
advance.
Floods are natural hazards. Their occurrence
can be predicted based on scientific
computations.
Flood controls are well within the hands of
human activities.

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It is our duty to plan the landscape and
human settlements to mitigate the loss due
to floods.
Conclusion:
Disasters are continuing natural hazards in
the earth and hence, it is necessary to study
the causes and effects and understand their
processes.

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This is needed not only to mitigate their
effects but also to save the life and the
environment.