Natural Hazards
HO Pui-sing
Contents
What are natural hazards
Flooding
Drought
Natural environment
Natural environment has provided man with
useful resources.
Natural environment may be classified into
four categories:
Physical: energy, mineral and soil resources;
Biotic: forest, fish, animal and other life resources
Geomorphic: flat plains and others
Locational: good location for transport,
communication and defence.
What are Natural Hazards
The natural environment is unstable.
People have been learning to deal with the
environmental risks and changes produced by the
unstable environment.
Extreme natural process (flood, earthquake or
landslide…..)has the potential to be a natural hazard
if it occurs where people live or have their property
(buildings or animals…).
Natural disaster when it exceeds normal
expectations of frequency or involved.
Venn Diagram of Natural Hazards
and Natural Disasters
 Responding to
those hazards,
society may seek
to modify the
natural events
system and the
human use system
of locations,
livelihoods, and
social organization.
Classification of hazards
Geophysical
Meteorological
Biological
Geomorphic
Floral
Faunal
Blizzard and snow
Avalanche – rock
Fungal disease
Bacterial, viral and
protozoal disease
Cold wave
Avalanche – snow
Hay fever
Malaria
Drought
Earthquake
Infestation
Foot and mouth disease
Flood
Erosion
Weeds
Infestation
Fog
Expansive soil
Red tide
Rabbits
Frost
Landslide
Locusts
Hailstorm
Shifting sand
Grasshoppers
Heat wave
Tsunami
Lightning strike and fire
Volcanic eruption
Temperature inversion
Tornado
Tropical Cyclone
Windstorm
Measuring and analyzing natural
hazards
Six indicators to measure and analyze the
extreme geophysical events that comprise
natural hazards. They are
Magnitude
Speed of onset
Duration
Frequency
Areal extent
Areal reliability
Six Indicators
Magnitude
It is the most important
indicator.
Magnitude rate as
‘extreme’ happenings
and pose sufficient
threats to the human
system to be
considered natural
hazards.
Example:
The Richter Scale
Speed of onset
It refers to the length of
time between the first
appearance or warning
of an event and its peak.
Example:
Slow-onset hazards
(drought and soil erosion)
Rapid-onset hazards
(floods and earthquakes)
Six Indicators – (cont’d)
Duration
It refers to the period of
time over which it
occurs.
Example:
Droughts: seasons or
years
Floods: days or weeks
Tornados: minutes or
hours
Frequency
It refers to how often an
event of a given
magnitude may be
expected to re-occur.
‘Return period’: a
recurrence interval of
10 years is to say it has
in any year a 10%
chance of occurring.
Six Indicators – (cont’d)
Areal extent
The area of natural
hazards affect.
Example:
Avalanche: short and
narrow belt of the
landscape.
Drought or flood: several
thousand km.
Areal reliability
Predictability of an
natural hazards occur in
an given area.
Predictable and
possible for planning.
Example:
Volcanic eruptions: fixed
point (volcano)
Floods: channels and
flood plains
Tropical cyclones:
several erratic paths
Earthquakes: unknown
Hazards Profile
Impacts of natural hazards and
level of economic development
Bangladesh (1970) vs United States (1969)
Tropical
cyclone
GNP per
capita
($US)
Population
affected
(millions)
Lives
lost
Estimated
value of
damage ($US)
Unnamed,
Bangladesh
12-13
November
1970
1971: $90
1980: $130
4.7
30000
$64 million
Gamille
USA
17-20
August
1969
1971:
$5000
1980:
$11360
19
256
$1422 million
Type of damage suffered
Loss of crops, especially rice; loss of cattle;
400,000 peasant houses damaged or
destroyed; small fishing boats destroyed;
trees down
Houses destroyed or damaged; shipping
damaged or destroyed (both fishing and
cargo); shopping centers severely damaged;
highways and railroads damaged; telephone
cables down; cars and lorries damaged;
trucking terminal destroyed; oil rigs
damaged; loss of cattle and ruin of crop land;
schools and colleges damaged; community
buildings damaged; cinemas and
entertainment centers damaged
Impacts of natural hazards and
level of economic development
" About 95% of disaster related deaths occur among the
two thirds of the world's population that occupy
developing countries. In contrast to (these) ... differences
in death rate, economic loss from natural disaster is
commensurate with income distribution. About threequarters of absolute global loss occurs in the wealthy
countries... (However) the ratio of loss to income is much
higher in the developing countries ... In developing
countries, disasters may be less frequent but are more
catastrophic and more costly in lives and relative wealth,
whereas they are increasingly costly in absolute wealth
in industrial nations."
Flooding
What is Flooding
Nature of flood
Casuses of floods
Impacts of floods
People’s responses to floods
Prevention of floods
What is Flooding?
Flood:
Coastal flooding: (above average sea level)
Unusual atmospheric conditions (eg. Onshore
hurricane, tornado…)
Earthquake or volcanic eruption that set up huge
tidal surges.
River flooding (flow exceeds bank-full capacity)
It is a common hazard and occur anywhere in a
river channel at lower course
May be caused by
Heavy rain
Rapidly melting snow
Natural or man-made dams collapse
others
Nature of flood
Floods are the most common of all natural hazards
Covers the largest spatial area
Causes the greatest loss of life and property
Human beings settle on flood plains
Abundant water supply
Fertile soil for farming
Flat relief for development
Navigator (transportation)
Damage of floods
Water inundating (flooding) land, utilities, buildings, crops,
communication and transport facilities.
After flooding, debris block streets, reservoirs and cover fields.
Disrupt normal supply of water, food, shelter and medicine,
which cause health and pollution problems.
Causes of Floods
Natural
Man-made
Many tributaries
Poor farming methods
Flood plain
Over grazing
Snow melting
Cut ditches to improve the drainage
of their moorland
Heavy rain storms
Erosion of peat
The land sinking / Sea level rising
No vegetation cover in highland
Few lakes
Poor mining method to enhance the
land sinks
Poor management of embankments
Impacts of floods
Date
Place
Deaths
Property Damage
1968
Gujarat, India
1000
1963
Belluno, Italy
> 2000
1955
Pakistan and India
1954
Kavin, Iran
> 2000
1953
Northern Europe
> 2000
1951
Northern China
> 5000
1939
Tianjin, China
1000
Million homeless
1933
Huang He, China
18000
3.6 million affected
1911
Chang Jiang, China
100000
1887
Henan, China
900000
Huang he overflowed, communities
destroyed
1642
Huang He, China
340000
Kaifeng city was completely destroyed
1700
Vaiont dam overtopped
2.27 million crop ha at loss of $63 million
Impacts of Floods – cont’d
What were the impacts of floods on people and the
environment?
1. Loss of life and homeless
2. Crops damaged and loss of livestock
3. Disrupting transport system / network
4. Fertile soil wash away from farmland (soil erosion)
5. Rivers become shallow due to soil deposition (not suitable for navigation)
6. Reduce storage capacity of reservoirs
People’s responses
Perceptions
How people will interpret various hazards
A range of responses
Accepting the hazards – Act of God
Trying to predict
Taking appropriate action to reduce damage
potential
To offsetting the losses through insurance
People’s perception
Hazard Perception
Common Responses
1. Deny the hazard exists at all
Do noting
eg. 'It can't happen here.'
'I know it flooded here 20 year ago, but lightning
never strikes in the same place twice.'
2. Accept the hazard as a natural and
inevitable event
Do nothing
eg. 'We get floods here every five years' Pray
eg. 'It's all in the hands of God.'
3. Hazards are inevitable, but
controllable.
Modify the causes of flooding
eg. forest planting in catchment areas to absorb runoff
People’s perception – cont’d
4. Hazards are inevitable, but the
effects can be controlled
Reduce the damage potential
eg. warning systems; community awareness programmes;
evacuation procedures; special building designs; low intensity
land use zoning on flood plains
Leave the area:
eg. seasonal nomadism; move in with friends or family;
permanent migration.
Plan for the damage and losses
eg. flood insurance
Spread the losses across the community
eg. disaster relief funds; government subsidized insurance;
international emergency relief
Bear the losses
eg. use savings
5. Some hazards are made or
intensified by people
Alter human behaviour and land use patterns
eg. reduce stocking levels; introduce low intensity land use
zoning and bans on living in flood-prone areas; stop dredging
that interferes with the river channel pattern; after political,
social and economic factors that force the poorest people to
live in flood-prone areas.
People’s perception – cont’d
Perception is influenced by the following:
The past record of hazards (magnitude and frequency)
The strength of traditional cultures (attitudes to the
environment)
Education standard of the community
Wealth and economic development
Community awareness and preparations
Willingness of local and national government to spend
money on
Long-term hazard prevention and damage reduction schemes, or
Short-term emergency relief after a disaster
People’s perception – cont’d
These factors emphasized the social or
cultural framework in which people live.
In fact, the range of choices open to people is
very limited and controlled by the social,
economic and political conditions and
pressures.
From this perspective, it can explain why
people often do seemingly irrational things.
Bounded rationality vs Satisfying behaviour
People’s perception – cont’d
Bounded rationality
People like to make a
rational choice of
responses.
However,
Few people have access to
full information
Many are just not aware of
all the alternative
responses available
People differ in their ability
Few like to forget previous
painful history
Satisfying behaviour
People make choices
that help them achieve
a satisfactory level of
reward, but
Stop short of striving for
the highest possible
level.
Satisficer
Accept ‘tolerable’ levels
of hazard
Avoid the worst of the
impact
Their property is insured
anyway.
People’s Choices
Only those people have a range of choices of
response to hazards
Access to full information.
Strong networked of family
Strong friendship support
Strong wealth and political power
Prevention of floods
Flood management strategies
Methods of floods prevention
Behavioural
Structural
Example
Flood management strategies
Individual can do little on the manage the impact
of hazards except personal preparations and
insurance.
Flood management should be a collective action
and ought to be coordinated by government
(local and national)
Most governments only provide emergency relief and
reconstruction after the hazards for facing many
conflicting demands on the public purse with limited
resources.
Economically richer nations, governments have
adopted a range of actions to predict, prevent or
minimize the effects of hazards and providing disaster
relief and reconstruction.
Twelve actions to manage the
hazards
 1.
 2.
 3.
 4.
 5.
 6.

Subsidized flood insurance for farmers and
industrialist
Use of existing government laws to ensure local
planners take proper and consistent account of flood
hazards.
Development of a uniform method of determining
flood frequency.
An improved system of flood forecasting.
Compulsory household flood insurance scheme
similar to compulsory third party car insurance.
Government support for local council flood control
projects, surveys, and engineering works.
Twelve actions to manage the
hazards – cont’d
 7.
Community education programmes to disseminate flood
hazard information and alternative methods of reducing
flood losses.
 8. Research to delimit major flood areas and to provide flood
hazard information in the form of maps, chart, graphs and
narrative descriptions.
 9. Use of zoning regulations to locate low intensity land uses in
flood-prone areas.
 10. Development of flood warning system, based upon agreed
lines of responsibility and communication, using established
flood heights as the determinants of action.
 11. Funding of local emergency relief services
 12. Funding to support relocations and flood-proofing as
alternatives to repetitive rebuilding.
Management
Strategy
Local Council
3, 4, 8, 10
National
Government
3, 4, 8
2, 6, 8
6, 8
1, 5, 7, 9, 10
1, 5, 7
11, 12
11, 12
Prediction
Hazard prevention
Impact Reduction
Relief and
Reconstruction
Methods of floods prevention
Behavioural
Accepting loss (Third
World)
Public relief funds
Flood insurance
Flood forecasting
and warning
Structural
Reserviors (dams
building)
Channel enlargement
Channel straightening
Embankments
Flood relief channels
Barrages
Flood plain zoning
Reforestation
Case Study: Thames flood
barrier scheme
 Location: Central London
 Painful events: 1928 (14 people died), 1953 (300 people
died)
 Reasons:
 Southern Britain is slowly sinking
 Very slow rise in world sea level
 Special weather conditions (north-easterly winds from North Sea
make a surge to London)
 Time: October to March (high tides and surges of water)
Case Study: Thames flood
barrier scheme
 Scheme: completed in 1982
 Location: River Thames at Woolwich
 Construction:
 Four main gates which can swing up from the river bed to form
a continuous steel wall against the incoming flood.
 The gates lie on the river bed when not in use for ships sailing
up and down river.
 River banks were also raised and strengthened.
 Warming system:
 4 hours before flooding is expected, announcements will be
made on TV and radio.
 1 hour to London’s flood, sirens will be sounded in riverside
areas, and
 Police will warn people with loud hailers to go to safety places.
Thames barriers
Thames barriers
Thames barriers
Thames barriers
Drought
What is drought?
Problems presented to man.
Role of man in causing the drought
hazard
People’s perception and responses to
drought
What is drought?
 More than 1/3 of land is dry or very dry.
 Desert: annual rainfall < 250 mm
 Definition of Drought:
 “a period of unusually or unexpectedly low rainfall, which
upsets the ecological balance.”
 A condition in which the amount of water needed for
transpiration and direct evaporation exceeds the amount
available in the soil.
 In term of the water need of a particular crop growing under a
specific combination of environmental conditions.
 Three classes of drought can be identified:
 Permanent drought associated with arid climate.
 Seasonal drought: annual periods of dry weather
 Drought due to precipitation variability or unreliable
Problems to man
Effects of droughts:
People themselves and their way of life
Crops and livestock
Natural vegetation and wildlife
Soil
Population size and population redistribution
Role of man in causing the
drought hazard
 The human context in which hazards offer is more
important than the geophysical causes of the event.
 Some factors influencing the human impact of natural
hazards
 Population density in the area affected,
 Prior experiences of hazards in the area,
 Traditional methods of coping with hazards
 The degree of accuracy in predicting the hazards,
 The effects of any warning, preparation and /or evacuation
procedures,
 The speed and effectiveness of local, national and
international emergency and long-term relief services,
 The overall level of economic development in the area
affected
Population Growth Effects
Population growth has put increasing pressure
on the environment.
This pressure increases the risk of humaninduced hazards and disasters.
For examples
The hazard of affecting the balance of world
climates by extensive clearance of forest. (Amazon)
The hazards of disturbing natural ecosystems and
food webs by the clearance of vegetation, the use of
chemicals in the atmosphere, and pollution.
The hazard of increasing the risk of drought, floods
and soil erosion by farming marginal areas
particularly in semi-arid regions.
People’s perception and
responses to drought
Governments’ View
Nomads’ View
1.
The nomads just wander aimlessly in a never-ending
search for pasture
1.
We follow traditional routes that make the best use of
a risky environment.
2.
We must limit this needless movement so we can
collect taxes and use their labour.
2.
We must be able to move freely as our routes change
according to climatic conditions.
3.
We could encourage the nomads to raise herds for
profit and benefit ourselves from trading with them.
3.
We are not interested in selling our animals. They
provide all our needs and are a sign of wealth.
A Major Drought Occurs
4.
The loss of human and animal life is shocking. We
must solve the problem of drought.
4.
Droughts and famines have always happened. We
accept them as part of the risks we face.
5.
We can use our superior technology to overcome the
shortage of water.
5.
The new water supplies are much more reliable and
convenient. We must change our routes.
6.
The nomads’ herds are very unhealthy and prone to
disease. We can improve their quality.
6.
The new medicines will protect our animals. Now we
can own more because fewer will die from disease.
Overgrazing and Climatic Change lead to increasing Desertification
7.
The nomads herds are too large for the pastures
available. We must reduce them.
7.
The pastures are getting worse. We must keep as
many animals as possible to allow for those that die.
Major Drought 1968-73
8.
The nomads are to blame for the advancing desert.
They must be settled permanently.
8.
Conditions in our traditional lands are hopeless. We
must find better pastures.
9.
Now that the nomads are helpless we can make sure
that they do not go back to their old ways.
9.
Our herds have been destroyed. We have no option
but to get food in the relief camps.
People’s perception and
responses to drought
Governments’ Action
Nomads’ Action
Large-scale vaccination programmes are carried
out
They fall back on traditional methods of surviving
the drought
Ways of improving the natural water supply are
investigated.
They move to relief camps makes them dependent
on food
Nomads are persuaded to become settled farmers
when crisis is over.
The size of their herds increases, causing
overgrazing
Attempts are made to control nomads.
No changes are made in their traditional lifestyle.
Nomadic tribes are ignored by early colonists.
Fewer animals than usual are sold
Deep wells and storage tanks are dug
Herds are concentrated on new wells, causing
overgrazing on surrounding land.
Territorial frontiers established by colonists.
Moving several hundred km further south, they
clash with permanent farmers.
Attempts are made by colonist to set up livestock
markets. Nomads are offered high prices to
encourage them to sell animals.
Only a few animals are sold, usually the worst.
They become aggressive over loss of land.