Lecture Note Hazard .pdf
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Title: HAZARDS AND RISK ANALYSIS
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ES 5131: Natural Hazards and Disaster Management
Hazard assessment; Hydrological Hazards (River & Coastal Floods, Tropical Cyclones), Geological Hazards
(Earthquakes, volcanic hazards, landslides); Vulnerability analysis: (areas affected by hydrological and geological
hazards); Risk assessment; (Specific risk and measurement methods, risk reduction measures for areas affected
by hydrological and geological hazards, cost of risk reduction measures). Disaster Management measures:
Prevention, Mitigation, Preparedness, Response Recovery and Logistic supports (Training, Public awareness,
The significance of disaster; The disaster threat; National Disaster management Policy; Major requirements for
coping with disaster; The disaster management cycle; Disaster legislation; Counter Disaster Resources;
International Disaster Assistance; Leadership, plans and utilization of resources.
Hazard means the occurrence, within a specific period of time in a given area, of
potentially damaging natural or man-made phenomena.
Hazards – natural and man-made are phenomenal in Bangladesh. The most
devastating cyclones and floods of the world occur in Bangladesh. The victims know best
on how to cope with these hazards. Hazards are not just cyclones and floods. These are
major disasters, but there are other ones too and some of them are as devastating as the
earlier mentioned ones. For example, cyclones are not as bad as storm surges, which
accompany a cyclone. Drought leaves a permanent damage and encourages the
desertification process that is going on in some parts of northern Bangladesh. River
erosion takes away thousands of hectors of land every year in a country where land is the
most scares resource. Earthquakes may cause millions and billions of Taka worth of
Perhaps the most disturbing but ignored fact about hazards is that they are all
linked to each other. So diminishing the effects of hazards would involve a total approach
rather than tackling one or the other hazards. But people are unfortunately becoming
increasingly unconcerned about the consequences of the chain of hazards.
Hazard and Risk
Hazard is best viewed as a naturally occurring or human-induced process or event
with the potential to create loss, i.e., a general source of danger. Risk is the actual
exposure of something of human value to a hazard and is often regarded as the
combination of probability and loss. Thus we may define hazard as a potential threat to
humans and their welfare and risk as the probability of a specific hazard consequence.
The distinction can be illustrated as:
Two people crossing an ocean. One in a small boat and other in a large boat. The
main hazard (deep water and large waves) is the same in both cases, but the risk
(probability of drowning) is very much greater for the person in the small boat.
Thus, while an earthquake hazard can exist in an uninhabited region, an
earthquake risk can occur only in an area where people and their possession exist. People,
and what they value, are the essential point of reference for all risk assessment and for all
When large numbers of people exposed to hazard are killed, injured or damaged
in some way, the event is termed as a disaster. So a disaster may be seen as the realization
of hazard, although there is no universally agreed definition of the scale on which loss
has to occur in order to qualify as a disaster.
Natural Event System
Human Use System
Natural hazard can be viewed in an ecological framework. This distinguishes
between natural events system and their interpretation as natural hazard or resources.
Both hazard and resource exist at the interface between natural event system and human
use system. Human response to hazard can modify both the natural event system and
human use system.
Hazard and Resource
Since the earth is a highly dynamic planet, most natural events show a wide range
of variation through time in the use of energy and materials for environmental processes.
The outer limits of this behaviour we call ‘extremes’ and certain statistical measures,
notably magnitude-frequency relationships, are used to describe such extremes. But
extreme natural events are not considered as hazards unless they cause large-scale death
or damage to humans.
Hazards, therefore, result from the conflict of geophysical processes with people.
This interpretation gives humans a central role. First through location, because it is only
when people, their possessions and what they value get in the way of natural processes
that a risk of hazard exists. Second, through perception, because humans place subjective
judgments on natural processes as part of a general environmental appraisal whenever
they settle and use land. In other words, many hazardous processes merely represent the
extremes of a distribution of events that, in a slightly different context, would be regarded
as a resource.
In the figure above, the slight variation beyond the average line represents an
acceptable range of variation for the magnitude of the physical variable which can be any
environmental element relevant to human survival, such as rainfall. Most social and
economic activities are geared to some expectation of the ‘average’ conditions. As long
as the variation of the environmental element remains fairly close to this expected
performance, the element will be perceived as mainly beneficial. However, whenever the
variability exceeds some ‘critical value’ beyond the ‘normal’ band of tolerance, the same
variable starts to impose damage and becomes a hazard. Thus very high or very low
rainfall will be deemed to create a flood or a drought respectively. The exceedance of a
critical value immediately enables two basic dimensions of a hazard to be identified. The
hazard ‘intensity’ is determined by the peak deviation beyond the critical value on the
vertical scale and the hazard ‘duration’ is determined by the length of time the critical
value is exceeded on the horizontal scale
There is often a fine line between hazards and resources, e.g., between water out
of control (flood hazard) and water under control (reservoir resource). The atmosphere is
‘nice’ when it produces sunshine but ‘hostile’ when it produces damaging storms. In
reality, the environment is neither nice nor hostile. It is ‘neutral’ and it is only human
location, actions and perception which identify resources and hazards within the range of
natural events. The same natural event (with similar intensity) can be hazard or resource
depending on geographical setting and local preparedness for dealing with such an event.
Hazards, therefore, result from the conflict of processes (natural or man-made) with the
people. This gives human as a central role.
Human Sensitivity to Hazard
It depends on a combination of physical exposure and human vulnerability.
Physical exposure reflects the range of potentially damaging events and their statistical
variability at a particular location. Human vulnerability reflects the breadth of social and
economic tolerance to such hazardous events at the same site. It follows that the risk of
hazards may vary through time according to changes in either (or both) physical exposure
or human vulnerability in a given area.
Figure above shows a schematic set of stage-damage curves which may be used to
assess the economic losses from flooding according to the depth of water entering
individual houses. It can be seen that no damage occurs until the water rises above some
level but then losses rise abruptly for comparatively small depth increments. For any
given depth, actual losses are related to the economic status of the housing.
Chain of Hazard Development
Hazards may be viewed as a sequence of events leading from human needs and wants to
the selection of a particular technology through to harmful consequences. This chain of
hazard evolution can be employed more widely, as shown in Figure above, for drought.
The top line indicates seven stages of hazard development. The stages are identified
generically at the top of each box and in terms of a sample development of hazard in the
bottom. The stages are linked by causal pathways (cause and effect) denoted by arrows.
Six control stages are linked to pathways between hazard states by vertical arrows. Each
is described generically as well as by specific control actions designed to eliminate or
reduce the evolving hazard.
Although community loss is the major characteristic of hazard, all these definitions
ignore the fact that, in virtually every hazard, some gains also arise. Therefore it is
necessary to categorize hazard impacts, not only into gains and losses, but also into other
effects, as shown in Figure above. Direct effects are those first order consequences which
occur immediately after an event, such as the deaths and damage. Indirect effects emerge
later and may be much more difficult to attribute directly to the event. These include
factors such as mental illness resulting from shock etc. Tangible effects are those to
which it is possible to assign reasonably reliable monetary values, such as replacement of
damaged property. Intangible effects, although real, can not be satisfactorily assessed in
Direct losses are the most visible consequence of hazard. They may be
comparatively easy to measure, but they are not always the most significant outcome.
They are caused by the immediate damage done to humans, goods and the environment.
Direct gains represent benefits which may flow to surviving residents in the area after a
hazard. These can include various forms of aid and even some longer-term enhancement
of the environment.
Indirect losses arise mainly through the second order consequences of hazard,
such as the disruption of economic and social activities in a community. After the hazard,
an ‘inverse multiplier’ effect may occur whereby property values fall, consumers save
rather than spend, business become less profitable and unemployment rises. These effects
often outlast those of the direct losses and can be highly intangible. There is a growing
realization, for example, that ill-health is a major indirect effect of hazard. Indirect gains
are even less understood. They represent the very long-term benefits enjoyed by a
community as a result of its hazard-prone location. Very little systematic research has
been undertaken in this regard.
Hazard Impact Pyramid