Comprehensive Risk Assessment for Natural Hazards - Planat

CHAPTER 8
STRATEGIES FOR RISK ASSESSMENT — CASE STUDIES
According to the United Nations Department of
Humanitarian Affairs (UNDHA, 1992), assessment involves
a survey of a real or potential disaster to estimate the actual
or expected damages and to make recommendations for
prevention, preparedness and response. The survey of the
expected damages for a potential disaster essentially consists
of a risk evaluation. Risk is defined as the expected
losses (of lives, persons injured, property damaged and economic
activity disrupted) due to a particular hazard for a
given area and reference period (UNDHA, 1992). Based on
mathematical calculations, risk is the product of hazard and
vulnerability (UNDHA, 1992).
Risk evaluations should be the basis of the design and
establishment of methods to prevent, reduce and mitigate
damages from natural disasters. Methods to evaluate meteorological,
hydrological, volcanic and seismic hazards are
available and have been presented in Chapters 2 to 5,
respectively. Methods also are available to develop a commensurate
rating system for the possible occurrence of
multiple potentially damaging natural phenomena (e.g.,
landslides and floods) and to present equivalent hazard
levels to land-use planners in a single map, as illustrated by
the example given in Chapter 6. Methods also have been
proposed to evaluate the economic damages resulting from
natural disasters some of which are presented in Chapter 7.
However, despite the availability of the methods to evaluate
the damages resulting from natural disasters, most societies
have preferred to set somewhat arbitrary standards on the
acceptable hazard level as the basis for mitigation of risks
from natural disasters. Without a detailed evaluation of the
damages resulting from natural disasters and the direct
consideration of societally acceptable damage levels
(including loss of life), society is sure to inadequately allocate
natural-disaster risk-mitigation funds and, as a result,
is guaranteed to encounter damage that is deemed
unacceptable by society.
In recent years, several countries have started to apply
risk evaluations in the design and establishment of methods
to prevent, reduce and mitigate damages from natural disasters.
This chapter includes reviews of examples of these
methods applied to: (1) the design of coastal protection
works in The Netherlands, earthquake resistant structures
in Mexico and Japan, and flood-protection works in the
USA; and (2) the establishment of flood mitigation via
land-use planning in France. This chapter does not include
an exhaustive review of risk evaluations, but rather presents
examples to illustrate that the methods are available and
have been successfully applied. This review provides a
framework for the development and application of similar
methods for mitigation of other natural disasters, as appropriate,
to conditions in other countries. Thus, in this
chapter, assessment is defined as a survey and evaluation to
estimate the expected damages from a potential disaster and
to recommend designs or measures to reduce damages to
societally acceptable levels, if possible.
8.1 IMPLICIT SOCIETALLY ACCEPTABLE
HAZARDS
It is valuable to review the history of the determination of
societally acceptable hazards in order to understand the
need for risk assessment in the design and establishment of
mitigation programmes for risks from natural disasters. In
the design of structures and the establishment of land-use
management practices to prevent and/or reduce damages
resulting from natural disasters, the risk or damage assessment
typically has been implicit. An example can be taken
from the area of flood protection where the earliest structures
or land-use management practices were designed or
established on the basis of the ability to withstand previous
disastrous floods. Chow (1962) notes that the Dun waterway
table used to design railroad bridges in the early 1900s
was primarily determined from channel areas corresponding
to high-water marks studied during and after floods.
Using this approach, previous large floods of unknown frequency
would safely pass through the designed bridges.
Also, after a devastating flood on the Mississippi River in
1790, a homeowner in Saint Genieve, Missouri, rebuilt his
house outside the boundary of that flood. Similar rules were
applied in the design of coastal-protection works in The
Netherlands at the time the Zuiderzee was closed (1927-32)
(Vrijling, 1993).
In some cases, rules based on previous experience work
well. For example, the house in Missouri was not flooded
until the 1993 flood on the Mississippi River and the
Zuiderzee protection works survived the 1953 storm that
devastated the southwestern part of The Netherlands.
However, in most cases these methods are inadequate
because human experience with floods and other natural
hazards do not include a broad enough range of events nor
can they take into account changing conditions that could
exacerbate natural disasters. As noted by Vrijling (1993)
“one is always one step behind when a policy is only based
on historical facts.”
In the early part of the twentieth century, the concept of
frequency analysis began to emerge as a method to extend
limited data on extreme events. These probabilistically
based approaches allow estimates of the magnitude of rarely
occurring events. Frequency analysis is a key aspect of meteorological,
hydrological and seismic hazard analyses as
described in Chapters 2, 3 and 5, respectively. Thus, using
frequency-analysis methods, it is possible to estimate events
with magnitudes beyond those that have been observed.
This necessitates the selection of a societally acceptable hazard
frequency.
In the USA, the societally acceptable frequency of occurrence
of flood damage was formally set to once on average in
100 years (the so-called 100-year flood) in the Flood Disaster
and Protection Act of 1973. However, the 100-year flood had
been used in engineering design for many years before 1973.
In this Act, the US Congress specified the 100-year flood as the