Comprehensive Risk Assessment for Natural Hazards - Planat

Comprehensive risk assessment for natural hazards
(e) the probability distribution of storm strength, and
therefore of surge height;
(f) possible coincidence of storm-induced surge and astronomical
high tide; and
(g) the propagation of the surge over the land surface. This
can be calculated using two-dimensional routing models.
For large shallow lakes, storm surge may be more easily
computed based on maximum sustained wind speed and
direction and the corresponding fetch length.Assessment of
tsunami hazards is particularly difficult because the
causative event may occur in numerous places, far or near.
Coastal morphology has a significant effect on run-up, but
the effect can be different for tsunami arriving from
different directions. In the Pacific basin there is a
programme for detecting tsunami, predicting their
propagation, and warning affected coastal areas (Bryant,
1991).
3.6 FLOOD RISK ASSESSMENT
Assessment of flood risk, which is the expected flood losses,
is important both for planning mitigation measures and for
knowing how to cope with an emergency situation.
Mitigation measures are designed to reduce the effect of
future hazards on society. Risk assessment can also be used
to evaluate the net benefits of a proposed flood-mitigation
programme. For example, expected losses would be calculated
with and without the proposed dam, or with and
without the flood-forecasting programme. Risk assessments
are rarely undertaken, however, because of their particular
requirements for data and the fact that many countries have
selected target flood return periods assuming an implicit
vulnerability level for the surrounding land. This may
change in the near future in the USA because the US Army
Corps of Engineers has mandated that a risk-based
approach must be applied to all flood-damage reduction
studies as described in section 8.4.1. Petak and Atkisson
(1982) give an example of a multi-hazard risk analysis performed
for the entire USA; many of the following methods
are drawn from their example.
Risk assessment includes four basic steps:
(a) Estimation of the hazard: this includes location, frequency
and severity;
(b) Estimation of the exposure: this includes the number of
people,buildings,factories,etc.exposed to the hazard:
these are sometimes called “elements at risk”;
(c) Estimation of the vulnerability of the elements at risk:
this is usually expressed as percentage losses of people,
buildings, crops, etc.; and
(d) Multiplication of the hazard, exposure and vulnerability
to obtain the expected losses.
Up-to-date census data on the geographic distribution
of the population are essential for accurate estimation of the
exposure. Aerial photographs or satellite images may be
helpful in updating older information or determining the
density of housing or factories. Economic data are needed
to transform the building count into an economic value as
described in Chapter 7. Some risk analyses attempt to estimate
the cost of indirect flood damages such as
unemployment and disruption of economic markets also
discussed in Chapter 7.
Experience from previous floods has been used to construct
curves of percentage losses versus flood depth. Grigg
and Helweg (1975) developed such loss curves for the USA.
Such information is not readily transferable to other
regions, however, due to differences in construction practices,
lifestyles and economic factors.
In the risk assessment performed by Petak and
Atkisson (1982), the flood-frequency curve was transformed
into a depth-frequency curve. Monte Carlo
simulation (repeated random selection of a flood depth
from the frequency distribution) was used to find the
expected losses. Estimation of losses over a time horizon
was obtained using projections of the future population and
future economic variables.
3.7 DATA REQUIREMENTS AND SOURCES
The types of data that are potentially useful in flood-hazard
assessment are listed in this section. The list begins with the
types of data used in data-intensive methods, and ends with
less commonly used types of data or types of data that are
used in data-poor watersheds. Therefore, it is not necessary
to have all these types of data for every flood-hazard
assessment.
(a) Systematic streamflow measurements. For flood-frequency
analysis, only peaks are needed. The water stage
associated with these peaks may also be useful.
Whereas it is preferable to use streamflow data collected
continuously in time, it is also possible to use
data from peak-flow gauges. These devices measure the
highest stage, but not the entire hydrograph or the time
of occurrence. Streamflow data are normally obtained
from the national hydrological services, or from agencies
operating reservoirs or managing water-supply
systems.
(b) Topographic data. These data are used for four purposes:
(i)
to determine the width and location of the flooded
area for a given water-surface elevation;
(ii) to determine the longitudinal (downriver) profile
of the water-surface elevation for a given discharge
rate;
(iii) to determine watershed size; and
(iv) to display results of the hazard analysis. The
required spatial resolution of the topographic data
will vary with how they are to be used and the size
of the river. It is therefore difficult to specify a
priori the required scale of the topographic data.
For detailed studies in developed countries, it is common
to supplement ordinary topographic data (at scales of
1:25 000 to 1:250 000) with specialized data (1:5 000 and
contour intervals of 0.5 metres) obtained specifically for
flood studies in selected watercourses. Ground surveys conducted
by trained surveyors are used to determine the slope
of the channel bed and the shape of the channel crosssection
at a number of sections. High-resolution topographic
data may also be produced from specialized data
taken from low-elevation aircraft flights. Very coarse
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