Comprehensive Risk Assessment for Natural Hazards - Planat

8
(a) Agriculture: considerable damage to crops, which sustain
the economy and population.
(b) Industries: production and export markets become
severely disrupted.
(c) Electricity and telephone network: damages to poles and
overhead cables become inevitable.
(d) Water distribution system: clogging of filter system and
overflow of sewage into potable surface and underground
water reservoirs.
(e) Cattle: these are often decimated with all the ensuing consequences
to meat, milk production and human health.
(f) Commercial activity: stock and supply of food and other
materials are obviously damaged and disrupted.
(g) Security: population and property become insecure and
looting and violence follow.
Furthermore, often-human casualties and damage to
property result from the secondary effects of hazards: flood,
landslide, storm surge and ponding (stagnant water for days)
that destroy plantations and breed waterborne diseases.
2.3.1.1 Tropical storms
In assessing tropical storm hazard,it is important to consider the
subject in its global perspective, since storms, at the peak of
their intensities, in most cases, are out over oceans.
Furthermore, tropical storms may within hours change their
characteristics depending on the type of terrain they pass over
during their lifetimes.It is,perhaps,paramount to consider
cases region by region, since say, the Philippines and the areas
surrounding the Bay of Bengal are more prone to damage by
flood and storm surges than by wind, whereas other countries,
such as mountainous islands (like Mauritius and Réunion),
would be more worried by the wind from tropical storms than
by rains. Therefore, storm data for each area must be carefully
compiled since the most common and comprehensible diagnostic
approach for tropical storm impact and risk assessment
is the “case-study” approach.
Furthermore, assessment of tropical storm events must
consider the different components that actually represent
the risks occurring either separately or all together. These
components are flood,landslide,storm surge,tornado and
wind. The flood aspect of torrential rain will not be considered
in this chapter as it is discussed in detail in Chapter 3
on Hydrological Hazards.
2.3.1.2 Extratropical storms
As mentioned earlier, extratropical storms originate in subtropical
and polar regions over colder seas than do tropical
storms. Their salient feature is that they form over a frontal
surface where air masses, with differing properties (essentially
warm and cold), which originate in subtropical and
polar regions, meet. At this point in space with a small perturbation
on a quasistationary front, warm air encroaches
slightly on the cold air, causing a pressure fall. This process,
once triggered and bolstered by other elements, may accentuate
the cyclonic circulation, with further reduction of
pressure at the storm centre.
Chapter 2 — Meteorological hazards
Extratropical storms have been observed to enter the west
coast of Europe and the United States of America and Canada
in the northern hemisphere, whereas in the southern hemisphere
the southern coast of Australia and New Zealand are
mostly hit in quick and fairly regular succession. These storms
are sometimes described as travelling in whole families.
Some extratropical storms are particularly violent with
winds exceeding 100 km/h. Rarely, some storms have been
reported to have winds of 200 km/h or more, but when this
happens the havoc caused can be as disastrous as with tropical
storms. After a long lull such a storm did take place one
day in October 1987 and western Europe woke up and was
caught by surprise to see blocked roads and buildings and
infrastructures damaged by uprooted trees and swollen
rivers as a result of strong winds and heavy precipitation. A
similar scenario repeated itself in October 1998.
The comforting aspects of extratropical storms are their
fairly regular speed and direction of movement, which render
them relatively easy to forecast and follow by weather
prediction models.
2.3.2 Wind
Of all the damaging factors of tropical and extratropical
storms, strong winds are perhaps the best understood and
fortunately so, since the winds largely determine the other
damaging factors. Damage caused by wind pressure on regular
shaped structures increases with the square of the
maximum sustained wind. However, due to high gust factors,
the total damage may considerably increase to vary
with even the cube of the speed (Southern, 1987).
With the help of satellite imagery and with aircraft
reconnaissance flights, it has been possible to reconstruct
the wind distribution near ground level in meteorological
hazards. It has also been found that wind distribution on the
poleward side of the storm is stronger than on the equator
side. This is due to the increasing value of the Coriolis parameter
towards the poles.
2.3.3 Rain loads
Rain driven by strong winds and deflected from the vertical,
is known as “driving rain” and represents a serious threat to
walls of buildings and other structures. Walls, made of
porous material, succumb to driving rains. Door and window
joints which do not have to be airtight in the tropics,
most often cannot withstand driving rain. This is not considered
as a standard storm parameter and is not
systematically measured at meteorological stations.
Experimental measurements have been conducted only at a
few places and because of the scarcity of such observations,
driving rain values typically are computed. Kobysheva
(1987) provides details of the procedures for computing values
of rain loads.
The same author suggests the following as being some
of the basic climatic parameters of driving rain:
(a) Highest amount of precipitation and corresponding
wind speed and rain intensity.