Comprehensive Risk Assessment for Natural Hazards - Planat

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Comprehensive risk assessment for natural hazards 7 (a) Sufficiently wide ocean areas with relatively high surface temperatures (greater than 26°C). (b) Significant value of the Coriolis parameter. This automatically excludes the belt of 4 to 5 degrees latitude on both sides of the equator. The influence of the earth’s rotation appears to be of primary importance. (c) Weak vertical change in the speed (i.e. weak vertical shear) of the horizontal wind between the lower and upper troposphere. Sea surface temperatures are significantly high during mid-summer, but tropical storms are not as widespread over the North Indian Ocean, being limited to the northern Bay of Bengal and the South China Sea. This is due to the large vertical wind shear prevailing in these regions. (d) A pre-existing low-level disturbance and a region of upper-level outflow above the surface disturbance. (e) The existence of high seasonal values of middle level humidity. Stage I of tropical storm growth (Figure 2.1) shows enhanced convection in an area with initially a weak lowpressure system at sea level. With gradual increase in convective activity (stages II and III), the upper tropospheric high becomes well-established (stage IV). The fourth stage also often includes eye formation. Tropical storms start to dissipate when energy from the earth’s surface becomes negligibly small. This happens when either the storm moves inland or over cold seas. Stage I Stage II Stage III Stage IV PRESSURE RISE WARMING WEAK LOW WARMING PRESSURE FALL EYE STRONG PRESSURE FALL EYE WEAK HIGH Upper-tropospheric pressure and wind HIGH LOW RING HIGH LOW RING HIGH LOW 2.3 METEOROLOGICAL HAZARDS ASSESSMENT VERY LOW RING 2.3.1 Physical characteristics Havoc caused by tropical and extratropical storms, heavy precipitation and tornadoes differs from region to region and from country to country. It depends on the configuration of the area — whether flatland or mountainous, whether the sea is shallow or has a steep ocean shelf, whether rivers and deltas are large and whether the coastlines are bare or forested. Human casualties are highly dependent on the ability of the authorities to issue timely warnings; to access the community to which the warnings apply; to provide proper guidance and information; and, most significant of all, the preparedness of the community to move to relatively safer places when the situation demands it. The passage of tropical storms over land and along coastal areas is only relatively ephemeral, but they cause widespread damage to life and property and wreck the morale of nations as indicated by the following examples. In 1992, 31 storm formations were detected by the Tokyo-Typhoon Centre and of these, 16 reached tropical storm intensity. In the Arabian Sea and Bay of Bengal area, out of 12 storm formations depicted, only one reached tropical storm intensity, whereas in the South-West Indian Ocean 11 disturbances were named, and four of these reached peak intensity. In 1960, tropical storm Donna left strong imprints in the Florida region. Cleo and Betsy then came in the midsixties, but afterward, until August 1992, this region did not see any major storm. In 1992, Andrew came with all its Figure 2.1 — Schematic stages of formation of tropical storm (after Palmen and Newton, 1969) ferocity, claiming 43 lives and an estimated US $30 billion worth of property (Gore, 1993). In 1970, one single storm caused the death of 500 000 in Bangladesh. Many more died in the ensuing aftermath. Although warnings for storm Tracy in 1974 (Australia) were accurate and timely, barely 400 of Darwin’s 8 000 modern timber-framed houses were spared. This was due to inadequate design specifications for wind velocities and apparent noncompliance with building codes. In February 1994, storm Hollanda hit the unprepared country of Mauritius claiming two dead and inflicting widespread wreckage to overhead electricity and telephone lines and other utilities. Losses were evaluated at over US $ 100 million. In October 1998, hurricane Mitch caused utter devastation in Nicaragua and the Honduras, claiming over 30 000 lives, wrecking infrastructure, devastating crops and causing widespread flooding. Consequences of tropical storms can be felt months and even years after their passage. Even if, as in an idealized scenario, the number of dead and with severe injuries can be minimized by efficient communication means, storm warnings, and proper evacuation and refugee systems, it is extremely difficult to avoid other sectors undergoing the dire effects of the hazards. These sectors are:
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.
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Page 4 and 5: CONTENTS AUTHORS AND EDITORS . . .
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Chapter 6 HAZARD ASSESSMENT AND LAN
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CHAPTER 8 STRATEGIES FOR RISK ASSES
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