|Natural Disasters in South East Asia and Bangladesh - Vulnerability Risks and Consequences (Centre for Research on the Epidemiology of Disasters - International Center for Training Exchanges in the Geosciences, 1998, 83 p.)|
|PART II - NATURAL HAZARDS AND DISASTERS: DISTRIBUTION AND FREQUENCES|
Figure 14 shows data relating to cyclones, floods, earthquakes and volcanic eruptions likely to cause damage. Besides the principal cyclone tracks, lines of equal frequency built out of data standardised over a hundred years have been represented (after NEUMANN, 1993 in ROUX & VILTARD, 1997).
Statistically two types of cyclone tracks dominate: the ESE-WNW ones that mainly affect the Philippines, Vietnam: the cyclones of the ESE sector strengthening as they go over the positive thermal anomaly in the South China Sea; the others correspond to a displacement of cyclones in the SSW sector and affect preferentially Bangladesh or the northern Myanmar coast. The cyclones also grow stronger as a result of intense evaporation arising from the positive thermal anomaly in the Gulf of Bengal waters. Figure 15 illustrates the high frequency of the cyclones which affect preferentially the low deltaic coast of Bangladesh. The cyclones also affect the Arakan plains and mountains causing strong rains because of the orographic effect. The effective tracks of certain cyclones like the Forrest typhoon (November 1992) and to a certain extent the Colleen typhoon (October 1992) recall that major events can be left out by statistical plottings.
In addition to wind and rain effects, cyclones are characterised by destructive storm surges which affect the more highly exposed coastal fringes in Vietnam, Myanmar and particularly Bangladesh and the Philippines. Figure 15 shows that cyclones cause storm surges which can go upstream for more than 100 kilometres. Certain parts of the coast are more frequently subject to storm surge effects than others; this depending on the topography and the possibilities of shelter. Figures 16 and 17 illustrate this phenomenon in the Philippines and Vietnam.
The easily inundated areas and, moreover, those prone to mass movements or droughts cannot easily be represented at this scale, hence the drawing up of national maps. Figures 17 and 18 give some of the examples in Vietnam and Bangladesh. Floods and landslides are favoured in sectors statistically most affected by cyclones though they depend notably on the relief, lithology and the development of vegetation.
The areas prone to earthquakes with a magnitude greater than 5 are given by the Swiss Reinsurance Company (1992) and four zones of decreasing exposure have been identified. All the coasts of the region under study are exposed to tsunamis and particularly those nearest to earthquake epicentres. Here again, as shown in figure 19, the topography and shelter conditions may cause locally differences as concerns exposure. The data related to volcanism have also been taken from the Swiss Reinsurance Company and RANTUCCI (1994). The affected zones, are directly related to the regional geodynamic framework. South East Asia lies in the area of the planet where the Indian, Eurasian and Pacific plates come into contact. These contacts correspond to the destruction of the lithosphere by subduction, hence a high deep focus earthquake frequency. Their epicentres surround this southern extension of the Eurasian plate constituted by the continental part of South East Asia.
The map of potential threats (figure 14) and figure 20 showing the possibilities in terms of variety and hazard intensity , highlight three groups of countries.
Key to figure 14
1. Lines of equal frequency of cyclones using data normalized over 100 years.
2. Principal tracks of the major cyclones .
3. Observed and/or partly estimated paths of three
4. Areas vulnerable to river floods.
Exposure to earthquakes
* The Philippines and Bangladesh
These two countries both have a very high degree of exposure to several threats but show notable differences. Bangladesh is affected almost only by hydro-meteorological phenomena (cyclones, floods, and droughts), while all hazard types are potentially present in the Philippines.
* Myanmar, Vietnam and Laos
Myanmar, Vietnam and to a certain extent Laos are affected by most of hydro-meteorological and induced (mass movements) phenomena and have a potentially high degree of exposure. The earthquake threat is low except in Myanmar. Volcanic activity does not exist.
* Thailand and Cambodia
The principal threat in these countries is flooding.
Historical data on natural disasters in the areas affected by hazards will supplement the information given above.