Natural disasters represent the effects that a natural hazard affects the social economic system which has a certain level of vulnerability. This vulnerability prevents the affected society from fully addressing impacts. Natural hazard itself does not necessarily cause disasters, and it is the interaction among natural hazards, human beings as well as the environment that may have some influences which may even expand into catastrophes. The strategy of disaster reduction can be taken into consideration only when the technological and environmental disasters are caused by natural hazards. The definition of disaster comes as a serious disruption which destroys the normal operation of the community or society inflicting heavy losses on human beings and environments and the losses usually exceed the ability of the affected society to cope using its own resources.
There are some features of natural disasters: they have inevitability and universality over time and space; they have multitudious types and have a great impact; some may suddenlly burst out and some may take it gradually; they have randomicity and irregular periodicity; they may be generated in a group or in a chain; they can be contral to some extend, etc.
Worldwide, the number of people who died of natural and non-natural disasters in 1980s (86328 people / year) was more than in 1990s (7525 people / year). But the affected population in 1990s increased from an average of 147 million in 1980s to 211 million per year in 1990s. The number of geological hazards remained relatively stable, and the number of hydro meteorological disasters (caused by water and weather) was increased correspondingly. In 1990s, more than 90% of the population died of natural disasters died of hydro meteorological accidents, such as drought, storms and floods. And among the population died of natural disasters, people died of floods made up more than 2/3, while comparing with other disasters, the proportion of deaths by floods was relatively low, which accounted for only 15% of the total number of deaths.
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