مهندسی بلایای طبیعی شهری و کاهش آنها On civil engineering disasters and their mitigation

1 Introduction Disasters have been a part of human experience from earliest times and have been signifi cantly impacting human development and civilization. From a modern scientifi c viewpoint, a disaster is an abrupt event that leads to loss of human lives, properties, resources, or environmental wellbeing, exceeding the capacity of the hazard-bearing body, a term that is used herein to referred to any exposure of human society to a hazard. The following four characteristics are inherent to the above defi nition of disasters. Firstly, disasters are consequent to the presence of human beings and communities as hazard-bearing bodies. There would be no disasters if there were no humans; violent changes and movements have occurred since the beginning of the earth, but did not constitute disasters until the appearance of man. Secondly, disasters are uniquely expressed in terms of losses by human beings and communities. Such losses are not limited to life and property, but also include natural resources and the environment. Thirdly, there is a threshold of the extent of loss due to an event for it to be considered a disaster. In other words, not all losscausing events are considered disasters. For example, the collapse of an ordinary warehouse, everyday car accidents, and robberies may cause losses, but such are not generally categorized as disasters. The loss threshold for categorizing an event as a disaster primarily depends on the capacity of the hazard-bearing body to resist the disastrous event and accommodate the loss. While a car accident may constitute a disaster for a family, it is far from a disaster to the city. Moreover, there are always fortunate individuals or families that remain intact during major earthquakes, which may otherwise constitute devastating disasters to local communities. Hence, the aim of disaster mitigation is not necessarily to eliminate loss entirely, but to decrease the loss below the disaster threshold, which is often a more practical strategy. Finally, the above defi nition of disaster emphasizes the abruptness of the event. Disastrous events are typically abrupt, such as earthquakes, landslides, aviation accidents, and terrorist attacks. However, some disastrous events may occur gradually, such as global warming due to excessive and extended carbon emission, metropolitan smog due to air pollution, and the desertifi cation of a forest or prairie. In contrast to such gradual events, which are to some extent expected and observed during their development, abrupt disasters take place suddenly without effective forecast or prediction. Their durations are short, but they cause signifi cant and often lasting consequences.