There are no accurate estimates of the population in the world's floodplains. Even in the United States, only broad estimates are available, but the trends to increased vulnerability are clear. In 1955 U.S. floodplains had 10 million occupants. Thirty years later the number doubled to 20 million and by the mid-1990s about 12% of the national population lived in areas of periodic inundation. One sixth of the nation's floodplains are urbanized, and they contain more than 20,000 communities susceptible to flooding. Half of these communities have been developed since the early 1970s (Burby, 1985; Montz and Gruntfest, 1986; Alexander, 1993).
Many of the people at risk do not understand the potential consequences of the hazards they face. In the United States, flood damages exceed $2 billion annually. Only 20 to 30% of eligible structures are insured against flooding. Federal and state disaster assistance accounts for most of the difference. In the United States, almost two-thirds of the residential flood losses result from events that occur once every 1 to 10 years, even though the 100-year floodplain regulation is standard (Alexander, 1993).
In the United States, floods tend to be repetitive phenomena. From 1972 to 1979, 1900 communities were declared disaster areas by the federal government more than once, 351 were inundated at least three times, 46 at least four times and 4 at least five times. As of 1993, the United States was said to spend $9 billion a year on flood control and $300 million on flood forecasting (Alexander, 1993; Conrad, 1998).
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