Floods are among the most frequent and costly natural disasters in terms of human hardship and economic loss. As much as 90% of the damage related to all natural disasters (excluding droughts) is caused by floods and associated debris flows. Most communities in the United States can experience some kind of flooding. Over the 10-year period from 1988 to 1997, floods cost the nation, on average, $3.7 billion annually. The long-term (1940 to 1999) annual average of lives lost is 110 per year, mostly as a result of flash floods.