There are few places on Earth where people don't have to worry about flooding. Any place where rain falls is vulnerable, even if rain is not the only factor in flooding. Flooding occurs when water overflows or floods normally dry land. This can happen in multiple ways. The most common occurs when rivers or streams overflow their banks. Excessive rain, the failure of a dam or levee, rapid melting of ice in the mountains, or even a poorly placed beaver dam can overwhelm a river and cause it to spread onto adjacent land, called floodplains. Coastal flooding occurs when a large storm or tsunami causes the sea to rise inland. Most floods take hours or even days to develop, giving residents plenty of time to prepare or evacuate. Others generate quickly and with little warning. These flash floods can be extremely dangerous, instantly turning a babbling stream into a thundering wall of water and sweeping away everything in its path downstream. Disaster experts classify floods based on their likelihood of occurring within a given time period. A 100-year flood, for example, is an extra...