Flood Safety Awareness Week [100-year flood]

Flood Safety Awareness Week aims to raise public attention to the dangers of flooding and ways to protect life and property. In the USA, on average, flooding kills more people per year than any other severe weather phenomenon. Flooding causes damage averaging 8 billion dollars a year and an average of 95 fatalities per year. Three quarters of all presidential declared disasters result from floods. Brochure includes detailed explanation of the meaning of the 100-year flood:

"You hear it all the time. The river flooded again and it was in excess of the 100-year flood. If you are not mistaken, this was the 2nd 100-year flood in the last 5 years. That just doesn’t make sense! If it was a 100-year flood, shouldn’t I expect another in 100 years?"

"This question has perplexed many a homeowner who has been flooded by the 100-year flood…the 250-year flood…or even the 500-year flood. Actually, the term “100-year flood” is often incorrectly used and can be very misleading. The term does not mean that only one flood of that size will occur every 100 years. The term is actually a statement of probability that scientist and engineers use to describe how one flood compares to others that are likely to occur. The official phrase of the National Weather Service for what was once known as the 100-year flood is now the “1% annual chance flood”. This means that there is a 1% chance of a flood of that size occurring in any year. Over a 100-year period, it has a 63.5% chance of occurring. Even more surprising is that over a 30 year period (a typical mortgage period) the 1% annual chance flood has a 26% chance of occurring. This means a home in the mapped flood hazard area is five times more likely to be damaged by flood than to have a major fire!"

10-year flood is now the 10% annual chance flood - Chance of occurring over a 30 year period: 96%
50-year flood is now the 2% annual chance flood - Chance of occurring over a 30 year period: 46%
100-year flood is now the 1% annual chance flood - Chance of occurring over a 30 year period: 26%
250-year flood is now the .4% annual chance flood - Chance of occurring over a 30 year period: 11%
500-year flood is now the .2% annual chance flood - Chance of occurring over a 30 year period: 6%

"So when you think you are in the 100-, 250-, or 500-year flood plain and you think you are safe…think twice. Statistical probability can be misleading. You may want to have flood insurance. That is the safe choice!"