Public Works

100-Year Flood Explained

There's no way to predict when the next flood will come or how big it will be. However, past flooding gives some clues as to what to expect. Engineers studying past floods use statistics to estimate the chance that floods of various sizes will occur.

For example, a relatively common small flood of a certain size might be expected, from experience, to occur 33 times over a 100-year period. It would be expected to happen on an average of once every third year or, said another way, have a one in three or 33 percent chance of happening in any particular year. It would be called the three-year flood or the 33 percent chance flood.

A larger (more unusual and less frequent) flood found to occur on the average of 10 times in 100 years would be called the 10 percent chance flood or the 10-year flood.

The flood so large and unusual that it only occurs on the average of once every hundred years would have a one percent chance of occurring in any particular year and be called the 100-year flood or one percent chance flood.

This doesn't mean that a 10 percent flood occurs exactly once every 10 years. A rainy year might have several 10 percent floods and then there might not be another for many years. Similarly, two or more large floods, like the 100-year flood or even the 500-year flood, could occur back to back.

The percentage chance of a flood occurring is based on the average of what is expected over a long period of time.

The chance of a flood of a certain size occurring and then the same or bigger flood happening right away is like flipping a coin. Just because heads comes up doesn't mean that the next try has to be tails. Each time the coin is flipped there is a 50/50 chance for either heads or tails. In the same way, when one flood has passed, the chances are reset. A one percent flood has a one percent chance of occurring in any one year. And, as soon as it does happen, the chances are still one percent that it will occur again sometime during the following 365 days.