100 Year Flood, 500 Year Flood; What’s the Difference?

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100 Year Flood, 500 Year Flood; What’s the Difference?

100 Year Flood, 500 Year Flood; What’s the Difference?

We likely all have recent memories of news coverage of more than one “100 year flood” in our lifetimes.  How can this be if a “100 year flood” occurs once every 100 years?  While climate change could certainly have a role, the fact is that it is an oversimplification of statistics.

A common misunderstanding (rightfully so based upon the name) is that a 100 year flood means the level of flooding that is expected to occur once every 100 years, and a 500 year flood occurs once every 500 years.  However, this in not the case.
A 100 year flood is the level of flooding that has a 1% chance of occurring in any given year, and has an equal chance of occurring every year, regardless of whether or not it occurred in previous years.  It’s similar to buying a lottery ticket – the chances are slim of you getting the winning numbers, and winning one week’s lottery drawing does not impact your chances of winning the next week’s lottery drawing.  So the luck of the draw may result in a community getting hit by 100 year floods 2, 3, 4 years in a row.
Similarly, a 500 year flood is flood levels that have a 0.2% chance of occurring in any given year.  And again, it happening one year does not change the likelihood of it happening the next year, or the next, or the next.
We also call large storms the “100-year storm”….  but in actuality, just like the “100 year flood”, it too has a 1% chance of happening in any given year.
You can learn more at the USGS website here:  100 year flood
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