What is a “100 Year Flood”?

So this may sound like a simple question with an obvious answer, but in reality it is a lesson in redefining the way to explain flooding.  As we have been witness to first hand here in the Badger State over the past couple of weeks due to heavy rain in southern Wisconsin , many streams and rivers have overflowed their banks.  For example in southeastern Waushara County, the Fox River is currently 3 feet above flood stage, causing major flooding and it has been termed by the National Weather Service as a “25 year flood”.  In other parts of the Midwest , especially along the Mississippi River , such phrases as “100 year” and “500 year flood” have been batted around.  Does this mean that once every 100 or 500 years that flooding of this magnitude happens?  Were there even any ways of knowing accurately the flood levels from 100 years ago, let along 500 years ago?  The answer is probably not.

For starters, the most accurate and detailed weather records may stretch as far back as 150 years and that includes not only rainfall/snowfall and river levels, but also temperatures and sky conditions.  The more important item to answer is that the terms for describing floods as being “X number of years” is wrong.  The better way to describe the flood levels along say the Fox River is that there is a 1 in 25 chance in a year that this could happen.  That translates to a 4% chance that the flood levels would top out at 16 feet near Berlin along the Fox River in 2008.  The same idea holds true for those folks downstream along the Mississippi River where a “1 in 100 year” flood would be better explained as having only a 1 in 100 chance of occurring in any given year (or 1%).  You can follow the math from there for floods that are worst, that could be 1 in 500 or 1 in 1,000 chance of developing.  So where did this terminology for a certain number year flood come from?  That would be thanks to the folks in the insurance industry who use all kinds of statistical probabilities to determine things from flooding, to fires, to car accidents, to theft.  Putting this into insurance industry lingo, a major “100 year flood is a 1 in 100 chance of occurrence of flooding in any given year or return period of once every 100 years.”  For more, check out the fine print on a flood insurance policy.

That being said, when we talk about flooding that may be substantial in the Wisconsin River Valley , our goal as meteorologists is to describe it as accurately as possible.  In essence…"the Fox River is 3 feet above flood stage, at 16 feet, and there is only 4% chance that this could happen in a given year.”  Nevertheless, the main idea to keep in mind is that flooding does happen, no matter if you live next to the Wisconsin River, or in a low lying spot off the beaten path. All we need is a drenching downpour that adds up to an inch or more in a short span of time and a place for that water to run off to, whether that is your yard, the main highway, or the local river.  Thus it is always important to avoid flooded roadways and move to higher ground if your home is threatened by flood waters.

For more info, here are some links:
Flood Safety.com-Assessing Your Flood Risk 
NWS Milwaukee-Flooding in Southern Wisconsin
The National Flood Insurance Program


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