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Misnomer: 100-Year Flood

May 13, 2011

Guest post by Jason Gunderson.

In light of recent and frequent flooding from the Mississippi delta to my home in Eastern North Dakota, I thought I would address a common misconception among people who live in flood-prone areas: the 100-year flood standard. When most of us hear the term, “100-year flood,” we think of a flood that is so bad it will only come around every 100 years or so. In other words, it’s not something we should worry about, since it would be such a rare event. Nothing could be further from the truth.

If you live within a 100-year flood plain, or are protected by a levee built to withstand the 100-year flood, you need to understand that your chances of losing everything to a flood are greater than 50:50. Why? Because the term “100-year flood” is a dumbed down label for what is really termed a “1% flood.” The 1% flood is a flood that is so high it only has a 1% chance of occurring each year. That still doesn’t sound very dangerous, but the distinction between the terms is important. A 100-year flood won’t happen this year because, well, where are we in those 100 years? Heck, we’re probably only in the 30s. No problem. Oh wait, there was a record flood in 1997? Great! We ought to be safe until 2097! On the other hand, with a 1% flood, there is a small chance for devastating floodseach and every year. That chance seems small at only 1%, but it is real, and it’s not as small as you think.

When looking at the chances of something happening, it’s often useful to examine the opposite and then back out the answer you are really after. Let’s do that in this case. So, if there’s a 1% chance of something happening, what is the chance that it WON’T happen? 99% of course. Now let’s bring time into the mix, since we’re talking about your chances of getting flooded out over the years. What are the chances of your NOT getting flooded two years in a row? It’s the chance each year, multiplied together: 99% x 99% = 98%. Three years in a row? 99% x 99% x 99% = 97%. You get the idea. What about 15 years? Rather than writing out 99% fifteen times, I’ll use a little shorthand: 99% ^ 15. Take 0.99 (the decimal form of 99%) and multiply it times itself 15 times. That ends up being slightly over 86%. So if you live for 15 years in a flood-prone area, protected by a levee constructed to meet the 100-year flood standard, there is an 86% chance you will be fine, based on statistics alone. To put it another way–to back out the answer you are looking for–if there is an 86% chance of the flood NOT happening, there is a 14% chance it will.

What happens to your odds if you spend your whole life in a flood-prone area? I have three words for you: buy flood insurance. The average life expectancy in the U.S. is 78 years. If you were to spend all 78 of them in our locale in question (could be Fargo, St. Louis, Memphis, New Orleans, or any of thousands of other riverside communities), your chances of NOT experiencing a 1% flood are only 46%. In other words, there is a 54% chance you will lose everything if you are protected only to the 100-year flood standard. There is a fifty-four percent chance you will get hit! What about twice in your life? That’s not all that uncommon, either: 54% x 54% = 30%.

If there was a 54% chance you would lose all of, say, your retirement savings, and a 30% chance you might lose it TWICE, I have a feeling you’d seriously consider protecting it. You’ll likely take the same outlook with your worldly possessions. If you want to live in an area prone to flooding, buy flood insurance. Chances are, you’re going to need it.

The term “100-Year Flood” is misleading. It almost mythologizes the flood into a great, watery beast that only stirs once in a century. In fact, the 100-year flood is more accurately called a 1% flood, as there is a small chance it can come along at any time. And if you live in an area long enough, chances are you will experience it. Maybe twice.

Jason Gunderson is Vice President at contractor Terra Pacific Midwest, Inc., and earned an MBA from Vanderbilt University.  He can be reached at .


From → Natural Hazards

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