The 100 Year Flood

With the intense storms and flooding that have been experienced lately people are talking more about 100 year events. There are other number events too, such as 10 year, 50 year, and 500 year events. These events can be floods or storms, like a 100 year flood.

I first heard these terms a couple years ago while taking an emergency management course. We were talking about flooding at the time. We learned in class that the number, 100, 50, etc, actually refers to the statistical chance that the event will occur every year. So, a 100 year flood means that every year there is a 1 in a hundred, or 1%, chance that a large scale flooding event will occur. If there is a 1 in a hundred chance that the flood might occur, then one should happen about every 100 years. I found a great image, see below, that helps illustrate the water levels and occurrences of the different events.

Ohio Department of Natural Resources

According to FEMA in their handout titled The 100 Year Flood Myth all different levels of flooding occur and can come with their own label. Lesser floods will most likely occur more often and be labeled 1 year floods or 5 year floods. As the floods grow in intensity the likely-hood of them occurring decreases. So, a 50 year flood won't be as bad as a 100 year flood but is twice as likely to occur. However, the handout points out that floods occurring every couple years, or even every ten years, will end up doing more damage then a flood that occurs only once every 100 years.

The other interesting point made in the handout is that the term 100 year flood is being used more frequently than it should. Not every flood is a 100 year flood! The term sounds good, and catches peoples attention, so the press like to use it. It may seem like 100 year floods are occurring more than they should, but that is most likely due to the floods being mislabeled.

By using the term “100 year flood” inaccurately to describe
floods that are actually much lesser floods, the misconception is
spread that 100 year floods are occurring every few years! In
fact, floods close to this are occurring, but they are only 5 or 10
year events that look so similar to the “big one” that they get
labeled with the same name. This misapplication of the 100 year
term leads to misplaced criticism of the analysis that produced it
and of those engineers who generated it.

Floods are very important to understand, as they can cause a lot of damage. I know people who live by me don't expect flooding and are not prepared for it. I certainly am not. 

When Hurricane Sandy hit New Jersey and New York we were horrified, and still are, at the devastation that the flood waters caused.

PHOTOS: Hurricane Sandy Wipes Out Seaside Heights

Here's The Jersey Shore Devastation That Made
Chris Christie Get So Emotional

I remember seeing the damage that Hurricane Irene caused in Vermont. It was horrible. 

Who would have imagined that flooding in Vermont? 

Massive snowstorms, sure. 

Ice storms... remember the Ice Storm of '98? 

I sure do. But flooding? 

From Waterbury to Wilmington,
Vermonters shocked by Irene's destruction

Photos from The Republican

Flooding always seemed an event reserved for people living by the Mississippi River, in Florida, or New Orleans. The Northeast sure got a rude awakening. 

With climate change leading to more intense storms, will there end up being an adjustment to the 100 year floods?

It is definitely something worth watching.

There is a lot more to discuss about flood waters and flood safety. I might even take a look at flood insurance. So check back soon for more information on this topic!