Indian River Bay

Flood FAQ: What is a 100-year flood?

Large floods are often designated as 100-year flood events, but the term 100-year flood does not mean that such a flood occurs once every 100 years.

Instead, it means that there is a one in one hundred (or 1 percent) chance of such a flood being equaled or exceeded during any given year. In fact, two 100-year floods could occur a year apart or even a month apart.

Another way of considering the 1 percent annual chance of occurrence is that the 100-year flood has a 26 percent chance of occurring during a 30-year period, the length of many mortgages.

The 100-year flood is a regulatory standard used by Federal agencies and most states to administer floodplain management programs. The 100-year flood is used by the National Flood Insurance Program (NFIP) as the basis for insurance requirements nationwide. Thus, a structure located within a special flood hazard area shown on an NFIP map has a 26 percent or greater chance of suffering flood damage during the term of a 30-year mortgage.

A concern about the confusion associated with using the term, 100-year flood, is that floodplain residents may under-react to or be unprepared for the threat of future flooding. If the last flood experienced is assumed to be a 100-year flood, then a resident might reasonably assume that the recurrence of a flood that bad is a long way off – theoretically, around 100 years, but certainly at least 40 or 50 years away. This is actually very misleading.  If a flood victim believes that the last one was the “big one” and that he will not likely see a similar event again soon, he will not feel compelled to mitigate his damage potential.

What is a flood?

Even the term “flood” needs to be defined.  A flood is normally and broadly defined as the temporary inundation of normally dry land.  The NFIP further defines flood as being an event that affects two or more properties.  For example, a dam failure that inundates many properties is a flood according to the NFIP.  If an above ground pool or water tank fails but the inundation is limited to one property, this would not be considered a flood.  Since land inundation during rainfall events and coastal storms affect areas larger than one property, these events are almost always considered floods.  For specific questions about insurance coverage, you should consult with an insurance agent.

Understanding chance and probability - a bag of marbles

The probability of the 100-year flood occurring can be illustrated with a bag of marbles.  If you have a bag with 100 marbles—of which 99 are white, and one is blue—every time you stick your hand in the bag and pull out the blue marble you will find yourself experiencing a 100-year flood.

Of course, each time you grab the blue marble you have to put it back and shake the bag up before picking again. The laws of probability say that there is always a chance of picking the blue marble one, two, or even three times in a row.  And when consideration of flood probabilities (which are never known with perfect certainty) are added into the mix, that bag of marbles could also contain two or three additional blue marbles, thereby increasing the probability of picking a blue marble each time.

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Page Updated on October 13, 2014