From Massicecert.com (Josh Price)
I use the term “100-year flood zone daily for elevation certificates, LOMA’s, and explaining flood maps. But it doesn't mean what you might think. It means there is a 1% chance you will see a flood like the one on the FEMA flood map each and every year. Since 1% is also "1 out of 100", the term "100-year flood" was adopted because that's easier to talk about than rattling off a bunch of statistics.
FEMA sometimes shows a 500-year flood on their maps and that is technically the 0.2% annual chance flood. Try saying "zero point two percent annual chance flood zone" two dozen times a day and you can see why we use the short version.
People say all the time that their property hasn't flooded in 100 years so there's no way it's in a 100-year flood zone. And that's a pretty reasonable thing to say when we use the "100-year-flood" term all the time rather than putting the meaning behind it. Unfortunately, it's wrong.
You could see many "100-year" floods in the same town in the same year because it's just a statistical average based on past precipitation records and we never know what mother nature will do. You could roll snake eyes twice in a row or never see it in 100 rolls.
This is also dangerous. You might look at a FEMA flood map and think that because you are on the outside of the 100-year flood zone line (e.g. the 1% flood) that you are safe from flooding. But maybe you are now in the 0.99% annual chance flood zone or the 0.75% annual chance flood zone. With enough rain, anything will flood, and we still can’t predict the weather very well. So we are really all in a flood zone of some sort and it's just a matter of which one. If you are close to the 100-year flood, your risk may not have been significantly reduced.
A line had to be drawn somewhere to make the FEMA flood maps. A 1% annual risk of being flooded was the line FEMA drew. Being in or out of the 100-year flood zone is just the requirement for mandatory flood insurance purchase. It’s a bare minimum standard and it doesn’t mean you won’t flood.
So, even if you are outside of the 100-year flood zone, you are likely close to the 100-year flood zone, and therefore you are carrying almost the same risk but the insurance cost is very low. We recommend you get a flood insurance policy to cover you and your family.
There’s one more factor to consider. If your structure is right on top of the 100-year flood line on the map, you carry a 1% annual risk of flooding. But if you are closer to the flooding source (e.g. river, stream, pond, etc.) you have more risk. The closer you get to the flooding source, the more the ‘years’ go down. You could be in the 50-year floodplain, or the 25-year floodplain, or the 10-year floodplain as you move closer to the flooding source. FEMA frequently reports these other floodplain frequencies in the Flood Insurance Study (FIS), which is a report that accompanies the maps, but they aren’t on the maps because it’s too much information to clearly draw on the map.
If we use those same statistics, over the course of a 30-year mortgage, this is the flood risk you could be exposed to…
The 25-year flood zone gives you a 71% chance of being flooded
The 50-year flood zone gives you a 45% chance of being flooded
The 100-year flood zone gives you a 26% chance of being flooded
The 500-year flood zone gives you a 6% chance of being flooded
When you look at the 100-year flood zone that way, you have a 1 in 4 chance of experiencing flood damage during your 30-year mortgage. And you can see that being just a bit on one side or the other of the 100-year line doesn’t eliminate the risk to flooding even if it may eliminate the mandatory insurance requirement.
So, the 100-year flood might not have meant what you thought it meant.
Stay safe.