100-year floodplain

Any area that has a 1% chance of experiencing a base flood in any given year; also called base floodplains

Flooded plain

What is a 100-year flood?

The term “100-year flood” sounds like it’s a flood that only happens every 100 years, but it actually refers to a statistical probability. Specifically, it describes a flood that has a 1% chance of hitting an area in any given year. An area that has a 1% chance of flood is on the 100-year floodplain.

One part of protecting areas near water sources is to figure out the probability of different-sized floods. That way, people know how much flooding to expect and can take measures to guard against damage. The 100-year flood is simply one way of describing the likelihood of an area having a flood of a certain size, and it’s a baseline the National Flood Insurance Program (NFIP) uses for determining when flood insurance is mandatory for an area. For this reason, the 100-year flood is also called the base flood.

It’s important to keep in mind that while the 100-year flood is a statistical expectation, floods can happen more or less frequently. They can also be more severe than anticipated, with water going much higher than expected.

Where is the 100-year floodplain?

NFIP flood maps identify 100-year floodplains throughout the US and group them in flood zones based on their risk. In addition to floodplain boundaries, flood maps show any base flood elevation or flood depths for the area. The following flood zones are in a 100-year floodplain:

Because these zones are on a 100-year floodplain, they are labeled as Special Flood Hazard Areas (SPHA). This makes flood insurance mandatory for anyone with a federally backed-mortgage.

Not every flood zone has a 100-year floodplain. For example, areas with minimal-to-moderate risk for floods, like Zone C and Zone X, are above the 500-year flood level, meaning their chance of seeing a base flood is 0.20% per year. Another example is Zone D which has an undetermined flood risk and may or may not meet the 100-year flood designation.

You can search for your property’s flood zone at the Federal Emergency Management Agency’s Flood Map Service Center.

What is base flood elevation?

The base flood elevation (BFE) is the height the water surface is expected to reach during a 100-year flood in a given area. The NFIP and private insurance companies use BFEs to determine how much to charge for flood insurance, and building contractors also consider BFEs when building homes in flood zones. In fact, the NFIP mandates builders follow certain regulations on 100-year floodplains, such as elevating living spaces above the BFE.

What does being in the 100-year floodplain mean to homeowners?

Most importantly, federally backed and regulated mortgage lenders can only loan money to homeowners in a 100-year floodplain if the homeowner gets flood insurance. That may seem extreme if you focus on the area’s 1% risk for a base flood in a given year. However, you can also think about what that means over a 30-year mortgage, where your chances for a 100-year flood is 26%

A 100-year flood doesn’t happen very often, but when it does it’s catastrophic. Moreover, they aren’t the only flood you have to worry about. Other floods may not be as severe, but they occur more often, particularly if they have a lower elevation or are closer to the water, and can still cause significant damage. For example, if you’re close to the water, you may be subject to a:

  • 45% chance of seeing a 50-year flood.
  • 71% chance of seeing a 25-year flood.
  • 96% chance of seeing a 10-year flood.

Flood risk for owning a home can be very high – especially if you consider the chance of your home catching fire is usually around 1% or 2%.

Protecting homes in 100-year floodplains

Most people with homes in the 100-year floodplain go to the NFIP for flood insurance. However, all NFIP policies have a:

At Kin, we can offer flood insurance attached to your homeowners insurance policy with none of these limitations. In fact, your policy starts the moment you pay for it.


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