What exactly is a 100-Year Flood?
Drainage/Storm Drains
The phrase "100-year flood" still seems to cause confusion among the public, lenders, and insurers. Many continue to believe it is a description of a flood that occurs only once every 100 years. In fact, "100-year flood" is an abbreviated way of describing the magnitude of a rainfall and subsequent flood event that has a 1-percent chance of occurring. It is important to note that the same statistical chances apply for any storm at any time in each year.  

Floods are classified according to their frequency and depth. For instance, there are 10-year, 25-year, 50-year, 100-year, and 500-year floods. A 100-year flood occurs less frequently than a 10-year flood, but because it has larger volume and greater depth of water, is far more destructive and damaging, and is a more serious threat to human safety. The NFIP adopted as a national standard a "100-year floodplain" to describe Special Flood Hazard Areas (SFHAs) that are depicted on the Flood Insurance Rate Maps (FIRMs) as Zone A. Due to the confusion it created, use of the term "100-year floodplain" has been replaced with the newer designation of "base flood". Base Flood Elevation (BFE) requirements are listed on FIRMs and are used on Elevation Certificates to indicate the expected depth of water should a flood occur. New buildings constructed in SFHAs are required to have their lowest floors at or above the BFE listed for that location on the current FIRM.  

Buildings located in 100-year flood areas are required to have flood insurance as a condition of receiving a federally backed mortgage loan or home equity loan. Given that many mortgages have a repayment period of 30 years, buildings in areas subject to a 100-year flood have a 26 percent chance of experiencing that flood during the life of the loan. However, during that same period, there is only a 4 percent chance of a fire.  

So, the next time someone who lives in a 100-year floodplain--and the last 100-year flood was three years ago--says they don't have to worry about another flood for 97 years, refer them to this information to explain the real meaning of a 100-year flood.  

Should you have any further questions, please contact Kamran Dadbeh at (714) 229-6740.

(Information excerpted from Watermark, Fall/Winter 1998, a FEMA/National Flood Insurance Program newsletter.) 
Created on July 28, 2002 at 11:55 PM (PDT). Last updated by CityEngineer on May  4, 2010 at  9:58 AM (PDT). Owned by cmcmahon@ci.cypress.ca.us.
Catherine McMahon
Public Works Director
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