You may have been told that your home is in “the 100-year” floodplain, or you may have heard that term used to describe a specific flood. You may also have heard similar terms used, such as “50-year flood” or “500-year flood.” These terms can be misleading if not interpreted correctly. Flood frequencies are usually determined through statistics and engineering analyses performed by floodplain management agencies and other organizations responsible for implementation of flood control programs and floodplain regulation. The results of those analyses define the probability, expressed as a percentage, that a flood of a specific size on a specific stream will be equaled or exceeded in any year.
For example, the flood that has a 1-percent probability (1 in 100) of being equaled or exceeded in any year is sometimes referred to as the 100-year flood. This term is simply a convenient way to express probability. It should not be interpreted to mean a flood that happens exactly once every 100 years. Nor does it imply
that, once a 100-year flood occurs, there is little risk of another 100-year flood occurring in the near future. To the contrary, changes in climatic conditions, such as those caused by El Niño, often result in “clusters” of floods that occur over relatively short times at the same location. In this publication, the term 1-percent annual chance flood is used to describe the 100-year flood.
For most homeowners, the value of these terms is that they indicate relative frequencies and sizes. Over time, a 1-percent annual chance (100-year) flood is expected to occur less often than a 2-percent annual chance (50-year) flood and more often than a 0.2-percent annual chance (500-year) flood. In addition, a 1-percent annual chance flood will be more severe than a 2-percent annual chance flood and less severe than a 0.2-percent annual chance flood. For example, if your home is in the 1-percent annual chance floodplain of a nearby stream or river, the 1percent annual chance flood elevation at your home will be lower than the 0.2-percent annual chance flood elevation, and the water from a 2-percent annual chance flood might not even reach your home.
The 1-percent annual chance flood is particularly important for homeowners because it is the basis of NFIP flood insurance rates and regulatory floodplain management requirements. These requirements are discussed in detail in Section 2.6. In the NFIP, the 1-percent annual chance flood is referred to as the “base flood,” the 1-percent annual chance flood elevation as the “base flood elevation” (BFE), and the floodplain associated with the base flood as the Special Flood Hazard Area (SFHA). Other Federal agencies, such as the U.S. Army Corps of Engineers (USACE), use the 1-percent annual chance flood for planning and engineering design, as do many State and local agencies. These agencies often have their own names for the 1-percent annual chance flood.