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What do the letters on a FEMA Flood Insurance Rate Map (FIRM) mean?

This page explains them.

A 1-percent-annual-chance flood event area is otherwise known as a 100-year flood zone.   A 0.2-percent-annual-chance flood event area is otherwise known as a 500-year flood zone.

 

If an area is not in a designated flood zone, does that mean that it will not flood?

No.  Note the following statement from the FEMA website:

"Areas of moderate or minimal hazard are studied based upon the principal source of flood in the area. However, buildings in these zones could be flooded by severe, concentrated rainfall coupled with inadequate local drainage systems. Local stormwater drainage systems are not normally considered in a community’s flood insurance study. The failure of a local drainage system can create areas of high flood risk within these zones. Flood insurance is available in participating communities, but is not required by regulation in these zones. Nearly 25-percent of all flood claims filed are for structures located within these zones."

 

Floodplain Management in Tennessee

This document gives a very thorough overview of floodplain management concepts, the National Flood Insurance Program (NFIP), Special Flood Hazard Areas (SFHA's), and building in or near flood-prone areas.