Flood Maps 101 - Understanding the Basics
FEMA uses the best available statistical data (such as river flow, storm tides, hydrologic/hydraulic analyses, and rainfall and topographic surveys) to determine risk and outline a community’s flood map.

Additional considerations in determining a community’s flood risk include: rainfall and river flow data, topography, wind velocity, tidal surge, flood control measures, building development (existing and planned) and community maps.

Floodplains and areas subject to coastal storm surge are shown as high-risk zones or Special Flood Hazard Areas (SFHAs). Some parts of floodplains may experience frequent flooding while others are only affected by severe storms.
How Flood Risk is Determined
  • Moderate- to low-risk areas are labeled Zone X (or Zones B and C on older maps)
  • High-risk areas begin with the letters A or V
  • Areas where the risk is not known are shown with the letter D
FIRMs reflect the designated flood zones as follows:
  • The height that flood waters can be expected to reach during a major flood
  • A factor used in determining flood insurance premiums
  • Used by participating communities in making floodplain management decisions
  • The regulatory requirement for the elevation or flood-proofing of structures
FIRMs also reflect Base Flood Elevations (BFEs), which are: