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FEMA Flood Hazard Zone Definitions

Special Flood Hazard Areas Subject to Inundation by the 1% Annual Chance Flood

The 1% annual flood (100-year flood), also known as the base flood, is the flood that has a 1% chance of being equaled or exceeded in any given year. The Special Flood Hazard Area is the area subject to flooding by the 1% annual chance flood. Areas of Special Flood Hazard in Pima County include Zones A, AE, AH, and AO. The Base Flood Elevation is the water-surface elevation of the 1% annual chance flood.

The Flood Disaster Protection Act of 1973 requires federally insured lenders to have flood insurance on loans with structures located even partially within a federally mapped Special Flood Hazard Area (SFHA) floodplain. The District recommends that property owners purchase flood insurance for any structure within or immediately adjacent to a federal SHFA, federal Other Flood Area (OFA) or locally mapped floodplain, regardless of whether or not there is a federally backed loan on the structure.

Other Flood Areas

Flood insurance is not federally mandated in Other Flood Areas (OFA), but the District recommends the purchase of flood insurance for structures within OFAs.

Other Areas

Flood insurance is not federally mandated in Other Areas, but the District recommends that property owners contact their local jurisdiction to determine whether they are impacted by a locally mapped floodplain that is not shown on the DFIRMs. If a structure is impacted by a locally mapped floodplain not shown on the DFIRMs, the District recommends the purchase of flood insurance for the structure.

Special Flood Hazard Areas

FEMA FIRM Zone Designation FIRM Definition Additional Information
ZONE A No Base Flood Elevations determined. The local floodplain jurisdictions may have additional information including Base Flood Elevations.
ZONE AE Base Flood Elevations determined.
ZONE AH Flood depths generally between 1 to 3 feet (usually areas of ponding); Base Flood Elevations determined. In some locations the elevations within channels may be deeper than 3 feet.
ZONE AO 1
ZONE AO 2
ZONE AO 3
Flood depths of 1 to 3 feet (usually sheet flow on sloping terrain); average depths determined. For areas of alluvial fan flooding, velocities also determined.
ZONE AO - ALLUVIAL FAN 1
ZONE AO - ALLUVIAL FAN 2
ZONE AO - ALLUVIAL FAN 3
ZONE AO - ALLUVIAL FAN 4
Same as ZONE AO Alluvial fan flood zones may also be susceptible to erosion, deposition and channel avulsions. Letters of Map Amendment (LOMAs) and Letters of Map Revision (LOMRs) are more difficult to obtain from FEMA within alluvial fan flood zones.

Other Flood Areas

FEMA FIRM Zone Designation FIRM Definition Additional Information
ZONE X - SHADED Areas of 0.2% annual chance flood; areas of 1% annual chance flood with average depths of less than 1 foot or with drainage areas less than 1 square mile; and areas protected by levees from 1% annual chance flood.
ZONE X - PROTECTED BY LEVEE Areas protected by levees from 1% annual chance flood. Areas protected by levees from the 1% annual chance flood will either be shown as "Provisionally Accredited" or "Accredited". If a "Provisionally Accredited" levee can not be certified, future floodplain mapping will no longer show areas behind the levee as being protected by the 1% annual chance flood.

Other Areas

FEMA FIRM Zone Designation FIRM Definition Additional Information
ZONE X Areas determined to be outside the 0.2% annual chance floodplain. ZONE X includes areas where the flood hazard has not been mapped by FEMA but may have been mapped by a local jurisdiction; therefore ZONE X does not necessarily mean there are no flood hazards.
ZONE D Areas in which flood hazards are undetermined, but possible. Generally federal or tribal lands.