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Flood Zones and Terms

Base Flood: The flood having a 1% chance of being equaled or exceeded in any given year, also known as the "100-year" or "1% chance" flood. The base flood is a statistical concept used to ensure that all properties subject to the National Flood Insurance Program are protected to the same degree against flooding.

BFE: Base flood elevation. The elevation of the base of 100-year flood.

SFHA: Special Flood Hazard Area. The base floodplain delineated on a Flood Insurance Rate Map. The SFHA is mapped as Zone A. The SFHA may not encompass all of the community’s flood problems.

Zone A

The SFHA shown on a community’s FIRM.

There are 7 types of A Zones.

  1. A: SFHA where no base flood elevation is provided.
  2. A#: Numbered A Zones (e.g., A7 or A14), SFHA where the FIRM shows a base flood elevation in relation to National Geodetic Vertical Datum (NGVD) of 1929.
  3. AE: SFHA where the base flood elevations are provided. AE Zone delineations are now used on new FIRMs instead of A# Zones.
  4. AO: SFHA with sheet flow, ponding, or shallow flooding. Base flood depths (feet above grade) are provided.
  5. AH: Shallow flooding SFHA. Base flood elevations in relation to NGVD provided.
  6. AR: a temporary designation for an area where a flood control is expected to be improved so it will provide protection to the base flood again in the future. This zone is considered a SPHA or "regulatory floodplain" for CRS purposes.
  7. A99: A mapped floodplain that will be protected by a federal flood protection system where construction has reached specified statutory milestones. This zone is considered a SPHA or "regulatory floodplain" for CRS purposes.

Zone B

Area of moderate flood hazard usually depicted on FIRMs as between the limits of the base and 500-year floods of the primary source of flooding. B Zones may have local, shallow flooding problems. B Zones are also used to designate area protected by levees and base floodplains of little hazard, such as those with average depths of less than 1 foot.

Zone C

Area of minimal flood hazard, usually depicted on FIRMs as above the 500-year flood level of the primary source of flooding. C Zones may have local, shallow flooding problems. B and C Zones may have flooding that does not meet the criteria to be mapped as a SPHA, especially ponding and local drainage problems.

Zone D

Area of undetermined but possible flood hazard.

Zone X

Newer FIRMs show Zones B and C as Zone X. The shaded Zone X corresponds to Zone B and the unshaded Zone X corresponds to Zone C.