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
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
The SFHA shown on a community’s FIRM.
There are 7 types of A Zones.
- A: SFHA where no base flood elevation is provided.
- 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.
- AE: SFHA where the base flood elevations are provided.
AE Zone delineations are now used on new FIRMs instead of
- AO: SFHA with sheet flow, ponding, or shallow flooding.
Base flood depths (feet above grade) are provided.
- AH: Shallow flooding SFHA. Base flood elevations in
relation to NGVD provided.
- 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.
- 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
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.
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
Area of undetermined but possible flood hazard.
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