Flood Map Terminology

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FEMA and the National Flood Insurance Program use quite a few terms that can be at some times quite technical to describe flood processes. For a complete list of terms, please see the FEMA Glossary document links at the bottom of the screen.

Here are a select few terms which are used regularly within our area:

Base flood depth: A measurement of the base flood in feet above ground, used for shallow flooding.

Base flood: The flood having a 1% chance of being equaled exceeded in any given year. Also referred to as the 100-year flood. The base flood is used by the NFIP as the basis for mapping, insurance rating, and regulating new construction.

Base floodplain: The area of water and land inundated by the base flood.

Coastal high hazard area: That part of the coastal floodplain extending from offshore to the inland limit of the primary coastal dune along an open coast and any other area subject to high velocity wave action from storms and seismic sources. Wave heights during the base flood will generally be three feet or more in height above the stillwater elevation.

Community Rating System: A program that provides a flood insurance premium rate reduction based on a community’s floodplain management activities.

FIRM: Flood Insurance Rate Map. An official map of a community, on which the Federal Emergency Management Agency has delineated both the Special Flood Hazard Areas and the risk premium zones applicable to the community.

Flood: A general and temporary condition of partial or complete inundation of normally dry land areas.

Flood fringe: The portion of the floodplain lying outside of the floodway.

Flood hazard mitigation: All actions that can be taken to reduce property damage and the threat to life and public health from flooding.

Flood Insurance Study: A report published by FEMA for a community issued along with the community's Flood Insurance Rate Map (FIRM). The study contains such background data as the base flood discharges and water surface elevations that were used to prepare the FIRM.

Floodplain: Any land area susceptible to being inundated by flood waters from any source.

Floodway: The channel of a river or other watercourse and that portion of the adjacent floodplain that must remain open to permit passage of the base flood without cumulatively increasing the water surface elevation more than a designated height (usually one foot).

Freeboard: A margin of safety added to the base flood elevation to account for waves, debris, miscalculations, or lack of data.

Letter of Map Amendment (LOMA): An official revision to a FEMA map done by describing the property affected. LOMAs are generally issued when properties have been inadvertently included in the floodplain.

Letter of Map Revision (LOMR): An official revision to a FEMA map done by describing the property affected.

Special Flood Hazard Area (SFHA): the base floodplain displayed on FEMA maps. It includes the A and V zones.

Storm surge: Water that is pushed toward shore by persistent high wind and changes in air pressure. Storm surges can result from hurricanes and other coastal storms.

Stormwater management: Efforts to reduce the impact of increased runoff that results from new development.

Substantial improvement: Any reconstruction, rehabilitation, addition or other improvement to a structure, the total cost of which equals or exceeds 50 percent of the market value of the structure before the start of construction of the improvement. The definition of “substantial improvement” includes buildings that have incurred “substantial damage” regardless of the actual repair work performed. 

Watershed: An area that drains into a lake, stream or other body of water.

Zone A: The Special Flood Hazard Area (except coastal V Zones) shown on a community's Flood Insurance Rate Map. There are five types of A Zones:

A: SFHA where no base flood elevation is provided.
A1-30: Numbered A Zones (e.g., A7 or A14), SFHA where the FIRM shows a base flood elevation in relation to NGVD.
AE: SFHA where base flood elevations are provided. AE Zone delineations are now used on new FIRMs instead of A# Zones.
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 are provided.

Zone B: Area of moderate flood hazard, usually depicted on Flood Insurance Rate Maps as between the limits of the base and 500-year floods. B Zones are also used to designate 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 Flood Insurance Rate Maps as above the 500-year flood level. B and C Zones may have flooding that does not meet the criteria to be mapped as a Special Flood Hazard Area, especially ponding and local drainage problems.

Zone D: Area of undetermined but possible flood hazard.

Zone V: The Special Flood Hazard Area subject to coastal high hazard flooding. There are three types of V Zones: V, V1-30, and VE, and they correspond to the A Zone designations.

Zone X: Newer Flood Insurance Rate Maps show Zones B and C (see above) as Zone X. Unit 3, Section F.