Flooding Terms & Definitions

FEMA Flood Insurance Rate Map (FIRM) Key
Zone A 
The Special Flood Hazard Areas subject to inundation by the one (1%) percent annual chance (100-year) flood event generally determined using approximate methodologies. Because detailed hydraulic analyses have not been performed, no base flood elevations or flood depths are shown.
Zone AE
The Special Flood Hazard Areas subject to inundation by the one (1%) percent annual chance (100-year) flood event determined by detailed methods. Base Flood Elevation are shown.

Zone AO

The Special Flood Hazard Areas subject to inundation by one (1%) percent annual chance (100-year) shall flooding (usually sheet flow on sloping terrain) where average depths are between one and three feet.
Zone V
Coastal flood zone with velocity hazard (wave action); no Base Flood Elevations determined.

Zone VE 
The Special Flood Hazard Areas subject to inundation by the one (1%) percent annual chance (100-year) flood event with additional hazards due to storm-induced velocity wave action three feet or greater. Base flood elevations derived from detailed hydraulic analyses are shown.
Zone X (Shaded) or 0.2% Annual Chance 
Areas of 0.2% annual chance flood.

Zone X

Areas determined to be outside the 0.2% annual chance floodplain.

Flood Hazard Terms

  • 0.2 Percent AnnualChance Flood: The flood that has a 0.2% chance of being equaled or exceeded in any given year.
  • 1 Percent Annual Chance Flood: The flood that has a 1% chance of being equaled or exceeded in any given year.
  • Appeal: The formal objection to proposed or proposed modified Base Flood Elevations (BFEs), submitted by a community official or an owner or lessee of real property within the community during the statutory 90-day appeal period. An appeal must be based on data that show the proposed or proposed modified BFEs are scientifically or technically incorrect.
  • Appeal Period: The statutorily required 90-day period that is initiated when FEMA formally notifies community officials, residents, and other interested parties that FEMA is proposing new or modified Base Flood Elevations (BFEs) and/or new or modified base flood depths. To start the appeal period, FEMA sends a letter to the Chief Executive Officer and floodplain administrator of the affected communities, publishes a notice twice in the local newspaper(s) with wide circulation, and publishes a notice (called a Proposed Rule) in the Federal Register. During the 90-day appeal period, which begins on the date of the second publication of the BFE notice in the local newspaper(s), community officials or owners or lessees of real property within the community may appeal the proposed or proposed modified BFEs and/or base flood depths by submitting scientific or technical data to show that those BFEs or base flood depths are scientifically or technically incorrect. The 90-day appeal period is statutory and cannot be extended for any reason.
  • Back-Flow Valve: A device used to prevent sanitary sewer lines from backing up into the home when the system becomes flooded.
  • Base Flood: The flood that has a 1% chance of being equaled or exceeded in any given year.
  • Base Flood Elevation (BFE): The elevation of a flood having a 1% chance of being equaled or exceeded in any given year.
  • Base Flood Depth: The depth shown on the FIRM that indicates the depth of water above highest adjacent grade resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year.
  • Biggert-Waters Flood Insurance Reform Act of 2012 (BW-12): An act passed by Congress requiring FEMA to make changes to the NFIP in order to make it more financially sound.
  • Break-Away Wall: A wall that is not part of the structural support of the building and is intended through its design and construction to collapse under specific lateral loading forces, without causing damage to the elevated portion of the building or supporting foundation system.
  • Coastal A Zone: This is an area of special flood hazard extending inland to the limit of the 1.5-foot breaking wave.
  • Coastal Base Flood Elevations (BFEs): The 1% annual chance flood elevations shown on a FIRM or DFIRM within the coastal high hazard area. Coastal BFEs can be calculated using the following equation: Stillwater Elevation + Wave Height = Coastal BFE.
  • Coastal Flooding: This is flooding that occurs along the Great Lakes, the Atlantic and Pacific Oceans, and the Gulf of Mexico.
  • Coastal Flood Study: The term used to describe the engineering analysis of flood hazards along the Gulf or Atlantic coast performed by FEMA using the ADCIRC computer model, with the results of that analysis being reflected on the Digital Flood Insurance Rate Maps and associated Flood Insurance Study reports for the affected counties.
  • Comment: An objection to any information, other than BFEs, shown on an NFIP map that is submitted by community officials or interested citizens through the community officials during the appeal period.
  • Community: Any State or area or political subdivision thereof, or any Indian tribe or authorized tribal organization, or Alaska Native village or authorized native organization, which has the authority to adopt and enforce floodplain management regulations for the areas within its jurisdiction.
  • Community Coordination Meeting: A meeting during which Project Team members discuss plans for a study/mapping project, interim results of a study/mapping project, and final results of a study/mapping project for a particular community or group of communities.
  • Community Rating System (CRS): A FEMA initiative, established under the NFIP, to recognize and reward communities that have implemented floodplain management measures beyond the minimum NFIP requirements. Under the CRS, those communities that choose to participate voluntarily may reduce the flood insurance premium rates for property owners in the community by taking these additional actions.
  • Compliance/Adoption Period: The period that begins with the issuance of a Letter of Final Determination and ends when a new or revised DFIRM or FIRM becomes effective, during which a community must enact and adopt new or revised floodplain management ordinances required for participation in the NFIP.
  • Digital Flood Insurance Rate Map (DFIRM): A Flood Insurance Rate Map that has been prepared as a digital product, which may involve converting an existing manually produced FIRM to digital format, or creating a product from new digital data sources using a Geographic Information System environment. The DFIRM product allows for the creation of interactive, multi-hazard digital maps. Linkages are built into an associated database to allow users options to access the engineering backup material used to develop the DFIRM, such as hydrologic and hydraulic models, Flood Profiles, data tables, Digital Elevation Models, and structure-specific data, such as digital elevation certificates and digital photographs of bridges and culverts.
  • Duration: In wave forecasting, the length of time the wind blows in nearly the same direction over the fetch, or "generating area."
  • Effective Base Flood Elevations (BFEs): The BFEs that are shown on the effective map (FIRM or DFIRM) that is in effect for a community for flood insurance and floodplain management purposes.
  • Effective Date: The date on which the NFIP map for a community becomes effective and all sanctions of the NFIP apply.
  • Effective Map: The NFIP map issued by FEMA, usually a FIRM or DFIRM, that is in effect as of the date shown in the title block of the map as "Effective Date” "Revised" or "Map Revised" and is to be used by the community and others for flood insurance and floodplain management purposes.
  • Elevation Certificate: The form, developed by FEMA, to be used to provide elevation information necessary to ensure compliance with community floodplain management ordinances, to determine the proper insurance premium rate, and to support a request for a Letter of Map Amendment or Letter of Map Revision Based on Fill.
  • Federal Emergency Management Agency (FEMA): An agency of the U.S. Department of Homeland Security that oversees the administration of the NFIP.
  • Federal Insurance Administration: An agency of the U.S. Department of Homeland Security and FEMA that, among other responsibilities, administers the NFIP.
  • Flood: A general and temporary condition of partial or complete inundation of normally dry land areas from: 1) the overflow of inland or tidal waters; or, 2) the unusual and rapid accumulation or runoff of surface waters from any source.
  • Flood Boundary and Floodway Map (FBFM): The floodplain management map issued by FEMA that depicts, based on detailed flood hazard analyses, the boundaries of the 1 percent annual chance (100-year) and 0.2% annual chance (500-year) floodplains and, when appropriate, the regulatory floodway. The FBFM does not show flood insurance risk zone. or BFEs.
  • Flood Insurance Rate Map (FIRM): The insurance and floodplain management map produced by FEMA that identifies, based on detailed or approximate analyses, the areas subject to flooding during a 1% annual chance (100-year) flood event in a community. Flood insurance risk zones, which are used to compute actuarial flood insurance rates, also are shown. In areas studied by detailed analyses, the FIRM shows BFEs to reflect the elevations of the 1% annual chance flood. For many communities, when detailed analyses are performed, the FIRM also may show areas inundated by 0.2% annual chance (500-year) flood and regulatory floodway areas.
  • Flood Insurance Study (FIS) Report: A document, prepared and issued by FEMA, that documents the results of the detailed flood hazard assessment performed for a community. The primary components of the FIS report are text, data tables, photographs, and Flood Profiles.
  • Flood Vent: A permanent opening in a wall that allows flood waters to flow freely, reducing damage caused by water pressure.
  • Floodplain: Any land area that is susceptible to being inundated by water from any source.
  • Freeboard: A factor of safety usually expressed in feet above a flood level for purposes of floodplain management.
  • High-Velocity Wave Action: A condition in which wave heights or wave runup depths are greater than or equal to 3.0 feet.
  • Letter of Final Determination (LFD): The letter in which FEMA announces its final determination regarding the flood hazard information, including (when appropriate) proposed and proposed modified BFEs and base flood depths, presented on a new or revised FIRM, FIS report, and (when appropriate) FBFM for a particular community. In the LFD, FEMA begins the adoption/compliance period and establishes the effective date for the new or revised FIRM/DFIRM, FIS report, and/or FBFM.
  • Letter of Map Amendment (LOMA): An official determination by FEMA that a property has been inadvertently included in an SFHA as shown on an effective FHBM, FIRM, or DFIRM and is not subject to inundation by the 1 percent annual chance flood. Generally, the property is located on natural high ground at or above the BFE or on fill placed prior to the effective date of the first NFIP map designating the property as within an SFHA. Limitations of map scale and development of topographic data more accurately reflecting the existing ground elevations at the time the maps were prepared are the 2 most common bases for LOMA requests.
  • Limit of Moderate Wave Action (LimWA): A line within the SFHA designated Zone AE on a DFIRM that marks the inland limit of the area inundated by the 1% annual chance, 1.5-foot breaking wave. The LimWA is provided on the DFIRM, for informational purposes, because these moderate waves can cause damage to structures; the damage would not be as severe as the damage caused by the 1% annual chance , 3-foot breaking waves.
  • National Flood Insurance Program: Federal Program under which flood-prone areas are identified and flood insurance is made available to the owners of the property in participating communities.
  • Preliminary DFIRM: The NFIP map that reflects the initial results of a Flood Map Project that is performed by or for FEMA. The Preliminary DFIRM is provided to CEOs and floodplain administrators of all affected communities before the 90-day appeal period is initiated.
  • Special Flood Hazard Area (SFHA): The area delineated on a NFIP map as being subject to inundation by the base flood. SFHAs are determined using statistical analyses of records of river flow, storm tides, and rainfall; information obtained through consultation with a community; floodplain topographic surveys; and hydrologic and hydraulic analyses.
  • Virginia Department of Emergency Management (VDEM): An agency of the State department that provides resources and expertise to local, state, and federal agencies regarding emergency management.
  • Wave: A ridge, deformation, or undulation of the water surface.
  • Wave Height: The vertical distance between the wave crest and the wave trough.
  • Wave Runup: The rush of wave water up a slope or structure.
Definitions taken from FEMA - Region III Coastal Analysis and Mapping, Glossary and Acronyms