flood (44 CFR 64.3) and SFHAs that are either small (drainage areas of less than 1 square mile), expected to flood less than 1 foot, or protected by levees from the 1 percent annual chance flood. Flood insurance is voluntary, although lenders may require flood insurance for structures. In addition, communities may choose to regulate land use and siting of critical services and emergency response facilities in these areas.

  1. Areas in which flood hazards are minimal (e.g., less than a 0.2 percent annual chance of flooding) or undetermined, but still possible. These areas are not subject to federal regulations on insurance or land use, although communities and lenders may impose such requirements.

Each of these areas is divided into flood insurance rate zones, which designate the level and type of flood hazard (Box 2.1). The majority of SFHAs are either riverine and lacustrine (area along the shore of a lake or closed water basin) A zones (subject to a 1 percent annual chance flood) or coastal A zones and V zones (subject to storm surge where wave heights for the 1 percent annual chance flood are 3 feet or greater). Moderate flood areas are designated as shaded Zone X, and areas of minimal flood hazard include unshaded Zone X and zones for which flood hazard has not been determined. Example portions of FIRMs showing some of these zones in a riverine area and a coastal area are shown in Figures 2.1 and 2.2.

FEMA’s Map Modernization Program was intended to produce digital FIRMs for all of the nation’s 1 percent annual chance floodplains, but a midcourse adjustment gave priority to densely populated areas, where more lives and property are at risk (FEMA, 2006a). Risk-related priorities were based on total population, rate of population growth, number of housing units, number of flood insurance policies and claims, number of repetitive loss properties and claims, and number of declared flood disasters. This decision shifted emphasis from the risk of occurrence of a 1 percent annual chance flood to the risk of more significant flood damage.

BOX 2.1

Definitions of the Most Common Flood Insurance Rate Zones

Zone A: Special Flood Hazard Area (SFHA), defined as land subject to a 1 percent annual chance of flooding. The zone is divided into several subtypes, including

  • A (or unnumbered or approximate A): SFHA in which detailed analyses were not carried out and the base flood elevation is not shown.

  • AE, A1 through A30: SFHA in which the water surface elevation has been determined and is shown on the map.

Zone V: Coastal SFHA subject to high velocity wave action from storms or seismic sources. The zone is divided into several subtypes, including

  • V (or unnumbered V): Coastal SFHA for which water surface elevations are not shown.

  • V1 through V30, VE: Coastal SFHA with velocity hazard and water surface elevation determined and shown on the map. The VE designation is replacing the earlier numbered V designations.

Shaded Zone X, Zone B: Area of moderate flood hazard or future conditions flood hazard, generally defined as the 0.2 percent annual chance flood.


Unshaded Zone X, Zone C: Area of minimal flood hazard, commonly understood to have a lower probability of flooding than the moderate hazard area.


The numbers for zones A1 through A30 were determined by computing the difference between the 1 percent annual chance and 10 percent annual chance flood elevation, multiplying by 10, then applying a conversion factor (FEMA, 1983). The process was similar for numbered V zones, although different multiplication and conversion factors were used. Modernized maps have replaced the A1 through A30 designations with an AE designation, and the B and C designations with an X designation.


SOURCE: 44 CFR 59.1 and 44 CFR 64.3.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement