Understanding FEMA Flood Maps
Flood Zones Used to Determine Flood Insurances Rates and Requirement
Jul 14, 2009
FEMA has nationwide responsibility for reducing damage from flooding, and for making flood insurance available to flood-prone properties through the National Flood Insurance Program (NFIP). A large part of this is creation of flood maps (called Flood Insurance Rate Maps, or F.I.R.M.) which show flood-prone areas. To fully achieve this, FEMA has created a number of flood zones, each having a different impact on the property owner.
FEMA Maps Show Special Flood Hazard Areas
The layman’s term is “floodplain”. FEMA calls them “Special Flood Hazard Areas”. These are any location that has a fairly good chance of flooding over the life of a structure. FEMA established the 100-year flood (1 percent chance of occurring in any given year) as the base flood, and the elevation to which water will rise in that flood is called the base flood elevation, or BFE. During the term of a 30 year mortgage, a property in a 100-year flood zone has a 26 percent chance of being flooded.
To help establish flood insurance rates and applicability, FEMA defines several different flood hazard areas. As the NFIP has matured since it was founded by Federal legislation in 1968 and 1973, FEMA has tended to update the definitions of flood zones, to use older zones less and newer zones more as flood maps are modernized. The flood zones that might be seen on a F.I.R.M. are summarized as follows. Each of these cannot be discussed in detail in this article.
- Zones B, C, and X: Areas that are outside of the 100-year floodplain. This is actually most of a typical F.I.R.M. Zone X is sometimes used to define the area in the 500-year floodplain, but not the 100-year floodplain.
- Zone A: Areas in the 100-year floodplain, but in an area where a detailed flood study has not yet been prepared and no calculations of BFE have been performed.
- Zone AE, A1-A30: Areas in the 100-year flood plain and where a detailed flood study has been performed. Zones A1-A30 are rarely used any more. Zone AE is the preferred zone designation for modernized flood maps.
- Zone AH: Areas subject to flooding from a 100-year rainfall event, with that flooding coming from a pond or lake. These areas have been studied and BFE’s determined.
- Zone AO: Areas subject to river or stream flooding, and areas that are subject to shallow flooding (1 to 3 feet deep). Average flood depths, rather than BFEs, will have been determined by a detailed study. This flood zone is rarely used.
- Zone AR: Areas with a temporarily increased flood risk as a result of a flood control system under construction or repair.
- Zone A99: Areas subject to the 100-year flood, but which will be protected by a flood control structure, such as a levee. These zones require that the levee have been constructed to specified legal requirements.
- Zone V: Coastal areas subject to a 100-year flood, with additional hazard from storm waves. These areas are determined by approximate analysis, not by detailed calculations.
- Zones VE, V1-V30: Coastal areas subject to a 100-year flood, where a detailed study was performed to determine BFEs.
- Zone D: Any other area that FEMA believes could be subject to flood hazards. These areas have not been analyzed with detailed calculations.
Zone AE is Used for Most Modern Flood Maps
FEMA is constantly updating flood maps through a flood map modernization program. This is especially true in urbanizing areas, where conditions in the floodplain change significantly over a few years. For new or updated maps in inland areas, the preferred flood zone is Zone AE.
The regulations concerning this zone are well defined. Rigorous hydrological and hydraulic modeling are required using computer programs approved by FEMA. The goal of using this zone is to put a well-defined tool in the hands of the local Floodplain Administrator to direct development away from flood-prone areas and provide orderly development for what must be built in a flood hazard zone.
On a flood map, each zone will be shown in gray shading, as shown in the accompanying extract from a flood map. Sometimes different shades of gray will be used if two different zones are adjacent to each other. Flood zones B and C, when used on a flood map, will not be shaded. Zone X is sometimes shaded, sometimes not.
Flood insurance rate maps and the flood zones they show are excellent tools for homeowners, business owners, local floodplain administrators, real estate brokers, and insurance brokers.
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