Understanding Flood Maps and Flooding Factors

Is your property in a designated flood zone?

  • Many properties are within a flood zone, but the risk varies based on your location and the building’s finished floor elevation (FFE).

  • Floods maps are often redrawn by the Federal Emergency Management Agency (FEMA) to reflect new information. You may be among those now considered to be in a newly defined flood zone.

  • Obtain the FEMA flood map for your location, see “Determine your Flood Zone
    • C (unshaded) and X (unshaded), as defined by FEMA are areas of minimal flood hazard, usually depicted on FIRMs as above the 500-year flood level.
      • These zones are considered to be outside of flood prone areas. However, one-third of flooding occurs outside flood zones, according to the National Flood Insurance Program.
    • Zone C (unshaded) may have ponding and local drainage problems that don’t warrant a detailed study or designation as base floodplain.
    • Zone X (unshaded) is the area determined to be outside the 500-year flood level and protected by levee from the 100-year flood level.

If you are located in a flood-prone area:

If you aren’t sure:

  • Check with your city or county building authority, your insurance agent, or your mortgage lender to see what flood risk exists.

  • Floods maps are often redrawn by FEMA to reflect new information and recommendations. Consult your city or county building department to determine how the redrawing affects your property. You may be among those now considered to be in a newly defined flood zone.

What is your building’s Base Flood Elevation (BFE)?

  • The BFE is the elevation at which your building has a 1 percent chance of flooding annually.

  • Refer to city or county building department records or your own property survey to determine if the elevation of your building’s lowest floor is below a published BFE for your site.

  • If needed, hire a licensed surveyor to determine the elevation of your building’s lowest floor relative to a published BFE for the site.

Is your property in a Special Flood Hazard Area?

  • Your city or county building authority should be able to identify the zone you live in and allow you to plan accordingly.

Understanding flood zones and risks

If your property is in Flood Zone A:

  • The lowest floor of your home or business is likely below the base flood elevation unless it is sufficiently elevated above grade.

  • Your property is in proximity to a body of water that is subject to rising levels due to heavy rainfall or other factors.

If your property is in Flood Zone V, which typically applies to beachfront buildings:

  • These buildings are vulnerable to not only rising waters, but also wind-driven waves.

Do you have a basement?

  • Basement flooding can be a problem in some buildings, and there are steps to take to minimize potential damage. See the project Reduce Basement Flood Risks for more information.

Determine estimated flood depth for your property

  • Compare the estimated flood water heights with your building’s lowest finished floor elevation.

  • The term 100 year flood event actually means a 1 percent chance of the area flooding annually.

  • The BFE for a property, also known as the 100 year flood event, is the elevation at which your building has a 1 percent chance of flooding annually.

  • The BFE is included in the zone designations shown below.

  • A detailed description of each flood zone is also available at the FEMA Map Service Center.

The following is a summary of the flood zones:

  • Zone AE – The base floodplain where base flood elevations are provided. AE Zones are now used on new format FIRMs instead of A1-A30 Zones.

  • Zone A1-30 – These are known as numbered A Zones (e.g., A7 or A14). This is the base floodplain where the FIRM shows a BFE (old format).

  • Zone AH – Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

  • Zone AO – Average flood depths derived from detailed analyses are shown within these zones.

  • Zones VE, V1-30 – Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

The BFE is not shown for flood-prone areas such as:

  • Zone A – Areas with a 1% annual chance of flooding. Because detailed analyses are not performed for such areas; no depths or base flood elevations are shown within these zones.

  • Zone AR – Areas with a temporarily increased flood risk due to the building or restoration of a flood control system (such as a levee or a dam). Mandatory flood insurance purchase requirements will apply, but rates will not exceed the rates for unnumbered A zones if the structure is built or restored in compliance with Zone AR floodplain management regulations.

  • Zone V – Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves. No base flood elevations are shown within these zones.

  • B (shaded) and X (shaded) – Area of moderate flood hazard, usually the area between the limits of the 100-year and 500-year floods. B Zones are also used to designate base floodplains of lesser hazards, such as areas protected by levees from 100-year flood, or shallow flooding areas with average depths of less than one foot or drainage areas less than one-square-mile.

What is your building’s Finished Floor Elevation (FFE)?

To obtain your FFE, you have the following options:

  • The FFE can be obtained from the original AS-BUILT drawings.

  • If the AS-BUILT drawings are not available, contact the builder / architect to obtain the drawings.

  • Contact the city or county building department for the records on your property.

  • Hire a licensed surveyor to determine the elevation of your building’s lowest floor.

Compare the BFE / 100 year with the FFE

  • If the estimated BFE is greater than the FFE, your facility has a risk of more than 1% per year that it will be inundated by flood waters.

  • Even a small amount of flood water in a building can cause extensive damage.

Compare the 500 year with the FFE

  • If the estimated 500 year (0.2 % annual probability flood height) is greater than the FFE, flood protection should be considered.

Special Flood Hazard Considerations

Although buildings may be located outside of flood-prone areas such as C and X unshaded, they may still be vulnerable to some form of flooding because:

  • FEMA flood maps may be outdated due to construction and build up of the area.

  • Heavy rains and downpours, combined with surface and storm water runoff may create localized flooding and ponding.

  • Poor, clogged, or insufficient drainage systems can also lead to localized flooding.

  • Terrain sloping (topography) can create areas of localized flooding.

Local Area Flood Protection (LAFP)

  • LAFP includes various waterway systems that can control the amount of flow of water to adjust and maintain water levels in lakes, reservoirs, preservation areas, rivers, canals, etc.

  • LAFP can consist of levees, dams, dikes, flood gates, etc.

  • LAFP can be made of earth or man made products.

Is there a levee nearby?

  • Is it part of the U.S. Army Corps of Engineers’ Levee Inspection Program?

  • Is it part of the National Levee Database?

  • Obtain this information from the US Army Corps of Engineers.