National Flood Docs

Your glossary for all things flood related.

Flood Zone

What is flooding?

Flooding is a general and temporary condition of partial or complete inundation of normally dry land areas from:

  • Overflow of inland or tidal waters;
  • The nusual and rapid accumulation or runoff of surface waters from any source;
  • Mudslides, caused by flooding and are akin to a river of liquid and flowing mud on the surfaces of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current

For an event to be considered flooding, two acres or at least two properties must be affected (Federal Emergency Management Agency, 2018).

What is a flood zone?

All over the USA, local and federal agencies have rain gauges that monitor rainfall intensities year-round and they use these results to make statistical projections, eventually finding what is the storm intensity that relates to 1% chance of occurrence of being exceeded within a year’s timeframe. As this is based on historical data, these are always changing and getting updated. You could get 1, 2 or 6 100-year storms within single year or you could go 300 years without getting a 100 year storm; this is just a probabilistic reference used to calculate and mitigate flood risk.

The land area covered by the floodwaters of the base flood is called the “base floodplain”. On the NFIP maps this is referred to as the Special Flood Hazard Area (SFHA) and they have Flood Zone designations.

See BFE for more information on how rainfall is monitored.

How does a flood zone determine flood risk?

Broadly put, flood zones can be broken up into two categories: flood zones that where flood insurance is not mandatory and flood zones where insurance is mandatory.

Flood Risk

Moderate to Low Risk Areas:

  • Zone C, Zone X (unshaded) - Areas of minimal flood hazard, which are the areas outside the SFHA and higher than the elevation of the 0.2-percent-annual-chance flood.
  • Zone B, Zone X500, X (shaded) - Moderate flood hazard areas, between the limits of the base flood and the 0.2-percent-annual-chance (or 500-year) flood.
  • Zone D - An area of undetermined but possible flood hazards.
  • Undescribed (UNDES) - Area of Undesignated Flood Hazard. A body of open water, such as a pond, lake, ocean, etc., located within a community's jurisdictional limits, that has no defined flood hazard.

High Risk Areas

High Risk Areas are areas with a 1% annual chance flooding (100-year floodplain). This means that Flood Insurance is required.

  • Zone A - No BFEs have been determined.
  • Zone AE - BFEs have been determined.
  • Zone AH - Usually an area of ponding, for which BFEs have been determined; flood depths range from 1 to 3 feet.
  • Zone AO - Usually sheet flow on sloping terrain, for which average depths have been determined; flood depths range from 1 to 3 feet.
  • Zone AR - BFEs or average depths have been determined. This area is protected from the 1% annual chance of flood by a Federal flood protection system whose restoration is Federally funded and underway.
  • Zone A1 - A30-BFEs have been determined. Similar to regular AE.

High Risk Costal Areas

  • Zone VE - An area inundated by 1% annual chance flooding with velocity hazard (wave action); BFEs have been determined.
  • Zone V - An area inundated by 1% annual chance flooding with velocity hazard (wave action); for which no BFEs have been determined.

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