Coastal DFIRM Information
The North Carolina Flood Mapping Program is in the process of revising the Digital Flood Insurance Rate
Maps (DFIRM) for all of the areas subject to coastal flood hazards. The purpose of this fact sheet is to
describe how coastal flood hazard data is depicted on a DFIRM panel and facilitate the determination of the
Base Flood Elevations for properties located in coastal areas.
How are Base Flood Elevations determined in coastal areas?
The depth of a Base Flood Elevation (BFE) in coastal areas is determined through three main components:
• The depth of the 1% annual chance stillwater elevation (SWEL) (generated by storm surge);
• Wave setup (determined as an increase in water elevation due to the contribution of waves breaking);
and
• Wave height (determined as overland wave crest or wave runup, whichever is the greatest).
What is the difference between a VE Zone and an AE Zone?
In the coastal areas, there are 2 primary flood zones designation based on wave action: Zone VE and Zone
AE. Areas subject to a wave height 3 feet or greater are designated as VE Zones. The “V” stands for “velocity
wave action”, indicating that waves in these areas will be powerful enough to break the wall panel of a
residential structure away from the floor to which it has been nailed. The inland limit of the VE Zone is
determined by finding the most landward of the following:
•
•
•
The point where a 3 feet wave height can occur;
The point where the eroded ground profile is 3 feet below the computed wave runup elevation; and
The inland limit of the Primary Frontal Dune (PFD), a continuous or nearly continuous mound or ridge
of sand.
Structures within a Zone VE are required to conform to V-Zone construction standards, which addresses the
combined effects of strong winds and waves impacting the foundation of buildings. Coastal flood areas with
wave heights less than 3 feet are mapped as AE Zones. Recent building sciences studies have shown that a
1.5 foot wave can still cause structural damage, so a new feature to the coastal DFIRM panels is the depiction
of the Limit of Moderate Wave Action (LiMWA) line, which determines the inland limit of the 1.5 foot
wave. For more information on the PFD and LiMWA, see the corresponding fact sheets.
How is the floodplain boundary determined in coastal areas?
Unless storm induced erosion or wave runup is expected to occur, the inland limit of the coastal flood hazard
area will be that point where the surge (SWEL or SWEL plus wave setup) elevation intersects existing ground
elevations. This is typically the case for bays, estuaries, and other areas not directly exposed to large waves
or the open ocean. For open coasts where wave runup and storm-induced erosion have been considered, the
inland limit of the coastal flood hazard area will most likely fall landward of this surge/ground interface. Its
inland extent is determined by the location of the PFD, the height of wave runup, and the expected amount of
storm-induced erosion.
What if my property lies within a Coastal Barrier Resource System (CBRS) or
Otherwise Protected Area (OPA)?
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A “COBRA” Zone is the colloquial term for areas
mapped and designated as Coastal Barrier
Resources System (CBRS) units. The Coastal
Barriers Resources Act (CBRA) of 1982 removed most
Federal government expenditures for building and
development in undeveloped portions of hazardous and
environmentally fragile ecosystems in coastal areas.
The Act includes a ban on new NFIP insurance policies
for structures located in mapped CBRS Zones.
Additional legislation in 1990 increased the area under
CBRS protection and extended the ban on sale of NFIP
insurance to Otherwise Protected Areas (OPAs),
which are also shown on DFIRMs. The U.S Department
of Interior, Fish and Wildlife Service is responsible for
interpreting the boundaries of CBRS units and OPAs.
Figure 1: Coastal Barrier as Shown on DFIRM Panel
You may be able to purchase flood insurance if your house was built on the property before the date shown
on the effective DFIRM for the specific CBRS Zone or OPA where your property is located. If a house was
newly built or substantially improved after the date of CBRA designation, NFIP flood insurance cannot be
purchased, although a homeowner may be able to purchase (more expensive) private flood insurance.
How do I determine the Base Flood Elevation for my
property in a coastal area?
When dealing with coastal flood hazards, it is important to use all
of the information available to make an accurate Base Flood
Elevation (BFE) determination. This will necessitate referring to
both the tabular data found in the Flood Insurance Study
(FIS) Report, as well as the DFIRM panel. The FIS Report
provides a detailed account of the coastal methodologies, base
data and components that make up a flood study. The mapping
information displayed on a DFIRM is the result of performing the
coastal hazard analyses along coastal transects. A transects is a
profile line, drawn perpendicular to the shoreline; the topographic
elevations extracted along a transect are used as ground base
data to the coastal analyses. Transects are selected with
consideration to physical and cultural characteristics of the land
and are considered representative of the area surrounding it.
Transects are displaced on DFIRM as well as in the “Transect
Location Map” included in the FIS Report. The end product of the
modeling is the determination of Special Flood Hazard Areas
(SFHAs) and Base Flood Elevations (BFEs).
SFHA
Transect
Gutter
Zone Label and BFE
Figure 2: Example of Coastal DFIRM Panel
On the DFIRM panel a flood zone (i.e. SFHAs) is assigned to each BFE, based on the model output. Different
flood zones and different BFEs are separated by white lines, called gutters (for example: Zone VE (EL 14),
Zone VE (EL 13), Zone AE (EL 12) and Zone AE (EL 11) are all different zones). The inland extent of the
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floodplain is usually identified by a light blue line. An example of DFIRM for a coastal area is provided in Figure
2.
In the VE and coastal AE zones, the elevations
associated with each zone represents the total of the
1% annual chance stillwater, wave setup and wave
height elevations, averaged to the whole foot. In
areas where only the stillwater elevation is mapped
the stillwater to the nearest whole foot elevation is
shown on the DFIRM panel.
To make a correct determination of the 1%
SWEL applicable to a specific property, it is
necessary to look into the FIS Report that is
associated with the DFIRM panel(s) where the
property is located. In particular, the “Summary of
Coastal Stillwater Elevation Table” provides flooding
sources, and the 10%, 2%, 1% and 0.2% annual
chance stillwater elevations, in feet, for each panel
Figure 3: Example of Summary of Coastal Stillwater Elevations
Table from the FIS Report
(see Figure 3). These elevations do not include any
wave setup or wave heights.
In order to establish correctly how SFHAs and BFEs are determined for a specific property, both
DFIRM panel and FIS report need to be consulted. On a DFIRM panel, if an area is mapped as an AE (EL 12),
12 could represent an elevation ranging from 11.5 ft to 12.4 ft. If your property lies within a coastal VE Zone
or AE Zone, the BFE for the property or structure is the most conservative elevation associated with the
structure or property. For example, if your structure lies in an AE 12 ft and AE 13 ft, the BFE for that structure
would be 13 ft. To determine whether the BFE elevation of your property is generated by surge only or
includes wave components, one can refer to the “Summary of Coastal Analyses” table included in the FIS
Report (see Figure 4).
For each transect, the “Summary of Coastal
Analyses” table provides:
• Name of the flooding source;
• A description of the transect
location;
• The 10%, 2%, 1% and 0.2% annual
chance stillwater elevations. If one specific
transect includes waves contribution, the
1% annual chance stillwater elevation will
include wave setup (superscript is present
and a foot note is at the end of the table to
indicate the magnitude of the wave setup);
• Zone type and BFE elevation for
transect dominated by wave runup (BFE
determined as 1% annual chance stillwater
Figure 4: Example of Summary of Coastal Analyses Table from the FIS Report elevation + 2% wave runup height);
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•
•
Zone type and BFE elevation for transect including overland waves (BFE determined as 1% annual chance
Stillwater + wave setup + wave crest);
Whether or not a Primary Frontal Dune (PFD) has been identified. If yes, the inland extent of the VE Zone
has been, at a minimum, extended to the PFD location at that transect.
It is only through the integration of the information present on the FIS tables and on the DFIRM panel that a
precise identification of the zone designation and its associated BFE can be determined.
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