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Primary Frontal Dune Coastal High Hazard Area Mapping Requirements
Darryl Hatheway, Senior Coastal Scientist, BakerAECOM
In the California Coastal Analysis and Mapping Project Open Pacific Coast study area there are many Primary Frontal Dunes (PFD) which will be included in the Coastal High Hazard Area (CHHA), also known as a VE Zone on FEMA flood maps. Although this is not a new FEMA hazard zone designation, these areas have not previously been incorporated within FEMA coastal flood studies in California. The mapping criteria for these zones was adopted by FEMA in 1988, after most California coastal flood studies were last updated.
 
Prior to 1988, coastal dunes and especially the PFD were typically mapped outside of any regulatory flood hazard zone. Figure 1 illustrates the coastal flood hazard zones prior to the 1988 PFD requirement. The VE and AE Zone limits were a function of the calculated wave height or wave run-up above the 100-year stillwater elevation (SWEL). The remainder of the dune ridge would typically be mapped outside the special flood hazard area. This increased the vulnerability of these critical natural resources and allowed new structures to be constructed on the dunes without specific building standards or consideration of the risk of potential erosion and overtopping.
 

Figure 1_PFD_Coastal Beat.png
Figure 1. Illustration of coastal flood hazard zones prior to the 1988 PFD requirement.

Because the PFD is a natural frontline defense during coastal flood events, FEMA identified a need to establish definitive guidance for dune erosion and protection. In May 1988, FEMA adopted new regulations and VE Zone mapping requirements for the PFD that consider wave action and erosion processes. The National Flood Insurance Program (NFIP) regulations were revised to define the PFD and the requirement to map it within the CHHA. This revision redefined the CHHA to include the PFD to a specific landward limit.
 
The PFD is defined as “a continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms.” Therefore, the PFD is now included in the CHHA by administrative definition and the CHHA is defined as “an area of special flood hazard extending from offshore to the inland limit of the primary frontal dune along an open coast and any other area subject to high velocity wave action from storms or seismic sources.” Figure 2 shows typical zone designations following the May 1988 regulation changes and inclusion of the PFD as part of the VE Zone hazard mapping criteria. Under the PFD criteria, the mapped VE Zone is extended inland based on definitions of the PFD and not on detailed wave calculations or modeling results.
 
Figure 2_PFD_Coastal Beat.png
Figure 2. Illustration of coastal flood hazard zones after the 1988 PFD requirement was adopted.
 
While the NFIP regulations provide a definition of the PFD, identification in the field is not always easy. Dunes are often irregular features that may not be homogeneous. The figure below shows an example of an area with a PFD in Santa Cruz County near the Pajaro Dunes development. More details on PFD identification and mapping requirements can be found in the Guidelines for Coastal Flood Hazard Analysis and Mapping for the Pacific Coast of the United States, available on FEMA’s website.

 

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