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Translating Coastal Flood Hazard Modeling Results into Floodplain Mapping
Krista Conner
FEMA coastal flood hazard mapping under CCAMP follows guidelines set forth in FEMA’s Final Draft Guidelines for Coastal Flood Hazard Analysis and Mapping for the Pacific Coast of the United States (2005). This article summarizes some of the important considerations that go into coastal floodplain mapping. 
 
Coastal flood hazards often include both elevated water levels and storm-induced wave action.  Elevated water levels can inundate low-lying areas and allow larger waves to reach beyond the shoreline.  The primary wave hazards evaluated in a FEMA coastal flood hazard study are wave runup and overland wave propagation
 
Wave runup is the dominant wave hazard along shorelines with steep terrain, including natural features such as coastal bluffs and dunes, and engineered structures such as revetments, levees and flood walls.  Wave runup occurs when waves break against the shore and water rushes up the steep shoreline.  If the crest elevation of the feature or structure is relatively low, the wave runup can exceed the crest resulting in overtopping (see Figure 1 below).  The primary characteristics of a study area that impact wave runup results are incident wave conditions, shoreline slope, and the crest elevation of the shoreline feature or structure.
 
Overland wave propagation occurs when elevated water levels, often referred to as the stillwater elevation (SWEL), inundate normally dry land and waves are able to propagate farther inland.  Analysis of overland wave propagation evaluates how wave energy changes as the waves propagate inland and are attenuated by obstructions, such as vegetation and buildings, or augmented by wind energy over open water fetches, such as ponds or lagoons.  The primary characteristics of a study area that impact overland wave propagation results are incident wave conditions, depth of inundation, and density of obstructions.
 
CB 4_Coastal Hazard Modeling Photo 2.JPG
Figure 1. Wave runup and over topping
 
Once complete, the results of the coastal flood hazard assessment are translated into floodplain mapping that includes base flood elevations (BFEs) and flood zone hazard designations such as VE and AE Zones.  VE Zones are coastal high hazard areas where wave action and/or high-velocity water can cause structural damage during the 1-percent-annual-chance flood.  AE Zones are areas with less hazardous wave action and/or areas of inundation. 
 
 CB 4_Coastal Hazard Modeling Photo 3.JPG
Figure 2. Innundation and overland wave propagation
 
Reaches of shoreline where wave runup is the dominant wave hazard are mapped with shore-perpendicular flood zone break lines separating segments of shoreline with differing runup elevations.  The zone break lines are located at changes in the slope of the ground and orientation of the shoreline.  The BFE for a runup reach is the 1-percent-annual-chance wave runup elevation.  The flood zone designation of Zone VE or Zone AE is based on the magnitude of wave runup above the stillwater level.  A VE Zone is mapped for transects with runup heights greater than or equal to 3 feet; an AE Zone is mapped for transects with runup heights less than 3 feet.  The floodplain is typically narrow with a single flood zone designation and BFE for each transect. 
 
Overland wave propagation is typically mapped with zone break lines that are approximately shore parallel and that follow ground topography.  The BFE in areas of overland wave propagation is the wave crest elevation, or in areas of inundation with minimal wave action, the BFE is based on the 1-percent- annual-chance stillwater elevation.  Zone VE is mapped for areas with wave heights greater than or equal to 3 feet; Zone AE is mapped for areas with wave heights less than 3 feet.  The floodplain is typically broad with changes in zone designation and BFEs along the transect.
 
BFEs in coastal high hazard areas are shown as whole foot values on FEMA Flood Insurance Rate Maps.  A single BFE is shown for each area bounded by the zone breaklines.  Although the 1-percent-annual-chance water surface elevation can vary between 0.5 feet higher and 0.5 feet lower than the whole foot BFE with the bounded area, FEMA considers the whole foot value to be valid for enforcement of community floodplain management regulations and for flood insurance policy rating purposes.
 
The figure below is an example of overland wave propagation and wave runup mapping (from draft Marin County, San Francisco Bay Study).
 

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