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Analyzing and Mapping Wave Overtopping in FEMA Coastal Flood Studies
Jeremy Mull

​Wave overtopping is a common coastal hazard and source of flooding along the Pacific coast. During severe coastal storms, high waves and wave runup can overtop coastal barriers including bluffs, dunes, seawalls, revetments, and beach berms. When this happens, the overtopping water floods the area immediately behind the barrier. In some cases, the overtopping water has enough energy and force to damage structures, including homes. Therefore, analysis of wave overtopping is typically an important part of a detailed coastal flood study. It is also useful for communities to understand the process of wave overtopping so they can identify where it is included on their coastal flood maps. Learn more about some of the physical aspects of wave overtopping, how it was analyzed and why it was mapped in the California Coastal Analysis and Mapping Project (CCAMP) Open Pacific Coast (OPC) Study in this article.

Photo_Overtopping.jpg
Figure 1. Wave overtopping during a coastal storm in Pacifica, CA. Note that the wave overtops the seawall, but the range of the splash is limited to a small area behind the seawall. Photo by Justin Vandever.

The Wave Overtopping Process
Wave overtopping occurs during coastal storms when the highest tides, storm surge, and wave effects combine to generate splash or a bore (i.e., a broken wave) that exceeds a particular coastal barrier elevation (Figure 2). The overtopping water floods areas immediately landward of the coastal barrier. Flooding can be exacerbated at sites that have poor drainage where water can pond throughout the storm. In addition to flooding, the overtopping water may have enough mass, velocity, and hydraulic force to damage buildings. There are many documented cases of structures that have been damaged and flooded due to wave overtopping along the California coast. Damages typically include smashed or overturned walls, broken windows, and/or collapse of structural components due to settlement from wave scour.

F2_Wave Overtopping Process.jpg

Figure 2. A conceptual diagram of the wave overtopping process. Wave overtopping occurs during coastal storms when the highest tides, storm surge, and wave effects combine to generate splash or a bore that exceeds a coastal barrier elevation and floods the local area.

Wave overtopping is different from inland coastal inundation. During inland coastal inundation, high tides and storm surge combine to completely overwhelm natural barriers and coastal defenses, and inundate low-lying areas. Inundation can extend far inland in wide, flat coastal floodplains. While common during severe hurricanes that strike the Atlantic and Gulf coasts, inundation flooding is rare along the Pacific coast, where storm surge is relatively small and shoreline topography is relatively steep. Along the Pacific Coast, the volume of overtopped water is limited and the overtopping splash or bore typically loses energy just inland of the barrier. But it can and does flood and damage homes built near the shoreline. 

Wave Overtopping Technical Analysis
The CCAMP OPC Study included a detailed technical analysis of wave overtopping. A set of engineering equations in the FEMA Pacific Guidelines were used to calculate the height, velocity, splash trajectory, and inland distance of overtopping at each analysis transect (Figure 3). The calculations included site specific characteristics such as the barrier crest elevation and slope, maximum wave runup height, and wave period. The hydraulic force throughout the entire overtopping zone was calculated using the mass and velocity of the overtopping water. The calculation results define the specific zone where the overtopping water has enough hydraulic force to potentially damage buildings and the specific zone where the force of the overtopping water is below the structural damage threshold.

Figure 3_Wave Overtopping Analysis.jpg
Figure 3. A conceptual diagram of the wave overtopping analysis presented in the FEMA Pacific Guidelines. A set of engineering equations are applied to calculate the wave overtopping trajectory, height, depth, velocity, and hydraulic force.

Wave Overtopping Mapping
Once the detailed wave overtopping characteristics have been calculated, the resulting hazard zones are delineated onto the flood maps. Analysis transects with wave overtopping hazard zones are generally mapped with three distinct zones as shown in Figure 4. Starting at the shoreline (also referred to as the 0.0 NAVD88 ft. contour), a VE Zone is mapped that extends to the coastal barrier.  The 1-percent-annual-chance wave runup elevation at the coastal barrier is taken as the Base Flood Elevation (BFE) in this zone. Another VE Zone is mapped landward of the overtopped coastal barrier. This is the high energy zone where the overtopping splash or bore has enough hydraulic force to potentially damage structures. The maximum height of the overtopping splash or bore is taken as the BFE in this zone. An AO Zone is mapped immediately landward of the VE overtopping zone. This AO Zone area is expected to be flooded; however the overtopping splash or bore has low energy and insufficient hydraulic force to damage a structure. The depth of flooding (typically 1 to 3 ft.) is specified for the AO Zone area rather than a BFE.  The landward limit of the AO Zone is where the overtopping energy completely dissipates and there is no hydraulic force.

F4_Flood Hazard Zone Mapping for Overtopping Transects.jpg

Figure 4. Diagram showing FEMA flood hazard zone mapping for overtopping transects. Each zone must be at least 35 feet wide to distinctly appear on the flood maps.

The overtopping and AO flood zones in Figure 4 must be at least 35 feet wide to appear as a distinct zone on the flood maps. If the AO Zone is less than 35 feet wide, the zone is incorporated into the VE Zone immediately landward of the overtopped barrier. This larger VE Zone is mapped with the overtopping BFE. If the overtopping VE Zone is less than 35 feet wide (with or without the incorporated AO Zone) it is incorporated into the VE Zone seaward of the overtopped barrier. This VE Zone is mapped with the 1-perent-annual-chance wave runup elevation as the BFE.
 
Communities may notice that some areas of ground immediately landward of the overtopping flood zones have lower elevations than the mapped BFEs, but are not incorporated within the flood zones. Although the wave overtopping elevations exceed coastal barrier elevations, it would be inaccurate to simply project the BFEs inland until they intersect higher ground elevations. This approach would completely ignore the physics of wave overtopping and would reflect inland coastal inundation rather than overtopping. Instead, the detailed technical analysis and mapping procedure used in the CCAMP OPC Study more realistically captures the physics of wave overtopping and more accurate flood maps.


 

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