As of the 11:00 AM (Sept 11, 2004) National Hurricane Center (NHC) forecast, the west coast of Florida is within the cone of uncertainty for the path of Hurricane Ivan. The long, thin barrier islands that comprise the Gulf of Mexico coast of west Florida are particularly vulnerable to inundation during hurricanes because of their low elevation. In a cooperative research program between the U.S. Geological Survey and NASA, these islands have been surveyed using airborne laser mapping (lidar) providing, for the first time, detailed elevation maps of the island's 'first line of defense', essentially the Gulf-front dune (or in the absence of dunes, the crest of the beach berm or seawall). The average Florida west coast 'first line of defense' elevation is less than half that of the Florida central east coast where Hurricane Frances made landfall six days ago (2.1 m compared to 4.6 m).

Maps showing the spatial variability of where worst-case storm surge exceeds the 'first line of defense' elevations for Saffir-Simpson Categories 1 through 5 hurricanes for the west coast of Florida. The red areas (positive inundation) indicate where the potential storm surge is higher than the 'first line of defense' elevation, suggesting those parts of the coast may be inundated by storm surge. Green (negative inundation) indicates where surge is lower than the 'first line of defense' elevations, suggesting those areas are less vulnerable to inundation. The red areas will be inundated only if impacted by the indicated surge, which on the open coast typically occurs to the south of the eye at landfall and will decrease with distance along the coast from the eye wall. These estimates assume landfall occurs at mean astronomical tide and do not include changes in island elevation due to erosion. [larger version]

The maps above show the proportion of the 'first line of defense' that would be inundated by worst-case storm surge associated with different category hurricanes. The storm-surge elevations were obtained from NOAA and represent the maximum simulated surge that will result along the open coast from hurricanes of a given category approaching from different directions and at different speeds. On the west coast of Florida's barrier islands, the maximum surge will typically occur to the south of landfall under the eyewall and will decrease with distance away from the eyewall. Consequently, the worst case for any given storm is localized and does not occur along the entire coast.

When the dune is overtopped by storm surge—such as would occur for most of the Florida west coast if impacted by the south eyewall of a Category 3, 4, or 5 hurricane—strong currents drive potentially massive amounts of sand landward across the island (Storm Impact Scale for Barrier Islands, Impact level 4 of the Coastal Change Hazard Scale). These currents sometimes scour new inlets, as happened during 2003's Hurricane Isabel on the North Carolina coast and 2004's Hurricane Charley on the southwest coast of Florida.

Superimposed on the storm surge would be waves whose height would be roughly the same height as the water is deep. Hence, to the south of landfall of a Category 3 storm, the surge would submerge the 'first line of defense' with 1.3 m of water. On top of the surge there would be roughly 1.3-m-high waves for a total vertical reach of 2.6 meters. For a Category 5 storm, the 'first line of defense' would be submerged by a 3.2 meter surge. The waves would be an additional 3.2 m high giving a total vertical reach of 6.4 m. These are conservative estimates in that dune elevations would likely erode lower making the water deeper and the waves higher.

Note: This experimental product is based on research results of the USGS National Assessment of Coastal Change Project and applies to potential changes to the coast (e.g. erosion) caused by storm surge, ocean waves and associated currents. The vulnerability assessment does not directly consider potential property damage or the impacts of high wind speeds and heavy rain. The actual changes that occur to the coast during extreme storms are complex functions of a number of processes and variables. This discussion simplifies the problem to some of the most important aspects, but a full analysis of all processes will be required to fully understand the magnitudes and spatial variability of storm-induced coastal change of Hurricane Ivan.

See also:

• Coastal Change Hazard Scale
• http://cars.er.usgs.gov/

Source: USGS Coastal & Marine Geology Program