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Vulnerability of the U.S. South Atlantic Coast to Hurricane Frances: Most Recent Discussion12:00 PM, Friday, September 3, 2004 -- Based on the 11:00 AM National Hurricane Center (NHC) forecast, Hurricane Frances will make landfall along the U.S. south Atlantic coast during Labor Day weekend within the track uncertainty cone that extends from southern South Carolina southward to south Florida (inset map). As the storm draws closer to the coast, the cone of uncertainty will narrow, and potentially change location. See the National Hurricane Center's web site for the most recent forecast.
The coast from southern South Carolina to south Florida consists predominantly of barrier islands, thin strips of sand that separate the ocean from back bays and the mainland. These islands are commonly fronted with sand dunes that serve as 'first lines of defense' to protect buildings and infrastructure. The elevations of these dunes represent thresholds that, if exceeded by wave runup and/or storm surge, lead to the coast undergoing significant net coastal change (four-panel Storm Impact Scale). The thresholds are liberal estimates of the protection afforded by the 'first line of defense' as they do not include storm-induced lowering by dune erosion. At present, Frances is forecast to come ashore as a Category-3 hurricane. According to the Saffir-Simpson Scale, a Category-3 storm will force a storm surge on the open coast that ranges from 2.7 to 3.7 m (9 to 12 ft.) above sea level depending upon a variety of factors such as the slope of the continental shelf (the more gentle the slope, the higher the surge). This estimated range is a 'rule of thumb' for the maximum open coast surge and is applicable only to areas to the north of the hurricane's eye at landfall. (To the south of the eye, the wind will be directed offshore and sea level will be lowered.) The 'rule of thumb' does not include all the contributions to storm surge and, in some storms, like Hurricane Charley, the 'rule of thumb' has overestimated the surge. Numerical modeling for the specific landfall location, and its unique bathymetry and topography, will be required to accurately estimate the surge's magnitude and spatial variability. Below are percentages of coast where the 'first line of defense' will be overtopped by the 'rule of thumb' Category-3 storm surge impacting at mean (astronomical) tide. Also shown are results for Category-4 and Category-2 storms, should Frances intensify or weaken. The data used in this analysis are shown on the 'first line of defense' map.
These statistics refer to the percentage of coast that has the potential to be inundated (Storm Impact Scale Level 4) if impacted by the indicated surge, which should only occur on the open coast to the north of the eye at landfall and will decrease with distance along the coast from the eye wall. In general, more of the South Carolina and Georgia coast has the potential to be inundated, and impacted at Level 4, than the other coasts in, or near, the cone of track uncertainty. All areas have substantial regions that will be impacted at Level 4. At the time of this discussion, the NHC's consensus landfall forecast is in the Central East Florida region where the average 'first line of defense' elevation is 4.6 m (15 ft). Here, 6% to 35% of the coast has the potential to be inundated and subjected to Level 4 impacts by a Category 3 storm. If Frances develops a surge closer to Saffir-Simpson Category 4, significantly more of the East Central coast has the potential to be overtopped by surge and Level 4 impacts will dominate. If Frances develops a surge closer to Saffir-Simpson Category 2, significantly less of the East Central coast will have the potential for Level 4, suggesting that dune erosion (Level 2) and overwash (Level 3) will dominate. The maps at the top of this page show the spatial distribution of Level-4 impacts for Category 2, 3, and 4 storms for Cape Canaveral to near West Palm Beach on the central east coast of Florida. The red areas indicate where the potential storm surge is higher than the 'first line of defense' elevation, suggesting inundation and Level 4 impacts and green indicates where surge is lower than the 'first line of defense' elevation, indicating Level 2 (dune erosion) or Level 3 (overwash) impacts. Again, the red areas represent the potential to be inundated (Storm Impact Scale Level 4) if impacted by the indicated surge, which will only occur on the open coast to the north of the eye at landfall and will decrease with distance along the coast from the eye wall. These maps do not include the affect of astronomical tides to either increase (high tide) or decrease (low tide) the surge elevation. 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. The Storm Impact Scale, and 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 Frances. 12:00 PM, Thursday, September 2, 2004 -- Based on the 11:00 AM National Hurricane Center forecast, Hurricane Frances will make landfall along the U.S. south Atlantic coast during Labor Day weekend within the track uncertainty cone that extends from central South Carolina southward to the Florida Keys (inset map). As the storm draws closer to the coast, the cone of uncertainty will narrow, and potentially change location. See the National Hurricane Center's web site for the most recent forecast. The coast from central South Carolina to Miami, Florida consists predominantly of barrier islands, thin strips of sand that separate the ocean from back bays and the mainland. These islands are commonly fronted with sand dunes that serve as 'first lines of defense' to protect buildings and infrastructure. The elevations of these dunes represent thresholds that, if exceeded by wave runup and/or storm surge, lead to the coast undergoing significant net coastal change (four-panel Storm Impact Scale). The thresholds are liberal estimates of the protection afforded by the 'first line of defense' as they do not include storm-induced lowering by dune erosion. At present, Frances is forecast to come ashore as a Category-4 hurricane. According to the Saffir-Simpson Scale, a Category-4 storm will force a storm surge on the open coast that ranges from 4 to 5.5 m (13 to 18 ft.) above sea level depending upon a variety of factors such as the slope of the continental shelf (the more gentle the slope, the higher the surge). This estimated range is a 'rule of thumb' for the maximum open coast surge and is applicable only to areas to the right of the hurricane's eye at landfall. (To the left of the eye, the wind will be directed offshore and sea level will be lowered.) The 'rule of thumb' does not include all the contributions to storm surge such as wave setup against the coast and, in some storms, like Hurricane Charley, the 'rule of thumb' has overestimated the surge. Numerical modeling for the specific landfall location, and its unique bathymetry and topography, will be required to accurately estimate the surge's magnitude and spatial variability. Below are percentages of coast where the 'first line of defense' will be overtopped by the 'rule of thumb' Category-4 storm surge impacting at mean (astronomical) tide. Also shown are results for Category-3 and Category-2 storms, should the Saffir-Simpson Scale overestimate Frances' surge. The data used in this analysis are shown on the 'first line of defense' map.
These statistics refer to the percentage of coast that has the potential to be inundated (Storm Impact Scale Level 4) if impacted by the indicated surge, which will only occur on the open coast to the right of the eye at landfall and will decrease with distance along the coast from the eye wall. In general, more of the South Carolina and Georgia coast has the potential to be inundated, and impacted at Level 4, than the other coasts in, or near, the cone of track uncertainty. All areas have substantial regions that will be impacted at Level 4. At the time of this discussion, the consensus landfall forecast is in the East Central Florida region where the average 'first line of defense' elevation is 4.6 m (15 ft). Here, 49 to 78% of the coast has the potential to be inundated and subjected to Level 4 impacts by a Category 4 storm. If Frances develops a surge closer to Saffir-Simpson Category 3 or 2, significantly less of the East Central coast will have the potential for Level 4, suggesting that dune erosion (Level 2) and overwash (Level 3) will dominate. These statistics and discussion will be updated daily as Hurricane Frances approaches shore. Note: This experimental product is based on research results from 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. The Storm Impact Scale, and this discussion, simplifies the problem to some of the most important aspects. A full analysis of all processes will be required to fully understand the magnitudes and spatial variability of storm-induced coastal change of Hurricane Frances. 12:00 PM, Wednesday, September 1, 2004 -- Based on the 11:00 AM National Hurricane Center forecast, Hurricane Frances will make landfall along the U.S. south Atlantic coast during Labor Day weekend within the track uncertainty cone that extends from southern North Carolina southward to the Florida Keys (inset map). As the storm draws closer to the coast, the cone of uncertainty will narrow, and potentially change location. See the National Hurricane Center's web site for the most current forecast. The coast from southern North Carolina to Miami, Florida consists predominantly of barrier islands, thin strips of sand that separate the ocean from back bays and the mainland. These islands are commonly fronted with sand dunes that serve as 'first lines of defense' to protect buildings and infrastructure. The elevations of these dunes represent thresholds that, if exceeded by wave runup and/or storm surge, lead to the coast undergoing significant net coastal change (four-panel Storm Impact Scale). The thresholds are liberal estimates of the protection afforded by the 'first line of defense' as they do not include storm-induced lowering by dune erosion. At present, Frances is forecast to come ashore as a Category-4 hurricane. According to the Saffir-Simpson Scale, a Category-4 storm will force a storm surge on the open coast that ranges from 4 to 5.5 m (13 to 18 ft.) above sea level depending upon a variety of factors such as the slope of the continental shelf (the more gentle the slope, the higher the surge). This estimated range is a 'rule of thumb' for the maximum open coast surge and is applicable only to areas to the right of the hurricane's eye at landfall. (To the left of the eye, the wind will be directed offshore and sea level will be lowered.) The 'rule of thumb' does not include all the contributions to storm surge such as wave setup against the coast and, in some storms, like Hurricane Charley, the 'rule of thumb' has overestimated the surge. Numerical modeling for the specific landfall location, and its unique bathymetry and topography, will be required to accurately estimate the surge's magnitude and spatial variability. Below are percentages of coast where the 'first line of defense' will be overtopped by the 'rule of thumb' Category-4 storm surge impacting at mean (astronomical) tide. Also shown are results for a Category-3 storm, should the Saffir-Simpson Scale overestimate Frances' surge. The data used in this analysis are shown on the 'first line of defense' map.
These statistics refer to the percentage of coast that has the potential to be inundated (Storm Impact Scale Level 4) if impacted by a surge of 4 – 5.5 m (2.7 – 3.7 m), which will only occur on the open coast to the right of the eye at landfall and will decrease with distance from the eye wall. In general, more of the South Carolina and Georgia coast has the potential to be inundated, and impacted at Level 4, than the other coasts in, or near, the cone of track uncertainty. All areas have substantial regions that will be impacted at Level 4. If Frances develops a surge closer to Saffir-Simpson Category 3, significantly less of the coast will have the potential for Level 4, suggesting that dune erosion (Level 2) and overwash (Level 3) will dominate. These statistics and discussion will be updated daily as Hurricane Frances approaches shore. Note: This experimental product is based on research results from 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. The Storm Impact Scale, and this discussion, simplifies the problem to some of the most important aspects. A full analysis of all processes will be required to fully understand the magnitudes and spatial variability of storm-induced coastal change of Hurricane Frances. |
U. S. Department of the Interior | U.S. Geological Survey
