Coastal Change Hazards: Hurricanes and Extreme Storms

Hurricane Frances

Vulnerability of the U.S. South Atlantic Coast to Hurricane Frances: Methodology & Data

5-Day Forecast Track from the National Hurricane Center's Sept. 3, 2004, 11 AM advisory. For a larger map and the most current storm track, go to the National Hurricane Center.

Methodology & Data

The potential impact of Hurricane Frances on the south Atlantic's barrier islands depends not only on storm parameters, such as storm surge and wave height, but also on the elevation of the coast at landfall. A coastal change hazard scale (see four-panel illustration below) has been developed that is based on how high waves will run up the beach relative to island elevation thresholds, such as dune elevation (for more information, see Storm Impact Scale for Barrier Islands). For example, if wave runup elevation exceeds the dune elevation ), the dune will be overwashed and there will be net landward transport of sand. If the storm surge completely and continuously submerges the dune ), the changes are extreme, sometimes breaching an island and forming new inlets. Island breaching occurred on the North Carolina coast during 2003's Hurricane Isabel and on the southwest Florida coast during 2004's Hurricane Charley.

No Net Change to the System Net Dune Erosion Net Onshore Transport, Order 100 meters Net Onshore Transport, Order 1,000 meters   Coastal Change Hazard Scale: A storm-impact scale has been developed that is based on the elevation of wave runup on the beach relative to threshold values like dune crest elevation. Click on an image or impact level to learn more about each level (opens in a new window).

Dune crest and dune base are elevation thresholds. The elevation of wave runup relative to these thresholds is an important factor in determining how the system will change.

The map below shows elevations of the 'first line of defense' of the barrier-island system from North Carolina to south Florida (for more information, see USGS Open File Reports 02-287 and 02-288). The 'first line of defense' is typically the dune closest to the ocean that commonly protects houses, roads, and infrastructure that lie immediately landward. The elevations are measured every two meters along the coast using airborne lidar, a laser mapping system, in a cooperative program with NASA investigating the coastal impacts of severe storms (for more information, see USGS Open File Report 01-98).

The hot colors on the map indicate lower elevations that are more vulnerable to overwashing and inundation than the cool colors, representing higher elevations. Where dunes are absent and seawalls are present, for example at the base of some high-rise condominiums, the structure becomes the 'first line of defense' and its crest elevation is mapped. However, most of the south Atlantic coast is lined with dunes and the map represents the relative vulnerabilities of these areas to overwash and inundation for a hurricane of the same nearshore wave and storm surge characteristics hitting the coast at approximately the mid-astronomical-tide level.

The latest discussion of coastal vulnerability, based upon Hurricane Frances' most recent predicted storm track, is available via the Most Recent Discussion [revised 9/3/2004 12:00PM].

Coastal 'first line of defense' elevations. The map above illustrates the elevations of the 'first line of defense' of the beach system, typically the first dune ridge. The elevations represent the relative vulnerability of the South Atlantic coast to change from a storm of the same wave runup elevation on the beach, hitting the coast at approximately mid-tide level; the hotter colors indicate greater vulnerability. The histograms show the distribution of elevations for each region; click on each region to view a larger version. Inset map shows predicted track of Hurricane Frances.