Assessment of Potential Coastal Impacts
|For more information about the Storm Impact scale and the first line of defense, see the Storm-Impact Scale section.
Posted Saturday, July 9, 2005
As of the 11:00 a.m. National Hurricane Center (NHC) forecast, the barrier island chain extending from Mobile Bay, Alabama, to Destin, Florida, is within the cone of uncertainty for the path of Hurricane Dennis. These long, thin islands are particularly vulnerable to inundation during hurricanes because of their low elevation and erosion caused by Hurricane Ivan in September 2004. Using a USGS-developed coastal-impact scale, potential impacts of Hurricane Dennis on these barrier islands are determined. In this model, four impact regimes are defined by comparing elevations of storm surge and wave runup to elevations of barrier island topography. For Hurricane Dennis, vulnerability assessments focus on the most extreme impact regime (level 4), i.e. where storm surge exceeds the elevation of the seaward dune or berm and the barrier islands approach complete inundation.
In a cooperative research program between the U.S. Geological Survey and the U.S. Army Corps of Engineers, these islands were surveyed in December 2004 using airborne laser mapping (lidar). Lidar surveys provide detailed topography of the islands, which are used to measure the elevation of the seaward dune or berm. The December 2004 survey provides insight into coastal changes that have occurred since the impact of Hurricane Ivan. These changes include beach renourishment and berm construction as well as natural recovery of the beach. This survey does not reflect additional changes that have occurred between December and the time of impact of Hurricane Dennis. For example, a 3.5 meter sand berm was constructed along the 13 kilometer length of Pensacola Beach after the December lidar survey.
Predictions of storm surge for each hurricane category are obtained from the National Oceanic and Atmospheric Administration (NOAA). These predictions represent the maximum surge for each location along the beach, assuming landfall at that specific location. Areas susceptible to inundation during landfall of Hurricane Dennis are assessed by comparing lidar-derived dune elevations to simulated storm surge values. Maps indicating the vulnerability of the islands to inundation are created for each category storm (see figure below that shows Category 1 to 5). As the hurricane intensifies, storm surge elevations rise, increasing the potential for inundation and the associated coastal change. For example, inundation can cause sea level differences across the barrier islands that cause the landward transport of massive volumes of sand and the opening of new breaches that sever the islands. Examples of hurricanes that have caused barrier island breaching include Hurricane Isabel in 2003, Hurricane Charley in 2004, and Hurricane Ivan in 2004.
When interpreting these maps, it is important to understand that the assessments assume landfall at each location. That is, a red area will inundate only if it is located near, or immediately to the right (east) of, landfall.
Note: This experimental product is based on research results of the USGS National Assessment of Coastal Change Project and applies to potential inundation caused by storm surge only. This vulnerability assessment does not directly consider potential property damage or the impacts of high wind speeds and heavy rain. The actual changes that occur during extreme storms are complex functions of a number of processes and variables including ocean waves, currents, and tides. This discussion simplifies the problem to some of the most important aspects, but a full analysis of all processes would be required to fully understand the magnitude and spatial variability of storm-induced coastal change of Hurricane Dennis.
|Map showing storm surge minus dune elevation, which indicates the potential for level-4 coastal change (inundation), for Categories 1 through 5 hurricanes. Red indicates that simulated worst-case storm surge elevations are higher than the seaward dunes, which suggests greater vulnerability to inundation and associated coastal change impacts (see text above). Green indicates that the simulated surge is lower than the elevation of the dune, suggesting less vulnerability to inundation. Note: Red areas will inundate only for direct landfall at each location (see above paragraphs for further discussion).
Blue lines indicate areas of beach renourishment and/or berm construction completed after the December 2004 lidar survey. Therefore, these areas will be less vulnerable to inundation than indicated on the maps. For example, a 3.5 meter sand berm was constructed along the 13 kilometer length of Pensacola Beach after the December lidar survey.