U.S. Geological Survey (USGS) scientists focused their attention on the Gulf of Mexico coast from Louisiana to the Florida panhandle as Hurricane Ivan approached in September. The U.S. Gulf Coast shoreline is particularly vulnerable to storm surge and coastal change during hurricanes because of low elevation, shoreline retreat, and subsidence in the Mississippi Delta area. In June, the USGS released a new assessment of shoreline change on the Gulf Coast showing that 61 percent of the shoreline is eroding. Some areas are losing sand more rapidly than others, and some areas are actually gaining sand. The assessmenta 44-page full-color report available at URL http://pubs.usgs.gov/of/2004/1043/was designed to help coastal managers at all levels of government make more-informed decisions.
"At the beginning of hurricane season, coastal residents recognize how important their beaches are, not just for enjoyment but also for protection from mighty coastal storms," said Robert Morton, a USGS coastal geologist and the assessment's lead author. "Beach erosion is a chronic problem along most open-ocean shores of the United States. As coastal populations grow and community infrastructures are threatened by erosion, there is increased demand for accurate information regarding past and present trends and rates of shoreline movement."
In a cooperative research program, the USGS and the National Aeronautics and Space Administration (NASA) recently surveyed the shoreline by using airborne laser mapping, providing for the first time detailed elevation maps of the "first line of defense" at the shoreline. An example of the "first line of defense" is a sand dune protecting an ocean-front cottage or road. The average Florida west coast "first line of defense" elevation is about 6 ft less than half the 13-ft average of the Florida east coast where Hurricane Frances made landfall on September 5.
USGS scientists have prepared maps showing where the "first line of defense" would be inundated by worst-case-scenario storm surges associated with categories 1 through 5 hurricanes; the maps can be viewed on the USGS Hurricane Ivan's Potential Impacts Web page.
Storm-surge elevations, simulated by the National Oceanic and Atmospheric Administration (NOAA), represent the maximum surge that results along the open coast from hurricanes of a given category, approaching from different directions and at different speeds. On Florida's west-coast barrier islands, the maximum surge typically occurs to the south of landfall under the eye wall and decreases in elevation with distance away from the eye wall.
"Where the storm surge exceeds the elevation of the dunes, currents will flow across the barrier islands, potentially driving massive amounts of sand landward," says Abby Sallenger, USGS oceanographer. "In some places where barrier islands are low and narrow, the currents will carve new inlets, like what happened in 2003 on the Outer Banks of North Carolina during Hurricane Isabel and this year on North Captiva Island, Florida, during Hurricane Charley" (see Sound Waves article "USGS Scientists Gather Images and Information About Recent Hurricanes").
Data generated by the USGS National Assessment of Shoreline Change Project, including vector shorelines and transects, associated short- and long-term rates of change, statistical uncertainties, and areas of beach nourishment, have been compiled on the USGS Coastal and Marine Geology Program Internet Map Server (IMS). The IMS brings the usefulness of geographic information systems (GIS) to a Web browser, allowing the user to view and manipulate data layers interactively. The IMS can be found on the U.S. Gulf of Mexico Shoreline Change IMS Web site.
A data catalog complements the IMS and the assessment report mentioned earlier by offering downloadable data layers complete with Federal Geographic Data Committee-compliant metadata. These data can be found in USGS Open File Report 2004-1089.
in this issue:
Gulf of Mexico Vulnerable to Hurricanes