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Storm Impact, Sea-Floor Change, and Barrier-Island Evolution: Scientists Map the Sea Floor and Stratigraphy Around Ship and Horn Islands, Northern Gulf of Mexico
Gulf Islands National Seashore is a series of barrier islands that stretch from Florida to Louisiana. The white sandy beaches of the barrier-island chain make up the longest national seashore in the United States, and a unique variety of vegetation—including dune grasses, marshes, and forests—provides habitat for migratory birds and endangered mammals. More than 80 percent of the Gulf Islands National Seashore's resources are submerged; these include inlets, lagoons, and more than 20 km2 of seagrass habitat. The seagrass beds are in a state of decline, and the islands' land areas have been severely affected over the past century by storms, sea-level rise, and human alteration. Heavy damage was inflicted in 2005 by Hurricane Katrina, which made landfall as a category 3 storm and battered the islands with winds of more than 160 km/h (100 mph) and a storm surge of 9 m (30 ft).
Coastal processes such as storms affect the islands in many ways. A 2007 report by U.S. Geological Survey (USGS) geologist Bob Morton shows that the islands off Mississippi are undergoing rapid land loss and translocation at widely different rates. The study estimates that since the mid-1800s, Horn Island has lost 24 percent of its land area, while neighboring Ship Island has lost a staggering 64 percent. The National Park Service, which manages Gulf Islands National Seashore, has made it a priority to characterize this change through a comprehensive program of elevation and habitat mapping. The USGS has collaborated on these efforts by using airborne lidar (light detection and ranging) data and aerial photographs to construct coastal classification maps. In addition, the USGS is investigating the geology and morphology of this coastal environment through the Northern Gulf of Mexico Ecosystem Change and Hazard Susceptibility Project. One task of this project is to identify the influence of the geologic framework—for example, variations in underlying sediment composition—on the observed variability in land-loss and breach locations. As an example, Ship and Horn Islands are subjected to the same climate and wave action, yet Ship Island appears to undergo chronic breaching and more extensive island narrowing. Are differences in the underlying geology influencing the fate of these islands? To find out, scientists conducted high-resolution sea-floor and subbottom surveys around the islands in July 2008 to map their morphology and stratigraphy. In addition to producing a modern topobathymetric map of the Mississippi barrier-island chain, the data will be used to identify sea-floor change since the 1800s. These data are also of tremendous value to the U.S. Army Corps of Engineers (USACE), which is studying the feasibility of adding sediment to restore the islands to their previous sizes.
The survey with two vessels began in July 2008. Nancy DeWitt, BJ Reynolds, Dana Wiese, and Jordan Sanford of the USGS Florida Integrated Science Center (FISC) office in St. Petersburg used a 10-m Glacier Bay catamaran to conduct operations in water as shallow as 1 m. On the research vessel G.K. Gilbert, Captains Rich Young and Dave Bennett teamed with Jim Flocks (FISC-St. Petersburg) and Chuck Worley (USGS Woods Hole Science Center) to cover the deeper waters (2-15 m). Additional participants included USGS Gulf of Mexico science coordinator Dawn Lavoie and University of New Orleans student Kathryn Rose, who not only assisted in boat operations but also helped define the "zero contour" by walking the shoreline with a mobile Global Positioning System (GPS) unit.
The acoustic surveys were conducted by using Edgetech 512i and 424 subbottom chirp profilers, an SEA Submetrix interferometric-sonar (swath) bathymetric system, and a Marimatech E-Sea-103 (single-beam) bathymetric system. All these systems send and receive acoustic (sound) signals that bounce off surfaces separating materials of different density—for example, the boundary between water and sediment (the sea floor), or the boundary between sediment layers of differing density. The chirp systems, which employ an acoustic signal whose frequency increases or decreases over time, were used to provide an image of the subbottom as far as 50 m below the sea floor, with 1-m precision. Such images are commonly called "seismic" profiles because the sound signals travel through the sediment and rock of the subbottom like certain types of earthquake, or "seismic," waves. The interferometric-sonar system collects two types of data along a swath of sea floor: bathymetric data, which are sea-floor-depth measurements; and backscatter data, which are measurements of the amount of acoustic energy returned from the sea floor—a proxy for bottom roughness or texture. Backscatter is useful for identifying changes in bottom type that may cause only a subtle change in bathymetry. The single-beam system was used to obtain depth measurements in waters as shallow as 1 m. Vessel position was constrained by three GPS base stations. One station was maintained on a historical benchmark on Fort Massachusetts, an 1800s-era fortification on West Ship Island. A benchmark on the mainland was also occupied, and the survey team established a third station on Horn Island. The survey area, along with tracklines, is shown on map below.
Like all marine operations, the surveys were interrupted by moments of uninvited excitement: self-deployment of expensive equipment, propulsion failure at sea, ambush by storm, and the inevitable jellyfish attack. The team recovered, repaired, rode out, and remedied these challenges and acquired more than 1,050 km of survey lines around the islands in 3 weeks. Initial observations of the sea floor show breach enlargement, shoal migration, and sand waves. The process of natural alongshore sediment transport that built and maintains the islands is revealed by seismic profiles, which show Horn Island building out (prograding) westward into the adjacent Dog Keys Pass. Strong tidal currents in the pass are diverting the sediment offshore, reducing the amount that continues alongshore to neighboring Ship Island. Below the sea floor, distinct and persistent seismic patterns reflect the barrier platform, shoaling, and inlet scour. Elsewhere around the islands, the surveys detected a shoreface that is impressively steep, possibly a product of recent storm activity. Initial comparison of the bathymetric data with older navigational charts from the National Oceanic and Atmospheric Administration (NOAA) indicates a general loss of 1 m in nearshore elevation on the Gulf of Mexico side of the islands over the past few decades. The Mississippi Sound side shows no obvious overwash deposits associated with Hurricane Katrina, but submerged aquatic vegetation is depleted and occurs only in patches. This information is being used in a collaborative project with the National Park Service to produce a benthic-habitat map of the area around the islands.
One month after our survey, Hurricanes Gustav and Ike passed by the islands, bringing surges higher than 2.5 m and 1.5 m, respectively, to Ship Island. To monitor storm impact, the USGS used GPS coordinates to reoccupy positions along the survey tracklines collected in July. This rare opportunity allowed scientists to measure the immediate impact of storms on this ecosystem before natural recovery began. In addition, the post-storm comparison will provide the USACE with valuable insight into the possible effects of future storms on restoration material.
Data processing and interpretation are ongoing, and completion of a preliminary bathymetric map is anticipated by spring 2009. To further characterize the submerged lands of Gulf Islands National Seashore, the USGS anticipates completing investigation of the submerged parts of Horn Island and adjacent Petit Bois Island in summer 2009 through the Northern Gulf of Mexico Ecosystem Change and Hazard Susceptibility Project.
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U. S. Department of the Interior | U.S. Geological Survey
