Imagine a shoreline already subject to the highest erosion rates in the country. Now imagine that coastline being slammed by two major hurricanes in one season. That's what happened in 2005 to the south Louisiana coast. In summer 2006, U.S. Geological Survey (USGS) scientists and collaborators conducted extensive surveys of Louisiana shorelines to evaluate the changes caused by the hurricanes and to provide baseline data for assessing the effects of future hurricanes and other events.
The barrier and headland shorelines of the Mississippi Delta plain are rapidly retreating; the main contributors to shoreline loss are a subsiding delta plain and a high rate of eustatic (worldwide) sea-level rise. These and other factors result in as much as 4 m of shoreline retreat per year, for an annual land loss of more than 60 km2. The Caminada-Moreau headland, southwest of New Orleans, is subject to some of the highest erosion rates within the delta plain, and so its shoreline has been intensively monitored. Comparison of historical charts and aerial photographs indicates that the shoreline in the area has retreated as much as 3 km during the past 100 years. Repeated bathymetric surveys over the same time period indicate significant erosion, not only of the shoreface but also at water depths below the average wavebase (approx 3 m). The results of these analyses are published in a widely referenced USGS atlas on sea-floor change, "Louisiana Barrier Island Erosion Study: Atlas of Sea-Floor Changes from 1878 to 1989," by Jeff List and others (USGS Miscellaneous Investigations Series Map I-2150-B, currently out of print but available at USGS libraries; see URL http://library.usgs.gov/).
Monitoring the shoreline of the Caminada-Moreau headland is critically important because it protects a petroleum-industry infrastructure that supports the production of 75 percent of the oil produced from the deep Gulf of Mexico and 25 percent of the oil and gas consumed in the United States.
In July and August 2005, USGS scientists Nancy Dewitt, BJ Reynolds, Nick Ferina, and Jim Flocks of the USGS Florida Integrated Science Center office in St. Petersburg, Fla., conducted geophysical, sediment-coring, and high-resolution bathymetric surveys along this 20-km stretch of coast, with bathymetric coverage extending from the shoreline to 7 km offshore. The surveys were part of an erosion study conducted in collaboration with the Louisiana Department of Natural Resources (LDNR) and the University of New Orleans (UNO). The scientists were assisted by captains Richard Young, Dave Bennett, and Keith Ludwig and electronics technician Dana Wiese onboard the USGS research vessel G.K. Gilbert. Results of their investigation, in comparison with a bathymetric survey conducted in 1989 (from the atlas by List and others, cited above), show continued elevation loss of as much as 4 m near the shoreline. In addition, shore-normal profiles indicate continued removal of material from the shoreface.
Just after the 2005 fieldwork, on August 29, 2005, Hurricane Katrina rumbled ashore 70 km east of the Caminada-Moreau headland, becoming the third-strongest hurricane ever to make U.S. landfall. While over the Gulf of Mexico, Katrina had been the most intense hurricane ever recorded, until it was surpassed 1 month later by Hurricane Rita as that storm's eye passed within 200 km of the Caminada-Moreau headland. Hurricane Rita continued to the Texas-Louisiana State boundary, completely obliterating coastal towns along the way. Significant loss to infrastructure, shoreline, and wetlands occurred across the headland during both storms.
To monitor the impact of the devastating 2005 hurricane season on the shoreline and sea floor, Dawn Lavoie, USGS science coordinator for the Gulf of Mexico, and Jack Kindinger, associate center director for the USGS Florida Integrated Science Center, initiated a revisit of the Caminada-Moreau headland and promoted a base-level assessment of the Chandeleur Islands.
The USGS scientists who surveyed the Caminada-Moreau headland in 2005 returned in June 2006 to resurvey the same area, this time supported by a UNO survey team. In addition, the survey area was expanded to encompass the entire barrier-island shoreline of southern Louisiana, from Raccoon Island to the modern Mississippi Delta, including the northern Chandeleur barrier-island chain, amounting to more than 500 km2 of sea floor surveyed in 2 months.
The scale of this survey added to the logistic difficulty of working in a region still recovering from last year's damage. The scope required three survey vessels to operate simultaneously, in water depths ranging from 1 to 15 m. In addition to collecting bathymetric data, the three survey teams also acquired subbottom-profiling (compressed high-intensity radar pulse [CHIRP]) data to 40 m below the sea floor. The bathymetric data will be combined with high-resolution elevation data from lidar (light detection and ranging) surveys flown along the shoreline by Wayne Wright of the National Aeronautics and Space Administration (NASA), using NASA's Experimental Airborne Advanced Research Lidar (EAARL), and by Jeff Lillycrop of the U.S. Army Corps of Engineers (USACE), using USACE's Compact Hydrographic Airborne Rapid Total Survey (CHARTS) system. The data sets will be combined to create a comprehensive topobathymetric map for use in coastal-zone management and as a baseline for assessing future shoreline changes.
In July, the scientists moved to the remote Chandeleur Islands. Long-term geologic models suggest that this island chain will eventually disappear and become an inner-shelf shoal in response to the combined effects of sand loss during storms and continuing sea-level rise. Hurricane Katrina accelerated this process; at present, the south half of the chain is mostly submerged shoal. The severity of damage caused by Katrina brings into question whether the islands will recover from this storm. The USGS scientists and their collaborators want to know how much influence the geologic setting has on the long-term evolution and ultimate fate of this island chain, and in the short term they want to determine where the sand lost from the islands went and whether sand sources exist that will allow this island chain to recover or be restored.
The Chandeleur Islands survey was an especially challenging phase of the project. Complete submergence during Hurricane Katrina had removed all landmarks from the islands, and tasks ranging from refueling the vessels to establishing geographic benchmarks required exceptional effort. The scientists from UNO and those from the USGS office in St. Petersburg, Fla. (Dewitt, Reynolds, Ferina, and Flocks), deployed from a houseboat moored behind what remains of the islands. They surveyed from the shoreline to 5-m water depth on the gulf side and out to 5 km from shore on the shallow bay side of the islands. A fourth survey crew from the USGS office in Woods Hole, Mass., operated from the Louisiana Universities Marine Consortium (LUMCON)'s research vessel Acadiana in deeper water off the gulf side of the islands. The Woods Hole group (Dave Twichell, Chuck Worley, Wayne Baldwin, and Emile Bergeron) conducted 24-hour operations, covering the offshore areas to a 15-m water depth with multibeam bathymetry, sidescan sonar, and CHIRP subbottom profiling. All the bathymetric data will be combined with lidar data being acquired along the shorelines.
Initial observation of the data shows tremendous storm impact to the islands, with large scours offshore and numerous breaches to the shoreface. Researchers hope that this survey will launch a comprehensive monitoring effort for the beleaguered barrier islands. Watch for more data and discussion about these scientific activities.
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