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Using Shallow-Water Seafloor Mapping to Understand Sediment Movement in the Northern Chandeleur Islands, Louisiana

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Scientists from the U.S. Geological Survey (USGS) Coastal and Marine Geology Program’s science center in St. Petersburg, Florida, in cooperation with the U.S. Fish and Wildlife Service (FWS), have been studying the storm-related evolution of the Chandeleur Islands in eastern Louisiana for many years. This work was undertaken to support research objectives of the National Assessment of Coastal Change Hazards Project (http://coastal.er.usgs.gov/national-assessment/) and will also support a new project launched in October 2012: the Barrier Island Evolution Research Project. The latter seeks to address a research gap between the short time scale of individual storms (hours to days) and the longer time scales associated with the historic and geologic evolution of the coastal system (decades to millennia). The new project is an exciting challenge because it requires integration of two of the Coastal and Marine Geology Program’s strengths in studying coastal-change hazards—assessment of storm impacts and characterization of coastal geologic framework (the composition and geometry of rocks and sediment underlying coastal areas). Combining these strengths with modeling of morphology (the shape of the seafloor and land surface) will make possible predictions of barrier-island behavior over time scales useful to resource managers (1–5 years).

seafloor-mapping team
Above: The fearless seafloor-mapping team enjoys a post-survey sunset aboard service charter vessel The VI. Using a service charter vessel for room and board enabled the team to use small research vessels to access the shallow depths around the islands and simultaneously eliminate a 3- to 4-hour round trip to the mainland each day. Left to right, Kyle Kelso, BJ Reynolds, Julie Bernier, Will Pfeiffer, Jennifer Miselis, Dana Wiese, and Nancy DeWitt. [larger version]

Geologic variability (that is, changes in stratigraphy, surface-sediment distribution and composition, and morphologic features) has long been associated with barrier-island evolution over centennial and millennial time scales; however, the relative importance of geologic variability over shorter time scales (days to years) is poorly understood. Regional-scale research, while helpful for establishing the geologic framework in which barrier islands evolve, lacks the finer scale resolution necessary for addressing seasonal and interannual (over several years) system response. Furthermore, many models of morphologic evolution are ill equipped to incorporate the complexity of natural geologic variability and commonly assume uniformity in sediment distribution, composition, and availability that may not exist. Such assumptions can lead to model results that are not consistent with observations. In order to address medium-term relations between geologic variability and storm and nonstorm processes, the geographic extent of the observations must be reduced, high-resolution information from highly dynamic areas of the nearshore and surf zone must be obtained, and observed geologic variability must be suitably simplified for integration with predictive models.

To this end, scientists from the Seafloor Mapping Group at the USGS St. Petersburg Coastal and Marine Science Center are conducting a series of seafloor surveys of the submerged regions around the north end of the Chandeleur Islands. (For example, see “USGS Scientists Study an Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands, Louisiana,” Sound Waves, July/August 2012, http://soundwaves.usgs.gov/2012/08/.) The most recent survey took place in July 2012 and spanned 9 days. Because the region around the islands is very shallow (less than 10 meters [30 feet] deep), two small research vessels were used in lieu of one large vessel typically employed for this type of survey. A suite of acoustic sensors was deployed from the St. Petersburg center’s research vessel (R/V) Survey Cat to measure the surface and subsurface variability of the nearshore. A high-resolution swath bathymetry system (468 kHz) measured depths across a wide swath of the seafloor, a sidescan-sonar system (900 kHz) provided information about variations in seafloor-sediment type, and a chirp subbottom profiler (4–24 kHz) was used to measure the geometry of subseafloor sedimentary layers. The center’s R/V Twin Vee was outfitted with a dual-frequency single-beam echosounder (2 and 28 kHz) to reveal the morphology of the surf zone and of the submerged Hewes Point shoal north of the islands (see map below). In just 9 days, the R/V Survey Cat and R/V Twin Vee collected seafloor-mapping data along more than 900 kilometers (560 miles) of survey tracklines.

Analysis of data from the survey described above and from other surveys conducted in 2011 and 2012 will reveal patterns of cumulative change resulting from seasonal processes (for example, winter storms, summer tropical disturbances, and intervening fair-weather periods) and will help to identify annual sediment-transport pathways. The recent data will also be compared with regional-scale data collected in 2006 and 2007 and published in USGS Scientific Investigations Report 2009–5252 (http://pubs.usgs.gov/sir/2009/5252/). This comparison will help us better understand interannual changes in the submerged island platform caused by natural processes, as well as alterations to the system stemming from the construction of the Deepwater Horizon oil-spill-mitigation sand berm, begun in June 2010 and completed in March 2011. Furthermore, by combining seafloor-mapping data with island elevations derived from airborne lidar (light detection and ranging) surveys, we can establish links between the annual and interannual evolution of the Chandeleur Islands and the geologic variability just offshore. Establishing such links is essential for understanding not just the magnitude of surface-area changes to the islands, but also the response of the entire barrier-island system. Finally, data from 2012 were collected less than one month before the August 29 landfall of Hurricane Isaac, which directly affected the Chandeleur Islands. Data from airborne optical sensors can tell us the volume of sand lost from the islands. (Most of the oil-spill-mitigation sand berm was swept away; see http://coastal.er.usgs.gov/hurricanes/isaac/photo-comparisons/ and http://www.nola.com/hurricane/index.ssf/2012/09/isaacs_surge_waves_wiped_out_b.html.) A third seafloor survey planned for 2013 will help us pinpoint where lost sediment was deposited and whether or not it will be naturally available to the islands for post-storm recovery. This information is essential for models that will predict the evolution of the Chandeleur Island system over the next 1 to 5 years.

Morphology of submerged island platform surrounding the northernmost Chandeleur Islands
Above: Morphology of submerged island platform surrounding the northernmost Chandeleur Islands, in 2011 (A) and 2012 (B). Extent of the islands (dark green) in February 2011 (A) and February 2012 (B) is derived from airborne lidar (light detection and ranging) data and includes oil-spill-mitigation sand berm constructed along northernmost 12 kilometers (7 miles) of the chain on the Gulf of Mexico side. Hewes Point, a large, submerged sand deposit just north of the islands, was a source of sediment for berm construction. Note location and morphology of borrow pit and changes to the shape of Hewes Point itself. Photograph B in set below shows installation of the benchmark "BERM" (which is not on the berm but on an island west of the berm) before the 2012 survey. TMRK, another benchmark on the islands. Grayscale bathymetry in background from Love, M.R., Amante, C.J., Carignan, K.S., Eakins, B.W., and Taylor, L.A., 2010, Digital elevation models of the northern Gulf Coast—procedures, data sources and analysis: NOAA National Geophysical Data Center technical report, Boulder, Colo., 37 p. (http://www.ngdc.noaa.gov/dem/squareCellGrid/download/731). [larger version]

Information derived from these repeated shallow-water seafloor-mapping surveys allows the USGS to remain at the forefront of understanding coastal-change hazards. These surveys will expand our expertise from short- and long-term assessments to quantifying medium-term changes, filling a gap where scientists lack fundamental information about the relations between surf-zone and nearshore geologic variability and coastal evolution over time scales relevant to coastal-resource management. Datasets such as this one enable an assessment of those relations and allow the USGS to honor its commitment to applying cutting-edge science to resource-management issues. By integrating temporally relevant and simplified geologic observations into morphologic models of coastal change, we seek to refine predictions of barrier-island and coastal evolution and to support more informed management of the nation’s coasts.

Views of the survey area
Above: Views of the survey area. A, Low-elevation Chandeleur Islands from the research vessel (R/V) Twin Vee. B, (Left to right) Kyle Kelso, Nancy Dewitt, and Julie Bernier install a Global Positioning System (GPS) benchmark (BERM) on the northernmost part of the Chandeleur Islands. C, Julie Bernier surveys a transect from the Gulf of Mexico, across the berm, and onto the island with GPS to provide data for ground-truthing seafloor-mapping elevations in the Gulf and lidar elevations acquired after the geophysical survey. D, Will Pfeiffer surveys island elevations in different types of vegetation to provide data for ground-truthing lidar. [larger version]

Related Sound Waves Stories
USGS Scientists Study an Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands, Louisiana
August 2012

Related Web Sites
National Assessment of Coastal Change Hazards
Hurricane Isaac Potential Coastal-Change Impacts
Hurricane Isaac Pre- and Post-Storm Photo Comparisons
Sand Resources, Regional Geology, and Coastal Processes of the Chandeleur Islands Coastal System: an Evaluation of the Breton National Wildlife Refuge
Hurricane Isaac's Surge, Waves, Wipe out BP Oil Berm Along Northern Chandeleur Islands
The Times-Picayune

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cover story:
Scientists Predict, Measure Sandy's Impacts

Post-Tropical Cyclone Sandy

Sediment Movement in the Northern Chandeleur Islands

Recovery Slows for California's Sea Otters

Mapping the Georges Bank Seabed

Native Youth in Science—Preserving Our Homelands

2011 Excellence in Partnering Award

Staff Pacific Coastal and Marine Science Center Welcomes Andy O'Neill

Olivia Cheriton Joins Pacific Coastal and Marine Science Center

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