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Fieldwork

USGS Scientists Study an Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands, Louisiana



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The Chandeleur Islands and surrounding Breton National Wildlife Refuge provide habitat for a variety of threatened wildlife species, function as recreational areas, and act as storm protection for the vast human infrastructure in coastal Louisiana. Over the past two decades, the islands have become more susceptible to storm-induced breaching and erosion and are undergoing the highest rate of land loss among barrier islands in the Gulf of Mexico. For this reason, the fate of the islands has been the focus of intense study by the U.S. Geological Survey (USGS) and collaborators over the past decade. The most comprehensive studies to date occurred in 2006 and 2007, when projects sponsored by the Louisiana Department of Natural Resources, the U.S. Army Corps of Engineers, and the U.S. Fish and Wildlife Service, in collaboration with the USGS and the University of New Orleans, were conducted to provide a comprehensive characterization of the geology and morphology (shape) of the islands. Results are compiled in a USGS Scientific Investigations Report, “Sand Resources, Regional Geology, and Coastal Processes of the Chandeleur Islands Coastal System: An Evaluation of the Breton National Wildlife Refuge.”

On April 20, 2010, approximately 130 kilometers (80 miles) southeast of the Chandeleur Islands, the drilling rig Deepwater Horizon exploded and oil began discharging from the Macondo MC252 well beneath it. Within 2 weeks, oil was observed at the islands and elsewhere along the Louisiana coastline (see satellite image). By May 2010, in an attempt to protect mainland wetlands, the State of Louisiana had requested emergency authorization to construct sand berms along the coast to block the movement of oil (see map). At Breton National Wildlife Refuge, the original plan called for the construction of three lengths of berms seaward of the islands that stretched end-to-end from the northernmost island 48 kilometers south to the Mississippi River Delta. To construct the berms, sand would be excavated from a continuous trench 1 kilometer offshore and placed on the island shoreface to produce a mound of sand 182 meters wide at the base and 1.8 meters high at its apex. It was estimated that more than 6.5 million cubic meters of sand would be necessary for construction, making this one of the most ambitious coastal construction efforts in U.S. history. The challenge at this point was to locate such a large amount of suitable sandy material in an otherwise muddy Mississippi River Delta plain.

satellite imagery from May 24, 2010, showing the Deepwater Horizon oil spill Southern Louisiana and the site of the Deepwater Horizon oil-spill incident.
Above Left: MODIS (Moderate Resolution Imaging Spectroradiometer) satellite imagery from May 24, 2010, showing the Deepwater Horizon oil spill (light color) approaching the Chandeleur Islands and Louisiana coast. [larger version]

Above Right: Southern Louisiana and the site of the Deepwater Horizon oil-spill incident. The State of Louisiana originally planned to build a series of sand berms along the barrier islands to protect the interior marsh from oil contamination. Open segments between individual berms would allow tidal exchange. Black lines represent berms that were proposed but not built; thicker, red lines represent berms that were actually constructed. [larger version]

Through consultation with the USGS and other agencies, it was determined that excavation of material along a linear trench 1 kilometer offshore was not practical and that other sources for the material, such as offshore shoals, were more viable options. This and other concerns were outlined in the USGS Open-File Report, “Effects of Building a Sand Barrier Berm to Mitigate the Effects of the Deepwater Horizon Oil Spill on Louisiana Marshes.” On the basis of data from earlier studies, a large sand resource was identified at Hewes Point, a sand spit building outward from the north end of the barrier-island platform (see deposit-thickness map). Geophysical and coring surveys indicate that the Hewes Point deposit is approximately 27 square kilometers in area and 8 meters thick, and is composed of 97 percent well-sorted fine sand. The formation of Hewes Point at the northern terminus of the Chandeleur Islands was the result of unique geologic conditions. The combination of the island’s orientation parallel to the prevailing waves, a centralized source of sandy deposits, and deeper water that allowed sediment to accumulate (called “accommodation space”) produced an efficient natural sediment trap that rapidly formed the largest sand body among Louisiana’s barrier islands, with a volume of approximately 258 million cubic meters.

Sediment-thickness map
Above: Sediment-thickness map in perspective view (looking south), showing thickness of the sand deposit at Hewes Point, north of the Chandeleur Island chain. Sand used to construct the E-4 berm was excavated from the side of the deposit, about 3 kilometers north of the islands. [larger version]

With Hewes Point as a source, construction of the sand berm began in June 2010 and continued through March 2011, long after the Macondo MC252 well had been capped (July15, 2010) and observations of surface oil within the Gulf had ceased (August 2010). In its post-construction form, only the first length (known as E-4) of the originally proposed trio of berms was completed (see aerial photograph, below left). The E-4 berm extended along the submerged axis of the northernmost Chandeleur Island chain—detached from the islands—for approximately 8 kilometers and joined the island shoreface for an additional 4 kilometers. About 4 million cubic meters of sandy sediment was used in the construction. The berm was engineered as a temporary structure and underwent significant change both during and after construction. Despite a relatively uneventful tropical cyclone season and few significant winter cold fronts, winds and overwash from waves have reduced its elevation and segmented the berm at numerous locations, significantly reducing its subaerial extent (see aerial photograph below right).

completed E-4 berm Oblique aerial photograph showing significant breaching of the E-4 berm at several locations
Above Left: Photograph taken on April 13, 2011, of the completed E-4 berm, which was constructed along the Gulf of Mexico side of the Chandeleur Islands. The berm was detached from the islands for the first 8 kilometers (13 miles), beyond which it was constructed on the beach to reduce required sand volume. Breaches occurred during construction; the one labeled on the photograph occurred January 2011 at the site of a natural inlet. The breach expanded 600 meters in less than one month. Photograph courtesy of the Coastal Protection and Restoration Authority of Louisiana. [larger version]

Above Right: Oblique aerial photograph from a survey flown by the USGS on March 6, 2012, showing significant breaching of the E-4 berm at several locations and reduced elevations along its entire extent. Image courtesy of Karen Morgan, USGS. [larger version]

The berm and the northern Chandeleur Islands provide a natural laboratory of unusually large scale to observe how sudden changes in morphology (for example, due to storms or renourishment projects) and geologic processes (such as erosion, deposition, and rollover—the landward movement of a barrier island as sediment is eroded from the seaward side and deposited on the landward side) will affect barrier-island evolution. With the wealth of scientific data already available for the islands and the fact that the berm will interact with the barrier-island system on observable time scales, the USGS hopes to answer fundamental questions about how climatic and geologic variables influence the present and future morphology of coastal systems. Understanding the physical interactions that drive coastal evolution provides a framework of knowledge for effective management of coastal planning, protection, and restoration. This need prompted a new USGS Coastal and Marine Geology Program project, led by Nathaniel Plant, that includes a comprehensive spatial and temporal characterization of the E-4 berm and adjacent waters and islands.

Using satellite imagery, lidar (light detection and ranging) mapping of topography, bathymetric investigations, direct sampling, and numerical modeling, scientists have been monitoring changes along the berm and surrounding environment. From March 22 to 26, 2012, through collaboration with the U.S. Fish and Wildlife Service, a team accessed the berm and islands for direct sediment sampling. On hand were Kyle Kelso, Julie Bernier, Marci Marot, Carl Taylor, Christopher Smith, and Jim Flocks from the USGS St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida. Several sampling methods were used—surface grab samples, short (approximately 30 centimeter) hand-auger cores, ponar and ekman grab samples, and short (approximately 1 meter) push cores (see photographs at http://woodshole.er.usgs.gov/openfile/of2005-1001/htmldocs/grab.htm for examples). In addition, a modified version of the “poking eyeball” camera system was provided by the USGS Pacific Coastal and Marine Science Center (Santa Cruz, California) and the USGS Woods Hole Science Center (Woods Hole, Massachusetts) to collect closeup images of the sediments.

sampling team Marci Marot, Kyle Kelso, and Chris Smith of the USGS subsampling a push core collected from the Sound side of the Chandeleur Island chain
Above Left: USGS sampling team preparing to deploy equipment on the Chandeleur Islands. Rapid static Global Positioning System (GPS) instruments mounted on posts recorded sampling positions, normalized to a GPS base station on one of the islands (not shown). Shown in photograph (left to right) are Kyle Kelso, Julie Bernier, Christopher Smith, Marci Marot, and Carl Taylor. Photograph by Jim Flocks, USGS. [larger version]

Above Right: (Left to right) Marci Marot, Kyle Kelso, and Chris Smith of the USGS subsampling a push core collected from the Sound side of the Chandeleur Island chain. After sectioning, the samples were kept cool until returning to the laboratory. There, radioisotope and other analyses will determine accretion rates, microfossil assemblages, and physical characteristics. Sampling was conducted from aboard the floating fish camp Pelican. Photograph by Jim Flocks, USGS. [larger version]

USGS team members examining sediment on the E-4 berm
Above: USGS team members examining sediment on the E-4 berm. A, Carl Taylor (left) collects a closeup photograph of the berm sediments using the modified "poking eyeball" camera system, while Julie Bernier records position. B and C, Example photographs of in-place berm sediments deposited by water (B) and wind (C). D, Short core being extruded from a hand-auger piston. Photographs by Jim Flocks, USGS. [larger version]

The sampling strategy included several components with the intent to monitor change to the berm and islands over time. In the back barrier, short push cores were collected to quantify short-term (seasonal to annual) and long-term (decadal to centennial) sediment movement, as well as to assess sediment storage in the back-barrier environments (marshes, tidal flats, and so on; see photograph of core sample). Repeat sampling of these back-barrier environments through the duration of the project will provide critical information on how the presence of the berm may influence the storage of sediments. Similarly, grab-sample transects along the axis of the berm were collected to provide snapshots of the physical characteristics of the berm. Repeated sampling of these sites will provide information on how washover and aeolian (wind) processes redistribute the berm sediment. Closeup photographs were also collected to supplement the sampling dataset. Sample transects across the berm and onto the adjacent barrier island will ultimately measure the potential contribution of sediment from the berm to the island shoreface. And finally, reoccupying several sites that were sampled during surveys in 2006 and 2008 will enable comparison of pre- and post-berm physical characteristics (see sample-site map).

Landsat imagery from January 15, 2012, with the remnants of the E-4 berm
Above: Landsat imagery from January 15, 2012, with the remnants of the E-4 berm outlined in black dashed lines. Sampling sites shown by sample type. Grab-sample sites (green pentagons) extended along the axis of the berm; grab-sample collection was accompanied by photo surveys. Several grab sites along the Sound side reoccupied sample sites from a pre-berm survey (2008). Short hand-auger cores were collected in transects across the berm and onto the islands. In addition to grab samples, 1-meter push cores (orange and yellow dots) were collected in the back-barrier bay and marsh environments. 12BIM01, project's USGS activity ID. [larger version]

Fortunately, distinguishing the berm sediments from the original island sediments is not difficult. The source deposit at Hewes Point is remarkably well sorted, of uniform texture, and devoid of shells. In contrast, the beach sands have high shell content, are mixed with heavy minerals and organic particles, and are finer grained. These characteristics make it possible to differentiate the berm from the island platform. One goal of the study is to identify how these variations between sediment composition of the berm and the island change the natural response of the island system to physical processes. In an ironic twist, since the berm’s completion in March 2011, the erosion of the berm is being influenced by the island chain. The northernmost segment of the berm was constructed landward of the now-submerged footprint of the island platform. Hurricanes Georges, Ivan, and Katrina submerged this northern chain of islands at Hewes Point, yet the chain still provides a breakwater that is protecting the berm from overwash. Sands driven by longshore currents are presently prograding (building outward) from the berm along the submerged breakwater, and in places the island platform is re-emerging. Although it contains significant breaches, this segment of the berm has the highest remaining elevation, and aeolian processes currently dominate. South of this segment, where the healthiest islands exist, the central berm is rolling over into the manmade trough that formed between the berm and the islands during construction. This segment exhibits the most promising beach accretion if the unconsolidated berm sediments can resist removal during storms. The southernmost segment of the berm exhibits the highest reduction in elevation. Along this reach, islands and dunes are fewer, and overwash splays and inlets are wider. Virtually all of the berm along this reach has been overwashed and eroded and in places has been completely removed.

Photographs of the E-4 berm
Above: Photographs of the E-4 berm. A, View across the berm to the adjacent island. The berm was constructed immediately offshore from the original island shoreface, creating a trough that is now infilling with berm sediments through washover and aeolian processes. This segment of the berm is supported by healthy dunes with few inlets and is accreting to the original shoreface, producing an expansive beach. USGS team members Carl Taylor (left) and Julie Bernier are labeling samples and registering the sample site. B, The southernmost extent of the berm is distinguishable from the island by the berm's lighter sand and little-to-no shell material. Wind and waves have reduced the elevation to original beach height. Photographs by Jim Flocks, USGS. [larger version]

During their 4-day stay on the islands, the team operated from the floating fish camp Pelican, which always welcomes USGS scientists and provides outstanding views of the sunrise and sunset. The team used skiffs to navigate the shallow waters of the back barrier, although sometimes the shallow draft of these boats was not shallow enough. The team successfully circumvented the March 2012 cold fronts, invasion by Portuguese man o’ war (Physalia physalis), and other challenges typical of work in remote natural environments to complete the survey. The samples the team collected are currently being analyzed, and the team looks forward to integrating results of the study with the other components of the project.

Sunrise over the Chandeleur Islands Portuguese man o' war
Above Left: Sunrise over the Chandeleur Islands. The remote islands never lack stunning natural views; the marsh and surrounding waters provide habitat for numerous fowl and aquatic species. Photograph by Jim Flocks, USGS. [larger version]

Above Right: On the Sound side of the islands, Portuguese man o' war (Physalia physalis), whose venomous tentacles can deliver a powerful sting, were washing up onto the shoreline by the thousands. Photograph by Jim Flocks, USGS. [larger version]

Small skiff aground
Above: Small skiffs were used for transportation around the shallow back-barrier waters, and sometimes even their drafts were not shallow enough. The USGS team walked back to the fish camp the day this photograph was taken. Photographs by Jim Flocks, USGS. [larger version]


Related Sound Waves Stories
The "Poking Eyeball"—a Prototype Underwater Camera System
April 2003

Related Web Sites
Effects of Building a Sand Barrier Berm to Mitigate the Effects of the Deepwater Horizon Oil Spill on Louisiana Marshes
USGS
Sand Resources, Regional Geology, and Coastal Processes of the Chandeleur Islands Coastal System: an Evaluation of the Breton National Wildlife Refuge
USGS
Breton National Wildlife Refuge
U.S. Fish and Wildlife Service

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Fieldwork
cover story:
Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands

Ocean-Circulation and Sediment-Transport Data Offshore of Fire Island

Outreach
Open House in Menlo Park, California

Meetings
Workshop on Probability of Landslide-Generated Tsunamis

Key Drivers of Central California Coastal Change and Inundation Due to Climate Change

Awards
James V. Gardner, 2012 Shepard Medalist for Excellence in Marine Geology

Staff Team MarFac Completes Century Bicycle Ride

Publications July / August 2012 Publications

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