|Home||Archived February 20, 2019||(i)|
USGS Scientists Study an Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands, Louisiana
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.
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.
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).
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.
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).
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.
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.
in this issue:
|Home||Archived February 20, 2019|