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Spotlight on Sandy

Tracking Oil—USGS Tools and Analysis Inform Response to Deepwater Horizon and Future Oil Spills



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U.S. Geological Survey (USGS) scientists have developed a new computer model to track the movement of residual oil that persists along the northern Gulf of Mexico coast 4 years after the Deepwater Horizon disaster released several million barrels of gas and oil into the gulf. This new model is being used to help guide ongoing cleanup efforts and can be used to aid the response to future oil spills.

In the weeks and months after the Deepwater Horizon oil spill, oil mousse—a frothy blend of oil and seawater—was transported by waves and currents toward northern Gulf of Mexico beaches, where it mixed with sand in the shallow surf zone to form rounded clumps or mats of material, sometimes as much as 10s of meters long and several centimeters thick. In some cases, smaller clumps, called surface residual balls (SRBs), formed in the surf zone or broke free from the larger mats and caused a re-oiling of the beach. During the response to the spill, the U.S. Coast Guard attempted to locate and, where possible, remove mats and SRBs. Response was hampered, however, by the difficulty in determining the depth range over which mats and SRBs may have formed within the surf zone, and by not knowing when and where SRBs would be transported.

Surface residual ball (SRB) made of a mix of oil and sand on Gulf of Mexico beach Formation of sand/oil agglomerates requires the interaction of sediment and floating oil

Above Left: Surface residual ball (SRB) made of a mix of oil and sand on Gulf of Mexico beach. Photograph taken May 8, 2011, by the U.S. Coast Guard. [larger version]

Above Right: Formation of sand/oil agglomerates requires the interaction of sediment and floating oil. Analysis of sediment suspension indicates that such interaction is likely in the shallow swash zone, but at deeper depths can only occur in the surf zone under plunging breaking waves. [larger version]

As part of the Operational Science Advisory Team (OSAT3)—one of several multiagency working groups formed to provide scientific information to assist the government response to the Deepwater Horizon oil spill—researchers at the USGS St. Petersburg Coastal and Marine Science Center in St. Petersburg, Florida, developed a methodology for determining at what depths waves could suspend enough sediment to the water’s surface to mix with floating oil and form agglomerates. The analysis indicated that formation of oil/sand mats was likely in the swash zone and possible within the surf zone under plunging breaking waves (see diagram, upper right). The results of this analysis are included in the OSAT3 report .

Numerical-model results showing mobility of sand and surface residual balls

Above: Numerical-model results showing mobility of sand and surface residual balls (SRBs—clumps of oil and sand) in small waves (left) and large (storm) waves (right). Warmer colors indicate greater mobility, with magenta indicating the greatest mobility. Model results demonstrate that sand (top panels) is more mobile than SRBs (bottom panels), making burial and exhumation of SRBs likely. (2.5 centimeters [cm] is approximately 1 inch.) [larger version]

A numerical modeling approach was also developed to analyze the seafloor interaction and alongshore movement of SRBs. This work determined that centimeter-size SRBs are not mobilized during the low-energy wave conditions typically observed in the northern Gulf of Mexico, but that during storm events SRBs as much as 10 centimeters in diameter could be transported. Because sand is more mobile than the larger SRBs, burial by sand and subsequent exhumation are key processes of SRB dynamics, and larger SRBs may be found by response teams under lower energy wave conditions if the sand covering them is swept away. The analysis also found that inlets will trap SRBs, inhibiting their alongshore transport. Comparison with field data collected by the Coast Guard supported the key findings of the study (see article in Marine Pollution Bulletin).

Pseudo-SRBs (surface residual balls) made of paraffin wax and sand were deployed in the shallow surf zone (left panels) and tested in an oscillatory-flow tank (right panels)

Above: Pseudo-SRBs (surface residual balls) made of paraffin wax and sand were deployed in the shallow surf zone (left panels) and tested in an oscillatory-flow tank (right panels). Initial findings support the numerical-modeling conclusion that centimeter-size SRBs are not mobilized under low-energy conditions, and that burial and exhumation are important processes. For example, see lower right panel, where a sand ripple, moving from right to left, has uncovered an SRB to its right and is beginning to migrate over several SRBs to its left. View video footage of one of the oscillatory-flow tank experiments. [larger version]

Recent research aims to provide additional observational data to quantify the accuracy of the results of the numerical-modeling analysis. In December 2013, pseudo-SRBs constructed out of paraffin wax and sand were deployed in the shallow surf zone near St. Petersburg, Florida. Wave conditions were quantified with a pressure gauge, and the movement of the pseudo-SRBs was tracked with the USGS Along-Track Reef Imaging System (ATRIS) and commercial video camera systems. In March 2014, the pseudo-SRBs were tested in an oscillatory flow tank at the Sediment Dynamics Laboratory of the Naval Research Laboratory at Stennis Space Center, Mississippi, to further observe and quantify, under more controlled conditions, their thresholds of incipient motion and interaction with the seafloor. (View video footage of one of the oscillatory-flow tank experiments below.) Ongoing analysis of the field and laboratory data supports the previous findings of the importance of seafloor interaction and burial and exhumation, and may provide the basis for more refined estimates of SRB movement.

The methodologies developed here informed the Coast Guard response to the Deepwater Horizon oil spill, and they can be applied to future disasters to help inform response and cleanup.

Sand ripples cover and uncover pseudo-SRBs (surface residual balls) in this experiment in an oscillatory-flow tank at the Sediment Dynamics Laboratory of the Naval Research Laboratory at Stennis Space Center, Mississippi. Pseudo-SRBs made of paraffin wax and sand sit on a layer of sand. The back-and-forth motion of the water, driven by an offscreen piston, increases throughout the clip, from low-energy conditions typical of the northern Gulf of Mexico to high-energy conditions expected during storms. Sand grains move more readily than the pseudo-SRBs, which are covered and uncovered by the sand ripples that form and migrate in response to the flow. In the real world, covered SRBs are harder for cleanup crews to detect, and newly uncovered SRBs may re-pollute coastal areas.


Related Sound Waves Stories
USGS Responds to Deepwater Horizon Oil Spill in the Gulf of Mexico
May / June 2010
USGS Scientist Honored with Prestigious Federal Employee of the Year Medal for Role in EndingDeepwater Horizon Oil Spill
Nov. / Dec. 2011
Long-Lived, Slow-Growing Corals in Deep Waters of the Gulf of Mexico
March 2011
Corals Damaged in the Deep Gulf of Mexico by Deepwater Horizon Oil Spill
Sept. / Oct. 2012
USGS Scientists Study an Oil-Spill-Mitigation Sand Berm in the Chandeleur Islands, Louisiana
July / August 2012

Related Websites
Operational Science Advisory Team Report III
RestoreTheGulf.gov
Open-File Report 2012–1234
USGS
Assessing mobility and redistribution patterns of sand and oil agglomerates in the surf zone
Marine Pollution Bulletin
Along-Track Reef Imaging System (ATRIS)
USGS
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Fieldwork
cover story:
Earthquake, Landslide, and Tsunami Hazards in the Caribbean

Through the Eyes of a Polar Bear—First "Point of View" Video

Spotlight on Sandy
Hurricane Sandy Impacts Did Not Contribute to Subsequent Storm Flooding

New Personnel Study Estuarine Response to Storms

Summer Hires Assist Studies of Coastal Sediment Transport

Research
Tracking Oil—USGS Tools and Analysis Inform Oil-Spill Response

Help Identify Coastal Hazards with Aerial Photographs on "iCoast" Website

Coral Reefs Provide Critical Protection to Coastal Inhabitants

Staff
New Postdoctoral Researchers at USGS in Woods Hole, Massachusetts

Summer Intern at USGS in Woods Hole, Massachusetts

Publications New USGS Coastal and Marine Geology Web Pages

Facilitating Identification of Coastal and Undersea Features

May / June Publications

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