Tracking Debris to Forecast Coastal Impacts
The findings were published Jan. 19 in Environmental Research Letters.
On 5th May 2010, fifteen days after the Deepwater Horizon explosion, oceanographer William Graham and marine technicians from the Dauphin Island Sea Lab were working from a boat about 32 miles south of Dauphin Island, AL., when they saw a 6-mile-long, east-west line containing more than 50 pieces of white material interspersed with sargassum weed. The porous material was uniformly embedded with black spheres about a centimetre in diameter. No oil slick was in sight, but there was a halo of oil sheen around the honeycomb clumps.
Two days later, the researchers also collected similar samples about 25 miles south of Dauphin Island. Nobody knew what the material was, with some hypothesising at first that it could be coral or other substance made by marine plants or animals. Graham sent samples to WHOI chemist Chris Reddy, whose lab confirmed that the material was not biological. But the material's source remained unconfirmed.
In January 2011, Reddy and WHOI researcher Catherine Carmichael, lead author of the new study, collected a piece of the same unknown material of Elmer's Beach, Grand Isle, LA. In April 2011, they found several large pieces, ranging from 3 to 10 feet, of the honeycomb debris on the Chandeleur Islands off Louisiana.
Oil on all these samples was analysed at WHOI using two-dimensional gas chromatography. The technique identifies the thousands of individual chemical compounds that comprise different oils from different reservoirs. The chemistry of the oil on the debris matched that of oil sampled directly from the broken pipe from the Macondo well above the Deepwater Horizon rig.
In addition, one piece of debris from the Chandeleur Islands retained a weathered red sticker that read "Cuming" with the numbers 75-1059 below it. Reddy found a company called Cuming Corporation in Avon, MA, which manufactures syntactic foam flotation equipment for the oil and gas industry. He e-mailed photos of the specimen to the company, and within hours, a Cuming engineer confirmed from the serial number that the foam came from a buoyancy module from Deepwater Horizon.
The scientists overlaid the locations where they found honeycomb debris on May 5 and 7 with daily forecasts produced by the National Oceanic and Atmospheric Administration (NOAA) of the trajectory of the spreading oil slick. NOAA used a model that incorporated currents and wind speeds, along with data from planes and satellites. On both days, the debris was about 6.2 miles ahead of the spreading slick.
The explanation, the scientists said, is the principle of leeway, a measure of how fast wind or waves push materials. The leeway for fresh oil is 3 to 3.3 percent, but the scientists suspected that "the protruding profile of the buoyant material" acted acting like a sail, allowing wind to drive it faster than and ahead of the floating oil.
In this case, the flotsam served as a harbinger for the oncoming slick, but because different materials can have different leeways, oil spill models may not accurately forecast where oiled debris will head.
The Coast Guard has a long history of calculating the leeway of various materials, from life jackets to bodies of various sizes and weights, to improve forecasts of where the materials would drift if a ship sank or a plane crashed into the sea. But calculating leeways has not been standard practice in oil spills.
The study's authors wrote that these results provide insights into the fate of debris fields deriving from damaged marine materials and should be incorporated into emergency response efforts and forecasting of coastal impacts during future offshore oil spills.
