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Thursday, May 27, 2010

Jeff Masters' Wunderblog: What would a hurricane do to the Deepwater Horizon oil spill?

What would a hurricane do to the Deepwater Horizon oil spill?

by Jeff Masters, Wunderblog, May 26, 2010   
Hurricane season is upon us next week, and the Deepwater Horizon blowout is still spewing a geyser of oil into the Gulf of Mexico. With this year's hurricane season likely to be a severe one, with much above average numbers of hurricanes and intense hurricanes, we have the unwholesome prospect of a hurricane churning through the largest accidental oil spill in history. A hurricane has never passed over a sizable oil spill before, so there are a lot of unknowns about what might happen. The closest call came in 1979, after the greatest accidental oil spill in history, the massive Ixtoc I blowout. That disaster dumped 3 million barrels (126 million gallons) of oil into the Southern Gulf of Mexico between June 1979 and March 1980. Category 1 Hurricane Henri passed just north of the main portion of the oil spill on September 16 and 17, generating 15 foot seas and southwest winds of 15-25 knots over the spill region on the 16th. Interestingly, the NOAA/AOML report on the spill found that the winds did not blow long enough or strongly enough to control the direction of oil flow, as evidenced by the fact that the wind direction was often 180° to the direction of plume flow. The main impact of the wind was to dilute the oil and weather it, converting it to a thick "mousse."

Oil and beaches
During the Ixtoc spill, prevailing currents circulating clockwise from the blowout carried a 60 x 70 mile patch of sheen containing a 300 x 500 foot patch of heavy crude 900 miles to the South Texas coast. On August 6, 1979, tarballs from the spill impacted a 17 mile stretch of Texas beach. Mousse patches impacted the shoreline north of Port Mansfield Channel on August 15 and again on August 18. On August 24, mousse impacted shoreline south of Aransas Pass. By August 26, most of North Padre Island was covered with moderate amounts of oil. By September 1, all of the south Texas coast had been impacted by oil. However, Hurricane Henri formed in the Gulf of Mexico's Bay of Campeche on September 17-18. At the same time, a strong non-tropical low-pressure system formed along the Texas coast, bringing gale-force winds and rainfall amounts in excess of ten inches to the coast. The combination of swells from Hurricane Henri and wind-driven waves from the non-tropical low-pressure system scoured the oiled beaches of over 90% of their oil (Gundlach et al., 1981). The oil washed over the barrier islands into the estuaries behind them, and much of it sank to the bottom of the ocean. According to NOAA, impacts to the estuaries were minor. However, Payne and McNabb (1984) noted that selected regions of the coast, most of the beached oil was heavily resistant to transport during storms. Oil/sediment mats were ultimately covered by clean sand, but the oil/sediment mats were re-exposed and washed into the lagoon behind the barrier islands one year later when Category 3 Hurricane Allen battered the coast. No transport of the oil/sediment mats from the lagoon bottom was observed in the 3-year period following Hurricane Allen.

So, the Ixtoc blowout experience shows us that if a sandy beach is already fouled by oil, a hurricane can help clean up the mess. However, the situation is different along shores with marshlands, where the many shoreline plants offer crevices and tangled roots for the oil to accumulate in. A hurricane will help scour some of the oil out of marshlands, but the majority of it will probably remain stuck. This is also true of rocky beaches. Rocky shores fouled by the great Exxon Valdez oil spill in Alaska in 1989 have been pounded by many hurricane-strength storms over the years, but these storms were not able to clean the beaches of oil like Hurricane Henri did for Texas' beaches in 1979.

Transport of oil by hurricanes
Shores that are already fouled by oil will probably benefit from a hurricane, but the oil cleaned off of those shores then becomes someone else's problem. The strong winds and powerful ocean currents that a hurricane's winds drive will bring oil to large stretches of coast that otherwise would not have gotten oil. This is my chief concern regarding a hurricane moving through the Deepwater Horizon oil spill. Consider the case of the Exxon Valdez disaster in 1989. The ill-fated tanker split open in Prince William Sound on March 24, and oil spill response crews were initially able to contain the spill behind booms and make good progress removing it. However, two days later, a powerful Gulf of Alaska storm with 70 mph winds roared through, overwhelming the containment booms and distributing the oil along a 90-mile stretch of coast. The oil went on to foul over 400 miles of Alaska coast, a far larger disaster than would have occurred than if the storm had not passed by. Similarly, a hurricane moving through the Gulf of Mexico spill will very likely make the disaster much worse, spreading out the oil over a larger region, and bringing the oil to shores that otherwise might not have seen oil. It is true that the oil will be diluted some by being spread out over a larger area, so some shores will not see a substantial oiling. But overall, a hurricane passing through the oil spill is likely to result in much higher damage to the coast.

I expect that during the peak portion of hurricane season (August-October), the clockwise-rotating eddy that is attempting to cut off from the Loop Current this week will be fully separated from the Loop Current. The separation of this eddy will substantially reduce the possibility that significant amounts of oil will reach the Florida Keys and Southeast U.S. coast, since the Loop Current will be much farther south, flowing more due east towards the Keys from the Yucatan Channel. Oil moving southwards from the spill location due to a hurricane's winds will tend to get trapped in the 250-mile-wide eddy, potentially covering most of the surface of the eddy with oil. Thus we might have a 250-mile-wide spinning oil slick in the middle of the Gulf of Mexico for days or weeks after a hurricane. This could potentially have a significant warming effect on the Gulf waters, since the oil is dark and will absorb sunlight, and the oil will prevent evaporation from cooling the waters underneath it. Since Loop Current eddies contain a large amount of very warm water that extend to great depth, they often act as high-octane fuel for hurricanes that pass over. The rapid intensifications of Hurricanes Katrina and Rita were both aided by the passage of those storms over Loop Current eddies. Thus the warming of the Loop Current Eddy by oil pulled into it by a passing hurricane or tropical storm could lead to explosive intensification of the next hurricane that passes over the eddy.

The Loop Current Eddy will move slowly westwards toward Texas at about 4 miles per day after it fully cuts off. When it reaches the shallow waters near the Texas coast in early 2011, the eddy will turn northwards and gradually dissipate, By then, I expect that the vast majority of the oil in the eddy will have dispersed, sunk, or evaporated.

Storm surge and oil
One of the more unnerving prospects to consider if a hurricane hits the oil spill is what the hurricane's storm surge might do with the oil/dispersant mixture. The foul mix would ride inland on top of the surge, potentially fouling residential areas and hundreds of square miles of sensitive ecosystems with the toxic stew. The impacts of the oil and dispersant on vegetation may be too low to cause significant damage, since the hurricane would dilute the mixture with a large amount of sea water, and wash much of the toxic brew off the vegetation with heavy rain. We do have some limited experience with oil spills during Hurricane Katrina's storm surge to shed light on the subject. Katrina's storm surge caused over 8 million gallons of oil to spill into the storm surge waters. The largest spill occurred when the storm surge hit the Murphy Oil refinery in St. Bernard Parish, Louisiana. According to Santella et al. (2010), The refinery was inundated with 12 feet of water, and a partially filled 250,000-barrel above-ground storage tank was dislodged and ruptured, releasing 25,100 barrels (1.05 million gallons) of mixed crude oil. Dikes surrounding the oil tanks at the refinery were flooded and breached, and oil from the spill covered a residential area of approximately one square mile affecting approximately 1,800 homes. Front-end loaders were needed to remove the oily sediments from the area. A class action lawsuit resulted from the spill, ending in a $330 million settlement with a buy-out of properties closest to the spill and graded compensation in a larger zone. Katrina also caused a 139,000-gallon crude oil leak from a 20-inch pipeline at Shell Nairn Pipeline Company in Port Sulphur, Louisiana. Approximately 10,500 gallons of the spill reached the shoreline and coastal marshes, and only 10,700 gallons were recovered. This release resulted in a $5.5 million class action settlement to nearby property owners ( I haven't been able to find any information on how the marshlands fared after getting oiled by this spill.

Katrina's storm surge also destroyed an oil tank at Chevron's Empire facility, releasing oil into a retention pond in a region surrounded by marshland. Three and half weeks later, Hurricane Rita's storm surge hit the oily mess in the retention pond, washing 4,000-8,000 gallons of oil into nearby marshlands, which were heavily or moderately oiled. According to the EPA and Merten et al. (2008), the oiled marshlands were set on fire six weeks after the spill, resulting in 80-90% removal of the oil and contaminated vegetation. The marshland recovered fairly quickly, as seen in aerial photos taken five months after the burn (Figure 1) -- though oil still remained in the roots, affecting burrowing crabs and the wildlife that feed on them. So, oiled marshes can recover somewhat from a storm-surge driven oiling, but it is uncertain if burning could be successfully used to restore a 100+ square mile region of marshland oiled by the storm surge from a major hurricane. Another big unknown is how toxic BP's dispersants might be to the vegetation.

Figure 1. Upper left: oiled marshlands as seen on October 10, 2005, near Chevron's Empire facility, after the storm surges of Katrina and Rita. Right: The marshlands on March 16, 2006, five months after the controlled burn. The marshlands had largely recovered. Bottom: the controlled burn in progress (October 12, 2005.. Image credit: A.A. Merten, C. Henry, and J. Michel, 2008, Decision-making process to use in-situ burning to restore an oiled intermediate marsh following hurricanes Katrina and Rita, 2008 International Oil Spill Conference.

Wind and oil
The winds from a hurricane hurl ocean sea spray miles inland, often causing major defoliation and tree damage far beyond where the storm surge penetrates. For example, Category 2 Hurricane Bob of 1991 blew sea spray inland 4 miles (7 km) inland over Cape Cod. The salt deposited defoliated nearly all the deciduous trees along the coast. Kerr (2000) documented the case of Category 2 Typhoon Gay of November 23, 1992, which hit the 15-km wide island of Guam with 95-100 mph winds. Interaction with another typhoon disrupted Gay's thunderstorm activity, resulting in a nearly rainless typhoon for Guam. As a result, heavy amounts of salt coated the entire island, resulting in nearly complete defoliation. The salt didn't actually kill many plants, and the island re-greened within a year. The Category 3 New England Hurricane of 1938 was able to cause salt damage to trees as far as 45 miles inland, due to wind-blown sea spray. Thus we can anticipate that a hurricane passing over the oil spill will be able to hurl oil and toxic dispersants many miles inland during landfall. In regions where little rain falls, the concentrations of the oil and dispersants may be a problem. Again, we have no experience with this sort of situation, so the potential risks are unknown.

Rain and oil
Hurricanes evaporate huge amounts of water from the ocean and convert it to rain. In general, we do not need to worry about oil dissolving into the rain, since the oil and water don't mix. Furthermore, about 50-70% of the oil that is going to evaporate from the spill does so in the first 12 hours that the oil reaches the surface, so the volatile oil compounds that could potentially get dissolved into rain water won't be around. Hurricanes are known to carry sea salt and microscopic marine plankton hundreds of miles inland, since the strong updrafts of the storm can put these substances high in the troposphere where they can be carried far inland as the hurricane makes landfall. The Eastern Pacific's Hurricane Nora of 1997, whose remnants passed over Southern California, brought traces of sea salt and marine microorganisms to clouds over the central U.S. similarly, we can expect any landfalling hurricanes that pass over the oil spill to pick up traces of Gulf of Mexico crude and transport it hundreds of miles inland. However, I doubt that these traces would be detectable in rainwater except by laboratory analysis and would not cause any harm to plants or animals.

Lightning and oil
Could a lightning strike from a hurricane ignite oil from the spill, and the hurricane's winds hurl the flaming oil inland, creating a fiery maelstrom of water, wind, and flame? This would make a great scene in a typical bad Hollywood disaster movie, but it's not going to happen with the universe's current laws of physics. Lightning could set an oil slick on fire, in regions where the oil is most dense and very fresh. About 50-70% of the evaporation of oil's most flammable volatile compounds occurs in the first 12 hours after release, so fresh oil is the most likely to ignite. However, the winds of a hurricane are so fierce that any surface oil slick of flaming oil would quickly be disrupted and doused by wave action and sea spray. Heavy rain would further dampen any lightning-caused oil slick fires.

Bringing oil at depth to the surface
Hurricanes act like huge blenders that plow through the ocean, thoroughly mixing surface waters to depths as great as 200 meters (650 feet), and pulling waters from depth to the surface. Thus if sub-surface plumes of oil are located within 200 meters of the surface, a hurricane could potentially bring them to the surface. However, the huge sub-surface plumes of oil found by the research vessel Pelican were at depths of 2,300-4,200 feet, and a hurricane will not affect the ocean circulation at those depths.

Comparisons of the Deepwater Horizon blowout with Exxon Valdez
One footnote to consider when comparing the Deepwater Horizon blowout to the disastrous March 24, 1989, Exxon Valdez spill: the amount of oil spilled in that disaster is usually quoted as 11 million gallons (260,000 barrels). However, this is the number given by Exxon Mobil, and independent assessments by the State of Alaska came up with a much higher figure -- 24-36 million gallons, with state investigators stressing that the lower number was very unlikely. I'd be inclined to believe Exxon grossly understated the actual severity of the spill, much like BP is attempting to do with the Deepwater Horizon blowout. Steven Wereley, an associate professor at Purdue University, used a computer analysis (particle image velocimetry) to arrive at a rate of 95,000 barrels (4 million gallons) per day since the April 20 blowout, nearly 20 times greater than the 5,000 barrel a day estimate BP and government scientists have been citing. If he is correct, and the State of Alaska's figures on the Exxon Valdez disaster are correct, the Deepwater Horizon blowout so far has spilled five times the oil Exxon Valdez did.


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