ANN ARBOR—More than 700 miles of shoreline in lakes Huron and Michigan are potentially vulnerable to oil spills if the pipeline beneath the Straits of Mackinac ruptures, according to a new University of Michigan computer-modeling study.

U-M hydrodynamics expert David Schwab ran 840 simulations of a release from the Enbridge Line 5 pipeline under the Straits of Mackinac. The study, which was supported by the National Wildlife Federation, was released Thursday by the U-M Water Center and pinpoints areas of highest risk along the shores of lakes Huron and Michigan, and on their islands.

A total of 720 miles (1,162 km) of shoreline are considered potentially vulnerable to Straits of Mackinac oil spills. This map shows the probability that beached oil concentrations would exceed the threshold needed to trigger shoreline cleanup following the release of 25,000 barrels at the Straits of Mackinac. Image credit: David Schwab, U-M Water CenterUp to 152 miles (245 km) of coastline in lakes Huron and Michigan could be fouled by a single oil spill at the straits, according to the simulations. When all 840 simulated spills are plotted on a map, a total of 720 miles (1,162 km) of shoreline in the U.S. and Canada are considered potentially vulnerable to spills that would require cleanup. Seven hundred twenty miles is roughly the distance from Detroit to Atlanta.

Areas at highest risk include Mackinac and Bois Blanc islands, as well as locations directly east and west of Mackinaw City. Communities also at risk include Beaver Island, Cross Village, Harbor Springs, Cheboygan and other places along the lakes Huron-Michigan shoreline.

The study is a detailed follow-up to a 2014 pilot project by Schwab, which used computer simulations to determine how far and how fast an oil spill could spread from the Straits of Mackinac. The new, expanded study is the most comprehensive report on the topic available to the general public.

“Until now, no one knew exactly how much shoreline was vulnerable to spills in the Straits of Mackinac,” said Schwab, a research scientist at the U-M Water Center. “These findings show that under the right conditions, a spill in the Straits of Mackinac could affect a significant amount of shoreline and open-water areas in either Lake Michigan or Lake Huron, or both, very quickly.

“We hope this information will inform spill-response planning and will help government officials make sound decisions about the oil pipeline beneath the straits.”

The study uses a high-resolution hydrodynamic model developed by Schwab and colleague Eric Anderson of the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory in Ann Arbor. The model was run using weather and water-current data measured in the Straits of Mackinac during the ice-free season.

The simulations incorporate realistic estimates for a worst-case discharge just south of the halfway point in the 5-mile-wide Straits of Mackinac, which separates Michigan’s upper and lower peninsulas and connects lakes Michigan and Huron.

Other key findings:

• Nearly 60 percent of Lake Huron’s open water and more than 15 percent of Lake Michigan’s open water showed visible oil in at least one of the 840 spill simulations. The total area of those vulnerable open waters—17,318 square miles (44,405 square km)—is roughly equivalent to the combined surface areas of lakes Erie and Ontario.
• The maximum open-water area covered by a single hypothetical spill is 624 square miles (1,600 square km), an area larger than Lake St. Clair.
• The shortest arrival times for visible surface oil—2.5 hours—occur on the south shore of the straits, near Mackinaw City. Mackinac Island could be impacted in nine hours, Bois Blanc Island in 10 hours. Oil could reach Cheboygan in 30 hours.

The 63-year-old Enbridge Line 5 pipeline moves up to 20 million gallons of light crude oil, light synthetic crude oil and natural gas liquids across the straits each day. Line 5 splits into two 20-inch-diameter pipes to cross the Straits of Mackinac, just west of the Mackinac Bridge. The area around the straits is considered ecologically sensitive and is a major tourist draw.

The 2014 pilot project used two computer simulations to show that the strong currents in the Straits of Mackinac—which reverse direction every few days—would quickly contaminate shorelines miles away in both lakes Huron and Michigan. Based on that study, Schwab concluded that the Straits of Mackinac is “the worst possible place for an oil spill in the Great Lakes.” The new study reaffirms that characterization.

This study goes beyond the 2014 pilot project in several ways.

Instead of two simulations, there are now 840. They cover a wide range of weather and water-current conditions and account for various characteristics of light crude oil, including specific gravity, evaporation rate and dispersion properties.

Three spill volumes are considered in the latest report: 5,000 barrels (210,000 gallons), 10,000 barrels (420,000 gallons) and 25,000 barrels (1.05 million gallons).

The 10,000-barrel volume is a bit more than the 8,583-barrel “worst-case discharge at the Straits” estimated by Enbridge in its response to an April 2014 information request from Michigan’s attorney general. The estimated amount of oil released into the Kalamazoo River from Enbridge Line 6B in 2010 was about 25,000 barrels.

“It surprised me that the impacts of a spill—in terms of vulnerable shoreline and open-water extent—were much closer than I expected for the three spill volumes of 5,000, 10,000 and 25,000 barrels,” Schwab said.

More than 800 oil spills were simulated using a computer model with real weather and water-current data measured at the Straits of Mackinac. This map shows the percentage of cases in which oil is present at a given location at any time after a Straits of Mackinac release. Image credit: Dave Schwab, U-M Water CenterThe 840 simulations were used to create maps and animations that show oil location at hourly intervals for the first five days after a spill at the straits. They show that the potentially impacted area stretches from the Leelanau Peninsula near Traverse City in Lake Michigan, to Canadian islands along the north shore of Lake Huron, and south in Lake Huron past the Thumb, nearly to Port Huron.

The National Oceanographic and Atmospheric Administration’s threshold value for the oil density that triggers beach cleanup, 1 gram of oil per square meter of land, was used to determine the length of “impacted shoreline” following a breach of the straits pipeline.

It should be noted that these simulations assume that no actions are taken to contain the spread of an oil spill at the Straits of Mackinac, which is unlikely. However, the effects of containment actions were beyond the scope of the study.

“This report is a great example of how research universities can provide impartial expertise that helps inform discussions about critical public policy issues,” said Water Center Director Jennifer Read.

The new report comes as the state of Michigan studies the potential risks of underwater pipelines, as well as alternatives to them. The U-M study is a critical component of assessing Line 5’s risks to the Great Lakes, said Mike Shriberg, regional executive director of the National Wildlife Federation’s Great Lakes Regional Center.

“Michigan public officials have an important decision to make about how to protect our communities, economy, wildlife and Great Lakes from an oil spill disaster,” said Shriberg, who serves on the Michigan Pipeline Safety Advisory Board. “This report underscores how critical it is to act in the best interest of the millions of people who depend on the Great Lakes for their jobs, water and way of life.”

The new report is titled “Statistical analysis of Straits of Mackinac Line 5 worst case spill scenarios.” The U-M Water Center is part of the Graham Sustainability Institute, which fosters sustainability through translational knowledge, transformative learning and institutional leadership.

More information:

• U-M Water Center

U-M Sustainability fosters a more sustainable world through collaborations across campus and beyond aimed at educating students, generating new knowledge, and minimizing our environmental footprint. Learn more at sustainability.umich.edu.