The Three Sisters

Our article last month about the sinking of the Edmund Fitzgerald led to one discovery from the HIS website: Wisconsin Sea Grant researchers are learning more about the type of waves suspected in this legendary Great Lakes disaster.

Chin Wu, professor of civil and environmental engineering at the University of Wisconsin-Madison, and his research assistant Josh Anderson studied rogue waves in the Apostle Islands National Lakeshore in Lake Superior.

Rogue waves are defined as waves with a height more than double that of other waves occurring around them. They can be caused by multiple factors, such as wind, strong currents or shoreline geography. Also known as freak or killer waves, their tendency to occur unexpectedly and with huge force makes them especially dangerous.

On Lake Superior, a group of three rogue waves, colloquially called “three sisters,” is suspected as one of several causes for the sinking of the Fitzgerald on Nov. 10, 1975. Twenty-nine crew members were lost.

Because the waves follow each other closely, ships can’t recover and shed the water from the first before the others strike, which leads to sinkings. The captain of a ship near the Fitzgerald reported that his ship was hit by two 30- to 35-foot waves. These waves, possibly followed by a third, continued in the direction of the Fitzgerald and may have struck it about the same time it sank.

For their experiment, Wu and Anderson deployed wave and current-measuring instruments throughout the Apostle Islands National Lakeshore. They examined the data for rogue wave patterns, looking at three possible causes: refraction on shoals, diffraction around islands and reflection off the sandstone bluffs so prevalent in the area, and which make up the mainland sea caves near Cornucopia, Wis.

Anderson says that although the study is still in progress, preliminary results show an increased probability for rogue waves near reflecting walls. The duo also found that if one rogue wave was observed, others can’t be far behind. “They group together during certain wave conditions,” says Anderson. “You might get three or four in an hour and then you won’t get one for the rest of the day.”

Although the rogue waves observed in the Apostles aren’t nearly as large as the offshore ones that may have sunk the Fitzgerald, “they’re still dangerous to kayakers or sailboaters,” says Anderson. “Waves are hazardous and we still don’t know everything about them, so we’re doing this research for public safety and to understand them better.”

To document other wave and current patterns for their study, Wu and Anderson developed a computer model and calculated 35 years of conditions. “We found that basically, the overall wave climate has been increasing on Lake Superior due to less ice cover and stronger winds in the winter, which generates larger waves,” Anderson says. This can impact how sediment is transported around the islands and can change how bluffs erode.

To contact the author of this article, email engineering360editors@ihs.com

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