The data buoy at Granite Island is one of the two that captured the Great Lakes record 28.8-foot significant wave height. (Credit: John Lenters)
A pair of lonesome data buoys bobbing off Michigan’s storm-whipped Lake Superior shore were suddenly the stars of the state this fall when they captured the largest waves ever measured on the Great Lakes.
The buoys, near Granite Island and Munising, each recorded 28.8-foot significant wave heights during a storm that caused hundreds of thousands of dollars of damage along the coast. The record wave height exceeded the previous 27.6-foot record set by a Michigan Tech buoy near Houghton, Mich., in 2012. To give some perspective on the rarity of these types of events, waves at the record-capturing buoys only climbed above 12 feet four times throughout 2015 and 2016.
A headline-grabbing event like that shows how buoy data can help the public wrap their minds around the otherwise unimaginable power of a wild Lake Superior storm — one that claimed the lives of two people swept off the rocks at a popular overlook. But it also highlights the utility that these beacons have throughout the field season when the waves aren’t quite so eye-popping.
The lake freighters that pass through Superior are concerned with the big 10- to 15-foot waves, said John Lenters of Lentic Environmental Services, who helps manage the buoys with Northern Michigan University and the Superior Watershed Partnership. But in terms of public safety, kayakers and recreational boaters generally aren’t crazy enough to go out on those days. Managers want to know whether the waves are going to stay below 2 feet or get closer to 3 or 4, because that’s when it gets dangerous.
“It’s those 3- to 5-foot waves where people go out and test their limits and get caught in waves that are too big for them to handle,” Lenters said.
Even if the Oct. 24 storm’s immense wave action kept the boaters off the water, it still drew spectators.
“It was like people were lining up for the circus,” Lenters said of the crowds he encountered in Marquette on his way into the office for some buoy-related work. Waves were already washing over Lakeshore Boulevard that hugs the Upper Peninsula city’s waterfront. He turned around and headed for Munising, stopping along the way to capture some footage of the waves himself.
Lenters kept his eye on the data from the buoys throughout the storm, sometimes wondering whether the moorings or electronics would survive the ride. Previous storms had, wave by wave, shifted a buoy a quarter-mile from its original location. Previous buoys larger than the NexSens CB-450 deployed last year had broken free from their moorings. With enough force, water could also squeeze into the modem case, shorting out the system and draining the battery.
This year the buoys stayed put, save for a little nudge of the Granite Island buoy, despite its anchoring in the rocks. And the Munising buoy shut down shortly after measuring the 28.8-foot wave.
The gap in the data after the shutdown makes Lenters wonder whether an even larger wave group could have moved through after the buoy went quiet. Even if it continued to report, it is possible that the 28.8-foot record doesn’t even reflect the biggest wave from this storm.
An accelerometer in the buoy measures every wave, but the buoy only reports the average of the largest third of those waves over a certain time period. This metric, called the significant wave height, could easily mask a single rogue wave two or three times larger than what the buoy reports, Lenters said.
It’s also possible a larger group of waves slipped between the two buoys that each captured the 28.8-foot measurement. A visualization of modeled wave heights put together by National Weather Service meteorologist Greg Mann at the National Weather Service showed a purple bullseye representing the highest wave action.
“It was about a 30-foot wave group moving across the lake and it happened to just slam right in between those two buoys,” Lenters said. “From models, we estimate there were higher waves. It got me thinking: clearly this isn’t the biggest wave ever on the Great Lakes,” Lenters said. “It’s just the biggest ever measured.”
Sensor-based wave measurements in the Great Lakes only go back to 1979 when NOAA first placed buoys in the lakes to aid freighters, said Ed Verhamme, an engineer with LimnoTech, a consulting firm that helped initially deploy the Granite Island and Munising buoys back in 2015. Before 1979, wave heights were mostly based on human observations.
The NOAA buoy record begins just five years after the Edmund Fitzgerald sank on a night that the captain of another nearby freighter reported 30-to-35-foot waves. A NOAA simulation of wave heights based on weather conditions from that storm estimated waves averaging over 25 feet and one in a hundred waves climbing to 36 feet.
The NOAA buoys are near the centerline of each lake — an area that will rarely see the largest waves because it isn’t subject to as much fetch as the nearshore areas, Verhamme said. And until recently, NOAA’s National Data Buoy Center set a cutoff of 25 feet for its wave height records because they assumed anything larger meant a sensor was on the fritz. NOAA has since raised the cutoff to 50 feet.
While the true height of the Great Lakes’ tallest wave is up to speculation, there’s no question that weather on Lake Superior during that week this past October was one for the record books. Just three days after the record wave from the north, another storm blew in from the east that churned waves up to 19.4 feet at Granite Island. Mixed with the lake’s near-record water levels at the time, Lenters said we’re unlikely to see anything like that one-two punch again soon.
“When I think about two major wave events in the same week combined with high water levels, I would say a million-to-one that that’s probably unprecedented in the historical record,” Lenters said, meaning that “it probably hasn’t happened in a couple hundred years.”