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More rogue waves, but size limited

May 3, 2017
The ConocoPhillips North Sea oil structures where the oceanographic research laser sensors are located.

The ConocoPhillips North Sea oil structures where the oceanographic research laser sensors are located.

Courtesy ConocoPhillips

For ocean voyagers there are some undefined hazards that lie in wait in the hazy margins. One is the oft-discussed but rarely quantified danger of floating containers. Another rare beast is the rogue wave, the massive outsized graybeard with the potential to damage or sink vessels. Oceanographers at University of Miami’s Rosenstiel School of Marine and Atmospheric Science recently published a study titled “The Making of the Andrea Wave and other Rogues,” which suggests that massive rogue waves aren’t as rare as previously thought.

That’s a finding sure to grab the attention of an ocean sailor. The idea that there are more rogue waves lying in wait than anybody imagined might give a voyager pause. Is there a real danger here?

The study, co-authored by Mark Donelan, professor emeritus of ocean sciences at the UM Rosenstiel School, and Anne-Karin Magnusson from the Norwegian Meteorological Institute, involved measurements made by an array of lasers mounted to a bridge that crosses between two ConocoPhillips oil platforms in the North Sea. On the bridge, researchers placed four infrared lasers that pointed straight down to the sea surface. The lasers pulsed five times per second with 80 pulses in each group. They were set up in a square formation, according to co-author Mark Donelan. “Three lasers in a right triangle are enough to provide directional information,” Donelan said. “A fourth was included in case one of the lasers failed.”

The lasers sent pulses toward the water that were reflected back toward sensors. As the crests of waves passed underneath, the laser reflection returned to the sensor faster because it had a shorter distance to travel. Similarly, a wave trough could be detected because the reflection took a longer time to arrive due to the longer distance it had to travel.

Donelan and his co-author Magnusson took data acquired by the laser array and looked for signs of large “rogue” waves. After some detailed processing of the data, they were able to find an example of a rogue wave that occurred during the North Sea named storm “Andrea” in 2007. This Andrea wave was large and had a short period, which means it had a steep leading edge. The Andrea wave was calculated to be roughly 50 feet high — a steep, high wave that few voyagers would want to see up close.

Another aspect of the study involved the number of such waves you might see. According to Donelan, the data showed large waves to be more plentiful than the standard theory would suggest. “Typically in the North Sea, in a typical storm,” Donelan said, “you would get a wave like Andrea twice a day.” For this to be true, of course, the wave-making wind would need to be blowing at storm level.

According to Donelan, another important finding of the study was the self-limiting effect of breaking waves. Once a wave gets big and steep, it would break at the top. “What we’ve shown is that they are limited by breaking,” Donelan said. This self-limiting effect means that waves will not get any larger than 1.7 times the significant wave height. Significant wave height is the average height of the largest third of all waves. This means that 250-foot-high rogue waves are not possible.

This last fact might not produce much comfort when a steep 50-footer happens to be bearing down on you!

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