By now, many of you have seen the video of a "rogue wave" that slammed into spectators at the weekend's Mavericks competition, sending several to the hospital. That being said, it takes more for a wave to be determined "rogue" than apparent malevolence in causing harm to people who wandered close to shore. And, according to a pair of San Francisco oceanographers, this wave, while "big," was no "rogue."
Professor Tim Janssen of San Francisco State is about as good a source as a journalist could want for a story like this: He's an expert in rogue waves who also surfs (but not Mavericks). He points out that the loose definition of "rogue wave" is pretty much big wave I didn't see coming. But the scientific definition is more exact: "It is, loosely defined, as a wave that is three times bigger than the average wave height."
A wave three times the size of a Mavericks-sized wave, by the way, would have likely flooded basements in North Beach. "That would have been huge," affirms Janssen. "You can do the math."
Janssen's SFSU colleague, professor Toby Garfield, has extensively studied why Mavericks' waves are so big. He, too, did not think the weekend's wave could be categorized as a "rogue."
"It probably ranks in the category of normal waves that are larger than expected," he said. "A rogue wave is when you get a wave that's way outside the range you have been experiencing. Not just a little bit but way outside."
This happens, he continues, when two or more waves converge to create an amplified monster wave. While it's possible that the weekend's fan-dousing wave was, in fact, a convergence of several waves, it just wasn't big enough to truly go rogue.
"All of those signs along the coast that say 'beware rogue waves,' what they really are saying is 'beware of waves that are bigger than you expect,'" says Garfield.
Janssen, meanwhile, says a good rule of thumb is to gauge the "average" wave size and estimate 'will a wave twice as big hit me? If so, you might not want to build something there.'"