|Big wave of October 28, 2013 from Surfertoday.com|
This wave is at the so-called Nazare North Canyon (Praia do Norte) Portugal
What generates such waves? In my book "The Dynamics of Disaster" published last week by W.W. Norton Press, I have a chapter on ocean waves, and specifically on rogue waves. I'll highlight a few points here: What is a rogue wave? What happens as they approach shore?
Rogue waves are quite ephemeral, and that has made scientific documentation difficult. In 1861, a wave broke glass windows 85 feet above the ground in an English lighthouse--after climbing up a 130-foot high cliff! This would imply that the wave was 215 feet high, but as of now, no wave near this height has been documented by eyewitnesses or with instruments. Rogue waves are generated by storms, and they are a danger to shipping, fishing, tourism, and oil and gas production on the ocean.
Here's a quoted footnote from my book that defines rogue waves: "To provide a reference for defining rogue waves, oceanographers have introduced the concept of a “significant wave height.” The significant wave height is the average wave height of the one-third highest waves in a time period (typically taken as 10-30 minutes). Surfers might find the following exercise useful--and sobering: ignoring the small stuff, sit on a beach and make a list of the heights of all incoming waves for 10-30 minutes. For example, 1 foot (1’), 2 feet (2’), 3’, 5’, 3’, 4’, etc. Organize this list from biggest to smallest: 5’, 4’, 3’, 3, ’2’, 1.’ Keep the highest one-third of the values, 5’ and 4’. Then, take their average--4.5.’ This is the significant wave height. A rogue wave is defined as one whose height is two or more times the significant wave height--9’ in this example. In fact, rogue waves with a height more than four times the significant wave height have been documented. In this example, that would be an 18’-high wave. Surfers who are comfortable with 2’ or 3’ foot waves, perhaps an occasional 5’ wave, but not with 9’ or 18’ waves need to be aware that such waves can appear at any time. This is apparently what happened when a wall of water collapsed on and killed the experienced big wave surfer, Sion Milosky, at Half Moon Bay, California, in March, 2011. After nearly an hour of relatively small swells (18-20’) for that day, a rogue wave “bomb” rolled him to the bottom, where he was held down not only by this wave, but by a second as well, in what is known as a “two wave hold down” in surfing jargon. He was found too late, 20 minutes later."
|Map showing the likelihood of encountering|
a rogue wave within any 24-hour period.
Courtesy Burkard Baschek
Broad patterns of wind and ocean currents determine the zones of hazardous rogue waves on the ocean (as discussed more broadly in the book). Four factors can operate simultaneously to determine the height of he waves on the open ocean and near-shore: winds from hurricanes and storms that churn up the ocean surface; the interaction of strong waves moving in opposite directions, such as storm waves interacting with strong oceanic currents or strong opposing winds; constructive interference (addition of wave heights) of random waves; and piling up of waves from the deep ocean into shallow depths along the continental shelves. See book for details! And, it's that last effect that makes surfing so exciting!