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Safe at Sea: Rogue waves

By PAT SCHMIDT - | Jan 12, 2021

Recently, I remembered a “rogue wave” that hit Captiva at the Castaway Cottages. A “meteotsunami” — caused by rapid atmospheric changes — struck the area swiftly and unexpectedly. While not a tsunami one typically envisions, the force and speed of a “rogue wave” experience is surprising. You can view the local phenomenon at www.youtube.com/watch?v=OFdKhzb48U4.

This, of course, led to the topic of rogue waves at sea. Waves normally come along in a regular pattern of height, length and direction, with one following another. This repetitious pattern is named a wave “train.”

When two (or more) wave trains meet — clash? — in the same place at the same time, they produce rogue waves. By definition, a rogue wave is usually defined as a wave that is two times the “significant” wave height in the area. (The “significant” wave height is the average of the highest one-third of waves that occur over a given period of time.) Important to know: When two waves meet in the same place, the height of the resulting waves is the combined height of the two waves. Therefore, two 5-footers will become a 10-footer. Knowing this makes it easy to realize how more extreme wave heights can occur.

Theories of how rogue waves originate are varied. The most recent theory relates to deep depressions in the weather, the depressions that cause storms. The bigger the wave, the faster it travels. Thus, when a depression is deepening, there are smaller and slower waves being generated at first; then these are followed by larger and faster waves. The theory is that when the larger waves catch up with slower waves, the two crests combine — a rogue wave that combines the height of the two crests.

Another potential cause is when two (or more) wave trains are crossing during turbulent weather. Again, the resulting rogue is the combined height of the two waves. In both cases, the rogue wave will be transient, appearing and disappearing. Nevertheless, the danger is underscored in the wave’s “trough.” The trough is deeper below the median line than the wave crest above it. Wave height is the distance between the trough and crest. Most likely, the deep trough that precedes or follows a rogue wave does the damage.

While reported encounters with extreme rogue waves have been relatively rare, one cannot help but wonder just how large rogue waves can be. According to the Guinness World Book of Records, the largest recorded rogue wave was 84 feet high, which hit an oil platform in the North Sea in 1995. (A side note: The largest wave ever ridden by a surfer, Rodrigo Koxa, was an 80-foot wave in 2017 in Portugal.)

All of this is to say, before heading out in your boat on a windy day, visit the Sanibel-Captiva Conservation Foundation’s River, Estuary and Coastal Observing Network at http://recon.sccf.org/news, which provides wave height information in real time. As I wrote this, the wave height was 2 feet at Redfish Pass, a great day to be “safe at sea.”

Pat Schmidt is a member of America’s Boating Club of Sanibel-Captiva. For more about the chapter and its boating education courses, visit www.sancapboating.club or contact education@sanibelcaptivasps.org or 612-987-2125.