Variations in seafloor create freak ocean waves

Posted: February 3, 2019 by oldbrew in Maths, modelling, Ocean dynamics, research

Image credit: NASA


Researchers have an ambition to use ‘new mathematics’ to try and predict where and when these extreme events will occur.

Florida State University researchers have found that abrupt variations in the seafloor can cause dangerous ocean waves known as rogue or freak waves—waves so catastrophic that they were once thought to be the figments of seafarers’ imaginations, Phys.org reports.

“These are huge waves that can cause massive destruction to ships or infrastructure, but they are not precisely understood,” said Nick Moore, assistant professor of mathematics at Florida State and author of a new study on rogue waves.

The study is published in the journal Physical Review Fluids.

Once regarded as a myth, these waves have stumped the scientific community for several decades.

Over the years, researchers across the globe have examined a number of different factors they thought might contribute to these waves, including the seafloor, wind excitation and a phenomenon called Benjamin-Feir where deviations from a periodic waveform are reinforced by nonlinearity.

Most of the studies that focused on the seafloor considered only gentle slopes, and the few studies that pushed the slopes to greater extremes relied primarily on computer simulations.

“There was a relative underrepresentation of real-world data that you can get from laboratory experiments, where you can carefully control the various factors,” Moore said. “Often you need this real-world data to see whether the computer simulations are giving you sensible predictions at all.”

Moore’s laboratory experiments were the first to examine the effect of abrupt seafloor variations on wave statistics.

Along with FSU’s Geophysical Fluid Dynamics Institute Director Kevin Speer and now-former FSU student Tyler Bolles, Moore created a long chamber with a variable bottom. Using a motor to generate randomized waves, the research team tracked thousands of waves to see if any patterns emerged.

After the waves passed through several feet of a constant depth, they encountered a step in the bottom of the tank that represented an abrupt change in the seafloor. Moore and his colleagues found that initially the waves appeared normal, following a traditional bell curve. But when they passed over the step, the structures of the waves significantly changed.

The altered waves followed what’s called a gamma distribution, a mathematics function describing certain patterns that defy the bell curve in a particular way.

Full report here.

Comments
  1. E.M.Smith says:

    I thought it was already known these waves were from constructive interference from different wave sets.

  2. oldbrew says:

    EMS – that’s just one of the theories listed here.

    https://en.wikipedia.org/wiki/Rogue_wave#Causes

  3. Kip Hansen says:

    Yes, there is a lot of research being done on “rouge waves” — a simple web search for “recent studies rogue waves” reveals quite a few — even a review study.

    They are mostly concerned with 20-30 meter waves — but from my personal experience, small rogue waves (outliers in wave size by 200-300 %) are common enough that I have run into several in 15 years at sea in a 40 foot catamaran.

    The funniest one (in that it did not cause a disaster, thus we could see the humor) was when our youngest son was at the helm and my wife and I were napping in our cabins with overhead hatches opened and angled forward to catch the breeze — we were riding smoothly over 3 foot swells and the helmsman was bored with the conditions — failed to notice the 10-12 foot swell approaching. The rogue wave broke over the bow, with the curl coming down on the high cabin top, dumping 50 gallons of sea water into each forward cabin and washing the upper decks clean.

    Had the boat been of a different configuration, we could have been in real trouble with that much sea water coming aboard.

    On a different occasion, while towing our inflatable dinghy through the Vieques Channel, between Puerto Rico and Isla Vieques, a rouge wave from astern broke into the inflatable – filling it completely. The inflatable stopped while we sailed on running with the storm. The sound was horrendous, and I was sure we had snapped a mast stay …. alas, when looking astern, I saw we were towing just the inflatable painter — no boat. As far was we could tell, the dinghy never resurfaced (not that we could see) — nor did it wash ashore and get reported to the Coast Guard.

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