Look beneath the waves on a beach or in a bay, and you’ll find ripples in the sand. Passing waves shape these sandforms and can even build them to heights that require dredging to keep waterways passable to large ships. To better understand how the sand interacts with the flow, researchers build computer models that couple the flow of the water with the behavior of individual sand grains. One recent study found that sand grains experienced the most shear stress as the flow first accelerates and then again when a vortex forms near the crest of the ripple. (Image credit: D. Hall; research credit: S. DeVoe et al.; via Eos)
Tag: ocean waves
Predicting Sea States
Transferring cargo between ships and landing aircraft on carriers requires predicting how the waves will behave for the next few minutes. That’s a notoriously difficult task for several reasons: rough seas can hide a ship radar’s view and the inherent nonlinearity of ocean waves means that they can occasionally coalesce unexpectedly large (“rogue“) waves, seemingly from nowhere.
A new study describes a technique for improving sea state predictions. In their model, the team first use multiple radar returns to average out gaps in the current wave state data, then feed that interpolated data into a prediction algorithm that includes nonlinearities up to the third-order. The results, they found, gave far better predictions than current techniques, some of which had errors 3 times as high. (Image credit: R. Ding; research credit: J. Yao et al.; via APS News)
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Rip Currents and Hurricanes
When it comes to the beach, looks can be deceiving. That calm-looking water to the side of big crashing waves may actually be a rip current that carries water back out to the ocean. Rip currents are a result of conservation of mass; just as waves carry water to the shore, something has to carry that incoming water back out to the ocean. Depending on the local topography, that outflow could be below the water surface, creating an undertow, or along the surface, as a rip current.
Even when far offshore, hurricanes can trigger unexpected and strong rip currents, largely because they create bigger waves that travel shoreward. Those waves can also change the depth and layout of the underwater shoreline, potentially exacerbating rip currents.
For more on rip currents, including the latest guidance on how to escape one, check out this article. (Image credit: A. Marlowe; via SciAm)
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A Sandy Spine
Where sea and sand meet, Gaia’s spine rises. Photographer Satheesh Nair captured this striking image in western Australia, where wind and wave action have dragged a dune into vertebrae-like cusps. Notice how the size and shape of the curves differs between the under- and above-water sections. Those differences reflect the differing forces that shape them — just water for one set, water and air for the other. (Image credit: S. Nair/IAPOTY; via Colossal)
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“Surfing on the Other Side”
Surfers come in many forms — humans, robots, birds, and even honeybees. Most of the time, though, we see surfers above the water. In this award-winning photo, on the other hand, the surfing penguin shoots by beneath the water, riding beneath the wave’s crest. Keeping pace with the breaking wave should be no trouble for a penguin. They waddle awkwardly on land, but they have incredible speed in the water. Years ago, a penguin streaked past me in the water like a rocket to my paper airplane. (Image credit: L. Fitze/BPOTY)
Waves Break Up Floating Rafts
Small particles can float on a liquid, held together as a raft through capillary action. But those rafts — like the tea skin below — break up when waves jostle them. In this study, researchers looked at how standing waves broke up a raft of graphite powder. Although the raft’s break-up resembles fields of sea ice breaking apart, the researchers found that different mechanisms were responsible. In their experiment, waves pushed and pulled horizontally at the raft, causing it to fracture. But that push-and-pull is negligible in sea ice, where sheets instead break from the up-and-down motion of waves vertically bending the ice. Nevertheless, the new insights are valuable for various biofilms and some ice floes. (Image and research credit: L. Saddier et al.; via APS Physics)
The skin atop a cup of tea breaks up into polygons after stirring with a spoon. Although the effect resembles sea ice breakup, the specific wave mechanism differs. 
Breaking in Rogue Seas
Many models for forecasting ocean waves simplify the physics by assuming that waves are essentially two-dimensional, like a long breaker heading toward shore. But in the open ocean, waves often come from more than one direction; crossing seas are a good example. When waves from different directions combine, a recent study shows, the resulting wave can grow far larger and steeper than expected. These monstrous rogue waves are especially dangerous for offshore infrastructure like oil rigs and wind turbines, which must be built to withstand rare but extreme waves. (Image credit: O. Мороз; research credit: M. McAllister et al.; via Gizmodo)
The Crashing Waves of French Polynesia
Surfer and photographer Tim McKenna lives in the village of Teahupo’o on Tahiti’s southeastern coast. The area’s shallow coral reef system creates some of the world’s biggest barreling waves, which attract surfers from around the world. McKenna captures the majestic power of these surges in these black-and-white photographs; you can find more of his work on his website and Instagram. (Image credit: T. McKenna; via Colossal)
Beneath the Surf
A surfer duck-dives beneath a passing wave in this image from photographer John Barton. I always love seeing big waves from this underwater perspective. The turbulent surf looks like storm clouds, and sometimes you see features that are invisible from the surface. Barton’s shot captures the dichotomy of serenity and chaos in the breaking surf. (Image credit: J. Barton/OPOTY; via Colossal)
Seeking Rogue Wave Origins
Rogue waves — rare waves much larger than any surrounding waves — have long been a part of sailors’ tales, but their existence has only been confirmed relatively recently. The exact mechanisms behind them are still a matter of debate. Laboratory experiments with mechanically-produced waves have created miniature rogue waves, but we still lack real-world observations of their formation.
To that end, researchers sailed the Southern Ocean, known for its rough waves, during austral winter and observed the state of the wind and waves nearby using stereo cameras. They found that young wind-driven waves tend to be steeper, and they move slower than the wind, as they’re still drawing energy from it. Older waves, in contrast, were shorter, less steep, and less likely have white caps from breaking. Overall, they found that strong winds could more easily drive young waves into the nonlinear growth that leads to rogue waves. (Image credit: S. Baisch; research credit: A. Toffoli et al.; via APS Physics)
