Thursday, September 22, 2011

Those aren't waves, they are mountains on the move

kw: book reviews, nonfiction, oceanography, waves

You've probably seen this picture before, of a surfer on a 50-foot wave known as Jaws. What isn't shown is the jet ski that towed him into position at high speed, and is waiting to rescue him when (not if) he wipes out. Somewhere in the 40-foot range, waves arrive too fast to be caught by swimming or paddling, even on a stand-up board using a paddle. Jet ski towing, first promoted by Laird Hamilton and others, is the only way to catch the really big ones.

I don't pay a great deal of attention to surfing. I'm a mediocre body-surfer, nothing over twelve feet. So I took the chance to catch up on my education by reading The Wave: In Pursuit of the Rogues, Freaks, and Giants of the Ocean by Susan Casey. It is a rather eclectic book, because she investigated all the world's largest waves, not just surfing waves, but also mid-ocean rogues and tsunamis.

Nonetheless, more than half the chapters in the book relate to big surfing waves and the men (and one woman) who obsessively pursue them. One chapter reports on a mix, where a salvage operator flew some surfers and their jet skis into the Agulhas Current off South Africa to catch some 100-foot rogues waves. Trouble is, big oceanic rogues are not nearly as steep-faced as breakers coming up on the beach. The guys got some rides, but it was rather anticlimactic.

Of course, when you are on the bridge of a big ship, facing a wave even higher than your viewpoint, they are mighty intimidating. This wave is probably in the 80-foot range. A few ships have encountered waves over 100 feet and managed to return to port, or have been salvaged. A large container ship or two is lost every week, and the size of the waves that do them in is never recorded; they typically vanish without a trace. A visit by the author to Lloyd's of London verified such figures.

For decades, oceanographers contended that oceanic waves of 100 feet and larger were impossible. That was before photographic evidence was gathered, showing that waves three and four times as large as the seas around them could pop up. It was also before radar-bearing satellites began routinely measuring wave heights of 100 feet and more in the heart of storms.

The secret to this is nonlinear dynamics, a common natural phenomenon which is behind the "mixing" that allows an AM or FM radio to work. Scientists use "linear" mathematics to model natural systems whenever they can, because linear systems can be solved mathematically. Nonlinear systems are typically impossible solve, or even to calculate accurately without lots of costly supercomputer time. But nature is full of nonlinear effects.

The air-water interface introduces nonlinear effects. I have seen a simple experiment using a wave tank. When waves with two different wavelengths, coming from different angles, pass through one another, you get wave trains with four wavelengths. One of the extra trains has a frequency that is the sum of frequencies going in, and the other one's frequency is their difference. Out on the open ocean, a storm produces a variety of wave trains, called swells. These can mix in unexpected ways. As Ms Casey writes, sometimes one plus one equals seventeen. That is a bit exaggerated. In reality, energy from several wave trains can "mix" nonlinearly to produce a sudden surge, or train of surges, that can rise to several times the height of the swells around it. That is a rogue wave. Researchers discussed in the book have been able to create small versions of nonlinear rogues in wave tanks.

Surf breakers and tsunamis behave differently from oceanic rogue waves, because they interact with the sea bottom. When a swell approaches shallowing water, it begins to "feel the bottom" when the water depth is comparable to the wavelength, and typically begin to grow in height as they slow down once the depth is half the wavelength. By the time a wave is in water shallower than its height, it rises fast and then either tumbles down or breaks over, depending on how steep the shore is. Of course, the shore is seldom a smooth plane. Curvature of all kinds can either dissipate or focus wave energy. Famous wave locales such as Jaws have curved bottoms that focus waves that are already large because they arise from powerful oceanic storms that created large, powerful long-wavelength swells.

The longer a swell's wavelength, the more energetic it is, and the faster it moves. The longest swells result from sub-ocean-floor earthquakes and ocean floor landslides. These can have wavelengths of a number of miles, and travel at speeds exceeding 600 mph (960 kph). On a steeply dipping shore with some focusing, they rear up into tsunamis that can rise 100 feet or more and travel far inland.

The biggest verified wave, 1,740 feet (2,800 m), occurred in Lituya Bay, Alaska on July 9, 1958. This was a splash wave caused by an enormous land-and-ice fall from one of the extra-steep mountainsides that flank the landward end of the bay. Lituya bay is shaped like a double funnel, with Cenotaph island smack in the center. It is tailor-made to amplify any wave that arises within it. Aerial photos show that no trees grow within about 2,000 feet (3,200 m), vertically, of the shoreline. The 1,740-foot monster is not the biggest that has ever occurred, just the biggest one witnessed and, somehow, lived through. If a surfer managed to catch up with a wave this size, he would soon find himself smacked into the mountainside at a speed in excess of 100 mph. There really are limits to what can be surfed!

But the author, and the book, return again and again to Maui, the home of Jaws, Egypt and Hookipa, which among them just about corner the market on large waves that can be surfed with some chance of living through it. Big-wave surfers share one characteristic with professional hockey players: an oft-repaired body and long familiarity with emergency rooms. The author's hero, Laird Hamilton, stated that he quit counting at 1,000 stitches. Their obsession sends one to an early grave every year or two. In a late chapter, the author relates how Laird Hamilton asked her to ride along on his jet ski while he pulled a fellow surfer into position at Jaws. Instead of swinging away right after, he then surfed the wave on the jet ski with her aboard. Scared out of her mind, she was immediately ready to repeat the experience: "Again? How about ten more times!" Adrenaline is quite addicting.

No comments: