Saturday, March 24, 2007

All too often, the Ocean wins

kw: book reviews, nonfiction, oceanography, waves, ship design

I know nearly nothing about sailing or cruising, and have only a very amateurish body-surfer's acquaintance with waves. I don't think I've ever been more than a mile from shore, except on one ferry ride from Kobe to Tokushima, Japan. Thus, I was quite interested to read Extreme Waves by Craig B. Smith.

A lifelong sailor and student of the sea, a highly respected engineer, particularly of marine engineering, Smith has not just the experience, but the access to professional and governmental sources, to present a highly readable and informative account of the four kinds of extra-large waves, and the conditions that spawn them.

Large storms on large seas produce large waves. Statistically, a few of these will be two or three times as large (with as much as ten times the energy) as the "significant wave height", defined as the average of the largest one-third of waves in a Rayleigh distribution. Thus, a storm with winds of 50 knots (58 mph) will, over a time of two or three days, if it has a long "fetch" of a thousand miles or more, produce waves whose significant height is 50 feet or more. The "expected maximum" height, exceeded by fewer than one percent of the waves, is 99 feet, but a very few waves over 100 feet can be expected to occur. Note that this is quite a bit less than the winds of a hurricane, which exceed 64 knots (74 mph).

Tsunami were suddenly much in the news since the day after Christmas, 2004, when a series of huge tsunami took nearly 300,000 lives in the Sumatra-Thailand area of the Indian ocean. These waves are triggered when an undersea earthquake or landslide causes a sudden shift of a few cubic miles of seawater. The Sumatra earthquake of 2004 ruptured along a thrust fault 170 miles long, suddenly lifting a large area as much as twenty feet. A long swell raced outward at 300+ knots, and its interaction with sloping beaches pushed up waves that ran up 200 feet above sea level in some places, and carried objects far inland, over ridges and structures fifty and more feet high. One hotel situated on a "safe" ledge fifty feet above high tide was demolished.

Confused seas result from the interaction of storm waves. Two storms within a few hundred miles of one another, each producing waves whose significant height is forty feet or so, will interact at distances of a thousand miles or more to produce a confused sea, with waves from two directions leading to a choppy sea. Some waves will cancel each other out, while others will add together, producing waves of eighty or more feet...perhaps under a blue sky!

Finally, rogue waves can arise out of nowhere, it seems; actually they also result from the interaction of multiple wave trains. Modern radar measurements from satellites show that these are more numerous than once thought.

From all these sources, there are just a handful of reports of actual measurements of waves exceeding 100 feet, and one "post-mortem" measurement, based on a run-up of 1,700 feet (!) that indicates the wave(s) responsible exceeded 500 feet...this was in 1958, in Alaska.

The second message of the book is the human and economic toll of such waves. I was surprised to learn that several large ships are lost every week! Of tens of thousands of cargo ships in use, it seems we need to build about a thousand new ones every year, just to keep up with founderings and sinkings.

The author has numerous descriptions of the ways large waves overturn, break up, or swamp ships, even supertankers and 'super Panamax' ships, those too big to go through the Panama Canal, which must then traverse the most dangerous waters north of Antarctica, to get around South America. He ends the book with a call for better design, in a time of increasing "false economics" of shaving ship design. Deep-sea sailing is seen to be riskier than coal mining or warfare, often thought to be the most dangerous professions. Sadly, it is getting riskier, not less, because of too great a focus on the 'bottom line' in this global economy. We're consuming the blood of sailors to buy our fruit from Argentina and our shoes from Malaysia.

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