Charting the probabilities of superwaves in storms
11/12/2000“Look at those swells. Somebody's getting wind somewhere.”
Keith Poland, a veteran Great Lakes fishing guide, was remarking about the low, crestless waves on western Lake Ontario during a salmon fishing trip in September.
We had been pinned to the dock for two days during a classic, howling northeaster. The storm had driven 5 to 8-foot waves over the breakwall at the mouth of the Oak Orchard River at Kent, N.Y.
But a bright sunny sunrise had revealed an all but calm lake - save for the swells we initially encountered as we set out to troll offshore for king salmon.
As it turned out, the swells that the guide had noticed at first light were prophetic. The “wind elsewhere” must have been down-east on the lake and headed west, for by noon we were rolling through three to five-footers. Lake Ontario was threatening to return to its unruly behavior of the previous two days.
So it is with water and waves. It is, if you will, a fluid situation.
What Great Lakes fisherman - or boater - of any experience has not found himself or herself out on the water in a steady sea of constant-height waves only to encounter a rare rogue roller of tooth-jarring proportions?
Many is the time that a walleye fishing crew on Lake Erie, intent on casting and retrieving lures and oblivious to all else - has heard a veteran lookout on board holler, “Hang on! Here comes a big one!”
As it turns out, the occasional huge wave, or superwave, has been documented on oceans, seas, and large lakes around the world. Oceanographic engineers even have formulas to describe them.
Indeed, one of the prevailing theories on the cause of the sinking of the Great Lakes freighter Edmund Fitzgerald and her crew of 29 - 25 years ago last Friday - is that a giant wave caused it to submarine to the bottom of Lake Superior.
In fact, Capt. Bernie Cooper, master of the Arthur M. Anderson, which trailed the Fitzgerald by about 10 miles in a hurricane-force winter storm, made a point of reporting an encounter with superwaves.
About 6:55 p.m., Nov. 10, 1975, Mr. Cooper and his crew in the Anderson's pilothouse felt a bump, felt the ship lurch, and turned to watch an incredible wave swallow their vessel from astern.
The wave surged and rolled forward, engulfing the Anderson's pilothouse and driving her bow into the sea. But the plucky ship raised her bow and shook off the gigantic freak, like a Labrador retriever waggling off water after a swim in a pond. Then, another monster wave hit the Anderson, as big as or bigger than the first.
“I watched those two waves head down the lake toward the Fitzgerald, and I think those were the two that sent him [Capt. Ernest McSorley] under,” Captain Cooper later stated. Fifteen minutes after the Anderson weathered the two rogue waves, the Fitz disappeared from the Anderson's radar screen, forever.
Whether the Fitzgerald was sunk by a monstrous wave may never be conclusively determined. But the huge wave was no phantom, no figment of a stressed-out seaman's imagination or an act in a Hollywood model-ship tank.
“Even in storms with lower velocity winds there is always a statistical chance of a very high wave,” wrote Willard Bascom in Waves and Beaches, a science text. Mr. Bascom was a prominent oceanographic engineer and author.
“No one can predict when or where or how high, but superwaves must exist because of the random nature of waves,” the engineer wrote. “For example, if 1,000 waves were observed on 20 different occasions, the highest of the thousand waves will be 2.22 times the significant height,” he added, referring to a mathematical series of equations.
Mr. Bascom said that if the significant height of a wave in the ocean is 44 feet, “as it would be in a fully developed 40-knot wind,” the exceptionally high wave would be 97 feet.
The engineer describes how such a superwave would exist only for a short while in a storm and would be very unstable: Because it towers above all others around, its crest would be blown off, forming a dangerous breaking wave of tons of wind-driven water. “It is these breaking storm waves ... that do serious damage to ships that are unlucky enough to be hit.”
Mr. Bascom notes that great storm waves are very hard to measure and often are exaggerated in storytelling after the fact. However, 45-footers are said to be fairly common on the North Atlantic and several well-authenticated examples exist of much larger waves.
In 1933, an officer aboard the U.S. Navy tanker Ramapo documented a storm wave in the Pacific that reached 112 feet.
Many factors contribute to creation of waves, from earthquakes and landslides to changes in atmospheric pressure and the passage of the moon. But most waves are raised by the wind.
Three factors affect the magnitude of wind-waves. Wind velocity, the duration of the time the wind blows, and the fetch. The latter refers to the extent of open ocean, or lake, across which a wind blows.
On Lake Erie, for example, the greatest fetch lies along the lake's southwest-northeast axis. Thus for Toledo-area sailors, the greatest wind-wave potential lies from winds that blow straight down the lake from the northeast for several days or more.
Engineer Bascom even developed a chart of what he called “fully developed seas,” that is, the maximum waves that a given wind is capable of generating. For example, a 20-knot wind must blow at least 10 hours along a minimum fetch-length of 75 miles to raise fully the waves it is capable of making.
When the sea from a 20-knot is fully developed, the average height of the waves is 5 feet, the average height of the highest 10 per cent of the waves is 10 feet, and the “significant height” is 8 feet.
For all that, the ocean-engineer offers some comfort to those who work or play on the water, at least in deepwater seas of non-breaking waves:
“Objects in the water, such as ships, tend to make the same motion as the water they displace. A ship at sea in large waves will describe orbital circles that are roughly the same size as the water in that part of the wave.
“There is little relative motion between the bulk of the ship and the surrounding water. This motion of a ship may be uncomfortable, but it is safe.”
Breaking waves, however, can be destructive. “If the crest breaks off a wave, the water moves faster than the wave-form and independently of the orbiting water (and ship). While moving in different directions the two may collide with disastrous results.”
Believe it or not, oil has a calming effect on the sea surface, though Engineer Bascom notes that animal or fish oils are best and that petroleum products have little calming effect. It has to do with increasing surface tension of the surrounding sea thanks to a very thin, elastic slick of oil.
“Properly used, oil can be very helpful to the small-boat operator in emergency conditions.
“The time-honored method is to put cod-liver oil, for example, in a canvas bag filled with old rags and hang the bag over the side in the water. The oil oozes drop by drop through the canvas and spreads out on the sea surface. Even a small quantity - say, a half gallon an hour - will calm the area around the boat to a distance of as much as 100 feet.”
Mr. Bascom adds a qualification against overconfidence: “There is no doubt whatever that this method works, but one must not expect too much. A thin film of oil could hardly be expected to have any effect on large waves or swell, but it does quickly extinguish small waves.
“Moreover, as the sea surface becomes slick, the wind has less effect on it, no spray is blown about, and the wave crests become more rounded. The boatman can see better and there is less chance of a wave's breaking as its crest is blown off by the wind.”