Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 47
Extreme Waves 3 Over the Bounding Main One summer night I anchored Dreams at Johnson’s Lee, a sheltered cove on the west side of Santa Rosa Island, 120 nautical miles northwest of our home port at Newport Beach. Johnson’s Lee was a submarine base during the Second World War, but the navy has since removed the last vestiges of the base. Santa Rosa Island is one of the northern Channel Islands and lies a little south and east of Point Arguello, where the California coast takes a sharp dogleg to the southeast. Here, the prevailing winds from the northwest funnel into Santa Barbara Channel between the islands and the mainland, often creating rough seas. This area is known to local mariners as Windy Alley. After we finished dinner, the wind picked up noticeably, gusting on occasion to 35 knots. I watched small wavelets race across the cove, marking the passage of the wind. It was blowing out of the north, swooping over the island and down across the anchorage. I could hear the wind vibrating the rig and could feel the boat ride back as the force of the wind pulled up the slack in the chain that led to our 45-pound anchor and then move forward as the gusts died out. I thought briefly of setting a second anchor, but then decided we were hooked solidly. Bad decision!
OCR for page 48
Extreme Waves Somewhere around 2 A.M. there was a huge gust—the captain of a neighboring boat thought that it registered 50 knots—and Dreams’ anchor pulled free. We rode backward toward the open sea, dragging the anchor. Words are inadequate to describe the chaotic scene that ensued: me jumping out of bed in my underwear, probing the darkness with a flashlight—the wind howling, the boat rocking, another crew member running forward to release the windlass and let out more chain, hoping the anchor would catch before we were blown completely out of the cove. Finally the anchor caught and I had a moment to clamber below for pants, shirt, and a jacket. After that, I maintained watch on deck until the sun rose and the wind died. In the morning I surveyed the scene. The cove was surrounded by a dense growth of kelp. We had entered to one side where there was no kelp and anchored in a clearing broad enough for several boats. The wind had blown us backward several hundred yards through the kelp. Our path was clearly marked; it was as if a giant lawn mower had mowed a swath through the kelp. A mariner’s worst nightmare is to be blown onto a lee shore. We did not face that risk, since the wind was blowing offshore. Had the wind direction been reversed, I would have certainly set two anchors or, more likely, left the cove entirely for the open ocean. Still, it was enough—another dramatic and lasting lesson of how quickly the weather can change at sea. The wind shifts, waves build, and suddenly conditions are very different. I spoke earlier of linear models used to analyze the “ideal wave.” While these models lack the sophistication to accurately portray the random nature of real waves, they are useful in explaining certain phenomena we see in the ocean, and they are helpful in understanding how giant waves can suddenly emerge in seas that are average in size. A realistic situation is that one or more storms occur in distant parts of the ocean. Waves are created, and these disperse, the longer wavelengths running at faster speeds and leaving the shorter waves behind. The sudden appearance of these long, low swells that outrace the storm that caused them is sometimes a warning that a storm is approaching.
OCR for page 49
Extreme Waves WAVE ADDITION Suppose that at some point in the ocean the sea consists of only two types of waves with different wavelengths. When these two waves come together, they are said to either destructively interfere (cancel) or constructively interfere (superimpose, add together to create a larger wave). Figure 6 illustrates these concepts. Destructive interference occurs when the wavelength and phase of the two waves are such that they effectively cancel out. On the contrary, if the two waves have similar wavelengths and are in phase (meaning that each successive crest of each wave lines up with the other), a wave with crests that are the sum of the crests of the two interacting waves will be produced. This is known as constructive interference. Extend this reasoning to the real oceans, where there are countless unique waves coming from all points of the compass, originating from near and distant storms that happen every day over the world’s oceans—you can see how it is possible for just the precise combination to occur occasionally, forming bigger waves. And at rare moments, even a wave so large that it will be called an extreme wave can form. Now that we’ve ventured into the mathematics of waves, there are a few additional points that I want to make in the next page or two. After that, we’ll leave mathematics behind us forever, cast overboard and bobbing in our wake, so to speak. COMPOSITE WAVES Since two waves can add together to form a larger wave, it is not difficult to imagine the opposite process—that of breaking down an irregularly shaped wave into a series of idealized sine waves, each with a different period, which added together exactly duplicate the original wave. Fourier, the French mathematician introduced in Chapter 1, developed this method, known as the Fourier transform. The only requirement is that the wave be periodic—that is, that it repeat itself at some interval. For simple and regular mathematical functions, Fourier’s method is straightforward to apply, but for a real ocean wave, application of the method before the advent of digital computers in the 1960s was te-
OCR for page 50
Extreme Waves FIGURE 6A Destructive interference of waves. dious. Computers made it possible to digitize the record of such a wave and then apply a new mathematical technique called the fast Fourier transform to break down the digitized wave into its constituent wave components. For simplicity, the composite wave in this hypothetical example has only seven constituent waves (Figure 7). The transform process determines the frequency (reciprocal of the period) of each component as well as its amplitude or wave height. Since we know that the energy carried by the wave is proportional to its height squared, if we use the computer to calculate this quantity and plot it versus frequency, we can determine where most of the energy in the wave lies. This graph is called an energy spectrum of the wave and from it we can determine the significant period of the composite wave—that is, the wave period where most of the energy is concentrated. As noted in Chapter 2, this
OCR for page 51
Extreme Waves FIGURE 6B Constructive interference of waves. method is similar to that used in the analysis of earthquakes and in calculating the response of buildings to a specific earthquake. We’ve now seen how the seemingly endless variety of waves can be approximated by considering them to be the sum of a large number of uniform sine waves having different periods. In uncomplicated terms, this method is useful for forecasting sea conditions if some things—such as wind speed and duration—are known. However, as you might expect, this method is not useful for forecasting extreme waves.
OCR for page 52
Extreme Waves FIGURE 7 A composite wave with seven components. If one storm originates in close proximity to another, the combination can produce large waves, especially in the area of a strong current. This situation was tragically demonstrated in late October 1980, when a crew member on a Japanese tanker spotted a lifeboat floating in the sea. Inspection revealed that the damaged lifeboat had been torn from its supporting davits by a violent force. The lifeboat belonged to
OCR for page 53
Extreme Waves the Derbyshire, a bulk carrier—specifically, a type known as a combination carrier, which can transport ore, bulk goods such as grain, or oil. She was one of the largest ships of this type when launched in 1976. The Derbyshire was 957 feet long with a beam of 145 feet. When fully loaded, her freeboard, or distance from the water line to the top of the deck rail, was about 23 feet. In September 1980, the Derbyshire was nearing the end of a voyage hauling 158,000 metric tons of iron ore concentrate from Canada to Japan. She crossed the Atlantic, stopped at Cape Town, and then proceeded for Japan under the guidance of a weather routing service. Approaching Japan in normal weather, the ship learned of an impending tropical depression and the weather routing service recommended a northerly course to avoid it. The Derbyshire increased speed to get ahead of the storm. On the following day, September 6, she reduced speed, apparently believing the danger had passed. Meanwhile, a second storm, Typhoon Orchid, arose close to the tropical depression, but the weather routing service apparently failed to advise the Derbyshire of this new threat.1 Typhoon Orchid, with winds of 85 knots and 60-foot-high seas, bore down on the vessel, now somewhere off Okinawa, and headed north in the Kuroshio Current. On September 9, the Derbyshire radioed that she was hove to due to a severe tropical storm. That was the last word that was ever heard from the Derbyshire or the 42 crew members and two spouses onboard. No Mayday or other distress signal was heard, suggesting that the foundering of the Derbyshire was sudden and cataclysmic. When the Derbyshire failed to arrive in Kawasaki, a search was initiated. Search vessels and aircraft found a large oil slick around 300 nautical miles southeast of Japan but no other sign of the missing vessel or her crew. In 1987 the British government declared the vessel missing and presumed lost—“probably overcome by the forces of nature in Typhoon Orchid.” Shortly after the disappearance of the Derbyshire, several of her six sister ships were found to exhibit cracks in structural members and in the deck—in the aft portion of the ship, an area identified as “frame 65.” In March 1982 the Tyne Bridge encountered severe weather in the
OCR for page 54
Extreme Waves North Sea and, because several cracks were found in the deck, had to return to port. In November 1986, the Kowloon Bridge developed deck cracks in the same location while crossing the North Atlantic in a severe storm. She returned to port, slipped anchor, went aground, and broke apart near frame 65. These events suggested a flaw in the design of these bulk carriers and prompted family members of the Derbyshire crew to organize and fund an investigation to locate and survey the lost ship. In May 1994, the Derbyshire was located in waters 13,700 feet (2.6 miles) deep, the bow section partially buried in an impact crater, the stern section some 2,000 feet away, and debris and iron ore scattered over the intervening area. With this new information, the British government launched a formal investigation into the loss of the ship, including two additional underwater surveys of the wreckage. A court hearing took place in April-May 2000. The court found evidence indicating that heavy seas destroyed ventilators and air pipes on the foredeck, allowing water to enter the forward section of the vessel. The added weight forward caused the vessel to ride “bow down.” Then with heavy seas crashing on hatch cover number 1, it collapsed, allowing more seawater to flood the vessel. Next, hatch cover number 2 likewise failed, and perhaps number 3, until the doomed vessel nose-dived out of control into the depths of the sea, finally impacting on the ocean floor at a speed of around 15 miles per hour. The case of the Derbyshire illustrates how rapidly a storm can build and move. In the case in which the seas have already been built up by a previous storm, large waves can be produced. In the case of an extreme wave, a vessel could go bow down into the wave and never come out the back side. It was estimated that waves that hit the Derbyshire were at least 60 feet high. More significant, the Derbyshire was but another in a long list of bulk carriers that have been broken up or sunk by large waves. For additional details, see Chapter 10. PROBABILITY OF LARGE WAVES Wave heights are probabilistic—in other words, wave behavior can be analyzed statistically but cannot be predicted precisely. Weather fore-
OCR for page 55
Extreme Waves casters are unable to say: “A 60-foot wave will occur at such and such a location at such and such a time,” but they can say something like this: “There is likelihood that 8 percent of the waves that occur in a year at this location will be 60 feet or higher.” I stated earlier that weather forecasters report the significant wave height (HS), which is defined as the average of the highest one-third measured wave heights (or the average of the highest one-third forecasted wave heights). Moderate waves in deep water can be modeled by a Rayleigh distribution (see Figure 8). The vertical axis in Figure 8 is the probability that a particular wave height will occur, while the horizontal axis is the wave height. The figure shows the lowest 10 percent of the waves, the most probable wave height, labeled HP, the average wave height HA, the significant wave height HS, and the highest 10 percent of the wave heights.3 While it may seem a little complicated, Figure 8 is one of the most important illustrations in this book. You can see that the significant wave height is equal to 1.6 times the average wave height. But still greater waves are possible, although they occur infrequently. The prob- FIGURE 8 Rayleigh distribution of wave heights.2
OCR for page 56
Extreme Waves ability of a wave greater than the significant wave is shown by the shaded area under the curve to the right of HS. If the mathematics is carried out, this area is found to be 13.5 percent of the total, meaning there is a 13.5 percent chance that a wave higher than the significant wave will occur, or that roughly one in seven waves will be larger than the significant wave. Compare this to surfer’s lore, as described in Chapter 6, or to the oft-stated mariner’s view that every seventh wave in a set is larger.4 The maximum expected wave height is approximately 1.8 times the significant wave height. A mathematician would recognize that the Rayleigh distribution goes on forever. In other words, there is a vanishingly small probability of very, very large waves. If the wave height is much greater than this value, it falls outside the range we might reasonably expect. This is what has given rise to the term “rogue” or “freak” wave. In a random sea, how many waves would it take before you experience the maximum wave? In 20 waves there is about a 5 percent chance of reaching a maximum wave, and in 200 waves a 5 percent chance of reaching a height twice as great as the significant wave height. An extreme wave—one greater than 2.2 to 2.4 times the significant wave—has a 5 percent chance of occurring in 1,000 to 4,000 waves if a constant sea is assumed.5 Under typical conditions, a vessel encounters 5 waves per minute, 300 waves per hour (waves with a 12-second period). Thus, in traveling 3.3 to 13.3 hours in such a sea, a vessel would have a 5 percent chance of experiencing an extreme wave. However, during this time, at 10 knots the vessel would have moved 33 to 133 nautical miles, hopefully to an area where the significant wave was smaller and the consequences of encountering an extreme wave less severe. For larger waves, with variable wind conditions such as those associated with hurricanes, or for waves in shallow water, more complex methods are required and the analysis must be handled differently.6 Waves in shallow waters and very large waves are nonlinear, meaning they have crests that are several times higher than the distance from the centerline to the bottom of the trough, and they are steeper than other waves.
OCR for page 57
Extreme Waves FIGURE 9 Typical NOAA marine weather wind-wave forecast. Figure 9 is an actual U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA) marine weather forecast for the North Pacific on March 7, 2005. Note the storm centered at latitude 30 degrees north, longitude 150 degrees west, northeast of the Hawaiian Islands. The forecast shows 40-knot winds and waves with a
OCR for page 60
Extreme Waves move up and down but will advance forward very slowly. Each circular orbit advances the wave only a short distance. This form of wave motion, which occurs in deep water, is called an inverted cycloid. To visualize it, imagine a point on the rim of a bicycle wheel—for example, the valve stem. Now picture the motion of the valve stem as the wheel rolls along a horizontal surface. From the top position it can be seen to roll forward initially, then when in the bottom position, to move backward relative to the bicycle’s forward movement. In shallow water, the orbits of an individual particle of water change, becoming elliptical in shape. This results from the fact that as the water becomes very shallow the vertical component of motion decreases. The same process occurs when a boat moves in the same direction as the predominant waves. The crest of the wave pushes the vessel forward, in effect adding the speed of the wave to the speed of the vessel. (Of course, due to the inertia of the vessel, it does not immediately reach the speed of the wave, which is generally moving much faster than the vessel.) As this crest passes under the hull, the boat enters the trough of the wave, where the direction of water flow is now opposite to the direction of the vessel, and the boat is slowed somewhat until accelerated again by the next crest. However, since the waves are propagating in the forward direction, the net effect is to increase the speed of the vessel over what it would be in calm water. Conversely, if the boat was moving against the direction of the waves, its speed would be reduced. A wave rises up and then, as it collapses, displaces water, creating a new wave. Think of the ocean as a long tube completely filled with marbles, each marble representing a wave. Now jam another marble in one end of the tube, and what happens? A marble pops out of the other end—but each marble only moves a distance equal to its diameter. Likewise, one wave initiates another, which causes another, and the disturbance propagates at a speed we call the wave group or wave system speed. Waves travel long distances, gradually dissipating energy as they move and also gaining energy from new sources of wind. As waves progress away from a storm or other initiating event, they gradually separate, the faster waves (those with longer wavelengths and periods)
OCR for page 61
Extreme Waves traveling farther in a given period of time. At long distances, waves will appear in groups, called sets—the long wavelengths appearing first, followed by the shorter wavelengths. This process is known as dispersion. As waves propagate they also attenuate, which is to say that they gradually lose energy, the short wavelengths attenuating more rapidly. A wave system actually travels at half the speed of an individual wave. If you look carefully at waves approaching a vessel, you will see the oncoming wave crest and then disappear, transferring its energy to a wave that forms behind it; the speed of the entire wave system is only half that of an individual wave.8 Consider the North Pacific storm of March 7, 2005, described earlier. The wave height in the vicinity of Southern California’s Channel Islands on March 7 was 6 feet. I checked the National Oceanic and Atmospheric Administration marine weather report for the same area on March 10. By that date the storm had moved northeast into the Gulf of Alaska and was located south of Valdez about 1,700 nautical miles distant from the Channel Islands, where the waves were now 12 feet. Since 72 hours had elapsed, these waves had traveled at around 24 knots to reach the coast of California. Actual seas consist of many separate wave trains arising from different storms at varying distances, modified by refraction or reflection from nearby landmasses and by wave interference or superposition as described earlier. This is why the sea at any given time has a noticeably irregular surface, although superimposed on it are seemingly semi-regular patterns of the dominant waves emerging from dispersion. When wave systems cross at angles to each other, irregular peaks can be produced, or you might see the effect of two waves running into each other. When the water becomes shallower—a process called shoaling—the wavelength and wave speed decrease and the wave height increases. In shallow water, defined as water where the depth is very much less than the wavelength, the wave speed and wavelength depend on the depth rather than on the wave period. In the nearshore area, as the wave slows, not only do the crests become higher, they also come closer together.9
OCR for page 62
Extreme Waves BREAKING WAVES As waves slow and become higher and steeper, their crests—less constrained by the ocean’s bottom—move faster than their troughs. The result: the top of the wave topples forward, like a bucket full of water that has been tipped over. As any surfer knows, this process is called breaking. Breaking waves are plunging waves—the type seen frequently in surfing films—the top curling over and plunging into the valley, forming a horizontal “tube” perpendicular to the wave direction; or spilling waves, wherein the wave shape is concave on each side, but rather than curling over, the crest slides down the face and disintegrates into a welter of foam. If waves do not break completely, they can reform and break again. The shore is not the only place waves break. Strong winds can cause them to break in the open ocean. When the wind’s speed is 30 knots, it blows spray from the tops of the waves. At Beaufort Force 9 wind conditions (41 to 47 knots), wave crests begin to topple over. Waves breaking in the open ocean are a definite hazard to vessels. If a 3.3-foot (1-meter) high column of water lands on a ship’s deck, it exerts a static force of 1 ton per square meter (204 pounds per square foot); the dynamic load is even greater and damage can result. Another category is a surging breaker, one that rises to a crest but overruns the beach without plunging or spilling.10 The height of breaking waves is influenced by shore conditions, the direction of the incident wave, and the wavelength of the incoming wave. Longer waves will break in deeper water. On steeply sloping beaches, waves tend to be of the plunging type. On a given day, walking between the Balboa Pier and the Newport Pier (both in Newport Beach, California), it is possible to see each type of breaker. There are sections of the beach where the bottom rises sharply, waves form plunging breakers, and there is a strong backwash of water returning to the sea, sometimes accompanied by riptides. Farther down the beach toward the Newport Pier, the bottom slopes more gradually and spilling breakers occur. In Chapter 1, I stated that theoretical calculations predict that waves will break once their height reaches one-seventh of their wave-
OCR for page 63
Extreme Waves length. This suggests that to produce an extreme breaking wave with a height of 98 feet, the wavelength would have to be 686 feet or greater. An idealized (hypothetical) wave with this wavelength would have a theoretical speed of 35 knots and a period of 12 seconds. Such a wave would easily overtake most vessels in the ocean. There would be no outrunning it! Another important wave parameter is the steepness, defined earlier as the height divided by the wavelength: Steepness = Height/Wavelength. Thus sailors are not concerned about high waves with long wavelengths because their vessel will ride up the crest and slide down the back side, with no perceptible discomfort if the wave is sufficiently long. It is the short-wavelength waves (or short-period waves) that create problems. In this case, a vessel climbing the crest of a wave will ride up and over, burying its bow in the trough of the next wave, taking on water and subjecting the vessel to severe impacts. Under extreme conditions, a vessel can pitchpole, or flip end over end in these conditions, often with fatal consequences (Figure 11). While the speed of propagation of wind waves in deep water is determined by the wavelength, a different condition applies to tsunami waves. The initiating event for a tsunami is an underwater earthquake, landslide, or volcanic eruption that results in sudden displacement of a massive volume of water. Rather than the surface layer of water moving as in the case of wind waves, the entire volume of water from the bottom to the surface is accelerated by the initiating event. Tsunami have extraordinarily long wavelengths (hundreds of kilometers) and periods of up to an hour. The speed is determined only by the depth of the ocean, and tsunami can cross vast expanses of ocean at speeds of 380 to 430 knots (437 to 495 miles per hour), or as fast as a jet plane. In the open ocean, the wave height may be 3 feet or less. However, the volume of water that is put in motion is enormous. In a manner analogous to wind waves, when a tsunami approaches shoaling waters the wave slows, but because of its high speed, slowing has a much greater effect and the height of the wave can become 50
OCR for page 64
Extreme Waves FIGURE 11 Wave steepness.11 feet, 60 feet, or more, causing extensive damage and loss of life. Usually there is more than one crest; the first crest may not be the largest. The arrival of the first wave may be signaled by a large and unusual backwash that drains beaches and harbors, uncovering normally inundated surfaces, but this may occur after the first wave hits. In either case, spectators coming to view this unusual scene do so at considerable peril. (Tsunami are discussed further in Chapter 6.) THE PULL OF THE MOON: TIDES Imagine a wave that stretches halfway around the world, has a period of 12 hours and 25 minutes, and is moving at hundreds of miles per hour in the open sea.12 As surprising as it might seem, you’ve experienced such a wave if you have spent any time on or near the ocean. The crests of this wave are known as high tides; its troughs, as low tides. Tide waves are not to be confused with tsunami although you will hear tsunami mistakenly referred to as “tidal waves”; however, they have nothing to do with the tides. Tides do not usually cause extreme waves,
OCR for page 65
Extreme Waves but there are two situations in which tidal forces can cause large waves. These are known as seiches and tidal bores. Seiching can occur when any external force disturbs an enclosed body of water. Waves move back and forth from one end to the other. The period of the waves depends on the size (length and depth) of the body of water. Standing waves can also occur. Seiches can be caused in bays and harbors by tidal currents, by the arrival of a distant swell with just the right period, or by storms or a tsunami. Sometimes these will oscillate for days. Seiches are generally not a problem and are detectable only by means of careful measurements. However, in the case of harbor design, engineers determine the predominant wave periods in the area and ensure that the harbor dimensions do not create a condition in which large seiches can occur, since this could cause excessive movement of floating docks and straining of vessel mooring lines. On a large scale, seiches have proved dangerous, damaging boats at dock and occasionally killing people fishing near shores or on breakwaters. Such an incident occurred on June 26, 1954, when a 10-foot-high wave suddenly rolled in from Lake Michigan and swept eight fishermen off a breakwater, drowning them.13 TIDAL BORES Tidal currents in the open ocean are weak—say, 0.1 to 0.2 knots—but near the coast or in bays and river mouths they can reach 5 knots or more. In shallow rivers, when currents exceed a critical speed, tidal bores appear.14 This is one tidal phenomenon that can create unusual if not large waves—a wave that moves forward as a wall of water. If large tides occur in narrow harbors or river mouths, the rapidly changing height of the water is compressed into a narrow channel, the current flowing much faster than the current of the tide wave in the open ocean. This can cause a fast-moving wave to sweep up the channel, sometimes creating hazardous conditions for boats entering or leaving the area. The wave can be a breaking wave or just an abrupt wave front; it is called a tidal bore. Usually tidal bores are around 3.3 feet in height, but they can be as high as 26 feet—for example, on the Qiantang River in China. Plate 6 is a photograph of the Qiantang bore
OCR for page 66
Extreme Waves taken in late August 1986 from a location near the city of Hangzhou. The bore forms below the city and then travels at a rate of 10 to 20 knots upstream for about 25 miles. In September and October, visitors arrive from all over China to watch the spectacle. A number have ventured too close and have been swept to their deaths. Local boatmen know to get their vessels out of the way! WAVE BUILDUP NEAR SHORE We had an unusual amount of rain in Newport Beach during the winter of 2005, and that plus a lot of travel kept me away from Dreams and offshore waters for most of the winter and early spring. Finally I saw an opening, and early in May, Nancy and I sailed to Emerald Bay, a small cove on the north side of Catalina Island near the west end. We planned to spend a few days before the Mother’s Day weekend, when once again other commitments would prevail. Emerald Bay is a favorite spot of mine for a short trip because it is relatively isolated. The nearest neighbor is a youth camp on shore at the far west end of the anchorage. It is a pleasant spot for diving or hiking to Stony Point and an isolated beach called Parson’s Landing. The weather was forecast to be mild but with a probability of showers, which was undoubtedly why we found ourselves the only boat in the anchorage. The cove has an entrance channel (fairway) about 1,500 feet long and moorings or room to anchor for about 100 small boats. Near the opening is a prominent rock—Indian Rock, about 1,000 feet from shore—marking the east end of a largely submerged reef that runs along the north side of the cove, giving some protection from swells coming in from the north. Just outside the reef the water depth drops quickly to 13 fathoms; and half a mile beyond that, it drops to 60 fathoms; and then another 2.5 miles out, it descends to depths of around 490 fathoms in the San Pedro Channel. We arrived in the early afternoon, got the boat squared away on a mooring, launched the dinghy, and relaxed—me down below writing, Nancy on deck with her watercolors, sketching the island, now a brilliant green after all the rain. The moorings are in long rows; Dreams was on the second row, in about 3 fathoms of water, and about 160 feet from a rocky cliff that divides the beach into two sections. There was
OCR for page 67
Extreme Waves no wind and only a gentle swell from the northwest when we went to sleep. At 3:00 A.M. I awoke as the motion of the boat shifted. The dinghy tied alongside so as not to bang all night was in fact banging and the boat was rearing up and down like an impatient horse. I went on deck to check the mooring lines and investigate. A breeze had come up—blowing over the island from the south—and this accounted for part of the noise. The main culprit, however, was a new swell coming from the north directly into the harbor. It was not terribly large—say, around a foot high, but the wavelength was about 33 feet—the same as the length of Dreams. As each wave came in, the boat would rise and then drop down into the face of the next incoming wave, creating the bucking motion that had awakened me. It was also just past low tide, and I knew the tide was rising. After making certain that all lines were secure and repositioning the dinghy so that it would make less noise, I went back to bed, making a mental note to wake up again if the swell increased, because it might become advisable to move out of the harbor. I was remembering the story of the wreck of the Memphis. In Chapter 2, I mentioned the buildup of waves approaching shallow shores and bays. The height that waves can attain under these conditions and the tremendous energy they carry was graphically and unforgettably illustrated in the wreck of the USS Memphis, a 500-foot-long armored cruiser. On August 29, 1916, the Memphis was anchored in Santo Domingo Harbor in what is now the Dominican Republic. She was anchored in 7.5 fathoms (45 feet) of water, about a half-mile from a rocky beach. The harbor opens directly to the south and thus is vulnerable to storms. August is hurricane season in the Caribbean, so the Memphis maintained 2 of its 16 boilers in operation in case it had to leave the harbor in a hurry. A previous storm warning had shown that steam could be raised in 40 minutes when two boilers were left in operation.15 Shortly after noon on August 29, one of the ship’s officers noted that the cruiser was rolling more than usual—about plus or minus 10 degrees. Captain Edward L. Beach, Sr., took a look at the horizon and could see a heavy swell starting to set in. The rocky shore now showed breaking waves. No storm warnings had been received, but the captain was so concerned that he issued the order to fire the boilers, rig the
OCR for page 68
Extreme Waves vessel for heavy weather, and be prepared to leave the anchorage. A second anchor was readied as a backup measure. A smaller naval vessel, the gunboat USS Castine, also made preparations to leave the harbor. The engine room informed the captain that sufficient steam would be available for the vessel to get under way at 4:35 P.M. The timing of the following events is important. By 3:30 P.M. it was evident to the captain and other officers of the Memphis that conditions in the harbor were deteriorating. When they looked out to sea at around 3:45 P.M. they could see a huge wave of yellow water approaching them. The wave spanned the entire horizon and was estimated to be 75 feet high. By 4:00 P.M. the Memphis was rolling so violently that seawater was coming in through the ventilators 50 feet above the waterline. By 4:25 P.M., water was coming down the stacks 70 feet above the waterline—and was snuffing out the fires in the boilers. Not only was the Memphis being battered by the waves, her only means of escape was being quenched by them as well. While all of this was transpiring, the Castine miraculously got under way and managed to get out of the harbor, pummeled and damaged by the large waves, but somehow avoiding capsizing. By 4:00 P.M., the bridge had noted that the huge “ocher-colored wave” was nearer and had grown in size to 100 feet or more. Now the Memphis was rolling 45 degrees, causing the gun ports to go underwater and letting torrents of seawater into the ship. At 4:40 P.M. the ship started striking the bottom of the harbor. Finally the engines began to respond, but it was too little, too late. Waves were rolling the vessel and battering her on the rocky bottom, damaging the propeller shafts, and soon the engines lost steam pressure. Around the same time, the Memphis rolled over into a deep trough and then was hit by a succession of three large waves—the last one the largest. Water washed crew members overboard and flowed over all but the highest points on the ship. By 5:00 P.M. she had been carried near the cliffs and was resting on the rocky bottom. The photograph in Plate 7, taken by a bystander on the afternoon of August 29, 1916, shows waves pouring over the Memphis. In an hour and a half, she had been battered into a total wreck. In the definitive account of the wreck, written by Captain Beach’s
OCR for page 69
Extreme Waves son, the loss of the Memphis is attributed to a Caribbean tsunami. This did not seem plausible to me, given the time sequence summarized above. The succession of waves that hit the ship seemed to have periods more typical of storm waves. Also, the elapsed time from the sighting of the large wave and its arrival—something between half an hour and one hour—did not seem to indicate a tsunami. If you could see a 75-foot-high wave at a distance of 18 nautical miles, a tsunami traveling at 400 knots would have hit the ship in a little less than three minutes, and if it had slowed to 200 knots, in about five minutes.16 On the other hand, if it was a storm wave traveling the same distance, the speed of 18 to 36 knots is about what you would expect. I could find no record of a Caribbean tsunami on August 29, 1916, so I sought the help of Professor George Pararas-Carayannis. Pararas-Carayannis is one of the world’s foremost authorities on tsunami, having served as the director of the International Tsunami Information Center (under the auspices of the United Nations Educational, Scientific, and Cultural Organization-Intergovernmental Oceanographic Commission) and as chief scientist for various missions sponsored by the United Nations development program. When we spoke he told me that a search of his databases also failed to locate a tsunami occurring on August 29, 1916, in an area that would impact Santo Domingo. However, several hurricanes had occurred in the Caribbean on that date and on days immediately preceding. One went west to hit Yucatan; another curved northwest and hit Corpus Christi, Texas. A tropical depression arose on August 27 at latitude 46 degrees west, longitude 14 degrees north. It subsequently became a category one hurricane and, by August 29, a category two. At 12:00 GMT (4:00 P.M. Santo Domingo time) it was located at latitude 15.6 degrees north, longitude 67.6 degrees west. As it traveled west, the counterclockwise pattern of winds (as viewed from above) would send swells to the northwest, in the direction of Santo Domingo. Waves would build and move—the faster, long-period waves arriving first. Given the early storms, these waves could have encountered others and might have added to create the three large successive waves (a phenomenon now known as the “Three Sisters”) that hit the Memphis. In summary, Pararas-Carayannis stated that he doubted that a tsunami caused the loss of the vessel.17
OCR for page 70
Extreme Waves Regardless of the source of the wave, it is clear that several waves—one of which was at least 70 feet high—hit the Memphis, rolled her, lifted her up, dropped her down on the bottom of the bay, and surfed her to ruin on the beach about a half-mile distant. This is something to remember when selecting an anchoring spot.
Representative terms from entire chapter: