The Power of the Sea: Tsunamis, Storm Surges, Rogue Waves, and Our Quest to Predict Disastersby Bruce Parker
The Power of the Sea describes our struggle to understand the physics of the sea, so we can use that knowledge to predict when the sea will unleash its fury against us. In a wide-sweeping narrative spanning much of human history, Bruce Parker, former chief scientist of the National Ocean Service, interweaves thrilling and often moving stories of unpredicted natural… See more details below
The Power of the Sea describes our struggle to understand the physics of the sea, so we can use that knowledge to predict when the sea will unleash its fury against us. In a wide-sweeping narrative spanning much of human history, Bruce Parker, former chief scientist of the National Ocean Service, interweaves thrilling and often moving stories of unpredicted natural disaster with an accessible account of scientific discovery. The result is a compelling scientific journey, from ancient man's first crude tide predictions to today's advanced early warning ability based on the Global Ocean Observing System. It is a journey still underway, as we search for ways to predict tsunamis and rogue waves and critical aspects of El Niño and climate change caused by global warming.
An appealing overview of sea movements.
Former National Ocean Service chief scientist Parker begins with the familiar: tides, which turn out to be more complicated than readers may have learned in high school. Lunar gravity pulls the ocean, but so does the sun, plus a contribution from the centrifugal force of the earth's rotation and another from the tilt of its axis. Gravity powers tides, but geography and weather determine how high they rise. This varies from almost no tides in the Mediterranean to more than 50 feet around the Bay of Fundy in eastern Canada. The author then moves on to a discussion of more violent movements—when a river narrows as it approaches the sea, incoming tide is compressed and amplified, often producing a "tidal bore," a destructive wave that races upstream twice a day. Parker devotes the most attention to tsunamis, which are produced by undersea earthquakes or landslides, but similar phenomena, storm surges, occur more often. Winds from tropical and nontropical cyclones can push an immense wall of water across the shore. Surges produced the 1900 Galveston and 2005 New Orleans catastrophes, but these were modest compared with those in the Bay of Bengal, which have killed hundreds of thousands. Surges, not tsunamis or heavy rains, undoubtedly gave rise to flood myths present in almost all cultures. Parker mixes hair-raising descriptions of disasters with efforts to understand them, followed by advances, mostly since 1800, in predicting sea movements, a complex process that today involves satellites, supercomputers and worldwide warning networks.
Focusing on water alone—leaving marine life to Rachel Carson and others—the author provides a lucid, original contribution to popular-science writing.
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The Power of the Sea
Tsunamis, Storm Surges, Rogue Waves, and Our Quest to Predict Disasters
By Bruce Parker
Palgrave MacmillanCopyright © 2012 Bruce Parker
All rights reserved.
The Earliest Predictions for the Sea
On July 25, 1798, Napoleon Bonaparte entered Cairo as the master of all Egypt after defeating the Egyptian Mamluks in the Battle of the Pyramids. Such victories are not always long lasting, however, and Napoleon realized that another battle was imminent, this time against the British and the Turks. While he prepared for that battle, Napoleon immersed himself in the local culture, exploring his newly conquered territory and establishing an institute of arts and sciences at Cairo, staffing it with the 167 scientists and artists he had brought with him from France. In December Napoleon visited Suez to examine the remnants of a canal built many centuries earlier by Egyptian Pharaohs to connect the Nile River with the Red Sea, back when sea level had been higher. He also inspected the site of a planned new canal that was to connect the Mediterranean Sea to the Red Sea. On the morning of December 28 Napoleon intended to take a small band of soldiers to visit the Wells of Moses, on the other side of the Gulf of Suez at the northern end of the Red Sea. The Gulf of Suez extended about three miles farther north of the port of Suez in 1798 than it does today. The point where Napoleon expected to cross the Gulf of Suez was about one mile wide and always dry, or at least fordable, at low tide. Caravans from Tor and Mount Sinai regularly crossed at this spot.
Predicting the time of low tide at that location on the morning of December 28 would have been no problem for Napoleon's scientists. By 1798 tide prediction had finally become a scientific endeavor. But even before then the French, the English, and other peoples living by the sea had developed their own approximate methods for predicting the twice-daily rise and fall of the sea. Tidal ranges were large along all the coasts of France except the Mediterranean coast. The largest were along the coasts of the English Channel, where the rise from low tide to high tide was greater than twenty feet and at a few locations even as great as forty-five feet. In such areas, beaches that were little more than a few yards wide at high tide could become a mile wide at low tide, six and a quarter hours later. Thus, it had been out of necessity that methods of tide prediction were developed, for such large tides affected the lives of those who lived along the coast, as did strong tidal currents, the oscillating horizontal water flow that accompanies the rise and fall of the tide. A fisherman who went to dig up oysters or scallops from mudflats that were uncovered at low tide had to know when the water would come rushing back, or he would drown. A sea captain leaving a harbor had to know the time of high tide, so his ship would not run aground and sink. Or his ship might not be able to buck a strong tidal current flowing into the harbor (referred to as the flood current). He would thus have to know the time when the current would reverse and flow outward (the ebb current). Monks working in a tide mill, a mill that used the power of the tide to grind wheat, planned their entire work schedule according to the two times of low tide each day, when the tidal power would be maximum.
The older tide-prediction techniques were developed when mariners noticed that the oscillating tide correlated with the phases and movement of the moon. They observed the surface of the sea rising to a highest level and then six and a quarter hours later falling to a lowest level, and then after another six and a quarter hours the sea returned to the highest level again. Then the whole cycle repeated, there being roughly two such cycles in a day. Then they noticed that the height of the high waters changed throughout the month, from cycle to cycle. The highest high waters (as well as the lowest low waters) occurred near the time when the moon was full and round and at its brightest. The tidal range, the vertical distance from low water to high water, was largest near this time of full moon, now referred to as spring tide. But two weeks later, when the moon was totally dark, referred to as new moon, the tide again had its largest tidal range, another spring tide. Those trying to figure out how the moon caused the tides were confused by a dark new moon causing tidal ranges as large as a bright full moon. Surely, they thought, a bigger, brighter moon should be able to pull more water with it than a dark moon, but it didn't work that way. The smallest tidal range, called a neap tide, occurred halfway between the times of new moon and full moon, when half the moon shone. Mariners had also observed at night, when the moon rose in the east and moved across the sky, that the sea rose, reaching high water around the time when the moon was highest in the night sky. Then, as the moon fell toward the western horizon, the waters along the shore fell back. Coastal dwellers found that the tide changed with a pattern they could rely on, day after day, month after month, year after year—a pattern that allowed them to roughly predict the changing height of the sea's surface.
For those living on the coast, the timetable of their lives was usually determined by the tide, rather than by the solar timetable of days and nights. The tide has a lunar timetable. The two high waters that occur each day occur a little later than they did the day before, and the next day they are later still. And, of course, the same is true for the low waters, so when oyster fishermen journeyed out to the mudflats at low water, it was a little later each day. On a particular day the first low water might be at noon, but the next day it would be almost an hour later, and the day after that almost another hour later. After six or seven days, low water would be in the evening and the fishermen would be digging up oysters in the dark, or they would shift to the next tidal cycle and dig up oysters early the next morning.
Most French people, including Napoleon, had heard the stories about the dangerous tides near the cone-shaped rocky isle of Mont-Saint-Michel at the southeastern end of the Gulf of St. Malo, which is connected to the English Channel. The tide at Mont-Saint-Michel rises forty-five feet from low water to high water in roughly six and a quarter hours, which means that the water rises more than seven feet every hour. That doesn't leave a fisherman much time to dig oysters on mudflats revealed at low tide and safely leave before the sea returns. But it's not just how fast the water rises that is of concern; it's also the way the sea returns to cover the mudflats. Throughout the centuries the locals living along this French coast have described the incoming tide as arriving like galloping horses, swiftly encircling the Benedictine abbey that has been on Mont-Saint-Michel since AD 708. Fishermen caught on this expanse of mudflats at the wrong time found themselves suddenly surrounded by water rushing in at them on all sides. Many drowned.
The tidal range at the northern end of the Red Sea was not nearly as large as the tidal range near Mont-Saint-Michel, but even an eight-foot range could cause problems for one caught on a mud or a sand flat when the tide comes in. So an accurate tide prediction was important for Napoleon. When he and his men reached the shore of the Gulf of Suez at the scheduled time, he found a mile-long expanse of sea bottom exposed at low tide as his scientists had predicted. His small band of soldiers on horseback easily crossed the dry flats to the other coast before the tide began to rise.
Napoleon reached the Wells of Moses in good time. He stayed throughout the afternoon, meeting with Cenobites from the convent of Mount Sinai and with some Arabian chiefs from Tor. Then late in the afternoon he and his men left and began their return trip to Suez. The sun had set by the time Napoleon and his soldiers reached the seashore. The tide seemed to be out far enough for them to begin crossing the exposed sea bottom. But the sea bottom did not stay exposed for long. Suddenly the tide began rushing in at them, seemingly from all directions. Surrounded by rapidly rising water, and with darkness adding to their confusion, they were thrown into disorder and panic. They could not see a shoreline in any direction. As the tide rose, the water quickly became deeper and threatened to engulf them. Their only chance was to find a shoal where the water might still be shallow enough to walk on. Napoleon calmed his men and ordered them to form concentric circles around him, each horseman facing outward as part of several straight lines pointing in different directions, like the spokes of a wheel. He then ordered each line of horsemen to advance outward. When the lead horse of a line reached deeper water and had to begin desperately swimming, that column drew back and followed one of the columns still walking on the sea bottom. Eventually, each of the columns lost their footing until only one remained, which everyone followed to an ultimate escape from the Red Sea. In spite of the rising water surface on which wind waves rode and crashed against them in the darkness, only one of Napoleon's contingent came close to being lost, a general with a wooden leg who had trouble sitting firmly on his horse with water up to his waist.
Back on shore Napoleon is said to have remarked, "Had I perished in that manner, like Pharaoh, it would have furnished all the preachers of Christendom with a magnificent text against me." He was referring to the famous Exodus from Egypt by the Children of Israel led by Moses. Napoleon had been told that this route was purportedly the same route across the Red Sea used by the Israelites.
Three thousand years earlier the Children of Israel had been camped on the shore of the Gulf of Suez at the northern end of the Red Sea, or the Sea of Reeds, as many believe it was called back then, because of the reedy marshes growing in brackish water at its northern end. Their location was probably farther north than where Napoleon almost met his demise, because in the time of Moses sea level was higher than it was in Napoleon's time. The Israelites had left Egypt behind and, they hoped, their lives as slaves under the rule of Pharaoh. But Moses knew that Pharaoh would send his army after them. When the dust clouds raised by their chariots were finally seen, still miles in the distance, they filled the Israelites with fear, for now they were trapped between Pharaoh's army and the Sea of Reeds.
The dust clouds from Pharaoh's chariots were, however, probably an important part of Moses' plan, for they would have allowed him to calculate how soon Pharaoh's army would arrive at the seacoast. For Moses surely had a plan. One does not attempt such a massive undertaking as leading an entire people into freedom without a plan, even if hoping for help from on high. And Moses must have known that his key to success was timing—exact timing. In his earlier years Moses had lived in the wilderness. He knew the area by the Sea of Reeds. He knew the night sky. And he must have known where caravans crossed the Sea of Reeds at low tide. Pharaoh, on the other hand, living along the Nile River, which was connected to the almost tideless Mediterranean Sea, probably had little experience with the tide in the Sea of Reeds. It would not have occurred to him that the tide would soon wash over what had been the Israelites' path to freedom.
Knowing when low tide would occur, how long the sea bottom would remain dry, and when the waters would rush back in, Moses could plan an escape across the Sea of Reeds that took advantage of the tide. Choosing a full moon for their escape (perhaps merely to light their way on a nighttime crossing) would have given them a spring tide and a larger tidal range. That would have meant a lower low water, and thus a longer-lasting dry sea bottom and more time for the Israelites to cross, followed by a higher high water to better engulf Pharaoh's pursuing army. Timing was crucial. The last of the Israelites had to cross the dry sea bottom just before the tide returned, enticing Pharaoh's army of chariots onto the exposed sea bottom where they would drown in the returning tidal waters.
The Bible mentions a strong east wind that blew all night and pushed back the waters. Ocean physics tells us that wind blowing over a shallow waterway pushes back more water than a wind blowing over a deep waterway. Thus, if a wind did by chance fortuitously blow as the Israelites were crossing the Red Sea, it would have had more effect at low water than at any other time, uncovering even more sea bottom. Such a wind would surely have been assigned to divine intervention, and as the story of the Exodus was retold, that aspect would have overshadowed Moses' planning their escape to take advantage of a predicted low tide. But Moses could not have predicted a suddenly beneficial wind, and therefore he could not have based his plan on such a wind. He could have predicted only the tide. And thus the successful Exodus of the Israelites would have depended on timing based on that tide prediction.
Today at the northern end of the Gulf of Suez the spring tidal range averages just under five feet and reaches six feet at certain times of the year, not counting wind effects. Tide measurements did not begin in the Red Sea until the 1890s, but a hundred years earlier, at the time of Napoleon's escape from the onrushing tide, his private secretary and the author of his memoirs, Louis de Bourrienne, said that at high tide the water rose five or six feet and as much as nine or ten feet with the wind blowing in the right direction. We know from astronomical calculations that the tide that engulfed Napoleon and his men was only a neap tide. The etching in figure 1.1 is correct; the moon was in its third quarter. Napoleon might not have escaped the Red Sea if there had been a nearly full or new moon, which brings the larger spring tide. Napoleon's sudden encounter with the onrushing tide was shocking enough, and such a relatively small tide during the Exodus might have been enough to defeat Pharaoh's army, especially with chariot wheels getting stuck in wet sand. If that is what happened, the Exodus story would have certainly become more and more dramatic with each retelling, until it included walls of water drowning Pharaoh's army of chariots.
However, there is a very good chance that the tidal range at the northernmost end of the Red Sea was larger in Moses' time. Three thousand years before Napoleon, at the time the Exodus is believed to have taken place, there is evidence that sea level was higher than it is now and that the Gulf of Suez extended farther north. A lengthened Gulf of Suez, or a smaller basin connected to the Gulf of Suez, would most likely have increased the amplification of the tide, producing tidal ranges at the time of the Exodus that were larger than those seen today or in Napoleon's time. In that case the Red Sea event wouldn't have needed that much exaggeration as it was passed down from generation to generation and then finally written into the Bible (with different pieces of the story contributed by at least three different authors, we are told by biblical scholars). If the tide was indeed involved in Moses' "parting" of the Red Sea, one might rightly say that this was the most dramatic tide prediction in history.
But as it turns out, my suggestion that Moses might have planned to cross the Sea of Reeds at the predicted time of a low tide turns out not to be a new suggestion. In Praeparatio Evangelica (Preparation for the Gospel), Eusebius of Caesarea (ca. AD 263–339) quotes from a book by the Hellenistic historian Artapanus (80–40 BC). In that quote Artapanus gives two versions of the crossing of the Red Sea. One is a story similar to what appears in the Bible, which he attributes to the people of Heliopolis. The second version he summarizes thus: "Now the people of Memphis say, that Moses being acquainted with the country waited for the ebb, and took the people across the sea when dry."
As one might expect, the earliest written references to the tides are from geographic areas where large tidal ranges coincided with early civilizations. Along the west coast of India in the Gulf of Cambay north of what is today Mumbai, formerly Bombay, the tidal range can reach over thirty feet. Farther north in the Gulf of Kutch the range is still over twenty feet. The ancient Harappan civilization that developed in the Indus Valley by 2300 BC reached south to include these two areas. It is not surprising then that the first known written reference to the tides was in the S maveda, a series of ancient Indian hymns probably written around 1100 BC. Not only are the tides described, but their main cause is correctly attributed to the moon—and with this understanding they were most likely able to crudely predict the tide. There is, however, evidence that the Harappans were well aware of the tides a thousand years before the S maveda was written. The Archaeological Survey of India carried out excavations at Lothal, at the northern end of the Gulf of Cambay, that revealed a dockyard for berthing ships that was built around 2300 BC. The amazing thing about this dockyard was its lock system. A ship would enter the channel at high tide, at which time a wooden door was inserted in grooves on both sides of the channel and lowered to close off the channel, keeping the water in the basin as the tide fell and thus allowing the ship to stay afloat. The Harappans who lived farther north, in the Indus Valley, were also familiar with a dramatic manifestation of the tide, the tidal bore. Twice a day a tumultuous wall of water charged up the western branch of the river—a distorted, perpetually breaking wave, which, as we shall see shortly, eight hundred years later would come as an almost fatal surprise to a famous visitor from the Mediterranean Sea.
Excerpted from The Power of the Sea by Bruce Parker. Copyright © 2012 Bruce Parker. Excerpted by permission of Palgrave Macmillan.
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