Peahi Beach, on the north shore of Maui, Hawaii, is famous for some of the largest breaking waves in the world. Several times each winter, storms near Alaska produce swells that cruise thousands of kilometers to crash as surf at Maui. About half a kilometer offshore lies a deepwater reef, and when a 10-m swell hits the reef, the waves rise to heights of 20m and more.
It’s hard to describe how powerful and majestic these waves appear to an observer on the beach. Each one seems to rise slowly, implacably out of the offshore swell. Then there is that spine-tingling moment when the crest hovers on the brink of breaking. As the crest finally begins to curl over at the highest point of the wave, it forms an enormous tunnel of air. This tunnel propagates rapidly along the front of the wave. And then the wave collapses in a thundering mass of foam and surges far up the beach. What a thrill you get just by watching!
But the ultimate thrills are experienced only by the madcap crew that come to surf these monsters. They are members of the exclusive club of “extreme” tow-in surfers, who arrive every winter to compete in riding the highest possible wave and setting a world record. These surfers need to be towed by Jet Skis beyond the breaking waves because the waves move too fast (48km/h) to catch by paddling. Indeed, the waves are so awesome and so dangerous that the beach was nicknamed “Jaws” by the surfers who first discovered these immense breakers.
How high a wave is it possible to surf? Professional surfers travel the world to find out. Over time, as their skills and daring—and surfing technology—have improved, they have tackled taller and taller waves. Back in 1969 Greg Noll was credited with surfing the highest wave ever ridden at Makaha on the western shore of Oahu, Hawaii. It was “only” 10m high.
But by 2001, tow-in surfing had made it possible to reach the bigger waves, and the record had jumped quickly to just under 20m. Mike Parsons, a Californian surfer, traveled to the Cortez Bank, 160km off the San Diego coast, to find a higher wave. The bank is an underwater mountain range that rises to within a meter of the surface. When a high swell hits the bank, huge waves are created. On January 19, 2001, Parsons caught a fabulous wave and set a new record: 20.1m (66ft). He received an award of $66,000 for the feat, the largest prize ever won by a professional surfer. Parsons was 36 years old at the time, a ripe old age for a surfer.
Parsons’s record didn’t last very long. On January 10, 2004, Pete Cabrinha, another professional, set a new world record of 21.3m (70ft) at Jaws, Maui. He earned $70,000 for a few minutes of work and for risking his life. Mike Parsons was not about to be overtaken, however. On January 5, 2008, Parsons broke his own record at Cortez Bank by riding a wave estimated at more than 70 feet. Then later in 2008, Parsons set the official world record at Cortez Bank by riding a wave 23.5m (77ft) high. That’s as tall as a seven-story building. “I couldn’t believe it was that big. The drop just never ended. It went down, down, down, down,” Parsons said in an ESPN interview after his ride.
Nonetheless, back in 1998 Ken Bradshaw was credited unofficially with surfing an 80-foot wave (24.4m) at Outer Log Cabins on the north shore of Oahu, Hawaii. Could others beat that record?
Parsons’s official world record of 23.5m remained secure for a decade. Then on November 9, 2011, Garrett McNamara was videotaped surfing a wave off the coast of Nazaré, Portugal, where an underwater canyon focuses high swells to impressive heights. His monster wave was rated unofficially at a jaw-dropping 27.4m, or 90 feet (fig. 7.1 shows him exiting the wave). Then on January 28, 2013, McNamara may have broken his own record with an estimated 100-foot wave, also at Nazaré.
In December 2011, a board of expert judges was convened to examine the videos of McNamara’s November 2011 ride. Using McNamara’s shin bone as a length scale, the judges concluded the wave was actually “only” 25.7m (78 ft) high. It was not 90 feet, but it was still a world record, topping the old mark by a whole foot. McNamara’s wave was accepted by Guinness World Records in May 2012. In an interview on ESPN Action Sports, the 44-year-old McNamara said he had been on a mission for the previous 10 years to catch the “biggest, best waves on the planet.” McNamara’s feat will be hard to beat. Who will ever ride a certified 80-foot wave?
Fig. 7.1 Garrett McNamara exiting his record-breaking 78-foot wave. (Photo 27758072, dreamstime.com.)
Giant surfing waves are created when a big swell, sometimes as high as 10m, meets some offshore obstruction like a steep slope or a reef. So to understand breaking waves we need to begin with the life cycle of a swell.
You’ll remember that swells are the remnants of the chaotic waves in a distant storm. As the wind blows over a long distance (the fetch) for several days, it creates tall waves with a broad spectrum of wavelengths, sharply peaked crests, and a wide range of directions. For example, a storm with winds of 50 knots (93km/h) that blow for several days over a fetch of 1,000km could generate waves 10m high. After the winds die down, the longest waves—with a period of, say, 15 seconds and a wavelength of 350m—could escape the disturbed area at a speed of 84km/h.
A swell like this can travel thousands of kilometers with hardly any decay. As it rides into shallower water near an island or continent, the swell is transformed, changing shape, speed, and height. Let’s see how this happens.
As a swell approaches a shore, it is affected by the varying depth of water over the bottom. One effect is a change in the direction of the swell. On our very first walk along the beach in chapter 1, we noticed how the wave fronts of a swell turned to face nearly parallel to the shore. This effect is an example of refraction, the bending of a wave front due to a variation of wave speed along the front.
In water shallower than half a wavelength, the speed of a wave decreases with decreasing depth of the bottom. So if, for example, the left side of an incoming wave front passes over shallower water than the right side, the left side will slow down and the wave front will turn toward the left.
At an actual beach or bay, where the sandy bottom may have a complex shape, incoming waves may be turned in different directions. For example, in the top drawing of figure 7.2, we see a headland that extends as an underwater ridge some distance from the shore (the dashed lines are contours of the bottom). Incoming waves will turn to climb the ridge. They are focused by the changing depth of the water on the flanks of the ridge. The opposite effect occurs in a bay with a concave bottom, as shown in the bottom drawing. In both of these cases, the waves tend to shape the areas on which they break: over time, the bay becomes more semicircular while the point of land gradually erodes away from the relentless pounding of the focused waves.
A change of direction is not the only change a swell incurs in shallow water. On the approach to shore, the swell grows taller (a process called shoaling) before it eventually breaks. How does this happen?
A simple explanation for the growth of a wave as it approaches the beach is that the wave preserves its rate of energy transport until the instant it breaks. Its energy transport rate, according to Airy’s theory, depends on the square of its height and on its speed. As we saw just now, a wave slows down in shallow water. So if the energy transport rate remains constant and the speed declines, the height must increase.
Another way of looking at shoaling is to imagine that the swell is riding up a steep, smooth slope toward the beach. The wave’s period would remain constant, but the wave speed would decrease as the wave encountered shallower water. Each crest would move slightly slower than the crests behind it. As a result the crests would bunch together, like a line of cars approaching a stalled vehicle on a highway. Therefore, the wavelength (the distance between crests) would decrease as the wave approaches the shore.
Now let’s look more closely inside the wave to understand why the wave grows taller. When the wave is in water deeper than half its wavelength, the blobs of water under the surface rotate in synchronized circular orbits, returning to nearly their original positions on each circuit (see fig. 2.3). The greater the depth, the smaller is the diameter of the orbit.
Fig. 7.2 Shoaling waves are refracted by the varying depths of water near the shore. In this illustration the long dashes show the contours of depth; the short dashes show the paths of waves. Incoming waves turn toward a headland that extends underwater (top) and diverge in a concave bay (bottom).
Fig. 7.3 As a swell moves into shallow water, its underwater orbits change size and shape. The wave’s height increases, as does the orbital speed of the topmost orbit. When the orbital speed exceeds the wave’s phase speed, the wave will topple over.
However, as the wave rides up the slope, the orbits change shape from circular to horizontally elliptical (fig. 7.3). At a depth of about half a wavelength, the deepest orbits touch the bottom, and their vertical motions are inhibited. The orbital blobs no longer trace out perfect circles in one place; rather, they trace elliptical orbits that start moving toward the shore. In still shallower water, the deepest orbits flatten into virtually linear orbits, in which blobs oscillate only horizontally. The energy they lose from their vertical oscillations is transferred to orbits closer to the surface (if we ignore friction). These more energetic top blobs now trace out increasingly larger ellipses as they move further and faster toward the shore. In effect, the kinetic energy within the wave shifts upward and forward.
As you can see in figure 7.3, the orbit closest to the surface determines the height of the wave—that is, the distance from crest to trough. As this orbit gains energy from its neighbors, it cannot spin faster because its rotation period is a fixed fundamental property of the wave. Therefore, the orbit absorbs additional energy by increasing both its vertical and horizontal dimensions. Hence, we get a towering wave just before it breaks.
Now, when does a wave break? One answer is that the wave becomes unstable when its steepness ratio (height to wavelength) is about 1 to 7. So when a wave with a wavelength of 7m reaches a height of 1m, it becomes unstable. Another criterion is the slope of front face of the crest: when it becomes vertical, the wave crashes. A rough rule of thumb, valid for very gentle slopes of the bottom, says the wave will break when its height reaches 80% of the depth of the water. But why doesn’t such an unstable wave break backwards out to sea? The dynamic reason for breaking forward is the acceleration of the crest. Here’s how it happens.
As described earlier, the topmost orbit of a shoaling wave absorbs energy from its lower neighbors. Consequently, its diameter increases while its rotation period remains the same. That means the water on top of this increasingly large orbit has to increase its forward linear speed. But the top water forms the crest. So the crest moves forward faster and faster than the water below it. At some point the crest overtakes the trough ahead of it and tips over. In other words, the orbital speed of the water in the crest becomes faster than the wave’s phase speed (the wavelength divided by the period), and the crest topples over.
In real life, friction with the bottom is not negligible. When a wave’s deepest orbits touch the bottom, friction will slow the base of the wave but not the crests. Therefore, the wave is sheared and will eventually break. The bottommost orbits still do move forward, albeit slower than the top, so that their contact with the bottom moves the sediment gradually toward the beach. Coastal engineers are vitally concerned about the transport of sand, the evolution of beaches, and the development of currents, especially during winter storms. We’ll return to this matter in a moment.
After riding in toward the shore and building in height, a wave does finally break. The shape of the breaking wave depends primarily on the slope of the bottom but also on the wave’s steepness. There are three main types of breaking waves: spilling, plunging, and surging.
Steep waves moving onto gentle slopes (those with, say, a rise of 1m in 100m) become spilling breakers (fig. 1.2, top), in which water tumbles from the crest down the front face as a beard of white foam. The wave’s height decreases slowly over a long distance as the wave moves toward the shore and dissipates its energy in turbulent froth. These breakers can give a surfer a nice, long, but undramatic ride.
Moderately steep waves riding on long, moderate slopes (say, a rise of 1m in 20m) generate plunging breakers. Alternatively, an abrupt change of slope, such as an underwater reef that faces deep water, can produce plunging breakers. Their crests curl over and fall forward of the front face (fig. 1.2, bottom). A spectacular splash or jet may result from the impact, and a tube of air may be trapped under the falling crest. These are the tubes (or barrels) in which daring surfers love to ride. Big barrels are called Mackers: you could drive a Mack truck through them.
The perfect wave for surfers is a wave that develops its tube by curling over progressively along its crest. The wave shown at the bottom of figure 1.2 is curling over from left to right. This happens when the wave is approaching the shore at a slight angle to the slope of the beach. If the wave were to hit the slope dead on, the whole crest would curl over at once, making it dangerous or impossible for surfers to ride.
A wave with low steepness that encounters a steep slope (say, a rise of 1m in 10–15m) can become a surging breaker. Its crest doesn’t crumble or pitch over as it approaches shore. Instead, the wave remains intact as it rides up the slope, until suddenly the front of the wave collapses all at once and the water surges far up the beach. Surging waves have steep, smooth, fast-moving front faces.
Surging waves can be dangerous because they don’t look threatening. A pair of newlyweds learned that on March 7, 2011. They were having their wedding portraits videotaped on the beach of Bodega Bay, California, and were posing with their backs to the sea. The bride was wearing her long white dress and veil; the groom was dressed more casually. Without warning, a big wave broke as a surging breaker and knocked them off their feet. Happily, they escaped with nothing more than a good soaking. As compensation, their video went “viral” on TV, and they enjoyed a touch of celebrity for a while.
Coastal engineers earn their living by designing the wharves and seawalls we see around a harbor. In order to build a safe structure, an engineer has to be able to predict the biggest breaker that can be expected to arrive at the structure. Until recently, the theory of breakers was inadequate to make reliable predictions. Therefore, engineers had to rely on observations near a coast or on laboratory experiments in a wave tank. From such data they were able to extract empirical rules, but without a clear understanding of the physics underlying the rules.
For instance, they devised an empirical breaking index to predict what type of breaker will form on a particular slope. The index, termed Xi, is dimensionless and equals the product of the steepness (height divided by wavelength) and the slope of the seabed. Here are some examples of the index:
Xi value |
Type of breaker |
>3.3 |
Surging |
0.5–3.3 |
Plunging |
<0.5 |
Spilling |
We’ll see that numerical simulations have refined these criteria.
Then in 1974, J. Richard Weggel (Coastal Research Center, Washington, D.C.) compiled a useful design tool for the coastal engineer. After analyzing all the available observations on breaking waves and extracting empirical formulas from the data, he used these formulas to create a set of graphs to help engineers estimate the maximum wave heights of breakers on a structure near the shore. His practical results are still in use today.
Specifically, Weggel found that the maximum wave height of a breaker at a structure depends on the depth of the water at the structure, the steepness and period of the incident wave, and the slope of the bottom. As an example, consider a breakwater in 20 feet of water which is fronted by a bottom with a slope of 1:20. How high will breakers form at the breakwater from a swell with a 10-second period? Dipping into Weggel’s graphs we find a maximum breaker height of 0.83 times the 20-foot depth of the water, or 16.6 feet.
Weggel wrote that “breakers higher than 16.6 feet will break further offshore from the structure and will have dissipated a sizable fraction of their energy before reaching the structure. While smaller breakers may reach the structure they will not exceed a critical design condition” (Maximum Breaker Height for Design, U.S. Army Coastal Engineering Research Center, 1973). Thus, the slope seaward of a structure acts as a filter for the wave spectrum, causing higher waves to break further offshore.
Thirty years after Weggel published his work, engineers are still relying on empirical rules to predict maximum wave heights at particular locations. Several experimental groups have continued to explore how waves break, however. For example, W. Alsop and his team at the Wallingford Laboratory in Oxford, U.K., use a long computer-controlled wave tank and sophisticated instrumentation to measure waves breaking on smooth slopes. They have obtained more reliable and detailed relationships among the wave parameters.
As powerful computers, sophisticated computer algorithms, and improved modeling techniques have become more available, researchers have tended to replace physical wave tank experiments with numerical simulations. These computer models are cheaper, faster, and more flexible (as well as drier) and can be used to test current knowledge of the physics of breaking waves. A good example is the 1997 investigation of Stephan T. Grilli, I. A. Svendsen, I. A. Subramanya, and R. Subramanya at the University of Rhode Island.
Grilli was born in Belgium and received his training in civil engineering and oceanography at the University of Liège. He joined the University of Rhode Island in 1987 and eventually became chair of the Department of Ocean Engineering. He has specialized in coastal engineering and has become expert in numerical modeling of dynamic fluid behavior. Since 1998 he has been studying tsunamis caused by underwater landslides. Following the 1994 Indian Ocean tsunami he organized an expedition to obtain images of the seafloor at the epicenter. The team used a remotely operated vehicle to photograph the rupture zone at a depth of 4,500m.
Grilli’s team calculated the evolution of a solitary wave as it shoals and breaks over a mild or moderate slope. The wave arrives in deep water with an assumed initial depth at the foot of the slope. The computer model, which contains the two-dimensional, nonlinear equations of hydrodynamics, calculates the changes in the shape and speed of the wave, taking into account the gradual decrease of water depth. Figure 7.4 shows a typical example of a plunging wave. The curling lip at the front is realistic even though the model does not assume any friction on the slope.
From numerous trials with these models, using different slopes and initial wave heights, Grilli was able to define a breaking criterion that predicts the type of breaker—spilling, plunging, or surging—that will appear. He found that the criterion is proportional to the slope and inversely proportional to the square root of the ratio of initial wave height and initial water depth. Translated, this means that the various types of breakers are associated with the breaking criterion as follows:
Breaking criterion |
Type of breaker |
0.3–0.37 |
Surging |
0.025–0.3 |
Plunging |
<0.025 |
Spilling |
Fig. 7.4 A numerical simulation of a breaking wave. The wave is shown at successive stages as it climbs a slope of 1:35. (Drawn after S. T. Grilli et al., Journal of Waterway, Port, Coastal, and Ocean Engineering, May/June 1997, p. 107.)
This result provides engineers with even more accurate ways of predicting the types and strengths of waves that might hit breakwaters and other structures.
Grilli was also able to determine how the maximum height of a breaking wave depends on the slope. Contrary to intuition, the flatter the slope, the higher the maximum height and the deeper the water where the wave breaks. So, for example, on a shallow slope of 1:100 (i.e., a 1-m rise over a 100-m distance), a wave reaches twice its initial height and breaks at a depth equal to 40% of the initial water depth. On a steeper slope of 1:15, however, the wave rises to only 1.4 times its initial height and breaks at a depth of one-seventh the depth at the foot of the slope.
Grilli’s model has an impressive ability to follow the development of the curling lip of a plunging breaker, including the air tube underneath the curl. In examples published in 2003, Grilli and coworkers tracked the impact of the curl on the water and the splash that follows. These results encourage us to think that the basic physics is reasonably well understood.
There is still much more to be explored with numerical simulations, and several groups have embarked on ambitious projects. For example, Qun J. Richard Zhao and his team at the University of Delaware are working to include turbulence in their models of breaking waves. P. Higuera and a Spanish team at the University of Cantabria are investigating the movement of gravel on a slope as a wave shoals and breaks. Their results so far compare favorably with laboratory results. Oceanographers at Delft University in the Netherlands have also upgraded the sophisticated SWAN (Simulating Waves Nearshore) computer program that was discussed in chapter 6. SWAN now takes into account bottom friction and whitecapping, wave diffraction around obstacles, and refraction due to currents and depth. An extended three-dimensional version of the program (Delft3D) can also forecast currents and sediment transport.
When a wave breaks, the water under it surges forward as surf, a foaming mass of moving water. All the energy contained in the wave is dissipated in turbulence and in the kinetic energy of the water. The last, dying gasp of the wave is a thick layer of water that rides up the beach in the “swash zone” to a high point, stops, and drains back to the sea. This backwash water has to go somewhere, so it flows under or between the incoming waves in a system of currents. Oceanographers recognize three main types of beach currents: rip, undertow, and longshore.
Rip currents flow in narrow channels out from the shore. These channels are typically trenches through sand bars or along jetties. In order to drain the water that constantly arrives at the beach, these narrow rip currents must be very fast, with speeds that range from 0.5m/s to over 2.5m/s (9km/h). They can easily pose a threat to a careless swimmer, who can be carried swiftly out to sea with no chance of swimming against the rip. But because the rip currents are narrow, the smart swimmer knows to turn parallel to the shore and swim sideways out of the channel, where they can again turn to swim with the surf to the beach. Rip currents are actually stronger on their surfaces, and so do not tend to pull swimmers underwater.
As the name indicates, an undertow is a broad deep current that returns the excess of water directly under the breaking waves back to the sea, rather than in narrow channels. Undertows are slower than rip currents as a rule because they flow in broad paths back to the sea. They can still be dangerous to weak swimmers, but they are not nearly as threatening as rip currents.
Most waves approach the shore at an angle. When they break, they push water along the shore as a longshore current that carries a heavy load of sediments—primarily sand, but also pebbles, gravel, and other debris. At some point along the shore, the turbulent current dies out, and the load starts to drop. The flow patterns depend upon the wind direction, the wave strength within the swash zone, and the type and strength of the currents flowing back to the sea.
Longshore currents can transport sand long distances along the shore. If waves arrive from different directions relative to the shore, they can generate longshore currents in opposite directions. Thus, large quantities of sand may oscillate along the shore, sculpting the beach.
A beach may grow, move, or disappear entirely because of wave action. If, for example, waves erode a cliff at one end of the shore, the rubble may be carried by a longshore current for some distance and deposited in a bare place. A new beach will build up at this place. Or if the source of sand is another beach, that beach may be transported along the shore to a new location.
A beach normally goes through a seasonal cycle. In summer, gentle waves move sand from offshore and drop it on the beach in a thick layer. A flat terrace, called a “berm” may be formed up the beach. Then in winter, high storm waves erase the summer berm and deposit its sand in a winter berm higher up the beach. At the same time the strong backwash of the winter waves pulls sand off the lower part of the beach and builds a sand bar offshore. The beach as a whole is left bare or covered with gravel. This cycle can maintain a beach in oscillating equilibrium for many years.
Some beaches are strongly affected by storm-driven waves, especially waves with a very long fetch. These “storm beaches” usually have very steep slopes (up to 45 degrees) composed of rounded cobbles, shingle, and only occasionally sand. As might be expected, these components are sorted along the slope of the beach. The smallest pebbles lie within the swash zone, while the largest cobbles lie in berms at the top of the beach. The ferocious storm waves wash away any sand, while the backwash trickles through the larger cobbles rather than washing them away. A number of such beaches can be found on Cape Hatteras, North Carolina, and on the western coasts of England and Scotland.
Breaking waves can leave a permanent pattern in the sand of a beach. One distinct pattern is called beach cusps, but a better name might be chain of arcs. Each arc has two “horns” that point toward the sea and a bow that points up the slope of the beach. The horns of neighboring arcs touch in “cusps.” An arc measures a few meters to perhaps 60m between horns. Each arc in the pattern has about the same width and height.
The arcs are formed as water from a breaking wave sloshes up the beach slope and drains back to the sea. These cusps are self-sustaining—that is, once they have been formed, they maintain themselves. As waves crash first on the horns, they start slowing down, first dropping the heavier pebbles at the horn and then splitting in half to roll into the two arcs on either side of the horn. As the wave enters into the arc, it slows down even more and eventually collides with the half wave that was split at the next horn down, causing the remaining finer sediments to fall out until remnants wash up exhausted on the swash zone. Finally, the backwash drains downslope at the center of the arc in a kind of rip current.
Two theories have been proposed to explain the formation of these interesting cusp patterns: self-organization and standing offshore waves. In self-organization, a slight initial depression in the sand is enhanced because it attracts and accelerates the flow of water, which erodes the top of the arc. Once formed, the pattern maintains itself. Alternatively, a system of standing waves in the near offshore region may produce a variation in the height of the slosh along the shore, which creates the chain of arcs. Despite numerous simulations and field studies, investigators have not been able to demonstrate a clear choice between these theories. The subject is not purely academic because it bears on beach erosion and repair. And beach cusps appear on beaches around the world, so studying them is useful.
In recent years oceanographers have carried out a number of campaigns to measure waves and currents on a coast and to improve forecasting models such as SWAN and Delft3D. A favorite site for such work is on the North Carolina coast. Off the Outer Banks town of Duck, the continental shelf rises slowly and smoothly toward the shore for about 100km. It provides an ideal outdoor laboratory. The Army Corps of Engineers has maintained a coastal research station there for many years.
In the summer and fall of 1997, the Corps hosted several hundred scientists in a field experiment called Sandy Duck. Their goal was to better understand how sediment is moved by breaking waves and how beaches evolve. The scientists deployed a large array of buoys and sensors along the shore as well as along a line perpendicular to shore. In addition, they set up an array of fixed sensors in the surf zone to measure currents and sediment concentration. They carried out 30 different experiments, ranging from the measurement of waves, currents, and bottom topography, to the swash of broken waves up the beach.
During this campaign a major storm that raged for four days increased the maximum wave height to 4m. During similar storms in 1985 and 1990, a sand-bar had formed and moved offshore, leaving a deep trough behind it. As each storm passed, a system of rip channels had torn through the bar. But in 1997, a long bar formed without a deep trough close to the beach. The source of the sand remained a mystery. Moreover, the bar contained rip channels throughout the whole period rather than just after a storm. Several explanations were proposed for this behavior, but it became clear that too much was still unknown.
The coast at Duck is battered by hurricanes every year from September to December. A team from the Naval Postgraduate School at Monterey Bay, California, decided to monitor the waves at Duck during the hurricane season of 1999, as part of a larger exercise called the Shoaling Wave Experiment (SHOWEX). They set up a line of six waverider buoys between depths of 21 and 195m. These buoys measure horizontal and vertical displacements at the sea surface; from these measurements, wave height spectra and directional spectra can be determined. In addition, the team laid down six bottom pressure sensors in a line parallel to the shore at a depth of 24m to measure wave directions accurately.
The Navy team was extremely lucky. In three months they experienced four hurricanes—Floyd, Gertrude, Irene, and Jose (categories 5, 4, 3, and 2, respectively). Their equipment worked continuously for three months without a flaw. They scooped up a ton of data.
During Hurricane Floyd, the significant wave height reached 8m at 100km offshore but only 4m near the shore. A similar effect was seen during Hurricane Gert. Thus, the wave energy decayed by a factor of 4 as the waves crossed the inner shelf. Sonar images of the sandy bottom showed the cause: a pattern of rough, sandy ripples. Evidently, the rough bottom interfered dramatically with the propagation of the waves. This result raised an intriguing possibility: that the movement of sand affects the movement of waves as they approach the shore, just as the waves affect the sand. In other words, there is a dynamic interaction involved in the erosion and buildup of a beach.
For many years scientists at Duck had assumed that the behavior of waves, sand, and currents was the same anywhere along the straight, smooth shore-line. But recent research demonstrated that in fact, waves at two locations separated by only a mile had a very different behavior. What could account for this? Several explanations were proposed. Perhaps the geology of the shore varies more than has been realized. Or perhaps the size of sand grains varies along the shore. Or perhaps the drainage properties of the shore differ from one location to another.
Britt Raubenheimer and her team from the Woods Hole Oceanographic Institution decided to test a different but related idea: are the locations of underwater sandbars the prime factors in determining where erosion occurs during a storm? When a sandbar forms offshore, it could shield the beach from storm waves. In contrast, a sandbar near the shore could allow waves to flood the beach and cause erosion of berms and cliff faces. The scientists went to Duck in 2000 and measured waves and mean water levels as storms attacked the shore. The data strongly supported the sandbar explanation, but several other factors still needed to be explored. Only new experiments at a more complex site could decide the issue.
By the fall of 2003, the community of nearshore scientists was ready to mount a massive attack on the problems of nearshore dynamics. Two famous research organizations, Woods Hole Oceanographic Institution and Scripps Institution of Oceanography, agreed to coordinate the efforts of three dozen scientists, engineers, and students from a dozen institutions. They were now equipped with more advanced technology as well as improved forecasting tools such as SWAN and Delft3D. For their Nearshore Canyon Experiment (NCEX), they chose two submarine canyons off the California coast at La Jolla. The site, close to Scripps, had been studied by Scripps scientists in earlier years. This time an all-out effort was planned.
The heads of these canyons are only a few hundred meters off the shoreline. Their complex bottom topography strongly disturbs the persistent swell from the Pacific and generates strong surf, reflected waves, and currents. Conditions vary dramatically along the coast. At Black Beach the waves are reliably high enough to give surfers a good ride. Just two miles down the coast at La Jolla, the waves are consistently gentle. So the primary goals of the NCEX campaign were to observe the shoaling waves and the currents they produce at different locations, to map the movements of sand, and to test the reliability of the Delft3D program.
As an example of the types of hardware deployed by the group, consider the efforts of a team from the U.S. Navy Ocean Waves Laboratory. They set up five waverider buoys to measure wave heights and speeds at the head of Scripps Canyon, where the strongest effects of the bottom topography were expected. A string of 17 pressure-velocity sensors was set up along the 10-m-depth contour north of the canyon, to measure alongshore currents outside the surf zone. A region just south of the canyon is completely sheltered from the direct swell by the submarine topography. To monitor this quiet region, an array of bottom pressure sensors was established. The team gathered data for a full three months, from September to December 2003.
Another team from the Naval Research Laboratory wanted to learn how sensitive the models of currents and surf were to the actual topography of the ocean bottom. They deployed a huge variety of instruments to gather new measurements. Aside from the usual waveriders and pressure gauges, they used autonomous underwater vehicles, airborne video cameras, commercial satellite imagery, digital motion imagery, and sonar equipment mounted on Jet Skis.
Britt Raubenheimer and her husband Steve Elgar, both scientists at Woods Hole, were primarily interested in how wave reflection and refraction redirect the flow of water in a complex canyon system. They observed how wave “setup”—the piling up of a giant hill of water during a storm—generates large alongshore and cross-shore currents. Variations of setup along the coast could account for much of the variation in current strength. But reflection and refraction of waves also affects the strength of currents. They also measured the momentum of onshore waves, the surge of water up the beach, the return of water in rip and undertow currents, and the transport of sediment. Their primary tool was an acoustic Doppler velocimeter—an instrument analogous to sonar. The device emits a stream of high-frequency sonic pulses focused at a single point, receives the echoes, and records the data. Twenty-five of these devices were deployed along 3km of shore, at depths ranging from 2.5 to 15m. The team gathered a mountain of data that will occupy them for years to come.
Several teams reported their preliminary findings at the fall 2004 meeting of the American Geophysical Union. A team from the U.S. Naval Research Laboratory, led by K. Todd Holland, had used an array of color digital cameras along 2km of shore to monitor waves and surf out to a depth of 25 m. The images obtained were analyzed in real time with an automatic process to obtain hourly estimates of wave period, wave direction, breaker height, and the effects of bottom topography. Estimates were also made of directional wave spectra and surf flow speeds. These estimates were used to fine-tune the Delft3D forecasting program, which was also provided with the canyon topography and incoming swell heights. Delft3D’s predictions were compared with the actual observations. The researchers commented that there were significant correlations between the model’s predictions and the observations via video. Translating from science-speak, this was a very good result. The agreement was particularly good for rip currents. The team concluded that their method of incorporating constraining data at the boundaries of the site resulted in marked improvement in the accuracy and resolution of the predictions.
We can look forward to a time when oceanographers can predict with some accuracy the impact of storm waves on a complicated shoreline. This knowledge will allow coastal engineers to armor the coast to protect homes and lives.
