Recent events have shown what kind of havoc waves can cause. We may never calm the seas, but advances in coastal engineering could reduce their damage.By Dale Keiger
Illustration by Michael Gibbs
John Hopkins Magazine
http://www.jhu.edu/~jhumag/0406web/force.htmlWhen Robert Anthony Dalrymple watches the surf, he sees the approaching swell, the foaming curl, the cascade of water, the sea sliding up the beach toward his feet. He's no less mesmerized than the rest of us. But where most people look at plunging surf and see chaos, he sees structure, eddies and vortices, wave forms and fluid mechanics. "I see things I can predict," he says, "and things I can't predict." He smiles and adds, "I can be very boring at the beach. My wife doesn't enjoy going with me anymore. She makes me go to the mountains now on vacation."
Tony Dalrymple is a wave guy. A professor in the Department of Civil Engineering in Johns Hopkins' Whiting School, he works as a coastal engineer, trying to better predict the behavior of the shoreline during short-term events like a hurricane, and over longer durations, like the next century of human development. His contributions to the field resulted in his election last February to the National Academy of Engineering. As a scientist, he analyzes and models waves. As an engineer, he studies the human implications of all that moving water.
The seriousness of those implications has become obvious during the first years of the 21st century. Massive hurricanes have pummeled the southeastern United States, and "pummeled" doesn't begin to describe what Katrina did to New Orleans and the Mississippi Gulf Coast last year. The day after Christmas 2004, the seabed of the Indian Ocean heaved and generated a tsunami that killed more than 283,000 people. In the years ahead there likely will be more storms, more floods, more changes to coastlines, more work for Dalrymple and his colleagues around the world. As the planet warms and sea levels rise and storms become more severe, he anticipates no shortage of work.
He laughs when called a civil engineering action figure, but he is part of a response team assembled by the American Society of Civil Engineers (ASCE) to fly into disaster areas and assess what happened. He surveyed Thailand after the tsunami, and was in Louisiana after Katrina. By going in four or five weeks after a debacle, the engineers avoid hindering search-and-rescue operations, but arrive in time to gather evidence of what happened, where civil engineering succeeded, and where it failed.
Mostly, Dalrymple looks for the unexpected on the monitor of a computer running simulations based on his models. But in January 2005, he went into the field to investigate an event for which good models, and therefore useful predictions, are problematic.
For centuries, a massive portion of the Earth's crust, the India tectonic plate, had been sliding toward the north-northwest under the Indian Ocean at a rate of about 2.5 inches per year. It was trying to subduct, or dive under, what scientists call the Burma microplate. But tectonic plates are not on bearings and they're not greased. They snag on each other. In this case, the India plate was snagged on and bending the Burma plate, forcing down its leading western edge and tilting up its eastern edge, building immense potential energy. The sea bottom was a huge cocked spring, and starting at 7:59 a.m. local time on December 26, 2004, the spring let go. Seismologists call what happened a mega-thrust event.
The first slippage was small, but then there occurred a sudden violent spasm as the India plate slid under the Burma plate, lurching 50 feet toward Indonesia. The front edge of the Burma plate heaved upwards more than 6 feet while its eastern section dropped, releasing even more energy. Along an underwater fault line, the subduction traveled like a giant zipper for 750 miles, the length of California. The mega-thrust event went on for 8 minutes, the most powerful temblor since the 1964 Alaska earthquake. But for hundreds of thousands of people, the event generated a deadlier number: It displaced 200 billion cubic meters of water. A tsunami.
Energy let loose by the earthquake sped east and west of the rupture at the speed of a jet airliner. In 30 minutes, massive waves traveled 300 miles and hit Banda Aceh, Indonesia, wiping out 90,000 people. Though the colloquial term for a tsunami is "tidal wave," the actual phenomenon is much more complex (and has nothing to do with tides). Four waves hit parts of Thailand. To the west, the main wave was so long it wrapped around the island of Sri Lanka before striking coastal areas of India. After barreling into Kenya and Somalia, waves bounced back and hit Thailand and Indonesia again. The whole Indian Ocean reverberated. Says Dalrymple, "Waves were just banging back and forth. It was like hitting a bell."
Before its energy was spent, the tsunami propagated into all the world's oceans. The National Geophysical Data Center recorded 302 run-ups — tsunami waves striking coastlines around the world. Wave energy, shunted and steered by underwater ridges, shot between Australia and Antarctica and headed east and north, eventually landing on the Canadian Pacific coast. Westward, waves wrapped around Africa, then split, with energy headed for North and South America. Twenty-one hours after the quake, tide monitors near Rio de Janeiro registered a 3-foot offshore swell. Eight hours after that, 1.5 feet of water came ashore at Nova Scotia. Six inches of tsunami lapped at Cornwall, England; 4 inches at Los Angeles; 9 inches at Atlantic City, New Jersey. No tsunami had reached so far since Krakatoa erupted in 1883. In the coastal areas that define the Indian Ocean, at least 283,100 people died — drowned or battered to death by floating debris.
Dalrymple shows a sheetpile wall that failed during Katrina, pointing out the difference in height between it and the concrete flood wall to the left. "Reasonable design says that the walls should all be at the same elevation," he says. A month later, Dalrymple and an ASCE team flew into Phuket, Thailand, one of the wrecked beach resorts, and for five days traveled to various devastated places along the Thai coast. As a coastal engineer, Dalrymple was surprised by how much the beaches had already recovered. Not the people or the houses or the businesses, but the sand.
Immediately after the waves, Thailand's western beaches were literally gone, sucked out and washed away. But in a single month the action of the surf had rebuilt them. How did that happen? "Interesting question," Dalrymple says. Sand is resilient, he notes, hard to destroy. In Thailand, he says, the tsunami carried some of the beach landward, but then dragged sand out of river beds as the waves receded. This sand was deposited offshore, then later moved to shore again by the surf. Dalrymple was startled by the speed with which these processes restored Thailand's beaches.
As an engineer, he was interested in how structures fared during the inundation, and if any protective measures had worked. Some buildings set on raised pilings survived; the sea surged through the pilings and left the structure intact. Seawalls in areas developed for tourism, like Phuket's Patong Beach, blunted the force of the water. They had been overtopped, for sure, but still mitigated damage to landward structures. At Patong, the engineers observed, the wall had wide openings, every block or so, to provide pedestrian access to the beach. Here, of course, the water simply poured through, scouring the beach around the openings when it retreated.
The shape of a wall changed its effectiveness. Concave barriers turned the water back on itself, providing additional protection. At the north end of the beach, the engineers found a wall that had not been so effective. This one, for some reason, had been built with a landward incline. During the tsunami, the water hit the incline and flew into the air as if off a ramp — straight into the second story of a beach house. The first story emerged relatively intact. The ASCE report notes, with just a touch of wryness, "The lesson here is that seawalls should not be sloped landward."
Wherever he went, Dalrymple found evidence of how big the waves had been. On his computer in his office, he calls up a photo included in the ASCE report. It shows a van parked under a lone tree on the sand, near water's edge at Khao Lak. An arrow superimposed on the image points to debris snagged by the tree, at least 25 feet above the ground. At Patong Beach, the engineers saw cars stacked atop each other or resting on structures.
As a modeler, Dalrymple was curious how well computer models of tsunami behavior matched reality. He calls the 2004 disaster the most recorded tsunami in history. Instrument stations all over the world collected data. For the first time, radar from satellites measured the amplitude and progress of the tsunami waves as they propagated. On the eyewitness level, dozens of tourists shot video of water slamming into Thailand and Indonesia. Within 24 hours, a remarkable amount of information about the disaster was available on the Internet, and scientists had an abundance of data. When they matched that data to models of tsunami behavior in deep water, Dalrymple says, the models proved accurate. But simulating a tsunami crashing into land or structures is far more complicated, and here the models, though better than expected, need a lot more refinement to be useful for anticipating structural damage and creating safeguards. Dalrymple is working on modeling the action of waves flowing through buildings — or lifting them from their foundations and carrying them off.
Protecting Against Future Tsunamis - After surveying areas of Thailand ravaged by the December 2004 tsunami, the American Society of Civil Engineers' response team made the following recommendations:
- Keep sources of debris and cars away from the front of structures.
- Sand dunes, especially vegetated dunes, can lessen the effects of tsunami run-up on inland development. So can properly built seawalls. These walls should be vertical or concave, with few or no low sections such as pedestrian openings.
- Buildings in the hazard zone should be built on deep foundation piles. Those buildings should be designed to allow water to flow through.
- Design of structures should take into account the erosive effects of tsunami waves receding. Water returning to the sea undermines structures and foundations.
"There are just so many interesting problems," Dalrymple says. The Department of Defense has just granted $5 million to a consortium of scientists, including him, to study what happens when waves come ashore over a muddy sea floor. Mud tends to dissipate wave energy. A better understanding of how that works might allow defense analysts, using satellite images of waves, to determine the composition of sea coasts in parts of the world where an on-site survey would be hazardous, if not impossible.
Lately, Dalrymple has been thinking about how waves break, and about surf zone turbulence, neither of which is well understood. People who work on off-shore oil rigs, or have to worry about the next hurricane or the migration of a resort's precious beach, would benefit from better understanding of both. He has a new model that employs smooth particle hydrodynamics. The basic idea is to consider a wave as if it were composed of 100,000 tennis balls, each ball a wave particle programmed to behave according to the laws of physics. On his computer, Dalrymple sets in motion an animation of a breaking wave, with the particles — the tennis balls — represented as a slew of tumbling red dots. As the wave breaks, a portion of the dots forms a sort of jet that bounces off the water's surface. Dalrymple points this out, then adds the puzzler — water doesn't bounce. Point a hose at a wall and yes, the water will splash, but it doesn't bounce back at you. Yet here are a bunch of red dots on his computer screen, bouncing like . . . like tennis balls.
And the model is right — he has found the corresponding phenomenon in actual waves. "You make the model, and it predicts what you want," Dalrymple says. "Then all of a sudden something new happens. You can observe that and forget about it, or you can observe that and ask, Why does it do that? And that leads you into a whole new area."
Rip currents, those powerful seaward currents that can unexpectedly pull swimmers away from the beach, have long fascinated him, and he's been studying them, too. He says, "During a study in the laboratory, my then-student Merrick Haller and I noticed that the rip currents flowing seaward from the beach were waving back and forth, like a flag in the breeze." Previous, simpler models had predicted that the currents would be steady and fixed in place. Dalrymple and Haller, who is now an assistant professor at Oregon State, rechecked their work. "Rather than being a mistake in the laboratory procedure, we found something cool: Rip currents can be unsteady. By using more sophisticated numerical models, we were able to predict this unexpected behavior."
Though he does the bulk of his work in his faculty office and on the university's powerful parallel computers, Dalrymple does occasionally go to the beach and watch the surf. "It's kind of humbling," he says. Does he look at waves and see equations? "Not equations. I see the fact that I can model it." He pauses. "But then I see something really bizarre, like a wave bouncing off the beach and heading back to sea, and it smacks into another wave and there's huge spray and I think, Ooooo . . . I can't model that."
Dale Keiger is a senior writer at Johns Hopkins Magazine.
