Coast Management Can Limit Tsunami Effect

July 27, 1998

ITHACA, N.Y. -- A Cornell University engineer believes it is possible to limit the destruction from the type of tsunami that slammed into the coast of Papua New Guinea on July 17 with proper coastal management, such as building structures like sea walls, and creating zoning policies banning building in high-risk areas.

Philip Liu, professor of civil and environmental engineering, believes that the thousands of deaths and terrible destruction on the island by the 30-foot-high ocean wave was due both to the flatness of the land -- basically lowland jungle -- as well as the flimsy nature of the buildings. "However, it is questionable that in a country like Papua New Guinea such policies could be implemented," because of limited economic resources, he says.

Liu has just returned from a conference in Japan, whose partial aim was to study the recovery from the tsunami that struck Okushiri Island and southwest Hokkaido on July 12, 1993, killing 120 people and causing widespread destruction, estimated at $600 billion. The conference moved to Okushiri, Liu says, to inspect the $60 million in coastal improvements made by the Japanese government in the five years since the tsunami. These include building a sea wall of reinforced concrete, nearly 50 feet high, around areas of the island that received most of the damage from the ocean wave. In addition, the southern tip of the island, where the entire village of Aonae was washed away by the tsunami, is now a memorial park with no new building permitted.

The Cornell researcher, who has written extensively on the causes and effects of tsunamis, notes that the Okushiri tsunami was generated by a 7.8 magnitude earthquake with its epicenter on the sea floor very close to the island. "So the damage could have been caused first by the earthquake, then only minutes later by the tsunami," he says.

Liu speculates that much the same thing could have happened in Papua New Guinea, the only difference being that the epicenter of the 7.1 earthquake that generated the tsunami was located inland in northern New Guinea near the coast. The effect this had on the ocean floor produced a tsunami of exceptional force, he says. "This was not a common tsunami."

The greatest wave height reported, says Liu, was more than 30 feet, but sea floor displacement caused by the earthquake exaggerated the force of the wave. He explains that the earthquake caused a "significant subsidence in the sea floor, which then rebounded like a drum, creating a huge hump of water." He says: "This was a little unusual because usually after an earthquake you have strong positive vertical motions on the ocean side and land subsidence on the land side. This seems to have been reversed."

To demonstrate the generation and effects of a tsunami, Liu and his fellow Cornell researchers have created a computer simulation model of the massive ocean wave as it struck the coast of Okushiri Island. This model has been successfully used to simulate waves on a sloping beach, Liu reported to the tsunami conference in Japan.

The simulation clearly shows the surging movement of the wave as it rises quickly and rushes landwards. Liu explains that while the wave is traveling through the deep ocean water, the length of the wave and its small wave height from trough to crest has the effect of spreading its energy out over hundreds of miles. Thus the local effect, on a ship traveling on the surface for example, is hardly noticeable. But as the wave approaches shallower offshore water, its length becomes greatly compressed and the wave height, or amplitude, increases. The force of this energy, which researchers call the runup, comes crashing down on the coast.

Liu's research is aimed at calculating this interaction between a tsunami and individual buildings and other structures. The destruction, he says, is caused both by the water and the debris that is picked up by the wave and hurled at buildings. He told the conference in Japan, "In order to design the coastal structure properly and to reduce the damages which might be caused by a future tsunami, it is essential to be able to estimate the impact force acting on the structure."

For a Quicktime movie of Liu's computer simulation model of the tsunami that struck Okushiri Island and southwest Hokkaido, Japan, on July 12, 1993, go to http://www.news.cornell.edu/releases/July98/tsunamiVid.html. For a U.S. Geological Survey animation of the tsunami that struck Papua New Guinea on July 17, go to http://walrus.wr.usgs.gov/docs/tsunami/PNGhome.html.

-30-

Cornell University

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