Tsunami damage can be limited

July 31, 1998
Webposted at 11:30 AM EDT

By Environmental News Network staff

Tsunami destruction is caused both by water and debris picked up by the wave and hurled at buildings.

(ENN) -- It is possible to limit the destruction from the type of tsunami that slammed into the coast of Papua New Guinea on July 17, according to a professor of civil and environmental engineering at Cornell University.

By constructing sea walls and creating zoning policies that ban building in high-risk areas, Philip Liu believes that tsunami destruction can be limited. "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 says that the destruction on Papua New Guinea was due to both the flatness of the land -- basically lowland jungle -- as well as the flimsy nature of the buildings.

Liu recently returned from a conference in Japan where he inspected the coastal improvements made on Okushiri Island and southwest Hokkaido in the wake of a July 12, 1993, tsunami that killed 120 people and caused $600 billion in damage.

Improvements include 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.

Liu says that the Papua New Guinea tsunami was exceptionally strong due to the nature of the earthquake that spawned the enormous wave. With the epicenter of the 7.1 earthquake inland in northern New Guinea near the coast, its effect of on the ocean floor produced a tsunami of exceptional force, he said. "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 says the earthquake caused a "significant subsidence in the sea floor, which then rebounded like a drum, creating a huge hump of water."

"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."

Liu and his colleagues have created a computer simulation model of the massive ocean wave as it struck the coast of Okushiri Island to demonstrate the generation and effects of a tsunami..

The simulation clearly shows the surging movement of the wave as it rises quickly and rushes land ward. 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.

The tsunami destruction, Liu says, is caused both by the water and the debris that is picked up by the wave and hurled at buildings. "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," he said at the conference in Japan.

Copyright 1998, Environmental News Network, All Rights Reserved