OREGON STATE UNIVERSITY

Beaches Ravaged by Tsunami Still Eroding

01/18/2008

CORVALLIS, Ore. – The catastrophic damage of the 2004 Indian Ocean earthquake was mostly done within a few hours, but that was just the beginning of a different process that may take up to a decade or more to complete – the stabilization of new beaches and landforms in areas ravaged by this disaster.

In continued studies, researchers at Oregon State University and the U.S. Geological Survey are finding that the beaches may continue to shift and change for several more years, as the lands adjust both to the tsunami impacts and the sudden drop of some nearby land by three to six feet.

Meanwhile, some roads constructed in the push for recovery after the disaster already are being threatened by eroding beaches and lapping water. Houses have been rebuilt on stilts in areas that properly should now be considered ocean, not land. And beach experts are closely studying this process – not only to learn more, but to make sure that when the Pacific Northwest tries some day to recover from its own massive tsunami, the decisions will be informed by good science.

“In Banda Aceh, the city most severely hit by the tsunami, some people are using fill, raised roadbeds and stilts to build their homes in what is essentially an intertidal zone,” said Peter Ruggiero, an OSU assistant professor of geosciences. “It’s amazing the energy they are putting into this, but it is apparent that much of this land may just disappear. You can see palm trees in the ocean, on what used to be dry land. Right now some areas look like a little Venice.

“And with some regularity, when we were looking at a new highway in Indonesia that already needs rip-rap to prevent the ocean from claiming it, I was visualizing Highway 101 on the Oregon Coast,” Ruggiero said. “Our input may be too late to help the Indonesians in their recovery from this disaster, but hopefully we may learn a lot here that will some day help the U.S. recover from the tsunami in our future.”

There’s little realization, Ruggiero said, that after a tectonic and geologic event the magnitude of the one in Indonesia, the ocean will take years to adjust to a new equilibrium, one in which shorelines are largely stable and neither eroding or building. And predicting exactly what the ocean will give, and what it will take away, is a very new science – one that will get a major boost from what is now being learned during the Indonesian recovery. The research is being funded by the U.S. Geological Survey.

“Never before with modern scientific monitoring tools have we been able to so rigorously study a tsunami disaster such as this, literally on film from the moment it occurred to many years later,” Ruggiero said. “There have already been some surprises in places, where we’ve found the sand being moved and then re-distributed, and we’ll find out more as we go. But this process is nowhere near complete yet.”

On Dec. 26, 2004, a 9.2 magnitude earthquake triggered one of the deadliest natural disasters in modern world history, which included tsunami waves that reached up to 100 feet high. The waves scoured the ocean floor with their enormous energy and deposited huge amounts of sand and debris some distances inland, while other material was stripped away and washed out to sea. More than 225,000 people died in 11 countries.

“We have been able to study both the ocean floor and the inland effects, and are monitoring changes through time,” Ruggiero said. “We’ve discovered sandbars created by the tsunami in some shallow ocean waters that otherwise would not be there, and they appear to be gradually rebuilding some of the beaches.”

But receiving less attention at the time of the disaster, Ruggiero said, was the subsidence of some nearby areas by three to six feet – a result of land that had been “pushed up” by a subduction zone for centuries, only to drop back down suddenly during the earthquake. This resulted in an instant change of sea level over broad areas of coastal Indonesia that is still working itself out.

“As a result of the coastal subsidence, we’re still seeing beaches in retreat, which may continue for some time,” Ruggiero said. “Spits and inlets will form. We’re going to try to understand the forces at work and make predictions about where the ultimate shoreline will be, then come back in later years and see if we were right or not. We should learn a lot from this process.”

That knowledge, he said, may help scientists not only to better understand past tsunamis, but also to assist in recovery from those yet to occur. And one of the prime candidates for such an event is the Pacific Northwest coast of the United States and Canada, where the Cascadia Subduction Zone is nearly a geologic twin to its Indonesian counterpart. It’s believed that this zone has had several subduction zone earthquakes in the past 1,000 years, the last of which may have occurred in 1700.

“We’re already learning, just from what we’ve observed in Indonesia, that you must be very cautious what setbacks to allow for new construction and rebuilding after a major tsunami or land subsidence,” Ruggiero said. “Hopefully we’ll be able to develop computer models that will allow us to predict the final shape of the shorelines with more accuracy. And those shores may be dramatically different that the ones we now have.”