Tsunami May Have Carried Some Permanently to Sea
Dec. 30, 2004 — -- Some of the people who were swept out to sea by the giant tsunamis that crashed against the shorelines of Asia and East Africa may sadly remain at sea forever, in an ocean grave, scientists say.
The bulk of the force from a tsunami is released as the wave crashes into shore. This means most debris and people may have been carried further inland, say oceanographers. But a giant reverse force is also created as gravity pulls the waves back to the ocean. Like a rip tide on steroids, the rapid return flow of ocean water can suck people and debris out to sea at rapid speeds.
Depending on how far out people or debris are carried, they may be caught up by local coastal currents, which could eventually wash them ashore, or by broader ocean currents that dominate water flow further out at sea.
"These very strong reverse currents don't last long, but they can take things out to sea very fast," said Ricardo Matano, an oceanographer at Oregon State University. "So whatever was taken out to sea may remain there. The question is, how long?"
Lynne Talley, an oceanographer at Scripps Institution of Oceanography at the University of California in San Diego, tracks currents in the affected regions of the Indian Ocean and says one of the main active currents in the Bay of Bengal -- where much of the devastation occurred -- is now the East India coastal current, which flows south and west toward Africa's east coast.
"The currents are complicated in this area because of the monsoons that shift from summer to winter," she said. "The main question is whether debris escapes from the currents along the coastal interior."
While a current such as the East India coastal current could carry some debris for hundreds of miles, human victims may eventually sink, according to Philip Liu, an oceanographer and environmental engineer at Cornell University in Ithaca, N.Y.
Deadly Impact and Retreat
The death toll continued to climb as the waters that pulled many out to sea then deposited them on beaches along the coasts of Sri Lanka, India, Sumatra and Indonesia. Rescue workers expected the number of counted dead to rise as more bodies are recovered and disease spreads among survivors.
Despite the rising death toll, oceanographer Dan Cox of Oregon State University said most of the deadly waves' victims were likely killed as the waters churned up debris and large objects such as cars and homes and turned them into fast-moving projectiles.
"Once the wave overturns it becomes turbulent on land and it carries debris as it moves inland -- this creates the deadliest force," said Cox.
It is possible, however, that some were snarled by strong rip currents that carried them out to sea. Liu explains that the mechanical energy of a crashing wave goes through an up-and-down cycle that is exaggerated in force in the case of tsunamis.
"It hits the shore with a lot of energy and then slows down," Liu said. "Then the water retreats and gains momentum again because gravity pulls the wave back into the ocean. So some debris can be carried back to sea, but most is pushed further on shore."
Liu plans to travel to affected regions this week as part of an international team of scientists who will survey the damage wreaked by the deadly swells. In areas where there were no instruments to record data from the event, Liu and the other scientists will rely on signs including water marks on buildings, how far debris was carried, and information from local people about when the waves struck to try and track the path of the deadly tsunamis.
All of the data will then be pumped into mathematical models to refine and improve them for researchers working to predict future tsunami events.
"The more confidence we can gain in our models, the more likely we can provide people with better warning systems," he said. "Better models can help us prevent this kind of tragedy from happening again."