Tsunami mapped with laser scanners

Ashley N. Paddock, ashley.paddock@photonics.com

Using eyewitness video and terrestrial lasers to map the epic March 2011 Tohoku tsunami could produce flooding forecasts that influence future evacuation plans and building designs and could help prevent disasters of similar magnitude from taking such a huge toll.

Researchers from Georgia Institute of Technology used the equipment from atop the tallest buildings that survived the disaster to map the tsunami’s height and flood zone and to better understand the flow of its currents.

“The ultimate goal is to save lives,” said associate professor Hermann Fritz. “In order to do so, we have to have a better understanding of what worked and didn’t work.”

Professor Hermann Fritz used terrestrial laser scanners to map the height of the March 2011 Tohoku tsunami and learn more about the flow of its destructive currents. His team determined that the tsunami reached a maximum height of 9 m, followed by outflow currents of 11 m/s less than 10 minutes later, a speed that Fritz said is impossible to survive or navigate by vessel. Courtesy of Georgia Tech.

The tsunami was Japan’s deadliest in more than 100 years. Although the country took extraordinary measures to prepare for it, the disaster caused more than 90 percent of the almost 20,000 fatalities that occurred last March.

The researchers surveyed the impact of the tsunami on a fishing town in Kesennuma Bay, where 1500 people died. The bay had been hit by historic tsunamis in 1896, 1933, 1960 and 2010, making it the most vulnerable spot in Japan. Its coastal structures and other mitigation measures were designed based on conservative, historic high-water marks rather than on probable maximum tsunamis.

Fritz’s reconnaissance team used lasers to scan the port and bay entrance, creating a 3-D topographic model of the flood zone. The group determined that the tsunami reached a maximum height of 9 m, followed by outflow currents of 11 m/s less than 10 minutes later, a speed impossible to survive or navigate by vessels, Fritz said.

“What we can learn from the hydrograph is confirmation that the water goes out first, drawing down to more than negative three meters on the landward side of the trench, which can make vessels hit ground inside harbors,” Fritz said. “During the subsequent arrival of the main tsunami wave, the water rushing back in changed the water level by 40 feet, engulfing the entire city in 12 minutes.”

Understanding the impact of tsunamis will help prepare for future disasters – whether it is designing seawalls and breakwaters strong enough to control the flow of water or erecting buildings high enough to serve as vertical evacuation points.

Besides these mitigation measures, Fritz emphasizes tsunami education.

“People need to be tsunami-aware,” he said.

Fritz worked with teams of researchers from the University of Southern California and the University of Tokyo, the Tokyo University of Marine Science and Technology, and the Port and Airport Research Institute, in coordination with the UNESCO-organized International Tsunami Survey Team and Tohoku University in Sendai.

For more information on the effect of the tsunami and earthquake on the photonics industry, see “Optics industry on steady ground after quake,” on p. 38 of the March 2012 issue of Photonics Spectra.