Tsunami Mapped with Laser Scanners

Using terrestrial lasers and eyewitness video to map the epic March 2011 Tohoku tsunami could influence future evacuation plans and building designs, and help prevent disasters of similar magnitude from taking such a huge toll, particularly in lives lost.

The Tohoku tsunami was Japan’s deadliest in more than 100 years. Although the country took extraordinary measures to prepare for the disaster, the tsunami caused more than 90 percent of the almost 20,000 fatalities that occurred last March. (See also: Optics industry on steady ground after quake)

To prevent such devastation from happening again, researchers at the Georgia Institute of Technology used eyewitness video and terrestrial laser scanners from atop the highest buildings that survived the tsunami to map its height and flood zone and to better understand the flow of its currents.

Hermann Fritz used terrestrial laser scanners to map the height of the tsunami and learn more about the flow of the destructive currents. Fritz’s team determined that the tsunami reached a maximum height of 9 m, followed by outflow currents of 11 m/s less than 10 min later, a speed that Fritz said is impossible to survive or navigate by vessels. (Image: Georgia Tech)

The mapping could produce flooding forecasts that help reduce lives lost and property damage, not only in Japan but in other areas of the world susceptible to tsunamis.

“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.”

The researchers surveyed the impact of the tsunami on a fishing town in Kesennuma Bay, where 1500 people died. The bay has 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 probable maximum tsunamis, Fritz said.

Hermann Fritz, associate professor in Georgia Tech’s School of Civil and Environmental Engineering, has traveled to Japan several times since last year’s historic earthquake and tsunami. He used eyewitness video and terrestrial laser scanners from atop buildings to map the tsunami’s height and flood zone to learn more about the flow of the devastating currents. Although the wave killed nearly 20,000 people, Fritz said Japan’s preparedness may have saved hundreds of thousands of additional lives.

His reconnaissance team used lasers to scan the port and bay entrance, creating a three-dimensional topographic model of the flood zone. They 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 3 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’s designing sea walls and breakwaters strong enough to control the flow of water or erecting buildings high enough to serve as vertical evacuation points. In addition to these mitigation measures, Fritz emphasizes tsunami education.

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

Fritz worked with 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 the Tohoku University in Sendai.

For more information, visit: www.gatech.edu