The Japanese and American engineers stood atop a tsunami “refuge hill” near Sendai, Japan, and looked across an endless, muddy landscape of structures wrecked by the March 11 tsunami. One of the investigators stretched a long measuring pole into a surviving tree. Its branches apparently had been broken by a massive stone monument—commemorating a previous tsunami of 77 years ago—that was launched from its hilltop pedestal by the latest tsunami. “Three meters,” he said, indicating how much higher the water must have been than the hill. “About 10 feet.”
The 9.0-magnitude quake and tsunami that devastated northeast Honshu Island left 14,775 people dead and 10,706 missing. However, it also left an unprecedented wealth of data about how structures and societies respond to the loads of extreme earthquakes and tsunamis. To mine that data, the American Society of Civil Engineers is sending a series of seven expert reconnaissance teams to join their counterparts in the Japan Society of Civil Engineers to gather evidence.
The leader of the first team, Gary Chock, president of Honolulu-based engineer Martin & Chock Inc., says the time-sensitive opportunity to gather data and validate assumptions about tsunami loading is “unparalleled, absolutely, by far.” He adds, “It's extraordinary—the level of detail, the amount of photography, the availability of data. It would be impossible to ignore the lessons that come from here and that would apply in the U.S. and Canada.”
It is now understood by people in coastal Japan and increasingly by residents of Hawaii and the northwestern coast of the U.S. and Canada that tsunamis are not abstract threats. Devastating tsunamis have hit those coasts before and have the very real potential to strike again.
The first two ASCE teams will focus on tsunami loads on structures and coastal engineering. They started in mid-April. ENR met them in the field to observe. Follow-on teams will look at other earthquake consequences, including the effects on ports, harbors, industrial and nuclear facilities, and critical infrastructure, such as transportation, communications and power distribution.
The first ASCE team started at the northern end of the damage zone on the Tohoku Coast of Japan in Iwate Prefecture and worked its way south to Sendai in Myagi Prefecture.
The timing was extraordinary. In January, a number of engineers with tsunami reconnaissance experience proposed to the ASCE that the next revision of the standard reference “ASCE 7 Minimum Design Loads for Buildings and Other Structures” include a tsunami chapter.
“The ASCE flood loads exclude tsunamis,” explains Chock. “There was a gap in information, so a number of us proposed the tsunami loads committee. It was approved in mid-February,” he says.
“The timing was almost freaky,” says Ian Robertson, a civil engineering professor at the University of Hawaii at Manao. He is a veteran of tsunami fieldwork and one of the committee proposers. “We had just formed the committee, and it wasn't an easy sale. ‘Why would you need that?' we were asked.”
Then the tsunami struck.
“As far as destruction, this is 10 times the losses in Chile—$300 billion or more,” says Robertson, who with Chock led the team that studied the tsunami there in 2010. “Chile is $30 billion to $40 billion [in damage], and a lot of that was seismic. Here, seismic, not that much. The majority of the damage was [tsunami-related].”
Tsunamis are very different from hurricane storm surge, Chock and Robertson explain. Rather than a storm surge's bulge on the ocean topped by wind waves, tsunamis are a set of deep, fast-moving, long-frequency waves generated by an abrupt change of seabed elevation. Their velocity and dimensions change as they begin to drag the bottom and slow down. Height increases as speed decreases. The wave breaks as it nears shore, sending a turbulent bore racing 10 meters per second or faster across the land. The velocity, momentum and run-up elevation varies depending on topography, and the water's retreat may be even more violent than the initial assault.
Including a number of U.S. and Japanese investigators, the field teams vary as individuals cycle in and out. However, the methodology is the same: Analyze video and satellite imagery of the damage area, then search for specific structures and reference points to validate estimates of wave height, velocity, land momentum and the impact of water and debris.
In Natori, just south of Sendai, a broad coastal plain let the bore smash through many kilometers of coastal communities, blowing out rear walls of reinforced concrete structures and obliterating almost all buildings framed in wood. Using videos and satellite photos, the team spotted several concrete structures that were apparently damaged but remained standing. The team visited those sites to collect concrete and rebar samples, measuring deformations so they could model them accurately and calculate the loads that caused that level of damage. “There will be a lot of hypotheses on the ground. But if we take enough hard data, we can test those hypotheses,” explains Chock.
At the container port of Sendai, one goal was to examine strike damage to structures from wave-borne shipping containers, the main cause of incapacitation of port facilities in extreme flood events. “We hope to validate modeling that says the load inside the container is not a significant factor,” Chock explained. Hypothetically, looseness in the load creates a secondary impact, but it does not significantly amplify the initial strike, he says.
Targets of opportunity were found as well. They consisted of any structure that could be measured and modeled which had been deformed, but not destroyed, by the rush of water or the impact of identifiable debris. A cargo truck stuffed into the bent frame of a steel building or a line of metal flagpoles folded down against the ground by the press of rushing water became objects for analysis.
“Engineers are skeptical of things based on theories,” Chock says. “They like to see practical demonstrations. If we can validate at full scale, that would be a good thing, because we can't do full-scale tests on any of this.”