Seawalls may give false sense of security during a tsunami

If a coastal city's seawall is higher than a forecast tsunami, residents are less likely to evacuate promptly, suggests a new study based on interviews with survivors of the 2011 tsunami that hit eastern Japan.

The Great East Japan Earthquake struck about 40 miles off the country's northeast coast on March 11, 2011 with a magnitude of 9.0. It generated tsunami waves more than 30 meters tall(about 100 feet) that claimed nearly 16,000 lives and destroyed 122,000 buildings.

Since then, researchers have looked for ways to better communicate the importance of evacuations, tsunami warnings and preparedness.

"Seawalls have been a key tsunami countermeasure in Japan," Giancarlos Troncoso Parady of the University of Tokyo told Reuters Health by email. "However, while seawalls might delay the flooding of a coastal town and ideally provide residents with precious time to evacuate, they might also have the undesirable effect of delaying evacuation."

Troncoso Parady and colleagues analyzed data from a survey of 6,600 residents from 23 municipalities of the hardest hit Iwate and Miyagi prefectures.

Tsunami height forecasts were issued 3, 28, 44, and 90 minutes after the earthquake, with earlier forecasts underestimating the wave height, the research team writes in the journal Injury Prevention.

According to news reports, the first tsunami waves hit Sendai Airport in Miyagi Prefecture just over an hour after the earthquake.

The research team found that regardless of where survey participants where when the earthquake occurred, on average, 68 percent evacuated promptly. Among those who lived within one kilometer (0.6 miles) of the coast, 78 percent evacuated promptly.

However, if the municipality had a sea wall higher than the most recent wave height forecast, the odds of prompt evacuation fell by 30 percent, the researchers estimated.

Critics have debated seawall reconstruction, which often creates walls larger than the previous ones without environmental impact assessments. Seawalls are also associated with environmental disruption of coastal ecosystems and scenery destruction, Troncoso Parady said.

"Of course, this is a very politically complex issue, but I believe there is a discussion to be had on how high is high enough for a seawall," he said. "Building higher and higher seawalls is not the most adequate solution. We should be thinking about this problem from a more holistic perspective."

"More work is needed in disaster preparedness education and in the way tsunami warnings are given, taking into consideration the risk of forecast error," his team concludes. "Priority should be given to promoting prompt evacuation and educating residents as to the uncertainty of tsunami forecasting, to ensure that residents do not ignore evacuation warnings due to false impressions of the safety provided by seawalls."

SOURCE: Injury Prevention, online November 17, 2018.