City adds ‘extreme tsunami’ zones

The discovery of a sinkhole at Poipu, Kauai, more than 100 yards from the shoreline and about 24 feet above sea level was all the evidence that geophysicist Rhett Butler needed to prove that Hawaii needs to prepare for an extreme tsunami like the one in 2011 in Japan.

"I looked it up and it was an epiphany," Butler recalled, citing research by colleague David Burney that showed shells and other marine deposits in the hole.

The new scientific and geological information suggests that at some point in the past 500 years, a massive 9.0 earthquake in the eastern Aleutian Trench may have generated a tsunami that far exceeded inundations that are known to have occurred during tsunami events through Hawaii’s recorded history. Such an extreme tsunami may occur once every 1,000 years.

The research has prompted Honolulu to add an "extreme tsunami" evacuation zone to its maps and change its evacuation plans, including for high-rises in Waikiki.

The maps represent "an unlikely worst-case scenario" and will not replace current, standard tsunami evacuation maps, Department of Emergency Management officials said. Instead, the maps "add a second evacuation zone for a worst-case, extreme, once-in-a-thousand-years tsunami event"that would complement the existing tsunami evacuation maps.

The draft maps will be circulated at public meetings on Oahu starting at 7 p.m. Monday at the Aina Haina Community Park meeting room. There will be 17 city-sponsored public outreach workshops.

"We are changing the ‘regular’ vertical evacuation to accommodate both regular and extreme events," said Peter Hirai, deputy director of the city Emergency Management Department.

Hirai said that in all cases, his department is recommending people evacuate up to the fourth floor or above in 10-story buildings in Waikiki.

"The building must be 10 stories tall in order to withstand the force of the waves at the ground levels," Hirai said.

The city had been recommending going to the third floor or above in a minimum six-story building constructed of reinforced concrete or structural steel. Hirai said that after the 2011 Japan tsunami, engineers determined the fourth floor would be more prudent.

Residents in crowded urban areas such as Waikiki have the option of relocating to higher ground outside the area or "vertical evacuation" to higher floors in buildings. In Waikiki, vertical evacuation is recommended because there would be insufficient time to evacuate everyone out of the area.

Hirai said the maps are not expected to have an impact on homeowner insurance rates. The new material "does not change flood zone designations" in flood insurance maps issued by the Federal Emergency Management Agency that are used by insurance companies.

"No one’s rates should change because the flood insurance rate maps won’t change,"Hirai said.

FEMA’s flood insurance rate maps are based on historical data, while the new extreme tsunami maps are based on hypothetical data drawn from computer models.

To see the maps, go to bit.ly/1u7dUzb or the department’s official site at www.honolulu.gov/dem.html.

"People should be aware of the change,"Hirai said. "Now we’ve got two separate zones. But they should keep in mind that the second zone is just for an extreme event, extremely unlikely, but we have to plan for it. We don’t want to get caught like Japan did," he said. The March 2011 earthquake and tsunami disaster that struck Japan killed more than 18,000 and displaced hundreds of thousands.

"We don’t want the same thing happening to us,"said Hirai. "So we’re planning for what we call our unlikely worst-case scenario."

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