More hurricanes are predicted to reach Hawaii

Hawaii will be under increasing risk of flooding and other damaging storm impacts as climate change shifts the historical pattern of hurricane tracks and raises the ocean level to make the islands more vulnerable, according to a study led by researchers at the University of Hawaii at Manoa.

Researchers at the university’s School of Ocean and Earth Science and Technology studied the probability of hurricane impacts in the last two decades of this century on urban Honolulu.

While the study didn’t find a significant increase in the number of cyclones, the data did show that hurricanes that historically pass to the south of the islands will instead approach closer to Hawaii with more landfalls.

A team of researchers, led by UH professor Kwok Fai Cheung and lead author Ning Li of UH’s Department of Ocean and Resources Engineering, mapped future hurricane flooding from storm surge and waves on top of the projected sea level rise.

The maps show substantial hurricane flooding across large sections of Oahu’s South Shore, including parts of Waikiki, Kakaako, Honolulu Harbor and downtown, and Daniel K. Inouye Inter­national Airport.

Particularly vulnerable is the Ala Wai Canal, where the Army Corps of Engineers is proposing to build a 4-foot-tall wall along one or both sides of nearly two miles of the Waikiki waterway, among other anti-flooding measures.

“Most of the flooding in Waikiki does not come directly from the ocean, but via the Ala Wai Canal,” Cheung said.

One of the study’s authors is Honolulu structural engineer Gary Chock, who is a consultant to the City and County of Honolulu in its ongoing effort to update its Honolulu Hazard Pre-Disaster Mitigation Plan while factoring in the future effects of climate change.

Chock said the study’s inundation maps have significant implications for engineering practice and land-use planning and will help Honolulu adapt to a growing risk of storm flooding.

“Our objective was to come up with a flood hazard map that can be used until the end of the 21st century,” Cheung said. “When you design a building now, the building will probably stay there for 100 years. So you need to know what the building will experience. I won’t live that long but the building will, so we want to account for all foreseeable scenarios with the useful life.”

To estimate future vulnerability, the research team gathered information from computer models estimating future weather conditions under the influence of climate change, along with data about hurricane formation and intensity, storm surge, waves, sea level rise and tides.

Historically, tropical cyclones traveling from the Eastern Pacific to the Central Pacific weaken and move to the south when approaching the Hawaiian Islands due to a high- pressure system to the northeast, strong wind shear and relatively cool waters.

But this study indicates those conditions are unlikely to be prevalent toward the end of the century.

“Nowadays they all seem to be moving closer to the islands,” said Cheung, who has lived in Hawaii for 25 years. “In the last few years, we’ve had several major storm events approaching the Hawaiian Islands from the east, so we already feel a change in the pattern.”

Cheung said Hawaii may have gotten a glimpse of the future in 2015, when powerful El Nino conditions warmed ocean waters around the islands and helped to fuel 15 cyclones in the Central Pacific — roughly triple the basin’s average.

The study joins a growing body of scientific evidence suggesting that climate change will bring greater impacts from hurricanes in the coming decades.

Five years ago a study by UH meteorologists also found that more tropical cyclones will threaten the islands toward the end of the century. Other studies have shown that hurricanes are increasingly tracking toward higher latitudes.

Li said that from nearly 2,500 future scenarios in the latest study, the team selected 24 major storm events that track near the islands to assess the probability of coastal flooding and create detailed flood maps with 100-, 200- and 500-year return periods.

The 500-year flood zone extends more than a mile inland and covers the most heavily developed and populated area of Hawaii, according to the study, and the inundation far exceeds the flooding observed on southern Kauai from landfall of powerful Hurricane Iniki in 1992.

“A flood event of this magnitude along with hurricane wind damage will paralyze the local economy with a long recovery time,” the study says.

The 200- and 100-year flood maps show significant flooding along the coastal area as well.

Appearing in the journal Coastal Engineering earlier this year, the research was motivated by a pilot study sponsored by the state Office of Planning and the Department of Business, Economic Development and Tourism. Other authors include UH researchers Yoshiki Yamazaki and Volker Roeber.

Cheung said his team is hoping to map future flooding in other populated coastal communities in Hawaii.

To see the study, go to 808ne.ws/climatestudy.