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In 2016, heavy surf pounding Oahu’s Windward Coast tore off two large chunks of Kamehameha Highway near Kaaawa in a three-week period.

Residents had already watched their beach  but they’d never seen the waters claim part of the highway.

Crews rushed to repair the road — a scenic, vital passage that hugs the eastern edge of Hawaii’s most populous island. State transportation officials said the fixes would better protect the highway against erosion.

But they  it wasn’t a permanent solution.

In fact, Hawaii’s Department of Transportation faces huge, costly challenges if it’s to protect the state’s coastal highways from a rising ocean that is already taking its toll.

The agency has a grim estimate of how much that will cost: $15 billion.

The figure, provided by DOT’s deputy director for highways, Ed Sniffen, assumes the state will need $7.5 million for every mile of highway road that must either be raised, pushed back or relocated entirely to escape erosion and flooding in the next 50 to 100 years  — and $40 million for every mile of bridge.

It’s a baseline estimate, Sniffen said, as his agency and University of Hawaii researchers investigate how global climate change caused by human activity will cripple the low-lying highways that trace the nation’s only island state. They’re scheduled to release a plan in detailing what they can do about it.

About 15 percent of all state roads are vulnerable to the rising surf and tides, according to Sniffen, including critical commuter corridors on Oahu’s Leeward side, west Maui and Molokai.

“It’s something that’s coming, so we’ve got to address it,” he said.

The expense will likely limit the state to saving — and possibly rerouting — the roads it has instead of building entirely new ones, Sniffen said.

Under policies set by Gov. David Ige, the state spends most of its highway dollars preserving and maintaining roads. It often focuses on overdue and badly needed paving and reconstruction projects.

But if that policy continues, “preservation” will mean staving off erosion, Sniffen said.

Costs Spread Over Decades

The $15 billion dollar estimate to protect Hawaii’s low-lying state highways is nearly twice the latest, official $8.16 billion price tag to build Honolulu rail, which excludes that project’s financing costs.

However, rail officials are scrambling to finish the already delayed transit system in the next seven years. The billions in highway costs, on the other hand, would spread over the next several decades or longer.

Rep. Sylvia Luke, who chairs the House Finance Committee, said that makes those billions in highway costs less daunting. The state spends as much as $1.5 billion in capital projects during its two-year budget cycle, she said, so it could gradually absorb the highway spending over time.

“It is not an impossible fix,” Luke said Monday. That strategy, however, would likely force the state to forego other infrastructure spending to make room for the highway fixes, she said.

No one knows how severe sea level rise will be. The Hawaii Climate Change Mitigation and Adaptation Commission expects the island chain , largely based on data from the Intergovernmental Panel on Climate Change.

A more recent report from the National Oceanic and Atmospheric Administration indicates that ocean levels are rising even faster than the IPCC’s data suggests.

“Watching the scientific literature, things get worse and worse with every report that comes out,” said Horst Brandes, a UH professor of geotechnical engineering who’s researching how sea level rise will affect the highways for DOT.

In its to the Legislature last year, Hawaii’s interagency climate commission estimated that sea level rise could cost the state’s economy $19 billion.

That estimate doesn’t include the damage to roads and highways, according to officials with the state’s Department of Land and Natural Resources.

In its report, the Hawaii commission further noted that it may have to revise the 3-foot projection upward.

‘It’s Already Occurring’

Windward Oahu isn’t the only local spot coping with severe erosion, as many Hawaii residents can attest.

Property owners on the North Shore have seen their homes by the surf. Last year, emergency officials declared the erosion at Sunset Beach had reached “” Eventually, the bike path makai of the highway there collapsed.

In mid-February, the city cut down seven date palm trees lining the road amid concerns the worsening erosion might uproot them and cause them to fall, potentially harming passersby.

And in Waikiki, officials have spent millions of dollars to fight beach erosion at Hawaii’s largest tourist hub.

“It’s already occurring. You can see it pretty clearly,” Brandes said of the damage to Hawaii’s roads. “They’re already doing maintenance, but it’s going to get much worse in the years to come.”

Currently, DOT essentially performs triage on the highways that are already getting hit — fortifying the roads in Hauula, Kaaawa, Makaha and elsewhere with boulders grouted together and other temporary solutions.

But those fixes won’t hold forever, and DOT no longer builds walls in such spots because that amounts to “millions of dollars that we’re putting in an area that we’re not going to be in very soon,” Sniffen said.

The report due next year will lay out how his agency proposes to permanently protect each vulnerable road and, significantly, in what order of priority.

A road’s priority level will depend on several key factors, including how expensive the proposed fix is, how many nearby residents there are, how much traffic the highway gets, and how much economic activity hangs in the balance, Sniffen said.

“It’s all based on data,” Sniffen said. “I’m not going to go out there and do something because I like one community better than another. It’s not going to be like that.”

“I’m sure every resident along (Kamehameha Highway) thinks that’s the most important route for us to fix,” Sniffen said. “But so does west Maui. So does Makaha.”

Elevating the highway on Oahu’s Windward side would probably involve raising the road as high as 9 feet, but that would also be too steep of an incline for residents on the mauka side of the road to access it, Sniffen said.

Instead, the solution might be to relocate the highway farther inland — and to tunnel through parts of the Koolau mountain range, both Sniffen and Brandes said. But tunneling would be tremendously costly. Plus, officials say they’d also have to consider the environmental and cultural impacts, similar to when the H-3 freeway was carved through Oahu’s Halawa and Haiku valleys.

That controversial 16-mile freeway opened in 1997, taking nearly four decades to build and costing about .

“I don’t think we’ll ever have the funding necessary to say we can build everything we need. I don’t think we’ll ever be there,” Sniffen said. “So, it’ll get harder and harder to prioritize those portions of our roadways which we need to fix right away.”

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