As sea level rise gets worse, will all of Hawaii’s vulnerable coastal highways be worth saving?

It’s a tough question, but one that Hawaii policymakers will have to grapple with now if they’re to sufficiently prepare the nation’s lone island state for climate change in the coming decades, according to the state’s top roads director.

“I don’t want to scare people, but I want to put out there that we need to look long term,” Ed Sniffen, the state’s deputy director for highways, said in an interview Tuesday.

In some vulnerable shoreline areas, such as Kaaawa, abandoning the two-lane coastal Kamehameha Highway altogether might make the most sense later this century if the local community is eventually displaced by the encroaching ocean, Sniffen said.

Climate scientists project at least 3 feet of sea level rise by the year 2100. Rising levels are already making the flooding on Hawaii’s coastal highways more severe, they say.

“If the road is gone, if the coast is gone,” then it might be more reasonable to provide ferry service or some other means of access to that Windward area, Sniffen added.

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Sniffen’s comments follow the release of a new strategic plan, prepared by the University of Hawaii Manoa’s College of Engineering for the DOT, that flags the state roads most urgently in need of protection against erosion and sea level rise.

The State Coastal Highway Program Report uses a new, detailed formula developed at UH to rank those nearshore roads in order of urgency.

It considers beach and coast conditions, tidal changes and wave heights, road conditions, existing buildings and seawalls, among other factors.

The report found Kamehameha Highway in Hauula, around the road’s 22nd mile marker, to be the most critical road to address.

Stretches of the same highway in Kaaawa, Kualoa and Waimanalo also made the top 10, using what researchers have dubbed “Coastal Erosion Susceptibility Index,” or CRESI. The higher the score, the more vulnerable the road is.

Statewide Threat

It’s not just Oahu. Second on the report’s list was a critical bend of Maui’s Honoapiilani Highway near Olowalu that connects communities such as Lahaina and Kaanapali to the rest of the island.

Stretches of highway on Molokai also made the top 10.

This interactive map simulates 2 feet of sea level rise using NOAA’s Sea Level Rise Viewer . Zoom in to see in greater detail how far the ocean is expected to encroach along the coastline of the Hawaiian islands. The map also includes the top 10 sections of road found to be in most urgent need of protection from sea level rise according to a new UH Manoa report.

For most of these areas, the report generally recommends either “hardening” the roads or relocating them all together.

What the report doesn’t consider is eliminating roadways if they’re no longer viable. Instead, it assumes everything is to remain and be serviced.

That’s where the difficult policy decisions among the state’s elected leaders come in, Sniffen said. So far, DOT has only discussed with the state’s Office of Planning the possibility of eventually eliminating some roads, he added.

Now that the UH report is out, DOT plans to discuss the prospect further with lawmakers and communities most threatened by sea level rise.

For the next two decades or so, the plan is to protect the road system that’s in place. For repairs and improvements beyond that, state leaders will have to consider what’s viable given the impacts of climate change, Sniffen said.

As an example, he brought up the much-needed repairs to the Waiahole Stream bridge on Oahu’s Windward side.

The bridge is listed in poor condition and ranked 28th out of some 741 state bridges for repairs. The state could consider relocating the bridge further inland as the seas encroach, but that would have a huge impact on nearby homes and cost around $200 million, according to Sniffen.

With sea level rise, “for me it makes the most sense” to strengthen the bridge and keep it where it is for the next 20 years, Sniffen said. “Those are the types of decisions we need to make based on the data.”

An Urgent Discussion That’s ‘Entirely Appropriate’

So far, the world’s observed warming has largely matched the scientific projections on climate change, according to Chip Fletcher, a UH climate scientist and associate dean at the School of Ocean and Earth Science and Technology.

What’s surprising climate scientists, however, is how rapidly the natural world is changing in response to that warming, he said.

As more warming and melting occurs in Antarctica, Greenland and mountain glaciers, those trends are likely to accelerate — and they’re all “going to be driving sea level rise well past 1 meter by the end of the century,” Fletcher said.

“To recognize this reality, which is fully accepted by the scientific community … and place it on the table for policy discussion is entirely appropriate,” Fletcher added Tuesday. “I really want to praise Ed Sniffen for having the courage to put this up for public discussion.”

The UH report doesn’t include a price tag, but Sniffen previously estimated it would cost around $15 billion to protect all of the state’s coastal highways from the rising seas. The figure assumed $7.5 million for every mile of road that will need to escape erosion in the next 50 to 100 years and $40 million for every mile of bridge.

The threatened stretches represent about 15% of all state roads.

The roughly $15 billion estimate can’t be revised, however, until policymakers decide whether it’ll be worth saving all the roads, he added this week.

In Kaaawa, residents have already seen local beaches disappear and chunks of Kamehameha Highway collapse into the ocean.

CRESI, significantly, can be applied to the conditions in other states — and that’s significant because it can help coastal states work together to secure federal funding, Sniffen said.

This week, California’s state transportation department, Caltrans, is expected to release a report similar to UH’s on how to maintain that state’s roadway system amid climate change.

Cory Lum/Civil Beat

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 last year.

An alternative solution there — relocating the highway further inland — would likely include tunneling through parts of the Koolau mountains, both Sniffen and UH engineering professor Host Brandes said last year. But such a plan would be tremendously costly.

“Just out of a sense of social equity, we need to begin discussing this problem with a great deal of seriousness. These are working class families that are already sitting in areas that flood dramatically when there’s a heavy rain storm, and sea level rise is already making that flooding more severe,” Fletcher said of the 5,300 or so residents that live in Hauula and Kaaawa, based on 2017 U.S. Census numbers.

“There needs to be a policy discussion on how to equitably treat this community so that they are not made homeless by this problem,” whether that eventually means eventually relocating them to escape the floods, or adapting the area to withstand more than a meter of sea level rise, he added.

This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.