Nearly 60% of Hawaii’s roads and over 75% of its bridges are threatened by possible landslides, coastal erosion, sea level rise, storm surges, tsunamis, wildfires or other natural hazards, according to a state Department of Transportation report that identifies vulnerable areas along state highways and recommends actions to make them more resilient to climate-related effects.

The newly released Hawai‘i Highways Climate Adaptation Action Plan uses historical data and climate projections from other agencies to examine potential risks to 917 miles of state-owned roads on six islands, including 397 bridges, 73 culverts and six tunnels.

The action plan is a “first step to act comprehensively across the agency in recognizing and considering more fully these changing climatic conditions,” DOT Deputy Director for Highways Ed Sniffen said in his message accompanying the report.

He added that the state agency “needs to be more resilient, adaptive, and engaged in climate change risks to our highway network,” which is vital to community access, economic well-being, national security and overall mobility.

“These changing conditions have important implications to Hawai‘i — inundated shorelines, coastal erosion, floods from rainfall, a higher water table, more saturated soils and greater risks of landslides, and increased wildfires,” Sniffen said.

“Given our understanding of what is changing, we need to make some tough decisions to ensure the long-term viability of the State. It no longer makes sense to continue the same approaches we have been using to maintain and improve the State Highway Network, which assume historic environmental conditions will remain unchanged.”

Due to Hawaii’s topography, highways typically circle each island in low-lying coastal areas, while steep mountain ranges limit the number of cross-island routes. This makes the highway network especially vulnerable to natural hazards and offers limited alternatives when access is cut off in certain areas, the report said.

Although not climate- related, lava flows were included as a potential hazard.

According to the action plan, 564 miles — or 58% of total miles of road under DOT jurisdiction — are vulnerable to potential climate- change stresses or lava flows. Additionally, 303 bridges, or 76%, were similarly exposed, as were 48 culverts, or 66%, and all six of the assessed tunnels.

Tsunamis were the biggest hazard, with the potential to inundate approxi­- mately 178 miles of roadway across the islands, or 18% of the total, and 135 bridges, or 34% of the total.

Rockfalls and landslides are other major concerns, threatening 168 miles of roads, or 17%; 126 bridges, or 32%; 11 culverts, 15%; and all six tunnels.

The DOT Highways Division acknowledges in the report that recent extreme weather events have resulted in significant damage and disruption that caught the agency unaware, “partly because the impacts to highways were much greater than had been expected when the road was designed.”

Recent major events include heavy rainfall in April 2018 that severely damaged Kuhio Highway on Kauai’s north shore, isolating residents of Wainiha and Haena and restricting traffic for about a year while the road was reconstructed and stabilized.

And emergency repairs are ongoing for a section of the same highway north of Hanalei Hill leading to Hanalei Bridge that was buried in early March by a landslide.

Other vulnerable areas on Kauai include Kaumualii Highway in Kekaha and Waimea, and Kuhio Highway over the Wailua River and through Kapaa, according to the action plan.

On Oahu a large rockslide in February 2019 on the Windward side of Pali Highway completely or partially closed the highway for 10 months while crews removed loose material from the slopes, put up fencing and netting, and created a structure over one of the tunnel entrances where the rockfall occurred.

Natural hazards also threaten portions of Farrington Highway on the Waia­­nae Coast; Kameha­meha Highway on the North Shore and Windward shore from Kualoa to Kahuku; Kalanianaole Highway in Hawaii Kai and Waimanalo; Sand Island; and Ala Moana Boulevard.

Vulnerable areas in Maui County include portions of Honoapiilani Highway in West Maui from Papalaua to Lahaina; North Kihei Road; roads surrounding Kahului Harbor; and portions of Kamehameha V Highway on the south coast of Molokai.

On Hawaii island, storm surge and tsunamis threaten roads along Hilo Bay and Kawaihae Harbor, the action plan noted, and lava flows are potential threats to portions of Mamalahoa Highway and Hawaii Belt Road through Hilo, Puna and Volcano to Kailua-Kona, and Queen Kaahumanu Highway and Mamalahoa Highway mauka of Waikoloa Village.

The action plan mentions several steps already taken by DOT toward highway resiliency, including integrating potential climate hazards into its Transportation Asset Management Plan, installing an adaptive design framework in the Waipa and Waikoko bridge repair project on Kauai to withstand tsunamis, and preparation of a Statewide Coastal Highway Program Report in 2019.

Recommendations for further action include partnering with the University of Hawaii and other local and national organizations to expand data and research to help state officials prioritize climate adaptation measures.

The action plan also calls for adjusting internal DOT policies and practices “to ensure that all decisions are mindful of expected future climate change conditions”; creating an environmental office at the branch level or higher with broad responsibility for DOT’s environmental, climate adaptation and system resilience efforts; and hiring more staff and establishing a funding program for highway system resilience.