On Oahu, well over half of sandy shorelines are now subject to chronic erosion, leaving stretches of coastal roadways vulnerable to crumbling and collapse.

Sizing up that problem and suggesting fixes is the State Coastal Highway Program Report, prepared for the Department of Transportation by University of Hawaii-Manoa’s College of Engineering. The report’s assessment of some 300 mileposts statewide ranks a stretch of Kamehameha Highway in Hauula as most in need of attention.

It’s getting plenty of that now. Last weekend DOT closed a portion of the highway’s makai lane in Hauula after a chunk of shoulder and lane fell into the ocean. Repairs to about 270 feet of undermined area started Sunday, with crews filling the weak spot with cobblestones — topped with cement and highway repaving.

Another 1,200 feet of repairs were added this week to guard against collapse that could be touched off by a swell or storm system. This sort of short-term “loose fill” repair — expected to last five to 10 years — is a huge red flag, signaling that it’s time for Hawaii residents and policymakers to launch plans for long-term fixes as more ocean-related hazards are expected.

Since 1960, sea level has risen between 2 inches and 8 inches relative to Hawaii’s shoreline. In beachfront places like Hauula, residents have watched strips of shoreline makai of coastal roads slowly disappear, with wave action on high surf days sending spray onto the roads. Climate scientists project at least 3 feet of sea level rise here by the year 2100.

Elsewhere on Kamehameha Highway, milepost areas in Kaaawa, Kualoa and Waimanalo are on the Coastal Highway Program report’s top 10 most-vulnerable list. After Hauula, in the No. 2 slot is a section of Maui’s Honoapiilani Highway near Olowalu.

The report’s remediation suggestions — roadway hardening, elevation and relocation — are accompanied by a warning that the cost of delaying improvements for the top 20 sites until severe damage develops may be 50 times higher than enacting preventive measures now.

Alongside a climbing price tag would be traffic interruption, safety concerns and business losses.

Clearly, to avoid such a fate, state lawmakers and communities across the islands should start pressing DOT officials and others now about how to prepare for the decades ahead.

California’s recent prepping for climate change has included moving inland — by up to 475 feet — nearly 3 miles of Highway 1 near San Luis Obispo. The realignment project, which cost nearly $20 million, aims to prevent encroaching ocean waters from causing major highway closures there over the next century. By the year 2100 California’s coastline in some places could rise by as much as 10 feet, according to projections.

Given Hawaii’s growing count of immediate funding needs — from repair of dilapidated public facilities to efforts alleviating homelessness — rallying support for big-ticket state spending on a problem materializing at a steady but seemingly slow pace is a daunting task.

Still, it would be foolhardy to postpone serious debate about options, including the pros and cons of erecting seawalls, along with subsequent policy- making and investing in long-term solutions.

Among the projections tied to dithering delays: With 3 feet of sea level rise on Oahu, 18 miles of coastal roads will become impassable and 13,000 residents will be displaced.

Hauula’s Kamehameha Highway predicament underscores the rising urgency to move forward with a statewide strategy to fortify, relocate — and, perhaps along some coastal areas, even surrender to Mother Nature. The challenge ahead must be met with broad public support and political will.