As much as 40% of Oahu’s beaches could disappear over the next 30 years if coastal management policies are not changed to better protect sandy shorelines retreating under the pressure of rising sea levels, a new study found.

The study, published in the journal Scientific Reports, is the latest work by the researchers of the Coastal Geology Group at the University of Hawaii- Manoa School of Ocean and Earth Science and Technology.

The Coastal Geology Group, led by Chip Fletcher, professor and associate dean, has been documenting the detrimental effects of sea walls, revetments and other forms of coastal hardening on Hawaii’s beaches for years.

In 2018 a study by the research group showed how sea walls work not only to diminish the beach in front of the structure, but accelerate erosion on neighboring properties, often leading to further shoreline hardening.

Now the researchers have found that some 25 miles of Oahu’s beaches could be lost as property owners seek to protect their oceanfront land under the pressure of escalating sea levels, estimated to rise by nearly 10 inches by midcentury.

The team of scientists, led by graduate student Kammie Tavares, assessed the shoreline around Oahu that would be most vulnerable to hardening and beach loss under three scenarios of sea level rise.

All of the scenarios pointed to the worst of the beach loss occurring before or just around midcentury.

“Hardening the shoreline prevents property erosion and protects back-shore development from erosional hazards,” the study says. “However, it causes ‘coastal squeeze’ or beach narrowing and eventual loss on chronically retreating shorelines, an inevitability with sea level rise.”

In Hawaii, hardening can be triggered when evidence of erosion is within 20 feet of certain structures, allowing an applicant to request emergency protection.

Using computer modeling to predict the beach migration caused by the 10 inches of sea level rise, the researchers found that emergency permit applications for sea walls and other structures designed to protect beachfront property likely will substantially increase.

According to the study, almost 30% of all present-day sandy shorelines on Oahu already have hardening, with hot spots on the Windward coast near Punaluu and shores on the southeast side of the island, including Waikiki, which survives with help from costly sand replenishment projects.

Another 3.5% of sandy beaches were found to be so threatened that those areas qualify for emergency permits today.

Additionally, by midcentury another 8% of sandy shoreline will be at risk of hardening to protect homes, roads and beach parks all across the island, according to the paper.

“At that point, nearly 40 percent of Oahu’s sandy beaches could be lost in favor of hardened shorelines,” said co-author Tiffany Anderson, assistant researcher in the Department of Earth Sciences.

This study, like the 2018 paper by the same research group, puts much of the blame on shoreline management, calling it a largely reactive, parcel-by-parcel endeavor that diminishes the role of proactive decision-making.

“It is clear that management decisions made today, and during the careers of most of today’s natural resource managers, will be critical in determining if future generations will inherit a healthy shoreline, or one that has been ruined by seawalls, and other types of shoreline hardening,” Fletcher said in a statement.

The 2018 study pointed out that Congress enacted the U.S. Coastal Zone Management Act in 1972, in part, to preserve and protect the nation’s coastal zone. The Hawaii Coastal Zone Management Program was established five years later as the local entity charged with overseeing the federal law through state and county agencies.

Hawaii has received millions of dollars in support of those goals but has been granting “hardship variances” that allow oceanfront property owners threatened by erosion to build sea walls and other forms of shoreline armoring to protect their property.

According to the study, the construction of initial sea walls destabilizes neighboring shorelines and triggers a domino effect of coastal hardening that condemns entire stretches of sandy beach to narrowing and eventual loss.

Fletcher said hardening continues to be the preferred policy choice of management agencies largely because they have failed to formulate any kind of assisted transition plans for beachfront landowners who are caught in “a tightening vice” because of accelerating sea level rise.

“In fact, directly to the contrary, beachfront lands continue to be sold to unwitting buyers with no appreciation for the expensive and frustrating situation they are entering into,” he said.

Fletcher urged people to lobby county and state policymakers to collaborate on providing ways to help beachfront parcel owners to transition out of their doomed locations to save the sandy shoreline.

“There are creative and not costly economic incentives that could be tailored to individual owners,” he said. “We need a government program to develop these ideas and work them into a sustainable long-term plan to get out of the way of our moving beaches.”

Tavares, the new paper’s lead author, started working on this project while she was a UH undergrad, and is now a geospatial analyst with the department.

Tavares said she grew up near the beach in Waianae but was unaware of Hawaii’s beach loss problem until she was in college. She said she became inspired to bring about awareness of the issue.

“A lot of people think sea level rise is something to plan for in the long term — like at the end of the century. But it’s going to happen a lot sooner than we might have originally thought,” she said.

Tavares acknowledged that saving all the beaches will be expensive and frankly unrealistic, but priorities should be set and something must be done to help stem a growing wave of beach loss sooner rather than later.

“The time to act is now,” she said.