Oahu could lose 40 percent of its beaches to coastal erosion

The measures taken to manage beaches in Hawaii will not sufficiently protect them from rising sea levels and coastal erosion. According to a new study from the University of Hawaii at Mānoa, up to 40 percent of Oahu’s beaches could be lost in the next three decades if policies are not changed. 

The study was led by graduate researcher Kammie Tavares from the Coastal Geology Group in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST).

“By assessing computer models of the beach migration caused by 9.8 inches (0.25 meters) of sea level rise, an amount with a high probability of occurring before mid-century, we found that emergency permit applications for shoreline hardening to protect beachfront property will substantially increase,” said Tavares.

As sea levels rise, beaches must migrate toward land so that they are not inundated. This beach migration, also known as shoreline retreat, causes coastal erosion of private and public beachfront property. 

When the shoreline is modified or “hardened” by barriers such as seawalls, this interferes with the beach’s natural ability to migrate. Coastal erosion is then accelerated on neighboring properties, and the beach will gradually drown under the rising sea. 

“In another study published in 2018, we showed that accelerated erosion on neighboring properties, called flanking, usually leads to additional shoreline hardening, and condemns entire beaches,” said co-author Dr. Chip Fletcher.

“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 inherent a healthy shoreline, or one that has been ruined by seawalls, and other types of shoreline hardening.”

The researchers set out to identify areas of Oahu’s shoreline that would be most vulnerable to erosion under three scenarios of sea level rise, all of which are predicted to occur before or shortly after mid-century.

For each location, the team assessed the risk of shoreline hardening and beach loss, and created a potential timeline for the increase in erosion. 

The most threatened properties fall into an “administrative erosion hazard zone,” an area likely to experience erosion hazards and qualify for the emergency permitting process to harden the shoreline.

“We determined that almost 30 percent of all present-day sandy shoreline on Oahu is already hardened, with another 3.5 percent found to be so threatened that those areas qualify for an emergency permit today,” said study co-author Dr. Tiffany Anderson.

“Our modeling indicates that, as sea level rises about 10 inches by mid-century, an additional nearly eight percent of sandy shoreline will be at risk of hardening – meaning at that point, nearly 40% of Oahu’s sandy beaches could be lost in favor of hardened shorelines.”

The global mean sea level has been rising for decades and is accelerating, which means that further coastal erosion is inevitable. The Intergovernmental Panel on Climate Change has projected continued sea level rise for many centuries, even if greenhouse gas emissions are completely eliminated. 

“Despite these facts, we continue to see shoreline hardening as the preferred policy choice, largely because management agencies have failed to develop assisted transition plans for beachfront landowners who are caught in a tightening vice because of accelerating sea level rise,” said Dr. Fletcher. “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.”

“Because coastal zone management laws continue to allow hardship variances in this era of rising sea level, despite widespread knowledge that seawalls kill beaches under these conditions, the same legal system designed to protect public trust lands, is responsible for destroying them.”

“Government agencies must develop creative and socially equitable programs to rescue beachfront owners and free the sandy ecosystem so that it can migrate landward as it must in an era of rising seas. It is urgent that options are developed soon for beachfront landowners and resource managers to avoid further destructive management decisions.”

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer