40% of Oʻahu beaches could be lost by mid-century

Shoreline hardening protects a road on Oʻahu and has led to beach loss. Credit: K Tavares

Residential areas experience beach loss due to erosion and shoreline hardening. Credit: K Tavares.

Erosion at Rocky Point, Oahu is visible in this drone image. Credit: Shellie Habel.

Link to video and sound (details below): https://bit.ly/2RDdanO

The reactive and piecemeal approach historically used to manage beaches in Hawaiʻi has failed to protect them, according to a new study by researchers in the Coastal Geology Group at the University of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology (SOEST). The study also found that if policies are not changed, as much as 40 percent of all beaches on Oʻahu could be lost before mid-century.

In an era of rising sea-level, beaches need to migrate landward, otherwise they drown. Beach migration, also known as shoreline retreat, causes coastal erosion of private and public beachfront property. Shoreline hardening, the construction of seawalls or revetments, interrupts natural beach migration—causing waves to erode the sand, accelerating coastal erosion on neighboring properties, and dooming a beach to drown in place as the ocean continues to rise.

The team of scientists, led by graduate researcher Kammie Tavares in SOEST’s Department of Earth Sciences, assessed the shoreline around Oʻahu that would be most vulnerable to erosion under three scenarios of sea-level rise—all estimated to occur around mid-century.

Shoreline hardening and beach loss

They identified the location and severity of risk of shoreline hardening and beach loss, and a potential timeline for the increase in erosion hazards. 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.

“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.

According to co-author Tiffany Anderson, assistant researcher in the Department of Earth Sciences, “We determined that almost 30 percent of all present-day sandy shoreline on Oʻahu is already hardened, with another 3.5 percent found to be so threatened that those areas qualify for an emergency permit today. Our modeling indicates that, as sea-level rises about 10 inches (0.25 meters) by mid-century, an additional nearly eight percent of sandy shoreline will be at risk of hardening—meaning at that point, nearly 40 percent of Oʻahu’s sandy beaches could be lost in favor of hardened shorelines.”

“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 Chip Fletcher, associate dean and professor in SOEST. “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.”

Transitioning beachfront landowners

Coastal erosion is inevitable when sea-level is rising and global mean sea-level has been rising for decades and is accelerating. The Intergovernmental Panel on Climate Change has projected continued sea-level rise for many centuries, even if greenhouse gas emissions are reduced or stopped altogether. However, economists are projecting that greenhouse gas emissions will likely continue into the mid-century and we will see more years like 2019, when the use of fossil fuels rose faster than the use of renewable forms of energy.

“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 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,” according to Fletcher. “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.”

“Beaches are critical ecosystems to native plants and animals, offer protection from storms, are an essential cultural setting, and attract tourists, who are important for Hawaiʻi’s current economy,” added Tavares. “This research shows that conversations on the future of our beaches and how we will care for them must happen now rather than later, if we are to protect our sandy beaches.”

VIDEO BROLL: (1 minute, 50 seconds)

• 0:00-1:04 - Shots of Waikīkī Beach erosion and shoreline hardening

• 1:04-1:28 - Shots of Haleʻiwa Beach erosion and shoreline hardening

• 1:28-1:50 - Drone footage of erosion and shoreline hardening on Rocky Point (Video credit: Dr. Shellie Habel, Sea Grant Extension and Coastal Geologist, DLNR)

SOUNDBITES:

Kammie Tavares, Department of Earth Sciences, Graduate Researcher (9 seconds) 

“One of the most striking things that we found was that 40 percent of our beaches could be threatened by beach loss if current management practices continue.”

Tavares (11 seconds) 

“We developed a timeline of locations and different development types of areas that would be most vulnerable to erosion, shoreline hardening and thus beach loss as sea-level rises.”

Tavares (12 seconds)

“It’s important for us to plan for mid-century, which is a lot sooner, to plan for those near term sea-level rise scenarios because that’s when we’re going to see a rapid increase in pressure along our shoreline.” 

Link to video and sound (details below): https://bit.ly/2RDdanO