Fronting the Halekulani Hotel in Waikiki, a narrow concrete boardwalk stretches across what was once a sandy shoreline at Kawehewehe — one of four ancient Hawaiian healing sites in the area. Known as the Halekulani Boardwalk, the path connects beachgoers between the Halekulani and Outrigger hotels.

At first glance it appears walkable, drawing tourists and residents hoping to cross the shallow channel. But the walkway quickly becomes slick and uneven. A warning sign is posted nearby, but there are no railings or physical barriers. Rick Egged, president of the Waikiki Improvement Association and the Waikiki Beach Special Improvement District Association, said there have been several instances where beachgoers — mistaking the walkway for a safe passage — have slipped and fallen.

Dolan Eversole, the University of Hawaii at Manoa’s coastal processes specialist at the School of Ocean and Earth Science and Technology, said that over the past six years the beach adjacent to the walkway has lost approximately five feet of sand and the shoreline has receded about 30 feet.

The city Department of Parks and Recreation previously had installed fences to block the beach access path between
the Halekulani and Outrigger hotels, but Eversole said those fences have since
disappeared — taken out by erosion as well.

Egged said the state Department of Land and Natural Resources recently issued the Halekulani a permit to make improvements to the walkway and railing. He said DLNR is working on getting a contractor to fix the broken slabs that are near the walkway area, and at some point will close the walkway for construction.

The deteriorating Waikiki walkway is just one example of why some Waikiki stakeholders are saying immediate action is needed to keep one of Hawaii’s most iconic coastlines from steadily slipping away. From tidal back-flow in storm drains to collapsing walkways, Waikiki’s shoreline faces a multifaceted crisis: chronic erosion, sea level rise, land subsidence (sinking) and pressure from infrastructure built along an unstable coast.

The urgency to address the sinking is greater in some parts of Hawaii like Waikiki, which is sinking faster than other places. That discovery, published recently in a study by researchers at UH Manoa, highlights that as sea level rises, low-lying areas in Hawaii are at risk of flooding sooner than scientists anticipated. Certain parts of urban Oahu are sinking up to 25 millimeters per year — nearly 40 times the average rate. This dramatically shortens the timeline for flood risk in low-lying coastal communities, and will cost cost tens of millions of dollars to fix.

Eversole emphasizes that Waikiki’s erosion is not solely due to climate change or sinking land. Much of it stems from an engineered coastline built atop terrain that never naturally supported beaches.

“We’re forcing beaches into areas that wouldn’t have had beaches that large, if any beach at all,” Eversole said. “So they’re inherently unstable and tend to erode. That requires regular maintenance to maintain a beach.”

Waikiki’s beach is almost entirely artificial. Historically, the area consisted of mudflats and rocky outcrops. Over the past
century, sand has been repeatedly imported from neighbor islands or offshore deposits to construct and replenish the beach. But that sand doesn’t stay in place.

Eversole said, “With the exception of a couple small places, pretty much all of Waikiki is classified as chronically eroding.”

On average, Waikiki’s shoreline recedes by about a foot per year, based on 60 to 70 years of U.S. Geological Survey data. Seasonal wave patterns and storms contribute to the erosion. Ironically, large south swells and Kona storms often add sand to the beach, while persistent tradewind swells are the main force behind erosion.

The crisis also extends
inland. Kyle Murray, a researcher and lead author of a UH Manoa study on land
subsidence, pointed to places like Mapunapuna, where storm drains bubble over during king tides (the highest high tide of the year).

SOEST researchers now are studying how to respond to the varying causes of the compounding flood issue such as beach loss, stormwater flooding and groundwater rise, all of which require different solutions.

“Subsidence is a major, yet often overlooked, factor in assessments of future flood exposure,” said Murray. “In rapidly subsiding areas, sea level rise impacts will be felt much sooner than previously estimated.”

Most Hawaiian Islands naturally subside at a slow rate due to tectonic movement. On Oahu it’s around 0.6 mm per year. But localized areas like Mapunapuna and Kahauiki Village are sinking much faster due to development practices.

“These areas were built with artificial fill — sediments, coral, even tires,” Murray said. “These materials are loose, full of voids, and compact over time.”

In Mapunapuna, subsidence could increase flood exposure by over 50% by 2050 — compressing preparation timelines by decades, Murray said. His team used nearly 20 years of satellite radar and a new high-­resolution elevation model to estimate how combined sinking and rising seas will worsen flooding.

“We’re seeing significant subsidence in Kahauiki Village,” Murray said. “It’s not dangerous in itself, but flood risk comes sooner.”

In Pearl Harbor, floodwaters are reaching farther inland nearly every other year. Subsidence also is an issue on Hawaii island, though it’s mostly tied to volcanic
activity.

Meanwhile, erosion continues to eat away at Oahu’s coastline. A separate UH study on extreme erosion on Oahu’s shores estimates that 81% of the island’s shoreline could experience erosion by 2100 — with 40% of that loss occurring by 2030.

With the inclusion of additional satellite data and seasonal modeling, the study indicated that erosion is expected to increase by nearly 44% above earlier
estimates.

The consequences are serious. Roads, homes, buried utilities and entire beaches are at risk. Scientists warn that armored areas like
Oahu’s north and east shores could face total beach loss. But experts say the studies will help build resilience by providing crucial data for long-term planning.

Chip Fletcher, director of the Coastal Research Collaborative and interim dean of SOEST, noted in the UH flooding study, “This work directly serves the people
of Hawaii by ensuring that local adaptation strategies are based on the best available science, ultimately helping to protect homes, businesses, and cultural areas.”

UH researchers plan to expand their models to other islands and integrate
erosion, flooding and groundwater data.

“These things are inevitable,” said Murray. “But it’s a slow process. That means we have time — if we start now.”