Diamond Head, the extinct volcano that has become synonymous with Wai­kiki Beach, isn’t going anywhere, but much of the sand that both locals and visitors enjoy has vanished.

"It was so much more sand here before, and now, as you can see, everything is almost gone," said Pia Wackerl, a Hawaii Pacific University student who moved to Oahu in August.

Over the past several months, severe erosion has caused a concrete slab near the water’s edge at Kuhio Beach to become exposed. The concrete is uneven with sharp edges, and several pieces of rusty rebar pose a potential hazard to beachgoers.

One of the nearby beachboy stands erected more than 100 feet of plastic chain to highlight the growing problem and keep people away.

"It’s kind of dangerous for the kids when they’re surfing in because if they can’t stop on the high tide, the wave will double up and they come right into this wall," said Willie Grace, who works as a surf instructor for Star Beachboys. "I’ve seen it happen a couple of times."

In June 2012 the state’s Department of Land and Natural Resources completed a beach replenishment project that added 24,000 cubic yards of offshore sand across 1,730 feet of Wai­kiki Beach. The $2.2 million project was funded by the state and Wai­kiki hotels, but a University of Hawaii study found one-fourth of the replenished sand was gone a year after its completion.

"I guess trial and error," Grace said of the project. "That’s a lot of money that they wasted, and this is Wai­kiki Beach. It should be covered with some of the nicest whiter sand."

Waikiki resident Tom Wentworth says the erosion that led to concrete being exposed at Kuhio Beach got progressively worse after a jetty was removed, something locals warned the state not to do.

"That’s why the sand stayed right there, because it just circled around, from this jetty to the little one," said Wentworth. "Now, without that little jetty, it’s just back out."

Beach erosion in Wai­kiki has been a problem for decades and has resulted in at least 10 sand replenishment projects since 1939. According to DLNR, 300,000 cubic yards of sand has been placed onto Wai­kiki Beach over the past 77 years.

DLNR spokeswoman Debo­rah Ward said the state is working with the city to study the recent erosion along Wai­kiki Beach and come up with possible solutions. Hono­lulu City Council members Ikaika Anderson, Stanley Chang and Ernie Martin have also proposed the creation of a nonprofit Wai­kiki improvement district under Bill 82, which would raise funds for restoration efforts.

The bill passed first reading in December and was referred to the Zoning and Planning Committee for further debate.