KAILUA — Erosion at Kailua Beach has reached startling proportions, revealing a 60-foot-long sand trap installed 30 years ago at its southernmost end and buried until recently, and raising concern that the problem could work its way north along the entire beach if something isn't done.

The shore at the end of Kailua Bay, where a boat ramp juts out into the sea, is gone. In its place is a sheer dropoff to a narrow beach lined with a "sand grabber" placed there in the 1970s with the intention of catching and holding sand in place.

It was government's response to the problem and it seemed to work, until recently.

The boat ramp is also taking a hit where water is undermining its left edge, exposing its boulder foundation and causing parts of the asphalt surface to collapse.

About 100 feet of sandy shore from the dropoff out to sea is no longer there. For hundreds of feet along the shore, the roots of ironwood trees are exposed and several ironwood trees have had to be removed for safety reasons.

People thought the sand grabber was working but there was always seasonal movement of the sand, said Raul Fallau, who has lived in Kailua for 50 years and seen it go through many changes. Eventually the sand grabber was totally covered and beachgoers forgot it was there.

"As time went on it started eroding but it would always come back. But this time it isn't coming back," Fallau said. "They brought in sand and tried to fill it up, but you can't compete with Mother Nature."

But leaving the beach as it is could be dangerous to tourists and kids, said Steven and Carol-Ann Inouye, who heard about the problem and were at the beach recently to see the damage for themselves.

"I was surprised to see the tiles (of the sand grabber) and the erosion," Steven Inouye said. He had thought the sand grabber, a sort of wall made up of concrete tile blocks, was recently installed. "I don't think the tiles will stop big water from taking the sand."

The beach is part of the city's Kailua Beach Park. But the ocean and up to the high-water mark are the jurisdiction of the state.

The city has asked the state for recommendations about the sand grabber and whether to remove it or repair it, said Lester Chang, director of the city Department of Parks and Recreation. Chang said he's not sure that the sand grabber really worked but is deferring to the state because his department has no expertise on the issue.

When the erosion began, the city moved sand piling up at Kaelepulu Stream to the boat ramp area. However, the state Department of Health halted that, saying the sand, once moved, could not be allowed to come into contact with ocean water.

Kailua isn't the only beach that is eroding, Chang said. Areas in Hau'ula and Hale'iwa are experiencing similar problems. Over the years and at other beaches, he said, he's noticed that sand comes and goes — but what's happening at Kailua is different.

"Usually most of it comes back, but this last two years it seems like it's taking one step forward and three backward," he said.

At a workshop last week hosted by the Army Corps of Engineers on the broader subject of coastal sediment transport, Kailua's erosion problem was raised, said Joseph Bonfiglio, spokesman for the Army Corps.

"The workshop participants agreed on the need to form a focus group with all agencies involved to further discuss issues and alternatives to sediment management at the Kaelepulu Stream mouth," Bonfiglio said in an e-mail.

Chip Fletcher, a University of Hawai'i professor who is conducting a study of the historical shoreline changes of every beach on O'ahu, said the group needs to help develop best management practices for clearing sand from the stream mouth.

Fletcher said the erosion is chronic and creeping north but a plan to manage sand clearing might stem that progress.

"The longer we wait, the longer the erosion will work its way north and affect the entire beach," he said.